׉?<ׁB!בCט u׉׉ 7cassandra://6HMZEr-5bUv7VErgX07dIl4nuQawNwvnFCD1SWly6yA\`׉ 7cassandra://CU1xFz99l2ir2ZvIUjDJTubvM6VQRt2Tu8fm8ExPG78̈́`w׉ 7cassandra://KckTYOdk0py9zsB-ne6a__QtwgoYnlPXIOAaVolFwcs0`׉ 7cassandra://_zX3MmVHvU2RE_zlkSfcmqBOArVFror_fAoaq5WsAKUZ"͠mcZ%Eb™׈EcZ%Eb׉E׉ 7cassandra://KckTYOdk0py9zsB-ne6a__QtwgoYnlPXIOAaVolFwcs0`cZ%EbcZ%Eb בCטu׉׉ 7cassandra://Bfc7MK0fYvZXhGxDSiI2ZGsioNPRtBltUHjeHqalLPMvE`׉ 7cassandra://YY6t0Edv_vVJDUX-YyuLpLrtQkviNjohlFN9dTmanwY7d`׉ 7cassandra://XavIMwNp5Pyj9DI1AkLSJ8dW1Ug5pW4tV7YN5PgnVzY`׉ 7cassandra://nUUMCHWyW56uRFHoZjebG4IF3WGTWSZXscHOjG0uBiw:(͠cZ%EbNטu׉׉ 7cassandra://WlEC6t6wB7_6Cdze3F6f8RJPLeW2TNBQuRHU86ouTGkt`׉ 7cassandra://sg5AXRrF0HAXn9obGQT6eROIlPXvHkHHh9a5mGkS7tgW`v׉ 7cassandra://CWwsfW4aS0Deu7Y1xD1OGRG086lv5IrqriByO0zBcT8q`׉ 7cassandra://M2sPRJTnGZmW0cxwhCC-nyqsZfoh7EPX6qrwAmXsEs0O͠hcZ%EbOנcZ%Ebp9׉H)https://support.pearson.com/getsupport/s/Gׁ ׁ rנcZ%Eb_f̵9׉H$http://www.pearsonglobaleditions.comGׁ ׁ rנcZ%EbX}9׉H%http://www.pearsoned.com/permissions/Gׁ ׁ rנcZ%EbiH9׉H%http://www.pearsoned.com/permissions/Gׁ ׁ rנcZ%EbT? | 9ׁHhttp://www.pearsoned.com/ׁ ׁ ЈנcZ%EbS.̴ 9ׁH$http://www.pearsonglobaleditions.comׁ ׁ Ј׉EManufacturing Engineering and Technology •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• EIGHTH EDITION IN SI UNITS Serope Kalpakjian Illinois Institute ofTechnology Steven R. Schmid The University ofNotre Dame SI Contributions by K. S. Vijay Sekar Sri Sivasubramaniya Nadar College ofEngineering ׉ 7cassandra://XavIMwNp5Pyj9DI1AkLSJ8dW1Ug5pW4tV7YN5PgnVzY`cZ%Eb׉E ••••••••••• Hockham Way Harlow CM17 9SR United Kingdom and Associated Companies throughout the world Visit us on the World Wide Web at: www.pearsonglobaleditions.com © Pearson Education Limited 2023 The rights of Serope Kalpakjian and Steven R. Schmid to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. Authorized adaptation from the United States edition, entitled Manufacturing Engineering and Technology, 8th Edition, ISBN 978-0-13-668165-6 by Serope Kalpakjian and Steven R. Schmid published by Pearson Education © 2020. All rights reserved. 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Ltd. ׉ 7cassandra://CWwsfW4aS0Deu7Y1xD1OGRG086lv5IrqriByO0zBcT8q`cZ%EbcZ%EbבCט#u׉׉ 7cassandra://teHiBgRSynwwCYiBhVduyOHUSMiDI6L-fPnV6QjDeMc?`׉ 7cassandra://CdNJFUAYZpAbIHNbhIhkQ6f7cn2833t6EVbpiLGPABII`s׉ 7cassandra://dELp7riZtDo3Ov7XXsfiP8hzQEkX_on4X5JkkaUuyp8`׉ 7cassandra://m7mxRJz9fn7E_ZvJq6_WSZ54yHsj-rSfqqMt78ncPEI ͠^cZ%EbRט#u׉׉ 7cassandra://hXYgR1OpE7CSa1oQzjhbJBzm86sZZJzHnxHUFxaHW5E2`׉ 7cassandra://5W_YCYlhommakFewFzM4ZXa2wR33QbI-qf9prkhPrCI@`s׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`׉ 7cassandra://pVeKeFZqP0XFOMqtwzeYiv2erJZVLKiZJEVyk8w6SAM͠^cZ%EbU׉EODedicated to our families, whose patience and support made this book possible. ׉ 7cassandra://dELp7riZtDo3Ov7XXsfiP8hzQEkX_on4X5JkkaUuyp8`cZ%Eb׉E&This page is intentionally left blank ׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HaT7_QdfmZIsJSqJet6WPYS-W6RK29YhEdKeoj1vGeA)`׉ 7cassandra://K_YbOB8hLou-0zlMb2iGpyFgA0euZqcRuedmfb92PVs<`s׉ 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•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Preface 22 About the Authors 25 General Introduction 27 I.1 What Is Manufacturing? 27 I.2 Product Design and Concurrent Engineering 34 I.3 Design for Manufacture, Assembly, Disassembly, and Service 39 I.4 Environmentally Conscious Design, Sustainable Manufacturing, and Product Life Cycle 40 I.5 Selection of Materials 42 I.6 Selection of Manufacturing Processes 44 I.7 Computer-integrated Manufacturing 52 I.8 Quality Assurance and Total Quality Management 55 I.9 Lean Production and Agile Manufacturing 57 I.10 Manufacturing Costs and Global Competition 57 I.11 Trends in Manufacturing 59 Part I: Fundamentals of Materials: Behavior and Manufacturing Properties 61 1 The Structure of Metals 63 1.1 Introduction 63 1.2 Types of Atomic Bonds 65 1.3 The Crystal Structure of Metals 65 1.4 Deformation and Strength of Single Crystals 67 1.5 Grains and Grain Boundaries 71 1.6 Plastic Deformation of Polycrystalline Metals 73 1.7 Recovery, Recrystallization, and Grain Growth 75 1.8 Cold, Warm, and Hot Working 76 Summary 77 Key Terms 77 Bibliography 78 Review Questions 78 Qualitative Problems 79 Quantitative Problems 80 Synthesis, Design, and Projects 81 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 82 2.1 Introduction 83 2.2 Tension 83 5 ׉ 7cassandra://7cF947D-3vt7xQ1Q3cP0Eq0uZd6IqOK0InA5gLuVi50`cZ%Ebl׉Eu•••••••• 6 Contents 2.3 Compression 94 2.4 Torsion 96 2.5 Bending (Flexure) 96 2.6 Hardness 97 2.7 Fatigue 102 2.8 Creep 105 2.9 Impact 105 2.10 Failure and Fracture of Materials 106 2.11 Residual Stresses 112 2.12 Work, Heat, and Temperature 114 Summary 114 Key Terms 115 Bibliography 116 Review Questions 116 Qualitative Problems 117 Quantitative Problems 119 Synthesis, Design, and Projects 121 3 Physical Properties of Materials 123 3.1 Introduction 123 3.2 Density 124 3.3 Melting Point 124 3.4 Specific Heat 125 3.5 Thermal Conductivity 127 3.6 Thermal Expansion 127 3.7 Electrical, Magnetic, and Optical Properties 129 3.8 Corrosion Resistance 130 Summary 132 Key Terms 132 Bibliography 133 Review Questions 133 Qualitative Problems 133 Quantitative Problems 134 Synthesis, Design, and Projects 135 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 136 4.1 Introduction 137 4.2 Structure of Alloys 138 4.3 Phase Diagrams 139 4.4 The Iron–Carbon System 142 4.5 The Iron–Iron-carbide Phase Diagram and the Development of Microstructures in Steels 143 4.6 Cast Irons 145 4.7 Heat Treatment of Ferrous Alloys 147 4.8 Hardenability of Ferrous Alloys 149 4.9 Heat Treatment of Nonferrous Alloys and Stainless Steels 153 4.10 Case Hardening 155 4.11 Annealing 155 4.12 Heat Treating Furnaces and Equipment 159 4.13 Design Considerations for Heat Treating 160 ׉ 7cassandra://Uo-T_tl7N1IqDJZ31cfZooapYf9RaoyK5fH9nlZuRzs&`cZ%EbmcZ%Ebl#בCט#u׉׉ 7cassandra://8tzWSquu9RLASkoDXAyRDhmAxf-oHS9e9sL6IDDqy9od`׉ 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rנcZ&Eb̽9׉SG ׁ ׁ rנcZ&Ebe9׉SG ׁ ׁ rנcZ&EbN9׉SG ׁ ׁ rנcZ&Eb2b9׉S G ׁ ׁ rנcZ&EbJ9׉S G ׁ ׁ rנcZ&Eba.9׉SG ׁ ׁ rנcZ&Ebx9׉SG ׁ ׁ rנcZ&Ebl9׉SG ׁ ׁ rנcZ&Eb1q9׉SG ׁ ׁ rנcZ&Eb̀9׉SG ׁ ׁ rנcZ&Eb̤9׉SG ׁ ׁ rנcZ&Eb i̸9׉SG ׁ ׁ rנcZ&Eb 9׉SG ׁ ׁ rנcZ&Eb 9׉SG ׁ ׁ r׉EEContents 7 Summary 160 Key Terms 161 Bibliography 162 Review Questions 162 Qualitative Problems 163 Quantitative Problems 163 Synthesis, Design, and Projects 164 5 Ferrous Metals and Alloys: Production, General Properties, and Applications 165 5.1 Introduction 165 5.2 Production of Iron and Steel 166 5.3 Casting of Ingots 169 5.4 Continuous Casting 170 5.5 Carbon and Alloy Steels 172 5.6 Stainless Steels 179 5.7 Tool and Die Steels 181 Summary 183 Key Terms 184 Bibliography 184 Review Questions 185 Qualitative Problems 185 Quantitative Problems 186 Synthesis, Design, and Projects 186 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications 188 6.1 Introduction 189 6.2 Aluminum and Aluminum Alloys 191 6.3 Magnesium and Magnesium Alloys 194 6.4 Copper and Copper Alloys 195 6.5 Nickel and Nickel Alloys 196 6.6 Superalloys 197 6.7 Titanium and Titanium Alloys 198 6.8 Refractory Metals and Alloys 199 6.9 Beryllium 200 6.10 Zirconium 201 6.11 Lithium 201 6.12 Low-melting Alloys 201 6.13 Precious Metals 202 6.14 Shape-memory Alloys (Smart Materials) 203 6.15 Amorphous Alloys (Metallic Glasses) 203 6.16 Metal Foams and Metamaterials 203 6.17 Rare Earth Metals 205 Summary 205 Key Terms 206 Bibliography 206 Review Questions 207 Qualitative Problems 207 Quantitative Problems 208 Synthesis, Design, and Projects 208 ׉ 7cassandra://MyNMm6fOcTgRe4kQBHfayt5IRgOsSy3LI8aeMI2m0vM`cZ%Eb׉E8 Contents 7 Polymers: Structure, General Properties, and Applications 210 7.1 Introduction 211 7.2 The Structure of Polymers 211 7.3 Thermoplastics 220 7.4 Thermosetting Plastics 224 7.5 Additives in Plastics 224 7.6 General Properties and Applications of Thermoplastics 225 7.7 General Properties and Applications of Thermosetting Plastics 227 7.8 Biodegradable Plastics 229 7.9 Elastomers (Rubbers) 230 7.10 Gels 231 Summary 231 Key Terms 232 Bibliography 233 Review Questions 233 Qualitative Problems 234 Quantitative Problems 235 Synthesis, Design, and Projects 235 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications 237 8.1 Introduction 238 8.2 The Structure of Ceramics 238 8.3 General Properties and Applications of Ceramics 243 8.4 Glasses 247 8.5 Glass Ceramics 249 8.6 Graphite 249 8.7 Diamond 251 8.8 Nanomaterials 252 Summary 253 Key Terms 254 Bibliography 254 Review Questions 255 Qualitative Problems 256 Quantitative Problems 256 Synthesis, Design, and Projects 257 9 Composite Materials: Structure, General Properties, and Applications 258 9.1 Introduction 259 9.2 The Structure of Reinforced Plastics 259 9.3 Properties of Reinforced Plastics 264 9.4 Applications of Reinforced Plastics 267 9.5 Metal-matrix Composites 269 9.6 Ceramic-matrix Composites 271 9.7 Other Composites 271 Summary 272 Key Terms 273 Bibliography 273 Review Questions 274 Qualitative Problems 274 Quantitative Problems 275 Synthesis, Design, and Projects 276 ׉ 7cassandra://be1bXTcH9pqEzJMrhhATAGH2-R5Lm1GyvR9lBJBP9XAs`cZ%EbρcZ%Eb΁#בCט#u׉׉ 7cassandra://N5c4V6pxg426CMJcOsguCz4omEUbGW-VVqYjoCNOVlUl`׉ 7cassandra://nL0k2Ut-8CuZ4S5y9NXFDBOuaq75rIXYhLjF_T67-iA4`s׉ 7cassandra://hZtMUlaQ12YLnzRuDWSRnTlSPNzg3AKtEr0aLpFvXEkc`׉ 7cassandra://DZ7OWRug7LSr0jo4QFJoytMEkzvyjQDYSGvbh1hODH8KR͠^cZ&Ebmט#u׉׉ 7cassandra://SjWGiotpMh3R2onCOxqgpIa3y8Qq0oPT1clXRIcySn8`׉ 7cassandra://Adh1IGvyRGa5WLBt-vPr5Bb03ULJCqOuO9mHzEocY9M<``s׉ 7cassandra://h1hfLebH62O9_EJPZanTze9n2VauDDnXQ71Xv9zBSzM`׉ 7cassandra://GlD-J-dg4n6K7Qg38QeJugLEv2mmGAcu4_D3trSPnsEMy͠^cZ&Ebn^נcZ&Ebx̱=9׉SG ׁ ׁ rנcZ&Eby$9׉SG ׁ ׁ rנcZ&Eḇ(̽9׉SG ׁ ׁ rנcZ&Eb̰> 9׉SG ׁ ׁ rנcZ&EḇU̱9׉SG ׁ ׁ rנcZ&Eb̰m9׉S#G ׁ ׁ rנcZ&Eb̰9׉S$G ׁ ׁ rנcZ&Eḇ̕9׉S'G ׁ ׁ rנcZ&Eb̰n9׉S*G ׁ ׁ rנcZ&Eb6q9׉S+G ׁ ׁ rנcZ&Eb̀9׉S+G ׁ ׁ rנcZ&Eb̰ǁ̤9׉S,G ׁ ׁ rנcZ&Ebmǁ̹9׉S,G ׁ ׁ 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rנcZ&EbÙ9׉SG ׁ ׁ rנcZ&EbV̤9׉SG ׁ ׁ rנcZ&EbWi̸9׉SG ׁ ׁ rנcZ&EbX9׉SG ׁ ׁ rנcZ&EbY9׉SG ׁ ׁ rנcZ&EbZt9@9׉SG ׁ ׁ rנcZ&Eb[̼ 9׉SG ׁ ׁ rנcZ&Eb\ 9׉SG ׁ ׁ rנcZ&Eb] 9׉SG ׁ ׁ rנcZ&Eb^ʁ9׉SG ׁ ׁ rנcZ&Eb_ 9׉SG ׁ ׁ rנcZ&Eb`9׉SG ׁ ׁ rנcZ&Eba9׉SG ׁ ׁ rנcZ&Ebb'9׉SG ׁ ׁ rנcZ&Ebc?9׉SG ׁ ׁ rנcZ&EbdV}9׉SG ׁ ׁ rנcZ&Ebem9׉SG ׁ ׁ rנcZ&Ebfl9׉SG ׁ ׁ rנcZ&Ebg2q9׉SG ׁ ׁ rנcZ&Ebh9׉SG ׁ ׁ rנcZ&Ebi̤9׉SG ׁ ׁ rנcZ&Ebjj̸9׉SG ׁ ׁ rנcZ&Ebk9׉SG ׁ ׁ rנcZ&Ebl9׉SG ׁ ׁ r׉EContents 9 Part II: Metal-casting Processes and Equipment 279 10 Fundamentals of Metal Casting 281 10.1 Introduction 282 10.2 Solidification of Metals 282 10.3 Fluid Flow 287 10.4 Fluidity of Molten Metal 291 10.5 Heat Transfer 292 10.6 Defects 295 Summary 298 Key Terms 299 Bibliography 299 Review Questions 300 Qualitative Problems 300 Quantitative Problems 302 Synthesis, Design, and Projects 303 11 Metal-casting Processes and Equipment 304 11.1 Introduction 305 11.2 Expendable-mold, Permanent-pattern Casting Processes 306 11.3 Expendable-mold, Expendable-pattern Casting Processes 316 11.4 Permanent-mold Casting Processes 321 11.5 Casting Techniques for Single-crystal Components 331 11.6 Rapid Solidification 333 11.7 Inspection of Castings 333 11.8 Melting Practice and Furnaces 334 11.9 Foundries and Foundry Automation 335 Summary 335 Key Terms 336 Bibliography 336 Review Questions 337 Qualitative Problems 337 Quantitative Problems 338 Synthesis, Design, and Projects 340 12 Metal Casting: Design, Materials, and Economics 342 12.1 Introduction 342 12.2 Design Considerations in Casting 343 12.3 Casting Alloys 351 12.4 Economics of Casting 356 Summary 356 Key Terms 357 Bibliography 357 Review Questions 357 Qualitative Problems 358 Quantitative Problems 358 Synthesis, Design, and Projects 359 ׉ 7cassandra://hZtMUlaQ12YLnzRuDWSRnTlSPNzg3AKtEr0aLpFvXEkc`cZ%Eb.׉E10 Contents Part III: Forming and Shaping Processes and Equipment 363 13 Metal-rolling Processes and Equipment 365 13.1 Introduction 365 13.2 The Flat-rolling Process 367 13.3 Flat-rolling Practice 373 13.4 Rolling Mills 376 13.5 Various Rolling Processes and Mills 378 Summary 383 Key Terms 384 Bibliography 384 Review Questions 385 Qualitative Problems 385 Quantitative Problems 386 Synthesis, Design, and Projects 387 14 Metal-forging Processes and Equipment 388 14.1 Introduction 389 14.2 Open-die Forging 390 14.3 Impression-die and Closed-die Forging 393 14.4 Various Forging Operations 397 14.5 Forgeability of Metals; Forging Defects 401 14.6 Die Design, Die Materials, and Lubrication 403 14.7 Die-manufacturing Methods and Die Failures 405 14.8 Forging Machines 407 14.9 Economics of Forging 408 Summary 412 Key Terms 412 Bibliography 413 Review Questions 413 Qualitative Problems 413 Quantitative Problems 414 Synthesis, Design, and Projects 415 15 Metal Extrusion and Drawing Processes and Equipment 417 15.1 Introduction 418 15.2 The Extrusion Process 419 15.3 Hot Extrusion 422 15.4 Cold Extrusion 425 15.5 Extrusion Defects 428 15.6 Design Considerations 430 15.7 Extrusion Equipment 431 15.8 The Drawing Process 433 15.9 Drawing Practice 434 15.10 Drawing Defects and Residual Stresses 436 15.11 Drawing Equipment 436 Summary 438 Key Terms 438 Bibliography 438 Review Questions 439 Qualitative Problems 439 Quantitative Problems 440 Synthesis, Design, and Projects 441 ׉ 7cassandra://h1hfLebH62O9_EJPZanTze9n2VauDDnXQ71Xv9zBSzM`cZ%Eb/cZ%Eb.#בCט#u׉׉ 7cassandra://SRgXxaTAN8TGi7kG-UILE9F92CcsiYVioK3LofryKT0z`׉ 7cassandra://PQQRFUJvYcQLKk7RkBF749aRF3v6eoGxALsziUKvQDc=$`s׉ 7cassandra://Nx552ocSJW_QC3faL6EFCP-Wl3OFW7T8rgY1TCkDMv8`׉ 7cassandra://f5eOa8DL28ghZASCR8cEg-xpuvEy1-e2MDEuPws-Sg4K͠^cZ&Ebט#u׉׉ 7cassandra://WSTZY1GqBsKySmaC65YxZo50k_Pq1j_ZuO-orxTkEz8`׉ 7cassandra://LSZUhokIIa-OmHzYS8wYIkDAFPcXjS8-BOzFI7yMamQ7`s׉ 7cassandra://fTyf87pWJkh2lTq4hIg_rTXIgSaoH7wPoOc5G6qHuig`׉ 7cassandra://opTg0AbDB5bEtxtM2j-Zavvdu1ybV3voYc5SHmvRKnAK͠^cZ&EbaנcZ&Ebpy̱?9׉SG ׁ ׁ rנcZ&Ebq̱̻ 9׉SG ׁ ׁ rנcZ&Ebṟ̠9׉SG ׁ ׁ rנcZ&Ebs̱+9׉SŁG ׁ ׁ rנcZ&EbṯBY9׉SȁG ׁ ׁ 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Plates, and Tubes 458 16.6 Miscellaneous Bending and Related Forming Operations 463 16.7 Deep Drawing 469 16.8 Rubber Forming and Hydroforming 476 16.9 Spinning 481 16.10 Superplastic Forming 484 16.11 Hot Stamping 485 16.12 Specialized Forming Processes 487 16.13 Manufacturing of Metal Honeycomb Structures 491 16.14 Design Considerations in Sheet-metal Forming 493 16.15 Equipment for Sheet-metal Forming 496 16.16 Economics of Sheet-forming Operations 497 Summary 498 Key Terms 499 Bibliography 500 Review Questions 500 Qualitative Problems 501 Quantitative Problems 502 Synthesis, Design, and Projects 504 17 Powder-metal Processes and Equipment 506 17.1 Introduction 507 17.2 Production of Metal Powders 508 17.3 Compaction of Metal Powders 514 17.4 Sintering 524 17.5 Secondary and Finishing Operations 526 17.6 Design Considerations 528 17.7 Economics of Powder Metallurgy 530 Summary 534 Key Terms 534 Bibliography 535 Review Questions 535 Qualitative Problems 536 Quantitative Problems 536 Synthesis, Design, and Projects 537 18 Ceramics, Glasses, and Superconductors: Processing and Equipment 539 18.1 Introduction 540 18.2 Shaping Ceramics 540 18.3 Forming and Shaping of Glass 547 18.4 Techniques for Strengthening and Annealing Glass 551 18.5 Design Considerations for Ceramics and Glasses 553 18.6 Processing of Superconductors 554 ׉ 7cassandra://Nx552ocSJW_QC3faL6EFCP-Wl3OFW7T8rgY1TCkDMv8`cZ%Eb׉E12 Contents Summary 555 Key Terms 556 Bibliography 556 Review Questions 556 Qualitative Problems 557 Quantitative Problems 558 Synthesis, Design, and Projects 558 19 Plastics and Composite Materials: Forming and Shaping 559 19.1 Introduction 560 19.2 Extrusion 562 19.3 Injection Molding 569 19.4 Blow Molding 574 19.5 Rotational Molding 574 19.6 Thermoforming 576 19.7 Compression Molding 577 19.8 Transfer Molding 578 19.9 Casting 579 19.10 Foam Molding 580 19.11 Cold Forming and Solid-phase Forming 581 19.12 Processing Elastomers 581 19.13 Processing Polymer-matrix Composites 582 19.14 Processing Metal-matrix and Ceramic-matrix Composites 590 19.15 Design Considerations 592 19.16 Economics of Processing Plastics and Composite Materials 594 Summary 595 Key Terms 595 Bibliography 596 Review Questions 597 Qualitative Problems 597 Quantitative Problems 598 Synthesis, Design, and Projects 599 20 Additive Manufacturing 600 20.1 Introduction 601 20.2 Additive Manufacturing Methodology 603 20.3 Extrusion-based Processes 606 20.4 Photopolymerization 608 20.5 Material Jetting 611 20.6 Powder Bed Processes 612 20.7 Laminated-object Manufacturing 617 20.8 Miscellaneous Processes 617 20.9 Emerging AM Applications 619 20.10 Direct Manufacturing and Rapid Tooling 619 20.11 Design for Additive Manufacturing 624 20.12 Additive Manufacturing Economics 626 Summary 629 Key Terms 629 Bibliography 630 Review Questions 630 Qualitative Problems 631 Quantitative Problems 631 Synthesis, Design, and Projects 632 ׉ 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ׁ ׁ rנcZ&Eb#iX̸9׉S+G ׁ ׁ rנcZ&Eb$m9׉S,G ׁ ׁ rנcZ&Eb%m9׉S,G ׁ ׁ rנcZ&Eb&tB9׉S-G ׁ ׁ rנcZ&Eb'ށ̼9׉S.G ׁ ׁ rנcZ&Eb(T9׉S0G ׁ ׁ rנcZ&Eb) 9׉S6G ׁ ׁ rנcZ&Eb*$b9׉S?G ׁ ׁ rנcZ&Eb+:g9׉SHG ׁ ׁ rנcZ&Eb,R9׉SHG ׁ ׁ rנcZ&Eb-i9׉SIG ׁ ׁ rנcZ&Eb.9׉SNG ׁ ׁ rנcZ&Eb/9׉SPG ׁ ׁ r׉EContents 13 Part IV: Machining Processes and Machine Tools 633 21 Fundamentals of Machining 636 21.1 Introduction 637 21.2 Mechanics of Cutting 639 21.3 Cutting Forces and Power 648 21.4 Temperatures in Cutting 652 21.5 Tool Life: Wear and Failure 654 21.6 Surface Finish and Integrity 661 21.7 Machinability 664 Summary 667 Key Terms 668 Bibliography 668 Review Questions 669 Qualitative Problems 669 Quantitative Problems 671 Synthesis, Design, and Projects 673 22 Cutting-tool Materials and Cutting Fluids 674 22.1 Introduction 675 22.2 High-speed Steels 678 22.3 Cast-cobalt Alloys 678 22.4 Carbides 679 22.5 Coated Tools 682 22.6 Alumina-based Ceramics 686 22.7 Cubic Boron Nitride 687 22.8 Silicon-Nitride-based Ceramics 687 22.9 Diamond 688 22.10 Whisker-reinforced Materials and Nanomaterials 688 22.11 Tool Costs and Reconditioning of Tools 689 22.12 Cutting Fluids 689 Summary 694 Key Terms 695 Bibliography 695 Review Questions 696 Qualitative Problems 696 Quantitative Problems 697 Synthesis, Design, and Projects 698 23 Machining Processes: Turning and Hole Making 699 23.1 Introduction 700 23.2 The Turning Process 701 23.3 Lathes and Lathe Operations 713 23.4 Boring and Boring Machines 727 23.5 Drilling, Drills, and Drilling Machines 728 23.6 Reaming and Reamers 737 23.7 Tapping and Taps 738 ׉ 7cassandra://w69wAIO1hDL_ojTSoOO39h973Md1dVqcyfFGrk3MqTA`cZ%Eb׉E14 Contents Summary 740 Key Terms 741 Bibliography 741 Review Questions 741 Qualitative Problems 742 Quantitative Problems 743 Synthesis, Design, and Projects 744 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 745 24.1 Introduction 746 24.2 Milling and Milling Machines 746 24.3 Planing and Shaping 763 24.4 Broaching and Broaching Machines 763 24.5 Sawing 766 24.6 Filing 769 24.7 Gear Manufacturing by Machining 770 Summary 780 Key Terms 781 Bibliography 781 Review Questions 781 Qualitative Problems 782 Quantitative Problems 783 Synthesis, Design, and Projects 784 25 Machining Centers, Machine-tool Structures, and Machining 785 25.1 Introduction 785 25.2 Machining Centers 786 25.3 Machine-tool Structures 793 25.4 Vibration and Chatter in Machining Operations 798 25.5 High-speed Machining 803 25.6 Hard Machining 804 25.7 Ultraprecision Machining 804 25.8 Machining Economics 805 Summary 809 Key Terms 809 Bibliography 810 Review Questions 810 Qualitative Problems 811 Quantitative Problems 812 Synthesis, Design, and Projects 812 26 Abrasive Machining and Finishing Operations 813 26.1 Introduction 814 26.2 Abrasives and Bonded Abrasives 816 26.3 The Grinding Process 822 26.4 Grinding Operations and Machines 831 26.5 Design Considerations for Grinding 840 26.6 Ultrasonic Machining 840 26.7 Finishing 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Machining 875 27.8 Water-jet Machining 876 27.9 Abrasive-jet Machining 878 27.10 Hybrid Machining Systems 878 27.11 Economics of Advanced Machining Processes 879 Summary 881 Key Terms 883 Bibliography 883 Review Questions 884 Qualitative Problems 884 Quantitative Problems 885 Synthesis, Design, and Projects 885 Part V: Micromanufacturing and Fabrication of Microelectronic Devices 887 28 Fabrication of Microelectronic Devices 890 28.1 Introduction 891 28.2 Clean Rooms 891 28.3 Semiconductors and Silicon 895 28.4 Crystal Growing and Wafer Preparation 896 28.5 Film Deposition 897 28.6 Oxidation 900 28.7 Lithography 901 28.8 Etching 908 28.9 Diffusion and Ion Implantation 916 28.10 Metallization and Testing 918 28.11 Wire Bonding and Packaging 920 28.12 Yield and Reliability 925 28.13 Printed Circuit Boards 927 28.14 Roll-to-Roll Printing of Flexible Electronics 928 28.15 Flexible Hybrid Electronics 930 Summary 932 Key Terms 932 Bibliography 933 Review Questions 934 Qualitative Problems 934 Quantitative Problems 935 Synthesis, Design, and Projects 936 ׉ 7cassandra://UXGzTuxFYTSU4PIHJ1kwanhvYYG3L-v5FvnbHWryh-k `cZ%EbT׉E16 Contents 29 Fabrication of Microelectromechanical Devices and Systems and Nanoscale Manufacturing 938 29.1 Introduction 939 29.2 Micromachining of MEMS Devices 940 29.3 Electroforming-based Processes 952 29.4 Solid Free-form Fabrication of Devices 957 29.5 Mesoscale Manufacturing 960 29.6 Nanoscale Manufacturing 962 Summary 964 Key Terms 964 Bibliography 964 Review Questions 965 Qualitative Problems 965 Quantitative Problems 966 Synthesis, Design, and Projects 966 Part VI: Joining Processes and Equipment 968 30 Fusion Welding Processes 972 30.1 Introduction 973 30.2 Oxyfuel–Gas Welding 973 30.3 Arc-welding Processes: Nonconsumable Electrode 977 30.4 Arc-welding Processes: Consumable Electrode 980 30.5 Electrodes for Arc Welding 985 30.6 Electron-beam Welding 987 30.7 Laser-beam Welding 988 30.8 Cutting 990 30.9 The Weld Joint, Quality and Testing 991 30.10 Joint Design and Process Selection 1001 Summary 1004 Key Terms 1005 Bibliography 1006 Review Questions 1006 Qualitative Problems 1007 Quantitative Problems 1008 Synthesis, Design, and Projects 1008 31 Solid-state Welding Processes 1011 31.1 Introduction 1012 31.2 Cold Welding and Roll Bonding 1012 31.3 Ultrasonic Welding 1013 31.4 Friction Welding 1014 31.5 Resistance Welding 1016 31.6 Explosion Welding 1025 31.7 Diffusion Bonding 1025 31.8 Economics of Welding Operations 1028 Summary 1030 Key Terms 1031 Bibliography 1031 Review Questions 1031 Qualitative Problems 1032 Quantitative Problems 1033 Synthesis, Design, and Projects 1034 ׉ 7cassandra://gseyaD_Y2IMXKsCqEqQ7TS4TDPlh8YQ1Prpvssd-fdE`cZ%EbUcZ%EbT#בCט#u׉׉ 7cassandra://KOyKGpk7_AIfcpYEmVR95j7zF6NqFaAnQnavsMEz-n8`׉ 7cassandra://UmJWPr2spauGvscU9QsDxO0CPoOA6GKEPGAIOwGYAoA:`s׉ 7cassandra://axIitHH3YTdmi7cqldoZ05u71CA8xI2kQl01Nh30f440`׉ 7cassandra://BG2w19VVjNbf6dpFzZbebAF7W7tllR5djNBV1dwtiZwQ$͠^cZ'Ebט#u׉׉ 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rנcZ'Eb̰N9׉SG ׁ ׁ rנcZ'Eb̰e9׉SG ׁ ׁ rנcZ'EbÁ̰|9׉SG ׁ ׁ rנcZ'Ebā̰9׉SG ׁ ׁ rנcZ'EbŁ̵9׉SG ׁ ׁ rנcZ'EbƁ́9׉SG ׁ ׁ rנcZ'Ebǁ9׉SG ׁ ׁ rנcZ'Ebȁ̦9׉SG ׁ ׁ rנcZ'EbɁ9׉SG ׁ ׁ rנcZ'Ebʁ*t9׉SG ׁ ׁ rנcZ'Ebˁ:*y9׉SG ׁ ׁ rנcZ'Eb́)̇9׉SG ׁ ׁ rנcZ'Eb́>̫9׉SG ׁ ׁ rנcZ'Eb΁r=̿9׉SG ׁ ׁ rנcZ'EbρR9׉SG ׁ ׁ rנcZ'EbЁQ9׉SG ׁ ׁ r׉EkContents 17 32 Brazing, Soldering, Adhesive-bonding, and Mechanical Fastening Processes 1035 32.1 Introduction 1036 32.2 Brazing 1037 32.3 Soldering 1040 32.4 Adhesive Bonding 1045 32.5 Mechanical Fastening 1053 32.6 Joining Plastics, Ceramics, and Glasses 1057 32.7 Economics of Joining Operations 1060 Summary 1061 Key Terms 1061 Bibliography 1062 Review Questions 1062 Qualitative Problems 1063 Quantitative Problems 1063 Synthesis, Design, and Projects 1064 Part VII: Surface Technology 1065 33 Surface Roughness and Measurement; Friction, Wear, and Lubrication 1067 33.1 Introduction 1068 33.2 Surface Structure and Integrity 1068 33.3 Surface Texture and Roughness 1070 33.4 Friction 1073 33.5 Wear 1077 33.6 Lubrication 1081 33.7 Metalworking Fluids and Their Selection 1083 Summary 1086 Key Terms 1086 Bibliography 1087 Review Questions 1088 Qualitative Problems 1088 Quantitative Problems 1089 Synthesis, Design, and Projects 1089 34 Surface Treatments, Coatings, and Cleaning 1091 34.1 Introduction 1092 34.2 Mechanical Surface Treatments 1092 34.3 Mechanical Plating and Cladding 1094 34.4 Case Hardening and Hard Facing 1094 34.5 Thermal Spraying 1095 34.6 Vapor Deposition 1096 34.7 Ion Implantation and Diffusion Coating 1099 34.8 Laser Treatments 1099 34.9 Electroplating, Electroless Plating, and Electroforming 1100 34.10 Conversion Coatings 1104 34.11 Hot Dipping 1104 ׉ 7cassandra://axIitHH3YTdmi7cqldoZ05u71CA8xI2kQl01Nh30f440`cZ%Eb׉E18 Contents 34.12 Porcelain Enameling; Ceramic and Organic Coatings 1104 34.13 Diamond Coating and Diamondlike Carbon 1106 34.14 Surface Texturing 1107 34.15 Painting 1107 34.16 Cleaning of Surfaces 1108 Summary 1109 Key Terms 1109 Bibliography 1110 Review Questions 1110 Qualitative Problems 1111 Quantitative Problems 1111 Synthesis, Design, and Projects 1111 Part VIII: Engineering Metrology, Instrumentation, and Quality Assurance 1113 35 Surface Treatments, Coatings, and Cleaning 1114 35.1 Introduction 1115 35.2 Measurement Standards 1115 35.3 Geometric Features of Parts: Analog and Digital Measurements 1116 35.4 Traditional Measuring Methods and Instruments 1116 35.5 Modern Measuring Instruments and Machines 1123 35.6 Automated Measurement 1126 35.7 General Characteristics and Selection of Measuring Instruments 1128 35.8 Geometric Dimensioning and Tolerancing 1129 Summary 1134 Key Terms 1135 Bibliography 1135 Review Questions 1135 Qualitative Problems 1136 Quantitative Problems 1137 Synthesis, Design, and Projects 1137 36 Quality Assurance, Testing, and Inspection 1139 36.1 Introduction 1140 36.2 Product Quality 1140 36.3 Quality Assurance 1141 36.4 Total Quality Management 1141 36.5 Taguchi Methods 1143 36.6 The ISO and QS Standards 1147 36.7 Statistical Methods of Quality Control 1148 36.8 Statistical Process Control 1151 36.9 Reliability of Products and Processes 1157 36.10 Nondestructive Testing 1158 36.11 Destructive Testing 1162 36.12 Automated Inspection 1162 ׉ 7cassandra://XiXcCPKVZnS2-mSu0i5TYJX6jkgV3SukrXTCY9UEpgM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://W8me1Bc-OZgba20ymms7qfj9BCgWlGHfKqM-1H_eayA`׉ 7cassandra://1CskIgRvNdt40wzVZA1XkrfT92HzFnZVgrOwc-G4P_A;u`s׉ 7cassandra://dJ4UAvaTDCgsmYymAlpemg1UquoOAbhYEnVv1j-qof0`׉ 7cassandra://-y02otSpN2O--IEye6bgNxtkRouihvwHurTlCe8HSG4Ob͠^cZ'EbMט#u׉׉ 7cassandra://Eu8FLW9oReC-U7_sq4mpTu0Dmp73RAPZ2aaXoxn3YIo`׉ 7cassandra://PmBejyD005caZ5dFnLEeot6GRVIOC3RgDMv1-Ii2K1Y9`s׉ 7cassandra://LpWNxt1VRcx8wFiU6VLNoHAt2oGfZbUoBdCoRT0uJRI`׉ 7cassandra://lMRz2fvpaCCQ-oLrsHpZf8DK-bInVumGK__dzNIWXsIUZ͠^cZ'EbNנcZ'Eb>t,̉9׉SG ׁ ׁ rנcZ'Eb?]9׉S&G ׁ ׁ rנcZ'Eb@̠9׉S,G ׁ ׁ rנcZ'EbA 9׉S4G ׁ ׁ rנcZ'EbBˁ39׉S̃G ׁ ׁ rנcZ'EbC߁9׉S̳G ׁ ׁ rנcZ'EbD19׉S̵G ׁ ׁ rנcZ'EbE9׉SG ׁ ׁ rנcZ'EbF9׉SG ׁ ׁ rנcZ'EbG.F9׉SفG ׁ ׁ rנcZ'EbHBk9׉SG ׁ ׁ rנcZ'EbIW9׉SG ׁ ׁ rנcZ'EbJk@9׉SG ׁ ׁ rנcZ'EbK9׉S G ׁ ׁ rנcZ'EbL9׉SG ׁ ׁ r׉EContents 19 Summary 1163 Key Terms 1163 Bibliography 1164 Review Questions 1164 Qualitative Problems 1165 Quantitative Problems 1165 Synthesis, Design, and Projects 1166 Part IX: Manufacturing in a Competitive Environment 1167 37 Automation of Manufacturing Processes and Operations 1168 37.1 Introduction 1169 37.2 Automation 1170 37.3 Numerical Control 1177 37.4 Adaptive Control 1184 37.5 Material Handling and Movement 1186 37.6 Industrial Robots 1188 37.7 Sensor Technology 1195 37.8 Flexible Fixturing 1199 37.9 Assembly Systems 1200 37.10 Design Considerations for Fixturing, Assembly, Disassembly, and Servicing 1203 37.11 Economic Considerations 1206 Summary 1207 Key Terms 1207 Bibliography 1208 Review Questions 1209 Qualitative Problems 1209 Quantitative Problems 1210 Synthesis, Design, and Projects 1210 38 Computer-aided Manufacturing 1212 38.1 Introduction 1213 38.2 Manufacturing Systems 1213 38.3 Computer-integrated Manufacturing 1214 38.4 Computer-aided Design and Engineering 1216 38.5 Computer-aided Manufacturing 1220 38.6 Computer-aided Process Planning 1221 38.7 Computer Simulation of Manufacturing Processes and Systems 1223 38.8 Group Technology 1224 Summary 1231 Key Terms 1231 Bibliography 1232 Review Questions 1232 Qualitative Problems 1233 Synthesis, Design, and Projects 1233 39 Computer-integrated Manufacturing Systems 1234 39.1 Introduction 1235 39.2 Cellular Manufacturing 1235 39.3 Flexible Manufacturing Systems 1236 ׉ 7cassandra://dJ4UAvaTDCgsmYymAlpemg1UquoOAbhYEnVv1j-qof0`cZ%Eb׉EC20 Contents 39.4 Mass Customization 1239 39.5 Holonic Manufacturing 1240 39.6 Just-in-time Production 1241 39.7 Lean Manufacturing 1243 39.8 Communications Networks in Manufacturing 1244 39.9 Artificial Intelligence and Machine Learning 1246 39.10 Economic Considerations 1248 Summary 1249 Key Terms 1249 Bibliography 1250 Review Questions 1250 Qualitative Problems 1251 Synthesis, Design, and Projects 1251 40 Product Design and Manufacturing in a Competitive Environment 1253 40.1 Introduction 1254 40.2 Product Design 1254 40.3 Product Quality 1257 40.4 Life-cycle Assessment and Sustainable Manufacturing 1258 40.5 Energy Consumption in Manufacturing 1259 40.6 Material Selection for Products 1262 40.7 Material Substitution 1265 40.8 Manufacturing Process Capabilities 1267 40.9 Process Selection 1269 40.10 Manufacturing Costs and Cost Reduction 1272 Summary 1277 Key Terms 1277 Bibliography 1278 Review Questions 1278 Qualitative Problems 1279 Synthesis, Design, and Projects 1280 Index 1283 Case Studies I.1: Three-dimensional Printing of Guitars 38 I.2: U.S. Pennies 44 I.3: Saltshaker and Pepper Mill 52 3.1: Selection of Materials for Coins 131 5.1: Advanced High-strength Steels in Automobiles 179 5.2: Stainless Steels in Automobiles 181 6.1: Aluminum Production from Ore: The Hall-H´eroult Process 192 6.2: The Tesla Model S 60 193 7.1: Dental and Medical Bone Cement 217 7.2: Materials for a Refrigerator Door Liner 228 8.1: Ceramic Knives 241 8.2: Ceramic Ball and Roller Bearings 246 9.1: Composite Military Helmets and Body Armor 268 9.2: Aluminum-matrix Composite Brake Calipers 270 ׉ 7cassandra://LpWNxt1VRcx8wFiU6VLNoHAt2oGfZbUoBdCoRT0uJRI`cZ%Eb cZ%Eb#בCט#u׉׉ 7cassandra://JZFnG7HZyGwHgHtSZnwXTY6_nuXItoxcoFNRV9KNSy4+`׉ 7cassandra://YxlJvdiqcdq7tW9_pr5KZHIT1HykutjdCkjQ3VNQUggA`s׉ 7cassandra://Dld9K33-sZG1YxZwCZ0iRfUD5oMZh3tuVeVfAHxXLWo`׉ 7cassandra://MmFvyJrYGn-slrPxZg-jV3pwC4aROQOY_9XlCs-8ew4:͠^cZ(Eb{ט#u׉׉ 7cassandra://08tBY4hxAborNvV8smEh7CFiEVoa7XH4S9k6_7pk2JM`׉ 7cassandra://-WuVU5B952iQTVDp6ORX4PAHjiSrhUyPEpMpolJ5IFAh+`s׉ 7cassandra://HPLhUy64nGOHogDIkCJ1vYSa40MV71cs_X0d1z4P25k`׉ 7cassandra://0fa9WcW9g8GjyVvbLDHTqCg6DCK-5g4ia7oGg9slLNUK͠^cZ(Eb|*נcZ'EbQ̵U9׉SG ׁ ׁ rנcZ'EbR̹Ɂ\9׉S>G ׁ ׁ rנcZ'EbS̹݁9׉S@G ׁ ׁ rנcZ'EbT̺P9׉SGG ׁ ׁ rנcZ'EbU̺-9׉SHG ׁ ׁ rנcZ'EbV̹9׉S]G ׁ ׁ rנcZ'EbW̺,B9׉SG ׁ ׁ rנcZ'EbX̺A9׉SG ׁ ׁ rנcZ'EbY̹Ts9׉SG ׁ ׁ 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Operations 399 14.2: Suspension Components for the Lotus Elise Automobile 410 15.1: Manufacture of Aluminum Heat Sinks 425 15.2: Cold-extruded Part 427 16.1: Tailor-welded Sheet Metal for Automotive Applications 449 16.2: Manufacture of Food and Beverage Cans 476 16.3: Tube Hydroforming of an Automotive Radiator Closure 478 16.4: Computer-aided Incremental Sheet Forming 483 16.5: Cymbal Manufacture 490 17.1: Hot Isostatic Pressing of a Valve Lifter 520 17.2: Production of Tungsten Carbide for Tools and Dies 527 17.3: Powder Metallurgy Parts in a Snowblower 532 18.1: Production of High-temperature Superconducting Tapes 554 19.1: Polymer Automotive-body Panels Shaped by Various Processes 588 19.2: Manufacturing of Head Protector R Tennis Racquets 588 19.3: Metal-matrix Composite Brake Rotors and Cylinder Liners 592 20.1: Functional Prototyping 602 20.2: Production of Athletic Shoes 610 20.3: Casting of Plumbing Fixtures 622 20.4: Implications of Powder Reuse 627 23.1: Brake Disk Machining 707 23.2: Bone Screw Retainer 739 24.1: Broaching Internal Splines 766 24.2: Ping Golf Putters 776 24.3: Machining of Aerospace Structures from Monolithic Extruded Aluminum 778 25.1: Machining Outer Bearing Races on a Turning Center 793 26.1: Gear Grinding with Engineered Abrasives 849 27.1: Electrochemical Machining of a Biomedical Implant 865 27.2: Manufacturing of Small Satellites 879 28.1: A Flexible Arduino R 931 29.1: Digital Micromirror Device 944 29.2: Photonic Integrated Circuits 959 31.1: Linear Friction Welding of Blanes and Blisks in a Jet Engine 1029 32.1: Light Curing Acrylic Adhesives for Medical Products 1059 36.1: Manufacture of Television Sets by Sony Corporation 1146 37.1: Robotic Deburring of a Blow-molded Toboggan 1195 ׉ 7cassandra://Dld9K33-sZG1YxZwCZ0iRfUD5oMZh3tuVeVfAHxXLWo`cZ%Eb4׉EPreface •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Since the mid-2000s, manufacturing has undergone a rebirth in its development and research activities. With the recognition that manufacturing adds value to products, and wealth to national economies, governments around the world have been investing in their infrastructure and are now partnering with industry to bring new manufacturing capabilities to the global marketplace. Manufacturing continues to be a dynamic activity, providing all-encompassing opportunities for contributions from several traditional disciplines. The proliferation of powerful software and Internet communication tools, especially their wireless capabilities, has made new approaches possible. The engineering terminology now includes such terms as Big Data, mass customization, cobots, and cybersecurity. Traditional manufacturing approaches and the materials involved are constantly being adjusted for ever-increasing efficiency and continuous improvements in performance. In view of the advances being made in all aspects of manufacturing, the authors have continued their efforts to present a comprehensive, balanced, and, most importantly, an up-to-date coverage of the science, engineering, and technology of manufacturing. As in its previous editions, this text maintains the same number of chapters, while continuing to emphasize the interdisciplinary nature of all manufacturing activities, including complex interactions among materials, design, and manufacturing processes and operations. Every attempt has been made to motivate and challenge students to understand and develop an appreciation of the vital importance of manufacturing in the modern global economy. The extensive questions and problems, at the end of each chapter, are designed to encourage students to explore viable solutions to a wide variety of challenges, giving them an opportunity to describe and assess the capabilities as well as limitations of all manufacturing processes and operations. These challenges include economic considerations and the competitive aspects in a global marketplace. The numerous examples and case studies throughout the book also help give students a perspective on real-world applications of the topics described throughout the book. What’s New in This Edition • The eighth edition has been thoroughly updated, with numerous new topics and illustrations relevant to all aspects of manufacturing. See the table on page 23 for specifics. • Wherever appropriate, illustrations and examples have been replaced, indicating recent advances in manufacturing. • The text contains more cross references to other relevant sections, tables, and illustrations in the book. • The Questions, Qualitative Problems, Quantitative Problems, and Design/Projects at the end of each chapter have been expanded. • The Bibliographies at the end of each chapter have been thoroughly updated. • Manufacturing Engineering and Technology is also available as an eText. Pearson eText offers a simpleto-use, mobile-optimized, personalized reading experience. It lets students add bookmarks, highlight, and take notes all in one place, even when offline. Seamlessly integrated videos engage students and give them access to the help they need, when they need it. Educators can easily schedule readings and share their own notes with students so they see the connection between their eText and what they learn in class—motivating them to keep reading, and keep learning. And, reading analytics offer insight into how students use the eText, helping educators tailor their instruction. 22 ׉ 7cassandra://HPLhUy64nGOHogDIkCJ1vYSa40MV71cs_X0d1z4P25k`cZ%Eb5cZ%Eb4#בCט#u׉׉ 7cassandra://d8EwgzOg_M3FlazkkSo9XlqgP_8uqJFhC4AdRsCl1TYJ`׉ 7cassandra://oGKxureZJmgA90CTMc7Bg2q-4M2WAvUhDHxlgVtKKxsYa`s׉ 7cassandra://R3g6X2WhHiWsUG12O5V_gZE0fJ7Ifzs5C6q8xa61V6U%`׉ 7cassandra://SOV3uD4-kwBwyCD7ctwPuQMu6zYXVPyMmGktVfRISbkB ͠^cZ(Eb~ט#u׉׉ 7cassandra://HhRlxCeBMOysS4ZBlq-OztuZN_QC3UouAaqGSMQnQPkz`׉ 7cassandra://wOlRqYLIZCOpzaJs4WDvST5aFhTw2nWXCcwfdye7leAM1`s׉ 7cassandra://lSu8bOTnR8nUK_EoLqSxXc3f76APEKx55V-Z94Xp36kg`׉ 7cassandra://056jZ66e9G60jIVDStaw1wI1X6FrkG_y0ULqVWDQmFYH0͠^cZ(Eb׉E Preface 23 • The Solutions Manual, available for use by instructors, has been expanded; it now provides MATLAB code for numerous problems, allowing instructors to easily be able change relevant parameters. • Reflecting the rapid advances in additive manufacturing, Chapter 20 has been thoroughly revised to include the latest technologies. New or expanded topics in this edition are: Chapter Topics Introduction Complexity of products; definition of Technology Readiness Level and Manufacturing Readiness Level, to show the stages in product development and manufacturing at scale; case study on three-dimensional printing of guitars; expansion and update of the section on Trends in Manufacturing. ISO for grain size number. 1 5 6 7 8 10 11 13 15 16 17 20 Second- and third-generation high-strength steels; nano-structured steels; and new case study on high-strength steels in automobiles. Addition of the Hall-H´eroult process for aluminum manufacture; new sections on lithium and rare earth metals; a case study on Tesla automobile design and manufacture; metamaterials. Electrically conductive and semi-conductive polymers; gels and aerogels. Porous ceramics; graphene; carbon and graphite foam. Freeze casting. Integrated computational materials engineering (ICME); machining of sand molds; new case study on a die-cast magnesium liftgate. Tailor-rolled blanks and tailored coils. Friction stir extrusion. Expansion of hot stamping of sheet metal; camera-based forming-limit diagrams; electrically assisted forming; new case study on single-point incremental forming. Expansion of powder morphology effects. Additive Manufacturing: mass customization; distributed manufacturing; composite AM; projection stereolithography; continuous liquid interface production (CLIPS); new case study on AM of athletic shoes; powder bed processes; JetFusion; wire and arc AM; bioprinting; architectural applications of AM; conformal cooling; expanded section on design for AM; topology optimization; economic considerations; new case study on the implications of powder reuse. 23-24 25 26 27 28 29 31 37 38 39 40 Design considerations in machining; new case study on machining aerospace parts from monolithic aluminum. Stability lobes in chatter. Engineered abrasives; new case study on gear grinding with engineered abrasives. Electrolytic trepanning; shaped-tube electrolytic machining. Roll-to-roll printing; flexible electronics; conductive and semi-conductive inks; rotogravure, flexography, flat and rotary screen printing; self-aligned imprint lithography; flexible hybrid electronics; new case study on a flexible Arduino. Photonic integrated circuits; mesoscale manufacturing. Friction stir spot welding; expansion of linear friction welding. Microcontrollers; cloud computing; cybersecurity; gain scheduling; cobot; design considerations. Cloud storage; expansion of ERP and MES. Mass customization; Internet of Things; cloud computing; MTConnect; Big Data; digital twin. Life-cycle engineering; energy use in manufacturing. ׉ 7cassandra://R3g6X2WhHiWsUG12O5V_gZE0fJ7Ifzs5C6q8xa61V6U%`cZ%Eb6׉E ]24 Preface Acknowledgments We gratefully acknowledge individuals for their contributions to various sections in the book, as well as for their reviews, comments, and constructive suggestions for this revision. We are happy to present below as list of those individuals, in academic institutions, as well as in industrial and research organizations, who, in one way or another, have made various contributions to this and the recent editions of this book. Kent M. Kalpakjian, Micron Technology, Inc., was the original author of the sections on fabrication of microelectronic devices. D. Adams W. Bair N. Banu G. Boothroyd D. Bourell J. Cesarone A. Cinar D. Dods D.A. Dornfeld M. Dugger D.R. Durham M. Giordano M. Hawkins T. Jaramillo K. Jones R. Kassing K. Kozlovsky K.M. Kulkarni M. Madou H. Malkani J. Marsh M. Molnar S. Mostovoy C. Nair P.G. Nash J. Neidig C. Petronis M. Prygoski P. Saha K. Savala J. Schiltz T. Schmitz R. Shivpuri K.S. Smith A. Taub B.S. Thakkar J.E. Wang K.R. Williams Robert Ivester John Vickers P.K. Wright We would also like to acknowledge the dedication and continued help and cooperation of Holly Stark, Executive Portfolio Manager at Pearson, and Amanda Brands, Content Producer at Pearson.We are grateful to numerous organizations that supplied us with illustrations, videos, and various materials for the text. These contributions have specifically been acknowledged throughout the text. SEROPE KALPAKJIAN STEVEN R. SCHMID Acknowledgments for the Global Edition Pearson would like to acknowledge and thank the following for the Global Edition: Contributor K. S. Vijay Sekar graduated from the University of Madras, Chennai, with a Bachelor’s in Production Engineering and a Master’s in Computer-aided Design. He completed his PhD in Mechanical Engineering from Anna University, Chennai, specializing in finite element analysis in machining. He has over twenty-five years of teaching experience and more than fifteen years of research experience. His areas of research interest include materials, materials processing, and application of finite elements in manufacturing. At present, he is a professor at the Department of Mechanical Engineering, Sri Sivasubramaniya Nadar College of Engineering (SSNCE), Kalavakkam, Tamil Nadu, India. Reviewers K. Baxevanakis, Loughborough University C. Chatwin, University of Sussex X. Song, The Chinese University of Hong Kong ׉ 7cassandra://lSu8bOTnR8nUK_EoLqSxXc3f76APEKx55V-Z94Xp36kg`cZ%Eb7cZ%Eb6#בCט#u׉׉ 7cassandra://QUIUM7RjA2BTZ3R6z4iJWukc1u3Ge3oDz0EqRR18eNA`׉ 7cassandra://keGxOsknzAcgTLmzD0ALbvWkAA38dKOMC6qt8yURtoQ^Q`s׉ 7cassandra://zGdIeYPX3hBJuDdX655v4249DzjsdLjTv1qk-nBlsJs`׉ 7cassandra://h7SizG1L_FJzRHP-ffttsAROI7aWMsiKYWDOjSzoEJ0X͠^cZ(EbƁט#u׉׉ 7cassandra://hXYgR1OpE7CSa1oQzjhbJBzm86sZZJzHnxHUFxaHW5E2`׉ 7cassandra://5W_YCYlhommakFewFzM4ZXa2wR33QbI-qf9prkhPrCI@`s׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`׉ 7cassandra://pVeKeFZqP0XFOMqtwzeYiv2erJZVLKiZJEVyk8w6SAM͠^cZ(EbƂ׉E About the Authors •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• Serope Kalpakjian is professor emeritus of Mechanical and Materials Engineering, the Illinois Institute of Technology. He is the author of Mechanical Processing ofMaterials and co-author of Lubricants and Lubrication in Metalworking Operations (with E.S. Nachtman). The first editions of his textbooks Manufacturing Processes for Engineering Materials and Manufacturing Engineering and Technology have received the M. Eugene Merchant Manufacturing Textbook Award. He has conducted research in various areas of manufacturing, is the author of numerous technical papers and articles in handbooks and encyclopedias, and has edited a number of conference proceedings. He also has been editor and co-editor of several technical journals and has served on various editorial boards, including the Encyclopedia Americana. Among various awards, Professor Kalpakjian has received the Forging Industry Educational and Research Foundation Best Paper Award, the Excellence in Teaching Award from lIT, the ASME Centennial Medallion, the International Education Award from SME, A Person of the Millennium Award from lIT, the Albert Easton White Outstanding Teacher Award from ASM International, and the 2016 SME Gold Medal Award. The Outstanding Young Manufacturing Engineer Award of SME, for 2001, was named after him. Professor Kalpakjian is a Life Fellow ASME, Fellow SME, Fellow and Life Member ASM International, Fellow Emeritus International Academy for Production Engineering (CIRP), and is a founding member and past president of NAMRI. He is a graduate of Robert College (High Honor, Istanbul), Harvard University, and the Massachusetts Institute Technology. Steven R. Schmid is professor of Aerospace and Mechanical Engineering, the University of Notre Dame, where he teaches and conducts research in the general areas of manufacturing, machine design, and tribology. From 2016 to 2018, he served as a Program Director of the Advanced Manufacturing Program at the National Science Foundation. He received his B.S. degree from Illinois Institute of Technology (with Honors) and Master’s and Ph.D. degrees from Northwestern University, all in mechanical engineering. He has received numerous awards, including the John T. Parsons Award from SME, the Newkirk Award from ASME, the Kaneb Center Teaching Award (three times), and the Ruth and Joel Spira Award for Excellence in Teaching. Professor Schmid served as President of the North American Manufacturing Research Institution (NAMRI, 2015–2016) and was appointed the first Academic Fellow at the Advanced Manufacturing National Program Office, U.S. Department of Commerce, where he helped design the Manufacturing USA program. Dr. Schmid is the author of over 140 technical papers, and has co-authored Fundamentals of Machine Elements, Fundamentals of Fluid Film Lubrication, Manufacturing Processes for Engineering Materials,and Manufacturing Engineering and Technology. He is a Fellow of the American Society of Mechanical Engineers and the Society of Manufacturing Engineers. He was elected to the Board of the Society of Manufacturing Engineers in 2018. 25 ׉ 7cassandra://zGdIeYPX3hBJuDdX655v4249DzjsdLjTv1qk-nBlsJs`cZ%Eb8׉E?•••••••• This page is intentionally left blank ׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`cZ%Eb9cZ%Eb8#בCט#u׉׉ 7cassandra://xlHZrD1JqPMAcijKXpr5eSFD_hYqw9i7auhgz7gRQNE?`׉ 7cassandra://Y8CctaYq0cHBTgq7nsBOjiofZMUCkgCqCnXT8AkyM-oB`s׉ 7cassandra://HpS4eyV-T1oajlnbRJkYXolpkQXKzmZdh04p85-vC1c`׉ 7cassandra://moluwtnUe3ZSTDyRLzByqq1YaAaYWc4SQ0drorkV_eU<͠^cZ(EbƄט#u׉׉ 7cassandra://qKaeg_49NT6qpP1OQGezgAwsferykASn2A9ooZAljN05T`׉ 7cassandra://TAWr20GBuRSLb1NjO_dWYTG3E1cOr8_-5LgcqDTe8zYE`s׉ 7cassandra://PFdFvI6cUZ-vj-bEHf3R89kIzWoZAFhl7aNW6_eh8T4`׉ 7cassandra://BgY9weK9dO5qDUe2TZ9uhQrWrf9XSKHF2S6cmCPwNOIo͠^cZ)Ebƅ׉EoChapter I General Introduction I.1 What Is Manufacturing? 27 I.2 Product Design and Concurrent Engineering 34 I.3 Design for Manufacture, Assembly, Disassembly, and Service 39 I.4 Environmentally Conscious Design, Sustainable Manufacturing, and Product Life Cycle 40 I.5 Selection of Materials 42 I.6 Selection of Manufacturing Processes 44 I.7 Computer-integrated Manufacturing 52 I.8 Quality Assurance and Total Quality Management 55 I.9 Lean Production and Agile Manufacturing 57 I.10 Manufacturing Costs and Global Competition 57 I.11 Trends in Manufacturing 59 Case Studies: I.1 Three-dimensional Printing of Guitars 38 I.2 U.S. Pennies 44 I.3 Saltshaker and Pepper Mill 52 I.1 What Is Manufacturing? As you begin to read this chapter, take a few moments to inspect various objects around you: mechanical pencil, light fixture, chair, cell phone, and computer. You soon will note that all these objects, and their numerous individual components, are made from a variety of materials and have been produced and assembled into the products you now see. You also will note that some objects, such as a paper clip, nail, spoon, and door key, are made of a single component. However, as shown in Table I.1 and Fig. I.1, the vast majority of objects around us consist of numerous individual parts that are built and assembled by a combination of processes called manufacturing (Fig. I.2). 27 ׉ 7cassandra://HpS4eyV-T1oajlnbRJkYXolpkQXKzmZdh04p85-vC1c`cZ%Eb:׉E28 General Introduction 1010 108 Space shuttle 747-400 C-5A 106 104 Steam locomotive 102 Musket 100 1800 1840 1880 1920 Year 1960 2000 2040 Bicycle Manual lawnmower Aircraft 737 DC 3 Ford Model T Lawnmower Dreamliner 787 Modern car Figure I.1: Increasing complexity of products since the Industrial Revolution. Source: After J. Jeswiet. The word manufacture first appeared in English in 1567, and is derived from the Latin manu factus, meaning made by hand. The word manufacturing first appeared in 1683, and the word production, which is often used interchangeably with the word manufacturing, first appeared sometime during the 15th century. Manufacturing is making products. A manufactured product may, in turn, itself be used to make other products, such as (a) a large press, to shape flat sheet metal into appliances, (b) a drill, for producing holes, and (c) machinery, to produce a countless variety of individual items. Nails, bolts, screws, nuts, paper clips, tires are discrete products, meaning individual items. By contrast, a spool of wire, metal or plastic tubing, and a roll of aluminum foil are continuous products, which are then cut into individual pieces of various lengths for specific purposes. Because a manufactured item typically begins with raw materials, which are then subjected to a sequence of processes to make individual products, it has a certain value. Clay, for example, has some value as mined, but when made into pottery, electrical insulator, or cutting tool, it has added value. Similarly, a nail has a value over and above the cost of a short piece of wire from which it is made. Products such as computer chips, electric motors, medical implants, machine tools, and aircraft are known as high-value-added products. Table I.1: Approximate Number of Parts in Products. Common pencil 4 Rotary lawn mower Grand piano Automobile Boeing 747–400 300 12,000 15,000 6,000,000 Number of parts ׉ 7cassandra://PFdFvI6cUZ-vj-bEHf3R89kIzWoZAFhl7aNW6_eh8T4`cZ%Eb;cZ%Eb:#בCט#u׉׉ 7cassandra://FqfzJEhn8aqnDnUewicQowhUJnnD_YcfoIRBwQFFXroD`׉ 7cassandra://u-T8ZIg8GB4i6VNvRqOJPHjfqqUaaXrRqUuTH4gCYC0fV`s׉ 7cassandra://Y-_TGc7UbVLli79wtFgVVA-DvpmryAKIEcKH_yhkf8A`׉ 7cassandra://7sCWUfERs8M9FjWuV7w1L2rGpbCiq1ZHbH926HmXw1U{͠^cZ)EbƇט#u׉׉ 7cassandra://QvacDsocss2qcTxDXRX6PNlvk4PcN4jIJT_0cpw3vYE`׉ 7cassandra://BtVDBur4E_UgDLfACHFu9Pv0D9BBVe-Kzob8xyXL1gEF"`s׉ 7cassandra://b6Uz7JU2g4xoCc9Ld44iL0jepGcK-41FA7MO06ECHJM=`׉ 7cassandra://c4aEd9m1plnT_cOImN_VxeHE8jiYyRf9X5SmhpJsoZw]!͠^cZ)EbƈנcZ)EbƋng/9ׁHhttp://B.C.toׁ ׁ ЈנcZ)EbƊO|/9ׁHhttp://B.C.toׁ ׁ Ј׉E UWhat Is Manufacturing? 29 Steel pistons with friction-welded skirt and crown and chrome/ molybdenum disulfide coated piston ring Fiberglass hood Copper tube, Aluminum fin coolers Lexan windshield Silicon microprocessors with gold-plated connectors High strength steel bolts Figure I.2: John Deere tractor showing the variety of materials and processes incorporated. Source: Shutterstock/Nils Versemann. A Brief History of Manufacturing. Manufacturing dates back to the period 5000 to 4000 B.C. (Table I.2). Thus, it is older than recorded history, which dates back to the Sumerians, around 3500 B.C. Primitive cave drawings, as well as markings on clay tablets and stones, needed (a) some form of a brush and some sort of pigment, as in the prehistoric cave paintings in Lascaux, France, estimated to be 16,000 years old; (b) a means of first scratching the clay tablets and then baking them, as in cuneiform scripts and pictograms of 3000 B.C.; and (c) simple tools for making incisions and carvings on the surfaces of stone, as in the hieroglyphs in ancient Egypt. Manufacturing items for specific uses began with the production of household artifacts, typically made of wood, stone, or metal. The materials first used in making utensils and ornamental objects included gold, copper, and iron, followed by silver, lead, tin, bronze, and brass. The processing methods first employed involved mostly casting and hammering, because they were relatively easy to perform. Over the centuries, these simple processes gradually began to be developed into more and more complex operations, at increasing rates of production, and at higher levels of product quality. Note from Table I.2 that, for example, lathes for cutting screw threads already were available during the period from 1600 to 1700, but it was not until three centuries later that automatic screw machines were developed. Although ironmaking began in about 1100 B.C. in the Middle East, a major milestone was the production of steel, in Asia, during the period 600 to 800 A.D. A wide variety of materials then began to be developed. Today, countless metallic and nonmetallic materials with unique properties are available, including engineered materials and other advanced materials. Among the available materials now are industrial ceramics, composite materials (often in the form of fiber reinforced plastics), and nanomaterials ׉ 7cassandra://Y-_TGc7UbVLli79wtFgVVA-DvpmryAKIEcKH_yhkf8A`cZ%Eb<׉E30 General Introduction Table I.2: Historical Development of Materials and Manufacturing Processes. Period Dates Metals and casting Before 4000 B.C. Gold, copper, meteoric iron 4000-3000 B.C. Copper casting, stone and metal molds, lost-wax process, silver, lead, tin, bronze 3000-2000 B.C. Bronze casting and drawing, gold leaf 2000-1000 B.C. Wrought iron, brass 1000-1 B.C. Cast iron, cast steel 1-1000 A.D. 1000-1500 Zinc, steel Blast furnace, type metals, casting of bells, pewter 1500-1600 Cast-iron cannon, tinplate 1600-1700 Permanentmold casting, brass from copper and metallic zinc Glass pressing and blowing Venetian glass Stamping of coins Armor, coining, forging, steel swords Crystal glass Wire drawing, gold- and silversmith work Cast plate glass, flint glass Porcelain Water power for metalworking, rolling mill for coinage strips Rolling (lead, gold, silver), shape rolling (lead) Forge welding of iron and steel, gluing Improved chisels, saws, files, woodworking lathes Etching of armor Sandpaper, windmill-driven saw Hand lathe for wood Boring, turning, screw-cutting lathe, drill press Glass beads, potter’s wheel, glass vessels Various materials and composites Earthenware, glazing, natural fibers Forming and shaping Hammering Stamping, jewelry Wire by slitting sheet metal Soldering (CuAu, Cu-Pb, Pb-Sn) Riveting, brazing Joining Tools, machining and manufacturing systems Tools of stone, flint, wood, bone, ivory, composite tools Corundum (alumina, emery) Hoe making, hammered axes, tools for ironmaking and carpentry Middle Ages: ∼ 476 to 1492. Egypt: ∼ 3100 B.C.to ∼ 300 B.C. Renaissance: 14th to 16th centuries. Greece: ∼ 1100 B.C.to ∼ 146 B.C. Roman Empire: ∼ 500 B.C. to 476 A.D. ׉ 7cassandra://b6Uz7JU2g4xoCc9Ld44iL0jepGcK-41FA7MO06ECHJM=`cZ%Eb=cZ%Eb<#בCט#u׉׉ 7cassandra://fvFvmvqCatmeGCjaWYCSCwvECUDDHr4htACQICczcwQX`׉ 7cassandra://qxi-kIkBCpNrtF26sQL52Pxc_dzueeBuPauZ3L6n-JIK2`s׉ 7cassandra://FCgqyLkag92yjOz4upsckt73N_Gks8TKTeWrhJIJuWs6`׉ 7cassandra://xob1fynppHntYZrFjA-Nzcvzch0tfVdq7U01EE5nBOk[̰͠^cZ)Ebƌט#u׉׉ 7cassandra://Lj3aTwmlOhcVjcDxM11XihX802d6nqvqjYM5ij3dRb8`׉ 7cassandra://u4sW1HKclKlcsW4ISObyWnGBffMcbcX_y6_8-mwi8PwP`s׉ 7cassandra://qreEZefHVC6h9G8nIMLduRiiQ3lluQiBreX2GwphbfAK`׉ 7cassandra://qeRlSV_7e20CtnCqSVFRznEsLeqHMm-SuGHpMw8lG5sN ̔͠^cZ)Ebƍ׉EWhat Is Manufacturing? 31 Table I.2: Historical Development of Materials and Manufacturing Processes (continued). Period Dates Metals and casting 1700-1800 Malleable cast iron, crucible steel (iron bars and rods) 1800-1900 Centrifugal casting, Bessemer process, electrolytic aluminum, nickel steel, babbitt, galvanized steel, powder metallurgy, open-hearth steel 1900-1920 Window glass from slit cylinder, light bulb, vulcanization, rubber processing, polyester, styrene, celluloid, rubber extrusion, molding Automatic bottle making, bakelite, borosilicate glass Various materials and composites Forming and shaping Extrusion (lead pipe), deep drawing, rolling Steam hammer, steel rolling, seamless tube, steelrail rolling, continuous rolling, electroplating Tube rolling, hot extrusion Oxyacetylene; arc, electrical-resistance, and thermit welding Shaping, milling, copying lathe for gunstocks, turret lathe, universal milling machine, vitrified grinding wheel Geared lathe, automatic screw machine, hobbing, high-speed steel tools, aluminum oxide and silicon carbide (synthetic) 1920-1940 Die casting Development of plastics, casting, molding, polyvinyl chloride, cellulose acetate, polyethylene, glass fibers 1940-1950 Lost-wax process for engineering parts 1950-1960 Ceramic mold, nodular iron, semiconductors, continuous casting Acrylics, synthetic rubber, epoxies, photosensitive glass Acrylonitrilebutadiene-styrene, silicones, fluorocarbons, polyurethane, float glass, tempered glass, glass ceramics Extrusion (steel), swaging, powder metals for engineering parts Cold extrusion (steel), explosive forming, thermochemical processing Submerged arc welding Gas metal arc, gas tungsten arc, and electroslag welding; explosion welding Phosphate conversion coatings, total quality control Electrical and chemical machining, automatic control. Tungsten wire from metal powder Coated electrodes Tungsten carbide, mass production, transfer machines Joining Tools, machining and manufacturing systems WWII Industrial Revolution: ∼ 1750 to 1850 WWI ׉ 7cassandra://FCgqyLkag92yjOz4upsckt73N_Gks8TKTeWrhJIJuWs6`cZ%Eb>׉E(concludes on next page) 32 General Introduction Table I.2: Historical Development of Materials and Manufacturing Processes (concluded). Period Dates Metals and casting 1960-1970 Squeeze casting, single-crystal turbine blades 1970-1990 Compacted graphite, vacuum casting, organicallybonded sand, automation of molding and pouring, rapid solidification, metal-matrix composites, semi- solid metalworking, amorphous metals, shape-memory alloys 1990-2000 Rheocasting, computer-aided design of molds and dies, rapid tooling Various materials and composites Acetals, polycarbonate, cold forming of plastics, reinforced plastics, filament winding Adhesives, composite materials, semiconductors, optical fibers, structural ceramics, ceramic-matrix composites, biodegradable plastics, electricallyconducting polymers Forming and shaping Hydroforming, hydrostatic extrusion, electroforming Precision forging, isothermal forging, superplasticforming, dies made by computer-aided design manufacturing, net-shape Joining Plasma-arc and electron -beam welding, adhesive bonding and forging and forming, computersimulation Laser beam, diffusion bonding (also combined with superplastic forming), surface-mount soldering Tools, machining and manufacturing systems Titanium carbide, synthetic diamond, numerical control, integrated circuit chip Cubic boron nitride, coated tools, diamond turning, ultraprecision machining, computer-integrated manufacturing, industrial robots, machining and turning centers, flexible manufacturing systems, sensor technology, automated inspection, computer simulation and optimization Nanophase materials, metal foams, high-temperature superconductors. machinableceramics, diamond-like carbon 2000-2010s TRIP and TWIP steels Carbon nanotubes, graphene Additive manufacturing, rapid tooling, environmentallyfriendly metalworking fluids Single point incremental forming, hot stamping, electrically assisted forming Friction stir welding, leadfree solders, laser butt-welded (tailored) sheet-metal blanks linear friction welding Micro- and nanofabrication, LIGA, dry etching, linear motor drives, artificial neural networks, six sigma Digital manufacturing, three-dimensional computer chips, blue-arc machining, soft lithography, flexible electronics Information Age Space Age ׉ 7cassandra://qreEZefHVC6h9G8nIMLduRiiQ3lluQiBreX2GwphbfAK`cZ%Eb?cZ%Eb>#בCט#u׉׉ 7cassandra://qEltrREPcL2P_bzwQUlJ2u93kmOdQKgY1Febu8P_C9A^`׉ 7cassandra://vlshS_2f8K9KK3uIqo7WUHjn2v86APyISdvnCRIAf8Q~R`s׉ 7cassandra://xyIjWiTZkVtZwF1iB8CWmJidzETibv-BPy3Au8fqRCM`׉ 7cassandra://KlDh89bRKEtoon4YDP8iFtn9X8QbKsax5GyP2RvN6f0o͠^cZ*EbƏט#u׉׉ 7cassandra://sBj0JKQzzf0Z_PMGTM9eydpzZgbqK917oS9xcfelSUQ:`׉ 7cassandra://PrllG74RBftpsuRJD5Pm5qsAxk1pUx_BXsb5boX6fGQ\H`s׉ 7cassandra://PzoPIGCu38l6jbvTh3knVxQiGS_JPNBwtO9To-vqbkU`׉ 7cassandra://TtlfGCjkoKe6SYys_zPsGIdhu6Eun4I4jgd-tS-QERIͫs"͠^cZ*EbƐנcZ*Ebƒ/9ׁHhttp://B.C.toׁ ׁ Ј׉EWhat Is Manufacturing? 33 that are now used in an extensive variety of products, ranging from prosthetic devices and computers to supersonic aircraft. Until the Industrial Revolution, which began in England in the 1750s (also called the First Industrial Revolution), goods had been produced in batches, which required high reliance on manual labor in all phases of production. The Second Industrial Revolution is regarded, by some, as having begun in the mid-1900s, with the development of solid-state electronic devices and computers (Table I.2). Mechanization began in England and other countries of Europe with the development of textile machinery and machine tools for cutting metal. Mechanization soon moved to the United States, where it continued to be further developed. A major advance in manufacturing began in the early 1800s, with the design, production, and use of interchangeable parts, conceived by the American manufacturer and inventor E. Whitney (1765–1825). Prior to the introduction of interchangeable parts, much hand fitting was necessary, because no two parts could be made exactly alike. By contrast, it is now taken for granted that a broken bolt can easily be replaced with an identical one produced decades after the original was made. Further developments soon followed, resulting in countless consumer and industrial products which we now cannot imagine being without. Beginning in the early 1940s, several milestones were reached in all aspects of manufacturing, as can be observed by a review of Table I.2. Note particularly the progress that has been made during the 20th century, as compared with those achieved during the 40-century long period from 4000 B.C.to 1 B.C. For example, in the Roman Empire (around 500 B.C. to 476 A.D.), factories were available for mass production of glassware; the methods used were generally very slow, and much manpower was required in handling the parts and operating the machinery. Today, production methods have advanced to such an extent that (a) aluminum beverage cans are made at rates of more than 500 per minute, with each can costing about four cents to make; (b) holes in sheet metal can be punched at rates of 800 holes per minute; and (c) light bulbs are made at rates of more than 2000 bulbs per minute, each costing less than one dollar. The period from the 1940s to the 1990s was characterized by mass production and expanding global markets. Initially, the United States had a dominant position, as it was the only developed nation with an intact infrastructure following World War II; however, this advantage dissipated by the 1960s. The quality revolution began to change manufacturing in the 1960s and 1970s, and in the 1980s, programmable computers became widely used. The digital manufacturing era began around 1990. As a fundamental change in manufacturing operations, powerful computers and software are now fully integrated across the design and manufacturing enterprise. Advances in communications, some Internet-based, have led to further improvements in organizations and their capabilities. The effects are most striking when considering the origin and proliferation of additive manufacturing, described in Chapter 20. Prior to 1990, the prototype of a part could be produced only through intensive effort and costly manufacturing approaches, requiring significant operator skill. Today, a part can first be drafted in a CAD program, then produced generally in a matter of minutes or hours (depending on size and part complexity) without the need for hard tools or skilled labor. Prototyping systems have become more economical, faster, and with improved raw materials. The term digital manufacturing has been applied to reflect the notion that manufacturing parts and components can take place completely through such computer-driven CAD and production machinery. Recent innovations are the proliferation of communications protocols, sensors and controls throughout the manufacturing enterprise. Referred to as Industry 4.0 (Section 37.2.2) or Digital Manufacturing, some of the key developments are the following: 1. Sensors and smart device designs can be printed from conductive and insulate inks, or use very thin silicon integrated circuits in flexible hybrid designs (Fig. I.3). These low-cost devices include communication ability and are central to the Internet of Things (Section 39.8.1). 2. Machines can be monitored at all times, using Internet-based communications protocols such as MTConnect, so that precise information is available at all times for every machine in an organization. 3. The wide application of sensors to all aspects of manufacturing has led to the development of Big Data, where trends and conditions of manufacturing systems can be accurately measured at all times. ׉ 7cassandra://xyIjWiTZkVtZwF1iB8CWmJidzETibv-BPy3Au8fqRCM`cZ%Eb@׉E l34 General Introduction Patch substrate Oxygen generator Oxygen sensor Wound region communication Sensor readings Bluetooth Electronics module Wound area Control signals (e.g., for drug release) Skin Figure I.3: A flexible hybrid electronic wound care system (see Section 28.15). The device integrates communication ability (in this case a Bluetooth ability), sensors to detect the oxygen levels at the wound, and oxygen generating devices to increase oxygen level to optimize healing. These devices use combinations of printed electronic devices and silicon-based integrated circuits. Source: Courtesy of NextFlex. 4. Machine learning algorithms, and the incorporation of physics-based mathematical models of manufacturing processes and systems, has led to the ability to apply advanced controls to the manufacturing enterprise. 5. Advanced models of manufacturing processes, combined with the detailed measurement of the manufacturing and service environment of a product (Big Data), lead to the computer-based representation of the product, referred to as a digital twin. The digital twin represents a virtual model of the part, and accurate performance models applied to the virtual twin can predict failure or required service of the actual part. These developments are a natural extension of the computer revolution that started in the 1990s, and developments are certain to continue. I.2 Product Design and Concurrent Engineering Product design involves the creative and systematic prescription of the shape and characteristics of a product to achieve specified objectives, while simultaneously satisfying several important constraints. Design is a critical activity, because it has been estimated that as much as 80% of the cost of product development and manufacturing is determined by the decisions made in the initial stages of design. The product design process has been studied extensively; it is briefly introduced here because of the strong interactions among manufacturing and design activities. Innovative approaches are essential in successful product design, as are clearly specified functions and a clear statement of the performance expected of the product. The market for a product, which may be new or a modified version of an existing product, and its anticipated use or uses, also must be clearly defined at this stage. This aspect also involves the assistance of market analysts and sales personnel who will bring valuable and timely input to the manufacturer, especially regarding market needs and trends. Generated oxygen Wound oxygen Oxygen-permeable collagen matrix INTEGRA® Paper-ply bilayer with microfluidics O2 sensors O2 generators ׉ 7cassandra://PzoPIGCu38l6jbvTh3knVxQiGS_JPNBwtO9To-vqbkU`cZ%EbAcZ%Eb@#בCט#u׉׉ 7cassandra://70BH4U5E4EFTLJWo6GIBd_V-hHj-rwZ-TOqjx8QJO_AU`׉ 7cassandra://BwYu4y1RgbGZWzzLBRmae8RrHcLNo4BzbF0zQkwjZ6Ej`s׉ 7cassandra://PIfqYXT8FFdmECDYDY5doNWa1GKGpPjAAi51cYpYibw`׉ 7cassandra://t6aUTJEIVCLlCwfKvkz1W8lwCJysWPtCxr9djtEOJTYhM\͠^cZ*EbƓט#u׉׉ 7cassandra://Ti8qCikCcIfMHj3xMtNkPdYv9TOURf33V9knlAFko_U/`׉ 7cassandra://c4NgSKKX5Kg49YnymMVMeYb9tbmhuLaRZ947fN9D6UoG`s׉ 7cassandra://ItBisS60k-tExbfgkIlNxnQaPEEtCC5MqNkslHmXtigQ`׉ 7cassandra://guua4hOCT_6IU9PeqeKVCLUD-2UvGB_yKtwGOdnjrqIͩ- ͠^cZ*EbƔ׉EUProduct Design and Concurrent Engineering 35 Table I.3: Definitions of Technology Readiness Level (TRL) and Manufacturing Readiness Level (MRL). TRL Description MRL Description 1 Basic principles observed and reported 2 4 Technology concept and/or application formulated 3 Analytical and experimental critical function and/or characteristic proof of concept Component and/or breadboard validation in a laboratory environment 5 Component or breadboard validation in a relevant environment 6 System/subsystem model or prototype demonstration in a relevant environment 7 System prototype demonstration in an operational environment 8 Actual system completed and qualified through test and demonstration 9 Actual system proven through successful mission operations 1 Manufacturing feasibility assessed Manufacturing concepts defined 3 Manufacturing concepts developed 4 Capability to produce the technology in a laboratory environment 5 Capability to produce prototype components in a production relevant environment 6 Capability to produce a prototype system or subsystem in a production relevant environment 7 Capability to produce systems, subsystems or components in a production representative environment 8 Pilot line capability demonstrated; Ready to begin low rate initial production 9 Low rate production demonstrated; capability in place to begin full rate production 2 Technology Readiness Level and Manufacturing Readiness Level. Product development generally follows the flow outlined in Table I.3. Technology readiness level (TRL) and Manufacturing readiness level (MRL) are measures of a products ability to be produced, marketed, and sold. In practice, all technologies must progress from some starting point up to a TRL and MRL of 9. A new scientific discovery or a product idea begins at a TRL of 1, and it may or may not ever be suitable for commercial application. New versions of existing products may start at some higher TRL or MRL level, but the flow of its development is always the same. Note that each stage of a products development typically requires different skills and resources. Demonstrating a new concept in a laboratory environment (TRL 3) and demonstrating it in a new system in a real environment (TRL 7) are very different tasks. Similarly, producing a laboratory prototype (MRL 4) is very different from demonstrating manufacturing strategies for producing a product at scale (MRL 7), which is also very different from having a production facility in place. The Design Process. Traditionally, design and manufacturing activities took place sequentially, as shown in Fig. I.4a. This methodology may, at first, appear to be straightforward and logical; in practice, however, it is wasteful of resources. Consider the case of a manufacturing engineer who, for example, determines that, for a variety of reasons, it would be more desirable to (a) use a different material, such as a polymer or a ceramic instead of metal; (b) use the same material but in a different condition, such as a softer instead of a harder or one with a smoother surface finish; or (c) modify the design of a component in order to make it easier, faster, and less costly to manufacture. Note that these decisions must take place at the material-specification stage (the sixth box from the top in Fig. I.4a). Each of the modifications just described will necessitate a repeat of the design analysis stage (the third box from the top in Fig. I.4a) and the subsequent stages. This approach is to ensure that the product will still meet all specified requirements and will function satisfactorily. A later change from, say, a forged, cast, or machined component will, likewise, necessitate a repeat analysis. Such iterations obviously waste both time and the resources of a company. Concurrent Engineering. Driven primarily by the consumer electronics industry, a continuing trend has been to bring products to the marketplace as rapidly as possible, so as to gain a higher percentage share of the market and thus higher profits. An important methodology aimed at achieving this end is concurrent engineering, which involves the product-development approach shown in Fig. I.4b. ׉ 7cassandra://PIfqYXT8FFdmECDYDY5doNWa1GKGpPjAAi51cYpYibw`cZ%EbB׉E36 General Introduction Definition of product need; marketing information Conceptual design and evaluation; feasibility study Design analysis; codes/standards review; physical and analytical models Prototype production; testing and evaluation Production drawings; instruction manuals Material specification; process and equipment selection; safety review Pilot production Production Inspection and quality assurance Packaging; marketing and sales literature Product (a) Manufacture Computer-aided manufacturing and process planning (CAM and CAPP) Computer-aided design (CAD) Market Specification Iterations Concept design Computer-integrated manufacturing (CIM) Detail design Sell (b) Figure I.4: (a) Chart showing various steps involved in traditional design and manufacture of a product. Depending on the complexity of the product and the type of materials used, the time span between the original concept and the marketing of the product may range from a few months to several years. (b) Chart showing general product flow in concurrent engineering, from market analysis to marketing the product. Source: After S. Pugh. Although this concept still has the same general product-flow sequence as in the traditional approach, shown in Fig. I.4a, it now includes several deliberate modifications. From the earliest stages of product design and engineering, all relevant disciplines are now simultaneously considered. As a result, any iterations that may have to be made will require a smaller effort, resulting in much less wasted time than occurs in the traditional approach to design. It should be apparent that a critical feature of this approach is the recognition of the importance of communication among and within all disciplines. Main design Flow ׉ 7cassandra://ItBisS60k-tExbfgkIlNxnQaPEEtCC5MqNkslHmXtigQ`cZ%EbCcZ%EbB#בCט#u׉׉ 7cassandra://3BcrhYth95Mg1_1sm72x9mA7Vz64iC-d_D1VHJW1d6ss]`׉ 7cassandra://IYMX19Qz-00pOkvCopUGH9a9mXZZwmAizjPncx4e45ono`s׉ 7cassandra://l-GAYHCSXNUYu4z-VmWnSVI_JklG0uxRDLFqfWXKN1wQ`׉ 7cassandra://_GaRJfAPRWHvczOESLFUswr_mUgpFPhozFtgdKNTjNU`͠^cZ+EbƖט#u׉׉ 7cassandra://UfbaBvKPK_LnorJnRtTtXsWrMKivShE0rvyIHXU5eJM˘`׉ 7cassandra://fquSoxDj41lZlAs5f2r2rRjidRWCm7wpyN4y2JRkz9EY`s׉ 7cassandra://uLIQQRsNILwKNw_0OW9rVUlLfSoQsEgnN4XbdoPGi2E`׉ 7cassandra://oM7XUqdmO8wmCIpr6xJGKEbkk59Z1lUix2wAREr85Xk͙͠^cZ+EbƗ׉EProduct Design and Concurrent Engineering 37 Concurrent engineering can be implemented in companies large or small, which is particularly significant because 98% of all U.S. manufacturing companies have fewer than 500 employees; the companies are generally referred to as small businesses or small manufacturing enterprises (SMEs). As an example of the benefits of concurrent engineering, one automotive company reduced the number of components in one of its engines by 30%, decreased the engine weight by 25%, and reduced its manufacturing time by 50%. Life Cycle. In concurrent engineering, the design and manufacture of products are integrated, with a view toward optimizing all elements involved in the life cycle of the product (see Section I.4). The life cycle of a new product generally consists of four stages: 1. Product start-up 2. Rapid growth of the product in the marketplace 3. Product maturity 4. Decline. Consequently, life-cycle engineering requires that the entire life of a product be considered, beginning with the design stage and on through production, distribution, use, and, finally, recycling or the disposal of the product. Role of Computers in Product Design. Typically, product design first requires the preparation of analytical and physical models of the product, for the purposes of visualization and engineering analysis. Although the need for such models depends on product complexity, constructing and studying these models are now done using computer-aided design (CAD) and computer-aided engineering (CAE) techniques. CAD systems are capable of rapid and complete analyses of designs, whether it is a simple part in large and complex structures. The Boeing 777 passenger airplane, for example, was designed completely by computers, in a process called paperless design, with 2000 workstations linked to eight design servers. Unlike previous mock-ups of aircraft, no prototypes or mock-ups were built and the 777 was built and assembled directly from the CAD/CAM software that had been developed. Through computer-aided engineering, the performance of structures subjected, for example, to static or fluctuating loads or to temperature gradients also can be simulated, analyzed, and tested, rapidly and accurately. The information gathered is stored, and it can be retrieved, displayed, printed, and transferred anytime and anywhere within an organization. Design modifications can be made and optimized directly, easily, and at any time. Computer-aided manufacturing involves all phases of manufacturing, by utilizing and processing large amounts of information on materials and processes gathered and stored in the organization’s database. Computers greatly assist in such tasks as (a) programming for numerical-control machines and for robots for material-handling and assembly operations (Chapter 37), (b) designing tools, dies, molds, fixtures, and work-holding devices (Parts II, III, and IV), and (c) maintaining quality control throughout the total operation (Chapter 36). On the basis of the models developed and analyzed, product designers finalize the geometric features of each of the product’s components, including specifying their dimensional tolerances and surface characteristics. Because all components, regardless of their size, eventually have to be assembled into the final product, dimensional tolerances are a major consideration in manufacturing (Chapter 35). The models developed also allow the specification of the mechanical and physical properties required, which in turn affect the selection of materials. (Section I.5). Prototypes. A prototype is a physical model of an individual component or product. The prototypes developed are carefully reviewed for possible modifications to the original design, materials, or production methods. An important and continuously evolving technology is additive manufacturing (Chapter 20). Using CAD/CAM and various specialized technologies, designers make prototypes rapidly and at low cost, from a variety of metallic or nonmetallic materials. ׉ 7cassandra://l-GAYHCSXNUYu4z-VmWnSVI_JklG0uxRDLFqfWXKN1wQ`cZ%EbD׉E38 General Introduction Additive manufacturing significantly reduces costs and associated product-development times. The technology has now advanced to such a level that it is used for low-volume economical production of a variety of actual and functional parts. (a) (b) Figure I.5: Guitars produced through additive manufacturing. (a) Spider design being removed from a powder bed. Note that the support material, or cake, has some strength and needs to be carefully removed. (b) Finished Spider guitars. Source: Courtesy of O. Diegel. Case Study I.1 Three-dimensional Printing of Guitars The design flexibility of additive manufacturing is illustrated by the custom guitars produced by ODD, Inc. These guitars are designed in CAD programs, with full artistic freedom to pursue innovative designs; those in Fig. I.5 are only a selection of the many available. The CAD file is then sent to a three-dimensional printer, using the selective laser sintering process and produced from nylon (Duraform PA). As printed, the guitars are white; they are first dyed to a new base color, then hand-painted and sprayed with a clear satin lacquer. The customer-specified hardware (pickups, bridges, necks, tuning heads, etc.) are then mounted to produce the electric guitar. Virtual Prototyping. This is a software-based method that uses advanced graphics and virtual-reality environments to allow designers to view and examine a part in detail. This technology, also known as simulation-based design, uses CAD packages to render a part such that, in a 3-D interactive virtual environment, designers can observe and evaluate the part as it is being developed. Virtual prototyping has been gaining importance, especially because of the availability of low-cost computers and simulation and analysis tools. ׉ 7cassandra://uLIQQRsNILwKNw_0OW9rVUlLfSoQsEgnN4XbdoPGi2E`cZ%EbEcZ%EbD#בCט#u׉׉ 7cassandra://V0-ECY2iIxRsdPNlt_9CuscFKnxsQ5x4B_3U3j40DzMf`׉ 7cassandra://_F8X0nxpc4TF5MVPwTjVRkmqtFTTUjVH6AHNWa-Q66c_w`s׉ 7cassandra://sNFFkMKjoyqSLjstf3385hQQFXyLrDjKnS391PTasVw2`׉ 7cassandra://s_Z9GoQhQWeLNa4xlejoduwpsqMb_2UhiqOqFUW3GMY uz͠^cZ,Ebƚט#u׉׉ 7cassandra://-CN9dTT3WBz-KQ0Y5kvpYZDASGt8FPAKgWkCbCmYHakc`׉ 7cassandra://ZsDO05sIaAVd6SbLIkYHjrnwzXgN9ipr0CRBV2W3X1se`s׉ 7cassandra://juteukLiaVF-phZs4iMGjoB1b4Gin4AFEY2cyjG5zFo`׉ 7cassandra://9JMqtGmaUFWJ2DLbx6lx9kpOIEqXFIcDh5Xiiaw0hb4^?͠^cZ,EbƜנcZ-Ebƞ\9ׁHhttp://landfills.Thׁ ׁ Ј׉E Design for Manufacture, Assembly, Disassembly, and Service 39 Poor Good Poor Good Poor Good Parts can hang up Chamfer allows part to fall into place (a) Part must be released before it is located (b) Poor Poor Difficult to feed—parts overlap (d) Good Easy to feed Insertion difficult Air-relief hole in workpiece (e) Figure I.6: Redesign of parts to facilitate assembly. Source: After G. Boothroyd and P. Dewhurst. I.3 Design for Manufacture, Assembly, Disassembly, and Service Design for manufacture (DFM) is a comprehensive approach to integrating the design process with production methods, materials, process planning, assembly, testing, and quality assurance. DFM requires a fundamental understanding of (a) the characteristics and capabilities of materials, manufacturing processes, machinery, equipment, and tooling, and (b) variability in machine performance, dimensional accuracy and surface finish of the workpiece, processing time, and the effect of processing methods employed on product quality. Establishing quantitative relationships is essential in order to be able to analyze and optimize a design, for ease of manufacturing and assembly at the lowest cost. The concepts of design for assembly (DFA), design for manufacture and assembly (DFMA), and design for disassembly (DFD) are all important considerations in manufacturing. Methodologies and computer software are available for design for assembly, utilizing 3-D conceptual designs and solid models. Subassembly, assembly, and disassembly times and costs are minimized, while maintaining product integrity and performance.A product which is easy to assemble is usually also easy to disassemble. Assembly is an important phase of manufacturing, requiring considerations of the ease, speed, and cost of putting together the numerous individual components of a product (Fig. I.6). Assembly costs in manufacturing can be substantial, typically ranging from 20% to 60% of the total product cost. Disassembly of a product, for such activities as maintenance, servicing, and eventual recycling of its individual components, is an equally important consideration. As described in Part VI, there are several methods of assembly of components, including the use of a wide variety of fasteners, adhesives, or joining techniques, such as welding, brazing, or soldering. As is the case in all types of manufacturing, each of these assembly operations has its own specific characteristics, times, advantages, limitations, associated costs, and design considerations. Individual parts may be assembled by hand or by a variety of automatic equipment and industrial robots. The choice depends on several Air-relief hole in pin Air-relief flat on pin Part is located before release Can easily tangle Will tangle only under pressure (c) Good ׉ 7cassandra://sNFFkMKjoyqSLjstf3385hQQFXyLrDjKnS391PTasVw2`cZ%EbF׉E o40 General Introduction factors, such as product complexity, number of components to be assembled, care and protection required to prevent damage to the parts, and relative cost of labor compared with the cost of machinery required for automated assembly. Design for Service. In addition to design for assembly and for disassembly, design for service is an important aspect of product design. Products often have to be disassembled, to varying degrees, in order to service them and, if necessary, repair them. The design should take into account the concept that, for ease of access, components that are most likely to be in need of servicing be placed, as much as possible, at the outer layers of the product. This methodology can be appreciated by anyone who has had the experience of servicing machinery. I.4 Environmentally Conscious Design, Sustainable Manufacturing, and Product Life Cycle In the United States alone, more than 30 billion kg of plastic products are discarded each year, and 62 billion kg of paper products. Every three months, U.S. industries and consumers discard enough aluminum to rebuild the country’s commercial air fleet. Globally, countless metric tons of automobiles, televisions, appliances, and computers are discarded each year. Metalworking fluids, such as lubricants and coolants, and fluids and solvents, such as those used in cleaning manufactured products, can pollute the air and waters, unless they are recycled or disposed of properly. Likewise, there are numerous byproducts from manufacturing plants: (a) sand with additives from foundries; (b) water, oil, and various other fluids from heat-treating and facilities; (c) slag from foundries and welding operations; and (d) a wide variety of metallic and nonmetallic scrap produced in such operations as sheet forming, casting, and molding. Consider also the various effects of water and air pollution, acid rain, ozone depletion, hazardous wastes, landfill seepage, and global warming. Recycling efforts have gained increasing momentum over the years; aluminum cans, for example, are now recycled at a rate of 67% and plastics at around 9%. Note that, as indicated below, the term discarding suggests that the product has reached the end of its useful life; however, it does not necessarily indicate that it has to be dumped into landfills.The particular manufacturing process and the operation of machinery can each have a significant environmental impact. Manufacturing operations generally produce some waste, such as: 1. Chips from machining and trimmed materials from sheet forming, casting, and molding operations 2. Slag from foundries and welding operations 3. Additives in sand used in sand-casting operations 4. Hazardous waste and toxic materials used in various products 5. Lubricants and coolants in metalworking and machining operations 6. Liquids from such processes as heat treating and plating 7. Solvents from cleaning operations 8. Smoke and pollutants from furnaces and gases from burning fossil fuels. The adverse effects of these activities, their damage to the environment and to the Earth’s ecosystem, and, ultimately, their effect on the quality of human life are now widely recognized. Major concerns involve global warming, greenhouse gases (carbon dioxide, methane, and nitrous oxide), acid rain, ozone depletion, hazardous wastes, water and air pollution, and contaminant seepage into water sources. One measure ׉ 7cassandra://juteukLiaVF-phZs4iMGjoB1b4Gin4AFEY2cyjG5zFo`cZ%EbGcZ%EbF#בCט#u׉׉ 7cassandra://awvQLRBzzCHCz3lkIbuTG_1g4jhB0fmh6KgDBtoEphAe`׉ 7cassandra://qH4uH7R1UrFqELwFOwkkQDrcLflR4to9w-QSZRKE6aYd `s׉ 7cassandra://Jp2qAqF-4QYkGp_jXUoH5EA1JYYzcvHASNzrVVueV28`׉ 7cassandra://GBHEgVsXbRWMaKVM4Easei39hxiIjLxE6mBvaf0kC-Meb͠^cZ-EbƟט#u׉׉ 7cassandra://Jkx0NMRdNjcpXtm6h6zu_t3htdrSVF0O3zQ07ZGMuTAr`׉ 7cassandra://L-LqY5akunjxj0YkMofv93K6qWKkegQ3KnZsKFQfD74a`s׉ 7cassandra://DAYICdUeb3YOd96TqIVX6NsgHT8g6XdZId48r-Q1u7Q`׉ 7cassandra://fvJzVtEIm7RL7kbXuZShfMBI2gmS2jOZmPmt6tyFzjU]o͠^cZ-EbƠ׉E Environmentally Conscious Design, Sustainable Manufacturing,and Product Life Cycle 41 of the adverse impact of human activities is called the carbon footprint, which quantifies the amount of greenhouse gases produced in our daily activities. The term sustainable design and manufacturing has become in common usage in all industrial activities, with major emphasis on design for the environment (DFE). Also called environmentally conscious design and manufacturing and green design, this approach considers all possible adverse environmental impacts of materials, processes, operations, and products, so that they can all be taken into account at the earliest stages of their design and production. These goals also have led to the concept of design for recycling (DFR). Recycling may involve one of two basic activities: • Biological cycle: Organic materials degrade naturally, and in the simplest version of a biological cycle, they lead to new soil that can sustain life. Thus, product design involves the use of organic materials, as well as ensuring that products function well for their intended life and can then be safely disposed of. • Industrial cycle: The materials in the product are recycled and reused continuously. To demonstrate the economic benefits of this approach, it has been estimated that producing aluminum from scrap, instead of from bauxite ore, reduces production costs by as much as 66% and reduces energy consumption and pollution by more than 90%. A basic principle of design for recycling is the use of materials and product design features that facilitate biological or industrial recycling. In the U.S. automotive industry, for example, about 75% of automotive parts (mostly metal) are now recycled, and there are continuing plans to recycle the rest as well, including plastics, glass, rubber, and foam. About 80% of the 300 million discarded automobile tires are reused in various ways. Cradle-to-cradle Production. Also called cradle-to-cradle (C2C), manufacturing considers the impact of each stage of a product’s life cycle, from the time natural resources are mined and processed into raw materials, through each stage of manufacturing products, their use and, finally, recycling. Cradle-to-grave production, also called womb-to-tomb production, has a similar approach, but it does not necessarily consider or take on the responsibility of recycling. Cradle-to-cradle production emphasizes 1. Sustainable and efficient manufacturing activities, using clean technologies 2. Waste-free production 3. Using recyclable and nonhazardous materials 4. Reducing energy consumption 5. Using renewable energy, such as wind, solar, and ocean waves 6. Maintaining ecosystems by minimizing the environmental impact of all manufacturing activities 7. Using materials and energy sources that are available locally, so as to reduce energy use associated with their transport which, by and large, has an inherently high carbon footprint 8. Continuously exploring the reuse and recycling of materials, and perpetually trying to recirculate materials; also included is investigating the composting of materials whenever appropriate or necessary, instead of dumping them into landfills. Guidelines for Sustainable Design and Manufacturing. In reviewing the activities described thus far, it can be noted that there are overarching relationships among the basic concepts of DFMA, DFD, DFE, and DFR. These relationships can be summarized as guidelines, rapidly accepted worldwide: ׉ 7cassandra://Jp2qAqF-4QYkGp_jXUoH5EA1JYYzcvHASNzrVVueV28`cZ%EbH׉E 42 General Introduction 1. Reduce waste of materials, by refining product design, reducing the amount of materials in products, and select manufacturing processes that minimize scrap (such as forming instead of machining). 2. Reduce the use of hazardous materials in products and processes. 3. Investigate manufacturing technologies that make environmentally friendly and safe products and by-products. 4. Make improvements in methods of recycling, waste treatment, and reuse of materials. 5. Minimize energy use; whenever possible, encourage the use of renewable sources of energy. Select materials can have a major impact on the latent energy in products, as described in Section 40.5. 6. Encourage recycling by using materials that are a part of either industrial or biological cycling, but not both in the same product. Ensure proper handling and disposal of all waste of materials that are used in products, but are not appropriate for industrial or biological cycling. I.5 Selection of Materials An increasingly wide variety of materials are now available, each type having its own properties and manufacturing characteristics, advantages, limitations, and costs (Part I). The selection of materials for products (consumer or industrial) and their components is typically made in consultation with materials engineers; design engineers may also be sufficiently experienced and qualified to assist. The general types of materials used, either individually or in combination with other materials, are the following: • Ferrous metals: Carbon, alloy, stainless, and tool and die steels (Chapter 5) • Nonferrous metals: Aluminum, magnesium, copper, nickel, titanium, superalloys, refractory metals, beryllium, zirconium, low-melting-point alloys, and precious metals (Chapter 6) • Plastics (polymers): Thermoplastics, thermosets, and elastomers (Chapter 7) • Ceramics, glasses, glass ceramics, graphite, diamond, and diamond-like materials (Chapter 8) • Composite materials: Reinforced plastics and metal-matrix and ceramic-matrix composites (Chapter 9) • Nanomaterials (Section 8.8) • Shape-memory alloys (smart materials), amorphous alloys, semiconductors,and superconductors (Chapters 6, 18 and 28) As new developments continue, selection of an appropriate material for a particular application from a very large variety of materials has become even more challenging. Furthermore, there are continuously shifting trends in the substitution of materials, driven not only by technological considerations, but also by economics. Properties of Materials. Mechanical properties of interest in manufacturing generally include strength, ductility, hardness, toughness, elasticity, fatigue, and creep resistance (Chapter 2). Physical properties are density, specific heat, thermal expansion and conductivity, melting point, and electrical and magnetic properties (Chapter 3). Optimum designs often require a consideration of a combination of mechanical and physical properties. A typical example is the strength-to-weight and stiffness-to-weight ratios of materials for minimizing the weight of structural members. Weight minimization is particularly important for aerospace and automotive applications, in order to improve performance and fuel economy. ׉ 7cassandra://DAYICdUeb3YOd96TqIVX6NsgHT8g6XdZId48r-Q1u7Q`cZ%EbIcZ%EbH#בCט#u׉׉ 7cassandra://-FdeX4FfjoQDL61wiFmInZTTpQ-8mBtd68pqm6p0vns`׉ 7cassandra://fsEx-tmmLl295Gl9ohLV6R_ckG7kpwykCRBxpo2NIuM_`s׉ 7cassandra://tYixPXCOnvsvZfp8R8qbbFeKRksHt4IMFr_haB0zlLs`׉ 7cassandra://vU1ybOq44nyX8-zksfEgMA7SWqIlw24Zh0cJ5Wb4btEb^L͠^cZ-EbƢט#u׉׉ 7cassandra://ZOEghZvDWoE9xyx0vpvRks9SU_5IMBLKRywY7Qnot3st[`׉ 7cassandra://5eivN2_f40t15zXzIeKzyv-QR1FVKzJ3Oq6XOCdrVqcVd`s׉ 7cassandra://qb6sn5z9SUY06eufN1BxpFYHSjaUsr3CgPe3t2WyAwg0`׉ 7cassandra://NibLzasH-zqJUxFoHWz7xoGyUPUhwTVrfB5ARWisnsgLG͠^cZ-EbƣנcZ-EbƥDů 9ׁHhttp://verypoor.Thׁ ׁ Ј׉E Selection of Materials 43 Table I.4: General Manufacturing Characteristics of Various Materials. Alloy Aluminum Copper Gray cast iron White cast iron Nickel Steels Zinc Castability Weldability E G–F E G F F E F F D VP F E D Machinability E–G G–F G VP F F E Note: E, excellent;G,good;F,fair;D, difficult; VP,verypoor.Theratings shown depend greatly on the particular material, its alloys, and its processing history. Chemical properties include oxidation, corrosion, degradation, toxicity, and flammability. These properties play a significant role under both hostile (such as corrosive) and normal environments. Manufacturing properties indicate whether a particular material can be cast, formed, shaped, machined, joined, and heat treated with relative ease. As Table I.4 illustrates, no one material has the same manufacturing characteristics. Another important consideration is appearance, which includes such characteristics as surface texture, color, and feel, all of which can play a significant role in a product’s acceptance by the public. Availability. As emphasized throughout this book, the economic aspect of material selection is as important as technological considerations (Chapter 40). Availability of materials is a major concern in manufacturing. Furthermore, if materials are not available in the shapes needed, dimensions, surface texture, and quantities, materials substitution or additional processing of a particular material may well be required, all of which can contribute significantly to product cost. Reliability of supply is important in order to meet production schedules. In automotive industries, for example, materials must arrive at a plant at appropriate time intervals (see also just in time, Section I.7). Reliability of supply is also important, considering the fact that most countries import numerous raw materials. The United States, for example, imports most of the cobalt, titanium, chromium, aluminum, nickel, natural rubber, and diamond that it needs. A country’s self-reliance on resources, especially energy, is an often-expressed political goal, but challenging to achieve. Geopolitics (defined briefly as the study of the influence of a nation’s physical geography on its foreign policy) also must thus be a consideration, particularly during periods of global instability or hostility. Service Life. Everyone has directly experienced a shortened service life of a product, which often can be traced to one or more of the following: (a) improper selection of materials, (b) improper selection of production methods, (c) insufficient control of processing variables, (d) defective raw materials or parts, or manufacturing-induced defects, (e) poor maintenance of machinery and equipment, and (f) improper use of the product. Generally, a product is considered to have failed when it • stops functioning, due to the failure of one or more of its components, such as a broken shaft, gear, turbine blade, or a burned-out electric motor • does not function properly or perform within its required specifications, due, for example, to worn gears or bearings • becomes unreliable or unsafe for further use, as in the erratic behavior of a switch, poor connections in a printed-circuit board, or delamination of a composite material. Material Substitution in Products. For a variety of reasons, numerous substitutions are often made in materials, as evidenced by a routine inspection and comparison of common products, such as home appliances, ׉ 7cassandra://tYixPXCOnvsvZfp8R8qbbFeKRksHt4IMFr_haB0zlLs`cZ%EbJ׉E 44 General Introduction sports equipment, and automobiles. As a measure of the challenges faced in material substitution, consider the following examples: (a) metal vs. wooden handle for a hammer, (b) aluminum vs. cast-iron lawn chair, (c) copper vs. aluminum electrical wire, and (d) alloy steel vs. titanium submarine hull. The following two case studies describe some details of the major factors involved in material substitution in common products. Case Study I.2 U.S. Pennies Billions of pennies are produced and put into circulation each year by the U.S. Mint. The materials used have undergone significant changes throughout their history, largely because of periodic material shortages and the resulting fluctuating cost of appropriate raw materials. The following table shows the chronological development of material substitutions in pennies: 1793–1837 1837–1857 1857–1863 1864–1962 1982–present 100% copper 95% copper, 5% tin and zinc 88% copper, 12% nickel 1943 (WW II years) Steel, plated with zinc 1962–1982 95% copper, 5% tin and zinc 95% copper, 5% zinc 97.5% zinc, plated with copper I.6 Selection of Manufacturing Processes There is often more than one method that can be employed to produce a part from a given material. The following broad categories of manufacturing methods are all applicable for metallic as well as nonmetallic materials: 1. Casting (Fig. I.7a): Expendable mold and permanent mold (Part II). 2. Forming and shaping (Figs. I.7b through I.7d): Rolling, forging, extrusion, drawing, sheet forming, powder metallurgy, and molding (Part III). 3. Machining (Fig. I.7e): Turning, boring, drilling, milling, planing, shaping, broaching; grinding; ultrasonic machining; chemical, electrical, and electrochemical machining; and high-energy-beam machining (Part IV). This broad category also includes micromachining for producing ultraprecision parts (Part V). 4. Joining (Fig. I.7f): Welding, brazing, soldering, diffusion bonding, adhesive bonding, and mechanical joining (Part VI). 5. Finishing: Honing, lapping, polishing, burnishing, deburring, surface treating, coating, and plating (Chapters 26 and 34). 6. Microfabrication and nanofabrication: Technologies that are capable of producing parts with dimensions at the micro (one-millionth of a meter) and nano (one-billionth of a meter) levels; fabrication of microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS), typically involving processes such as lithography, micromachining, etching, LIGA, and various specialized processes (Chapters 28 and 29). Process Selection. The selection of a particular manufacturing process or, more often, sequence of processes, depends on the geometric features of the parts to be made, including the dimensional tolerances ׉ 7cassandra://qb6sn5z9SUY06eufN1BxpFYHSjaUsr3CgPe3t2WyAwg0`cZ%EbKcZ%EbJ#בCט#u׉׉ 7cassandra://H0zL_zzLj6MbYiXL4VVSxn7X4hDqGvzXMGcOkeyUUZct`׉ 7cassandra://ytvtvDAcbxO9Kf4ag-Kyg1cMzp3Zj65zhP4k2T47WCUG`s׉ 7cassandra://w5oEZfAgj52QEvA-7v8Xcqi3Xp8eerzYyU48O9HJB_c`׉ 7cassandra://Br-urNum5MCpaKJJTma_1c8DNVvIO6uyEScFF9YuYDYm͠^cZ1Ebƨט#u׉׉ 7cassandra://jW2dIJ0BVGqZYDnXzP3bgeQDbuX0wgrdSfjxxUrgiZM`׉ 7cassandra://UnE9YWA8uBwmp4ST2r4igytoEwDybVL55c95lGbVDWoT`s׉ 7cassandra://kWnd4F4fby_XEJN4h2Ejv5f6fFf3rOZFo9EIedAeQMI`׉ 7cassandra://0rvpoewqtP8QwPPRjLglWCPiQd6TMQX29xWhJL9HwVU=n͠^cZ2EbƬ׉ENSelection of Manufacturing Processes 45 Casting processes Expendable pattern and mold and other Expendable mold, permanent pattern Permanent mold Investment casting Sand casting Permanent mold casting Lost-foam casting Shell-mold casting Die casting Single-crystal casting Ceramic-mold casting Centrifugal casting Melt-spinning process Squeeze casting (a) Figure I.7: (a) Schematic illustrations of various casting processes. and surface texture required, and on numerous factors pertaining to the particular workpiece material and its manufacturing properties. To emphasize the challenges involved, consider the following two cases: 1. Brittle and hard materials cannot be shaped or formed without the risk of fracture, unless they performed at elevated temperatures, whereas these materials can be cast, machined, or ground with relative ease. ׉ 7cassandra://w5oEZfAgj52QEvA-7v8Xcqi3Xp8eerzYyU48O9HJB_c`cZ%EbL׉E 46 General Introduction Bulk-deformation processes Rolling Forging Extrusion and drawing Flat rolling Open-die forging Direct extrusion Shape rolling Closed-die forging Cold extrusion Ring rolling Heading Drawing Roll forging Piercing (b) Figure I.7: (b) Schematic illustrations of various bulk-deformation processes. 2. Metals that have been preshaped at room temperature become less formable during subsequent processing, which, in practice, is often necessary to complete the part. This is because the metals have become stronger, harder, and less ductile than they were prior to processing them further. There is a constant demand for new approaches to production challenges and, especially, for manufacturing cost reduction. For example, sheet-metal parts traditionally have been cut and fabricated using common mechanical tools, such as punches and dies. Although still widely used, some of these operations have been replaced by laser cutting (Fig. I.8). This method eliminates the need for hard tools, which typically have only fixed shapes, and can be expensive and time consuming to make. The laser path in this operation is computer controlled, thereby increasing the operation’s flexibility and its capability for accurately producing an infinite variety of shapes, repeatedly, and economically. Because of the high heat involved in using lasers, however, the surfaces produced have very different characteristics (such as texture and discoloration) than those produced by traditional methods. This difference can have Tube drawing ׉ 7cassandra://kWnd4F4fby_XEJN4h2Ejv5f6fFf3rOZFo9EIedAeQMI`cZ%EbMcZ%EbL#בCט#u׉׉ 7cassandra://MI_WWHTRWC2XW7sn4h0E_jFawB30guvgaG2PhUaPmWM`׉ 7cassandra://Et4o8V1QoA8oVRTk3d5wTjPIPZg9ro4cq5hnXlGTxaAJ`s׉ 7cassandra://jFnYsmmBwepWaYjv4Mov692hkjemCnu3ws4dNIxDBfI`׉ 7cassandra://hbWQ1W7fhQCZa7s9UHwTbtRnPyfvFMC7P62J4z_5-RIͳH͠^cZ4EbƮט#u׉׉ 7cassandra://0rDeBr1uBM1InI0EWWeifB_LG_xetnv1vK7IZEA0F0Mw<`׉ 7cassandra://_t639MWkd-DwO4p9eLmGdAIkT8elFL3zh0ypFZac8FEWy`s׉ 7cassandra://Zn5sOfmNlSNPz11QyebH7WJJ0Ih8e_Js42oBV3rcyjw`׉ 7cassandra://cEoN9m2jwWY7BDanEei37BVyac3L3jooJnBSNQ6kqwo8͠^cZ5EbƯ׉EtSelection of Manufacturing Processes 47 Sheet-metal forming processes Shearing Bending and drawing Forming Blanking Bending Stretch forming Slitting Hemming Hydroforming Punching Roll forming Spinning Piercing Deep drawing (c) Magnetic-pulse forming Figure I.7: (c) Schematic illustrations of various sheet-metal forming processes. significant adverse effects, not only on appearance, but especially on its subsequent processing and in the service life of the product. Moreover, the inherent flexibility of the laser cutting process is countered by the fact that it is slower than traditional punching operations. Several factors can have a major role in process selection, such as part size, shape complexity, and dimensional accuracy and surface finish required. For example: • Flat parts and thin cross sections can be difficult to cast. • Complex parts generally cannot be shaped easily and economically by such metalworking techniques as forging, whereas, depending on part size and level of complexity, the parts may be precision cast, fabricated and assembled from individual pieces, or produced by powder-metallurgy techniques. ׉ 7cassandra://jFnYsmmBwepWaYjv4Mov692hkjemCnu3ws4dNIxDBfI`cZ%EbN׉E48 General Introduction Polymer-processing processes Thermoplastics Thermosets Rapid prototyping Extrusion Compression molding Stereolithography Injection molding Pultrusion Fused-deposition modeling Blow molding Vacuum-bag forming Three-dimensional printing Thermoforming Transfer molding (d) Laminated-object manufacturing Figure I.7: (d) Schematic illustrations of various polymer-processing methods. • Dimensional tolerances and surface finish in hot-working operations are not as fine as those obtained in operations performed at room temperature (cold working), because of the dimensional changes, distortion, warping, and surface oxidation due to elevated temperatures. Part size and dimensional accuracy. The size, thickness, and shape complexity of a part have a major bearing on the process selected. Complex parts, for example, may not be formed easily and economically, whereas they may be produced by casting, injection molding, and powder metallurgy, or they may be fabricated and assembled from individual pieces. Likewise, flat parts with thin cross sections may not be cast easily. Dimensional tolerances and surface finish in hot-working operations cannot be as fine as those in cold-working operations, because dimensional changes, warping, and surface oxidation occur during processing at elevated temperatures. Also, some casting processes produce a better surface finish than others, ׉ 7cassandra://Zn5sOfmNlSNPz11QyebH7WJJ0Ih8e_Js42oBV3rcyjw`cZ%EbOcZ%EbN#בCט#u׉׉ 7cassandra://l--Fm-cCu59YDkNky7JEVPWutaNYGiWuih5p-FbREbA`׉ 7cassandra://p3Msj0UPthMQBU0Vr9Gb50K93mzgcUknfeH4lmcPm2YV8`s׉ 7cassandra://a5NTpTVLn6_HLoGJrDHuVCyUfQAduEhq8qmhKo5EIxY`׉ 7cassandra://2zUuI7uT873bJxXBAyxdyNgX798L4TxpEGtGSMnymSo*͠^cZ8EbƵט#u׉׉ 7cassandra://LKbrPU1XKYxHuy6SeRMt0A0pb2NoNQn0txjJ51kXVcM`׉ 7cassandra://lYhiV9tM0SRV730oVdUORRHKJ-2kanBQmh0Tj_86kDsOX`s׉ 7cassandra://-Ux01KAD9fyElIjN1eOLPLH7ZcguxDNxXU3EpJntNBo`׉ 7cassandra://jKUtsMNCYlyiD47BD7YS4hLlMXkBACGHMfG7J_cHygMv͠^cZ:Ebƻ׉ESelection of Manufacturing Processes 49 Machining and finishing processes Machining Advanced machining Finishing Turning Wire EDM Surface grinding Drilling Chemical machining Centerless grinding Milling Laser machining Lapping Broaching Water-jet machining (e) Electrochemical polishing Figure I.7: (e) Schematic illustrations of various machining and finishing processes. because of the different types of mold materials used. Moreover, the appearance of materials after they have been manufactured into products greatly influences their appeal to the consumer; color, surface texture, and feel are characteristics typically are considered when making a purchasing decision. The size and shape of manufactured products vary widely. The main landing gear for the twinengine, 400-passenger Boeing 777 jetliner, for example, is 4.3 m tall, and has three axles and six wheels. The main structure of the landing gear is made by forging, followed by several machining operations ׉ 7cassandra://a5NTpTVLn6_HLoGJrDHuVCyUfQAduEhq8qmhKo5EIxY`cZ%EbP׉E50 General Introduction Joining processes Fusion welding Other welding Fastening and bonding Shielded metal-arc welding Friction-stir welding Adhesive bonding Gas–metal arc welding Resistance welding Bolted connection Flux-cored arc welding Explosion welding Wave soldering Gas–tungsten arc welding Cold welding (f) Brazing Figure I.7: (f) Schematic illustrations of various joining processes. (Chapters 6, 8, and 9). At the other extreme is manufacturing microscopic parts and mechanisms. These components are produced through surface micromachining operations, typically using electron beam, laser beam, and wet and dry etching techniques, on materials such as silicon. Process Substitution. It is common practice in industry that, for a variety of reasons and after a review of all appropriate and applicable processes, a particular production method, which may have been employed in the past, may well have to be substituted with another method. Consider, for example, the following ׉ 7cassandra://-Ux01KAD9fyElIjN1eOLPLH7ZcguxDNxXU3EpJntNBo`cZ%EbQcZ%EbP#בCט#u׉׉ 7cassandra://eLgLQcxKn-yICJPvaaVKqJTE6olpLVcYkjgyYNMOuxg= `׉ 7cassandra://bMY5wMql2BIAODsf1FbUwm7cp-rbN8y1iEefX6UhBkcT`s׉ 7cassandra://SvrkJqVFEhQNmF6z6TeVjngifRNprMey1W6daxYFDwYY`׉ 7cassandra://f0AYYPecxhzbM6IkfabxerMcnP-MH1qrSKKnuKstHmYj+͠^cZ:Ebƽט#u׉׉ 7cassandra://jCNr7iRSDSoAUbShNPhwC6uBBizcQaA9br7CbPagjik:f`׉ 7cassandra://3FeBmjzgIfrbbshCJBVM8vegacFqiI7aTj68uVXs1Wk^u`s׉ 7cassandra://ZEzvIB74V62dAIjwrhUprRrFrumkL9pofiy86DqKz_c`׉ 7cassandra://vOIU8GOdGo_m618k8DpR2eu-xlJMpgNZzj9Qrs42W9UX͠^cZ:Ebƾ׉E Selection of Manufacturing Processes 51 Figure I.8: Cutting sheet metal with a laser beam. Source: Courtesy of SPI Lasers UK Ltd. products which can be produced by any of the sets of the following processes: (a) Cast vs. forged crankshaft, (b) stamped sheet-metal vs. forged or cast automobile wheels, (c) cast vs. stamped sheet-metal frying pan, (d) selective laser sintered vs. extruded or cast polymer bracket, and (e) welded vs. riveted sheet-metal safety hood for a machine. Criteria for process selection include such factors as cost, maintenance required, whether the product is for industrial or consumer use, parameters to which the product will be subjected (such as external forces, impact, temperatures, and chemicals), environmental concerns that have to be addressed, and the product’s appeal to the customer. Net-shape and Near-net-shape Manufacturing. Net-shape and near-net-shape manufacturing together constitute an important methodology, by which a part is made in only one operation at or close to the final specific dimensions, tolerances, and surface finish. The difference between net shape and near net shape is a matter of degree, of how close the product is to its final dimensional surface-finish characteristics. The necessity for and benefits of net-shape manufacturing can be appreciated from the fact that, in the majority of cases, more than one additional manufacturing operation or step is often necessary to produce the part. For example, a cast or forged crankshaft generally will not have the necessary dimensional surface finish characteristics, and will typically require additional processing, such as machining or grinding. These additional operations can contribute significantly to the cost of a product. Typical examples of net-shape manufacturing include precision casting (Chapter 11), forging (Chapter 14), forming sheet metal (Chapter 16), powder metallurgy and injection molding of metal powders (Chapter 17), and injection molding of plastics (Chapter 19). Ultraprecision Manufacturing. Dimensional accuracies for some modern equipment and instrumentation have now reached the magnitude of the atomic lattice (below 1 nm). Several techniques, including the use of highly sophisticated technologies (see micromechanical and microelectromechanical device fabrication, Chapter 29), are rapidly being developed to attain extreme accuracy. Also, mirror-like surfaces on metals can now be produced by machining, using a very sharp diamond with a nose radius of 250 μmasthe cutting tool. The equipment is highly specialized, with very high stiffness, to minimize deflections, as well as vibration and chatter, during machining. It is operated in a room where the ambient temperature is controlled to within 1◦C, in order to avoid thermal distortions of the machine. ׉ 7cassandra://SvrkJqVFEhQNmF6z6TeVjngifRNprMey1W6daxYFDwYY`cZ%EbR׉E 752 General Introduction Types of Production. The number of parts to be produced (such as the annual quantity) and the rate (the number of pieces made per unit time) are important economic considerations in determining optimum processes and types of machinery required. Note, for example, that beverage cans, door locks, and spark plugs are produced in numbers and at rates that are much higher than those for jet engines ship propellers. A brief outline of the general types of production, in increasing order of annual quantities produced, are: 1. Job shops: Small lot sizes, typically less than 100, using general-purpose machines, such as lathes, milling machines, drill presses, and grinders, many now typically equipped with computer controls. 2. Small-batch production: Quantities from about 10 to 100, using machines similar to those in job shops. 3. Batch production: Lot sizes typically between 100 and 5000, using more advanced machinery with computer control. 4. Mass production: Lot sizes generally over 100,000, using special-purpose machinery, known as dedicated machines, and various automated equipment in a plant for transferring materials and parts in progress. Case Study I.3 Saltshaker and Pepper Mill The saltshaker and pepper mill set shown in Fig. I.9 consists of metallic and nonmetallic components. The main parts (the body) of the set are made by injection molding of a thermoplastic (Chapter 19), such as an acrylic, which has both transparency and other characteristics for this particular application and also is easy to mold. The round metal top of the saltshaker is made of sheet metal, has punched holes (Chapter 16), and is electroplated for improved appearance corrosion resistance (Section 34.9). The knob on the top of the pepper mill is made by machining (Chapter 23) and is threaded on the inside to allow it to be screwed and unscrewed. The square rod connecting the top portion of the pepper mill to the two pieces shown at the bottom of the figure is made by rolling (Chapter 13). The two grinder components, shown at the bottom of the figure, are made of stainless steel. A design for manufacturing analysis indicated that casting or machining the two components would be too costly; consequently, it was determined that an appropriate and economical method would be the powder-metallurgy technique (Chapter 17). I.7 Computer-integrated Manufacturing Computer-integrated manufacturing (CIM), as the name suggests, integrates the software and the hardware needed for computer graphics, computer-aided modeling, and computer-aided design and manufacturing activities, from initial product concept through its production and distribution in the marketplace. This comprehensive and integrated approach began in the 1970s, and has been particularly effective because of its capability of making possible the following tasks: • Responsiveness to rapid changes in product design modifications and to varying market demands • Better use of materials, machinery, and personnel • Reduction in inventory • Better control of production and management of the total manufacturing operation. ׉ 7cassandra://ZEzvIB74V62dAIjwrhUprRrFrumkL9pofiy86DqKz_c`cZ%EbScZ%EbR#בCט#u׉׉ 7cassandra://XWJIfeAMqENGPCUaDG9bP3y_8Hum3ruqCNREL-36RhM=`׉ 7cassandra://LvPDEk25xs-6mIeU4fT368ccasW7VB18NSBYWZjGOCsPa`s׉ 7cassandra://i8V6hxV4hG3Y7H2qAxnS4xO0-11K_Sr3uvmi-GqTmmUd`׉ 7cassandra://beLS2_9neaZ2G8q1nhzCy5P8FlEoct28n577ISq58eg` ͠^cZ;Ebט#u׉׉ 7cassandra://DQtW277StBdcXTNQEA0DHfz-QNuInK5yOxAny8kcqh4`׉ 7cassandra://ZyyypLf0ogx7cIaqwcZht1FBEnqkVGi_2azjAry0VCUV`s׉ 7cassandra://NUFXD3HaA-VH8PzTv-jS1n_nBnS9hJCmIipzKOraq_s`׉ 7cassandra://B-yTbCbqGnchxXDbqjlJCR-Qe0K2iYLtaYjugQb7sB0t ͠^cZ;Eb׉E Computer-integrated Manufacturing 53 Figure I.9: A saltshaker and pepper mill set. The two metal pieces (at the bottom) for the pepper mill are made by powder-metallurgy techniques. Source: Courtesy of the Metal Powder Industries Federation. The following is a brief outline of the various elements in CIM, all described in detail in Chapters 38 and 39: 1. Computer numerical control (CNC). First implemented in the early 1950s, this is a method of controlling the movements of machine components by direct insertion of coded instructions in the form of numerical data. 2. Adaptive control (AC). The processing parameters in an operation are automatically adjusted to optimize the production rate and product quality and to minimize manufacturing costs. For example, in machining, forces, temperature, surface finish, and dimensions of the part are constantly monitored. If they move outside the specified range, the system automatically adjusts the relevant variables until all the parameters are within the specified range. 3. Industrial robots. Introduced in the early 1960s, industrial robots have rapidly been replacing humans, especially in operations that are repetitive, dangerous, and boring. As a result, variability in product quality decreases and productivity is improved. Robots are particularly effective in assembly operations; intelligent robots have been developed with sensory perception capabilities and movements that simulate those of humans. Recent innovations involve cobots, which are designed to work and interact with humans. 4. Automated materials handling. Computers have made possible highly efficient handling of materials and parts in various stages of completion (work in progress), as in moving a part from one machine to another, and then to points of inspection, to inventory, and, finally, to shipment. 5. Automated assembly systems. These systems have been developed to replace assembly by human operators, although humans still have to perform some of the operations. Depending on the type of product, assembly costs can be high; thus, products must be designed such that they can be assembled more easily and faster by automated machinery. 6. Computer-aided process planning (CAPP). By optimizing process planning, this system is capable of improving productivity, product quality, and consistency, thus reducing costs. Functions such as cost estimating and monitoring work standards (time required to perform a certain operation) are also incorporated into the system. ׉ 7cassandra://i8V6hxV4hG3Y7H2qAxnS4xO0-11K_Sr3uvmi-GqTmmUd`cZ%EbT׉E X54 General Introduction 7. Group technology (GT). The concept behind group technology is that numerous parts can be grouped and produced by classifying them into families according to similarities in (a) design and (b) the manufacturing processes employed to produce them. In this way, part designs and processing plans can be standardized, and new parts, based on similar parts made previously, can be produced efficiently and economically. 8. Just-in-time production (JIT). The principle behind JIT is that (a) supplies of raw materials and parts are delivered to the manufacturer just in time to be used, (b) parts and components are produced just in time to be made into subassemblies, and (c) products are assembled and finished just in time to be delivered to the customer. As a result, inventory carrying costs are minimal, defects in components are detected right away, productivity is increased, and high-quality products are made and at low cost. 9. Cellular manufacturing (CM). This system utilizes workstations that consist of a number of manufacturing cells, each containing various production machines, all controlled by a central robot, with each machine performing a specific operation on the part, including inspection (Fig. I.10). 10. Flexible manufacturing systems (FMS). These systems integrate manufacturing cells into a large production facility, in which all cells are interfaced with a central computer. Although very costly, flexible manufacturing systems are capable of producing parts efficiently (although in relatively small quantities, because hard automation is still most efficient for mass production) and of quickly changing manufacturing sequences required for making different types of parts. Flexibility enables these systems to meet rapid changes in market demand for all types of products. 11. Expert systems (ES). Consisting basically of complex computer programs, these systems have the capability of performing a variety of tasks and solving difficult real-life problems (much as human experts would), including expediting the traditional iterative process. 12. Artificial intelligence (AI). Computer-controlled systems are capable of learning from experience and of making decisions that optimize operations and minimize costs, ultimately replacing human intelligence. Figure I.10: Robotic arm production line. Source: Shutterstock/Andrey Armyagov. ׉ 7cassandra://NUFXD3HaA-VH8PzTv-jS1n_nBnS9hJCmIipzKOraq_s`cZ%EbUcZ%EbT#בCט#u׉׉ 7cassandra://e86P0mE3fr2MOgOYOxUsRQZckrcYGNY2dQhg_GqUnY0`׉ 7cassandra://-khwXju6Lu9EbkqSpQWfBYCg5-ZYIe8_IPS8uc0Sh1IM`s׉ 7cassandra://mYgXbuEppf6wDpAfyX6jG7TlM0HjnIl1fiKmhqKthGM`׉ 7cassandra://UfibspE7ZZxk3tmGALv36c5NErjoIKhKu1tOhJDD3T8j%T͠^cZ;Ebט#u׉׉ 7cassandra://DfI_bwMoL7lcEhgP8cNwIf2lmS2dRbitBdEvvcrK1pgh`׉ 7cassandra://i_dEw_-n2eHU4QYXcDEa-zEHqSUY-BJ6tEaOYFtc2U0i`s׉ 7cassandra://tn7A3KKM8Rkwfs6-xTFXDo48vYMB-cDTMmyfBtDV0Dg`׉ 7cassandra://raPqtA0QEyPd7dXkb8SJMmcBtsDrx0qRjMkYxStII94j$͠^cZ;Eb׉E Quality Assurance and Total Quality Management 55 13. Artificial neural networks (ANN). These networks are designed to simulate the thought processes of the human brain, with such capabilities as modeling and simulating production facilities, monitoring and controlling manufacturing operations, diagnosing problems in machine performance, and conducting financial planning and managing a company’s manufacturing strategy. I.8 Quality Assurance and Total Quality Management Product quality is one of the most critical considerations in manufacturing, because it directly influences customer satisfaction, thus playing a crucial role in determining a product’s success in the marketplace (Chapter 36). The traditional approach of inspecting products after they were made has largely been replaced by the recognition that quality must be built into the product, from its initial design through all subsequent steps of manufacturing and assembly operations. Even small products typically undergo several manufacturing steps, and each step involves its own variations in performance, which can occur within a relatively short time. A production machine, for example, may perform differently when it is first turned on than after it begins to warm up or when the ambient temperature in the plant fluctuates. Consequently, continuous control of processes online monitoring) is a critical factor in maintaining product quality. The objective is to control processes, not products. Quality assurance and total quality management (TQM) are widely recognized as being the responsibility of everyone involved in the design and manufacturing of products and their components. Product integrity is a term generally used to define the degree to which a product • Functions reliably during its life expectancy (Table I.5) • Is suitable for its intended purposes • Can be maintained with relative ease. Producing and marketing defective products can be very costly to the manufacturer, with costs varying by orders of magnitude, as shown in Table I.6. Table I.5: Average Life Expectancy of Various Products. Type of product U.S. dollar bill Personal computer Car battery Hair dryer Automobile Dishwasher Kitchen disposal unit Vacuum cleaner Water heater (gas) Clothes dryer (gas) Clothes washer Air-conditioning unit (central) Manufacturing cell Refrigerator Furnace (gas) Machinery Nuclear reactor Life expectancy (years) 1.5 2 4 5 8 10 10 10 12 13 13 15 15 17 18 30 40 Note: Significant variations can be expected, depending on the quality of the product and how well it has been maintained. ׉ 7cassandra://mYgXbuEppf6wDpAfyX6jG7TlM0HjnIl1fiKmhqKthGM`cZ%EbV׉E56 General Introduction Table I.6: Relative Cost of Repair at Various Stages of Product Development and Sale. Stage When part is being made Subassembly of the product Assembly of the product Product at the dealership Product at the customer Relative cost of repair 1 10 100 1000 10,000 Pioneers in quality control, particularly W.E. Deming (1900–1993), J.M. Juran (1904–2008), and G. Taguchi (1924–2012), all emphasized the importance of management’s commitment to (a) product quality, (b) pride of workmanship at all levels of production, and (c) the necessity of using statistical process control (SPC) and control charts (Chapter 36). They also pointed out the importance of online monitoring and rapidly identifying the sources ofquality problems in production, before even another defective part is produced. The major goal of control is to prevent defective parts from ever being made, rather than to inspect, detect, and reject defective parts after they have been made. As an example of strict quality control, computer chips are now produced with such high quality that only a few out of a million chips may be defective. The level of defects is identified in terms of standard deviation, denoted by the symbol σ (the Greek letter sigma). Three sigma would result in 2700 defective parts per million, which is unacceptable in modern manufacturing. In fact, it has been estimated that at this level, no modern computer would function reliably. At six sigma, defective parts are reduced to only 3.4 per million parts made. This level has been reached through major improvements in manufacturing process capabilities in order to reduce variability in product quality. Important developments in quality assurance include the implementation of experimental design,a technique by which the factors involved in a manufacturing operation and their interactions are studied simultaneously. For example, the variables affecting dimensional accuracy or surface finish in a machining operation can readily be identified, thus making it possible for appropriate on-time preventive adjustments to be taken. Quality Standards. Global manufacturing and competitiveness have led to an obvious need for international conformity and consensus in establishing quality control methods. This need resulted in the establishment of the ISO 9000 standards series on quality management and quality assurance standards, as well as of the QS 9000 standards (Section 36.6), introduced in 1994. A company’s registration for these standards, which is a quality process certification and not a product certification, means that the company conforms to consistent practices as specified by its own quality system. ISO 9000 and QS 9000 have permanently influenced the manner in which companies conduct business in world trade. Human-factors Engineering. This topic deals with human–machine interactions, and thus it is an important aspect of manufacturing operations in a plant, as well as of products in their expected use. The humanfactors approach is essential in the design and manufacture of safe products. It emphasizes ergonomics, defined as the study of how a workplace and the machinery and equipment in it, can best be designed and arranged for comfort, safety, efficiency, and productivity. Examples of the need for proper ergonomic considerations are the following: (a) a mechanism that is difficult to operate manually, causing injury to the employee; (b) a poorly designed keyboard that causes pain to the user’s hands and arms during its normal use (repetitive stress syndrome); and (c) a control panel on a machine which is difficult to reach or use safely and comfortably. Product Liability. Designing and manufacturing safe products is an essential responsibility of the manufacturer. All those involved with product design, manufacture, and marketing must fully recognize the consequences of a product’s failure, including failure due to foreseeable misuse of the product. ׉ 7cassandra://tn7A3KKM8Rkwfs6-xTFXDo48vYMB-cDTMmyfBtDV0Dg`cZ%EbWcZ%EbV#בCט#u׉׉ 7cassandra://mS4lYPE6eCVPYcqV3FluKthmW2Qe6pG0Ns4F0PM133I:`׉ 7cassandra://ewc4y-Si1hTwAKSIll73CLTOmDU6emfOm7M7A5JNlNU[{`s׉ 7cassandra://NHfm6FxrDXG1H63g59ZR_NfoYSAO0hzOSL8UHW750F8Y`׉ 7cassandra://vVpcOxEd2e5zouIOYdhp0mMoiiHti38pXNw76UMg1Ys`͠^cZz`s׉ 7cassandra://fIf-hITW6266hylTLph5fSP0SBr7g68QZYXYkbzwX4s`׉ 7cassandra://yBnNZHH5zgMdNaufs8k2QJI5pZo-rAc_Hf8iN_7-5L8=͠^cZ=Ebט#u׉׉ 7cassandra://ke_8XhHAjeTTMyFxVRuI91RCDQojLg-uEDPc2JcMQ2oJ+`׉ 7cassandra://Yum0tuEqqvGFB-xL5LrP5vhtZt5dSGGeN_cFDCRzGSgJ6`s׉ 7cassandra://ron7XCycFd8Ao6h4KUHTqHMQgyvy14sN99NkPe6G60k``׉ 7cassandra://r4w4DJ1h78o99ePc_ajEMq-Ibk_EizyI0mNt37e8VuM}l͠^cZ=Eb׉E,Chapter 1 The Structure of Metals 1.1 Introduction 63 1.2 Types of Atomic Bonds 65 1.3 The Crystal Structure of Metals 65 1.4 Deformation and Strength of Single Crystals 67 1.5 Grains and Grain Boundaries 71 1.6 Plastic Deformation of Polycrystalline Metals 73 1.7 Recovery, Recrystallization, and Grain Growth 75 1.8 Cold, Warm, and Hot Working 76 Example: 1.1 Number of Grains in a Paper Clip 72 • This chapter describes the crystalline structure of metals and explains how crystal structure determines properties and behavior. • It begins with a review of the types of atomic bonds and their characteristics: ionic, covalent, and metallic. • Metal structures and the arrangement of atoms within the structure are then examined; the types of imperfections in the crystal structure and their effects on material behavior are presented. • The effects of grains and grain boundaries are examined, followed by a description of strain hardening and anisotropy of metals. 1.1 Introduction Why are some metals hard and others soft? Why are some metals brittle, while others are ductile and can be shaped easily without fracture? Why can some metals withstand high temperatures while others cannot? Why does a piece of sheet metal behave differently when stretched in one direction versus another? These questions can be answered by studying the atomic structure of metals—that is, the arrangement of the atoms within the metals. This knowledge then serves as a guide to controlling and predicting the behavior and performance of metals in various manufacturing processes. 63 ׉ 7cassandra://fIf-hITW6266hylTLph5fSP0SBr7g68QZYXYkbzwX4s`cZ%Eb^׉E64 Chapter 1 The Structure of Metals Figure 1.1: Turbine blades for jet engines, manufactured by three different methods: left: conventionally cast; center: directionally solidified, with columnar grains as can be seen from the vertical streaks, and right: single crystal. Although more expensive, single-crystal blades have properties at high temperatures that are superior to those of other blades. Source: Courtesy of NASA. Understanding the structure of metals allows prediction and evaluation of their properties such as strength and stiffness, whereby appropriate selection for specific applications can then be made. For example, single-crystal turbine blades (Fig. 1.1) for use in jet engines have properties that are better than those for conventional blades. In addition to their atomic structure, several other factors also influence the properties and behavior of metals. These include the composition of the particular metal, impurities and vacancies in their atomic structure, grain size, grain boundaries, the presence of impurities and inclusions, the environment, surface condition of the metal, and the methods by which they are made into specific products. The topics described in this chapter are outlined in Fig. 1.2. The structure and general properties of materials other than metals are described in Chapter 7 (on polymers), Chapter 8 (ceramics and glasses), and Chapter 9 (composite materials). The structure of metal alloys, the control of their structure, and heattreatment processes are described in Chapter 4. Single Liquid metal Solidification Unit cell Body-centered cubic Face-centered cubic Hexagonal close-packed Allotropism Crystals Polycrystals Lattice Imperfections Dislocations Grain boundaries Plastic deformation Anisotropy Figure 1.2: An outline of the topics described in Chapter 1. crystals Products: solid-state devices, turbine blades Products: paper clips, bolts, springs, I-beams, aircraft fuselage ׉ 7cassandra://ron7XCycFd8Ao6h4KUHTqHMQgyvy14sN99NkPe6G60k``cZ%Eb_cZ%Eb^#בCט#u׉׉ 7cassandra://zm8JcxMfG-UXzNhl1MXPxRypdUbHxOiAdyS4y3gQq1kG`׉ 7cassandra://koXIA18tQYU56vS06-XfoGbw1zqchVMGZcMFdhWJrkwj`s׉ 7cassandra://ka34CAfLF04fgZkkDsi6BFTd9lr16n8fkAS21olw9KM_`׉ 7cassandra://mwwHevY0QPHMYU_PBP1DDCWCuH0e-z_E-Ry0qLpPTk0dV͠^cZMEbט#u׉׉ 7cassandra://WvecSBhVcpqppOidBC6C62SbrPz2isWe0waOkOcjiRQA`׉ 7cassandra://qUfDoQD4MhFymueuwj6t5XFiG8kyNISRZ-vwHvrF_9gIX`s׉ 7cassandra://pIrq1SQMzTX2TJ_Td_djnzSfGxILr0OynbQUR5Cnnoo=`׉ 7cassandra://n1BPwNVBNr_y4KKOCqSafH5ZXIIOXGahkmbjSRMK5jwe͠^cZdEbנcZdEbBj9ׁHhttp://structure.Thׁ ׁ Ј׉EThe Crystal Structure of Metals 65 1.2 Types of Atomic Bonds Allmatterismadeupof atoms consisting of a nucleus of protons and neutrons and surrounding clouds, or orbits, of electrons. The number of protons in the nucleus determines whether a particular atom will be metallic, nonmetallic, or semi-metallic. An atom with a balanced charge has the same number of electrons as protons; when there are too many or too few electrons, the atom is called an ion. An excess of electrons results in a negatively charged atom, referred to as an anion, while too few electrons results in a positively charged atom, called a cation. The number of electrons in the outermost orbit of an atom determines the chemical affinity of that atom for other atoms. Atoms can transfer or share electrons; in doing so, multiple atoms can combine to form molecules. Molecules are held together by attractive forces called bonds, which act through electron interaction. The basic types of atomic attraction associated with electron transfer, called primary bonds or strong bonds, are the following: • Ionic bonds. When one or more electrons from an outer orbit are transferred from one material to another, a strong attractive force develops between the two ions. An example is that of sodium (Na) and chlorine (Cl) in common table salt; it consists of Na+ and Cl− ions (hence the term ionic bond), which are strongly attracted to each other. The attraction is between all adjacent ions, allowing crystalline structures to be formed, as described in Section 1.3. Molecules with ionic bonds generally have low ductility and low thermal and electrical conductivity. Ionic bonding is the predominant bond in ceramic materials (Chapter 18). • Covalent bonds. In a covalent bond, the electrons in outer orbits are shared by atoms to form molecules. The number of electrons shared is reflected by terms such as “single bond,” “double bond.” Polymers (Chapter 7) consist of large molecules covalently bonded together. Water (H2O) and nitrogen gas (N2) are common examples of molecules formed by covalent bonds. Solids formed by covalent bonding typically have low electrical conductivity and can have high hardness; diamond, a form of covalently bonded carbon, is an example. • Metallic bonds. Metals have relatively few electrons in their outer orbits, and thus they cannot complete the outer shell when self-mated. Instead, metals and alloys form metallic bonds, whereby the available electrons are shared by all atoms in contact. The resultant electron cloud provides the attractive forces to hold the atoms together, resulting in generally high thermal and electrical conductivity. In addition to the strong attractive forces associated with electrons, weak or secondary attractions exist between molecules; also referred to as van der Waals forces, these forces arise from the attraction of opposite charges without electron transfer. Water molecules, for example, consist of one oxygen atom and two smaller hydrogen atoms, located around 104◦ from each other. Although each molecule has a balanced, or neutral, charge, there are more hydrogen atoms on one side of the molecule (i.e., it is a dipole), so that the molecule develops a weak attraction to nearby oxygen atoms on that side. 1.3 The Crystal Structure of Metals When metals solidify from a molten state (Section 10.2), the atoms arrange themselves into various orderly configurations, called crystals; this arrangement is called crystal structure or crystalline structure.The smallest group of atoms exhibiting the characteristic lattice structure of a particular metal is known as a unit cell. The three basic atomic arrangements in metals, and some examples of each, are 1. Body-centered cubic (bcc); alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium. ׉ 7cassandra://ka34CAfLF04fgZkkDsi6BFTd9lr16n8fkAS21olw9KM_`cZ%Eb`׉E366 Chapter 1 The Structure of Metals 2. Face-centered cubic (fcc); gamma iron, aluminum, copper, nickel, lead, silver, gold, and platinum. 3. Hexagonal close-packed (hcp); beryllium, cadmium, cobalt, magnesium, alpha titanium, zinc, and zirconium. These structures are represented by the illustrations given in Figs. 1.3 through 1.5, in which each sphere represents an atom. The distance between the atoms in these crystal structures is on the order of 0.1 nm. The models shown are known as hard-ball or hard-sphere models; they can be likened to tennis balls arranged in various configurations in a box. a a a a R a (a) (b) (c) Figure 1.3: The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. a a 2R a a (a) (b) (c) Figure 1.4: The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. a a c (a) (b) Figure 1.5: The hexagonal close-packed (hcp) crystal structure: (a) unit cell and (b) single crystal with many unit cells. a ׉ 7cassandra://pIrq1SQMzTX2TJ_Td_djnzSfGxILr0OynbQUR5Cnnoo=`cZ%EbacZ%Eb`#בCט#u׉׉ 7cassandra://DjInv0Jgy3PVZCKASVlLrXtpBaKyYwmTgCEhiAvTs2UT`׉ 7cassandra://N2SNlAiTKYJ0a1Px7S88yuti60bSqpWVB8QMrLyAUrg^N`s׉ 7cassandra://wLIYq-Mhai4_SZIhgZHmANGCaCBztTf7kEPpZSzV7IM`׉ 7cassandra://mAbVee-BnEGBzRD5ss3QTj-uiFUXFKCjdxE0ApGBEDw2͠^cZEbHט#u׉׉ 7cassandra://ATyZkuHJFFvVP2e4w0M1ppFadkaaz2kaiA-UV8GCvNcl`׉ 7cassandra://dTZVncrOaJxmTKSC2G1HlQAgg_VnuuqC3LkR7L_VPPcq(`s׉ 7cassandra://eTClw2zY6hrCaEE_yRDGHXL6kWkr_b_CaDLULRqecOIK`׉ 7cassandra://XSMqZmtC69dWNqXuasBMGvxGev_WYTgjH3eBJzKbDAQi͠^cZEbIנcZEbL1ʁQ9ׁHhttp://planes.Alׁ ׁ Ј׉E WDeformation and Strength of Single Crystals 67 In the three structures described earlier, the hcp crystals have the most densely packed configurations, followed by fcc and then bcc. In the hcp structure, the top and bottom planes are called basal planes.All three arrangements can be modified by adding atoms of some other metal or metals, known as alloying, and often improving various properties of the metal. The presence of more than one type of crystal structure in metals is known as allotropism or polymorphism, meaning “many shapes.” Because the properties and behavior of a particular metal depend greatly on its crystal structure, allotropism is important in the heat treatment of metals and in metalworking and welding operations, described in Parts III and V, respectively. Single crystals of metals are now produced as ingots in sizes on the order of 1.5 m long and up to 300 mm in diameter, with such applications as gas turbine blades and semiconductors (Sections 11.5 and 28.4). Most metals used in manufacturing operations are polycrystalline, as described in Section 1.5. 1.4 Deformation and Strength of Single Crystals When a single crystal is subjected to an external force, it first undergoes elastic deformation (Chapter 2); that is, it returns to its original shape when the force is removed. A simple analogy to this type of behavior is a helical spring that stretches when loaded and returns to its original shape when the load is removed. If the force is increased sufficiently, the crystal undergoes plastic deformation or permanent deformation; that is, it does not return to its original shape when the force is removed (see also shape-memory alloys, Section 6.14). There are two basic mechanisms by which plastic deformation takes place in crystal structures. One mechanism involves a plane of atoms slipping over an adjacent plane (called the slip plane) under a shear stress (Fig. 1.6a). This behavior is much like sliding of playing cards against each other. Shear stress is defined as the ratio of the applied shearing force to the cross-sectional area being sheared. b a Atomic planes Shear stress Twinning plane Slip plane (a) (b) Figure 1.6: Permanent deformation of a single crystal under a tensile load. The highlighted grid of atoms emphasizes the motion that occurs within the lattice. (a) Deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip. (b) Deformation by twinning, involving the generation of a “twin” around a line of symmetry subjected to shear. Note that the tensile load results in a shear stress in the plane illustrated. Shear stress ׉ 7cassandra://wLIYq-Mhai4_SZIhgZHmANGCaCBztTf7kEPpZSzV7IM`cZ%Ebb׉E<68 Chapter 1 The Structure of Metals Just as it takes a certain force to slide playing cards against each other, a single-crystal metal requires a certain magnitude of shear stress (called critical shear stress) to undergo permanent deformation. Thus, there must be a shear stress of sufficient magnitude for plastic deformation to occur; otherwise, the deformation remains elastic. The shear stress required to cause slip in single crystals is directly proportional to the ratio b/a in Fig. 1.6a, where a is the spacing of the atomic planes and b is inversely proportional to the atomic density in the atomic plane. As the ratio b/a decreases, the shear stress required to cause slip decreases; thus, slip in a single crystal takes place along planes of maximum atomic density. In other words, slip takes place in closely packed planes and in closely packed directions. Because the b/a ratio varies for different directions within the crystal, a single crystal exhibits different properties when tested in different directions, a property called anisotropy. An example is the behavior of plywood, which is much stronger in the planar direction than it is along its thickness direction. The second and less common mechanism of plastic deformation in crystals is twinning, in which a portion of the crystal forms a mirror image of itself across the plane of twinning (Fig. 1.6b). Twins form abruptly and are the cause of the creaking sound (called “tin cry”) that occurs when a tin or zinc rod is bent at room temperature; twinning usually occurs in hcp metals. Slip Systems. The combination of a slip plane and slip direction is known as a slip system. Metals with five or more slip systems are generally ductile. 1. In body-centered cubic crystals, there are 48 possible slip systems; therefore, the probability is high that an externally applied shear stress will operate on one of these systems and cause slip. Because of the relatively high b/a ratio in this type of crystal, the required shear stress is high. Metals with bcc structures (such as titanium, molybdenum, and tungsten) generally have good strength and moderate ductility, but can have high ductility at elevated temperatures. 2. In face-centered cubic crystals, there are 12 slip systems. The probability of slip is moderate, and the shear stress required to cause slip is low because of the relatively low b/a ratio. These metals, such as aluminum, gold, copper, and silver, generally have moderate strength and good ductility. 3. The hexagonal close-packed crystal has three slip systems and, therefore, has a low probability of slip; however, additional slip systems become active at elevated temperatures. Metals with hcp structures, such as beryllium, magnesium, and zinc, are generally brittle at room temperature. Note in Fig. 1.6a that the portions of the single crystal that have undergone slip have rotated from their original angular position toward the direction of the tensile force; note also that slip has taken place only along certain planes. It can be observed from electron microscopy that what appears to be a single slip plane is actually a slip band, consisting of several slip planes (Fig. 1.7). 1.4.1 Imperfections in the Crystal Structure of Metals The actual strength of metals is approximately one to two orders of magnitude lower than the strength levels obtained from theoretical calculations. This discrepancy is explained in terms of defects and imperfections in the crystal structure. Unlike in idealized models described earlier, actual metal crystals contain a large number of defects and imperfections, generally categorized as: 1. Point defects, such as a vacancy (missing atom), an interstitial atom (extra atom in the lattice), or an impurity (foreign atom that has replaced the atom of the pure metal) (Fig. 1.8); 2. Linear, or one-dimensional, defects, called dislocations (Fig. 1.9); 3. Planar,or two-dimensional, imperfections, such as grain boundaries and phase boundaries (see Section 1.5); 4. Volume,or bulk, imperfections, such as voids, inclusions (nonmetallic elements, such as oxides, sulfides, and silicates), other phases,or cracks. ׉ 7cassandra://eTClw2zY6hrCaEE_yRDGHXL6kWkr_b_CaDLULRqecOIK`cZ%EbccZ%Ebb#בCט#u׉׉ 7cassandra://NhA9jmc1D-HAAEy1PNp6Mg8GukSSRFfyMfRmbkS2NZ01`׉ 7cassandra://WZlLehbW6eYY-1EsBJaWFHwNsPGmxRRF_UUyhSePJ2MA5`s׉ 7cassandra://laFpwhNZfyrHEkzniQjrlOvPr4U-ccDqfKNnPiVuYj4v`׉ 7cassandra://cP6HKcFP0xqqTgacca8gpc_9V-wG8QGLElilflv8FQo͠^cZEbNט#u׉׉ 7cassandra://PPN7eeweqJr8bgBfhJYgSWLw0zvnUiF959E-v7siAlEę`׉ 7cassandra://TeDNTyw1-lrURYaHWQ3klRQC8MhGnVb6Q_tqcRYtsyM\`s׉ 7cassandra://Vbsh_405njTg9d6uPpEHqKBfkLikaXksBDK1ekw3Nz0C`׉ 7cassandra://N8rNsA9UtcRn6EJ3t3O1n1ei97nRugydG_z7prSBhOQ9g͠^cZEbQ׉EDeformation and Strength of Single Crystals 69 Single crystal (grain) Grain boundaries Approximately 1000 atomic diameters Slip band ~10,000 atomic diameters Figure 1.7: Schematic illustration of slip lines and slip bands in a single crystal (grain) subjected to a shear stress. A slip band consists of a number of slip planes. The crystal at the center of the upper illustration is an individual grain surrounded by several other grains. Self-interstitial atom Vacancy Interstitial impurity atom Substitutional impurity atom Slip lines approximately 100 atomic diameters Figure 1.8: Schematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional. Mechanical and electrical properties of metals, such as yield stress, fracture strength, and electrical conductivity, are adversely affected by the presence of defects; these properties are known as structure sensitive. By contrast, physical and chemical properties, such as melting point, specific heat, coefficient of thermal expansion, and elastic constants such as modulus of elasticity and modulus of rigidity (see Sections 2.2.1 and 2.4) are not sensitive to these defects; these properties are known as structure insensitive. Shear stress ׉ 7cassandra://laFpwhNZfyrHEkzniQjrlOvPr4U-ccDqfKNnPiVuYj4v`cZ%Ebd׉E `70 Chapter 1 The Structure of Metals Screw dislocation (a) (b) Figure 1.9: Types of dislocations in a single crystal: (a) edge dislocation and (b) screw dislocation. Slip plane Figure 1.10: Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals is much lower than that predicted by theory. Dislocations. First observed in the 1930s, dislocations are defects in the orderly arrangement of the atomic structure of a metal. Because a slip plane containing a dislocation (Fig. 1.10) requires much lower shear stress to allow slip than does a plane in a perfect lattice, dislocations are the most significant defects that explain the discrepancy between the actual and the theoretical strengths of metals. There are two types of dislocations: edge and screw (Fig. 1.9). An analogy to the movement of an edge dislocation is the progress of an earthworm, which moves forward by means of a hump that starts at its tail and moves toward its head. Another analogy is moving a large carpet on a floor by first forming a hump at one end and gradually moving the hump to the other end. (Recall that the force required to move a carpet in this way is much lower than that required to slide the whole carpet along the floor.) Screw dislocations are so named because the atomic planes form a spiral ramp, like the threads on a screw or bolt. 1.4.2 Work Hardening (Strain Hardening) Although the presence of a dislocation lowers the shear stress required to cause slip, dislocations can be: 1. Entangled and interfere with each other, and 2. Impeded by barriers, such as grain boundaries, impurities, and inclusions in the material. The higher shear stress required to overcome entanglements and impediments thus results in an increase in the overall strength and the hardness of the metal, and is known as work hardening or strain hardening. The greater the deformation, the greater is the number of entanglements and, hence, the higher the increase in strength. Work hardening is a mechanism for strengthening of metals in metalworking processes at low to moderate temperatures. Typical examples are producing sheet metal for appliances and aircraft fuselages by cold rolling (Chapter 13), producing the head of a bolt by heading (Chapter 14), and strengthening wire by drawing it through a die at room temperature (Chapter 15). ׉ 7cassandra://Vbsh_405njTg9d6uPpEHqKBfkLikaXksBDK1ekw3Nz0C`cZ%EbecZ%Ebd#בCט#u׉׉ 7cassandra://1Wkwyk26kf-Qcs70klMOO0eZT0l2GK_T5VT8ZVFGz_Q8`׉ 7cassandra://bMkzHGUJ-M7hQv8EdZgVFP5fBTC61cZ9_J64Tqayy2UdF`s׉ 7cassandra://10l3Vx1dFqf1zn6OnTEE6U-pgrDd-Obumg1ShQnuv18"`׉ 7cassandra://1ExYdbUzb4DadPOeQ1HjIu3RfQkBcuDA6E8JJ5-NMwgT6͠^cZEbTט#u׉׉ 7cassandra://9i2Qf2QtuEHN5cG4ElR8cd97UsZKOxCdj8ym8gqL-M0r`׉ 7cassandra://WQ8oQwSU-etR_93hT-RRR5bB2SaX5eSioMV0MFfuBF0a`s׉ 7cassandra://2ptLufLolOnEmfo-nxZw5wQ6AAlj9laXx0atUTKPlk0`׉ 7cassandra://va3Su3pfTSexTN87Z0ITKw1gJJs-gSew69ydxxyjo2gyF ͠^cZEbUנcZEbX#U9ׁHhttp://sliding.Thׁ ׁ Ј׉E Grains and Grain Boundaries 71 1.5 Grains and Grain Boundaries When a mass of molten metal begins to solidify, crystals form independently of each other at various locations within the liquid mass, and thus have random and unrelated orientations (Fig. 1.11). Each of these crystals eventually grows into a crystalline structure, or grain. Each grain consists of either a single crystal (for pure metals) or a polycrystalline aggregate (for alloys). The number and size of the grains developed in a unit volume of the metal depends on the rate at which nucleation (the initial stage of crystal formation) takes place. The median size of the grains depends on (a) the number of different sites at which individual crystals begin to form (note that there are seven in Fig. 1.11a) and (b) the rate at which these crystals grow. If the nucleation rate is high, the number of grains in a unit volume of metal will be large, and thus grain size will be small. Conversely, if the crystal growth rate is high (as compared with their nucleation rate), there will be fewer grains per unit volume, and thus grain size will be larger. Generally, rapid cooling produces smaller grains, whereas slow cooling produces larger grains. Note in Fig. 1.11d that the growing grains eventually interfere with and impinge upon one another; the interfaces that separate the individual grains are called grain boundaries. Note also that the crystallographic orientation changes abruptly from one grain to the next across the grain boundaries. Recall, from Section 1.4, that the behavior of a single crystal or a single grain is anisotropic. Because its many grains have random crystallographic orientations, the behavior of a polycrystalline metal is thus essentially isotropic; that is, its properties do not vary with direction. 1.5.1 Grain Size Grain size has a major influence on the mechanical properties of metals. At room temperature, for example, a large grain size is generally associated with low strength, low hardness, and low ductility. Grains can be so large as to be visible with the naked eye; zinc grains on the surface of galvanized sheet steels are an example. Large grains also cause a rough surface appearance after the material has been plastically deformed, particularly in the stretching of sheet metals (see orange peel, Section 1.7). The yield strength, Sy, of the metal is the most sensitive property and is related to grain size by the empirical formula (known as the Hall-Petch equation) Sy = Syi + kd−1/2 (1.1) where Syi is the yield stress for a large grained material, k is a constant, and d is the mean grain diameter. Equation (1.1) is valid for a temperature below the recrystallization temperature of the metal. (a) (b) (c) (d) Figure 1.11: Schematic illustration of the stages during the solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other. ׉ 7cassandra://10l3Vx1dFqf1zn6OnTEE6U-pgrDd-Obumg1ShQnuv18"`cZ%Ebf׉E 72 Chapter 1 The Structure of Metals Grain size is usually measured either by counting the number of grains in a given area or by counting the number of grains that intersect a prescribed length of a line (drawn randomly on an enlarged image of the grains), taken under a microscope on a polished and etched specimen. Software is available to automate these tasks. Grain size may also be determined by comparing such an image against a standard chart. The American Society for Testing and Materials (ASTM) grain size number, n, is related to the number of grains, N, per square inch at a magnification of 100× (equal to 0.0645 mm2 of actual area) by the formula N =2n−1 An International Standards Organization (ISO) equivalent to Eq. (1.2) is given by N =8  2Gm  where Gm is the metric grain size number and N is the number of grains per square millimeter at 1× magnification. Because grains are typically extremely small, numerous grains may occupy a small volume of metal (Table 1.1). Grain sizes between 5 and 8 are generally considered fine grained; a grain size of 7 is typically acceptable for sheet metals for making automotive bodies, appliances, and kitchen utensils (Chapter 16). Example 1.1 Number of Grains in a Paper Clip Given: A paper clip is made of wire that is 120 mm long and 0.75 mm in diameter, with an ASTM grain size of 9. Find: Calculate the number of grains in the paper clip. Solution: A metal with an ASTM grain size of 9 has 185,000 grains per mm3 (see Table 1.1). The volume of the paper clip is V = d2l = π 4 π 4 (0.75)2 (120) = 53.0 mm3 The total number of grains is calculated by multiplying the volume by the grains per mm3, or No. of grains = (53.0 mm3) (185,000 grains/mm3) = 9.81 million. 1.5.2 Influence of Grain Boundaries Grain boundaries have an important influence on the strength and ductility of metals, and because they interfere with dislocation movement also influence strain hardening. The magnitude of these effects depends on temperature, deformation rate, and the type and amount of impurities present along the grain boundaries. Because the atoms along the grain boundaries are more disordered and hence packed less efficiently, grain boundaries are more reactive than the grains themselves. As a result, the boundaries have lower energy than the atoms in the orderly lattice within the grains; thus, they can be more easily removed or chemically bonded to another atom. For example, the surface of a piece of metal becomes rougher when etched or it is subjected to corrosive environments. (See also end grains in forging, Section 14.5.) At elevated temperature, and in metals whose properties depend on the rate at which they are deformed, plastic deformation also takes place by means of grain-boundary sliding.The creep mechanism (elongation under stress over time, usually at elevated temperatures) involves grain-boundary sliding (see Section 2.8). Grain-boundary embrittlement. When exposed to certain low-melting-point metals, a normally ductile and strong metal may crack when subjected to very low external stresses. Two examples of such behavior are (a) aluminum wetted with a mercury–zinc amalgam or with liquid gallium and (b) copper at elevated (1.2) ׉ 7cassandra://2ptLufLolOnEmfo-nxZw5wQ6AAlj9laXx0atUTKPlk0`cZ%EbgcZ%Ebf#בCט#u׉׉ 7cassandra://SAqRPWHWE3sRmSDujnHoDC3D0xU8mXh-gnh6rUXgZu8 `׉ 7cassandra://SqhZtmYcbm_ZiPhfScMKHi6gSkLs493EtWdGHIfxvHYQX`s׉ 7cassandra://mApBoHmXwJXym2IoUvcP1BUT4uStyhVCbgtafSRFm3Q`׉ 7cassandra://4nhzOuLWp959LmP5aDlJmI9qd_uI5wB1tvLOENrNwWo_ol͠^cZEbYט#u׉׉ 7cassandra://V6eGtYU5WRxKop77LRuFylgZz9kwgnMwwfmkDXGkgtQ`׉ 7cassandra://U4kFCMJ2TJ0Ge_FVw3P2lqSPWQv-Qvz5ATWObuRRv9sS`s׉ 7cassandra://dUq5ul6eYr0kRwnmvJwNXrpISHah2_G9LWVGGdfFGro`׉ 7cassandra://LESYqQGB8ofvLEvycW1TEA8Ht1So2qnPiz8SjDCmraIW[͠^cZEbZ׉E DPlastic Deformation of Polycrystalline Metals 73 Table 1.1: Grain Sizes. ASTM No. Grains/mm2 2 8 Grains/mm3 −31 0.7 −2 2 −14 5.6 0 116 2 3 4 5 6 7 8 9 10 11 16 45 32 64 128 256 512 1024 2048 4096 8200 16,400 128 360 1020 2900 8200 23,000 65,000 185,000 520,000 1,500,000 temperature wetted with lead or bismuth; these elements weaken the grain boundaries of the metal by embrittlement. The term liquid-metal embrittlement is used to describe such phenomena because the embrittling element is in a liquid state. However, embrittlement can also occur at temperatures well below the melting point of the embrittling element, known as solid-metal embrittlement. Another embrittlement phenomenon is hot shortness, caused by local melting of a constituent or of an impurity along a grain boundary at a temperature below the melting point of the metal itself. When subjected to plastic deformation at elevated temperatures (hot working), the metal crumbles along its grain boundaries; examples are (a) antimony in copper, (b) leaded steels (Section 21.7.1), and (c) leaded brass. To avoid hot shortness, the metal is usually worked at a lower temperature to prevent softening and melting along the grain boundaries. Temper embrittlement in alloy steels is another form of embrittlement, caused by segregation (movement) of impurities to the grain boundaries (Section 4.11). 1.6 Plastic Deformation of Polycrystalline Metals When a polycrystalline metal with uniform equiaxed grains (grains having equal dimensions in all directions) is subjected to plastic deformation at room temperature (called cold working), the grains become deformed and elongated, as shown schematically in Fig. 1.12. Deformation may be carried out by, for example, compressing the metal, as is done in a forging operation to make a turbine disk (Chapter 14) or by subjecting it to tension, as is done in stretch forming of sheet metal (Section 16.6). The deformation within each grain takes place by the mechanisms described in Section 1.4 for a single crystal. During plastic deformation, the grain boundaries remain intact and mass continuity is maintained. The deformed metal exhibits higher strength than before, because of entanglement of dislocations with grain boundaries and with each other. The increase in strength depends on the degree of deformation (strain) to which the metal is subjected; the higher the deformation, the stronger the metal becomes. The strength increase is higher for metals with smaller grains, because they have a larger grain-boundary surface area per unit volume of metal and, hence, more entanglement of dislocations. ׉ 7cassandra://mApBoHmXwJXym2IoUvcP1BUT4uStyhVCbgtafSRFm3Q`cZ%Ebh׉E 874 Chapter 1 The Structure of Metals (a) (b) Figure 1.12: Plastic deformation of idealized (equiaxed) grains in a specimen subjected to compression (such as occurs in the forging or rolling of metals): (a) before deformation and (b) after deformation. Note the alignment of grain boundaries along a horizontal direction; this effect is known as preferred orientation. Top view Crack Sheet Side view (a) (b) Figure 1.13: (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused, for example, by pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. (b) Aluminum sheet with a crack (vertical dark line at the center) developed in a bulge test; the rolling direction of the sheet was vertical. Source: After J.S. Kallend, Illinois Institute of Technology. Anisotropy (Texture). Note in Fig. 1.12b that, as a result of plastic deformation, the grains have elongated in one direction and contracted in the other direction. Consequently, the metal has become anisotropic,and thus its properties in the vertical direction are different from those in the horizontal direction. The degree of anisotropy depends on the temperature at which deformation has taken place and on how uniformly the metal is deformed. Note from the crack direction shown in Fig. 1.13, for example, that the ductility of the cold-rolled sheet in the transverse direction is lower than in its rolling direction (see also Section 16.5). Anisotropy influences both mechanical and physical properties of metals, described in Chapter 3. For example, sheet steel for electrical transformers is rolled in such a way that the resulting deformation imparts anisotropic magnetic properties to the sheet. This operation then reduces magnetic-hysteresis losses, thus improving the efficiency of transformers (see also amorphous alloys, Section 6.15). There are two general types of anisotropy in metals: preferred orientation and mechanical fibering. Preferred Orientation. Also called crystallographic anisotropy, preferred orientation can be best described by referring to Fig. 1.6a. When a single-crystal metal is subjected to tension, the sliding blocks rotate toward the direction of the tensile force; as a result, slip planes and slip bands tend to align themselves with the general direction of deformation. Similarly, for a polycrystalline metal, with grains in random orientations, all slip directions tend to align themselves with the direction of the tensile force being applied. By contrast, Rolling direction ׉ 7cassandra://dUq5ul6eYr0kRwnmvJwNXrpISHah2_G9LWVGGdfFGro`cZ%EbicZ%Ebh#בCט#u׉׉ 7cassandra://c-6ondiFxYngfQTJ1H07tnFZP5gttrui8qObjLqVjtE9`׉ 7cassandra://UdJocvXjAXfkayu7YnrGfm66arwZCJjNFXaFkgtf0JkkJ`s׉ 7cassandra://_83u5FpKqQSkyRH1GvTfLYcyg55wBQznZbAgx-hcoiE)`׉ 7cassandra://p9dxyGB1TsuE_w49otXc2yffMrKhpy0h5d2TrLXzWX8Z`͠^cZEb]ט#u׉׉ 7cassandra://S5ZsSz3vValFFtZ0LJSIDzn4K5PFQc7FuLVUVwnAyJw`׉ 7cassandra://unqNTaRNThwxzMBTMpRJpQIf28ycuY1lEvAGbQ43HlQJ`s׉ 7cassandra://xv_SjSNJ-IVVDbN0S9muLW3137hzlXszQ7SqXRAJVmo`׉ 7cassandra://r_vxIpy1gY-VPIZdAb8r4dr-iyvm0gOn2xC2Uy99ePst ͠^cZEb^׉ERecovery, Recrystallization, and Grain Growth 75 slip planes under compression tend to align themselves in a direction perpendicular to the direction of the applied compressive force. Mechanical Fibering. This is a type of anisotropy that results from the alignment of inclusions (stringers), impurities, and voids in the metal during deformation. Note that if the spherical grains shown in Fig. 1.12a were coated with impurities, these impurities would align themselves in a generally horizontal direction after deformation. Because the impurities weaken the grain boundaries, this piece of metal will now be less strong and less ductile when tested in the vertical direction. As an analogy, consider plywood, which is strong in tension along its planar direction but splits easily when subjected to tension in its thickness direction. 1.7 Recovery, Recrystallization, and Grain Growth Recall that plastic deformation at room temperature causes (a) distortion of the grains and grain boundaries, leading to anisotropic behavior, (b) a general increase in strength, and (c) a decrease in ductility. These effects can be reversed and the properties of the metal brought back to their original levels by heating the metal to a specific temperature range for a given period of time—a process called annealing (described in detail in Section 4.11). Three events take place, consecutively, during this process: 1. Recovery.During recovery, which occurs at a certain temperature range below the recrystallization temperature of the metal (described next), the stresses in the highly deformed regions of the metal are relieved. Subgrain boundaries begin to form (called polygonization), with no significant change in mechanical properties (such as hardness and strength, Fig. 1.14). 2. Recrystallization. This is the process in which, within a certain temperature range, new equiaxed and strain-free grains are formed, replacing the older grains. The temperature required for recrystallization ranges approximately between 0.3Tm and 0.5Tm, where Tm is the melting point of the metal on the absolute scale. Generally, the recrystallization temperature is defined as the temperature at which complete recrystallization occurs within approximately one hour. Recrystallization decreases the density of dislocations, lowers the strength, and raises the ductility of the metal (Fig. 1.14). Lead, tin, cadmium, and zinc recrystallize at about room temperature; consequently, they do not usually work harden. The recrystallization temperature depends on the degree of prior cold work (work hardening): the more the cold work, the lower the temperature required for recrystallization. The reason is that, as the amount of cold work increases, the number of dislocations and the amount of energy stored in dislocations (called stored energy) also increase. This energy supplies some of the work required for recrystallization. Recrystallization is also a function of time, because it involves diffusion—the movement and exchange of atoms across grain boundaries. The effects of temperature, time, and the degree of plastic deformation by cold working on recrystallization are as follows: (a) For a constant degree of deformation by cold working, the time required for recrystallization decreases with increasing temperature. (b) The higher the prior cold work, the lower the temperature required for recrystallization. (c) The higher the amount of deformation, the smaller the grain size becomes during recrystallization; this effect is a commonly used method for converting a coarse-grained structure to one having a finer grain, and with improved properties. (d) Some anisotropy (due to preferred orientation) usually persists after recrystallization; to restore isotropy, a temperature higher than that required for recrystallization may be necessary. ׉ 7cassandra://_83u5FpKqQSkyRH1GvTfLYcyg55wBQznZbAgx-hcoiE)`cZ%Ebj׉E176 Chapter 1 The Structure of Metals Residual stresses Strength, hardness, ductility Strength Ductility Hardness Cold-worked and recovered Grain size Recovery Temperature Recrystallization Grain growth New grains Figure 1.14: Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: After G. Sachs. 3. Grain growth. If the temperature is raised higher, the grains begin to grow (see lower part of Fig. 1.14), and their size may eventually exceed the original grain size. Called grain growth, this phenomenon adversely affects mechanical properties (Fig. 1.14). Large grains also produce a rough surface appearance on sheet metals, called orange peel, when they are stretched to form a part, or on the surfaces of a piece of metal when subjected to bulk deformation, such as compression in forging (Chapter 14). 1.8 Cold, Warm, and Hot Working Cold working refers to plastic deformation carried out at room temperature. When deformation occurs above the recrystallization temperature, it is called hot working. Cold and hot are relative terms, as can be seen from the fact that deforming lead at room temperature is a hot-working process, because the recrystallization temperature of lead is about room temperature. As the name implies, warm working is carried out at intermediate temperatures; thus, warm working is a compromise between cold and hot working. The important technological differences in products that are processed by cold, warm, and hot working are described in Part III. The temperature ranges for these three categories of plastic deformation are given in Table 1.2 in terms of a ratio, T/Tm, where T is the working temperature and Tm is the melting point of the metal, both on the absolute scale. Although it is dimensionless, this ratio is known as the homologous temperature. Table 1.2: Homologous Temperature Ranges for Various Processes. Process T/Tm Cold working <0.3 Warm working 0.3–0.5 Hot working >0.6 ׉ 7cassandra://xv_SjSNJ-IVVDbN0S9muLW3137hzlXszQ7SqXRAJVmo`cZ%EbkcZ%Ebj#בCט#u׉׉ 7cassandra://GGahRU8g93PfXAngPzmMwtCZn259-pIEJeyS5FSAKcs`׉ 7cassandra://v976c5JJMX5MUbVCUmGfcSLPnAYRoL89eyyNgqNaAaIQ.`s׉ 7cassandra://-wC1lZdQBzWdFknjFGROY0mB_66MTEJ1I8wPSw16_mo`׉ 7cassandra://T8vgMkiWERBGvQJbrnPF0MGxbyHCmG7T8EJ3dKw2AR8KI͠^cZEbaט#u׉׉ 7cassandra://mSR7CSnpQNxR-ICj_-kxmCtGqf6qMIU5Hjg5W-Vo0t8`׉ 7cassandra://anmmSvYrlA3O4dxvMg5aAma1Hlue59pu6EaRPYes1C0X`s׉ 7cassandra://wa7g0EAiazBMO-wsSNjxPcU0eCcsMvVgDUJqrGGWvlU[`׉ 7cassandra://YrAxJXux4-JDOgWKuC4ldiecPlQKDJ9s2Djh5ZlV50AX#͠^cZEbb׉E 7Key Terms 77 Summary • There are three basic crystal structures in metals: body-centered cubic (bcc), face-centered cubic (fcc), and hexagonal close-packed (hcp). Grains made of these crystals typically contain various defects and imperfections, such as dislocations, vacancies, impurities, inclusions, and grain boundaries. Polycrystalline metals consist of many crystals, or grains, in random orientations. • Plastic deformation in metals takes place by a slip mechanism. Although the theoretical shear stress required to cause slip is very high, actual required stresses are much lower because of the presence of dislocations (edge or screw type). Dislocations become entangled with one another or are impeded by barriers such as grain boundaries, impurities, and inclusions. As a result, the shear stress required to cause further slip is increased; consequently, the overall strength and hardness of the metal is also increased (through work hardening or strain hardening). • Grain size has a significant effect on the strength of metals: the smaller the size, the stronger is the metal, and the larger the size, the more ductile is the metal. However, excessively large grains are generally associated with brittle behavior. • Grain boundaries have a major influence on the behavior of metals, as boundaries can undergo embrittlement, severely reducing ductility at elevated temperatures (hot shortness). They are also responsible for the creep phenomenon, due to grain boundary sliding. • Metals may be plastically deformed at room, warm, or high temperatures; their behavior and workability depend largely on whether deformation takes place below or above the recrystallization temperature of the metal. Deformation at room temperature (cold working) results in higher strength, but reduced ductility; generally, it also causes anisotropy (either preferred orientation or mechanical fibering), whereby the properties are different in different directions. • The effects of cold working can be reversed by annealing; that is, heating the metal to a specific temperature range for a given period of time, thereby allowing the successive stages of recovery, recrystallization, and grain growth to take place. Key Terms Allotropism Anisotropy Basal plane Body-centered cubic Cold working Covalent bond Creep Crystals Dislocations Elastic deformation Embrittlement Face-centered cubic Grains Grain boundaries Grain growth Grain size Hall-Petch effect Hexagonal close-packed Homologous temperature Hot shortness Hot working Imperfections Ionic bond Lattice structure Mechanical fibering Metallic bond ׉ 7cassandra://-wC1lZdQBzWdFknjFGROY0mB_66MTEJ1I8wPSw16_mo`cZ%Ebl׉EN78 Chapter 1 The Structure of Metals Nucleation Orange peel Plastic deformation Polycrystals Polygonization Polymorphism Preferred orientation Primary bond Recovery Recrystallization Secondary bond Shear stress Slip band Bibliography Ashby, M.F., and Jones, D.R.H., Engineering Materials,Vol.1, An Introduction to Their Properties and Applications, 3rd ed., Pergamon, 2005; Vol. 2, An Introduction to Microstructures, Processing and Design, Pergamon, 2005. Ashby, M.F., Materials Selection in Mechanical Design, 4th ed., Pergamon, 2010. Askeland, D.R., andWright,W.J., The Science and Engineering of Materials, 7th ed., CENGAGE Learning, 2015. Brandt, D.A., and Warner, J.C., Metallurgy Fundamentals, 5th ed., Goodheart-Wilcox, 2009. Callister, W.D., Jr., and Rethwisch, D.G., Materials Science and Engineering: An Introduction,9thed., Wiley, 2013. Hosford, W.F., Physical Metallurgy, 2nd ed., Taylor & Francis, 2010. Shackelford, J.F., Introduction to Materials Science for Engineers, 8th ed., Pearson, 2014. Review Questions 1.1. What is the difference between an atom and a molecule, and a molecule and a crystal? 1.2. Describe ionic, covalent, and metallic bonds. 1.3. Explain the difference between a unit cell and a single crystal. 1.4. In tables on crystal structures, iron is listed as having both a bcc and an fcc structure. Why? 1.5. Define anisotropy. What is its significance? 1.6. What effects does recrystallization have on the properties of metals? 1.7. What is strain hardening, and what effects does it have on the properties of metals? 1.8. Explain what is meant by structure-sensitive and structure-insensitive properties of metals. 1.9. Make a list of each of the major kinds of imperfection in the crystal structure of metals, and describe them. 1.10. What influence does grain size have on the mechanical properties of metals? 1.11. What is the relationship between the nucleation rate and the number of grains per unit volume of a metal? Slip plane Slip system Strain hardening Structure insensitive Structure sensitive Texture Twinning Unit cell Vacancy van der Waals force Warm working Work hardening ׉ 7cassandra://wa7g0EAiazBMO-wsSNjxPcU0eCcsMvVgDUJqrGGWvlU[`cZ%EbmcZ%Ebl#בCט#u׉׉ 7cassandra://yq5xgTwe92EB9N72rbEwZk4z8dxwa3UA3eIDwojOZH04`׉ 7cassandra://Y10Xxs4Ve_Zqa-XzEoi1mnkXqpiUaVGWkGKhgFF7R8cnS`s׉ 7cassandra://_4pBS7en-JUMAwGedu-UTp3LnS0A38d0_ef-KBPd4ro`׉ 7cassandra://amwgbPCBRxk5Ubygo6GDJefcuPhOlw1RJ0Qyq-TvixUC ͠^cZEbdט#u׉׉ 7cassandra://_IXiNSJ3ALZQoPPixiuZkYPdCggx25d2YOHRv172Ck0`׉ 7cassandra://qBLwC_YMAdHaD7BE-X5tA5epYbjN0DgGuNYMBEbDgSQS`s׉ 7cassandra://QMnjFH-c3AUvMXMYFZ1DEB8mwgVPZOp3308OAXNsfp4`׉ 7cassandra://5ADychuxq6Z84i82tVuQSeh6WlcBpZRHcMO8wtQQd2wl0͠^cZEbe׉E Qualitative Problems 79 1.12. What is a slip system, and what is its significance? 1.13. Explain the difference between recovery and recrystallization. 1.14. What is hot shortness, and what is its significance? 1.15. Explain the advantages and limitations of cold, warm, and hot working, respectively. 1.16. What is a slip band? Explain why they can be seen on the surface of a crystal. 1.17. Describe what the orange peel effect is. Explain why we may have to be concerned with the orangepeel effect on metal surfaces. 1.18. Some metals, such as lead, do not become stronger when worked at room temperature. Explain the reason. 1.19. Describe the difference between preferred orientation and mechanical fibering. 1.20. Differentiate between stress relaxation and stress relieving. 1.21. What is twinning? How does it differ from slip? 1.22. What is annealing? 1.23. Which one is larger—a unit cell or a grain? 1.24. Describe the different approaches used to measure grain size. Qualitative Problems 1.25. Explain your understanding of why the study of the crystal structure of metals is important. 1.26. What is the significance of the fact that some metals undergo allotropism? 1.27. Is it possible for two pieces of the same metal to have different recrystallization temperatures? Is it possible for recrystallization to take place in some regions of a part before it does in other regions of the same part? Explain. 1.28. Describe your understanding of why different crystal structures exhibit different strengths and ductilities. 1.29. A cold-worked piece of metal has been recrystallized. When tested, it is found to be anisotropic. Explain the probable reason. 1.30. What materials and structures can you think of (other than metals) that exhibit anisotropic behavior? 1.31. Two parts have been made of the same material, but one was formed by cold working and the other by hot working. Explain the differences you might observe between the two. 1.32. Explain the importance of homologous temperature. 1.33. Do you think it might be important to know whether a raw material to be used in a manufacturing process has anisotropic properties? What about anisotropy in the finished product? Explain. 1.34. What is the difference between an interstitial atom and a substitutional atom? 1.35. Explain why the strength of a polycrystalline metal at room temperature decreases as its grain size increases. 1.36. Describe the technique you would use to reduce the orange-peel effect on the surface of workpieces. 1.37. What is the significance of the fact that such metals as lead and tin have a recrystallization temperature that is about room temperature? 1.38. It was stated in this chapter that twinning usually occurs in hcp materials, but Fig. 1.6b shows twinning in a rectangular array of atoms. Can you explain the discrepancy? 1.39. It has been noted that the more a metal has been cold worked, the less it strain hardens. Explain why. ׉ 7cassandra://_4pBS7en-JUMAwGedu-UTp3LnS0A38d0_ef-KBPd4ro`cZ%Ebn׉E 80 Chapter 1 The Structure of Metals 1.40. Is it possible to cold work a metal at temperatures above the boiling point of water? Explain. 1.41. Comment on your observations regarding Fig. 1.14. 1.42. Is it possible for a metal to be completely isotropic? Explain. 1.43. Referring to Fig. 1.1, assume you can make a ball bearing from a single crystal. What advantages and disadvantages would such a bearing have? 1.44. Referring to Fig. 1.10, explain why edge dislocations cannot cross grain boundaries using appropriate sketches. Quantitative Problems 1.45. How many atoms are in a single repeating cell of an bcc crystal structure? How many in a repeating cell of an hcp structure? 1.46. The atomic weight of gold is 196.97, meaning that 6.023 × 1023 atoms weigh 196.97 g. The density of gold is 19,320 kg/m3, and pure golf forms fcc crystals. Estimate the diameter of a gold atom. 1.47. Plot the data given in Table 1.1 in terms of grains/mm2 observations. vs. grains/mm3, and discuss your 1.48. A strip of metal is reduced from 50 mm in thickness to 25 mm by cold working; a similar strip is reduced from 50 mm to 30 mm. Which of these cold-worked strips will recrystallize at a lower temperature? Why? 1.49. The ball of a ballpoint pen is 1.6 mm in diameter and has an ASTM grain size of 12. How many grains are there in the ball? 1.50. How many grains are on the surface of the head of a pin? Assume that the head of a pin is spherical with a 2-mm diameter and has an ASTM grain size of 12. 1.51. The unit cells shown in Figs. 1.3 through 1.5 can be represented by tennis balls arranged in various configurations in a box. In such an arrangement, the atomic packing factor (APF) is defined as the ratio of the sum of the volumes of the atoms to the volume of the unit cell. Show that the APF is 0.68 for the bcc structure and 0.74 for the fcc structure. 1.52. Show that the lattice constant a in Fig. 1.4a is related to the atomic radius by the formula α =2 2R, where R is the radius of the atom as depicted by the tennis-ball model. √ 1.53. Show that, for the fcc unit cell, the radius r of the largest hole is given by r =0.414R. Determine the size of the largest hole for the iron atoms in the fcc structure. 1.54. A technician determines that the grain size of a certain etched specimen is 8. Upon further checking, it is found that the magnification used was 150×, instead of the 100× that is required by the ASTM standards. Determine the correct grain size. 1.55. If the diameter of the aluminum atom is 0.28 nm, how many atoms are there in a grain of ASTM grain size 10? 1.56. The following data are obtained in tension tests of brass: Grain size Yield strength (µm) 30 40 100 150 200 (MPa) 150 140 105 90 75 Does the material follow the Hall-Petch effect? If so, what is the value of k? ׉ 7cassandra://QMnjFH-c3AUvMXMYFZ1DEB8mwgVPZOp3308OAXNsfp4`cZ%EbocZ%Ebn#בCט#u׉׉ 7cassandra://N90-cgtM6Q5kz66nv8fjmACKauJDY-bRSukS8afCPJI`׉ 7cassandra://mpoH9CveaB12yijEYhZybRKyiWu99ba-w8jp5-D7WwYZ`s׉ 7cassandra://PwrARkY0lJLev_K6OHWC9XJNMRUyHTsX-LfIXDC4t2UE`׉ 7cassandra://8EiqnRMCqcYdcWNlcIPe0IQVgVHaPhSJKiEBrsnrcew]͠^cZEbhט#u׉׉ 7cassandra://2w3wSdST5d2mWUWqAVDLtthIcMBanMvAV5I-sfliRacU[`׉ 7cassandra://8X9sawDmaiYFBIA4FvUVbSPiVTnG_he_3yObXFxX5A46`s׉ 7cassandra://Udez9l_m6ZyO8n6JAGZaLWB_pNmUr2aj1KsfLu7yP3E`׉ 7cassandra://sGYFRQ0lT1tnDOiHI_JaR4cZKMpd5i00QzTa5BXiaRY7 ͠^cZEbi׉E Synthesis, Design, and Projects 81 1.57. Does water have a homologous temperature? What is the highest temperature where water (ice) can get cold worked? 1.58. The atomic radius of iron is 0.125 nm, while that of a carbon atom is 0.070 nm. Can a carbon atom fit inside a steel bcc structure without distorting the neighboring atoms? 1.59. Estimate the atomic radius for the following materials and data: (a) Aluminum (atomic weight = 26.98 g/mol, density = 2700 kg/m3); (b) silver (atomic weight = 107.87 g/mol, density = 10,500 kg/m3); (c) titanium (atomic weight = 47.87 g/mol, density = 4506 kg/m3). 1.60. A simple cubic structure involves atoms located at the cube corners that are in contact with each other along the cube edges. Make a sketch of a simple cubic structure, and calculate its atomic packing factor. 1.61. Estimate the ASTM grain size number for a 300-mm silicon wafer used to produce computer chips. 1.62. Pure copper and pure titanium follow the Hall–Petch equation. For copper, Syi =24 MPa and k =0.12 MPa-m1/2. For titanium, Syi =80 MPa and k =0.40 MPa-m1/2. (a) Plot the yield strength of these metals as a function of grain size for ASTM grain sizes of −3 to 11. (b) Explain which material would see greater strengthening from a reduction in grain size, as in cold working. Synthesis, Design, and Projects 1.63. By stretching a thin strip of polished metal, as in a tension-testing machine, demonstrate and comment on what happens to its reflectivity as the strip is being stretched. 1.64. Draw some analogies to mechanical fibering—for example, layers of thin dough sprinkled with flour or melted butter between each layer. 1.65. Draw some analogies to the phenomenon of hot shortness. 1.66. Obtain a number of small balls made of plastic, wood, marble, or metal, and arrange them with your hands or glue them together to represent the crystal structures shown in Figs. 1.3–1.5. Comment on your observations. 1.67. Take a deck of playing cards, place a rubber band around it, and then slip the cards against each other to represent Figs. 1.6a and 1.7. If you repeat the same experiment with more and more rubber bands around the same deck, what are you accomplishing as far as the behavior of the deck is concerned? 1.68. Give examples in which anisotropy is scale dependent. For example, a wire rope can contain annealed wires that are isotropic on a microscopic scale, but the rope as a whole is anisotropic. 1.69. The movement of an edge dislocation was described in Section 1.4.1 by means of an analogy involving a hump in a carpet on the floor and how the whole carpet can eventually be moved by moving the hump forward. Recall that the entanglement of dislocations was described in terms of two humps at different angles. Use a piece of cloth placed on a flat table to demonstrate these phenomena. 1.70. If you want to strengthen a material, would you wish to have it consist of one grain, or would you want it to have grains that contain the minimum number of atoms? Explain. ׉ 7cassandra://PwrARkY0lJLev_K6OHWC9XJNMRUyHTsX-LfIXDC4t2UE`cZ%Ebp׉EChapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.1 Introduction 83 2.2 Tension 83 2.3 Compression 94 2.4 Torsion 96 2.5 Bending (Flexure) 96 2.6 Hardness 97 2.7 Fatigue 102 2.8 Creep 105 2.9 Impact 105 2.10 Failure and Fracture of Materials 106 2.11 Residual Stresses 112 2.12 Work, Heat, and Temperature 114 Examples: 2.1 Calculation of Ultimate Tensile Strength 90 2.2 Calculation of Modulus of Resilience from Hardness 102 • This chapter examines the effects of external forces on the behavior of materials, and the test methods employed in determining mechanical properties. • The tension test, described first, is commonly used for quantifying a number of material parameters, including elastic modulus, yield stress, ultimate strength, ductility, and toughness. • Compression tests are important because they more closely simulate some metalworking processes; however, they have the unavoidable drawback of contributing the effects of friction to the test results. 82 ׉ 7cassandra://Udez9l_m6ZyO8n6JAGZaLWB_pNmUr2aj1KsfLu7yP3E`cZ%EbqcZ%Ebp#בCט#u׉׉ 7cassandra://3wUhhiZPB4cy7BNFFzKA2krtN_4CTPGoXs5yyJscxAI`׉ 7cassandra://UW0jwnpj2XI8JtaEQSSeAgjjsPIL5qhyXQGoYhhWBFgW `s׉ 7cassandra://7h2ETOCGdnZgmG-2u4lGi515stJDRT-BhK8iVKfTtw0F`׉ 7cassandra://jtv4eu0xtTPLfF4HOs6--p2TYYiTqgNEUmBM_5aBFw0S̜͠^cZEbkט#u׉׉ 7cassandra://3x-oI7CtOfoyXILozKb6zPlsbuxvV4d6j1gA27leU_U`׉ 7cassandra://w-huB5z1j_wlORvEd2lH5CtW64rM2Bsr_LFb8qlngbAB`s׉ 7cassandra://jqMeXSHfnHVdKKHFzwnFrcSfh-OAx95bpS6lHmGO0FM`׉ 7cassandra://h9PEOq9t7MeKUZplwAsC3vKJnZsOJTK2gH37EGNldqY͌ ͠^cZEbl׉E Tension 83 • Bending tests are particularly useful for brittle materials; three- and four-point tests are in common use. • Hardness and the variety of hardness tests and their range of applicability are then explored. • Fatigue involves the failure of materials subjected to cyclic or repeating loads; creep is deformation due to the application of a constant load over an extended period of time. These phenomena are also discussed. • This chapter ends with descriptions of the types of and the factors involved in failure and fracture of materials. 2.1 Introduction In manufacturing operations, parts and components are formed into a wide variety of shapes by applying external forces to the workpiece, typically by means of a variety of tools and dies. Common examples of such operations are forging of turbine disks, extruding various components for aluminum ladders, drawing wire for making nails, and rolling metal to make sheets for appliances. Forming operations may be carried out at room temperature or at elevated temperatures, and at a low or a high rate of deformation. Many of these operations are also used in forming and shaping nonmetallic materials, such as plastics, ceramics, and composite materials. As indicated in Tables 1.2 and 2.1, a wide variety of metallic and nonmetallic materials is now available, with an equally wide range of properties and characteristics. This chapter covers those aspects of mechanical properties and behavior of metals that are relevant to the design and manufacturing of products, and includes commonly used test methods employed in assessing various material properties. 2.2 Tension The tension test is the most commonly used method for determining the mechanical properties of materials such as strength, ductility, toughness, elastic modulus, and strain-hardening exponent. The test first requires the preparation of a test specimen, as shown in Fig. 2.1a. Although most specimens are solid and Table 2.1: Relative Mechanical Properties of Various Materials at Room Temperature, in Decreasing Order. Metals Are in Their Alloy Form. Strength Glass fibers Carbon fibers Kevlar fibers Carbides Molybdenum Steels Tantalum Titanium Copper Reinforced thermosets Lead Hardness Diamond Toughness Ductile metals Hardened steels Wood Titanium Cast irons Copper Thermosets Magnesium Thermoplastics Reinforced thermoplastics Tin Thermoplastics Lead Stiffness Diamond Cubic boron nitride Reinforced plastics Carbides Carbides Thermosets Ceramics Glass Copper Titanium Aluminum Ceramics Reinforced plastics Wood Thermosets Thermoplastics Rubbers Thermoplastics Tungsten Steel Strength/Density Reinforced plastics Titanium Steel Aluminum Magnesium Beryllium Copper Tantalum ׉ 7cassandra://7h2ETOCGdnZgmG-2u4lGi515stJDRT-BhK8iVKfTtw0F`cZ%Ebr׉E84 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Elastic Stress Sy Sy Sut Offset lo Ao Original gage length, lo Fracture Total elongation lf Af (a) (b) Figure 2.1: (a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) Stages in specimen behavior in a tension test. round in shape, they can also be flat or tubular. The specimen is prepared generally according to American Society for Testing and Materials (ASTM) specifications, although various other specifications are also available from corresponding organizations around the world. Typically, the specimen has an original gage length, lo, generally 50 mm, and a cross-sectional area, Ao, usually with a diameter of 12.5 mm. The specimen is mounted in the jaws of a tension-testing machine, equipped with various accessories and controls so that it can be tested at different temperatures and rates of deformation. 2.2.1 Stress–Strain Curves A typical sequence of events in a tension test is shown in Fig. 2.1b. When the load is first applied, the specimen elongates in proportion to the load, a behavior called linear elastic (Fig. 2.2). If at this stage the load is removed, the specimen returns to its original length and shape, in a manner similar to stretching a rubber band and releasing it. The engineering stress (also called nominal stress) is defined as the ratio of the applied load, P,tothe original cross-sectional area, Ao, of the specimen: σ = The engineering strain is defined as e = where l is the instantaneous length of the specimen. l − lo lo , (2.2) P Ao . (2.1) Post-uniform elongation le lf Uniform elongation Neck lu 0 tan{1 E eo Plastic Fracture eu ef Strain (for lo 5 1) Dl ׉ 7cassandra://jqMeXSHfnHVdKKHFzwnFrcSfh-OAx95bpS6lHmGO0FM`cZ%EbscZ%Ebr#בCט#u׉׉ 7cassandra://MFRTNyfuNxYKs2VbSXYcOlkZ8SHjW3v0_ugAfsOJNCY`׉ 7cassandra://lThjMtB34ctBPKw0cFAlbWLT6rk6rpJC-71nfe2t3fQMp`s׉ 7cassandra://-IXwNVxxgW8XMMOJ-uuB9stVoB7BesARVPBa9lmI_Rk[`׉ 7cassandra://h6wiWsXN8l4QOKpdH3VCAHoZgV1C4mIfrRLAGzVEaeMt͠^cZEbnט#u׉׉ 7cassandra://eyEGbQ8kL1fIvLXMguNIKl_e44_ajdlCcF898cAIbIgs`׉ 7cassandra://tLHNweHPMVQ5GeO4AAezQjVyzBc8snWmtEl74GmABWwO`s׉ 7cassandra://2y7vq0BpgZlU_fmwTHwyYFv1IwexhlpTlvmJU-ExFkk`׉ 7cassandra://x0gx7N0Ldwr-XNZx0RD-_wcrLp53cZB64K6NGyph2kch͠^cZEbo׉E uTension 85 Stress, s 5 P Ao Ultimate tensile strength (Sut) Yield stress (Sy) Uniform elongation Necking Fracture Elastic Plastic Offset Strain, e 5 l 2 lo lo Figure 2.2: A typical stress–strain curve obtained from a tension test, showing various features. As the load is increased further, the specimen begins to undergo nonlinear elastic deformation at a stress level called the proportional limit. At that point, the stress and strain are no longer proportional, as they were in the linear elastic region, but when unloaded, the specimen still returns to its original shape. Permanent (plastic) deformation occurs when the yield stress, Sy, of the material is reached. (The yield strength and other properties of various metallic and nonmetallic materials are given in Table 2.2.) For soft and ductile materials, it may be difficult to determine the exact location on the stress–strain curve at which yielding occurs, because the slope of the curve begins to decrease slowly above the proportional limit. For such materials, Sy is usually defined by drawing a line with the same slope as the linear elastic curve, but that it is offset by a strain of 0.002, or 0.2% elongation. The yield strength is then defined as the stress where the offset line intersects the stress–strain curve. This simple procedure is shown on the left side in Fig. 2.2. As the specimen begins to elongate under a continuously increasing load, its cross-sectional area decreases permanently and uniformly within its gage length. If the specimen is unloaded (from a stress level higher than the yield stress), the curve follows a straight line downward and parallel to the original slope of the curve, as shown in Fig. 2.3. As the load is increased further, the engineering stress eventually reaches a maximum and then begins to decrease (Fig. 2.2). The maximum engineering stress is called the tensile strength,or ultimate tensile strength, Sut, of the material. (Values for Sut for a variety of materials are given in Table 2.2.) If the specimen is loaded beyond its ultimate tensile strength, it begins to neck,or neck down. The crosssectional area of the specimen is no longer uniform along the gage length and is smaller in the necked region. As the test progresses, the engineering stress drops further and the specimen fractures at the necked region (Fig. 2.1a). The engineering stress at fracture is known as the breaking or fracture strength. The ratio of stress to strain in the elastic region is called the modulus of elasticity, E,or Young’s modulus (after T. Young, 1773–1829): E = . σ e This linear relationship is known as Hooke’s law (after R. Hooke, 1635–1703). (2.3) ׉ 7cassandra://-IXwNVxxgW8XMMOJ-uuB9stVoB7BesARVPBa9lmI_Rk[`cZ%Ebt׉E86 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Table 2.2: Mechanical Properties of Various Materials at Room Temperature. Ultimate Elastic Materials Metals (wrought) Aluminum and its alloys Copper and its alloys Lead and its alloys Magnesium and its alloys Steels Titanium and its alloys Tungsten and its alloys Zinc and its alloys Nonmetallic materials Ceramics Diamond Glass and porcelain Silicon carbide (SiC) Silicon nitride (Si2N4) Rubbers Thermoplastics Thermoplastics, reinforced Thermosets Boron fibers Carbon fibers Glass fibers Kevlar fibers Spectra Fibers Yield modulus strength (GPa) (MPa) 69–79 35–550 14 tensile strength (MPa) 90–600 105–150 76–110 140–1310 14 20–55 Molybdenum and its alloys 330–360 Nickel and its alloys 41–45 130–305 240–380 90–2340 80–2070 180–214 105–1200 345–1450 190–210 205–1725 415–1750 80–130 344–1380 415–1450 350–400 50 550–690 620–760 25–180 240–550 70–1000 — 140–2600 820–1050 — 60,000 70–80 — 140 200–500 — — 310–400 1.4–3.4 — 7–80 2–50 — — Elongation Poisson’s in 50 mm ratio, (%) ν 45–4 65–3 50–9 21–5 40–30 60–5 65–2 25–7 0 65–5 0 — 0 — — 0.31–0.34 0.33–0.35 0.43 0.29–0.35 0.32 0.31 0.28–0.33 0.31–0.34 0.27 0.27 0.2 0.2 0.24 0.19 280–310 — 160–580 — 0.26 0.01–0.1 — 0.5 20–120 3.5–17 — 35–170 380 3500 275–415 — 2000–3000 — 3500–4600 73–85 62–117 — 2800 73–100 — 2400–2800 1000–5 0.32–0.40 10–1 0 0 0 0 0 3 0–0.5 0.34–0.5 0.27 0.21–0.28 0.22–0.26 0.36 0.46 Note: In the upper part of the table, the lowest values for E, Sy,and Sut and the highest values for elongation are for pure metals. Note in Eq. (2.3) that, because engineering strain is dimensionless, E hasthe same unitsasstress. The modulus of elasticity is the slope of the elastic portion of the curve and indicates the stiffness of the material. The higher the elastic modulus, the higher is the load required to stretch the specimen to the same extent, and thus the stiffer is the material. Compare, for example, the stiffness of metal wire with that of a rubber band or plastic when they are both stretched. ׉ 7cassandra://2y7vq0BpgZlU_fmwTHwyYFv1IwexhlpTlvmJU-ExFkk`cZ%EbucZ%Ebt#בCט#u׉׉ 7cassandra://TjBf0455wfpUB4b_qXqSJH1ZiP-rvKafyW6_w9-VoQo`׉ 7cassandra://ujr2HvbOgbPg2jOcpD2I_CwX_NapEaGWRoh7IysJpzE;`s׉ 7cassandra://7h0EWwqpo0lBSA09_mevjIXvraj1fmdvQw7_rMGKsXc`׉ 7cassandra://Jm09lfbY2gbmGKP7HgCrPgtMPqYWWYkx59IdHPWsCv8\aN͠^cZEbqט#u׉׉ 7cassandra://uqGDG9W7p3AyOEL6r7M-qzSQ1synvtcIXQkJLEjK9MM%/`׉ 7cassandra://k8KcSsrHTg0zJgAn2l_yfOX_pNbYE2mouGFDBvzV3GIjI`s׉ 7cassandra://Df3CdFYGdmBK_Kh9ANf6IiE84_5ge1y2kHYSWLaENK0`׉ 7cassandra://UjF6Usqs-BmA8sgIocbfDnP76GMU2HkX0ovwYfpnbkUz ͠^cZEbr׉ETension 87 Unload Load Strain Elastic recovery Permanent deformation Figure 2.3: Schematic illustration of the loading and the unloading of a tensile-test specimen. Note that, during unloading, the curve follows a path parallel to the original elastic slope. The elongation of the specimen under tension is accompanied by lateral contraction; this effect can easily be observed by stretching a rubber band. The absolute value of the ratio of the lateral strain to the longitudinal strain is known as Poisson’s ratio (after S.D. Poisson, 1781–1840) and is denoted by the symbol ν. 2.2.2 Ductility An important behavior observed during a tension test is ductility—the extent of plastic deformation that the material can undergo prior to fracture. There are two common measures of ductility. The first is the total elongation of the specimen, given by Elongation = lf − lo lo × 100, (2.4) where lf and lo are measured as shown in Fig. 2.1a. Note that the elongation is based on the original gage length of the specimen, and that it is calculated as a percentage. The second measure of ductility is the reduction of area, given by Reduction of area = Ao −Af Ao × 100, (2.5) where Ao and Af are, respectively, the original and final (fracture) cross-sectional areas of the test specimen. Thus, the ductility of a piece of chalk is zero, because it does not stretch at all or reduce in its cross section. By contrast, a ductile specimen, such as a pure metal (such as copper or gold) or thermoplastic, stretches and necks considerably before it fractures. 2.2.3 True Stress and True Strain Recall that engineering stress is based on the original cross-sectional area, Ao, of the specimen. However, the instantaneous cross-sectional area of the specimen becomes smaller as it elongates, just as the area of a Stress ׉ 7cassandra://7h0EWwqpo0lBSA09_mevjIXvraj1fmdvQw7_rMGKsXc`cZ%Ebv׉E88 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials rubber band does when stretched. Thus, engineering stress does not represent the actual (or true) stress to which the specimen is subjected. True stress is defined as the ratio of the applied load, P, to the actual (instantaneous, hence true) crosssectional area, A, of the specimen: σ = P A . (2.6) For true strain, first consider the elongation of the specimen as consisting of increments of instantaneous change in length. Then, using calculus, it can be shown that the true strain (natural or logarithmic strain)is calculated as =ln   l lo . (2.7) Note from Eqs. (2.2) and (2.7) that, for small values of strain, the engineering and true strains are essentially equal; however, they diverge rapidly as the strain increases. For example, when e =0.1, =0.095,and when e =1, =0.69. Unlike engineering strains, true strains indicate actual physical phenomena in the deformation of materials. For example, consider a hypothetical situation where a compression specimen 50 mm in height is reduced, between flat platens, to a final height of zero. In other words, the specimen has been deformed to an infinite diameter. According to their definitions, the engineering strain that the specimen undergoes is (0 − 50)/50 = −1, but the true strain is −∞. Note that the answer will be the same regardless of the original height of the specimen. Clearly, then, true strain describes the extent of deformation correctly, since the deformation is indeed infinite. 2.2.4 Construction of Stress–Strain Curves The procedure for constructing an engineering stress–strain curve is to take the load–elongation curve (Fig. 2.4a; also, Fig. 2.2) and divide (1) the load (vertical axis) by the original cross-sectional area, Ao,and (2) the elongation (horizontal axis) by the original gage length, lo. Because Ao and lo are constants, the engineering stress–strain curve obtained, shown in Fig. 2.4b, has the same shape as the load–elongation curve shown in Fig. 2.4a. (In this example, Ao =36.1 m2 and Af =10.3 m2.) True stress–true strain curves are obtained similarly, by dividing the load by the instantaneous crosssectional area, with the true strain calculated from Eq. (2.7); the result is shown in Fig. 2.4c. Note the correction to the curve, reflecting the fact that the specimen’s necked region is subjected to three-dimensional tensile stresses, as described in more advanced texts. This state of stress gives higher stress values than the actual true stress; thus, to compensate for it, the curve must be corrected downward. The true stress–true strain curve in Fig. 2.4c can be represented by the equation σ = Kn, (2.8) where K is the strength coefficient and n is the strain-hardening (or work-hardening) exponent. Typical values for K and n for several metals are given in Table 2.3. When the curve shown in Fig. 2.4c is a log–log plot, it will be found that the curve is approximately a straight line (Fig. 2.4d). The slope of the curve is the exponent n. Thus, the higher the slope, the greater is the strain-hardening capacity of the material—that is, the stronger and the harder it becomes as it is strained. True stress–true strain curves for a variety of metals are given in Fig. 2.5. When reviewed in detail, some differences between Table 2.3 and Fig. 2.5 will be noted. These discrepancies are due to the different sources of data and different specimens that have been involved in obtaining them. Note also that the elastic regions in the curves have been deleted, because the slope of the curve in this region is very high. Consequently, the point of intersection of each curve with the vertical axis in this figure can be considered to be the yield strength, Sy, of the material. The area under the true stress–true strain curve at a particular strain is the energy per unit volume (specific energy) of the deformed material, and it indicates the work required to plastically deform a ׉ 7cassandra://Df3CdFYGdmBK_Kh9ANf6IiE84_5ge1y2kHYSWLaENK0`cZ%EbwcZ%Ebv#בCט#u׉׉ 7cassandra://tDUlxIFTPmRn1Zd-njEzJGlLE8c0AKKgAIq_VfOPPsMr`׉ 7cassandra://6VpG3UUuJbaYe4gQDtBp61KSBFmKyR7leeEkmTkdjVI=`s׉ 7cassandra://tZH6yqQmJMiRw9uVYCRwq-64vrg-FBB5UB4DkPvVxSM`׉ 7cassandra://o1GpCH6Ql_Xk2RMZZOXsrMOPAhMsB94kWw7JDvbQ780[<<͠^cZEbtט#u׉׉ 7cassandra://96UHsIUyvunyv4fv57VPBFXMl1p07f1C-PsW3PkKJIoN[`׉ 7cassandra://rGsTPA1TY7KxiGJ3H6OmNUD5BzNf8XbANaueADvI1PUCY`s׉ 7cassandra://JZdpb-fSGXZM3LRPzJAxPjpK6H5Cx_cCfAU56B5HpT4`׉ 7cassandra://mQRhih_TAVxk0BwH40zr0Fnq7n6UVCtVtp48mKEL960[S̪͠^cZEbu׉ETension 89 0 1 2 3 0 Extension, Dl (mm) 10 20 30 (a) 350 700 1050 1400 0 0 0.5 1.0 1.5 True strain ( ) (c) 350 525 700 225 0 0 0.2 0.4 Strain ( ) (b) 0.6 Maximum load 7000 Corrected for necking n 700 70 0.01 0.1 True strain ( ) (d) Figure 2.4: (a) Load–elongation curve in tension testing of a stainless steel specimen. (b) Engineering stress– engineering strain curve, drawn from the data in Fig. 2.4a. (c) True stress–true strain curve, drawn from the data in Fig. 2.4b. Note that this curve has a positive slope, indicating that the material is becoming stronger as it is strained. (d) True stress–true strain curve plotted on log–log paper and based on the corrected curve in Fig. 2.4c. The correction is due to the triaxial state of stress that exists in the necked region of the specimen. unit volume of the material to that strain. The area under the true stress–true strain curve up to fracture is known as the material’s toughness, that is, the amount of energy per unit volume that the material dissipates prior to fracture. Note that toughness involves both the height and width of the stress–strain curve of the material, whereas strength is related only to the height of the curve and ductility is related only to the width of the curve. 2.2.5 Strain at Necking in a Tension Test As stated earlier, the onset of necking in a tension-test specimen corresponds to the ultimate tensile strength of the material. Note that the slope of the load–elongation curve at this point is zero, and it is there that the specimen begins to neck. The specimen cannot support the load being applied because the cross-sectional 1.0 True stress (MPa) Load, P (kg 3 103) True stress (MPa) Stress (MPa) ׉ 7cassandra://tZH6yqQmJMiRw9uVYCRwq-64vrg-FBB5UB4DkPvVxSM`cZ%Ebx׉E90 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Table 2.3: Typical Values for K and n for Selected Metals. Material K (MPa) Aluminum 1100–O 2024–T4 5052-O 6061–O 6061–T6 7075–O Brass 70–30, annealed 85–15, cold-rolled Steel Low-C, annealed 1020, annealed 4135, annealed 4135, cold-rolled 4340, annealed 304 stainless, annealed 410 stainless, annealed Titanium Ti-6Al-4V, annealed, 20◦C 530 745 1015 0.26 0.20 0.17 1100 0.14 640 960 0.15 1275 0.45 0.10 1400 650 0.015 Ti-6Al-4V, annealed, 200◦C 1040 0.026 Ti-6Al-4V, annealed, 600◦C 0.064 Ti-6Al-4V, annealed, 800◦C 350 0.146 900 580 315 0.49 0.34 Cobalt-base alloy, heat-treated 2070 0.50 Copper, annealed 0.54 180 690 202 205 410 400 n 0.20 0.16 0.13 0.20 0.05 0.17 area of the neck is becoming smaller at a rate that is higher than the rate at which the material becomes stronger (strain hardens). The true strain at the onset of necking is numerically equal to the strain-hardening exponent, n,of the material. Thus, the higher the value of n, the higher the strain that a material can experience before it begins to neck. This observation is important, particularly in regard to sheet-metal-forming operations (Chapter 16) that involve the stretching of the workpiece material. It can be seen in Table 2.3 that annealed copper, brass, and stainless steel, for example, have high n values; this means that they can be stretched uniformly to a greater extent than the other metals listed in the table. Example 2.1 Calculation of Ultimate Tensile Strength Given: This example shows that the ultimate tensile strength, Sut, of a material can be calculated from its strength coefficient, K, and strain hardening exponent, n. Assume that a material has a true stress–true strain curve given by σ = 7000.5 MPa ׉ 7cassandra://JZdpb-fSGXZM3LRPzJAxPjpK6H5Cx_cCfAU56B5HpT4`cZ%EbycZ%Ebx#בCט#u׉׉ 7cassandra://FBdjlGTZCt1_L7pSEIDk-qqbTnqZ3FoFr66nMRFe5Jw6`׉ 7cassandra://EgP_3pRzrGwPuOdG-N_-fadyzei7_Q4wzW3-lLPJWFU6`s׉ 7cassandra://ELi0RqP_0aj7IAjfxOxjCdRcuOhcZ9i9YFGv7mmlBbgM`׉ 7cassandra://MdEb9z7MSDxmSJFRxZ4ZjZnDYYMzCD44WixX2K-6KI8w͠^cZEbwט#u׉׉ 7cassandra://I9bBpV9zU0uNyVHN3o4kOO5dVx-XUY-v4ECm5W-7fJ8O`׉ 7cassandra://ixJQA0jIqb-4mX8_75GELTP2THKNokhRMj8OjIEReLkX:`s׉ 7cassandra://oV8Eq_ifMitoynDMlT-YZS8WTQzf53pIiykpiHBJxxg`׉ 7cassandra://unWshb5isbEI65vkwL142lkjyIr37ENz6Kj7lmk1Fv0́\n͠^cZEbx׉ETension 91 Find: Calculate the true ultimate tensile strength and the engineering Sut of this material. Solution: Recall that the necking strain corresponds to the maximum load; thus, the necking strain for this material is = n =0.5. Therefore, the true ultimate tensile strength is σ = Knn = 700(0.5)0.5 = 495 MPa. The true area at the onset of necking is obtained from ln  Thus, Aneck = Ao−0.5, and the maximum load, P,is P = σAneck = σAoe−0.5, where σ is the true ultimate tensile strength. Hence, P = (495)(0.606)(Ao) = 300Ao MN Since Sut = P/Ao, Sut = 300 MPa 1200 1000 800 600 400 200 1100–H14 Al 0 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 True strain ( ) Figure 2.5: True stress–true strain curves in tension at room temperature for various metals. The curves start at a finite level of stress: The elastic regions have too steep a slope to be shown in this figure; thus, each curve starts at the yield strength, Sy, of the material. 304 Stainless steel 8650 Steel 1112 Steel, cold rolled 70230 Brass, as received 70230 Brass, annealed 1020 Steel 4130 Steel Copper, annealed 20242T36 Al 20242O Al 6061–O Al 1100–O Al Ao Aneck  = n =0.5. MPa ׉ 7cassandra://ELi0RqP_0aj7IAjfxOxjCdRcuOhcZ9i9YFGv7mmlBbgM`cZ%Ebz׉E U92 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials eT 600 400 200 0 0 200 400 Temperature (8C) Figure 2.6: Effect of temperature on mechanical properties of carbon steel. Most materials display a similar temperature sensitivity for elastic modulus, yield strength, ultimate strength, and ductility. 2.2.6 Temperature Effects Increasing the temperature generally has the following effects on stress–strain curves (Fig. 2.6): a) The ductility and toughness increase b) The yield strength and the modulus of elasticity decrease. Temperature has a major influence on the magnitude of the strain-hardening exponent, n, for most metals, in that it increases with increasing temperature. However, this behavior is best described in conjunction with the rate of deformation, because increasing strain rate tends to decrease n. 2.2.7 Effects of Rate of Deformation and Strain Rate Just as a balloon can be inflated or a rubber band stretched at different rates, materials in manufacturing processes can be shaped at different speeds. Some machines, such as hydraulic presses, form materials at low speeds, while others, such as mechanical presses, form them at high speeds. The deformation rate in a tension test is the speed at which the specimen is being stretched, in units such as m/s. The strain rate, on the other hand, is a function of the specimen’s length. For example, consider two rubber bands, one 20 mm and the other 100 mm long, respectively, that are stretched by 10 mm within a period of one second. The engineering strain in the shorter specimen is 10 100 =0.1. Thus, the strain rates are 0.5 s−1 and 0.1 s−1, respectively. Although they are both being stretched at the same deformation rate, the short one is being stretched at a strain rate five times higher than that for the long one. 10 Deformation rates typically employed in various testing and metalworking processes, and the true strains involved, are given in Table 2.4. Because of the wide ranges encountered in practice, strain rates are generally stated in terms of orders of magnitude, such as 102 s−1, 104 s−1,and soon. The typical effects that temperature and strain rate jointly have on the strength of metals are shown in Fig. 2.7. Note that increasing the strain rate increases the strength of the material, called strain-rate hardening. The slope of these curves is the strain-rate sensitivity exponent, m. The magnitude of m is determined from log–log plots, provided that the vertical and horizontal scales are the same (unlike those shown in Fig. 2.7). A slope of 45◦, for example, would indicate a value of m =1. The relationship is given by the equation σ = C˙m (2.9) 20 =0.5; the strain in the longer is 600 200 150 100 50 0 60 40 20 0 Stress (MPa) Elastic modulus (GPa) Elongation (%) Elastic modulus h t g n h E l o n g a t i o n e l i s n e r t s Yield strengt ׉ 7cassandra://oV8Eq_ifMitoynDMlT-YZS8WTQzf53pIiykpiHBJxxg`cZ%Eb{cZ%Ebz#בCט#u׉׉ 7cassandra://hc33uDTnq4wcihnhM80sOEhPeP-VYwqWGdPqSpRa8esL`׉ 7cassandra://0borQY1trdoh5UlG4GaliDqgcKTmPdAI-TN9LwD7gHU@F`s׉ 7cassandra://GE0X2KcTpE8vtuyW-cXiTeq5-31oRv6Z5-ebApyYrEoZ`׉ 7cassandra://2YugWeEOuIBqW2IiBKf5mn9ZoCIK78Hym3pRSOcyhZUns̔͠^cZEbzט#u׉׉ 7cassandra://5XKjVw8WVsb0r46SvOLMThXFwdVWz8EIbBhkVyeRf4Ir`׉ 7cassandra://l_oBNTze_Drqd18fCBP7xjnXPROa-MHFtLx4hd2Jco4o `s׉ 7cassandra://8H9sqbbQTj5FApvweyvnhPvPbPe82RGoNpzPEbKlEa0)`׉ 7cassandra://yTklxDXto89JgoiaoQL659h0YjKE4PCgG-o5WwWUvHcl3͠^cZEb{׉EyTension 93 Table 2.4: Typical Ranges of Strain and Deformation Rate in Manufacturing Processes. Process True Strain Deformation rate (m/s) Cold working Forging, rolling Wire and tube drawing Explosive forming Hot working and warm working Forging, rolling Extrusion Machining Sheet-metal forming Superplastic forming 0.1–0.5 0.05–0.5 0.05–0.2 0.1–0.5 2–5 1–10 0.1–0.5 0.2–3 0.1–100 0.1–100 10–100 0.1–30 0.1–1 0.1–100 0.05–2 10−4–10−2 where C is the strength coefficient and ˙ is the true strain rate, defined as the true strain that the material undergoes per unit time. Note that C has the units of stress and is similar to, but not to be confused with, the strength coefficient K in Eq. (2.8). From Fig. 2.7, it can be seen that the sensitivity of strength of the material to strain rate increases with temperature; in other words, m increases with increasing temperature. Also note that the slope is relatively flat at room temperature; that is, m is very low. This condition holds for most metals, but not for those that recrystallize at room temperature, such as lead and tin. Typical ranges of m for metals are up to 0.05 for cold-working, 0.05 to 0.4 for hot-working, and 0.3 to 0.85 for super-plastic materials (see below). 200 100 50 10 10{6 10{4 10{2 100 102 104 106 Strain rate (s]1) Figure 2.7: The effect of strain rate on the ultimate tensile strength for aluminum. Note that, as the temperature increases, the slopes of the curves increase; thus, strength becomes more and more sensitive to strain rate as temperature increases. Source: J.H. Hollomon. 8008 6008 4008 200° Room temperature 308C MPa 10008 ׉ 7cassandra://GE0X2KcTpE8vtuyW-cXiTeq5-31oRv6Z5-ebApyYrEoZ`cZ%Eb|׉EM94 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials The magnitude of the strain-rate sensitivity exponent significantly influences necking in a tension test. With increasing m, the material stretches further before it fails; thus, increasing m delays necking. Ductility enhancement caused by the high strain-rate sensitivity of some materials has been exploited in superplastic forming of sheet metal, as described in Section 16.10. Superplasticity. The term superplasticity refers to the capability of some materials to undergo large uniform elongations prior to necking and to fracture in tension. The elongation may range from a few hundred percent to as much as 2000%. Common nonmetallic materials exhibiting superplastic behavior are bubble gum, glass at elevated temperatures (Section 8.4), and thermoplastics (Section 7.3). Thus, glass and thermoplastics, as examples, can successfully be formed into a wide variety of complex shapes. Among metals exhibiting superplastic behavior are very fine grained (10 to 15 μm) titanium alloys and alloys of zinc–aluminum; when heated, they can elongate to several times their original length. 2.2.8 Hydrostatic Pressure Effects A variety of tests can be performed to determine the effect of hydrostatic pressure on the mechanical properties of materials. Tests at pressures up to 3.5 GPa indicate that increasing the hydrostatic pressure substantially increases the strain at fracture, both for ductile and for brittle materials. The beneficial effect of hydrostatic pressure has been exploited in metalworking processes, especially in hydrostatic extrusion (Section 15.4.2) and in compaction of metal powders (Section 17.3). 2.2.9 Radiation Effects In view of the use of various metals and alloys in nuclear applications, extensive studies have been conducted on radiation’s effects on mechanical properties. Typical changes in the properties of steels and other metals exposed to doses of high radiation are increased yield strength, tensile strength, hardness, and decreased ductility and toughness. 2.3 Compression Numerous metalworking processes in manufacturing, such as forging, rolling, and extrusion (Part III), are performed whereby the workpiece is subjected to compressive forces. The compression test, in which the specimen is subjected to a compressive load, gives information that is essential in estimating forces and power requirements in these processes. The test is usually carried out by compressing a solid cylindrical specimen between two well-lubricated flat dies (platens). Because of friction between the test specimen and the platens, the specimen’s cylindrical surface bulges, an effect called barreling (see Fig. 2.8). The height-todiameter ratio of the specimen should be typically less than 3:1 in order to avoid buckling during the test (see also Section 14.4 on heading). Because of barreling, the specimen’s cross-sectional area varies along its height, and thus developing stress–strain curves in compression can be challenging. Furthermore, since friction dissipates energy (Section 33.4), the compressive force is higher than it otherwise would be in order to overcome friction. With effective lubrication, friction can be minimized and thus a reasonably constant cross-sectional area can be maintained during the test. When the results of compression and tension tests on ductile metals are compared, the true stress–true strain curves coincide. This behavior, however, is not the case for brittle materials, which are generally much stronger and more ductile in compression than in tension (see Table 8.2). If a specimen is subjected first to tension, deformed plastically, the load is released, and then a compressive load is applied, the yield strength in compression is found to be lower than that in tension, a behavior known as the Bauschinger effect (after J. Bauschinger, reported in 1881). This behavior is exhibited to varying degrees by all metals and alloys. The phenomenon is also called strain softening or work softening, because of the lowered yield strength in the direction opposite to that of the original load application. ׉ 7cassandra://8H9sqbbQTj5FApvweyvnhPvPbPe82RGoNpzPEbKlEa0)`cZ%Eb}cZ%Eb|#בCט#u׉׉ 7cassandra://ER-aYOQuJ7ipnxqDR7y0Uj8nO_Rt2oTPQkueXfh-4JAR`׉ 7cassandra://dlb3DnF-_3041YKvBZmXYxo-Ph8PTXPahAYhF4R9wIQ4`s׉ 7cassandra://feXlIX2mKAnrh53JaMRc2VmJ6GTmnFZnMO0ySHIEia8d`׉ 7cassandra://fQPjW3UspsuBAuPQrg5ZatH2ceW7mDZCTLnUeq4ifcUpr͠^cZEb}ט#u׉׉ 7cassandra://n4p9mv4ajIyYFcsAIf2SZmAl-imZW32Z73NUFVOWZVA`׉ 7cassandra://p2_CAQNbCbWlU3kXBPMueIiIdZquLmDo1oR-a06fJq4H:`s׉ 7cassandra://DwHqYzl3EeJ9k8CHWdqzHOXSILV3AP8e9cA2whltxDA`׉ 7cassandra://wHIDGEMzfVqwaRAFr8OBklfKxI6q5DXE-ONKX4KnpK4qH͠^cZEb~׉E"Compression 95 Figure 2.8: Barreling in compressing a round solid cylindrical specimen (7075-O aluminum) between flat dies. Barreling is caused by friction at the die–specimen interfaces, which retards the free flow of the material (see also Fig. 14.3). Disk Test. For brittle materials such as ceramics and glasses (Chapter 8), the disk test can be used, in which a disk is subjected to a diametral compression force between two hardened flat platens (Fig. 2.9). When the specimen is loaded as shown, tensile stresses are developed perpendicular to the vertical centerline along the disk. Fracture then initiates and the disk splits vertically in half. The tensile stress, σ, in the disk is uniform along the centerline; it can be calculated from the formula σ = 2P πdt , (2.10) where P is the load at fracture, d is the diameter of the disk, and t is its thickness. In order to avoid premature failure at the contact points, thin strips of soft metal are placed between the disk and the two platens. The strips also protect the platens from being damaged during the test. The phenomenon of fracture at the centerline of the specimen has been utilized in making seamless tubing (Section 13.5). P Fracture P Figure 2.9: Disk test on a brittle material, showing the direction of loading and the fracture path. ׉ 7cassandra://feXlIX2mKAnrh53JaMRc2VmJ6GTmnFZnMO0ySHIEia8d`cZ%Eb~׉E[96 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.4 Torsion In addition to undergoing tension and compression, a workpiece may be subjected to shear strains (Fig. 2.10), such as in the punching of holes in sheet metals (Section 16.2), in swaging (Section 14.4), and in machining operations (Section 21.2). The method generally used to directly determine properties of materials in shear is the torsion test. This test is usually performed on a thin tubular specimen, in order to develop an approximately uniform stress and strain distribution along its cross section. A torsion test specimen typically has a reduced cross section in order to confine the deformation to a narrow zone. The shear stress can then be calculated from the formula τ = T 2πr2t , (2.11) where T is the torque applied, r is the average radius of the tube, and t is the thickness of the tube at its narrow cross section. The shear strain can be calculated from the formula γ = rφ l , (2.12) where l is the length of the tube section and φ the angle of twist in radians. The ratio of the shear stress to the shear strain in the elastic range is known as the shear modulus or modulus of rigidity, G. The modulus, G, is a quantity related to the modulus of elasticity, E, by the formula G = E 2(1 + ν) . (2.13) The angle of twist, φ, to fracture in the torsion of solid round bars at elevated temperatures has been found to be useful in estimating the forgeability of metals (Section 14.5). The greater the number of twists prior to failure, the better is the forgeability. 2.5 Bending (Flexure) Preparing specimens from brittle materials can be difficult because of the challenges involved in shaping, machining, and finishing them to appropriate final dimensions. Furthermore, the specimens are typically sensitive to surface defects (such as scratches and notches), and clamping brittle specimens for testing can f l r r rf t rf l Figure 2.10: A typical torsion-test specimen, mounted between the two heads of a testing machine and twisted. Note the shear deformation of an element in the reduced section of the specimen. ׉ 7cassandra://DwHqYzl3EeJ9k8CHWdqzHOXSILV3AP8e9cA2whltxDA`cZ%EbcZ%Eb~#בCט#u׉׉ 7cassandra://THGqVJZrmRrmbBdduWu2HkquGLczKQvGVDfuDIQ_KbYJ`׉ 7cassandra://CBWmLdXwStZVTELxxG2neL6U4KabCdPv97YCQiFlQuQS`s׉ 7cassandra://yMJe2GS2puhcZoVy2BrYNc-AH0Wc7BW8v7IQkAq5n4w`׉ 7cassandra://1zzjm8mHGv14Dn3tdfcRauxNcEzzFIWqj0_GKU1uVZAy)͠^cZEbǀט#u׉׉ 7cassandra://LsNGPe61tAQq_mmW8wjtBHpvujN1fo-D37st7jTqh2o`׉ 7cassandra://Tpdbbj_A5EQLrBV5GvSldmRFtAv7m4Nu0InVW-zT8X470`s׉ 7cassandra://ZC__W_TNEmtxhfjMjwBBVWTEVpHTCfhbjrT2rP_jqpAI`׉ 7cassandra://3oHHpBEJI1q7GIazdLHd4qw4SYH1Nc-6CI8xeE8OPiAh ͠^cZEbǁ׉E Hardness 97 Maximum bending moment (a) (b) Figure 2.11: Two bend-test methods for brittle materials: (a) three-point bending, and (b) four-point bending. The two areas shown above the beams represent the bending-moment diagrams, described in texts on the mechanics of solids. Note the region of constant maximum bending moment in (b); by contrast, the maximum bending moment occurs only at the center of the specimen in (a). be difficult. Improper alignment of the test specimen also can result in nonuniform stress distribution along its cross section. A commonly used test method for brittle materials is the bend or flexure test; it usually involves a specimen that has a rectangular cross section and supported in the manner shown in Fig. 2.11. The load is applied vertically, at either one point or two points. Consequently, these tests are referred to as three-point and four-point bending, respectively. The longitudinal stresses in the specimens are tensile at their lower surfaces and compressive at their upper surfaces. The stresses developed can be calculated using simple beam equations, described in texts on the mechanics of solids. The stress at fracture in bending is known as the modulus of rupture or transverse rupture strength (see Table 8.2). Note that, because of the larger volume of material subjected to the same bending moment in Fig. 2.11b, there is a higher probability that defects exist within this volume than exist in the point shown in Fig. 2.11a. Consequently, the four-point test predicts a lower modulus of rupture than the three-point test. 2.6 Hardness Hardness is generally defined as resistance to permanent indentation; thus, steel is harder than aluminum, and aluminum is harder than lead. Hardness is not a fundamental property, because the resistance to indentation depends on the shape of the indenter and on the load applied. Hardness is a commonly used property; it gives a general indication of the strength of the material and of its resistance to scratching and to wear. 2.6.1 Hardness Tests Several test methods, using different indenter materials and shapes (Fig. 2.12), have been developed to measure the hardness of materials. The most commonly used hardness tests are described next. Brinell Test. Introduced by J.A. Brinell, in 1900, this test involves pressing a steel or tungsten-carbide ball, 10 mm in diameter, against a surface (Fig. 2.13). The Brinell hardness number (HB) is defined as the ratio of the applied load, P, to the curved surface area of the indentation. The harder the material tested, the smaller is the impression; a 1500 kg or 3000 kg load is usually recommended in order to obtain impressions sufficiently large for accurate measurement of hardness. ׉ 7cassandra://yMJe2GS2puhcZoVy2BrYNc-AH0Wc7BW8v7IQkAq5n4w`cZ%Eb׉EU98 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials (a) (b) (c) Figure 2.12: A selection of hardness testers. (a) A Micro Vickers hardness tester; (b) Rockwell hardness tester; (c) Leeb tester. Source: (a) and (b) Courtesy of Buehler (c) Courtesy of Wilson R Instruments. Depending on the surface condition of the material tested, two types of impression develop (Fig. 2.14). In annealed metals, the impression generally has a rounded profile along its periphery (Fig. 2.14a). In cold-worked metals, they usually have a sharp profile (Fig. 2.14b). The correct method of measuring the indentation diameter, d, is shown in the figure. The indenter has a finite elastic modulus, hence it undergoes elastic deformation under the applied load. As a result, hardness measurements may not be as accurate as expected, depending on the indenter material. One method for minimizing this effect is using tungsten-carbide balls (Section 22.4), which, because of their higher modulus of elasticity, distort less than steel balls do. These indenters are usually recommended for materials with a Brinell hardness number higher than 500. Rockwell Test. Developed by S.P. Rockwell, in 1922, this test measures the depth of penetration instead of the diameter of the indentation. The indenter is pressed onto the surface, first with a minor load and ׉ 7cassandra://ZC__W_TNEmtxhfjMjwBBVWTEVpHTCfhbjrT2rP_jqpAI`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://gaDGY2bEt7gHDKqxYns5vYiIblA-_t_Syg-hF2PiLw8_`׉ 7cassandra://m4m1QRaK9KI9OpVWdH1D7lPskWU2akq7g3yNwHsllxMRb`s׉ 7cassandra://WWW2BgPerxB8S89Y8SdNYQfZs81-LdoVvJOhA6vOZTQv`׉ 7cassandra://oHuFzfOcM3DLQYmI5K76c1y-yQeokXVvP560vsKWTJIm͠^cZEbǃט#u׉׉ 7cassandra://sFBk1_mtAOx4nSYX6GrLzb-rOpONjTiHRe23kmf_zBo`׉ 7cassandra://eaHCzpU2mxSaf2M7oBZspSGv1diDISnYbRxsbS5ff9MHm`s׉ 7cassandra://CYt2_PxNY3i84vBSA1ZVOT3gbPZgLkpAG1XSGUy_KR0`׉ 7cassandra://9c30ib6qpbswi7UjplALvM-WiLASPM9dBKcvhYcMFrYm9"͠^cZEbDŽ׉EHardness 99 Shape of indentation Test Brinell Vickers Indenter 10-mm steel or tungstencarbide ball Diamond pyramid b Knoop Rockwell A C D B F G E 1208 Diamond cone t 5 mm 1.6-mm diameter steel ball t 5 mm 3.2-mm diameter steel ball 60 kg 150 kg 100 kg 100 kg 60 kg 150 kg 100 kg Figure 2.13: General characteristics of hardness-testing methods and formulas for calculating hardness. HRA HRC HRD HRB HRF HRG HRE 5 100 2 500t Diamond pyramid L/b 5 7.11 b/t 5 4.00 25 g25 kg t L HK 5 14.2P L2 Side view d D 1368 L Top view d Load, P 500 kg 1500 kg 3000 kg 12120 kg Hardness number HB 5 2P (pD)(D 2 D2 2 d2 HV 5 1.854P L2 ) 5 130 2 500t then with a major load. The difference in the depths of penetration is a measure of the hardness of the material. Figure 2.13 shows some of the common Rockwell hardness scales for engineering materials and the indenters used. Rockwell superficial hardness tests, using the same type of indenters but at lighter loads, also are available. Vickers Test. This test, developed in 1922 and previously known as the diamond pyramid hardness test, uses a pyramid-shaped diamond indenter (Fig. 2.13) and at a load that ranges from 1 kg to 120 kg. The Vickers hardness number is indicated as HV. The impressions are typically less than 0.5 mmon the diagonal. This test gives essentially the same hardness number regardless of the load, and is suitable for testing materials with a wide range of hardness, including heat-treated steels. Test procedures have been developed to perform tests using atomic force microscopes and nanoindenters, where indentation depths are as low as 20 nm. Knoop Test. This test, developed by F. Knoop, in 1939, uses a diamond indenter in the shape of an elongated pyramid (Fig. 2.13). The applied load generally ranges from 25 g to 5 kg. The Knoop hardness number is indicated as HK. Because of the light loads applied, this test is a microhardness test, and is suitable for very small or very thin specimens, and for such brittle materials as carbides, ceramics, and glass. The Knoop test is also used for measuring the hardness of individual grains or components in a metal alloy. Because the size of the indentation is typically in the range of 0.01–0.10 mm, surface preparation is ׉ 7cassandra://WWW2BgPerxB8S89Y8SdNYQfZs81-LdoVvJOhA6vOZTQv`cZ%Eb׉E100 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials d d (a) (b) (c) Figure 2.14: Indentation geometry in Brinell hardness testing: (a) annealed metal; (b) work-hardened metal; (c) deformation of mild steel under a spherical indenter. Note that the depth of the permanently deformed zone is about one order of magnitude larger than the depth of indentation. For a hardness test to be valid, this zone should be fully developed in the material. Source: After M.C. Shaw and C.T. Yang. important. The hardness number obtained depends on the applied load, therefore test results should always cite the load employed. Scleroscope and Leeb Tests. The scleroscope (from the Greek skleros, meaning hard) is an instrument in which a diamond-tipped indenter (hammer), enclosed in a glass tube, is dropped onto the specimen from a certain height. The hardness is related to the rebound of the indenter: the higher the rebound, the harder is the material tested. The impression made by a scleroscope is very small. Since reliable results with a scleroscope can be difficult to obtain, an electronic version, called a Leeb, or Equotip, test, has been developed (Fig. 2.12d). A carbide hammer impacts the surface, and the incident and the rebound velocities are electronically measured. The Leeb number is then calculated and is usually converted to Rockwell or Vickers hardness. Mohs Hardness. Developed in 1822 by F. Mohs, this test is based on the capability of one material to scratch another. The Mohs hardness number is based on a scale from 1 to 10, with 1 being the measure for talc and 10 for diamond (the hardest substance known; see also Section 8.7). Thus, a material with a higher Mohs hardness number always scratches the one with a lower number. Soft materials typically have a number between 2 and 3, hardened steels about 6, and aluminum oxide (used in cutting tools and abrasives) of 9. Although the Mohs scale is qualitative and is used mainly by mineralogists, it correlates well with Knoop hardness. ׉ 7cassandra://CYt2_PxNY3i84vBSA1ZVOT3gbPZgLkpAG1XSGUy_KR0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://WWOpvd5boN7HgD-H8m_Gzv-1o_coZmpaorAb3JpTcKw]j`׉ 7cassandra://xeiOg1A5BrOjrYdRXDxfxvy7DkN9UidUSZjAMpqXoUQY`s׉ 7cassandra://yCnKAnsBD1XpT3MrspDWChwWVzNok1ROp7y4qH_d7ww`׉ 7cassandra://i1TQGNEZSO0s9J_P4txRwWGPlolsOYZ0wtRTKLLzWTE]͠^cZEbdžט#u׉׉ 7cassandra://2_1BR-MRtm5SZclwLIVAugmrfk60L5qth89TNIlgkNAC`׉ 7cassandra://5g5qQCD4-9kMuw4xotiOp2joRTEHVCgLfKGJzGCG_tsW`s׉ 7cassandra://mW0WcqdYWFUM8IitjUx5BIH8yQoZ-RdMifXf7eDF29U`׉ 7cassandra://-4erW9Kgpn-INocKMiTvQpV4fTWU7BGzfi9F4pK-u_cr͠^cZEbLJ׉E Hardness 101 Shore Test. The hardness of such materials as rubbers, plastics, and soft and elastic nonmetallic materials is generally measured by a Shore test, with an instrument called a durometer (from the Latin durus, meaning hard). An indenter is first pressed against the surface and a constant load is rapidly applied. The depth of penetration is then measured after one second. There are two different scales for this test. For Type A, a blunt indenter is used at an applied load of 1 kg. This method is typically used for softer materials. Type D has a sharper indenter and at a load of 5 kg; it is used for harder materials. The hardness numbers in these two tests range from 0 to 100. Hot Hardness. The hardness of materials at elevated temperatures (see Fig. 22.1) is an important factor in such applications as cutting tools and for dies in hot-working and casting operations. Hardness tests can be performed using conventional testers, with some modifications such as enclosing the specimen and indenter in a small electric furnace. 2.6.2 Hardness and Strength Because hardness is the resistance to permanent indentation, it can be likened to performing a compression test on a small area on a material’s surface (Fig. 2.14c). It has been shown that the hardness of a cold-worked metal, for example, is about three times its yield strength Sy (using the same units). For annealed metals, the hardness is about five times Sy. A relationship has been established between the ultimate tensile strength (Sut) and the Brinell hardness number (HB) for steels, measured for a load of 3000 kg. In SI units, the relationship is given by Sut =3.5(HB), where Sut is in MPa. (2.14) 2.6.3 Hardness-testing Procedures For a hardness test to be reliable, the zone of deformation under the indenter (Fig. 2.14c) must be allowed to develop freely. Consequently, the location of the indenter (with respect to the location of the edges of the specimen to be tested) and the thickness of the specimen are important considerations. Generally, the location should be at least two diameters of the indentation from the edge of the specimen, and the thickness of the specimen should be at least 10 times the depth of penetration of the indenter. Successive indentations on the same surface of the workpiece should be far enough apart so as not to interfere with each other. The indentation should be sufficiently large to give a representative hardness value for the bulk material. If (a) hardness variations, if any, must be detected in a small surface area, or (b) the hardness of individual constituents in a matrix or in an alloy is to be determined, the indentations must be very small (such as those obtained in Knoop or Vickers tests, using light loads). While surface preparation is not critical for the Brinell test, it is important for the Rockwell test and even more important for the other hardness tests, because of the small sizes of the indentations. Surfaces may have to be polished to allow correct measurement of the impression’s dimensions. The hardness values obtained from different tests can be interrelated, and converted, using Fig. 2.15. Care should be exercised in using these charts because of the variables involved in material characteristics and in the shape of the indentation used. ׉ 7cassandra://yCnKAnsBD1XpT3MrspDWChwWVzNok1ROp7y4qH_d7ww`cZ%Eb׉E 102 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Example 2.2 Calculation of Modulus of Resilience from Hardness Given: A piece of steel is highly deformed at room temperature. Its hardness is found to be 300 HB. Find: Estimate the area of the elastic portion of the stress–strain curve up to the yield point (that is, the resilience) for this material if the yield strength is one-third the Brinell hardness. Solution: Since the steel has been subjected to large strains at room temperature, it may be assumed that its stress–strain curve has flattened considerably, thus approaching the shape of a perfectly plastic curve. Since the yield strength is one-third the Brinell hardness, Sy = 300 3 The area under the stress–strain curve is S2 Modulus of resilience = From Table 2.2, E = 210 GPa for steel. Hence, Modulus of resilience = (981 × 106)2 2 (210 × 109) y . 2E =2.29 MNm/m3. = 100 kg/mm2 = 981 MPa. 2.7 Fatigue The components of manufacturing equipment, such as tools, dies, gears, cams, shafts, and springs, are often subjected to rapidly fluctuating (cyclic or periodic) loads, in addition to static loads. Cyclic stresses develop by fluctuating mechanical loads, such as (a) on gear teeth or in reciprocating sliders, (b) by rotating machine elements under constant bending stresses, as is commonly encountered in shafts, or (c) by thermal stresses, as when a die at room temperature comes into repeated contact with hot workpieces, and then begins to cool down between successive contacts. Under any of these conditions, the component may fail at a stress level below that at which failure would occur under static loading. Upon inspection, failure is found to be associated with cracks that develop and grow with every stress cycle. The cracks propagate through the part until a critical crack length is reached and the part fractures. Known as fatigue failure, this phenomenon is responsible for the majority of failures in mechanical components. Fatigue test methods involve testing specimens under a variety of states of stress, usually in a combination of tension and bending. The test is carried out at various stress amplitudes (S); the number of cycles (N) to cause total failure of the specimen or part is then recorded. Stress amplitude is defined as the maximum stress, in tension and compression, to which the specimen is subjected. Typical plots, called S-N curves, are shown in Fig. 2.16. These curves are based on complete reversal of the stress—that is, maximum tension, then maximum compression, then maximum tension, and so on— such as that imposed by bending a piece of wire alternately in one direction and then the other. Tests may also be performed on a rotating shaft in four-point bending (Fig. 2.11b.) With some materials, the S-N curve becomes horizontal at low stress levels; the maximum stress to which the material can be subjected without fatigue failure, regardless of the number of cycles, is known as the endurance limit or fatigue limit. ׉ 7cassandra://mW0WcqdYWFUM8IitjUx5BIH8yQoZ-RdMifXf7eDF29U`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://yfQj3cCGxULtePH-Qy7ih7sruCd4GNccRM-EQjwOEZka`׉ 7cassandra://arAgI3V4_io3Kyfvi0BBHyhBpnrceAOVVF7oWdO4DIQ:`s׉ 7cassandra://5V-rdO_siamzA5J2u7MbeLrP6vNhMT7fErRbKAg1kzo)`׉ 7cassandra://gt1K0HezsSbPAzTTGGd3XebR4SvW5MtVrdxRpA8ceqk_͠^cZEbljט#u׉׉ 7cassandra://h_Uho-D8mwSN1eRVKD9EXY4h7ft3lkIjbs6qcUYj5hEo`׉ 7cassandra://FlM-Y3NQFWJ1FZgru7yXLDTPET_dk3NbLXzeAUviNZY=z`s׉ 7cassandra://6gOZrAkoQVxwp4SzitxCplOKImgx5CWdLQqetJVK1vA`׉ 7cassandra://-vxIF8bfGc9AHYUy3rhLyareVmQUvRix9hSOqay_m6osu^͠^cZEbNJנcZEbnj>>9ׁHhttp://107.Thׁ ׁ Ј׉EUFatigue 103 10000 8000 10 Diamond Diamond 3000 4000 5000 3000 2000 1100 1400 400 500 600 700 800 900 1000 100 150 200 300 60 70 80 90 30 40 50 20 40 50 20 30 400 500 600 700 800 900 1000 200 300 85 70 75 80 45 50 55 60 65 100 40 100 110 120 70 80 90 20 30 40 50 60 0 10 20 30 40 50 60 70 80 0 100 40 50 60 70 80 90 20 30 400 500 600 700 800 100 150 200 300 1 2 3 4 5 6 7 8 9 Corundum Topaz Quartz Orthoclase Apatite Fluorite Calcite Gypsum Talc 2100 1800 1400 1000 700 500 Cubic boron nitride Boron carbide Titanium carbide Tungsten carbide Cementite Hard chromium plate Martensite Fully hardened tool steel Glass Cold-drawn 0.5 C steel Cold-drawn 0.25 C steel Titanium Gray iron Malleable iron Annealed mild steel 70–30 brass Nickel Pure iron Bakelite Zinc, gold, magnesium Silver Polystyrene Pure aluminum Vinyl resin 10 9 7 5 4 3 2 10 Graphite Tin 4 5 2 3 Lead 1 1 Figure 2.15: Chart for converting various hardness scales; note the limited range of most of the scales. Because of the many factors involved, these conversions are approximate. HB HV HRA HRB HRC Scleroscope HK Mohs scale UTS (steels) MPa ׉ 7cassandra://5V-rdO_siamzA5J2u7MbeLrP6vNhMT7fErRbKAg1kzo)`cZ%Eb׉E104 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 500 400 300 200 100 0 Endurance limit 10 20 30 40 50 60 0 103 104 105 106 107 108 109 1010 Number of cycles, N (a) 103 104 105 106 Number of cycles, N (b) Figure 2.16: (a) Typical S-N curves for two metals. Note that, unlike steel, aluminum does not have an endurance limit. (b) S-N curves for common polymers. 107 l a t e Although several materials, especially steels, have a specific endurance limit, others, especially aluminum alloys, do not have such a limit and the S-N curve continues its downward trend. For metals exhibiting such behavior, the fatigue strength is specified at a certain number of cycles, such as 107.The useful service life of the component can then be specified. The endurance limit for metals can be approximately related to their ultimate tensile strength (Fig. 2.17). For carbon steels, for example, the endurance limit is usually 0.4–0.5 times the tensile strength. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.1 0 Titanium Steels Cast irons Copper alloys Cast magnesium alloys Aluminum alloys Wrought magnesium alloys 0 200 400 600 800 1000 1200 MPa Figure 2.17: Ratio of endurance limit to tensile strength for various metals, as a function of tensile strength. Because aluminum does not have an endurance limit, the correlations for aluminum are based on a specific number of cycles, as is seen in Fig. 2.16. l y o 1045 Steel - 4 Stress amplitude, S (MPa) Endurance limit/tensile strength Stress amplitude, S (MPa) D i e P l o y a l l y l - p T F h t h a E e p Nylon (dry) o l f n m i P 2 o P 1 0 c l T a y s r A 6 P E b u h l u o o x l i c n o n n u y a t e m a l ׉ 7cassandra://6gOZrAkoQVxwp4SzitxCplOKImgx5CWdLQqetJVK1vA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://CpsCLxEovFZufqriV5qGo9a3ZcwZ-85BF3tuw9ZrxMk_`׉ 7cassandra://7uQRwtyrdwXUuDKut4WjIjwwE2yPKoBEDoIc32jeG78X`s׉ 7cassandra://LVRE72zavsDtyCC45usQ5JRd3twsdGqqaqtUZGP-2uEQ`׉ 7cassandra://zdve6HhyjwfajNHftwJeQzw4WB0V7hXREdtH0CmnorMe͠^cZEbǍט#u׉׉ 7cassandra://Ik1QACrOsur1mlpELZLDnUT4jtJzwtdduvphF6S2xN8*`׉ 7cassandra://bGSD2-hihcZJKCSi0zrU5K1wAsrHtKIKQLZAWMiW80IX+`s׉ 7cassandra://8L8p4q6z8aoHoIbC0DJqs8wAjtehNUAaOc0qUVQVDwQn`׉ 7cassandra://6TCyvnDB-Y75l-AcBKAwQTadDuDyrTGogAEeGEDVn1g͒[͠^cZEbǎ׉E Impact 105 Rupture Primary Tertiary Secondary Instantaneous deformation Time Figure 2.18: Schematic illustration of a typical creep curve. The linear segment of the curve (secondary) is used in designing components for a specific creep life. 2.8 Creep Creep is the permanent elongation of a material under a static load maintained for a period of time. This phenomenon occurs in metals as well as nonmetallic materials, such as thermoplastics and rubbers, and it can occur at any temperature; lead, for example, creeps under a constant tensile load at room temperature. For metals and their alloys, creep of any significance occurs at elevated temperatures, beginning at about 200◦C for aluminum alloys and at about 1500◦C for refractory alloys. The mechanism of creep at elevated temperature in metals is generally attributed to grain-boundary sliding (see Section 1.5). It is especially important in high-temperature applications, such as gas-turbine blades and various components in jet engines and rocket motors. High-pressure steam lines, nuclear-fuel elements, and furnace components are likewise subject to creep. Creep can also occur in tools and dies that are subjected to high stresses at elevated temperatures during hot-working operations, such as forging and extrusion. The creep test typically consists of subjecting a specimen to a constant tensile load (hence constant engineering stress) at elevated temperature and measuring the changes in length at various time increments. A creep curve typically consists of primary, secondary, and tertiary stages (Fig. 2.18). During the test, the specimen eventually fails by necking and fracture, called rupture or creep rupture. As expected, the creep rate increases with specimen temperature and the applied load. Design against creep usually requires knowledge of the secondary (linear) range and its slope, because the creep rate can be determined reliably only when the curve has a constant slope. Resistance to creep generally increases with the melting temperature of the material. Thus, stainless steels, superalloys, and refractory metals and their alloys are commonly used in applications where resistance to creep is required. Stress Relaxation. Closely related to creep, the stresses resulting from external loading of a structural component decrease in magnitude over a period of time, even though the dimensions of the component remain constant. A typical example is the decrease in tensile stress of a wire in tension between two fixed points (as in the wires in a piano or a violin). Other examples include stress relaxation in rivets, bolts, guy wires, and various similar parts, either under tension, compression, or flexure. Stress relaxation is particularly common in thermoplastics (Section 7.3) and at a microscale in hot isostatic pressing (Section 17.3.2). 2.9 Impact In numerous machinery components and manufacturing operations, materials are subjected to impact or dynamic loading, such as in the heading operation for nails and bolt heads (Section 14.4). A typical impact Strain ׉ 7cassandra://LVRE72zavsDtyCC45usQ5JRd3twsdGqqaqtUZGP-2uEQ`cZ%Eb׉E 106 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Scale Specimen (10 3 10 3 55 mm) Pendulum Notch Anvil lzod (a) Figure 2.19: Impact test specimens. (a) Izod; (b) Charpy. test for determining high-strain-rate properties of materials consists of placing a notched specimen in an impact tester and breaking it with a swinging pendulum (Fig. 2.19). In the Charpy test, the specimen is supported at both ends, while in the Izod test it is supported at one end, as in a cantilever beam. From the swing of the pendulum, the energy dissipated in breaking the specimen can be obtained and is known as the impact toughness of the material. Unlike hardness-test conversions (Fig. 2.15), no quantitative relationships have yet been established between the Charpy and the Izod tests. Impact tests are particularly useful in determining the ductile–brittle transition temperature of materials (Section 2.10.1). Materials that have high impact resistance generally also have high strength and high ductility, hence high toughness. Sensitivity of materials to surface defects (notch sensitivity) is an important factor, as it can significantly lower impact toughness, particularly in heat-treated metals and in ceramics and glasses. 2.10 Failure and Fracture of Materials Failure is one of the most important aspects of material behavior, because it directly influences material selection and the method(s) of manufacture, and determines the service life of a component. Because of the many factors involved, failure and fracture of materials is a complex area of study. This section focuses only on those aspects of failure that are of particular significance to selecting and processing materials. There are two general types of failure: 1. Fracture, through either internal or external cracking. Fracture is further subclassified into two general categories: ductile and brittle (Figs. 2.21 and 2.22). 2. Buckling, as shown in Fig. 2.20b. Although failure of materials is generally regarded as undesirable, some products are designed so that failure is essential for their proper function. Typical examples are (a) beverage or food containers, with pop tops which are opened by shearing the sheet metal along a scored profile; (b) shear pins on shafts, to prevent damage of machinery in the case of overloads; (c) perforated sheet, to ease tearing along a specific path, as in packaging; and (d) metal or plastic screw caps for beverage bottles, to ease their removal. Charpy (b) End of swing Hammer Pendulum Specimen Starting position Specimen (10 3 10 3 75 mm) ׉ 7cassandra://8L8p4q6z8aoHoIbC0DJqs8wAjtehNUAaOc0qUVQVDwQn`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://XCzInQiOiFKfIvZD4_qBujn6BcfKaUJic6aY0riligU#`׉ 7cassandra://97v9DDvM7C5S3SHH0vceJxtTS_l0H8RI1dJ0JRG4ThM5`s׉ 7cassandra://vKS1Gzt2dDxfj5_kwCwanj-0QtsvLspA0sXrK7QcN1M1`׉ 7cassandra://0GLIwsYfAITaHmY38Tm5HTIuxLRng4foWMlf0lEFds8͵d ͠^cZEbǐט#u׉׉ 7cassandra://G4zuq56OZHo5JWRcQjOi05jz_hGi5E4PFTljAZwVc48[`׉ 7cassandra://EZRsr3ILIKcxcIrIzVi9raLGWWjx2KEJ_1nqTYvgJj0fv`s׉ 7cassandra://4DvMGhPaH3zz2ytAiTdxRP815-ZUVv9HH5RZpAqeZEI`׉ 7cassandra://1kd9N5GhzP9aU-OwQ-t5w1w9IX3cEJe6M0DxE-FM3p4͗x z͠^cZEbǑ׉EFailure and Fracture of Materials 107 Barreling Cracks (a) (b) (c) (d) Figure 2.20: Schematic illustration of types of failures in materials: (a) necking and fracture of ductile materials; (b) buckling of ductile materials under a compressive load; (c) fracture of brittle materials in compression; (d) cracking on the barreled surface of ductile materials in compression. (a) (b) (c) (d) Figure 2.21: Schematic illustration of the types of fracture in tension: (a) brittle fracture in polycrystalline metals; (b) shear fracture in ductile single crystals—see also Fig. 1.6a; (c) ductile cup-and-cone fracture in polycrystalline metals; (d) complete ductile fracture in polycrystalline metals, with 100% reduction of area. Figure 2.22: Surface of ductile fracture in low-carbon steel, showing dimples. Fracture is usually initiated at impurities, inclusions, or preexisting voids (microporosity) in the metal. Source: After K.-H. Habig and D. Klaffke. ׉ 7cassandra://vKS1Gzt2dDxfj5_kwCwanj-0QtsvLspA0sXrK7QcN1M1`cZ%Eb׉E108 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.10.1 Ductile Fracture Ductile fracture is characterized by plastic deformation, which precedes failure (Fig. 2.20a). In a tension test, highly ductile materials such as gold and lead may neck down to a point before failing (Fig. 2.21d). Most metals and alloys, however, neck down to a finite cross-sectional area and then fail. Ductile fracture typically takes place along planes on which the shear stress is a maximum. In torsion, for example, a ductile metal fractures along a plane perpendicular to the axis of twist; that is, the plane on which the shear stress is a maximum. Fracture in shear, by contrast, is due to extensive slip along slip planes within the grains (see Fig. 1.7). Close examination of the surface of ductile fracture (Fig. 2.22) shows a fibrous pattern with dimples,as if a number of very small tension tests have been carried out over the fracture surface. Failure is initiated with the formation of tiny voids, usually around small inclusions or preexisting voids, which then grow and coalesce, developing into microcracks which then continue to grow in size, eventually leading to fracture. In a tension-test specimen, fracture begins at the center of the necked region, resulting in the growth and coalescence of cavities (Fig. 2.23). The central region thus becomes one large crack, as can be seen in the midsection of the tension-test specimen in Fig. 2.23d. The crack then propagates to the periphery of the necked region and results in total failure. Because of its appearance, the fracture surface of a tension-test ductile specimen is called a cup-and-cone fracture. Effects of Inclusions. Because they are nucleation sites for voids, inclusions influence ductile fracture and, consequently, the workability of metals. Inclusions may consist of impurities of various kinds and of second-phase particles, such as oxides, carbides, and sulfides. The extent of their influence depends on such factors as their shape, hardness, distribution, and their fraction of the total volume. The greater the volume fraction of inclusions, the lower will be the ductility of the material. Voids and porosity can also develop during processing of metals, such as porosity in castings (Section 10.6.1) and metalworking processes, such as drawing and extrusion (Chapter 15). Two factors affect void formation: 1. The strength of the bond at the interface between an inclusion and the matrix. If the bond is strong, there is lower tendency for void formation during plastic deformation. 2. The hardness of the inclusion. If the inclusion is soft, such as manganese sulfide, it will conform to the overall shape change of the workpiece during plastic deformation. If the inclusion is hard (as, for example, in carbides and oxides; see also Section 8.2), it could lead to void formation (Fig. 2.24). Hard and brittle inclusions may also break up into smaller particles during plastic deformation. The alignment of inclusions during plastic deformation leads to mechanical fibering (Section 1.6). Subsequent processing of such a material must therefore involve considerations of the proper direction of working the material in order to develop maximum ductility and strength. Shear (a) (b) (c) (d) (e) Figure 2.23: Sequence of events in the necking and fracture of a tensile-test specimen: (a) early stage of necking; (b) small voids begin to form within the necked region; (c) voids coalesce, producing an internal crack; (d) the rest of the cross section begins to fail at the periphery, by shearing; (e) the final fracture, known as a cup- (top fracture surface) and-cone- (bottom surface) fracture, surfaces. Fibrous ׉ 7cassandra://4DvMGhPaH3zz2ytAiTdxRP815-ZUVv9HH5RZpAqeZEI`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://YJSzhjwUqeoC5Zli00Mt1ZwwhHfdieYBgOeEWobo5VwU`׉ 7cassandra://uJZwYlW8COrIj5ePp7FGk5RSHZO_JTIzt2dR3WjyHYQQ`s׉ 7cassandra://AxB1Ue6wwEVCjXW_XGbyC8RfIBajGAdMo7rPan-b2CQo`׉ 7cassandra://beDwACgc-WTx1hUfDct58iQEk0t1vPOT-LV8dFwnbgAy>͠^cZEbǓט#u׉׉ 7cassandra://0T16KQYrGg-OyrNZ6ekoxrq0TyQAX7_p5hmRUlZtBbk~`׉ 7cassandra://UqtBohMTCRTotYtmVWibOBhFfkVqERDj2_kJgcGzMe0_A`s׉ 7cassandra://ZYAMVIwfJ26s1Bs0DqSYtiOc26UGcipfGCTX-YCQ_Vk `׉ 7cassandra://r55iwKIsNPEUI3l-aw320gMYniUB4NYivcdklYA6Rqg͞#͠^cZEbǔנcZEbǖ=F9ׁHhttp://aging.Anׁ ׁ Ј׉E Failure and Fracture of Materials 109 Void Matrix Inclusion Soft inclusion Hard inclusion (a) Before deformation or Hard inclusion of deformed metal (b) After deformation Figure 2.24: Schematic illustration of the deformation of soft and hard inclusions and of their effect on void formation in plastic deformation. Note that, because they do not conform to the overall deformation of the ductile matrix, hard inclusions can cause internal voids. Transition Temperature. Metals may undergo a sharp change in ductility and toughness across a narrow temperature range, called the transition temperature (Fig. 2.25). This phenomenon occurs mostly in bodycentered cubic and in some hexagonal close-packed metals; it is rarely exhibited by face-centered cubic metals. The transition temperature depends on such factors as (a) the composition, microstructure, and grain size of the material, (b) the surface finish and the shape of the specimen, and (c) the deformation rate. High rates, abrupt changes in workpiece shape, and the presence of surface notches raise the transition temperature of a material. Strain Aging. Strain aging is a phenomenon in which carbon atoms in steels segregate to dislocations, thereby pinning the dislocations and, in this way, increasing the resistance to their movement. As a result, strength is increased and ductility is reduced. Instead of taking place over several days at room temperature, strain aging can occur in just a few hours at a higher temperature, called accelerated strain aging.An example of accelerated strain aging in steels is blue brittleness, so named because it occurs in the blue-heat range, where the steel develops a bluish oxide film. Blue brittleness causes a significant decrease in ductility and toughness, and an increase in the strength of plain-carbon and of some alloy steels. 2.10.2 Brittle Fracture Brittle fracture occurs with little or no significant plastic deformation. In tension, brittle fracture takes place along the crystallographic plane (cleavage plane) on which the normal tensile stress is a maximum. Brittle fracture does not usually occur with face-centered cubic metals, but it is not uncommon with body-centered cubic and some hexagonal close-packed metals. In general, low temperature and a high rate of deformation promote brittle fracture. In a polycrystalline metal under tension, the fracture surface has a bright granular appearance (unlike the fibrous appearance in ductile fracture), because of the changes in the direction of the cleavage planes as Weak direction Voids Strong direction Transition temperature Temperature Figure 2.25: Schematic illustration of transition temperature in metals. Ductility, toughness ׉ 7cassandra://AxB1Ue6wwEVCjXW_XGbyC8RfIBajGAdMo7rPan-b2CQo`cZ%Eb׉E j110 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Figure 2.26: Fracture surface of steel that has failed in a brittle manner. The fracture path is transgranular (through the grains). Magnification: 200×. Source: After B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc. the crack propagates from one grain to another (Fig. 2.26). Brittle fracture, in compression, is more complex; fracture may even follow a path that is theoretically at an angle of 45◦ to the direction of the applied force. Examples of fracture along a cleavage plane are the splitting of rock salt and the peeling of layers of mica. With mica, graphite, and other lamellar solids, tensile stresses normal to the cleavage plane, caused by pulling, initiate and control the propagation of fracture. Materials such as chalk, gray cast iron, and concrete fail in tension in the manner shown in Fig. 2.21a. In torsion, they fail along a plane at an angle of 45◦ to the axis of twist (Fig. 2.10)—that is, along a plane on which the tensile stress is a maximum. Defects. An important factor in fracture is the presence of defects, such as scratches, flaws, and preexisting external or internal cracks. Under tension, the sharp tip of the crack is subjected to high tensile stresses, which then lead the crack to propagate rapidly. The presence of defects explains why brittle materials exhibit weakness in tension as compared to their strength in compression (see Table 8.2). For example, the ratio of compressive to tensile strength is on the order of 10 for rocks and similar materials, 5 for glass, and 3 for gray cast iron. Under tensile stresses, cracks propagate rapidly, causing what is known as catastrophic failure. In polycrystalline metals, the fracture paths most commonly observed are transgranular (transcrystalline or intragranular); that is, the crack propagates through the grain. In intergranular fracture, the crack propagates along the grain boundaries (Fig. 2.27). It generally occurs when the grain boundaries are soft, contain a brittle phase, or they have been weakened by liquid- or solid-metal embrittlement (Section 1.5.2). Fatigue Fracture. Fatigue fracture generally occurs in a brittle manner and is associated with cyclic loads. Minute external or internal cracks develop at preexisting flaws or at various defects in the material. These cracks then propagate with each load cycle and eventually lead to total and sudden failure of the part. The surface in fatigue fracture is generally characterized by the term beach marks, because of its appearance (Fig. 2.28). Under high magnification (typically at more than 1000×), a series of striations can be seen on fracture surfaces, each beach mark consisting of several striations. Improving Fatigue Strength. Fatigue life is greatly influenced by the method of surface preparation (Fig. 2.29). The fatigue strength of manufactured products can be improved by the following methods: 1. Inducing compressive residual stresses on surfaces—for example, by shot peening or roller burnishing (Section 34.2). 2. Case hardening (surface hardening) by various means (Section 4.10). ׉ 7cassandra://ZYAMVIwfJ26s1Bs0DqSYtiOc26UGcipfGCTX-YCQ_Vk `cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://8hBESFTUzxYyiDsd5vv2L5qNV_V3UoL0kQLLFe_wXGcG`׉ 7cassandra://99YY6DtxDgU5UhBy25SYVb_kyhn113Ire8n4Sc2-Tgk>`s׉ 7cassandra://4DRxpdn7BFiqkoCu6aLppbVKiinLqJ_K34krxJUvih8`׉ 7cassandra://Wb9Gmk3cEZ6MppWTv-l8-C4HHv3atg8dg4v4WQ8EAAcË͠^cZEbǗט#u׉׉ 7cassandra://RU0JU478fzc2xPrMHTyDMadpcKhCtgtW784aH7XdDlw`׉ 7cassandra://3J1zrCEvLQSI6EylavN07FbbvGwZw0UKaFi2DyUzRDIL*`s׉ 7cassandra://ZvaPGdLf4KITMhxL7GYmMI1VQF-3HDOumBK7VXneCwY`׉ 7cassandra://k8KPUToQ3irzH3ywVXWu_F4YUYN7E8F0PlbQkhMkdCAs̶͠^cZEbǘ׉E1Failure and Fracture of Materials 111 Figure 2.27: Intergranular fracture, at two different magnifications. Grains and grain boundaries are clearly visible in this micrograph. The fracture path is along the grain boundaries. Magnification: left, 100×;right, 500×. Source: After B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc. 3. Producing a fine surface finish, thereby reducing the detrimental effects of scratches, notches, and other surface imperfections. 4. Selecting appropriate materials and ensuring that they are free from significant amounts of inclusions, voids, and impurities. Conversely, the following factors and processes can reduce fatigue strength: 1. Tensile residual stresses on the surface (see Section 2.11). 2. Decarburization. 3. Surface pits (such as due to corrosion), that act as stress raiser. 4. Hydrogen embrittlement. 5. Galvanizing. 6. Electroplating. Stress–Corrosion Cracking. An otherwise ductile metal can fail in a brittle manner by stress–corrosion cracking (also called stress cracking or season cracking). Parts that are free from defects may develop cracks, Figure 2.28: Typical fatigue-fracture surface on metals, showing beach marks. Magnification: left, 500×; right, 1000×. Source: After B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc. ׉ 7cassandra://4DRxpdn7BFiqkoCu6aLppbVKiinLqJ_K34krxJUvih8`cZ%Eb׉E 112 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 50 40 30 20 10 0 60 70 500 800 1000 MPa Figure 2.29: Reductions in the fatigue strength of cast steels subjected to various surface-finishing operations. Note that the reduction becomes greater as the surface roughness and the strength of the steel increase. Source: M.R. Mitchell. either over time or even soon after they are made. Crack propagation may be either intergranular or transgranular. The susceptibility of metals to stress–corrosion cracking depends mainly on the material, the presence and magnitude of tensile residual stresses, and the environment (such corrosive media as salt water or chemicals). Brass and austenitic stainless steels are among metals that are highly susceptible to stress cracking. The usual procedure to avoid stress–corrosion cracking is to stress relieve the part just after it is formed. Full annealing (Section 4.11) may also be done, but this treatment reduces the strength of cold-worked parts. Hydrogen Embrittlement. The presence of hydrogen can reduce ductility and can cause severe embrittlement and premature failure in metals and their alloys, as well as in nonmetallic materials. Called hydrogen embrittlement, this phenomenon is especially severe in high-strength steels. Possible sources of hydrogen arise during melting of the metal in preparation for casting, pickling (Section 13.3), and electrolysis in electroplating (Section 34.9). Other sources of hydrogen are water vapor in the atmosphere and moisture on electrodes and in fluxes used during welding. Oxygen also can cause embrittlement, particularly in copper alloys. 1300 Gr Polishing inding Fine polishing 2.11 Residual Stresses Residual stresses are stresses that remain within a part after it has been shaped and all the external forces (applied through tools and dies) are removed. Residual stresses also may develop when workpieces are subjected to plastic deformation that is not uniform throughout the part. A typical example is the bending of a metal bar (Fig. 2.30). Note that the external bending moment first produces a stress distribution that varies linearly through the thickness (Fig. 2.30a). As the moment is increased, the outer fibers in the bar Reduction in fatigue strength (%) Fine turning Rough turning As cast ׉ 7cassandra://ZvaPGdLf4KITMhxL7GYmMI1VQF-3HDOumBK7VXneCwY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://dFVhT1GoHaWimIwi6_u0BpaGJY_pjhVJJ-KyMBYdr2AP<`׉ 7cassandra://7rJKYlLaox3Wte8zwi8NPpAIHn-icso5hvCd6EqwufYW`s׉ 7cassandra://O9855JnMGUKTs9FUL9v9exNHqBBJgGiOD-wuPzPPcfo`׉ 7cassandra://hNQSy8F6oeTEQ8zaTJYCOA8YZqDtdMVU2zsEAjyWArUͿ=͠^cZEbǛט#u׉׉ 7cassandra://uKvVOS14N311AALdxRGJfX6P5a3elAorSKyRa8ZuCQo`׉ 7cassandra://WyKfy7LX3_YjqsK2qjOJrQo7oiIWvZvjmoevBVIYEUIi`s׉ 7cassandra://nyTyPDFLEkP0_NDK91nRMixNQ8I1uXKMkO6U2Q6OWUE`׉ 7cassandra://Mwu9zuB6nYpjJxbZTKuxqpfIqvH6GjyiPBtNrmYPwFIq͠^cZEbǞ׉E Residual Stresses 113 Tensile Compressive (a) (b) ab c o a d o e f (c) (d) Figure 2.30: Residual stresses developed in bending a beam having a rectangular cross section. Note that the horizontal forces and moments caused by residual stresses in the beam must be balanced internally. Because of nonuniform deformation, especially during cold-metalworking operations, most parts develop residual stresses. reach a stress level sufficiently high to cause yielding. For a typical strain-hardening material, the stress distribution shown in Fig. 2.30b is eventually reached, and the bar has now undergone permanent bending. Consider the effects of removing the external bending moment on the bar. This operation is equivalent to applying an equal but opposite moment to the bar; thus, the moments of the areas oab and oac in Fig. 2.30c must be equal. Line oc, which represents the opposite bending moment, is linear, because all unloading and recovery is elastic (see Fig. 2.3). The difference between the two stress distributions gives the residual stress pattern within the bar, as is shown in Fig. 2.30d. Note the presence of compressive residual stresses in layers ad and oe, and tensile residual stresses in layers do and ef. Because there are now no external forces applied on the bar, the internal forces resulting from these residual stresses must be in static equilibrium. It should be noted that although this example involves residual stresses only in the longitudinal direction of the bar, in most cases residual stresses are three dimensional, and more difficult to analyze. The removal of a layer of material from the surfaces of the bar, such as by machining or grinding, will disturb the equilibrium of the residual stresses shown in Fig. 2.30d. The bar will then acquire a new radius of curvature in order to balance the internal forces. Such disturbances of residual stresses cause warping of parts (Fig. 2.31). (Residual stresses may also be disturbed by relaxation of these stresses over a period of time; see below.) Residual stresses also can be developed by temperature gradients within the part, such as occur during cooling of castings or a hot forgings. The local expansions and contractions caused by temperature gradients within the part will produce a nonuniform deformation, such as described above in the permanent bending of a beam. Before After (a) (b) (c) Figure 2.31: Distortion of parts with residual stresses after cutting or slitting: (a) flat sheet or plate; (b) solid round rod; (c) thin-walled tubing or pipe. ׉ 7cassandra://O9855JnMGUKTs9FUL9v9exNHqBBJgGiOD-wuPzPPcfo`cZ%Eb׉Ec114 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Tensile residual stresses on the surface of a part are generally undesirable, because they lower the fatigue life and fracture strength of the parts made. This is because a surface with tensile residual stresses cannot sustain as large of tensile stresses from external forces before fracture as if it were stress free. The reduction in strength is particularly characteristic of brittle or less ductile materials, in which fracture takes place with little or no plastic deformation preceding fracture. Tensile residual stresses can also lead, over a period of time, to stress cracking or stress–corrosion cracking of parts made (Section 2.10.2). Compressive residual stresses on a surface, on the other hand, are generally desirable. In fact, in order to increase the fatigue life of components, compressive residual stresses can be purposefully imparted to surfaces by techniques such as shot peening or surface rolling (Section 34.2). Reduction and Elimination of Residual Stresses. Residual stresses can be reduced or eliminated either by stress-relief annealing (Section 4.11) or by further plastic deformation of the part, such as by stretching. Given sufficient time, residual stresses may also diminish at room temperature by relaxation. The time required for relaxation can be greatly reduced by raising the workpiece temperature. 2.12 Work, Heat, and Temperature Almost all the mechanical work in plastic deformation is converted into heat. However, the conversion is not complete, because a portion of the work is stored within the deformed material as elastic energy, known as stored energy (Section 1.7). This energy is generally 5–10% of the total energy input. In some metal alloys, however, it may be as high as 30%. In a simple frictionless deformation process, and assuming that work is completely converted into heat, the theoretical (adiabatic) temperature rise, ΔT, in the workpiece is given by ΔT = u , ρc where u is the specific energy (work of deformation per unit volume), ρ is the density, and c is the specific heat of the material. It can be noted that higher temperatures are associated with large areas under the stress–strain curve and with smaller values of specific heat. However, such physical properties as specific heat and thermal conductivity (Chapter 3) may also depend on temperature; thus, they must be taken into account in the calculations. The temperature rise for a true strain of 1 (such as occurs in a 27 mm-high specimen when it is compressed down to 10 mm) can be calculated as: for aluminum aluminum, 75◦C; copper, 140◦C; low-carbon steel, 280◦C; and titanium 570◦C. In actual metalworking operations, however, heat is lost to the environment, to the tools and dies, and to the lubricants or coolants used, if any, in the process. If deformation is performed at high speed, the heat losses will be relatively low over that brief period. If, on the other hand, the process is carried out slowly, the actual temperature rise will be only a fraction of the calculated value. Summary • Manufacturing processes include shaping materials by plastic deformation; consequently, such mechanical properties as strength (yield strength, Sy, and ultimate tensile strength, Sut); modulus of elasticity, E; ductility (total elongation and reduction of area); hardness; and the energy required for plastic deformation are important factors. These properties depend on the particular material and on its condition, temperature, deformation rate, surface condition, and the environment. • The tensile test is the most commonly used to determine mechanical properties. From these tests, true stress–true strain curves are constructed to then determine the strength coefficient (K), the strainhardening exponent (n), the strain-rate sensitivity exponent (m), and the toughness of materials. (2.15) ׉ 7cassandra://nyTyPDFLEkP0_NDK91nRMixNQ8I1uXKMkO6U2Q6OWUE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://h1JZGOacVp7pVMWoPxON84spuCDYqblLEAwgrzeUH5cf`׉ 7cassandra://Ys-e0BEjTZtntbw-IiirPRfEoh83AQvSn6rie3T1z18L`s׉ 7cassandra://7APrjoVCW3-XvVQUBXGUo3j-usgNSw71kZrBwWAeg6UF`׉ 7cassandra://nD8hDl7jE7JeA8cAwq9PN9c0cDbkpnKPnjhHFuP138cB͠^cZEbǠט#u׉׉ 7cassandra://SVNEAY-2WKT20GdhRWPwOSBHrfu9ejWp5WlQ6uQhewoW`׉ 7cassandra://JrkHrupTqmOdnDGXHR1OwXojLVYMIDCJdeHWR1oG3eE]`s׉ 7cassandra://w7g4hD9TUZ4wleoZdXoZQ-Uv6vusPJOrPwJ73ct67YwA`׉ 7cassandra://nwVK1X6kjwR1bw5Sieau4B25V5gjGVty0Dzw2VsMhP8k͠^cZEbǡ׉E Key Terms 115 • Compression tests are subject to inaccuracy due to the presence of friction and barreling of the specimen. Torsion tests typically are conducted on tubular specimens and subjected to twisting. Bending or flexure tests are commonly used for brittle materials to determine their modulus of rupture or the transverse rupture strength. • Several hardness tests are available to determine the resistance of a material to permanent indentation or to scratching. Hardness is related to strength and wear resistance of a material, but hardness is not a fundamental property. • Fatigue tests indicate the endurance limit or fatigue limit of materials, i.e., the maximum stress to which a material can be subjected without fatigue failure, regardless of the number of cycles. Some materials have no endurance limit; instead, their allowable stress is reported with respect to the number of loading cycles. • Creep is the permanent elongation of a component under a static load maintained for a period of time; failure is by rupture (necking and fracturing). • Impact tests determine the energy required to completely break a specimen, called the impact toughness of the material. These tests are also useful for determining the transition temperature of materials. • Failure and fracture constitute an important aspect of a material’s behavior when subjected to deformation during manufacturing operations. Ductile fracture is characterized by plastic deformation preceding fracture; it requires a considerable amount of energy. Brittle fracture can be catastrophic, because it is not preceded by plastic deformation; however, it requires much less energy than does ductile fracture. Impurities, inclusions, and voids play a major role in the fracture of metals and alloys. • Residual stresses are those that remain in a workpiece after it has been plastically deformed and then has all external forces removed. Surface tensile residual stresses are generally undesirable; they may be reduced or eliminated by stress-relief annealing, further plastic deformation, or by relaxation over a period of time. Key Terms Bauschinger effect Blue brittleness Brittle fracture Buckling Charpy test Compression Creep Defects Deformation rate Disk test Ductile fracture Ductility Durometer Elongation Engineering strain Engineering stress Fatigue Fatigue failure Flexural test Fracture Hardness Impact loading Inclusions Izod test Leeb test Microhardness Modulus of elasticity Modulus of rigidity ׉ 7cassandra://7APrjoVCW3-XvVQUBXGUo3j-usgNSw71kZrBwWAeg6UF`cZ%Eb׉E116 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Modulus of rupture Poisson’s ratio Reduction of area Residual stresses Rupture Shear Shear modulus Shore test Strain aging Strain-hardening exponent Strain rate Strain-rate sensitivity exponent Strain softening Bibliography Ashby, M.F., Materials Selection in Mechanical Design, 4th ed., Pergamon, 2010. ASM Handbook,Vol.8: Mechanical Testing and Evaluation, ASM International, 2000. Atlas of Stress–Strain Curves, 2nd ed., ASM International, 2002. Beer, F.P., Johnston, E.R., DeWolf, J.T., and Mazurek, D., Mechanics of Materials, 7th ed., McGraw-Hill, 2014. Courtney, T.H., Mechanical Behavior of Materials, 2nd ed., Waveland Press, 2005. Dowling, N.E., Mechanical Behavior of Materials: Engineering Methods for Deformation, Fracture, and Fatigue, 4th ed., Prentice Hall, 2012. Herrmann, K. (ed.), Hardness, Testing: Principles and Applications, ASM International, 2011. Hosford, W.F., Mechanical Behavior of Materials, 2nd ed., Cambridge, 2009. Wulpi, D.J., Understanding How Components Fail, 2nd ed., ASM International, 1999. Review Questions 2.1. What is tension? What is shear? 2.2. Distinguish between engineering stress and true stress. 2.3. In a stress–strain curve, what is the proportional limit? Is it different than the yield point? 2.4. Describe the events that occur when a specimen undergoes a tension test. Sketch a plausible stress– strain curve, and identify all significant regions and points between them. Assume that loading continues up to fracture. 2.5. What is ductility, and how is it measured? 2.6. In the equation σ = Kn, which represents the stress–strain curve for a material, and what is the significance of the exponent n? 2.7. What is strain-rate sensitivity, and how is it measured? 2.8. What test can measure the properties of a material undergoing shear strain? 2.9. What kind of loading is applied by scissors while cutting paper? 2.10. What testing procedures can be used to measure the properties of brittle materials, such as ceramics and carbides? Strength coefficient Stress–corrosion cracking Stress relaxation Superplasticity Tension Torsion test Toughness Transition temperature True strain True stress Ultimate tensile strength Yield stress ׉ 7cassandra://w7g4hD9TUZ4wleoZdXoZQ-Uv6vusPJOrPwJ73ct67YwA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://g7Hf3lVxM9KggNd8Wg_l-ioQ7H6-cJarJrpyE0AOJOs`׉ 7cassandra://66j5Nn3WCGCQuGIRVDnZZ3VJyQA_v_HSHkQvxbxTUvA\`s׉ 7cassandra://-DVvcNWejlXFI5vscMIc1vpQhzInOZb3dNls97UeKCs`׉ 7cassandra://0Qufcyu0zCRwOOVIx9CptYS2mSumvhYBnIuBA_SOMg8H͠^cZEbǣט#u׉׉ 7cassandra://RyBlY5x_PP0c2dU5cWJNuJUMZFhWcMgwrm3d2o3XV5As`׉ 7cassandra://O9qiAEAyfBcUTxfrICZsEmre0w9cM4D_w14lmeOdS7Mfv`s׉ 7cassandra://nLiJ_lB6uDtKTOp-v4LaESoYLGVMjEAHSDsHKopjMwo`׉ 7cassandra://TWQbHYiCd5na-Uf1DnO_HaoPOI1uj8X5wPEAdttbMzYF͠^cZEbǤ׉E Qualitative Problems 117 2.11. Describe the differences between brittle and ductile fracture. 2.12. What is hardness? Is it different from hardness number? Explain. 2.13. Describe the features of a Rockwell hardness test. 2.14. What is a Leeb test? How is it different from a Rockwell A test? 2.15. Differentiate between stress relaxation and creep. 2.16. Describe the difference between elastic and plastic behavior. 2.17. Explain what uniform elongation means in tension testing. 2.18. Describe the difference between deformation rate and strain rate. What unit does each one have? 2.19. Describe the difficulties involved in conducting a compression test. 2.20. What is Hooke’s law, Young’s modulus, and Poisson’s ratio? 2.21. Describe the difference between transgranular and intergranular fracture. 2.22. What is the reason that yield strength is generally defined as a 0.2% offset strength? 2.23. Why does the fatigue strength of a specimen or part depend on its surface finish? 2.24. Explain how you would determine whether or not a material has an endurance limit. 2.25. If striations are observed under microscopic examination of a fracture surface, what do they suggest regarding the mode of fracture? 2.26. What is an Izod test? Why are Izod tests useful? 2.27. Why does temperature increase during deformation? 2.28. What is a residual stress? How can residual stresses be removed? Qualitative Problems 2.29. On the same scale for stress, the tensile true stress–true strain curve is higher than the engineering stress–engineering strain curve. Explain whether this condition also holds for a compression test. 2.30. Explain why it is difficult to break a sheet of paper in tension, but easy to cut it with scissors. 2.31. What are the similarities and differences between deformation and strain? 2.32. Can a material have a negative Poisson’s ratio? Give a rationale for your answer. 2.33. Referring to Table 2.2, explain why there can be so much variation in the strength and elongation in a class of metal alloys. 2.34. Referring to Table 2.2, explain why the stiffness of diamond has so much variation. 2.35. It has been stated that the higher the value of m, the more diffuse the neck is, and likewise, the lower the value of m, the more localized the neck is. Explain the reason for this behavior. 2.36. Explain why materials with highmvalues, such as hot glass and taffy, when stretched slowly, undergo large elongations before failure. Consider events taking place in the necked region of the specimen. 2.37. Explain if it is possible for stress–strain curves in tension tests to reach 0% elongation as the gage length is increased further. 2.38. With a simple sketch, explain whether it is necessary to use the offset method to determine the yield stress, Sy, of a material that has been highly cold worked. 2.39. Explain why the difference between engineering strain and true strain becomes larger as strain increases. Does this difference occur for both tensile and compressive strains? Explain. ׉ 7cassandra://-DVvcNWejlXFI5vscMIc1vpQhzInOZb3dNls97UeKCs`cZ%Eb׉E 8118 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.40. Consider an elastomer, such as a rubber band. This material can undergo a large elastic deformation before failure, but after fracture it recovers completely to its original shape. Is this material brittle or ductile? Explain. 2.41. If a material (such as aluminum) does not have an endurance limit, how then would you estimate its fatigue life? 2.42. What role, if any, does friction play in a hardness test? Explain. 2.43. Which hardness tests and scales would you use for very thin strips of metal, such as aluminum foil? Explain. 2.44. Consider the circumstance where a Vickers hardness test is conducted on a material. Sketch the resulting indentation shape if there is a residual stress on the surface. 2.45. Which of the two tests, tension or compression, would require a higher capacity of testing machine, and why? 2.46. In a Brinell hardness test, the resulting impression is found to be an ellipse. Give possible explanations for this result. 2.47. List and explain briefly the conditions that induce brittle fracture in an otherwise ductile metal. 2.48. List the factors that you would consider in selecting a hardness test. Explain why. 2.49. List two situations where a material’s toughness is important from a design standpoint. 2.50. On the basis of Fig. 2.5, if a metal tension-test specimen is pulled and broken rapidly, where would the temperature be highest, and why? 2.51. Comment on the temperature distribution if the specimen in Question 2.50 is pulled very slowly. 2.52. Comment on your observations regarding the contents of Table 2.2. 2.53. Is the disk test applicable to a ductile material? Why or why not? 2.54. Refer to Table 2.4, and note the true strain encountered by a material in different manufacturing processes. Explain why some typical strains are large and others are small. 2.55. Refer to Table 2.4, and sketch the original and deformed shape of a 25 mm specimen subjected to the largest typical strain for each process. What are your observations regarding strains that can be achieved? 2.56. If a tension test on carbon steel is conducted at room temperature, and then with a bath of boiling water, would you expect the strength to be different? Explain. 2.57. What hardness test is suitable for determining the hardness of a thin ceramic coating on a piece of metal? 2.58. Wire rope consists of many wires that bend and unbend as the rope is run over a sheave. A wire-rope failure is investigated, and it is found that some of the wires, when examined under a scanning electron microscope, display cup-and-cone failure surfaces, while others display transgranular fracture surfaces. Explain these observations. 2.59. A statistical sampling of Rockwell C hardness tests are conducted on a material, and it is determined that the material is defective because of insufficient hardness. The supplier claims that the tests are flawed because the diamond-cone indenter was probably dull. Is this a valid claim? Explain. 2.60. In a Brinell hardness test, the resulting impression is found to be elliptical. Give possible explanations for this result. 2.61. In the machining of an extruded aluminum block to produce a smart phone case, it is seen that there is significant warpage after machining. Explain why. What would you do to reduce this warpage? ׉ 7cassandra://nLiJ_lB6uDtKTOp-v4LaESoYLGVMjEAHSDsHKopjMwo`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Z4ZED-QdYDHCpWsZJ-CmROMhHMFg1t1FZuHFjVdCxLYڬ`׉ 7cassandra://0b9qJ_klX_QO7U-UotAHiZtq1dozva915tbIdMUWbU8W`s׉ 7cassandra://ih16UwJNKz8f9LZxLGSCOax0mhFWaJy_OoUDwZShctk`׉ 7cassandra://XnAJ3LWSm3b91xxYJ3jdZ0rGxsV52_rvRCWrrWpDjW8W$͠^cZEbǦט#u׉׉ 7cassandra://6hJenvw2SUDlxdxV92Gjov5c_p6rAe3BNRGFmNE2D0o`׉ 7cassandra://obldrEFmg6Efn2Zwc9uT82Fa-kZpPgqd2aQpX9ygZSsf`s׉ 7cassandra://-KlmW5n8ZYQZo0gvqREThIJxWIDTlHXN5sbkUSdteP8`׉ 7cassandra://yXde6v7GuGkBhDT8grXWbraLfxYoBU4-2t6XK-2TQvs_͠^cZEbǧ׉E 3Quantitative Problems 119 2.62. Some coatings are extremely thin—some as thin as a few nanometers. Explain why even the Knoop test is not able to obtain reliable results for such coatings. Recent investigations have attempted to use highly polished diamonds (with a tip radius around 5 nm) to indent such coatings in atomic force microscopes. What concerns would you have regarding the appropriateness of the results? 2.63. Select an appropriate hardness test for each of the following materials, and justify your answer: 1. Cubic boron nitride 2. Lead 3. Cold-drawn 0.5%C steel 4. Diamond 5. Caramel candy 6. Granite. 2.64. Referring to Fig. 2.13, the material for testers is either steel, tungsten carbide, or diamond. Why isn’t diamond used for all of the tests? Quantitative Problems 2.65. A paper clip is made of wire 1 mm in diameter. If the original material from which the wire is made is a rod 50 mm in diameter, calculate the longitudinal engineering and true strains that the wire has undergone during processing. 2.66. A 150-mm-long strip of metal is stretched in two steps, first to 250 mm and then to 500 mm. Show that the total true strain is the sum of the true strains in each step; in other words, the true strains are additive. Show that, in the case of engineering strains, the strains cannot be added to obtain the total strain. 2.67. Identify the two materials in Fig. 2.5 that have the lowest and the highest uniform elongations. Calculate these quantities as percentages of the original gage lengths. 2.68. Plot the ultimate strength versus stiffness for the materials listed in Table 2.2, and prepare a threedimensional plot for these materials where the third axis is their maximum elongation in 50 mm. 2.69. If you remove the layer of material ad from the part shown in Fig. 2.30d—for instance, by machining or grinding—which way will the specimen curve? (Hint: Assume that the part shown in sketch d in the figure is composed of four horizontal springs held at the ends. Thus, from the top down, you have compression, tension, compression, and tension springs.) 2.70. Prove that the true strain at necking equals the strain hardening exponent. 2.71. Percent elongation is always defined in terms of the original gage length, such as 50 mm. Explain how percent elongation would vary as the gage length of the tensile-test specimen increases. (Hint: Recall that necking is a local phenomenon.) 2.72. Make a sketch showing the nature and distribution of residual stresses in Fig. 2.31a and b, prior to the materials being cut. (Hint: Assume that the split parts are free from any stresses; then force these parts back to the shape they originally had.) 2.73. You are given the K and n values of two different metals. Is this information sufficient to determine which metal is tougher? If not, what additional information do you need? ׉ 7cassandra://ih16UwJNKz8f9LZxLGSCOax0mhFWaJy_OoUDwZShctk`cZ%Eb׉E 120 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.74. A cable is made of two strands of different materials, A and B, and cross sections as follows: For material A, K = 500 MPa, n =0.6, Ao =0.00060 m2. For material B, K = 300 MPa, n =0.6, Ao =0.00030 m2. Calculate the maximum tensile force that this cable can withstand prior to necking. 2.75. On the basis of the information given in Fig. 2.5, calculate the ultimate tensile strength (engineering) of 304 stainless steel. 2.76. In a disk test performed on a specimen 30 mm in diameter and 8 mm thick, the specimen fractures at a stress of 180 MPa. What was the load on at fracture? 2.77. A piece of steel has a hardness of 275 HB. Calculate its tensile strength, in MPa. 2.78. A metal has the following properties: Sut = 500 MPa and n =0.25. Calculate its strength coefficient, K. 2.79. Using only Fig. 2.5, calculate the maximum load in tension testing of an annealed copper specimen with an original diameter of 10 mm. 2.80. Estimate the modulus of resilience for a highly cold worked piece of steel having a hardness of 300 HB; for a piece of highly cold worked copper with a hardness of 100 HRB. 2.81. A metal has a strength coefficient K = 600 MPa and n =0.25. Assuming that a tensile-test specimen made from this metal begins to neck at a true strain of 0.25, show that the ultimate tensile strength is 362 MPa. 2.82. Plot the true stress–true strain curves for the materials listed in Table 2.3. 2.83. The design specification for a metal requires a minimum hardness of 80 HRA. If a Rockwell test is performed and the depth of penetration is 80 μm, is the material acceptable? 2.84. Calculate the major and minor pyramid angles for a Knoop indenter, and compare your results with those obtained from Vickers and Rockwell A indenters. 2.85. If a material has a target hardness of 300 HB, what is the expected indentation diameter? 2.86. A Rockwell A test was conducted on a material and a penetration depth of 0.15 mm was recorded. What is the hardness of the material? What material would typically have such a hardness value? If a Brinell hardness test were to be conducted on this material, give an estimate of the indentation diameter if the load used was 1500 kg. 2.87. For a cold-drawn 0.5% carbon steel, will a Rockwell C test or a Brinell test at 500 kg result in a deeper penetration? 2.88. A material is tested in tension. Over a 25-mm gage length, the engineering strain measurements are 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.15, 0.2, 0.5, and 1.0. Plot the true strain versus engineering strain for these readings. 2.89. Calculate the work done in frictionless compression of a solid cylinder 40 mm high and 15 mm in diameter to a reduction in height of 50% for the following materials: (a) 1100-O aluminum; (b) annealed copper; (c) annealed 304 stainless steel; and (d) annealed 70-30 brass. 2.90. A bar 2 m long is bent and then stress relieved. The radius of curvature to the neutral axis is 1 m. The bar is 30 mm thick and is made of an elastic, perfectly plastic material with Sy = 500 MPa and E = 207 GPa. Calculate the length to which this bar should be stretched so that, after unloading, it will become and remain straight. 2.91. Take a cubic piece of metal with a side length lo and deform it plastically to the shape of a rectangular parallelepiped of dimensions l1, l2,and l3. Assuming that the material is rigid and perfectly plastic, show that volume constancy requires that the following expression be satisfied:1 +2 +3 =0. ׉ 7cassandra://-KlmW5n8ZYQZo0gvqREThIJxWIDTlHXN5sbkUSdteP8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://0xQNIzwTxFTEyPFt_szq0ooIh7KH98M9OnOjhFUVP8sNJ`׉ 7cassandra://bmGUwMEvLa8OOurG0S0z3W16GguLxNyvAV6RCkfF_X8oQ`s׉ 7cassandra://jJuRD3s_okIs9svVokx7DWClhkj9ALKkWd1wm2YD6R4`׉ 7cassandra://IbYpNWTuou8SHpjm8ACaWcBeDHqQfm_2p1s0g2N04soXQ͠^cZEbǫט#u׉׉ 7cassandra://GU79_rHtSHLfoiImEu30UX4f0Y3tCPrUPW29LKHKBXIOt`׉ 7cassandra://UZtqMu6_1wmtK4DI_n6T5ydnz-4mHWDUlC9zuFqf9a0J,`s׉ 7cassandra://DGH286UxItQZZ2rY8AS91S8DjfudkPGCfXYj-3j7Stk)`׉ 7cassandra://jYa6scjAvYruWwfCdjKyqRIXftK0z4WnM6U0jx2itTIB͠^cZEbǬ׉ESynthesis, Design, and Projects 121 Synthesis, Design, and Projects 2.92. List and explain the desirable mechanical properties of (a) an automobile body panel, (b) a paper clip, (c) a leaf spring for a truck, (d) a bracket for a bookshelf, (e) a backpack shoulder strap, (f) a wire coat hanger, (g) the clip for a pen, and (h) a lens for an optical microscope. 2.93. When making a hamburger, you may have observed the type of cracks shown in Fig. 2.20d. What would you do to avoid such cracks? [Note: Test hamburger patties by compressing them at different temperatures, and observe the crack path (i.e., the path through the fat particles, the meat particles, or their interface).] 2.94. An inexpensive claylike material called Silly Putty R is generally available in stores that sell toys and games. Obtain a sample and perform the following experiments: (a) Shape it into a ball, and drop it onto a flat surface. (b) Reround the ball and place a heavy book on it for one minute. (c) Shape the putty into a long rod, and pull on it—first slowly, then very quickly. Describe your observations, referring to the specific sections in this chapter where each particular observation is relevant. 2.95. Make individual sketches of the mechanisms of testing machines that, in your opinion, would be appropriate for tension, for torsion, and for compression testing of specimens at different rates of deformation. What modifications would you make on these machines to include the effects of temperature on material properties? 2.96. In tension testing of specimens, mechanical and electronic instruments are typically used to measure elongation. Make sketches of instruments that would be suitable for this purpose, commenting on their accuracy. What modifications would you make to these instruments to include the use of specimens at elevated temperatures? 2.97. Obtain small pieces of different metallic and nonmetallic materials, including stones. Rub them against each other, observe the scratches made, and order them in a manner similar to the Mohs hardness numbering system. 2.98. Demonstrate the stress-relaxation phenomenon by tightly stretching thin plastic strings between two nails placed at the ends of a long piece of wood. Pluck the strings frequently, to test the tension as a function of time. Repeat the test at a higher temperature by placing the fixture in an oven set on low. 2.99. Demonstrate the impact toughness of a piece of round chalk by first using a triangular file to produce a V-notch on the cylindrical surface (as shown in Fig. 2.19a) and then bending the chalk to break it. 2.100. Using a large rubber band and a set of weights, obtain the force–displacement curve for the rubber band. Is the result different from the stress–strain curves shown in Fig. 2.4? Explain. 2.101. Design a test protocol to obtain the work of plastic deformation by measuring the temperature rise in a workpiece, assuming that there is no heat loss and that the temperature distribution is uniform throughout. If the specific heat of the material decreases with increasing temperature, will the work of deformation calculated using the specific heat at room temperature be higher or lower than the actual work done? Explain. 2.102. Find or prepare some solid circular pieces of brittle materials, such as chalk, ceramics, etc. and subject them to the type of test shown in Fig. 2.9 by using the jaws of a simple vise. Describe your observations as to how the materials fracture. Repeat the tests, using ductile materials, such as clay, soft metals, and describe your observations. 2.103. Take several rubber bands and pull them at different temperatures, including from a frozen state. Comment on their behavior such as ductile or brittle. 2.104. Devise a simple fixture for conducting the bend tests shown in Fig. 2.11. Test sticks of various brittle materials by loading them with dead weights until they break. Verify the statement in the text that the specimens on the right in the figure will fracture sooner than the ones on the left. ׉ 7cassandra://jJuRD3s_okIs9svVokx7DWClhkj9ALKkWd1wm2YD6R4`cZ%Eb׉E 122 Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials 2.105. By pushing a small ball bearing against the top surfaces of various materials, such as clay and dough, observe the shape of the indentation with a magnifier, referring to those shapes shown in Fig. 2.14a and b. 2.106. Describe your observations regarding Fig. 2.14c. 2.107. Embed a small steel ball in a soft block of material such as clay, and compress the clay as shown in Fig. 2.24a. Then cut the clay carefully along the center plane and observe the deformation of the material. Repeat the experiment by embedding a small round jelly bean in the clay and deforming the material. Comment on your observations. 2.108. A penny-shaped piece of soft metal is brazed to the ends of two flat, round steel rods of the same diameter as the piece. The assembly is then subjected to uniaxial tension. What is the state of stress to which the soft metal is subjected? Explain. 2.109. Devise a simple experiment, and perform tests on materials commonly found around the house by bending them at different temperatures for a qualitative assessment of their transition temperature, as shown in Fig. 2.25. 2.110. Obtain some solid and some tubular metal pieces, and slit them as shown in Fig. 2.31. Comment on whether there are any residual stresses in the parts prior to slitting them. 2.111. Explain how you would obtain an estimate of the hardness for a carbon nanotube (see Section 8.6.2). 2.112. Without using the words “stress” or “strain,” define elastic modulus. 2.113. We know that it is relatively easy to subject a specimen to hydrostatic compression, such as by using a chamber filled with a liquid. Devise a means whereby the specimen (say, in the shape of a cube or a round disk) can be subjected to hydrostatic tension, or one approaching this state of stress. (Note that a thin-walled, internally pressurized spherical shell is not a correct answer, because it is subjected only to a state of plane stress.) 2.114. Assume that you are running four-point bending tests on a number of identical specimens of the same length and cross section, but with increasing distance between the upper points of loading. (See Fig. 2.19b.) What changes, if any, would you expect in the test results? Explain. 2.115. Describe a test protocol, complete with forces and geometry, that you execute to determine the quality of food. Consider the crispness of an apple or snack chip, and then consider the softness of cake. ׉ 7cassandra://DGH286UxItQZZ2rY8AS91S8DjfudkPGCfXYj-3j7Stk)`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://E2gjwrxMbDEu4_o8mGw1PgjmS6ujCkGD6Th1tx4vb8sق`׉ 7cassandra://VLKojhnue9b8RhnZUVv89VSNU_oy8eAt2l-1q2imXTs:`s׉ 7cassandra://pI18DrNO5rdTukp17cSrvMysdKVwlR7tqJZ3rAwrO6Q*`׉ 7cassandra://W_sjYDjWij9NuvhtZ_rNXoApFHZmim8bQgGBWBVDBjk9͠^cZEbǮט#u׉׉ 7cassandra://wmXlUZigPuYbdlLHB1AdZuq4r9a9En7d0iv2H1HY2c4 `׉ 7cassandra://NXi8b0vTOHn2rFjk5A7DKAgHJx6jPkv8b_JGghkYUmotK`s׉ 7cassandra://CoAf69FvMG_ZfoVKbljo4GJXTD6Inpmy4p5vIrLvA4A:`׉ 7cassandra://b_Fdefjbg98v7jpmYAC7NnpvS-D3EzGEfAFu9w5JuIgU ͠^cZEbǯ׉EjChapter 3 Physical Properties of Materials 3.1 Introduction 123 3.2 Density 124 3.3 Melting Point 124 3.4 Specific Heat 125 3.5 Thermal Conductivity 127 3.6 Thermal Expansion 127 3.7 Electrical, Magnetic, and Optical Properties 129 3.8 Corrosion Resistance 130 Case Study: 3.1 Selection of Materials for Coins 131 • Physical properties have several significant roles in the selection, processing, and use of materials. They can also be key factors in determining a material’s suitability for specific applications, especially when considered simultaneously with mechanical properties. • Strength-to-weight and stiffness-to-weight ratios, as examples, are described in the context of lightweight designs, an important consideration especially in aerospace and automotive industries. • Thermal, electrical, magnetic, and optical properties are presented. • The importance of corrosion and corrosion-resistant materials are then described. • Design and manufacturing implications of each physical property is considered, with specific examples given. 3.1 Introduction Why is electrical wiring generally made of copper? Why are stainless steel, aluminum, and copper commonly used in cookware? Why are the handles of cookware usually made of plastic, while other types of handles are made of metal? What kind of material should be chosen for the heating elements in toasters 123 ׉ 7cassandra://pI18DrNO5rdTukp17cSrvMysdKVwlR7tqJZ3rAwrO6Q*`cZ%Eb׉E7124 Chapter 3 Physical Properties of Materials and toaster ovens? Why does metal feel colder to the touch than plastic even though both are at room temperature? Why are the metallic components in some machines being replaced with ceramics? Why are commercial airplane bodies generally made of aluminum while others are made of reinforced plastics? From these questions it is apparent that a major criterion in material selection requires consideration of physical properties: density, melting point, specific heat, thermal conductivity, thermal expansion, electrical and magnetic properties, and resistance to oxidation and corrosion. Combinations of mechanical and physical properties, such as the strength-to-weight and stiffness-to-weight ratios of materials, are equally important, particularly for aircraft and aerospace structures. Also, high-speed equipment, such as textile and printing machinery, and forming and cutting machines for high-speed operations, require lightweight components to reduce inertial forces and prevent the machines from being subjected to excessive vibration. Several other examples of the importance of physical properties are described in this chapter. 3.2 Density The density of a material is its mass per unit volume; another term is specific gravity, which expresses a material’s density relative to that of water; specific gravity has therefore no units. The range of densities for a variety of materials at room temperature, along with other properties, is given in Tables 3.1 and 3.2. Weight saving is particularly important for aircraft, aerospace, automotive structures, sports equipment, and for various other products where energy consumption and power limitations are significant concerns. Substitution of materials for weight savings and fuel economy is a major factor in the design both of advanced equipment and machinery and of consumer products, such as sporting goods, portable computers, and bicycles. A significant role that density plays is in the strength-to-weight ratio (specific strength)and stiffnessto-weight ratio (specific stiffness) of materials. Figure 3.1 shows the ratio of maximum yield stress to density for a variety of metal alloys. Note that titanium and aluminum are at the top of the list; consequently, they are among the most commonly used metals for aircraft and aerospace applications. The ranges for specific tensile strength and specific stiffness at room temperature for a variety of metallic and nonmetallic materials are given in Fig. 3.2. Note the positions of composite materials, as compared to those of metals, with respect to these properties. These advantages have led composites to become among the most important materials, as described in Chapter 9. At elevated temperatures, specific strength and specific stiffness are likewise important considerations, especially for components operating at these temperatures, such as automotive and jet engines, gas turbines, and furnaces. Typical ranges for a variety of materials are given in Fig. 3.3. Density is an important factor in the selection of materials for high-speed equipment, such as magnesium in printing and textile machinery, many components of which typically operate at very high speeds. Aluminum is used with some digital cameras for better performance in cold weather. Because of their low density, ceramics (Chapter 8) are being used for components in high-speed automated machinery and machine tools. On the other hand, there are applications where weight is desirable. Examples are counterweights for various mechanisms (using lead or steel), flywheels, ballasts on ships and aircraft, and weights on golf clubs (using high-density materials such as tungsten). 3.3 Melting Point The temperature range within which a component or structure is designed to function is an important consideration in selection of materials. Plastics, for example, have the lowest useful temperature range (Table 7.2), while ceramics, graphite, and refractory-metal alloys have the highest useful range. Pure metals have a definite melting point, whereas the melting temperature of a metal alloy can have a wide range (Table 3.1), depending on its composition. The melting point has several indirect effects on manufacturing operations. Because the recrystallization temperature of a metal is related to its melting point (Section 1.7), operations such as annealing and ׉ 7cassandra://CoAf69FvMG_ZfoVKbljo4GJXTD6Inpmy4p5vIrLvA4A:`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://iNhOmoLKkcGTmkrpjQI3hWl5BUhQTxyzND3qShJSDiQs`׉ 7cassandra://-Mb9tyi3uMg7uhoqtB-cdIyvocne4YP67MkBKQ_4BVcQ`s׉ 7cassandra://V1vVcBuPqYt0FDGR4nBJn3Whuiv8kz0V9vVj7H7Pdhc`׉ 7cassandra://0jmk_8TyAl-bZqsAO2t7XG3f6wjvLMfgOJ85B7X7IN4n{6͠^cZEbDZט#u׉׉ 7cassandra://nR5pVb1a1jIma6ggjJf7NGz_wjwvZ78sSp7_6SWL9W8F`׉ 7cassandra://G77HGiKAOiPFmY9xQiw58lo2XdvfABzHoPBw9twHv6k5`s׉ 7cassandra://NnVIr8iurtfaO7M__4GA9UiW4C3RWhlQgsN24HDSwl8`׉ 7cassandra://SUcOsxTx3lB2oochQFkSF22klMmPA7hT8Pre5Kh3z5cf8͠^cZEbDz׉E :Specific Heat 125 Table 3.1: Physical Properties of Selected Materials at Room Temperature Coefficient Material Metallic Aluminum Aluminum alloys Beryllium Niobium(columbium) Copper Copper alloys Gold Iron Steels Lead Lead alloys Magnesium Magnesium alloys Molybdenum alloys Nickel Nickel alloys Platinum Silicon Silver Tantalum alloys Tin Titanium Titanium alloys Tungsten Zinc Zinc alloys Nonmetallic Ceramics Glasses Graphite Plastics Wood Density (kg/m3) 2700 Melting point (◦C) 660 1278 2468 1082 7860 6920–9130 11,350 8850–11,350 1745 1770 10,210 8910 2330 10,500 16,600 7310 4510 4430–4700 19,290 7140 1063 1537 Specific heat (J/kg K) 900 2630–2820 476–654 880–920 1854 8580 8970 1884 272 385 7470–8940 885–1260 377–435 19,300 129 460 1371–1532 327 182–326 650 610–621 2610 1453 1768 1423 961 448–502 130 126–188 1025 1046 276 440 7750–8850 1110–1454 381–544 21,450 2996 232 1668 1549–1649 3410 419 6640–7200 386–525 580–1540 133 712 235 142 217 519 502–544 138 385 402 2300–5500 — 750–950 2400–2700 1900–2200 — 840 110–330 900–2000 500–850 1000–2000 400–700 — 2400–2800 Thermal conductivity (W/m-K) 222 121–239 146 52 393 219–234 317 74 15–52 35 24–46 154 75–138 142 92 12–63 71.6 148 429 54 67 17 8–12 166 113 105–113 10–17 0.6–1.7 5–10 0.1–0.4 0.1–0.4 of thermal expansion (µm/m-◦C) 23.6 23.0–23.6 8.5 7.1 16.5 16.5–20 19.3 11.5 11.7–17.3 29.4 27.1–31.1 26.0 26.0 5.1 13.3 12.7–18.4 8.8 7.63 19.3 6.5 22 8.35 8.1–9.5 4.5 32.5 32.5–35 5.5–13.5 4.6–70 7.86 72–200 2–60 Electrical resistivity (Ω-m) 2.8 10−8 2.8–4.0 ×10−8 4.0 ×10−8 15 ×10−8 1.7 ×10−8 1.7–5.9 ×10−8 2.4 ×10−8 9.5 ×10−8 17.0 ×10−8 20.6 ×10−8 20.6–24 ×10−8 4.5 ×10−8 4.5–15.9 ×10−8 5.3 ×10−8 6.2 ×10−8 6.2–110 ×10−8 10.5 ×10−8 1.0 ×10−3 1.6 ×10−8 13.5 ×10−8 11.5 ×10−8 42 ×10−8 40–171 ×10−8 5.0 ×10−8 5.45 ×10−8 6.06–6.89 ×10−8 — — — — — heat treating (Chapter 4) and hot working (Part III) require knowledge of the melting points of the materials involved. These considerations are also important in the selection of tool and die materials. In casting operations (Part II), melting point plays a major role in the selection of the equipment and the melting practice employed. In the electrical-discharge machining process (Section 27.5), the melting points of metals are related to the rate of material removal and of electrode wear. 3.4 Specific Heat A material’s specific heat is the energy required to raise the temperature of a unit mass by one degree. Alloying elements have a relatively minor effect on the specific heat of metals. The temperature rise in a workpiece, such as those in forming or machining operations (Parts III and IV, respectively), is a function of ׉ 7cassandra://V1vVcBuPqYt0FDGR4nBJn3Whuiv8kz0V9vVj7H7Pdhc`cZ%Eb׉Em126 Chapter 3 Physical Properties of Materials Table 3.2: Physical Properties of Materials, in Descending Order Melting Density Platinum Gold Tungsten Tantalum Lead Silver Steel Titanium Aluminum Beryllium Glass Magnesium Plastics point Specific heat Tungsten Wood Tantalum Titanium Iron Molybdenum Beryllium Copper Copper Gold Silver Magnesium Lead Tin Plastics Beryllium Molybdenum Porcelain Niobium Titanium Iron Copper Molybdenum Aluminum Tungsten Lead Aluminum Thermal conductivity Silver Copper Gold Aluminum Graphite Tungsten Beryllium Zinc Steel Titanium Glass Plastics Thermal expansion Plastics Lead Tin Magnesium Copper Steel Gold Ceramics Glass Tantalum Tungsten Ceramics Electrical conductivity Silver Copper Gold Aluminum Graphite Magnesium Aluminum Magnesium Glass Tungsten Beryllium Steel Tin Graphite Ceramics Glass Plastics Quartz Titanium Aluminum Steels Magnesium Nickel Copper Tantalum Molybdenum Lead 0 0.75 1.50 2.25 3.00 Yield strength/density (m 3 104) Figure 3.1: Ratio of maximum yield stress to density for selected metals. the work done and of the specific heat of the workpiece material (Section 2.12). An excessive temperature rise in a workpiece can a) decrease product quality by adversely affecting its surface finish and dimensional accuracy, b) cause excessive tool and die wear, and c) result in undesirable metallurgical changes in the material. 3.75 ׉ 7cassandra://NnVIr8iurtfaO7M__4GA9UiW4C3RWhlQgsN24HDSwl8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://w-6E5y7kQ47RweAAz52qvooPK7YzH3jlC0w34tbelBov`׉ 7cassandra://n98Y9TwAVliEcZpeacdfHsbjip7lsKVARAu8bK0cvKsM `s׉ 7cassandra://kLXijJKq2RuyrAnQy_WPLIRVRA3rhBM83pScaqNIC-U`׉ 7cassandra://J2vjr3kjLp50jTBNW00JP37b1PLczGomwLzrpOH6J1M`P͠^cZEbǴט#u׉׉ 7cassandra://fH4IZU-h5YcUGm0Dpo7XM_z9M1Om0MYEJjU-HtGPMroɑ`׉ 7cassandra://TM8lCHdApnCBJU-YXbdWAEPfQYRU69hYwO1XNBF-BbI]`s׉ 7cassandra://AsSRBUqaUHWzIwSeVdMbXKa-CqqGjgxm9YCXan6UbAE*`׉ 7cassandra://izZp_S7FGsGyaVIZBnkZtc3VLXWFLf4M2MUpWhc9MW4ͅ ͠^cZEbǵ׉EThermal Expansion 127 Kevlar 49 S-glass High-tensile graphite Boron High-modulus graphite E-glass Graphite/epoxy Boron/epoxy Glass/epoxy Titanium Steel Aluminum 0 5 10 15 20 25 Tensile strength/density (m 3 104) (a) High-modulus graphite Boron High-tensile graphite Graphite/epoxy Boron/epoxy Kevlar 49 Aluminum E-glass S-glass Titanium Steel Glass/epoxy 0 5 10 15 20 Elastic modulus/density (m 3 106) (b) Figure 3.2: Specific strength (tensile strength/density) and specific stiffness (elastic modulus/density) for various materials at room temperature (see also Chapter 9). 3.5 Thermal Conductivity Thermal conductivity indicates the rate at which heat flows within and through a material. Metallically bonded materials (metals) generally have high thermal conductivity, while ionically or covalently bonded materials (ceramics and plastics) have poor conductivity (Table 3.2). Alloying elements can have a significant effect on the thermal conductivity of alloys, as can be seen by comparing pure metals with their alloys in Table 3.1. In general, materials with high electrical conductivity also have high thermal conductivity. Thermal conductivity is an important consideration in numerous applications. For example, high thermal conductivity is desirable in cooling fins, cutting tools, and die-casting molds to extract heat quickly. In contrast, materials with low thermal conductivity are used, for instance, in furnace linings, insulation, coffee cups, and handles for pots and pans. One function of a lubricant (Section 33.7) in hot metalworking is to act as an insulator to keep workpieces hot and formable. 3.6 Thermal Expansion The thermal expansion of materials can have several significant effects, particularly the relative expansion or contraction of different materials in assemblies, such as electronic and computer components, glass-tometal seals, struts on jet engines, coatings on cutting tools (Section 22.5), and moving parts in machinery that require certain clearances for proper functioning. The use of ceramic components in cast-iron engines, for example, also requires consideration of their relative expansion and contraction during their operation. ׉ 7cassandra://kLXijJKq2RuyrAnQy_WPLIRVRA3rhBM83pScaqNIC-U`cZ%Eb׉E 128 Chapter 3 Physical Properties of Materials 2 3 4 5 6 7 8 1 0 High-modulus carbon FRP Intermediate-modulus carbon FRP Al MMCs Titanium MMCs Ti alloys Al alloys Mg alloys 500 8C Figure 3.3: Specific strength (tensile strength/density) for a variety of materials as a function of temperature. Note the useful temperature range for these materials and the high values for composite materials. MMC = metal-matrix composite; FRP = fiber-reinforced plastic. Shrink fits utilize thermal expansion and contraction. A shrink fit is a part, often a sleeve or a hub, that is to be installed over a shaft. The part is first heated and then slipped over the shaft or spindle; when allowed to cool, the hub shrinks and the assembly becomes an integral component. Typical coefficients of thermal expansion are given in Table 3.1 (see also Invar below). Generally, the coefficient of thermal expansion is inversely proportional to the melting point of the material. Alloying elements have a relatively minor effect on the thermal expansion of metals. Thermal expansion in conjunction with thermal conductivity plays the most significant role in the development of thermal stresses (due to temperature gradients), both in manufactured components and in tools and dies, and molds for casting operations. This consideration is particularly important in, for example, a forging operation during which hot workpieces are repeatedly placed over a relatively cool die, thus subjecting the die surfaces to thermal cycling. To reduce thermal stresses, a combination of high thermal conductivity and low thermal expansion is desirable. Thermal stresses also can be caused by anisotropy of thermal expansion; that is, the material expands differently in different directions, a property generally observed in hexagonal close-packed metals, ceramics, and composite materials. Thermal expansion and contraction can lead to cracking, warping, or loosening of assembled components during their service life, as well as cracking of ceramic parts and in tools and dies made of relatively brittle materials. Thermal fatigue results from thermal cycling and causes a number of surface cracks, especially in tools and dies for casting and in metalworking operations (heat checking). Thermal shock is the term generally used to describe the development of a crack or cracks after being subjected to a single thermal cycle. To alleviate some of the problems caused by thermal expansion a family of iron-nickel alloys with very low thermal-expansion coefficients are available, called low-expansion alloys. The low thermal expansion characteristic of these alloys is often referred to as the Invar effect, after the metal Invar. Their thermal coefficient of expansion is typically in the range of from 2×10−6 to 9×10−6 per ◦C (compare with those given in Table 3.1). Typical compositions are 64% Fe–36% Ni for Invar and 54% Fe–28% Ni–18% Co for Kovar. Low-expansion alloys also have good thermal-fatigue resistance, and because of their good ductility they can easily be formed into various shapes. Applications include (a) bimetallic strips, which consist Beryllium Ni-based alloys High-temperature MMCs Cb- and Mobased alloys 1000 Carbon/carbon composites Tungsten 1500 Tensile strength/density (m 3 106) ׉ 7cassandra://AsSRBUqaUHWzIwSeVdMbXKa-CqqGjgxm9YCXan6UbAE*`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://xT5LXwezvdUZ5r0gMHTBm41Y25Lc0xnVj2I0J1qxzhkJ`׉ 7cassandra://M4vYAOw-X4V5o6eIOj1FAtZVZX6xK2spyb1ncruz7YYm!`s׉ 7cassandra://TWSEggDlsPSETuOCHW2fEmTePqTEKSccCzN4jQrFo6Q!`׉ 7cassandra://EgkKv9fRsfODLL57wwtDXkOjB6wHrqACiBzRG-wcSoki͠^cZEbǷט#u׉׉ 7cassandra://b5fp15x3-Owlz10tXuNZ25ErYmx3e5eLvL6lXtttMFAy`׉ 7cassandra://QkKJ3UI22t4UY2ckxMQXnsVXBPxR0MQ6f559XUKpdAEp`s׉ 7cassandra://s5KMmNn2obMFwM-l72ztg0RhJnT1QTD7LMOUFoAzybs`׉ 7cassandra://5WIkxsUqdQ7aEbwixodCebbogMgJUyIzFhCB_KwVU68e\͠^cZEbǸ׉EElectrical, Magnetic, and Optical Properties 129 of a low-expansion alloy bonded to a high-expansion alloy (thus, the strip develops a curvature when subjected to temperature variations), and (b) glass-to-metal seals, in which the thermal expansions of the two materials are matched. 3.7 Electrical, Magnetic, and Optical Properties Electrical conductivity and the dielectric properties of materials are important not only in various electrical equipment and machinery, but also in such manufacturing processes as magnetic-pulse forming (Section 16.12), resistance welding (Section 31.5), and electrical-discharge machining and electrochemical grinding of hard and brittle materials (Chapter 27). The units of electrical conductivity are mho/m, where mho is the reciprocal of ohm, the unit of electrical resistance. Alloying elements have a major effect on the electrical conductivity of metals: The higher the conductivity of the alloying element, the higher is the electrical conductivity of the alloy. Dielectric Strength. An electrically insulating material’s dielectric strength is the largest electric field to which it can be subjected without degrading or losing its insulating properties. This property is defined as the voltage required per unit distance for electrical breakdown, and has the units of V/m. Conductors. Materials with high electrical conductivity, such as metals, are generally referred to as conductors. Electrical resistivity is the inverse of electrical conductivity. Materials with high electrical resistivity are referred to as dielectrics or insulators. Superconductors. Superconductivity is the phenomenon of near-zero electrical resistivity that occurs in some metals and alloys below a critical temperature. The temperatures involved often are near absolute zero (0 K, –273◦C). The highest temperature at which superconductivity has to date been exhibited, at about –123◦C, is with an alloy of thallium, barium, calcium, copper, and oxygen; other material compositions are continuously being investigated. The main application of superconductors is largely for high-power magnets. Superconductors are the enabling technology for magnetic resonance imaging (MRI), used for medical imaging. Other applications envisioned for superconductors include magnetic levitation (maglev) trains, efficient power transmission lines, and extremely fast computer components. Semiconductors. The electrical properties of semiconductors, such as single-crystal silicon, germanium, and gallium arsenide, are extremely sensitive to temperature and to the presence and type of minute impurities. Thus, by controlling the concentration and type of impurities (called dopants), such as phosphorus and boron in silicon, electrical conductivity can be controlled. This property is utilized in semiconductor (solid-state) devices, used extensively in miniaturized electronic circuitry (Chapter 28). Ferromagnetism and Ferrimagnetism. Ferromagnetism is a phenomenon characterized by high permeability and permanent magnetization that are due to the alignment of iron, nickel, and cobalt atoms into domains. It is important in such applications as electric motors, electric generators, electric transformers, and microwave devices. Ferrimagnetism is a permanent and large magnetization exhibited by some ceramic materials such as cubic ferrites. Piezoelectric Effect. The piezoelectric effect (piezo from Greek, meaning to press) is exhibited by smart materials. Two basic behaviors are involved: (a) When subjected to an electric current, they undergo a reversible change in shape, by as much as 4% and (b) when deformed by an external force, they emit a small electric current. Piezoelectric materials include quartz crystals and some ceramics and polymers. The piezoelectric effect is utilized in making transducers, which are devices that convert the strain from an external force into electrical energy. Typical applications are sensors, force or pressure transducers, inkjet printers, strain gages, ׉ 7cassandra://TWSEggDlsPSETuOCHW2fEmTePqTEKSccCzN4jQrFo6Q!`cZ%Eb׉E130 Chapter 3 Physical Properties of Materials sonar detectors, and microphones. As an example, an air bag in an automobile has a sensor that, when subjected to an impact force, sends an electric charge that then deploys the bag. Magnetostriction. The phenomenon of expansion or contraction of a material when subjected to a magnetic field is called magnetostriction. Pure nickel and some iron–nickel alloys exhibit this behavior. Magnetostriction is the principle behind ultrasonic machining equipment (Section 26.6). Magnetorheostatic and Electrorheostatic Effects. When subjected to magnetic or electric fields, some fluids undergo a major and reversible change in their viscosity within a fraction of a second, turning from a liquid to an almost solid state. For example, magnetorheostatic behavior is attained by mixing very fine iron filings with oil. Called smart fluids, they are being developed for such applications as vibration dampeners, engine mounts, prosthetic devices, clutches, and valves. Optical Properties. Among various other properties, color and opacity are particularly relevant to polymers and glasses (see Sections 7.2.2 and 8.4.3, respectively). 3.8 Corrosion Resistance Corrosion not only leads to surface deterioration of components and structures, such as bridges and ships, but also reduces their strength and structural integrity. The direct cost of corrosion to the U.S. economy alone has been estimated to be over $400 billion per year, approximately 2% of the gross domestic product; indirect costs of corrosion are estimated at twice this amount. Metals, ceramics, and plastics are all subject to forms of corrosion. The word corrosion itself usually refers to the deterioration of metals and ceramics, while similar phenomena in plastics (Chapter 7) are generally called degradation. Corrosion resistance is an important aspect of material selection for applications in the chemical, petroleum, and food industries, as well as in manufacturing operations. In addition to various possible chemical reactions from the elements and compounds present, environmental oxidation and corrosion of a wide range of components and structures is a major concern, particularly at elevated temperatures. Resistance to corrosion depends on the composition of the material and on its particular environment. Corrosive media may consist of various chemicals (acids, alkalis, and salts) and the environment (oxygen, moisture, pollution, and acid rain), including water (fresh or salt water). Nonferrous metals, stainless steels, and nonmetallic materials generally have high corrosion resistance. Steels and some cast irons generally have poor resistance and must be protected by a variety of coatings and surface treatments (Chapter 34). Corrosion can occur over an entire surface or it can be localized, called pitting. Pitting is a term that is also used for fatigue wear or failure of gears and in forging (see Section 33.5). Corrosion can also occur along grain boundaries of metals as intergranular corrosion, and at the interface of bolted or riveted joints as crevice corrosion. Two dissimilar metals may form a galvanic cell (after L. Galvani, 1737–1798); that is, two electrodes in an electrolyte in a corrosive environment that includes moisture and cause galvanic corrosion. Two-phase alloys (Section 4.2) are more susceptible to galvanic corrosion (because of the physical separation of the two different metals involved) than are single-phase alloys or pure metals. As a result, heat treatment can have a significant influence on corrosion resistance. Stress-corrosion cracking (Section 2.10.2) is an example of the effect of a corrosive environment on the integrity of a product that, as manufactured, contained residual stresses. Likewise, cold-worked metals are likely to have residual stresses, thus making them more susceptible to corrosion than are hot-worked or annealed metals. Tool and die materials also can be susceptible to chemical attack by lubricants and coolants. The chemical reaction alters their surface finish and adversely influences the metalworking operation. One example is carbide tools and dies with cobalt as a binder (Section 22.4); the cobalt is attacked by elements in the ׉ 7cassandra://s5KMmNn2obMFwM-l72ztg0RhJnT1QTD7LMOUFoAzybs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://GHsKDXYmg-jvqjM5Ak38xKfJ8NnI3s1p0Ink9t1pF7Uo`׉ 7cassandra://isUmaxGdGIx0sP6hZOFBLyIWeAuRXhqYGbmWmPJtH4g]`s׉ 7cassandra://07w_GbtMfnB-TZjcJnk4yx8_eH440tmemc5smHeiyIg`׉ 7cassandra://PJQkM7gwPMzBFu3jjTROlK98RWHKGHcE3oIFlImLMDgp3͠^cZEbǺט#u׉׉ 7cassandra://hD0rJ1E8I-o4IuNVRb6jUnx61m7tIQORosXQkyOj1Y4)`׉ 7cassandra://tkdGdT7PBdMQ-SzAB48vnxTdtmVw6LKw4I6RyCx-tiMJn`s׉ 7cassandra://7kRvkK8ILHOAQhhqAuDC8rPSLI0n1C2QmmLEo23oQnU)`׉ 7cassandra://k_eakTuVjkc3j44y14bzuVLuc81mTqKeh5dA6FCPraYL͠^cZEbǻ׉E nCorrosion Resistance 131 Figure 3.4: A selection of coins, manufactured from different metal alloys of copper, nickel, tin, zinc, and aluminum. Valuable metals such as gold and silver are used for coins, but are not used for general currency. Source: Scott Gibson/Corbis RF/Alamy Stock Photo. metalworking fluid, called selective leaching. The compatibility of the tool, die, and workpiece materials with the metalworking fluid, under actual operating conditions, is thus an important consideration. Chemical reactions should not always be regarded as having only adverse effects. Advanced machining processes, such as chemical and electrochemical machining (Chapter 27), are indeed based on controlled chemical reactions. These processes remove material by chemical action in a manner similar to etching of metallurgical specimens. The usefulness of some level of oxidation is demonstrated also by the corrosion resistance of aluminum, titanium, and stainless steel. Aluminum, for example, develops a thin (a few atomic layers), strong and adherent hard-oxide film (Al2O3) that better protects the surface from further environmental corrosion. Titanium develops a film of titanium oxide (TiO2); a similar phenomenon occurs in stainless steels which, because of the chromium present in the alloy, develop a protective film. These processes are known as passivation. When the protective film is scratched and exposes the metal underneath, a new oxide film begins to form. Case Study 3.1 Selection of Materials for Coins There are six general criteria in the selection of materials for coins (Fig. 3.4). 1. The subjective factors, such as the appearance of the coin, its color, weight, and its ring (the sound made when striking). Also included in this criterion is the feel of the coin. This term is similar in effect to the feel of a fine piece of wood, polished stone, or tableware; It is difficult to quantify because it combines several human factors. 2. The intended life of the coin is also a consideration; this duration will reflect resistance to corrosion and to wear (Chapter 33) while the coin is in circulation. These two factors basically determine the span over which the surface imprint of the coin will remain identifiable as well as the ability of the coin to retain its original luster. 3. The manufacturing of the coin includes factors such as the formability of the candidate coin materials, the life of the dies used in the coining operation (Section 14.4), and the capability of the materials and processes to resist counterfeiting. 4. Another consideration is the suitability for use in coin-operated devices, such as vending machines and turnstiles. These machines are generally equipped with detection devices that test the coins – first, for proper diameter, thickness, and surface condition, and second, for electrical conductivity and density. The coin is rejected if it fails any of these tests. ׉ 7cassandra://07w_GbtMfnB-TZjcJnk4yx8_eH440tmemc5smHeiyIg`cZ%Eb׉E132 Chapter 3 Physical Properties of Materials 5. Health issues must be considered. For example, given the large number of population with nickel allergies, Euro coins are minted from nickel-free alloys. 6. A final consideration is the cost of raw materials and processing, and whether there is a sufficient supply of the coin materials. For example, Canada recently decided it would eliminate the penny because of the high cost of production and its limited currency value. The United States has similar concerns, since a penny (one cent) costs around 1.6 cents to manufacture. Summary • Physical properties can have several important influences on materials selection, manufacturing, and on the service life of components. These properties and other relevant characteristics should be considered, because of their possible effects on product design, service requirements, and compatibility with other materials, including tools, dies, and workpieces. • Combined properties, such as strength-to-weight and stiffness-to-weight ratios, are important factors in selecting materials for lightweight and high-performance structures. • Thermal conductivity and thermal expansion are major factors in the development of thermal stresses and thermal fatigue and shock, effects that are important in tool and die life in manufacturing operations. • Chemical reactions, including oxidation and corrosion, are important factors in material selection, design, and manufacturing, as well as in the service life of components. Passivation and stress-corrosion cracking are additional phenomena to be considered. • Some physical properties are utilized in manufacturing processes and their control, such as the magnetostriction effect (in ultrasonic machining of materials) and the piezoelectric effect (in force transducers and various other sensors). Key Terms Conductors Corrosion Degradation Density Dielectric Electrical conductivity Electrical resistivity Electrorheostatic Ferromagnetism Galvanic corrosion Heat checking Invar effect Magnetorheostatic Magnetostriction Melting point Oxidation Passivation Piezoelectric effect Selective leaching Semiconductors Smart fluids Smart materials Specific heat Specific stiffness Specific strength Stress-corrosion cracking ׉ 7cassandra://7kRvkK8ILHOAQhhqAuDC8rPSLI0n1C2QmmLEo23oQnU)`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Hw-F2NaB0GUp-5UssdTG4zD93k4lsyXV-IJ_MDGfRg8#`׉ 7cassandra://BWkg1D0rHuOc9zqqKMpIjErw4hR2_x-PJPHNc45GU5MS`s׉ 7cassandra://1i0To5u5JGdx9Sx7I8TrQoQ5dz7tWfBPpxMXVN5Ym1c`׉ 7cassandra://iqE9sh9CqfsEUz3_hm8A7htCHO70AcTRnm8tzuRtpUYgO͠^cZEbǽט#u׉׉ 7cassandra://LQeIipRCTuFJgqFBaWRNhgztcZfe7Z9hgSiLTzYPoyEs`׉ 7cassandra://KEAgejnuSF0uGNFHrtit_h8WvMfpfbG39Ycc5MiqbuQdw`s׉ 7cassandra://wBh92DTgb_YrxaWAvkz62eHI_BtQjFHgucwr6Zz4JG0`׉ 7cassandra://XAh3FttDAuEKrZacWPAn0XeLX8Vrk1QPRw1dF0Krmwk]͠^cZEbǾ׉E :Qualitative Problems 133 Superconductivity Thermal conductivity Thermal expansion Thermal fatigue Thermal stresses Bibliography Ashby, M.F., Materials Selection in Mechanical Design, 4th ed., Pergamon, 2010. ASM Handbook, Vol. 13A: Corrosion: Fundamentals, Testing, and Protection, 2003. —, Vol. 13B: Corrosion Materials, 2005. —, Vol. 13B: Corrosion: Environments and Industries, 2006, ASM International. Budinski, K.G., Engineering Materials: Properties and Selection, 9th ed., Prentice Hall, 2009. Fox, M., Optical Properties of Solids, Oxford, 2010. Fulay, P., and Lee, J.-K., Electronic, Magnetic, and Optical Materials, CRC Press, 2016. Hummel, R.E., Electronic Properties of Materials, 4th ed., Springer, 2011 Revie, R.W. (ed.), Uhlig’s Corrosion Handbook, 3rd ed., Wiley-Interscience, 2011. Shackelford, J. F., Introduction to Materials Science for Engineers, 8th ed., Macmillan, 2014. Solymar, L., Walsh, D., and R.R.A. Syms, Electrical Properties of Materials, 9th ed., Oxford, 2014. White, M.A., Physical Properties of Materials, 2nd ed., CRC Press, 2011. Review Questions 3.1. List several reasons that density is an important material property. 3.2. Explain why the melting point of a material can be an important factor in material selection. 3.3. What adverse effects can be caused by thermal expansion of materials? Give some examples. 3.4. Is thermal cracking the same as thermal shock? Why or why not? 3.5. What is the piezoelectric effect? 3.6. Describe the factors that can lead to the corrosion of a metal. 3.7. What is a superconductor? Describe two applications of superconducting materials. 3.8. What is the difference between thermal conductivity and thermal expansion? 3.9. What is corrosion? How can it be prevented or accelerated? 3.10. What is specific heat? Why is it important in manufacturing? 3.11. Explain stress-corrosion cracking. Why is it also called season cracking? 3.12. What is the difference between a superconductor and a semiconductor? 3.13. What are smart materials? 3.14. What is magnetostriction? What are its uses? Qualitative Problems 3.15. What is the fundamental difference between mechanical properties of materials, discussed in Chapter 2, and physical properties of materials described in this chapter? 3.16. Describe the significance of structures and machine components made of two materials with different coefficients of thermal expansion. 3.17. Which of the properties described in this chapter are important for (a) pots and pans, (b) cookie sheets for baking, (c) rulers, (d) paper clips, (e) music wire, and (f) beverage cans? Explain your answers. ׉ 7cassandra://1i0To5u5JGdx9Sx7I8TrQoQ5dz7tWfBPpxMXVN5Ym1c`cZ%Eb׉E d134 Chapter 3 Physical Properties of Materials 3.18. Note in Table 3.1 that the properties of the alloys of metals have a wide range compared with the properties of the pure metals. Explain why. 3.19. Rank the following in order of increasing thermal conductivity: aluminum, copper, silicon, titanium, ceramics, and plastics. Comment on how this ranking influences applications of these materials. 3.20. Does corrosion have any beneficial effects? Explain. 3.21. Explain how thermal conductivity can play a role in the development of residual stresses in metals. 3.22. List examples of products where materials that are transparent are desired. List applications for opaque materials. 3.23. Refer to Fig. 3.2 and explain why the trends seen are to be expected. 3.24. Two physical properties that have a major influence on the cracking of workpieces, tools, or dies during thermal cycling are thermal conductivity and thermal expansion. Explain why. 3.25. Which of the materials described in this chapter has the highest (a) density, (b) electrical conductivity, (c) thermal conductivity, (d) specific heat, (e) melting point, and (f) cost. 3.26. Is oxidation beneficial with respect to corrosion prevention, or is it part of the corrosion process? Explain. 3.27. Which properties described in this chapter can be affected by applying a coating? Quantitative Problems 3.28. If we assume that all the work done in plastic deformation is converted into heat, the temperature rise in a workpiece is (1) directly proportional to the work done per unit volume and (2) inversely proportional to the product of the specific heat and the density of the workpiece. Using Fig. 2.5, and letting the areas under the curves be the unit work done, calculate the temperature rise for (a) 8650 steel, (b) 304 stainless steel, and (c) 1100-H14 aluminum. 3.29. The natural frequency, f, of a cantilever beam is given by f =0.56  EIg wL4 , where E is the modulus of elasticity, I is the moment of inertia, g is the gravitational constant, w is the weight of the beam per unit length, and L is the length of the beam. How does the natural frequency of the beam change, if at all, as its temperature is increased? Assume that the material is steel. 3.30. Plot the following for the materials described in this chapter: elastic modulus versus density, yield stress versus density, thermal conductivity versus density. Comment on the implications of these plots. 3.31. It can be shown that thermal distortion in precision devices is low for high values of thermal conductivity divided by the thermal expansion coefficient. Rank the materials in Table 3.1 according to their ability to resist thermal distortion. 3.32. Add a column to Table 3.1 that lists the volumetric heat capacity of the materials listed, expressed in units of J/cm3 K. Compare the results to the value for liquid water (4.184 J/cm3 K). Note that the volumetric heat capacity of a material is the product of its density and specific heat. 3.33. Using strength and density data, determine the minimum weight of a 2 m-long tension member that must support a load of 8 kN, manufactured from (a) annealed 303 stainless steel; (b) normalized 8620 steel; (c) as-rolled 1080 steel; (d) 5052-O aluminum alloy; (e) AZ31B-F magnesium; and (f) pure copper. 3.34. Plot the thermal conductivity against electrical conductivity for the materials in Table 3.1. Is there a correlation? Explain. ׉ 7cassandra://wBh92DTgb_YrxaWAvkz62eHI_BtQjFHgucwr6Zz4JG0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://KqhCPPNEUWQhzN3R8k7RR7k2cNxembMHeDwACu8poH8H`׉ 7cassandra://R5vK2BCem7LhjX9-eQOjCzAvdZ7zOPkxSJ8cMPQ80D0Q`s׉ 7cassandra://5_Uz8yM1Ba2M2-_VlnokT9v8kjn7vcxwzgf8ExCsusMO`׉ 7cassandra://3TqkgGvE9bgu4kElLWdQ6gWZK51u_sm8ilkLKM9QQ4QE~͠^cZEbט#u׉׉ 7cassandra://v3l6RY4JTljkzRHjRKfAX5hAoS4gcVkpBTk4dhtBRwg`׉ 7cassandra://IGFRfHbXkMmgQkW2QUoqL2IxAPp60aphN6nzIrVIeg4A`s׉ 7cassandra://rzhBHbTeDzhySyXueTphhS_DGSZItlHhIQCourQxC6M`׉ 7cassandra://Wq8djzl6oRYqceCbcVuuoNWnmXD7hqiPhA2f9lEY1co:͠^cZEb׉E XSynthesis, Design, and Projects 135 Synthesis, Design, and Projects 3.35. Conduct a literature search and add the following materials to Table 3.1: cork, cement, ice, sugar, lithium, graphene, and chromium. 3.36. From your own experience, make a list of parts, components, or products that have corroded and have had to be replaced or discarded. 3.37. List applications where the following properties would be desirable: (a) high density, (b) low density, (c) high melting point, (d) low melting point, (e) high thermal conductivity, and (f) low thermal conductivity. 3.38. Describe several applications in which both specific strength and specific stiffness are important. 3.39. Design several mechanisms or instruments based on utilizing the differences in thermal expansion of materials, such as bimetallic strips that develop a curvature when heated. 3.40. For the materials listed in Table 3.1, determine the specific strength and specific stiffness. Describe your observations. 3.41. The maximum compressive force that a lightweight column can withstand before buckling depends on the ratio of the square root of the stiffness to the density for the material. For the materials listed in Table 2.2, determine (a) the ratio of tensile strength to density and (b) the ratio of elastic modulus to density. Comment on the suitability of each for being made into lightweight columns. 3.42. Describe possible applications and designs using alloys exhibiting the Invar effect of low thermal expansion. 3.43. Collect some pieces of different metallic and nonmetallic materials listed in Table 3.2. Using simple tests and/or instruments, determine the validity of the descending order of the physical properties shown in the table. 3.44. Add the following materials to Table 3.1: (a) uranium; (b) lithium; (c) sodium. 3.45. Design an actuator to turn on a switch when the temperature drops below a certain level. Use two materials with different coefficients of thermal expansion in your design. 3.46. Conduct an Internet and technical literature review and write a one-page paper highlighting applications of piezoelectric materials. 3.47. It has been widely reported that mechanical properties such as strength and ductility can be very different for micro-scale devices than are measured at normal length scales. Explain whether or not you would expect the physical properties described in this chapter to be scale dependent. 3.48. If you were given a metal (not an alloy), and asked to identify it, list (in order) the experiments or measurements you would perform. Explain what influence the shape of the metal would have on your prioritization. ׉ 7cassandra://5_Uz8yM1Ba2M2-_VlnokT9v8kjn7vcxwzgf8ExCsusMO`cZ%Eb׉EzChapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 4.1 Introduction 137 4.2 Structure of Alloys 138 4.3 Phase Diagrams 139 4.4 The Iron–Carbon System 142 4.5 The Iron–Iron-carbide Phase Diagram and the Development of Microstructures in Steels 143 4.6 Cast Irons 145 4.7 Heat Treatment of Ferrous Alloys 147 4.8 Hardenability of Ferrous Alloys 149 4.9 Heat Treatment of Nonferrous Alloys and Stainless Steels 153 4.10 Case Hardening 155 4.11 Annealing 155 4.12 Heat Treating Furnaces and Equipment 159 4.13 Design Considerations for Heat Treating 160 • This chapter reviews the structures of metal alloys, including solid solutions, intermetallic compounds, and two-phase systems. • Phase diagrams show graphically the phases that develop as a function of alloy composition and temperature. • The system of iron and carbon and the phases involved are described in detail. • Heat treatment of metals is a common method of improving mechanical properties; it involves controlled heating and cooling, transforming a microstructure into a different phase. • Some metals, such as aluminum and stainless steels, can be heat treated only by precipitation, hardening, or aging. • Improving the ductility of a material is at the expense of such properties as strength or hardness. • The chapter ends with a review of the characteristics of heat-treating equipment. 136 ׉ 7cassandra://rzhBHbTeDzhySyXueTphhS_DGSZItlHhIQCourQxC6M`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://yv6v9e3jtrhHmfuu2dhMS6gcvJFbTs9lknYmw1ZgpGQ `׉ 7cassandra://sMJuVPQxKwXDrj9bb81XyHxtYsNY3ezatp-1yLCEGPUB`s׉ 7cassandra://dZcwsrnH2D-w2lFoh2r268ztpLJgdWU16w87DKlLcWAp`׉ 7cassandra://VsVTQCqDpqbQokAbwG8L8jEivz3TUtucVZ13nVXgkeQp͠^cZEbט#u׉׉ 7cassandra://uD_9z1sFKNKFyz3EOd2tl9jAVTrudm-g7zfK14V7ahoB`׉ 7cassandra://okgcpm8HTa9pkZJk5q1rDI3XF5gBqdLfshrzrpPiaFcg`s׉ 7cassandra://vSAzKIh9rA9vgusYpjHN-kG9O_dMGxO_e2F3c882dssg`׉ 7cassandra://T9Fw9Y-aQskEdAtvBNSce55KFGyCl_4aDwSgfwWTnLAe͠^cZEb׉E[Introduction 137 4.1 Introduction The properties and behavior of metals and alloys during manufacturing and their performance during their service life depend on their composition, structure, and their processing history, including the heat treatment to which they have been subjected. Important properties, such as strength, hardness, ductility, toughness, and resistance to wear, are greatly influenced by alloying elements and the heat-treatment processes employed. The most common example of a process that improves properties is heat treatment (Sections 4.7–4.10), which modifies microstructures. A variety of mechanical properties important to manufacturing can then develop, such as improved formability, machinability, or increased strength and hardness to improve the performance of tools and dies. These properties also enhance service performance of machine components, such as gears, cams, and shafts (Fig. 4.1). This chapter follows the outline shown in Fig. 4.2, beginning with the role of various alloying elements, the solubility of one element in another, phases, equilibrium phase diagrams, and the influence of composition, temperature, and time. The chapter also describes methods and techniques of heating, quenching, tempering, and annealing of metals and alloys, and the characteristics of the equipment involved. Figure 4.1: Cross-section of gear teeth showing induction-hardened surfaces. Source: TOCCO Div., ParkOhio Industries, Inc. Iron–carbon system Alloying elements Pure metals Alloys Phase diagrams Microstructures Solid solutions Nonferrous, Stainless steels Substitutional solid solutions Interstitial solid solutions Intermetallics Figure 4.2: Outline of topics described in Chapter 4. Heat treatment Solution treatment, precipitation hardening, aging, etc. Ferrous, Cast irons Heat treatment Quenching, annealing, tempering, normalizing, etc. ׉ 7cassandra://dZcwsrnH2D-w2lFoh2r268ztpLJgdWU16w87DKlLcWAp`cZ%Eb׉E138 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 4.2 Structure of Alloys When describing the basic crystal structure of metals, in Chapter 1, it was noted that the atoms are all of the same type, except for the presence of impurity atoms. These metals are known as pure metals,even though they may not be completely pure. Commercially pure metals are used for various purposes, such as aluminum for foil, copper for electrical conductors, nickel or chromium for plating, and gold for electrical contacts. Pure metals have somewhat limited properties but they can be enhanced or modified by alloying. The majority of metals used in engineering applications are some form of alloy. An alloy consists of two or more chemical elements, at least one of which is a metal. Alloying consists of two basic forms: solid solutions and intermetallic compounds. 4.2.1 Solid Solutions Two terms are essential in describing alloys: solute and solvent. The solute is the minor element (such as salt or sugar) that is added to the solvent, which is the major element (such as water). In terms of the elements in a crystal structure, the solute (composed of solute atoms) is the element that is added to the solvent (composed of host atoms). When the particular crystal structure of the solvent is maintained during alloying, the alloy is called a solid solution. Substitutional Solid Solutions. If the size of the solute atom is similar to that of the solvent atom, the solute atoms can replace solvent atoms and form a substitutional solid solution (Fig. 1.8). An example is brass (Section 6.4), which is an alloy of zinc and copper in which zinc (the solute atom) is introduced into the lattice of copper (the solvent atom). The properties of brass can thus be modified by controlling the amount of zinc in copper. Interstitial Solid Solutions. If the size of the solute atom is much smaller than that of the solvent atom, each solute atom can occupy an interstitial position, forming an interstitial solid solution. An important family of interstitial solid solutions is steel (Chapter 5), which is an alloy of iron and carbon in which the carbon atoms are present in interstitial positions between iron atoms. The atomic radius of carbon is 0.071 nm, which is very small as compared to the 0.124 nm radius of the iron atom. The properties of carbon steels can be varied over a wide range by adjusting the ratio of carbon to iron. The ability to control this ratio is a major reason why steel is such a versatile and useful material with a very wide range of properties and applications. 4.2.2 Intermetallic Compounds Intermetallic compounds are complex structures, consisting of two metals in which solute atoms are present among solvent atoms in specific proportions. Typical examples are aluminides of titanium (Ti3Al), nickel (Ni3Al), and iron (Fe3Al). Some intermetallic compounds have solid solubility, and the type of their atomic bond may range from metallic to ionic. Intermetallic compounds are strong, hard, and brittle. Because of their high melting points, strength at elevated temperatures, good oxidation resistance, and relatively low density, intermetallic compounds are candidate materials for applications such as advanced gas-turbine engines. 4.2.3 Two-phase Systems Recall that a solid solution is one in which two or more elements form a single homogeneous solid phase, in which the elements are uniformly distributed throughout the solid mass. Such a system has a maximum concentration of solute atoms in the solvent-atom lattice, just as there is a solubility limit for sugar in water. Most alloys consist of two or more solid phases, and they may be regarded as mechanical mixtures; such a system with two solid phases is known as a two-phase system. ׉ 7cassandra://vSAzKIh9rA9vgusYpjHN-kG9O_dMGxO_e2F3c882dssg`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://eBvH3Ff98fCkhUhKC5pwN_vZ3fNWjmr7z1l1VY7KbhwKR`׉ 7cassandra://PR_LpBTZdFKbnpHLTcxTOw8jfz3YddwtyQ-FQpuX4rQj`s׉ 7cassandra://VIoFjnedKiZu5yXvTR-yEkunXlrUHp5P0ueEor4-sjo`׉ 7cassandra://LSSDHVaHLBWQGkZHd3Zs-UAoMxwyAuoFOSwPG7wI_1MN͠^cZEbט#u׉׉ 7cassandra://1vvllCQNiBzkvFjX3YhidI81qEDxovssAksZF9VcYv8$`׉ 7cassandra://muMD2bHPBc6koWy8gGDlBW3OAej6zRP12EVkpetxhhYTa`s׉ 7cassandra://i43kDe9SC-vKiiO8nszX_WRTsoGs4vubSLfm3-fTq-E`׉ 7cassandra://A6F26POKF165dvXt5UBcjF8bkAN54cumTW2afOnEplE.$͠^cZEb׉EUPhase Diagrams 139 (a) (b) Figure 4.3: (a) Schematic illustration of grains, grain boundaries, and particles dispersed throughout the structure of a two-phase system, such as a lead–copper alloy. The grains represent lead in solid solution in copper, and the particles are lead as a second phase. (b) Schematic illustration of a two-phase system consisting of two sets of grains: dark and light. The blue grains and the white grains have separate compositions and properties. A phase is defined as a physically distinct and homogeneous portion in a material; each phase is a homogeneous part of the total mass, and has its own characteristics and properties. Consider a mixture of sand and water as an example of a two-phase system; these two very different components have their own distinct structures, characteristics, and properties. There is a clear boundary in this mixture between the water (one phase) and the sand particles (the second phase). Another example is ice in water: the two phases have exactly the same chemical elements (hydrogen and oxygen) even though their properties are very different. Note that it is not necessary for one phase to be a liquid; for example, sand suspended in ice is also a two-phase system. An example of a two-phase system in metals occurs when lead is added to copper in the molten state. After the mixture solidifies, the structure consists of two phases: (a) one having a small amount of lead in solid solution in copper, and (b) the other having lead particles (roughly spherical in shape) dispersed throughout the structure (Fig. 4.3a). The lead particles are analogous to sand particles in water, described above. The copper–lead alloy has properties that are different from those of either copper or lead alone. Alloying with finely dispersed particles (the second-phase particles) is an important method of strengthening alloys and controlling their properties. In two-phase alloys, the second-phase particles become obstacles to dislocation movement, and thus increase the strength of the alloy. Figure 4.3b shows another example of a two-phase alloy, which is an aggregate structure where there are two sets of grains, each with its own composition and properties. The darker grains in the figure may, for example, have a different structure than the lighter grains; they may be brittle, while the lighter grains are ductile. Defects may develop during metalworking operations, such as forging or extrusion (Chapters 14 and 15). Such flaws may be due to the lack of ductility of one of the phases in the alloy. In general, two-phase alloys are stronger and less ductile than solid solutions. 4.3 Phase Diagrams Pure metals have clearly defined melting or freezing points, and solidification takes place at a constant temperature. When the temperature of a molten metal is reduced to the freezing point, the energy of the latent heat of solidification is given off while the temperature remains constant. Eventually, solidification is complete and the solid metal continues cooling to ambient (room) temperature. Unlike pure metals, alloys solidify over a range of temperatures (Fig. 4.4). Solidification begins when the temperature of the molten metal drops below the liquidus temperature, and is completed when the temperature reaches the solidus. Within this temperature range, the alloy is in a mushy or pasty state; its composition and state are then described by the particular alloys phase diagram. Ap ¯ ¯ hase diagram, also called an equilibrium or constitutional diagram, shows the relationships among temperature, the composition, and the phases present in a particular alloy system at equilibrium. ׉ 7cassandra://VIoFjnedKiZu5yXvTR-yEkunXlrUHp5P0ueEor4-sjo`cZ%Eb׉E 140 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Alloy composition First solid 1455 Liquid solution (36% Cu–64% Ni) Liquid (50% Cu–50% Ni) Solid 1288 1313 1249 Solid solution 1082 0 100 36 42 50 58 67 64 58 50 42 33 Composition (% by weight) CS C0 CL Fraction liquid 01 Figure 4.4: Phase diagram for nickel–copper alloy system obtained at a slow rate of solidification. Note that pure nickel and pure copper each have a specific freezing or melting temperature. The top circle on the right depicts the nucleation of crystals; the second circle shows the formation of dendrites (see Section 10.2.2). The bottom circle shows the solidified alloy, with grain boundaries. Equilibrium means that the state of a system does not vary with time. The word constitutional indicates the relationships among the structure, the composition, and the physical makeup of the alloy. As described in detail below, types of phase diagrams include those for (a) complete solid solutions; (b) eutectics, such as cast irons; and (c) eutectoids, such as steels. An example of a phase diagram is shown in Fig. 4.4 for the copper–nickel alloy; it is called a binary phase diagram because there are two elements (copper and nickel) present in the system. The left boundary of this diagram (100% Ni) indicates the melting point of pure nickel; the right boundary (100% Cu) indicates the melting point of pure copper. (All percentages are by weight, not by number of atoms.) Lever Rule. The composition of various phases in a phase diagram can be determined by a procedure called the lever rule. As shown in the lower portion of Fig. 4.4, the procedure is to first construct a lever between the solidus and liquidus lines (called tie line), balanced (on the triangular support) at the nominal weight composition Co of the alloy. The left end of the lever represents the composition CS of the solid phase and the right end of the composition CL of the liquid phase. Note from the graduated scale that the liquid fraction is also indicated along the tie line, ranging from 0 at the left (fully solid) to 1 at the right (fully liquid). The lever rule states that the weight fraction of solid is proportional to the distance between Co and CL: S = S + L Co −CL CS −CL (4.1) 1981 100 Copper (Cu) 0 Nickel (Ni) (42% Cu–58% Ni) Liquid (58% Cu–42% Ni) Solid solution (50% Cu–50% Ni) Solidus Liquidus L 1 S Temperature (8C) ׉ 7cassandra://i43kDe9SC-vKiiO8nszX_WRTsoGs4vubSLfm3-fTq-E`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://b9hP8SLrU8a45dysvUA2SlPg5soGBjk_Xuts3RFzSak`׉ 7cassandra://b-mJU1mozXPEIDYts23RJBw7frC5ynYdqWC51Fqg3ggQ`s׉ 7cassandra://_2xfp8uTrkXr4MJHLEpxGVSIEyjpeTEV4_ZPV-OzAf8k`׉ 7cassandra://zVlYULjn5cZPHbelYB-pftGg1Yhtbvwd9-TyV7ClV1w{ ͠^cZEbט#u׉׉ 7cassandra://nbAzUbQREdid0PtgYmCIKx7GNK0nO3wwEhix1Uesi2w i`׉ 7cassandra://U5li7GGbEfzMEvY3QjxnpzLWd2WGBLBCfbVflpP_6B8WX`s׉ 7cassandra://fkac9sES1pum3QzKUEKs2bfEkxevZqs8RqlnACYE97o`׉ 7cassandra://8a-pOT5tizCU-DQq6ff_uG7cP9cvQqYoHf5ktQDs1oǗn͠^cZEb׉E Phase Diagrams 141 Likewise, the weight fraction of liquid is proportional to the distance between CS and Co, hence L = S + L CS −Co CS −CL (4.2) Note that these quantities are fractions, and they must be multiplied by 100 to determine percentages. From inspection of the tie line in Fig. 4.4 (and for a nominal alloy composition of Co =50% Cu–50% Ni) it can be noted that, because Co is closer to CL than it is to CS, the solid phase contains less copper than does the liquid phase. By measurement on the phase diagram and using the lever-rule equations, it can be seen that the composition of the solid phase is 42% Cu and of the liquid phase is 58% Cu, as stated in the middle circle at the right in Fig. 4.4. These calculations refer to copper. Reversing the phase diagram in the figure, so that the left boundary is 0% nickel (whereby nickel now becomes the alloying element in copper) will give the compositions of the solid and liquid phases in terms of nickel. The lever rule is also known as the inverse lever rule because, as indicated by Eqs. (4.1) and (4.2), the amount of each phase is proportional to the length of the opposite end of the lever. The completely solidified alloy in the phase diagram shown in Fig. 4.4 is a solid solution, because the alloying element, Cu (the solute atom), is completely dissolved in the host metal, Ni (the solvent atom), and each grain has the same composition. The atomic radius of copper is 0.128 nm and that of nickel is 0.125 nm, and both elements have a face-centered cubic structure; thus, they readily form solid solutions. The mechanical properties of solid solutions of Cu–Ni depend on their composition (Fig. 4.5). The properties of pure copper are, up to a limit, improved upon by increasing the nickel content; thus, there is an optimal percentage of nickel that gives the highest strength and hardness to the Cu–Ni alloy. 300 400 500 200 0 100 25 75 50 50 75 25 Composition (%) (a) 100 120 Nickel 40 60 80 0 100 25 75 Zinc 50 50 75 25 Composition (%) (c) 100 0 Ni or Zn Cu 20 30 40 50 60 70 0 100 25 75 100 0 Ni or Zn Cu Nickel Zinc 100 200 300 0 0 100 25 75 50 50 75 25 Composition (%) (b) Zinc 100 0 Ni or Zn Cu Nickel Zinc Nickel 50 50 75 25 Composition (%) (d) Figure 4.5: Mechanical properties of copper–nickel and copper–zinc alloys as a function of their composition. The curves for zinc are short, because zinc has a maximum solid solubility of 40% in copper. 100 0 Ni or Zn Cu Hardness (HRF) Tensile strength (MPa) Elongation (% in 50 mm) Yield stress (MPa) ׉ 7cassandra://_2xfp8uTrkXr4MJHLEpxGVSIEyjpeTEV4_ZPV-OzAf8k`cZ%Eb׉E 142 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Figure 4.5 shows how zinc, as an alloying element in copper, affects the mechanical properties of the alloy. Note the maximum of 40% solid solubility for zinc (solute) in copper (solvent), whereas copper and nickel are completely soluble in each other. The improvements in properties are due to pinning (blocking) of dislocations (Section 1.4.1) at substitutional nickel or zinc atoms, which may also be regarded as impurity atoms. As a result, dislocations cannot move as freely, and thus the strength of the alloy increases. 4.4 The Iron–Carbon System Steels and cast irons are represented by the iron–carbon binary system. Commercially pure iron contains up to 0.008% C, steels up to 2.11% C, and cast irons up to 6.67% C, although most cast irons contain less than 4.5% C. The iron–carbon system is described in this section, including the techniques employed to evaluate and modify the properties of these important materials, for specific applications. The iron–iron-carbide phase diagram is shown in Fig. 4.6. Although this diagram can be extended to the right—to 100%C (pure graphite); see Fig. 4.10—the range that is significant to engineering applications is up to 6.67% C, because Fe3C is a stable phase. Pure iron melts at a temperature of 1538◦C, as shown at the left boundary in Fig. 4.6. As iron cools, it first forms delta ferrite, then austenite, and finally alpha ferrite. Ferrite. Alpha ferrite, also denoted α-ferrite or simply ferrite, is a solid solution of body-centered cubic (bcc) iron; it has a maximum solid solubility of 0.022% C at a temperature of 727◦C. Just as there is a solubility limit for salt in water (with any extra amount precipitating as solid salt at the bottom of a container), there is a solid solubility limit for carbon in iron. Ferrite is relatively soft and ductile; it is magnetic from room temperature to 768◦C, the so-called Curie temperature (after M. Curie, 1867–1934). Although very little carbon can dissolve interstitially in bcc iron, the amount of carbon can significantly affect the mechanical properties of ferrite. Furthermore, significant amounts of chromium, manganese, nickel, molybdenum, tungsten, and silicon can be contained in iron in solid solution, imparting special properties. 1600 d (ferrite) 1000 1200 1400 800 600 400 15388C 1495°C Liquid 13948C g 1 liquid 11488C g (austenite) 9128C a 1 g 7278C 0.77% 0.022% 0 123456 6.67 Cementite (Fe3C) a (ferrite) Carbon (% by weight) Figure 4.6: The iron–iron-carbide phase diagram. Because of the importance of steel as an engineering material, this diagram is one of the most important of all phase diagrams. a 1 cementite 2.11% 4.30% g 1 cementite Temperature (8C) ׉ 7cassandra://fkac9sES1pum3QzKUEKs2bfEkxevZqs8RqlnACYE97o`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Nv8p-pyKa_XVJmSj3BmJqsx4aX5sPsL56dufGixCLaE}`׉ 7cassandra://eIHBhYN_8juSMv3uoEv4sjbHA_CGL29CNvPlxFA9CkQd`s׉ 7cassandra://VytJ9ZJ0ixkTy2hkJjOEFvxXpsAQdFA4MmSamteqdb8x`׉ 7cassandra://cyUzsHxFWrTk4oPg1YT1mqxcDTxYhq5O77S6YBG0Xfo4͠^cZEbט#u׉׉ 7cassandra://523GskQ20_MP6AwNHX12Gm_i9kmMwPmTEl0wbE8TBCA\`׉ 7cassandra://ygBNPiFcf1OtnCPzUYboyGOVEdduPodkiFDLzGNYnMk@`s׉ 7cassandra://uVxSAsN2Ptjcf6c8CE9oo1X_bdWcmmdgKyk480rCMCk`׉ 7cassandra://ouYaqfn523JclL4erPNLs0kzftFNeAXGemA1MJLlQR4͠^cZEb׉E The Iron–Iron-carbide Phase Diagram and the Development of Microstructures in Steels 143 Fe atoms C atoms a c a Fe atoms Austenite Ferrite Martensite Carbon (%) 0 0.20 0.40 (a) (b) (c) c (nm) a (nm) 0.286 0.288 0.291 (d) Figure 4.7: The unit cells for (a) austenite, (b) ferrite, and (c) martensite. The effect of percentage of carbon (by weight) on lattice dimensions for martensite is shown in (d). Note the interstitial position of the carbon atoms (see Fig. 1.8). Note also the increase in dimension c with increasing carbon content; this effect causes the unit cell of martensite to be in the shape of a rectangular prism. Austenite. As shown in Fig. 4.6, within a certain temperature range iron undergoes a polymorphic transformation from a bcc to an fcc structure, becoming gamma iron (γ-iron) or, more commonly, austenite (after W.R. Austen, 1843–1902). This structure has a solid solubility of up to 2.11% C at 1148◦C. Because the fcc structure has more interstitial positions, the solid solubility of austenite is about two orders of magnitude higher than that of ferrite, with the carbon occupying the interstitial positions, as shown in Fig. 4.7a. Austenite is an important phase in heat treatment of steels (Section 4.7). It is denser than ferrite, and its single-phase fcc structure is ductile at elevated temperatures, thus possessing good formability. Large amounts of nickel and manganese can be dissolved in fcc iron, to impart various properties. Steel is nonmagnetic in austenitic form, either at high temperatures or, for austenitic stainless steels, at room temperature. Cementite. The right boundary of Fig. 4.6 represents cementite, which is 100% iron carbide (Fe3C) with a carbon content of 6.67%. Cementite, from the Latin caementum (meaning stone chips), is also called carbide (not be confused with other carbides, used as dies, cutting tools, and abrasives, such as tungsten carbide, titanium carbide, and silicon carbide; Chapters 8 and 22). Cementite is a very hard and brittle intermetallic compound and has a significant influence on the properties of steels. 4.5 The Iron–Iron-carbide Phase Diagram and the Development of Microstructures in Steels The region of the iron–iron-carbide phase diagram that is significant for steels is shown in Fig. 4.8 (an enlargement of the lower left-hand portion of Fig. 4.6). Various microstructures can be developed, depending on (a) carbon content, (b) amount of plastic deformation (working), and (c) method of heat treatment. For example, consider the eutectic point of iron with a 0.77% C content, while it is being cooled very slowly from a temperature of, say, 1100◦C in the austenite phase; the reason for very slow cooling is to maintain equilibrium. At 727◦C, a reaction takes place in which austenite is transformed into alpha ferrite (bcc) and cementite. Because the solid solubility of carbon in ferrite is only 0.022%, the extra carbon forms cementite; 0.286 0.2858 0.2856 C atom Fe Fe C a ׉ 7cassandra://VytJ9ZJ0ixkTy2hkJjOEFvxXpsAQdFA4MmSamteqdb8x`cZ%Eb׉Ez144 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 400 500 600 700 800 900 1000 1100 g g g g g 1 Fe3C a 1 g a Fe3C Ferrite a 1 Fe3C 0 0.5 1.0 1.5 2.0 2.5 Carbon (% by weight) Figure 4.8: Schematic illustration of the microstructures for an iron–carbon alloy of eutectoid composition (0.77% carbon), above and below the eutectoid temperature of 727◦C. this reaction is called a eutectoid (meaning eutecticlike). This reaction indicates that at a certain temperature, a single solid phase (austenite) is transformed into two other solid phases (ferrite and cementite). The structure of eutectoid steel is called pearlite, because, at low magnifications, it resembles mother-of-pearl (Fig. 4.9). The microstructure of pearlite consists of alternating layers (lamellae) of ferrite and cementite; consequently, the mechanical properties of pearlite are intermediate between those of ferrite (soft and ductile) and cementite (hard and brittle). 4.5.1 Effects of Alloying Elements in Iron Although carbon is the basic element that transforms iron into steel, other elements are added to impart a variety of desirable properties. The main effect of these alloying elements on the iron–iron-carbide phase diagram is to shift the eutectoid temperature and eutectoid composition (percentage of carbon in steel at the eutectoid point); these elements shift other phase boundaries as well. 7278C g Figure 4.9: Microstructure of pearlite in 1080 steel, formed from austenite of eutectoid composition. In this lamellar structure, the lighter regions are ferrite and the darker regions are carbide. Magnification: 2500×. Temperature (8C) ׉ 7cassandra://uVxSAsN2Ptjcf6c8CE9oo1X_bdWcmmdgKyk480rCMCk`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://uCwZXQDRDi0UmDWQWqC77JLWavR7bCdVn1pclmEWfVM``׉ 7cassandra://tb1u5UR_ZmjExIIr2y5TqKYxBr30-rLNTuFX-2ByWlQI`s׉ 7cassandra://CZh1YnKeCCJMR6XqrJ1lm1RZZ4BodqPy3ztEn7YqJzk9`׉ 7cassandra://r3reU2dU9S3LKdJWOS9uVyoBwGhS5NTtJkGwUtbQ8vcgU͠^cZEbט#u׉׉ 7cassandra://-ZYxBIhrI9SfpKzLW7sBa7sxY2PRqAkv8KVna8H3LhE`׉ 7cassandra://2WM6RpQbXPVmGuIkevIjECMD9scFnc5I4VXg0TquViceM`s׉ 7cassandra://TLT7VksZdc1ZGmPAs0Gy-NrFk4wQ3NGbS5W3_hfIqS0`׉ 7cassandra://StB43wkRMvJKRKY_1DmCFT6fnSWflip3UWSCFZxUE18͕ ͠^cZEb׉E Cast Irons 145 The eutectoid temperature may be raised or lowered from 727◦C, depending on the particular alloying element. Alloying elements always lower the eutectoid composition; that is, its carbon content is lower than 0.77%. Lowering the eutectoid temperature means increasing the austenite range; as a result, an alloying element such as nickel is known as an austenite former. Because nickel has an fcc structure, it favors the fcc structure of austenite. Conversely, chromium and molybdenum have a bcc structure, thus favoring the bcc structure of ferrite; these elements are known as ferrite stabilizers. 4.6 Cast Irons The term cast iron refers to a family of ferrous alloys composed of iron, carbon (ranging from 2.11% to about 4.5%), and silicon (up to about 3.5%). Cast irons are classified according to their solidification morphology from the eutectic temperature (see also Section 12.3.2): 1. Gray cast iron or gray iron 2. Ductile cast iron, also called nodular cast iron or spheroidal graphite cast iron 3. White cast iron 4. Malleable iron 5. Compacted graphite iron. Cast irons are also classified by their structure: ferritic, pearlitic, quenched and tempered, or austempered. The equilibrium phase diagram relevant to cast irons is shown in Fig. 4.10, in which the right boundary is 100% C—that is, pure graphite. Because the eutectic temperature is 1154◦C, cast irons are completely liquid at temperatures lower than those required for liquid steels; consequently, iron with high carbon content can be cast (see Part II) at lower temperatures than can steels. Cementite is metastable, not completely stable, with an extremely low rate of decomposition. It can, however, be made to decompose into alpha ferrite and graphite. The formation of graphite (graphitization) can be controlled, promoted, and accelerated by modifying the composition and the rate of cooling, and by the addition of silicon. 1600 1400 g 1 liquid 600 800 1000 1200 400 0 1 2 3 4 Composition (%) Figure 4.10: Phase diagram for the iron–carbon system with graphite (instead of cementite) as the stable phase. Note that this figure is an extended version of Fig. 4.6. 5 90 100 Graphite g (austenite) 11548C 2.11% 7388C 0.77% a (ferrite) a 1 graphite 4.30% g 1 graphite Liquid Liquid 1 graphite Temperature (8C) ׉ 7cassandra://CZh1YnKeCCJMR6XqrJ1lm1RZZ4BodqPy3ztEn7YqJzk9`cZ%Eb׉E s146 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment (a) (b) (c) Figure 4.11: Microstructure for cast irons. Magnification: 100×. (a) Ferritic gray iron with graphite flakes. (b) Ferritic ductile iron (nodular iron), with graphite in nodular form. (c) Ferritic malleable iron; this cast iron solidified as white cast iron, with the carbon present as cementite, and was heat treated to graphitize the carbon. Gray Cast Iron. In this structure, graphite exists largely in the form of flakes (Fig. 4.11a). It is called gray cast iron or gray iron, because, when broken, the fracture path is along the graphite flakes, and has a gray, sooty appearance. These flakes act as stress raisers; as a result, gray iron has negligible ductility and is weak in tension, although strong in compression. On the other hand, the presence of graphite flakes gives gray iron the important capacity to dampen vibrations (by internal friction); thus gray cast iron is a suitable and commonly used material for constructing machine-tool bases and machinery structures (Section 25.3). Three types of gray cast iron are ferritic, pearlitic, and martensitic. Because of their different structures, each has different properties and applications. In ferritic gray iron (also known as fully gray iron), the structure consists of graphite flakes in an alpha-ferrite matrix. Pearlitic gray iron has a structure of graphite in a matrix of pearlite, and although still brittle, it is stronger than fully gray iron. Martensitic gray iron is obtained by austenitizing a pearlitic gray iron and then quenching it rapidly to produce a structure of graphite in a martensite matrix; as a result, it is very hard. Ductile (Nodular) Iron. In this structure, graphite is in a nodular or spheroid form (Fig. 4.11b), which permits the material to be somewhat ductile and shock resistant. The shape of the graphite flakes can be modified into nodules (spheres) by small additions of magnesium and/or cerium to the molten metal prior to pouring. Ductile iron can be made ferritic or pearlitic by heat treatment; it can also be heat treated to obtain a structure of tempered martensite (Section 4.7). White Cast Iron. White cast iron is obtained either by cooling gray iron rapidly or by adjusting the composition by keeping the carbon and silicon content low. It is also called white iron because of the white crystalline appearance of the fracture surface. The structure is very hard, wear resistant, and brittle, because of the presence of large amounts of iron carbide, instead of graphite. Malleable Iron. Malleable iron is obtained by annealing white cast iron in an atmosphere of carbon monoxide and carbon dioxide, at between 800◦ and 900◦C, for up to several hours, depending on the size of the part. During this process, the cementite decomposes (dissociates) into iron and graphite. The graphite exists as clusters or rosettes (Fig. 4.11c) in a ferrite or pearlite matrix. Consequently, malleable iron has a structure similar to that of nodular iron, promoting good ductility, strength, and shock resistance—hence, the term malleable (from the Latin malleus meaning it can be hammered). ׉ 7cassandra://TLT7VksZdc1ZGmPAs0Gy-NrFk4wQ3NGbS5W3_hfIqS0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://0rOcf4FZK8MNAQepZ9JbGea6rl6-Qj59wP3ODVYypig{`׉ 7cassandra://BfJdjTZ4g1bOG489hop3H6jySQqr66IfKrEPFK7Sm8s[`s׉ 7cassandra://9HTTEdOAJP8GbxIq8VS7VG6kb1i-U_QguCXoB0VWvs8`׉ 7cassandra://whavoRHkiZgQRzg3SW3I3q59pOxeWvkYV0QB1RkYYDgg͠^cZEbט#u׉׉ 7cassandra://IRHEOAaoWjEINAw6cikgn1U-aJN67iTa3Ia24Rb6hfM`׉ 7cassandra://imOFUashCl4P3BEZ-G6aZ2X_1-RQY_j7pPxincjMgIsO;`s׉ 7cassandra://CqZKHuMlHOSBHs8tLm4Fn4qBKr0ktM-wYAvjcL_gjFw`׉ 7cassandra://ZLkI2xWUw_pdGzmlk_UdOgz1LYr-IqwkritFTdqFFmY͇I6͠^cZEb׉E Heat Treatment of Ferrous Alloys 147 Compacted-graphite Iron. The graphite in this structure is in the form of short, thick, interconnected flakes with undulating surfaces and rounded extremities. The mechanical and physical properties of this cast iron are intermediate between those of flake-graphite and nodular-graphite cast irons. 4.7 Heat Treatment of Ferrous Alloys The microstructures described thus far can be modified by heat-treatment—that is, by controlled heating and cooling of the alloys at various rates. These treatments induce phase transformations, which greatly influence such mechanical properties as strength, hardness, ductility, toughness, and wear resistance. The specific effects of thermal treatment depend on the particular alloy, its composition and microstructure, the degree of prior cold work, and the rates of heating and cooling during heat treatment. This section focuses on the microstructural changes in the iron–carbon system. Because of their technological significance, the structures considered are pearlite, spheroidite, bainite, martensite, and tempered martensite. The heat-treatment processes described are annealing, quenching, and tempering. Pearlite. If the ferrite and cementite lamellae in the pearlite structure of the eutectoid steel, shown in Fig. 4.9, are thin and closely packed, the microstructure is called fine pearlite; if they are thick and widely spaced, it is called coarse pearlite. The difference between the two depends on the rate of cooling through the eutectoid temperature, which is the site of a reaction in which austenite is transformed into pearlite. If the rate of cooling is relatively high (as in air), the structure is fine pearlite; if cooling is slow (as in a furnace), coarse pearlite is produced. Spheroidite. When pearlite is heated to just below the eutectoid temperature and then held at that temperature for a period of time (called subcritical annealing, Section 4.11), such as at 700◦C for a day, the cementite lamellae transform to roughly spherical shapes (Fig. 4.12). Unlike the lamellar shapes of cementite, which act as stress raisers, spheroidites (spherical particles) have smaller stress concentrations because of their rounded shapes. Consequently, this structure has higher toughness and lower hardness than the pearlite structure. It can be cold worked, because the ductile ferrite has high toughness and the spheroidal carbide particles prevent the initiation of cracks within the the structure. Bainite. Visible only through electron microscopy, bainite is a very fine microstructure, consisting of ferrite and cementite, similar to pearlite, but with a different morphology. Bainite can be produced in steels adding alloying elements and at cooling rates higher than those required for pearlite. This structure, called bainitic Figure 4.12: Microstructure of eutectoid steel. Spheroidite is formed by tempering the steel at 700◦C. Magnification: 1000×. ׉ 7cassandra://9HTTEdOAJP8GbxIq8VS7VG6kb1i-U_QguCXoB0VWvs8`cZ%Eb׉E148 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 60 70 50 40 30 20 10 0 0 0.2 0.4 0.6 0.8 Composition (% C) (a) 1.0 (b) Figure 4.13: (a) Hardness of martensite as a function of carbon content. (b) Micrograph of martensite containing 0.8% carbon. The gray platelike regions are martensite; they have the same composition as the original austenite (white regions). Magnification: 1000×. steel (after E.C. Bain, 1891–1971), is generally stronger and more ductile than pearlitic steels at the same hardness level. Martensite. When austenite is cooled at a high rate, such as by quenching in water, its fcc structure is transformed into a body-centered tetragonal (bct) structure. This structure can be described as a body-centered rectangular prism that is slightly elongated along one of its principal axes (see Fig. 4.7d). This microstructure is called martensite (after A. Martens, 1850–1914). Because martensite does not have as many slip systems as a bcc structure, and the carbon is in interstitial positions, it is extremely hard and brittle (Fig. 4.13). Martensite transformation takes place almost instantaneously, because it involves not the diffusion process but a slip mechanism, and thus allowing plastic deformation. This is a time-dependent phenomenon that is the mechanism in other transformations as well. Retained Austenite. If the temperature to which the alloy is quenched is not sufficiently low, only a portion of the structure is transformed to martensite. The rest is retained austenite, which is visible as white areas in the structure, along with the dark, needlelike martensite. Retained austenite can cause dimensional instability and cracking, and lower the hardness and strength of the alloy. 20 30 40 50 60 70 102 Initial hardness of martensite 3008 2008C 4008 6008 10 min 1 hr 103 104 Time (s) Figure 4.14: Hardness of tempered martensite as a function of tempering time for 1080 steel quenched to 65 HRC. Hardness decreases because the carbide particles coalesce and grow in size, thereby increasing the interparticle distance of the softer ferrite. 5008 10 hr 105 Hardness (HRC) Hardness (HRC) Martensite Ferrite 1 pearlite Carbide coalescence ׉ 7cassandra://CqZKHuMlHOSBHs8tLm4Fn4qBKr0ktM-wYAvjcL_gjFw`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://w5Nn0vG07ALMZnfzeVZCJ5wYKKPdnYm2uMrcn6OfGSIs8`׉ 7cassandra://yWzv5hMIAS92wK3U6FrdNKOTFhKGd4ABlybCuh8CQrko`s׉ 7cassandra://6AGKSGDTnPv-u3fxJFA7DDbd5EIIRbndMs1MoJswu5E~`׉ 7cassandra://tyirxNaSbo341-BpnQGZIphZGK2pRVYQQh8uEsQ-bqggD͠^cZEbט#u׉׉ 7cassandra://ZXSS7ul50Fk4eNTr2VQzsH0MLiG3gWYVHdOKR-2Zoy0`׉ 7cassandra://oc78NAGGNQIdXsdxV0Wiv1Jh3f9Ov_ASmc1z0IXqpVYD2`s׉ 7cassandra://zunSpyN6tM-_CvX2jz8PCa9BLGtGhmZTyjN2ik7x3pk2`׉ 7cassandra://gw8bozXvp4daZHDlFGdlHialRC3wp6lS0hYgBtRaaNU͊f͠^cZEbנcZEb.fT9ׁHhttp://band.Noׁ ׁ Ј׉EHardenability of Ferrous Alloys 149 Tempered Martensite. Martensite is tempered to improve its mechanical properties. Tempering is a process by which hardness is reduced and toughness is increased. The body-centered tetragonal martensite is heated to an intermediate temperature, typically 150◦ to 650◦C, where it decomposes to a two-phase microstructure, consisting of body-centered cubic alpha ferrite and small particles of cementite. With increasing tempering time and temperature, the hardness of tempered martensite decreases (Fig. 4.14). The reason is that the cementite particles coalesce and grow, and the distance between the particles in the soft ferrite matrix increases as the less stable and smaller carbide particles dissolve. 4.7.1 Time–Temperature–Transformation Diagrams The percentage of austenite transformed into pearlite as a function of temperature and time (Fig. 4.15a). This transformation is best illustrated by Fig. 4.15b and c in diagrams called isothermal transformation (IT) diagrams,or time–temperature–transformation (TTT) diagrams, constructed from the data given in Fig. 4.15a. The higher the temperature or the longer the time, the more austenite is transformed into pearlite. Note that, for each temperature, there is a minimum time for the transformation to begin; this time period defines the critical cooling rate; with longer times, austenite begins to transform into pearlite, as can be traced in Figs. 4.15b and c. The TTT diagrams shown allow the design of heat treatment schedules to obtain desirable microstructures. Consider, for example, the TTT curves shown in Fig. 4.15c. The steel can be raised to a very high temperature (above the eutectic temperature) to start with a state of austenite. If the steel is cooled rapidly, it can follow the 140◦C/s cooling rate trajectory shown, resulting in complete martensite. On the other hand, it can be more slowly cooled (in a molten salt bath) to develop pearlite- or bainite-containing steels. For tempered martensite, the heat treatment and quench stages will be followed by a tempering process. The differences in hardness and toughness of the various structures developed are shown in Fig. 4.16; note that fine pearlite is harder and less ductile than coarse pearlite. The effects of various percentages of carbon, cementite, and pearlite on other mechanical properties of steels are shown in Fig. 4.17. 4.8 Hardenability of Ferrous Alloys The capability of an alloy to be hardened by heat treatment is called its hardenability, and is a measure of the depth of hardness that can be obtained by heating and subsequent quenching. Note that the term hardenability should not be confused with hardness, which is the resistance of a material to indentation or scratching (Section 2.6). It can be seen that hardenability of ferrous alloys depends on their (a) carbon content, (b) grain size of the austenite, (c) alloying elements present in the material, and (d) the cooling rate. 4.8.1 The End-quench Hardenability Test In this commonly used Jominy test (afterW.E. Jominy, 1893–1976), a round test bar 100mmlong, made from the particular alloy, is austenitized—that is, heated to the proper temperature to form 100% austenite. It is then quenched directly at one end (Fig. 4.18a) with a stream of water at 24◦C. The cooling rate thus varies throughout the length of the bar, the rate being highest at the lower end, being in direct contact with the water. The hardness along the length of the bar is then measured at various distances from the quenched end. As expected from the description of the effects of cooling rates in Section 4.7, hardness decreases away from the quenched end of the bar (Fig. 4.18b). The greater the depth to which hardness increases, the greater is the hardenability of the alloy. Each composition of an alloy has its particular hardenability band.Note that the hardness at the quenched end increases with increasing carbon content, and that 1040, 4140, and 4340 steels have the same carbon content (0.40%), and thus they have the same hardness (57 HRC) at the quenched end. ׉ 7cassandra://6AGKSGDTnPv-u3fxJFA7DDbd5EIIRbndMs1MoJswu5E~`cZ%Eb׉E150 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 100 75 50 25 0 110 Time (s) (a) 100 Transformation temperature 6758C 50 0 Transformation begins 100 110 102 103 104 105 Time (s) Austenite (stable) 700 600 400 500 Austenite (unstable) 50% Completion curve Pearlite Completion curve (~100% pearlite) Begin curve (~0% pearlite) 110 102 103 104 105 Time (s) (b) Eutectoid temperature Transformation ends 50 0 300 400 500 600 700 800 200 100 0 102 0 6008C 650 675 25 50 75 100 103 Eutectoid temperature Austenite pearlite Critical cooling rate Martensite (start) Martensite 1 pearlite Martensite 1021 110 102 Time (s) (c) Figure 4.15: (a) Austenite-to-pearlite transformation of iron–carbon alloy as a function of time and temperature. (b) Isothermal transformation diagram obtained from (a) for a transformation temperature of 675◦C. (c) Microstructures obtained for a eutectoid iron–carbon alloy as a function of cooling rate. 4.8.2 Quenching Media The fluid used for quenching the heated specimen also has an effect on hardenability. Quenching may be carried out in water, brine (salt water), oil, molten salt, or air; caustic solutions, polymer solutions, and gases may also be used. Because of the differences in thermal conductivity, specific heat, and heat of vaporization of these media, the rate of cooling of the specimen (severity of quench) is also different. In relative terms and in decreasing order, the cooling capacities of various quenching media are: agitated brine, 5; still water, 1; still oil, 0.3; cold gas, 0.1; and still air, 0.02. 103 104 105 Pearlite Temperature (8C) Percent of austenite transformed to pearlite Pearlite (%) Austenite (%) Temperature (8C) Austenite (%) 358 s /C 140 C8 / s ׉ 7cassandra://zunSpyN6tM-_CvX2jz8PCa9BLGtGhmZTyjN2ik7x3pk2`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://LW5OQbDnd1-HAc3nmG6QBGemf7UFeyIPD1SzUTmfpQc`׉ 7cassandra://pFqOq2F2ulq78FKKLjdmpki796zKqfp-bbEdkC_xvVY;`s׉ 7cassandra://jVj6AmDP02ZkF9EChc_VTWMDDQ6V71p4aTAvVrBet4E`׉ 7cassandra://lsv1kE3zudIclbCeBOZ2_zr1d7_QmPyYzG7zjOksFBUp͠^cZEbט#u׉׉ 7cassandra://cDvb2JT4MbBt1X3dQ_NXIgUC0bGJrYyYKXa3usvMTLMa`׉ 7cassandra://vbOv84mfVjus65R-b0hBdHIskef4p86d7c7I_9_stKgG1`s׉ 7cassandra://uPAjoBv8c33p1Wgdnj7418gBo6Bn1UIz_IE45tNSEIs`׉ 7cassandra://wMfVGepzZxITovUnIwtgbj1mY7evXBDwPu0jHOY8HFg͔ ͠^cZEb׉EHardenability of Ferrous Alloys 151 400 Ferrite 1 pearlite 200 Spheroidite 0 0 0 0 0.2 3 25 0.4 6 50 0.6 9 (a) Spheroidite 100 50 0 0 0 0 0.2 3 25 0.4 6 50 0.6 9 (c) Figure 4.16: (a) and (b) Hardness and (c) toughness for annealed plain-carbon steels as a function of carbide shape. Carbides in the pearlite are lamellar. Fine pearlite is obtained by increasing the cooling rate. The spheroidite structure has sphere-like carbide particles. Pearlite 1 carbide Ferrite 1 pearlite 160 240 320 80 0 0 120 160 200 40 80 0 0.2 0.4 0.6 0.8 % C 015369 12 097 1.0 % Fe3C 25 50 75 100 % Pearlite (a) 80 50 40 0 0 0 0 0 0.2 3 25 0.4 6 50 (b) Figure 4.17: Mechanical properties of annealed steels as a function of composition and microstructure. Note in (a) the increase in hardness and strength, and in (b), the decrease in ductility and toughness, with increasing amounts of pearlite and iron carbide. 0.6 9 75 0.8 12 100 1.0 15 97 % C % Fe3C % Pearlite Ferrite 1 pearlite 100 Pearlite 1 carbide 0.8 12 75 100 1.0 15 97 % Pearlite % Fe3C % C Ferrite 1 pearlite 0.8 12 75 100 1.0 15 97 % Pearlite % Fe3C % C 0 0 0 0 0.2 3 25 0.4 6 50 0.6 9 (b) 0.8 12 75 100 1.0 15 97 % Pearlite % Fe3C % C 200 Ferrite 1 coarse pearlite 400 Ferrite 1 fine pearlite Hardness (HB) Hardness (HB) J Strength (MPa) Hardness (HB) Ductility (%, 50 mm) J Y i o n e l t i Hardness U d stren a r e g a a l t gth gth o f E l o n i d u c t i o n n mate stre Toughness R e ׉ 7cassandra://jVj6AmDP02ZkF9EChc_VTWMDDQ6V71p4aTAvVrBet4E`cZ%Eb׉E152 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Distance from quenched end 100 mm 25 mm Water at 248C 12.5 mm 12.5 mm orifice (a) 300 600 10 20 30 40 50 60 70 150 Cooling rate at 7008C (8C/s) 55 25 12.5 8 5.5 4.5 3 2.5 10 10 102 3 1 Cooling rate at 7008C (8C/s) Jominy bar 80 mm 20 40 60 0 4340 4140 1060 1020 0 10 20 1040 30 40 Distance from quenched end (b) Figure 4.18: (a) End-quench test and cooling rate. (b) Hardenability curves for five different steels, as obtained from the end-quench test. Small variations in composition can change the shape of these curves. Each curve is actually a band, and its exact determination is important in the heat treatment of metals, for better control of properties. 50 mm Agitation is a significant factor in the rate of cooling; the more vigorous the agitation, the higher is the rate of cooling. In tool steels, the quenching medium is specified by a letter (see Table 5.7), such as W for water hardening, O for oil hardening, and A for air hardening. The cooling rate also depends on the surface-area-to-thickness or surface-area-to-volume ratio of the part; the higher this ratio, the higher is the cooling rate. For example, a thick plate cools more slowly than a thin plate with the same surface area. These considerations are also significant in the cooling of metals and of plastics in casting and in molding processes (see Sections 10.5.1 and 19.3). Water is a common medium for rapid cooling; however, the heated specimen may form a vapor blanket along its surfaces, due to the water-vapor bubbles that form when water boils at the metal–water interfaces. This blanket creates a barrier to heat conduction, because of the lower thermal conductivity of the vapor. Agitating the fluid or the part helps to reduce or eliminate the blanket; also, water may be sprayed onto the part under the high pressure. Brine is an effective quenching medium, because salt helps to nucleate bubbles at the interfaces, which improves agitation; note, however, that brine can corrode the part. Hardness (HRC) ׉ 7cassandra://uPAjoBv8c33p1Wgdnj7418gBo6Bn1UIz_IE45tNSEIs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://27lhAJ30jDq3Ol8sT9bD2AxFdPRtBf5HrtJX4hbQUbAc0`׉ 7cassandra://najdQjCOo-llNBc0_jG-qGZnjr1mayTez4qtgFjXyeI[`s׉ 7cassandra://9RvKEz1bRJ4d9iIjwmi6eHRobRWL61I51bEUi8juw0o `׉ 7cassandra://R5mghz37bNA2UPGwLzj9pMPFjrRJx2zmhRQLHm0AUKkͤ^T͠^cZEbט#u׉׉ 7cassandra://AgQra62fM8-kWHhKXnMeZ52c_BroTxzkSypEowF3aFc8`׉ 7cassandra://PnxU8oQLMzhJMPBMgVbty3eiZn6F9yahXQftwYNOJ6Y^`s׉ 7cassandra://kD-a0LupbdgYhHk5kd7Db0FL617g2Skzg8pj6iAQ9Ik`׉ 7cassandra://Im_PBNCh7fV9qL5xWafzinmtBWiOqTE4u-jBNOPzpaoz>̠͠^cZEbנcZEb9ׁHhttp://hardening.Intׁ ׁ Ј׉E Heat Treatment of Nonferrous Alloys and Stainless Steels 153 Polymer quenchants can be used for ferrous as well as for nonferrous alloys, and have cooling characteristics that generally are between those of water and petroleum oils. Typical polymer quenchants are polyvinyl alcohol, polyalkaline oxide, polyvinyl pyrrolidone, and polyethyl oxazoline. These quenchants have such advantages as better control of hardness, elimination of fumes and fire (as may occur when oils are used as a quenchant), and reduction of corrosion (as may occur when water is used). 4.9 Heat Treatment of Nonferrous Alloys and Stainless Steels Nonferrous alloys and some stainless steels cannot be heat treated by the techniques described for ferrous alloys. The reason is that nonferrous alloys do not undergo phase transformations as do steels. The hardening and strengthening mechanisms for these alloys are therefore fundamentally different. Heat-treatable aluminum alloys, copper alloys, martensitic, and some stainless steels are hardened and strengthened by precipitation hardening. In this process, small particles of a different phase, called precipitates,areuniformly dispersed in the matrix of the original phase (Fig. 4.3a). Precipitates form because the solid solubility of one element (one component of the alloy) in the other is exceeded. Three stages are involved in precipitation hardening, which can best be described by reference to the phase diagram for the aluminum–copper system (Fig. 4.19a). For a composition of 95.5% Al–4.5% Cu, a single-phase (kappa phase) substitutional solid solution of copper (solute) in aluminum (solvent) exists between 500◦ and 570◦C. The kappa phase is aluminum rich, has an fcc structure, and is ductile. Below the lower temperature (that is, below the lower solubility curve) there are two phases: kappa (κ)and theta (θ), which is a hard intermetallic compound of CuAl2. This alloy can be heat treated, and its properties modified by two different methods: solution treatment and precipitation hardening. 4.9.1 Solution Treatment In solution treatment, the alloy is heated to within the solid-solution kappa phase (say, 540◦C), and then cooled rapidly such as by quenching it in water. The structure developed soon after quenching 500 600 700 Liquid a 1 liquid a X X—solid solution XA—quenched, solid solution retained AB—age hardened, precipitation starts (submicroscopic) AC—overaging, precipitate agglomerates 200 a 1 b 20 100 95 90 Aluminum (Al) 0 5 10 Copper (Cu) Composition (% by weight) (a) (b) Figure 4.19: (a) Phase diagram for the aluminum–copper alloy system. (b) Various microstructures obtained during the age-hardening process. AB Time C Temperature (8C) Temperature ׉ 7cassandra://9RvKEz1bRJ4d9iIjwmi6eHRobRWL61I51bEUi8juw0o `cZ%Eb׉E 154 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment (A in Fig. 4.19b) consists only of the single phase kappa; this alloy has moderate strength and considerable ductility. 4.9.2 Precipitation Hardening The structure developed in A in Fig. 4.19b can be made stronger by precipitation hardening.Inthisprocess, the alloy is first reheated to an intermediate temperature, and then held for a period of time, during which precipitation takes place; the copper atoms diffuse to nucleation sites and combine with aluminum atoms. This process develops the theta phase, which forms as submicroscopic precipitates (shown in B by the small dots within the grains of the kappa phase). The resulting structure is stronger than that in A, although it is less ductile; the increase in strength is due to the increased resistance to dislocation movement in the region of the precipitates. Aging. Because the precipitation process is one of time and temperature, it is also called aging, and the property improvement is known as age hardening. If carried out above room temperature, the process is called artificial aging. However, several aluminum alloys harden and become stronger over a period of time and at room temperature; this process is then called natural aging. Such alloys are first quenched and then, if desired, they are shaped by plastic deformation at room temperature; finally, they are allowed to develop strength and hardness by aging naturally. The rate of natural aging can be slowed by refrigerating the quenched alloy (cryogenic treatment). In the precipitation process, if the reheated alloy is held at elevated temperature for an extended period of time, the precipitates begin to coalesce and grow. They become larger, but fewer in number, as shown by the larger dots in C in Fig. 4.19b. This process is called over-aging, whereby the alloy becomes softer and less strong. There is an optimal time–temperature relationship in the aging process that must be observed in order to obtain the desired properties (Fig. 4.20). It is apparent that an aged alloy can be used only up to a certain maximum temperature in service, as otherwise it will over-age and lose some of its strength and hardness, although it will have better dimensional stability. Maraging. This is a precipitation-hardening treatment for a special group of high strength, iron-base alloys. The term maraging is derived from the words martensite age hardening, a process in which one or more intermetallic compounds are precipitated in a matrix of low-carbon martensite. A typical maraging steel may contain 18% Ni, in addition to other elements, and aging takes place at 480◦C. Because hardening by maraging does not depend on the cooling rate; uniform and full hardness can be developed throughout large parts, and with minimal distortion. Typical uses of maraging steels are in dies and tooling for casting, molding, forging, and extrusion (Parts II and III). 100 200 300 400 1508C 1908 230° 0.01 0.1 1.0 10 102 103 104 3458 2608 Aging time (hr) Figure 4.20: The effect of aging time and temperature on the yield stress of 2014-T4 aluminum alloy. Note that, for each temperature, there is an optimal aging time for maximum strength. Yield stress (MPa) ׉ 7cassandra://kD-a0LupbdgYhHk5kd7Db0FL617g2Skzg8pj6iAQ9Ik`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://BST7j1gsi77VRwdsxbONCG0DjmwAG70ieETdOmOaFDw>`׉ 7cassandra://4zNSaBTF72cTRVzpbrpDKl9GwSHfTkboIr2jg-Dy8x4]%`s׉ 7cassandra://ttS20bdxoqhmRIPvp8OuXB17wppqPC1Mfw5T_u0CDxk`׉ 7cassandra://Rdwi7iM8O-W0fanFOpcrqUqMnqVhAw7TJ_nzhMO9uUMS͠^cZEbט#u׉׉ 7cassandra://Hlhgcm8pKHPwVOmJaUPH9HNBDPX5ARdIVtUghYt1Gk8`׉ 7cassandra://K1BXeycHuGrpGCRVLJFr-9L8cNLHbgURQMpdorm5HTYF`s׉ 7cassandra://8kpw7FRKV8Ax_HoNgo9_RS1GaNYt655bMrMR53BIIaE`׉ 7cassandra://dLH42u_EXTflDmGz1OzVwSDnbqepAjPrKp8l9OMODnc[̜͠^cZEb׉E Annealing 155 4.10 Case Hardening The heat-treatment processes described thus far involve microstructural alterations and property changes in the bulk of the component by means of through hardening. It is not desirable to through harden parts, because a hard part lacks the required toughness for some applications. For example, a small surface crack could propagate rapidly through a part and cause sudden and total failure. In many cases, modification of only the surface properties of a part is desirable (hence, the term surface or case hardening). This widely used method is particularly useful for improving resistance to surface indentation, fatigue, and wear; typical applications are gear teeth, cams, shafts, bearings, fasteners, pins, automotive clutch plates, tools, and dies. Several case-hardening processes are available (Table 4.1): 1. Carburizing (gas, liquid, and pack carburizing) 2. Carbonitriding 3. Cyaniding 4. Nitriding 5. Boronizing 6. Flame hardening 7. Induction hardening 8. Laser-beam hardening. Basically, these are operations where the component is heated in an atmosphere containing such elements as carbon, nitrogen, or boron, which modify the composition, microstructure, and properties of surfaces. For steels with sufficiently high carbon content, surface hardening takes place without the use of any of these additional elements. Only the heat-treatment processes described in Section 4.7 are required to modify the microstructures, usually by either flame hardening or induction hardening, as outlined in Table 4.1. Laser beams and electron beams (Sections 27.6 and 27.7) are used effectively to harden small and as well as large surfaces, such as gears, valves, punches, and engine cylinders. The depth of the case-hardened layer is typically less than 2.5 mm; these methods are also used for through hardening of relatively small parts. The main advantages of laser surface hardening are close control of power input, low part distortion, and the ability to reach areas that would otherwise be inaccessible by other means. Because case hardening involves a localized surface layer, the parts have a hardness gradient. Typically, the hardness is a maximum at the surface and decreases inward, the rate of decrease depending on the composition and physical properties of the metal and processing variables. Surface-hardening techniques can also be used for tempering (Section 4.11), for modifying the properties of surfaces that have been subjected to heat treatment. Several other processes and techniques for surface hardening, such as shot peening and surface rolling to improve wear resistance and other characteristics, are described in Section 34.2. Decarburization is the phenomenon in which alloys lose carbon from their surfaces, as a result of heat treatment or of hot working in a medium, usually oxygen, which reacts with the carbon. Decarburization is undesirable because it affects the hardenability of surfaces, by lowering their carbon content; it also adversely affects the hardness, strength, and fatigue life of steels, significantly lowering their endurance limit. 4.11 Annealing Annealing is a general term used to describe the restoration of a cold-worked or heat-treated alloy to its original properties–for instance, to increase ductility (and hence formability) and reduce hardness and ׉ 7cassandra://ttS20bdxoqhmRIPvp8OuXB17wppqPC1Mfw5T_u0CDxk`cZ%Eb׉E R156 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Table 4.1: Outline of Heat-treatment Processes for Surface Hardening Element Metals Process hardened Carburizing Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C) added to surface General Procedure C Heat steel at 870◦–950◦C in an atmosphere of carbonaceous gases (gas carburizing) or carbon-containing solids (pack carburizing). Then quench. Carbonitriding Low-carbon steel Cand N Heat steel at 700◦–800◦C in an atmosphere of carbonaceous gas and ammonia. Then quench in oil. Cyaniding Low-carbon steel (0.2% C), alloy steels (0.08–0.2% C) Nitriding Steels (1% Al, 1.5% Cr, 0.3% Mo), alloy steels (Cr, Mo), stainless steels, high-speed tool steels Boronizing Steels Flame hardening Induction hardening Mediumcarbon steels, cast irons C and N Heat steel at 760◦–845◦C in a molten bath of solutions of cyanide (e.g., 30% sodium cyanide) and other salts. N Heat steel at 500◦–600◦C in an atmosphere of ammonia gas or mixtures of molten cyanide salts. No further treatment. B None Part is heated using boron-containing gas or solid in contact with part. Surface is heated with an oxyacetylene torch, then quenched with water spray or other quenching methods. Same as above None Metal part is placed in copper induction coils andisheatedbyhigh frequency current, then quenched. characteristics A hard, high-carbon surface is produced. Hardness55to65HRC. Case depth < 0.5 to 1.5 mm. Some distortion of part during heat treatment. Surface hardness 55 to 62 HRC. Case depth 0.07 to 0.5 mm. Less distortion than in carburizing. Surface hardness up to 65 HRC. Case depth 0.025 to 0.25 mm. Some distortion. Surface hardness up to 1100 HV. Case depth 0.1 to 0.6 mm and 0.02 to 0.07 mm for high speed steel. Extremely hard and wear resistant surface. Case depth 0.025 to 0.075 mm. Surface hardness 50 to 60 HRC. Case depth 0.7 to 6 mm. Little distortion. Same as above Gear and sprocket teeth, axles, crankshafts, piston rods, lathe beds and centers Same as above Typical applications Gears, cams, shafts, bearings, piston pins, sprockets, clutch plates Bolts, nuts, gears Bolts, nuts, screws, small gears Gears, shafts, sprockets, valves, cutters, boring bars, fuel-injection pump parts Tool and die steels strength, or to modify the microstructure of an alloy. The annealing process is also used to relieve residual stresses in a manufactured part, as well as to improve machinability and dimensional stability. The term annealing also applies to the thermal treatment of glass and similar products (Section 18.4), and for castings and weldments. The annealing process typically consists of the following steps: 1. Heating the workpiece to a specific temperature range in a furnace 2. Holding it at that temperature for a period of time (soaking) 3. Cooling the workpiece, in air or in a furnace ׉ 7cassandra://8kpw7FRKV8Ax_HoNgo9_RS1GaNYt655bMrMR53BIIaE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://FkBKbDk88I2qUoG7D4omy9Y8V5-PKcCQqewixFN333M=`׉ 7cassandra://e6YFpKWzgVDmJoU3a3WExnf0L5d3rnJIRwS5Cbewyr4``s׉ 7cassandra://r-VmEmsu1Z_Fo8RcmdHLGw_BAbgt1HSFho5iLNTw1X0)`׉ 7cassandra://2saLZ3k4-MmMPZIRMOZG5dHVlkmq7OBpoLGcqCex9s8}'͠^cZEbט#u׉׉ 7cassandra://5HKQnvVErArywUf7ENKNFGlQJSVzkiSvruNa56zKnZI[`׉ 7cassandra://EPLNdYWKcsGY5vp7_l3phb02vG_N6-T1xfKR5mLDYIcF `s׉ 7cassandra://__AHsSaqTIRmnzxCZlARFLG4wvXO5g334nSvuD1m8XQ`׉ 7cassandra://5xl2j-6vw1R3HQx9ciTkO3SmXPHVwCOOib0lisEASjY{͠^cZEb׉E Annealing 157 600 700 800 900 1000 Normalizing 7388C A3 A1 annealing Acm Full Spheroidizing 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Composition (% C) Figure 4.21: Heat-treating temperature ranges for plain-carbon steels, as indicated on the iron–iron carbide phase diagram. Annealing may be carried out in an inert or a controlled atmosphere, or at lower temperatures to minimize or prevent surface oxidation. The annealing temperature may be higher than the metal’s recrystallization temperature, depending on the degree of cold work. For example, the recrystallization temperature for copper ranges between 200◦ and 300◦C, whereas the temperature required to fully recover the original properties ranges from 260◦ to 650◦C, depending on the degree of prior cold work (see also Section 1.7). Full annealing is a term applied to annealing of ferrous alloys. The steel is heated to above A1 or A3 (Fig. 4.21), then cooling takes place slowly [typically at 10◦C per hour], in a furnace, after which it is turned off. The structure developed through full annealing is coarse pearlite, which is soft and ductile and has small, uniform grains. To avoid excessive softness, the cooling cycle may be done completely in still air. In normalizing,the part is heated to a temperature above A3 or Acm in order to transform the structure to austenite. The process results in somewhat higher strength and hardness, and lower ductility than in full annealing (Fig. 4.22). The structure developed is fine pearlite, with small, uniform grains. Normalizing is generally carried out to refine the grain structure, obtain uniform structure (homogenization), decrease residual stresses, and improve machinability. The structure of spheroidites and the procedure for obtaining them are described in Section 4.7 and shown in Figs. 4.12 and 4.21. Spheroidizing annealing improves the cold workability (Section 14.5) and the machinability of steels (Section 21.7). Stress-relief Annealing. To reduce or eliminate residual stresses, the part is typically subjected to stressrelief annealing or stress relieving. The temperature and the time required for this process depend on the material and on the magnitude of the residual stresses present. Residual stresses may have been induced during forming, machining, or other shaping processes, or they may have been caused by volume changes during phase transformations. Tempering. If steels are hardened by heat treatment, tempering or drawing is used in order to reduce brittleness, increase ductility and toughness, and reduce residual stresses. The term tempering is also used for glasses (Section 18.4). In tempering, the steel is heated to a specific temperature, depending on its composition, and then cooled at a prescribed rate. The results of tempering for an oil-quenched AISI 4340 steel are shown in Fig. 4.23. Alloy steels may undergo temper embrittlement, which is caused by the segregation of impurities along grain boundaries, at temperatures between 480◦ and 590◦C. Austempering. In this process, the heated steel is rapidly quenched from the austenitizing temperature, to avoid formation of ferrite or pearlite. The part is held at a certain specific temperature until isothermal Temperature (8C) ׉ 7cassandra://r-VmEmsu1Z_Fo8RcmdHLGw_BAbgt1HSFho5iLNTw1X0)`cZ%Eb׉E158 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment 200 300 400 500 600 700 800 100 0 0 Quenched 70 60 50 40 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Carbon content (%) Figure 4.22: Hardness of steels in the quenched and normalized conditions as a function of carbon content. 800 1000 1200 1400 1600 1800 Tensile strength Yield stress Reduction of area 200 300 400 500 600 Tempering temperature (8C) Figure 4.23: Mechanical properties of oil-quenched 4340 steel as a function of tempering temperature. transformation from austenite to bainite is complete. It is then cooled to room temperature, usually in still air and at a moderate rate, in order to avoid thermal gradients within the part. The quenching medium most commonly used is molten salt, at temperatures ranging from 160◦ to 750◦C. Austempering is often substituted for conventional quenching and tempering, either to reduce the tendency for cracking and distortion during quenching or to improve ductility and toughness while maintaining hardness. Because of the shorter cycle time involved, this process is economical. Martempering (Marquenching). In martempering, steel or cast iron is first quenched from the austenitizing temperature in a hot-fluid medium, such as hot oil or molten salt. It is then held at that temperature until the temperature is uniform throughout the part. It is cooled at a moderate rate, such as in air in order to avoid excessive temperature gradients within the part. The part is subsequently tempered, because the structure developed is otherwise primarily untempered martensite, thus not suitable for most applications. Martempered steels have lower tendency to crack, distort, or develop residual stresses during heat treatment. In modified martempering, the quenching temperature is lower, and the cooling rate is higher; the process is suitable for steels with lower hardenability. 60 50 40 30 Hardness (HB) Strength (MPa) Reduction of area (%) HRC Normalized ׉ 7cassandra://__AHsSaqTIRmnzxCZlARFLG4wvXO5g334nSvuD1m8XQ`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://PTZgMwY5jh0CWjfwqaicFQL7AtCnhJvN5XVuyDx8FC8`׉ 7cassandra://FmpEAiPILIeVpsK_3evuvLXYheSsZctQ6KtdyWfMesEjp`s׉ 7cassandra://0K9hjSuCPT7HcRSTNG49iCGcNMREJrVIfJ0Vik3V7gA@`׉ 7cassandra://fsewX_QwMoWqhLg39g4hh2XSvzX9N31Rricm4wRfZ9g\u͠^cZEbט#u׉׉ 7cassandra://BqeKTVPfs22tsntFL49GRtGJ7qT3T3VIqJtO-7qLRh8.`׉ 7cassandra://t8dti145k3TbrQpuHsQ4gISQILXdYIQDey51tqL0J2YX`s׉ 7cassandra://2NpG0ZBVw1qHCBbmtfhXwz4h9dDZCJjDx3D1MLm_Mp4`׉ 7cassandra://e-IJtjyUycgQq4ls-w1EzDJcTGv3urbCFvkIfPmk4GUE͠^cZEb׉E Heat Treating Furnaces and Equipment 159 Ausforming. In this process, also called thermomechanical processing, the steel is formed into desired shapes within controlled ranges of temperature and time to avoid the formation of nonmartensitic transformation products. The part is then cooled at various rates to develop the desired microstructures. 4.12 Heat Treating Furnaces and Equipment Two basic types of furnaces are used for heat treating: batch furnaces and continuous furnaces. Because they consume much energy, their insulation and efficiency are important design considerations, as are their initial cost, the personnel needed for their operation and for maintenance, and their safe use. Uniform temperature and accurate control of temperature–time cycles are important. Modern furnaces are equipped with various electronic controls, including computer-controlled systems, programmed to run through a complete heat-treating cycle repeatedly and with reproducible accuracy. The fuels used are usually natural gas, oil, or electricity (for resistance or induction heating); the type of fuel affects the furnaces atmosphere: Unlike electric heating, gas or oil introduces combustion products into the furnace (a disadvantage). Electrical heating, however, has a slower start-up time and is more difficult to adjust and control. Batch Furnaces. In a batch furnace, the parts to be heat treated are loaded into and unloaded from the furnace in individual batches. The furnace basically consists of an insulated chamber, a heating system, and an access door or doors. Batch furnaces are of the following basic types: 1. A box furnace is a horizontal rectangular chamber with one or two access doors through which parts are loaded. 2. A pit furnace is a vertical pit below ground level into which the parts are lowered. 3. A bell furnace is a round or rectangular box furnace without a bottom, and is lowered over stacked parts that are to be heat treated. This type of furnace is particularly suitable for coils of wire, rods, and sheet metal. 4. In an elevator furnace, the parts are loaded onto a car platform, rolled into position, and then raised into the furnace. Continuous Furnaces. In this type of furnace, the parts to be heat treated move continuously through the furnace on conveyors of various designs. Salt-bath Furnaces. Because of their high heating rates and better control of uniformity of temperature, salt baths are commonly used in various heat-treating operations, particularly for nonferrous strip or wire. Heating rates are high because of the higher thermal conductivity of liquid salts compared with that of air or gases. Fluidized Beds. Dry, fine, and loose solid particles, usually aluminum oxide, are heated and suspended in a chamber by the upward flow of hot gas at various speeds. The parts to be heat treated are then placed within the floating particles, hence the term fluidized bed. Induction Heating. In this method, the part is heated rapidly by the electromagnetic field generated by an induction coil carrying alternating current, which induces eddy currents in the part. The coil, which can be shaped to fit the contour of the part to be heat treated (Fig. 4.24), an important consideration, is made of copper or a copper-base alloy. The coil, which is usually water cooled, may be designed to quench the part as well. Furnace Atmospheres. The atmospheres in furnaces can be controlled so as to avoid oxidation, tarnishing, and decarburization of ferrous alloys heated to elevated temperatures. Oxygen causes corrosion, rusting, ׉ 7cassandra://0K9hjSuCPT7HcRSTNG49iCGcNMREJrVIfJ0Vik3V7gA@`cZ%Eb׉E 160 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Cooling water Induction coils Shaped coil Travel Part being heated and quenched (a) (b) (c) Figure 4.24: Types of coils used in induction heating of various surfaces of parts. and scaling. Carbon dioxide, which has various effects, may be neutral or decarburizing (see Fig. 30.1), depending on its concentration in the furnace atmosphere. Nitrogen is a common neutral atmosphere, and a vacuum provides a completely neutral atmosphere. Water vapor in the furnace causes oxidation, resulting in a blue color. The term bluing is used to describe formation of a thin, blue film of oxide on finished parts, for the purpose of improving their appearance and their resistance to oxidation. 4.13 Design Considerations for Heat Treating In addition to metallurgical factors, successful heat treating involves several design considerations for avoiding problems such as cracking, distortion, and nonuniformity of properties throughout and among heat-treated parts. The rate of cooling during quenching may not be uniform, particularly in parts of complex shapes with varying cross-sections and thicknesses; the nonuniformity may produce severe temperature gradients in the part. Lack of uniformity can lead to variations in part contraction, resulting in thermal stresses that may cause warping or cracking. Nonuniform cooling also causes residual stresses in the part, which then can lead to stress-corrosion cracking (Section 2.10.2). The quenching method selected, the care taken during the process, and the selection of a proper quenching medium and temperature are important considerations. As a general guideline for part design for heat treating: (a) Sharp internal or external corners should be avoided, as otherwise stress concentrations at these corners may raise the level of stresses high enough to cause cracking. (b) The part should have its thicknesses as nearly uniform as possible. (c) The transition between regions of different thicknesses should be made smooth. (d) Parts with holes, grooves, keyways, splines, and asymmetrical shapes may be difficult to heat treat, because they may crack during quenching. (e) Large surfaces with thin cross-sections are likely to warp. (f) Hot forgings and hot steel-mill products may have a decarburized skin (a layer that has lost its carbon, Section 4.10) that may not respond successfully to heat treatment. Summary • Commercially pure metals generally do not have sufficient strength for most engineering applications; consequently, they are alloyed with various elements to alter their structures and properties. Important concepts in alloying are the solubility of alloying elements in a host metal and the phases present at various ranges of temperature and composition. Slideway being surface hardened ׉ 7cassandra://2NpG0ZBVw1qHCBbmtfhXwz4h9dDZCJjDx3D1MLm_Mp4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://2qrBJPJ1H9KycsgmtftZLZN2cEbSdCmKPTfTXb3kK9Uq`׉ 7cassandra://ZY59WG2ypa_ixYH9llcaUQYHbGn1wZnEewlVyNPwyTAN`s׉ 7cassandra://aCOnkNY1GaNisbS5i_SloznxQM1vA49pGw5GkTIVmzs`׉ 7cassandra://e0kH9gOWhT1eBXYajajpz_RI3j-s0sx_43cD3y9JZdM?͠^cZEbט#u׉׉ 7cassandra://3GHzIG3EI-GBsSGIzT4KGeIxhUtIh4i1Dyd5Tn1jMF8`׉ 7cassandra://g4zdGssm89vh7lSWU3ykOrb8Tc3bMV8ce3My4T3CJsgU{`s׉ 7cassandra://9r8fmygBxMF7ySTwrBG9LJwLuamMNPQoi2GIHpdMM_8`׉ 7cassandra://pJ7aSvPcrkD8mcnhMrNc9pp7-PtRQUhjf2H2w0ThMo4]͠^cZEb׉E Key Terms 161 • Alloys basically have two forms: solid solutions and intermetallic compounds; solid solutions may be substitutional or interstitial. There are certain conditions pertaining to the crystal structure and atomic radii that have to be met to develop these structures. • Phase diagrams show the relationships among the temperature, composition, and phases present in a particular alloy system. As temperature is decreased at various rates, correspondingly various transformations take place, resulting in microstructures that have widely different characteristics and properties. • Among binary systems, the most important is the iron–carbon system, which includes a wide range of steels and cast irons. Important components in this system are ferrite, austenite, and cementite. The basic types of cast irons are gray iron, ductile (nodular) iron, white iron, malleable iron, and compacted-graphite iron. • The mechanisms for hardening and strengthening metal alloys basically involve heating the alloy and subsequently quenching it at varying cooling rates. As a result, important phase transformations take place, producing various structures such as pearlite (fine or coarse), spheroidite, bainite, and martensite. Heat treating of nonferrous alloys and stainless steels involves solution treatment and precipitation hardening. • Furnace atmosphere, the quenchants used, the control and characteristics of the equipment, and the shape of the parts to be heat treated are important considerations. • Hardenability is the capability of an alloy to be hardened by heat treatment. The Jominy end-quench test is a method commonly used to determine hardenability bands for alloys. • Case hardening is an important process for improving the wear and fatigue resistance of parts. Several methods are available, such as carburizing, nitriding, induction hardening, and laser hardening. • Annealing includes normalizing, process annealing, stress relieving, tempering, austempering, and martempering, each with the purpose of enhancing the ductility and toughness of heat-treated parts. Key Terms Age hardening Aging Alloy Annealing Austenite Austempering Bainite Case hardening Cast iron Cementite Curie temperature Decarburization End-quench test Equilibrium diagram Eutectic Eutectoid reaction Ferrite Furnaces Hardenability Heat treatment Intermetallic compounds Iron–Carbon system Jominy test Maraging Martempering Martensite Normalizing Pearlite ׉ 7cassandra://aCOnkNY1GaNisbS5i_SloznxQM1vA49pGw5GkTIVmzs`cZ%Eb׉E 162 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Phase diagram Phase transformations Precipitation hardening Pure metals Retained austenite Solid solution Solute Solution treatment Bibliography ASM Handbook, Vol. 3: Alloy Phase Diagrams, ASM International, 2016. ASM Handbook, Vol. 4A: Steel Heat Treating Fundamentals and Processes, ASM International, 2013. —, Vol. 4B: Steel Heat Treating Technologies, ASM International, 2014. —, Vol. 4C: Induction Heating and Heat Treatment, ASM International, 2014. —, Vol. 4D: Heat Treating of Irons and Steels, ASM International, 2014. —, Vol. 4E: Heat Treating of Nonferrous Alloys, ASM International, 2016. —, Vol. 9: Metallography and Microstructures, ASM International, 2004. Brandt, D.A., and Warner, J.C., Metallurgy Fundamentals, 5th ed., Goodheart-Wilcox, 2009. Bryson, W.E., Heat Treatment, Selection, and Application of Tool Steels, 2nd ed., Hanser Gardner, 2005. Callister, W.D., Jr., and Rethwisch, D.G., Materials Science and Engineering, 9th ed., Wiley, 2013. Campbell, F,C., (ed.), Phase Diagrams: Understanding the Basics, ASM International, 2012. Hosford, W.F., Physical Metallurgy, 2nd ed., Taylor & Francis, 2010. Krauss, G., Steels: Processing, Structure, and Performance, ASM International, 2005. Shackelford, J. F., Introduction to Materials Science for Engineers, 8th ed., Macmillan, 2014. Review Questions 4.1. Describe the difference between a solute and a solvent. 4.2. What is a solid solution? 4.3. What are the conditions for obtaining (a) substitutional and (b) interstitial solid solutions? 4.4. Describe the difference between a single-phase and a two-phase system. 4.5. What is an induction heater? What kind of part shapes can be heated by induction heating? 4.6. Describe the major features of a phase diagram. 4.7. What do the terms “equilibrium” and “constitutional,” as applied to phase diagrams, indicate? 4.8. What is the difference between “eutectic” and “eutectoid”? 4.9. What is tempering? Why is it performed? 4.10. Explain what is meant by “severity of quenching.” 4.11. What are precipitates? Why are they significant in precipitation hardening? 4.12. What is the difference between natural and artificial aging? 4.13. Describe the characteristics of ferrite, austenite, and cementite. 4.14. What is the purpose of annealing? 4.15. What is a Time–Temperature–Transformation diagram? How is it used? Solvent Spheroidite Stress relieving Tempered martensite Tempering Time–Temperature–transformation diagrams Two-phase systems ׉ 7cassandra://9r8fmygBxMF7ySTwrBG9LJwLuamMNPQoi2GIHpdMM_8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://a6HajkFLif-v06G7gBjyQbn-c6MVfPNRAyBhRVAymyU`׉ 7cassandra://Wlw9Tf8SfoliE9qDfmmGWspiYOABIVlhXlDlOsDHG6QjC`s׉ 7cassandra://QpEZtZMkBK443rbTGKmYgd7fj6O5Q85YYD0_VeBx1mc1`׉ 7cassandra://qGxoXmQv_T9wjgNSZWJs2rcG4B5YdXsKh3wFeOVMSqwL͠^cZEbט#u׉׉ 7cassandra://zXB0H-y_ffElmGH5vQPWbhUOU1IIacUQ7Zh9E-1eWnwF`׉ 7cassandra://VjI-r8yw7D9IfpNCCK3hE8VriLCoG_RhYi0LUggIb_ME`s׉ 7cassandra://reQWj5UY9YRr4NOeOdjhoRH1ccVmrneuQOR8JXSk12Ic`׉ 7cassandra://znxkwDAPqVdvErovBInKm3XdXPW-1z70gxP0qdfa13QG͠^cZEb׉E Quantitative Problems 163 Qualitative Problems 4.16. You may have seen some technical literature on products stating that certain parts in those products are “heat treated.” Describe briefly your understanding of this term and why the manufacturer includes it. 4.17. Describe the engineering significance of the existence of a eutectic point in phase diagrams. 4.18. What is the difference between hardness and hardenability? 4.19. Referring to Table 4.1, explain why the items listed under typical applications are suitable for surface hardening. 4.20. It generally is not desirable to use steels in their as-quenched condition. Explain why. 4.21. Describe the differences between case hardening and through hardening, insofar as engineering applications of metals are concerned. 4.22. Describe the characteristics of (a) an alloy, (b) pearlite, (c) austenite, (d) martensite, and (e) cementite. 4.23. Explain why carbon, among all elements, is so effective in imparting strength to iron in the form of steel. 4.24. How does the shape of graphite in cast iron affect its properties? 4.25. In Section 4.8.2, several fluids are listed in terms of their cooling capacity in quenching. Which physical properties of these fluids influence their cooling capacity? 4.26. Why is it important to know the characteristics of heat-treating furnaces? Explain. 4.27. Explain why, in the abscissa of Fig. 4.16c, the percentage of pearlite begins to decrease after 0.8% carbon content is reached. 4.28. What is the significance of decarburization? Give some examples. 4.29. Explain your understanding of size distortion and shape distortion in heat-treated parts, and describe their causes. 4.30. Comment on your observations regarding Fig. 4.18b. 4.31. In Fig. 4.1, the hardened surface at the tip of the gear teeth is much higher than at the root. Explain why. 4.32. List the methods by which (a) steel and (b) aluminum can be hardened. Indicate the methods that are common to both materials. Quantitative Problems 4.33. Design a heat-treating cycle for carbon steel, including temperature and exposure times, to produce (a) pearlite–martensite steels and (b) bainite–martensite steels. 4.34. Using Fig. 4.4, estimate the following quantities for a 75% Cu–25% Ni alloy: (a) the liquidus temperature, (b) the solidus temperature, (c) the percentage of nickel in the liquid at 1150◦C, (d) the major phase at 1150◦C, and (e) the ratio of solid to liquid at 1150◦C. 4.35. Extrapolating the curves in Fig. 4.14, estimate the time that it would take for 1080 steel to soften to 40 HRC at (a) 300◦C and (b) 400◦C. 4.36. A typical steel for tubing is AISI 1040, and one for music wire is 1085. Considering their applications, explain the reason for the difference in carbon content. ׉ 7cassandra://QpEZtZMkBK443rbTGKmYgd7fj6O5Q85YYD0_VeBx1mc1`cZ%Eb׉E l164 Chapter 4 Metal Alloys: Their Structure and Strengthening by Heat Treatment Synthesis, Design, and Projects 4.37. It was stated in this chapter that, in parts design, sharp corners should be avoided in order to reduce the tendency toward cracking during heat treatment. If it is essential for a part to have sharp corners for functional purposes, and it still requires heat treatment, what method would you recommend for manufacturing this part? 4.38. The heat-treatment processes for surface hardening are summarized in Table 4.1. Each of these processes involves different equipment, procedures, and cycle times; as a result, each incurs different costs. Review the available literature, contact various companies, and then make a similar table outlining the costs involved in each process. 4.39. It can be seen that, as a result of heat treatment, parts can undergo size distortion and shape distortion to various degrees. By referring to the Bibliography at the end of this chapter, make a survey of the technical literature, and report quantitative data regarding the distortions of parts having different shapes. 4.40. Figure 4.18b shows hardness distributions in end-quench tests, as measured along the length of the round bar. Make a simple qualitative sketch showing the hardness distribution across the diameter of the bar. Would the shape of the curve depend on the bar’s carbon content? Explain. 4.41. Throughout this chapter, you have seen specific examples of the importance and the benefits of heat treating parts or certain regions of parts. Refer to the Bibliography in this chapter, make a survey of the heat-treating literature, and then compile several examples and illustrations of parts that have been heat treated. 4.42. Refer to Fig. 4.24, think of a variety of other part shapes to be heat treated, and design coils that are appropriate for these shapes. Describe how your designs would change if the parts have varying shapes along their length (such as from a square at one end to a round shape at the other end). 4.43. Inspect various parts in your car or home, and identify those that are likely to have been case hardened. Explain your reasons. 4.44. A vendor provides you heat-treated steel parts, but tension test data suggests the strength is not as high as desired. List investigations you can perform to determine if the low strength is caused by improper heat treating. ׉ 7cassandra://reQWj5UY9YRr4NOeOdjhoRH1ccVmrneuQOR8JXSk12Ic`cZ%EbÁcZ%Eb#בCט#u׉׉ 7cassandra://gNykdxvoN2p7nagFls93duJ330kSkmfiuClu3ghee5I`׉ 7cassandra://4xz_c933ipyD1_E6L1XCd9sWGd5VXkFrLtT0AHsPpEk@{`s׉ 7cassandra://9QaCXcY1bVPi_SyMRDT0mT1o2P-27D-cDyK-MkAUhDk`׉ 7cassandra://yrWu0G-HStiFZrGOqE8kpvN8FwqvsPaukHBpfICqHFYB<͠^cZEbט#u׉׉ 7cassandra://qtRi8ZICGzt3_CFaq2bogG9VCu0n0YBRIFlOSaYiPTE z`׉ 7cassandra://ZplY-zZcE1J8wmIw3TrETtMTsl8RzgDqFEDibtpCt98j`s׉ 7cassandra://unhBjkKjx1z6y-vzmMvcesPa26BGqyQuJmwi87XK5fAE`׉ 7cassandra://bWeMtfCmIrhGBq4mH-hP4E4-tDzC27p5a_vhHZmebBIj͠^cZEb׉EChapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications 5.1 Introduction 165 5.2 Production of Iron and Steel 166 5.3 Casting of Ingots 169 5.4 Continuous Casting 170 5.5 Carbon and Alloy Steels 172 5.6 Stainless Steels 179 5.7 Tool and Die Steels 181 Case Studies: 5.1 Advanced High-strength Steels in Automobiles 179 5.2 Stainless Steels in Automobiles 181 • Ferrous metals and alloys are the most widely used structural materials, generally because of their properties and performance, ease of manufacturing, and low cost. • The chapter opens with a brief outline of iron and steel production from ore, including descriptions of furnaces and foundry operations. • The casting of ingots is described, followed by continuous casting operations that are now into wide use for producing high-quality metals in large volumes. • The chapter then discusses in detail the properties and applications of ferrous metals, including cast irons, carbon and alloy steels, high strength steels, stainless steels, and tool and die steels. 5.1 Introduction By virtue of their relatively low cost and wide range of mechanical, physical, and chemical properties, ferrous metals and alloys are among the most useful of all metals. They contain iron as their base metal and are generally classified as carbon and alloy steels, stainless steels, tool and die steels, cast irons,and cast 165 ׉ 7cassandra://9QaCXcY1bVPi_SyMRDT0mT1o2P-27D-cDyK-MkAUhDk`cZ%Eb׉EB166 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications steels. Steel refers to a ferrous alloy, which can be as simple as a mixture of iron and carbon, but also often containing a number of alloying elements to impart various properties. Ferrous alloys are produced as • Sheet steel for automobiles, appliances, and containers • Plates for boilers, ships, and bridges • Structural members such as I-beams, bar products, axles, crankshafts, and railroad rails • Tools, dies, and molds • Rods and wire for fasteners such as bolts, rivets, nuts, and staples. Carbon steels are the least expensive of all structural metals. As an example of their widespread use, ferrous metals make up 70–85% by weight of structural members and mechanical components. The average U.S. passenger vehicle (including trucks and sport utility vehicles) contains about 1000 kg of steel, accounting for about 60% of its total weight. The use of iron and steel as structural materials has been one of the most important technological developments. Primitive ferrous tools, which first appeared about 4000 to 3000 B.C., were made from meteoritic iron, obtained from meteorites that had struck the earth. True ironworking began in Asia Minor in about 1100 B.C. and signaled the advent of the Iron Age. Invention of the blast furnace in about 1340 A.D. made possible the production of large quantities of high-quality iron and steel. (See Table I.2.) 5.2 Production of Iron and Steel 5.2.1 Raw Materials The three basic materials used in iron- and steelmaking are iron ore, limestone,and coke. Although it does not occur in a free state in nature, iron is one of the most abundant elements (in the form of various ores) in the world, making up about 5% of the earth’s crust. The principal iron ores are taconite (a black flintlike rock), hematite (an iron-oxide mineral), and limonite (an iron oxide containing water). After it is mined, the ore is crushed into fine particles, the impurities are removed (by various means, such as magnetic separation), and the ore is formed into pellets, balls, or briquettes, using water and various binders. Typically, pellets are about 65% pure iron and about 25 mm in diameter. The concentrated iron ore is referred to as beneficiated (as are other concentrated ores). Some iron-rich ores are used directly, without pelletizing. Coke is obtained from special grades of bituminous coal (a soft coal rich in volatile hydrocarbons and tars) that are heated in vertical ovens, to temperatures of up to 1150◦C, and then cooled with water in quenching towers. Coke has several functions in steelmaking, including (a) generating the high level of heat required for the chemical reactions in ironmaking to take place and (b) producing carbon monoxide (a reducing gas, meaning that it removes oxygen, thus reducing iron oxide to iron. The chemical by-products of coke are used in the synthesis of plastics and of chemical compounds. The function of limestone (calcium carbonate) is to remove impurities from molten iron. The limestone reacts chemically with impurities, acting like a flux (meaning to flow as a fluid) that causes the impurities to melt at a low temperature. The limestone combines with the impurities and forms a slag (which, being light, floats over the molten metal, and, subsequently, is removed). Dolomite (an ore of calcium magnesium carbonate) also is used as a flux. The slag is later used in making cement, fertilizers, glass, building materials, rock-wool insulation, and road ballast. 5.2.2 Ironmaking The three raw materials described above are first dumped into the top of a blast furnace (Fig. 5.1), an operation called charging the furnace. A blast furnace is basically a large steel cylinder lined with refractory (heat-resistant) brick; it has a height of about a 10-story building. The charge mixture is then melted in a ׉ 7cassandra://unhBjkKjx1z6y-vzmMvcesPa26BGqyQuJmwi87XK5fAE`cZ%EbŁcZ%Ebā#בCט#u׉׉ 7cassandra://foSw3h9qAy1MCGgU_2iAIcSN4DHkznuTQ0TVdWxnwPk# `׉ 7cassandra://5PIyQeysEAzGw2qZ7IvZnsy2vSFRlwV65i8A4FQuBkoW`s׉ 7cassandra://To2HIpr9qdlKMbYztWH7bH4nML1g4eOcWs5FvxDgWQs`׉ 7cassandra://C9LEzZkbVG2TPWyJpsIm2Bmbpbqx_1UBo5BUoC48V2Y7̀͠^cZEbט#u׉׉ 7cassandra://6oUKBfjQ1NamumKSmdhle65pbn5gCTsa5YWaC29axJYa`׉ 7cassandra://U9TFpejLFAAVoFI4xXrm9HhWSYpRCRwMXq2mUKLdWRkj`s׉ 7cassandra://8YB3NCyIW6_KY0zd4LQRYVOy8i8EwqW2S0VwOis2vvkm`׉ 7cassandra://susbK8MIbvaVJI0wk5570t4eeamACa9-r9jVIe98_xM͢d8͠^cZEb׉E Production of Iron and Steel 167 Pellets Blast Furnace Iron ore Sinter Gas uptakes Exhaust gas Layers of iron ore, coke, and limestone Limestone Crushed Steel shell Refractory lining Bustle pipe Tuyere Hot blast air Coal Coke ovens Figure 5.1: Schematic illustration of a blast furnace. Slag Molten iron reaction at 1650◦C, with the air preheated to about 1100◦Cand blasted into the furnace (hence the term “blast furnace”), through nozzles called tuyeres. Although a number of other reactions may take place, the basic reaction is that of oxygen combining with carbon to produce carbon monoxide, which, in turn, reacts with the iron oxide, reducing it to iron. Preheating the incoming air is necessary because the burning coke alone does not produce sufficiently high temperatures for these reactions to take place. The molten metal accumulates at the bottom of the blast furnace, while the impurities float to the top. At intervals of four to five hours, the molten metal is drawn off (tapped) into ladle cars, each holding as much as 145 metric tons of molten iron. The molten metal at this stage is called pig iron,orsimply hot metal, and has a typical composition of 4% C, 1.5% Si, 1% Mn, 0.04% S, 0.4% P, the rest being iron. The word pig comes from the early practice of pouring the molten iron into small sand molds, arranged around a main channel. These closely packed molds reminded early ironworkers of a litter of small pigs crowding against their mother sow. The solidified metal is later used in making iron and steels. 5.2.3 Steelmaking Steel was first produced in China and Japan about 600 to 800 A.D. The steelmaking process is essentially one of refining the pig iron by (a) reducing the percentages of manganese, silicon, carbon, and other elements and (b) controlling the composition of the output through the addition of various elements. The molten metal from the blast furnace is then transported into one of four types of furnaces: open-hearth, electric, vacuum,or basic-oxygen. The name “open-hearth” is derived from the shallow hearth shape open directly to the flames that melt the metal. Developed in the 1860s, the open-hearth furnace has now been replaced by electric furnaces and by the basic-oxygen process, because they are more efficient and produce steels of better quality. ׉ 7cassandra://To2HIpr9qdlKMbYztWH7bH4nML1g4eOcWs5FvxDgWQs`cZ%Eb׉E168 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Carbon electrodes Power leads Door Roller Trunnion Rammed hearth Metal Slag (a) Metal Electrodes Refractory cement Crucible Molten metal Copper induction coils (b) (c) Figure 5.2: Schematic illustration of types of electric furnaces: (a) direct arc, (b) indirect arc, and (c) induction. Electric Furnace. The source of heat in this type of furnace is a continuous electric arc formed between the electrodes and the charged metal (Fig. 5.2a and b). Temperatures as high as 1925◦C are generated in this type of furnace. There are usually three graphite electrodes that can be as large as 750 mm in diameter and 1.5 to 2.5 m long. Their height in the furnace can be adjusted to the amount of metal present and of wear of the electrodes. Steel scrap and small amounts of carbon and limestone are first dropped into the electric furnace through the open roof. The roof is then closed and the electrodes are lowered. The power is turned on, and within about two hours, the temperature increases sufficiently to melt the metal. The current is then shut off, the electrodes are raised, the furnace is tilted, and the molten metal is poured into a ladle (a receptacle used for transferring and pouring molten metal). Furnace capacities range from 55 to 80 metric tons of steel per day; the quality of steel produced is better than that from either open-hearth or basic-oxygen process. For smaller quantities, electric furnaces can be of the induction type (Fig. 5.2c). The metal is placed in a crucible – a large pot made of refractory material and surrounded with a copper coil through which alternating current is passed. The induced current in the charge generates heat and melts the metal. Basic-oxygen Furnace. The basic-oxygen furnace (BOF) is the fastest and by far the most common steelmaking furnace. Typically, 180 metric tons of molten pig iron and 80 metric tons of scrap are charged into a vessel (Fig. 5.3a); some units can hold as much as 360 metric tons. Pure oxygen is then blown into the furnace, for about 20 minutes, through a water-cooled lance (a long tube) and under a pressure of about 1250 kPa, as shown in Fig. 5.3b. Fluxing agents (such as calcium or magnesium oxide) are added through a chute. The process is known as basic because of the pH of these fluxing agents. The vigorous agitation of the oxygen refines the molten metal by an oxidation process, in which iron oxide is produced. The oxide reacts with the carbon in the molten metal, producing carbon monoxide and carbon dioxide. The lance is then retracted, and the furnace is tapped by tilting it (note the opening in Fig. 5.3c for the molten metal). The slag is removed by tilting the furnace in the opposite direction. The BOF process is capable of refining 225 metric tons of steel in 35 to 50 minutes. Most BOF steels have low impurity levels and are of better quality than open-hearth furnace steels. They are then processed into plates, sheets, and various structural shapes, such as I-beams and channels (see Fig. 13.1). Vacuum Furnace. Steel may also be melted in induction furnaces from which the air has been removed (hence the vacuum), similar to the one shown in Fig. 5.2c. Cooling is accomplished by injecting an inert gas, typically argon, at high pressure into the furnace. Because the process removes gaseous impurities from the molten metal and prevents oxidation, vacuum furnaces produce high-quality steels. Vacuum furnaces are also commonly used for heat treating (Section 4.7) and brazing (Section 32.2). ׉ 7cassandra://8YB3NCyIW6_KY0zd4LQRYVOy8i8EwqW2S0VwOis2vvkm`cZ%EbǁcZ%EbƁ#בCט#u׉׉ 7cassandra://O8-oHnWki4j_aQOy01XGRHXH6sHvc6dJWmMvgYt_vLg`׉ 7cassandra://w9pzEArCMYnbVOWKFgatQ1iJpbWSo0A1LdxTJK2Gp6AP`s׉ 7cassandra://fRJEZTyF8wTqAiitk8HjIC_AYdl8jrkOrV5T01QV0BUJ`׉ 7cassandra://BQneiVhaHuHJt8Uuwcjj69SP9JYeiW83ntOYZF8qzCQDP͠^cZEbט#u׉׉ 7cassandra://XEPk_oh4tpPm1VOMGlmGhCRg9EompK_DQBOOGawyz_sw`׉ 7cassandra://u4XdHB66eOxi1ASwtQ_EqzcnKszQY9i6DvDwJAB5wzInV`s׉ 7cassandra://K6EUXNh_GLgf4PWPSfsbocQw154xrqqljbjWkjUky0o`׉ 7cassandra://lHrdGQ4ZjQd8Gi5AGUYT2YRo1PJOQ2xkPYg3gEhGwNgiu͠^cZEb׉E`׉ 7cassandra://hzKBGIgfj5jI7n9JrnmqFaDwUnPqgQUgcZifm2t6lggS\d͠^cZEb ׉EPContinuous Casting 171 Electric furnace Tundish Meniscus Support rolls Platform; 20 m above ground level Oil Tundish Cooling water X-ray transmitter Molten metal Solidified metal Air gap Tundish Catch basin Pinch rolls Nip pulley Starting dummy (a) Oxygen lance (for cutting) Edge dam blocks Water nozzle Backup rolls Top belt (carbon steel) High-velocity cooling water jets Tension pulley Synchronized pinch rolls Bottom belt Water gutters (c) Figure 5.4: (a) The continuous-casting process for steel. Typically, the solidified metal descends at a speed of 25 mm/s. Note that the platform is about 20 m above ground level. Source: Metalcaster’s Reference and Guide, American Foundrymen’s Society. (b) Continuous casting using support or guide rollers to allow transition from a vertical pour zone to horizontal conveyors. (c) Continuous strip casting of nonferrous metal strip. Source: (a) Courtesy of American Foundry Society and (b) Courtesy of Hazelett. Argon X-ray receiver (Controls pouring rate) (b) Solidiying shell or similar solid lubricants, in order to reduce both friction and adhesion at the mold–metal interfaces. The molds may also be vibrated to reduce friction and sticking. The continuously cast metal may then be cut into specific lengths by shearing or computer-controlled torch cutting, or it may be fed directly into a rolling mill for further reduction in thickness and for the shaping into specific shapes, such as channels and I-beams. In addition to lower cost, continuously cast metals have more uniform compositions and properties than those obtained by traditional ingot casting. ׉ 7cassandra://33rik59ngzbRWBtr7FWSLpzcIgeFCVkpgc27Yi4IKi4`cZ%Eb׉E o172 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Modern facilities are computer-controlled, on continuously cast strands, with final sheet thicknesses on the order of 2–6 mm by 750–1250 mm wide, for carbon and stainless steels. They have capabilities for a rapid switchover from one type of steel to another. Steel plates or other shapes undergo one or more further processing, such as (a) cleaning and pickling by chemicals to remove surface oxides, (b) cold rolling to improve strength and surface finish, (c) annealing, and (d) galvanizing or aluminizing to improve resistance to corrosion. In strip casting, thin slabs or strips are produced directly from molten metal. The metal solidifies in similar manner to strand casting, but the hot solid is then rolled to form the final shape (Fig. 5.4b). The compressive stresses in rolling (see Section 13.2) serve to reduce porosity and to provide better material properties. Thus, strip casting eliminates a hot-rolling operation in the production of metal strips or slabs. In modern facilities, final thicknesses on the order of 2 to 6 mm can be obtained, for carbon, stainless, and electrical steels as well as other metals. 5.5 Carbon and Alloy Steels Steel is an alloy that consists primarily of iron with a carbon content between 0.2 and 2.1% by weight. Alloys with higher than 2.1% carbon are known as cast irons (described in Section 12.3.2), and have a lower melting point than other steels and good castability. Carbon and alloy steels are among the most commonly used metals and have a wide variety of compositions, processing options, and applications (Table 5.1). They are available in variety of basic product shapes: plate, sheet, strip, bar, wire, tube, castings, and forgings. Table 5.1: Applications for Selected Carbon and Alloy Steels. Product Steel Aircraft forgings, tubing, fittings 4140, 8740 Automobile bodies 1010 Axles Ball bearings and races Bolts Camshafts Chains (transmission) Coil springs Connecting rods Crankshafts (forged) Differential gears Gears (car and truck) Landing gear Lock washers Nuts Railroad rails and wheels Springs (coil) Springs (leaf) Tubing Wire Wire (music) 1040, 4140 52100 1035, 4042, 4815 1020, 1040 3135, 3140 4063 1040, 3141, 4340 1045, 1145, 3135, 3140 4023 4027, 4032 4140, 4340, 8740 1060 3130 1080 1095, 4063, 6150 1085, 4063, 9260, 6150 1040 1045, 1055 1085 ׉ 7cassandra://GytZABEkDSAAIYjaSbLugcsMmQzzTopTkzYqs6cEXlI>`cZ%EbˁcZ%Ebʁ#בCט#u׉׉ 7cassandra://_rEsPdOU6p71uCxqMgHDF7nK26EwqyD66TVQ5ILGpZg4`׉ 7cassandra://V32__2vtFL09BFeWbKnddYZVFeg-Cw8oV2Dbsvk4kMkO`s׉ 7cassandra://IGo9fQP49ZDc440-5sIhe2_9iAxcLxisoI-G60TIezE`׉ 7cassandra://5TIfNYrvE9eSsatihtkTgUtjizptteB5920hgapch3sSl͠^cZEbט#u׉׉ 7cassandra://kfa3kwz1OtlfGPh4ebTlE-6krO_4Z-h0PcOrP9fRXFU.h`׉ 7cassandra://SuPxUsZfYS6459-uUJClvESpurhBKtTAs60xNd8SCfoZ`s׉ 7cassandra://HDXwXEkIU-6xoR-dQOml7A6w3Psk2q7p3U3G4ebn4I4/`׉ 7cassandra://XJFhGLjl13Ct-CQSyfDK1XeI8Hm8Xh4oy4nPr1p7zPsaD͠^cZEb׉E Carbon and Alloy Steels 173 5.5.1 Effects of Various Elements in Steels Various elements are added to steels in order to impart specific properties, such as hardenability, strength, hardness, toughness, wear resistance, workability, weldability, and machinability. These elements are listed in Table 5.2, with summaries of their beneficial and detrimental effects. Generally, the higher the percentages of these elements, the greater are the particular properties that they impart. For example, the higher the carbon content, the greater the hardenability of the steel and the greater its strength, hardness, and wear resistance. On the other hand, ductility, weldability, and toughness are reduced with increasing carbon content. Some residual elements, called trace elements, may remain after production, refining, and processing of steels. Although the elements in Table 5.2 may also be considered as residuals, the following generally are considered unwanted residual elements: Table 5.2: Effect of Various Elements in Steels. Element Aluminum Bismuth Boron Calcium Carbon Cerium, magnesium, zirconium Chromium Cobalt Copper Lead Manganese Molybdenum Nickel Niobium, tantalum Phosphorus Selenium Silicon Sulfur Tellurium Titanium Tungsten Vanadium Improves toughness, hardenability, wear and corrosion resistance, and high-temperature strength; promotes carburization and depth of hardening in heat treatment. Improves strength and hardness at elevated temperatures. Improves resistance to atmospheric corrosion; can increase strength without loss in ductility; adversely affects hot workability and surface quality. Improves machinability; can cause liquid metal embrittlement. Deoxidizes steel, improves hardenability, strength, abrasion resistance, and machinability; reduces hot shortness, and decreases weldability. Improves hardenability, wear resistance, toughness, elevated-temperature strength, creep resistance, and hardness; it minimizes temper embrittlement. Improves strength, toughness, corrosion resistance and hardenability. Improves strength and impact toughness; it lowers transition temperature and may decrease hardenability. Improves strength, hardenability, corrosion resistance, and machinability; it severely reduces ductility and toughness. Improves machinability. Improves strength, hardness, corrosion resistance, and electrical conductivity; decreases machinability and cold formability. Improves machinability when combined with manganese; decreases impact strength, ductility and weldability. Improves machinability, formability and toughness. Deoxidizes steel; improves hardenability. Improves hardness, especially at elevated temperature. Improves strength, toughness, abrasion resistance, and hardness at elevated temperatures; it inhibits grain growth during heat treatment. Effect Deoxidizes nitriding steels, limits austenite grain growth, increases hardness of nitriding steels Improves machinability Improves hardness without loss of (and perhaps some improvement in) machinability and formability. Deoxidizes steel; improves toughness; may improve formability and machinability. Improves hardenability, strength, hardness, and wear resistance; reduces ductility, weldability and toughness. Deoxidizes steel, improves toughness in HSLA steels; controls shape of inclusions. ׉ 7cassandra://IGo9fQP49ZDc440-5sIhe2_9iAxcLxisoI-G60TIezE`cZ%Eb׉E 174 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Antimony and arsenic cause temper embrittlement. Hydrogen severely embrittles steels; however, heating during processing drives out most of the hydrogen. Nitrogen improves strength, hardness, and machinability; in aluminum-deoxidized steels, it controls the size of inclusions. Nitrogen can increase or decrease strength, ductility, and toughness, depending on the presence of other elements. Oxygen slightly increases the strength of rimmed steels; it severely reduces toughness. Tin causes hot shortness and temper embrittlement. 5.5.2 Designations for Steels Traditionally, the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE) have designated carbon and alloy steels by four digits. The first two digits indicate the alloying elements and their percentages, and the last two digits indicate the carbon content by weight. The American Society for Testing and Materials (ASTM) has a designation system that incorporates the AISI and SAE designations and includes standard specifications for steel products. For ferrous metals, the designation consists of the letter A followed by numbers (generally three). The current standard numbering system is known as the Unified Numbering System (UNS) and has been widely adopted by the ferrous and nonferrous industries. It consists of a letter, indicating the general class of the alloy, followed by five digits, designating its chemical composition. Typical letter designations are: G–AISI and SAE carbon and alloy steels J–cast steels K–miscellaneous steels and ferrous alloys S–stainless steels and superalloys T–tool steels Two examples are: G41300 for AISI 4130 alloy steel, and T30108 for AISI A-8 tool steel. 5.5.3 Carbon Steels Carbon steels generally are classified by their proportion, by weight, of carbon content. The general mechanical properties of carbon and alloy steels are given in Table 5.3, and the effect of carbon on the properties of steel is shown in Fig. 5.5 and summarized as: • Low-carbon steel, also called mild steel, has less than 0.30% C. It often is used for common industrial products (such as bolts, nuts, sheets, plates, and tubes) and for machine components that do not require high strength. • Medium-carbon steel has 0.30–0.60% C. It generally is used in applications requiring higher strength than is available in low-carbon steels, such as in machinery, automotive and agricultural parts (gears, axles, connecting rods, and crankshafts), railroad equipment, and parts for metalworking machinery. • High-carbon steel has more than 0.60% C. Generally, high-carbon steel is used for applications requiring strength, hardness, and wear resistance, such as cutting tools, cable, music wire, springs, and cutlery. After being manufactured into shapes, the parts usually are heat treated and tempered (Chapter 4). The higher the carbon content of the steel, the higher is its hardness, strength, and wear resistance after heat treatment. ׉ 7cassandra://HDXwXEkIU-6xoR-dQOml7A6w3Psk2q7p3U3G4ebn4I4/`cZ%Eb́cZ%Eb́#בCט#u׉׉ 7cassandra://_l-8xRFf0EWKqQ_uwtMnkBoq1liJFVZDYib5gyD3rf0`׉ 7cassandra://Xy3xus7gVmN2GCOdqitxfNkwR-0CtUZuNAXoMfYK6_EX`s׉ 7cassandra://PAznpMlyDTqQ0FefDmNDVrlMDnJVHoLs-Hdyw2tKbWI`׉ 7cassandra://twBOXP239xljaLYS7LlK10sWcnafVAjn9efe-u64ybUpY͠^cZEbט#u׉׉ 7cassandra://_G_GMwrqWXun8plZeTANmfkdVwT_iJ2z8UuUrfPVsxA`׉ 7cassandra://0qTbZW1ySQ-1k1AP5SkuFh_AttExFiF-q1OSGCjvgLsA`s׉ 7cassandra://1X9opqTh7lgotoNxH3iloDrUMsn3G2L5xLa1_NMb1JI`׉ 7cassandra://TRilDrAC9noJCTHvv1kmp_NNUQRSX0b991D5nA3Gb68us$͠^cZEb׉E Carbon and Alloy Steels 175 Table 5.3: Typical Mechanical Properties of Selected Carbon and Alloy Steels. Ultimate tensile AISI Condition 1020 As-rolled 1080 Normalized Annealed As-rolled Normalized Annealed 3140 Normalized Annealed 8620 Normalized Annealed strength (MPa) 448 441 393 1010 965 615 891 689 4340 Normalized 1279 Annealed 744 632 536 Yield strength (MPa) 346 330 294 586 524 375 599 422 861 472 385 357 Elongation Reduction of area in 50 mm (%) 36 35 36 12 11 24 19 24 12 22 26 31 (%) 59 67 66 17 20 45 57 50 36 49 59 62 Typical hardness (HB) 143 131 111 293 293 174 262 197 363 217 183 149 • Carbon steels containing sulfur and phosphorus are known as resulfurized carbon steels (11xx series) and rephosphorized and resulfurized carbon steels (12xx series). For example, 1112 steel is a resulfurized steel with a carbon content of 0.12%. These steels have improved machinability, as described in Section 21.7. 5.5.4 Alloy Steels Steels containing significant amounts of alloying elements are called alloy steels. Structural-grade alloy steels are used mainly in the construction and transportation industries, because of their high strength. Other types of alloy steels are used in applications where strength, hardness, creep and fatigue resistance, and toughness are required. They can be heat treated to obtain the specific desired properties. 5.5.5 High-strength Low-alloy Steels In order to improve the strength-to-weight ratio of steels, several high-strength, low-alloy steels (HSLA) have been developed. These steels have low carbon content (usually less than 0.30%) and are characterized by a microstructure consisting of fine-grain ferrite as one phase and a hard second phase of martensite and austenite. The mechanical properties for selected HSLA steels are given in Table 5.4. These steels have high strength and energy-absorption capabilities as compared to conventional steels. The ductility, formability, and weldability of HSLA steels are, however, generally inferior to those of conventional low-alloy steels (see Fig. 5.6). To improve these properties, several ultra-high-strength steels have been developed, as described in Section 5.5.6. Sheet products of HSLA steels are used typically for parts of truck bodies and other transportation equipment (in order to reduce weight and hence fuel consumption) and in mining, agricultural, and various other industrial applications. Plates are used in ships, bridges, building construction, and for shapes such as I-beams, channels, and angles used in buildings and in various structures. Designations. Three steel categories comprise the system of AISI designations for high-strength sheet steel (Table 5.5). Structural quality (S) includes C, Mn, P, and N. Low alloys (X) contain Nb, Cr, Cu, Mo, Ni, Si, Ti, V, and Zr, either singly or in combination. Weathering steels (W) have environmental-corrosion resistance approximately four times higher than that of conventional low-carbon steels and contain Si, P, Cu, Ni, and ׉ 7cassandra://PAznpMlyDTqQ0FefDmNDVrlMDnJVHoLs-Hdyw2tKbWI`cZ%Eb׉E176 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Low carbon 1000 Normalized Annealed 800 Impact energy 600 400 200 0 0 0.2 0.4 0.6 % carbon Figure 5.5: Effect of carbon content on the mechanical properties of carbon steel. Cr in various combinations. The formability of these sheet steels is graded by the letters F (excellent), K (good), and O (fair). Another designation scheme in wide use is that defined by the Ultralight Steel Auto Body Consortium (ULSAB). The ULSAB practice is to define both the type of steel and its yield and tensile strengths in a compact designation, in the form XX aaa/bbb, where XX is the type of steel, aaa is the yield strength in MPa, and bbb is the ultimate tensile strength in MPa. These types of steel are BH–Bake-hardenable HSLA–High-strength low-alloy DP–Dual-phase TRIP–Transformation-induced plasticity TWIP–Twinning-induced plasticity MART–Martensitic CP–Complex phase. Thus, HSLA 350/450 would be a high-strength low-alloy steel with a minimum yield strength of 350 MPa and a minimum ultimate tensile strength of 450 MPa. Microalloyed Steels. These steels provide superior properties and can eliminate the need for heat treatment. They have a ferrite–pearlite microstructure, with fine dispersed particles of carbonitride. When 0.8 Yield strength % elongation 20 40 60 80 0 1.0 Tensile strength 100 140 120 100 80 60 40 Medium carbon High carbon Yield and tensile strength (MPa) % elongation Izod impact energy (Nm) ׉ 7cassandra://1X9opqTh7lgotoNxH3iloDrUMsn3G2L5xLa1_NMb1JI`cZ%EbρcZ%Eb΁#בCט#u׉׉ 7cassandra://ASeHUUBBh8Iirrl9OqrXnCvZgklmWqfGG8NTrSvViO8q`׉ 7cassandra://NU802-UMk9c6UdtSttCl_y7dtDdo-5J5AGoWcxJ5Pp4>q`s׉ 7cassandra://UQAiCJ73VSqUgzFf_i_kWOZh3ZBk6EGsV_HQmHtSbBM`׉ 7cassandra://SgTw8HBbIfgLolf9RIThMY8IwENtN6SX5bltWNwguXYͤnB͠^cZEbט#u׉׉ 7cassandra://ENiCmxxq46-Wz2UzaLin2gjcGg0W57SClbPDlggyNsc|`׉ 7cassandra://lSw1n-XFYAguFdMGhrg61L4mReUxm22pe7m1tYl-2_AX`s׉ 7cassandra://mdgKeiBMP2OrKu9-yt_LVeP2UeyN_cydf126HsLXSzo`׉ 7cassandra://6ytnsgujSP7tyRygmLZ7Rhhrr2e-S35pBjC08E0Y2sM^Zb͠^cZEb׉ECarbon and Alloy Steels 177 Table 5.4: Mechanical Properties of Selected Advanced High-strength Steels. Minimum Minimum ultimate yield Steel BH 210/340 BH 260/370 HSLA 350/450 DP 350/600 DP 500/800 DP 700/1000 DP 1180 TRIP 450/800 TRIP 400/600 CP 700/800 CP 1000 MART 950/1200 MART 1250/1520 22MnB5, hot stamped 27MnCrB5, as rolled 27MnCrB5, hot stamped 37MnB4, as rolled 37MnB4, hot stamped strength (MPa) 340 370 450 600 800 1000 1180 800 600 800 950 1200 1520 1500 967 1350 810 2040 strength (MPa) 210 260 350 350 500 700 1000 450 400 700 875 950 1250 1100 478 1097 580 1378 Elongation in 50 mm (%) 36 32 25 27 17 15 5 29 30 12 10 6 5 3 12 5 12 4 Strain-hardening exponent, n 0.18 0.13 0.14 0.14 0.14 0.13 — 0.24 0.23 0.13 — 0.07 0.065 — 0.06 0.06 0.06 0.06 80 Lowstrength steels (UTS,270 MPa) 60 TWIP Interstitial free Interstital free/ high strength 40 Mild CMn 20 Bake hardenable 0 0 500 1000 Tensile strength (MPa) Figure 5.6: Comparison of advanced high-strength steels. 1500 2000 HSLA Dual phase/ complex phase Martensitic TRIP Third Generation AHSS (under development) Heat treated boron steels Nano-structured steels High-strength steels Ultra-high-strength steels (UTS.700 MPa) Total elongation (%) ׉ 7cassandra://UQAiCJ73VSqUgzFf_i_kWOZh3ZBk6EGsV_HQmHtSbBM`cZ%Eb׉E 178 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Table 5.5: AISI Designations for High-strength Sheet Steel. Yield strength MPa 240 275 310 350 415 485 550 690 830 970 Example: 350XF 350 350 MPa yield strength X Low alloy F Killed plus sulfide inclusion control. Chemical composition Deoxidation practice S = structural quality F = killed plus sulfide inclusion control X = low alloy K = killed W = weathering O = nonkilled D = dual-phase subjected to controlled cooling (usually in air), these steels develop improved and uniform strength. Compared to medium-carbon steels, microalloyed steels also can provide cost savings of as much as 10%, since the steps of quenching, tempering, and stress relieving are not required. Nanoalloyed Steels. These steels have extremely small grain sizes (10–100 nm), and are produced using metallic glasses (Section 6.15) as a precursor or with special alloying-element combinations that result in a nanostructured combination of constituents. The latter is a leading approach for the production of third generation high-strength steels (see below). 5.5.6 Ultra-high-strength Steels These steels are defined by AISI as those with an ultimate tensile strength higher than 700 MPa. There are five important types of ultra-high-strength steel: dual-phase, TRIP, TWIP, complex phase, and martensitic. The main application of these steels is for crashworthy design of automobiles. The use of stronger steels allows for smaller cross sections in structural components, thus resulting in weight savings and fuel economy increases without compromising safety. The significant drawbacks of all these steels are higher cost, higher tool and die wear, higher forming loads, and more springback. Dual-phase steels are processed specially for a mixed ferrite and martensite structure. They have a high work-hardening exponent [n in Eq. (2.8)], which improves their ductility and formability. TRIP steels consist of a ferrite–bainite matrix and 5–20% retained austenite. During forming, the austenite progressively transforms into martensite. Thus, TRIP steels have both excellent ductility because of the austenite and high strength after forming. As a result, these steels can be used to produce more complex parts than other high-strength steels. TWIP steels (from TWinning-Induced Plasticity) are austenitic and have high manganese content (17– 20%). These steels derive their properties from the generation of twins during deformation (see Section 1.4) without a phase change, resulting in very high strain hardening and avoiding necking during forming. As can be seen in Fig. 5.6, TWIP steels combine high strength and high formability. Complex-phase grades (CP grades) are very fine-grained microstructures of ferrite and a high volume fraction of hard phases (martensite and bainite). These steels can have ultimate tensile strengths as high as 800 MPa, and are therefore of interest for automotive crash considerations, such in bumpers and ׉ 7cassandra://mdgKeiBMP2OrKu9-yt_LVeP2UeyN_cydf126HsLXSzo`cZ%EbсcZ%EbЁ#בCט#u׉׉ 7cassandra://z73UKWWz9_D6y6Qx1yjg1545z6uWqr0gSWBz5Xd1dSY`׉ 7cassandra://TVP3SdofBJlE2Taqoj3t1bj-A22mZVtsxrp4UhREh9Mh`s׉ 7cassandra://P17dyUTZysKR5hPESpdpohnrnBNy6eKu2o4nD_e5Jmo`׉ 7cassandra://fgOzDCvD2f3x2tzVMumGpDK63FIeVGj8MYbFBx7ayHM\,͠^cZEbט#u׉׉ 7cassandra://jeij9N19FLi7lCgRBvHkS0e3nPIxjQWuC4_QvMwnBGw}U`׉ 7cassandra://v3ahO6llk_3TkGRvQJrFAII0V47mMPmkoJx_C6fvyIsO`s׉ 7cassandra://go_KX6kOEZ9Gw_aqvM2-YzYdOAlnL_0VcjZ6PHM5Co4"`׉ 7cassandra://VhGPulMNXMDO3aWFet4SZAXKXquQr_4DIcFCjRhbhOM͠C͠^cZEb׉E>Stainless Steels 179 roof supports. Martensitic grades are also available, consisting of high fractions of martensite and attaining tensile strengths as high as 1500 MPa. Terminology has been developed to refer to the generations of advanced high strength steels (AHSS), differentiated by their color shading in Fig. 5.6: • Conventional high-strength steels include the traditional mild grades, bake hardenable, and HSLA grades. • First Generation AHSS refer to dual phase, complex phase, TRIP, and martensitic steels. • TWIP steels are part of a class of materials referred to as Second Generation AHSS. Because of their very high strength, springback (see Section 16.5) is a concern, hence these materials must be hot stamped (see Section 16.11). • Third Generation AHSS are now becoming commercially available. They combine the high strength of Second Generation AHSS with the improved formability of First Generation AHSS, through careful control of microstructures and phases. For example, a high strength phase such as martensite or ultra fine-grained ferrite may be mixed with a constituent that is highly formable and ductile, such as austenite. These materials can be cold-formed, thereby eliminating the challenges introduced by hot stamping (see Section 16.11). Case Study 5.1 Advanced High-strength Steels in Automobiles Increasing fuel economy in automobiles has received considerable attention in recent years for both environmental and economic reasons. Regulatory requirements call for automobile manufacturers to achieve corporate average fuel economy (CAFE) standards. To achieve higher fuel economy without compromising performance or safety, manufacturers have increasingly used advanced high-strength steels in structural elements of automobiles. For example, note the application of steel in the 2016 Honda Civic automobile shown in Fig. 5.7. Compared to the 2013 model, the use of hot stamped steel has increased from 1 to 14% of the body weight, and high strength steel makes up 58% of the body weight. 5.6 Stainless Steels Stainless steels are characterized primarily by their corrosion resistance and high strength and ductility. They are called stainless because, in the presence of oxygen (air), they develop a thin, hard, adherent film of chromium oxide that protects the metal from corrosion (passivation; see Section 3.8). This protective film builds up again in the event that the surface is scratched. For passivation to occur, the minimum chromium content should be 10–12% by weight. In addition to chromium and carbon, other alloying elements in stainless steels are nickel, molybdenum, copper, titanium, silicon, manganese, columbium, aluminum, nitrogen, and sulfur. The higher the carbon content, the lower is the corrosion resistance of stainless steels. The reason is that the carbon combines with the chromium in the steel and forms chromium carbide; the reduced presence of chromium oxide lowers the passivity of the steel. In addition, the chromium carbide introduces a second phase, thereby promoting galvanic corrosion. Developed in the early 1900s, stainless steels are produced in electric furnaces or by the basic-oxygen process, and by techniques similar to those used in other types of steelmaking processes. The level of purity is controlled through various refining techniques. Stainless steels are available in a wide variety of shapes. Typical applications include cutlery, kitchen equipment, health care and surgical equipment, and applications in the chemical, food-processing, and petroleum industries. Stainless steels generally are divided into five types (see also Table 5.6). ׉ 7cassandra://P17dyUTZysKR5hPESpdpohnrnBNy6eKu2o4nD_e5Jmo`cZ%Eb׉E180 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Others 1% FRP 3% Al 4% LSS 34% Sut 5 270 MPa AHSS 23% Sut 5 700 MPa Hot Stamped 14% Sut 5 1500 MPa UHSS 12% Sut 5 1100 MPa HSS 9% Sut 5 400 MPa (a) (b) Figure 5.7: Advanced high-strength steel applications in the 2016 Honda Civic. (a) Structural components and alloy used; (b) Classes of material and weight contribution, with typical ultimate strength range. The color schemes are the same in both images; note: UHSS = ultra-high–strength steel; AHSS = Advanced highstrength steel; HSS = high-strength steel; LSS = low-strength steel; Al is 6000 series aluminum components; FRP = fiber-reinforced composite. Source: Courtesy of the American Iron and Steel Institute. Table 5.6: Mechanical Properties and Typical Applications of Selected Annealed Stainless Steels at Room Temperature. Ultimate tensile AISI (UNS) designation strength (MPa) Yield strength (MPa) 303 (S30300) 550–620 240–260 304 (S30400) 5–620 240–290 316 (S31600) 50–590 210–290 Elongation in 50 mm (%) 53–50 60–55 Characteristics and typical applications Screw machine products (shafts, valves, bolts, bushings, and nuts) and aircraft fittings (bolts, nuts, rivets, screws, and studs) Chemical and food-processing equipment, brewing equipment, cryogenic vessels, gutters, downspouts, and flashings 60–55 High corrosion resistance and high creep strength, chemicaland pulp-handling equipment, photographic equipment, brandy vats, fertilizer parts, ketchup-cooking kettles, and yeast tubs 410 (S41000) 480–520 416 (S41600) 480–520 240–310 275 35–25 30–20 Machine parts, pump shafts, bolts, bushings, coal chutes, cutlery, tackle, hardware, jet engine parts, mining machinery, rifle barrels, screws, and valves Aircraft fittings, bolts, nuts, fire extinguisher inserts, rivets, and screws Austenitic (200 and 300 series). These steels generally are composed of chromium, nickel, and manganese in iron. They are nonmagnetic and have excellent corrosion resistance, but they are susceptible to stresscorrosion cracking (Section 3.8). Austenitic stainless steels, which are hardenable by cold working, are ׉ 7cassandra://go_KX6kOEZ9Gw_aqvM2-YzYdOAlnL_0VcjZ6PHM5Co4"`cZ%EbӁcZ%Ebҁ#בCט#u׉׉ 7cassandra://q1C3qZCCQvJM9LPyyRncaxqaCC6T4_YvJ07Ob3xWcQE`׉ 7cassandra://1b7SBYzUO9ZW_nN-kTIxPMv5IP4XOAJr-DHoOdYY2ckd`s׉ 7cassandra://y-K4YiGheREi6SFt0lH0EJirkckkrHuyWa4IggyeqZY`׉ 7cassandra://_E_eDl3Y7quFFm1kXkrA4XHFxYdDk63X4gHJyWusYQ4Q͠^cZEbט#u׉׉ 7cassandra://855nkd0eio6Axn4VYOhkN-BHx2tfvvxTcDLbWJQRklg`׉ 7cassandra://mduSivd9RaEbY2flLJtUXJkmpef45reghl3iVx1UnesD`s׉ 7cassandra://NWW2rTlefvgjb9Or9gNLyfhoV13U_r_DkRmfCB2CO9E`׉ 7cassandra://yGrH1vy6BbjC8EpFqhz3a1TCriOw1tjmstOrY1n4S3IY N͠^cZEb׉E Tool and Die Steels 181 the most ductile of all stainless steels and can be easily formed . These steels are used in a wide variety of applications, such as kitchenware, fittings, welded construction, lightweight transportation equipment, furnace and heat-exchanger parts, and as components for severe chemical environments. Ferritic (400 series). These steels have high chromium content. They are magnetic and have good corrosion resistance, but have lower ductility than austenitic stainless steels. Ferritic stainless steels are hardenable by cold working. They generally are used for nonstructural applications, such as kitchen equipment and automotive trim. Martensitic (400 and 500 series). Most martensitic stainless steels do not contain nickel and are hardenable only by heat treatment. These steels are magnetic, and have high strength, hardness, and fatigue resistance, good ductility, and moderate corrosion resistance. They typically are used for cutlery, surgical tools, instruments, valves, and springs. Precipitation-hardening (PH). These stainless steels contain chromium and nickel, along with copper, aluminum, titanium, or molybdenum. They have good corrosion resistance and ductility, and have high strength at elevated temperatures. Their main applications are in aircraft and aerospace structural components. Duplex Structure. These stainless steels have a mixture of austenite and ferrite. They have good strength and higher resistance to both corrosion (in most environments) and stress-corrosion cracking than do the 300 series of austenitic steels. Typical applications are in water-treatment plants and for heat-exchanger components. Case Study 5.2 Stainless Steels in Automobiles The types of stainless steel usually selected by materials engineers for use in automobile parts are 301, 409, 430, and 434. Because of its good corrosion resistance and mechanical properties, type 301 is used for wheel covers. Cold working during the forming process increases its yield strength and gives the wheel cover a springlike action. Type 409 is used extensively for catalytic converters. In addition to being corrosion resistant, type 434 closely resembles the color of chromium plating, thus an attractive alternative to 430. Stainless steels are also well suited for use in various automobile components, such as exhaust manifolds (replacing cast-iron manifolds to reduce weight, and increasing durability, providing higher thermal conductivity, and reduced emissions), mufflers, tailpipes, and brake tubing. 5.7 Tool and Die Steels Tool and die steels are specially alloyed steels (Tables 5.7 and 5.8), designed for tool and die requirements such as high strength, impact toughness, and wear resistance at room and elevated temperatures. They commonly are used in the forming and machining of metals (Parts III and IV). 5.7.1 High-speed Steels High-speed steels (HSS) are the most highly alloyed tool and die steels. First developed in the early 1900s, they maintain their hardness and strength at elevated operating temperatures. There are two basic types of high-speed steels: the molybdenum type (M-series) and the tungsten type (T-series). The M-series steels contain up to about 10% molybdenum, with chromium, vanadium, tungsten, and cobalt as other alloying elements. The T-series steels contain 12–18% tungsten, with chromium, vanadium, and cobalt as other alloying elements. The M-series steels generally have higher abrasion resistance than ׉ 7cassandra://y-K4YiGheREi6SFt0lH0EJirkckkrHuyWa4IggyeqZY`cZ%Eb׉E182 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications Table 5.7: Basic Types of Tool and Die Steels. Type AISI High speed M (molybdenum base) T (tungsten base) Hot work Cold work H1 to H19 (chromium base) H20 to H39 (tungsten base) H40 to H59 (molybdenum base) D (high carbon, high chromium) A (medium alloy, air hardening) O (oil hardening) Shock resisting S Mold steels Special purpose P1 to P19 (low carbon) P20 to P39 (others) L (low alloy) F (carbon–tungsten) Water hardening W Table 5.8: Processing and Service Characteristics of Common Tool and Die Steels. Resistance AISI designation M2 H11, 12, 13 A2 A9 D2 D3 H21 P20 P21 W1, W2 to decarburization Resistance to cracking Highest Highest Medium High High Highest High High High Medium Highest Medium Medium Highest Medium Highest Medium Approx. hardness (HRC) Medium Medium 60–65 Medium 38–55 57–62 35–56 54–61 54–61 36–54 28–37 30–40 50–64 Machinability Toughness Medium Medium to high Medium Medium Low Low Medium Medium to high Medium Highest Very high Medium High Low Low High High Resistance to softening Resistance to wear Low Very high Very high High High High Medium High Medium to high High High to very high High Very high High Medium to high Low to medium Low Medium Medium High Low Medium Low to medium T-series, undergo less distortion in heat treatment, and are less expensive. High-speed steel tools can be coated with titanium nitride and titanium carbide for improved wear resistance (see Chapter 34). 5.7.2 Die Steels Hot-work steels (H-series) are designed for use at elevated temperatures; they have high toughness and high resistance to wear and cracking. The alloying elements are generally tungsten, molybdenum, chromium, and vanadium. Cold-work steels (A-, D-, and O-series) are used for cold-working operations. They generally have high resistance to wear and cracking, and are available as oil-hardening or airhardening types. Shock-resisting steels (S-series) have impact toughness and are used in applications such as header dies, punches, and chisels. Various tool and die materials for a variety of manufacturing applications are given in Table 5.9. ׉ 7cassandra://NWW2rTlefvgjb9Or9gNLyfhoV13U_r_DkRmfCB2CO9E`cZ%EbՁcZ%Ebԁ#בCט#u׉׉ 7cassandra://cUDwlXXbDyUjsb4SWL4NhJcNp6L2c1gfKtSQOlpoeYk`׉ 7cassandra://7C8uVLqWc4wbKR7q4OTGhZvRrXdA9iDnV3lMNhTfK6sI`s׉ 7cassandra://XnnA3VlIpTBlSzrXwyg4uMA_bds_PkzasfVeBcgmJX4m`׉ 7cassandra://3zT0Kj6zuZLaEtBtP2q-dBOSepofQJHaUv-ZM_oxvnUK|͠^cZEbט#u׉׉ 7cassandra://2iTmzjQkCs9DhOB8Z6ffVosknZfHcYwKXhjlN0EQcv4`׉ 7cassandra://krda6MdbL-AlZWQMBMGmOdbuAA2t5OSgRBXBjQEWQ9UQv`s׉ 7cassandra://Pwn5xXK6pEhQ9QEiaHMfTzXEQmxpmWG80v8YNb1wiPc8`׉ 7cassandra://6ggHi3L4cPuzZhRQbF2YEyKQs2-d1lu4NYD3HkDiPWUc͠^cZEb׉E MSummary 183 Process Table 5.9: Typical Tool and Die Materials for Metalworking Processes. Material Die casting Powder metallurgy Punches Dies Hot extrusion Cold heading Cold extrusion Punches Dies Coining Drawing Wire Shapes Bar and tubing Rolls Rolling Thread rolling Shear spinning Sheet metals Cold Shearing Hot Shearing Pressworking Deep drawing Machining Cast iron, cast steel, forged steel, WC A2, D2, M2 A2, D2, D3 D2, A2, A9, S2, S5, S7 H11, H12, H13 Zinc alloys, 4140 steel, cast iron, epoxy composites, A2, D2, O1 W1, O1, cast iron, A2, D2 Carbides, high-speed steels, ceramics, diamond, cubic boron nitride Notes: Tool and die materials usually are hardened 55 to 65 HRC for cold working and 30 to 55 HRC for hot working. Tool and die steels contain one or more of the following major alloying elements: chromium, molybdenum, tungsten, and vanadium. (For further details, see the bibliography at the end of this chapter.) Summary • The major categories of ferrous metals and alloys are carbon steels, alloy steels, stainless steels, and tool and die steels. Their wide range of properties, availability, and their generally low cost have made them among the most useful of all metallic materials. • Steelmaking processes increasingly involve continuous-casting and secondary-refining techniques, resulting in higher quality steels and higher productivity. • Carbon steels are generally classified as low-carbon (mild steel), medium-carbon, and highcarbon steels. Alloy steels contain several alloying elements, particularly chromium, nickel, and molybdenum. • High-strength low-alloy (HSLA) steels have a low carbon content and consist of fine-grained ferrite as one phase and a second phase of martensite and austenite. Micro- and nanoalloyed steels are finegrained, high-strength low-alloy steels that provide superior properties without the need for heat treatment. • Second-generation high-strength steels have been developed; they provide exceptional strength and are used for applications where a combination of high strength and stiffness are required, such as H13, P20 A2, S7, D2, D3, M2 WC, D2, M2 Molds for plastics and rubber S1, O1, A2, D2, 6F5, 6F6, P6, P20, P21, H13 Hot forging 6F2, 6G, H11, H12 H11, H12, H13, H21 W1, W2, M1, M2, D2, WC A2, D2, M2, M4 O1,W1, A2,D2 52100, W1, O1, A2, D2, D3, D4, H11, H12, H13 WC, diamond WC, D2, M2 WC, W1, D2 ׉ 7cassandra://XnnA3VlIpTBlSzrXwyg4uMA_bds_PkzasfVeBcgmJX4m`cZ%Eb׉E C184 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications in structural components protecting operators of automobiles. Third-generation steels are becoming available, combining the high strength of second generation steels with higher ductility; they are cold formable, whereas second-generation steels are mainly hot stamped. • Stainless steels have chromium as the major alloying element; they are called stainless because they form a passivating chromium-oxide layer on their surface. These steels are generally classified as austenitic, ferritic, martensitic, and precipitation-hardening steels. • Tool and die steels are among the most important metallic materials, and are used widely in casting, forming, and machining operations. They generally consist of high-speed steels, hot- and cold-work steels, and shock-resisting steels. Key Terms Alloy steels Basic-oxygen furnace Blast furnace Carbon steels Complex-phase steels Continuous casting Dual-phase steels Electric furnace High speed steel High-strength low-alloy steels Ingot Killed steel Martensitic steel Microalloyed steels Nanoalloyed steels Bibliography ASM Handbook,Vol.1: Properties and Selection: Iron, Steels, and High-Performance Alloys,ASM International, 1990. ASM Specialty Handbook: Carbon and Alloy Steels, ASM International, 1995. ASM Specialty Handbook: Stainless Steels, ASM International, 1994. ASM Specialty Handbook: Tool Materials, ASM International, 1995. Beddoes, J., and Parr, J.G., Introduction to Stainless Steels, 3rd ed., ASM International, 1999. Bhadeshia, H., and Honeycombe, R., Steels: Microstructure and Properties, 4th ed., ButterworthHeinemann, 2017. Demeri, M.Y., Advanced High-Strength Steels: Science, Technology and Applicstions, ASM International, 2013. Krauss, G., Steels: Processing, Structure, and Performance, 2nd ed., ASM International, 2015. Mandal, S.K., Steel Metallurgy: Properties, Specifications and Applications, McGraw-Hill, 2015. Reed, C., The Superalloys: Fundamentals and Applications, Cambridge University Press, 2008. Roberts, G.A., Krauss, G., and Kennedy, R., Tool Steels, 5th ed., ASM International, 1998. Open-hearth furnace Pig iron Refining Rimmed steel Semi-killed steel Stainless steels Steel Strand casting Third generation steel Tool and die steels Trace elements TRIP steels TWIP steels Vacuum furnace ׉ 7cassandra://Pwn5xXK6pEhQ9QEiaHMfTzXEQmxpmWG80v8YNb1wiPc8`cZ%EbׁcZ%Ebց#בCט#u׉׉ 7cassandra://KjvdoGj2c8laM9RfegnnTMpc5ksQCqyDtfGDai7FkP4`׉ 7cassandra://G3mOGMN9IDT7h2fQXGm_q8U3e6hcAV5u4uAK5INzQRc``s׉ 7cassandra://kfWM5DtX9LGPKZnb9ANaHdPGsbLpkawg6nM3K70QO_w+`׉ 7cassandra://6vYiLycbYGlPYQ-BvzC7BbuGEPe0jzlstje8kIQHRWs?͠^cZEb ט#u׉׉ 7cassandra://0PulwAfGu1eUZaYyLseKGwMJkCp8eLCSrDobOMYqNCA`׉ 7cassandra://l29T4l_bi9wQlVUeoXMJ5kOxFStdyokPSRBf36LpQHwg`s׉ 7cassandra://5nHBQMeAJzCa7Exjr5yOqboes9v4DLonHYGGtAwrUCgu`׉ 7cassandra://ikIr2dFkmWM4wZmM_2H3jQpf1zhmyau88LVMpo0TNHkZ͠^cZEb!׉E Qualitative Problems 185 Review Questions 5.1. What are the major categories of ferrous alloys? 5.2. Why is steel so commonly used? 5.3. List the basic raw materials used in making iron and steel, and explain their functions. 5.4. List the types of furnaces commonly used in steelmaking, and describe their characteristics. 5.5. List and explain the characteristics of the types of steel ingots. 5.6. What does refining mean? How is it done? 5.7. What is continuous casting? What advantages does continuous casting have over casting into ingots? 5.8. What is the role of a tundish in continuous casting? 5.9. Name the four alloying elements that have the greatest effect on the properties of steels. 5.10. What are trace elements? 5.11. What are the percentage carbon contents of low-carbon, medium-carbon, and high-carbon steels? 5.12. How do stainless steels become stainless? 5.13. What are the major alloying elements in tool and die steels and in high-speed steels? 5.14. How does chromium affect the surface characteristics of stainless steels? 5.15. What kinds of furnaces are used to refine steels? 5.16. What is high-speed steel? 5.17. What is TRIP? TWIP? 5.18. What are the applications of advanced high-strength steels? 5.19. What characteristics are common among die steels? 5.20. What effect does carbon content have on mechanical properties of steel? What effects does it have on physical properties? 5.21. What is killed steel? In this context, what does ‘killed’ refer to? Qualitative Problems 5.22. Identify several different products that are made of stainless steel, and explain why they are made of that material. 5.23. Professional cooks generally prefer carbon-steel to stainless-steel knives, even though the latter are more popular with consumers. Explain the reasons for those preferences. 5.24. Why is the control of the structure of an ingot important? 5.25. Explain why continuous casting has been such an important technological advancement. 5.26. Describe applications in which you would not want to use carbon steels. 5.27. Explain what would happen if the speed of the continuous-casting process shown in Fig. 5.4a is (a) higher or (b) lower than that indicated, typically 25 mm/s. 5.28. The cost of mill products of metals increases with decreasing thickness and section size. Explain why. 5.29. Describe your observations regarding the information given in Table 5.9. 5.30. How do trace elements affect the ductility of steels? 5.31. Comment on your observations regarding Table 5.1. ׉ 7cassandra://kfWM5DtX9LGPKZnb9ANaHdPGsbLpkawg6nM3K70QO_w+`cZ%Eb׉ES186 Chapter 5 Ferrous Metals and Alloys: Production, General Properties, and Applications 5.32. In Table 5.9, D2 steel is listed as a more common tool and die material for most applications. Why is this so? 5.33. List the common impurities in steel. Which of these are the ones most likely to be minimized if the steel is melted in a vacuum furnace? 5.34. Explain the purpose of the oil shown at the top left of Fig. 5.4a given that the molten-steel temperatures are far above the ignition temperatures of the oil. 5.35. Recent research has identified mold-surface textures that will either (a) inhibit a solidified steel from separating from the mold or (b) force it to stay in contact in continuous casting. What is the advantage of a mold that maintains intimate contact with the steel? 5.36. Identify products that cannot be made of steel, and explain why this is so. (For example, electrical contacts commonly are made of gold or copper, because their softness results in low contact resistance, whereas for steel, the contact resistance would be very high.) 5.37. List and explain the advantages and disadvantages of using advanced high-strength steels. Quantitative Problems 5.38. Conduct an internet search and determine the chemical composition of (a) TRIP 450/800; (b) 304 stainless steel; (c) 4140 steel. If a foundry ladle will pour 60,000 kg, calculate the weight of each element in the ladle. 5.39. Refer to the available literature, and estimate the cost of the raw materials for (a) an aluminum beverage can, (b) a stainless-steel two-quart cooking pot, and (c) the steel hood of a car. 5.40. In Table 5.1, more than one type of steel is listed for some applications. Refer to data available in the technical literature listed in the bibliography, and determine the range of properties for these steels in various conditions, such as cold worked, hot worked, and annealed. 5.41. Some soft drinks are now available in steel cans (with aluminum tops) that look similar to aluminum cans. Obtain one of each type, weigh them when empty, and determine their respective wall thicknesses. 5.42. Using strength and density data, determine the minimum weight of a 1 m-long tension member that must support a load of 4 kN, manufactured from (a) annealed 303 stainless steel, (b) normalized 8620 steel, (c) as-rolled 1080 steel, (d) any two aluminum alloys, (e) any brass alloy, and (f) pure copper. 5.43. The endurance limit (fatigue life) of steel is approximately one-half the ultimate tensile strength (see Fig. 2.16), but never higher than 700 MPa. For iron, the endurance limit is 40% of the ultimate strength, but never higher than 170 MPa. Plot the endurance limit vs. the ultimate strength for the steels described in this chapter and for the cast irons shown in Table 12.3. On the same plot, show the effect of surface finish by plotting the endurance limit, assuming that the material is in the as-cast state (see Fig. 2.29). 5.44. Using the data given in Table 5.4, obtain the power-law curves for the advanced high-strength steels shown and plot the curves. Compare these materials with those given in Table 2.3. Synthesis, Design, and Projects 5.45. Based on the information given in Section 5.5.1, make a table with columns for each improved property, such as hardenability, strength, toughness, and machinability. In each column, list the elements that improve that particular property and identify the element that has the most influence. 5.46. Assume that you are in charge of public relations for a large steel-producing company. Outline all of the attractive characteristics of steels that you would like your customers to be informed about. ׉ 7cassandra://5nHBQMeAJzCa7Exjr5yOqboes9v4DLonHYGGtAwrUCgu`cZ%EbفcZ%Eb؁#בCט#u׉׉ 7cassandra://KqHAjamYGB0xXChHjLwI6YMODFfUXFPazjO-RKI2wDo`׉ 7cassandra://IprpF6jtBpWzNcwftP8mF2aCVVHOWoMxBOgrWoAcQV48`s׉ 7cassandra://7rNoMLELXaCfgORb0_iVaQlUiOLbtM79s3M0MnHmMi8`׉ 7cassandra://Duz7rITwIJchTb5UeSJ6svluKJIZnyVkjwQpYlmAoj0@j͠^cZEb%ט#u׉׉ 7cassandra://rqZmHKKsxK9AA44b06HcVbsKwRjJVPi93BoGmhkdf5U{`׉ 7cassandra://IopTIQ2DmyrYUk5gHxFRvEXJfo2H8NITXBFRxW6IBe42`s׉ 7cassandra://jVs7DUbfuexGRIV9UMzxNoNZoKaWNruTomFg9zwFxLM`׉ 7cassandra://C4BhOVCJku7iQDFCS3xD22ihAdqKo3nIwGAAx2wNVqI&Q͠^cZEb'׉ESynthesis, Design, and Projects 187 5.47. Assume that you are in competition with the steel industry and are asked to list all of the characteristics of steels that are not attractive. Make a list of those characteristics and explain their relevance to engineering applications. 5.48. Section 5.5.1 noted the effects of various individual elements, such as lead alone or sulfur alone, on the properties and characteristics of steels. What was not discussed, however, was the role of combinations of these elements (such as lead and sulfur together). Review the technical literature, and prepare a table indicating the combined effects of several elements on steels. 5.49. In the past, waterfowl hunters used lead shot in their shotguns, but this practice resulted in lead poisoning of unshot birds that ingested lead pellets (along with gravel) to help them digest food. Steel and tungsten are being used as replacement materials. If all pellets have the same velocity upon exiting the shotgun barrel, what concerns would you have regarding this substitution of materials? Consider both performance and environmental effects. 5.50. Aluminum is being used as a substitute material for steel in automobiles. Describe your concerns, if any, in purchasing an aluminum automobile. 5.51. In the 1940s (The Second World War), the Yamato and its sister ship, the Musashi, were the largest battleships ever built. Find out the weight of these ships, and estimate the number of automobiles that could have been built from the steel used in just one such ship. Estimate the time it would take to cast that much steel by continuous casting. 5.52. Search the technical literature, and add more parts and materials to those shown in Table 5.1. 5.53. Referring to Fig. 5.4, note that the mold has cooling channels incorporated to remove heat. Can continuous casting be done without such cooling channels? Can it be done with a heated mold? Explain your answer. ׉ 7cassandra://7rNoMLELXaCfgORb0_iVaQlUiOLbtM79s3M0MnHmMi8`cZ%Eb׉EChapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications 6.1 Introduction 189 6.2 Aluminum and Aluminum Alloys 191 6.3 Magnesium and Magnesium Alloys 194 6.4 Copper and Copper Alloys 195 6.5 Nickel and Nickel Alloys 196 6.6 Superalloys 197 6.7 Titanium and Titanium Alloys 198 6.8 Refractory Metals and Alloys 199 6.9 Beryllium 200 6.10 Zirconium 201 6.11 Lithium 201 6.12 Low-melting Alloys 201 6.13 Precious Metals 202 6.14 Shape-memory Alloys (Smart Materials) 203 6.15 Amorphous Alloys (Metallic Glasses) 203 6.16 Metal Foams and Metamaterials 203 6.17 Rare Earth Metals 205 Case Studies: 6.1 Aluminum Production from Ore: The Hall-H´eroult Process 192 6.2 The Tesla Model S 60 193 188 ׉ 7cassandra://jVs7DUbfuexGRIV9UMzxNoNZoKaWNruTomFg9zwFxLM`cZ%EbہcZ%Ebځ#בCט#u׉׉ 7cassandra://URtM5PxrC4EqCMfXtTNYdUoT3D_YedtsknTrSBLUFf0<`׉ 7cassandra://PJIGIbJVRfKHHr-mYI4yI2YZnflxKwb-XPqJpjS4PboF`s׉ 7cassandra://sz8sCrDnhu6fH1KFPBO6U3Lm6Pk7-P5qjPuQKk6tiEc@`׉ 7cassandra://XUjcL1vwOikMd2aS67r4Pb-uM4TdSKY5j-l3r-jny6sNX͠^cZEb)ט#u׉׉ 7cassandra://h3Va8L9lsmZ7nBEmAC-e-0IuseAn-4EOw4vcmLMrjjAW`׉ 7cassandra://z7oaxmzSKZUmqZLRfw9N6uWwdpfZN7LvXUM5QUXCPcgH`s׉ 7cassandra://u_7Xj1E1kCw9lnhaU_f8FurzMaJb2S7NLZq3Iq0lQ1A&`׉ 7cassandra://bvIY-C5ptecVbVHS7T7XVdxpc722ys8wROwcm_aEu8Q͐p͠^cZEb*׉EIntroduction 189 • Nonferrous metals include a wide variety of materials, with special properties that are indispensable in most products. • This chapter introduces each class of nonferrous metal and its alloys, and briefly describes their methods of production. • Their physical and mechanical properties are then summarized, along with general guidelines for their selection and applications. • Shape-memory alloys, amorphous alloys, metal foams, and rare earth metals are also described, with examples of their unique applications. 6.1 Introduction Nonferrous metals and alloys cover a very wide range, from the more common metals (such as aluminum, copper, and magnesium) to high-strength, high-temperature alloys (such as those of tungsten, tantalum, and molybdenum). Although generally more expensive than ferrous metals (Table 6.1), nonferrous metals have numerous important applications because of such properties as corrosion resistance, high thermal and electrical conductivity, low density, and ease of fabrication (Table 6.2). Typical examples of nonferrous metal and alloy applications include aluminum for aircraft bodies and cooking utensils, copper wire for electrical power cords, zinc for galvanized sheet metal for car bodies, titanium for jet-engine turbine blades and for orthopedic implants, and tantalum for rocket engine components. As an example, the turbofan jet engine (Fig. 6.1) for the Boeing 757 aircraft typically contains the following nonferrous metals and alloys: 38% Ti, 37% Ni, 12% Cr, 6% Co, 5% Al, 1% Nb, and 0.02% Ta. Table 6.1: Approximate Cost-per-unit-volume for Wrought Metals and Plastics Relative to the Cost of Carbon Steel. Table data are representative of 25-mm diameter bar stock. Material Gold Silver Molybdenum alloys Nickel Titanium alloys Copper alloys Zinc alloys Stainless steels Magnesium alloys Aluminum alloys High-strength low-alloy steels Gray cast iron Carbon steel Nylons, acetals, and silicon rubber∗ Rubber∗ Other plastics and elastomers* Relative cost 70,000 680 200–250 40 25–40 8–10 1.5–3.5 2–9 2–4 1.5–3 1.4 1.2 1 1.1–2 0.2–1 0.2–2 ∗ As molding compounds. Note: Costs vary significantly with quantity of purchase, supply and demand, size and shape, and various other factors. ׉ 7cassandra://sz8sCrDnhu6fH1KFPBO6U3Lm6Pk7-P5qjPuQKk6tiEc@`cZ%Eb׉E190 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Table 6.2: General Characteristics of Nonferrous Metals and Alloys. Material Aluminum Magnesium Copper Superalloys Tin Titanium Characteristics Nonferrous alloys More expensive than steels and plastics; wide range of mechanical, physical, and electrical properties; good corrosion resistance; high-temperature applications Alloys have high strength-to-weight ratio; high thermal and electrical conductivity; good corrosion resistance; good manufacturing properties Lightest metal; good strength-to-weight ratio High electrical and thermal conductivity; good corrosion resistance; good manufacturing properties Good strength and resistance to corrosion at elevated temperatures; can be iron-, cobalt-, and nickelbased alloys Good corrosion resistance and bright appearance; used also in solders and as bearing materials. Highest strength-to-weight ratio of all metals; good strength and corrosion resistance at high temperatures Refractory metals Molybdenum, niobium, tungsten, and tantalum; high strength at elevated temperatures Precious metals Zinc Rare earths Gold, silver, and platinum; generally good corrosion resistance and aesthetic characteristics. Very good corrosion resistance; commonly used in castings and galvanizing steel sheet for corrosion protection. Unique combinations of magnetic and electrical properties; commonly used in magnetic devices, highcapacity storage batteries and microelectronics. Low-pressure turbine Ni alloy Turbine exhaust case Ni alloy Combustion chamber Ni alloy High-pressure compressor Ti or Ni alloy Low-pressure compressor Ti or Al alloy High-pressure turbine Ni alloy Turbine blades Ni alloy Accessory section Al alloy or Fe alloy Inlet case Al alloy Fan Ti alloy Figure 6.1: Cross section of a jet engine (PW2037) showing various components and the alloys used in manufacturing them. Source: Shutterstock/Chesky. ׉ 7cassandra://u_7Xj1E1kCw9lnhaU_f8FurzMaJb2S7NLZq3Iq0lQ1A&`cZ%Eb݁cZ%Eb܁#בCט#u׉׉ 7cassandra://pAo7A1Q03w89w3okLBurQHj8OZIuTdXTGC2Ij_POAB0Y`׉ 7cassandra://lFWTDxdFa4lraeosQdsKHXgKfpapj2YfKzZFIsjyRhY_`s׉ 7cassandra://QAja5I0ZqrjQVbeIKaYhMad9bY3tDSSPDmRo3xIiK30`׉ 7cassandra://EPAvuEGHrBJnbspLsnptgjgBe6YD2v8JxumyDnVBQMw[P̾͠^cZEb-ט#u׉׉ 7cassandra://EmzoENZFhptUhU4WxaO2d3pMnfiFgX7aedzzZYci484G`׉ 7cassandra://IIp0qFEUBlnY7GvhvQ-_G8aRWZJum-0qhCg6lpGaTok``s׉ 7cassandra://eZI7roM3nVeuZ0aJrtaSBo1_00PXL3lHa-bQAkjoQgk`׉ 7cassandra://11C2h9N6ObU0FQBD7jJzebSPTSCbGAt3xWcVVRxnmYUmMx͠^cZEb/׉E Aluminum and Aluminum Alloys 191 This chapter introduces the general properties, production methods, and engineering applications of nonferrous metals and their alloys. The manufacturing properties of these materials, such as formability, machinability, and weldability, are described in various chapters throughout this text. 6.2 Aluminum and Aluminum Alloys The important characteristics of aluminum (Al) and its alloys are their high strength-to-weight ratios, resistance to corrosion, high thermal and electrical conductivities, nontoxicity, reflectivity, appearance, and ease of formability and machinability; also, they are nonmagnetic. The principal uses of aluminum and its alloys, in decreasing order of consumption, are in containers and packaging (aluminum beverage cans and foil); architectural and structural applications; transportation (aircraft and aerospace applications, buses, automobiles, railroad cars, and marine craft); electrical applications (as economical and nonmagnetic electrical conductors); computer, tablets, and smart phone casings; consumer durables (appliances, cooking utensils, and furniture), and portable tools (Tables 6.3 and 6.4). Nearly all high-voltage transmission wiring is made of aluminum. In its structural (load-bearing) components, 82% of a Boeing 747 aircraft and 70% of a Boeing 777 aircraft is aluminum. Although the Boeing 787 Dreamliner (first placed into service in late 2011) is well recognized for its carbon fiber-reinforced composite fuselage, it still uses 20% aluminum, by weight, as compared to 15% titanium. The frame and the body panels of the Rolls Royce Phantom coupe are made of aluminum, improving the car’s strength-to-weight and torsional rigidity-to-weight ratios. Aluminum alloys are available as mill products, that is, as wrought products made into various shapes by rolling, extrusion, drawing, and forging (Chapters 13 through 15). Aluminum ingots are available for casting, as is aluminum in powder form for powder-metallurgy applications (Chapter 17). Most aluminum alloys can be machined, formed, and welded with relative ease. There are two types of wrought alloys of aluminum: (a) Alloys that can be hardened by mechanical processing and are not heat treatable, and (b) alloys that can be hardened by heat treatment. Unified Numbering System. Aluminum and other nonferrous metals and alloys are identified internationally by the Unified Numbering System (UNS), consisting of a letter, indicating the general class of the alloy, followed by five digits, indicating its chemical composition. For example, A for aluminum, C for copper, N for nickel alloys, P for precious metals, and Z for zinc. Also, in the UNS designation, 2024 wrought aluminum alloy is A92024. Table 6.3: Properties of Selected Aluminum Alloys at Room Temperature. Ultimate tensile Yield strength Alloy (UNS) Temper 1100 (A91100) O 1100 2099 2099 H14 2024 (A92024) O 2024 3003 (A93003) O 3003 T4 T83 T8E67 H14 5052 (A95052) O 5052 6061 (A96061) O 6061 7075 H34 T6 7075 (A97075) O T6 strength (MPa) 90 125 190 470 560 530 110 150 190 260 125 310 230 570 (MPa) 35 120 75 325 525 485 40 145 90 215 55 275 105 500 Elongation in 50 mm (%) 35–45 9–20 20–22 19–20 9 10 30–40 8–16 25–30 10–14 25–30 12–17 16–17 11 ׉ 7cassandra://QAja5I0ZqrjQVbeIKaYhMad9bY3tDSSPDmRo3xIiK30`cZ%Eb׉E 192 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Table 6.4: Manufacturing Characteristics and Typical Applications of Selected Wrought Aluminum Alloys. Characteristics∗ Corrosion Alloy resistance A 1100 2024 2099 3003 5052 6061 C A A A B Machinability Weldability Typical Applications C–D B–C B–C C–D C–D C–D B–C A Sheet-metal work, spun hollowware, tin stock, power transmission lines Truck wheels, screw machine products, most widely used aluminum alloy for aircraft structures A Lightweight lithium alloy with limited fracture toughness, used in aerospace and lightweight applications. A Cooking utensils, chemical equipment, pressure vessels, sheet-metal work, builders’ hardware, storage tanks A Sheet-metal work, hydraulic tubes, and appliances; pressure vessels; bus, truck, and marine uses A Heavy-duty structures where corrosion resistance is needed; truck and marine structures, railroad cars, furniture, pipelines, bridge railings, hydraulic tubing. Used increasingly for hydroformed tubing and extrusions for lightweight vehicles 7075 C B–D D Aircraft and other structures, keys, hydraulic fittings ∗A, excellent; D, poor. Aluminum is widely seen as an essential metal for lightweight applications, such as vehicles where fuel economy goals have to be met. While the 2000 and 6000 series aluminum alloys are widely used for vehicles, new alloys, especially those containing lithium as the main alloying element, are now receiving significant interest. Aluminum 2099, for example, contains up to 3.0% copper and 2.0% lithium, along with other elements, and provides exceptional corrosion resistance and high strength, with a 4% reduction in density and 3% increase in stiffness as compared to other aluminum alloys. The main drawbacks to aluminum– lithium alloys are cost and limited fatigue and fracture strengths, although material innovations continue. Porous Aluminum. Blocks of aluminum are produced that are 37% lighter than solid aluminum and have uniform permeability and microporosity. This characteristic allows their use in applications where a vacuum or differential pressure has to be maintained. Examples are vacuum holding of fixtures for assembly and automation (Section 37.8), and vacuum forming or thermoforming of plastics (Section 19.6). The blocks are 70–90% aluminum powder; the rest is epoxy resin. They can be machined with relative ease and can be joined using adhesives or special welding processes. Case Study 6.1 Aluminum Production from Ore: The Hall-H´eroult Process First produced in 1825, aluminum is the most abundant metallic element, making up about 8% of the earth’s crust, and is produced in a quantity second only to iron. The principal ore for aluminum is bauxite, which is hydrous (water-containing) aluminum oxide and includes other oxides. Bauxite generally contains 30–60% aluminum oxide (alumina, Al2O3) combined with other elements such as iron. The first step in producing aluminum is to extract the aluminum oxide from the bauxite. This is done in a multi-step process: 1. The bauxite is first crushed in a comminution mill to an aggregate form. 2. The ore is then heated to a temperature of 150–200◦C in a sodium hydroxide (NaOH) solution, this results in the production of sodium aluminate (NaAlO2). 3. The sodium aluminate produces aluminum hydroxide, Al(OH)3, which is then converted to alumina in rotary kilns at temperatures up to 1000◦C. ׉ 7cassandra://eZI7roM3nVeuZ0aJrtaSBo1_00PXL3lHa-bQAkjoQgk`cZ%Eb߁cZ%Ebށ#בCט#u׉׉ 7cassandra://MOwHDqhNPDqhgPjDT1HjAc6TfYV9ISOME8coAdDLQQ4k`׉ 7cassandra://Ql_UhN5YP-UFVfkWuO2AmloB_DxpLDaj9kolmrvHrwY^`s׉ 7cassandra://KJRgh8XCjYiwe6CqtOAVE8GcoJWz3n6UxsGiTTD0x3wc`׉ 7cassandra://QpQlgtmB2r-Ob0Bu7SGzEQxT5Xv_gMQ1osmTTRIsehQI͠^cZEb2ט#u׉׉ 7cassandra://S8S23MyqVuOTw2bJak0JAoSlTj5-pjhvHBMp-X_n_G8@w`׉ 7cassandra://By8G9Vg1CENmeNh_nsgGRMF_DQAi4srmcfKuSz8uwlQ[`s׉ 7cassandra://01ir_gFLMQUzGYMvWfvkHRFWllz_977tW9esfDuheQU`׉ 7cassandra://xxlTM6nDHQPuSPMRbQHO855D0DQ50TNejzpE24mBnLs͋͠^cZEb5׉E Aluminum and Aluminum Alloys 193 1 2 Consumable graphite anode Graphite-lined reduction pot (cathode) Molten Al2O3 and Na3AlF6 CO2 bubbles Molten aluminum Figure 6.2: Schematic illustration of a Hall-H´eroult reduction pot, used to produce molten aluminum from aluminum oxide. The Hall-H´eroult process (Fig. 6.2) produces pure aluminum from the alumina in a carbon- or graphitelined container, referred to as a reduction pot. In the reduction pot, a small voltage, as low as 6 volts, is used, but the current can be extremely high-over 150,000 amperes. The Hall-H´eroult process uses a carbon anode that combines with the oxygen in the alumina, forming a combination of carbon dioxide and carbon monoxide, leaving molten aluminum to collect in the bottom of the pot, where it is siphoned periodically. The Hall-H´eroult process enabled aluminum to be an inexpensive commodity metal instead of a precious metal. The demand for aluminum is so high that it has been estimated that 5% of all electricity generated in the United States is used in Hall-H´eroult cells. This has serious implications, since it has been estimated that 11.4 metric tons of CO2 emissions are generated per ton of aluminum produced (see Section 40.5). Case Study 6.2 The Tesla Model S 60 Aluminum use in automobiles and in light trucks has been increasing steadily. As recently as 1990, there were no aluminum-structured passenger cars in production in the United States. It is expected that the automotive industry will need to reduce the mass of vehicles by around 9%, by 2028, in order to achieve mandated fuel efficiency (CAF´E) standards, requiring material advances with most metals. However, its light weight, high strength, formability, weldability and corrosion resistance of aluminum makes it very attractive for the production of lightweight vehicles. This would require the average aluminum content in vehicles to increase from the current weight of around 187 to 212 kg. The Tesla Model S is an electric vehicle, requiring around 455 kg of batteries to supply power to its electric motors. In order to achieve a curb weight of around 1900 kg, while seating up to seven passengers, a number of design innovations were required, including a lightweight aluminum space frame and an aluminum body (though with some selected steel reinforcement in some pillars for the operator zone). The aluminum sections are held together using a mix of cold-metal transfer welding, conventional spot welding, self-piercing rivets, and structural adhesives. The design also allows the batteries to be mounted low in the car, thus lowering the center of gravity and maximizing interior space. Many of the design innovations involve non-structural features such as several cameras to allow near self-driving, numerous airbags and a unique chassis design that places the battery at the base of the vehicle to maximize stability. However, since the electric vehicle does not have a large motor, it can use advanced extrusions and castings to produce an energy-absorbing frame. ׉ 7cassandra://KJRgh8XCjYiwe6CqtOAVE8GcoJWz3n6UxsGiTTD0x3wc`cZ%Eb׉E &194 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications (a) (b) Figure 6.3: The Tesla S 60. (a) Image of the Tesla S 60 electric automobile, with an aluminum alloy body; (b) Chasis showing the battery supported and protected by a titanium underbody. Source: (a) Shutterstock/Dimitris Leonidas (b) Shutterstock/Alexander Kondratenko. 6.3 Magnesium and Magnesium Alloys Magnesium (Mg) is the lightest engineering metal and has good vibration-damping characteristics. Its alloys are used in structural as well as nonstructural applications wherever weight is of primary importance. Magnesium is also an alloying element in various nonferrous metals. The main drawback to magnesium is its high cost (Table 6.1). Typical uses of magnesium alloys are in aircraft and missile components, material-handling equipment, portable power tools, ladders, luggage, bicycles, sporting goods, and general lightweight components. Magnesium is finding increased use in the automotive sector, mainly for weight savings. Magnesium alloys are available either as castings (such as die-cast camera frames) or as wrought products (such as extruded bars and shapes, forgings, and rolled plates and sheet). Its alloys are also used in printing and textile machinery to minimize inertial forces in high-speed components of machinery Because it is not sufficiently strong in its pure form, magnesium is alloyed with various elements (Table 6.5) in order to impart certain specific properties, particularly high strength-to-weight ratio. A variety of magnesium alloys have good casting, forming, and machining characteristics. Because magnesium as powder or in chip forms (as from machining) oxidizes rapidly (i.e., they are pyrophoric), a fire hazard exists; thus precautions must be taken when machining, grinding, or sand-casting magnesium alloys. Products made of magnesium and its alloys are, however, not a fire hazard during their normal use. Table 6.5: Properties and Typical Forms of Selected Wrought Magnesium Alloys. Ultimate tensile Nominal Alloy composition Condition AZ31B 3.0 Al, 1.0 Zn, 0.2 Mn F H24 AZ80A 8.5 Al, 0.5 Zn, 0.2 Mn T5 9.0 Al, 0.03 Cu 0.7 Zr, 3 Th F 1.0 Zn, 1.0 Ce 5.7 Zn, 0.55 Zr ZEK199 1.0 Zn, 0.3 Zr, 1.0 Ce ZK60A H24 F F T5 AZ91D HK31A ZE10 strength (MPa) 260 290 380 230 255 263 311 365 Yield strength (MPa) 200 220 275 160 200 163 308 300 Elongation in 50 mm (%) 15 15 7 3 8 16 19 11 Typical forms Extrusions Sheet and plate Extrusions and forgings Most common die cast alloy Sheet and plates Sheet and plates Extrusions and sheet Extrusions and forgings ׉ 7cassandra://01ir_gFLMQUzGYMvWfvkHRFWllz_977tW9esfDuheQU`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://aHFJkq6U-MpmOm6KPik82-ZygWouFWiwSNbfLyNcrd4`׉ 7cassandra://_dF1W9z8MoP_ak-EAFDbtcOUV_xCobxQc9u5kIdqMc8p`s׉ 7cassandra://PU0Xm-eGmuGtnoWS1Mldpx0PyU7ICvy4aZQjtQDoPWQ`׉ 7cassandra://bPKpEJy5Ptyl0ozRVHlZFblPRNaPIktyab3vB51MfTAa͠^cZEb8ט#u׉׉ 7cassandra://kopZ2F_mVXD7CvMJ2k8rk-S8TfhccW5nxpyfY0R2qw03`׉ 7cassandra://6uKilg_vXVbUPx1yEFEwxUoqlDjn-qd0BqNnulAfAAkW `s׉ 7cassandra://T-0Yn1aX_pe4QPHcuhKT9n-3olkagDM6eZfAx4pjJfc`׉ 7cassandra://zH-95Pp2n3FM7W9pKpB_DanQvjdzCF_AzKm_2NAgrZ4jR͠^cZEb9׉ExCopper and Copper Alloys 195 Magnesium is easy to cast but normally difficult to form. Efforts have been made to promote the increased use of magnesium in automobiles through improved welding and sheet formability. Alloys ZEK100, AZ31 and ZE10 are of current high interest. Production. Magnesium is the third-most-abundant metallic element (2%) in the earth’s crust, after iron and aluminum. Most magnesium comes from seawater, which contains 0.13% magnesium as magnesium chloride. First produced in 1808, magnesium metal can be obtained either electrolytically or by thermal reduction. In the electrolytic method, seawater is mixed with lime (calcium hydroxide) in settling tanks. The magnesium hydroxide developed precipitates to the bottom, and is then filtered and mixed with hydrochloric acid. The resulting solution is subjected to electrolysis (as is done with aluminum), producing magnesium metal; it is then cast into ingots for further processing into various shapes. In the thermal-reduction method, magnesium ores (dolomite, magnesite, and others) are broken down with reducing agents (such as powdered ferrosilicon, an alloy of iron and silicon) by heating the mixture in a vacuum chamber. As a result of this reaction, vapors of magnesium form, and they condense into magnesium crystals; they are then melted, refined, and poured into ingots to be processed further into various shapes. 6.4 Copper and Copper Alloys First produced in about 4000 B.C., copper (Cu, from the Latin cuprum) and its alloys have properties somewhat similar to those of aluminum and its alloys. In addition, they have good corrosion resistance and are among the best conductors of electricity and heat (Tables 3.1 and 3.2). Copper and its alloys can be processed easily by forming, machining, casting, and joining techniques. Copper alloys often are attractive for applications in which a combination of qualities, such as electrical, mechanical, nonmagnetic, corrosion-resistance, thermally conductivity, and wear-resistance are required. Applications include electrical and electronic components, springs, coins, plumbing components, heat exchangers, marine hardware, and consumer goods (such as cooking utensils, jewelry, and decorative objects). Although aluminum is the most common material for dies in polymer injection molding (Section 19.3), copper is often used because of its better thermal properties. Pure copper can be used as a solid lubricant in hot metal-forming operations (Section 33.7.6). Copper alloys now have improved manufacturing characteristics, and can be heat treated to improve their mechanical properties. The most common copper alloys are brasses and bronzes. Brass (an alloy of copper and zinc) is one of the earliest alloys developed and has numerous applications, including decorative objects (Table 6.6). Bronze is an alloy of copper and tin (Table 6.7); there are also other bronzes, such as aluminum bronze (an alloy of copper and aluminum) and bismuth bronze. Beryllium copper (or beryllium bronze) and phosphor bronze have good strength and hardness, with applications such as springs and bearings. Other major copper alloys are copper nickels and nickel silvers. Production. Copper is found in several types of ores, the most common being sulfide ores. The ores are generally of low grade (containing typically less than 5% copper) and usually are obtained from open-pit mines. The ore is ground into fine particles in ball mills (rotating cylinders with metal balls inside to crush the ore, as illustrated in Fig. 17.6b); the resulting particles are then suspended in water to form a slurry. Reducing chemicals and oil are added, and the mixture is agitated. The mineral particles form a froth, which is scraped and dried. The dry copper concentrate (as much as one-third of which is copper) is traditionally smelted (melted and fused) and refined, a process known as pyrometallurgy, because heat is used to refine the metal. For such applications as electrical conductors, the copper is further refined electrolytically to a purity of at least 99.95% (oxygen-free electrolytic copper). Copper is also processed by hydrometallurgy, involving both chemical and electrolytic reactions. ׉ 7cassandra://PU0Xm-eGmuGtnoWS1Mldpx0PyU7ICvy4aZQjtQDoPWQ`cZ%Eb׉E 196 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Table 6.6: Properties and Typical Applications of Selected Wrought Copper and Brasses. Nominal Type and UNS number Electrolytic toughpitch copper (C11000) Red brass, 85% (C23000) Cartridge brass, 70% (C26000) Free-cutting brass (C36000) Naval brass (C46400 to C46700) composition (%) Ultimate tensile strength (MPa) 99.90 Cu, 0.04 O 220–450 85.0 Cu, 15.0 Zn 70.0 Cu, 30.0 Zn 270–725 300–900 Yield strength (MPa) 70–365 70–435 75–450 Elongation in 50 mm (%) 55–4 Typical applications Downspouts, gutters, roofing, gaskets, auto radiators, bus bars, nails, printing rolls, rivets 55–3 Weather stripping, conduits, sockets, fasteners, fire extinguishers, condenser and heat-exchanger tubing 66–3 Radiator cores and tanks, flashlight shells, lamp fixtures, fasteners, locks, hinges, ammunition components, plumbing accessories 61.5 Cu, 3.0 Pb, 35.5 Zn 60.0 Cu, 39.25 Zn, 0.75 Sn 340–470 380–610 125–310 170–455 53–18 50–17 Gears, pinions, automatic high-speed screw machine parts Aircraft: turnbuckle barrels, balls, bolts; marine hardware: propeller shafts, rivets, valve stems, condenser plates Table 6.7: Properties and Typical Applications of Selected Wrought Bronzes. Nominal Type and UNS number Architectural bronze (C38500) Phosphor bronze, 5% A (C51000) composition (%) 95.0 Cu, 5.0 Sn, trace P Free-cutting phosphor 88.0 Cu, 4.0 Pb, bronze (C54400) Low-silicon bronze, (C65100) Nickel silver, 65–10 (C74500) 4.0 Zn, 4.0 Sn 98.5 Cu, 1.5 Si 65.0 Cu, 25.0 Zn, 10.0 Ni Ultimate tensile strength (MPa) 57.0 Cu, 3.0 Pb, 415 (as extruded) 40.0 Zn 325–960 300–520 275–655 340–900 Yield strength (MPa) 140 130–550 130–435 100–475 125–525 Elongation in 50 mm (%) 30 64–2 50–15 55–11 50–1 Typical applications Architectural extrusions, storefronts, thresholds, trim, butts, hinges Bellows, clutch disks, cotter pins, diaphragms, fasteners, wire brushes, chemical hardware, textile machinery Bearings, bushings, gears, pinions, shafts, thrust washers, valve parts Hydraulic pressure lines, bolts, marine hardware, electrical conduits, heat-exchanger tubing Rivets, screws, slide fasteners, hollowware, nameplates 6.5 Nickel and Nickel Alloys Nickel (Ni) is a silver-white metal and a major alloying element in metals imparting strength, toughness, and corrosion resistance. It is used extensively in stainless steels and in nickel-based alloys (also called superalloys). Nickel alloys are used in high-temperature applications (such as jet engine components, rockets, and nuclear power plants), food-handling and in chemical-processing equipment, coins, and marine applications. Because nickel is magnetic, its alloys are used in electromagnetic applications, such as solenoids. The principal use of nickel as a metal is in the electroplating of parts for their appearance and for improvement of their corrosion and wear resistance. Nickel alloys have high strength and corrosion resistance at elevated temperatures. Common alloying elements in nickel are chromium, cobalt, and molybdenum. The behavior of nickel alloys in machining, forming, casting, and welding can be modified by various other alloying elements. ׉ 7cassandra://T-0Yn1aX_pe4QPHcuhKT9n-3olkagDM6eZfAx4pjJfc`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://jWJ103kbDe98kxrFt_hcwD9smbcY833Wjt5HJhVwHiI`׉ 7cassandra://qF_R9i64TxEsHvD-MZOwkLqXtyoVrV7DrYLY_I3LfOkW`s׉ 7cassandra://JAKqeDmmjPNnV6ZBsJHV_3Y_UZbfDQLZ8yyzMDazrTk`׉ 7cassandra://Ggnb2QHpZT8UswhIQMl6ezfW9WjoWo-YzQI7j82Wg8wjm̬͠^cZEb;ט#u׉׉ 7cassandra://2RMoPMg9mWh30earITQWeG-f9-kMejnQRfiNvFALFz8#`׉ 7cassandra://3XjED3IvaZFmdIVDU5vehDFexHXgqYS2G7n2fr3h_o4^`s׉ 7cassandra://1vAIzANNolf8xiRFFXPFzl6us1tohOSQHCf3u6ToPNs`׉ 7cassandra://_PlY0UndNhostEIwxeu5vPOgxpWRnrvb37oEw-2PAFIw͠^cZEb<׉E Superalloys 197 Table 6.8: Properties and Typical Applications of Selected Nickel Alloys (All Are Trade Names). Ultimate Nominal Type and UNS number Nickel 200 (annealed) Duranickel 301 (age hardened) Monel R-405 (hot rolled) Monel K-500 (age hardened) Inconel 600 (annealed) Hastelloy C-4 (solution treated and quenched) composition (%) – 4.4 Al, 0.6 Ti 30 Cu 29 Cu, 3 Al 15 Cr, 8 Fe 16 Cr, 15 Mo tensile strength (MPa) 1300 525 1050 640 785 Yield strength (MPa) 380–550 100–275 900 230 750 210 400 Elongation in 50 mm (%) 60–40 28 35 30 48 54 Typical applications Chemical and food processing industry, aerospace equipment, electronic parts Springs, plastics extrusion equipment, molds for glass, diaphragms Screw-machine products, water meter parts Pump shafts, valve stems, springs Gas turbine parts, heat-treating equipment, electronic parts, nuclear reactors Parts requiring high-temperature stability and resistance to stress-corrosion cracking A variety of nickel alloys, with a wide range of strengths at different temperatures, have been developed (Table 6.8). Although trade names are still in wide use, nickel alloys are identified in the UNS system with the letter N; thus, for example, Hastelloy G is N06007. Other common trade names are as follow: • Monel is a nickel–copper alloy. • Hastelloy (also a nickel–chromium alloy) has good corrosion resistance and high strength at elevated temperatures. • Nichrome (an alloy of nickel, chromium, and iron) has high electrical resistance and high resistance to oxidation and is used for electrical heating elements. • Invar and Kovar (alloys of iron and nickel) have relatively low sensitivity to temperature changes (Section 3.6). Production. The main sources of nickel are sulfide and oxide ores, all of which have low concentrations of nickel. The metal is produced by sedimentary and thermal processes, followed by electrolysis; this sequence yields 99.95% pure nickel. 6.6 Superalloys Superalloys are important in high-temperature applications, hence they are also known as heat-resistant or high-temperature alloys. Superalloys generally have good resistance to corrosion, mechanical and thermal fatigue, mechanical and thermal shock, and creep and erosion at elevated temperatures. Major applications of superalloys are in jet engines and gas turbines; other applications are in reciprocating engines, rocket engines, tools and dies for hot working operations, and in the nuclear, chemical, and petrochemical industries. Generally, superalloys are identified by trade names or by special numbering systems, and are available in a variety of shapes. Most superalloys have a maximum service temperature of about 1000◦C in structural applications. For non-load bearing components, temperatures can be as high as 1200◦C. Superalloys are generally referred to as iron-based, cobalt-based, or nickel-based. • Iron-based superalloys generally contain from 32 to 67% Fe, 15 to 22% Cr, and 9 to 38% Ni. Common alloys in this group are the Incoloy series. ׉ 7cassandra://JAKqeDmmjPNnV6ZBsJHV_3Y_UZbfDQLZ8yyzMDazrTk`cZ%Eb׉E 198 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Table 6.9: Properties and Typical Applications of Selected Superalloys at 870◦C (All Are Trade Names). Ultimate tensile Alloy Condition Astroloy Wrought Co-28Cr-6Mo Cast Hastelloy X Wrought Cast IN-100 IN-102 Wrought Inconel 625 Inconel 718 Wrought Cast MAR-M 200 MAR-M 432 Ren´e41 Cast Wrought Udimet 700 Wrought Waspaloy Wrought Wrought strength (MPa) 770 655 255 885 215 285 340 840 730 620 690 525 Yield strength (MPa) 690 450 180 695 200 275 330 760 605 550 635 515 Elongation in 50 mm (%) 25 8 50 6 110 125 88 4 8 19 27 35 Typical applications Forgings for high-temperature use Surgical implants Jet engine sheet parts Jet engine blades and wheels Superheater and jet engine parts Aircraft engines and structures, chemical processing equipment Jet engine and rocket parts Jet engine blades Integrally cast turbine wheels Jet engine parts Jet engine parts Jet engine parts • Cobalt-based superalloys generally contain from 35 to 65% Co, 19 to 30% Cr, and up to 35% Ni. These superalloys are not as strong as nickel-based superalloys, but they retain their strength at higher temperatures. • Nickel-based superalloys are the most common of the superalloys and are available in a wide variety of compositions (Table 6.9). The proportion of nickel is from 38 to 76% and also contain up to 27% Cr and 20% Co. Common alloys in this group are the Hastelloy, Inconel, Nimonic, Ren´e, Udimet, Astroloy, and Waspaloy series. 6.7 Titanium and Titanium Alloys Titanium (Ti, named after the Greek god Titan) is a silvery white metal discovered in 1791, but not produced commercially until the 1950s. Although titanium is expensive, its high strength-to-weight ratio and corrosion resistance at room and elevated temperatures make it attractive for many applications, including aircraft; jet engines (see Fig. 6.1); racing cars; golf clubs; chemical, petrochemical, and marine components; submarine hulls; armor plate; and medical applications, such as orthopedic implants (Table 6.10). Titanium alloys are available for service at 550◦C for long periods of time, and service at up to 750◦C for shorter periods. Unalloyed titanium, known as commercially pure titanium, has excellent corrosion resistance for applications where strength considerations are secondary. Aluminum, vanadium, molybdenum, manganese, and other alloying elements impart special properties, such as improved workability, strength, and hardenability. The properties and manufacturing characteristics of titanium alloys are extremely sensitive to small variations in both alloying and residual elements. The control of composition and processing are therefore important, especially for the prevention of surface contamination by hydrogen, oxygen, or nitrogen during processing. These elements cause embrittlement (Section 1.5.2) of titanium and, consequently, reduce toughness and ductility. The body-centered cubic structure of titanium (beta-titanium) is above 880◦C and is ductile, whereas its hexagonal close-packed structure (alpha-titanium) is somewhat brittle and is very sensitive to stress corrosion. A variety of other structures (alpha, near-alpha, alpha–beta, and beta) can be obtained by alloying ׉ 7cassandra://1vAIzANNolf8xiRFFXPFzl6us1tohOSQHCf3u6ToPNs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Q_VeSQTzTgMU1I8ORhn8p0ud7phGRHWYWdr-OIOcaUw`׉ 7cassandra://HttB6p0xZICpoPG1CV8AR8cI55DGSyAVMkrLkviic6I[ `s׉ 7cassandra://9rd7gRKyTYb039RZH7YR_qtgVmGPWzl5BHMxrGSWOeU>`׉ 7cassandra://bqLrdbzUXU15h4uICoUtGrTFm0GiyCc5GBn47MbDXPgk͠^cZEbBט#u׉׉ 7cassandra://_N5wKA9zmEUJK6l8spiGTxIM1cVkIC8cfYfhAnmEgLE`׉ 7cassandra://vDOxaQ58AHPll-RDMuMNfS3ZQDnROdOKyA7AR9JasP0gT`s׉ 7cassandra://IFbGAFOD5MsXjT6KblICCxDdXya4mLmEC6W44-kAUzs\`׉ 7cassandra://rPkEpOypQ31xdeXTCFrEbxzqy-MkQbAOEazHdU6Gtwwf͠^cZEbC׉E Refractory Metals and Alloys 199 Table 6.10: Properties and Typical Applications of Selected Wrought Titanium Alloys at Various Temperatures. Nominal UNS number R50250 R54520 R56400 composition (%) 99.5 Ti 5 Al, 2.5 Sn 6 Al, 4 V Condition Annealed Annealed Annealed Solution + age R58010 13 V, 11 Cr, 3 Al Solution + age Temperature (◦C) 25 300 25 300 25 300 25 300 25 425 Ultimate tensile strength (MPa) 330 150 860 565 1000 725 1175 980 1275 1100 Yield strength (MPa) 240 95 810 450 925 650 1100 900 1210 830 Elongation (%) 30 16 14 10 8 Reduction in area (%) 55 40 30 20 – and heat treating, so that the properties can be optimized for specific applications. Titanium aluminide intermetallics (TiAl and Ti3Al; see Section 4.2.2) have higher stiffness and lower density than conventional titanium alloys, and can withstand higher temperatures. By far, the most widely used alloy of titanium is titanium—6% aluminum and 4% vanadium, or Ti-6-4; it is for aerospace structural applications and medical implants, because of its high strength, good fatigue and corrosion resistance, and (within the body) for its high biocompatibility. Production. Ores containing titanium are first reduced to titanium tetrachloride in an arc furnace, then converted to titanium chloride in a chlorine atmosphere. The compound is reduced further to titanium metal by distillation and leaching (dissolving). This sequence forms sponge titanium, which is then pressed into billets, melted, and poured into ingots to be later processed into various shapes. The complexity of these multistep thermochemical operations (the Kroll process) adds considerably to the cost of titanium. 6.8 Refractory Metals and Alloys There are four refractory metals: molybdenum, niobium, tungsten, and tantalum; they are called refractory because of their high melting points; they are also important alloying elements in steels and superalloys. More than most other metals and alloys, refractory metals maintain their strength at elevated temperatures, and thus are of great importance in rocket engines, gas turbines, and various other aerospace applications; in the electronic, nuclear-power, and chemical industries; and as tool and die materials. The temperature range for some of these applications is on the order of 1100 to 2200◦C, where strength and oxidation are of major concern. 6.8.1 Molybdenum Molybdenum (Mo) is a silvery white metal, with a high melting point, high modulus of elasticity, good resistance to thermal shock, and good electrical and thermal conductivity. It is used in higher amounts than any other refractory metal, in applications such as solid-propellant rockets, jet engines, honeycomb structures, electronic components, heating elements, and dies for die casting. The principal alloying elements for molybdenum are titanium and zirconium. Molybdenum is itself also an important alloying element in cast and wrought alloy steels and in heat-resistant alloys, imparting strength, toughness, and corrosion ׉ 7cassandra://9rd7gRKyTYb039RZH7YR_qtgVmGPWzl5BHMxrGSWOeU>`cZ%Eb׉E200 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications resistance. A major limitation of molybdenum alloys is their low resistance to oxidation at temperatures above 500◦C, necessitating the need for protective coatings. Production. The main source of molybdenum is the mineral molybdenite (molybdenum disulfide). The ore is first processed whereby the molybdenum is concentrated; it is then chemically reduced, first with oxygen and then with hydrogen. Powder-metallurgy techniques (Chapter 17) also are used to produce ingots, for further processing into various shapes. 6.8.2 Niobium (Columbium) Niobium (Nb, for niobium, after Niobe, the daughter of the mythical Greek king Tantalus) was first identified in 1801; it is also called columbium (after its source mineral, columbite). Niobium possesses good ductility and formability, and has higher oxidation resistance than other refractory metals. With various alloying elements, niobium alloys can be produced with moderate strength and good fabrication characteristics. These alloys are generally used in rockets and missiles and in nuclear, chemical, and superconductor applications. Niobium is also an alloying element in various alloys and superalloys. The metal is processed from ores by reduction and refinement, and from powder by first melting and shaping into ingots. 6.8.3 Tungsten Tungsten (W, for wolfram, its European name, and from its source mineral, wolframite; in Swedish, tung means heavy and sten means stone) is the most abundant of all the refractory metals. Tungsten has the highest melting point of any metal (3410◦C) and is notable for its high strength at elevated temperatures. However, it has high density, and hence used for balancing weights and counterbalances in mechanical systems, including self-winding watches. It is brittle at low temperatures and has poor resistance to oxidation. As an alloying element, tungsten imparts elevated-temperature strength and hardness to steels. Tungsten alloys are used for applications involving temperatures above 1650◦C, such as nozzle throat liners in missiles and in the hottest parts of jet and rocket engines, circuit breakers, welding electrodes, tooling for electrical-discharge machining, and spark-plug electrodes. Tungsten carbide, with cobalt as a binder for the carbide particles, is one of the most important tool and die materials (Chapter 22). Tungsten is processed from ore concentrates by chemical decomposition, then reduced, and further processed by powder-metallurgy techniques in a hydrogen atmosphere. 6.8.4 Tantalum Tantalum (Ta, after the mythical Greek king, Tantalus) is characterized by its high melting point (3000◦C), high density, good ductility, and resistance to corrosion. However, it has poor chemical resistance at temperatures above 150◦C. Tantalum is used extensively in electrolytic capacitors and in various components in the electrical, electronic, medical product (see Section 6.16), and chemical industries. It also is used for thermal applications, such as in furnaces and acid-resistant heat exchangers, and as an alloying element. A variety of tantalum-based alloys are available for use in missiles and aircraft. It is processed by techniques similar to those used for processing niobium. 6.9 Beryllium Steel gray in color, beryllium (Be, from the ore beryl) has a high strength-to-weight ratio. Unalloyed beryllium is used in rocket nozzles, space and missile structures, aircraft disc brakes, and precision instruments and mirrors. It is also used in nuclear and X-ray applications because of its low neutron absorption. Beryllium is also an alloying element, and its alloys of copper and nickel are used in various applications, ׉ 7cassandra://IFbGAFOD5MsXjT6KblICCxDdXya4mLmEC6W44-kAUzs\`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ZQ-L5Letq5W7cr0P63qmbcWBHNo2WzQD6wWtpMzQAAYH`׉ 7cassandra://2PG2OPe6KonB0Og02tqF6aweuM9eFT-DGnYTg1gPeTg\C`s׉ 7cassandra://kRQha_rAJRBBa7OPbnasu46fwetlse-HjXc2ITI3bLsz`׉ 7cassandra://6K7ZA644tPQjG9LoLmy51nRWdvk-CCGCZ0ox0kc7HtAV͠^cZEbEט#u׉׉ 7cassandra://NErJ89q_T0XLRYr-UG1TPHlfTSV-xwy3pc8P7LRV1V8`׉ 7cassandra://gTC-wlMpGoYRxlN6aSB3HT-HquoQyMUvEQaHVZIeTGUg`s׉ 7cassandra://peiu_OZkhYCCo65K8IARK-GcIIPG4rSSjbVrNtGJLSA`׉ 7cassandra://L_9L186yJ-2GMbCMqUXK1npD35Vx465m6QrOMm9KbHMb͠^cZEbF׉E Low-melting Alloys 201 including springs (beryllium copper), electrical contacts, and in non-sparking tools for use in explosive environments, such as mines and metal-powder production. Beryllium and its oxide are toxic. 6.10 Zirconium Zirconium (Zr) is silvery in appearance; it has good strength and ductility at elevated temperatures and has good corrosion resistance because of an adherent oxide film. Zirconium is used in electronic components and in nuclear-power reactor applications because of its low neutron absorption. 6.11 Lithium Lithium, from the Greek lithos or stone, is the lightest metal, with a silvery-white appearance. It is widely used in rechargeable batteries, with applications ranging from computers and cell phones to automobiles. It is also used to form alloys with aluminum and magnesium for lightweighting applications. Lithium carbonate is added to glass to improve its mechanical properties. Lithium stearate is an effective lubricating grease. Lithium is produced from ocean water brines or by crushing ore and converting the lithium compounds to lithium chloride, which is then melted and a current passed through the material to produce lithium and chlorine gas. 6.12 Low-melting Alloys Low-melting alloys are so named because of their relatively low melting temperatures. The major metals in this category are lead, zinc, tin, and their alloys. 6.12.1 Lead Lead (Pb, after plumbum, the root of the word plumber) has the properties of high density, resistance to corrosion (by virtue of the stable lead-oxide layer that forms to protect the surface), softness, low strength, ductility, and good workability. Alloying it with various elements (such as antimony and tin) enhances its desirable properties, making it suitable for piping, collapsible tubing, bearing alloys (Babbitt), cable sheathing, foil (as thin as 0.01 mm), roofing, and lead–acid storage batteries. Lead also is used for damping vibrations, radiation shielding against X-rays, ammunition, and in the chemical industry. The oldest known lead artifacts were made in about 3000 B.C. Lead pipes made by the Romans and installed in the Roman baths in Bath, England, two millennia ago, are still in use. An additional use of lead is as a solid lubricant for hot metal-forming operations. Because of its toxicity, however, major efforts continue to be made to replace lead with other elements, such as lead-free solders (Section 32.3.1). The most important mineral source of lead is galena (PbS); it is mined, smelted, and refined by chemical treatments. 6.12.2 Zinc Zinc (Zn) is bluish white in color and is the metal that is fourth most utilized industrially, after iron, aluminum, and copper. It has three major uses: (1) for galvanizing iron, steel sheet, and wire, (2) as an alloying element in other metals, and (3) as a metal for castings. In galvanizing, zinc serves as an anode and protects steel (cathode) from corrosive attack should the coating be scratched or punctured. Zinc is also used as an alloying element; brass, for example, is an alloy of copper and zinc. In zinc-based alloys, the major alloying elements are aluminum, copper, and magnesium; they impart strength and provide dimensional control during casting of the metal. ׉ 7cassandra://kRQha_rAJRBBa7OPbnasu46fwetlse-HjXc2ITI3bLsz`cZ%Eb׉E!202 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Zinc-based alloys are used extensively in die casting (Section 11.4.5), for making such products as fuel pumps and grills for automobiles, components for household appliances such as vacuum cleaners and washing machines, kitchen equipment, machinery parts, and photoengraving equipment. Another use for zinc is in superplastic alloys (Section 2.2.7). A very-fine grained 78% Zn–22% Al sheet is a common example of a superplastic zinc alloy that can be formed by methods used for forming plastics or metals (Part III). Production. The principal mineral source for zinc is zinc sulfide, also called zincblende. The ore is first roasted in air and converted to zinc oxide. It is then reduced to zinc either electrolytically (using sulfuric acid) or by heating it in a furnace with coal, which causes the molten zinc to separate. 6.12.3 Tin Although used in small amounts, as compared to iron, aluminum, or copper, tin (Sn, from the Latin stannum) is an important metal. A silver-white, lustrous metal, its most extensive use is as a protective coating on steel sheets (tin plates) used in making containers (tin cans), for food and for various other products. The low shear strength of the tin coatings on steel sheet improves its deep drawability (Section 16.7.1). Unlike galvanized steels, if this coating is punctured or destroyed, the steel corrodes because the tin is cathodic. Unalloyed tin is used in such applications as lining for water distillation plants and as a molten layer of metal in the production of float glass plate (Section 18.3.1). Tin-based alloys (also called white metals) generally contain copper, antimony, and lead. These alloying elements impart hardness, strength, and corrosion resistance. Tin itself is an alloying element for dental alloys and for bronze (copper–tin alloy), titanium, and zirconium alloys. Tin–lead alloys are common soldering materials (Section 32.3), with a wide range of compositions and melting points. Because of their low friction coefficients (which result from low shear strength and low adhesion), some tin alloys are used as journal-bearing materials. Known as babbitts (after I. Babbitt, 1799–1862), these alloys contain tin, copper, and antimony. Pewter, an alloy of tin, copper, and antimony, is used for tableware, hollowware, and decorative artifacts. Tin alloys are also used in making organ pipes. The most important tin mineral is cassiterite (a low grade tin oxide). The ore is first mined, then concentrated using various techniques, smelted, refined, and cast into ingots for further processing. 6.13 Precious Metals The most important precious (costly) metals, also called noble metals, are the following: • Gold (Au, from the Latin aurum) is soft and ductile, and has good corrosion resistance at any temperature. Typical applications include jewelry, coinage, reflectors, gold leaf for decorative purposes, dental work, electroplating, and electrical contacts and terminals. • Silver (Ag, from the Latin argentum) is ductile and has the highest electrical and thermal conductivity of any metal (see Table 3.2). However, it develops an oxide film that adversely affects its surface characteristics and appearance. Typical applications for silver include tableware, jewelry, coinage, electroplating, solders, bearing linings, and food and chemical equipment. Sterling silver is an alloy of silver and 7.5% copper. • Platinum (Pt) is a soft, ductile, grayish-white metal that has good corrosion resistance, even at elevated temperatures. Platinum alloys are used as electrical contacts; for spark-plug electrodes; as catalysts for automobile pollution-control devices; in filaments and nozzles; in dies for extruding glass fibers (Section 18.3.4); in thermocouples; and in jewelry and dental work. ׉ 7cassandra://peiu_OZkhYCCo65K8IARK-GcIIPG4rSSjbVrNtGJLSA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://XXUvz9k0SiAs8A4cQrfFUY06slw0NUsq4VyosXybXv0Y`׉ 7cassandra://JvRxV10nksrfOpod0LR09UL8HYLYua7ho7p_YK610Zks`s׉ 7cassandra://Ah40fsTyv9mrzoB1GltwGiyhax9LcsPd6BAV9gXIqwYr`׉ 7cassandra://P0_4uWNHNYVH3HVtbtTUXS_RJ9d0LjVaNlBufZEZcyYb͠^cZEbHט#u׉׉ 7cassandra://f_wCuEgqqImaVy6FUvK_X2ACmDWUY_5_wtMo5lfu4SA`׉ 7cassandra://3kfou3Go280EcFmJTHVVVcieFXPZ6H6a6hZhAwx53xwR`s׉ 7cassandra://kCMcoHtclwtFGNs2Q1FEJTGB8EU1OZDatla2psIgKsE6`׉ 7cassandra://rL9EazG60ciS4l3uhbfgqawnen3W8YxnFq_mIVpo21Q͖͠^cZEbI׉EMetal Foams and Metamaterials 203 6.14 Shape-memory Alloys (Smart Materials) Shape-memory alloys are unique in that, after being plastically deformed at room temperature into various shapes, they return to their original shape upon heating. For example, a piece of straight wire made of such a material, can be wound into the shape of a helical spring; when heated, the spring uncoils and returns to its original straight shape. Shape-memory alloys can be used to generate motion and/or force in temperature-sensitive actuators. The behavior of these alloys, also called smart materials, can be reversible; that is, the shape can switch back and forth repeatedly upon application and removal of heat. A typical shape-memory alloy is 55% Ni–45% Ti (Nitinol); other alloys are copper–aluminum–nickel, copper–zinc–aluminum, iron–manganese–silicon, and titanium–nickel–hafnium. Shape-memory alloys generally also have such properties as good ductility, corrosion resistance, and high electrical conductivity. Applications of shape-memory alloys include sensors, relays, pumps, switches, connectors, clamps, fasteners, seals, and stents for blocked arteries. As an example, a nickel–titanium valve has been made to protect people from being scalded in sinks, tubs, and showers. It is installed directly into the piping system and brings the water flow down to a trickle within three seconds after the water temperature reaches 47◦C. More recent developments include thin-film shape-memory alloys deposited on polished silicon substrates for use in microelectromechanical (MEMS) devices (Chapter 29). 6.15 Amorphous Alloys (Metallic Glasses) A class of metal alloys that, unlike metals, do not have a long-range crystalline structure is called amorphous alloys. They have no grain boundaries, and their atoms are packed randomly and tightly. The amorphous structure was first obtained in the late 1960s by rapid solidification of a molten alloy (Section 11.6). Because their structure resembles that of glasses, these alloys are also called metallic glasses. Amorphous alloys typically contain iron, nickel, and chromium, alloyed with carbon, phosphorus, boron, aluminum, and silicon. They are available as wire, ribbon, strip, and powder: One application is for faceplate inserts on golf-club heads; the alloy has a composition of zirconium, beryllium, copper, titanium, and nickel and is made by die casting. Another application is in hollow aluminum baseball bats, coated with a composite of amorphous metal by thermal spraying, and is said to improve the performance of the bat. Amorphous alloys exhibit excellent corrosion resistance, good ductility, high strength, and very low magnetic hysteresis (utilized in the production of magnetic steel cores for transformers, generators, motors, lamp ballasts, magnetic amplifiers, and linear accelerators). They have low magnetic hysteresis loss, providing greatly improved efficiency; however, fabrication costs are significant. Amorphous steels have been demonstrated to have strengths twice those of high-strength steels, and have potential applications in large structures; however, they are presently cost prohibitive. A major application for the superalloys of rapidly solidified powders is the consolidation into near-net shapes for parts used in aerospace engines. 6.16 Metal Foams and Metamaterials Metal foams are structures where the metal consists of only 5–20% of the structure’s volume, as shown in Fig. 6.4. Usually made of aluminum alloys (but also of titanium, tantalum, and others), metal foams can be produced by blowing air into molten metal and tapping the froth that forms at the surface. The froth then solidifies into a foam. Other approaches to producing metal foam include (a) chemical vapor deposition (Section 34.6.2) onto a carbon foam lattice, (b) depositing metal powders from a slurry onto a polymer foam lattice, followed by sintering (Section 17.4) to fuse the metals and burn off the polymer, (c) doping molten or powder metals (Chapter 17) with titanium hydride (TiH2), which then releases hydrogen gas at the elevated casting or sintering temperatures, and (d) pouring molten metal into a porous salt and, upon cooling, leaching out the salt with acid. ׉ 7cassandra://Ah40fsTyv9mrzoB1GltwGiyhax9LcsPd6BAV9gXIqwYr`cZ%Eb׉E204 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications Figure 6.4: Structure of a metal foam used in orthopedic implants to encourage bone ingrowth. Source: Courtesy of Zimmer, Inc. Metal foams have unique combinations of strength-to-density and stiffness-to-density ratios, although these ratios are not as high as the base metals themselves. However, metal foams are very lightweight and thus are attractive materials, especially for aerospace applications. Because of their porosity, other applications of metal foams are filters and orthopedic implants. More recent developments include nickel–manganese–gallium metal foams with shape-memory characteristics (Section 6.14). Metamaterials are similar to metal foams, in that they have a very high porosity; the difference is that metamaterials are designed to achieve certain mechanical, thermal, or electrical properties. The metamaterial shown in Fig. 6.5 consists of a number of struts, just as with a metal foam. However, the struts do not have a random orientation or location; as is common, this metamaterial has a unit cell that is repeated to fill a volume. By changing the geometry, it is possible to obtain desired characteristics such as high strength-toweight or stiffness-to-weight ratios, or zero (or negative) Poisson’s ratio. An application for metamaterials is described in Case Study 20.2. Figure 6.5: Nickel alloy hierarchical metamaterial and critical features across two orders of magnitude in length scale. (a) Large-area, high-resolution additive manufacturing of hierarchical metamaterials. (b)–(c) Optical microscope images of bulk hierarchical lattice material with a network of hierarchical stretch-dominated octet unit cells. Source: Xiaoyu (Rayne) Zheng, Virginia Tech. ׉ 7cassandra://kCMcoHtclwtFGNs2Q1FEJTGB8EU1OZDatla2psIgKsE6`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://5Og2oCl2u20m5vDpnFzymypZxsG6G89sgI6YZ0XtkgQO`׉ 7cassandra://rduWIiT9DgvzXkHhl5BfkOfajL4eB0rwiz7M1ZrpwFsb`s׉ 7cassandra://vbJALbTmK24tfWZ25kgbrmUMho1uX3gKghNW0tENsZM`׉ 7cassandra://TzKteJxUj-IHhTSkWuXg_rG7BlnTq1fzRBnVxYg8R6U[͠^cZEbKט#u׉׉ 7cassandra://AHr9tccNQmIH-4v_dk1mORxtgJWZ8HDi02Yav5APvRY`׉ 7cassandra://7s7JDfd2Dq8KJVYDQ97bWrskKAxIiRWHQaAFtRAWICAWR`s׉ 7cassandra://Spj-B31tK7v0EA9skDncRM8XfWpxfHnV9s85ETFKC5kZ`׉ 7cassandra://FaUIhAzyhamSzg9NiqAR1hfhtkAFYSPO_fAUo-205h4d͠^cZEbL׉E hSummary 205 Metamaterials have mainly been manufactured by additive manufacturing (Chapter 20) because of the ability to design the structure; an alternative approach uses a additive manufactured structure that is then investment cast to produce a metamaterial. The slender nature of the struts in Fig. 6.5 would not be suitable for investment casting, however, and the struts cell dimensions of such metamaterials are much larger. 6.17 Rare Earth Metals Rare earth metals. Rare earth metals are so named because they are generally difficult to mine and are available only in small quantities. However, they are often more common in the Earth’s crust than precious metals, such as gold and platinum. Because of their unique magnetic, luminescent, and electrical properties, rare earth metals are essential in modern technologies (see below). Among the more important rare earth metals are: 1. Yttrium, which is used in compact fluorescent lamps, light-emitting diodes, flat-panel monitors, laser technology, and superconductor applications. 2. Lanthanum, which is used in catalytic converters and anode materials in high-performance batteries. Most hybrid automobiles depend on lanthanum anodes in their batteries; for example, it is estimated that each Toyota Prius uses 10–15 kg of lanthanum. 3. Dysprosium, which is used in the production of lasers and commercial lighting, as well as in dosimeters to measure radiation exposure. When exposed to radiation, dysprosium emits light, which can be measured and correlated to radiation strength; it is also highly magnetostrictive, deforming under a magnetic field, thus making it useful for transducers and resonators. 4. Neodymium More common than cobalt, nickel, and copper, neodymium is the second most common rare earth metal (after cerium). Its alloy, Nd2Fe14B, produces the strongest permanent magnets known. As such, it is used where small but powerful magnets are required, such as in-ear headphones and microphones. 5. Cerium, which is mainly used in catalytic converters in automobiles, for the oxidation of carbon monoxide and nitrous oxide; it is also used in glass manufacture, permanent magnets, fuel cells, and in polishing optical components. 6. Samarium, which is used in compounds of cobalt (usually SmCo5 or SmCo17) and is the second strongest permanent magnet known, next to neodymium magnets. However, samarium-cobalt magnets have better stability and can be used to temperatures as high as 700◦C, whereas neodymium magnets are limited to 300◦Corso. 7. Terbium, which is used as a dopant in solid-state electronic devices and in various sensors. It has the highest magnetostriction (Section 3.7) of any alloy. Several other rare-earth metals have been used in industrial applications, usually for magnets (praseodymium, holmium), computer and portable electronic device displays (scandium, europium), and in radiation shielding (gadolinium, erbium) or in generation of radiation (thulium). Summary • Nonferrous metals and alloys include a very broad range of materials. The most common are aluminum, magnesium, and copper and their alloys, with a wide range of applications. For high temperature service, nonferrous metals include nickel, titanium, refractory alloys (molybdenum, niobium, tungsten, tantalum), and superalloys. Other nonferrous metal categories include low-melting alloys (lead, zinc, tin) and precious metals (gold, silver, platinum). ׉ 7cassandra://vbJALbTmK24tfWZ25kgbrmUMho1uX3gKghNW0tENsZM`cZ%Eb׉E 206 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications • Nonferrous alloys have a wide variety of desirable properties, such as strength, toughness, hardness, and ductility; resistance to high temperature, creep, and oxidation; a wide range of physical, thermal, and chemical properties; and high strength-to-weight and stiffness-to-weight ratios (particularly for aluminum and titanium). Nonferrous alloys can be heat treated to impart certain specific properties. • Shape-memory alloys (smart materials) have unique properties, with numerous applications in a variety of products as well as in manufacturing operations. • Amorphous alloys (metallic glasses) have properties that are superior to other materials; available in various forms, they have numerous applications. • Metal foams are very lightweight and thus are attractive for aerospace as well as various other applications. • Rare earth metals are not actually rare, but are difficult to mine in large amounts. These metals have widespread applications, including permanent magnets and devices that exploit them, computer and portable electronic displays, batteries and fuel cells, and radiation shielding and generation. • As with all materials, the selection of a nonferrous material for a particular application requires a careful consideration of several factors, including design and service requirements, long-term effects, chemical affinity to other materials, environmental attack, and cost. Key Terms Amorphous alloys Babbitts Brass Bronze Galvanizing Low-melting alloys Metal foam Metallic glasses Nonferrous Bibliography Ashby, M.F., Materials Selection in Mechanical Design, 4th ed., Pergamon, 2010. ASM Handbook,Vol.2: Properties and Selection: Nonferrous Alloys and Special-Purpose Materials,ASM International, 1990. ASM Specialty Handbook: Aluminum and Aluminum Alloys, ASM International, 1993. ASM Specialty Handbook: Copper and Copper Alloys, ASM International, 2001. ASM Specialty Handbook: Heat-Resistant Materials, ASM International, 1997. ASM Specialty Handbook: Magnesium and Magnesium Alloys, ASM International, 1999. ASM Specialty Handbook: Nickel, Cobalt, and Their Alloys, ASM International, 2000. Donachie, M.J. (ed.), Titanium: A Technical Guide, 2nd ed., ASM International, 2000. Donachie, M.J., and Donachie, S.J., Superalloys: A Technical Guide, 2nd ed., ASM International, 2002. Geddes, B., Leon, H., and Huang, X., Superalloys: Alloying and Performance, ASM International, 2010. Kaufman, J.G., Introduction to Aluminum Alloys and Tempers, ASM International, 2000. Lexcellent, C., Shape-memory Alloys Handbook, Wiley, 2013. Pewter Precious metals Pyrometallurgy Refractory metals Shape-memory alloys Smart materials Smelting Superalloys Temper designation ׉ 7cassandra://Spj-B31tK7v0EA9skDncRM8XfWpxfHnV9s85ETFKC5kZ`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://vEgiY4Cl09ehWnTDDJIEEbzyAMiKqRA-VurUjM-g7TA`׉ 7cassandra://SDm3D6jgd6kANZKMl5xcfqQvav91mNWqPFVDJKpagToXm`s׉ 7cassandra://vEAOlIGPwc0YrTq_DKQzHQukEm_NfMw6UfHYyxR9UjQ`׉ 7cassandra://Y0xQO408lLYrq_vh1u2rpoRL9dKGsc6dyDKGWbtPPq0T[͠^cZEbNט#u׉׉ 7cassandra://fomZAUgYbBXgfEzEvfYnhTJ3p7DA0AtcF4aB27aKa0Ql/`׉ 7cassandra://wkyXLWFD6sza_eHHrrIdTOvPEByrfZ94X6nrNb17-EMa`s׉ 7cassandra://8L6fuVCroOrOHA39t9HyciuDnaXRLC-HfRzXrPx77ro`׉ 7cassandra://ZpPG_vdc1YZGMi7KZav4m0u_naK5cCk-8Yr9aUQ90oEZ+͠^cZEbO׉E Qualitative Problems 207 Lutjering, G., and Williams, J.C., Titanium, 2nd ed., Springer, 2007. Reed, R.C,. The Superalloys: Fundamentals and Applications, Cambridge, 2006. Russew, K., and Stojanova, L. Glassy Metals, Springer, 2016. Schwartz, M., Smart Materials, CRC Press, 2008. Solanski, K.N., and Orlov, D. (eds.), Magnesium Technology, Springer, 2017. Totten, G. and MacKenzie, D.S. (eds.), Handbook of Aluminum, Vol. 1: Physical Metallurgy and Processes, CRC Press, 2003. Review Questions 6.1. Given the abundance of aluminum in the earth’s crust, explain why it is more expensive than steel. 6.2. Why is magnesium often used as a structural material in power hand tools? Why are its alloys used instead of pure magnesium? 6.3. What are the major uses of copper? What are the alloying elements in brass and bronze, respectively? 6.4. What are superalloys? Why are they so named? 6.5. What properties of titanium make it attractive for use in race-car and jet-engine components? Why is titanium not used widely for engine components in passenger cars? 6.6. Which properties of each of the major refractory metals define their most useful applications? 6.7. What are the main industrial uses of lithium? 6.8. What are metallic glasses? Why is the word “glass” used for these materials? 6.9. What is the composition of (a) babbitts, (b) pewter, and (c) sterling silver? 6.10. Name the materials described in this chapter that have the highest (a) density, (b) electrical conductivity, (c) thermal conductivity, (d) strength, and (e) cost. 6.11. What are the major uses of gold and silver, other than in jewelry? 6.12. Describe the advantages to using zinc as a coating for steel. 6.13. What are nanomaterials? Why are they being developed? 6.14. Why are aircraft fuselages made of aluminum alloys, even though magnesium is a lighter metal? Why isn’t lithium used for such applications? 6.15. How is metal foam produced? 6.16. What metals have the lowest melting points? What applications for these metals take advantage of their low melting points? 6.17. What are the main applications of rare earth metals? Qualitative Problems 6.18. Explain why cooking utensils generally are made of stainless steels, aluminum, or copper. 6.19. Would it be advantageous to plot the data in Table 6.1 in terms of cost per unit weight rather than cost per unit volume? Explain and give some examples. 6.20. Compare the contents of Table 6.3 with those in various other tables and data on materials in this book, and then comment on which of the two hardening processes (heat treating and work hardening) is more effective in improving the strength of aluminum alloys. 6.21. What factors other than mechanical strength should be considered in selecting metals and alloys for high-temperature applications? Explain. ׉ 7cassandra://vEAOlIGPwc0YrTq_DKQzHQukEm_NfMw6UfHYyxR9UjQ`cZ%Eb׉E 208 Chapter 6 Nonferrous Metals and Alloys: Production, General Properties, and Applications 6.22. Assume that, for geopolitical reasons, the price of copper increases rapidly. Name two metals with similar mechanical and physical properties that can be substituted for copper. Comment on your selection and any observations you make. 6.23. If aircraft, such as a Boeing 757, are made of 79% aluminum, why are automobiles made predominantly of steel? 6.24. Portable (notebook) computers and digital cameras can have their housing made of magnesium. Why? 6.25. Most household wiring is made of copper wire. By contrast, grounding wire leading to satellite dishes and the like is made of aluminum. Explain the reason. 6.26. The example in this chapter showed the benefits of making cars from aluminum alloys. However, the average amount of steel in cars has increased in the past decade. List reasons to explain these two observations. 6.27. If tungsten is the highest melting-point metal, why are no high temperature parts in Fig. 6.1 made from tungsten? Quantitative Problems 6.28. A simply supported rectangular beam is 50 mm wide and 2 m long, and it is subjected to a vertical load of 50 kg at its center. Assume that this beam could be made of any of the materials listed in Table 6.1. Select three different materials, and for each, calculate the beam height that would cause each beam to have the same maximum deflection. Calculate the ratio of the cost for each of the three beams. 6.29. Obtain a few aluminum beverage cans, cut them, and measure their wall thicknesses. Using data in this chapter and simple formulas for thin-walled, closed-end pressure vessels, calculate the maximum internal pressure these cans can withstand before yielding. (Assume that the can is a thin-walled, closed-end, internally pressurized vessel.) 6.30. Beverage cans usually are stacked on top of each other in stores. Use the information from Problem 6.24, and, referring to textbooks on the mechanics of solids, estimate the crushing load each of these cans can withstand. 6.31. Using strength and density data, determine the minimum weight of a 1 m-long tension member that must support 3000 N if it is manufactured from (a) 3003-O aluminum, (b) 5052-H34 aluminum, (c) AZ31B-F magnesium, (d) any brass alloy, and (e) any bronze alloy. 6.32. Plot the following for the materials described in this chapter: (a) yield strength vs. density, (b) modulus of elasticity vs. strength, (c) modulus of elasticity vs. relative cost, and (d) electrical conductivity vs. density. Synthesis, Design, and Projects 6.33. Because of the number of processes involved in making metals, the cost of raw materials depends on the condition (hot or cold rolled), shape (plate, sheet, bar, tubing), and size of the metals. Make a survey of the technical literature, obtain price lists or get in touch with suppliers, and prepare a list indicating the cost per 100 kg of the nonferrous materials described in this chapter, available in different conditions, shapes, and sizes. 6.34. The materials described in this chapter have numerous applications. Make a survey of the available literature in the bibliography, and prepare a list of several specific parts or components and applications, indicating the types of materials used. ׉ 7cassandra://8L6fuVCroOrOHA39t9HyciuDnaXRLC-HfRzXrPx77ro`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://PKzkyfKIUWevafGp2t-XvXNK9hij68k96qqlfepZ0FET`׉ 7cassandra://yP0Ynbd1aAURcydPHCn2xwkCdD0qg_Jm_59vLjl2eNoA}`s׉ 7cassandra://x1VhFPFAeXgisZUldPDLNGjBr9oQzqdzpLUGFHlZ5NAl`׉ 7cassandra://12SnFtMVB3_4GcKCRyh3MTumXHBH30kOxELw74ULbps7W͠^cZEbQט#u׉׉ 7cassandra://OnmKR8eTS23sxowdY_gCQo0S55conpCxjbns--YI5KM<`׉ 7cassandra://YKpPwEphoRuaTtzlZWKu_jPplhjJHbMoRIb0yNrjXD0<`s׉ 7cassandra://PNexLxfahkftt_fNkjD0-fG2XqW78QONl_I84XBkdvw `׉ 7cassandra://6hcB-yebA7SsWv_z3nRwad_UlXbAG3kO__OPfWP6T849R͠^cZEbR׉ESynthesis, Design, and Projects 209 6.35. Name products that would not have been developed to their advanced stages (as we find them today) if alloys having high strength, high corrosion resistance, and high creep resistance (all at elevated temperatures) had not been developed. 6.36. Assume that you are the technical sales manager of a company that produces nonferrous metals. Choose any one of the metals and alloys described in this chapter, and prepare a brochure, including some illustrations, for use as sales literature by your staff in their contact with potential customers. 6.37. Give some applications for (a) amorphous metals, (b) precious metals, (c) low-melting alloys, and (d) nanomaterials. 6.38. Describe the advantages of making products with multilayer materials. (For example, aluminum bonded to the bottom of stainless-steel pots.) 6.39. In the text, magnesium was described as the lightest engineering metal. Is it also the lightest metal? Explain. 6.40. Review the technical literature and the Internet and summarize the rare earth metals, their sources, and their main applications. 6.41. Review the technical literature, and write a detailed description of how magnesium is produced from sea water. 6.42. If you were to design an implant for use in the human body, what materials would you exclude? Which metals are possible for such applications? Of these, list three that you feel are best. 6.43. Perform an Internet search and obtain a typical design for an “ear bud,” intended to play sound in a device inserted into an ear. What role do rare earth elements play in ear buds? Estimate the required size of ear buds if rare earth elements were unavailable. ׉ 7cassandra://x1VhFPFAeXgisZUldPDLNGjBr9oQzqdzpLUGFHlZ5NAl`cZ%Eb׉E0Chapter 7 Polymers: Structure, General Properties, and Applications 7.1 Introduction 211 7.2 The Structure of Polymers 211 7.3 Thermoplastics 220 7.4 Thermosetting Plastics 224 7.5 Additives in Plastics 224 7.6 General Properties and Applications of Thermoplastics 225 7.7 General Properties and Applications of Thermosetting Plastics 227 7.8 Biodegradable Plastics 229 7.9 Elastomers (Rubbers) 230 7.10 Gels 231 Case Studies: 7.1 Dental and Medical Bone Cement 217 7.2 Materials for a Refrigerator Door Liner 228 • Polymers display a very wide range of properties and have several advantages over metallic materials, including low cost and ease of manufacturing; they continue to be among the most commonly used materials. • This chapter first describes the structure of polymers, the polymerization process, crystallinity, and the glass-transition temperature. • Mechanical properties and how they are affected by temperature and deformation rate are then described. • Two basic types of polymers are thermoplastics and thermosets. Thermoplastics follow a basic manufacturing procedure of heating them until they soften or melt, and then shaping them into the desired product. Thermosets involve precursors that are formed to a desired shape and set through polymerization or cross-linking between polymer chains. 210 ׉ 7cassandra://PNexLxfahkftt_fNkjD0-fG2XqW78QONl_I84XBkdvw `cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Qud-J6WvHuOAsmVtuzIA7afljznb5uYFt9ipPH0SozQ?`׉ 7cassandra://WSDiFDfnCE3fEbV1QsztREZXdUS2oWM0WmwpiX3L1G0Vz`s׉ 7cassandra://mhG78dtajDEL2O_Lu1nrTXJwUKEt7405ywLaK2mW3Jo`׉ 7cassandra://ZWbGQycUooRx8XgOCLse-Lu2yWiIXDEv0HvvD0duQyAV ͠^cZEbTט#u׉׉ 7cassandra://6-PNZNTpifdADSm7ff3HYlvBfU2F6ijrV6-Qf8y45yQ `׉ 7cassandra://8Xq8Xy7ytMswFiDijA0eUB-flntogdCAeDv2yq0XF-MIv`s׉ 7cassandra://UqnXgIWwUhOCExrYuBgCQZK1SU9DEFxJLuhX3dIkfls`׉ 7cassandra://oVyEcx8uiFcDK8IOOwJcA3JuQ-NEuu3H0HenGJVZSwsi͠^cZEbU׉E {The Structure of Polymers 211 • The chapter then describes the properties and uses of elastomers. • The general properties, typical applications, advantages, and limitations of polymers are all described throughout the chapter, with several specific examples. 7.1 Introduction The word plastics was first used as a noun in 1909, and is commonly interchanged as a synonym for polymers, a term first used in 1866. Plastics are unique in that they have extremely large molecules (macromolecules or giant molecules). Consumer and industrial products made of plastics include food and beverage containers, packaging, signs, housewares, housings for computers and monitors, textiles (clothing), medical devices, foams, paints, safety shields, toys, appliances, lenses, gears, electronic and electrical products, and automobile and aircraft bodies and numerous components. Because of their unique and diverse properties, polymers increasingly have replaced metallic components, reflecting the advantages of polymers in terms of the following characteristics: • Relatively low cost (Table 6.1) and ease of manufacture • Resistance to chemicals • Low electrical and thermal conductivity • Low density • High strength-to-weight ratio, particularly when reinforced • Noise reduction • Wide choice of colors and transparencies • Complex design possibilities and ease of manufacturing Characteristics of polymers that may or may not be desirable are low strength and stiffness (Table 7.1), high coefficient of thermal expansion, low useful-temperature range, and lower dimensional stability in service over a period of time. The word plastic is from the Greek word plastikos, meaning capable of being molded and shaped. Plastics can be formed, cast, machined, and joined into various shapes with relative ease. Little or no additional surface finishing is required; this characteristic provides an important advantage over metals. Plastics are available as film, sheet, plate, rod, and tubing of various cross sections. An outline of the basic process for making synthetic polymers is given in Fig. 7.1. In polyethylene, only carbon and hydrogen atoms are present, but other polymer compounds can be made by including chlorine, fluorine, sulfur, silicon, nitrogen, and oxygen. As a result, an extremely wide range of polymers, with an equally wide range of properties, has been developed. 7.2 The Structure of Polymers The properties of polymers depend largely on the structures of individual polymer molecules, molecule shape and size, and the arrangement of molecules to form a polymer structure. Polymer molecules are characterized by their very large size, a feature that distinguishes them from most other organic chemical compositions. Polymers are long-chain molecules formed by polymerization, that is, by the linking and cross-linking of different monomers. A monomer is the basic building block of a polymer. The word mer ׉ 7cassandra://mhG78dtajDEL2O_Lu1nrTXJwUKEt7405ywLaK2mW3Jo`cZ%Eb׉E 212 Chapter 7 Polymers: Structure, General Properties, and Applications Table 7.1: Range of Mechanical Properties for Various Engineering Plastics at Room Temperature. Ultimate tensile Material Thermoplastics: Acrylonitrile-butadiene-styrene (ABS) ABS, reinforced Acetal Acetal, reinforced Acrylic Cellulosic Fluorocarbon Nylon Nylon, reinforced Polycarbonate Polycarbonate, reinforced Polyester Polyester, reinforced Polyethylene Polypropylene Polypropylene, reinforced Polystyrene Polyvinyl chloride Thermosets: Epoxy Epoxy, reinforced Phenolic Polyester, unsaturated Elastomers: Chloroprene (neoprene) Natural rubber Silicone Styrene-butadiene Urethane 28–55 100 55–70 135 40–75 10–48 7–48 55–83 70–210 55–70 110 55 110–160 7–40 20–35 40–100 14–83 35–140 70–1400 28–70 30 15–25 17–25 5–8 10–25 20–30 1.4–2.8 1.4–3.5 10 1.4–3.5 0.4–1.4 0.7–2 1.4–2.8 2–10 2.5–3 6 2 8.3–12 75–5 0.35 7.5 — 0.35 75–25 50–5 100–5 0.35 — 0.35–0.40 0.37 0.39 300–100 200–60 10–1 125–10 6–4 300–5 3–1 0.1–1.4 1000–15 0.7–1.2 3.5–6 1.4–4 60–1 7–55 0.014–4 3.5–17 21–52 2.8–21 5–9 1–2 1.3 1–5 2–10 2–10 450–40 10–1 4–2 2–0 1–0 100–500 75–650 100–1100 250–700 300–450 500–10 0.46–0.48 0.32–0.40 — 0.38 — 0.38 — 0.46 0.43 4–2 — 0.35 0.40 0.30–0.35 — 0.41 0.40 0.5 0.5 0.5 0.5 0.5 strength (MPa) Elastic modulus Elongation (GPa) (%) Poisson’s ratio, ν (from the Greek word meros, meaning part) indicates the smallest repetitive unit, thus the term is similar to that of unit cell in crystal structures of metals (Section 1.3). The word polymer means many mers, repeated hundreds or thousands of times in a chainlike structure. Most monomers are organic materials, in which carbon atoms are joined through covalent (electron sharing) bonds with other atoms (such as hydrogen, oxygen, nitrogen, fluorine, chlorine, silicon, and sulfur). An ethylene molecule (Fig. 7.2) is an example of a simple monomer, consisting of carbon and hydrogen atoms. 7.2.1 Polymerization Monomers can be linked in repeating units to make longer and larger molecules by a chemical process called a polymerization reaction. Although there are several variations, two polymerization processes are important: condensation and addition polymerization. In condensation polymerization (Fig. 7.3a), polymers are produced by the formation of bonds between two types of reacting mers. A characteristic of this reaction is that the reaction by-products (such ׉ 7cassandra://UqnXgIWwUhOCExrYuBgCQZK1SU9DEFxJLuhX3dIkfls`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://QN9fCDa-eRjSQzsG9G4vy1rOImM3GxWIkeNx2dI8fjs9`׉ 7cassandra://tZAxmz6X-eal2khKhyMdjrHE5SDGEx4pDVTtoncWbCA/`s׉ 7cassandra://cDCGc03aWJG8UNJ-RBuIiunpOXIc-CM2-AY-JQunL6k`׉ 7cassandra://j-KDmkFayTftczmJsEc7cp17tAvUmf5ZVMADyKFqKRYZM"͠^cZEbWט#u׉׉ 7cassandra://Kz5c73M8cDZGAlfi2jR3a8WDs8TyxXZkgkBl7yCT_ck9O`׉ 7cassandra://TxhASbQMbmbcEd3OhnJCPv9aP7aqtVxDJ3CxGp1Ew_cY`s׉ 7cassandra://9iMBW3Q3M41WRd0rHtFMvRwerLC6iMAX1w6qszrFEsUM`׉ 7cassandra://qez9awNOk6gc5haaKFv-Kth93IQLTJ-DQsFpvTfmG-s͊?͠^cZEbX׉EThe Structure of Polymers 213 Fillers Plasticizers Stabilizers Colorants Flame retardants Lubricants Mer or mers Heat, pressure, catalyst Polymerization: condensation, addition Polymer Amorphous Partly crystalline Linear Branched Homopolymer Copolymer Terpolymer Figure 7.1: Outline of the topics described in Chapter 7. Monomer H C H H C H H C H H C H Fl C Fl H C H H C CH3 H C Cl H C C6H5 Fl C Fl H C H H C H H C H H C H Fl C Fl H C H H C CH n3 H C Cl H C C6H5 n Fl C Fl Polytetrafluoroethylene (PTFE) (Teflon) n Figure 7.2: Molecular structure of various polymers. These are examples of the basic building blocks for plastics. Polyvinyl chloride (PVC) n Polystyrene (PS) Polymer repeating unit Polyethylene (PETE) n Polypropylene (PP) Thermoplastics: Acrylics, ABS, nylons, polycarbonates, polyethylenes, polyvinyl chloride, etc. Thermosets: Epoxies, phenolics, polyimides, etc. Elastomers: Natural and synthetic rubbers, silicones, polyurethanes, etc. Cross-linking ׉ 7cassandra://cDCGc03aWJG8UNJ-RBuIiunpOXIc-CM2-AY-JQunL6k`cZ%Eb׉E 214 Chapter 7 Polymers: Structure, General Properties, and Applications Cl C CH2 CH2 CH2 CH2 C O O Cl Adipoyl chloride 1 H H N CH2 CH2 CH2 CH2 CH2 CH2 N Hexamethylene diamine H H C CH2 CH2 CH2 CH2 O O C NCH2 CH2 CH2 CH2 CH2 CH2 H Nylon 6,6 (a) C H C H H H Heat, pressure, catalyst C H H CC H H H (b) Figure 7.3: Examples of polymerization. (a) Condensation polymerization of nylon 6,6 and (b) addition polymerization of polyethylene molecules from ethylene mers. as water) are condensed out (hence the word condensation). This process is also known as step-growth or step-reaction polymerization, because the polymer molecule grows step-by-step until all of one reactant is consumed. In addition polymerization, also called chain-growth or chain-reaction polymerization, bonding takes place without reaction by-products, as shown in Fig. 7.3b. It is called chain reaction because of the high rate at which long molecules form simultaneously, usually within a few seconds, a rate much higher than that in condensation polymerization. In addition polymerization, an initiator is added to open the double bond between two carbon atoms, which then begins the linking process by adding several more monomers to a growing chain. For example, ethylene monomers (Fig. 7.3b) link to produce polyethylene; other examples of addition-formed polymers are given in Fig. 7.2. Molecular Weight. The sum of the molecular weights of the mers in a representative chain is known as the molecular weight of the polymer. The higher the molecular weight of a given polymer, the greater is the average chain length. Most commercial polymers have a molecular weight between 10,000 and 10,000,000. Because polymerization is a random event, the polymer chains produced are not all of equal length, although the chain lengths fall into a traditional distribution curve (described in Section 36.7). The molecular weight of a polymer is determined on a statistical basis by averaging. The spread of the molecular weights in a chain is called the molecular weight distribution (MWD). A polymer’s molecular weight and its distribution have a major influence on its properties. For example, the tensile and the impact strength, the resistance to cracking, and the viscosity (in the molten state) of the polymer all increase with increasing molecular weight (Fig. 7.4). Degree of Polymerization. It is convenient to express the size of a polymer chain in terms of the degree of polymerization (DP), defined as the ratio of the molecular weight of the polymer to the molecular weight of the repeating unit. For example, polyvinyl chloride (PVC) has a mer weight of 62.5; thus, with a molecular weight of 50,000 its DP is 50,000/62.5 = 800. In terms of polymer processing (Chapter 19), the higher the DP, C H H H Mer C H H C H H n N H 1 HCl Condensate Polyethylene ׉ 7cassandra://9iMBW3Q3M41WRd0rHtFMvRwerLC6iMAX1w6qszrFEsUM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Y_fGItQRa1CToNgPAsKPopDcP-zCFakmRzvQ8X5zBaE00`׉ 7cassandra://IF0TO7xwhyQKqrBqQSr-Ty9PFCHxxzac4yCkva5iHjAS`s׉ 7cassandra://kzzxXcwMJGFGNdu39BTIcCoHba21oPlq7X_MW37CfJU`׉ 7cassandra://KszoGd9YEv-XTvHPN5T7hTF3IOsCe2Pwsy26adIKCmsgo͠^cZEbaט#u׉׉ 7cassandra://rRXB5aBXKrmx5XtX0TDWDU4VzDvWqsjlgFJ9YFwZo7kw `׉ 7cassandra://4CD6JM1IGUOie0pD4VZJuqrJq2kk7idyFyn4A38U_o0H`s׉ 7cassandra://zVDUFRskIHO62_Cpa_m496IILBj2NGY-3fP7oYiz6gwn`׉ 7cassandra://2TUq8kH6MOjdqHiaCSGMDjmj4VESjwA0NDZ_Xki3O1oJ͠^cZEbȇ׉E :The Structure of Polymers 215 Commercial polymers Tensile and impact strength Viscosity 104 107 Molecular weight, degree of polymerization Figure 7.4: Effect of molecular weight and degree of polymerization on the strength and viscosity of polymers. the higher is the polymer’s viscosity, or its resistance to flow (Fig. 7.4). High viscosity adversely affects the ease of shaping the polymer, thus raising the cost of processing; however, high DP can result in stronger polymers. Bonding. During polymerization, the monomers are linked together by covalent bonds (Section 1.2), forming a polymer chain; because of their strength, covalent bonds are also called primary bonds. The polymer chains are held together by secondary bonds, such as van der Waals bonds, hydrogen bonds, and ionic bonds (Section 1.2). Secondary bonds are weaker than primary bonds by one to two orders of magnitude. In a given polymer, the increase in strength and viscosity with molecular weight is due, in part, to the fact that the longer the polymer chain, the greater is the energy needed to overcome the combined strength of the secondary bonds. For example, ethylene polymers having DPs of 1, 6, 35, 140, and 1350, at room temperature, are, respectively, in the form of gas, liquid, grease, wax, and hard plastic. Linear Polymers. The chainlike polymers shown in Fig. 7.2 are called linear polymers because of their sequential structure (Fig. 7.5a); however, a linear molecule is not necessarily straight in structure. In addition to those shown in this figure, other linear polymers include polyamides (nylon 6,6) and polyvinyl fluoride. Generally, a polymer consists of more than one type of structure; thus, a linear polymer may contain some branched and some cross-linked chains. As a result , the polymer’s properties are changed significantly. Branched Polymers. The properties of a polymer depend not only on the type of monomers but also on their arrangement in the molecular structure. In branched polymers (Fig. 7.5b), side-branch chains are attached to the main chain during the synthesis of the polymer. Branching interferes with the relative movement of the molecular chains; as a result, their resistance to deformation and stress cracking is increased. The density of branched polymers is lower than that of linear-chain polymers, because the branches interfere with the packing efficiency of the polymer chains. The behavior of branched polymers can be compared to that of linear-chain polymers, by making an analogy with a pile of tree branches (branched polymers) and a bundle of straight logs (linear polymers). Note that it is more difficult to move a branch within the pile of branches than to move a log within its bundle. The three-dimensional entanglements of branches make movements more difficult, a phenomenon akin to increased strength of the polymer. Property ׉ 7cassandra://kzzxXcwMJGFGNdu39BTIcCoHba21oPlq7X_MW37CfJU`cZ%Eb׉E'216 Chapter 7 Polymers: Structure, General Properties, and Applications (a) Linear (b) Branched (c) Cross-linked (d) Network Figure 7.5: Schematic illustration of polymer chains. (a) Linear structure-thermoplastics such as acrylics, nylons, polyethylene, and polyvinyl chloride have linear structures. (b) Branched structure, such as in polyethylene. (c) Cross-linked structure—many rubbers, or elastomers, have this structure, and the vulcanization of rubber produces this structure. (d) Network structure, which is basically highly cross-linked. Examples are thermosetting plastics, such as epoxies and phenolics. Cross-linked Polymers. Generally three-dimensional in structure, cross-linked polymers have adjacent chains linked by covalent bonds (Fig. 7.5c). Polymers with a cross-linked structure are called thermosets or thermosetting plastics, such as epoxies, phenolics, and silicones. Cross-linking has a major influence on the properties of polymers, typically imparting hardness, strength, stiffness, brittleness, and better dimensional stability (see Fig. 7.6) and the vulcanization of rubber (Section 7.9). Glassy Glassy Leathery Increasing crystallinity Rubbery Viscous Temperature Tg (a) Tm Temperature (b) Figure 7.6: Behavior of polymers as a function of temperature and (a) degree of crystallinity and (b) crosslinking. The combined elastic and viscous behavior of polymers is known as viscoelasticity. Leathery Rubbery Viscous Tm Increasing cross-linking Amorphous Elastic modulus (log scale) Elastic modulus (log scale) 100% crystalline No cross-linking ׉ 7cassandra://zVDUFRskIHO62_Cpa_m496IILBj2NGY-3fP7oYiz6gwn`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://hrun7d5B3cG-G73TvsVBY1aqP2RVAc4iLzVo6HXdd-A`׉ 7cassandra://mJxvfKUEz6CUqRP1xeiGmoHDtX_nip7vt6sdSzIwNjErZ`s׉ 7cassandra://uWmc16NXaHW2_PFbY9rB5cBN_1VJ5ClfzMtwySXBYns`׉ 7cassandra://2cB6noriCaDqfGj80SKBFrKCyij-bIxJ9c7admYdC1Ig͠^cZ+Ebȗט#u׉׉ 7cassandra://k-Wb5HenDxexQLPMjHQBCeuuEH9wVK9_2ELgReNyLcI`׉ 7cassandra://T5HZqtvFNAqEV6Y21XCx-gmOJBPLkJaUPVYjG1N1xSELV`s׉ 7cassandra://EdrpMc0Da75grL8M0YAm4q-rMGt7-l2KDgzjYGjvTzs`׉ 7cassandra://XKBvX_Dj-Mu0QVZ2FZPF34gFCCLWwPHCGh33b1ORHIw͠^cZPEbȿ׉EvThe Structure of Polymers 217 Network Polymers. These polymers consist of spatial (three-dimensional) networks consisting of three or more active covalent bonds (Fig. 7.5d); a highly cross-linked polymer also is considered a network polymer. Thermoplastic polymers that already have been formed or shaped can be cross-linked to induce higher strength, by subjecting them to high-energy radiation, such as ultraviolet light, X-rays, or electron beams. Excessive radiation can, however, cause degradation of the polymer. Copolymers and Terpolymers. If the repeating units in a polymer chain are all of the same type, the molecule is called a homopolymer. As with solid-solution metal alloys (Section 4.2), two or three different types of monomers can be combined to develop specific properties and characteristics, such as improved strength, toughness, and formability of the polymer. Copolymers contain two types of polymers, such as styrene-butadiene, which is used widely for automobile tires. Terpolymers contain three types of polymers, such as acrylonitrile-butadiene-styrene (ABS), which is used for helmets, telephones, and refrigerator liners. Case Study 7.1 Dental and Medical Bone Cement Polymethylmethacrylate (PMMA) is an acrylic polymer, commonly used in dental and medical applications as an adhesive, often referred to as bone cement. There are several forms of PMMA, but the adhesive is one common form undergoing an addition-polymerization reaction. PMMA is delivered to the manufacturer in two parts: a powder and a liquid, which are hand-mixed. The liquid wets and partially dissolves the powder, resulting in a liquid with a viscosity similar to that of vegetable oil. The viscosity increases significantly until a doughy state is reached in about five minutes; it fully hardens in an additional five minutes. The powder consists of high-molecular-weight poly[(methylmethacrylate)-costyrene] particles, about 50 µm in diameter, containing a small volume fraction of benzoyl peroxide. The liquid consists of methyl methacrylate (MMA) monomer, with a small amount of dissolved n, n dimethyl-p-toluidine (DMPT). When the liquid and the powder are mixed, the DMPT cleaves the benzoyl peroxide molecule into two parts, forming a catalyst with a free electron (also referred to as a free radical). This catalyst causes rapid growth of PMMA from the MMAmers, so that the final material is a composite of high-molecular-weight PMMA particles interconnected by PMMA chains. An illustration of a fully set bone cement is given in Fig. 7.7. 7.2.2 Crystallinity Polymers such as PMMA, polycarbonate, and polystyrene are generally amorphous; that is, the polymer chains exist without long-range order (see also amorphous alloys, Section 6.15). The amorphous arrangement of polymer chains is often described as being like a bowl of spaghetti, or like worms in a bucket, all intertwined with each other. In some polymers, however, it is possible to impart some crystallinity and, thereby, modify their characteristics. This arrangement may be fostered either during the synthesis of the polymer or by deformation during its subsequent processing. The crystalline regions in polymers are called crystallites (Fig. 7.8). They are formed when the long molecules arrange themselves in an orderly manner, similar to the folding of a fire hose in a cabinet or of facial tissues in a box. A partially crystalline (semicrystalline) polymer can be regarded as a two-phase material; one phase being crystalline and the other amorphous. By controlling the chain structure and the rate of solidification during cooling, it is possible to impart different degrees of crystallinity to polymers, although never 100%. Crystallinity ranges from an almost complete crystal (up to about 95% by volume in the case of polyethylene) to slightly crystallized (and mostly amorphous) polymers. The degree of crystallinity is also affected by branching. A linear polymer can become highly crystalline; a highly branched polymer cannot, although it may develop some low level of crystallinity. It will never achieve high crystallite content, because the branches interfere with the alignment of the chains into a regular crystal array. ׉ 7cassandra://uWmc16NXaHW2_PFbY9rB5cBN_1VJ5ClfzMtwySXBYns`cZ%Eb׉E218 Chapter 7 Polymers: Structure, General Properties, and Applications PMMA particles Polymerized MMA matrix PMMA dissolved in monomer Porosity Figure 7.7: Schematic illustration of the microstructure of polymethylmethacrylate cement used in dental and medical applications. Effects of Crystallinity. The mechanical and physical properties of polymers are greatly influenced by the degree of crystallinity. As crystallinity increases, polymers become stiffer, harder, less ductile, denser, less rubbery, and more resistant to solvents and heat (Fig. 7.6). The increase in density with increasing crystallinity is called crystallization shrinkage, and is caused by more efficient packing of the molecules in the crystal lattice. For example, the highly crystalline form of polyethylene, known as high-density polyethylene (HDPE), has a specific gravity in the range of 0.941–0.970 (80–95% crystalline). It is stronger, stiffer, tougher, and less ductile than low-density polyethylene (LDPE), which is about 60–70% crystalline and has a specific gravity in the range of 0.910–0.925. Optical properties of polymers also are affected by the degree of crystallinity. The reflection of light from the boundaries between the crystalline and the amorphous regions in the polymer (Fig. 7.8) causes Amorphous region Crystalline region Figure 7.8: Amorphous and crystalline regions in a polymer. The crystalline region (crystallite) has an orderly arrangement of molecules. The higher the crystallinity, the harder, stiffer, and less ductile the polymer. ׉ 7cassandra://EdrpMc0Da75grL8M0YAm4q-rMGt7-l2KDgzjYGjvTzs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://-sKzcosgDOF4khwXmagu-mqI__SYSNtQZEzQ065vrpY`׉ 7cassandra://ygiqQy0n3EqswukR8Y853f2PZ-5BXgT3CV3XIq6A-7UM`s׉ 7cassandra://5VghJ3Sps8UfE_rrTOnt5qPlZWgsJWY_XKBGTzbAn4g`׉ 7cassandra://j4EIsLNCTfVWIf-pxC__xhAC06iaDGhsjby5Q4PXM7ol\͠^cZPEbט#u׉׉ 7cassandra://re_yntwbRteNLnE30IwWIVXGehe-n0-L_doPlFj17Xs`׉ 7cassandra://Se_hl4K2e3fClDD2kNwgO_iPjJZQdju1ECuBEJxtRh8\Z`s׉ 7cassandra://QDdw0n8M9szIsIFiY75QZ8ev0OveUEuF3p8FOHXD2do`׉ 7cassandra://qZWWcyW0r4YPTh6aEcKpynq6IgM1aiPi1oOJ3-DNjY4l ^͠^cZPEb‘נcZPEbā cw9ׁHhttp://temperature.Thׁ ׁ Ј׉E 3The Structure of Polymers 219 Amorphous polymers Partly crystalline polymers Temperature Tg Tm Figure 7.9: Specific volume of polymers as a function of temperature. Amorphous polymers, such as acrylic and polycarbonate, have a glass-transition temperature, but do not have a specific melting point. Partly crystalline polymers, such as polyethylene and nylons, contract sharply while passing through their melting temperatures during cooling. opaqueness. Furthermore, because the index of refraction is proportional to density, the greater the density difference between the amorphous and the crystalline phases, the greater is the opaqueness of the polymer. Polymers that are completely amorphous can be transparent, such as polycarbonate and acrylics. 7.2.3 Glass-transition Temperature Although amorphous polymers do not have a specific melting point, they undergo a distinct change in their mechanical behavior across a narrow range of temperatures. At low temperatures, they are hard, rigid, brittle, and glassy; at high temperatures, they are rubbery or leathery. The temperature at which a transition occurs is called the glass-transition temperature (Tg), also called the glass point or glass temperature.The term glass is used in this description because glasses, which are amorphous solids, behave in the same manner (see metallic glasses, Section 6.15 and glass, Section 8.4). Although most amorphous polymers exhibit this behavior, an exception is polycarbonate, which is neither rigid nor brittle below its glass-transition temperature. Polycarbonate is tough at ambient temperatures and is used for safety helmets and shields. To determine Tg, a plot of the specific volume of the polymer as a function of temperature is produced; Tg occurs where there is a sharp change in the slope of the curve (Fig. 7.9). For highly cross-linked polymers, the slope of the curve changes gradually near Tg, making it difficult to determine their Tg. Glass-transition temperature varies with the type of polymer (Table 7.2), and it can be above or below room temperature. Unlike amorphous polymers, partly crystalline polymers have a distinct melting point, Tm (Fig. 7.9; see also Table 7.2). Because of the structural changes (called first-order changes) that occur, the specific volume of the polymer drops rapidly as its temperature is reduced. 7.2.4 Polymer Blends The brittle behavior of amorphous polymers below their glass-transition temperature can be reduced by blending them, usually with small quantities of an elastomer (Section 7.9). The tiny particles that make Cooling: rapid slow Specific volume ׉ 7cassandra://5VghJ3Sps8UfE_rrTOnt5qPlZWgsJWY_XKBGTzbAn4g`cZ%Eb׉E 220 Chapter 7 Polymers: Structure, General Properties, and Applications Table 7.2: Glass-transition and Melting Temperatures of Some Polymers. Material Nylon 6,6 Polycarbonate Polyester Polyethylene High density Low density Polymethylmethacrylate Polypropylene Polystyrene Polytetrafluoroethylene Polyvinyl chloride Rubber Tg (◦C) Tm (◦C) 57 150 73 –90 –110 265 265 265 137 115 105 — –14 100 –90 87 –73 176 239 327 212 — up the elastomer are dispersed throughout the amorphous polymer, enhancing its toughness and impact strength by improving its resistance to crack propagation; these polymer blends are known as rubbermodified polymers. Blending involves combining several components, developing polyblends that utilize the favorable properties of different polymers. Miscible blends (meaning mixing without separation of two phases) are produced by a process similar to the alloying of metals that enables polymer blends to become more ductile. Polymer blends account for about 20% of all polymer production. 7.3 Thermoplastics It was noted above that within each molecule, the bonds between adjacent long-chain molecules (secondary bonds) are much weaker than the covalent bonds between mers (primary bonds). It is the strength of the secondary bonds that determines the overall strength of the polymer; linear and branched polymers have weak secondary bonds. As the temperature is raised above the glass-transition temperature, Tg, or melting point, Tm, some polymers become easier to shape or to mold into desired shapes. When the polymer is cooled, it returns to its original hardness and strength; in other words, the process is reversible. Polymers that exhibit this behavior are known as thermoplastics, common examples of which are acrylics, cellulosics, nylons, polyethylenes, and polyvinyl chloride. The behavior of thermoplastics also depends on other variables, including their structure and composition; among the most important are temperature and deformation rate. Below the glass-transition temperature, most polymers are glassy (brittle) and they behave like an elastic solid. The relationship between stress and strain is linear, as shown in Fig. 2.2. The behavior also depends on the particular polymer; for example, PMMA is glassy below its Tg, whereas polycarbonate is not. When the applied stress is increased further, polycarbonate eventually fractures, just as a piece of glass does at room temperature. Typical stress–strain curves for some thermoplastics and thermosets at room temperature are shown in Fig. 7.10; their behavior may be described as rigid, soft, brittle, flexible, and so on. As can be noted from the mechanical properties of the polymers listed in Table 7.1, thermoplastics are about two orders of magnitude less stiff than metals, and their ultimate tensile strength is about one order of magnitude lower than that of metals (see Table 2.2). Effects of Temperature. If the temperature of a thermoplastic polymer is raised above its Tg, it first becomes leathery and then, with increasing temperature, rubbery (Fig. 7.6). Finally, at higher temperatures (above Tm for crystalline thermoplastics), the polymer becomes a viscous fluid, and its viscosity decreases with ׉ 7cassandra://QDdw0n8M9szIsIFiY75QZ8ev0OveUEuF3p8FOHXD2do`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://uyV-Hw1YotyiADtO9pOO_tcFKDEryEaXZOQbt1zFc5gk`׉ 7cassandra://j7naUqrI18PnfmB_czFFPVnAJz7k9Vjy5hiumY1AOVo?`s׉ 7cassandra://audpAvhEsfFSPmt9CoN8u6MglB7EmG2_DCVQjZZwYAQ>`׉ 7cassandra://JRIE4O5Wk7h6d83Ec1w-yFC5vXCl_5NWlrMSdo_rPSQzX͠^cZQEbט#u׉׉ 7cassandra://1lcEw8dtU1reuUVVcY70gt2aW3BH2FgKFuQh-WePbSc`׉ 7cassandra://xtqQRYnp2l-oi7CDpFSJnWcleSj01j4pjDAfvG-amck* `s׉ 7cassandra://xpSvHHjW8GB8eHoDF74SXLwn9bgVTl9Jk5Kyfe1FoV4[`׉ 7cassandra://8q5J_paNbIimJQ42Px179abCJ4DBPZrewTs4-FqDCMol:͠^cZQEb׉E+Thermoplastics 221 Rigid and brittle (melamine, phenolic) Tough and ductile (ABS, nylon) Soft and flexible (polyethylene, PTFE) 0 Strain Figure 7.10: General terminology describing the behavior of three types of plastics. PTFE (polytetrafluoroethylene) has Teflon as its trade name. Source: After R.L.E. Brown. increasing temperature. As a viscous fluid, it can be softened, molded into shapes, resolidified, remelted, and remolded several times. In practice, however, repeated heating and cooling causes degradation or thermal aging of thermoplastics. As with metals, the strength and the modulus of elasticity of thermoplastics both decrease with increasing temperature and the ductility increases (Fig. 7.11). The effect of temperature on impact strength is shown in Fig. 7.12; note the large difference in the impact behavior among various polymers. Effect of Rate of Deformation. When deformed rapidly, the behavior of thermoplastics is somewhat similar to metals, as shown by the strain-rate sensitivity exponent, m, in Eq. (2.9). Thermoplastics, in general, have high m values, indicating that they can undergo large uniform deformation in tension before they fracture. Note in Fig. 7.13 how, unlike in common metals, the necked region of the specimen elongates considerably. This phenomenon can easily be demonstrated by stretching a piece of the plastic holder for a six-pack of beverage cans, and observing the sequence of necking and stretching behavior shown in Fig. 7.13a. 10 8 6 4 2 0 0 2258C 08 258 508 658 808 5 1015202530 Strain (%) Figure 7.11: Effect of temperature on the stress–strain curve for cellulose acetate, a thermoplastic. Note the large drop in strength and the large increase in ductility with a relatively small increase in temperature. Source: After T.S. Carswell and H.K. Nason. Stress Stress (MPa) ׉ 7cassandra://audpAvhEsfFSPmt9CoN8u6MglB7EmG2_DCVQjZZwYAQ>`cZ%Eb׉E222 Chapter 7 Polymers: Structure, General Properties, and Applications Low-density polyethylene High-impact polypropylene Polyvinyl chloride Polymethylmethacrylate 218 0 32 Temperature (8C) Figure 7.12: Effect of temperature on the impact strength of various plastics. Small changes in temperature can have a significant effect on impact strength. Source: After P.C. Powell. 100 80 60 40 20 0 Molecules are being oriented 0 25 50 75 100 125 Elongation (mm) (a) (b) Figure 7.13: (a) Stress–elongation curve for polycarbonate, a thermoplastic. Source: Courtesy of R.P. Kambour and R.E. Robertson. (b) High-density polyethylene tensile-test specimen, showing uniform elongation (the long, narrow region in the specimen). Stress (MPa) Impact strength ׉ 7cassandra://xpSvHHjW8GB8eHoDF74SXLwn9bgVTl9Jk5Kyfe1FoV4[`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://sVvgU2E3B6UaU0oZ3rlhsOiWD8C9PYeap_rfUJYCbvsEV`׉ 7cassandra://ZivtKh28D_GZA_bIXPgdX-aisyguuVNa6bw4RHDPOjw|O`s׉ 7cassandra://BYVcjgIt246AyJJgWa7tKeYAfkNYbOcQjJDgO4uSv64`׉ 7cassandra://5UE8AkbNkWopo6RF9Gtjr-0zsiPKl3qKiLs4Y5rqNNsg͠^cZQEbט#u׉׉ 7cassandra://nEgVDPrq7CkmnZyHs3LEoYcQn7GPkh5r4c4p-FwaroQ`׉ 7cassandra://r_GkiASCEqRRvW5bqj-AepMe7iFlCCdFEcTR-NKNp-op`s׉ 7cassandra://OZ92Zs4bLDgGqklUF3JLY0y8MKMG8untr7_AG3dTLeI`׉ 7cassandra://ZM3lhG85E4vfxIYfDYp1YoYEluPnOwr7PlRhU1IYnbYf͠^cZQEbɑנcZQEbˁ0R9ׁHhttp://crazing.Alׁ ׁ Ј׉EThermoplastics 223 This characteristic, which is the same in the superplastic metals (Section 2.2.7), enables the thermoforming of thermoplastics (Section 19.6) into such shapes as candy trays, lighted signs, and packaging. Orientation. When thermoplastics are deformed (say by stretching), the long-chain molecules tend to align themselves in the general direction of the elongation, a behavior called orientation. As in metals, the polymer becomes anisotropic (see also Section 1.6), and the specimen becomes stronger and stiffer in the elongated (stretched) direction than in its transverse direction. Stretching is an important technique for enhancing the strength and toughness of polymers, and is especially exploited in producing high-strength fibers for use in reinforced plastics (Chapter 9). Creep and Stress Relaxation. Because of their viscoelastic behavior, thermoplastics are particularly susceptible to creep and stress relaxation (Section 2.8), and to a larger extent than metals. The extent of these phenomena depends on the particular polymer, stress level, temperature, and time. Thermoplastics exhibit creep and stress relaxation at room temperature, whereas most metals do so only at elevated temperatures. Crazing. When subjected to tensile or bending stresses, some thermoplastics (such as polystyrene and PMMA) develop localized, wedge-shaped narrow regions of highly deformed material, called crazing.Although they may appear to be cracks, crazes are spongy material, typically containing about 50% voids. With increasing tensile stress, these voids coalesce and form a crack, which eventually can lead to fracture of the polymer. Crazing has been observed both in transparent, glassy polymers and in other types. The environment, particularly the presence of solvents, lubricants, or water vapor, can enhance the formation of crazes, called environmental-stress cracking and solvent crazing. Residual stresses in the material also contribute to crazing and cracking of the polymer. A phenomenon related to crazing is stress whitening. When subjected to tensile stresses, such as those caused by folding or bending, the polymer becomes lighter in color, usually attributed to the formation of microvoids in the material. As a result, the polymer becomes less translucent (transmits less light), or more opaque. This behavior can easily be demonstrated by bending plastic components commonly found in colored binder strips for report covers, household products, and toys. Water Absorption. An important characteristic of some polymers, such as nylons, is their ability to absorb water (hygroscopy). Water acts as a plasticizing agent, making the polymer more plastic (see Section 7.5); in a sense, it lubricates the chains in the amorphous regions. With increasing moisture absorption, the glasstransition temperature, the yield stress, and the elastic modulus of the polymer typically become rapidly lower. Dimensional changes also occur, especially in a humid environment. Thermal and Electrical Properties. Compared to metals, plastics generally are characterized by low thermal and electrical conductivity, low specific gravity (ranging from 0.90 to 2.2), and high coefficient of thermal expansion (about an order of magnitude higher, as shown in Tables 3.1 and 3.2. Because most polymers have low electrical conductivity, they can be used for insulators and as packaging material for electronic components. The electrical conductivity of some polymers can be increased by doping (introducing impurities, such as metal powders, salts, and iodides, into the polymer). Discovered in the late 1970s, electrically conducting polymers include polyethylene oxide, polyacetylene, polyaniline, polypyrrole, polythiophene, and poly(3,4-ethylene dioxitiophene), or PEDOT, commonly used for flexible electronics (see Section 28.14). The electrical conductivity of polymers increases with moisture absorption. Their electronic properties also can be changed by irradiation. Applications for conducting polymers include adhesives, microelectronic devices, rechargeable batteries, capacitors, catalysts, fuel cells, fuel-level sensors, deicer panels, radar dishes, antistatic coatings, and thermoactuating motors (used in linear-motion applications such as for power antennae, sun roofs, and power windows). One of the earliest applications of conducting polymers was in rechargeable batteries. Modern lithium rechargeable batteries use lithium or an oxide of lithium as the cathode, and lithium carbide (LiyC6) as the anode, separated by a conducting polymer layer. Thermally conducting polymers are for applications requiring dimensional stability and heat transfer, such as heat sinks, and for reducing cycle times in molding and processing of thermoplastics. These ׉ 7cassandra://BYVcjgIt246AyJJgWa7tKeYAfkNYbOcQjJDgO4uSv64`cZ%Eb׉E224 Chapter 7 Polymers: Structure, General Properties, and Applications polymers are typically thermoplastics, such as polypropylene, polycarbonate, and nylon, embedded with thermally conducting particles. Their conductivity can be as much as 100 times that of conventional plastics (see also sprayed-metal tooling, Section 20.10). Shape-memory Polymers. Polymers also can behave in a manner similar to shape-memory alloys (Section 6.14). They can be stretched or compressed to very large strains, and then, when subjected to heat, light, or a chemical environment, they recover to their original shape. The potential applications for these polymers are similar to those for shape-memory metals, such as in opening blocked arteries, probing neurons in the brain, and improving the toughness of spines. 7.4 Thermosetting Plastics When the long-chain molecules of a polymer are cross-linked in a three-dimensional arrangement, the structure in effect becomes one giant molecule, with strong covalent bonds. These polymers are called thermosetting polymers or thermosets, because, during polymerization, the network is completed and the shape of the part is permanently set. The curing (cross-linking) reaction, unlike that of thermoplastics, is irreversible. The response of a thermosetting plastic to a sufficiently elevated temperature can be likened to what happens when baking a cake or boiling an egg: Once the cake is baked and cooled, or the egg boiled and cooled, reheating it will not change its shape. Some thermosets, such as epoxy, polyester, and urethane, cure at room temperature, because the heat produced by the exothermic reaction is sufficient to cure the plastic. Acommon thermoset is phenolic, which is a product of the reaction between phenol and formaldehyde. Typical products made of this polymer are the handles and knobs on cooking pots and pans and components of light switches and outlets. The polymerization process for thermosets generally takes place in two stages. The first occurs at the chemical plant, where the molecules are partially polymerized into linear chains. The second stage occurs during the final step of part production, where cross-linking is completed under heat and pressure during molding and shaping of the part (Chapter 19). Thermosetting polymers do not have a sharply defined glass-transition temperature. Because of the nature of the bonds, the strength and hardness of a thermoset are not affected by temperature or by rate of deformation, unlike those for thermoplastics. If the temperature is increased sufficiently, the thermosetting polymer instead will begin to burn, degrade, and char. Thermosets generally have better mechanical, thermal, and chemical properties; electrical resistance; and dimensional stability than do thermoplastics. 7.5 Additives in Plastics Polymers usually are compounded with additives, which modify and improve specific characteristics of the polymer, such as stiffness, strength, color, weatherability, flammability, electric arc resistance, and ease of subsequent processing. Additives may consist of: • Plasticizers. These are added to polymers to impart flexibility and softness, by lowering their glass-transition temperature. They are low-molecular-weight solvents, with high boiling points (nonvolatile); they reduce the strength of the secondary bonds between the long-chain molecules, and thus make the polymer flexible and soft. The most common use of plasticizers is in polyvinyl chloride (PVC), which remains flexible during its numerous uses, such as thin sheets, films, tubing, shower curtains, and clothing materials. • Colorants. The wide variety of colors available in plastics is obtained by adding colorants, which are either dyes (organic) or pigments (inorganic). • Fillers. Because of their low cost, fillers are important in reducing the cost of polymers. Depending on their type, fillers may also improve the strength, hardness, toughness, abrasion resistance, dimensional stability, and stiffness of plastics. These properties are greatest at specific percentages of ׉ 7cassandra://OZ92Zs4bLDgGqklUF3JLY0y8MKMG8untr7_AG3dTLeI`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://RBahYAFd7X6Y2_s9qfK7ufORGTnpcPLJGjnSsSaO0zEm`׉ 7cassandra://yScwo1NyfGY-DN7LO9gDg-viGEsKNnJMqDWB-om_xB8aF`s׉ 7cassandra://f_M_-_XY_15QgTwrUSnQf8mlYz0Crg_olxZ_G5UfQGM`׉ 7cassandra://g526rAKCc9xwk0gMAgGbl3nXMj5t2PSKlRNHUFpIcn8h1H͠^cZQEbט#u׉׉ 7cassandra://B7T9QJU-1fPfXchQXR34-9CSCTsrSLWqqgAuIDN3R9MU`׉ 7cassandra://CcDieIh9xRCt8csP4dKg3LNMdakC6CwCInavZsRNUzEd'`s׉ 7cassandra://MP9D6Z_tnwCnb3Bfd5-g5fReoOJC97o80vOzriAGXh8`׉ 7cassandra://irlUZRNl73Kvv5PmR0ust5YmNG18wLtfcz6bDFPnRJkg0͠^cZREb׉EGeneral Properties and Applications of Thermoplastics 225 different types of polymer-filler combinations. Fillers are generally wood flour (fine sawdust), silica flour (fine silica powder), clay, powdered mica, talc, calcium carbonate, and short fibers of cellulose, glass, and asbestos. • Flame retardants. If the temperature is sufficiently high, most polymers will ignite and burn. The flammability (ability to support combustion) of polymers varies considerably, depending on their composition and especially on their chlorine and fluorine content. The flammability of polymers can be reduced either by making them from less flammable raw materials or by adding flame retardants, such as compounds of chlorine, bromine, and phosphorus. Cross-linking also reduces polymer flammability. • Other additives. Most polymers are affected adversely by ultraviolet radiation, such as from sunlight, and by oxygen; they weaken and break the primary bonds and cause the scission (splitting) of the longchain molecules. The polymer then degrades and becomes stiff and brittle. An example of protection against ultraviolet radiation is the compounding of certain polymers and rubber with carbon black (soot). Protection against degradation caused by oxidation, particularly at elevated temperatures, is achieved by adding antioxidants to the polymer. 7.6 General Properties and Applications of Thermoplastics The general characteristics and typical applications of major classes of thermoplastics, particularly as they relate to the manufacturing and service life of plastic products and components, are outlined in this section. General recommendations for various plastics applications are given in Table 7.3, and Table 7.4 lists some of the more common trade names for thermoplastics. Acetals (from acetic and alcohol) have good strength, good stiffness, and good resistance to creep, abrasion, moisture, heat, and chemicals. Typical applications include mechanical parts and components Table 7.3: General Recommendations for Plastic Products. Design requirement Mechanical strength Wear resistance Frictional properties High Low Electrical resistance Chemical resistance Heat resistance Functional and decorative Functional and transparent Typical applications Gears, cams, rolls, valves, fan blades, impellers, pistons Gears, wear strips and liners, bearings, bushings, roller blades Tires, nonskid surfaces, footware, flooring Sliding surfaces, artificial joints All types of electrical components and equipment, appliances, electrical fixtures Containers for chemicals, laboratory equipment, components for chemical industry, food and beverage containers Appliances, cookware, electrical components Handles, knobs, camera and battery cases, trim moldings, pipe fittings Lenses, goggles, safety glazing, signs, foodprocessing equipment, laboratory hardware Housings and hollow shapes Power tools, housings, sport helmets, telephone cases Plastics Acetals, nylon, phenolics, polycarbonates, polyesters, polypropylenes, epoxies, polyimides Acetals, nylon, phenolics, polyimides, polyurethane, ultrahigh-molecular-weight polyethylene Elastomers, rubbers Fluorocarbons, polyesters, polyimides Polymethylmethacrylate, ABS, fluorocarbons, nylon, polycarbonate, polyester, polypropylenes, ureas, phenolics, silicones, rubbers Acetals, ABS, epoxies, polymethylmethacrylate, fluorocarbons, nylon, polycarbonate, polyester, polypropylene, ureas, silicones Fluorocarbons, polyimides, silicones, acetals, polysulfones, phenolics, epoxies ABS, acrylics, cellulosics, phenolics, polyethylenes, polypropylenes, polystyrenes, polyvinyl chloride Acrylics, polycarbonates, polystyrenes, polysulfones ABS, cellulosics, phenolics, polycarbonates, polyethylenes, polypropylene, polystyrenes ׉ 7cassandra://f_M_-_XY_15QgTwrUSnQf8mlYz0Crg_olxZ_G5UfQGM`cZ%Eb׉Ei226 Chapter 7 Polymers: Structure, General Properties, and Applications Table 7.4: Trade Names for Thermoplastic Polymers. Acetal: Delrin, Duracon, Lupital, Ultraform Acrylic: Lucite, Acrylite, Acrysteel, Cyrolite, Diakon, Implex, Kamax, Korad, Plexiglass, XT, Zylar Acrylic-polyvinyl chloride: Kydex Acrylonitrile-butadiene-styrene: Cycolac, Delta, Denka, Magnum, Novodur, Royalite, Terluran Aramid: Kevlar Fluorocarbon: Teflon (polytetrafluoroethylene) Polyamide: Capron, Celanese, Durethan, Grilamid, Maranyl, Nylon, Rilsan, Ultramid, Vespel, Vydyne, Zytel Polycarbonate: APEC, Calibre, Hyzod, Lexan, Makrolon, Merlon Polyester: Dacron, Eastpac, Ektar, Kodel, Mylar, Rynite Polyetherimide: Ultem Polyethylene: Alathon, Dowlex, Forar, Fortiflex, Hostalen, Marlex, Petrothene Polyimide: Aurum, Avimid, Estamid, Envex, Kapton, Lenzing, VTEC Polyphenylene: Forton, Fortron, Noryl Polypropylene: Fortilene, Oleplate, Olevac, Pro-Fax Polystyrene: Dylene, Fosta Tuf-Flex, Fostalite, Fostarene, Lustrex, Polystrol, Styron, Syrofoam Polysulfone: Mindel, Udel Polyurethane: Estane, Isoplast, Pellethane Polyvinyl chloride: Fiberloc, Geon, Saran, Sintra, Tygon Polyvinylidene fluoride: Foraflon, Kynar Styrene-methylmethacrylate: Zerlon requiring high performance over a long period (e.g., bearings, cams, gears, bushings, and rolls), impellers, wear surfaces, pipes, valves, shower heads, and housings. Acrylics, such as PMMA, possess moderate strength, good optical properties, and weather resistance. They are transparent (but can be made opaque), are generally resistant to chemicals, and have good electrical resistance. Typical applications include lenses, lighted signs, displays, window glazing, skylights, bubble tops, automotive lenses, windshields, lighting fixtures, and furniture. Acrylonitrile-butadiene-styrene (ABS) is rigid and dimensionally stable. It has good impact, abrasion, and chemical resistance; good strength and toughness; good low-temperature properties; and high electrical resistance. Typical applications include pipes, fittings, chrome-plated plumbing supplies, helmets, tool handles, automotive components, boat hulls, telephones, luggage, housing, appliances, refrigerator liners, and decorative panels. Cellulosics have a wide range of mechanical properties, depending on their composition. They can be made rigid, strong, and tough; however, they weather poorly and are affected by heat and chemicals. Typical applications include tool handles, pens, knobs, frames for eyeglasses, safety goggles, machine guards, helmets, tubing and pipes, lighting fixtures, rigid containers, steering wheels, packaging film, signs, billiard balls, toys, and decorative parts. Fluorocarbons possess good resistance to high temperature (Teflon, for example, has a melting point of 327◦C), chemicals, weather, and electricity. They also have unique non-adhesive properties and low friction. Typical applications include linings for chemical-processing equipment, nonstick coatings for cookware, electrical insulation for high-temperature wire and cable, gaskets, low-friction surfaces, bearings, and seals. Polyamides (from the words poly, amine,and carboxyl acid) are available in two main types: nylons and aramids: • Nylons, a coined word, have good mechanical properties and abrasion resistance; they are also selflubricating and resistant to most chemicals. All nylons are hygroscopic (absorb water); the moisture absorption reduces desirable mechanical properties and increases part dimensions. Typical applications include gears, bearings, bushings, rolls, fasteners, zippers, electrical parts, combs, tubing, wear-resistant surfaces, guides, and surgical equipment. ׉ 7cassandra://MP9D6Z_tnwCnb3Bfd5-g5fReoOJC97o80vOzriAGXh8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://wiPx616dECqxLQu5Einvyt-kSmHYGdsfNvdev_Jfz44`׉ 7cassandra://jHwTOHdRs8XmUGhryQRdFmMJNIDTYkZiDevB6tAxDrAx=`s׉ 7cassandra://BIHunINQMDvFc01DLnjnN1xLIlJGMVW5UsU80KpSjYUv`׉ 7cassandra://sYP1zsasJFTFnoAhEUvk6ueqYUfqwN6VPqnt78tu11Ef͠^cZREbט#u׉׉ 7cassandra://n6-46IAfJSIgEXeR2MjGfvfDnAVDshME1S8p4DG9630&`׉ 7cassandra://DMJC5ShGvb0ACRf1sfNMnqJQQAOB9DBdqoK_upV7cG8j`s׉ 7cassandra://7wp_6-qWCoGsDnN2LJNGKb_Qka9wciVA2SetQcojg14`׉ 7cassandra://ZJNf8me-R-yaSgXe8zzugKPwb_qcCM_KkNWpJC2X6c4c͠^cZREb׉EGeneral Properties and Applications of Thermosetting Plastics 227 • Aramids (aromatic polyamides) have very high tensile strength and stiffness. Typical applications include fibers for reinforced plastics, bulletproof vests, cables, and radial tires. Polycarbonates have good mechanical and electrical properties, high impact resistance, and they can be made resistant to chemicals. Typical applications include safety helmets, optical lenses, bullet-resistant window glazing, signs, bottles, food-processing equipment, windshields, load-bearing electrical components, electrical insulators, medical apparatus, business machine components, guards for machinery, and parts requiring dimensional stability. Polyesters (thermoplastic polyesters; see also Section 7.7) have good mechanical, electrical, and chemical properties, good abrasion resistance, and low friction. Typical applications include gears, cams, rolls, load-bearing members, pumps, and electromechanical components. Polyethylenes possess good electrical and chemical properties; their mechanical properties depend on composition and structure. Three major polyethylene classes are: (1) low density (LDPE), (2) high density (HDPE), and (3) ultrahigh molecular weight (UHMWPE). Typical applications for LDPE and HDPE are housewares, bottles, garbage cans, ducts, bumpers, luggage, toys, tubing, bottles, and packaging materials. UHMWPE is used in parts requiring high-impact toughness and resistance to abrasive wear; examples include artificial knee and hip joints. Polyimides have the structure of a thermoplastic but the nonmelting characteristic of a thermoset (see also Section 7.7). Polypropylenes have good mechanical, electrical, and chemical properties and good resistance to tearing. Typical applications include automotive trim and components, medical devices, appliance parts, wire insulation, TV cabinets, pipes, fittings, drinking cups, dairy-product and juice containers, luggage, ropes, and weather stripping. Polystyrenes generally have average properties and are somewhat brittle, but inexpensive. Typical applications include disposable containers; packaging; trays for meats, cookies, and candy; foam insulation; appliances; automotive and radio/TV components; housewares; and toys and furniture parts (as a substitute for wood). Polysulfones have excellent resistance to heat, water, and steam; they have dielectric properties that remain virtually unaffected by humidity, are highly resistant to some chemicals, but are attacked by organic solvents. Typical applications include steam irons, coffeemakers, hot-water containers, medical equipment that requires sterilization, power-tool and appliance housings, aircraft cabin interiors, and electrical insulators. Polyvinyl chloride has a wide range of properties, is water resistant, inexpensive, and can be made rigid or flexible. It is not suitable for applications requiring strength and heat resistance. Rigid PVC is tough and hard; it is used for signs and in the construction industry. Flexible PVC is used in wire and cable coatings, in low-pressure flexible tubing and hose, and in footwear, imitation leather, upholstery, records, gaskets, seals, trim, film, sheet, and coatings. 7.7 General Properties and Applications of Thermosetting Plastics This section outlines the general characteristics and typical applications of the major thermosetting plastics. Alkyds (from alkyl, meaning alcohol, and acid) possess good electrical insulating properties, impact resistance, dimensional stability, and low water absorption. Typical applications are in electrical and electronic components. Aminos have properties that depend on composition; generally, they are hard, rigid, and resistant to abrasion, creep, and electric arcing. Typical applications include small-appliance housings, countertops, toilet seats, handles, and distributor caps. Urea typically is used for electrical and electronic components; and melamine for dinnerware. Epoxies have excellent mechanical and electrical properties, good dimensional stability, strong adhesive properties, and good resistance to heat and chemicals. Typical applications include electrical components requiring mechanical strength and high insulation, tools and dies, and adhesives. ׉ 7cassandra://BIHunINQMDvFc01DLnjnN1xLIlJGMVW5UsU80KpSjYUv`cZ%Eb׉E228 Chapter 7 Polymers: Structure, General Properties, and Applications Fiber-reinforced epoxies have excellent mechanical properties and are used in pressure vessels, rocketmotor casings, tanks, and similar structural components. Phenolics are rigid, though brittle, and dimensionally stable, and they have high resistance to heat, water, electricity, and chemicals. Typical applications include knobs, handles, laminated panels, and telephones; bonding material to hold abrasive grains together in grinding wheels; and electrical components (such as wiring devices, connectors, and insulators). Polyesters (thermosetting polyesters; see also Section 7.7) have good mechanical, chemical, and electrical properties. They generally are reinforced with glass (or other) fibers and also are available as casting resins. Typical applications include boats, luggage, chairs, automotive bodies, swimming pools, and materials for impregnating cloth and paper. Polyimides possess good mechanical, physical, and electrical properties at elevated temperatures; they also have good creep resistance, low friction, and low wear characteristics. Polyimides have the nonmelting characteristic of a thermoset, but the structure of a thermoplastic. Typical applications include pump components (bearings, seals, valve seats, retainer rings, and piston rings), electrical connectors for high-temperature use, aerospace parts, high-strength impact-resistant structures, sports equipment, and safety vests. Silicones have properties that depend on their composition; generally, they weather well, possess excellent electrical properties over a wide range of humidity and temperature, and resist chemicals and heat (Section 7.9). Typical applications include electrical components requiring strength at elevated temperatures, oven gaskets, heat seals, and waterproof materials. Health Hazards. Some of the chemicals used in polymers may present health hazards, especially in products such as polycarbonate water containers and baby bottles, and also medical devices, sports safety equipment, and eating utensils. The chemical that is of particular concern is bisphenol A (BPA), which is widely used. Case Study 7.2 Materials for a Refrigerator Door Liner In selecting candidate materials for a refrigerator door liner (where eggs, butter, salad dressings, and small bottles are stored), the following factors should be considered: 1. Mechanical requirements: strength, toughness (to withstand impacts, door slamming, and racking), stiffness, resilience, and resistance to scratching and wear at operating temperatures. 2. Physical requirements: dimensional stability and electrical insulation. 3. Chemical requirements: resistance to staining, odor, chemical reactions with food and beverages, and cleaning fluids. 4. Appearance: color, stability of color over time, surface finish, texture, and feel. 5. Manufacturing properties: methods of manufacturing and assembly, effects of processing on material properties and behavior over a period of time, compatibility with other components in the door, and cost of materials and manufacturing. Considering all of the factors involved, a study identified two candidate materials for door liners: ABS (acrylonitrile-butadiene-styrene) and HIPS (high-impact polystyrene). One aspect of the study concerned the effect of vegetable oils, such as from salad dressing stored in the door shelf, on the strength of these two plastics. Experiments showed that the presence of vegetable oils significantly reduced the loadbearing capacity of HIPS. It was also found that it becomes brittle in the presence of oils (a case of solvent-stress cracking), whereas ABS is not affected to any significant extent. ׉ 7cassandra://7wp_6-qWCoGsDnN2LJNGKb_Qka9wciVA2SetQcojg14`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://E_mubhZjtn9oPnA4s5U1Kam64eDXOot4D-3FTALJmJ4`׉ 7cassandra://iY-Mp4Wdc47OOCMRnw2mvKPts2iDg-pLd_lO1IqKiZMk`s׉ 7cassandra://APnJOw0U6CVkdGS_gMBTml4r8fkgiqo8xsYc3uRvFuIb`׉ 7cassandra://pBWav2gg-zy-49-Y6HFgZUQ6pr8jCxvXd4d5FMDEFAw[*͠^cZREbט#u׉׉ 7cassandra://qkuopT4iLzPQgPfZTBAoaOtyh-y4jBmrKeCX4sp63IMÌ`׉ 7cassandra://WlCuTxCgZmnTmIYKDP2moCPmVEzn-HsCxVVvZ2Z22bkKr`s׉ 7cassandra://SQCj--N3UcXLn7pvz-getClcpHB8CXv4fS_reEAsjlY`׉ 7cassandra://NYSj3iLFr-W31VS_RILhJxt1FBv8UF-v7l0KWpcx4-4_̆͠^cZREb׉EBiodegradable Plastics 229 7.8 Biodegradable Plastics Plastic wastes contribute about 16% of municipal solid waste by weight, and make up 50–80% of waste littering beaches, oceans, and sea beds. On a volume basis, they contribute between two and three times their weight. Only about one-third of plastic production goes into disposable products, such as bottles, packaging, and garbage bags. With the growing use of plastics and continuing concern over environmental issues regarding the disposal of plastic products and the shortage of landfills, major efforts continue to develop completely biodegradable plastics. Traditionally, most plastic products have been made from synthetic polymers that are (a) derived from nonrenewable natural resources, (b) not biodegradable, and (c) difficult to recycle. Biodegradability means that microbial species in the environment (e.g., microorganisms in soil and water) will degrade all or part of the polymeric material under the proper environmental conditions, without producing toxic by-products. The end products of the degradation of the biodegradable portion of the material are carbon dioxide and water. Because of the variety of constituents in biodegradable plastics, these plastics can be regarded as composite materials (Chapter 9); consequently, only a portion of these products may be truly biodegradable. Three main biodegradable plastics have thus far been developed. They have different degradability characteristics, and they degrade over different periods of time, from a few months to a few years. 1. The starch-based system is the farthest along in terms of production capacity. Starch may be extracted from potatoes, wheat, rice, or corn. The starch granules are processed into a powder, which is heated and becomes a sticky liquid. The liquid is then cooled, shaped into pellets, and processed in conventional plastic-processing equipment (Chapter 19). Various additives and binders are blended with the starch to impart specific characteristics to the bioplastic materials. For example, a composite of polyethylene and starch is produced commercially as degradable garbage bags. 2. In the lactic-based system, fermenting feedstocks produce lactic acid, which is then polymerized to form a polyester resin. Typical uses include medical and pharmaceutical applications. 3. In fermentation of sugar, organic acids are added to a sugar feedstock. The resulting reaction produces a highly crystalline and very stiff polymer, which, after further processing, behaves in a manner similar to polymers developed from petroleum. Studies continue to be conducted on producing fully biodegradable plastics by using various agricultural waste (agrowastes), plant carbohydrates, plant proteins, and vegetable oils. Typical applications of this approach include the following: • Disposable tableware made from a cereal substitute, such as rice grains or wheat flour. • Plastics made almost entirely from starch extracted from potatoes, wheat, rice, or corn. • Plastic articles made from coffee beans and rice hulls that are dehydrated and molded under high pressure and temperature. • Water-soluble and compostable polymers for medical and surgical use. • Food and beverage containers made from potato starch, limestone, cellulose, and water, which can dissolve in storm sewers and oceans without affecting wildlife or marine life. Recycling of Plastics. Much effort continues to be expended globally on collecting and recycling of used plastic products. Thermoplastics are recycled by melting, blending, and reforming them into other products. Recycling symbols, in the shape of a triangle outlined by three clockwise arrows and with a number in the middle, are now commonly used. These numbers identify the following plastics: ׉ 7cassandra://APnJOw0U6CVkdGS_gMBTml4r8fkgiqo8xsYc3uRvFuIb`cZ%Eb׉E 230 Chapter 7 Polymers: Structure, General Properties, and Applications 1. PETE (polyethylene) 2. HDPE (high-density polyethylene) 3. V (vinyl) 4. LDPE (low-density polyethylene) 5. PP (polypropylene) 6. PS (polystyrene) 7. Other 7.9 Elastomers (Rubbers) Elastomers, derived from the words elastic and mer, consist of a large family of amorphous polymers (Section 7.2.1) with a low glass-transition temperature. They have the characteristic ability to undergo large elastic deformations without rupture; they are soft and have low elastic modulus. The structure of elastomer molecules is highly kinked (tightly twisted or curled): they stretch but then return to their original shape after the load is removed (Fig. 7.14). Elastomer can also be cross-linked, the best example of which is the elevated-temperature vulcanization of rubber with sulfur (discovered by C. Goodyear, in 1839, and named for Vulcan, the Roman god of fire). Once the elastomer is cross-linked, it cannot be reshaped. An automobile tire (a giant molecule) cannot be softened and reshaped. The terms elastomer and rubber often are used interchangeably. Generally, however, an elastomer is defined as being capable of recovering substantially in shape and size after the load has been removed. Rubber is defined as being capable of recovering quickly from large deformations. The hardness of elastomers, which is measured with a durometer (Section 2.6.1), increases with the crosslinking of the molecular chains. As with plastics, a variety of additives can be blended into elastomers to impart specific properties. Elastomers have a wide range of applications, such as high-friction and nonskid surfaces, protection against corrosion and abrasion, electrical insulation, and shock and vibration insulation. Examples include tires, hoses, weather stripping, footwear, linings, gaskets, seals, printing rolls, and flooring. An important property of elastomers is their hysteresis loss in stretching or compression (Fig. 7.14). The clockwise loop indicates energy loss, whereby mechanical energy is converted into heat. This property is important for absorbing vibrational energy (damping) and sound insulation. Elongation Figure 7.14: Typical load–elongation curve for rubbers. The clockwise loop, indicating the loading and the unloading paths, displays the hysteresis loss. Hysteresis gives rubbers their capacity to dissipate energy, damp vibration, and absorb shock loading, as is necessary in automobile tires and in vibration dampers placed under machinery. Loading Unloading Load ׉ 7cassandra://SQCj--N3UcXLn7pvz-getClcpHB8CXv4fS_reEAsjlY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://G16jCYHK-mFbc_n8mMJoyvYqpywjjsBydQGTTJ-7aOMj`׉ 7cassandra://h0UDcX5caxPqxExAKaNLiM_Kg2jQRpWxdzB_WJuA_AQeA`s׉ 7cassandra://27uyP-AIvxVnt4l1vNcTlEkEFa1tXPXp3rW6ewiHlgY`׉ 7cassandra://fgozaeiGFfKjrnjfywGhFIYnpdykD8WLNZyTi05PDBE[͠^cZSEbט#u׉׉ 7cassandra://xkm6M6IF2QM1WZ-peeGUxC_e-t9u-ir_RkuwTekg9bMY`׉ 7cassandra://ITFL82_YX0IcRrBdb4xEIckKs1S58oG0qWD4YKETjzMK"`s׉ 7cassandra://6qX7KbvaJ4Jv1PKVvdFMCagmhzUuI5KehO5nAIO8ZzI!`׉ 7cassandra://fqXQWpdu71ijkKMk7i1hCeTHTGhGJQa7kJ8sSgmu5QILO͠^cZSEbڑנcZSEb܁7XK9ׁHhttp://aerogel.Itׁ ׁ Ј׉ESummary 231 Natural Rubber. The source for natural rubber is latex, a milk-like sap obtained from the inner bark of a tropical tree. Natural rubber has good resistance to abrasion and fatigue, and high friction but low resistance to oil, heat, ozone, and sunlight. Typical applications are tires, seals, couplings, and engine mounts. Synthetic Rubbers. Examples of synthetic rubbers are butyl, styrene butadiene, polybutadiene, and ethylene propylene. Compared to natural rubber, they have better resistance to heat, gasoline, and chemicals, and they have a higher temperature range. Synthetic rubbers resistant to oils are neoprene, nitrile, urethane, and silicone. Typical applications are tires, shock absorbers, seals, and belts. Silicones. Silicones (Section 7.9) have the highest useful temperature range of elastomers (up to 315◦C), but properties such as strength and resistance to wear and oils generally are inferior to those in other elastomers. Typical applications of silicones are seals, gaskets, thermal insulation, high-temperature electrical switches, and electronic apparatus. Polyurethane. This elastomer has very good overall properties of high strength, stiffness, and hardness, and it also has exceptional resistance to abrasion, cutting, and tearing. Typical applications include seals, gaskets, cushioning, diaphragms for the rubber forming of sheet metals (Section 16.8), and auto body parts. 7.10 Gels A gel is mostly liquid by weight, but contains a cross-linked network within its structure. Edible jelly is a common example of a gel. The liquid in a gel is held by the network through surface tension, and physical and/or chemical bonds. Hydrogels have water as the contained liquid, and are highly absorbant. Common examples of hydrogels are polyvinyl alcohol, sodium polyacrylate, hydroxyethylmethacrylate (HEMA), polyethylene glycol (PEG), and several others. Because they can be made to have mechanical properties similar to that of soft tissue, there are numerous biomedical applications of hydrogels. The most common application is that of soft contact lenses, which are abrasive when dry, but absorb liquid from the user, providing a smooth interface between the lens and the eye. Aerogels are derived from gels, where the liquid has been replaced by air. The result is a soft material with extremely low density (160 g/m3). Some aerogels are over 98% air and have a density as low as 1900 g/m3. Aerogels are very effective at sound isolation and thermal insulation, and are also very effective desiccants (water absorbing agents). Common aerogels are produced from silica, graphite, and graphene (Section 8.6.3), or from organic polymers. The lightest material produced to date is graphene aerogel.Its density is around 1160 g/m3, including the air in the aerogel. Metal oxide aerogels are used as catalysts in chemical processing industries. Summary • Polymers are a major class of materials, and possess a very wide range of mechanical, physical, chemical, and optical properties. Compared to metals, polymers are generally characterized by (a) lower density, strength, elastic modulus, thermal and electrical conductivity, cost; (b) higher strength-toweight ratio, higher resistance to corrosion, higher thermal expansion, (c) wide choice of colors and transparencies; and (d) greater ease of manufacture into complex shapes. • Plastics are composed of polymer molecules and various additives. The smallest repetitive unit in a polymer chain is called a mer. Monomers are linked by polymerization processes (condensation or addition) to form larger molecules. The glass-transition temperature separates the region of brittle behavior in polymers from that of ductile behavior. ׉ 7cassandra://27uyP-AIvxVnt4l1vNcTlEkEFa1tXPXp3rW6ewiHlgY`cZ%Eb׉E232 Chapter 7 Polymers: Structure, General Properties, and Applications • The properties of polymers depend on their molecular weight, structure (linear, branched, crosslinked, or network), degrees of polymerization and crystallinity, and on additives present in their formulation. Additives have such functions as improving strength, flame retardation, lubrication, imparting flexibility and color, and providing stability against ultraviolet radiation and oxygen. Polymer structures can be modified by several means to impart a wide range of desirable properties. • Two major classes of polymers are thermoplastics and thermosets. Thermoplastics become soft and easy to form at elevated temperatures. Their behavior includes such phenomena as creep and stress relaxation, crazing, and water absorption. Thermosets are produced by cross-linking polymer chains; they do not become soft to any significant extent with increasing temperature, and are much more rigid and harder than thermoplastics. • Elastomers have a characteristic ability to undergo large elastic deformations and then return to their original shapes when unloaded. Consequently, they have important applications in tires, seals, footwear, hoses, belts, and shock absorbers. • Among important considerations in polymers are their recyclability and biodegradability. Several formulations of biodegradable plastics are available, and others are under continued development. • Gels are cross-linked polymers combined with water (hydrogels) or air (aerogels), yielding unique properties. Hydrogels are useful for soft biological applications (notably contact lenses) while aerogels are widely used in chemical process industries as catalysts. Key Terms Additives Biodegradable Blends Bonding Branched polymers Colorants Crazing Cross-linked polymers Crystallinity Curing Degradation Degree of crystallinity Degree of polymerization Doping Elastomer Fillers Flame retardants Gels Glass-transition temperature Latex Linear polymers Lubricants Mer Molecular weight Monomer Network polymers Orientation Plasticizers Plastics Polyblends Polymer Polymerization Primary bonds Recycling Rubber Secondary bonds Shape-memory polymers Silicones Stress whitening Thermal aging Thermoplastics Thermosets Vulcanization ׉ 7cassandra://6qX7KbvaJ4Jv1PKVvdFMCagmhzUuI5KehO5nAIO8ZzI!`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://bcTFeqBys3dYROrjvCFXLMtXPN-0Gr5cOKoauOPwl8E>]`׉ 7cassandra://v1BOAtgwlVrP1rf-bNCuDmT43iIyGY1fMMy6RrAnK_o^`s׉ 7cassandra://jOhHtewK4sb8606Fg7C5Zd87foXLtAF2_UvPVwhZe14`׉ 7cassandra://GGqlwRwwmR7OYm-FKKo9zBnudJBAWsZf6WOgEJWvvd4Z͠^cZSEbט#u׉׉ 7cassandra://9UHhne8gy2KFh6qBPnG-k91Zjbg41TbFxQuqVmjpcHM`׉ 7cassandra://S6zu4s2LgXobjDOnEMwGmYWY40xSr6Dg0SN8eV_YDqEnx`s׉ 7cassandra://0tAUzVygA5BOkOeL9qieAInw5Cyd_33KaPBOixX73uw`׉ 7cassandra://1SPkxf5HU6XodjjMuL4k2RfsO-6lSoCNgsUWywGj1f4O͠^cZSEb׉E ^Review Questions 233 Bibliography Bhowmick, A.K., and Stephens, H.L., Handbook of Elastomers, 2nd ed., CRC Press, 2000. Campo, E.A., Selection of Polymeric Materials, William Andrew, 2008. Chanda, M., Plastics Technology Handbook, 5th ed., CRC Press, 2017. Concise Encyclopedia of Medical Polymers and Polymeric Materials, CRC Press, 2017. Drobny, J.G., Handbook of Thermoplastic Elastomers, William Andrew, 2007. Ellis, B., and Smith, R., Plastics: A Property Database, 2nd ed., CRC Press, 2008. Fink, J.K., High Performance Polymers, 2nd ed., William Andrew, 2014. Flick, C.W., Plastics Additives, William Andrew, 2002. Gilbert, M., Brydson’s Plastics Materials, 8th ed., Butterworth-Heinemann, 2016. Goodship, V., Introduction to Plastics Recycling, 2nd ed., Smithers Rapra Press, 2008. Harper, C., Handbook of Plastics, Elastomers, and Composites, 4th ed., McGraw-Hill, 2003. Harper, C.A., Modern Plastics Handbook, McGraw-Hill, 2000. Khemani, K., and Scholz, C., Degradable Polymers and Materials: Principles and Practice, 2nd ed., American Chemical Society, 2013. Klingender, R.C. (ed.), Handbook of Specialty Elastomers, CRC Press, 2018. Kutz, M., Applied Plastics Engineering Handbook: Processing and Materials, 2nd ed., William Andrew, 2016. Margolis, J., Engineering Plastics Handbook, McGraw-Hill, 2006. Mark, J.E. (ed.), Physical Properties of Polymers Handbook, 2nd ed., Springer, 2006. Mills, N., Plastics: Microstructure and Engineering Applications, 3rd ed., Butterworth-Heinemann, 2006. Mittal, V. (ed.), High Performance Polymers and Engineering Plastics, 2nd ed., Wiley Scrinever, 2014. Osswald, T.A., and Menges, G., Materials Science of Polymers for Engineers, 3rd ed., Hanser, 2012, Strong, A.B., Plastics: Materials and Processing, 3rd ed., Prentice Hall, 2005. Ward, I.M., and Sweeny, J., An Introduction to the Mechanical Properties of Solid Polymers, 2nd ed., Wiley, 2004. Wypych, G., Handbook of Polymers, 2nd ed., ChemTech Publishing, 2016. Xanthos, M., Functional Fillers for Plastics, 2nd ed., Wiley-VCH, 2010. Young, R.J., and Lovell, P., Introduction to Polymers, 3rd ed., CRC Press, 2008. Zweifel, H., Maier, H., and Schiller, M., Plastics Additives Handbook, 6th ed., Hanser, 2009. Review Questions 7.1. Summarize the important mechanical and physical properties of plastics. 7.2. What are the major differences between (a) the mechanical and (b) the physical properties of plastics and metals? 7.3. List properties that are influenced by the degree of polymerization. 7.4. What is the difference between condensation polymerization and addition polymerization? 7.5. Explain the differences between linear, branched, and cross-linked polymers. 7.6. What is the glass-transition temperature? 7.7. List and explain the additives commonly used in plastics. 7.8. What is crazing? 7.9. What are polyblends? 7.10. List the major differences between thermoplastics and thermosets. ׉ 7cassandra://jOhHtewK4sb8606Fg7C5Zd87foXLtAF2_UvPVwhZe14`cZ%Eb׉E A234 Chapter 7 Polymers: Structure, General Properties, and Applications 7.11. What is an elastomer? 7.12. What effects does a plasticizing agent have on a polymer? 7.13. Define the following abbreviations: PMMA, PVC, ABS, HDPE, and LDPE. 7.14. Explain why it would be advantageous to produce a polymer with a high degree of crystallinity. 7.15. What are the differences and similarities of addition and condensation polymerization? 7.16. Are molecular weight and degree of polymerization related? Explain. 7.17. Why do polymers need to be dried before processing? Qualitative Problems 7.18. What characteristics of polymers make them attractive for clothing? 7.19. Do polymers strain harden more than metals or vice versa? Explain. 7.20. Inspect various plastic components in an automobile, and state whether they are made of thermoplastic materials or of thermosetting plastics. 7.21. Give applications for which flammability of plastics would be of major importance. 7.22. What characteristics make polymers advantageous for applications such as gears? What characteristics are drawbacks in such applications? 7.23. What properties do elastomers have that thermoplastics in general do not have? 7.24. Do you think that the substitution of plastics for metals in products traditionally made of metal may be viewed negatively by the public at large? If so, why? 7.25. Is it possible for a material to have a hysteresis behavior that is the opposite of that shown in Fig. 7.14, so that the two arrows run counterclockwise? Explain. 7.26. Observe the behavior of the specimen shown in Fig. 7.13, and state whether the material has a high or a low strain-rate sensitivity exponent, m (see Section 2.2.7). 7.27. Add more to the applications column in Table 7.3. 7.28. Discuss the significance of the glass-transition temperature, Tg, in engineering applications. 7.29. Describe how a rechargeable lithium battery works. 7.30. Explain how cross-linking improves the strength of polymers. 7.31. Describe the methods by which the optical properties of polymers can be altered. 7.32. How can polymers be made to conduct electricity? Explain. 7.33. Explain the reasons for which elastomers were developed. 7.34. Give several examples of plastic products or components in which creep and stress relaxation would be important considerations. 7.35. Describe your opinions regarding the recycling of plastics versus the development of plastics that are biodegradable. 7.36. Explain how you would go about determining the hardness of plastics. 7.37. Compare the values of the elastic modulus given in Table 7.1 to the values for metals given in Chapters 2, 5, and 6. 7.38. Why is there so much variation in the stiffness of products made of polymers? Explain. 7.39. Explain why thermoplastics are easier to recycle than thermosets. 7.40. Give an example of a process where crazing is desirable. ׉ 7cassandra://0tAUzVygA5BOkOeL9qieAInw5Cyd_33KaPBOixX73uw`cZ%Eb cZ%Eb#בCט#u׉׉ 7cassandra://kA2sPOSVE0ZPzygi02MXqp_ZXUM_3GFrpv4ROiTgKnUd`׉ 7cassandra://IaKjJdOgDXS_qQKorkUTl0QQoh_TlPIH54lvreTgrY4f3`s׉ 7cassandra://9Tnfe2wnQ0ebrI5dWcEnM54rdc7YZp08sxLv-nWeT1Y`׉ 7cassandra://KxG1u0W05XDFXWBmRxtWApMr5pqJDavb3JY-2ZnbKEoV+͠^cZSEbט#u׉׉ 7cassandra://qTyJWyNMDE_cO5ttQ7M-2YHVFX7wA2kL7Z5-iwBnC-U`׉ 7cassandra://bMKLQD29i34JSl4rfVOsDTlYC6oEOqV02T3Q_hjdbkQ6`s׉ 7cassandra://hb7a5Zn0abNUFf-2QcJRwzy0Hhz0XtsD9ck-tFqbfDw `׉ 7cassandra://ggnhyjy1udjYvHxARuXB8WyQTBj6-J9hCeLJ0R5XXWc3l͠^cZSEb׉E 3Synthesis, Design, and Projects 235 7.41. Describe the principle behind shrink wrapping. 7.42. List and explain some environmental pros and cons of using plastic shopping bags vs. paper bags. 7.43. List the characteristics required of a polymer for (a) a bucket, (b) a golf ball, (c) an automobile dashboard, (d) clothing, (e) flooring, and (f) fishing nets. 7.44. How can you tell whether a part is made of a thermoplastic or a thermoset? 7.45. As you know, there are plastic paper clips available in various colors. Why are there no plastic staples? 7.46. By incorporating small amounts of a blowing agent, it is possible to manufacture hollow polymer fibers with gas cores. List possible applications for such fibers. 7.47. In injection-molding operations, it is common practice to remove the part from its runner, to place the runner into a shredder, and to recycle the resultant pellets. List the concerns you would have in using such recycled pellets as opposed to so-called virgin pellets. 7.48. From an environmental standpoint, do you feel it is best to incorporate polymers or metals into designs? Explain your answer. Quantitative Problems 7.49. Calculate the areas under the stress–strain curve (toughness) for the materials shown in Fig. 7.11, plot them as a function of temperature, and describe your observations. 7.50. Note in Fig. 7.11 that, as expected, the elastic modulus of the polymer decreases as temperature increases. Using the stress–strain curves in the figure, make a plot of the modulus of elasticity versus the temperature. Comment on the shape of the curve. 7.51. A rectangular cantilever beam 75 mm high, 20 mm wide, and 1 m long is subjected to a concentrated load of 50 kg at its end. From Table 7.1, select three unreinforced and three reinforced materials and calculate the maximum deflection of the beam in each case. Then select aluminum and steel for the same beam dimensions, calculate the maximum deflection, and compare the results. 7.52. Estimate the number of molecules in a typical automobile tire. Estimate the number of atoms in the tire. 7.53. Using strength and density data, determine the minimum weight of a 2-m-long tension member that must support a load of 10,000 N if it is manufactured from (a) high-molecular-weight polyethylene, (b) polyester, (c) rigid PVC, (d) ABS, (e) polystyrene, and (e) reinforced nylon. 7.54. Plot the following for any five polymers described in this chapter: (a) ultimate tensile strength vs. density and (b) elastic modulus vs. ultimate tensile strength. Where appropriate, plot a range of values. Synthesis, Design, and Projects 7.55. Conduct an Internet search, and describe differential scanning calorimetry. What does this measure? 7.56. Describe the design considerations involved in replacing a metal beverage container with one made of plastic. 7.57. Assume that you are manufacturing a product in which all of the gears are made of metal. A salesperson visits you and asks you to consider replacing some of these metal gears with plastic ones. Make a list of the questions that you would raise before making a decision. 7.58. Assume you work for a company that produces polymer gears. You have arranged to meet with a potential new customer, who currently uses gears made of metal. Make a list of the benefits that plastic gears present, and prepare a presentation for the meeting. ׉ 7cassandra://9Tnfe2wnQ0ebrI5dWcEnM54rdc7YZp08sxLv-nWeT1Y`cZ%Eb ׉EU236 Chapter 7 Polymers: Structure, General Properties, and Applications 7.59. Sections 7.6 and 7.7 list several plastics and their applications. Rearrange this information by making a table of products (gears, helmets, luggage, electrical parts, etc.) that shows the types of plastic that can be used to make these products. 7.60. Make a list of products or parts that currently are not made of plastics and offer possible reasons why they are not. 7.61. Review the three curves shown in Fig. 7.10 and give some applications for each type of behavior. Explain your choices. 7.62. Repeat Problem 7.61 for the curves shown in Fig. 7.12. 7.63. In order to use a steel or aluminum container for an acidic liquid, such as tomato sauce, a polymeric barrier is usually placed between the container and its contents. Describe some methods of producing such a barrier. 7.64. Perform a study of plastics used for some products. Measure the hardness and stiffness of these plastics. (For example, dog chew toys use plastics with a range of properties.) 7.65. Add a column to Table 7.1 that describes the appearance of these plastics, including available colors and opaqueness. 7.66. With Table 7.3 as a guide, inspect various products both in a typical kitchen and in an automobile, and describe the types of plastics that could be used in making their individual components. ׉ 7cassandra://hb7a5Zn0abNUFf-2QcJRwzy0Hhz0XtsD9ck-tFqbfDw `cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://r1k1Uy1Hzk98fTZtCd9bFOWJel9xA6ksjP7E_YYavKs{`׉ 7cassandra://3SY2NBAUwCHZ1dAzncfUIa1hZoZ6BLL6Ti0aWp-7jsM>u`s׉ 7cassandra://h7gaw72qIaIEGlC5Pjv9bK7sz3Shvq3sZK47zvK6C90`׉ 7cassandra://yUIai1TFAxu3YDfhkp4iRIED3B7QT-yBk1LiaUj-PD89͠^cZSEbט#u׉׉ 7cassandra://7ImR60aWHFhLOI8z54TKLNNtZlPITH3c2HHTRBNKKI0i`׉ 7cassandra://kWVW_8iQODk-6Fz4IroL3mG-H-ilBwqEVBWth8ktVRo[?`s׉ 7cassandra://8h8D3XWDTYjIcycIqsYDhxCPn4j2xRdmEro_TshFAnU`׉ 7cassandra://jVVKNfpde_2vMH8NzerIUM848_fFDmETsExRBqlDLWYQ:͠^cZSEb׉EChapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials: Structure, General Properties, and Applications 8.1 Introduction 238 8.2 The Structure of Ceramics 238 8.3 General Properties and Applications of Ceramics 243 8.4 Glasses 247 8.5 Glass Ceramics 249 8.6 Graphite 249 8.7 Diamond 251 8.8 Nanomaterials 252 Case Studies: 8.1 Ceramic Knives 241 8.2 Ceramic Ball and Roller Bearings 246 • Ceramics, glass, and various forms of carbon present unique combinations of mechanical and physical properties that cannot be obtained with other metallic or nonmetallic materials. • Ceramic materials are first described in terms of their chemistry, microstructure, mechanical and physical properties, and applications. • The basic types of ceramics include oxide ceramics, such as aluminum or zirconium oxide, and carbides and nitrides. • Glasses have numerous formulations, all containing at least 50% silica. Their general properties and typical uses are described. • Several forms of carbon are commercially important; graphite is the most common, with numerous uses, including as reinforcement in composite materials, electrodes for electrical discharge machining, and solid lubricant. 237 ׉ 7cassandra://h7gaw72qIaIEGlC5Pjv9bK7sz3Shvq3sZK47zvK6C90`cZ%Eb ׉E 238 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials • Diamond is the hardest material known and, as such, is used for precision and abrasive machining and for polishing operations. • Nanomaterials, such as carbon nanotubes and graphene, are becoming increasingly important, with numerous applications for nanoscale electrical and microelectronic devices. 8.1 Introduction The various types of materials described in the preceding chapters are not suitable for certain engineering applications, including the following: 1. An electrical or thermal insulator for use at high temperatures 2. Floor tiles to resist scuffing and abrasion 3. A transparent baking dish 4. Small ball bearings that are light, rigid, hard, and resist high temperatures 5. Automobile windshields that are hard, abrasion resistant, and transparent 6. Cutting tools that remain hard and wear resistant at high pressure and temperature 7. Optics applications such as lenses and mirrors that require high levels of transparency. It is apparent from these examples that the specific properties required include high-temperature strength; hardness; desired optical properties of transparency; inertness to chemicals, foods, and the environment; resistance to wear and corrosion; and low electrical and thermal conductivity. The general characteristics and applications of those ceramics, glasses, and glass ceramics that are of importance in engineering applications and in manufacturing are first described. Because of their unique properties and uses, the various forms of carbon (graphite, diamond, carbon nanotubes, and graphene) are described next. The manufacturing of ceramic and of glass components and various shaping and finishing operations are detailed in Chapter 18. Composites, which contain combinations of the materials described, are described in Chapter 9. 8.2 The Structure of Ceramics Ceramics are compounds of metallic and nonmetallic elements. The term ceramics (from the Greek keramos, meaning potter’s clay, and keramikos, meaning clay products) refers both to the material and to the ceramic product itself. Because of the large number of possible combinations of various elements, a wide variety of ceramics is now available for a broad range of consumer and industrial applications. The earliest use of ceramics was in pottery and bricks, dating back to before 4000 B.C. They have become increasingly important in tool and die materials, medical products and automotive components (such as exhaust-port liners, automotive spark plugs, coated pistons, food processing equipment, and cylinder liners). Ceramics may be divided into two general categories: 1. Traditional ceramics, such as whiteware, tiles, brick, pottery, and abrasive wheels 2. Industrial ceramics (also called engineering, high-tech,or fine ceramics), such as automotive, turbine, structural, and aerospace components heat exchangers, semiconductors, and cutting tools. ׉ 7cassandra://8h8D3XWDTYjIcycIqsYDhxCPn4j2xRdmEro_TshFAnU`cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://UgY0LNqsWcUSFUe0yIeSQpWOwNsabVs_dQM-icdU7uE R`׉ 7cassandra://14d1pF621t6OUNjb6gvShKS2Dw7QDjyPAhBQKbxbOykC`s׉ 7cassandra://iGBPXAt5ptHgcezqwVF_B88coeFvVruduBx_qMqdLTwL`׉ 7cassandra://mDy0d-YmEDGZ01jJl56JP44ZMxgSjPehTmjDFsyZEnsͷ͠^cZTEbט#u׉׉ 7cassandra://TbR8MuaMRZay4OwxQoSKe2-WMEZJzXElXZh-zCbGMKk`׉ 7cassandra://D1pD13qOP2I-z81NLCKh-S2Fk7maI9GTEoMHEkQGYuk_`s׉ 7cassandra://bqs35lD2pWaX_tspFqfSE5b9ZAWZu68vwvWqqAv7suc`׉ 7cassandra://vDa596jo9MFSTdJr_BIU3KVHYSooOf4Zb8i3Wer53I8hd͠^cZTEb׉EThe Structure of Ceramics 239 (a) (b) Figure 8.1: (a) A ceramic total hip replacement. (b) Detail of the ceramic ball and cup produced from a zirconia-alumina blend. Ceramics perform well in such applications because of their high hardness and wear resistance. Source: Courtesy of DePuy, a Johnson & Johnson Company. The structure of ceramic crystals, containing various atoms of different sizes, is among the most complex of all material structures. The bonding between these atoms is generally covalent or ionic (Section 1.2), and as such are much stronger than metallic bonds. Consequently, properties such as hardness and thermal and electrical resistance are significantly higher in ceramics than in metals (Tables 3.1 and 3.2). Ceramics are available in single-crystal or polycrystalline form. Grain size has a major influence on the strength and properties of ceramics; the finer the grain size (hence the term fine ceramics), the higher the strength and toughness. 8.2.1 Raw Materials Among the oldest of the raw materials used for making ceramics is clay, which has a fine-grained sheetlike structure. The most common example is kaolinite (from Kaoling, a hill in China) which is a white clay consisting of silicate of aluminum, with alternating weakly–bonded layers of silicon and aluminum ions (Fig. 8.2). When blended with kaolinite, water attaches itself to these layers (adsorption); this makes the layers slippery and gives wet clay both its well-known softness and the plastic properties (hydroplasticity) that make it easily formable. Silicon ions Oxygen ions Aluminum ions OH ions Figure 8.2: The crystal structure of kaolinite, commonly known as clay; compare with Figs. 1.3–1.5 for metals. ׉ 7cassandra://iGBPXAt5ptHgcezqwVF_B88coeFvVruduBx_qMqdLTwL`cZ%Eb׉E240 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials Other major raw materials for ceramics that are found in nature include flint (a rock composed of very fine grained silica, SiO2)and feldspar (a group of crystalline minerals consisting of aluminum silicates and potassium, calcium, or sodium). Porcelain is a white ceramic composed of kaolin, quartz, and feldspar; its largest use is in appliances and kitchen and bath ware. In their natural state, these raw materials generally contain impurities of various types, which have to be removed prior to their further processing into specific products. 8.2.2 Oxide Ceramics There are two major types of oxide ceramics: alumina and zirconia (Table 8.1). Alumina. Also called corundum or emery, alumina (aluminum oxide, Al2O3) is the most widely used oxide ceramic, either in pure form or as a raw material to be blended with other oxides. It has high hardness and moderate strength. Although alumina exists in nature, it contains varying levels of impurities and Type Oxide ceramics Alumina Zirconia Table 8.1: Types, General Characteristics and Principal Uses of Ceramics. General characteristics and uses High hardness and moderate strength; most widely used ceramic; cutting tools; abrasives; electrical and thermal insulation. High strength and toughness; thermal expansion close to cast iron; suitable for high-temperature applications such as metallurgical furnace linings, jet engine components and nuclear fuel cladding. Carbides Tungsten carbide Hardness, strength, and wear resistance depend on cobalt binder content; commonly used for dies and cutting tools. Titanium carbide Silicon carbide Not as tough as tungsten carbide; has nickel and molybdenum as the binder; used as cutting tools. High-temperature strength and wear resistance; used for heat engines and as abrasives in grinding wheels. Nitrides Cubic boron nitride Second-hardest substance known, after diamond; used as abrasives and cutting tools. Titanium nitride Silicon nitride Gold in color; used as coatings because of low frictional characteristics. High resistance to creep and thermal shock; used in high-temperature applications such as turbocharger components, rolling element bearings and cutting tools. Sialon Cermets Silica Glasses Glass ceramics Graphite Diamond Carbon nanotubes Graphene Nanophase ceramics Consists of silicon nitrides and other oxides and carbides; used as cutting tools and feed tubes and linings for non-ferrous metal casting. Consist of oxides, carbides, and nitrides; used in high-temperature applications such as cutting tools and composite armor for military applications. High-temperature resistance; quartz exhibits piezoelectric effect; silicates containing various oxides are used in nonstructural applications such as fiber glass, plate glass, and optical glass. Contain at least 50% silica; amorphous structures; several types available with a wide range of mechanical and physical properties. Have a high crystalline component to their structure; good thermal-shock resistance and strong. Typical applications include glass-ceramic cooking tops for stoves and cookware. Crystalline form of carbon; high electrical and thermal conductivity; good thermal-shock resistance, used for structural reinforcement in composite materials, electrical discharge machining electrodes, piston rings. Hardest substance known; available as single crystal or in polycrystalline form; used as cutting tools and abrasives and as dies for drawing fine wire. Unique crystalline form of graphite, with high strength and electrical and thermal conductivity; in use in some structural composites and under investigation for MEMS and microelectronics applications. Single layer form of graphite with good electrical, magnetic and mechanical properties; used as cathodes in some batteries and fuel cells, and also under investigation for applications in display screens, microelectronics, and solar cells. Stronger and easier to fabricate and machine than conventional ceramics; used in automotive and jetengine applications. ׉ 7cassandra://bqs35lD2pWaX_tspFqfSE5b9ZAWZu68vwvWqqAv7suc`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Yz1wQJ3ELoe03pO0XWA08Bs4P0u3XthQVZwKou5dGng`׉ 7cassandra://IHM3YzgjE0o3zMHE-4sYnbs8sKO8olB6vw_CYsCtLN8m`s׉ 7cassandra://uHGcBP29S6wpo_4andgJYJ9YTFr59RT9L0XyXq3VuAg)`׉ 7cassandra://20sJW6X7xEq4qYLRVkdRh155e4zwHaykVCIOyKa3EHof0͠^cZTEbט#u׉׉ 7cassandra://SzSvkOeXKjdjwbTSxk2VexjLRLRt8HHljaTktA9kQ3EE`׉ 7cassandra://YNxublMGAORJvGumjqncvvouCVTwVR1_OWhKuodWVTIm`s׉ 7cassandra://glIcgkWdv3eujoh7QP0H7krpTrGKDZPmcIQckkoG8T0`׉ 7cassandra://ghdgQoJ8f4MUTcZ4Se5zISeoMBWHGwo3vEfRzdpWF4Qd=͠^cZTEb׉EThe Structure of Ceramics 241 possesses nonuniform properties; as a result, its performance also varies. Aluminum oxide, silicon carbide, and most other ceramics are now manufactured almost totally synthetically, so that their quality can be controlled at a consistently high level. First made in 1893, synthetic aluminum oxide is obtained from the fusion of molten bauxite (an aluminum-oxide ore, which is the principal source of aluminum), iron filings, and coke in electric furnaces. The cooled product is crushed and graded, by size, by passing it through standard screens. Aluminum oxide can also be blended with small amounts of other ceramics, such as titanium oxide and titanium carbide. Structures containing alumina and various other oxides are known as mullite and spinel, typically used as refractory materials for high-temperature applications. The mechanical and physical properties of alumina are suitable particularly in applications such as electrical and thermal insulation and in cutting tools and abrasives. Zirconia. Zirconia (zirconium oxide, ZrO2, white in color) has toughness, resistance to thermal shock, wear, and corrosion, low thermal conductivity, and a low friction coefficient. Partially stabilized zirconia (PSZ) has higher strength and toughness and better reliability in performance than zirconia. It is obtained by blending zirconia with oxides of calcium, yttrium, or magnesium. This process forms a material with fine particles of tetragonal zirconia in a cubic lattice. Typical applications include dies for the hot extrusion of metals, and zirconia beads used as grinding and polishing media for aerospace coatings, for automotive primers for paint and for fine glossy print on flexible food packaging. Two important characteristics of PSZ are its high coefficient of thermal expansion (only about 20% lower than that of cast iron), and its low thermal conductivity (about one-third that of other ceramics). Consequently, PSZ is highly suitable for heat-engine components, such as cylinder liners and valve bushings, to help keep the cast-iron engine assembly intact. Transformation-toughened zirconia (TTZ) has higher toughness because of dispersed tough phases in the ceramic matrix. Case Study 8.1 Ceramic Knives Generally made of zirconium oxide, ceramic knives are produced by a process described in Section 18.2. It starts with a blend of ceramic powder mixed with various binders, and compacted (molded) into blanks under high pressure. The blanks are then fired or sintered at temperatures above 1000◦C for several days. An optional hot isostatic pressing operation (Section 17.3.2) can be applied to densify and toughen the ceramic. Next, they are ground and polished on a diamond wheel to form a sharp edge, and the handle is attached. The Mohs hardness (Section 2.6) of the zirconium oxide ceramic is 8.2, as compared to 6 for hardened steel and a maximum of 10 for diamond. Among the advantages of ceramic knives over steel knives are: (a) Because of their very high hardness and wear resistance, ceramic knives can last months and even years before sharpening, depending on their frequency of use. (b) The knives are chemically inert; consequently, they do not stain and food does not stick to them, hence they are easy to clean, and leave no metallic taste or smell. (c) Because they are lightweight, they are easier to use. The knives should be stored in wooden knife blocks and handled carefully. Sharp impact against other objects (such as dishes or dropping it on its edge on a hard surface) should be avoided, as their sharp edges can chip. Also, they should be used only for cutting (not for prying), and in cutting meat, for example, contact with bones is not advisable. The knives have to be sharpened professionally to a precise edge, using diamond grinding wheels. Source: Courtesy of Kyocera Corporation. 8.2.3 Other Ceramics Carbides. Carbides are typically used as cutting tools and die materials, and as an abrasive, especially in grinding wheels. Common examples of carbides are: ׉ 7cassandra://uHGcBP29S6wpo_4andgJYJ9YTFr59RT9L0XyXq3VuAg)`cZ%Eb׉E242 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials • Tungsten carbide (WC) consists of tungsten-carbide particles with cobalt as a binder. The amount of binder has a major influence on the material’s properties; toughness increases with cobalt content, whereas hardness, strength, and wear resistance decrease. • Titanium carbide (TiC) has nickel and molybdenum as its binder, and is not as tough as tungsten carbide. • Silicon carbide (SiC) has high resistance to wear (thus suitable for use as an abrasive), thermal shock, and corrosion. It has a low friction coefficient and retains strength at elevated temperatures, thus suitable for high-temperature components in combustion and jet engines. First produced in 1891, synthetic silicon carbide is made from silica sand, coke, and small amounts of sodium chloride and sawdust. The process is similar to that for making synthetic aluminum oxide (Section 8.2.2). Nitrides. Common examples of nitrides are: • Cubic boron nitride (cBN) is the second-hardest known substance (after diamond), and has special applications, such as in cutting tools and as abrasives in grinding wheels. It does not exist in nature, and was first made synthetically in the 1970s, using techniques similar to those used in making synthetic diamond (Section 8.7). • Titanium nitride (TiN) is used widely as a coating on cutting tools; it improves tool life by virtue of its low friction characteristics. • Silicon nitride (Si3N4) has high resistance to creep at elevated temperatures, low thermal expansion, and high thermal conductivity, thus it resists thermal shock (Section 3.6). It is suitable for hightemperature structural applications, such as components in automotive engines and gas turbines, cam-follower rollers, bearings, sandblast nozzles, and components for the paper industry. Sialon. Derived from the words silicon, aluminum, oxygen, and nitrogen, sialon consists of silicon nitride, with various additions of aluminum oxide, yttrium oxide, and titanium carbide. It has higher strength and thermal-shock resistance than silicon nitride, and is used primarily as a cutting-tool material. Cermets. Cermets are combinations of a ceramic phase bonded with a metallic phase. Introduced in the 1960s and also called black ceramics or hot-pressed ceramics, they combine the high-temperature oxidation resistance of ceramics with the toughness, thermal-shock resistance, and ductility of metals. Acommon application of cermets is in cutting tools, with a typical composition being 70% Al2O3 and 30% TiC. Other cermets contain various oxides, carbides, and nitrides. Cermets have been developed for high-temperature applications, such as nozzles for jet engines and brakes for aircraft, as well as electrical components like resistors and capacitors subjected to high temperatures. Cermets can be regarded as composite materials (Chapter 9) and can be used in various combinations of ceramics and metals bonded by powder-metallurgy techniques (Chapter 17). 8.2.4 Silica Abundant in nature, silica is a polymorphic material; that is, it can have different crystal structures. The cubic structure is found in refractory bricks, used for high-temperature furnace applications. Most glasses contain more than 50% silica. The most common form of silica is quartz, a hard, abrasive hexagonal crystal, used extensively in communications applications, as an oscillating crystal of fixed frequency, because it exhibits the piezoelectric effect (Section 3.7). Silicates are products of the reaction of silica with oxides of aluminum, magnesium, calcium, potassium, sodium, and iron; examples are clay, asbestos, mica, and silicate glasses. Lithium aluminum silicate has very low thermal expansion and thermal conductivity, and high thermal-shock resistance. Because it has very low strength and fatigue life, it is suitable only for nonstructural applications, such as catalytic converters, regenerators, and heat-exchanger components. ׉ 7cassandra://glIcgkWdv3eujoh7QP0H7krpTrGKDZPmcIQckkoG8T0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://pA27_gcYt6PqhsoCQpfSYoqNhKSJVI1OgygPyt5zen8(4`׉ 7cassandra://8r4VaWANxHCNBami21WcVMGvSdSzg1zUHi8QF_k4PUokx`s׉ 7cassandra://v_gpeFvxl4EVSJwecvnxr1X8s-iXmGnBGM_27RZOscYg`׉ 7cassandra://EJ7mBle_Z44QYrGxzwHWFc-ZR0jvipCZnmE5oT3-ybAl͠^cZTEbט#u׉׉ 7cassandra://dkj_cY_H_36HLaZuJsf3W1FJPxqqP2qAAxnY_tKJ2Z8"`׉ 7cassandra://UfGiroJrI7B7dlmDpVZYHTRiGGp_z_5RIJEXKENlJ7YQ`s׉ 7cassandra://v0anfUZoGLsFGo0G5843B8sELDpTgNGoYtrimY3A4CIk`׉ 7cassandra://KDRZWbkyseTZsZJNTYr8xnJRCofQvjv9NOx2scDiXiw͍p͠^cZTEb׉EGeneral Properties and Applications of Ceramics 243 8.2.5 Nanoceramics and Composites In order to improve the ductility and manufacturing characteristics of ceramics, the particle size in ceramics can be reduced by means of various techniques, most commonly gas condensation, use of sol-gels, or by combustion synthesis. Called nanoceramics or nanophase ceramics, the structure of these materials consists of atomic clusters, each containing a few thousand atoms. Control of particle size, distribution, and contamination are important. Nanoceramics exhibit ductility at significantly lower temperatures than do conventional ceramics, and are stronger and easier to fabricate and machine, with fewer flaws. Applications of nanoceramics are in automotive components, such as valves, rocker arms, turbocharger rotors, and cylinder liners, and in jetengine components. Nanocrystalline second-phase particles (on the order of 100 nm or less) and fibers also are used as reinforcements in composites. These composites have enhanced properties, such as improved tensile strength and creep resistance. 8.2.6 Porous Ceramics Porous ceramics can be produced through a number of methods, including slip casting (Section 18.2.1) with a sacrificial insert or by freeze casting (Section 11.4.5). They have a combination of nano-scale and microscale structures, and are used in biomedical applications, heating elements, thermocouples, and diaphragms. Porous ceramics can be infiltrated with glass or polymers, such as in dental applications. Bioceramics. Because of their strength and inertness, ceramics are also used as biomaterials (bioceramics)to replace joints in the human body, as prosthetic devices, and in dental work. Commonly used bioceramics are aluminum oxide, hydroxyapatite, tricalcium phosphate, silicon nitride, and various compounds of silica. Ceramic implants can be made porous, so that bone can grow into the porous structure (as is the case with porous titanium implants for dental work), developing a strong bond with structural integrity. 8.3 General Properties and Applications of Ceramics Compared with metals, ceramics typically have the following relative characteristics: brittleness, high strength and hardness at elevated temperatures, high elastic modulus, low toughness, density, and thermal expansion, and low thermal and electrical conductivity. Because of the wide variety of compositions and grain size, the mechanical and physical properties of ceramics can vary considerably. Properties can also vary widely because of their sensitivity to flaws, defects, and surface or internal cracks. The presence of different types and levels of impurities and different methods of manufacturing also affect properties. 8.3.1 Mechanical Properties The mechanical properties of selected engineering ceramics are given in Fig. 8.3 and Table 8.2. Note that their strength in tension is approximately one order of magnitude lower than their compressive strength, because of their sensitivity to cracks, impurities, and porosity. Such defects lead to the initiation and propagation of cracks under tensile stresses, thus significantly reducing the tensile strength of the ceramic. Reproducibility and reliability are therefore important aspects in the service life of ceramic components. The tensile strength of polycrystalline ceramic increases with decreasing grain size and porosity. This relationship is represented approximately by the expression Sut = Sut,oe−nP, (8.1) where P is the volume fraction of pores in the solid (thus, if the porosity is 15%, P =0.15), Sut,o is the tensile strength at zero porosity; and the exponent n ranges between 4 and 7. The modulus of elasticity of ceramics is related to porosity by the expression E Eo 1 − 1.9P +0.9P2  where Eo is the elastic modulus at zero porosity. , (8.2) ׉ 7cassandra://v_gpeFvxl4EVSJwecvnxr1X8s-iXmGnBGM_27RZOscYg`cZ%Eb׉E 244 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials 600 500 400 300 200 100 0 High-purity silicon nitride (Fine grain) High-purity silicon nitride High-purity SiC Al2O3 SiC Sialon 116 Silicon nitride (reaction bonded) Glass ceramic Low-density SiC 0 200 400 600 800 1000 1200 1400 1600 Temperature (8C) (a) 400 300 200 100 0 Al2O3 ThO2 MgAl2O4 SrO2 TiC Si3N4 MgO 0 200 400 600 800 1000 1200 1400 1600 Temperature (8C) (b) Figure 8.3: (a) Effect of temperature on the strength of various engineering ceramics. Note that much of the strength is maintained at high temperatures. (b) Effect of temperature on the modulus of elasticity for various ceramics. These results can be compared with Fig. 2.6. Unlike most metals and thermoplastics, ceramics generally lack impact toughness and thermal-shock resistance, because of their inherent lack of ductility; once initiated, a crack propagates rapidly. Crack growth can be somewhat retarded by using a stabilizing platelet in their structure, thereby improving fatigue and toughness. In addition to undergoing fatigue failure under cyclic loading, ceramics exhibit a phenomenon called static fatigue, also exhibited by glasses. When subjected to a static tensile load over time, these materials may suddenly fail, a phenomenon that occurs in environments where water vapor is present. Static fatigue, which does not occur in a vacuum or in dry air, has been attributed to a mechanism similar to the stress– corrosion cracking of metals (Section 2.10.2). Table 8.2: Properties of Various Ceramics at Room Temperature. Transverse rupture Material Aluminum oxide Cubic boron nitride Diamond Silica, fused Silicon carbide Silicon nitride Titanium carbide Tungsten carbide Symbol Al2O3 cBN SiO2 SiC WC Partially stabilized zirconia PSZ strength (MPa) 140–240 725 — 1400 — Si3N4 TiC 100–750 480–600 Compressive strength (MPa) 7000 7000 1300 Elastic modulus Hardness Poisson’s (GPa) (HK) 1000–2900 310–410 2000–3000 4000–5000 850 550 700–3500 — 4100–5900 — 240–480 2100–3000 300–310 520–700 200 ratio, ν 0.26 — 830–1000 7000–8000 — 70 2000–2500 1800–2400 1100 0.25 0.14 0.24 — 0.30 Density (kg/m3) 4000–4500 3480 3500 — 3100 3300 1400–1900 3100–3850 310–410 1800–3200 — 5500–5800 1030–2600 620 Note: These properties vary widely depending on the condition of the material. 10,000–15,000 5800 Tensile strength (MPa) GPa Ci S ׉ 7cassandra://v0anfUZoGLsFGo0G5843B8sELDpTgNGoYtrimY3A4CIk`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Z00zRHoSJVqo1molAyvrMdCOAGIrCRZxjvg-MElAX3YK`׉ 7cassandra://GW-7r7-qK21XfrBY7RXtj9INpOk1gWyR6vjwWU3FrBkc`s׉ 7cassandra://ALQJ3m9Nif4jhJF2jdx5xVShPkWa0kYVMG-0Zu-1niY`׉ 7cassandra://-J_paj5mSSCEGNbHuy_F0y8YPfYx8hUEDZ3dqbVmGCY]͠^cZUEbט#u׉׉ 7cassandra://FpxEG7B1V1Va7OLZBkFUE47Ie-5Wof1xTmwsr7y4XT0G`׉ 7cassandra://B6pql3-U_us3Qg-JIvC24XyUd-pMOB0oDdcTBP7Mf30]`s׉ 7cassandra://d-h53_o2xRyg-Y5Aj-7K5csghIJlE9rc3P04YWflBjk`׉ 7cassandra://Ru5c0Ycx3ceze_CRyOCd7a0RjOYOmK9xPPLG8Bc3tmk͂ ͠^cZUEb׉EGeneral Properties and Applications of Ceramics 245 Ceramic components subjected to tensile stresses may be prestressed, in much the same way that concrete is prestressed by steel bars. Prestressing the shaped ceramic components subjects them to compressive stresses, with methods that include: • Heat treatment and chemical tempering (Section 18.4) • Laser treatment of surfaces (Section 34.8) • Coating with ceramics that have different thermal-expansion coefficients (Section 3.6) • Surface-finishing operations, such as grinding, in which compressive residual stresses are induced on the surfaces (Section 26.3). Major advances have been made in improving the toughness and other properties of ceramics, including the development of machinable and grindable ceramics. Among these advances are the proper selection and processing of raw materials, the control of purity and structure, and the use of reinforcements, with particular emphasis on advanced methods of stress analysis during the design of ceramic components. 8.3.2 Physical Properties Most ceramics have a relatively low specific gravity, ranging from about 3 to 5.8 for oxide ceramics as compared to 7.86 for iron (Table 3.1). They have very high melting or decomposition temperatures. The thermal conductivity of ceramics varies by as much as three orders of magnitude, depending on their composition, whereas in metals it varies by only one order. As with other materials, the thermal conductivity of ceramics decreases with increasing temperature and porosity, because air is a poor thermal conductor. The thermal conductivity, k, is related to porosity by the expression k = ko (1 −P) , (8.3) where ko is the thermal conductivity at zero porosity and P is the porosity, as a fraction of the total volume. Thermal expansion and thermal conductivity induce internal stresses that can then lead to thermal shock or to thermal fatigue in ceramics. The tendency toward thermal cracking (called spalling when a small piece or a layer from the surface breaks off) is lower with the combination of low thermal expansion and high thermal conductivity. For example, fused silica has high thermal-shock resistance, because of its virtually zero thermal expansion. The optical properties of ceramics can be controlled by using various formulations and by controlling their structure. These methods make possible the imparting of different degrees of transparency and translucency, and of different colors. For example, single-crystal sapphire is completely transparent, zirconia is white, and fine-grained polycrystalline aluminum oxide is translucent gray. Porosity influences the optical properties of ceramics in much the same way as air trapped in home-made ice cubes, making them less transparent and giving a white appearance. Although ceramics are basically resistors, they can be made electrically conducting by alloying them with certain elements in order to make the ceramic behave like a semiconductor or even like a superconductor. 8.3.3 Applications Ceramics have numerous consumer and industrial applications. Various types of ceramics are used in the electrical and electronics industries, because they have high electrical resistivity, high dielectric strength (voltage required for electrical breakdown per unit thickness), and magnetic properties suitable for such applications as magnets for speakers. The capability of ceramics to maintain their strength and stiffness at elevated temperatures makes them suitable for high-temperature applications. The higher operating temperatures made possible by the use of ׉ 7cassandra://ALQJ3m9Nif4jhJF2jdx5xVShPkWa0kYVMG-0Zu-1niY`cZ%Eb׉E .246 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials ceramic components results in more efficient combustion of fuel and the reduction of emissions in automobiles. Currently, internal combustion engines are only about 30% efficient, but with the use of ceramic components, the operating performance can be improved by at least 30%. The ceramics that are being used successfully, especially in automotive gas-turbine engine components, such as rotors, are: silicon nitride, silicon carbide, and partially stabilized zirconia. Other attractive properties of ceramics are their low density and high elastic modulus. They enable product weight to be reduced and allow the inertial forces generated by moving parts to be lower. Ceramic turbochargers, for example, are about 40% lighter than conventional ones. High-speed components for machine tools (Part IV) also are candidates for ceramics (Section 25.3). The high elastic modulus of ceramics makes them attractive for improving the stiffness of machines, while reducing the weight. Their high resistance to wear also makes them suitable for applications such as cylinder liners, bushings, seals, bearings, and liners for gun barrels. Coating metal with ceramics is another application, often done to reduce wear, prevent corrosion, or provide a thermal barrier. Case Study 8.2 Ceramic Ball and Roller Bearings Silicon-nitride ceramic ball and roller bearings are used when high temperature, high speed, or marginally lubricated conditions exist. The bearings can be made entirely from ceramics, or just the ball and rollers are ceramic and the races are metal, in which case they are referred to as hybrid bearings (Fig. 8.4). Examples of their applications include high-performance machine tool spindles, metal-can seaming heads, high-speed flow meters, and bearings for motorcycles, go karts and snowmobiles. The ceramic spheres have a diametral tolerance of 0.13 µm and a surface roughness of 0.02 µm. They have high wear resistance, high fracture toughness, low density, and perform well with little or no lubrication. The balls have a coefficient of thermal expansion one-fourth that of steel, and they can withstand temperatures of up to 1400◦C. Produced from titanium and carbon nitride by powder-metallurgy techniques, the full-density titanium carbonitride (TiCN) or silicon nitride (Si3N4) bearing-grade material can be twice as hard as chromium steel and 40% lighter. Components up to 300 mm in diameter have been produced. (a) (b) Figure 8.4: A selection of ceramic bearings and races. Source: Courtesy of The Timken Company. ׉ 7cassandra://d-h53_o2xRyg-Y5Aj-7K5csghIJlE9rc3P04YWflBjk`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://oBvovVi9Lzd7oIurRzI0OQ08Ai5gZkGX65auufb2Z8E2`׉ 7cassandra://6B1WMHIdVNPPIHmb0JiQP2QPidlNkBzJGyM7TaiggWMO`s׉ 7cassandra://serJOS7KRC57pDQMfOio__2nsOXMfooWi47RTPAE_YU[`׉ 7cassandra://fKYlcWBU6NNw1Lzv6H_xvVSVn75uVvhuUgwR9zRFNzI͠^cZVEbט#u׉׉ 7cassandra://FhskckMNAiJAavJSOpGy80YQHY-yxagUsSKhsGM127wK`׉ 7cassandra://dFdYDDKH7Jbjq9Kgh9_H-TRbStcZzsGrH8tFdmTSrTMZ`s׉ 7cassandra://By1y7UXWSQbKhH5V0F44-oZpuVn7_u6cn-PS_Hs1Fsw`׉ 7cassandra://TZjgomQxtepSJI_ZHXt8xolhtv0RxiPs_wpi1F5k3Usao̲͠^cZVEb׉E Glasses 247 Oxygen Silicon Sodium (a) (b) Figure 8.5: Schematic illustration of the structure of silica glass. (a) Pure silica glass, in the form of (SiO2)n random structure; (b) partially depolymerized glass. Note that a fourth bond for each silicon is outside the plane shown. 8.4 Glasses Glass is an amorphous solid with the structure of a liquid, as shown in Fig. 8.5, a condition that is obtained by supercooling (cooling at a rate too high to allow crystals to form). Technically, glass is defined as an inorganic product of fusion that has cooled to a rigid state without crystallizing. Glass has no distinct melting or freezing point, its behavior is thus similar to that of amorphous alloys (see metallic glasses, Section 6.15) and amorphous polymers (Section 7.2.2). Glass beads first were produced in about 2000 B.C., and the art of glassblowing began in about 200 B.C. Silica was used for all glass products until the late 1600s; rapid developments in glasses then began in the early 1900s. There are about 750 different types of commercially available glasses, with applications ranging from window glass to glass for containers, cookware, lighting, and mobile phones, and to glasses with special mechanical, electrical, high-temperature, chemical inertness , corrosion, and optical characteristics. Special glasses are used in fiber optics (for communication by light with little loss in signal power) and in glass fibers with very high strength (for use in reinforced plastics, Section 9.2.1). All glasses contain at least 50% silica, which is known as a glass former. The composition and properties of glasses can be modified by the addition of oxides of aluminum, sodium, calcium, barium, boron, magnesium, titanium, lithium, lead, and potassium. Depending on their function, these oxides are known as intermediates or modifiers. 8.4.1 Types of Glasses Almost all commercial glasses are categorized by the following types (Table 8.3): • Soda-lime glass (the most common type) • Lead-alkali glass • Borosilicate glass • Aluminosilicate glass • 96%-silica glass • Fused silica glass Glasses are also classified as colored, opaque (white and translucent), multiform (variety shapes), optical, photochromatic (darkens when exposed to light, as in some sunglasses), photosensitive (changing from ׉ 7cassandra://serJOS7KRC57pDQMfOio__2nsOXMfooWi47RTPAE_YU[`cZ%Eb׉E 248 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials Table 8.3: Properties of Various Glasses. Soda-lime Lead-alkali Property Density Strength Resistance to thermal shock Electrical resistivity Hot workability Heat treatability Chemical resistance Impact-abrasion resistance Ultraviolet-light transmission Relative cost glass High Low Low Moderate Good Good Poor Fair Poor Lowest glass Low Low Best Best Highest Medium Moderate Good Good Fair Good Fair Poor Poor Low Poor Good Good Fair Medium Borosilicate glass 96% silica Better Good Poor None Better Good Fused silica Low Lowest High Good Poorest None Best Best Good Good High Highest Highest Best clear to opaque), fibrous (drawn into long fibers, as in fiberglass), and foam or cellular (containing bubbles, thus a good thermal insulator). Glasses also can be referred to as hard or soft, usually in the sense of a thermal rather than mechanical property (see also hardness of glasses, Section 8.4.2). Thus, a soft glass softens at a lower temperature than does a hard glass. Soda-lime and lead-alkali glasses are considered soft, and the rest as hard. 8.4.2 Mechanical Properties The behavior of glass, like that of most ceramics, is generally regarded as perfectly elastic and brittle. The modulus of elasticity for commercial glasses typically ranges from 55 to 90 GPa, and their Poisson’s ratio from 0.16 to 0.28. The hardness of glasses, as a measure of resistance to scratching, ranges from 5 to 7 on the Mohs scale, which is equivalent to a range from around 350 to 500 HK (see Fig. 2.15). Glass in bulk form generally has a strength lower than 140 MPa. The relatively low strength of bulk glass is attributed to the presence of small flaws and microcracks on its surface, some or all of which may be introduced during normal handling of the glass by inadvertently abrading it. These defects reduce the strength of glass by two to three orders of magnitude, compared to its ideal (defect free) strength. Glasses can be strengthened by thermal or chemical treatments to obtain high strength and toughness (Section 18.4). The strength of glass theoretically can reach 35 GPa. When molten glass is drawn into fibers (fiberglass), its tensile strength ranges from about 0.2 to 7 GPa, with an average value of about 2 GPa. These fibers are stronger than steel, and are used to reinforce plastics in such applications as boats, automobile bodies, furniture, and sporting equipment (Tables 2.2 and 9.2). 8.4.3 Physical Properties Glasses are characterized by low thermal conductivity and high electrical resistivity and dielectric strength. Their coefficient of thermal expansion is lower than those for metals and plastics, and may even approach zero; titanium silicate glass (a clear, synthetic high-silica glass), for example, has a near-zero coefficient of thermal expansion. Fused silica (a clear, synthetic amorphous silicon dioxide of very high purity) also has a near-zero coefficient of expansion. The optical properties of glasses, such as reflection, absorption, transmission, and refraction, can be modified by varying their composition and treatment. Glasses generally are resistant to chemical attack, and are ranked by their resistance to corrosion by acids, alkalis, or water. ׉ 7cassandra://By1y7UXWSQbKhH5V0F44-oZpuVn7_u6cn-PS_Hs1Fsw`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://xBHen_wbborYfsbGRp9tLMWK3CiraDj1gziazgFHO5k!`׉ 7cassandra://raatbBI5JfxvIXIxvVQh1ZV4EPB2hLGmOUJx8dKDj-AZ`s׉ 7cassandra://G6vcCyPgPY1GianPP6dJmOAbD_F21BZoIpdi9cebRM0`׉ 7cassandra://6fCqPJ_k77NohxIcTLEhThJCugOm0H2fn5f6bi7CYWM͈͠^cZVEbט#u׉׉ 7cassandra://X9dsYFXeDk1HpMKdz9zduLB2CdGo-WDeLn614GpAGuA`׉ 7cassandra://RmF4eqTGpATMqp1brDNEV0F_XvNKTj9jgsQADE76ZH4t`s׉ 7cassandra://H5XJATHSAETywS0qwaUdEv2FP2tjZSU8-4WUPdE5zpM`׉ 7cassandra://RsEwZvtuaUbEGAnz0wcWTMiE1Px7gjumjQFva912cU0_͠^cZVEbנcZVEb̈9׉Hhttp://www.graphel.comGׁ ׁ rנcZVEbÁ̗9ׁHhttp://www.graphel.comׁ ׁ Ј׉EGGraphite 249 8.5 Glass Ceramics Although glasses are amorphous, glass ceramics have a high crystalline component to their microstructure. Glass ceramics, such as Pyroceram (a trade name) contain large proportions of several oxides; thus, their properties are a combination of those for glass and those for ceramics. Most glass ceramics are stronger than glass. These products are first shaped and then heat treated, whereby devitrification (recrystallization) of the glass occurs. Glass ceramics are generally white or gray in color. The hardness of glass ceramics ranges approximately from 520 to 650 HK. Because they have a nearzero coefficient of thermal expansion, they also have high thermal-shock resistance, and because of the absence of porosity usually found in conventional ceramics, they are also strong. The properties of glass ceramics can be improved by modifying their composition and by heattreatment techniques. First developed in 1957, glass ceramics are typically used for cookware, heat exchangers in gas-turbine engines, radomes (housings for radar antennas), and electrical and electronics components. Some electric stoves use glass-ceramic cooktops and infrared halogen or radiant-heating coils; the glass ceramic is advantageous because of its high hardness and low thermal conductivity. 8.6 Graphite Graphite is a crystalline form of carbon, and has a layered structure, with basal planes or sheets of closepacked carbon atoms (see Fig. 1.5); consequently, graphite is weak when sheared along the layers. This characteristic, in turn, gives graphite its low frictional properties behaving as a solid lubricant. However, its frictional properties are low only in an environment of air or moisture; in a vacuum, it is abrasive and thus a poor lubricant. Unlike with other materials, the strength and stiffness of graphite increase with temperature. (a) (b) Figure 8.6: (a) Various engineering components made of graphite. (b) Examples of graphite electrodes for electrical discharge machining. Source: Courtesy of (a) Poco Graphite, an Entegris Company (b) Graphel Corporation (www.graphel.com). ׉ 7cassandra://G6vcCyPgPY1GianPP6dJmOAbD_F21BZoIpdi9cebRM0`cZ%Eb׉E250 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials Amorphous graphite is known as lampblack (black soot) and is used as a pigment. Ordinary pencil lead is a mixture of graphite and clay. Although brittle, graphite has high electrical and thermal conductivity and good resistance to thermal shock and to high temperature (although it begins to oxidize at 500◦C). It is thus an important material for applications such as electrodes, heating elements, brushes for electric motors, high-temperature fixtures, furnace parts, mold materials (such as crucibles for melting and casting of metals), and seals (Fig. 8.6). An important use of graphite is as fibers in reinforced plastics and composite materials (Section 9.2). Another characteristic of graphite is its resistance to chemicals, thus it is also used in filters for corrosive fluids; its low absorption cross section and high scattering cross section for thermal neutrons make graphite also suitable for nuclear applications. Carbon and Graphite Foams. These foams have high service temperatures, chemical inertness, low thermal expansion, and thermal and electrical properties that can be tailored to specific applications. Carbon foams are available in either graphitic or nongraphitic structures. Graphitic foams (typically produced from petroleum, coal tar, and synthetic pitch) have low density, high thermal and electrical conductivity; however, they have lower mechanical strength and are much more expensive than nongraphitic foams (produced from coal or organic resins), which are highly amorphous. The foams have a cellular microstructure, with interconnected pores, thus their mechanical properties depend on density (see also Section 8.3). Blocks of foam can easily be machined into various complex shapes. Applications of carbon foams include their use as core materials for aircraft and ship interior panels, structural insulation, sound-absorption panels, substrates for spaceborne mirrors, lithium-ion batteries, and fire and thermal protection. 8.6.1 Fullerenes Carbon molecules (typically C60) are produced in the shape of soccer balls, called fullerenes or buckyballs, after B. Fuller (1895–1983), the inventor of the geodesic dome. These chemically inert spherical molecules are produced from soot, and act much like solid lubricant particles. When mixed with metals, fullerenes can become superconductors at low temperatures (around 40 K). Despite their promise, no commercial applications of buckyballs currently exist. 8.6.2 Nanotubes Carbon nanotubes can be thought of as tubular forms of graphite, and are of interest for the development of nanoscale devices (see also nanomaterials, Section 8.8). Nanotubes are produced most often by chemical vapor deposition (Section 34.6.2), or by laser ablation of graphite and by carbon-arc discharge. Carbon nanotubes have exceptional strength, thus making them attractive as reinforcing fibers for composite materials. However, because they have very low adhesion with most materials, delamination with a matrix can limit their reinforcing effectiveness. It is difficult to disperse nanotubes properly because they have a tendency to clump and this limits their effectiveness as a reinforcement. Some products have used carbon nanotubes, such as bicycle frames specialty baseball bats, golf clubs, and tennis racquets. They provide only a fraction of the reinforcing material (by volume), graphite fibers playing the major role. Another characteristic of carbon nanotubes is their very high electrical current carrying capability. They can be made as semiconductors or conductors, depending on the orientation of the graphite in the nanotube (see Fig. 8.7). Armchair nanotubes are theoretically capable of carrying a current density higher than 1000 times that for silver or copper, making them attractive for electrical connections in nanodevices (Section 29.6). Carbon nanotubes have been incorporated into polymers to improve their static-electricity discharge capability, especially in fuel lines for automotive and aerospace applications. Carbon nanotubes are used in filter membranes, as magnetic shielding in military hardware, and in advanced batteries and capacitors where their high surface area provides better current carrying ability combined with mechanical and electrical stability. Among numerous proposed uses for carbon nanotubes are flat-panel displays, tissue engineering, electrical cables for nano-scale circuitry, catalysts, and X-ray and microwave generators. ׉ 7cassandra://H5XJATHSAETywS0qwaUdEv2FP2tjZSU8-4WUPdE5zpM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://K4RQvJm78dw-odFNOJxWwyMDRp4dxZHae8jIC4hMLS8`׉ 7cassandra://P2Ro6i8yhfJ9Ink9qZr2iByCm9vblxy3JxyWnilyl8o^`s׉ 7cassandra://5v7RWQ2dt4QuVwhy6wwugux5zV0JfLVjJcY7XTnmg98v`׉ 7cassandra://wflSRQs3QvU2Hy5kepuxBeMrizCwTXojtD_-C6yi6kk(KRP͠^cZXEbט#u׉׉ 7cassandra://hiQ4pOWopJ5C6LyFidQP20M-mtevwz8Yguv3T4768yw`׉ 7cassandra://ztj-sPsbObPqAvHGicggum4QbOdvE5PGWHi5Y1MMPmYRr`s׉ 7cassandra://RNbXqaqPvVYHEECRS5WHooii7SaBVOTaXukU-IHIAro`׉ 7cassandra://nYNCl_KEyYLIRl_lyjTUnSJenMQ9Zx55FuljiVaeDG4l͠^cZXEb׉E Diamond 251 Graphene Armchair Zigzag Chiral Figure 8.7: Forms of carbon nanotubes produced from a section of graphene: armchair, zigzag, and chiral. Armchair nanotubes are noteworthy for their high electrical conductivity, whereas zigzag and chiral nanotubes are semiconductors. Annual carbon nanotube production worldwide is currently several thousand metric tons per year. They can be single-walled (SWNTs) or multi-walled (MWNTs) and can be doped with various species. 8.6.3 Graphene Graphene can be considered to be a single sheet of graphite, or an unwrapped nanotube, as shown in Fig. 8.7; it is one of the most commonly encountered materials. Although flakes of graphene are produced whenever a pencil is abraded by paper, its direct observation, using transmission electron microscopy, dates to the early 1960s. Several methods have now been developed for producing graphene, including epitaxy (Section 28.5) on silicon carbide or metal substrates. or by chemical reduction of graphite. Currently, graphene is used as a cathode coating in some batteries, and is in development for numerous microelectronic and photonic applications (see Case Study 29.2). 8.7 Diamond Diamond is a form of carbon, with a covalently bonded structure. It is the hardest substance known (7000–8000 HK); it is, however, brittle and begins to decompose in air at about 700◦C, but resists higher temperatures in a nonoxidizing environment. Synthetic (also called industrial) diamond was first produced in 1955. A common method of manufacturing it is to subject graphite to a hydrostatic pressure of 14 GPa and a temperature of 3000◦C, referred to as high-pressure, high-temperature (HPHT) synthesis. An alternative is to produce diamonds through a chemical vapor deposition process (CVD, Section 34.6.2), whereby carbon is deposited onto a starting seed of diamond powder. The CVD process is used most often for synthetic gemstones. Synthetic diamond has identical, and sometimes slightly superior, mechanical properties as natural diamond, because of its lack of impurities. The gemstones have a characteristic orange or yellow tint, due to impurities, resulting from the CVD process, whereas laser treatment of the diamond results changing the tint to pink or blue. However, since most of a gemstone’s cost is attributed to grinding and finishing (Chapter 26) to achieve a desired shape, synthetic diamonds are only slightly less expensive than natural ones. Synthetic diamond is available in a variety of sizes and shapes; for use in abrasive machining, the most common grit size is 0.01 mm in diameter. Diamond particles can be coated with nickel, copper, or titanium for improved performance in grinding operations. Diamond-like carbon also has been developed and is used as a diamond film coating, as described in Section 34.13. ׉ 7cassandra://5v7RWQ2dt4QuVwhy6wwugux5zV0JfLVjJcY7XTnmg98v`cZ%Eb׉E252 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials Figure 8.8: A collection of synthetic diamonds. Source: Shutterstock/ijp2726. In addition to its use in jewelry, gem-quality synthetic diamond has applications as heat sinks for computers, in telecommunications and integrated-circuit industries, and as windows for high-power lasers. Its electrical conductivity is 50 times higher than that of natural diamond, and it is 10 times more resistant to laser damage. Because of its favorable characteristics, diamond has numerous important applications, such as the following: • Cutting-tool materials, as a single crystal or in polycrystalline form • Abrasives in grinding wheels, for hard materials • Dressing of grinding wheels (i.e., sharpening of the abrasive grains) • Die inserts for drawing wire less than 0.06 mm in diameter • Coatings for cutting tools and dies. 8.8 Nanomaterials Important developments continue to take place in the production of materials as particles, fibers, wire, tube, films, and composites, with features typically on the order of 1 nm to up to 100 nm. First investigated in the early 1980s and generally called nanomaterials or nanostructured, nanocrystalline,or nanophase materials, they have certain properties that are often superior to traditional materials. These characteristics include high strength, hardness, ductility, toughness, resistance to wear and corrosion, and suitable for structural (load bearing) and nonstructural applications in combination with unique electrical, magnetic, thermal, and optical properties. The composition of a nanomaterial can be any combination of chemical elements; among the more important compositions are carbides, oxides, nitrides, metals and their alloys, organic polymers, semiconductors, and various composites. Nanometal-polymer hybrid nanomaterials have been developed for very lightweight components. More recent investigations include the development of nanopaper, with very high strength and toughness, produced from wood pulp with fibers rearranged into an entangled porous mesh. Production methods for nanomaterials include inert-gas condensation, sputtering, plasma synthesis, electrode position, sol–gel synthesis, and mechanical alloying or ball milling. The synthesized powders are ׉ 7cassandra://RNbXqaqPvVYHEECRS5WHooii7SaBVOTaXukU-IHIAro`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://MhE0Xotqt2oQeVjNdtZLVhj1G_xXRFJx6EUnLmN8SGQ`׉ 7cassandra://gcUj7LFEosAYt9eHYyMAlmkpTvV8IFql_0vRhj5NDEMb`s׉ 7cassandra://7W87HNpuAQ0JnvKFt749RQM9IKWtrCUcb0vEgzimjzg`׉ 7cassandra://frHGBhH8ygMm_sErb1zSYLscRQ-WRasI98iPIpnf_jkJ͠^cZXEbט#u׉׉ 7cassandra://FJtX_XogTv42lXqknZN063thk1v0kw2smOxLYvEEXFcC`׉ 7cassandra://jmdOvpLLc3HwO_WGYc5HCXXAoIkVhUe-IC6B2qt1ws0@p`s׉ 7cassandra://bVqiGmveCVlLIBGQfQkM2yrvD0WbJCrWX1Q6gt5vUEY`׉ 7cassandra://rU1kZ6-fP5-eXwVBgCzWv0AGM4HXWFV-BFDGCuVmdNAI͠^cZXEb׉E Summary 253 consolidated into bulk materials by various techniques, including compaction and sintering. Nanoparticles have a very high surface-area-to-volume ratio, thus affecting their behavior in processes such as diffusion and agglomeration (interaction of particles forming a cluster). Because the synthesis of nanomaterials is at atomic levels, their purity is on the order of 99.9999%, and their homogeneity and the uniformity of their microstructure are highly controlled. As a result, their mechanical, electrical, magnetic, optical, and chemical properties also can be controlled precisely. Nanomaterials are very expensive to produce and process into products, thus their cost-effectiveness is under continued study. Applications of Nanomaterials. The unique properties of nanomaterials enable manufacturing of products that are strong, and light. The following are some current and potential applications for nanomaterials: 1. Cutting tools and inserts, made of nanocrystalline carbides and other ceramics. 2. Nanophase ceramics that are ductile and machinable. 3. Specialty bicycle frames, baseball bats, and tennis racquets, using carbon nanotubes (see also Section 8.6.2). 4. Next-generation computer chips, using nanocrystalline starting materials with very high purity, better thermal conductivity, and more durable interconnections. 5. Flat-panel displays for laptop computers and televisions, made by synthesizing nanocrystalline phosphorus to improve screen resolution. 6. Spark-plug electrodes, igniters and fuels for rockets, medical implants, high-sensitivity sensors, catalysts for elimination of pollutants, high-power magnets, and high-energy-density batteries. 7. Switches, valves, motor, and pumps. 8. Coatings made of nanomaterials are being investigated for improved wear, abrasion, corrosion resistance and thermal insulation; nanocrystalline materials; nanophase materials because of their lower thermal conductivity. Health Hazards. Because of their extremely small size, nanoparticles can present various health hazards by their absorption through the skin, lungs, or the digestive track; they can also penetrate human cells. There is increasing evidence that nanoparticles can pollute the air, water, and the ground. Consequently, there is growing research on the risks of nanoparticles to humans and the environment. Summary • Ceramics, glasses, and various forms of carbon are of major importance in engineering applications and in manufacturing processes. Ceramics, which are compounds of metallic and nonmetallic elements, generally are characterized by high hardness, high compressive strength, high elastic modulus, low thermal expansion, high temperature resistance, good chemical inertness, low density, and low thermal and electrical conductivity. They are brittle and have low toughness. • Ceramics are generally classified as either traditional ceramics or industrial (or high-tech) ceramics; the latter are particularly attractive for applications such as engine components, cutting tools, and components requiring resistance against wear and corrosion. Ceramics of importance in design and manufacturing are the oxide ceramics (alumina and zirconia), tungsten and silicon carbides, nitrides, and cermets. • Glasses are supercooled liquids and are available in a wide variety of compositions and mechanical, physical, and optical properties. Glass ceramics are predominantly crystalline in structure, and have properties that are more desirable than those of glasses. ׉ 7cassandra://7W87HNpuAQ0JnvKFt749RQM9IKWtrCUcb0vEgzimjzg`cZ%Eb׉ED254 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials • Glasses in bulk form have relatively low strength, but they can be strengthened by thermal and chemical treatments. Glass fibers are used widely as a reinforcement in composite materials. Porous glass is used in biomedical and energy applications, because of its high surface area to volume ratio. • Graphite, fullerenes, carbon nanotubes, graphene, and diamond are forms of carbon that display unique combinations of properties. Graphite has high-temperature use and electrical applications; graphite fibers are used to reinforce plastics and other composite materials. • Diamond is used as cutting tools for precision machining operations, as dies for drawing of thin wire, and as abrasives for grinding wheels. Diamond-like carbon also has applications as a coating material for improved wear resistance. • Nanomaterials have physical, mechanical, optical, chemical, and thermal properties, with several unique applications. Carbon nanotubes are of continued research interest, particularly because of their relevance to nanoscale electrical and electromechanical systems. ‘ Key Terms Alumina Bioceramics Buckyballs Carbides Carbon Carbon foam Carbon nanotubes Ceramics Cermets Clay Devitrification Diamond Diamond-like carbon Feldspar Flint Fullerenes Glass Glass ceramics Glass fibers Bibliography Bansal, N.P. (ed.), Handbook of Ceramic Composites, Springer, 2005. Barsoum, M.W., Fundamentals of Ceramics, Institute of Physics Publishing, 2003. Bhushan, B. (ed.), Handbook of Nanotechnology, 3rd ed., Springer, 2010. Glass former Graphene Graphite Industrial ceramics Industrial diamond Nanoceramics Nanophase ceramics Nanotubes Nitrides Oxide ceramics Partially stabilized zirconia Porcelain Porosity Sialon Silica Static fatigue Transformation-toughened zirconia Zirconia ׉ 7cassandra://bVqiGmveCVlLIBGQfQkM2yrvD0WbJCrWX1Q6gt5vUEY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Y7aPK_Q1796FJYxIqiK4qlUOcFEvJkUIijTqJoEbKj0 `׉ 7cassandra://iF16KRujiT9xrHYr5iFEMhjx9DS0tUYzYbcNge-GeEc\`s׉ 7cassandra://89o6YIuxXNFuwYOYC5_Fjtac8JdmJJUb7qZvqxUbQ-4`׉ 7cassandra://e-NbFuItc9uP87VT_PWgjP2d8tm_ICV9J1rmYtGZq94Y)͠^cZYEbט#u׉׉ 7cassandra://UNY2WV5rEeRRRyjaNuviU62Gla5egwmp_P5K0mHDNj8<`׉ 7cassandra://Rs8zOkgcRXjsNMLbZFrmuyhSIMEkK3-khUSbMmHmahwi`s׉ 7cassandra://Dg2RQI3v4C8Hkb-zzmS69j6EVhiUWVkO_-T_0Rb3F4Y`׉ 7cassandra://ohGcODjtuas3ugQ68zGw549y505W6JWLvpunlHsC3FUV͠^cZYEb׉E 4Review Questions 255 Bhushan, B. (coordinating ed.), Handbook of Nanomaterials Properties, Springer, 2014. Carter, C.B., and Norton, M.G., Ceramic Materials: Science and Engineering, 2nd ed., Springer, 2013. Gogotsi, Y. and Presser, V., Carbon Nanomaterials, 2nd ed., CRC Press, 2013. Harper, C.A. (ed.), Handbook of Ceramics, Glasses, and Diamonds, McGraw-Hill, 2001. Hench, L.L., A Introduction to Bioceramics, 2nd ed., Imperial College Press, 2013. Holand,W., and Beall, G.H., Design and Properties of Glass Ceramics, 2nd ed., Wiley-American Chemical Society, 2012. Milne, W.I., and Gervasoni, J.L., Graphene Science Handbook, CRC Press, 2016. O’Connell, M.J. (ed), Carbon Nanotubes: Properties and Applications, CRC Press, 2006. Park, J., Bioceramics: Properties, Characterization and Applications, Springer, 2008. Parinov, I.A., Microstructure and Properties of High-Temperature Superconductors, 2nd ed., Springer, 2013. Pelleg, J., Mechanical Properties of Ceramics, Springer, 2014. Prelas, M.A., Popovichi, G., and Bigelow, L.K. (eds.), Handbook of Industrial Diamonds and Diamond Films, Marcel Dekker, 1998. Richerson, D.W. and Lee, W.E., Modern Ceramic Engineering: Properties, Processing, and Use in Design, 4th ed., Marcel Dekker, 2018. Sharma, K.R., Graphene Nanomaterials, Momentum Press, 2014. Shelby, J.E., Introduction to Glass Science and Technology, 2nd ed., Royal Society of Chemistry, 2005. Skakalova, V., and Kaiser, L.B. (eds.) Graphene: Properties, Preparation, Characterisation and Devices, Woodhead Publishing, 2014. Somiya, S. (ed.), Handbook of Advanced Ceramics: Materials, Applications, Processing and Properties, 2nd ed., Academic Press, 2013. Vajtai, R. (ed.), Handbook of Nanomaterials, Springer, 2013. Vollath, D., Nanomaterials: An Introduction to Synthesis, Properties and Applications, 2nd ed., WileyVCH, 2013. Wachtman, J.B., Cannon, W.R., and Matthewson, M.J., Mechanical Properties of Ceramics, Wiley, 2009. Review Questions 8.1. What is a ceramic? 8.2. List the major differences between the properties of ceramics and those of metals and plastics. 8.3. List the major types of ceramics that are useful in engineering applications. 8.4. What do the following materials typically consist of (a) carbides, (b) cermets, and (c) sialon? 8.5. What is porcelain? 8.6. What is glass? Why is it called a supercooled material? 8.7. How is glass different from a glass ceramic? 8.8. What is devitrification? 8.9. List the major types of glasses and their applications. 8.10. What is static fatigue? What is its significance? 8.11. Describe the major uses of graphite. 8.12. How are alumina ceramics produced? 8.13. What features of PSZ differentiate it from other ceramics? 8.14. What are buckyballs? 8.15. List the major uses of diamond. 8.16. What is a carbon nanotube? Explain why they are not as prevalent as other forms of carbon. ׉ 7cassandra://89o6YIuxXNFuwYOYC5_Fjtac8JdmJJUb7qZvqxUbQ-4`cZ%Eb׉E 256 Chapter 8 Ceramics, Glass, Graphite, Diamond, and Nanomaterials 8.17. What is graphene? How is it related to graphite? 8.18. What do the terms “armchair,” “zigzag,” and “chiral” have in common? 8.19. How do platelets improve fracture strength of ceramics? 8.20. What are nanomaterials? Why are they useful? Qualitative Problems 8.21. Explain why ceramics are weaker in tension than in compression. 8.22. What are the advantages of cermets? Suggest applications in addition to those given in this chapter. 8.23. Explain why the electrical and thermal conductivity of ceramics decreases with increasing porosity. 8.24. Explain why the mechanical property data given in Table 8.2 have such a broad range. What is the significance of this in engineering practice? 8.25. Describe the reasons that have encouraged the development of synthetic diamond. 8.26. Explain why the mechanical properties of ceramics generally differ from those of metals. 8.27. Explain how ceramics can be made tougher. 8.28. List and describe situations in which static fatigue can be important. 8.29. What properties are important in making heat-resistant ceramics for use on oven tops? Why? 8.30. A large variety of glasses are now available. Why is this so? 8.31. What is the difference between the structure of graphite and that of diamond? Is it important? Explain. 8.32. List and explain materials that are suitable for use as a coffee cup. 8.33. Aluminum oxide and PSZ are described as white in appearance. Can they be colored? If so, how would you accomplish this? 8.34. Why does the strength of a ceramic part depend on its size? 8.35. In old castles and churches in Europe, the glass windows display pronounced ripples and are thicker at the bottom than at the top. Explain. 8.36. Is a carbide an example of a composite material? Explain your answer. 8.37. Ceramics are hard and strong in both compression and shear. Why, then, are they not used as nails or other fasteners? Explain. 8.38. Perform an Internet search and determine the chemistry of glass used for (a) fiber-optic communication lines, (b) crystal glassware, and (c) high-strength glass fibers. 8.39. Investigate and list the ceramics used for high-temperature superconductor applications. 8.40. Explain why synthetic diamond gemstones are not appreciably less expensive than natural diamond gemstones. 8.41. Explain why ceramic glass is suitable for an electric stove cooktop. Quantitative Problems 8.42. In a fully dense ceramic, Sut,o = 250 MPa and Eo = 300 GPa. What are these properties at 15% porosity for values of n =4,5,6,and7,respectively? 8.43. Plot the Sut, E,and k values for ceramics as a function of porosity P, and describe and explain the trends that you observe in their behavior. ׉ 7cassandra://Dg2RQI3v4C8Hkb-zzmS69j6EVhiUWVkO_-T_0Rb3F4Y`cZ%Eb cZ%Eb#בCט#u׉׉ 7cassandra://Jt_ScZ3FtpvOBrTIiks52whX9YoZaKd6HrbM0ioeJZ4`׉ 7cassandra://koHCi0ztjgr9QKKdS7YAU8YrO8B5Stx_WnScYIuFR8Icu`s׉ 7cassandra://dP0-bRwsLrD27K01w9wE8Xsri9Y3FXVsbu3KIn2cLWA`׉ 7cassandra://gyYeJnCyMGqboeU-KlEcP0d5MgE4Dik1WLI6asW-kVsb͠^cZYEbט#u׉׉ 7cassandra://bP9_h2oS75FJiVHyVAIIjTL5LZxHGJ-cfO806rK8_wA`׉ 7cassandra://l36mutxDPEQg5Td7ERvKX8QRhVIyubvwGt2lK_hAaeA?L`s׉ 7cassandra://7D5yrGH19C03AHvd4XYgCPjZIcFYcMRnYEZh9GWghws`׉ 7cassandra://uBLTuREZLzvKXJNeBuHd3W1jh5KVwC3IlJz2h0UciY0?͠^cZYEb ׉E eSynthesis, Design, and Projects 257 8.44. What would be the tensile strength and the modulus of elasticity of the ceramic in Problem 8.42 for porosities of 20% and 40%, for the four n values given? 8.45. Calculate the thermal conductivities for ceramics at porosities of 10%, 20%, and 40% for ko =0.5 W/mK. 8.46. A ceramic has ko =0.75 W/mK. If this ceramic is shaped into a cylinder with a porosity distribution given by P =0.1(x/L)(1 − x/L) where x is the distance from one end of the cylinder and L is the total cylinder length, plot the porosity as a function of distance, evaluate the average porosity, and calculate the average thermal conductivity. 8.47. It can be shown that the minimum weight of a column which will support a given load depends on the ratio of the material’s stiffness to the square root of its density. Plot this property for a ceramic as a function of porosity. Synthesis, Design, and Projects 8.48. Make a list of the ceramic parts that you can find around your house or in your car. Give reasons why those parts are made of ceramics. 8.49. Assume that you are working in technical sales and are fully familiar with all the advantages and limitations of ceramics. Which of the markets traditionally using nonceramic materials do you think ceramics can penetrate? What would you like to talk about to your potential customers during your sales visits? What questions do you think they may ask you about ceramics? 8.50. Describe applications in which a ceramic material with a near-zero coefficient of thermal expansion would be desirable. 8.51. The modulus of elasticity of ceramics is typically maintained at elevated temperatures. What engineering applications could benefit from this characteristic? 8.52. List and discuss the factors that you would take into account when replacing a metal component with a ceramic component in a specific product. 8.53. Obtain some data from the technical literature in the Bibliography, and quantitatively show the effects of temperature on the strength and the modulus of elasticity of several ceramics. Comment on how the shape of these curves differs from those for metals. 8.54. Conduct a literature search and write a three page paper summarizing the properties and potential applications of graphene. 8.55. It was noted in Section 8.4.1 that there are several basic types of glasses available. Make a survey of the technical literature, and prepare a table for these glasses, indicating various mechanical, physical, and optical properties. 8.56. Ceramic pistons are being considered for high-speed combustion engines. List the benefits and concerns that you would have regarding this application. 8.57. It has been noted that the strength of brittle materials (such as ceramics and glasses) is very sensitive to surface defects, such as scratches (known as notch sensitivity). Obtain several pieces of these materials, scratch them, and test them by carefully clamping them in a vise and bending them. Comment on your observations. 8.58. Electric space heaters for home use commonly utilize a ceramic filament as the heating element. List the required mechanical properties for this filament, explain why a ceramic is a suitable material, and perform an Internet search to determine the specific ceramic material actually utilized in this application. 8.59. Conduct a literature search and write a summary of the uses of graphene in modern batteries. ׉ 7cassandra://dP0-bRwsLrD27K01w9wE8Xsri9Y3FXVsbu3KIn2cLWA`cZ%Eb!׉EChapter 9 Composite Materials: Structure, General Properties, and Applications 9.1 Introduction 259 9.2 The Structure of Reinforced Plastics 259 9.3 Properties of Reinforced Plastics 264 9.4 Applications of Reinforced Plastics 267 9.5 Metal-matrix Composites 269 9.6 Ceramic-matrix Composites 271 9.7 Other Composites 271 Example: 9.1 Calculation of Stiffness of a Composite and Load Supported by Fibers 267 Case Studies: 9.1 Composite Military Helmets and Body Armor 268 9.2 Aluminum-matrix Composite Brake Calipers 270 9.3 Composites in the Aircraft Industry 272 • With their high strength-to-weight and stiffness-to-weight ratios, composite materials are among the most important engineered materials. • Composites are widely used as structural components, especially in the aerospace and automotive industries, where weight savings are a major consideration. • This chapter describes the major types of composite materials, the characteristics of the commonly used reinforcing fibers, and their effect in improving mechanical properties. • The role of the matrix is then described, and the three principal classes of matrix materials (plastic, metal, and ceramic) are examined. • The chapter ends with the selection and applications of a variety of reinforced plastics and composites. 258 ׉ 7cassandra://7D5yrGH19C03AHvd4XYgCPjZIcFYcMRnYEZh9GWghws`cZ%Eb"cZ%Eb!#בCט#u׉׉ 7cassandra://eRjGWstPkg33RN7wvmv65g_6b_ritInjFY7T190pXiU`׉ 7cassandra://SPx1bXYTaTHGdDpRODXDd75jumas6eIRRkyd-TkiyIQT'`s׉ 7cassandra://sL1wmx_uTxU-f_XMq2Pd1chO9_oEIxieNwLEi503Bk4`׉ 7cassandra://P9Z8Y_cML6SQcCgEAgqbKfhm_oc1Kuw1X_9JszXWlAE͗I^͠^cZZEb ט#u׉׉ 7cassandra://-I86sBtdZ1LDmGpT8_5B1ovqfx0qHCriU2aR-Y5a3ks `׉ 7cassandra://mMbvxXVY-a686bEDS4sKol4yC7ZD0YRvXvvjGvyrIzQBT`s׉ 7cassandra://OXmjoKTqugUnRdEzfCElxyiQv5zG6bvJHmuT8WOEPkk`׉ 7cassandra://01I6tIgeF4d_eLETfgAqOLo5b3pZSLFYq1nd7yOgtBQ.͠^cZ^Eb׉E The Structure of Reinforced Plastics 259 9.1 Introduction A composite material is defined as a combination of two or more chemically distinct and insoluble phases with a recognizable interface, in such a manner that its properties and structural performance are superior to those of the constituents acting independently. These combinations are known as polymer-matrix, metalmatrix, and ceramic-matrix composites. As shown in Table 7.1, fiber reinforcements significantly improve the strength, stiffness, and creep resistance of plastics, particularly their strength-to-weight and stiffness-toweight ratios. Composite materials have found increasingly wider applications in aircraft (Fig. 9.1), space vehicles, satellites, offshore structures, piping, electronics, automobiles, boats, and sporting goods. The oldest example of composites, dating back to 4000 B.C., is the addition of straw to clay to make bricks for buildings. In this combination, the straws are the reinforcing fibers and the clay is the matrix. Another example of a composite material is reinforced concrete, developed in the 1800s. By itself, concrete is brittle and has little or no useful tensile strength; reinforcing steel rods (rebar) impart the necessary tensile strength to the concrete. Composites include a wide variety of materials, such as cermets (Section 8.2.3), two-phase alloys (Section 4.2), natural materials such as wood and bone, and reinforced or combined materials such as steelwire reinforced automobile tires or drive belts. This chapter describes the structure, properties, and applications of composite materials; processing and shaping of composite materials are described in Chapter 19. 9.2 The Structure of Reinforced Plastics Reinforced plastics, also known as polymer-matrix composites (PMC) and fiber-reinforced plastics (FRP), consist of fibers (the discontinuous, or dispersed, phase) in a polymer matrix (the continuous phase), as Vertical stabilizer (Carbon sandwich composite) Ailerons (Carbon laminate) Passenger floor panels (Carbon sandwich composite) Overwing panel (Carbon laminate) Fuselage (Carbon laminate) Belly fairing skins (Fiberglass) Spoilers (Carbon laminate) Radome (Fiberglass) Nose landing gear doors (Carbon laminate) Fan cowls and dynamic brace (Carbon laminate) Figure 9.1: Application of advanced composite materials in the Boeing 787-D (Dreamliner), which is 50% composite material by weight. The reinforcement type is shown. Source: Graphic image Courtesy of FlightGlobal. ׉ 7cassandra://sL1wmx_uTxU-f_XMq2Pd1chO9_oEIxieNwLEi503Bk4`cZ%Eb#׉E6260 Chapter 9 Composite Materials: Structure, General Properties, and Applications Laminate Particles (a) Foam Continuous fibers Short or long fibers, or flakes (b) (d) Figure 9.2: Schematic illustration of methods of reinforcing plastics (matrix) with (a) particles, (b) short or long fibers or flakes, and (c) continuous fibers. The laminate structures shown in (d) can be produced from layers of continuous fibers or sandwich structures using a foam or honeycomb core (see also Fig. 16.59). shown in Fig. 9.2. These fibers are strong and stiff (Table 9.2) and have high specific strength (strength-toweight ratio) and specific stiffness (stiffness-to-weight ratio), as shown in Fig. 9.3. In addition, reinforcedplastic structures have improved fatigue resistance, and higher toughness and creep resistance than those made of nonreinforced plastics. The fibers in reinforced plastics have, by themselves, little structural value; they are stiff in their longitudinal direction but have no transverse stiffness or strength. Although the plastic matrix is less strong 40 Thornel P-100 30 Kevlar 29 Kevlar 129 20 S-glass 10 E-glass Titanium 0 0 Steel Aluminum 5 10 Stiffness/density (m 3 106) Figure 9.3: Specific tensile strength (tensile-strength-to-density ratio) and specific stiffness (modulus-ofelasticity-to-density ratio) for various fibers used in reinforced plastics. Note the wide range of specific strength and stiffness. 15 20 High-tensile graphite Kevlar 49 Spectra 900 Celion 3000 Thornel P-55 Boron High-modulus graphite Spectra 2000 (c) Honeycomb Strength/density (m 3 104) ׉ 7cassandra://OXmjoKTqugUnRdEzfCElxyiQv5zG6bvJHmuT8WOEPkk`cZ%Eb$cZ%Eb##בCט#u׉׉ 7cassandra://3VxWyHmbPBGMNaHkyQXmv07aMCkwGVQy5Ehqmm7zsqA`׉ 7cassandra://Ej4f3Ba6nZIKMiDIOq1IjN_71hRfVkmhXrvmvSyXXOg``s׉ 7cassandra://k5OltEDkkjz2m5hwMoWmewAn2DJJrV7yD7Um7stO4Ygl`׉ 7cassandra://r24jA-j_FkZVSJqZf4vMe__qHc3vYeY7GmTJ_SoRV2Ip<͠^cZ^Ebט#u׉׉ 7cassandra://MbGHyi5zJQZ9l-UxA0B-XRKxQjv_pgfN0_iohUIcdRE`׉ 7cassandra://3seoGf7Dbno1zWc6zypfv9msCkwKVCoCv_qKQ6Ux7lQ;`s׉ 7cassandra://FLfbLTLPn8lYA3GoWycXdzoX7xKr4flJDiKuJsFuYa8 `׉ 7cassandra://qxaBx2_62kHs3u5DsOwHvQquWwV7dqsEp13syejqYK0<͠^cZ_Eb׉EThe Structure of Reinforced Plastics 261 and less stiff than the fibers, it is tougher and often more chemically inert; thus, reinforced plastics combine the advantages of each of the two constituents. The percentage of fibers (by volume) in reinforced plastics typically ranges between 10 and 60%. 9.2.1 Reinforcing Fibers Glass, carbon, ceramics, aramids, and boron are the most common reinforcing fibers for polymer-matrix composites (Table 9.1). Glass Fibers. Glass fibers are the most widely used and the least expensive of all fibers. The composite material is called glass-fiber reinforced plastic (GFRP), and may contain between 30 and 60% glass fibers. The fibers are made by drawing molten glass through small openings in a platinum die (Section 18.3.4); the glass is then elongated, cooled, and wound on a roll. The fibers are later treated with silane (a silicon hydride), as described in Section 9.3. The principal types of glass fibers are: • E-type: a calcium aluminoborosilicate glass, the type most commonly used • S-type: a magnesia aluminosilicate glass, offering higher strength and stiffness, but at a higher cost • E-CR-type: a high-performance glass fiber, with higher resistance to elevated temperatures and acid corrosion than does the E glass. Carbon Fibers. Carbon fibers (Fig. 9.4a), although more expensive than glass fibers, have a combination of low density, high strength, and high stiffness; the composite is called carbon-fiber reinforced plastic (CFRP). Although the two words are often used interchangeably, the difference between carbon and graphite depends on the temperature at which it was processed and resulting microstructure. A typical carbon fiber contains amorphous (noncrystalline) carbon and graphite (crystalline carbon). These fibers are classified by their elastic modulus, which ranges from 35 to 800 GPa, as standard, intermediate, high, and very high modulus. Some trade names for carbon fibers are Celion and Thornel (Fig. 9.3). Carbon nanotubes have also been used as reinforcement in composite materials (Section 8.6.2). All carbon fibers are made by pyrolysis of organic precursors. Pyrolysis is the process of inducing chemical changes by heat—for example, by burning a length of yarn, causing the material to carbonize and become black in color. A common precursor is polyacrylonitrile (PAN); rayon and pitch (the residue in Material Fibers Glass Carbon Boron Table 9.1: Types and General Characteristics of Composite Materials. Characteristics High strength, low stiffness, high density; lowest cost; E (calcium aluminoborosilicate) and S (magnesia aluminosilicate) types commonly used Available as high modulus or high strength; low cost; less dense than glass; sometimes used in combination with carbon nanotubes (see Section 8.6.2) High strength and stiffness; highest density; highest cost; has tungsten filament at its center Aramids (Kevlar) Highest strength-to-weight ratio of all fibers; high cost Other fibers Matrix materials Thermosets Nylon, silicon carbide, silicon nitride, aluminum oxide, boron carbide, boron nitride, tantalum carbide, steel, tungsten, molybdenum Epoxy and polyester, with the former most commonly used; others are phenolics, fluorocarbons, polyethersulfone, silicon, and polyimides Thermoplastics Polyetheretherketone; tougher than thermosets, but lower resistance to temperature Metals Ceramics Aluminum, aluminum–lithium, magnesium, and titanium; fibers are carbon, aluminum oxide, silicon carbide, and boron Silicon carbide, silicon nitride, aluminum oxide, and mullite; fibers are various ceramics ׉ 7cassandra://k5OltEDkkjz2m5hwMoWmewAn2DJJrV7yD7Um7stO4Ygl`cZ%Eb%׉EV262 Chapter 9 Composite Materials: Structure, General Properties, and Applications Table 9.2: Typical Properties of Reinforcing Fibers. Tensile Elastic Type Boron Carbon High strength High modulus Glass E-type S-type Kevlar 29 49 129 Nextel 312 610 Spectra 900 1000 2000 3500 4600 2920 3000 3200 1700 2770 2270 2670 3240 Alumina (Al2O3) 1900 Silicon carbide 3500 73 85 70.5 112.4 85 150 328 64 90 115 380 400 2480 2540 1440 1440 1440 2700 3960 970 970 970 3900 3200 Lowest Lowest High High High High High High High High High High Note: These properties vary significantly depending on the material and method of preparation. strength modulus Density (kg/m3) (MPa) 3500 3000 2000 (GPa) 380 275 415 2380 1900 1900 Relative cost Highest Low Low petroleum refining) also are used as precursors.With PAN, the fibers are partially cross-linked at a moderate temperature (in order to prevent melting during subsequent processing steps), and are elongated simultaneously. At this stage, the fibers are carburized, that is, they are exposed to elevated temperatures to expel the hydrogen (dehydrogenation) and the nitrogen (denitrogenation) from the PAN. The temperatures for carburizing range up to about 1500◦C for carbon fibers and up to 3000◦C for graphite fibers (graphitizing). Conductive Graphite Fibers. These fibers are produced to make it possible to enhance the electrical and thermal conductivity of reinforced plastic components. The fibers are coated with a metal (usually nickel) diameter 5 0.1 mm Tungsten Matrix Figure 9.4: Cross section of boron fiber-reinforced composite material. diameter 5 0.012 mm Boron ׉ 7cassandra://FLfbLTLPn8lYA3GoWycXdzoX7xKr4flJDiKuJsFuYa8 `cZ%Eb&cZ%Eb%#בCט#u׉׉ 7cassandra://xd3KenHC6WuykBAbCcn6nqnIFkNn_0S8QN7oIyJBnmIgX`׉ 7cassandra://s9U1G1j_0Ul_Mzmh_uYNUlsur9VIxGSG67db7bLxmQgl`s׉ 7cassandra://IolsgU98EgWQlcuEp0T8PE_T1sNnd1XQgQwkvGLoEwQ`׉ 7cassandra://lKIKM63rIB0JAkwKFbASsIdyY9BDYJiWPmZsb-btNNco͠^cZ_Ebט#u׉׉ 7cassandra://hP8pPa4p0s4QKv-Rm1Y7mmbJWLnhwp3nXdLbF56DYCMfJ`׉ 7cassandra://fTxRNXy0M63XXhrxaP4tltaI0oET19VXYtdQKXjpiQUny`s׉ 7cassandra://v-zlCOmrfK50yxUDsNUkD89FdxqSWewzucE7qAhXzZ0{`׉ 7cassandra://gbnHFYqOZHRjKUskPZSwWdNooJu1VFBOHBC4elJpuLkj͠^cZ`Eb׉EThe Structure of Reinforced Plastics 263 using a continuous electroplating process. The coating is typically 0.5-µmthick on a 7-µm-diameter graphite fiber core. Available in chopped or continuous form, the conductive fibers are incorporated directly into injection-molded plastic parts (Section 19.3). Applications include electromagnetic and radio-frequency shielding and lightning-strike protection. Ceramic Fibers. Ceramic fibers are advantageous for high temperature applications and metal matrix composites (Section 9.5). The fibers have low elongation, low thermal conductivity, and good chemical resistance, in addition to being suitable for high-temperature applications. One family of ceramic fibers is Nextel (a trade name). These fibers are oval in cross section and consist of alumina, silica, and boric oxide. Typical mechanical properties are given in Table 9.2. Polymer Fibers. Polymer fibers can be made of nylon, rayon, acrylics, or aramids; the most common are aramid fibers. Aramids (Section 7.6), such as Kevlar, are among the toughest fibers, with very high specific strength (Fig. 9.3). They can undergo some plastic deformation prior to fracturing, and thus have higher toughness than brittle fibers. However, aramids absorb moisture (hygroscopy), degrading their properties and complicating their application. Another high-performance polyethylene fiber is Spectra (a trade name); it has ultra-high molecular weight and high molecular-chain orientation. Spectra, a bright white polyethylene, has better abrasion resistance and flexural-fatigue strength than aramid fibers, and at a similar cost. In addition, because of its lower density (970 kg/m3), it has a higher specific strength and specific stiffness than aramid (Kevlar) fibers (Table 9.2). However, its low melting point and poor adhesion characteristics as compared to other polymers are major limitations to applications. The manufacture of polymer fibers is described in Section 19.2.2. Boron Fibers. These fibers consist of tungsten fibers with a layer of boron, deposited by chemical vapordeposition techniques (Fig. 9.4b). Boron also can be deposited onto carbon fibers. These fibers have high strength and stiffness, both in tension and in compression, and resistance to high temperatures; however, because of the high density of tungsten, they are heavy and expensive. Miscellaneous Fibers. Among other fibers used in composites are silicon carbide, silicon nitride, aluminum oxide, sapphire, steel, tungsten, molybdenum, boron carbide, boron nitride, and tantalum carbide. Whiskers also are used as reinforcing fibers (see also Section 22.10); they are tiny, needlelike single crystals that grow to 1–10 µm in diameter. They have high aspect ratios (ratio of fiber length to its diameter), ranging from 100 to 15,000. Because of their small size, whiskers are either free of imperfections or the imperfections they contain do not significantly affect their strength, which approaches the theoretical strength of the material. The elastic moduli of whiskers range between 400 and 700 GPa, and their tensile strength is on the order of 15 to 20 GPa, depending on the material. 9.2.2 Fiber Size and Length Fibers are very strong and stiff in tension, because (a) the molecules in the fibers are oriented in the longitudinal direction and (b) their cross section is so small (usually less than 0.01 mm in diameter), that the probability is low for any significant defects to exist in the fiber. Glass fibers can have tensile strengths as high as 4600 MPa, whereas the strength of glass in bulk form (Section 8.4.2) is much lower (see Table 2.2). Fibers are classified as short (discontinuous) or long (continuous). The designations short and long fiber are, in general, based on the following distinction: In a given type of fiber, if the mechanical properties improve as a result of increasing average fiber length, it is called a short fiber. If no such improvement in composite properties occurs, it is called a long fiber. Short fibers typically have aspect ratios between 20 and 60, and long fibers between 200 and 500. ׉ 7cassandra://IolsgU98EgWQlcuEp0T8PE_T1sNnd1XQgQwkvGLoEwQ`cZ%Eb'׉E$264 Chapter 9 Composite Materials: Structure, General Properties, and Applications Reinforcing elements in composites may also be in the shape of chopped fibers, particles, flakes, or as continuous roving (slightly twisted strands) fibers, woven fabric (similar to cloth), yarn (twisted strands), and mats of various combinations. 9.2.3 Matrix Materials The matrix in reinforced plastics has three principal functions: 1. Support the fibers in place and transfer the stresses to them, so that the fibers can carry most of the load (see Example 9.1). 2. Protect the fibers against physical damage, and environmentally caused chemical degradation. 3. Slow the propagation of cracks in the composite by virtue of the higher ductility and toughness of the plastic matrix. Matrix materials can be thermoplastics or thermosets such as epoxy, polyester, phenolic, fluorocarbon, polyethersulfone, or silicon. The most common are epoxies (80% of all reinforced plastics) and polyesters (less expensive than the epoxies). Polyimides, which resist exposure to temperatures in excess of 300◦C, are available for use as a matrix with carbon fibers. Some thermoplastics, such as polyetheretherketone (PEEK), are also used as matrix materials. They generally have higher toughness than thermosets, but their resistance to temperature is lower, being limited to 100◦ to 200◦C. 9.3 Properties of Reinforced Plastics The mechanical and physical properties of reinforced plastics depend on the type, shape, and orientation of the reinforcing fiber, their length, and the volume fraction (percentage) of the reinforcing material. Short fibers are less effective than long fibers (Fig. 9.5), and their properties are strongly influenced by temperature and time under load. Long fibers transmit the load through the matrix better, and are less likely to pull out of the matrix (caused by shear failure of the fiber–matrix interface); thus, they are used in critical applications, particularly at elevated temperatures. The physical properties of reinforced plastics and their resistance to fatigue, creep, and wear depend greatly on the type and amount of reinforcement. Composites can be tailored to impart specific properties, such as permeability and dimensional stability, and make processing easier and reducing production costs. Because the load is transmitted through the fiber–matrix interface, a critical factor in reinforced plastics is the strength of the interfacial bond. Weak bonding can cause fiber pullout and delamination of the composite, particularly under adverse environmental conditions. Adhesion at the interface can be improved by special surface treatments, such as coatings and coupling agents. Glass fibers, for example, are treated with silane for improved wetting and bonding between the fiber and the matrix. The importance of proper bonding can be appreciated by inspecting the fracture surfaces of reinforced plastics; note in Figs. 9.6a and b the separation between the fibers and the matrix. Generally, the highest stiffness and strength in reinforced plastics are achieved when the fibers are aligned in the direction of the tension force; the composite is then highly anisotropic. As a result, properties such as creep resistance, thermal and electrical conductivity, and thermal expansion, also are anisotropic. The transverse properties of a unidirectionally reinforced structure are much lower than their longitudinal properties. For example, note how strong a fiber-reinforced packaging tape is when pulled in tension, yet how easily it can split when pulled in the width direction. Because it is an engineered material, a part made of reinforced plastic can be given an optimal configuration for a specific service condition. If, for example, the part is to be subjected to forces in different directions, such as in thin-walled, pressurized vessels, (a) the fibers can be criss-crossed in the matrix or (b) the layers of fibers oriented in different directions can be built up into a laminate having improved properties in more than one direction (see filament winding and tape laying, Section 19.13). ׉ 7cassandra://v-zlCOmrfK50yxUDsNUkD89FdxqSWewzucE7qAhXzZ0{`cZ%Eb(cZ%Eb'#בCט#u׉׉ 7cassandra://I_y1uZOXEBe5HD32ZwVDuYEfVCUSrmjsVBv_1RIoA0k `׉ 7cassandra://LTPJNrk1exgivu-S4S5zgaQnu_dEw1PIXT-fHpMJ3E83o`s׉ 7cassandra://E000X201ge7whPCJ70fWESj42eZT5pzD7eFq68PLtXc`׉ 7cassandra://DtKlieTksPbGkeFlLEgnAzSIChtcUGP97JFnTJa7pnYs~͠^cZ`Ebט#u׉׉ 7cassandra://v8u1PHqdDWR_jvrLS7QtbB1OJG3cezzH93G9DvLaGxA`׉ 7cassandra://SdXZ4ed9UyrlxWWPOWu1C5RNM7fd3h1-tWo7ravqmEk>`s׉ 7cassandra://__gYey1dz3BtRuhqzwKiWpTPf5Dp57bwyJSDqioZtjw6`׉ 7cassandra://e3xDHhJjCobW2VqikHh-F09NJF_1knlu4oCHLp5LHbEͅ~͠^cZ`Eb ׉EProperties of Reinforced Plastics 265 100 200 300 400 0 0 10 20 30 Reinforcement (%) (a) 30 40 20 10 0 0 10 20 30 Reinforcement (%) (c) 40 200 300 400 100 0 0 10 20 30 40 Reinforcement (%) (d) Figure 9.5: The effect of the type of fiber on various properties of fiber-reinforced nylon (6,6). Source: Courtesy of NASA. 40 Carbon fibers 100 200 300 n 0 0 10 20 30 Reinforcement (%) (b) fibers o 40 (a) (b) Figure 9.6: (a) Fracture surface of a glass fiber-reinforced epoxy composite. The fibers are 10 µm in diameter and have random orientation. (b) Fracture surface of a graphite fiber-reinforced epoxy composite. The fibers, 9 to 11 µm in diameter, are in bundles and are all aligned in the same direction. Source: After L.J. Broutman. s r e Short glass fibers Short glass fibers Long and short glass fibers s b r C a Long glass fibers C a r o L Carbon fibers b o g g n fi Long glas n s s a l b s fibers b fi r e Short glass fibers GPa MPa MPa J/m ׉ 7cassandra://E000X201ge7whPCJ70fWESj42eZT5pzD7eFq68PLtXc`cZ%Eb)׉E2266 Chapter 9 Composite Materials: Structure, General Properties, and Applications Unidirectional 1000 Orthogonal Random 500 0 20 40 60 80 Glass content (% by weight) Figure 9.7: The tensile strength of glass-reinforced polyester as a function of fiber content and fiber direction in the matrix. 9.3.1 Strength and Elastic Modulus of Reinforced Plastics The strength and elastic modulus of a reinforced plastic, with unidirectional fibers, can be determined in terms of the strengths and moduli of the fibers and the matrix, and in terms of the volume fraction of fibers in the composite. In the following equations, c refers to the composite, f to the fiber, and m to the matrix. The total load, Pc, on the composite is shared by the fiber (Pf) and the matrix (Pm). Thus, Pc = Pf +Pm, which can be written as σcAc = σfAf + σmAm, (9.2) where Ac, Af, and Am are the cross-sectional areas of the composite, the fiber, and the matrix, respectively; thus, Ac = Af +Am. Let’s now denote x as the area fraction of the fibers in the composite. (Note that x also represents the volume fraction, because the fibers are uniformly longitudinal in the matrix.) Then Eq. (9.2) can be written as follows: σc = xσf + (1 − x)σm. (9.3) The fraction of the total load carried by the fibers can now be calculated. First, note that in the composite under a tensile load, the strains sustained by the fibers and the matrix are the same; that is, ec = ef = em. Next, recall from Eq. (2.3) that e = Consequently, Pf Pm AfEf = AmEm . (9.4) Since the relevant quantities for a specific situation are known by using Eq. (9.1), the fraction Pf/Pc can be found. Then, using the foregoing relationships, the elastic modulus, Ec, of the composite can be calculated by replacing σ in Eq. (9.3) with E. Thus, Ec = xEf + (1 − x)Em. (9.5) σ E P = AE . (9.1) MPa ׉ 7cassandra://__gYey1dz3BtRuhqzwKiWpTPf5Dp57bwyJSDqioZtjw6`cZ%Eb*cZ%Eb)#בCט#u׉׉ 7cassandra://7zgvsb8OmLHU98Ob2rw49uzeumk9ZL5nJEbB26MfYoc}`׉ 7cassandra://dZmygVXW0HnJ4q40_072sY2Akm91uKupXybNAVSe6gwU`s׉ 7cassandra://X8CrfEzDDpeHkpKb_UvGPeSV3I4hF1wEBnx7_OvAwCs`׉ 7cassandra://HArV_ze6GIiWcfq858RFBQW5DJ4K-jSjaahgbtbTXNEm͠^cZaEb"ט#u׉׉ 7cassandra://aU3iFUu5--3pPFC2L4ahkKLYazeygQz2G1j29Rwg_CU38`׉ 7cassandra://oL3Kf5RwPnVe06oCDnUNskgXkKmjZhSFDP50TtfOeYgVK`s׉ 7cassandra://xhfYMpla01i1ks14_laeQx-gDlE3VfhG_pL9iYVuAYkc`׉ 7cassandra://o4X7CGpvVGyEy3k-3Lf8ZoI1sbFsWtmremZ7Ts6gWZo )|p͠^cZfEb,׉E Applications of Reinforced Plastics 267 Example 9.1 Calculation of Stiffness of a Composite and Load Supported by Fibers Given: Assume that a graphite–epoxy reinforced plastic with longitudinal fibers contains 20% graphite fibers. The elastic modulus of the fibers is 300 GPa, and that of the epoxy matrix is 100 GPa. Find: Calculate the elastic modulus of the composite and the fraction of the load supported by the fibers. Solution: The data given are x = 0.2, Ef = 300 GPa, and Em = 100 GPa. Using Eq. (9.5), Ec = 0.2(300) + (1 − 0.2)100 = 60 + 80 = 140 GPa. From Eq. (9.4), the load fraction Pf/Pm is found to be Pf Pm Because Pc = Pf +Pm it can be seen that, Pc = Pf + Pf 0.75 = 2.33Pf, or Pf = 0.43Pc. Thus, the fibers support 43% of the load, even though they occupy only 20% of the cross-sectional area (and hence volume) of the composite. and Pm = Pf 0.75 , = 0.2(300) 0.8(100) = 0.75. 9.4 Applications of Reinforced Plastics The first engineering application of reinforced plastics was in 1907, for an acid-resistant tank made of a phenolic resin embedded with asbestos fibers. In the 1920s, formica (a trade name) was developed, and was used commonly for countertops. Epoxies were first used as a matrix material in the 1930s. Beginning in the 1940s, boats were made with fiberglass, and reinforced plastics were used for aircraft, electrical equipment, and sporting goods. Major developments in composites began in the 1970s, called advanced composites. Glass or carbon fiber-reinforced hybrid plastics were developed for high-temperature applications, with continuous use ranging up to about 300◦C. Reinforced plastics are typically used in commercial and military aircraft, rocket components, helicopter blades, automobile bodies, leaf springs, driveshafts, pipes, ladders, pressure vessels, sporting goods, helmets, boat hulls, and various other structures and components. About 50% (by weight) of the Boeing 787 Dreamliner is made of composites. By virtue of the resulting weight savings, reinforced plastics have reduced fuel consumption in aircraft by about 2%. The Airbus jumbo jet A380, with a capacity of up to 700 passengers, has horizontal stabilizers, ailerons, wing boxes and leading edges, secondary mounting brackets of the fuselage, and a deck structure made of composites with carbon fibers, thermosetting resins, and thermoplastics. The contoured frame of the Stealth bomber is made of composites, consisting of carbon and glass fibers, epoxy-resin matrices, high-temperature polyimides, and other advanced materials. Boron fiber-reinforced composites are used in military aircraft, golf-club shafts, tennis rackets, fishing rods, and sailboards (Fig. 9.8). Another example is the development of a small, all-composite ship (twin-hull catamaran design) for the U.S. Navy, capable of speeds of 93 kph. ׉ 7cassandra://X8CrfEzDDpeHkpKb_UvGPeSV3I4hF1wEBnx7_OvAwCs`cZ%Eb+׉E 268 Chapter 9 Composite Materials: Structure, General Properties, and Applications Extra tough three-layer multiaxial glass weave Double coating of polyurethane lacquer with nonskid finish Impact and ultraviolet-resistant printed polycarbonate skin Glass fiber reinforcement Laminated composite stringer Embedded honeycomb sheet (paper) Carbon or Kevlar fiber strips Glass reinforcement Ultralight expanded polystyrene foam core within honeycomb Compound layer Section A–A Figure 9.8: Cross section of a composite sailboard, an example of advanced materials construction. Source: K. Easterling, Tomorrow’s Materials, 2nd ed., Institute of Metals, 1990. Case Study 9.1 Composite Military Helmets and Body Armor Personal protective equipment, in the form of body armor and helmets, are widespread for military and police applications. Body armor relies on high-strength woven fibers to prevent the penetration of projectiles. To stop a bullet, a composite material must first plastically deform it or flatten it, a process that occurs when the bullet’s tip comes into contact with as many individual fibers of the composite as possible, without the fibers being pushed aside. The momentum associated with projectiles is felt by the user of the armor, but successful designs will contain bullets and shrapnel, preventing serious and fatal injuries. There are two main types of body armor: (a) soft armor, which relies upon several layers of highstrength, woven fibers, and is designed mainly to contain handgun bullets and (b) hard armor, which utilizes a metal, ceramic, or polymer plate, in addition to the woven fiber; it is intended to provide protection against rifle rounds and shrapnel. A schematic of a body armor is shown in Fig. 9.9. Several types of fiber meshes have been developed to be used in body armor applications. Different suppliers employ different combinations of fiber meshes, and may include additional layers to provide protection against blunt trauma. The first fiber used for flexible body armor was Kevlar 29 (an aramid), which has been improved through the years. Others include Kevlar 49, Kevlar 129, and Kevlar Protera— where tensile strength and energy-absorbing capabilities have been improved through the development of advanced spinning processes to produce the fibers. Aramid fibers are used very commonly in flexible body armor; other designs include over a thousand finely spun filaments that interact with each other to dissipate the impact energy. Glass weave reinforcement ׉ 7cassandra://xhfYMpla01i1ks14_laeQx-gDlE3VfhG_pL9iYVuAYkc`cZ%Eb,cZ%Eb+#בCט#u׉׉ 7cassandra://MVI4WdlKXnTmOkNTzowGh1ChYlU5FHjzOvd5WfYpi7Q`׉ 7cassandra://d88yGSy62E8ncsWmFUivc7qBNrYuZPnE030UcSYuYHQ?Q`s׉ 7cassandra://7Pop6rdnfe1F036VAUDo95BR8VT2l68r9qL_H-g9WB0`׉ 7cassandra://Zqd7--Sbj3cS__OuDdxpmChHsTnfda0wLYL3y3fN8uAd͠^cZgEb.ט#u׉׉ 7cassandra://434Z2T8vv33rgpninRBzFqrjmVXoS76srF3ZHGFLOA4 `׉ 7cassandra://klXTnkD-9GO5vrmd_MC-W-xfyv1eUGjWMtr3jDjV7fUS`s׉ 7cassandra://Z0qf4SIS5ar6MuakIW_qMxgCqmDfuxLIEQkFACK8n7Q4`׉ 7cassandra://jnHbBcDVReSxyz_dP-SC5jXDp06yBtGa_U5Kri7naWgc̊͠^cZgEb/׉E8Metal-matrix Composites 269 Spectra fiber is used to make body armor; a layer of Spectra Shield composite consists of two unidirectional layers of Spectra fiber, arranged to cross each other at 0- and 90-degree angles, and held in place by a flexible resin. Both the fiber and the resin layers are sealed between two thin sheets of polyethylene film. Hard armor uses several designs, but typically it consists of steel, ceramic (usually aluminum oxide and silica), or polyethylene plates that are strategically located to prevent the penetration of ballistic particles to critical areas. Source: Courtesy of Pinnacle Armor, Allied Signal Corp., and CGS Gallet SA. Carrier Plastic film Kevlar® Figure 9.9: Schematic illustration of body armor, showing the layers of woven fibers. 9.5 Metal-matrix Composites Matrix materials in metal-matrix composites (MMC) are usually aluminum, aluminum–lithium alloy (lighter than aluminum), magnesium, copper, titanium, or superalloys (Fig. 9.10). Fiber materials are graphite, aluminum oxide, silicon carbide, boron, molybdenum, or tungsten. The elastic modulus of nonmetallic fibers ranges between 200 and 400 GPa, with tensile strengths in the range from 2000 to 3000 MPa. The advantages Figure 9.10: Brake disc: An example of metal-matrix composite parts. Source: Shutterstock/Andrei Kholmov. ׉ 7cassandra://7Pop6rdnfe1F036VAUDo95BR8VT2l68r9qL_H-g9WB0`cZ%Eb-׉E 270 Chapter 9 Composite Materials: Structure, General Properties, and Applications Fiber Graphite Table 9.3: Metal-matrix Composite Materials and Applications. Applications Matrix Aluminum Magnesium Lead Copper Boron Alumina Silicon carbide Aluminum Magnesium Titanium Aluminum Lead Magnesium Aluminum, titanium Molybdenum, tungsten Superalloy Satellite, missile, and helicopter structures Space and satellite structures Storage-battery plates Electrical contacts and bearings Compressor blades and structural supports Antenna structures Jet-engine fan blades Superconductor restraints in fission power reactors Storage-battery plates Helicopter transmission structures High-temperature structures Superalloy (cobalt base) High-temperature engine components High-temperature engine components of a metal matrix over a polymer matrix are higher elastic modulus, toughness, ductility, and higher resistance to elevated temperatures. The limitations are higher density and a greater difficulty in processing the composite parts. Typical compositions and applications for metal-matrix composites are given in Table 9.3. Case Study 9.2 Aluminum-matrix Composite Brake Calipers A trend in automobile design and manufacture is the increased effort toward lighter weight designs in order to realize improved performance and/or fuel economy; this trend can be seen in the development of metal-matrix composite brake calipers. Traditional brake calipers are made of cast iron, and can weigh around 3 kg each in a small car, and up to 14 kg in a truck. The cast-iron caliper could be redesigned completely, using aluminum to achieve weight savings, but it would require a larger volume since the nominal strength of aluminum is lower than the cast iron, and the space available between the wheel and the rotor is very constrained. Anew brake caliper was designed, using an aluminum alloy locally reinforced with precast composite inserts using continuous ceramic fiber. The fiber is a nanocrystalline alumina, with a diameter of 10 to 12 µm and a fiber volume fraction of 65%. The fiber and the composite properties are summarized in Table 9.4. Finite element analysis confirmed the placement and amount of reinforcement, leading to a design that exceeded minimum design requirements, and also matched deflections of cast-iron calipers in a packaging-constrained environment. The new brake caliper is shown in Fig. 9.11. It has a weight savings of 50%, with the added benefits of corrosion resistance and ease of recyclability. Table 9.4: Summary of Fiber and Composite Properties for an Automotive Brake Caliper. Alumina Alumina-reinforced Property Density fiber Tensile strength 3100 MPa Elastic modulus 380 GPa 3.9 g/cm3 composite material 1.5 GPa 270 GPa 3.48 g/cm3 ׉ 7cassandra://Z0qf4SIS5ar6MuakIW_qMxgCqmDfuxLIEQkFACK8n7Q4`cZ%Eb.cZ%Eb-#בCט#u׉׉ 7cassandra://HIjtZVnTNXlxVe0_n-SQWFWWOpxUVUnoGcl8soyxFKY=d`׉ 7cassandra://aArO3uJbOzUS5KCmgwQw6JRNCxStCpGoQ4U0RQ7k6kYH^`s׉ 7cassandra://_yiZkpTkCDzU4YQN37qI07lmfrA-Iz9FUemjo_RzTzg`׉ 7cassandra://pp7fmJUWL3RE06o9Ty1WUZV52RY773HdVSoElGZGBgkb͠^cZhEb1ט#u׉׉ 7cassandra://iWcOuFKRegg3gI55hFb-OOnEOd3SwNuQDIkRph2I9vs۰`׉ 7cassandra://wc-f7ZlIatI-qHjVakTm9QsVyLa7BFLzlqvAr5blSIcF`s׉ 7cassandra://NWQ6PkSKF2JJk9gPBukFldX4uD0ntB2_EyvwmQNoTXE`׉ 7cassandra://rLRUJFaIbs37eX8xIvHkBVUB2Rhp6Bx9FpaNVL1JG4Qoh͠^cZhEb2׉EOther Composites 271 Figure 9.11: Aluminum-matrix composite brake caliper using nanocrystalline alumina fiber reinforcement. Source: Courtesy of 3M Specialty Materials Division. 9.6 Ceramic-matrix Composites Ceramic-matrix composites (CMC) are characterized by their resistance to high temperatures and corrosive environments. As described in Section 8.3.1, ceramics are strong and stiff; they resist high temperatures, but generally lack toughness. Matrix materials that retain their strength up to 1700◦C are silicon carbide, silicon nitride, aluminum oxide, and mullite (a compound of aluminum, silicon, and oxygen). Carbon/carbonmatrix composites retain much of their strength up to 2500◦C, although they lack oxidation resistance at high temperatures. Fiber materials are usually carbon and aluminum oxide. Applications of CMC include jet and automotive engine components, deep-sea mining equipment, pressure vessels, structural components, cutting tools, and dies for the extrusion and drawing of metals. 9.7 Other Composites Composites also may consist of coatings of various types, applied on base metals or substrates (Chapter 34). Examples are: • Plating of aluminum or other metals over plastics, generally for decorative purposes • Enamels, for wear resistance, hardness, and decorative purposes • Vitreous (glasslike) coatings on metal surfaces for various functional or ornamental purposes. Composites are made into cutting tools and dies, such as cemented carbides and cermets. Other composites are grinding wheels, made of aluminum oxide, silicon carbide, diamond, or cubic-boron-nitride abrasive particles, all held together with various organic, inorganic, or metallic binders (Section 26.2.2). A composite used in machine-tool beds for some precision grinders consists of granite particles in an epoxy matrix; it has high strength, good vibration-damping capacity (better than gray cast iron), and good frictional characteristics. ׉ 7cassandra://_yiZkpTkCDzU4YQN37qI07lmfrA-Iz9FUemjo_RzTzg`cZ%Eb/׉E272 Chapter 9 Composite Materials: Structure, General Properties, and Applications 60 B787 40 A380 20 A320 A310 B767 0 1980 1985 1990 1995 2000 2005 2010 2015 Introduction to market (year) Figure 9.12: Composite content in selected commercial aircraft as a function of time (measured by date first introduced into the market). A = Airbus, B = Boeing. Case Study 9.3 Composites in the Aircraft Industry Any design changes that would lead to increased efficiency and fuel economy continue to be aggressively pursued by aircraft manufacturers. One area where this effect is most dramatic is the increased composite content in commercial aircraft, as shown in Fig. 9.12. In addition to the amount of composite materials used, there are several design innovations in the types and applications of composite materials, including the following: • GLARE is a GLAss-REinforced aluminum consisting of several layers of glass fiber-reinforced polymer and sandwiched between thin sheets of aluminum. It is used on the upper fuselage of the Airbus A380 and the leading edges of the tail plane, and has been credited with over 500 kg of weight savings, as compared to previously used materials. GLARE also provides improved fatigue strength and corrosion resistance. • The Boeing 787 Dreamliner has an all-composite fuselage, constructed mainly from carbon-fiber reinforced plastic. In addition to weight savings, the fuselage is constructed in one piece and joined end to end, eliminating the need for an estimated 50,000 fasteners. Composites make up around 50% of the weight of the Dreamliner, as compared to 12% on the 777 aircraft, first introduced in 1994. A340-500/600 A340 B777 A350 Summary • Composites are an important class of engineered materials, with numerous attractive properties. Three major categories are fiber-reinforced plastics, metal-matrix composites, and ceramic-matrix composites. They have a wide range of applications in the aircraft, aerospace, and transportation industries, sporting goods, and structural components. Composite content by weight (%) ׉ 7cassandra://NWQ6PkSKF2JJk9gPBukFldX4uD0ntB2_EyvwmQNoTXE`cZ%Eb0cZ%Eb/#בCט#u׉׉ 7cassandra://A3J0rPThU4rZgnuDRwHHGFysUefqvMCHuroXxf6VSKg`׉ 7cassandra://PrqW9eYlaDSPrEG8t4KX0tqJWCSeOnn4JOj2HDADRyMV`s׉ 7cassandra://qQt7l4_6hOoa4xjEUksnPDLJY13-0GQa3XWRpVHZIJk`׉ 7cassandra://xbwfNzll2ar58_-ll39Sq0ZF4GUozuj4ygXuwUx8tjUd͠^cZhEb4ט#u׉׉ 7cassandra://1s37RjsJPlKc3FE1IM_7mRhwYPn2mC7uHKt_NJuqLEE`׉ 7cassandra://rSBqUlsPfIrFs9a7bdyz1381qgcMQv33qCZCI13QITMj`s׉ 7cassandra://vhN4OAXsKh5uvLrSOQP4-41UxW6c2QShUoE3NdD-kDM`׉ 7cassandra://4suoVwPlEBFJmQJDaCLcMzTEoAIyMiRmV2A6ucM1TR4[z͠^cZhEb5׉E wBibliography 273 • In fiber-reinforced plastics, the fibers are usually glass, graphite, aramids, or boron. Polyester and epoxies commonly are used as the matrix material. These composites have particularly high toughness and high strength-to-weight and stiffness-to-weight ratios. • In metal-matrix composites, the fibers are graphite, boron, aluminum oxide, silicon carbide, molybdenum, or tungsten. Matrix materials generally consist of aluminum, aluminum–lithium alloy, magnesium, copper, titanium, or superalloys. • In ceramic-matrix composites, the fibers are usually carbon and aluminum oxide, and the matrix materials are silicon carbide, silicon nitride, aluminum oxide, carbon, or mullite (a compound of aluminum, silicon, and oxygen). • In addition to the type and quality of the materials used, important factors in the structure and properties of composite materials are the size and length of the fibers, their volume percentage compared with that of the matrix, the strength of the bond at the fiber–matrix interface, and the orientation of the fibers in the matrix. Key Terms Advanced composites Ceramic matrix Composite materials Delamination Engineered materials Fiber pullout Fibers Hybrid Bibliography Agarwal, B.D., Broutman, L.J., and Chandrashekhara, K., Analysis and Performance of Fiber Composites, 3rd ed., Wiley, 2006. Antoniac, I.V. (ed.), Handbook of Bioceramics and Biocomposites, Springer, 2016. ASM Handbook, Vol. 21: Composites, ASM International, 2001. Bansal, N.P., and Lamon, G., Ceramic Matrix Composites, Wiley, 2014. Campbell, F.C., Structural Composite Materials, ASM International, 2010. Cantor, B., Dunne, F.P.E., and Stone, I.C. (eds.), Metal and Ceramic Matrix Composites, CRC Press, 2003. Chawla, N., and Chawla, K.K., Composite Materials: Science and Engineering, 2nd ed., Springer, 2013. —Metal Matrix Composites, Springer, 2006. Chung, D.D.L., Composite Materials: Science and Applications, Butterworth-Heinemann, 2nd ed., 2016. Daniel, I.M., and Ishai, O., Engineering Mechanics of Composite Materials, 2nd ed., Oxford, 2005. Davim, J.P., and Jarzabek, D.M., Micromechanics of Metal Matrix Composites, Springer, 2016. Dvorak, G., Micromechanics of Composite Materials, Springer, 2013. Gay, D., Composite Materials: Design and Applications, 3rd ed., CRC Press, 2014. Gibson, R.F., Principles of Composite Material Mechanics, 4th ed., CRC Press, 2016. Krenker, W. (ed.), Ceramic Matrix Composites: Fiber Reinforced Materials and Their Applications, Wiley, 2008. Loos, M., Carbon Nanotube Reinforced Composites, Elsevier, 2013. Matrix Metal matrix Polymer matrix Precursor Pyrolysis Reinforced plastics Silane Whiskers ׉ 7cassandra://qQt7l4_6hOoa4xjEUksnPDLJY13-0GQa3XWRpVHZIJk`cZ%Eb1׉E 274 Chapter 9 Composite Materials: Structure, General Properties, and Applications Low, I.M. (ed.), Advances in Ceramic Matrix Composites, Woodhead Publishing, 2013. Nishida, Y., Introduction to Metal Matrix Composites, Springer, 2013. Sapuan, S.M., Composite Materials: Concurrent Engineering Approach, Butterworth Heinemann, 2017. Shimpi, N.G., Biodegradable and Biocompatible Polymer Composites, Woodhead Publishing, 2017. Strong, A.B., Fundamentals of Composites Manufacturing: Materials, Methods and Applications, 2nd ed., Society of Manufacturing Engineers, 2007. Zweben, C.H., and Beaumont, P. (eds.), Comprehensive Composite Materials II, Elsevier, 2017. Review Questions 9.1. Distinguish between composites and metal alloys. 9.2. What is a fiber? What is a matrix? 9.3. Describe the functions of the matrix and the reinforcing fibers. What fundamental differences are there in the characteristics of the two materials? 9.4. Name the reinforcing fibers generally used to make composites. Which type of fiber is the strongest? Which type is the weakest? 9.5. What is the range in length and diameter of typical reinforcing fibers? 9.6. List the important factors that determine the properties of reinforced plastics. 9.7. Comment on the advantages and limitations of metal-matrix composites, reinforced plastics, and ceramic-matrix composites. 9.8. What are the most commonly used matrix materials? Why? 9.9. Describe the advantages of hybrid composites over other composites. 9.10. What material properties are improved by the addition of reinforcing fibers? 9.11. Describe the purpose of the matrix material. 9.12. What are the most common types of glass fibers? 9.13. Explain the difference between a carbon fiber and a graphite fiber. 9.14. How can a graphite fiber be made electrically and thermally conductive? 9.15. What is a whisker? What is the difference between a whisker and a fiber? 9.16. Explain the composition of boron fibers. Why are they heavy? 9.17. Give a succinct definition of fiber, yarn, and fabric. Qualitative Problems 9.18. How do you think the use of straw mixed with clay originally came about in making brick for dwellings? 9.19. What products have you personally seen that are made of reinforced plastics? How can you tell? 9.20. Describe applications that are not well suited for composite materials. 9.21. Is there a difference between a composite material and a coated material? Explain. 9.22. Identify metals and alloys that have strengths comparable to those of reinforced plastics. 9.23. What limitations or disadvantages do composite materials have? What suggestions would you make to overcome the limitations? 9.24. Give examples of composite materials other than those stated in this chapter. ׉ 7cassandra://vhN4OAXsKh5uvLrSOQP4-41UxW6c2QShUoE3NdD-kDM`cZ%Eb2cZ%Eb1#בCט#u׉׉ 7cassandra://BP4zZ4LcqdiX0CxteR5Mm0Q77YFqFACAlZn73xTnsww`׉ 7cassandra://YeUzjUUsb2mycMu5P9Iw0O7qArHqDRIhy9qK1g3M_VQd`s׉ 7cassandra://epKapT_8FtLUtbVySX2-6AYX4Yh4c2u2u0vLsbs58h4`׉ 7cassandra://kW3MqgvDughVrwEijSWK3LZEqQigkKiiwMExwCVC8JIa͠^cZiEb7ט#u׉׉ 7cassandra://uTmGILHvUZ1G_uSpiXHx-SbTjLoUArmzTedSTjFt39k}`׉ 7cassandra://tKu6ixBNwKpr5SPABhKXzT1-qZO_keEbWR0R2oenbhcj`s׉ 7cassandra://zduEcv3jZIJYFxOzAv1-eEIkM9aCAA7I7bH-CnlSYIs`׉ 7cassandra://Tr1URynmN03M-ik3hSqKVNT9doxL5hZgDsKhH42_zlU_h͠^cZiEb8׉E uQuantitative Problems 275 9.25. Explain why the behavior of the materials depicted in Fig. 9.5 is as shown. 9.26. Explain why fibers are so capable of supporting a major portion of the load in composite materials. 9.27. Do metal-matrix composites have any advantages over reinforced plastics? Explain. 9.28. Give reasons for the development of ceramic-matrix composites. Name some applications and explain why they should be effective. 9.29. Explain how you would go about determining the hardness of reinforced plastics and of composite materials. Are hardness measurements on these types of materials meaningful? Does the size of the indentation make any difference? Explain. 9.30. How would you go about trying to determine the strength of a fiber? 9.31. Glass fibers are said to be much stronger than bulk glass. Why is this so? 9.32. Describe situations in which a glass could be used as a matrix material. 9.33. When the American Plains states were settled, no trees existed for the construction of housing. Pioneers cut bricks from sod—basically, prairie soil as a matrix and grass and its root system as reinforcement. Explain why this approach was successful. Also, if you were a pioneer, would you stack the bricks with the grass horizontally or vertically? Explain. 9.34. By incorporating small amounts of a blowing agent, it is possible to manufacture hollow polymer fibers with gas cores. List possible applications for such fibers. 9.35. Referring to Fig. 9.2c, would there be an advantage in using layers of cloth (woven fibers) instead of continuous fiber stacks without weaving? Explain. 9.36. Is it possible to design a composite material that has a Poisson’s ratio of zero in a desired direction? Explain. Can a composite material be designed that has a thermal conductivity of zero in a desired direction? Explain. Quantitative Problems 9.37. Calculate the average increase in the properties of the plastics given in Table 7.1 as a result of their reinforcement, and describe your observations. 9.38. In Example 9.1, what would be the percentage of the load supported by the fibers if their strength were 1000 MPa and the matrix strength were 250 MPa? What would be the answer if the fiber stiffness were doubled and the matrix stiffness were halved? 9.39. Calculate the percent increase in the mechanical properties of reinforced nylon from the data shown in Fig. 9.5. 9.40. Calculate the elastic modulus and load supported by fibers in a composite with an epoxy matrix (E = 100 GPa), made up of 30% fibers made of (a) high-modulus carbon fiber and (b) Kevlar 29 fibers. 9.41. For a composite material consisting of high modulus carbon fibers (E = 415 GPa) and an epoxy matrix (E = 100 GPa), determine the volume fraction of fibers needed to produce a composite material with a stiffness equal to that of steel. 9.42. Plot E/ρ and E/ρ0.5 for the composite materials listed in Table 9.2, and compare your results with the properties of the materials described in Chapters 4 through 8 (see also Table 9.1). 9.43. Calculate the stress in the fibers and in the matrix in Example 9.1. Assume that the cross-sectional area is 300 m2 and Pc = 2250 N. 9.44. Repeat the calculations in Example 9.1 (a) if Nextel 610 fiber is used and (b) if Spectra 2000 is used. 9.45. Refer to the properties listed in Table 7.1. If acetal is reinforced with E-type glass fibers, what is the range of fiber content in glass-reinforced acetal? ׉ 7cassandra://epKapT_8FtLUtbVySX2-6AYX4Yh4c2u2u0vLsbs58h4`cZ%Eb3׉E276 Chapter 9 Composite Materials: Structure, General Properties, and Applications 9.46. Plot the elastic modulus and strength of an aluminum metal-matrix composite with high-modulus carbon fibers as a function of fiber content. 9.47. For the data in Example 9.1, what should be the fiber content so that the fibers and the matrix fail simultaneously? Use an allowable fiber stress of 250 MPa and a matrix strength of 50 MPa. 9.48. It is desired to obtain a composite material with a target stiffness of 5 GPa. If a high strength carbon fiber is to be used, determine the required fiber volume if the matrix is (a) nylon, (b) polyester, (c) acetal, and (d) polyethylene. 9.49. A rectangular cantilever beam 150 mm high, 30 mm wide, and 1.5 m long is subjected to a concentrated load of 60 kg at its end. (a) Consider a polymer reinforced by high modulus carbon fibers, with a fiber volume ratio of x = 15%. What is the maximum deflection of the beam if the matrix material is polyester? (b) Obtain the deflection of the beam if aluminum or steel was used, using the same beam dimensions. (c) What fiber volume ratio is needed to produce the same deflection as the aluminum or steel beams? (d) Determine the weight of the beams considered in parts (b) and (c) and compare them. 9.50. In Example 9.1, assume the strength of the fibers is 2800 MPa, and the load to be supported is 1.5 kN. (a) If the length of the tension member is 1.5 m, what is the weight of the composite? 9.51. In Example 9.1, graphite fibers are used to reinforce the composite material. If the fibers were produced from a different form of carbon, namely diamond, what percentage of load would be carried by the fibers? What volume percentage of fibers would be needed to maintain 43% of the load support by the fibers? 9.52. Consider a composite consisting of reinforcing fibers (Ef = 300 GPa) in an epoxy matrix (E = 100 GPa). If the allowable fiber stress is 225 MPa and the matrix strength is 90 MPa, what should be the fiber content so that the fibers and matrix fail simultaneously? Synthesis, Design, and Projects 9.53. What applications for composite materials can you think of other than those given in Section 9.4? Why do you think your applications would be suitable for these materials? 9.54. Using the information given in this chapter, develop special designs and shapes for possible new applications of composite materials. 9.55. Would a composite material with a strong and stiff matrix and a soft and flexible reinforcement have any practical uses? Explain. 9.56. Make a list of products for which the use of composite materials could be advantageous because of their anisotropic properties. 9.57. Inspect Fig. 9.1 and explain what other components of an aircraft, including the cabin, could be made of composites. 9.58. Name applications in which both specific strength and specific stiffness are important. 9.59. What applications for composite materials can you think of in which high thermal conductivity would be desirable? Explain. 9.60. As with other materials, the mechanical properties of composites are obtained by preparing appropriate specimens and then testing them. Explain what problems you might encounter in preparing specimens for testing in tension. Suggest methods for making appropriate specimens, including fashioning their shape and how they would be clamped into the jaws of testing machines. 9.61. Developments are taking place in techniques for three-dimensional reinforcement of composites. Describe (a) applications in which strength in the thickness direction of the composite is important and (b) your ideas on how to achieve this strength. Include simple sketches of the structure utilizing such reinforced plastics. ׉ 7cassandra://zduEcv3jZIJYFxOzAv1-eEIkM9aCAA7I7bH-CnlSYIs`cZ%Eb4cZ%Eb3#בCט#u׉׉ 7cassandra://4fYjSJra1PEvq7qfRZzopaf9FVV3Pl76mimcUXaF5lww}`׉ 7cassandra://0pqG8xKUjbz1Z5kTaKBBuhXHemXR_TQpevZIAIuJtlAZ`s׉ 7cassandra://oZcTHer7AwE3ZpI1YFeGq25qYmMVQyyi4UeZN1HOiHQ`׉ 7cassandra://zj9n2sQ4wh0yLmVNWKwzjKn7r_h0hC2WK8vXWvz8CDI ͠^cZoEbCט#u׉׉ 7cassandra://hXYgR1OpE7CSa1oQzjhbJBzm86sZZJzHnxHUFxaHW5E2`׉ 7cassandra://5W_YCYlhommakFewFzM4ZXa2wR33QbI-qf9prkhPrCI@`s׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`׉ 7cassandra://oeScU-BIOmAxXjZSg4RmHaIgcdwg2TLKD5ihmYRRa_g͠^cZoEbD׉E 6Synthesis, Design, and Projects 277 9.62. Design and describe a test method to determine the mechanical properties of reinforced plastics in their thickness direction. (Note, for example, that plywood is not particularly strong in its thickness direction.) 9.63. As described in this chapter, reinforced plastics can be adversely affected by the environment— in particular, moisture, chemicals, and temperature variations. Design and describe test methods to determine the mechanical properties of composite materials subjected to these environmental conditions. 9.64. Comment on your observations on the design of the sailboard illustrated in Fig. 9.8. 9.65. Make a survey of various sports equipment and identify the components made of composite materials. Explain the reasons for and the advantages of using composites in these specific applications. 9.66. Several material combinations and structures were described in this chapter. In relative terms, identify those that would be suitable for applications involving one of the following: (a) very low temperatures, (b) very high temperatures, (c) vibrations, and (d) high humidity. 9.67. Obtain a textbook on composite materials and investigate the effective stiffness of a continuous fiberreinforced polymer. Plot the stiffness of such a composite as a function of orientation with respect to the fiber direction. 9.68. Derive a general expression for the coefficient of thermal expansion for a continuous fiber-reinforced composite in the fiber direction. 9.69. It is possible to make fibers or whiskers with a varying cross section, or a “wavy” fiber. What advantages would such fibers have? 9.70. Describe how you can produce some simple composite materials using raw materials available around your home. Explain. 9.71. Gel spinning is a specialized process used in making fibers with high strength or special properties. Search the technical literature and write a brief paper on this subject. 9.72. The sketch shows a section of a three-dimensional weave that uses a binder yarn to tie layers of fibers together. Conduct a literature search and determine the advantages and disadvantages of using three-dimensional weaves as reinforcements in composite materials. Binder yarn 9.73. Review the functions of the matrix material in Section 9.2.3. Could diamond be a useful matrix material in a fiber-reinforced composite material? Explain. 9.74. Glass fibers are used as reinforcements in composite materials. Is there any benefit in using the glass fibers to transmit light, as in a fiber optic cable? List potential applications for this feature. ׉ 7cassandra://oZcTHer7AwE3ZpI1YFeGq25qYmMVQyyi4UeZN1HOiHQ`cZ%Eb5׉E&This page is intentionally left blank ׉ 7cassandra://LIy0nnbOoLaT3ZD3CzI7r7IehMbDJrImCAdbBMxkvsk`cZ%Eb6cZ%Eb5#בCט#u׉׉ 7cassandra://axA4OuekfItjdDyIpMdEvAZxMNucGmXFY7qejmXx3Q88`׉ 7cassandra://-P30UOnYuc1NofzVXNp_FToIdNIWP6_216vD4Cg0hvMI<`s׉ 7cassandra://Z2v5VoqkL-tdX1w2H8P6o7QsWCmropjUzvdgqw-QsAU-`׉ 7cassandra://Z3bhiTgnpYWKum7nK4V04JugST79DbvHdG89_RLctYA_͠^cZoEbHט#u׉׉ 7cassandra://ukO8ZAsZftH_K9hK_vRbM4XY1srPDpGnHpOBZ9C22EU`׉ 7cassandra://V2Pc55PIGWN4n-V7xNV6oiV1GFHCozn6w9ISYle9T6Y@?`s׉ 7cassandra://6wa9nIHhOJOKrvmma3Ipv9ycIE0OKFXNGaavXE3GX7I`׉ 7cassandra://EIP6WtZJsZaLXOB41CxHwWEhX7vgkJVVC4k1qZ5FimI͠^cZoEbI׉EPART II Metal-casting Processes and Equipment Several methods are available to shape metals into products. One of the oldest processes is casting, which basically involves pouring molten metal into a mold cavity; upon solidification, the metal takes the shape of the cavity. Two examples of cast parts are shown in Fig. II.1. Casting was first used around 4000 B.C. to make ornaments, arrowheads, and various other simple objects. The process is now capable of producing intricate shapes, in one piece, and including those with internal cavities, such as engine blocks. Figure II.2 shows cast components in a typical automobile, a product that was used in the introduction to Part I to illustrate the selection and use of a variety of materials. The casting processes developed over the years are shown in Fig. II.3. As in all manufacturing operations, each casting process has its own characteristics, applications, advantages, limitations, and costs involved. Casting is most often selected over other manufacturing methods for the following reasons: • Casting can produce complex shapes and can incorporate internal cavities or hollow parts. • Very large parts can be produced in one piece. • Casting can utilize materials that are difficult or uneconomical to process by other methods, such as hard metals that are difficult to machine or plastically deform. • The casting process is less expensive than other manufacturing processes for the particular application being considered. (a) (b) Figure II.1: (a) Examples of cast parts. (b) A tree of rings produced through investment casting. Source: (a) Shutterstock/Mr.1 (b) Courtesy of Romanoff, Inc. 279 ׉ 7cassandra://Z2v5VoqkL-tdX1w2H8P6o7QsWCmropjUzvdgqw-QsAU-`cZ%Eb7׉E280 Part II Metal-casting Processes and Equipment Alternator housing Grill Water pump, pulleys Intake and exhaust manifolds Wheels, disk brakes Transmission housing Door handles Locks Brake rotors Figure II.2: Cast parts in a typical automobile. Metal-casting processes Expendable mold Composite mold Sand Shell Evaporative pattern Plaster Ceramic Investment Slush Pressure Die Centrifugal Squeeze Semisolid Figure II.3: Outline of metal-casting processes described in Part II. Almost all metals can be cast in the final shape desired, or nearly so, often requiring only minor finishing operations. This capability places casting among the most important net-shape manufacturing technologies, along with net-shape forging (Chapter 14), stamping of sheet metal (Chapter 16), and powder metallurgy and metal-injection molding (Chapter 17). With modern processing techniques and control of chemical composition, mechanical properties of castings can equal those made by other manufacturing processes. Single crystals for microelectronics Single-crystal turbine blades Directional solidification Permanent mold Differential housing Engine block, piston Brake cylinders Foundries Single-crystal growing ׉ 7cassandra://6wa9nIHhOJOKrvmma3Ipv9ycIE0OKFXNGaavXE3GX7I`cZ%Eb8cZ%Eb7#בCט#u׉׉ 7cassandra://e5opouccl3eI4yZvc6kKzmNDmKEvOVCZ8lK50jAeJdY`׉ 7cassandra://T3vYFpEaXtvNaYbyOpaM5Rme0TOYbdOYn5P2W2TdlpU2/`s׉ 7cassandra://tWVQ_JA9LHWYlud8hR6uqbFzyhyHKsii1vMr47D3uac`׉ 7cassandra://noUEZdqyHtQVVwcU9S816IJbMpzqXiNqxpe1_ieE_jc;=͠^cZoEbKט#u׉׉ 7cassandra://x3F-L-eU6LpK0p4ODtIRrkx6PBVTJ7oR5KI1dpTF7_o; `׉ 7cassandra://WSIc3m0a7M8XRzt-6ZF-0PiROLJFoLBnemlCUawhXhkj!`s׉ 7cassandra://gmSE81ldxn_GmpJxB5dZSATtPNyBxFuJ-MhcQ0Pm87g>`׉ 7cassandra://S2VKvVufhh-zT9VRdsdTdL8cI10eyykacPa8lGpUixs\͠^cZoEbL׉E7Chapter 10 Fundamentals of Metal Casting 10.1 Introduction 282 10.2 Solidification of Metals 282 10.3 Fluid Flow 287 10.4 Fluidity of Molten Metal 291 10.5 Heat Transfer 292 10.6 Defects 295 Example: 10.1 Solidification Times for Various Shapes 294 • First used about 6000 years ago, casting continues to be an important manufacturing process for producing very small, very large, and complex parts. • The first topic described is solidification of molten metals, including the differences between solidification of pure metals and alloys. • Fluid flow in casting is then described, with Bernoulli’s and the continuity equations being applied to establish a framework for analyzing molten metal flow into the cavities of a mold. • The importance of turbulent versus laminar flow is introduced. • Heat transfer and shrinkage of castings are also described, including Chvorinov’s rule for solidification time. • The chapter ends with a description of the causes of porosity in castings and common methods of reducing them to improve the properties of castings. 281 ׉ 7cassandra://tWVQ_JA9LHWYlud8hR6uqbFzyhyHKsii1vMr47D3uac`cZ%Eb9׉EF282 Chapter 10 Fundamentals of Metal Casting 10.1 Introduction The casting process basically involves (a) pouring molten metal into a mold containing a cavity that produces the desired part shape, (b) allowing it to solidify, and (c) removing the part from the mold. As with all other manufacturing processes, an understanding of the underlying science is essential for producing high quality, economical castings, and for establishing proper techniques for mold design and casting practice. Important considerations in casting operations are: • Flow of the molten metal into the mold cavity, and design of gating systems or pathways for molten metal to fill the cavity • Solidification and cooling of the metal in the mold • Influence of the mold material. This chapter describes relationships among various relevant factors involved in casting. The flow of molten metal into the mold cavity is first described in terms of mold design and fluid-flow characteristics. Solidification and cooling of metals in the mold are affected by several factors, including the metallurgical and thermal properties of the metal and the type of mold because it affects the rate of cooling. The chapter ends with a description of the factors influencing defect formation in castings. Metal-casting processes, design considerations, and casting materials are described in Chapters 11 and 12. The casting of ceramics and plastics, which involve methods and procedures somewhat similar to those for metal, are described in Chapters 18 and 19, respectively. 10.2 Solidification of Metals After molten metal is poured into a mold, a sequence of events takes place during solidification and cooling of the metal to ambient temperature. These events greatly influence the size, shape, uniformity, and chemical composition of the grains formed throughout the casting, which, in turn, influence the overall properties of the casting. The significant factors affecting these events are the type of metal cast, the thermal properties of both the metal and the mold, the geometric relationship between volume and surface area of the casting, and the shape of the mold. 10.2.1 Pure Metals Because pure metal has a clearly defined melting, or freezing, point, it solidifies at a constant temperature, as shown in Fig. 10.1. Pure aluminum, for example, solidifies at 660◦C, iron at 1537◦C, and tungsten at 3410◦C (see also Table 3.1). After the molten metal temperature drops to its freezing point, its temperature remains constant while the latent heat offusion is given off. The solidification front (the solid–liquid interface) moves through the molten metal from the mold walls in toward the center. The solidified metal, now called the casting, is then removed from the mold and allowed to cool to ambient temperature. As shown in Fig. 10.1b and described in greater detail in Section 10.5.2, metals generally shrink when they solidify (Table 10.1) and shrink further while cooling. This behavior is an important consideration, because shrinkage can lead to microcracking and associated porosity, which can adversely affect the mechanical properties of the casting. As an example of the grain structure that develops in a casting, Fig. 10.2a shows a cross section of a boxshaped mold. At the mold walls, which are at ambient temperature at first or typically are much cooler than the molten metal, the metal cools rapidly, producing a solidified skin,or shell, of fine equiaxed grains. The grains generally grow in a direction opposite to that of the heat transfer out through the mold. Those grains that have favorable orientation grow preferentially, and are called columnar grains (Fig. 10.3). Those grains that have substantially different orientations are blocked from further growing. As the driving force of the heat transfer decreases away from the mold walls, the grains become equiaxed and coarse. This sequence ׉ 7cassandra://gmSE81ldxn_GmpJxB5dZSATtPNyBxFuJ-MhcQ0Pm87g>`cZ%Eb:cZ%Eb9#בCט#u׉׉ 7cassandra://dfFw9tl8OyaR6hSgb2YQNP-szyl3a33cQOoiBZIQZ6AX)`׉ 7cassandra://oISiWFfl3V8Ulc2ptKXQAT6uXyvjA6s8sToWIWFiAy8:`s׉ 7cassandra://HxdlOpWk8mH4SjnTp-iJFRXL_VxvExDjrYHI9g7BofI `׉ 7cassandra://49t8bYghlFf4Imecwzy3rxjn_-YT4gKbJ_fSCsJF_uAkK ͠^cZpEbNט#u׉׉ 7cassandra://4pXxTWpv1qckVZ8-PSAsWBQEWq7OGyziELA5gxejQmAH`׉ 7cassandra://0OK9HTzRpr-X5rxL7uAKyFTFZeudMDE2biabUV110TgIX`s׉ 7cassandra://LjoXbFkST6A0_1qDgX2PZqZcOpyxRqwOSLrPO-L8ShY2`׉ 7cassandra://QbZinj7bH63VyPhedMw_FujMfK9tI_Abrlw4AjUHB0E1"6͠^cZpEbO׉ESolidification of Metals 283 Cooling of liquid Shrinkage of solid Freezing begins Freezing ends A Freezing temperature Cooling of solid Liquid Liquid 1 solid Time (a) Time (b) Figure 10.1: (a) Temperature as a function of time for the solidification of pure metals; note that freezing takes place at a constant temperature. (b) Density as a function of time. Chill zone Columnar zone Equiaxed zone Equiaxed structure Solid Shrinkage of liquid B Solidification shrinkage (a) (b) (c) Figure 10.2: Schematic illustration of three cast structures of metals solidified in a square mold: (a) pure metals; (b) solid–solution alloys; and (c) structure obtained by using nucleating agents. Source: After G.W. Form, J.F. Wallace, J.L. Walker, and A. Cibula. Temperature Density ׉ 7cassandra://HxdlOpWk8mH4SjnTp-iJFRXL_VxvExDjrYHI9g7BofI `cZ%Eb;׉E284 Chapter 10 Fundamentals of Metal Casting Columnar zone Chill zone Mold Figure 10.3: Development of a preferred texture at a cool mold wall; note that only favorably oriented grains grow away from the surface of the mold. of grain development is known as homogenous nucleation, meaning that the grains (crystals) grow upon themselves, starting at the mold wall. 10.2.2 Alloys Solidification in alloys begins when the temperature drops below the liquidus, TL, and is complete when it reaches the solidus, TS (Fig. 10.4). Within this temperature range, the alloy is in a mushy or pasty state, consisting of columnar dendrites (from the Greek dendron, meaning akin to, and drys, meaning tree). Note in the figure that the spaces between the dendrite arms are taken up by the liquid metal. Dendrites have threedimensional arms and branches (secondary arms), which eventually interlock, as can be seen in Fig. 10.5. The study of dendritic structures, although complex, is important, because such structures can contribute to various detrimental factors, including compositional variations, segregation, and microporosity within a cast part. The width of the mushy zone, where both liquid and solid phases are present, is an important factor during solidification. This zone is described in terms of a temperature difference, known as the freezing range,as Freezing range = TL −TS. Liquid L S Solid S L Solid Alloying element (%) Pure metal Mold wall Mushy zone Solid Liquid Dendrites Figure 10.4: Schematic illustration of alloy solidification and temperature distribution in the solidifying metal. Note the formation of dendrites in the mushy zone. Liquid TS TL (10.1) Temperature uLiq i d su S olidus ׉ 7cassandra://LjoXbFkST6A0_1qDgX2PZqZcOpyxRqwOSLrPO-L8ShY2`cZ%Eb<cZ%Eb;#בCט#u׉׉ 7cassandra://McpEcjEPtodHzBZxXJd1uPpp-XvJvvSwlaST3pBc9l0`׉ 7cassandra://Zkc2U3pBW3AxdKCPnQz-2i3mkPRr9uEIEP5ixCbNvd4]`s׉ 7cassandra://NkComQmSmr4kYx_gOUkZaBMg2610MimzctnoIj7x_WY`׉ 7cassandra://vxtqgTfEPEcX3-pRYyLCkTuKgthaTaZw7K6qEWK3Ats̈́}D͠^cZqEbQט#u׉׉ 7cassandra://stHpp1NNWbaloQPt0cZ7C96k2dHqp_v_fk4LcjTPugMJ`׉ 7cassandra://XXc4IATFGI0XB8oc2bTz3BeBZxvOBzWBTlYngpyZ3eUW3`s׉ 7cassandra://7A8mQImjKHY_c2VL8Iqw5yp5ei4-qC_xS_iw0rkQZy4`׉ 7cassandra://wLJuGk94J6-EVabhKlk1ZbOinikMMFhOIDwCPJSVs9MXv͠^cZsEbR׉E Solidification of Metals 285 8 11 40 60 Minutes after pouring (a) 0.05–0.10% C Steel Sand mold Chill mold 0.25–0.30% C Steel Sand mold Chill mold 0.55–0.60% C Steel Sand mold Chill mold 90 102 52 15 2 16 Minutes after pouring (b) Figure 10.5: (a) Solidification patterns for gray cast iron in a 180-mm square casting. Note that after 11 minutes of cooling, dendrites begin to reach each other, but the casting is still mushy throughout. It takes about 2 hours more for this casting to solidify completely. (b) Solidification of carbon steels in sand and chill (metal) molds. Note the difference in solidification patterns as the carbon content of the metal increases. Source: After H.F. Bishop and W.S. Pellini. It can be noted in Fig. 10.4 that pure metals have a freezing range that approaches zero and that the solidification front moves as a plane without developing a mushy zone. Eutectics (Section 4.3) solidify in a similar manner, with an essentially plane front; the structure developed upon solidification depends on the composition of the eutectic. In alloys with a nearly symmetrical phase diagram (see Fig. 4.4), the structure is generally lamellar, with two or more solid phases present, depending on the alloy system. When the volume fraction of the minor phase of the alloy is less than about 25%, the structure generally becomes fibrous. These conditions are particularly significant for cast irons. For alloys, a short freezing range generally involves a temperature difference of less than 50◦C, and a long freezing range more than 110◦C. Ferrous castings typically have narrow mushy zones, whereas aluminum and magnesium alloys have wide mushy zones; consequently, these alloys are in a mushy state throughout most of their solidification cycle. Effects of Cooling Rates. Slow cooling rates, on the order of 102 K/s, or long local solidification times result in coarse dendritic structures, with large spacing between dendrite arms. For higher cooling rates, on the order of 104 K/s, or short local solidification times, the structure becomes finer, with smaller dendrite arm spacing. For still higher cooling rates, on the order of from 106 to 108, the structures developed are amorphous, as described in Section 6.15. The structures developed and the resulting grain sizes have an influence on the properties of the casting. As grain size decreases, the strength and ductility of the cast alloy increase, microporosity (interdendritic shrinkage voids) in the casting decreases, and the tendency for the casting to crack (hot tearing, see Fig. 10.14) during solidification decreases. Lack of uniformity in grain size and grain distribution produce castings that have anisotropic properties. 10.2.3 Structure–Property Relationships Because all castings are expected to meet design and service requirements, the relationships between properties and the structures developed during solidification are important. This section describes these 2 ׉ 7cassandra://NkComQmSmr4kYx_gOUkZaBMg2610MimzctnoIj7x_WY`cZ%Eb=׉E U286 Chapter 10 Fundamentals of Metal Casting Mold wall Solid Liquid Solid Liquid Solid Liquid (a) (b) (c) Figure 10.6: Schematic illustration of three basic types of cast structures: (a) columnar dendritic; (b) equiaxed dendritic; and (c) equiaxed nondendritic. Source: Courtesy of D. Apelian. relationships in terms of dendrite morphology and the concentration of alloying elements in various regions within a casting. The compositions of dendrites and the liquid metal are given by the phase diagram of the particular alloy. When the alloy is cooled very slowly, each dendrite develops a uniform composition; however, under the normally faster cooling rates encountered in practice, cored dendrites are formed. These dendrites have a surface composition different from that at their centers, a difference referred to as concentration gradient. The surface of the dendrite has a higher concentration of alloying elements than at its core, due to solute rejection from the core toward the surface during solidification of the dendrite (microsegregation). The darker shading in the interdendritic liquid near the dendrite roots, shown in Fig. 10.6, indicates that these regions have a higher solute concentration; microsegregation in these regions is much more pronounced than in others. There are several types of segregation. In contrast to microsegregation, macrosegregation involves differences in composition throughout the casting itself. In situations where the solidification front moves away from the surface of a casting as a plane (Fig. 10.7), lower melting-point constituents in the solidifying alloy are driven toward the center (normal segregation). Consequently, such a casting has a higher concentration of alloying elements at its center than at its surfaces. In dendritic structures such as those found in solid–solution alloys (see Fig. 10.2b), the opposite occurs; that is, the center of the casting has a lower concentration of alloying elements (inverse segregation) than does at its surface. The reason is that liquid metal (having a higher concentration of alloying elements) enters the cavities developed from solidification shrinkage in the dendrite arms, which have solidified sooner. Another form of segregation is due to gravity; called gravity segregation, it involves a process whereby higher density inclusions, or compounds, sink while lighter elements (such as antimony in an antimony-lead alloy) float to the surface. Mold wall Solid Liquid Liquid (a) (b) Figure 10.7: Schematic illustration of cast structures in (a) plane front, single phase, and (b) plane front, two phase. Source: Courtesy of D. Apelian. ׉ 7cassandra://7A8mQImjKHY_c2VL8Iqw5yp5ei4-qC_xS_iw0rkQZy4`cZ%Eb>cZ%Eb=#בCט#u׉׉ 7cassandra://QKTPDW4i4yWIvEukxKaKktJCQapYwe63iTxTf6PQiiw`׉ 7cassandra://uAQ-E5UMpRjaTlw2gqyR3ENxrGPAsPeU1YlctosVzVcoV`s׉ 7cassandra://vdXfAPQZPLJrYCaG6ZyhF75X7JjJ6duYg32UAHvYTl4`׉ 7cassandra://aST99UpZtZ_rrtFDxbJhD3173J06de2NXEAjr1rYetA]͠^cZtEbTט#u׉׉ 7cassandra://chiuG-JrgdaHdVxGm0rVi13xKP6Pief-x-MO3M3Whpcy`׉ 7cassandra://5eZeDuo2-wdy-Grs4OHONuVXDO8exKt4hbnl6F3uopEE%`s׉ 7cassandra://LIIh1ceiBDXiS-ilCHvITCOBXsRCHnvTdQ6OL1MCuQE`׉ 7cassandra://FlxHB_hWn0e42mzeMJGN_EfxfEU-r9RKadsiizZRmKo͞͠^cZuEbU׉EFluid Flow 287 A typical cast structure of a solid–solution alloy, with an inner zone of equiaxed grains, is shown in Fig. 10.2b. The inner zone can be extended throughout the casting, as shown in Fig. 10.2c, by adding an inoculant (nucleating agent) to the alloy. The inoculant induces nucleation of the grains throughout the liquid metal, called heterogeneous nucleation. Because of the presence of thermal gradients in a solidifying mass of liquid metal, and due to gravity and the resulting density differences, convection has a strong influence on the structures developed. Convection involves heat transfer by the movement of matter; in a casting, it usually is associated with the flow of the liquid metal. Convection promotes the formation of an outer chill zone, refines grain size, and accelerates the transition from columnar to equiaxed grains. The structure shown in Fig. 10.6b also can be obtained by increasing convection within the liquid metal, whereby dendrite arms separate (dendrite multiplication). Conversely, reducing or eliminating convection results in coarser and longer columnar dendritic grains. The dendrite arms are not particularly strong and can be broken up by agitation or by mechanical vibration in the early stages of solidification (as in semisolid metal forming and rheocasting, described in Section 11.4.7). This process results in finer grain size, with equiaxed nondendritic grains distributed more uniformly throughout the casting (Fig. 10.6c). A side benefit is the thixotropic behavior of alloys (that is, the viscosity decreases when the liquid metal is agitated), leading to improved castability of the metal. Another form of semisolid metal forming is thixotropic casting, where a solid billet is first heated to a semisolid state and then injected into a die-casting mold (Section 11.4.5). 10.2.4 Freeze Casting Dendrite formation can be used to produce porous metals, ceramics, or polymers through the freeze casting process. In this approach, a slurry (suspension or mixture of particles in a liquid, usually water) is frozen. As the liquid solidifies, the particles are not soluble in the solid and therefore segregate at the solidification front. Eventually, the fluid freezes fully, with the carrier fluid and particles as separate phases. The fluid can then be removed by lowering the pressure (in a vacuum), as shown in Fig. 10.8, and then raising the temperature. The result is a porous metal with a microstructure that is derived from the dendritic structure of the carrier fluid (Fig. 10.9). This structure generally has to be sintered to develop strength (Section 17.4). 10.3 Fluid Flow To emphasize the importance of fluid flow in casting, consider a basic gravity casting system, as shown in Fig. 10.10. The molten metal is poured through a pouring basin or cup; it then flows through the gating system (consisting of sprue, runners, and gates) into the mold cavity. As also illustrated in Fig. 11.3, the sprue is a tapered vertical channel through which the molten metal flows downward in the mold. Runners are the channels that carry the molten metal from the sprue into the mold cavity or they connect the sprue to the gate (that portion of the runner through which the molten metal enters the mold cavity). Risers,also called feeders, serve as reservoirs of molten metal; they supply sufficient molten metal necessary to prevent porosity due to shrinkage during solidification. Although such a gating system appears to be relatively simple, successful casting requires proper design and control of the solidification process to ensure adequate fluid flow in the system. For example, an important function of the gating system in sand casting is to trap contaminants (such as oxides and other inclusions) and remove them from the molten metal by having the contaminants adhere to the walls of the gating system, thereby preventing them from reaching the mold cavity. Furthermore, a properly designed gating system helps avoid or minimize such problems as premature cooling, turbulence, and gas entrapment. Even before it reaches the mold cavity, the molten metal must be handled carefully to avoid the formation of oxides on molten-metal surfaces from exposure to the environment or the introduction of impurities into the molten metal. ׉ 7cassandra://vdXfAPQZPLJrYCaG6ZyhF75X7JjJ6duYg32UAHvYTl4`cZ%Eb?׉E2288 Chapter 10 Fundamentals of Metal Casting Ice crystals Liquid Suspended particles 1. 2. Freezing Sublimation Solid Gas Temperature Figure 10.8: Freeze casting. (1) A slurry of a carrier liquid (commonly water) and insoluble particles is first reduced in temperature. (2) The liquid freezes in directional or dendritic fashion, forcing the particles away from the solidified volume where the particles are insoluble. (3) Decreasing pressure and increasing temperature evaporates the carrier fluid (freeze drying), leaving behind the particles that can be fused through sintering operations. 3. Freeze-cast structure Figure 10.9: Microstructure after freeze casting. The specimen shown is titanium oxide (TiO2)withpure water as a freezing agent. Source: Courtesy S. Naleway and T. Ogden, University of Utah. Pressure ׉ 7cassandra://LIIh1ceiBDXiS-ilCHvITCOBXsRCHnvTdQ6OL1MCuQE`cZ%Eb@cZ%Eb?#בCט#u׉׉ 7cassandra://kSKEF6SrH8IUYyIH4B9r75hS4cuOP4jT_ycXhDhznUI'x`׉ 7cassandra://pTLqGmryM-3DvX-R3AUqIMpuRPcOqLQlE8Fryz7MxnMU`s׉ 7cassandra://fxiUsbKQl5c2vIysC6IBvuMak4qaDq_7P3VtdIXtDbA`׉ 7cassandra://ks2p54H21pmh5klCIlwMdNru3iGpUkdSx5Zbq7e28uo0ͦT͠^cZvEbZט#u׉׉ 7cassandra://dpuo0xRjxNGZae00sBPMZQPkkUVK6bitFl8XEPVyuBw&`׉ 7cassandra://352bRpPwb-p7BEwjC2g1dVOkCFyC19DmJpcrC8xxs-sk`s׉ 7cassandra://k2gnAxL9WYvpkGkZq0PTpyt0bfFmfqIKmIFSoBhckbU5`׉ 7cassandra://tCXbBHayMJW3_s5IvlB_QXPvMBNNfVKQefMXiEz27ecnH͠^cZvEb[׉E Fluid Flow 289 Pouring cup Top riser Side riser Sprue Gate Well Runner Figure 10.10: Schematic illustration of a typical riser-gated casting. Risers serve as reservoirs, supplying molten metal to the casting as it shrinks during solidification. Two basic principles of fluid flow are relevant to gating design: Bernoulli’s theorem and the law of mass continuity. Bernoulli’s Theorem. This theorem is based on the principle of the conservation of energy, and it relates pressure, velocity, the elevation of the fluid at any location in the system, and the frictional losses in a fluid system. The Bernoulli equation is h + p ρg + v2 2g = constant, (10.2) where h is the elevation above a certain reference level, p is the pressure at that elevation, v is the velocity of the liquid at that elevation, ρ is the density of the fluid (assuming that it is incompressible), and g is the gravitational constant. Conservation of energy requires that the following relationship be satisfied: h1 + p1 ρg + v2 2g 1 = h2 + p2 ρg + v2 2g 2 + f, (10.3) where the subscripts 1 and 2 represent two different locations in the system and f represents the frictional loss in the liquid as it travels through the system. The frictional loss includes such factors as energy loss at the liquid-mold wall interfaces and turbulence in the liquid. Mass Continuity. The law of mass continuity states that, for incompressible liquids and in a system with impermeable walls, the rate of flow is constant. Thus, Q = A1v1 = A2v2, (10.4) where Q is the volume rate of flow (such as m3/s), A is the cross-sectional area of the liquid stream, and v is the average velocity of the liquid in that cross section. The subscripts 1 and 2 refer to two different locations in the system. According to this law, the flow rate must be maintained everywhere in the system. The wall permeability is important, because otherwise some liquid will escape through the walls (as occurs in sand molds); thus, the flow rate will decrease as the liquid moves through the system. Coatings are often used to inhibit such behavior in sand molds. A small amount of permeability is, however, useful to allow escape of gases and can aid in heat transfer. Sprue Design. An application of the two principles just described is the traditional tapered design of sprues (shown in Fig. 10.10). Note that in a free-falling liquid (such as water from a faucet), the cross-sectional area of the stream decreases as the liquid gains velocity. Thus, if a sprue has a constant cross-sectional area and molten metal is poured into it, regions may develop where the liquid loses contact with the sprue walls. As a result, aspiration (a process whereby air is entrapped in the liquid) may take place. One of two basic Casting ׉ 7cassandra://fxiUsbKQl5c2vIysC6IBvuMak4qaDq_7P3VtdIXtDbA`cZ%EbA׉E290 Chapter 10 Fundamentals of Metal Casting alternatives is used to prevent aspiration: a tapered sprue is used to prevent molten metal separation from the sprue wall, or straight-sided sprues are supplied with a choking mechanism at the bottom, consisting of either a choke core or a runner choke, as shown in Fig. 11.3. The choke slows the flow sufficiently to prevent aspiration in the sprue. The specific shape of a tapered sprue that prevents aspiration can be determined from Eqs. (10.3) and (10.4). Assuming that the pressure at the top of the sprue is equal to the pressure at the bottom, and that there are no frictional losses, the relationship between height and cross-sectional area at any point in the sprue is given by the parabolic relationship A1 A2 =  h2 h1 , (10.5) where, for example, the subscript 1 denotes the top of the sprue and 2 denotes the bottom. The distances h1 and h2 are measured from the liquid level in the pouring cup or basin (Fig. 10.10), so that h2 is larger than h1. Moving downward from the top, the cross-sectional area of the sprue must therefore decrease. The area at the bottom of the sprue, A2, is selected to allow for desired flow rates, as described below, and the profile is produced according to Eq. (10.5). Depending on the assumptions made, expressions other than Eq. (10.5) can also be obtained. For example, assume a certain molten-metal velocity, V1, at the top of the sprue. Then, using Eqs. (10.3) and (10.4), an expression can be obtained for the ratio A1/A2 as a function of h1, h2,and V1. Modeling. Another application of the foregoing equations is in the modeling of mold filling in casting. For example, consider the situation shown in Fig. 10.10, where molten metal is poured into a pouring cup or basin; it flows through a sprue to a runner and a gate and fills the mold cavity. If the pouring basin has a much larger cross-sectional area than the sprue bottom, then the velocity of the molten metal at the top of the pouring basin is very low, and it can be taken to be zero. If frictional losses are due to a viscous dissipation of energy, then f in Eq. (10.3) can be taken to be a function of the vertical distance, and is often approximated as a linear function. Therefore, the velocity of the molten metal leaving the gate is obtained from Eq. (10.3) as v = c  2gh, where h is the distance from the sprue base to the liquid metal height and c is a friction factor. For frictionless flow, c equals unity and for flows with friction, c is always between 0 and 1. The magnitude of c varies with mold material, runner layout, and channel size, and it can include energy losses due to turbulence, as well as to viscous effects. If the liquid level has reached a height of x at the gate, then the gate velocity is v = c  √ 2g h − x. The flow rate through the gate will be the product of this velocity and the gate area according to Eq. (10.4). The shape of the casting will determine the height as a function of time. Integrating Eq. (10.4) gives the mean fill time and flow rate, and dividing the casting volume by the mean flow rate gives the mold fill time. Simulation of mold filling assists designers in the specification of the runner diameter, as well as the size and number of sprues and pouring basins. To ensure that the runners remain open, the fill time must be a small fraction of the solidification time, but the velocity should not be so high as to erode the mold (referred to as mold wash) or to result in too high of a Reynolds number (see below). Otherwise, turbulence and associated air entrainment will result. Several computational tools are now available to evaluate gating designs and to assist in the sizing of components, such as Magmasoft, Flow 3D Cast, Wincast, ProCast, Quikcast, SolidCast, SUTCast, and PASSAGE/PowerCAST. Flow Characteristics. An important consideration of fluid flow in gating systems is turbulence, as opposed to laminar flow of fluids. Turbulence is flow that is highly chaotic; such flow can lead to aspiration in casting ׉ 7cassandra://k2gnAxL9WYvpkGkZq0PTpyt0bfFmfqIKmIFSoBhckbU5`cZ%EbBcZ%EbA#בCט#u׉׉ 7cassandra://57YWLavp1Edw-mEf5NIh64o2J1fQzI5USKToJKPuSf0@`׉ 7cassandra://kDlfeXe-IJ6SG6wyT1bv3yd9Nu5ooxiLqJK5i5kVEMob(`s׉ 7cassandra://VUwMDM7OFWe5iMUfa6Q-Pckfw-w3wLAhOttnS7dVQNY`׉ 7cassandra://dYQU-onL9_80f-sbBfUaI9PycRHfnS55o7EH9ckyUPI^H͠^cZwEb]ט#u׉׉ 7cassandra://pSW7G2ln_SSxuoAhDqGGN2ifFHuXut2CW1R3cHyUdCw`׉ 7cassandra://v_o9qIOWAhE7tJcRvzImEeRFg-qJljz9VSU5zVq3mY8R)`s׉ 7cassandra://WJMsdpIzA3Xd2MOo_nKORdUS4wnL1RzOoQvPXB3_fRwS`׉ 7cassandra://GUFvhS9OusTuBGidgGXg4A1piXT_BpJoSqzhNh16jjo͈~ ͠^cZwEb_׉E Fluidity of Molten Metal 291 systems. The Reynolds number, Re, is used to quantify this aspect of fluid flow. It represents the ratio of the inertia to the viscous forces in fluid flow and is defined as Re = vDρ η , (10.6) where v is the velocity of the liquid, D is the diameter of the channel, and ρ and η are the density and viscosity of the liquid, respectively. The higher the Reynolds number, the greater the tendency for turbulent flow. In gating systems, Re typically ranges from 2000 to 20,000, where a value of up to 2000 represents laminar flow; between 2000 and 20,000, it represents a mixture of laminar and turbulent flow. Such a mixture is generally regarded as harmless in gating systems. However, Re values in excess of 20,000 represent severe turbulence, resulting in significant air entrainment and the formation of dross (the scum that forms on the surface of molten metal) from the reaction of the liquid metal with air and other gases. Techniques for minimizing turbulence generally involve avoidance of sudden changes in flow direction and in the shape of channel cross sections in gating system design. Dross or slag can be eliminated only by vacuum casting (see Section 11.4.2). Conventional atmospheric casting relieves the problem of dross or slag by (a) skimming, (b) using properly designed pouring basins and runner systems, (c) tapping the molten metal from below the surface, such as in pressure casting (Fig. 11.18), or (d) using filters, which also can eliminate turbulent flow in the runner system. Filters are typically made of ceramics, mica, or fiberglass; their proper location and placement are important for effective filtering of dross and slag. 10.4 Fluidity of Molten Metal The capability of molten metal to fill mold cavities is called fluidity; it consists of two basic factors: (a) the molten metal and (b) casting parameters. The characteristics of the molten metal that influence fluidity are: Viscosity. As viscosity and its sensitivity to temperature increase, fluidity decreases. Surface Tension. A high surface tension of the liquid metal reduces fluidity; also, oxide films on the surface of the molten metal have a significantly adverse effect on fluidity. For example, an oxide film on the surface of pure molten aluminum triples the surface tension. Inclusions. Because they are insoluble, inclusions can have a significant effect on fluidity. This effect can easily be verified by observing the viscosity of a liquid (such as oil) with and without sand particles in it; a liquid with sand in it has a higher viscosity and thus lower fluidity. Solidification Pattern of the Alloy. The manner in which solidification takes place (Section 10.2) can influence fluidity. Fluidity is inversely proportional to the freezing range (see Eq. 10.1): The shorter the range, as in pure metals and eutectics, the higher the fluidity. Conversely, alloys with long freezing ranges, such as solid–solution alloys, have lower fluidity. The following casting parameters influence fluidity, fluid flow, and thermal characteristics of the system: Mold Design. The design and dimensions of the sprue, runners, and risers all influence fluidity. Mold Material and Its Surface Characteristics. The higher the thermal conductivity of the mold and the rougher its surfaces, the lower is the fluidity of the molten metal. Although heating the mold improves fluidity, it slows down solidification of the metal; thus, the casting develops coarse grains and hence has lower strength. ׉ 7cassandra://VUwMDM7OFWe5iMUfa6Q-Pckfw-w3wLAhOttnS7dVQNY`cZ%EbC׉E A292 Chapter 10 Fundamentals of Metal Casting Pouring cup Sprue Fluidity index Figure 10.11: A test method for fluidity using a spiral mold. The fluidity index is the length of the solidified metal in the spiral passage. The greater the length of the solidified metal, the greater is the metal’s fluidity. Degree of Superheat. Superheat (defined as the increment of temperature of an alloy above its melting point) improves fluidity by delaying solidification. The pouring temperature often is specified instead of the degree of superheat, because it is more easily measured and controlled. Rate of Pouring. The slower the rate of pouring molten metal into the mold, the lower the fluidity because of the higher rate of cooling when poured slowly. Heat Transfer. This factor directly affects the viscosity of the liquid metal (see below). Castability. Although complex, this term is generally used to describe the ease with which a metal can be cast to produce a part with good quality. It includes not only fluidity, but is also affected by casting practices. 10.4.1 Tests for Fluidity Several tests have been developed to quantify fluidity, although none has been accepted universally. In one common test, the molten metal is made to flow along a channel that is at room temperature (Fig. 10.11); the distance the metal flows before it solidifies and stops flowing is a measure of its fluidity. Obviously, the length is a function of the thermal properties of the metal and the mold, as well as of the design of the channel. 10.5 Heat Transfer The heat transfer during the complete cycle (from pouring, to solidification, and to cooling to room temperature) is an important consideration in metal casting. Heat flow at different locations in the system is a complex phenomenon and depends on several factors related to the material cast and the physical properties of the mold and the processing parameters. For instance, in casting thin sections, the metal flow rates must be high enough to avoid premature chilling and solidification of the metal. On the other hand, the flow rate must not be so high as to cause excessive turbulence, with its detrimental effects on the casting operation. A typical temperature distribution at the mold-liquid metal interface is shown in Fig. 10.12. Heat from the liquid metal is given off through the mold wall and to the surrounding air. The temperature drop at the air–mold and mold–metal interfaces is caused by the presence of boundary layers and imperfect contact at these interfaces. The shape of the curve depends on the thermal properties of the molten metal and the mold. ׉ 7cassandra://WJMsdpIzA3Xd2MOo_nKORdUS4wnL1RzOoQvPXB3_fRwS`cZ%EbDcZ%EbC#בCט#u׉׉ 7cassandra://rcWnUrEGopfcLUKNZUMEgToKSDiPCQHXYjARJQlblzo7`׉ 7cassandra://Wes458fzzOwyWP_3wSPXW4Jt3hCyErPfMifhQEG8Joo=`s׉ 7cassandra://WAwqNC-K1x-m0TniiiXtEUFMNfWWYlaA_KhR0mA_rD0R`׉ 7cassandra://5yA-nsIxkhOggLnDnQWIJwYSmNfgnL8s7UdjGjbMoQAʖR͠^cZwEbeט#u׉׉ 7cassandra://yyTGN3ZRrup20Q9vXsqeNA-5SHhURzcIhTi-CTIQPSw`׉ 7cassandra://1N6lCWYedkSOM8dVa_7bzOVQkx_QnDFzugBcWsdFtBsH*`s׉ 7cassandra://2NVsezyikZee3ySytExrlHEGbOPDIy7JKFZ27C97IHw/`׉ 7cassandra://LeoDYUbrg9dlWcZLG3J9tE5C0Nv3X1yhGKyBeP1jH4ct͠^cZxEbf׉E>Heat Transfer 293 Air Mold Solid Liquid Melting point DT Room temperature Distance Figure 10.12: Temperature distribution at the interface of the mold wall and the liquid metal during the solidification of metals in casting. 10.5.1 Solidification Time During the early stages of solidification of the metal, a thin skin begins to form at the relatively cool mold walls; as time passes, the thickness of the skin increases (Fig. 10.13). With flat mold walls, the thickness is proportional to the square root of time; thus, doubling the time will make the skin √ Solidification time = C  Volume Surface area n 2=1.41 times or 41% thicker. The solidification time is a function of the volume of a casting and its surface area (Chvorinov’s rule): , (10.7) where C is a constant reflecting (a) the mold material, (b) the metal properties, including latent heat, and (c) the temperature. The parameter n has a value between 1.5 and 2, but usually taken as 2. Thus, for example, a large solid sphere will solidify and cool to ambient temperature at a much slower rate than will a smaller solid sphere. Note that the volume of a sphere is proportional to the cube of its diameter, and its surface area AB DT at metal–mold interface at mold–air interface 5 s 1 min 2 min 6 min Figure 10.13: Solidified skin on a steel casting. The remaining molten metal is poured out at the times indicated in the figure. Hollow ornamental and decorative objects are made by a process called slush casting, which is based on this principle. Source: After H.F. Taylor, J. Wulff, and M.C. Flemings. Temperature ׉ 7cassandra://WAwqNC-K1x-m0TniiiXtEUFMNfWWYlaA_KhR0mA_rD0R`cZ%EbE׉E294 Chapter 10 Fundamentals of Metal Casting is proportional to the square of its diameter. Similarly, it can be shown that molten metal in a cube-shaped mold will solidify faster than in a spherical mold of the same volume (see Example 10.1). The effects of mold geometry and elapsed time on skin thickness and shape are shown in Fig. 10.13. As illustrated, the unsolidified molten metal has been poured from the mold at different time intervals ranging from 5 seconds to 6 minutes. As expected, the skin thickness increases with elapsed time, and the skin is thinner at internal angles (location A in the figure) than at external angles (location B). The latter condition is caused by slower cooling at internal angles than at external angles. Example 10.1 Solidification Times for Various Shapes Given: Three metal pieces being cast have the same volume, but different shapes: One is a sphere, one a cube, and the other a cylinder with its height equal to its diameter. Assume that n =2. Find: Which piece will solidify the fastest, and which one the slowest? Solution: The volume of the piece is taken as unity; thus from Eq. (10.7), Solidification time ∝ The respective surface areas are as follows: Sphere: V =   4 3 πr3,r= A =4πr2 =4π Cube: V = a3,a=1, Cylinder: V = πr2h =2πr3,r= A =2πr2 +2πrh =6πr2 =6π   1 2π 1 2π 1/3 2/3 , =5.54. The respective solidification times are therefore tsphere =0.043C, tcube =0.028C, tcylinder =0.033C. Hence, the cube-shaped piece will solidify the fastest, and the spherical piece will solidify the slowest. 10.5.2 Shrinkage Because of their thermal expansion characteristics, metals usually shrink (contract) during solidification and while cooling to room temperature. Shrinkage, which causes dimensional changes and sometimes warping and cracking, is the result of the following three sequential events: 1. Contraction of the molten metal as it cools prior to its solidification 2. Contraction of the metal during phase change from liquid to solid 3. Contraction of the solidified metal (the casting) as its temperature drops to ambient temperature. and A =6a2 =6  3 4π  2/3 3 4π 1/3 =4.84. , 1 (Surface area)2 . ׉ 7cassandra://2NVsezyikZee3ySytExrlHEGbOPDIy7JKFZ27C97IHw/`cZ%EbFcZ%EbE#בCט#u׉׉ 7cassandra://MJ0zSr0zZdsLUq-QYxSmJ4lANzUQ25kuAg9StVolqAU>`׉ 7cassandra://D_zNpxhDqGBiw60JH5FRKppznf6qysFAbhOhyKCNWdI>U`s׉ 7cassandra://-Bvj4Hr6QoUZJBUC4V64XM17gFCiL2W0VGn4b3OsLl4`׉ 7cassandra://c-947pEUK0pqzs4zYp89sElDVHRc7oy6kiUYOAgGJyY͞hK͠^cZzEbiט#u׉׉ 7cassandra://oo3vvqTMAAB52pD3dAu9BHcQFjYKyQMv8KInYGpA0nY0`׉ 7cassandra://tdUXsdBWCCQl_3KBF7Ajt6dvbTJlABexxjPk46y0HakN`s׉ 7cassandra://VOvVZUoPoYhbjJJPp0iHIGIMDIT-IJsUT3Mx8cwQvJ4`׉ 7cassandra://DF8Z7c5LIgofIwaR9tOHZzLL7suvUhLnsD7bDYO1QJ4h~8͠^cZzEbj׉EDefects 295 Table 10.1: Volumetric Solidification Contraction or Expansion for Various Cast Metals. Contraction (%) Expansion (%) Aluminum Zinc Al–4.5% Cu Gold White iron Copper Brass (70–30) Magnesium 7.1 Bismuth 3.3 Silicon 6.5 4–5.5 4.9 4.5 4.2 90% Cu–10% Al 4 Carbon steels Al–12% Si Lead 2.5–4 3.8 3.2 The largest amount of shrinkage occurs during the phase change of the material from liquid to solid; detrimental effects of this shrinkage can be reduced through the use of risers or by pressure-feeding of molten metal. The amount of contraction during solidification of various metals is shown in Table 10.1. Note that some metals, such as gray cast iron, expand. The reason for this expansion is that graphite has a relatively high specific volume, and when it precipitates as graphite flakes during solidification of the gray cast iron, it causes a net expansion of the metal. Shrinkage, especially that due to thermal contraction, is further described in Section 12.2.1 in connection with design considerations in casting. 10.6 Defects Depending on factors such as the quality of raw materials, product design, and control of processing parameters, several defects can develop in castings, as illustrated in Figs. 10.14 and 10.15. While some defects affect only the appearance of the parts made, others can have major adverse effects on their structural Hot tear Casting Hot tear Casting Core Hot tear Figure 10.14: Examples of hot tears in castings. These defects occur because the casting cannot shrink freely during cooling, owing to constraints in various portions of the molds and cores. Exothermic (heat producing) compounds may be used as exothermic padding to control cooling at critical regions to avoid hot tearing. Pouring cup Sprue Runner Hot tear 2.9 6.3 Gray iron 2.5 5.5 ׉ 7cassandra://-Bvj4Hr6QoUZJBUC4V64XM17gFCiL2W0VGn4b3OsLl4`cZ%EbG׉Ec296 Chapter 10 Fundamentals of Metal Casting Surface of casting Blow Scar Blister (a) (b) Scab Sprue Gate Wash Sand mold (d) Gate Cold shut (g) Figure 10.15: Examples of common defects in castings. These defects can be minimized or eliminated by proper design and preparation of molds and control of pouring procedures. Source: After J. Datsko. integrity. The International Committee of Foundry Technical Associations has developed a standardized nomenclature, consisting of seven basic categories of casting defects, identified with boldface capital letters: A—Metallic projections, consisting of fins, flash, or projections, such as swells and rough surfaces. B—Cavities, consisting of rounded or rough internal or exposed cavities, including blowholes, pinholes, and shrinkage cavities (see porosity, Section 10.6.1). C—Discontinuities, such as cracks, cold or hot tearing, and cold shuts. If the solidifying metal is constrained from shrinking freely, cracking and tearing may occur. Although several factors are involved in tearing, coarse grain size and the presence of low-melting-point segregates along the grain boundaries of the metal increase the tendency for hot tearing. Cold shut is an interface in a casting that lacks complete fusion, because of the meeting of two streams of liquid metal from different gates. D—Defective surface, such as surface folds, laps, scars, adhering sand layers, and oxide scale. E—Incomplete casting, such as misruns (due to premature solidification), insufficient volume of the metal poured, and runout (due to loss of metal from the mold after pouring). Incomplete castings also can result from the molten metal being at too low a temperature or from pouring the metal too slowly. F—Incorrect dimensions or shape, due to such factors as improper shrinkage allowance, pattern-mounting error, irregular contraction, deformed pattern, or warped casting. G—Inclusions, which form during melting, solidification, and molding. Generally nonmetallic, they are regarded as harmful because they act as stress raisers, reducing the strength of the casting. Inclusions (e) Gate Gate (f) Misrun (c) ׉ 7cassandra://VOvVZUoPoYhbjJJPp0iHIGIMDIT-IJsUT3Mx8cwQvJ4`cZ%EbHcZ%EbG#בCט#u׉׉ 7cassandra://sbxBz3l-M_r1zvLzVwgEsnOnLiprgtqjFtDbVj7Si6g*`׉ 7cassandra://j8hP1Zj0Dg6n22zuA522ecsoy1N0AMq5S2Lwvc6meHsJ`s׉ 7cassandra://fSC0cZDETU9YiUmckVgM8E6FgvM0Qe7mU1TOXMd1MVw`׉ 7cassandra://9fO6A10WWt7pGsxgWEmW5Q2MfCULnDVdq9SVn9LSjJMsb͠^cZEbmט#u׉׉ 7cassandra://ZBKRjA2ZSs4xNfUZHHYNstkaMhWpvN7vS9m6Ha5jgYQZ`׉ 7cassandra://NQdMWQHp-RG5i3D3fnRIonOZeNwetA1RBfUEvfNflKYM2`s׉ 7cassandra://6hr8JUrnQ_sy2NjkRDtw24JnPkR4tTQbDR3iHCp-b7k`׉ 7cassandra://QXRc-1rMNy388YSEPNHsqONHx9_puiSI0ZWkgibtDbAbn2͠^cZEbn׉EDefects 297 may form during melting when the molten metal reacts with the environment (usually oxygen), with the crucible or the mold material. Chemical reactions among components in the molten metal itself may produce inclusions. Slags and other foreign material entrapped in the molten metal can become inclusions, although filtering the molten metal can remove particles as small as 30 μm. Spalling of the mold and core surfaces also can produce inclusions, thus indicating the importance of the quality of molds and of their proper maintenance. 10.6.1 Porosity Porosity in a casting may be caused by shrinkage, entrained or dissolved gases, or both. Porous regions can develop in castings because of shrinkage of the solidified metal. Thin sections in a casting solidify sooner than thicker regions; consequently, molten metal flows into the thicker regions that have not yet solidified. Porous regions may develop at their centers because of contraction as the surfaces of the thicker regions begin to solidify first. Microporosity can develop when the liquid metal solidifies and shrinks between dendrites and between dendrite branches. Porosity is detrimental to the strength and ductility of a casting and its surface finish, potentially making the casting permeable, thus affecting the pressure tightness of a cast pressure vessel. Porosity caused by shrinkage can be reduced or eliminated by various means: • Adequate liquid metal should be provided to prevent cavities caused by shrinkage. • Internal or external chills, as those used in sand casting (Fig. 10.16), are an effective means of reducing shrinkage porosity. The function of chills is to increase the rate of solidification in critical regions. Internal chills are usually made from the same material as the casting itself, and are left within the casting. Problems may arise that involve proper fusion of the internal chills with the casting; thus, Sand Casting Chill Sand (a) Chill Porosity (b) Casting Boss Chill (c) Figure 10.16: Various types of (a) internal and (b) external chills (dark areas at corners) used in castings to eliminate porosity caused by shrinkage. Chills are placed in regions where there is a larger volume of metal, as shown in (c). ׉ 7cassandra://fSC0cZDETU9YiUmckVgM8E6FgvM0Qe7mU1TOXMd1MVw`cZ%EbI׉E 298 Chapter 10 Fundamentals of Metal Casting Fusion Melting point Temperature Figure 10.17: Solubility of hydrogen in aluminum. Note the sharp decrease in solubility as the molten metal begins to solidify. foundries generally avoid using internal chills. External chills may be made from the same material as the casting or may be made of iron, copper, or graphite. • Porosity in alloys can be reduced or eliminated by using high temperature gradients, that is, by increasing the cooling rate. For example, mold materials with higher thermal conductivity may be used. • Subjecting the casting to hot isostatic pressing is another method of reducing porosity (see Section 17.3.2). Gases are more soluble in liquid metals than in solid metals (Fig. 10.17); thus, when a metal begins to solidify, the dissolved gases are expelled. Gases also may be due to reactions of the molten metal with the mold materials. Gases either accumulate in regions of existing porosity (such as in interdendritic regions; see Section 10.2.3) or cause microporosity in the casting, particularly in cast iron, aluminum, and copper. Dissolved gases may be removed from the molten metal by flushing or purging with an inert gas or by melting and pouring the metal in a vacuum. If the dissolved gas is oxygen, the molten metal can be deoxidized. Whether microporosity is a result of shrinkage or is caused by gases may be difficult to determine. If the porosity is spherical and has smooth walls (similar to the shiny holes in Swiss cheese), it is generally from gases. On the other hand, if the walls are rough and angular, porosity is likely from shrinkage between dendrites. Gross porosity is from shrinkage and usually is called a shrinkage cavity. Summary • Casting is a solidification process in which molten metal is poured into a mold and allowed to cool. The metal may flow through a variety of passages (pouring basins, sprues, runners, risers, and gating systems) before reaching the final mold cavity. Bernoulli’s theorem, the continuity law, and the Reynolds number are the analytical tools used in designing castings, with the goals of achieving an appropriate flow rate and eliminating defects associated with fluid flow. • Solidification of pure metals takes place at a constant temperature, whereas solidification of alloys occurs over a range of temperatures. Phase diagrams are important tools for identifying the solidification point or points for technologically important metals. Hydrogen solubility Solid Liquid ׉ 7cassandra://6hr8JUrnQ_sy2NjkRDtw24JnPkR4tTQbDR3iHCp-b7k`cZ%EbJcZ%EbI#בCט#u׉׉ 7cassandra://f7YOEvrpVoGeNLF7DbBYOzMrD3FR5fHKDQsQyd_uHBY `׉ 7cassandra://xcrM3EsG3vJ4e55QHJMvt4C_IOr6HqGOs6HTEySUdlIE`s׉ 7cassandra://tRdMx4dqi5mz6u4w0HBwx7L5D_vPOORWKzvF6wvTuzcF`׉ 7cassandra://IHzlj0ocMnr6D1m98zfjvdzwgHO26DCp1BZyBLIuN1oSr͠^cZEbqט#u׉׉ 7cassandra://YtpnfG7FYYHPdW8121MB9NCHr3z8pimagZxfhup7EuQ`׉ 7cassandra://tUK6OglCgMSUL2_9G7rQMCWAZ_K10gHJQ8y7Hfsh-yQT2`s׉ 7cassandra://Dp0JkSA-7bxpkJBQXfLH9VX9bXH6jMktko_Tw0fPrLsh`׉ 7cassandra://kgQkCtLyXA3wX95tJX_NKZ_bznFN5xlIPfDWLrhDHp0k͠^cZEbr׉EBibliography 299 • The composition and cooling rates of the molten metal both affect the size and shape of the grains and the dendrites in the solidifying alloy. In turn, the size and structure of grains and dendrites influence properties of the solidified casting. Solidification time is a function of the volume and surface area of a casting (Chvorinov’s rule). • The grain structure of castings can be controlled by various means to obtain desired properties. Because most metals contract during solidification and cooling, cavities can form in the casting. Porosity caused by gases evolved during solidification can be a significant problem, particularly because of its adverse effect on the mechanical properties of castings. Various defects also can develop in castings from lack of control of material and process variables. • Dimensional changes and cracking (hot tearing) are difficulties that can arise during solidification and cooling. Several basic categories of casting defects have been identified. • Melting practices have a direct effect on the quality of castings, as do foundry operations such as pattern and mold making, pouring the molten metal, removing the cast parts from molds, cleaning, heat treatment, and inspection. Key Terms Aspiration Bernoulli’s theorem Casting Chills Columnar dendrite Columnar grain Cored dendrite Dendrite Fluidity Freeze casting Freezing range Gate Gating system Heterogeneous nucleation Homogenous nucleation Inoculant Macrosegregation Bibliography Analysis of Casting Defects, American Foundrymen’s Society, 2002. ASM Handbook,Vol.1A: Cast Iron Science and Technology, ASM International, 2017. ASM Handbook, Vol. 15: Casting, ASM International, 2008. Campbell, J., Castings, Butterworth-Heinemann, 2nd ed., 2003. Campbell, J., Complete Casting Handbook: Metal Casting Processes, Techniques and Design, Butterworth-Heinemann, 2011. Microsegregation Mold Mushy zone Normal segregation Porosity Pouring basin Reynolds number Rheocasting Riser Runner Segregation Shrinkage Skin Solidification Sprue Turbulence ׉ 7cassandra://tRdMx4dqi5mz6u4w0HBwx7L5D_vPOORWKzvF6wvTuzcF`cZ%EbK׉E 300 Chapter 10 Fundamentals of Metal Casting Cantor, B., and O’Reilly, K., Solidification and Casting, Taylor & Francis, 2002. Casting,in Tool and Manufacturing Engineers Handbook, Volume II: Forming, Society of Manufacturing Engineers, 1984. Glicksmann, M.E., Principles of Solidification: An Introduction to Modern Casting and Crystal Growth Concepts, Springer, 2010. Poirer, D.R., and Poirer, E.J., Heat Transfer Fundamentals for Metal Casting, Wiley, 1998. Reikher, A., and Barkhudarov, M., Casting: An Analytical Approach, Springer, 2008. Sahoo, M., and Sahu, S., Principles of Metal Casting, 3rd ed., McGraw-Hill, 2014. Steel Castings Handbook, 6th ed., Steel Founders’ Society of America, 1995. Stefanescu, D.M., Science and Engineering of Casting Solidification, 3rd ed., Springer, 2015. Review Questions 10.1. Explain why casting is an important manufacturing process. 10.2. Why do most metals shrink when they are cast? 10.3. What are the differences between the solidification of pure metals and metal alloys? 10.4. What are dendrites? Why are they called so? 10.5. Describe the difference between short and long freezing ranges. 10.6. What is superheat? Is it important? What are the consequences of excessive superheat? 10.7. Define shrinkage and porosity. How can you tell whether cavities in a casting are due to porosity or to shrinkage? 10.8. What is the function of chills? What are they made of? 10.9. Why is the Reynolds number important in casting? 10.10. What is a sprue? What shape should a sprue have if a mold has no other choking means? 10.11. How is fluidity defined? Why is it important? 10.12. Explain the reasons for hot tearing in castings. 10.13. Why is it important to remove dross or slag during the pouring of molten metal into the mold? What methods are used to remove them? 10.14. Why is Bernoulli’s equation important in casting? 10.15. Describe thixocasting and rheocasting. 10.16. What is Chvorinov’s Rule? 10.17. How is a blister related to a scab? Qualitative Problems 10.18. Is there porosity in a chocolate bar? In an ice cube? Explain. 10.19. Describe the stages involved in the contraction of metals during casting. 10.20. Explain the effects of mold materials on fluid flow and heat transfer in casting operations. 10.21. It is known that pouring metal at a high rate into a mold can have certain disadvantages. Are there any disadvantages to pouring it very slowly? 10.22. Describe the events depicted in Fig. 10.5. 10.23. Would you be concerned about the fact that portions of internal chills are left within the casting? Explain. ׉ 7cassandra://Dp0JkSA-7bxpkJBQXfLH9VX9bXH6jMktko_Tw0fPrLsh`cZ%EbLcZ%EbK#בCט#u׉׉ 7cassandra://CNQZu18lN-1m8-kPAGAHWKqtmY-sIzTS64QB5SABjMo&`׉ 7cassandra://-5dFxrhxIBeCG2ncT_J89X4rV1bSqZFEEO3aHTnpMzMf`s׉ 7cassandra://YfXVi5TgFsPPZLyPMjieSxg3qK1yQFQi6XL05yO9rgY`׉ 7cassandra://bGbQjUIKV-K5yYDHaCp4Y7NDpd1y6ypIzP32NY8gWpUE͠^cZEbtט#u׉׉ 7cassandra://kJyW9dRt4FdrdMdYUHmN_WwNO5_ajF9JWz2qdhEPZqot`׉ 7cassandra://-CHTPdgY2uitHe2jm3dyrgB-_x1h8ecI1-qatQnjvxcl?`s׉ 7cassandra://7NO9YaVYWN-HW993xsqAXtNpf6c_8wAS0xDDeb7Sn8Y`׉ 7cassandra://Bcp2ZzLAgSS1bnL5eOw2Ya31oadiszmghYjBr5hADDsU͠^cZEbu׉E Qualitative Problems 301 10.24. Review Fig. 10.10 and make a summary, explaining the purpose of each feature in shown and the consequences of omitting the feature from the mold design. 10.25. Make a sketch of volume vs. temperature for a metal that shrinks when it cools from the liquid state to room temperature. On the graph, mark the area where shrinkage is compensated by risers. 10.26. What practical demonstrations can you suggest to indicate the relationship of the solidification time to the volume and surface area of a casting? 10.27. Explain why a casting may have to be subjected to various heat treatments. 10.28. List and explain the reasons that porosity can develop in a casting. 10.29. Why does porosity have detrimental effects on the mechanical properties of castings? Would physical properties, such as thermal and electrical conductivity, also be adversely affected by porosity? Explain. 10.30. A spoked handwheel is to be cast in gray iron. In order to prevent hot tearing of the spokes, would you insulate the spokes or chill them? Explain. 10.31. Which of the following considerations are important for a riser to function properly? Must it: (a) have a surface area larger than the part being cast, (b) be kept open to atmospheric pressure, and/or (c) solidify first? Explain. 10.32. Explain why the constant C in Eq. (10.7) depends on mold material, metal properties, and temperature. 10.33. Are external chills as effective as internal chills? Explain. 10.34. Explain why, as shown in Table 10.1, gray cast iron undergoes expansion rather than contraction during solidification. 10.35. Referring to Fig. 10.13, explain why internal corners, such as A, develop a thinner skin than external corners, such as B, during solidification. 10.36. Note the shape of the two risers shown in Fig. 10.10, and discuss your observations with respect to Eq. (10.7). 10.37. Is there any difference in the tendency for shrinkage void formation in metals with short and long freezing ranges, respectively? Explain. 10.38. What is the influence of the cross-sectional area of the spiral channel shown in Fig. 10.11 on fluidity test results? What is the effect of sprue height? If this test is run with the entire test setup heated to elevated temperatures, would the results be more useful? Explain. 10.39. It has long been observed that (a) low pouring temperatures (i.e., low superheat) promote the formation of equiaxed grains over columnar grains and (b) equiaxed grains become finer as the pouring temperature decreases. Explain these two phenomena. 10.40. In casting metal alloys, what would you expect to occur if the mold were agitated (vibrated) aggressively after the molten metal had been in the mold for a sufficient amount of time to form a skin? 10.41. If you inspect a typical cube of ice, you are likely to see air pockets and cracks in the cube. Some ice cubes, however, are tubular in shape and do not have noticeable air pockets or cracks in their structure. Explain this phenomenon. 10.42. How can you tell whether cavities in a casting are due to shrinkage or entrained air bubbles? 10.43. Describe the drawbacks to having a riser that is (a) too large and (b) too small. 10.44. Reproduce Fig. 10.2 for a casting that is spherical in shape. 10.45. List the process variables that affect the fluidity index as shown in Fig. 10.11. 10.46. Assume that you have a method of measuring porosity in a casting. Could you use this information to accurately predict the strength of the casting? Explain. ׉ 7cassandra://YfXVi5TgFsPPZLyPMjieSxg3qK1yQFQi6XL05yO9rgY`cZ%EbM׉E302 Chapter 10 Fundamentals of Metal Casting Quantitative Problems 10.47. Derive Eq. (10.5). 10.48. Sketch a plot of specific volume versus temperature for a metal that shrinks as it cools from the liquid state to room temperature. On the graph, mark the area where shrinkage is compensated for by risers. 10.49. A round casting is 0.3 m in diameter and 1 m in length. Another casting of the same metal is elliptical in cross section, with a major-to-minor axis ratio of 2, and has the same length and cross sectional area as the round casting. Both pieces are cast under the same conditions. What is the difference in the solidification times of the two castings? 10.50. A 75-mm thick square plate and a right circular cylinder with a radius of 100 mm and a height of 25 mm have the same volume. If each is to be cast with the use of a cylindrical riser, will each part require the same-size riser to ensure proper feeding? Explain. 10.51. Assume that the top of a round sprue has a diameter of 75 mm and is at a height of 150 mm from the runner. Based on Eq. (10.5), plot the profile of the sprue diameter as a function of its height. Assume that the sprue has a diameter of 5 mm at the bottom. 10.52. Pure aluminum is poured into a sand mold. The metal level in the pouring basin is 250 mm above the metal level in the mold, and the runner is circular with a 7.5 mm diameter. What is the velocity and rate of the flow of the metal into the mold? Is the flow turbulent or laminar? 10.53. A cylinder with a diameter of 50 mm and height of 75 mm solidifies in 3 minutes in a sand casting operation. What is the solidification time if the cylinder height is doubled? What is the time if the diameter is doubled? 10.54. The volume flow rate of metal into a mold is 0.05 m3/s. The top of the sprue has a diameter of 30 mm, and its length is 200 mm. What diameter should be specified at the bottom of the sprue to prevent aspiration? What is the resultant velocity and Reynolds number at the bottom of the sprue if the metal being cast is aluminum with a viscosity of 0.004 Ns/m2? 10.55. A rectangular mold with dimensions 120 mm × 240 mm × 480 mm is filled with aluminum with no superheat. Determine the final dimensions of the part as it cools to room temperature. Repeat the analysis for gray cast iron. 10.56. The constant C in Chvorinov’s rule is given as 2.5 s/mm2 and is used to produce a cylindrical casting with a diameter of 60 mm and height of 130 mm. Estimate the time for the casting to fully solidify. The mold can be broken safely when the solidified shell is at least 20 mm. Assuming that the cylinder cools evenly, how much time must transpire after pouring the molten metal before the mold can be broken? 10.57. A sprue is 300mmlong and has a diameter of 75mm at the top. The molten metal level in the pouring basing (which is much larger than the top of the sprue) is taken to be 75 mm from the top of the sprue for design purposes. If a flow rate of 500 cm3/s is to be achieved, what should be the diameter at the bottom of the sprue? Will the sprue aspirate? Explain. 10.58. Pure copper is poured into a sand mold. The metal level in the pouring basin is 250 mm above the metal level in the mold, and the runner is circular with a 10 mm diameter. What are the velocity and rate of the flow of the metal into the mold? Is the flow turbulent or laminar? 10.59. For the sprue described in Problem 10.58, what runner diameter is needed to ensure a Reynolds number of 2000? How long will a 250 cm3 casting take to fill with such a runner? 10.60. How long would it take for the sprue in Problem 10.58 to feed a casting with a square cross section of 50 mm per side and a height of 100 mm? Assume that the sprue is frictionless. ׉ 7cassandra://7NO9YaVYWN-HW993xsqAXtNpf6c_8wAS0xDDeb7Sn8Y`cZ%EbNcZ%EbM#בCט#u׉׉ 7cassandra://UTY5w1fSNjZQj2mQ-RWrs3CxNwhI3jD8sjCpp8npnpQ`׉ 7cassandra://r3yR63QnwBZKoSZpwn5RuaUb31tj5wHsynDB-0VA8lQU`s׉ 7cassandra://zuRjK98ReFPRLNW6yiAi_wXEEBLOoo912q9bkil00vQP`׉ 7cassandra://a1iozVdO1O6Re2gacbRyooXdyrUgrIi5ROH5PjWb1c8{͠^cZEbwט#u׉׉ 7cassandra://yCMpgbZpA4L4ONQ0w4F4Bj6hiDQXSE0RUQ95CgexsIk`׉ 7cassandra://-hXy3MgCtCiePTGL4LhGP6mklbhskHB5NesoWJonJEg=`s׉ 7cassandra://HGFrvKc1VSJiIl5r7tdhvOoAhJW9C8q71D-Aa9LvSzw`׉ 7cassandra://CX2uUgaHGPSSHX4M772ohF7fEI4Syq5VDJCVsbaClxc>|͠^cZEbx׉E #Synthesis, Design, and Projects 303 10.61. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers to them. 10.62. When designing patterns for casting, pattern makers use special rulers that automatically incorporate solid shrinkage allowances into their designs. Therefore, a 100-mm patternmaker’s ruler is longer than 100 mm. How long should a patternmaker’s ruler be for (1) aluminum castings, (2) malleable cast iron, and (3) high manganese steel? Synthesis, Design, and Projects 10.63. Can you devise fluidity tests other than that shown in Fig. 10.11? Explain the features of your test methods. 10.64. The illustration indicates various defects and discontinuities in cast products. Review each defect and offer solutions to avoid it. Fracture Gate Casting (a) Sink mark (b) Riser (c) Cold tearing (d) 10.65. The fluidity test shown in Fig. 10.11 illustrates only the principle of this test. Design a setup for such a test, showing the type of materials and the equipment to be used. Explain the method by which you would determine the length of the solidified metal in the spiral passage. 10.66. Utilizing the equipment and materials available in a typical kitchen, design an experiment to reproduce results similar to those shown in Fig. 10.13. Comment on your observations. 10.67. One method of relieving stress concentrations in a part is to apply a small, uniform plastic deformation to it. Make a list of your concerns and recommendations if such an approach is suggested for a casting. 10.68. Describe the effects on mold design, including the required change in the size of the risers, runners, chokes, and sprues, for a casting of a given shape that is to be doubled in volume. 10.69. Small amounts of slag often persist after skimming and are introduced into the molten-metal flow in casting. Recognizing that the slag is much less dense than the metal, design mold features that will remove small amounts of slag before the metal reaches the mold cavity. 10.70. Figure II.2 shows a variety of components in a typical automobile that are produced by casting. Think of other products, such as power tools and small appliances, and prepare an illustration similar to the figure. 10.71. Design an experiment to measure the constants C and n in Chvorinov’s rule, Eq. (10.7). Describe the features of your design, and comment on any difficulties that might be encountered in running such an experiment. ׉ 7cassandra://zuRjK98ReFPRLNW6yiAi_wXEEBLOoo912q9bkil00vQP`cZ%EbO׉E;Chapter 11 Metal-casting Processes and Equipment 11.1 Introduction 305 11.2 Expendable-mold, Permanent-pattern Casting Processes 306 11.3 Expendable-mold, Expendable-pattern Casting Processes 316 11.4 Permanent-mold Casting Processes 321 11.5 Casting Techniques for Single-crystal Components 331 11.6 Rapid Solidification 333 11.7 Inspection of Castings 333 11.8 Melting Practice and Furnaces 334 11.9 Foundries and Foundry Automation 335 Case Studies: 11.1 Lost-foam Casting of Engine Blocks 318 11.2 Investment Casting of Total Knee Replacements 320 11.3 Die Casting of a Headlight Mount 327 11.4 Die Cast Magnesium Liftgate 328 • Building upon the fundamentals of solidification, fluid flow, and heat transfer described in the preceding chapter, this chapter presents the principles of industrial casting processes. • Casting processes are generally categorized as permanent-mold and expendable-mold processes; expendable-mold processes are further categorized as permanent-mold and expendable-pattern processes. • The characteristics of each process are described, together with typical applications, advantages, and limitation. • Special casting processes that produce single-crystal components as well as amorphous alloys are then described. • The chapter ends with a description of inspection techniques for castings. 304 ׉ 7cassandra://HGFrvKc1VSJiIl5r7tdhvOoAhJW9C8q71D-Aa9LvSzw`cZ%EbPcZ%EbO#בCט#u׉׉ 7cassandra://-ftYt6gopwFBgRBTWJRCMkLectVdbwgQBHpjyuFFTFk`׉ 7cassandra://R3vjEWH1deCY6C_ruONTKLRXXMp33dZuQ2IGj3xqxD8c`s׉ 7cassandra://4WUZz0LiRe1RMK4AnTu8ouCgnF6wk1GMKWKc-Ul6DYU`׉ 7cassandra://lk3VYUVGG9NNTdinlNzxLWvpLAdavdAh_1tOeAHWNxsY_L͠^cZEb}ט#u׉׉ 7cassandra://NjFN3w0b-IB0YN3_31hjphskGEotoxDQu-0UVVSlBREw`׉ 7cassandra://y3_h7Iscj2AKUDDJZqOP4heznMP4L_nSJV5rzasHpFcd`s׉ 7cassandra://PtyJ8DCC8Ix2YGYeYUgzEYjFLBi_fGPasOVZq1-uaIA`׉ 7cassandra://X0z2D7cWi7-HS8KTfv0_hHKqZR46DCDhtelB8z17mYc͈͠^cZEb~׉EIntroduction 305 Typical products made by casting: Engine blocks, crankshafts, power tool housings, turbine blades, plumbing parts, zipper teeth, dies and molds, gears, railroad wheels, propellers, and office equipment. Alternative processes: Forging, powder metallurgy, additive manufacturing, machining, and fabrication. 11.1 Introduction Metal castings were first made during the period 4000 to 3000 B.C., using stone and metal molds for casting copper. A variety of casting processes have been developed over time, each with its own characteristics and applications to meet specific design requirements (Table 11.1, see also Fig. I.6a). A very wide variety of parts and components are made by casting, such as frying pans, jewelry, engine blocks, crankshafts, automotive components and powertrains (Fig. 11.1), agricultural and railroad equipment, pipes, plumbing fixtures, power-tool housings, gun barrels, orthopedic implants, and very large components for hydraulic turbines. Four trends have had a major impact on the casting industry. (a) Mechanization and automation of the casting process, which has led to significant changes in the use of equipment and labor; advanced machinery and automated process-control systems have replaced or enhanced traditional methods of casting. (b) Increasing demand for high-quality castings with close dimensional tolerances. (c) Development of powerful modeling software, allowing predictive evaluation of dies and potential defects that result from poor design, as well as estimating a cast material’s mechanical properties and its microstructure. This Integrated Computer Materials Engineering (ICME) trend has positively affected most manufacturing processes. (d) Additive manufacturing (Chapter 20) is still evolving, but has greatly aided mold manufacturing. This chapter is organized around the major classifications of casting practices (see Fig. II.3 outlined in the Introduction to Part II), as they relate to mold materials, pattern production, molding processes, and methods of feeding the mold with molten metal. The major categories are: 1. Expendable molds, typically made of sand, plaster, ceramics, and similar materials, and generally mixed with various binders (bonding agents) for improved properties. A typical sand mold consists Process Sand Shell mold Advantages Table 11.1: Summary of Casting Processes. Limitations Almost any metal can be cast; no limit to part size, shape, or weight; low tooling cost Good dimensional accuracy and surface finish; high production rate Evaporative pattern Most metals can be cast, with no limit to size; complex part shapes Plaster mold Ceramic mold Investment Intricate part shapes; good dimensional accuracy and surface finish; low porosity Intricate part shapes; close-tolerance parts; good surface finish; low cooling rate Intricate part shapes; excellent surface finish and accuracy; almost any metal can be cast Permanent mold Good surface finish and dimensional accuracy; low porosity; high production rate Die Centrifugal Excellent dimensional accuracy and surface finish; high production rate Large cylindrical or tubular parts with good quality; high production rate Some finishing required; relatively coarse surface finish; wide tolerances Part size limited; expensive patterns and equipment Patterns have low strength and can be costly for low quantities Limited to nonferrous metals; limited part size and volume of production; mold-making time relatively long Limited part size Part size limited; expensive patterns, molds, and labor High mold cost; limited part shape and complexity; not suitable for high-melting-point metals High die cost; limited part size; generally limited to nonferrous metals; long lead time Expensive equipment; limited part shape ׉ 7cassandra://4WUZz0LiRe1RMK4AnTu8ouCgnF6wk1GMKWKc-Ul6DYU`cZ%EbQ׉E 306 Chapter 11 Metal-casting Processes and Equipment (a) (b) Figure 11.1: (a) Examples of stainless steel castings. Note the intricate part shapes. (b) Die-cast magnesium automobile wheels. Source: (a) Shutterstock/Mr.1 (b) Shutterstock/socrates471. of 90% sand, 7% clay, and 3% water. As described in Section 8.2, these materials are refractories,that is, they are capable of withstanding the high temperatures of molten metals. After the casting has solidified, the mold is broken up to remove the casting, hence the word expendable. The mold is produced from a pattern; in some processes, the mold is expendable but the pattern is reused to produce several molds; such processes are referred to as expendable-mold, permanent-pattern casting processes. On the other hand, investment casting requires a pattern for each mold produced, an example of an expendable-mold, expendable-pattern process. 2. Permanent molds, made of metals that maintain their strength at high temperatures. As the name implies, the molds are used repeatedly, and are designed in such a manner that the casting can be removed easily and the mold used for the next casting. Metal molds are better heat conductors than expendable nonmetallic molds (see Table 3.1), thus the casting is subjected to a higher rate of cooling during solidification. This in turn affects the microstructure and grain size within the casting. 3. Composite molds, made of two or more different materials (such as sand, graphite, and metal), combining the advantages of each material. These molds have a permanent and an expendable portion, and are used in some casting processes to improve mold strength, control the cooling rate, and optimize the overall economics of the casting operation. The general characteristics of sand casting and other casting processes are summarized in Table 11.2. Almost all commercial metals can be cast; the surface finish obtained is largely a function of the mold material, and can be very good, although, as expected, sand castings generally have rough, grainy surfaces. Dimensional tolerances generally are not as good as those in machining and other net-shape processes. However, intricate shapes, such as engine blocks and turbocharger impellers, can be made by casting. Because of their unique characteristics and applications, particularly in making microelectronic devices (Part V), basic crystal-growing techniques are included in this chapter, which concludes with a brief overview of modern foundries. 11.2 Expendable-mold, Permanent-pattern Casting Processes The major categories of expendable-mold, permanent-pattern casting processes are sand, shell mold, plaster mold, ceramic mold, and vacuum casting. ׉ 7cassandra://PtyJ8DCC8Ix2YGYeYUgzEYjFLBi_fGPasOVZq1-uaIA`cZ%EbRcZ%EbQ#בCט#u׉׉ 7cassandra://p4pwUn_nUKRaGvvvcoKhst2H8MJhh_vzAQ6pin1m_DAl`׉ 7cassandra://HE3FCbccQK1qxyZ3lhzr00AD1GXC8jh5jEupEj5yWz02\`s׉ 7cassandra://v00ZySRnkL15fANSB2-wyV50IDMASOjWPbOWA41t3rs `׉ 7cassandra://nYmyVW2jq592jKgfVPnADaNiKs5BuYxHbA-52zTrNQsl͠^cZEbɀט#u׉׉ 7cassandra://Ev_x1-AQQkBvPw7W9zAWEfpOERPAywv30iHR_XQmMM4`׉ 7cassandra://hn0ksBxFyxEscvQ7xO47PTAYYXqJaLRQOgNY6UFx5FY]`s׉ 7cassandra://fqul54PUf-G4Op3Zp85qNopWBipCVKC7TJI3B7UOZ20e`׉ 7cassandra://anmOEiXQMDbuqLPfrMJRXb8AkyF-xe78EA91vDxRMIc͋`8͠^cZEbɂ׉EExpendable-mold, Permanent-pattern Casting Processes 307 Table 11.2: General Characteristics of Casting Processes. Evaporative Typical materials cast Weight (kg): Minimum Maximum Typical surface finish (Ra in µm) Porosity1 Shape Complexity1 Dimensional accuracy1 Section thickness (mm): Minimum Maximum Typical dimensional tolerance (mm) Equipment Pattern/die Labor Typical lead time2 Typical production rate2 (parts/mold-hour) Minimum quantity2 Sand All 0.01 No limit 5–25 3–5 1–2 3 3 (0.25 mm for small parts) 3–5 3–5 1–3 Days 1–20 1 3 2–3 3 5–50 100 2–3 2–3 3 Weeks Weeks 1–20 500 3–5 3–5 1–2 Days 1–10 10 3–5 2–3 1–2 Weeks 1–1000 10 2 2 3 1 1 5 Weeks Weeks to months 5–50 2–200 1000 10,000 1 1 5 Months 1–1000 10–10,000 Notes: 1. Relative rating, from 1 (best) to 5 (worst). Note that, for example, a die casting has relatively low porosity, mid-to-low shape complexity, high dimensional accuracy, high equipment and die costs, and low labor costs. These ratings are general, as significant variations can occur, depending on the particular production method. 2. Approximate values, without using rapid prototyping technologies. Minimum quantity is 1 when applying rapid prototyping. Source: Data taken from J.A. Schey, Introduction to Manufacturing Processes, 3rd ed., McGraw-Hill, 2000. Shell All 0.01 100+ 1–3 4–5 2–3 2 2 pattern All 0.01 100+ 5–25 3–5 1–2 3 2 No limit — — 1.6–4 mm ±0.003 Plaster Nonferrous (Al, Mg, Zn, Cu) 0.01 50+ 1–2 4–5 1–2 2 1 — ±0.005 − 0.010 Investment All 0.001 100+ 0.3–2 5 1 1 1 75 ±0.005 Permanent mold All 0.1 300 2–6 2–3 2–3 1 2 50 ±0.015 Die Nonferrous (Al, Mg, Zn, Cu) ¡0.01 1–2 50 1–3 3–4 1 0.5 12 ±0.001 − 0.005 Centrifugal All 0.01 5000+ 2–10 1–2 3–4 3 2 100 0.015 ׉ 7cassandra://v00ZySRnkL15fANSB2-wyV50IDMASOjWPbOWA41t3rs `cZ%EbS׉E *308 Chapter 11 Metal-casting Processes and Equipment 11.2.1 Sand Casting The traditional method of casting metals is using sand molds, and has been done for millennia. Sand casting is still the most prevalent form of casting, led by China (about 45 million metric tons per year), followed by India and the U.S. Typical applications of sand casting include machine bases, large turbine impellers, propellers, and plumbing fixtures. The capabilities of sand casting are given in Table 11.2. Sand casting basically consists of (a) placing a pattern, having the shape of the part to be cast, in sand to make an imprint, (b) incorporating a gating system for molten metal flow, (c) removing the pattern and filling the mold cavity with molten metal, (d) allowing the metal to cool until it solidifies, (e) breaking away the sand mold, and (f) removing the casting (Fig. 11.2). Alternatively, a pattern can be machined directly into a sand preform. Since the strength of sand arises from its binders, machining can take place at high removal rates and can produce molds of high quality. An important and more recent development in mold and pattern making is the application of additive manufacturing (see Chapter 20) to directly produce molds. In sand casting, for example, a pattern can be binder-jet printed with complex shapes and self-supported cores to produce hollow sections, greatly easing mold assembly. There are several rapid prototyping techniques applicable to casting, and can produce molds or patterns; they are best suited for small production runs. Sands. Most sand-casting operations use silica sand (SiO2) as the mold material, although alternative sands and binders are under development, because of challenging health concerns associated with silica exposure in foundries. Sand is inexpensive and is suitable as a mold material because of its high-temperature characteristics and high melting point. There are two general types of sand: naturally bonded (bank sand)and synthetic (lake sand). Because its composition can be controlled more accurately, synthetic sand is preferred by most foundries. Several factors are important in the selection of sand for molds, and certain tradeoffs with respect to properties have to be considered. Sand having fine, round grains can be packed closely, forming a smooth mold surface. Although fine-grained sand enhances mold strength, the fine grains also lower mold permeability. Good permeability of molds and cores allows gases and steam evolved during the casting process to escape easily. The mold also should have good collapsibility, in order to allow the casting to shrink while it is cooling, and thus prevent defects in the casting, such as hot tearing and cracking, shown in Fig. 10.14. Pattern Sand Mold Heat Melting of metal Pouring into mold Solidification and cooling Furnaces Shakeout and removal of risers and gates treatment Cleaning and finishing Additional heat treatment Defects, Pressure tightness, Dimensions Figure 11.2: Outline of production steps in a typical sand-casting operation. Inspection Pattern making Core making Gating system ׉ 7cassandra://fqul54PUf-G4Op3Zp85qNopWBipCVKC7TJI3B7UOZ20e`cZ%EbTcZ%EbS#בCט#u׉׉ 7cassandra://VTSNTFkssP_jOSQZgYMFkPexq6BX2TB_WdaCNpbzdEE̷`׉ 7cassandra://VX4ZBToK3Qn8fRWGzUUtNj97akHUgKE5LRFEDJ54Rd4Y`s׉ 7cassandra://UxhRxN6Lqlz-vty_lC2ZAtOcJarHbJJbvSqnz0d1jAM`׉ 7cassandra://e01XSd8rew1NMst9k-4wVuqbVmJpo5Rvhpw8KT2de4c) ͠^cZEbɉט#u׉׉ 7cassandra://nZ8i9tsQCOmRMCo9mzepmQCXfLA-c4gbtB9fOsIvlewG`׉ 7cassandra://D4HzsqTnuAzgTND9TMcvvnCNZuWkLgaNf9YRos2VGtET`s׉ 7cassandra://CKsYdnyM0SdeGu9Q0DGwT6Ie9vb0Kwxj5L2lmgGO6gE`׉ 7cassandra://WlBXcLzZRWS7unY2Jtbud3WR_eIZ_YRSf2kNwi1RvG4=͠^cZEbɊ׉E Expendable-mold, Permanent-pattern Casting Processes 309 Types of Sand Molds. Sand molds (Fig. 11.3) are characterized by the types of sand and the methods used to produce them. There are three basic types of sand molds: (a) green-sand, (b) cold-box, and (c) no-bake molds. The most common mold material is green molding sand, a mixture of sand, clay, and water. The term green refers to the fact that the sand in the mold is moist or damp while the metal is being poured into it. Green-sand molding is the least expensive method of making molds, and the sand is recycled easily for subsequent reuse. In the skin-dried method, the mold surfaces are dried, either by storing the mold in air or by drying it with torches. Because of their higher strength, these molds are generally used for large castings. In the cold-box mold process, various organic and inorganic binders are blended into the sand to bond the grains chemically for greater strength. These molds are more dimensionally accurate than green-sand molds, but are more expensive. In the no-bake mold process, a synthetic liquid resin is mixed with the sand, and the mixture hardens at room temperature. Because the bonding of the mold in this and in the cold-box process takes place without applying heat, they are called cold-setting processes. Sand molds can be oven dried (baked) prior to pouring the molten metal; they then become stronger than green-sand molds and impart better dimensional accuracy and surface finish to the casting. However, this method has the drawbacks that (a) distortion of the mold is greater, (b) the castings are more susceptible to hot tearing, because of the lower collapsibility of the mold, and (c) production rate is lower, because of the significant drying time required. The major features of molds in sand casting are: 1. The flask, which supports the mold itself. Two-piece molds consist of a cope on top and a drag on the bottom; the seam between them is the parting line. When more than two pieces are used in a sand mold, the additional parts are called cheeks. 2. A pouring basin or pouring cup, into which the molten metal is poured. 3. A sprue, through which the molten metal flows downward by gravity. 4. The runner system, which has channels that carry the molten metal from the sprue to the mold cavity. Gates are the inlets into the mold cavity. 5. Risers, which supply additional molten metal to the casting as it shrinks during solidification. Two types of risers, a blind riser and an open riser, are shown in Fig. 11.3. Open riser Pouring basin (cup) Vent Cope Core (sand) Drag Mold cavity Gate Well Runner Sand Blind riser Flask Sand Sprue Choke Parting line Figure 11.3: Schematic illustration of a sand mold, showing various features. ׉ 7cassandra://UxhRxN6Lqlz-vty_lC2ZAtOcJarHbJJbvSqnz0d1jAM`cZ%EbU׉E P310 Chapter 11 Metal-casting Processes and Equipment 6. Cores, which are inserts made from sand and placed in the mold to form hollow regions or otherwise define the interior surface of the casting. Cores also are used on the outside of the casting to shape features, such as lettering and numbering. 7. Vents, which are placed in molds to carry off gases produced when the molten metal comes into contact with the sand in the mold and the core. They also exhaust air from the mold cavity as the molten metal flows into the mold. Patterns. Patterns are used to mold the sand mixture into the shape of the casting, and may be made of wood, plastic, metal, or a combination of materials. Their selection depends on the size and shape of the casting, the dimensional accuracy and the quantity of castings required, and the molding process. Because patterns are used repeatedly to make molds, the strength and durability of the material selected for a pattern must reflect the number of castings that the mold is expected to produce. Patterns made of a combination of materials reduce wear in critical regions; they usually are also coated with a parting agent to facilitate the removal of the casting from the mold. Patterns can be designed with a variety of features to fit specific applications and economic requirements. One-piece patterns, also called loose or solid patterns, are generally used for simpler shapes and low-quantity production; they generally are made of wood and are inexpensive. Split patterns have two pieces, made in such a way whereby each part forms a portion of the cavity for the casting; in this way, castings with complicated shapes can be produced. Match-plate patterns are a common type of mounted pattern in which two-piece patterns are constructed by securing each half of one or more split patterns to the opposite sides of a single plate (Fig. 11.4). In such constructions, the gating system can be mounted on the drag side of the pattern. This type is used most often in conjunction with molding machines and for large production runs for producing smaller castings. Pattern design is a critical aspect of the total casting operation. The design should provide for metal shrinkage, permit proper metal flow in the mold cavity, and allow the pattern to be easily removed from the sand mold by means of a taper or draft (Fig. 11.5) or some other geometric feature (see also Chapter 12). Cope side Plate Drag side Figure 11.4: A typical metal match-plate pattern used in sand casting. Damage Pattern Draft angle Flask Sand mold Poor Good Figure 11.5: Taper on patterns for ease of removal from the sand mold. ׉ 7cassandra://CKsYdnyM0SdeGu9Q0DGwT6Ie9vb0Kwxj5L2lmgGO6gE`cZ%EbVcZ%EbU#בCט#u׉׉ 7cassandra://fBWMOt2F-KUB-mqOeQ0W8PSgoJ1_EE-vnIlRP_bTQlw o`׉ 7cassandra://muLRo5Yv27BcBR-BoZUSQFBWd_LZvkL2OlWJAcbAec8o `s׉ 7cassandra://NaWLTRWtRE8sgXCozv8NMrDH81Os68LaGkcYhFLH9SY`׉ 7cassandra://PUXzTh93bwGR9FeCEismyziAArIX0e9koBMS8H0uP9c>͠^cZEbɌט#u׉׉ 7cassandra://SMavcMaDhWjtB7Xgi6DoXPZthJA5L-2asErITRz42e4"P`׉ 7cassandra://tDyNPfPPkTKU2ZdaVuvoUh712r5pYsg46MD9JG5IeVIE`s׉ 7cassandra://3GlaWGRuBzhT7C0yU67exS0af-1W7Auh-SrJujugrZo\`׉ 7cassandra://7ojZ4zWqtn8iQk8bhhPMtplPAJc08ZBhYASDJ8X5jkQ*~͠^cZEbɍ׉EExpendable-mold, Permanent-pattern Casting Processes 311 Cavity Core Cavity Chaplet Core Core prints Parting line Mold (a) (b) Figure 11.6: Examples of sand cores, showing core prints and chaplets to support the cores. Cores. For castings with internal cavities or passageways, such as those found in automotive engine blocks or valve bodies, cores are placed in the mold cavity to form the interior surfaces of the casting. They are then removed from the finished part during shakeout and further processing. Like molds, cores must possess strength, permeability, collapsibility, and the ability to withstand heat; hence, they are made of sand aggregates. The core is anchored by core prints, geometric features added to the pattern in order to locate and support the core and to provide vents for the escape of gases (Fig. 11.6a). A common difficulty with cores is that, for some casting requirements (as in the case where a recess is required), they may lack sufficient structural support in the cavity. To keep the core from shifting, metal supports (chaplets) may be used to anchor the core in place (Fig. 11.6b). Cores are generally made in a manner similar to that used in sand mold making; most are made using shell (see Section 11.2.2), no-bake, or cold-box processes. Cores are shaped in core boxes and used in much the same way that patterns are used to form sand molds. Sand-molding Machines. The oldest known method of molding, which is still used for simple castings and for small production runs, is to compact the sand by hand hammering (tamping) or ramming it around the pattern. For most operations, the sand mixture is compacted around the pattern by molding machines. These machines manipulate the mold in a controlled manner, offer high-quality casting by improving the application and distribution of forces, and increase production rate. In vertical flaskless molding, the pattern halves form a vertical chamber wall against which sand is blown and compacted (Fig. 11.7). The mold halves are then packed horizontally, with the parting line oriented vertically and moved along a pouring conveyor. The operation is simple and eliminates the need to handle flasks, allowing for very high production rates, particularly when other aspects of the operation (such as coring and pouring) are all automated. Sandslingers fill the flask uniformly with sand under a high-pressure stream; often automated, they are used to fill large flasks. An impeller in the machine throws sand from its blades or cups at such high speeds that the machine not only places the sand but also rams it sufficiently for proper packing. In impact molding, the sand is compacted by a controlled explosion or instantaneous release of compressed gases. This method produces molds with uniform strength and good permeability. In vacuum molding (also known as the Vprocess), shown in Fig. 11.8, the pattern is covered tightly with a thin sheet of plastic. A flask is placed over the covered pattern and is then filled with dry, binderless sand. A second sheet of plastic is then placed on top of the sand, and a vacuum action compacts the sand. Both halves of the mold are made in this manner and are subsequently assembled. During pouring of the molten metal, the mold remains under a vacuum, but not the casting cavity. When the hot metal has solidified, the vacuum is turned off and the sand falls away, releasing the casting. As shown in Fig. 11.8, vacuum molding does not require a draft in the part, and can be very economical because of the low tooling costs, long pattern life, and absence of binders in the sand (also simplifying sand recovery and reuse). Vacuum molding produces castings with high surface detail and dimensional accuracy; it is suited especially well for large, relatively flat (plane) castings. Core prints ׉ 7cassandra://NaWLTRWtRE8sgXCozv8NMrDH81Os68LaGkcYhFLH9SY`cZ%EbW׉E312 Chapter 11 Metal-casting Processes and Equipment Metal poured here Box Ram force Sand (a) (b) (c) Figure 11.7: Vertical flaskless molding. (a) Sand is squeezed between two halves of the pattern. (b) Assembled molds pass along an assembly line for pouring. (c) A photograph of a vertical flaskless molding line. Source: Courtesy of American Foundry Society. Heating element Pattern Plastic sheet Vacuum Vacuum (a) Sand (b) Vacuum Pouring basin and sprue Second plastic sheet Pattern Cope Ladle Vacuum Vacuum Drag (c) (d) Figure 11.8: The vacuum molding process. (a) A plastic sheet is first thermoformed (see Section 19.6) over a pattern; (b) a vacuum flask is then placed over the pattern, a pouring basin/sprue insert is located, and the flask is filled with sand. A second sheet is located on the top of the sand mold, and a vacuum is applied to tightly compact the sand against the pattern. (c) A drag is also produced, along with cheeks, cores, etc., as in conventional sand casting. The cope and drag can be carefully transported without vacuum being applied. (d) After the mold halves are joined, a vacuum is applied to ensure mold strength, and molten metal is poured into the mold. Vacuum Vacuum ׉ 7cassandra://3GlaWGRuBzhT7C0yU67exS0af-1W7Auh-SrJujugrZo\`cZ%EbXcZ%EbW#בCט#u׉׉ 7cassandra://j39gKSpUlFFmir84BfN4_9dACleoGcC-HTSaVtyWsvg>`׉ 7cassandra://MV1G6tgL40cdjQoYnnzX-5S82RSNDqx6dcIlTvMwaFA_L`s׉ 7cassandra://sm_Hg72B-N2u_KqHyzA7kipeOLnIBgQlg0sUJ_5EH9Q`׉ 7cassandra://Yv2REZv4jxOVLsl2NAGBIwEVpTR-qXuB3Kjf0-Y47fw[~͠^cZEbɖט#u׉׉ 7cassandra://jJGCiEkVEymkImSue5RQuKZCYgrvxdwgdNHrVIJdqcQ`׉ 7cassandra://E0ZYoVbBo1Ax7zAn0DvgCA0FZ9UGJwLHElClq4q9n0wtE`s׉ 7cassandra://nWzQPN__iem7snMEwZ4m4Dgxy8Znk4pYRwniviySdsY*`׉ 7cassandra://V9AgFeKs4redt8jcbnN1VoQyBllgjff7UXL9mhpf0mM_͠^cZEbɗ׉E Expendable-mold, Permanent-pattern Casting Processes 313 The Sand-casting Operation. After the mold has been shaped and the cores have been placed in their positions, the two mold halves (cope and drag) are closed, clamped, and weighted down, to prevent the separation of the mold sections under the pressure exerted when the molten metal is poured into the mold cavity. A complete sequence of operations in sand casting is shown in Fig. 11.9. After solidification, the casting is shaken out of its mold, and the sand and oxide layers adhering to the casting are removed (by means of vibration, using a shaker, or by sand blasting). Castings are also cleaned Core prints Core prints Mechanical drawing of part (a) Sprue Risers Flask Cope after ramming with sand and Core halves pasted together (e) Cope ready for sand (f) removing pattern, sprue, and risers (g) Drag ready for sand (h) Drag after removing pattern (i) Cope pattern plate (b) Gate Drag pattern plate (c) Core boxes (d) Cope Drag Closing pins Drag with core set in place (j) Cope and drag assembled and ready for pouring (k) Casting as removed from mold; heat treated (l) Casting ready for shipment (m) Figure 11.9: Schematic illustration of the sequence of operations for sand casting. (a) A mechanical drawing or CAD representation of the part is used to generate a design for the pattern. Considerations such as part shrinkage and draft must be included into the drawing. (b–c) Patterns have been mounted on plates equipped with pins for alignment. Note also the presence of core prints designed to hold the core in place. (d–e) Core boxes produce core halves, which are pasted together. The cores will be used to produce the hollow area of the part shown in (a). (f) The cope half of the mold is assembled by securing the cope pattern plate to the flask with aligning pins and attaching inserts to form the sprue and risers. (g) The flask is rammed with sand, and the plate and inserts are removed. (h) The drag half is produced in a similar manner, with the pattern inserted. A bottom board is placed below the drag and is aligned with pins. (i) The pattern, flask, and bottom board are inverted, and the pattern is withdrawn, leaving the appropriate imprint. (j) The core is set in place within the drag cavity. (k) The mold is closed by placing the cope on top of the drag and securing the assembly with pins. The flasks are then subjected to pressure to counteract buoyant forces in the molten metal, which might lift the cope. (l) After the metal solidifies, the casting is removed from the mold. (m) The sprue and risers are cut off and recycled, and the casting is cleaned, inspected, and heat treated (if necessary). Source: Courtesy of Steel Founders’ Society of America. ׉ 7cassandra://sm_Hg72B-N2u_KqHyzA7kipeOLnIBgQlg0sUJ_5EH9Q`cZ%EbY׉E_314 Chapter 11 Metal-casting Processes and Equipment by blasting with steel shot or grit (shot blasting; Section 26.8). The risers and gates are cut off either by oxyfuel-gas cutting, sawing, shearing, or abrasive wheels; they may also be trimmed in appropriate dies. Gates and risers on steel castings may also be removed with air carbon-arc cutting (Section 30.8) or torches. Castings may be further cleaned by electrochemical means or by pickling with chemicals to remove surface oxides (see Section 34.16). The casting may subsequently be heat treated (Chapter 4) to improve certain properties required for its intended use; heat-treatment is particularly important for steel castings. Finishing operations may involve machining, straightening, or forging with dies (sizing) to obtain final dimensions. Inspection is an important final step, and is carried out to ensure that the casting meets all design and quality-control requirements. Rammed-graphite Molding. In this process, rammed graphite (Section 8.6) is used to make molds for casting reactive metals, such as titanium and zirconium; sand cannot be used because these metals react vigorously with silica. The molds are packed like sand molds, air dried, baked at 175◦C, fired at 870◦C, and then stored under controlled humidity and temperature. The casting procedures are similar to those for sand molds. Mold Ablation. Ablation can be used to improve the mechanical properties and production rates in sand casting. In this process, a sand mold is filled with molten metal, and the mold is then immediately sprayed with a liquid and/or gas solvent to progressively erode the sand. As the mold is exposed, the liquid stream causes rapid and directional solidification of the metal.With properly designed risers, mold ablation results in significantly lower porosity than conventional sand casting, leading to higher strength and ductility; it has therefore been applied to normally difficult-to-cast materials or for metal-matrix composites. Since ablation speeds up solidification and also removes cores, significant productivity improvements can also be achieved. 11.2.2 Shell Molding Shell molding, first developed in the 1940s, has grown significantly because it can produce numerous types of castings with close dimensional tolerances and good surface finish, and at low cost. Shell-molding applications include small mechanical parts requiring high precision, such as gear housings, cylinder heads, and connecting rods. The process is also used widely in producing high-precision molding cores. The capabilities of shell-mold casting are given in Table 11.2. In this process, a mounted pattern, made of a ferrous metal or aluminum, is (a) heated to a range of 175◦–370◦C, (b) coated with a parting agent (such as silicone), and (c) clamped to a box or chamber. The box contains fine sand, mixed with 2.5–4% of a thermosetting resin binder (such as phenol-formaldehyde), which coats the sand particles. Either the box is rotated upside down (Fig. 11.10) or the sand mixture is blown over the pattern, allowing it to form a coating. The assembly is then placed in an oven for a short period of time to complete curing of the resin. In most shell-molding machines, the oven consists of a metal box, with gas-fired burners that swing over the shell mold and cure it. The shell hardens around the pattern and is removed by means of built-in ejector pins. Two half-shells are made in this manner and are bonded or clamped together to form a mold. The thickness of the shell can be determined accurately by controlling the time that the pattern is in contact with the mold. In this way, the shell can be formed with the required strength and rigidity to hold the weight of the molten liquid. The shells are light and thin, usually 5 to 10 mm, and, consequently, their thermal characteristics are different from those for thicker molds. Since a much smaller grain size is used in shell molding, shell sand has a much lower permeability than the sand for green-sand molding. The decomposition of the shell-sand binder produces a high volume of gas; consequently, unless the molds are vented properly, trapped air and gas can produce defects in shell molding of ferrous castings. The high quality of the finished casting can reduce cleaning, machining, and other finishing costs significantly. Complex shapes can be produced with less labor, and the process can be automated. ׉ 7cassandra://nWzQPN__iem7snMEwZ4m4Dgxy8Znk4pYRwniviySdsY*`cZ%EbZcZ%EbY#בCט#u׉׉ 7cassandra://_yacy9LYP2USImSbGUWjYBJq15hCRlruKwlJ8slb7rs`׉ 7cassandra://FTuZUL4vyrBN2SuWS3t7B6Yz1OJkTdBvj_ZtN0Ib8Ys`"`s׉ 7cassandra://g9_wkRjMNB_Ub2OeUD1KbjjyJGEACMZYloIVKbnPiwk)`׉ 7cassandra://KdJn0UXVKcbT28TmLSM_WXloGgMzPmE41BZItzEM81oLH͠^cZEbɚט#u׉׉ 7cassandra://nC_tBma7Af0H3jvxGpEiOz7LPTLG-XuQ8hfLjtLAxJIR`׉ 7cassandra://YmsKGKuC6KaZ0hPazM59zbDzp42_px7QOLvKt0QudSIi(`s׉ 7cassandra://wKS6dOQZA1HAXSbG86RgdwkTqoRZCmADjjdAuAYfum0`׉ 7cassandra://o7V8f8pAeTtVEGxFXql_2GiSvhvxyIvKaBYqikubjgQ"P͠^cZEbɜ׉E Expendable-mold, Permanent-pattern Casting Processes 315 Pattern Coated sand Coated sand Pattern Coated sand 1. Pattern rotated and clamped to dump box Dump box 2. Pattern and dump box rotated 3. Pattern and dump box in position for the investment Investment Shell Flask Shells Excess coated sand 4. Pattern and shell removed from dump box Adhesive 5. Mold halves joined together Sand or Clamps metal beads 6. Mold placed in flask and metal poured Figure 11.10: The shell-molding process, also called the dump-box technique. 11.2.3 Plaster-mold Casting This process, and the ceramic-mold and investment casting processes (described in Sections 11.2.4 and 11.3.2) are known as precision casting, because of the high dimensional accuracy and good surface finish obtained. Typical parts made are lock components, gears, valves, fittings, tooling, and ornaments. They weigh in the range of 125–250 g, although parts as light as 1 g have been made. The capabilities of plastermold casting are given in Table 11.2. In the plaster-molding process, the mold is made of plaster of paris (gypsum or calcium sulfate), with the addition of talc and silica powder to improve strength and to control the time required for the plaster to set. The three components are mixed with water and the slurry is poured over the pattern. After the plaster sets, usually within 15 minutes, it is removed and the mold is dried, at a typical temperature range of 120◦ to 260◦C. The mold halves are then assembled to form the mold cavity and are preheated to about 120◦C. The molten metal is then poured into the mold. Because plaster molds have very low permeability, gases evolved during solidification of the metal cannot escape; consequently, the molten metal is poured either in a vacuum or under pressure. Mold permeability can be increased significantly by the Antioch process, in which the molds are dehydrated in an autoclave (pressurized oven) for 6 to 12 hours, and then rehydrated in air for 14 hours. Another method of increasing the permeability of the mold is to use foamed plaster, containing trapped air bubbles. Patterns for plaster molding are generally made of aluminum alloys or thermosetting plastics, but brass or zinc alloys are also used. Since there is a limit to the maximum temperature that the plaster mold can withstand (generally about 1200◦C), plaster-mold casting is used only for aluminum, magnesium, zinc, and some copper-based alloys. The castings have a good surface finish with fine details. Also, because plaster molds have lower thermal conductivity than other mold materials, the castings cool slowly; a more uniform grain structure is obtained and with less warpage. The wall thickness of the parts made can be as thin as 1 to 2.5 mm. ׉ 7cassandra://g9_wkRjMNB_Ub2OeUD1KbjjyJGEACMZYloIVKbnPiwk)`cZ%Eb[׉E 316 Chapter 11 Metal-casting Processes and Equipment Transfer bowl Ceramic slurry Pattern Flask 1. Pouring slurry Green mold Torch Pattern Plate Flask 2. Stripping green mold Mold 3. Burn-off Figure 11.11: Sequence of operations in making a ceramic mold. Source: Metals Handbook,Vol.5,8thed,ASM International, 1970. 11.2.4 Ceramic-mold Casting This process, also called cope-and-drag investment casting, is similar to the plaster-mold process, except that it uses refractory mold materials suitable for high-temperature applications. A slurry is first produced from a mixture of fine-grained zircon (ZrSiO4), aluminum oxide, fused silica, and bonding agents; this slurry is then poured over the pattern (Fig. 11.11) which has been placed in a flask. Typical parts made are impellers, cutters for machining operations, dies for metalworking operations, and molds for casting plastic and rubber components. Parts weighing as much as 700 kg have been cast by this process. After setting, the molds (ceramic facings) are removed, dried, ignited to burn off volatile matter, and baked. The molds are then clamped firmly and used as all-ceramic molds. In the Shaw process, the ceramic facings are backed by fireclay (which resists high temperatures) to give strength to the mold. The facings are later assembled into a complete mold, ready to be used. The high-temperature resistance of refractory molding materials allows the molds to be used for casting ferrous and other high-temperature alloys, stainless steels, and tool steels. Although the process is somewhat expensive, the castings have good dimensional accuracy and surface finish over a wide range of sizes and intricate shapes. 11.3 Expendable-mold, Expendable-pattern Casting Processes Evaporative-pattern and investment casting are also referred to as expendable-pattern casting processes or expendable mold–expendable pattern processes. They are unique in that a mold and a pattern has to be produced for each casting, whereas the patterns in the processes described in the preceding section are all reusable. Typical applications of these processes are cylinder heads, engine blocks, crankshafts, brake components, and machine bases. 11.3.1 Evaporative-pattern Casting (Lost-foam Process) The evaporative-pattern casting (EPC) process uses a polystyrene pattern, which evaporates upon contact with molten metal to form a cavity for the casting; this process is also known as lost-foam casting,orthe full-mold casting (FMC) process. It has become one of the more important casting processes for ferrous and nonferrous metals, particularly for the automotive industry. In this process, polystyrene beads, containing 5–8% pentane (a volatile hydrocarbon), are placed in a preheated die that is usually made of aluminum. Complex patterns may be made by bonding various individual pattern sections, using a hot-melt adhesive (Section 32.4.1). Polymethylmethacrylate (PMMA) and polyalkylene carbonate also may be used as pattern materials for ferrous castings. The polystyrene expands and takes the shape of the die cavity; additional heat is applied to fuse and bond the beads together. The die is cooled and opened, and the polystyrene pattern is removed. The pattern is then coated with water-based refractory slurry, dried, and placed in a flask. The flask is filled with loose, ׉ 7cassandra://wKS6dOQZA1HAXSbG86RgdwkTqoRZCmADjjdAuAYfum0`cZ%Eb\cZ%Eb[#בCט#u׉׉ 7cassandra://WRvutIpadJ5Fm5yp53g5FrL3z19e7TOAabTWtcwC0XQY`׉ 7cassandra://Ban789SRqtHyx_8ocME9FUvG0zjicF4anHSOzRiIBW0R(`s׉ 7cassandra://xfMBFOAYi358MiUpPcUU9ze1oHKOByTdreicKdpVM34`׉ 7cassandra://nYqP7GrX17yTraHIJZxRxPbuxNf8oVKukQJwlcvnRw0x]͠^cZEbɠט#u׉׉ 7cassandra://nAQUI6JrrttOqgdhPjGHexfn0Em_jUI8WejHP6RFX28#`׉ 7cassandra://A0wf0uPx8zG2cXmJ57AjGU45jLcmVSRxBmraIIFgA9gp`s׉ 7cassandra://oxYBEHkTPr_Z0k_um2YvSODJElf1SEMaMsWCSWHIUew`׉ 7cassandra://0eQoRmTsBMQteIdZ17YIMAapd1taUKAAYXHkLC8sb9wh͠^cZEbɡ׉E[Expendable-mold, Expendable-pattern Casting Processes 317 1. Pattern molding 2. Cluster assembly 3. Coating Cluster Parts 4. Compacted in sand 5. Casting 6. Shakeout Figure 11.12: Schematic illustration of the expendable-pattern casting process, also known as lost-foam or evaporative-pattern casting. fine sand, which surrounds and supports the pattern (Fig. 11.12), and may be dried or mixed with bonding agents to give it additional strength. The sand is compacted periodically, without removing the polystyrene pattern; then the molten metal is poured into the mold. The molten metal vaporizes the pattern and fills the mold cavity, completely replacing the space previously occupied by the polystyrene. Any degradation by-products from the polystyrene are vented into the surrounding sand. Because the polymer requires considerable energy to degrade, large thermal gradients are present at the metal–polymer interfaces. In other words, the molten metal cools faster than it would if it were poured directly into an empty cavity; consequently, fluidity is less than in sand casting. This has important effects on the microstructure throughout the casting, and also leads to directional (columnar) solidification of the metal (see Section 10.2.3). The evaporative-pattern process has several advantages over other casting methods: • The process is relatively simple, because there are no parting lines, cores, or riser systems. • The flasks used for the process are inexpensive. • Polystyrene is inexpensive, and can be processed easily into patterns having complex shapes, various sizes, and fine surface detail. • The casting requires minimal finishing and cleaning operations. • The process can be automated and is economical for long production runs; however, the cost of producing the die and the need for two sets of tooling are significant factors to consider. ׉ 7cassandra://xfMBFOAYi358MiUpPcUU9ze1oHKOByTdreicKdpVM34`cZ%Eb]׉E2318 Chapter 11 Metal-casting Processes and Equipment In a modification of the evaporative-pattern process, called the Replicast R C-S process, a polystyrene pattern is surrounded by a ceramic shell; then the pattern is burned out prior to pouring the molten metal into the mold. Its principal advantage over investment casting (which uses wax patterns, Section 11.3.2) is that carbon pickup into the metal is avoided. Further developments in EPC include the production of metalmatrix composites (Sections 9.5 and 19.14). During molding of the polymer pattern, fibers or particles are embedded throughout the part, which then become an integral part of the casting. Other techniques include the modification and grain refinement of the casting, by using grain refiners and modifier master alloys. Case Study 11.1 Lost-foam Casting of Engine Blocks One of the most important components in an internal combustion engine is the engine block. Industry trends have focused upon high-quality, low-cost and lightweight designs. Economic benefits can be gained through casting more complex geometries and by incorporating multiple components into one part. Recognizing that EPC can simultaneously satisfy all of these requirements, Mercury Castings built a lost-foam casting line to produce aluminum engine blocks and cylinder heads. One example of a part produced through lost-foam casting is a 45-kW, three-cylinder engine block used for marine applications, such as an outboard motor on a small boat, and illustrated in Fig. 11.13c. Previously manufactured as eight separate die castings, the block was converted to a single 10-kg casting, with a weight and cost savings of 1 kg and $25, respectively, on each block. The casting chosen also allowed consolidation of the engine’s cylinder head and the exhaust and cooling systems into the block, thus eliminating the associated machining operations and fasteners required in sand-cast or die-cast designs. Moreover, since the pattern contained holes, which could be cast without the need for cores, numerous drilling operations were eliminated. Mercury Marine also was in the midst of developing a new V6 engine, utilizing a new corrosionresistant aluminum alloy with increased wear resistance. This engine design also required the integration of the cylinder block and the engine head, featuring hollow sections for water jacket cooling that could not be cored out in die casting or semipermanent mold processes (which were used for other V6 blocks). Based on the success that the foundry had with the three-cylinder lost-foam block, engineers applied this process for casting the V6 die block (Fig. 11.13b). The new engine block involves only one casting, that is lighter and less expensive than the previous designs. Produced with an integrated cylinder head and exhaust and cooling system, this component is cast hollow to develop more efficient water jacket cooling of the engine during its operation. The company also developed a pressurized lost-foam process. First, a foam pattern is made, placed in a flask, and surrounded by sand. Then the flask is inserted into a pressure vessel, where a robot pours molten aluminum onto the polystyrene pattern. A lid on the pressure vessel is closed, and a pressure of 1 MPa is applied to the casting until it solidifies, in about 15 minutes. The result is a casting with better dimensional accuracy, lower porosity, and improved strength compared to conventional lost-foam casting. Source: Courtesy of Mercury Marine. 11.3.2 Investment Casting The investment-casting process, also called the lost-wax process, was first used during the period from 4000 to 3000 B.C. Typical parts made are components for office equipment and mechanical components, such as gears, cams, valves, and ratchets. Parts up to 1.5 m in diameter and weighing as much as 1140 kg have been cast successfully. The capabilities of investment casting are given in Table 11.3. The sequence of operations involved in investment casting is shown in Fig. 11.14. The pattern is made of wax or of a plastic, such as polystyrene, by molding or rapid-prototyping techniques (Chapter 20). It ׉ 7cassandra://oxYBEHkTPr_Z0k_um2YvSODJElf1SEMaMsWCSWHIUew`cZ%Eb^cZ%Eb]#בCט#u׉׉ 7cassandra://NwSg02D7OjMZfHjJb7kvvE7ef2sBrYfCVuISnUEnsgY`׉ 7cassandra://JdDb2iVBxkbltfP-30msdnpgNf6b0aaq4eJRmcwF_2cK[`s׉ 7cassandra://7FhqKQBaCsKcSLdvRGzAdmA-XkQ3EzqNhmRU6IkgXI0)`׉ 7cassandra://Y7rGi3An9IX2_PjuE1bMY32aqe_ouUFTx0TB7wm6al4T͠^cZEbɣט#u׉׉ 7cassandra://_24OzkrAKuM62suuRsL3y3FO1y14oFzhk9nrs9YlMqw`׉ 7cassandra://WBaclhYiUQgg1CpydEGHY50p8yOHPGOAeXzIWTZ6r8Iw`s׉ 7cassandra://_Hmh2v94I_v8DFezpRLU5QQ_qieqS6zTq7rMn74_wmUF`׉ 7cassandra://erNW-jXNtTZ8g0aDp6WxzxzUtcZt1Km9mxKZDLofyjsbZ͠^cZEbɤנcZEbɦ8́D9ׁHhttp://casting.Itׁ ׁ Ј׉E9Expendable-mold, Expendable-pattern Casting Processes 319 (a) (b) (c) Figure 11.13: (a) Metal is poured into a mold for lost-foam casting of a 45-kW, three-cylinder marine engine; (b) finished engine block; (c) completed outboard motor. Source: Mercury Marine, a division of Brunswick Corporation. Mold to make pattern Wax pattern 1. Injecting wax or plastic pattern 2. Ejecting pattern Autoclaved Heat Heat Casting Molten wax or plastic 6. Completed mold 7. Pattern melt-out 8. Pouring 9. Shakeout 10. Pattern Figure 11.14: Schematic illustration of the investment-casting (lost-wax) process. Castings produced by this method can be made with very fine detail and from a variety of metals. Source: Courtesy of Steel Founders’ Society of America. 3. Pattern assembly (Tree) 4. Slurry coating Molten metal 5. Stucco coating ׉ 7cassandra://7FhqKQBaCsKcSLdvRGzAdmA-XkQ3EzqNhmRU6IkgXI0)`cZ%Eb_׉E320 Chapter 11 Metal-casting Processes and Equipment is then dipped into a slurry of refractory material, such as very fine silica and binders, including water, ethyl silicate, and acids. After this initial coating has dried, the pattern is coated repeatedly to increase its thickness, for higher strength. Note that smaller particles can be used for the initial coating to develop a better surface finish in the casting; subsequent layers use larger particles and are intended to increase the coating thickness quickly. The term investment derives from the fact that the pattern is invested (surrounded) with the refractory material. Wax patterns require careful handling because they are not sufficiently strong to withstand the forces encountered during mold making; unlike plastic patterns, however, wax can be recovered and reused. The one-piece mold is then dried in air and heated to a temperature of 90◦ to 175◦C. It is held in an inverted position for a few hours to melt out the wax. The mold is then fired to 650◦ to 1050◦C for about four hours (depending on the metal to be cast), to drive off the water of crystallization (chemically combined water) and to burn off any residual wax. After the metal has been poured and has solidified, the mold is broken up and the casting is removed. A number of patterns can be joined together to make one mold, called a tree (Fig. 11.14), significantly increasing the production rate. For small parts, the tree can be inserted into a permeable flask and filled with a liquid slurry. The investment is then placed into a chamber and evacuated (to remove any air bubbles) until the mold solidifies. The flask is usually placed in a vacuum-casting machine, so that the molten metal is drawn into the permeable mold and onto the part, thus producing fine detail. Although the mold materials and the labor involved make the lost-wax process costly, it is suitable for casting high-melting-point alloys, with good surface finish and close dimensional tolerances. Few or no finishing operations are required, which otherwise would add significantly to cost of the casting. The process is capable of producing intricate shapes from a wide variety of ferrous and nonferrous metals and alloys, with parts weighing from 1 g to 35 kg. Advances include the casting of titanium aircraft-engine and structural airframe components, with wall thicknesses on the order of 1.5 mm, thus competing with previously used sheet-metal structures (see Chapter 40). Ceramic-shell Investment Casting. A variation of the investment-casting process is ceramic-shell casting.It uses the same type of wax or plastic patterns, which is first dipped in ethyl silicate gel and, subsequently into a fluidized bed (see Section 4.12) of fine-grained fused silica or zircon powder. The pattern is then dipped into coarser grained silica, to build up additional coatings and develop a proper thickness so that the pattern can withstand the thermal shock due to pouring of the hot metal. The rest of the procedure is similar to investment casting. The process is economical and is used extensively for the precision casting of steels and high-temperature alloys. The sequence of operations involved in making a turbine disk by this method is shown in Fig. 11.25. If the cores are made of ceramics, they are later removed by leaching with caustic solutions under high pressure and temperature. The molten metal may also be poured in a vacuum, to extract evolved gases and reduce oxidation, thus improving the casting quality. To further reduce microporosity, the castings made by this, as well as other processes, are subjected to hot isostatic pressing. Case Study 11.2 Investment Casting of Total Knee Replacements With major advances in medical care, life expectancies have increased significantly, so the expectations for the quality of life in the later years of a person’s life remain high. One of the reasons for improvement has been the great success of orthopedic implants. Hip, knee, shoulder, spine, and other implants have resulted in greatly increased activity and reduced pain for millions worldwide. An example of an orthopedic implant that has greatly improved quality of life is total knee replacement (TKR), as shown in Fig. 11.15a. TKRs are very popular and reliable for the relief of osteoarthritis, a chronic and painful degenerative condition of the knee joint that typically sets in after middle age. TKRs consist of multiple parts, including femoral, tibial, and patellar components. Typical materials ׉ 7cassandra://_Hmh2v94I_v8DFezpRLU5QQ_qieqS6zTq7rMn74_wmUF`cZ%Eb`cZ%Eb_#בCט#u׉׉ 7cassandra://9tKb5l6_LqgUO6phreHUGdahHWuZ9ZFDhZ50Bsf9T9c`׉ 7cassandra://MMLJz1taxH8qw1r2FxxiB-s7Gp3LLeiZ7nbunvfTNTUk?`s׉ 7cassandra://f1kF9ToBwNheTaWEkI459QyPiDMXQ_jUdFc3VLzsH7Y`׉ 7cassandra://wSvGhoTy2GEkQbJVkZwQZ7NZMT0783B2aIEXvZj-b0YZ͠^cZEbɧט#u׉׉ 7cassandra://49iK-pKy7P_SCK8V8aWdKRCio1y1IvMqkHO8v3CHu_E~ `׉ 7cassandra://SvcMD7tTF_35OUSIYdVJsB9XMTi4PbFMB8WwFFtOho4D`s׉ 7cassandra://qWj6lgAwkcjGAzkP9FBqOUOdQUws37D7YyCoSuLDKv8`׉ 7cassandra://IiOYNcThv5LvSUtWduqePpNRS71bT2qqnlCs8ZPMVEU͍S͠^cZEbɨ׉EPermanent-mold Casting Processes 321 used include cobalt alloys, titanium alloys, and ultrahigh-molecular-weight polyethylene (UHMWPE). Each material is chosen for specific properties important in the application of the implant. This case study describes the investment casting of femoral components of TKRs, which are produced from cobalt–chrome alloy (Section 6.6). The manufacturing process begins with injection molding of the patterns, which are then hand assembled onto trees, as shown in Fig. 11.15b. The patterns are spaced properly on a central wax sprue; they are then welded in place by dipping them into molten wax and pressing them against the sprue until the patterns are held in place. The final assembled tree, shown in Fig. 11.16a, contains 12 knee implants arranged in four rows. The completed trees are then placed in a rack, where they form a queue and are then taken in order by an industrial robot (Section 37.6). The robot follows a set sequence in building up the mold. It first dips the pattern into dilute slurry, then rotates it under a sifting of fine particles. Next, the robot moves the tree beneath a blower to quickly dry the ceramic coating, and then it repeats the cycle. After a few cycles of such exposure to dilute slurry and fine particles, the details of the patterns are well produced, and good surface finish is ensured. The robot then dips the pattern into a thicker slurry which quickly builds up the mold thickness (Fig. 11.16c). The trees are then dried and placed into a furnace to melt out and burn the wax. The trees are placed into another furnace to preheat them in preparation for casting. Figure 11.16 shows the progression of investment casting, from tree, to investment, to casting. A mold, ready for investment casting, is placed into a casting machine. The mold is placed upside down on the machine, directly over a measured volume of molten cobalt chrome alloy. The machine then rotates so that the metal flows into the mold, as shown in Fig. 11.15d. The tree is allowed to cool and the mold is removed; the cast parts are machined from the tree and are further machined and polished to the required surface finish and dimensional tolerance. Source: Courtesy of M. Hawkins, Zimmer Biomet, Inc. 11.4 Permanent-mold Casting Processes Permanent-mold casting processes have certain advantages over other casting processes, as described below. 11.4.1 Permanent-mold Casting In permanent-mold casting (also called hard-mold casting), two halves of a mold are made from such materials as cast iron, steel, bronze, graphite, or refractory metal alloys, with high resistance to erosion and thermal fatigue. Typical parts made are automobile pistons, cylinder heads, connecting rods, gear blanks for appliances, and kitchenware. Parts that can be made economically typically weigh less than 25 kg, although special castings, weighing a few hundred kilograms, have been made using this process. The capabilities of permanent-mold casting are given in Table 11.3. The mold cavity and the gating system are machined into the mold and thus become an integral part of the mold. To produce castings with internal cavities, cores, made of metal or sand aggregate, are placed in the mold prior to casting. Typical core materials are oil-bonded or resin-bonded sand, plaster, graphite, gray iron, low-carbon steel, and hot-work die steel. Gray iron is used most commonly, particularly for large molds for aluminum and magnesium casting. Inserts also are used in various locations of the mold. In order to increase the life of permanent molds, the surfaces of the mold cavity are usually coated with a refractory slurry, such as sodium silicate and clay, or are sprayed with graphite every few castings. These coatings also serve as parting agents and as thermal barriers, thus controlling the rate of cooling of the casting. Mechanical ejectors, such as pins located in various parts of the mold, may be required for the removal of complex castings. Ejectors usually leave small round impressions, which generally are not significant. ׉ 7cassandra://f1kF9ToBwNheTaWEkI459QyPiDMXQ_jUdFc3VLzsH7Y`cZ%Eba׉E322 Chapter 11 Metal-casting Processes and Equipment (a) (b) (c) (d) Figure 11.15: Manufacture of total knee replacements. (a) The Zimmer NexGen mobile-bearing knee (MBK); the femoral portion (top component) of the total knee replacement is the subject of Case Study 11.2. (b) Assembly of patterns onto a central tree. (c) Dipping of the tree into slurry to develop a mold from investment. (d) Pouring of metal into a mold. Source: Courtesy of M. Hawkins, Zimmer, Inc. (a) (b) (c) Figure 11.16: Progression of the tree. (a) After assembly of blanks onto the tree; (b) after coating with investment; (c) after removal from the mold. Source: Courtesy of M. Hawkins, Zimmer Biomet, Inc. ׉ 7cassandra://qWj6lgAwkcjGAzkP9FBqOUOdQUws37D7YyCoSuLDKv8`cZ%EbbcZ%Eba#בCט#u׉׉ 7cassandra://1r8ygP-1QYHYQ4uOywV9EIe4ZopFuCoAJcGgjbq1IHM`׉ 7cassandra://qQC01tz8ciYOMiBq3iHMRnNHX6JmFToTM54MNl8xi6M]`s׉ 7cassandra://SogU29_Fvlk9Hd0kCU1t8BqSkgJpK0R35QoCOu8D3j8`׉ 7cassandra://nNLZgAP7Yyh5rT_UkcdxF_iiRtQ2sxX6EweNKAE4SXYn*̨͠^cZEbɪט#u׉׉ 7cassandra://_mVKGNgcic0p8WgOFSMDs_RLEjIJsEDhVVNQd2qq9HM`׉ 7cassandra://R1W_-S4oFCD1hukkJ6fmRv_zwY35te9_-MWkwYpYTesN`s׉ 7cassandra://WsBuBEnfMV0ftAtf3v5BA90EGeOOGn4iVyfZpbpVcO4`׉ 7cassandra://Eo32-3PeHhmqWNWErkVcbfy6Mzjiiq8Q4fRn_EeGgHst͠^cZEbɭ׉E Permanent-mold Casting Processes 323 Table 11.3: Properties and Typical Applications of Some Common Die-casting Alloys. Source: American Die Casting Institute. Ultimate tensile Alloy Aluminum 380 (3.5 Cu–8.5 Si) 13 (12 Si) Brass 858 (60 Cu) Magnesium AZ91 B (9 Al–0.7 Zn) Zinc No. 3 (4 Al) No. 5 (4 Al–1 Cu) strength (MPa) 320 300 380 230 Yield strength (MPa) 160 150 200 160 Elongation in 50 mm (%) 2.5 2.5 15 3 280 — 10 320 — 7 Applications Appliances, automotive components, electrical motor frames and housings Complex shapes with thin walls, parts requiring strength at elevated temperatures Plumbing fixtures, lock hardware, bushings, ornamental castings Power tools, automotive parts, sporting goods Automotive parts, office equipment, household utensils, building hardware, toys Appliances, automotive parts, building hardware, business equipment The two molds are clamped together by mechanical means, and heated to about 150◦ to 200◦C to facilitate metal flow and reduce thermal damage to the dies. Molten metal is then poured through the gating system; after solidification, the molds are opened and the casting is removed. The mold often incorporates special cooling features, such as a means for pumping cooling water through the channels located in the mold and the use of cooling fins. Although the permanent-mold casting operation can be performed manually, it is often automated for large production runs. This process is used mostly for aluminum, magnesium, and copper alloys, as well as for gray iron because of their generally lower melting points; however, steels also can be cast using graphite or heat-resistant metal molds. Permanent-mold casting produces castings with a good surface finish, close dimensional tolerances, uniform and good mechanical properties, and at high production rates. Although equipment costs can be high because of high die costs, labor costs are kept low through automation. The process is not economical for small production runs and is not suitable for intricate shapes, because of the difficulty in removing the casting from the mold. However, in a process called semipermanent mold casting, easily collapsible sand cores can be used, which are then removed from castings, leaving intricate internal cavities. 11.4.2 Vacuum Casting A schematic illustration of the vacuum-casting process, also called countergravity low-pressure (CL) process (not to be confused with the vacuum molding process described in Section 11.2.1) is shown in Fig. 11.17. Vacuum casting is an alternative to investment, shell-mold, and green-sand casting, and is suitable particularly for thin-walled complex shapes. With uniform properties, typical parts made are superalloy gas-turbine components with walls as thin as 0.5 mm. In this process, a mixture of fine sand and urethane is molded over metal dies, and cured with amine vapor. The mold is then held with a robot arm and immersed partially into molten metal held in an induction furnace. The metal may be melted in air (CLA process) or in a vacuum (CLV process). The vacuum reduces the air pressure inside the mold to about two-thirds of the atmospheric pressure, thus drawing the molten metal into the mold cavities through a gate in the bottom of the mold. The metal in the furnace is ׉ 7cassandra://SogU29_Fvlk9Hd0kCU1t8BqSkgJpK0R35QoCOu8D3j8`cZ%Ebc׉E 324 Chapter 11 Metal-casting Processes and Equipment Mold Gate Vacuum Casting Molten metal Induction furnace (a) (b) Figure 11.17: Schematic illustration of the vacuum-casting process. Note that the mold has a bottom gate. (a) Before and (b) after immersion of the mold into the molten metal. Source: After R. Blackburn. usually at a temperature of 55◦C above the liquidus temperature of the alloy; consequently, it begins to solidify within a very short time. This process can be automated, with production costs that are similar to those for green-sand casting. Carbon, low- and high-alloy steel, and stainless steel parts, weighing as much as 70 kg, have been vacuum cast by this method. CLA castings are made easily at high volume and relatively low cost; CLV parts usually involve reactive metals, such as aluminum, titanium, zirconium, and hafnium. 11.4.3 Slush Casting Note in Fig. 10.13 that a solidified skin develops in a casting, which becomes thicker with time. Thin-walled hollow castings can be made by permanent-mold casting using this principle, in a process called slush casting. The molten metal is poured into the metal mold; after the desired thickness of solidified skin is obtained, the mold is inverted (or slung) and the remaining liquid metal is poured out. The mold halves are then opened and the casting is removed. Note that this operation is similar to making hollow chocolate shapes and other confectionaries. Slush casting is suitable for small production runs, and is generally used for making ornamental and decorative objects, such as lamp bases and stems, and toys from low-meltingpoint metals, such as zinc, tin, and lead alloys. 11.4.4 Pressure Casting In the two permanent-mold processes described previously, the molten metal flows into the mold cavity by gravity. In pressure casting, also called pressure pouring or low-pressure casting, the molten metal is forced by gas pressure into a graphite or metal mold. The molten metal is tapped from below the surface, and thus avoiding entrainment of dross and oxides into the mold cavity. The pressure is maintained until the metal has completely solidified in the mold. The molten metal may be forced upward by a vacuum, which also removes dissolved gases and produces a casting with lower porosity. Pressure casting is generally used for high-quality castings, such as steel railroad-car wheels; these wheels also may be cast in sand molds or semipermanent molds made of graphite and sand. ׉ 7cassandra://WsBuBEnfMV0ftAtf3v5BA90EGeOOGn4iVyfZpbpVcO4`cZ%EbdcZ%Ebc#בCט#u׉׉ 7cassandra://mC4Sf_TJuCUviyEf1m6gn1PmH2xKTr00FP5VodeYN8ki`׉ 7cassandra://h-w5h2VKP4VXSG6tN1SuYBTJ3ktRD9Wsjz53PWrVfIc[`s׉ 7cassandra://R4aDViohrt_7XSNtjsmE6glfEp7fVn6CUivPqAZH5XUV`׉ 7cassandra://gMkvuwJjS2Rp2WRHvKdMtj0W8Q6Lq9AGm5rempm-WTQͶ ͠^cZEbɲט#u׉׉ 7cassandra://bRNnEOEc7U8CHmFv0XbzTlCbT0qH4KObSRpojY6BXSc#`׉ 7cassandra://vOLnoM3rAZ__AKZe1DEAd6gPhCUe0Gtbk3w5AJWgRoc\`s׉ 7cassandra://PLNOGbkzO4QVbPHNTGNrJUudBP0kQCAunA0a38xiA-o`׉ 7cassandra://jzHPvWe6a1YSK2geLEWdHLqgFcGbfB1hbpLCX9vr4cA& <͠^cZEbɴ׉E 2Permanent-mold Casting Processes 325 11.4.5 Die Casting The die-casting process, developed in the early 1900s, is a further example of permanent-mold casting. The European term for this process is pressure die casting, and should not be confused with pressure casting, described in Section 11.4.4. Typical parts made by die casting are housings for transmissions, businessmachine and appliance components, hand-tool components, and toys. The weight of most castings typically ranges from less than 90 g to about 25 kg. Equipment costs, particularly the cost of dies, are somewhat high, but labor costs are generally low when the process is semi- or fully automated. The capabilities of die casting are given in Table 11.3. In the die-casting process, molten metal is forced into the die cavity at pressures ranging from 0.7 to 700 MPa. There are two basic types of die-casting machines: hot- and cold-chamber. The hot-chamber process (Fig. 11.18) involves the use of a piston, which forces a specific volume of metal into the die cavity through a gooseneck and nozzle; pressures range up to 35 MPa, with an average of about 15 MPa. The metal is held under pressure until it solidifies in the die. To improve die life and to aid in rapid metal cooling (thereby reducing cycle time), dies are usually cooled by circulating water or oil through various passageways in the die block. Low-melting-point alloys, such as zinc, magnesium, tin, and lead, are commonly cast using this process. Cycle times usually range from 200 to 300 shots (individual injections) per hour for zinc, although very small components, such as zipper teeth, can be cast at rates of 18,000 shots per hour. In the cold-chamber process (Fig. 11.19), molten metal is poured into the injection cylinder (shot chamber). The chamber is not heated, hence the term cold chamber. The metal is forced into the die cavity at pressures usually ranging from 20 to 70 MPa, although they may be as high as 150 MPa. The machines may be horizontal, as shown in the figure, or vertical, in which case the shot chamber is vertical. High-melting-point alloys of aluminum, magnesium, and copper normally are cast using this method, although ferrous and other metals also can be cast. Molten-metal temperatures start at about 600◦C for aluminum and some magnesium alloys, and increase considerably for copper-based and iron-based alloys. Freeze casting. Die casting can be used for freeze casting (see Section 10.2.4) to produce porous metals. In this case, the die is maintained at room temperature, thereby freezing a carrier fluid and separating a suspended powder; the cast part is then sintered (see Section 17.4). Nozzle Hydraulic shot cylinder Ejector die Die cavity Cover die Plunger rod Plunger Gooseneck Molten metal Pot Furnace Figure 11.18: Schematic illustration of the hot-chamber die-casting process. ׉ 7cassandra://R4aDViohrt_7XSNtjsmE6glfEp7fVn6CUivPqAZH5XUV`cZ%Ebe׉E 326 Chapter 11 Metal-casting Processes and Equipment Cavity Ejector platen (moves) Ejector die half Ejector box Stationary die half Shot sleeve Pouring hole Plunger Plunger rod Plunger rod Stationary platen Ladle Hydraulic cylinder Closing cylinder Clamp mechanism Ejector box Cover disc Metal sleeve Shot cylinder Figure 11.19: Schematic illustration of the cold-chamber die-casting process. These machines are large as compared to the size of the casting, because high forces are required to keep the two halves of the dies closed under pressure. Process Capabilities and Machine Selection. Die casting has the capability for rapid production of highquality parts with complex shapes, especially with aluminum, brass, magnesium, and zinc (Table 11.3). It also produces good dimensional accuracy and surface details, so that parts require little or no subsequent finishing operations (net-shape forming; Section 1.6). Because of the high pressures involved, walls as thin as 0.38 mm are produced, which are thinner than those obtained by other casting methods. However, ejector marks remain on part surfaces, as may small amounts of flash (thin material squeezed out between the dies at the die parting line). Cycle time greatly depends on the ability of a die to extract heat from the molten metal. It is a common practice to incorporate cooling channels in the die, and to pump coolant through the cooling channels; the forced heat transfer keeps the die cool and allows continuous operation. Conformal or contoured cooling can be performed with dies produced in additive manufacturing; in this case, the cooling channels closely follow the contour of the mold to most efficiently extract heat from the desired location. Conformal cooling is described in greater detail in Section 20.10. A typical part made by die casting is the aluminum impeller shown in Fig. 11.1d; note the intricate shape and fine surface detail. For certain parts, die casting can compete favorably with other manufacturing methods (such as sheet-metal stamping and forging) or other casting processes. In addition, because the molten metal chills rapidly at the die walls, the casting has a fine-grained, hard skin with high strength; consequently, the strength-to-weight ratio of die-cast parts increases with decreasing wall thickness. With good ׉ 7cassandra://PLNOGbkzO4QVbPHNTGNrJUudBP0kQCAunA0a38xiA-o`cZ%EbfcZ%Ebe#בCט#u׉׉ 7cassandra://dRNJHN7L4mvk3JYf-WGsBjB1CH9EwsQDpgwNndrbrZYP`׉ 7cassandra://L6iio5OCtazVke3sY-q2hn9HXO5oE_AKHZneDy-g-M0gg`s׉ 7cassandra://dyI10sdPJ5X_HooEXN_Y1fOg1qnwnATHRQqYcRGfDus`׉ 7cassandra://T21NHSgCPlWR05oAWlzJG4OSPfQkadkKbYiay7v0wCUͫ b͠^cZEbɶט#u׉׉ 7cassandra://If7mt2zmH2oTJJdk99GhCW_064de3Q3fFnvx8qFBsp4`׉ 7cassandra://wj8AjzUzqbuEtSBPgIZeg93A1fE7PgnzMR9Pn53ktncKi`s׉ 7cassandra://wFvBbWMwgLK-eOcgCCvmbtcLi6nyIGoItyTbjksgWb4"`׉ 7cassandra://IBXioWxbUDxMdu5nLkywERAYAnh3y8pxmO1uZTYb_zQkm͠^cZEbɹ׉E Permanent-mold Casting Processes 327 (a) Single-cavity die (b) Multiple-cavity die (c) Combination die (d) Unit die Figure 11.20: Various types of cavities in a die-casting die. Source: Courtesy of American Die Casting Institute. surface finish and dimensional accuracy, die casting can produce smooth surfaces, such as for bearings, that otherwise would normally have to be machined. Multimaterial components, such as pins, shafts, and certain threaded fasteners, can be die cast integrally. Called insert casting, this process is similar to placing wooden sticks in popsicles prior to freezing (see also Section 19.3). For high interfacial strength, insert surfaces may be knurled (see Fig. 23.11), grooved, or splined. Steel, brass, and bronze inserts are commonly used with die-casting alloys. In selecting insert materials, the possibility of galvanic corrosion should be taken into account. To avoid this potential problem, the insert can be insulated, plated, or surface treated. Because of the high pressures involved, dies for die casting have a tendency to separate unless they are clamped together tightly (see Fig. 11.19). Die-casting machines are hence rated according to the clamping force that can be exerted to keep the dies closed during casting. The capacities of commercially available machines range from about 22.5 to 2700 metric tons. Other factors involved in the selection of die-casting machines are die size, piston stroke, shot pressure, various features, and cost. Die-casting dies (Fig. 11.20) may be single cavity, multiple cavity (several identical cavities), combination cavity (several different cavities), or unit dies (simple, small dies that can be combined in two or more units in a master holding die). Typically, the ratio of die weight to part weight is 1000 to 1. Thus, for example, the die for a casting weighing 2 kg would weigh about 2000 kg. The dies are usually made of hot-work die steels or mold steels (see Section 5.7). Heat checking of dies (surface cracking from cyclic heating and cooling of the die, described in Section 3.6) can be a problem. When the die materials are selected and maintained properly, however, dies can last more than a half million shots before any significant die wear takes place. Case Study 11.3 Die Casting of a Headlight Mount Figure 11.22 shows a die-cast aluminum component of a daytime running lamp and turn signal for an automobile. Aluminum was preferable to plastic because of its higher heat-sink characteristics and rigidity, and also because tight tolerances were required for mounting and providing wiring access to LED bulbs. The fin size, thickness, and spacing were determined from a heat transfer analysis. The fins were tapered to allow for easy removal from a die, and the corner radii were designed to prevent distortion during ejection. The part was then oriented so that mounting holes and pockets were coplanar to the die parting line to simplify die fabrication. Heating channels were incorporated into the die near the thin sections to slow cooling, while cooling channels were incorporated near the thick sections. The resulting thermal balance led to lower distortion in the final product. The final product was cast from 380 aluminum; it measures 100 × 75 × 100 mm for the turn signal and 250 × 100 × 50 mm for the daytime running light sub-assembly. ׉ 7cassandra://dyI10sdPJ5X_HooEXN_Y1fOg1qnwnATHRQqYcRGfDus`cZ%Ebg׉EC328 Chapter 11 Metal-casting Processes and Equipment Case Study 11.4 Die Cast Magnesium Liftgate Figure 11.21 shows a complex high-pressure die-casting produced from a 2017 Chrysler Pacifica, and represents the first high-volume magnesium application of its kind in the automotive industry. The casting forms part of a four-piece assembly, with aluminum sheet upper and lower outer panels and a wiper reinforcement. The AM60B magnesium alloy part reduced the weight of the liftgate by more than 10 kg, representing a 50% reduction over the previous generation design. However, the liftgate affected the designs of the motor, strut, hinge, and other mechanical components, so that the weight savings was actually much greater. Lightweighting strategies such as design optimization and exploitation of materials with high strength-to-weight ratios are common in automotive and aerospace applications so as to achieve ever increasing fuel economy goals. The original design was a weldment of seven steel stampings; the redesigned liftgate was combined into a single magnesium casting, with 10 spot welds and rivets compared to 84 in the original design. The thin-walled casting takes special care to extract the heat from the magnesium slowly, in order to prevent solidification in the mold and resulting underfills. 11.4.6 Centrifugal Casting As its name implies, the centrifugal-casting process utilizes inertia (caused by rotation) to force the molten metal into the mold cavities, a method that was first suggested in the early 1800s. The capabilities of centrifugal casting are given in Table 11.3. There are three types of centrifugal casting: true centrifugal casting, semicentrifugal casting, and centrifuging. Figure 11.21: A magnesium liftgate for a 2017 Chrysler Pacifica, saving more than 10 kg in weight over the previous generation steel stamping, and with reduction of welds from 84 to 10. Magnesium is increasingly used for vehicle lightweighting, and high-pressure die castings such as this one can be produced with thin walls and intricate part details. Source: Courtesy of American Foundry Society. ׉ 7cassandra://wFvBbWMwgLK-eOcgCCvmbtcLi6nyIGoItyTbjksgWb4"`cZ%EbhcZ%Ebg#בCט#u׉׉ 7cassandra://Mgk0sMxxuKk1LCpBxitOt-1mWpLBqrTwevi2WKFgEmMA`׉ 7cassandra://mszLa3SpczHc32AIK-v1vLunhphTLS3CwfM_td6rjHY]`s׉ 7cassandra://1LEsKqBe95EkKaixtfteNJEUK1pClYE41H2euU-HhbI`׉ 7cassandra://tocVwqqjB9VW1_kZyzaxauoiQ6I7KQRXnm10J5kzAgoI7͠^cZEbɼט#u׉׉ 7cassandra://pOdkzQg0Cj__TQ5nqsGC_hDu2iGUueTvV1ZS7UyNjLYv`׉ 7cassandra://Qvo_YoFbvFfzBR63ZeR1G2I7Iy2QnMVEzKn8rXar3DUR`s׉ 7cassandra://rtp7jVsYhk4c8OJUryLzoTRnZP788llK7xoKKz-Vvl0`׉ 7cassandra://hH-bGkUMvNX12gGWTXyP_HasA1lBBj6aKXhN3AIqSDQnz͠^cZEbɽ׉E Permanent-mold Casting Processes 329 Mold Mold Molten metal Ladle Drive roll Spout Driveshaft (a) Rolls (b) Figure 11.22: (a) Schematic illustration of the centrifugal-casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process. (b) Side view of the machine. True Centrifugal Casting. In true centrifugal casting, hollow cylindrical parts (such as pipes, gun barrels, bushings, engine-cylinder liners, bearing rings with or without flanges, and street lampposts) are produced by the technique shown in Fig. 11.22. In this process, molten metal is poured into a rotating mold; the axis of rotation is usually horizontal, but can be vertical for short workpieces. Molds are made of steel, iron, or graphite, and may be coated with a refractory lining to increase mold life. The mold surfaces can be shaped so that pipes with various external designs can be cast. The inner surface of the casting remains cylindrical, because the molten metal is distributed uniformly by the centrifugal forces. However, because of density differences, lighter elements (such as dross, impurities, and pieces of the refractory lining in the mold) tend to collect on the inner surface of the casting. Consequently, the properties of the casting can vary throughout its thickness. Cylindrical parts ranging from 13 mm to 3 m in diameter and 16 m long can be cast centrifugally, with wall thicknesses ranging from 6 to 125 mm. The pressure generated by the centrifugal force is high (the angular acceleration can be as much as 150 times gravity); such high pressure is necessary for casting thickwalled parts. Castings with good quality, dimensional accuracy, and external surface detail are produced by this process. Semicentrifugal Casting. An example of semicentrifugal casting is shown in Fig. 11.23. This method is used to cast parts with rotational symmetry, such as a wheel with spokes. Centrifuging. In centrifuging, also called centrifuge casting, mold cavities are placed at a certain distance from the axis of rotation. The molten metal is poured from the center, and is forced into the mold by centrifugal forces (Fig. 11.23). The properties of the castings can vary by distance from the axis of rotation, as in true centrifugal casting. 11.4.7 Squeeze Casting and Semisolid-metal Forming Two casting processes that incorporate the features of both casting and forging (Chapter 14) are squeeze casting and semisolid-metal forming. Squeeze Casting. The squeeze-casting or liquid-metal forging process was invented in the 1930s, but developed for industrial applications in the 1960s, and involves the solidification of molten metal under high pressure (Fig. 11.24). Typical products made are automotive components and mortar bodies (a cannon with a short body). The machinery includes a die, punch, and ejector pin. The pressure applied by the punch keeps the entrapped gases in solution, while the contact under high pressure at the die–metal interface Idling roll ׉ 7cassandra://1LEsKqBe95EkKaixtfteNJEUK1pClYE41H2euU-HhbI`cZ%Ebi׉EV330 Chapter 11 Metal-casting Processes and Equipment Pouring basin and gate Cope Flasks Holding fixture Drag Casting Molten metal Mold Casting Revolving table (a) (b) Figure 11.23: (a) Schematic illustration of the semicentrifugal casting process. Wheels with spokes can be cast by this process. (b) Schematic illustration of casting by centrifuging. The molds are placed at the periphery of the machine, and the molten metal is forced into the molds by centrifugal force. promotes rapid heat transfer, thus resulting in a casting with a fine microstructure and good mechanical properties. The application of pressure also overcomes hot-metal feeding difficulties that may arise when casting metals with a long freezing range (Section 10.2.2). Complex parts can be made to near-net shape, with fine surface detail from both nonferrous and ferrous alloys. Semisolid-metal Forming. Semisolid-metal forming, also called mushy-state processing (see Fig. 10.4) was developed in the 1970s. When it enters the die, the metal (consisting of liquid and solid components) is stirred so that all of the dendrites are broken into fine solids. When cooled in the die, a fine-grained structure is developed. The alloy exhibits thixotropic behavior, described in Section 10.2.3, hence the process is also called thixoforming or thixomolding, meaning its viscosity decreases when agitated. Thus, at rest and above its solidus temperature, the molten alloy has the consistency of butter at room temperature, but when agitated vigorously, its consistency becomes more like that of motor oil. Die Finished casting Cavity Ejector pin 1. Melt metal 2. Pour molten metal into die 3. Close die and apply pressure 4. Eject squeeze casting, charge melt stock, repeat cycle Figure 11.24: Sequence of operations in the squeeze-casting process. This process combines the advantages of casting and forging. ׉ 7cassandra://rtp7jVsYhk4c8OJUryLzoTRnZP788llK7xoKKz-Vvl0`cZ%EbjcZ%Ebi#בCט#u׉׉ 7cassandra://LVPiiezOsMY0lFLy0opWjPAWHw23KHCKsr2nhRQXWCws`׉ 7cassandra://I5oKEGlQq42o0_58WseEcIxdaJXac0JqSCGNd_g6Irco`s׉ 7cassandra://mQ_dWtqa55dpkOTg02Z20iWWQRZTfOxoDb66whLJzQc`׉ 7cassandra://F25i99pGxvDmVqNj04QBRGBJuYDc2iiECzFY4f9s48oiJ͠^cZEbɿט#u׉׉ 7cassandra://GPzH5GQNTYxfSY6cISAm1hEMKg2LYuJ9CKuKvdgttbc1`׉ 7cassandra://SyCgKy3wvK0s1zBzEbaWUWCdIP5apV8zgtayPpMe83MV`s׉ 7cassandra://83FJ68xxZoF_mWt85N-zwvAanbjxNcWewBWweaGeqlkx`׉ 7cassandra://VJanC2nDbjUo0Cu3VidJMgpiZHDNb2XpJDkpC23Zpgw͟4 ͠^cZEbנcZEb{xi9ׁHhttp://Blades.Intׁ ׁ Ј׉ECasting Techniques for Single-crystal Components 331 Processing metals in their mushy state also has led to developments in mushy-state extrusion, similar to injection molding (described in Section 19.3), forging,and rolling processes, hence the term semisolid metalworking. These processes are also used in making parts with specially designed casting alloys, wrought alloys, and metal-matrix composites (Section 9.5). They also have the capability for blending granules of different alloys, called thixoblending, for specific applications. Thixotropic behavior has also been utilized in developing technologies that combine casting and forging, using cast billets that are then forged when the metal is 30–40% liquid. Parts made include automotive control arms, brackets, and steering components. Processing steels by thixoforming has not yet reached the same stage as with aluminum and magnesium, largely because of the high temperatures involved (which adversely affect die life) and the difficulty in making complex shapes. The advantages of semisolid metal forming over die casting are: (a) the structures developed are homogeneous, with uniform properties, lower porosity, and high strength; (b) both thin and thick parts can be made; (c) casting alloys as well as wrought alloys can be used; (d) parts can subsequently be heat treated; and (e) the lower superheat results in shorter cycle times. However, material and overall costs are higher than those for die casting. Rheocasting. This technique, first investigated in the 1960s, is used for forming metals in their semisolid state. The metal is heated to just above its solidus temperature, and poured into a vessel to cool it down to the semisolid state. The slurry is then mixed and delivered to the mold or die. This process is being used successfully with aluminum and magnesium alloys. 11.4.8 Composite-mold Casting Operations Composite molds are made of two or more different materials and are used in shell molding and various other casting processes. They are generally employed in casting complex shapes, such as impellers for turbines. Composite molds increase the strength of the mold, improve the dimensional accuracy and surface finish of the castings, and can help reduce overall costs and processing time. Molding materials commonly used are shells (made as described in Section 11.2.2), plaster, sand with binder, metal, and graphite. These molds may include cores and chills to control the rate of solidification in critical areas of castings. 11.5 Casting Techniques for Single-crystal Components This section describes the techniques used to cast single-crystal components, such as gas turbine blades which generally are made of nickel-based superalloys, and used in the hot stages of the engine. Conventional Casting of Turbine Blades.Inthe conventional-casting process, the molten metal is poured into a ceramic mold, and begins to solidify at the mold walls. The grain structure developed is polycrystalline, similar to that shown in Fig. 10.2c. However, the presence of grain boundaries makes this structure susceptible to creep and cracking along the boundaries under the centrifugal forces and elevated temperatures commonly encountered in an operating gas turbine. Directionally Solidified Blades. The directional-solidification process (Fig. 11.25) was first developed in 1960. The ceramic mold, supported by a water-cooled chill plate, is preheated by radiant heating. After the metal is poured into the mold, the chill-plate assembly is lowered slowly. Crystals begin to grow at the chill-plate surface and on upward, like the columnar grains shown in Fig. 10.3. The blade is solidified directionally, with longitudinal but no transverse grain boundaries. The blade is thus stronger in the direction of centrifugal forces developed in the gas turbine. Single-crystal Blades. In crystal growing, developed in the late 1960s, the mold has a constriction in the shape of a corkscrew or helix (Figs. 11.25b and c); its cross section is so small that it allows only one crystal ׉ 7cassandra://mQ_dWtqa55dpkOTg02Z20iWWQRZTfOxoDb66whLJzQc`cZ%Ebk׉E 332 Chapter 11 Metal-casting Processes and Equipment Radiant heat Heat baffles Radiant heat Columnar crystals Columnar crystals Chill plate (a) Chill plate (b) (c) Figure 11.25: Methods of casting turbine blades: (a) directional solidification; (b) method to produce a single-crystal blade; (c) a single-crystal blade with the constriction portion still attached (see also Fig. 1.1). Source: (a) and (b) After B.H. Kear, (c) Courtesy of ASM International. to fit through. The mechanism of crystal growth is such that only the most favorably oriented crystals are able to grow through the helix (a situation similar to that shown in Fig. 10.3), because all other crystals are intercepted by the walls of the helical passage. As the assembly is slowly lowered, a single crystal grows upward through the constriction and begins to grow in the mold; strict control of the rate of movement is essential. Although single-crystal blades are more expensive than other types, the lack of grain boundaries makes them resistant to creep and thermal shock, hence they have a longer and more reliable service life. Single-crystal Growing. Single-crystal growing is a major activity in the semiconductor industry in the manufacture of the silicon wafers for microelectronic devices (Chapter 28). There are two basic methods of crystal growing: • In the crystal-pulling method, also known as the Czochralski (CZ) process (Fig. 11.26), a seed crystal is dipped into the molten metal and then pulled out slowly, at a rate of about 10 µm/s, while being rotated. The liquid metal begins to solidify on the seed, and the crystal structure of the seed continues throughout. Dopants (Section 28.3) may be added to the liquid metal to impart specific electrical properties. Single crystals of silicon, germanium, and various other elements are grown using this process. Single-crystal ingots up to 400 mm in diameter and over 2 m in length have been produced by this technique, although 200- and 300-mm ingots are common in the production of silicon wafers for integrated circuit manufacture (Part V). • The floating-zone method (Fig. 11.26b) starts with a rod of polycrystalline silicon resting on a single crystal; an induction coil then heats these two pieces while the coil moves slowly upward. The single crystal grows upward, while maintaining its orientation. Thin wafers are then cut from the rod (see Section 28.4), cleaned, and polished for use in microelectronic device fabrication. This process is suitable for producing diameters under 150 mm, with very low levels of impurities. Constriction ׉ 7cassandra://83FJ68xxZoF_mWt85N-zwvAanbjxNcWewBWweaGeqlkx`cZ%EblcZ%Ebk#בCט#u׉׉ 7cassandra://vtqOzjPvOZYg8uOPbeaBG4MXWlXNGGkz6pLaZcyGFG0ڧ`׉ 7cassandra://DCe79pRHvXXv8rXLrpWWvhe3ypswVP1hQdfC16Q_LvMQa`s׉ 7cassandra://QVxj7HrxcpSiTxs98msqyZYu5WMrXPrbY_YuOseIXYs`׉ 7cassandra://3N64-DlEiB1s4lojUuH0Gh6wShAqM83oy97m_PKYSHAͧ4L͠^cZEbט#u׉׉ 7cassandra://cYWXOh6gRH0NmnsHo2n3iTj49kxZhVgirY4ukkgv-nAl`׉ 7cassandra://LBBnxRyW94ZDd_WgatFwqSd7aFK-6iW00SaG67xW02EXa`s׉ 7cassandra://Z2Lh3QLopcl_pFf75dHCOtQmmnYgXluvA0ZUlHADga8a`׉ 7cassandra://vdCD-WGbgL7XahgDuLyLEgcKsdwcmijGF0PuPLOKxno͠^cZEb׉E Inspection of Castings 333 ~1 rev/s Induction coil Liquid (a) (b) (c) Figure 11.26: Two methods of crystal growing: (a) crystal pulling (Czochralski process) and (b) the floatingzone method. (c) A single-crystal ingot produced by the Czochralski process. Source: Courtesy of Intel Corp. 11.6 Rapid Solidification The properties of amorphous alloys, also known as metallic glasses, are described in Section 6.15. The technique for making these alloys, called rapid solidification, involves cooling the molten metal at rates as high as 106 K/s so that it does not have sufficient time to crystallize (see also Fig. 1.11). Rapid solidification results in a significant extension of solid solubility (Section 4.2), grain refinement, and reduced microsegregation (see Section 10.2.3). Metallic glasses have very high strength but limited ductility; this behavior can be thought of as an extension of the Hall–Petch effect (see Section 1.5.1), where the grain size is on the order of one atom. In a method called melt spinning (Fig. 19.6), the alloy is melted by induction in a ceramic crucible. It is then propelled, under high gas pressure, against a rotating copper disk (chill block), which rapidly chills the alloy (splat cooling), forming a metallic glass strip. Significant research is taking place to produce bulk forms of metallic glass. 11.7 Inspection of Castings Several methods can be used to inspect castings to determine their quality and the presence and types of any defects. Castings can be inspected visually,or optically, for surface defects. Subsurface and internal defects are investigated using various nondestructive techniques, described in Section 36.10. In destructive testing (Section 36.11), specimens are removed from various locations in a casting, and tested for strength, ductility, and various other mechanical properties, and to determine the presence, location, and distribution of porosity and other defects. Pressure tightness of cast components, such as valves, pumps, and pipes, is usually determined by sealing the openings in the casting, then pressurizing it with water, oil, or air. For leak tightness requirements in critical applications, pressurized helium or specially scented gases, with detectors (sniffers), are used. The casting is then inspected for leaks while the pressure is maintained; unacceptable or defective castings are remelted for reprocessing. 10 mm/s 20 mm/s Single crystal Polycrystalline feed ׉ 7cassandra://QVxj7HrxcpSiTxs98msqyZYu5WMrXPrbY_YuOseIXYs`cZ%Ebm׉E E334 Chapter 11 Metal-casting Processes and Equipment 11.8 Melting Practice and Furnaces Melting practice is an important aspect of casting operations, because it has a direct bearing on the quality of castings. Furnaces are charged with melting stock, consisting of metal, alloying elements, and various other materials, such as flux and slag-forming constituents. Fluxes are inorganic compounds that refine the molten metal by removing dissolved gases and various impurities. They may be added manually or can be injected automatically into the molten metal. Melting Furnaces. The melting furnaces commonly used in foundries are electric-arc furnaces, induction furnaces, crucible furnaces, and cupolas. 1. Electric arc furnaces, described in Section 5.2.3 and illustrated in Fig. 5.2, are used extensively in foundries, because of their high rate of melting (thus high-production rate), much less pollution than other types, and their ability to hold the molten metal (keeping it at a constant temperature for a period of time) for alloying purposes. 2. Induction furnaces (Fig. 5.2c) are especially useful in smaller foundries, and produce compositioncontrolled melts. There are two basic types. (a) The coreless induction furnace consists of a crucible, surrounded with a water-cooled copper coil through which high-frequency current passes. Because there is a strong electromagnetic stirring action during induction heating, this type of furnace has excellent mixing characteristics and is used for alloying and adding a new charge of metal into the furnace. (b) A core or channel furnace, uses low-frequency current (as low as 60 Hz), and has a coil that surrounds only a small portion of the unit. These furnaces are commonly used in nonferrous foundries, and are particularly suitable for superheating (heating above normal casting temperature to improve fluidity), for holding, which makes it suitable for die-casting applications, and for duplexing (using two furnaces: melting the metal in one furnace and then transferring it to another). 3. Crucible furnaces (Fig. 11.27a), which have been used extensively throughout history, are heated using various fuels, such as commercial gases, fuel oil, and fossil fuel, and with electricity. Crucible furnaces may be stationary, tilting, or movable. Spark arrester Charging floor Charging door Shell Crucible Gas 1 Air Slag spout Tuyere Wind box Bottom prop Bottom door in (a) dropped position (b) Figure 11.27: Two types of melting furnaces used in foundries: (a) crucible and (b) cupola. Stack Preheating zone Melting zone Superheating zone Well Tapping spout Breast Bott ׉ 7cassandra://Z2Lh3QLopcl_pFf75dHCOtQmmnYgXluvA0ZUlHADga8a`cZ%EbncZ%Ebm#בCט#u׉׉ 7cassandra://X7KR_EjDGuEcMwyf5fSSf69McqHkjZYOh0rnxq8ZwZ0۳`׉ 7cassandra://-DIN4OXs-OQzrTKVnLEH6sVzg8-QGSAbwlZktAVTpXAe`s׉ 7cassandra://NBIvDkhBi0UOT6ARqvsKXE-nkukIDCOdBUyJj414jqY`׉ 7cassandra://kSE0c7pUz25adHET9OwZnnXrUdYwYNJXGs00LNCvK-8T͠^cZEbט#u׉׉ 7cassandra://kdpwI7b4TfRpeJylwwKTC2FFEScSI7jVb354Q7tc0OA `׉ 7cassandra://0HjSWs2Es2oIq5kGtswxhYz8aZB47mFH_rWS5hKwZRUG`s׉ 7cassandra://oPKcAK4yEYAh0mDHbzmgsOR-k_HGuJ_ENqohtFhtgF0`׉ 7cassandra://gDy_nMAyH5ygbOzEhd1BbPmSO-3ojl0nlbhShrMLEqw`?͠^cZEb׉E Summary 335 4. Cupolas are basically vertical refractory-lined steel vessels, charged with alternating layers of metal, coke, and flux (Fig. 11.27b). Although they require major investments and are increasingly replaced by induction furnaces, cupolas operate continuously, have high melting rates, and produce large amounts of molten metal. 5. Levitation melting involves magnetic suspension of the molten metal. An induction coil simultaneously heats a solid billet and stirs and confines the melt, thus eliminating the need for a crucible (which could contaminate the molten metal with its oxide inclusions). The molten metal flows downward into an investment-casting mold, placed directly below the coil. Investment castings made by this method are free of refractory inclusions and of gas porosity, and have a uniform fine-grained structure. 11.9 Foundries and Foundry Automation Casting operations are carried out in foundries (from the Latin fundere, meaning melting and pouring). Although these operations traditionally have involved much manual labor, modern foundries have efficient automated and computer-integrated facilities for all aspects of their operations. As outlined in Fig. 11.2, foundry operations initially involve two separate groups of activities. The first group is pattern and mold making using computer-aided design and manufacturing (Chapter 38) and rapid-prototyping techniques (Chapter 20), thus improving efficiency and lowering costs. A variety of automated machinery is used to minimize labor costs, which can be significant in the production of castings. The second group of activities involves melting the metals, controlling their compositions and impurities, and pouring them into molds. The rest of the operations in a foundry, such as pouring into molds (some carried along conveyors), shakeout, cleaning, heat treatment, and inspection, also are automated. Automation minimizes labor, reduces the possibility of human error, increases the production rate, and attains higher quality levels. Industrial robots (Section 37.6) are used extensively, such as for cleaning, cutting risers, mold venting, mold spraying, pouring, sorting, and inspection. Other operations involve automatic storage and retrieval systems for cores and patterns, using automated guided vehicles (Section 37.5). Summary • Expendable-mold, permanent-pattern processes include sand, shell-mold, plaster-mold, and ceramicmold casting. These processes require the destruction of the mold for each casting produced, but mold production is facilitated by a reusable pattern. • Expendable-mold, expendable-pattern processes include lost-foam and investment casting. In these processes, a pattern is consumed for each mold produced, and the mold is destroyed after each casting. • Permanent-mold processes have molds or dies that can be used to produce castings at high production rates. Common permanent-mold processes include slush casting, pressure casting, die casting, and centrifugal casting. • The molds used in permanent-mold casting are made of metal or graphite, and are used repeatedly to produce a large number of parts. Because metals are good heat conductors but do not allow gases to escape, permanent molds have fundamentally different effects on castings than sand or other aggregate mold materials. • In permanent-mold casting, die and equipment costs are relatively high, but the processes are economical for large production runs. Scrap loss is low, dimensional accuracy is relatively high, and good surface details can be achieved. ׉ 7cassandra://NBIvDkhBi0UOT6ARqvsKXE-nkukIDCOdBUyJj414jqY`cZ%Ebo׉E336 Chapter 11 Metal-casting Processes and Equipment • Casting processes include squeeze casting (a combination of casting and forging), semisolid-metal forming, rapid solidification (for the production of amorphous alloys), and the casting of singlecrystal components (such as turbine blades and silicon ingots for making wafers in integrated-circuit manufacture). • Melting processes and their control are important factors in casting operations. They include proper melting of the metals, preparation for alloying and removal of slag and dross, and pouring the molten metal into the molds. Inspection of castings for possible internal or external defects also is essential. • Castings are generally subjected to subsequent processing, such as heat treatment and machining operations, to produce the final desired shapes, surface characteristics, and the required surface finish and dimensional accuracy. Key Terms Binders Centrifugal casting Ceramic-mold casting Chaplets Composite mold Core print Cores Crystal growing Die casting Evaporative-pattern casting Expendable mold Expendable-pattern casting Flux Foundry Green molding sand Insert casting Investment casting Levitation melting Lost-foam process Bibliography Analysis of Casting Defects, American Foundrymen’s Society, 2002. ASM Handbook, Vol. 1A: Cast Iron Science and Technology, ASM International, 2017. ASM Handbook, Vol. 15: Casting, ASM International, 2008. Beeley, P.R., Foundry Technology, Butterworth-Heinemann, 2002. Beeley, P.R., and Smart, R.F. (eds.), Investment Casting, Maney Publishing, 2008. Campbell, J., Complete Casting Handbook: Metal Casting Processes, Techniques and Design, 2nd ed., Butterworth-Heinemann, 2015. Hirt, G., and Kopp, R., Thixoforming, Wiley-VCH, 2009. Lost-wax process Parting agent Patterns Permanent mold Permanent-mold casting Plaster-mold casting Precision casting Pressure casting Rammed-graphite molding Rapid solidification Rheocasting Sand casting Semisolid-metal forming Shell-mold casting Slush casting Squeeze casting Thixotropic Vacuum casting ׉ 7cassandra://oPKcAK4yEYAh0mDHbzmgsOR-k_HGuJ_ENqohtFhtgF0`cZ%EbpcZ%Ebo#בCט#u׉׉ 7cassandra://L4OSOecrIDFQwzBxzNw_HjRLHd87kY7hzj5JZl_TuQ0`׉ 7cassandra://yc0e-FQ5GUZ6_BxHKueVEI5jZUUd0t4EIr6ua9vca4cP`s׉ 7cassandra://nUfpmP0a6j-XQDv7aVrLuJGr5MutXp3cr9FRQc7m4yYX`׉ 7cassandra://gSosa9S_zyTjTm8H910xtfF-AwNgcs6Bb61FD9S3ZH8R͠^cZEbט#u׉׉ 7cassandra://b0N42vbtX4-PjO157MQXCgUel26-5zeomjmukwhgb_AES`׉ 7cassandra://IuWNaywtKLJFETOJewFxjgfyNGvyyX0mZWdG5ibeowk``s׉ 7cassandra://qbDli53zWTuw0EYWtZO9nWrkMDveUikjnr3Qx-0X_Hc`׉ 7cassandra://QZwGsCEXf9sSYsGx68sChIAb1iKUR6bzbNgrjuvk9RAQ5͠^cZEb׉E Qualitative Problems 337 Investment Casting Handbook, Investment Casting Institute, 1997. Kaufman, J.G., and Rooy, E.L., Aluminum Alloy Castings: Properties, Processes and Applications,ASM International, 2004. Kirkwood, D.H., Suery, M., Kapranos, P., and Atkinson, H.V., Semi-solid Processing of Alloys,Springer, 2009. Nafisi, S., and Ghomashchi, R., Semi-solid Processing of Aluminum Alloys, Springer, 2016. Sias, F.R., Lost-wax Casting, Woodsmere Press, 2006. Steel Castings Handbook, 6th ed., ASM International, 1995. Vinarcik, E.J., High Integrity Die Casting, Wiley, 2002. Young, K.P., Semi-solid Processing, Chapman & Hall, 1997. Review Questions 11.1. Describe the differences between expendable and permanent molds. 11.2. Name the important factors in selecting sand for molds. 11.3. What are the major types of sand molds? What are their characteristics? 11.4. List important considerations when selecting pattern materials. 11.5. What is the function of a core? 11.6. What is the difference between sand-mold and shell-mold casting? 11.7. What are composite molds? Why are they used? 11.8. Describe the features of plaster-mold casting. 11.9. Name the type of materials typically used for permanent-mold casting processes. 11.10. What are the advantages of pressure casting over other processes? 11.11. List the advantages and limitations of die casting. 11.12. What is the purpose of a riser? What is a blind riser? 11.13. Explain the purpose of a vent and a runner in a casting mold. 11.14. How are shell molds produced? 11.15. What keeps the mold together in vacuum casting? 11.16. What is squeeze casting? What are its advantages? 11.17. What are the advantages of the lost-foam casting process? 11.18. How are single-crystal turbine blades produced? Qualitative Problems 11.19. What are the reasons for the large variety of casting processes that have been developed over the years? Explain with specific examples. 11.20. Why are risers not as useful in die casting as they are in sand casting? 11.21. Describe the drawbacks to having a riser that is (a) too large and (b) too small. 11.22. Why can blind risers be smaller than open-top risers? 11.23. Why does die casting produce the smallest cast parts? 11.24. Why is the investment-casting process capable of producing fine surface detail on castings? 11.25. What differences, if any, would you expect in the properties of castings made by permanent-mold versus sand-casting processes? ׉ 7cassandra://nUfpmP0a6j-XQDv7aVrLuJGr5MutXp3cr9FRQc7m4yYX`cZ%Ebq׉E m338 Chapter 11 Metal-casting Processes and Equipment 11.26. Recently, cores for sand casting have been produced from salt. What advantages and disadvantages would you expect from using salt cores? 11.27. Would you recommend preheating the molds used in permanent-mold casting? Would you remove the casting soon after it has solidified? Explain your reasons. 11.28. Give reasons for, and examples of, using die inserts. 11.29. Referring to Fig. 11.3, do you think it is necessary to weigh down or clamp the two halves of the mold? Explain your reasons. Do you think that the kind of metal cast, such as gray cast iron versus aluminum, should make a difference in the clamping force? Explain. 11.30. Explain why squeeze casting produces parts with better mechanical properties, dimensional accuracy, and surface finish than do expendable-mold processes. 11.31. How are the individual wax patterns attached on a “tree” in investment casting? 11.32. Describe the measures that you would take to reduce core shifting in sand casting. 11.33. You have seen that, even though die casting produces thin parts, there is a limit to how thin they can be. Why can’t even thinner parts be made by this process? 11.34. How are hollow parts with various cavities made by die casting? Are cores used? If so, how? Explain. 11.35. It was stated that the strength-to-weight ratio of die-cast parts increases with decreasing wall thickness. Explain why. 11.36. How are risers and sprues placed in sand molds? Explain, with appropriate sketches. 11.37. In shell-mold casting, the curing process is critical to the quality of the finished mold. In this stage of the process, the shell-mold assembly and cores are placed in an oven for a short period of time to complete the curing of the resin binder. List probable causes of unevenly cured cores or of uneven core thicknesses. 11.38. Why does the die-casting machine shown in Fig. 11.19 have such a large mechanism to close the dies? Explain. 11.39. Chocolate forms are available in hollow shapes. What process should be used to make these chocolates? 11.40. What are the benefits to heating the mold in investment casting before pouring in the molten metal? Are there any drawbacks? Explain. 11.41. The “slushy” state of alloys refers to that state between the solidus and liquidus temperatures, as described in Section 10.2.2. Pure metals do not have such a slushy state. Does this mean that pure metals cannot be slush cast? Explain. 11.42. Can a chaplet also act as a chill? Explain. 11.43. Rank the casting processes described in this chapter in terms of their solidification rate. (That is, which processes extract heat the fastest from a given volume of metal?) Quantitative Problems 11.44. Estimate the clamping force for a die-casting machine in which the casting is rectangular with projected dimensions of 125 mm× 175 mm.Would your answer depend on whether it is a hot-chamber or cold-chamber process? Explain. 11.45. The blank for the spool shown below is to be sand cast out of A-319, an aluminum casting alloy. Make a sketch of the wooden pattern for this part, and include all necessary allowances for shrinkage and machining. ׉ 7cassandra://qbDli53zWTuw0EYWtZO9nWrkMDveUikjnr3Qx-0X_Hc`cZ%EbrcZ%Ebq#בCט#u׉׉ 7cassandra://nPdwtiMvYTxKEErpSFBox9dtEnl0O5Lkmrw9f7mF9nA`׉ 7cassandra://JPkjiZSkuzqZYrdq_OWtI588zokZNuMWhOO-82knW4E0`s׉ 7cassandra://9vNN3fb4dPOCgoAoQm7Rr0CcHQxuIBQcej2mhYSRQMQ`׉ 7cassandra://LOmHpAF9Y3il1TWWDnk4p1NOT9Tr25u9SpaA-tCgo2Mj<͠^cZEbט#u׉׉ 7cassandra://6piMXjAHQJHUvN6lCUjohI0Wrf5XaDogvUvXw8yq97U`׉ 7cassandra://cRm2sGjKJkM5WeB0WJ4U6KHzAIRaZiw3MIsTs297wVI^`s׉ 7cassandra://wuoW1lPdbzwflS67-CdNM1yNWfp7NDv3DXZkCTsYZAA`׉ 7cassandra://yPp2AbeQEw6_FdLnW3Oa6QNK4VkqZbsOlvT_Gt310xAO͠^cZEb׉ELQuantitative Problems 339 10 mm 10 mm 75 mm 11.46. Repeat Problem 11.45, but assume that the aluminum spool is to be cast by expendable-pattern casting. Explain the important differences between the two patterns. 11.47. In sand casting, it is important that the cope-mold half be weighted down with sufficient force to keep it from floating when the molten metal is poured in. For the casting shown below, calculate the minimum amount of weight necessary to keep the cope from floating up as the molten metal is poured in. (Hint: The buoyancy force exerted by the molten metal on the cope is dependent on the effective height of the metal head above the cope.) AA 100 mm Section A–A 50 mm 25 mm 75 mm 50 mm R 5 18.5 mm 62 mm 25 mm 12 mm 25 mm 62 mm 100 mm 125 mm Material: Low-carbon steel Density: 7196 kg/m3 All dimensions in inches 100 mm ׉ 7cassandra://9vNN3fb4dPOCgoAoQm7Rr0CcHQxuIBQcej2mhYSRQMQ`cZ%Ebs׉E 340 Chapter 11 Metal-casting Processes and Equipment 11.48. If an acceleration of 125 g is necessary to produce a part in true centrifugal casting and the part has an inner diameter of 300 mm, a mean outer diameter of 400 mm, and a length of 8 m, what rotational speed is needed? 11.49. A jeweler wishes to produce 20 gold rings in one investment-casting operation. The wax parts are attached to a wax central sprue with a 20-mm diameter. The rings are located in four rows, each 15 mm from the other on the sprue. The rings require a 3-mm diameter, 12-mm long runner to the sprue. Estimate the weight of gold needed to completely fill the rings, runners, and sprues. Assume a typical ring has a 25-mm outer diameter, 19-mm inner diameter, and 5-mm width. The specific gravity of gold is 19.3. 11.50. Assume that you are an instructor covering the topics described in this chapter, and you are giving a quiz on the numerical aspects of casting processes to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 11.51. Describe the procedures that would be involved in making a large outdoor bronze statue. Which casting process(es) would be suitable? Why? 11.52. The optimum shape of a riser is spherical to ensure that it cools more slowly than the casting it feeds. However, spherically shaped risers are difficult to cast. (a) Sketch the shape of a blind riser that is easy to mold, but also has the smallest possible surface-area-to-volume ratio. (b) Compare the solidification time of the riser in part (a) with that of a riser shaped like a right circular cylinder. Assume that the volume of each riser is the same and the height of each is equal to the diameter. (See Example 10.1.) 11.53. Sketch and describe a casting line consisting of machinery, conveyors, robots, sensors, etc., that automatically could perform the expendable-pattern casting process. 11.54. Outline the casting processes that would be most suitable for making small toys. Explain your choices. 11.55. Make a list of the mold and die materials used in the casting processes described in this chapter. Under each type of material, list the casting processes that are employed and explain why these processes are suitable for that particular mold or die material. 11.56. Write a brief report on the permeability of molds and the techniques that are used to determine permeability. 11.57. Light metals commonly are cast in vulcanized rubber molds. Conduct a literature search and describe the mechanics of this process. 11.58. It sometimes is desirable to cool metals more slowly than they would be if the molds were maintained at room temperature. List and explain the methods you would use to slow down the cooling process. 11.59. The part shown below is a hemispherical shell used as an acetabular (mushroom-shaped) cup in a total hip replacement. Select a casting process for making this part, and provide a sketch of all the patterns or tooling needed if it is to be produced from a cobalt–chrome alloy. ׉ 7cassandra://wuoW1lPdbzwflS67-CdNM1yNWfp7NDv3DXZkCTsYZAA`cZ%EbtcZ%Ebs#בCט#u׉׉ 7cassandra://veUd6GNO1qUGHsI0t0rvQfSMJJYWAdQh-R7LvK6UZlIo `׉ 7cassandra://Edzjbc55uI2Mc367o9cbFXOXPIuIF5Lx4Pmrp_pGkvQ`s׉ 7cassandra://N_1SjrRFPA0pbC-lgTWV1v_LkIEi6OvpD7xQUll4E64 `׉ 7cassandra://idfySnMwzxglVfQxcGSx_9XActcGbRoGosHY-bRd3VgO͠^cZEbט#u׉׉ 7cassandra://_lC6AKf0gAfUtgbcswHx25bgO7j87N1Qy93ATjatZq8 `׉ 7cassandra://wURhYjPMiwwx96AvWsKwk1tCCdidgCBgyWVp42XruRoB`s׉ 7cassandra://3KDx1Ywu5vP86QnfJZX2BepOl-lV89fV-AVLQdwYUqE`׉ 7cassandra://WeAs49zwa7ghcC9kxlY_PePlPT7mbHmKkkte7tY7hBw7 ͠^cZEb׉E'Synthesis, Design, and Projects 341 3 5 R 528 57 20 25 Dimensions in mm 11.60. Porosity that has developed in the boss of a casting is illustrated below. Show that the porosity can be eliminated simply by repositioning the parting line of this casting. Boss Cope Riser Drag Part Core 11.61. Review Fig. II.1b, and note that the gemstones have been cast in place. Design a ring with a means of securing a gemstone in the wax pattern, such that it will remain in the mold as the wax is being melted. Could such an approach be used in lost foam casting? ׉ 7cassandra://N_1SjrRFPA0pbC-lgTWV1v_LkIEi6OvpD7xQUll4E64 `cZ%Ebu׉EChapter 12 Metal Casting: Design, Materials, and Economics 12.1 Introduction 342 12.2 Design Considerations in Casting 343 12.3 Casting Alloys 351 12.4 Economics of Casting 356 Case Studies: 12.1 Illustrations of Poor and Good Casting Designs 349 • This final chapter on metal casting serves as a general guide to the interrelationships among product design, material and process selection, and economic considerations in casting. • The chapter describes in detail the design considerations for casting operations, and discusses the general guidelines for successful casting practices. • The characteristics and applications of the most common ferrous and nonferrous alloys are then described. • The chapter ends with a brief review of casting economics. 12.1 Introduction In the preceding two chapters, it was noted that successful casting practice requires the proper control of a large number of variables. These variables pertain to the particular characteristics of the metals and alloys cast, method of casting, mold and die materials, mold design, and processing parameters. Factors such as the flow of the molten metal in the mold cavities, the gating systems, the rate of cooling, and the gases evolved all influence the quality of a casting. This chapter describes general design considerations for metal casting, and presents guidelines for avoiding defects. It then describes the characteristics of the metals and alloys that are commonly cast, together with their typical applications. Because the economics of casting operations are just as important as their technical aspects, the chapter also outlines the basic economic factors relevant to all casting operations. 342 ׉ 7cassandra://3KDx1Ywu5vP86QnfJZX2BepOl-lV89fV-AVLQdwYUqE`cZ%EbvcZ%Ebu#בCט#u׉׉ 7cassandra://82RkmPEQvEh0BjivgshHUG1UR1nlQl6nfZgx4rmpacs`׉ 7cassandra://0HSn1Oxa_YiBJRIxC1_K7Zp4-o7gsFtEwSIPGPMQGAsj`s׉ 7cassandra://O6372yhZ0lXXye-GLfiWuBwgB5kXdhgGtUhQP22flBY`׉ 7cassandra://nkoXROHG3IpvPUp2MioCmk974xXBkqmP2hSut1tNOYsB͠^cZEbט#u׉׉ 7cassandra://XI2nfNHFMKfSMC0TZiUcYzNlkFyMuXu-8aQofb_VXts`׉ 7cassandra://c3lrd9Dch0SNTbF2VFLWieWNz9X4LnPzMlZIu0lKJmoP7`s׉ 7cassandra://cpyFq_qGieL8t8d9YaDoOekcr3nTJdynIVlrvqvUTYs`׉ 7cassandra://nJcC9YodiSBBrTHx0AaDqcjvh9Q7kqj9pgLBfhWi6XIV͠^cZEb׉E;Design Considerations in Casting 343 12.2 Design Considerations in Casting As in all manufacturing operations, certain design principles pertaining to casting have been developed over the years. Although these principles have been established primarily through experience, analytical methods, process simulation and modeling, and computer-aided design and manufacturing techniques have now all come into wide use as well, improving the quality of castings and productivity, resulting in significant cost savings. All casting processes share several basic characteristics; consequently, a number of design considerations apply equally to, for example, sand casting and die casting. However, each process still has its own particular design considerations. Sand casting will require consideration of mold erosion and associated sand inclusions in the casting process, whereas die casting will not have this concern, but it has others, such as heat checking of dies, which significantly reduces die life. Troubleshooting the causes of defects in cast products can be complicated; the considerations presented in this chapter are to serve only as guidelines. Moreover, defects frequently are random and can be difficult to reproduce, thus complicating the implementation of corrective measures. In most situations, a given mold design will produce mostly good parts as well as some defective parts. For these reasons, strict quality control procedures have to be implemented, especially for critical applications. It should also be noted that many of the design rules developed over the years are now put in doubt or somewhat relaxed through the application of additive manufacturing (Chapter 20). For example, a sand mold produced through additive manufacturing need not be designed with certain accommodations, such as draft and corner radii, to allow for pattern removal. Additive manufacturing has its own design concerns, as described in Section 20.12, as well as its economic usefulness for short production runs. 12.2.1 General Design Considerations for Castings There are two types of design issues in casting: (a) geometric features, tolerances, etc., that should be incorporated into the part and (b) mold features that are needed to produce the desired casting. Robust design of castings usually involves the following steps: 1. Design the part so that the shape is cast as easily as possible. If secondary operations, such as machining, are required, include a machining allowance (that is, a slight oversize of the part), surfaces for fixturing, and reinforcement where necessary (to support the machining forces). Several design considerations are given in this chapter to assist in such efforts. 2. Select a casting process and a material suitable for the part, its size, the required production quantity, and mechanical properties. Often, the shape, the material, and the process(s) need to be specified simultaneously, which can be a demanding design challenge. 3. Locate the parting line of the mold in the part. 4. Design and locate the gates to allow uniform feeding of the mold cavity with molten metal. 5. Select an appropriate runner geometry for the system. 6. Locate mold features, such as sprues, screens, and risers, as appropriate. 7. Check that proper controls and good practices are in place. Design of Parts to Be Cast. The following considerations are important in designing castings, as outlined in Fig. 12.1: 1. Corners, angles, and section thickness. Sharp corners, angles, and fillets should be avoided as much as possible, because they act as stress raisers and may cause cracking and tearing of the metal (as ׉ 7cassandra://O6372yhZ0lXXye-GLfiWuBwgB5kXdhgGtUhQP22flBY`cZ%Ebw׉E344 Chapter 12 Metal Casting: Design, Materials, and Economics Poor Good Poor Good Use radii or fillets to avoid corners and provide uniform cross-section. Ribs and/or fillets improve bosses. Poor Good Poor Good Wall sections should be uniform. Poor Side cores can be eliminated with this hole design. Good Sloping bosses can be designed for straight die parting to simplify die design. Figure 12.1: Suggested design modifications to avoid defects in castings. Source: Courtesy of the American Die Casting Institute. well as of the dies) during solidification. Fillet radii should be selected so as to minimize stress concentrations and to ensure proper molten-metal flow during pouring. Fillet radii usually range from 3 to 25 mm, although smaller radii may be permissible in small castings and for specific applications. On the other hand, if the fillet radii are too large, the volume of the material in those regions also is large, and hence the cooling rate is lower. Section changes should be blended smoothly into each other. The location of the largest circle that can be inscribed in a particular region (Figs. 12.2a and b) is critical so far as shrinkage cavities are concerned. Because the cooling rate in regions with larger circles is lower, these regions are called hot spots, and can cause shrinkage cavities and porosity (Figs. 12.2c and d). Cavities at hot spots can be eliminated by using small cores; although they produce cored holes in the casting (Fig. 12.2e), these holes do not significantly affect its strength. It is also important to maintain uniform cross sections and wall thicknesses throughout the casting, in order to avoid or minimize shrinkage cavities. Although they increase the production cost, metal paddings or chills in the mold can eliminate or minimize hot spots (see Fig. 10.14). 2. Flat areas. Large flat areas (plane surfaces) should be avoided, since (a) they may warp during cooling because of temperature gradients or (b) result in poor surface finish because of uneven flow of the metal during pouring. One of the common techniques for avoiding these problems is to break up flat surfaces with staggered ribs and serrations, as described below. ׉ 7cassandra://cpyFq_qGieL8t8d9YaDoOekcr3nTJdynIVlrvqvUTYs`cZ%EbxcZ%Ebw#בCט#u׉׉ 7cassandra://mCtuNGXPN2vgtBEoswxBtzSsotHIU04kFpC4EiZ0C9c`׉ 7cassandra://bKLXA4qX7UKLkfz6QUrA6HzG_9l0o2geoqmUosWP76kO`s׉ 7cassandra://AKJsUAWhuHv40U8niHbXgm6sWdQbPAjPTCc1Ctw5lfk`׉ 7cassandra://qEbQUFIaDLpcBm16xzyY_a5WE0VMPeX_yqnaEDsf6Dkͫj͠^cZEbט#u׉׉ 7cassandra://GaJj86iunW4WQY1g2PEYaEkCTyIuiYNVS0KW1NksrVg`׉ 7cassandra://5OWcg-gshaCN4WTBesvnFud1YUVVGra-X0fjVUCxLX0[`s׉ 7cassandra://TQAOsLIVpUlyBwb69JRzlJKuA3I2uggQ1bd-xvd4DSU`׉ 7cassandra://uv08-6n5U5lvlpMwjj-CYlznWD5kdjJ0dtsAjZ_GLPk[<͠^cZEb׉EDesign Considerations in Casting 345 Poor Shrinkage cavity (a) Poor (b) Good Core (c) Good (d) (e) Figure 12.2: Examples of designs showing the importance of maintaining uniform cross sections in castings to avoid hot spots and shrinkage cavities. (a) Illustration of the method of inscribing the largest possible circle in a cross section; locations where abrupt changes in circle size occurs are concerns for hot spots and shrinkage pores. (b)–(e) Common geometries and strategies for reducing or eliminating pores. 3. Ribs. One method of producing parts with uniform thickness is to eliminate large, bulky volumes in the casting, as shown in Fig. 12.1; however, this can result in a loss in stiffness and, especially with flat regions, can lead to warping. One solution to these difficulties is to use ribs or a support structure on the casting, as shown in Fig. 12.3. They are usually placed on the side that is less visible. Ribs should, in general, have a thickness around 80% of the adjoining member thickness, and should be deeper than their strut thickness. It is beneficial to have the ribs solidify before the members they adjoin. Ribs should not be placed on both sides of a casting, and should not meet at acute angles, because of complications to molding. 4. Shrinkage. To avoid cracking of the casting during cooling, allowance should be provided for shrinkage during solidification and/or cooling to room temperature. In castings with intersecting ribs, the tensile stresses developed can be reduced by staggering the ribs or by modifying the intersection Poor Poor Good Best (a) (b) (c) (d) Figure 12.3: Rib designs for use on thin sections or flat surfaces to control or eliminate warping. Note the progression of designs: from left to right, the rib designs have improved castability and reliability. ׉ 7cassandra://AKJsUAWhuHv40U8niHbXgm6sWdQbPAjPTCc1Ctw5lfk`cZ%Eby׉E 346 Chapter 12 Metal Casting: Design, Materials, and Economics Table 12.1: Normal Shrinkage Allowance for Some Metals Cast in Sand Molds. Metal Shrinkage allowance (%) Cast Irons Gray cast iron White cast iron Malleable cast iron Aluminum alloys Magnesium alloys Copper alloys Yellow brass Phosphor bronze Aluminum bronze High-manganese steel 0.83–1.3 2.1 0.78–1.0 1.3 1.3 1.3–1.6 1.0–1.6 2.1 2.6 geometry. Pattern dimensions also should allow for shrinkage of the metal during solidification and cooling. Allowances for shrinkage, known as patternmaker’s shrinkage allowances, usually range from about 10 to 20 mm/m. Table 12.1 gives the normal shrinkage allowance for metals commonly sand cast. 5. Draft. A small draft (taper) is typically provided in sand-mold patterns to enable the removal of the pattern without damaging the mold (see Fig. 11.5). Drafts generally range from 5 to 15 mm/m. Depending on the quality of the pattern, draft angles usually range from 0.5◦ to 2◦. The angles on inside surfaces typically are twice this range; they have to be higher than those for outer surfaces because the casting shrinks inward toward the core. 6. Dimensional tolerances. Dimensional tolerances depend on the particular casting process employed, size of the casting, and type of pattern used. Tolerances should be as wide as possible, within the limits of good part performance, as otherwise the cost of the casting increases. In commercial practice, tolerances are typically in the range of ±0.8 mm for small castings, and increase with the size of the castings. For large castings, for instance, they may be as much as ±6 mm. 7. Lettering and markings. It is common practice to include some form of part identification, such as lettering, numbers, or company logos. These features can be sunk into the casting or can protrude from the casting surface; the more desirable one depends on the method of producing the molds. For example, in sand casting, a pattern plate is produced by machining on a computer numerically controlled milling machine (Section 24.2), because it is simpler to machine letters into the pattern plate, they well be recessed in the part. In die casting, it is simpler to machine letters into the mold, leading to letters that protrude. 8. Finishing operations. In casting design, it is important to consider the subsequent machining and finishing operations that may be required. For example, if a hole is to be drilled in a casting, it is better to locate it on a flat surface rather than on a curved surface, in order to prevent the drill from wandering. An even better design would incorporate a small dimple on the curved surface as a starting point for the drilling operation. Castings should also include features that allow them to be clamped easily on to machine tools, if secondary machining operations are necessary. 9. Integrated Computational Materials Engineering (ICME). The use of modern computational tools allows identification of design and manufacturing issues and allows the prediction of material properties and microstructure that results from a particular mold cavity design. The time spent in process simulation actually saves time that normally would be expended in tooling rework and cost associated with defects. ׉ 7cassandra://TQAOsLIVpUlyBwb69JRzlJKuA3I2uggQ1bd-xvd4DSU`cZ%EbzcZ%Eby#בCט#u׉׉ 7cassandra://3_PM4beSlG1EIK3keIxUE4wu0XLdhcHa2ZV8OrNSuB8A`׉ 7cassandra://sRciyadpD_nt7-Wkf6my1hAA3Kyfkeh4BCiFUrvG0vAm`s׉ 7cassandra://oufHMUWKYP66x2N0QBLhDwoCFNvoXQja52wh3qL2_wY`׉ 7cassandra://y5EQmJSYPkyfWIda0ZpadUR_REXyx0FZ1Wq-bSzach8NL͠^cZEbט#u׉׉ 7cassandra://oD1bFb3-3XtIAZdpApTFsbMb1nn_iIP1wfaI1Z4v1wk>|`׉ 7cassandra://WE70CpBYOFBrg2vQdnnmJc12OPelPSPV6ZUBHMd7X1Yl`s׉ 7cassandra://tGg_0D_iHOE4DsqrU0KoMjzh68bk78UpOWzsJEwEx1o`׉ 7cassandra://Vlf4kRQwE-SRG8wbU3AdT__UEqxtSJwk5omHgGLNRXc[͠^cZEb׉EDesign Considerations in Casting 347 Selecting a Casting Process. Casting processes cannot be selected separately from economic considerations, as described in Section 12.4. Table 11.2 lists some of the advantages and limitations of casting processes that have an impact on casting design. Locating the Parting Line. A casting should be oriented in a mold so that the large portion of the casting is relatively low and the height of the casting is minimized. Part orientation also determines the distribution of porosity; for example, in casting aluminum, hydrogen is soluble in liquid metal but is not soluble as the aluminum solidifies (see Fig. 10.17). Hydrogen bubbles can form during the casting of aluminum, which float upwards due to buoyancy and causing a higher porosity in the top regions of castings. Thus, critical surfaces should be oriented so that they face downwards. A properly oriented casting then can have the parting line determined; this is the line or plane separating the upper (cope) and lower (drag) halves of molds (see Fig. 11.3). In general, the parting line should be along a flat plane rather than be contoured. Whenever possible, the line should be at the corners or edges of castings rather than on flat surfaces in the middle of the casting, so that the flash (material squeezing out between the two halves of the mold) at the parting line will not be as visible. The location of the line is also important because it influences mold design, ease of molding, the number and shape of cores required, method of their support, and the gating system. The parting line should be placed as low as possible (relative to the casting) for metals with lower density (such as aluminum alloys) and be located at around mid-height for denser metals (such as steels). However, the molten metal should not be allowed to flow vertically, especially when unconstrained by a sprue. The placement of the parting line has a large effect on the remainder of the mold design. For example, in sand casting, it is common practice that the runners, gates, and sprue well are all placed in the drag on the parting line. Also, the placement of the parting line and orientation of the part determine the number of cores required, especially when it is preferable to avoid the use of cores whenever practical. Locating and Designing Gates. Gates are the connections between the runners and the part to be cast. Important considerations in gating system design are: • Multiple gates often are preferable, and are necessary for large parts; they have the benefits of allowing lower pouring temperature and reducing temperature gradients in the casting. • Gates should feed into thicker sections of castings. • A fillet should be used where a gate meets a casting; this feature produces less turbulence than abrupt junctions. • The gate closest to the sprue should be placed sufficiently away from the sprue, so that the gate can be easily removed. This distance may be as small as a few millimeters for small castings, and up to 500 mm for large ones. • The minimum gate length should be three to five times the gate diameter, depending on the metal being cast. The gate cross section should be large enough to allow the filling of the mold cavity and should be smaller than the runner cross section. • Curved gates should be avoided; when necessary, a straight section in the gate should be located immediately adjacent to the casting. Runner Design. The runner is a horizontal distribution channel that receives molten metal from the sprue and delivers it to the gates. They are used to trap dross (a mixture of oxide and metal that forms on the surface of metals) and keep it from entering the gates and mold cavity. Commonly, dross traps are placed at the ends of runners, and the runner projects above the gates to ensure that the metal in the gates is tapped from below the surface. A single runner is used for simple parts, but two-runner systems may be necessary for more complicated castings. ׉ 7cassandra://oufHMUWKYP66x2N0QBLhDwoCFNvoXQja52wh3qL2_wY`cZ%Eb{׉E348 Chapter 12 Metal Casting: Design, Materials, and Economics Designing Various Mold Features. The main goal in designing a sprue (described in Section 10.3) is to achieve the required molten-metal flow rates, while preventing aspiration (entrainment of air) or excessive dross formation. Flow rates are determined such that turbulence is avoided, but also that the mold is filled quickly as compared to the solidification time required. A pouring basin can be used to ensure that the metal flow into the sprue is uninterrupted; also, if molten metal is maintained in the pouring basin during pouring, the dross will float and will not enter the mold cavity. Filters are used to trap large contaminants, also serving to reduce the metal velocity and make the flow more laminar. Chills can be used to speed solidification of the metal in a particular region of a casting. Establishing Good Practices. It has been widely observed that a given mold design can produce acceptable castings as well as defective ones, and it rarely will produce only good or only defective castings. To check for defective ones, quality control procedures are necessary. Some common concerns are the following: • Starting with a high-quality molten metal is essential for producing superior castings. Pouring temperature, metal chemistry, gas entrainment, and handling procedures all can affect the quality of metal being poured into a mold. • The pouring of the molten metal should not be interrupted, because it can lead to dross entrainment and turbulence. The meniscus of the molten metal in the mold cavity should experience a continuous, uninterrupted, and upward advance. • The different cooling rates within the body of a casting can cause residual stresses. Thus, stress relieving (Section 4.11) may be necessary to avoid distortions of castings in critical applications. 12.2.2 Design for Expendable-mold Casting Expendable-mold processes have certain specific design requirements, mainly involving the mold material, part sizes, and the manufacturing method. Recall that a casting in an expendable-mold process, such investment casting, will cool much more slowly than it would in, say, die casting; this has important implications in the layout of molds. Important design considerations for expendable-mold casting are as follows. Mold Layout. The various features in a mold must be placed logically and compactly, with gates as necessary. One of the most important goals in mold layout is to have solidification initiate at one end of the mold and progress across the casting in a uniform front, with the risers solidifying last. Traditionally, mold layout has been based on experience and on considerations of fluid flow and heat transfer. Commercial computer programs have now become widely available assisting in the analysis of fluid flow and heat transfer. These programs simulate mold filling and allow the rapid evaluation and design of mold layouts. Riser Design. A major concern in the design of castings is the size of risers and their placement. Risers are very useful in affecting the solidification-front progression across a casting, and are an essential feature in mold layout described previously. Blind risers are good design features and maintain heat longer than open risers do. Risers are designed according to the following basic rules: 1. The riser must not solidify before the casting does. This rule usually is satisfied by avoiding the use of small risers and by using cylindrical risers with small aspect ratios (i.e., small ratios of height to cross section). Spherical risers are the most efficient shape, but are difficult to work with. 2. The riser volume must be sufficiently large to provide enough molten metal to compensate for shrinkage in the casting. 3. Junctions between the casting and the riser should not develop hot spots, where shrinkage porosity can occur. 4. Risers must be placed such that the molten metal can reach locations where it is most needed. ׉ 7cassandra://tGg_0D_iHOE4DsqrU0KoMjzh68bk78UpOWzsJEwEx1o`cZ%Eb|cZ%Eb{#בCט#u׉׉ 7cassandra://VQtigL1TQbr9rFWYxqvDE7pyUXzXF3LAYwiGXM76nKI0`׉ 7cassandra://E1dtF_c8798MMnIUutqbAS3s5XrcimVlFtrqto-c_fgco`s׉ 7cassandra://--SCZh_3CI_VOtg4sgJEtbhm-sMrALiMZnVRss1kR9I`׉ 7cassandra://YjDhj-DgUH3W7PtdkZrxrRhyhJa4zRRjXFYbo1hJ5TgW͠^cZEbט#u׉׉ 7cassandra://tAC2e3DPHx1_tgz3OXuhFGnlrriPChwF45wNE0QOkyc`׉ 7cassandra://Chtgl3_vfUMyg0FuLhz0Ecg7GoqrBFT7jpXQV7PEWvwCH`s׉ 7cassandra://Ojp-qnkMvwroHrZe4GQsga--_UA8dDa9mxaGiUZ_3dA`׉ 7cassandra://jdmUD2BWo5G_rIr-Z9nNeS4S6Y8riM20Kv3Ot6TVlo0 I͠^cZEb׉EDesign Considerations in Casting 349 5. There must be sufficient pressure to drive the molten metal into locations in the mold where it is needed. Risers are not as useful for metals with low density (such as aluminum alloys) as they are for those with higher density (such as steel and cast irons). 6. The pressure head from the riser should suppress cavity formation and encourage complete filling of the mold cavity. Machining Allowance. Most expendable-mold castings require some additional finishing operations, such as machining and grinding; allowances have to be included in casting design for these operations. Machining allowances, which are included in pattern dimensions, depend on the type of casting operation, noting also that they increase with the size and section thickness of the casting. Allowances usually range from about 2 to 5 mm for small castings to more than 25 mm for large castings. 12.2.3 Design for Permanent-mold Casting General design guidelines for permanent-mold casting are described in Example 12.1. Although designs may be modified to eliminate the draft, for better dimensional accuracy, a draft angle of 0.5◦ or even 0.25◦ is usually required; otherwise, galling (localized seizure or sticking of two surfaces, Section 33.5) may occur between the part and the dies, causing distortion of the casting. Die-cast parts are nearly net shaped, typically requiring only the removal of gates and minor trimming to remove flashing and other minor defects. The surface finish and dimensional accuracy of die-cast parts are very good (see Table 11.3) and, in general, they do not require a machining allowance. Case Study 12.1 Illustrations of Poor and Good Casting Designs Several examples of poor and good designs in permanent-mold and die casting are illustrated in Fig. 12.4. The significant differences in design are outlined here for each example: 1. The lower portion of the design on the left has a thin wall, with no apparent specific function; at this location, the part may fracture if subjected to high forces or to impact. The good design eliminates this possibility, and also may simplify die and mold making. 2. Large flat surfaces always present difficulties, as they tend to warp and develop uneven surfaces. A common practice to avoid this situation is to break up the surface with ribs (see Fig. 12.3) and serrations on the reverse side of the casting. This approach greatly reduces part distortion, while not adversely affecting the appearance and function of the flat surface. In addition to ribs, it is beneficial to use a textured surface, as shown in Fig. 12.4b, since very smooth surfaces are difficult to cast without objectionable aesthetic features. 3. This example of poor and good design is relevant not only to castings, but also to parts that are subsequently machined or ground. It is difficult to produce sharp internal radii or corners that may be required for functional purposes, such as inserts designed to reach the bottom of the part cavity. Also, in the case of lubricated cavities, the lubricant can accumulate at the bottom and, because it is incompressible, prevent full insertion of an insert. The placement of a small radius at the corners or periphery at the bottom of the part eliminates this problem. 4. A cast part could function, for instance, as a knob to be gripped and rotated, hence the outer features along its periphery. Note in the design on the left that the inner periphery of the knob also has features which are not functional but help save material; the die for the good design is easier to manufacture. ׉ 7cassandra://--SCZh_3CI_VOtg4sgJEtbhm-sMrALiMZnVRss1kR9I`cZ%Eb}׉E-350 Chapter 12 Metal Casting: Design, Materials, and Economics Poor Good Poor Good (a) (b) (c) (d) (e) Figure 12.4: Examples of undesirable (poor) and desirable (good) casting designs. 5. Note that the poor design has sharp fillets at the base of the longitudinal grooves, indicating that the die has sharp (knife-edge) protrusions. It is thus possible that, with overextended use of the die, these edges may chip off. 12.2.4 Computer Modeling of Casting Processes Because casting involves complex interactions among several material and process variables, a quantitative study of these interactions is essential to the proper design and production of high-quality castings. Rapid advances in modeling techniques have led to important innovations in modeling casting processes. These include fluid flow, heat transfer, and the microstructures developed during solidification under various casting conditions, as described in Section 10.3. Simulations are capable of predicting, for example, the width of the mushy zone (see Fig. 10.4) during solidification and the grain size in castings. Similarly, the capability to calculate isotherms (lines of equal temperature) give insight into possible hot spots and the subsequent development of shrinkage cavities. With the availability of user-friendly software and advances in computer-aided design and manufacturing (Chapter 38), modeling techniques have become easier to implement. The benefits of this approach are improved quality, easier planning and cost estimating, increased productivity, and faster response to design changes. ׉ 7cassandra://Ojp-qnkMvwroHrZe4GQsga--_UA8dDa9mxaGiUZ_3dA`cZ%Eb~cZ%Eb}#בCט#u׉׉ 7cassandra://i6Qi-2alvn3LTAMqmD86VSMCbi9tTVKaH0U3TfEYF8g9`׉ 7cassandra://BFG36Z3t8vD3AQJS4M-So7WIDJed2RpOlnjmsAwxnmQc{`s׉ 7cassandra://tkutPFFVLNf7T6L915_dkC6unMaKm4DlR8M8H1AXSAc`׉ 7cassandra://Ack7xZOlWbQpFzVjCAJy_mT9AgW_IwLC1dqGXR7rKWwN͠^cZEbט#u׉׉ 7cassandra://0b1zN4vYJfhi1EjZdrl1jyeXikdSZ1nRYQQjA3XDykE3`׉ 7cassandra://4ZG6XmfipH0-wAJ_cnpYqy3CccX-KoMDe1Vt6VSxCWg7`s׉ 7cassandra://C8tACHzEuO0Q9HlNO8igE6RtKeycTevZH99v5-DeXXY@`׉ 7cassandra://dr3OTwg64JcjFVu4v3ZnzmiF5Bb1gMCcSzY5tA-gnXox`͠^cZEb׉E Casting Alloys 351 12.3 Casting Alloys The general properties and typical applications of ferrous and nonferrous metals and alloys were presented in Chapters 5 and 6, respectively. This section describes the properties and applications of cast metals and alloys; their properties and casting and manufacturing characteristics are summarized in Fig. 12.5 and Tables 12.2 through 12.5. In addition to their casting characteristics, other important considerations in casting alloys include their machinability and weldability, since they are assembled with other components to produce the entire assembly. The most commonly used casting alloy (in tonnage) is gray iron, followed by ductile iron, malleable iron, steel, copper, aluminum, magnesium, and zinc. Shipments of castings in the United States alone are around 9.07 million metric tons per year. 12.3.1 Nonferrous Casting Alloys Common nonferrous casting alloys are as follows: Aluminum-based Alloys. Aluminum alloys have a wide range of mechanical properties, mainly because of various hardening mechanisms and heat treatments that can be used (Section 4.9). Parts made of aluminum and magnesium alloys are known as light-metal castings. They have high electrical conductivity and generally good atmospheric corrosion resistance; however, their resistance to all alkalines and some acids is poor, and care must be taken to prevent galvanic corrosion. Aluminum alloys are lightweight, nontoxic, and have good machinability. Except for alloys containing silicon, they generally have low resistance to wear and abrasion. They have numerous applications, including architectural and decorative purposes. An increasing trend is their use in automobiles, for components such as engine blocks, cylinder heads, intake manifolds, transmission cases, suspension components, wheels and brakes. Magnesium-based Alloys. These alloys have the lowest density of all commercial casting alloys. They have good corrosion resistance and moderate strength, depending on the particular heat treatment used. Typical applications include automotive wheels, housings, and air-cooled engine blocks. Because of their light weight, magnesium castings are being increasingly used in automobiles to increase fuel economy. Copper-based Alloys. These alloys have the advantages of good electrical and thermal conductivity, corrosion resistance, and nontoxicity, as well as wear properties suitable as bearing materials. A wide variety of copper-based alloys is available, including brasses, aluminum bronzes, phosphor bronzes, and tin bronzes. Zinc-based Alloys. A low-melting-point alloy group, zinc-based alloys have good corrosion resistance, good fluidity, and sufficient strength for structural applications. These alloys are commonly used in die casting, particularly for parts with thin walls and complex shapes. Tin-based Alloys. Although low in strength, these alloys have good corrosion resistance and are typically used for linings or bearing surfaces. High-temperature Alloys. These alloys have a wide range of properties, and typically require temperatures of up to 1650◦C for casting titanium and superalloys, and even higher for refractory alloys (Mo, Nb, W, and Ta). Special techniques are used to cast these alloys for nozzles and various jet- and rocket-engine components. Some high-temperature alloys are more suitable and economical for casting than for shaping by other manufacturing methods, such as forging and powder metallurgy techniques. ׉ 7cassandra://tkutPFFVLNf7T6L915_dkC6unMaKm4DlR8M8H1AXSAc`cZ%Eb׉E#352 Chapter 12 Metal Casting: Design, Materials, and Economics 2000 1800 1600 1400 1200 1000 800 600 400 200 (a) (b) Wrought Cast 200 150 100 50 (c) 0 (e) 800 700 600 500 400 300 200 100 0 (g) 70 60 50 40 30 20 10 0 (d) 10 12 8 6 4 2 0 2000 1800 1600 1400 1200 1000 800 600 400 200 (f) 90 80 70 60 50 40 30 20 10 0 (h) 70 60 50 40 30 20 10 0 Figure 12.5: Mechanical properties for various groups of cast alloys. Note that even within the same group, the properties vary over a wide range, particularly for cast steels. Source: Courtesy of Steel Founders’ Society of America. Elongation (%) Brinell hardness (HB) Steel Nodular iron Gray iron Malleable iron Aluminum base Copper base Magnesium base Nickel base Zinc base Zinc base Magnesium base Nickel base Nodular iron Gray iron Malleable iron Aluminum base Copper base Steel Young’s modulus (GPa) Magnesium base Aluminum base Zinc base Titanium metal Titanium alloys Copper base Nickel base Gray iron Nodular iron Malleable iron Cast steel Gray iron Malleable iron Aluminum base Copper base Magnesium base Nickel base Zinc base Tensile strength (MPa) Steel Nodular iron Reduction of area (%) Impact energy J Tensile strength/density ratio (m 3 103) Yield strength (MPa) Steel Steel Nodular iron Nodular iron Gray iron Malleable iron Copper base Nickel base Gray iron Malleable iron Copper base Nickel base Gray iron Nodular iron Malleable iron Steel Magnesium base Nickel base Zinc base Aluminum base Magnesium base Titanium metal Titanium alloy Steel Nodular iron Gray iron Malleable iron Aluminum base Copper base ׉ 7cassandra://C8tACHzEuO0Q9HlNO8igE6RtKeycTevZH99v5-DeXXY@`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://as5QUSOLKTxxfOF-bMp7HrqZRQPad-lDdCXN4Zv8Woc`׉ 7cassandra://nG7aTuOEYHd_Axma3k_8Oiq6vvS9vUrrOofac3Utbsk<`s׉ 7cassandra://GVjZ-WwmrpK5pfIkJctby3E7xVJz468bf_GH9bM_Q4w`׉ 7cassandra://PxruP1Hba7fVqNFmMreoWRRPHFrJms8eUAvmqavnq-M\t͠^cZEbט#u׉׉ 7cassandra://KNb1aakLz8a3dauvmXLAEpnqLUhwT_NwSwN7bdwFvgg8`׉ 7cassandra://pDNJOCbI3WsOBuu_z4OIeTq0_D56H3aw0TiQsrLySSwF`s׉ 7cassandra://8q1nYPG8B7rL8ibWEbrux2nUjsowWvOKun82Et-dzFc*`׉ 7cassandra://x9_TfmrF_OTdFTc5s4TujK7n9_v-dwqfI-0q6d6Emsco͠^cZEb׉E'Casting Alloys 353 Type of alloy Aluminum Copper Iron Ductile Gray Malleable iron White iron Magnesium Nickel Steel Carbon and low-alloy High-alloy Zinc F F E E E D F F E Note: ∗ E=excellent;G=good;F=fair; VP =verypoor;D=difficult Die blocks, heavy-duty gear blanks, aircraft undercarriage members, railroad wheels Gas-turbine housings, pump and valve components, rock-crusher jaws Door handles, radiator grills G E G G G–E F D D D VP G F G G G VP E F Crankshafts, heavy-duty gears Engine blocks, gears, brake disks and drums, machine bases Farm and construction machinery, heavy-duty bearings, railroad rolling stock Mill liners, shot-blasting nozzles, railroad brake shoes, crushers, and pulverizers Crankcase, transmission housings Gas turbine blades, pump and valve components for chemical plants Table 12.2: Typical Applications for Castings and Casting Characteristics. Castability∗ Weldability∗ E F–G F F Machinability∗ Typical applications G–E F–G Pistons, clutch housings, intake manifolds Pumps, valves, gear blanks, marine propellers Table 12.3: Properties and Typical Applications of Cast Irons. Ultimate tensile Cast iron Gray Ductile (Nodular) Type Ferritic Pearlitic Martensitic Ferritic Pearlitic Tempered martensite Malleable Ferritic Pearlitic Tempered martensite White Pearlitic strength (MPa) 170 275 550 415 550 825 365 450 700 275 Yield strength (MPa) 140 240 550 275 380 620 240 310 550 275 Elongation in 50 mm (%) 0.4 0.4 18 6 2 18 10 2 0 Typical applications Pipe, sanitary ware Engine blocks, machine tools 0 Wear surfaces Pipe, general service Crankshafts, highly stressed parts High-strength machine parts, wear-resistant parts Hardware, pipe fittings, general engineering service Railroad equipment, couplings Railroad equipment, gears, connecting rods Wear-resistant parts, mill rolls ׉ 7cassandra://GVjZ-WwmrpK5pfIkJctby3E7xVJz468bf_GH9bM_Q4w`cZ%Eb׉E354 Chapter 12 Metal Casting: Design, Materials, and Economics Table 12.4: Mechanical Properties of Gray Cast Irons. Ultimate tensile ASTM class 20 25 30 35 40 50 60 strength (MPa) 152 179 214 252 293 362 431 Compressive strength (MPa) 572 669 752 855 965 1130 1293 Elastic modulus Hardness (GPa) 66–97 79–102 90–113 100–119 110–138 130–157 141–162 (HB) 156 174 210 212 235 262 302 Table 12.5: Properties and Typical Applications of Nonferrous Cast Alloys. Ultimate tensile Alloys (UNS) Aluminum alloys 195 (AO1950) 319 (AO3190) 356 (AO3560) Copper alloys Red brass (C83600) Yellow brass (C86400) Manganese bronze (C86100) Sulfur tin bronze (C83470) Copper Bismuth (C89836) Gun metal (C90500) Nickel silver (C97600) Magnesium alloys AZ91A AZ63A AZ91C EZ33A HK31A QE22A Condition Heat treated strength (MPa) 220–280 260 Yield strength (MPa) 110–220 Heat treated 185–250 125–180 Heat treated 185 Annealed Annealed Annealed As cast As cast Annealed Annealed F T4 T6 T5 T6 T6 235 275 480 190 230 275 275 230 275 275 160 210 275 115 95 195 95 95 105 175 150 95 130 110 105 205 Elongation in 50 mm (%) 8.5–2 2–1.5 5 25 25 30 20 30 15 3 12 5 3 8 4 Typical applications Sand castings Sand castings Permanent mold castings Pipe fittings, gears Hardware, ornamental Propeller hubs, blades 15 Water supply piping and fittings, valves Antimicrobial; water supply and fittings Pump parts, fittings Marine parts, valves Die castings Sand and permanent mold castings High-strength parts Elevated-temperature parts Elevated-temperature parts Highest-strength parts 12.3.2 Ferrous Casting Alloys Commonly cast ferrous alloys are as follows: Cast Irons. Cast irons represent the largest quantity of all metals cast. They can easily be cast into intricate shapes, and generally possess several desirable properties, such as high hardness, wear resistance, and good machinability. The term cast iron refers to a family of alloys, and as described in Section 4.6, they are classified as gray cast iron (gray iron), ductile (nodular or spheroidal) iron, white cast iron, malleable iron, and compacted-graphite iron. Their general properties and typical applications are given in Tables 12.3 and 12.4. ׉ 7cassandra://8q1nYPG8B7rL8ibWEbrux2nUjsowWvOKun82Et-dzFc*`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://OprAIAJ0roWLeqrp4EtQMV19iu6bwWOyTr81XLwGbeUZ`׉ 7cassandra://w8hhmqaNWAm3bFtimJeL8g7Ayh82ZgUEBxAqmdtgpEUl`s׉ 7cassandra://k9EwJLhR4pJHLAE0v2Ewby8X52Zlu2vfeXnRYZac88I$`׉ 7cassandra://AO14VPL8lLvhqudRxp-7xTKAFhwMIBwipnnw_Y2KPhMVJ͠^cZEbט#u׉׉ 7cassandra://-byFIuLCtTfJHXzWdyCw-2kyU8Oo5Krk9rlQAv7o4I4`׉ 7cassandra://3PrUwQ4FC67jrSNFoVo2Tue-U8vYI-mx45Bd1JsTOX4_`s׉ 7cassandra://gdv4HYB2LIjDx1EQzc_jZN_id8TlsMpKSwx0uLSJKcc`׉ 7cassandra://hmNfVe25IRSGuXisOgAZ3kYeo_IViLg4EfcGVfx4IWwYj͠^cZEb׉ECasting Alloys 355 1. Gray cast iron. Gray iron castings have relatively few shrinkage cavities and low porosity. Various forms of gray cast iron are ferritic, pearlitic, and martensitic, and because of differences in their structures, each type has different properties (Table 12.4). Gray cast irons are specified by a two-digit ASTM designation; thus, for example, class 20 specifies that the material must have a minimum tensile strength of 140 MPa. Typical uses of gray cast iron are in engine blocks, electric-motor housings, pipes, and wear surfaces for machines. Also, because of its high damping capacity, gray iron is used widely for machine-tool bases (Section 25.3). 2. Ductile (nodular) iron. Typically used for machine parts, housings, gears, pipe, rolls for rolling mills, and automotive crankshafts, ductile irons are specified by a set of two-digit numbers. For example, class or grade 80-55-06 indicates that it has a minimum tensile strength of 550 MPa, a minimum yield strength of 380 MPa, and 6% elongation in 50 mm. 3. White cast iron. Because of its very high hardness and wear resistance, white cast iron is used typically for rolls for rolling mills, railroad-car brake shoes, and liners in machinery for processing abrasive materials. 4. Malleable iron. The principal use of malleable iron is for railroad equipment and various types of hardware, fittings, and components for electrical applications. Malleable irons are specified by a fivedigit designation. For example, 35018 indicates that the yield strength is 240 MPa and its elongation is 18% in 50 mm. 5. Compacted-graphite iron. First produced commercially in 1976, compacted-graphite iron (CGI) has properties that are between those of gray irons and ductile irons. Gray iron has good damping and thermal conductivity, but low ductility, whereas ductile iron has poor damping and thermal conductivity, but high tensile strength and fatigue resistance. Compacted-graphite iron has damping and thermal properties similar to gray iron and strength and stiffness that are comparable to those of ductile iron. Because of its strength, castings made of CGI can be smaller, thus lighter. This iron is easy to cast and has properties that are consistent throughout the casting. Moreover, its machinability is better than that of ductile iron (an important consideration since compacted-graphite iron is used for automotive engine blocks and cylinder heads, which require extensive machining). Cast Steels. Because of the high temperatures required to melt steels (up to about 1650◦C), casting steels requires special considerations. The high temperatures involved present difficulties in the selection of mold materials, particularly in view of the high reactivity of steels with oxygen during the melting and pouring of the metal. Steel castings possess properties that are more uniform (isotropic) than those made by mechanical working processes (Part III). Although they can be welded, welding alters the cast microstructure in the heat-affected zone (see Fig. 30.15), thus influencing the strength, ductility, and toughness of the base metal. Subsequent heat treatment would be required to restore the mechanical properties of the casting. Cast weldments have gained importance for assembling large machines and structures. Cast steels have important applications in mining, chemical plants, oil fields, heavy construction, and equipment for railroads. Cast Stainless Steels. Casting of stainless steels involves considerations similar to those for steels. Stainless steels generally have long freezing ranges (see Section 10.2.2) and high melting temperatures. They can develop several structures, depending on their composition and processing parameters. Cast stainless steels are available in various compositions, and they can be heat treated and welded. Cast stainless-steel parts have high heat and corrosion resistance, especially useful in the chemical and food industries. Nickel-based casting alloys are used for very corrosive environments and for very high temperature service. ׉ 7cassandra://k9EwJLhR4pJHLAE0v2Ewby8X52Zlu2vfeXnRYZac88I$`cZ%Eb׉E p356 Chapter 12 Metal Casting: Design, Materials, and Economics Table 12.6: General Cost Characteristics of Casting Processes. Cost∗ Casting process Sand Shell mold Plaster Investment Die L L–M Equipment Labor L M–H L–M H L–M L–M L–M M M–H M–H H Permanent mold M M L–M Die H L–M Centrifugal M H L–M ∗ L = low; M = medium; H = high. 12.4 Economics of Casting As in all manufacturing processes, the cost of each cast part (unit cost) depends on several factors, including materials, equipment, and labor. Recall that among various casting processes described in Chapter 11, some require more labor than others, some require expensive dies and machinery, and some require a long production times to produce the castings (Table 12.6). Each of these individual factors affects the overall cost of a casting operation and to varying degrees. As can be noted in Table 12.6, relatively little cost is involved in making molds for sand casting, whereas molds for other casting processes and especially dies for die-casting require expensive materials and manufacturing operations. There are also major costs involved in making patterns for casting, although much progress continues to be made in utilizing additive manufacturing techniques (Section 20.10) to reduce costs and production time. Costs are also incurred in melting and pouring the molten metal into molds, and in heat treating, cleaning, and inspecting the castings. Heat treating is an important part of the production of many alloy groups (especially ferrous castings), and may be necessary for improving the mechanical properties. However, heat treating may also introduce another set of production problems, such as scale formation on casting surfaces and warpage of the part, that can be a significant aspect of production costs. The labor and the skills required can vary considerably, depending on the particular casting operation and level of automation in the foundry. Investment casting, for example, requires much labor because of the several steps involved in the operation, although some automation in a plant can be implemented, such as using robots (Fig. 11.13a). On the other hand, operations such as in highly automated die-casting maintain high production rates, with little labor involved. Note also that the equipment cost per casting decreases as the number of parts cast increases. Sustained high production rates can justify the high cost of dies and machinery. However, if demand is relatively small, the cost per casting increases rapidly. It then becomes more economical to manufacture the parts either by other casting processes described in this chapter or by considering other manufacturing processes, described in detail in Parts III and IV, singly or in combination. Summary • General guidelines have been established to aid in the production of castings without defects, and to meet dimensional tolerances, surface finish, service requirements, and various specifications and standards. The guidelines concern the shape of the casting and the various techniques to minimize hot spots that could lead to shrinkage cavities. Because of the large number of variables involved, close control of all parameters is essential, particularly those related to the nature of liquid-metal flow into molds and dies, and the rate of cooling in different regions of the mold. Production rate (pieces/hr) < 20 < 10 < 10 < 1000 < 60 < 200 < 50 ׉ 7cassandra://gdv4HYB2LIjDx1EQzc_jZN_id8TlsMpKSwx0uLSJKcc`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://iU3drIEn7QvQUYEWHpyZl5jlE0kQ_CfsA6TOj4NgpNg[`׉ 7cassandra://qQyV_zZBUw1Da6jHCcSwEy4DTfEVhCKSa2q1znss0-cLt`s׉ 7cassandra://HbNkSGjD65EymIzSDJv0d7EzX7PPrNMo8JXx6i-sTRQ`׉ 7cassandra://otip8kceHeAw-s-5aWaWAkTV8d8pqhWIpCUjaTys4Qwen͠^cZEbט#u׉׉ 7cassandra://LOVOD9U6us72lPoOgukj-wCkk_NvmN7efAzP_3bBL2I`׉ 7cassandra://QJIEPJFy_9Xb1eiYb2bPyM2HwJcCwA26xvr0HlDw0bE\?`s׉ 7cassandra://nYL4VgG2DtbLT7O8kzsnwzkmoMyAPwylnDLQ3f3Qq64`׉ 7cassandra://_mgM-ArHLJ1Cmx6HV04wEcJ9zvIxWdgriWY5t7MfDVEJ ͠^cZEb׉EReview Questions 357 • Numerous nonferrous and ferrous casting alloys are available, with a wide range of properties, casting characteristics, and applications. Because many castings are designed and produced to be assembled with other mechanical components and structures (subassemblies), several other considerations, such as weldability, machinability, and surface characteristics, also are important. • Within the limits of good performance, the economics of casting is just as important as the technical considerations. Factors affecting the overall cost are the cost of materials, molds, dies, equipment, and labor, each of which varies with the particular casting operation. Key Terms Cast iron Compacted-graphite iron Design principles Draft Flash Hot spots Bibliography Abrasion-Resistant Cast Iron Handbook, American Foundry Society, 2000. ASM Handbook, Vol. 15: Casting, ASM International, 2008. ASM Specialty Handbook: Cast Irons, ASM International, 1996. Campbell, J., Complete Casting Handbook: Metal Casting Processes, Techniques and Design, Butterworth-Heinemann, 2011. Casting Design and Performance, ASM International, 2009. Casting Design Handbook, ASM International, 2012. Powell, G.W., Cheng, S.-H., and Mobley, C.E., Jr., A Fractography Atlas of Casting Alloys, Battelle Press, 1992. Product Design for Die Casting, Diecasting Development Council, 1988. Steel Castings Handbook, 6th ed., ASM International, 1995. Tuttle, R.B., Foundry Engineering: The Metallurgy and Design of Castings, CreateSpace Independent Publishing Platform, 2012. Review Questions 12.1. Why are steels more difficult to cast than cast irons? 12.2. What is the significance of hot spots in metal casting? 12.3. What is shrinkage allowance? Machining allowance? 12.4. Explain the reason for drafts in molds. 12.5. Why are ribs useful for flat surfaces? 12.6. What are light castings and where are they used most commonly? 12.7. Name the types of cast irons generally available, and list their major characteristics and applications. 12.8. Comment on your observations regarding Fig. 12.5. 12.9. Describe the difference between a runner and a gate. Machining allowance Parting line Patternmaker’s shrinkage allowance Porosity Shrinkage cavities Unit cost ׉ 7cassandra://HbNkSGjD65EymIzSDJv0d7EzX7PPrNMo8JXx6i-sTRQ`cZ%Eb׉E ,358 Chapter 12 Metal Casting: Design, Materials, and Economics 12.10. What is the difference between machining allowance and dimensional tolerance? 12.11. What is dross? Can it be eliminated? Qualitative Problems 12.12. Describe your observation concerning the design changes shown in Fig. 12.1. 12.13. If you need only a few castings of the same design, which three processes would be the most expensive per piece cast? 12.14. Do you generally agree with the cost ratings in Table 12.6? If so, why? 12.15. Describe the nature of the design differences shown in Fig. 12.4. What general principles do you observe in this figure? 12.16. Note in Fig. 12.5 that the ductility of some cast alloys is very low. Do you think that this should be a significant concern in engineering applications of castings? Explain. 12.17. Do you think that there will be fewer defects in a casting made by gravity pouring versus one made by pouring under pressure? Explain. 12.18. Explain the difference in the importance of drafts in green-sand casting versus permanent-mold casting. 12.19. What type of cast iron would be suitable for heavy-machine bases, such as presses and machine tools? Why? 12.20. Explain the advantages and limitations of sharp and rounded fillets, respectively, in casting design. 12.21. Explain why the elastic modulus, E, of gray cast iron varies so widely, as shown in Table 12.4. 12.22. If you were to incorporate lettering or numbers on a sand-cast part, would you make them protrude from the surface or recess them into the surface? What if the part were to be made by investment casting? Explain your answer. 12.23. The general design recommendations for a well in sand casting (see Fig. 11.3) are that (a) its diameter should be at least twice the exit diameter of the sprue and (b) its depth should be approximately twice the depth of the runner. Explain the consequences of deviating from these guidelines. 12.24. The heavy regions of parts typically are placed in the drag in sand casting and not in the cope. Explain why. 12.25. What are the benefits and drawbacks to having a pouring temperature that is much higher than the metal’s melting temperature? What are the advantages and disadvantages in having the pouring temperature remain close to the melting temperature? Quantitative Problems 12.26. When designing patterns for casting, patternmakers use special rulers that automatically incorporate solid shrinkage allowances into their designs. For example, a 300 mm patternmaker’s ruler is longer than 300 mm. How long should a patternmaker’s ruler be for making patterns for (a) aluminum castings and (b) high-manganese steel? 12.27. Using the data given in Table 12.2, develop approximate plots of (a) castability versus weldability and (b) castability versus machinability, for at least five of the materials listed in the table. ׉ 7cassandra://nYL4VgG2DtbLT7O8kzsnwzkmoMyAPwylnDLQ3f3Qq64`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://4hxwAvJ4AHqeb8bXb7G9e3tveEvGlBIiWQFbAXuzA3o`׉ 7cassandra://pib8tFRLQXzlnTiHKMNvi1f0eAQnH1nUCOWP5Qc9Zl4?`s׉ 7cassandra://W6ca-FLxAoAxUdUdQh72ucyCJJ3mnzm8emVY0ksA8K8`׉ 7cassandra://o5V3ft_N7L8XGKU8GhsHr6z0xPdz2uGb8zLRVtYH_vErR͠^cZEbט#u׉׉ 7cassandra://XanQyQRSSwJeU-9EkjceVBl_r-7K2V3H8nnnOO7N2xQ`׉ 7cassandra://3D4r1DDaQwgqjC9jEYrINPcueFn2gE8Ei3y0xu930qw*`s׉ 7cassandra://ywbyBcKUU3xopoLwUrrUHXi8zqwzz6tuXESju9Uz85sL`׉ 7cassandra://xHq1pyuOtcLZ57E9YgTiApo3D5vqx5qUz_JARqteuEIY͠^cZEb׉E]Synthesis, Design, and Projects 359 12.28. The part in the figure below is to be cast of 10% Sn bronze at the rate of 100 parts per month. To find an appropriate casting process, consider all casting processes, then reject those that are (a) technically inadmissible, (b) technically feasible but too expensive for the purpose, and (c) identify the most economical one. Write a rationale using common-sense assumptions about cost. 250 mm 25 mm 100 mm 250 mm 11±1 mm Ra=3 mm Ra=1.5 mm Synthesis, Design, and Projects 12.29. Describe the general design considerations pertaining to metal casting. 12.30. Add more examples to those shown in Fig. 12.2. 12.31. Explain how ribs and serrations are helpful in casting flat surfaces that otherwise may warp. Give a specific illustration. 12.32. List casting processes that are suitable for making hollow parts with (a) complex external features, (b) complex internal features, and (c) both complex external and complex internal features. Explain your choices. 12.33. Small amounts of slag and dross often persist after skimming and are introduced into the molten metal flow in casting. Recognizing that slag and dross are less dense than the molten metal, design mold features that will remove small amounts of slag before the metal reaches the mold cavity. 12.34. If you need only a few units of a particular casting, which process(es) would you use? Why? 12.35. For the cast metal wheel illustrated below, show how (a) riser placement, (b) core placement, (c) padding, and (d) chills may be used to help feed molten metal and eliminate porosity in the isolated hub boss. Rim Hub boss ׉ 7cassandra://W6ca-FLxAoAxUdUdQh72ucyCJJ3mnzm8emVY0ksA8K8`cZ%Eb׉Ec360 Chapter 12 Metal Casting: Design, Materials, and Economics 12.36. Assume that the introduction to this chapter is missing. Write a brief introduction to highlight the importance of the topics covered in it. 12.37. In the figure below, the original casting design shown in (a) was resized and modified to incorporate ribs in the design shown in (b). The casting is round and has a vertical axis of symmetry. What advantages do you think the new design has as a functional part over the old one? 25 mm 38 mm (a) 25 mm Ribs or brackets 25 mm (b) 12.38. An incorrect and a correct design for casting are shown below. Review the changes made and comment on their advantages. Outside core (a) Incorrect Outside core (b) Correct 12.39. Three sets of designs for die casting are shown below. Note the changes made to design 1 and comment on the reasons for then or them. ׉ 7cassandra://ywbyBcKUU3xopoLwUrrUHXi8zqwzz6tuXESju9Uz85sL`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://JAOgWinY4A_rzbwq_l0JMdMaCJjiquEVRgAVdYdKKO0!<`׉ 7cassandra://WQexTKaFQCaJuL0vc6NbswBGTfSd5DQRpMtNDf3kF-0>`s׉ 7cassandra://0BT7DHFnGG6PPYY1nsDRCGjK0r3zUEkXBTfMFOjdqck`׉ 7cassandra://l_U9w2rrdG0qDISHCt987EWrCS6ov4Uzk2891QXpVb0ͨ&͠^cZEbט#u׉׉ 7cassandra://alhhtnPbc-sLR8TOgVR7k5mCSp3HdhQUUHNDeALDDlY4`׉ 7cassandra://q0o0lrWO5ma2gqr2G-1kpUJQbGeSEdnKwyJZZU-ZBHI`s׉ 7cassandra://6yFbCwLxBySwswnkomhI5lhfSry_It4nLlkgbZNiq7I <`׉ 7cassandra://ILN7_cAnVev9JH_swslnUDz-WRw7gj5XZihjf5G_-Mc[ ͠^cZEb ׉E{Synthesis, Design, and Projects 361 1. 2. (a) Parting line Parting line 1. 2. (b) 3. Parting line 1. 2. (c) 12.40. Using the method of inscribed circles shown in Fig. 12.2, justify the trend shown in Fig. 12.3. 12.41. A growing trend is the production of patterns and molds through rapid prototyping approaches described in Chapter 20. Consider the case of an injection molding operation, where the patterns are produced by rapid prototyping, and then hand assembled onto trees and processed in traditional fashion. What design rules discussed in this chapter would still be valid, and which would not be as important in this case? 12.42. Repeat Problem 12.41 for the case where (a) a pattern for sand casting is produced by rapid prototyping; (b) a sand mold for sand casting is produced. 12.43. It is sometimes desirable to cool metals more slowly than they would be if the molds were maintained at room temperature. List and explain the methods you would use to slow down the cooling process. 12.44. The two illustrations shown are proposed designs of a gating system for an aluminum low-power water turbine blade. The first uses a conventional sprue-runner-gate system, while the second uses a ceramic filter underneath a pouring cup, but without gates (direct pour method). Evaluate the two designs, and list their advantages and disadvantages. Based on your analysis, select a preferred approach. ׉ 7cassandra://0BT7DHFnGG6PPYY1nsDRCGjK0r3zUEkXBTfMFOjdqck`cZ%Eb׉E362 Chapter 12 Metal Casting: Design, Materials, and Economics Thickness = 45 mm Thickness = 14 mm 900 mm Ceramic filter (a) Sprue (b) 12.45. Note that in cast jewelry, gemstones are usually cast in place; that is, they are not attached after the ring is cast, but are incorporated into the ring. Design a ring with a means of securing a gemstone in the wax pattern, such that it will remain in the mold as the wax is being melted. Could such an approach be used in lost foam casting? 300 mm ׉ 7cassandra://6yFbCwLxBySwswnkomhI5lhfSry_It4nLlkgbZNiq7I <`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://MC8HMmTOfY74bLY0f4AMgvqpoKv1o8aLvr5c1eiJ-AM`׉ 7cassandra://p7sc0ERLNPffILaI2_g6GDsrPlkzA-6WwqcCDVbScFkh]`s׉ 7cassandra://HbxlBcbOBQVCAMFKBs2c7dAlFhdhwH-sC7nYot2WNQI`׉ 7cassandra://34YAK8y7Dnp5mUz8iVh6vCov9K3nX-secz2sZ2cVlkkc͠^cZEb ט#u׉׉ 7cassandra://aHtTjDZpitPgyZC-jA4GqYF5wy_ysu7_HiGrkdODpB0n`׉ 7cassandra://Ai5hvl1-_zWM3IV-oRPZVSdGrKS5Ivr_A20vzpoBQjw]`s׉ 7cassandra://QQcjBhi73y7PIV03rcocnuM87xmYW-xOyB_1Lp1cuQg2`׉ 7cassandra://zkiZbD4rKtnC_m24tsEAniL-0Stx4-tjkTb3gilh114f<͠^cZEb ׉E ]PART III Forming and Shaping Processes and Equipment Examination of various products soon leads to the realization that a wide variety of materials and processes have been used in making them, as can also be seen from the example of the automobile shown in Fig. III.1. It will be noted that some products consist of only one part (screws, bolts, washers) or a few parts (pens, eyeglasses, microscopes), while others consist of hundreds or thousands of parts (automobiles, computers) or millions of parts (airplanes, ships). Some products are used for routine applications (paper clips, forks, door keys) while others are used in critical applications (elevator cables, stents, turbine blades). Some are very thin (aluminum foil, plastic film) whereas others are thick (boiler plates, submarine hulls). Note that the words forming and shaping are both used in the title of Part III of this book. Although there are not always clear distinctions between the two terms, forming generally indicates changing the shape of an existing solid body. Thus, in forming processes, the starting material (workpiece, stock, or blank) may be in the shape of a plate, sheet, bar, rod, wire, or tubing. For example, a common wire coat hanger is made by taking a straight piece of wire and bending and twisting it into the shape of a hanger. Likewise, the sheet-metal body for a washer or dryer is generally made of flat, cold-rolled steel (occasionally aluminum) sheet, which is then formed into various shapes. Shaping processes typically involve molding or casting, producing a part that generally is at or near the final desired shape. A plastic coat hanger, for example, is made by forcing molten plastic into a twopiece mold, with a cavity in the shape of the hanger. Computer mouse or video game console housings, refrigerator-door liners, some auto-body parts, and countless other plastic products are likewise shaped by forcing molten polymer into a mold, and removing it after it solidifies. Some forming and shaping operations produce long continuous products, such as plates, sheets, tubing, wire, and rod and bars, which then are shaped into specific products. Rolling, extrusion, and drawing processes (Chapters 13 and 15) are capable of making such long products, which then are cut into desired lengths. On the other hand, processes such as forging (Chapter 14), sheet metal forming and stamping (Chapter 16), powder metallurgy (Chapter 17), ceramic slip casting and glass pressing (Chapter 18), Shaped windshield Ceramic spark plug Powder-metal filters Forged valves Blow-molded windshieldwasher fluid tank Deep-drawn oil pan Stamped wheel covers Compression-molded tires Figure III.1: Formed and shaped parts in a typical automobile. 363 Molded dashboard Drawn tube for antenna Cold-rolled sheet metal or reinforced plastic for body Stamped license plates Injection-molded tail light lenses Fuel tank Springs ׉ 7cassandra://HbxlBcbOBQVCAMFKBs2c7dAlFhdhwH-sC7nYot2WNQI`cZ%Eb׉E364 Part III Forming and Shaping Processes and Equipment Process Rolling Flat Shape Forging Table III.1: General Characteristics of Forming and Shaping Processes. Characteristics Production of flat plate, sheet, and foil at high speeds; good surface finish, especially in cold rolling; very high capital investment; low-to-moderate labor cost Production of various structural shapes (such as I-beams and rails) at high speeds; includes thread rolling; requires shaped rolls and expensive equipment; low-to-moderate labor cost; requires moderate operator skill Production of discrete parts with a set of dies; some finishing operations usually required; usually performed at elevated temperatures, but also cold for smaller parts; die and equipment costs are high; moderate-to-high labor cost; requires moderate-to-high operator skill Extrusion Production of long lengths of solid or hollow shapes with constant cross section; product is then cut into desired lengths; usually performed at elevated temperatures; cold extrusion has similarities to forging and is used to make discrete products; moderate-to-high die and equipment cost; low-to-moderate labor cost; requires low-to-moderate operator skill Drawing Sheet-metal forming Powder metallurgy Processing of plastics and composite materials Forming and shaping of ceramics Additive manufacturing Production of long rod and wire with various cross sections; good surface finish; low-tomoderate die, equipment, and labor costs; requires low-to-moderate operator skill Production of a wide variety of shapes with thin walls and simple or complex geometries; generally low-to-moderate die, equipment, and labor costs; requires low-to-moderate operator skill Production of simple or complex shapes by compacting and sintering metal powders; moderate die and equipment cost; low labor cost and skill Production of a wide variety of continuous or discrete products by extrusion, molding, casting, and fabricating processes; moderate die and equipment costs; requires high operator skill in processing of composite materials Production of discrete products by various shaping, drying, and firing processes; low-tomoderate die and equipment cost; requires moderate-to-high operator skill Production of discrete parts by various computer-controlled methods; no dies involved; lowto-high equipment cost; requires high operator skill processes involving plastics and reinforced plastics (Chapter 19), and additive manufacturing (Chapter 20), typically produce discrete products. The initial raw material used in forming and shaping metals is usually molten metal, which is cast into individual ingots or continuously cast into slabs, rods, or pipes. Cast structures are converted to wrought structures by plastic-deformation processes. The raw material used also may consist of metal powders, which then are pressed and sintered (heated without melting) into individual parts. For plastics, the starting material is usually pellets, flakes, or powder; for ceramics, it is clay, powder, and oxides, obtained from ores or produced synthetically. In this part of the text, the important factors involved in each forming and shaping process are described, along with how material properties and processes affect the quality and integrity of the product made (Table III.1). Detailed mathematical models of processes are now available and can be found in the Bibliographies at the end of the chapters. This book provides only general models for the various forming and shaping processes considered. It will also become clear that some materials can be processed only by certain specific manufacturing methods, and why parts with particular shapes can only be processed by certain specific techniques and not by others. Also included are descriptions of the characteristics of the machinery and equipment used, as they can significantly affect product quality, production rate, and the economics of a particular manufacturing operation. ׉ 7cassandra://QQcjBhi73y7PIV03rcocnuM87xmYW-xOyB_1Lp1cuQg2`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://J2QbwqX2vRJly5Fc3Qt2dWUed8h-k2TeqMmg556aDKgҔ`׉ 7cassandra://eX_69fR5TKuGExSRyl_WiuSvWBhVjjGZ3WBTlthBvM0B`s׉ 7cassandra://q3KovlYH4JWcQZ4nFmJ_s1NxxEhMe-QuAdm-gxI9r-M`׉ 7cassandra://2c6iHOTspSM3L5DIDc8ge1R8f_0aFj8QWUXoa_nH4Y0>͠^cZEbט#u׉׉ 7cassandra://WU6r32iuiLewMqXfF7MxLNysnvVYurVlo2lnebi5PdUS`׉ 7cassandra://zqdeVF0AXz7kGQbO6mHEGXgC5pbUX-An-VavLqYdOOY<`s׉ 7cassandra://w_B8DJRHFfr-okKP-mNFpbGErXLM1_byH_ZwgB5xguY`׉ 7cassandra://QILJJw_ZHUHIzdyIgJ8K5DPj2tUxZs0t_6lwi9fj7Ss!Z ͠^cZEb׉EChapter 13 Metal-rolling Processes and Equipment 13.1 Introduction 365 13.2 The Flat-rolling Process 367 13.3 Flat-rolling Practice 373 13.4 Rolling Mills 376 13.5 Various Rolling Processes and Mills 378 Example: 13.1 Calculation of Roll Force and Torque in Flat-rolling 369 • This chapter describes the rolling of metals, which is the most important metal-forming operation based on the total volume of metals rolled. • The chapter begins with a description of the flat-rolling process and analysis of the force, torque, and power required, in terms of relevant material and process parameters; it also includes a review of defects and their causes in rolled products. • Shape-rolling processes are then described, where workpieces pass through a series of shaped rolls. • Special rolling processes such as cross rolling, ring rolling, thread rolling, tube rolling, and tube piercing are also described. • The chapter ends with a description of the characteristics of rolling mills and roll arrangements for making specific products. Typical products made by various rolling processes: Plates for ships, bridges, structures, large machines; sheet metal for car bodies, aircraft fuselages, appliances, containers; foil for packaging; I-beams, railroad rails, architectural shapes, large rings, seamless pipe and tubing; bolts, screws, and threaded components. Alternative processes: Continuous casting, extrusion, drawing, machining of threaded components. 13.1 Introduction Rolling is the process of reducing the thickness or changing the cross section of a long workpiece by compressive forces applied through a set of rolls (Fig. 13.1). Rolling, which accounts for about 90% of all metals 365 ׉ 7cassandra://q3KovlYH4JWcQZ4nFmJ_s1NxxEhMe-QuAdm-gxI9r-M`cZ%Eb׉EY366 Chapter 13 Metal-rolling Processes and Equipment Hot strip Pickling and oiling Cold strip Slab Skelp Welded pipe Plate Steel plates Hot-rolled bars Continuous casting or ingots Billet Rods Cold-drawn bars Wire and wire products Tube rounds Seamless pipe Structural shapes Bloom Rails Figure 13.1: Schematic outline of various flat-rolling and shape-rolling processes. Source: After American Iron and Steel Institute. produced by metalworking processes, was first developed in the late 1500s. Modern steelmaking practices and the production of various ferrous and nonferrous metals and alloys now generally integrate continuous casting with rolling processes. This method greatly improves productivity and lowers production costs, as described in Section 5.4. Nonmetallic materials also are rolled to reduce their thickness and enhance their properties. ׉ 7cassandra://w_B8DJRHFfr-okKP-mNFpbGErXLM1_byH_ZwgB5xguY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://4Tpk4bd4N38ThcPQenCiAjY41qgeuS3g2mzIrGCutVc`׉ 7cassandra://nG-FF3UQGnAEQON4NYy0qqXGHi2U92paY_2wzyDkaCQce`s׉ 7cassandra://mt4hDInRypHOzkISpazlQDDECD5mZ0cE9cZRx9Bl6FM@`׉ 7cassandra://-aK9nn_OsdTdvodnwtrlKj-d0AFLQxi7fmkQpQRtvTo#|:$͠^cZEbט#u׉׉ 7cassandra://L1GmzVD_T0RqLYkCNSVXmQgQzZAt2x3UjTAjHAAhkm8X`׉ 7cassandra://H8YXDg6NC20P8sBwpXZxr9Akpmw7d2KbnWBJrlxfg3A``s׉ 7cassandra://o5vqJmvqb-N0BOv14YPKvhdzZKs5bZlDMKh56lR_Vmg`׉ 7cassandra://Cd0I_iDHjTLT7iYL9uUJaHv22kxSb1jwTOOSa57No7cmo͠^cZEb׉E The Flat-rolling Process 367 Rolling is first carried out at elevated temperatures (hot rolling). During this stage, the coarse-grained, brittle, and porous structure of the ingot (or the continuously cast metal) is broken down into a wrought structure, having a finer grain size and enhanced properties, such as increased strength and hardness. Subsequent rolling is generally carried out at room temperature (cold rolling), whereby the rolled sheet has higher strength and hardness, and better surface finish. However, cold rolling will result in a product with anisotropic properties, due to preferred orientation or mechanical fibering, described in Section 1.6. Plates generally have a thickness of greater than 6 mm, and are used for structural applications, such as ship hulls, boilers, bridges, and heavy machinery. Plates can be as thick as 300 mm for large structural supports, 150 mm for reactor vessels, and 100 to 125 mm for machinery frames and warships. Sheets are generally less than 6 mm thick, and are typically provided to manufacturing facilities as coils, weighing as much as 30,000 kg, or as flat sheets for further processing into a wide variety of sheetmetal products. Sheets are typically used for aircraft bodies, appliances, food and beverage containers, and kitchen and office equipment. Commercial aircraft fuselages and trailer bodies are usually made of a minimum of 1-mm thick aluminum-alloy sheets. The skin thickness of a Boeing 747 fuselage, for example, is 1.8 mm; for a Lockheed L1011 it is 1.9 mm. Steel sheets used for appliances bodies are typically about 0.7 mm thick. Aluminum beverage cans are made from sheets 0.28 mm thick, which becomes a cylindrical body with a wall thickness of 0.1 mm after processing into a can (Section 16.7). Aluminum foil typically has a thickness of 0.008 mm, although thinner foils, down to 0.003 mm, also can be produced. 13.2 The Flat-rolling Process A schematic illustration of the flat-rolling process is shown in Fig. 13.2a. A metal strip of thickness ho enters the roll gap and is reduced to thickness hf by a pair of rotating rolls, each powered individually by electric motors. The surface speed of the rolls is Vr. The velocity of the strip increases from its entry value of Vo as it moves through the roll gap, and is highest at the exit from the roll gap, and is denoted as Vf. The metal accelerates in the roll gap, in the same manner as an incompressible fluid flows through a converging channel. Because the surface speed of the rigid roll is constant, there is relative sliding between the roll and the strip along the contact length, L. At one point, called the neutral point or no-slip point, the velocity of the strip is the same as that of the roll. To the left of this point, the roll moves faster than the strip; to the wo ho (Top roll removed) Vr Vo Strip Vf L Roll (a) Vr hf wf Entry zone Roll gap, L (b) Exit zone Torque (c) Figure 13.2: (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on strip surfaces. (c) Roll force, F,andtorque, T, acting on the rolls. The width of the strip, w, usually increases during rolling, as shown later in Fig. 13.5. F Vo Workpiece Friction forces R f (Bottom roll removed) Roll No-slip point Vf L F F F a ׉ 7cassandra://mt4hDInRypHOzkISpazlQDDECD5mZ0cE9cZRx9Bl6FM@`cZ%Eb׉E d368 Chapter 13 Metal-rolling Processes and Equipment right, the strip moves faster than the roll. Consequently, the frictional forces act on the strip as shown in Fig. 13.2b. Forward slip in rolling is defined in terms of the exit velocity of the strip, Vf, and the surface speed of the roll, Vr,as Forward slip = Vf −Vr Vr , (13.1) and is a measure of the relative velocities in the roll gap. Forward slip can easily be calculated by measuring the roll and workpiece velocities on a rolling mill, and gives a real-time indication of the neutral point location. Forward slip also correlates with the surface finish of the rolled strip, with low values being preferable to high values. The rolls pull the material into the roll gap through a net frictional force on the material; thus, a net frictional force must exist and be to the right in Fig. 13.2b. This also means that the frictional force to the left of the neutral point must be higher than the friction force to the right. Although friction is essential to enable rolling (just as it is in driving a car on a road), energy is dissipated in overcoming friction. Note that increasing friction also increases rolling forces and power requirements. Furthermore, high friction could damage the surface of the rolled product or cause sticking. A compromise is therefore made in practice through lubricant selection, leading to low and controlled levels of friction. The maximum possible draft is defined as the difference between the initial and final strip thicknesses, or (ho −hf). A large draft could cause the rolls to slip. It can be shown that the maximum draft is a function of the roll radius, R, and the coefficient of friction, μ, given by ho − hf = μ2R. (13.2) Thus, as expected, the higher the friction and the larger the roll radius, the greater the maximum possible draft. This is a situation similar to the use of large tires (hence high R) and rough treads (hence high μ) on farm tractors and off-road equipment, which allows the vehicles to travel over rough terrain without skidding. 13.2.1 Roll Force, Torque, and Power Requirements The rolls apply pressure on the flat strip, indicating the presence of a roll force, F, as shown in Fig. 13.2c. Note in the figure that this force appears to be perpendicular to the plane of the strip, rather than being at an angle. This is because, in practice, the arc of contact is very small compared with the roll radius, thus it can be assumed that the roll force is approximately perpendicular to the strip. The roll force in flat rolling can be estimated from the expression F = Lwσavg, (13.3) where w is the width of the strip, and σavg is the average true stress (see Section 2.2.3) of the strip in the roll gap. L is the length of contact and can be approximated as the projected length; thus L = RΔh, √ (13.4) where R istherollradiusand Δh is the difference between the original and final thicknesses of the strip (called draft). Equation (13.3) is for a frictionless condition; however, an estimate of the actual roll force, including friction, may be made by increasing this calculated force by about 20%. The torque on the roll is the product of F and a for frictionless rolling (see Fig. 13.2c). The power required per roll can then be estimated by assuming that F acts in the middle of the arc of contact, or a ≈ L/2;thus, in Fig. 13.2c, the total power (for two rolls), in S.I. units, is ׉ 7cassandra://o5vqJmvqb-N0BOv14YPKvhdzZKs5bZlDMKh56lR_Vmg`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HZzLxO4ebpGtqtZ3sYhbUA5dkfBD-wlV6V-O-Ouw7P8|`׉ 7cassandra://HJEP0ZwHiRRUv01J08ZO0EoQbds2VbXzeBNfbY4sjIQK`s׉ 7cassandra://wr2oKe_qIm_ZmSM1HY61WzaR1viPIScsxHePoSPjY-M,`׉ 7cassandra://z4QR-XmlT-W3SW6IK3mJaUX5uzm0yCLFd5-DiDFxN2smi͠^cZEbט#u׉׉ 7cassandra://rMGtVDJm0iitqLEgRlLrBt_WBqTtqw6ZgRKcpnLdwEAW`׉ 7cassandra://-IX_xBrFGwgH60QdUQaPD0NH2SVi1RywKnpN74uw0OY9`s׉ 7cassandra://mx993ZcqpkxwlZg5YnUTQg3MN2jfsuDd-x3siN2aiSE`׉ 7cassandra://9opizBCiXCUyAMzIrCw6ezQpHA9UdlhX5opldi-ffNA]j͠^cZEb׉E The Flat-rolling Process 369 Power (kW) = 2πFLN 60, 000 , where F is in newtons, L is in meters, and N is the revolutions per minute of the roll. Example 13.1 Calculation of Roll Force and Torque in Flat-rolling Given: An annealed copper strip 225 mm wide and 25 mm thick is being rolled to a thickness of 20 mm in one pass. The roll radius is 300 mm, and the rolls rotate at 100 rpm. Find: Calculate the roll force and the power required in this operation. Solution: The roll force is determined from Eq. (13.3), in which L is the roll-strip contact length. From Eq. (13.4), L =  R(ho − hf)=  (300)(25 − 20) = 38.7 mm =0.0387 m. The average true stress, σavg, for annealed copper is determined as follows: First note that the absolute value of the true strain that the strip undergoes in this operation is =ln  25 20  =0.223. Referring to Fig. 2.5, annealed copper has a true stress of about 80 MPa in the unstrained condition and at a true strain of 0.223, the true stress is around 275 MPa. Hence the average true stress in the roll gap is (80 + 275)/2 = 178 MPa. Thus, the roll force is F = Lwσavg =(0.0387)(0.225)(178 × 106)=1.55 MN. The total power is calculated from Eq. (13.5), with N = 100 rpm. Thus, Power = 2πFLN 60, 000 = 2π(1.55 × 106)(0.0387)(100) 60, 000 (13.5) = 628 kW. Exact calculation of the force and the power requirements in rolling can be difficult, because of the uncertainties involved in (a) determining the exact contact geometry between the roll and the strip and (b) accurately estimating both the coefficient of friction and the strength of the material in the roll gap. The calculation can be difficult, particularly for hot rolling because of the sensitivity of the strength of the material to temperature and strain rate (see Section 2.2.7). Reducing Roll Force. Roll forces can cause significant deflection and flattening of the rolls, as it does in a rubber tire. Such changes will, in turn, affect the rolling process and its ability to produce a uniform thickness in the rolled sheet (known as gage control, see Section 13.3.2). Also, the columns of the roll stand (including the housing, chocks, and bearings, as shown in Fig. 13.3) would deflect under high roll forces to such an extent that the roll gap may open up significantly. Consequently, the rolls have to be set closer than originally calculated in order to compensate for this deflection and to ensure the desired final thickness. ׉ 7cassandra://wr2oKe_qIm_ZmSM1HY61WzaR1viPIScsxHePoSPjY-M,`cZ%Eb׉E370 Chapter 13 Metal-rolling Processes and Equipment Screw or hydraulic mechanism Housing Back-up rolls Chocks Work rolls Back-up rolls (a) (b) (c) Support roll Cage Planetary rolls (d) (e) Backing bearing Driven roll Second intermediate roll Driven roll Driven roll Housing Bearing shaft First intermediate roll Work roll Strip Driven roll (f) Figure 13.3: Schematic illustration of various roll arrangements: (a) two-high mill; if a two-high mill is used for thick but short workpieces, it will commonly roll a billet back-and-forth in multiple passes, known as a reversing mill; (b) three-high mill with elevator for multiple passes; (c) four-high rolling mill showing various features. The stiffness of the housing, the rolls, and the roll bearings are all important in controlling and maintaining the thickness of the rolled strip; (d) tandem rolling, with three stands; (e) planetary mill; and (f) cluster mill, also known as a Sendzimir or Z-mill. ׉ 7cassandra://mx993ZcqpkxwlZg5YnUTQg3MN2jfsuDd-x3siN2aiSE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://5HGlweTTM8qkZugEVBVHM1BGVUsKvwTglAijUVLNZ18`׉ 7cassandra://-gLa045xJrPHtBDallqA0n0CSKHfO5KJsj22IfRVkqoN`s׉ 7cassandra://z1nxEGO94eUMFqSWBcW5A1xYDqabNaJ8nesVBprHyz8O`׉ 7cassandra://SWSOpln81FjpqsLIk6LC8LYteXDduMQmXUYlEyvOO5IZ͠^cZEbט#u׉׉ 7cassandra://C7G-qfW39hBBLbGdSRJHEi1EHuZ2gv1shbT6DsviTyM`׉ 7cassandra://xWQ0i0cpH2c7j_e5BaxpV502_kWPy5EeEihvGEXj0ZgH`s׉ 7cassandra://Oglb3sEaD6G3yiS3-jEdmaXbEqSMRBKdKz7nPvSwGEs`׉ 7cassandra://ZT_1cLM_sKoeLN9q2VBs4P2iSzM_xPCRxxaUgrgOUE4ͯ %͠^cZEb׉E _The Flat-rolling Process 371 Roll forces can be reduced by the following means: • Reducing friction at the roll-workpiece interface • Using smaller diameter rolls, to reduce the contact area • Taking smaller reductions per pass, to reduce the contact area • Rolling at elevated temperatures, to lower the strength of the material • Applying tensions to the strip to reduce the roll pressure, as a result of which the compressive stresses required to plastically deform the material become smaller. Because they require high roll forces, tensions are particularly important in rolling of high-strength metals. Tensions can be applied to the strip at either the entry zone (back tension), the exit zone (front tension), or both. Back tension is applied to the sheet by a braking action to the reel that supplies the sheet into the roll (pay-off reel). Front tension is applied by increasing the rotational speed of the take-up reel. Although it has limited and specialized applications, rolling also can be carried out by front tension only, with no power supplied to the rolls, known as Steckel rolling. 13.2.2 Geometric Considerations Just as a straight beam deflects under a transverse load, roll forces tend to elastically bend the rolls during rolling, as shown in Fig. 13.4a. The higher the elastic modulus of the roll material, the smaller is the roll deflection. As a result of roll deflections, the rolled strip will now be thicker at its center than at its edges, known as crown. A common method of avoiding this problem is to grind the rolls in such a way that their diameter at the center is slightly larger than at their edges (called camber). Thus, when the rolls bend, the strip being rolled will have a constant thickness along its width (Fig. 13.4b). For rolling sheet metals, the radius of the maximum camber is generally 0.25 mm greater than the radius at the ends of the roll. However, a particular camber is correct only for a specific load and strip width. To reduce deflection, the rolls can also be subjected to external bending by applying moments at their bearings. Rolls Strip thicker at center (a) Strip with uniform thickness (b) Figure 13.4: (a) Bending of straight cylindrical rolls caused by roll forces. (b) Bending of rolls ground with camber, producing a strip with uniform thickness through the strip width. Deflections have been exaggerated for clarity. ׉ 7cassandra://z1nxEGO94eUMFqSWBcW5A1xYDqabNaJ8nesVBprHyz8O`cZ%Eb׉E X372 Chapter 13 Metal-rolling Processes and Equipment Side view hf ho (a) Top view wo (b) Figure 13.5: Spreading in flat rolling; note that similar spreading can be observed when dough is rolled with a rolling pin. Because of the heat generated due to the work of plastic deformation during rolling, rolls can become slightly barrel shaped, known as thermal camber. Unless compensated for by some means, this condition will produce strips that are thinner at the center than at their edges. Thermal camber can be controlled by adjusting the location of coolants and their flow rate along the length of the rolls. Roll forces also tend to flatten the rolls elastically, producing an effect much like the flattening of automobile tires. Flattening is undesirable because it increases contact area and roll forces. Thin sheets and foils are especially difficult to roll because of roll flattening. Spreading. The increase in the width of strip during rolling is called spreading (Fig. 13.5). In rolling plates and sheets with high width-to-thickness ratios, the width of the strip remains effectively constant during rolling. However, with smaller ratios, such as a bar with a square cross section, its width increases significantly as it passes through the rolls. It can be shown that spreading increases with (a) decreasing width-to-thickness ratio of the entering strip, (b) increasing friction, and (c) decreasing ratio of roll radius to strip thickness. The last two effects are due to the increased longitudinal constraining force that the material experiences in the roll gap. Spreading can be prevented by using additional rolls, with vertical axes in contact with the edges of the strip; known as edger mills, the vertical rolls provide a physical barrier to spreading. 13.2.3 Vibration and Chatter Chatter is a complex phenomenon (see also Section 25.4), resulting from interactions between the structural dynamics of the mill stand and the dynamics of the rolling operation. Generally defined as self-excited vibration, chatter in rolling leads to periodic variations in the thickness of the rolled sheet and in its surface finish, and may lead to excessive scrap (see Table 40.4). Chatter in rolling is found predominantly in tandem mills (Fig. 13.3d). It has been estimated, for example, that modern rolling mills could operate at up to 50% higher speeds were it not for chatter. wf ׉ 7cassandra://Oglb3sEaD6G3yiS3-jEdmaXbEqSMRBKdKz7nPvSwGEs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://oUECn0Q6cSJPw7_ZxGZPV1NjafGXLS5d1bKhYB82u9MfY`׉ 7cassandra://IHy5__P0jA52N86iT62ghI763kc9cgjsRYgV9Drmv4Ad`s׉ 7cassandra://v18jFCxmPSCIt5miTCMzA_SLwySqO5K-jVuTAhN_xs8>`׉ 7cassandra://yw20oBlVFKIU5FdiYnW8CA-K4_RGXN4jbrwUCDzR4f0͝͠^cZEb!ט#u׉׉ 7cassandra://JTVU93FdR_XwQNlCT1jXpLPgxtcFkZ6Cgdsg6_8vmSYj`׉ 7cassandra://UPAWaYx7gnPY1gFw6cb4WUeFEE00SM_9o4tIZvWe0x0S`s׉ 7cassandra://A8w1viptX5mPdv2dhMvl_Gf29f6I4DedP8sP26yIjgk:`׉ 7cassandra://qGoXuwp49xwDHFVNnz42rCyWYuTDpD5SS7Q9FsmGcfMT͠^cZEb%׉E Flat-rolling Practice 373 Rolling speed and lubrication are found to be the two most significant parameters affecting chatter. Although not always practical to implement, chatter may be reduced by (a) increasing the distance between the stands of the rolling mill, (b) increasing the strip width, (c) decreasing the reduction per pass (draft), (d) increasing the roll radius, (e) increasing the strip-roll friction, and (f) incorporating external dampers in the roll supports. 13.3 Flat-rolling Practice The initial rolling steps (breaking down) of the material is usually done by hot rolling, above the recrystallization temperature of the metal (Section 1.7). As described in Section 10.2 and illustrated in Fig. 10.2, a cast structure typically is dendritic, consisting of coarse and nonuniform grains, a structure that is usually brittle, and may also be porous. Hot rolling converts the cast structure to a wrought structure (Fig. 13.6), with finer grains and enhanced ductility, both of which result from the breaking up of brittle grain boundaries and the closing up of internal defects, including porosity, during rolling. Typical temperature ranges for hot rolling are about 450◦C for aluminum alloys, up to 1250◦C for alloy steels, and up to 1650◦Cfor refractory alloys (see also Table 14.3). The rolled product of the first hot-rolling operation is called bloom, slab,or billet (see Fig. 13.1). A bloom typically has a square cross section, at least 150 mm on the side, whereas a slab is usually rectangular in cross section. Blooms are further processed by shape rolling into structural shapes, such as I-beams and railroad rails (Section 13.5). Slabs are rolled into plates and sheets. Billets usually are square (with a crosssectional area smaller than that for blooms), and are later rolled into various shapes, such as round rods and bars, using shaped rolls. Hot-rolled round rods, called wire rods, are commonly used as the starting material for rod- and wire-drawing operations (Chapter 15). In hot rolling of blooms, billets, and slabs, the surface of the material is usually conditioned (prepared for a subsequent operation) prior to rolling them. Conditioning is often done by means of a torch (scarfing), which removes heavy scale or by rough grinding, which smoothens surfaces. Prior to cold rolling, the scale developed during hot rolling may be removed by pickling with acids (acid etching), by such mechanical means as blasting with water or by grinding. Cold rolling is carried out near room temperature and, compared with hot rolling, it produces sheets and strips with a much better surface finish (because of lack of scale), better dimensional tolerances, and enhanced mechanical properties (because of strain hardening). Pack rolling is a flat-rolling operation in which two or more layers of sheet are rolled together, thus increasing productivity. Aluminum foil, for example, is pack rolled in two layers, where only the top and Hot rolling Wrought product with large grains Ingot with nonuniform grains (a) Deformed elongated grains (b) Figure 13.6: Changes in the grain structure of cast or of large-grain wrought metals during hot rolling. Hot rolling is an effective way of reducing grain size in metals for improved strength and ductility. The cast structures of ingots or of continuous castings are converted to a wrought structure by hot working. New grains forming New grains growing Recrystallization complete Wrought product with small, uniform grains ׉ 7cassandra://v18jFCxmPSCIt5miTCMzA_SLwySqO5K-jVuTAhN_xs8>`cZ%Eb׉E i374 Chapter 13 Metal-rolling Processes and Equipment Thickness sensor Computer-controlled dynamic force Contoured plate or strip Figure 13.7: Production of tailor rolled blanks, with varying thickness in the rolling direction. A desired contour can be rolled into the workpiece, allowing for optimum placement of material. bottom outer layers are in contact with the rolls, and hence is smoother. Note that one side of aluminum foil is matte, while the other side is shiny. The foil-to-foil side has a matte and satiny finish, whereas the foil-to-roll side is shiny and bright; this is because it has been in contact with the polished rolls during rolling. Rolled mild steel, when subsequently stretched during sheet-forming operations, undergoes yield-point elongation (Section 16.3), a phenomenon that causes surface irregularities, called stretcher strains or L¨uder’s bands. To prevent this situation, the sheet metal is subjected to a final light pass of 0.5–1.5% reduction (known as temper rolling or skin pass) shortly before stretching it in a subsequent forming operation. A rolled sheet may not be sufficiently flat as it exits the roll gap, due to factors such as variations in the incoming material or in the processing parameters during rolling. To improve flatness, the rolled strip typically passes through a series of leveling rolls. Several roll arrangements can be used, as shown in Fig. 13.8, in which the sheet is basically flexed in opposite directions as it passes through the sets of rolls. Tailor Rolled Blanks. The thickness of a rolled sheet can be varied by changing the roll forces during the rolling process (Fig. 13.7). Tailor rolled blanks have been used to produce sheet stock as well as tubes. The thickness can be periodic or complex, depending on the dynamic force applied during rolling. Tailor rolled blanks can place material where it is needed for subsequent manufacture or for design purposes. For example, a sheet can be made thicker in locations where sheet-metal forming strains are higher or where stresses are high in service of the part made. Tailored Coils are two or more continuously welded coils, intended to provide different materials or thicknesses at different locations across the coil width. Sheet Rod Leveling rolls (a) Rolls (b) Figure 13.8: (a) A method of roller leveling to flatten rolled sheets. (b) Roller leveling to straighten drawn bars. ׉ 7cassandra://A8w1viptX5mPdv2dhMvl_Gf29f6I4DedP8sP26yIjgk:`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://U7EtPYqrFhk1g4SSXon16RsxpLCzhB9ySgmKJMBYLhUL*`׉ 7cassandra://Kt8BJW2IQ6IulWinYlGzh1YiQ70fP9CHJK3OZz_-dnwW`s׉ 7cassandra://kCefUxgiMqjlSrCLaKNuotLWuyhnX6bx38uxplVv5T0K`׉ 7cassandra://ic99AIdJCpJcensw_dgoM5htl8OszHEP53VPUpyhAlk~x͠^cZEb'ט#u׉׉ 7cassandra://S1iPloRDl8vVI7cEgy3JofWTS6cOTZABoD9vcwF2yMcr`׉ 7cassandra://aOeQHK7ZJjV9KnwWqn0-qr0tRmDDj0NtC_xhQLvt_MM]Y`s׉ 7cassandra://uv4jjpZuT2gqGB6ORs6aKJnSxOM8_CBDKcKh5h9YYXA`׉ 7cassandra://0pPpjUH9O6_0MJ7G1w8JG711GZnU1dHTPK7DsXXBIp4ڧt͠^cZEb(׉E Flat-rolling Practice 375 Rolling direction (a) (b) (c) (d) Figure 13.9: Schematic illustration of typical defects in flat rolling: (a) wavy edges; (b) zipper cracks in the center of the strip; (c) edge cracks; and (d) alligatoring. 13.3.1 Defects in Rolled Plates and Sheets Defects may be present on the surfaces of rolled plates and sheets, or there may be internal structural defects. Defects are undesirable not only because they adversely affect surface appearance, but also because they may affect strength, formability, and other manufacturing characteristics of the rolled sheets. Surface defects such as scale, rust, scratches, gouges, pits, and cracks, may be caused by inclusions and impurities in the original cast material or by various other conditions related to material preparation and to the particular rolling operation itself. Wavy edges on sheets (Fig. 13.9a) are due to roll bending, whereby the strip becomes thinner along its edges than at its center (see Fig. 13.4a). Because of volume constancy in plastic deformation, the edges then have to elongate more than the material at the center; consequently, the edges buckle because they are constrained by the central region from expanding freely in the longitudinal (rolling) direction. The cracks shown in Fig. 13.9b and c are usually the result of low material ductility at the rolling temperature. Because the quality of the edges of the sheet is important in subsequent forming operations, edge defects in rolled sheets may have to be removed by shearing and slitting operations (Section 16.2). Alligatoring (Fig. 13.9d) is typically caused by nonuniform bulk deformation of the billet during rolling or by the presence of defects in the original cast material. 13.3.2 Other Characteristics of Rolled Metals Residual Stresses. Because of nonuniform deformation of the material within the roll gap, residual stresses can develop in rolled plates and sheets, especially in cold rolling. Small-diameter rolls or small thickness reductions per pass tend to plastically deform the metal to a higher degree at its surfaces than in its bulk (Fig. 13.10a). This situation then results in the development of compressive residual stresses on the surfaces and tensile stresses in the bulk. Conversely, large-diameter rolls or high reductions per pass tend to deform the bulk more than its surfaces (Fig. 13.10b). This is due to the higher frictional constraint at the surfaces along the arc of contact. Dimensional Tolerances. Thickness tolerances for cold-rolled sheets typically range from ±0.1 to 0.35 mm. Tolerances are much higher for hot-rolled plates, because of thermal effects. Flatness tolerances are usually within ±15 mm/m for cold rolling, and ±55 mm/m for hot rolling. Surface Roughness. The ranges of surface roughness in cold and hot rolling are given in Fig. 33.5 which also includes other manufacturing processes for comparison. Note that cold rolling can produce a very fine ׉ 7cassandra://kCefUxgiMqjlSrCLaKNuotLWuyhnX6bx38uxplVv5T0K`cZ%Eb׉E 376 Chapter 13 Metal-rolling Processes and Equipment Sheet thickness Tension Compression (a) Tension Compression (b) Figure 13.10: (a) Residual stresses developed in rolling with small-diameter rolls or at small reductions in thickness per pass. (b) Residual stresses developed in rolling with large-diameter rolls or at high reductions per pass. Note the reversal of the residual stress patterns. surface finish; thus, products made of cold-rolled sheets may not require additional finishing operations. Note also in the figure that hot rolling and sand casting produce the same range of surface roughness. Gage Numbers. The thickness of a sheet is identified by a gage number: the smaller the number, the thicker is the sheet. Several numbering systems are used in industry, depending on the type of sheet metal. Rolled sheets of copper and brass are generally identified by thickness changes during rolling, such as 1/4 hard, 1/2 hard, and so on. 13.4 Rolling Mills Several types of rolling mills and equipment are available, with a range of sizes and a variety of roll arrangements. Although the designs of equipment for hot and cold rolling are essentially the same, there are important differences in the roll materials, processing parameters, lubricants, and cooling systems. The design, construction, and operation of rolling mills (Fig. 13.11) require major investments. Highly automated mills now produce close-tolerance, high-quality plates and sheets, at high production rates and at low cost per unit weight, particularly when integrated with continuous casting (Section 5.4). The width of rolled products may range up to 5 m, and rolling speeds are up to 40 m/s. Two-high rolling mills (Fig. 13.3b) are used for hot rolling in initial breakdown passes (primary roughing or cogging mills) on cast ingots or in continuous casting, with roll diameters ranging from 0.6 to 1.4 m. In the three-high mill (reversing mill, Fig. 13.3c) the direction of material movement through the rolls is reversed after each pass, using an elevator mechanism and various manipulators. Four-high mills (Fig. 13.3a) and cluster mills (Sendzimir or Zmill, Fig. 13.3d) are based on the principle that small-diameter rolls involve lower roll forces because of smaller roll-strip contact area, and thus lower power requirements and reduced spreading. Moreover, when worn or broken, small rolls can easily be replaced and at much lower cost than can large ones. On the other hand, small rolls will deflect more under roll forces, and thus have to be supported by other large-diameter rolls, as is done in four-high and cluster mills. Although the cost of a Sendzimir mill facility is very high, the system is particularly suitable for cold rolling thin sheets of high-strength metals and alloys. Common rolled widths in this mill are 0.66 m, with a maximum of 1.5 m. In tandem rolling, the strip is rolled continuously, through a number of stands, to thinner gages with each pass (Fig. 13.12). Each stand consists of a set of rolls, with its own housing and controls. A group of ׉ 7cassandra://uv4jjpZuT2gqGB6ORs6aKJnSxOM8_CBDKcKh5h9YYXA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://F2mlbkGWdsJXIlueLAkJAor2pmiPn5n5IN9YyGAWHPQ`׉ 7cassandra://AXfEKHjCWYqDp5Hs4nsr8eNlEBmDr_ZhLq6xIo5ZYbo3`s׉ 7cassandra://tuHHp2gglVgI_HiVzW8yVlvcGln0N6fy2fw5LJXyEQQ`׉ 7cassandra://7fETEKAD7A9YOuvxnR90eDsA6xVTlhQfgcl3HAubrCI͜͠^cZEb*ט#u׉׉ 7cassandra://0SDrEuAjcePJm5G2klkZAzF25mxhYeMmG9Uu4hkrQic7`׉ 7cassandra://ynn0SJipuS3LsUY9gi9s2yJpB87OkK-x6mBg3pzRIVgU]`s׉ 7cassandra://UJSsGnu04Li4mtsJeDTF5DowhTLPSe6WuVQwJLM_1JE`׉ 7cassandra://YV86poiQMI4GR1N8KxeM5IrSoaCB_bFEVhEVQeTSO_kͨ$͠^cZEb-׉E|Rolling Mills 377 Coil storage Take-up reel Mill stands Operator controls Figure 13.11: View of a rolling mill. Source: Courtesy ofIspat Inland, an ArcelorMittal legacy company. Stand 1 23 4 30 Take-up reel 0.26 0.34 0.56 0.90 1.45 2.25 mm Stand 4 Stand 5 5 17.7 10.7 6.6 4.1 m/s 2.6 m/s Payoff reel 0.90 mm 1.45 mm 2.25 mm Figure 13.12: An example of a tandem-rolling operation. ׉ 7cassandra://tuHHp2gglVgI_HiVzW8yVlvcGln0N6fy2fw5LJXyEQQ`cZ%Eb׉E 378 Chapter 13 Metal-rolling Processes and Equipment stands is called a train. The control of the strip thickness and the speed at which the strip travels through each roll gap is critical. Extensive computer controls are used in these operations, particularly in precision rolling at high speeds. Roll Materials. The basic requirements for roll materials are strength and resistance to wear. Common roll materials are cast iron, cast steel, and forged steel; tungsten carbide is also used for small-diameter rolls, such as the working roll in the cluster mill (Fig. 13.3d). Forged-steel rolls, although more costly than cast rolls, have higher strength, stiffness, and toughness than cast-iron rolls. Rolls for cold rolling are ground to a fine finish; for special applications, they are also polished. Rolls made for cold rolling should not be used for hot rolling, because they may crack due to thermal cycling (heat checking)or spall (cracking or flaking of surface layers). Lubricants. Hot rolling of ferrous alloys is usually carried out without lubricants, although graphite may be used to reduce friction. Water-based solutions may be used to cool the rolls and to break up the scale on the rolled material. Nonferrous alloys are hot rolled using a variety of compounded oils, emulsions, and fatty acids. Cold rolling is carried out with water-soluble oils or low-viscosity lubricants, such as mineral oils, emulsions, paraffin, and fatty oils (see also Chapter 33). 13.5 Various Rolling Processes and Mills Several rolling processes and mills have been developed over the years to produce a specific family of product shapes. Shape Rolling. Straight and long structural shapes, such as channels, I-beams, railroad rails, and solid bars, are formed by shape rolling (profile rolling), in which the heated stock passes through a set of specially designed rolls (Fig. 13.13; see also Fig. 13.1). Cold shape rolling can be done for making rod or wire with various cross sections. Because the entering material’s cross section is reduced nonuniformly, the design of Stage 1: Blooming rolls Stage 2: Edging rolls Stage 3: Roughing horizontal and vertical rolls Stage 4: Intermediate horizontal and vertical rolls Stage 5: Edging rolls Stage 6: Finishing horizontal and vertical rolls Figure 13.13: Steps in the shape rolling of an I-beam. Various other structural sections, such as channels and rails, also are rolled by this kind of process. ׉ 7cassandra://UJSsGnu04Li4mtsJeDTF5DowhTLPSe6WuVQwJLM_1JE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://wDumBQDsRltt2_NOARvS5V-gDNcrSr_-WT_t-dRMmeAٮ`׉ 7cassandra://30KaGL9Vr66vG77TO9V67zYUeLFbiFftcxTbGXj8wpkY`s׉ 7cassandra://zCnBP8p1OooPs_ZMWOsw3M9f6EDZuivPWWbWAisn0-M`׉ 7cassandra://ZvXWbtJjQJ1LGg-WTktxk07I5Ut523lTAlXoKpuO6CM-͠^cZEb0ט#u׉׉ 7cassandra://ypeEepo04XpGBMsTJK2nTx-LhpE9qIZmEXhCSpVtxcEq`׉ 7cassandra://fc4Bd4XtYX9gQ9HS1l0Yqyz3QeyC30l-7joGxxgcecIh`s׉ 7cassandra://WTIOvMHiuYEthdNQdxdJ5Nm1q1849u7VcXPEbreL1ig`׉ 7cassandra://TqLSY9Gsc9VK1dmnqz9RHOKP6UFzjW-6wGQCnJHS5aMC͠^cZEb7׉EAVarious Rolling Processes and Mills 379 Shaped roll Workpiece Workpiece (a) (b) Figure 13.14: Two examples of the roll-forging operation, also known as cross rolling. Tapered leaf springs and knives can be made by this process. Source: After J. Holub. a sequence of rolls, called roll-pass design, is critical in order to prevent formation of external and internal defects, hold dimensional tolerances, and to reduce roll wear. Also, a coil can use varying thickness or material across the width by using a tailored coil (see Section 13.3). Roll Forging. In this operation, also called cross rolling, the cross section of a round bar is shaped by passing it through a pair of rolls with specially profiled grooves (Fig. 13.14). This process is typically used to produce tapered shafts and leaf springs, table knives, and hand tools. Roll forging may also be used as a preliminary forming operation, to be followed by other forging processes described in Chapter 14. Skew Rolling. This is a process similar to roll forging and is typically used for making ball bearings (Fig. 13.15a). Round wire or rod is fed into the roll gap, and spherical blanks are formed continuously by the action of the rotating rolls. Another method is illustrated in Fig. 13.15b, which is basically a combined forging and heading operation, described in Fig. 14.12. The balls, which require further finishing, are subsequently ground and polished in special machinery (see Fig. 26.17). Ring Rolling. In ring rolling, a thick ring is expanded into a larger diameter and thinner ring. The ringshaped blank is placed between two rolls, one of which is driven while the other is idle (Fig. 13.16a). The thickness of the ring is reduced by bringing the rolls closer together as they rotate. Since the volume of the Blank Stock Semifinished ball Die Ejector Die insert (a) (b) Figure 13.15: (a) Producing steel balls by the skew-rolling process. (b) Producing steel balls by upsetting a cylindrical blank. Note the formation of flash. The balls made by these processes are subsequently ground and polished for use in ball bearings. Ball Flash ׉ 7cassandra://zCnBP8p1OooPs_ZMWOsw3M9f6EDZuivPWWbWAisn0-M`cZ%Eb׉E 380 Chapter 13 Metal-rolling Processes and Equipment Edging roll Idler roll Rounding roll Main roll (driven) Workpiece (a) (b) (c) (d) Figure 13.16: (a) Schematic illustration of a ring-rolling operation. Thickness reduction results in an increase in the part diameter. (b) through (d) Examples of cross sections that can be formed by ring rolling. ring remains constant during deformation (volume constancy), the reduction in ring thickness results in its increase in diameter. The process can be carried out either at room or elevated temperature. The ring size can be up to 3 m in diameter. The blank may be produced by such means as cutting from a plate, piercing, shearing a thick-walled pipe. Various cross sections can be ring rolled using shaped rolls (Fig. 13.16). The thickness of rings also can be reduced by an open-die forging process, as illustrated in Fig. 14.4c; however, dimensional control and surface finish will not be as good as in ring rolling. Typical applications of ring rolling are large rings for rockets and turbines, jet-engine cases, ball-bearing and roller-bearing races, flanges, and reinforcing rings for pipes. Compared with other manufacturing processes that are capable of producing the same part, the advantages of ring rolling are short production times, material savings, close dimensional tolerances, and favorable grain flow in the product, thus enhancing its strength in the desired direction. Thread Rolling. Thread rolling is a cold-forming process by which straight or tapered threads are formed on round rods. The threads are formed with each stroke of a pair of flat reciprocating dies (Fig. 13.17a). In another method, threads are formed by using two rolls (Fig. 13.17b) or rotary or planetary dies (Fig. 13.17c), at production rates as high as 80 pieces per second. Typical parts made are screws, bolts, and threaded parts. Depending on die design, the major diameter of a rolled thread may or may not be larger than a machined thread (Fig. 13.18a), that is, the same as the blank diameter. The thread-rolling process has the advantages of generating threads with good strength (due to cold working) and without any scrap. The surface finish produced is very smooth, and the process induces compressive residual stresses on the surfaces, thus improving fatigue life. The process is superior to other methods of thread manufacturing, notably thread cutting, as illustrated in Fig. 23.1k. Machining the threads cuts through the grain-flow lines of the material, whereas rolling the threads results in a grain-flow pattern that improves thread strength (Fig. 13.18). Spur and helical gears can be produced by a cold-rolling process similar to thread rolling (see also Section 24.7). The operation may be carried out on solid cylindrical blanks or on precut gears. Cold rolling of gears has extensive applications in automatic transmissions and in power tools. Internal thread rolling can be carried out with a fluteless forming tap (Section 23.7), an operation that is similar to external thread rolling; it produces accurate internal threads with good strength. Lubrication is important in thread-rolling operations, in order to obtain a good surface finish and surface integrity, and minimize defects. Lubrication affects the manner in which the material deforms during ׉ 7cassandra://WTIOvMHiuYEthdNQdxdJ5Nm1q1849u7VcXPEbreL1ig`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://5ORUEQ9kQRrPE08eAeEq4CEgYpfS4NeheNUmc7YJLJg`׉ 7cassandra://SOPHxSv8ozUG96V2uejzyukhZoGDCz_bFZoO9WSpbxUDE`s׉ 7cassandra://b-qaOqZ36-qge1BEgn_alcTleTrlDQ9hbUBXIFWvGw8O`׉ 7cassandra://P5eIQLfwuXc_SLof85h5ms0lubhzyFIEdGtoSrt2irElv͠^cZEb;ט#u׉׉ 7cassandra://qu0mBCVMa4qloa9VJjtLT76ONm57b9pwp982xplpuqYͲ`׉ 7cassandra://rqPyh4NCr6DL0t4RSkUNr3okr6yTaYwNBpmcsxOvNSsX%`s׉ 7cassandra://QLIugj6b4xwWvJ6Lrb2TLALJNe9qkdpVpKJTRAceVb8`׉ 7cassandra://ZO6_3t84_i8I2ugDnelioClmYax0kqRJgvpRpS5s-as_͛L͠^cZEb<׉EOVarious Rolling Processes and Mills 381 Blank Work rest Moving die Workpiece Stationary cylindrical die Stationary die Threaded part Moving cylindrical die (a) (b) Stationary die (not rotating) Workpiece Rotating die (c) (d) Figure 13.17: Thread-rolling processes: (a) reciprocating flat dies used to produce a threaded fastener; (b) two-roll dies; (c) rotary or planetary die set; (d) A collection of thread-rolled parts made economically at high production rates. Source: Courtesy of Tesker Manufacturing Corp. processing, an important consideration because of the possibility of internal defects being developed (see, for example, Fig. 14.17). Typically made of hardened steel, rolling dies are expensive because of their complex shape, and usually cannot be reground after they are worn. With proper selection of die materials and preparation, die life may range up to millions of pieces. Rotary Tube Piercing. Also known as the Mannesmann process, this is a hot-working operation for producing long, thick-walled seamless pipe and tubing (Fig. 13.19). Developed in the 1880s, this process is based ׉ 7cassandra://b-qaOqZ36-qge1BEgn_alcTleTrlDQ9hbUBXIFWvGw8O`cZ%Eb׉E 382 Chapter 13 Metal-rolling Processes and Equipment Diameter of bar Minor Machined or rolled thread (a) Major diameter diameter Machined thread (b) Rolled thread (c) Figure 13.18: (a) Features of a machined or rolled thread. Grain flow in (b) machined and (c) rolled threads. Unlike machining, which cuts through the grains of the metal, the rolling of threads imparts improved strength because of cold working and favorable grain flow. on the principle that when a round bar is subjected to radial compressive forces, tensile stresses develop at its center (see Fig. 2.9). When continuously subjected to these cyclic compressive stresses (Fig. 13.19b), the bar begins to first develop a small cavity at its center, which then begins to grow. This phenomenon can be demonstrated with a short piece of round eraser, by rolling it back and forth on a hard flat surface, as shown in Fig. 13.19b. Rotary tube piercing is carried out using an arrangement of rotating rolls (Fig. 13.19c). The axes of the rolls are skewed in order to pull the round bar through the rolls by the axial component of the rotary motion. An internal mandrel assists the operation by expanding the hole and sizing the inside diameter of the tube. The mandrel may be held in place by a long rod or it may be a floating mandrel, without a support (see Fig. 15.21c for a similar floating mandrel used in drawing). Because of the severe deformation that the bar undergoes, the blank must be of high quality and free of inclusions. Tube Rolling. The diameter and thickness of pipes and tubing can be reduced by tube rolling, which utilizes shaped rolls arranged in various configurations (Fig. 13.20). These operations can be carried out with or without an internal mandrel. In the pilger mill, the tube and an internal mandrel undergo a reciprocating motion; the rolls are rotated continuously. During the gap cycle on the roll, the tube is advanced and rotated, starting another cycle of tube reduction, whereby the tube undergoes a reduction in both its diameter and its wall thickness. Steel tubing 265 mm in diameter has been produced by this process. Other operations for tube manufacturing are described in Chapter 15. Rolls Tensile stress Tube Cavity Void initiation (a) Mandrel Solid rod (b) (c) Figure 13.19: Cavity formation in a solid, round bar and its utilization in the rotary tube-piercing process for making seamless pipe and tubing (see also Fig. 2.9). ׉ 7cassandra://QLIugj6b4xwWvJ6Lrb2TLALJNe9qkdpVpKJTRAceVb8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://u0wrmBn9C86QxMkB8pmkfnFzxdc-IvSwvsygS7i4Abgd`׉ 7cassandra://ylC8d-Dw4q2qROZYcyK1dKKwNgIkmtLiZwOrVSh1W74S`s׉ 7cassandra://-fK5kQoJJPKEEnBMRqjvbD6Kdtr1vD9AzC7HPzz_0G0`׉ 7cassandra://6v39SUjHBekBUerh9miINoVwaUufFVq7-jRxwcKqyP0v͠^cZEb?ט#u׉׉ 7cassandra://OL1GjL7OrOaA-dF34RMN1v9cdC6gG4_5ADRalO2YKScl`׉ 7cassandra://4zkIhmg4f38q8cL0UD66yvVFoUDJ56rJuSTowKUC_6ME`s׉ 7cassandra://sxXPAhyCla8AJ4KqT4LSTR_2K7-9x12hERu9Q19J8FI`׉ 7cassandra://uZuAWuPSRWDZE4PP8pUXRE2e4XzHTI352zZ4GRH41lkOS͠^cZEb@׉E Summary 383 Roll Mandrel Workpiece (a) Roll Workpiece Mandrel rod (b) Pilger roll Workpiece Mandrel (c) (d) Figure 13.20: Schematic illustration of various tube-rolling processes: (a) with a fixed mandrel; (b) with a floating mandrel; (c) without a mandrel; and (d) pilger rolling over a mandrel and a pair of shaped rolls. Tube diameters and thicknesses also can be changed by other processes, such as drawing, extrusion, and spinning. 13.5.1 Integrated Mills and Minimills Integrated Mills. These mills are large facilities that involve complete integration of all activities, from the production of hot metal in a blast furnace to the casting and rolling of finished products, ready to be shipped to the customer. Minimills. In minimills, scrap metal is (a) melted in electric-arc furnaces, (b) cast continuously, and (c) rolled directly into specific lines of products. Each minimill produces essentially one type of rolled product (rod, bar, or structural sections such as angle iron), from basically one type of metal or alloy. The scrap metal, obtained locally to reduce transportation costs, is typically old machinery, cars, and farm equipment. Minimills have the economic advantage of lower capital equipment costs for each type of metal and product line, with low labor and energy costs. The products typically are aimed at markets in the mill’s particular geographic location. Summary • Rolling is the process of reducing the thickness or changing the cross section of a long strip by compressive forces applied through a set of rolls. Shape rolling is used to make products with various cross sections. Other rolling operations include ring rolling and thread rolling. • The process may be carried out at room temperature (cold rolling) or at elevated temperatures (hot rolling). Rolling involves several material and process variables, including roll diameter (relative to material thickness), reduction per pass, speed, lubrication, and temperature. Spreading, bending, and flattening are important considerations for controlling the dimensional accuracy of the rolled stock. • Rolling mills have a variety of roll configurations, such as two-high, three-high, four-high, cluster (Sendzimir), and tandem. Front and/or back tension may be applied to the material to reduce roll forces. Roll Workpiece Rod ׉ 7cassandra://-fK5kQoJJPKEEnBMRqjvbD6Kdtr1vD9AzC7HPzz_0G0`cZ%Eb׉E384 Chapter 13 Metal-rolling Processes and Equipment • Continuous casting and rolling of ferrous and of nonferrous metals into semi-finished products is a common practice because of the economic benefits. • Integrated mills are large facilities involving the total sequence of activities, from the production of hot metal in a blast furnace to the casting and the rolling of finished products and ready to be shipped to the customer. On a much smaller scale, minimills utilize scrap metal that is melted in electric-arc furnaces, cast, and continuously rolled into specific lines of products. Key Terms Alligatoring Back tension Billet Bloom Camber Cast structure Chatter Cogging mill Cold rolling Crown Draft Flat rolling Foil Front tension Gage number Hot rolling Mannesmann process Minimill Neutral point Pack rolling Pilger mill Plate Bibliography Ginzburg, V.B., and Ballas, R., Flat Rolling Fundamentals, CRC Press, 2000. Hosford, W.F., and Caddell, R.M., Metal Forming: Mechanics and Metallurgy, 4th ed., Cambridge University Press, 2014. Lee, Y., Rod and Bar Rolling: Theory and Applications, CRC Press, 2004. Lenard, J.G., Primer on Flat Rolling, Elsevier, 2007. Pittner, J., and Simaan, M.A., Tandem Cold Metal Rolling Mill Control: Using Practical Advanced Methods, Springer, 2010. Ring rolling Roll Roll forging Roll stand Rolling Rolling mill Rotary tube piercing Sendzimir mill Shape rolling Sheet Skew rolling Slab Spreading Stand Steckel rolling Tandem rolling Temper rolling Tailor welded coil Thread rolling Tube rolling Wrought structure ׉ 7cassandra://sxXPAhyCla8AJ4KqT4LSTR_2K7-9x12hERu9Q19J8FI`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HAcxR4Zxrg4t1FuCTZ7yCejMCC78Jrj5daO_MrmOBtg@`׉ 7cassandra://3pNYAVDTNh_GIZx9oTsiozJ_FMGuCnD4E3ktyQGePgMZ`s׉ 7cassandra://YXaketpXxrmW6Tbxs-0FNcfdTBPLnGXWK0Dmf8UMV78`׉ 7cassandra://EhxI4n4Oym8VCuhS6Bm1M6iKL0fLz0LizSndEIQpe3MM@͠^cZEbBט#u׉׉ 7cassandra://UigHKWXXU41DiEyI2H5kDg8UEEliu3aFzP8x5L8UXUY)`׉ 7cassandra://2bS7f7UX5U9EPXY9JnQio5X2yPmG_ZijQaNY7WrwVOgX`s׉ 7cassandra://I66is9H1bR0J_F2cydxTntYj_obnM6Cj_ooLSDGfgG8`׉ 7cassandra://Egj1sh7b1O4TRfkW5HHK8nZzloa9cKdI4txjRA3Pq34Z͠^cZEbC׉E 8Qualitative Problems 385 Review Questions 13.1. What is the difference between a plate and a sheet? 13.2. Define (a) roll gap,(b) neutral point,and(c) draft. 13.3. What factors contribute to spreading in flat rolling? 13.4. What is forward slip? Why is it important? 13.5. Explain the types of deflections that rolls undergo. 13.6. Describe the difference between a bloom, a slab, and a billet. 13.7. Why may roller leveling be a necessary operation? 13.8. List the defects commonly observed in flat rolling. 13.9. What are the advantages of tandem rolling? Pack rolling? 13.10. How are seamless tubes produced? 13.11. Why is the surface finish of a rolled product better in cold rolling than in hot rolling? 13.12. What is a Sendzimir mill? What are its important features? 13.13. What is the Mannesmann process? How is it different from tube rolling? 13.14. Describe ring rolling. Is there a neutral plane in ring rolling? 13.15. How is back tension generated? Qualitative Problems 13.16. Explain why the rolling process was invented and developed. 13.17. Flat rolling reduces the thickness of plates and sheets. It is possible, instead, to reduce their thickness simply by stretching the material? Would this be a feasible process? Explain. 13.18. Explain how the residual stress patterns shown in Fig. 13.10 become reversed when the roll radius or reduction-per-pass is changed. 13.19. Explain whether it would be practical to apply the roller-leveling technique shown in Fig. 13.8a to thick plates. 13.20. Describe the factors that influence the magnitude of the roll force, F, in Fig. 13.2c. 13.21. Explain how you would go about applying front and back tensions to sheet metals during rolling. How would you go about controlling these tensions? 13.22. What typically is done to make sure that the product in flat rolling is not crowned? 13.23. Make a list of some parts that can be made by (a) shape rolling and (b) thread rolling. 13.24. Describe the methods by which roll flattening can be reduced. Which property or properties of the roll material can be increased to reduce roll flattening? 13.25. In the chapter, it was stated that spreading in flat rolling increases with (a) a decreasing width-tothickness ratio of the entering material, (b) decreasing friction, and (c) a decreasing ratio of the roll radius to the strip thickness. Explain why. ׉ 7cassandra://YXaketpXxrmW6Tbxs-0FNcfdTBPLnGXWK0Dmf8UMV78`cZ%Eb׉E 386 Chapter 13 Metal-rolling Processes and Equipment 13.26. As stated in this chapter, flat rolling can be carried out by front tension only, using idling rolls (Steckel rolling). Since the torque on the rolls is now zero, where, then, is the energy coming from to supply the work of deformation in rolling? 13.27. Explain the consequence of applying too high a back tension in rolling. 13.28. Note in Fig. 13.3d that the driven rolls (powered rolls) are the third set from the work roll. Why isn’t power supplied through the work roll itself? Is it even possible? Explain. 13.29. Describe the importance of controlling roll speeds, roll gaps, temperature, and other process variables in a tandem-rolling operation, as shown in Fig. 13.12. Explain how you would go about determining the distance between the stands. 13.30. In Fig. 13.10a, if you remove the top compressive layer by, say, grinding, will the strip remain flat? If not, which way will it curve and why? 13.31. Name several products that can be made by each of the operations shown in Fig. 13.1. 13.32. List the possible consequences of rolling at (a) too high of a speed and (b) too low of a speed. 13.33. It is known that in thread rolling as illustrated in Fig. 13.17, a workpiece must make roughly six revolutions to form the thread. Under what conditions (process parameters, thread geometry or workpiece properties) can deviation from this rule take place? 13.34. If a rolling mill encounters chatter, what process parameters would you change, and in what order? Explain your answer. 13.35. Can the forward slip ever become negative? Why or why not? Quantitative Problems 13.36. In Example 13.1, calculate the roll force and the power for the case in which the workpiece material is 1100-O aluminum and the roll radius, R, is 500 mm. 13.37. Calculate the individual drafts in each of the stands in the tandem-rolling operation shown in Fig. 13.12. 13.38. Estimate the roll force, F, and the torque for an AISI 1020 carbon-steel strip that is 200 mm wide, 12 mm thick, and rolled to a thickness of 6 mm. The roll radius is 200 mm, and it rotates at 200 rpm. 13.39. A rolling operation takes place under the conditions shown in the accompanying figure. What is the position, xn, of the neutral point? Note that there are a front and back tension that have not been specified. Additional data are as follows: Material is 5052-O aluminum; hardened steel rolls; surface roughness of the rolls = 0.025 μm; rolling temperature = 210◦C. R 5 75 mm x 5 mm V 5 1.5 m/s 3 mm 2.0 m/s Sketch P13.39 13.40. Estimate the roll force and power for annealed low carbon steel strip 200 mm wide and 10 mm thick, rolled to a thickness of 6 mm. The roll radius is 200 mm, and the roll rotates at 200 rpm. Use μ =0.2. ׉ 7cassandra://I66is9H1bR0J_F2cydxTntYj_obnM6Cj_ooLSDGfgG8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://x5Z1tjaqaYne040j-6S0rsfo1D1xmKY2ZzrZEoOhWy0d`׉ 7cassandra://MoaM8FAG5UGUs7e18le12kpIRlRe_-kuXLcsPPv7C4QL`s׉ 7cassandra://tAa-Z4E6mQ2Bs3j6s0JTN6c0f_X9cIrdoD5q9QY0JXk`׉ 7cassandra://eUSv28YNoaAc39o8RwB-xWhv2nll9BYk5zhd6lVWTP0Y͠^cZEbEט#u׉׉ 7cassandra://L-jYWnTSIi6qx3LAmD4-OH1aNbq3rYXI0llVaaW2VgU`׉ 7cassandra://-_2nVPi8-7Q8ByK2qBQbqM3N15Ps0qTuHQtd89BGf3Q@ `s׉ 7cassandra://6Ygs7tUzZCA2527R4g_qOZOEUaJkgNnqIo9v6NuLcCM`׉ 7cassandra://9HxuEHJyS0ITt6qyZ0OVPru8M42HHad6moPYn1ZNJ389͠^cZEbF׉E Synthesis, Design, and Projects 387 13.41. A flat-rolling operation is being carried out where ho =6 mm, hf =5 mm, wo =300 mm, R=250 mm, μ=0.25, and the average flow stress of the material is 275 MPa. Estimate the roll force and the torque. 13.42. It can be shown that it is possible to determine μ in flat rolling without measuring torque or forces. By inspecting equations for rolling, describe an experimental procedure to do so. Note that you are allowed to measure any quantity other than torque or forces. 13.43. A U-channel of 85-15 brass will be shape formed, but first must be flat rolled to a thickness of 0.9 mm. The strip has a width of 25 mm and an initial thickness of ho =1.5 mm. A preliminary process design suggests a 40% reduction in a single pass on a rolling mill with 150 mm-radius rolls. If the roll surface speed is 1 m/s, and the coefficient of friction is μ =0.1, calculate the rolling force and power requirements. Repeat the problem if two passes were taken to achieve the desired reduction. 13.44. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 13.45. A simple sketch of a four-high mill stand is shown in Fig. 13.3c. Make a survey of the technical literature and present a more detailed sketch for such a stand, showing the major components. 13.46. Obtain a piece of soft, round rubber eraser, such as that at the end of a pencil, and duplicate the process shown in Fig. 13.19b. Note how the central portion of the eraser will begin to erode, producing ahole. 13.47. If you repeat the experiment in Problem 13.46 with a harder eraser, such as that used for erasing ink, you will note that the whole eraser will begin to crack and crumble. Explain why. 13.48. Design a set of rolls to produce cross sections other than those shown in Fig. 13.13. 13.49. Design an experimental procedure for determining the neutral point in a flat-rolling operation. 13.50. Using a rolling pin and any available dough (bread, cookie, etc.), measuring 100 by 100 by 8 mm, quantify the spreading in flat rolling for different reductions in thickness. 13.51. Derive an expression for the thickest workpiece that can be drawn between two rolls as a function of roll gap, roll radius and coefficient of friction. 13.52. Make an extensive list of products that could use the benefits of tailor welded coils, and for each product, explain why. ׉ 7cassandra://tAa-Z4E6mQ2Bs3j6s0JTN6c0f_X9cIrdoD5q9QY0JXk`cZ%Eb׉EOChapter 14 Metal-forging Processes and Equipment 14.1 Introduction 389 14.2 Open-die Forging 390 14.3 Impression-die and Closed-die Forging 393 14.4 Various Forging Operations 397 14.5 Forgeability of Metals; Forging Defects 401 14.6 Die Design, Die Materials, and Lubrication 403 14.7 Die-manufacturing Methods and Die Failures 405 14.8 Forging Machines 407 14.9 Economics of Forging 408 Example: 14.1 Calculation of Forging Force in Upsetting 392 Case Studies: 14.1 Manufacture of a Stepped Pin by Heading and Piercing Operations 399 14.2 Suspension Components for the Lotus Elise Automobile 410 • This chapter describes the fundamentals of forging and related processes, including design and economic considerations. • Open-die forging operations for producing simple shapes are described first, followed by impression-die and closed-die forging operations for producing more intricate shapes. • Various forging operations, such as heading, piercing, coining, swaging, and cold extrusion, are then introduced. • Factors involved in forging defects and die failures are explained. • The economics of forging, as it relates to process selection, is introduced. • The chapter ends with a review of forging design considerations, guidelines for die design and manufacturing, and selection of die materials and lubricants in forging operations. 388 ׉ 7cassandra://6Ygs7tUzZCA2527R4g_qOZOEUaJkgNnqIo9v6NuLcCM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://_7tC_3hcJtb74jyqfVGzVmU7KOWnQ4iGX_X4IgqKsCc_`׉ 7cassandra://NnkC_rMrCuFST_S-jvit16qGuhWU_aM41s8RNfjPIcIYG`s׉ 7cassandra://SY-omDuBj23SXei9F98lvvAxivL6pJTplvGyXDOfgPcy`׉ 7cassandra://sHe83-S_DNzDj7Wf4QOCgxqe4pE28d1e1eMosgA74Osq ͠^cZEbHט#u׉׉ 7cassandra://35iNnR0jZqz-gn5RfsFLMn9-M6_OfkgTorl3k0VrQCk`׉ 7cassandra://zJiSzj2OQmUGy9jWcChRTV5IDEWmJRWGWfFB82bbFpwZ`s׉ 7cassandra://s_pCmrXzzDOFQrDIYGYxBskCXcgdIRukRH1sF8ckMJE`׉ 7cassandra://00G9mXe4TV-rsSHPQCgW0FO_iV0HDbveOftXtwKb2e4͠^cZEbI׉E Introduction 389 Typical parts made by forging and related processes: Shafts, gears, bolts, turbine blades, hand tools, dies, and a wide variety of components for machinery, transportation, and farm equipment. Alternative processes: Casting, powder metallurgy, machining, additive manufacturing, and fabrication. 14.1 Introduction Forging is a basic process in which the workpiece is shaped by compressive forces applied through various dies and tooling. One of the oldest and most important metalworking operations, dating back at least to 4000 B.C., forging was first used to make jewelry, coins, and various implements, by hammering metal with tools made of stone. Forged parts now include large rotors for turbines, gears, cutlery (Fig. 14.1), hand tools, and miscellaneous components for machinery, aircraft, and transportation equipment. Unlike rolling operations (Chapter 13) that generally produce continuous plates, sheets, strips, and various structural cross sections, forging operations produce discrete parts. Because the metal flow in a die and the material’s grain structure can be controlled, forged parts have good strength and toughness, and are very reliable for highly stressed and critical applications (Fig. 14.2). Simple forging operations can be performed with a heavy hammer and an anvil, as has been done traditionally by blacksmiths for centuries. Most forgings require a set of dies and such equipment as presses or powered hammers. Forging may be carried out at room temperature (cold forging) or at elevated temperatures (warm or hot forging), depending on the homologous temperature, described in Section 1.8. Cold forging requires higher forces, because of the higher strength of the workpiece material. The workpiece material must possess sufficient ductility at room temperature to be able to undergo the required deformation without cracking. Cold-forged parts have good surface finish and dimensional accuracy. Hot forging requires lower forces, but the dimensional accuracy and surface finish of the parts are not as good as those in cold forging. Forgings generally are subjected to subsequent finishing operations, such as heat treating to modify properties, and machining for dimensional accuracy and good surface finish. The finishing operations can be minimized by precision forging, an important example of net-shape or near-net-shape forming. As described throughout this book, parts that can be forged successfully also may be manufactured economically by other methods, such as casting (Chapter 11), powder metallurgy (Chapter 17), additive (a) (b) Figure 14.1: (a) Illustration of the steps involved in forging a knife. (b) Open die forging of a steel billet. Source: (a) Courtesy of Mundial, Inc. (b) Shutterstock/Milos Zvicer. ׉ 7cassandra://SY-omDuBj23SXei9F98lvvAxivL6pJTplvGyXDOfgPcy`cZ%Eb׉E W390 Chapter 14 Metal-forging Processes and Equipment (a) (b) (c) Figure 14.2: Schematic illustration of a part (dragline chain link, approximately 2-m long) made by three different processes and showing grain flow. (a) Casting by the processes described in Chapter 11. (b) Machining from a blank, described in Part IV of this book, and (c) forging. Each process has its own advantages and limitations regarding external and internal characteristics, material properties, dimensional accuracy, surface finish, and the economics of production. Source: Courtesy of the Forging Industry Association. manufacturing (Chapter 20), or machining (Part IV). Each of these will produce a part having different characteristics, particularly with regard to strength, toughness, dimensional accuracy, surface finish, and the possibility of internal or external defects. 14.2 Open-die Forging Open-die forging is the simplest forging operation (Table 14.1). Although most open-die forgings generally weigh 15 to 500 kg, forgings as heavy as 270 metric tons have been made. Part sizes may range from very small (such as pins, nails, and screws) to very large [up to 23-m-long shafts for ship propellers]. In its simplest form, open-die forging can be described by a metal workpiece blank, placed between two flat dies (platens), and reduced in height by compressing it (Fig. 14.3), an operation that is also called upsetting or flat-die forging. The die surfaces may have shallow cavities or features to make relatively simple forgings (see also blocker dies, Section 14.3). The deformation of a solid cylindrical workpiece under frictionless conditions is shown in Fig. 14.3b. Because constancy of volume is to be maintained, any reduction in height increases the diameter of the forged part. Note that the workpiece is deformed uniformly. In an actual operation, however, there is friction at the die–workpiece interfaces, whereby the part develops a barrel shape (Fig. 14.3c), a deformation mode also called pancaking. Process Open die Closed die Advantages Table 14.1: General Characteristics of Forging Processes. Limitations Simple and inexpensive dies; wide range of part sizes; good strength characteristics; generally for small quantities Relatively good utilization of material; generally better properties than open-die forgings; good dimensional accuracy; high production rates; good reproducibility Blocker Low die costs; high production rates Conventional Requires much less machining than blocker type; high production rates; good utilization of material Precision Close dimensional tolerances; very thin webs and flanges possible; machining generally not necessary; very good material utilization Limited to simple shapes; difficult to hold close tolerances; machining to final shape necessary; low production rate; relatively poor utilization of material; high degree of skill required High die cost, not economical for small quantities; machining often necessary Machining to final shape necessary; parts with thick webs and large fillets Higher die cost than blocker type High forging forces, intricate dies, and provision for removing forging from dies ׉ 7cassandra://s_pCmrXzzDOFQrDIYGYxBskCXcgdIRukRH1sF8ckMJE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://hw0qRc8zMBE4wNzuC5jQM3POA8_JlABH33yZMU8YO6gf`׉ 7cassandra://NAWRrAkCvQ-g30jQzobV4e1wnDR-mVurkq8s7Jq7dX4Vm`s׉ 7cassandra://ckwlLPt_qBuTMzj6Qs3npEvQ0WEWsM7QuKwpJuUpZZI`׉ 7cassandra://yUA9wZ8v3nef3ZVyRX1VaDQrE0JMr3KPhbGk86XJ1LE\ ͠^cZEbLט#u׉׉ 7cassandra://-52U3pPOf8HMI4keoPaH-9on1FtUmiIYaXhppwlPP4s `׉ 7cassandra://ySBDgV8hWVf796muDo-47MzeW1m34mxt1LiUV8xYU7AJ`s׉ 7cassandra://zucZmyT9MY5tF7caTGrkwbSV4MoFmL9HDo9-5lk-Ppw `׉ 7cassandra://E_pB1mKzCBv40M4uBjtxc7GEhnnnU3UPxasX8y2CRNA͆12͠^cZEbN׉E Open-die Forging 391 Workpiece Die d0 Die (a) (b) (c) Figure 14.3: (a) Solid cylindrical billet upset between two flat dies. (b) Uniform deformation of the billet without friction. (c) Deformation with friction. Note barreling of the billet caused by friction forces at the billet–die interfaces. Barreling is caused primarily by frictional forces that oppose the outward flow of the workpiece at the die interfaces, thus it can be minimized by using an effective lubricant. Barreling also can develop in upsetting hot workpieces between cold dies; the material at the die surfaces cools rapidly, while the bulk remains relatively hot. Consequently, the material at the top and bottom of the workpiece has higher resistance to deformation than the material at the center. As a result, the central portion of the workpiece expands laterally to a greater extent than do the ends. Barreling from thermal effects can be reduced or eliminated by using heated dies. Thermal barriers, such as glass cloth placed at the two die–workpiece interfaces also can be used for this purpose. Cogging, also called drawing out, is basically an open-die forging operation in which the thickness of a bar is reduced by successive forging steps (bites) at specific intervals (Fig. 14.4a). The thickness of bars and rings can be reduced also by similar open-die forging techniques, as illustrated in Fig. 14.4b and c. Because the contact area between the die and the workpiece is now smaller, a long section of a bar can be reduced in thickness without requiring large forces or heavy machinery. Note that blacksmiths have been performed such an operation for centuries, using a hammer, an anvil, and a periodically heated workpiece. Cogging of larger workpieces is usually done on mechanized equipment and with computer controls. The lateral and the vertical movements of the dies are coordinated to produce the desired shape. h0 d1 h1 h2 Friction forces Barreling Workpiece Die Die Die Workpiece Die Workpiece Die Die (a) (b) (c) Figure 14.4: (a) Schematic illustration of a cogging operation on a rectangular bar. Blacksmiths use this process to reduce the thickness of bars by hammering the part on an anvil. Reduction in thickness is accompanied by barreling, as in Fig. 14.3c. (b) Reducing the diameter of a bar by open-die forging; note the movements of the dies and the workpiece. (c) The thickness of a ring being reduced by open-die forging. ׉ 7cassandra://ckwlLPt_qBuTMzj6Qs3npEvQ0WEWsM7QuKwpJuUpZZI`cZ%Eb׉E392 Chapter 14 Metal-forging Processes and Equipment p/sy e2ma/h 1 0 h Workpiece ro r Figure 14.5: Distribution of die pressure in upsetting with sliding friction. Note that the pressure at the outer radius is equal to the flow stress, σf, of the material. Sliding friction means that the frictional stress is directly proportional to the normal stress. Forging Force. The forging force, F,inan open-die forging operation on a solid cylindrical workpiece can be estimated from the formula F = σfπr2  1+ 2μr 3h  , (14.1) where σf is the flow stress of the material (see Example 14.1), μ is the coefficient of friction between the workpiece and the die, and r and h are the instantaneous radius and height of the workpiece, respectively. Friction Hill. Consider the upsetting of a cylinder, as depicted in Fig. 14.3. If the workpiece-die interfaces are frictionless, then the die pressure is the flow stress of the material. If friction is present, as is the case in actual operations, then the die pressure is calculated as follows. For upsetting of a cylinder with outer radius ro, height, h, and coefficient of friction, μ, the die pressure at any radius can be expressed as p = σfe2µ(ro−r)/h. (14.2) The die pressure distribution is shown in Fig. 14.5. Note that the pressure it is at a maximum at the center of the workpiece, and can be very high especially if the diameter-to-height ratio of the workpiece is high. Because of its shape, the pressure-distribution curve in Fig. 14.5 is referred to as the friction hill. Example 14.1 Calculation of Forging Force in Upsetting Given: A solid cylindrical workpiece made of 304 stainless steel is 150 mm in diameter and 100 mm in height. It is reduced in height by 50%, at room temperature, in an open-die forging operation with flat dies. Assume that the coefficient of friction is 0.2. Find: What is the forging force at the end of the stroke? Solution: The forging force at the end of the stroke is calculated using Eq. (14.1), in which the dimensions pertain to the final dimensions of the forging. The final height is h = 100/2=50 mm, and the final radius, r, is determined from volume constancy, by equating the volumes before and after deformation. Hence, (π)(75)2(100) = (π)(r)2(50). ׉ 7cassandra://zucZmyT9MY5tF7caTGrkwbSV4MoFmL9HDo9-5lk-Ppw `cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://8zY6tNG5ybSdWd_PiJOVIPt93nY5ANS0vF1WJ0AuY1o8`׉ 7cassandra://JuL2U0L3zEFsPGuh60BAcff4dmoCeq5d7oIyKsxqo5A]`s׉ 7cassandra://r23M84yipP7c3-6wsEdFWj7P-SYW1hmuVCSfEmH-yAA`׉ 7cassandra://scpu-LufL7HA2JUNq6XHHeNZ-YTJU4WZs-oXDifAdNgt ͠^cZEbPט#u׉׉ 7cassandra://Ax8Sa1n1OLTj9-HkCUzy69PSvX0hA7ZSQcFUIYSUe8o`׉ 7cassandra://xKXiZLdGkAJm-Z8hx6wcYZl3pIrvLk73hi5UkC277Eo>`s׉ 7cassandra://yqr8T0MFX5Twmql6opLNA3voBUOHD9orhvU7c2o_OhU`׉ 7cassandra://RG0B2--9hCvwcbwmsPjDsmyTp0Cp6bBeFOgaEETs15AK͠^cZEbT׉E Impression-die and Closed-die Forging 393 Thus, r = 106 mm. The quantity σf in Eq. (14.1) is the flow stress of the material, which is the stress required to continue plastic deformation of the workpiece at a particular true strain. The absolute value of the true strain that the workpiece has undergone at the end of the stroke in this operation is =ln  100 50  =0.69. The flow stress can be determined using by referring to Eq. (2.8) and noting from Table 2.3 that, for 304 stainless steel, K = 1275 MPa and n =0.45. Thus, for a true strain of 0.69, the flow stress is calculated to be 1100 MPa. Another calculation method is to refer to Fig. 2.5 and note that the flow stress for 304 stainless steel at a true strain of 0.69 is about 1000 MPa. The small difference between the two values is due to the fact that the data in Table 2.3 and Fig. 2.5 are from different sources. Taking the latter value for flow stress, the forging force can now be calculated, noting that in this problem the units in Eq. (14.1) must be in N and m. Thus, F =  1000 × 106 (π)(0.106)2(1) + (2)(0.2)(0.106) (3)(0.050) =4.5 × 107 N =45 MN = 4500 metric tons. 14.3 Impression-die and Closed-die Forging In impression-die forging, the workpiece takes the shape of the die cavity while being forged between two shaped dies (Figs. 14.6a through c). This process is usually carried out at elevated temperatures in order to lower the forging forces and to develop enhanced ductility of the workpiece. Note in Fig. 14.6c that, during deformation, some of the material flows outward and forms a flash. The flash has an important role in impression-die forging: The high pressure and the resulting high frictional resistance in the flash present a major constraint on the radially outward flow of the material in the die; this is due to the friction hill effect, described in Sec. 14.2. Thus, based on the principle that the material flows in the direction of least resistance (because it requires less energy), the material flows preferentially into the die cavity, eventually filling it completely. Instead of being made as one piece, forging dies may be made of two or more pieces (segmented), including die inserts (Fig. 14.7) and particularly for complex part shapes. The inserts can easily be replaced in case of wear or failure in a particular region of the die; they are usually made of stronger and harder wear-resistant materials (see Section 33.5). The blank to be forged can be prepared by (a) cropping (shearing, Section 16.2) from an extruded or drawn bar stock; (b) powder metallurgy or casting; or (c) it is a preformed blank from a prior forging operation. The blank is placed on the lower die, and as the upper die begins to descend, its shape gradually changes, as shown in Fig. 14.8a. Preforming operations (Figs.14.8b and c) are typically made to enhance the distribution of the material into various regions of the blank, using simple dies with various contours. In fullering, material is distributed away from a die region; in edging, it is gathered into a localized region. The part is then formed into a rough shape by a process called blocking,using blocker dies. The final operation consists of finishing of the forging in impression dies, giving the forging its final shape. The flash is later removed by a trimming operation (Fig. 14.9). ׉ 7cassandra://r23M84yipP7c3-6wsEdFWj7P-SYW1hmuVCSfEmH-yAA`cZ%Eb׉E394 Chapter 14 Metal-forging Processes and Equipment Die Blank Die Die (a) (b) Flash Flash (c) (c) (d) External and internal draft angles Flash Parting line Land Rib Web Fillet Corner Trim line (e) Figure 14.6: (a) through (d) Stages in impression-die forging of a solid round billet, with contours showing effective strain. Note the formation of flash, which is excess metal that is subsequently trimmed off. (e) Standard terminology for various features of a forging die. Forging Force. The forging force, F,requiredinan impression-die forging operation can be estimated from the formula F = kσfA, (14.3) where k is a multiplying factor, obtained from Table 14.2, σf is the flow stress of the material at the forging temperature, and A is the projected area of the forging, including the flash area. In hot-forging operations, the actual forging pressure for most metals typically ranges from 550 to 1000 MPa. As an example, assume that the flow stress of a material at the forging temperature is 300 MPa, and a part (such as that shown in Fig. 14.8a) has a projected area (with flash) of 0.05 m2. Taking a value of k =10 from Table 14.2, the forging force would be F = (10)(300 × 106)(0.05) = 150 MN. Gutter Parting line ׉ 7cassandra://yqr8T0MFX5Twmql6opLNA3voBUOHD9orhvU7c2o_OhU`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://b5T5P4j9SUhbe2MNc9GNc3u0j_A80z14OxUaf_hggPU 2`׉ 7cassandra://kXsW8-qTnJ3xgenaSaacvRnw-ik3PnPKFdf_KgFRl2oF`s׉ 7cassandra://7xKY5FhEMI1KO6oidFfwdmN3dbAFCeLyGhdn4m50LKQt`׉ 7cassandra://_q55hxPtpTOBwYDz2-WbeUPqBuPq9u9FZpsDy7_nSKkE͉d͠^cZEbVט#u׉׉ 7cassandra://Oggn8YNp5lcnXzdCH3n8T5i06aGNrwoB3PDzZknAi9o`׉ 7cassandra://xHXiIXkYK5VlVixQ3rmIvwaQbnYIavoVwF2hP4ochd8B`s׉ 7cassandra://1oObzE_u4VIRTemo5b9cWJ1jMG1gO8Hpi0WvihuDq_0`׉ 7cassandra://Fw98dX3JzOijZ-vwGRqcViawGK-z0dli-8WeXlvNvh0͝ 4͠^cZEbW׉EImpression-die and Closed-die Forging 395 Upper die block (6F2 or 6G) Insert (H12) Insert (H12) Workpiece Lower die block Insert (6F2 or 6G) Figure 14.7: Die inserts used in forging an automotive axle housing (see Section 5.7 for die materials). Closed-die Forging. The process shown in Fig. 14.6 is also referred to as closed-die forging. In true closed-die forging, however, a flash does not form (hence the term flashless forging), and the workpiece completely fills the die cavity (see right side of Fig. 14.10b). The accurate control of the blank volume and proper die design are essential to producing a forging with the required dimensional tolerances. Undersized blanks prevent the complete filling of the die cavity; conversely, oversized blanks generate excessive pressures and may cause dies to fail prematurely or the forging machine to jam. Precision Forging. In order to reduce the number of additional finishing operations, hence cost, the trend has been toward greater precision in forged products (net-shape forming). Typical precision-forged products are gears, connecting rods, and turbine blades. Precision forging requires (a) special and more complex dies, (b) precise control of the blank’s volume and shape, and (c) accurate positioning of the blank 1. Blank (bar stock) 2. Edging 3. Blocking 4. Finishing 5. Trimming (a) (b) (c) Figure 14.8: (a) Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash required to ensure proper filling of the die cavities. (b) Fullering and (c) edging operations to distribute the material properly when preshaping the blank for forging. Fullering Die Workpiece Die Edging ׉ 7cassandra://7xKY5FhEMI1KO6oidFfwdmN3dbAFCeLyGhdn4m50LKQt`cZ%Eb׉EG396 Before Punch Trimming die Slug Stationary punch Chapter 14 Metal-forging Processes and Equipment After trimming Flash (scrap) Figure 14.9: Trimming flash from a forged part. Note that the thin material at the center is removed by punching. Table 14.2: Range of k Values for Eq. (14.3). Shape k Simple shapes, without flash 3–5 Simple shapes, with flash 5–8 Complex shapes, with flash 8–12 in the die cavity. Because of the higher forces required to produce fine details on the part, precision forging requires higher capacity equipment. Aluminum and magnesium alloys are particularly suitable, because of the relatively low forging loads and forging temperatures that they require; however, steels and titanium also can be precision forged economically. Forging Practice and Product Quality. A hot forging operation typically involves the following sequence of steps: 1. Prepare a slug, billet, or preform; if necessary, clean surfaces by such means as shot blasting (see Section 34.16). Upper die Upper punch Billet Forging Flash Billet Forging Lower die 1. Start of stroke (a) 2. End of stroke Die 1. Start of stroke Lower punch Die 2. End of stroke (b) Figure 14.10: Comparison of (a) closed-die forging with flash and (b) precision or flashless forging of a round billet. Source: After H. Takemasu, V. Vazquez, B. Painter, and T. Altan. ׉ 7cassandra://1oObzE_u4VIRTemo5b9cWJ1jMG1gO8Hpi0WvihuDq_0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HprJ1qlo2VcEZIjWZwlYJOpaKJ8NtvQeABSE4qmWTbQ`׉ 7cassandra://jqrz_qhvtKrA8EyRYH2Mb8t4h1y4UynwzVYe_w7njJAR`s׉ 7cassandra://jXs3GQlPX90Gfz42drNVNT-W2cEjepFah2KMB2aE4mEf`׉ 7cassandra://MSpp2DZ4tCg2FmVWFN6xI_4nqjEBsBe0fDEfI0ooPnY͙ ͠^cZEbYט#u׉׉ 7cassandra://l1vazxYdRY_qZ9YWwhI-5ND_AoNgvp73SMYzfl4GL2Y~`׉ 7cassandra://VoLuHo11kAzy2df6eFaVyQ_7JXyJVQFCwbupYIaReV8["`s׉ 7cassandra://H1qfkN_2aVzB-yggIe0i4HBL59o7k7L4ttiTRwTDFmw`׉ 7cassandra://S9QPqAR5vBTt2F0H3aM3coO-rdejaCylf0qXKN9opjM͊<͠^cZEbZ׉EVarious Forging Operations 397 2. Heat the workpiece in a suitable furnace; then, if necessary, descale it with a wire brush, water jet, steam, or by scraping. Some descaling also may occur during the initial stages of forging, when the thick, brittle scale falls off during forging. 3. Preheat, if necessary, and lubricate the dies. 4. Forge the billet in appropriate dies and in the proper sequence. If necessary, remove any excess material, especially any flash, by trimming, machining, or grinding. 5. Clean the forging, check for dimensional accuracy; if necessary, machine or grind to final dimensions and specified tolerances and surface finish. 6. Perform additional finishing operations, such as straightening and heat treating, for improving mechanical properties. 7. Inspect the forging for any external and internal defects. The quality, dimensional tolerances, and surface finish of a forging depend on how well these operations have been performed. Generally, dimensional tolerances range between ±0.5 and ±1%ofthe dimensions of the forging. In good practice, tolerances for hot forging of steel are usually less than ±6 mm; in precision forging, they can be as low as ±0.25 mm. Other factors that contribute to dimensional inaccuracies are draft angles, radii, fillets, die wear, whether the dies have closed properly, and mismatching of the dies. 14.4 Various Forging Operations Several other operations related to the basic forging process are described below. Coining. Essentially a closed-die forging process, coining was originally used in the minting of coins, medallions, and jewelry (Fig. 14.11). It is also used to produce a wide variety of parts with high accuracy, such as precision gears, industrial seals, and medical devices. The blank or slug is coined in a completely closed die cavity, in order to produce fine details. The pressures required can be as high as five or six Retaining ring Die holder Upper die Die holder Workpiece Lower die Coined part (a) (b) Figure 14.11: (a) Schematic illustration of the coining process. (b) An example of a modern coining operation, showing the coins and tooling. Note the detail and superior surface finish that can be achieved in this process. Source: Courtesy of C &W Steel Stamp Co., Inc. ׉ 7cassandra://jXs3GQlPX90Gfz42drNVNT-W2cEjepFah2KMB2aE4mEf`cZ%Eb׉E 6398 Chapter 14 Metal-forging Processes and Equipment Kickout pin Die Blank Punch 1. 2. Head formed in punch 2. 3. Head formed in die 63 mm 3. 114 mm 4. (a) (b) Figure 14.12: (a) Heading operation to form heads on fasteners, such as nails and rivets. (b) Sequence of operations used to produce a typical bolt head by heading. times the strength of the material. On some parts, several coining operations may be required. Lubricants should not be used in coining because they can become entrapped in die cavities and, being incompressible, prevent the full reproduction of die-surface details and surface finish. Marking parts with letters and numbers (for identification) also can be done rapidly through coining. Sizing is a process used mainly with forged or powder metal blanks (see Chapter 17) and other processes to improve surface finish and to impart the desired dimensional accuracy, with little or no change in part size. Heading. Also called upset forging, heading is basically an upsetting operation, performed on the end of a rod or wire in order to increase the cross section. Typical products made are nails, bolt heads, screws, rivets, and fasteners (Fig. 14.12a). Heading can be carried out cold, warm, or hot, and can be combined with cold-extrusion processes to make various parts, as described in Section 15.4. Heading operations are performed on machines called headers; they are highly automated, with production rates of hundreds of pieces per minute for small parts. Hot heading operations on larger parts typically are performed on horizontal upsetters. An important consideration in heading is the tendency for the workpiece to buckle if its unsupported length-to-diameter ratio is too high. This ratio is generally limited to 3:1, but with appropriate dies, it can be higher. Higher ratios can be accommodated if the diameter of the die cavity is not more than 1.5 times the diameter of a round bar. Piercing. This is a process of indenting, but not breaking through, the surface of a workpiece with a punch, in order to produce a cavity or an impression (Fig. 14.13). The workpiece may be confined in a container, such as a die cavity, or may be unconstrained. The surface deformation of the workpiece will depend on how much it is constrained from flowing freely as the punch penetrates. Piercing may be followed by punching to produce a hole in the part; see the slug above the stationary punch in the central portion of Fig. 14.9. The piercing force depends on (a) the cross-sectional area and the tip geometry of the punch, (b) the strength of the workpiece material, and (c) friction at the punch-workpiece interfaces. The pressure may range from three to five times the strength of the material, which is about the same level of stress required to make an indentation in hardness testing (see Section 2.6). 34 mm 147 mm 1. 38 mm diameter ׉ 7cassandra://H1qfkN_2aVzB-yggIe0i4HBL59o7k7L4ttiTRwTDFmw`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://TnCAyQngVkfTyIsKBJLZ9fxOOFw6EtVrN8Ri65Q5WHoi`׉ 7cassandra://m-FKayCuKBeHn68bZahG5JI6xPq0tHgmDeJbqke9H80I^`s׉ 7cassandra://LkJ6rpGgHjEQ10uMjJpfPOXXjcugomVITVjwrB3W5U4g`׉ 7cassandra://yinnv6dVndU6N9BRGwHG_PewJ_eOEAGxEShcE8a1GSg͠^cZEbaט#u׉׉ 7cassandra://7Ob810IQzEX77-yr6lQkILv78yjOqOnDB1qnwOjrsak`׉ 7cassandra://Ad6_1F8h3hr3pcqdlCqhztY4XEOADD-juWcXjrxTvBcp3`s׉ 7cassandra://geAQWQiA3zj8z67VCIjEH2-bjif8lKVWmhyKSv26DV41`׉ 7cassandra://fADIm3AgIUzKM0G-eH77H5F9jz6EBJLMwAAx9ocTIt8p͠^cZEbb׉E?Various Forging Operations 399 Punch Workpiece Die (a) Figure 14.13: Examples of piercing operations. Case Study 14.1 Manufacture of a Stepped Pin by Heading and Piercing Operations Figure 14.14a shows a stepped pin made from SAE 1008 steel, and used as a portion of a roller assembly to adjust the position of a car seat. The part is complex and must be produced in a progressive manner, in order to produce the required details and fill the die completely. The cold-forging steps used to produce this part are shown in Fig. 14.14b. First, a solid, cylindrical blank is extruded (Chapter 15) in two operations, followed by upsetting. The upsetting operation uses a conical cross section in the die to produce the preform, and is oriented such that material is concentrated at the top of the part in order to ensure proper die filling. After impression-die forming, a piercing operation is performed to form the bore. Hubbing. This process consists of pressing a hardened punch, with a specific tip geometry, into the surface of a block of metal. The cavity produced is subsequently used as a die for forming operations, such as those employed in making tableware. The die cavity is usually shallow, but for deeper cavities some material may be removed from the surface of the block by machining prior to hubbing (see Figs. 24.2c and d). The hubbing force can be estimated from the equation Hubbing force =3(Sut)(A), (14.4) (a) (b) Figure 14.14: (a) The stepped pin used in Case Study 14.1. (b) Illustration of the steps used to produce the stepped pin. Source: Courtesy of National Machinery, LLC. ׉ 7cassandra://LkJ6rpGgHjEQ10uMjJpfPOXXjcugomVITVjwrB3W5U4g`cZ%Eb׉E400 Chapter 14 Metal-forging Processes and Equipment where Sut is taken from Table 2.2, and A is the projected area of the impression. As an example: for highstrength steel, with Sut = 1500 MPa and a part with a projected area of 400 mm2, the hubbing force is =1.8 MN = 183 metric tons. (3)  1500 N/mm2   400 mm2 Orbital Forging. In this process, the upper die moves along an orbital path and forms the part incrementally. The operation is similar to the action of a mortar and pestle, used for crushing herbs and seeds. Typical components made are disk-shaped and conical parts, such as bevel gears and gear blanks. The forging force is relatively small, because at any particular instant, the die contact is concentrated onto a small area of the workpiece (see also incremental forging next). The operation is relatively quiet, and parts can be formed within 10 to 20 cycles of the orbiting die. Incremental Forging. In this process, a tool forges a blank into a particular shape. The operation is somewhat similar to cogging (Fig. 14.4a), in which the die deforms the blank, in several small steps, to a different extent at different positions. Because of the small contact area with the die, the process requires much lower forces then in conventional impression-die forging; the tools are simpler and less costly. Isothermal Forging. Also known as hot-die forging, in this process the dies are heated to the same temperature as that of the hot workpiece. Because the workpiece remains hot, its flow stress and high ductility are maintained during forming. Thus, the forging load is low, and the material flow within the die cavity is improved. Complex parts can be isothermally forged, with good dimensional accuracy and to near-net shape by one stroke in a hydraulic press. The dies are usually are made of nickel or molybdenum alloys, because of their resistance to high temperature. The process is expensive and the production rate is low. It can, however, be economical for specialized, intricate forgings, made of such materials as superalloys and titanium. Rotary Swaging. In this process, also known as radial forging, rotary forging,or swaging, a solid rod or tube is subjected to radial impact forces, using a set of reciprocating dies of the machine (Figs. 14.15a and b). The dies are activated by means of a set of rolls within a cage, in an action similar to that of a roller bearing. The workpiece is stationary and the dies rotate while moving radially in their slots, striking the workpiece at ratesashighas20strokespersecond. In die-closing swaging machines, die movements are through the reciprocating motion of the wedges (Fig. 14.15c). The dies can be opened wider than those in rotary swagers, thereby accommodating largediameter or variable-diameter parts. In other arrangements, the dies do not rotate but move radially in and out. The swaging process also can be used to assemble fittings over cables and wire, in which case the tubular fitting is swaged directly onto the cable. The process is also used for operations such as pointing (tapering the tip of a round rod) and sizing (finalizing the dimensions of a part). Swaging generally is limited to a maximum workpiece diameter of about 150 mm, and parts as small as 0.5 mm have been swaged. Dimensional tolerances range from±0.05 to ±0.5 mm. The process is suitable for medium-to-high rates of production, as high as 50 parts per minute, depending on part complexity. Swaging is a versatile process and is limited in length only by the length of the bar supporting the mandrel, if one is needed (see Fig. 14.15b). Tube Swaging. In this process, the internal diameter and/or the thickness of a tube is reduced, with or without using internal mandrels (Figs. 14.16a and b). For small-diameter tubing, high-strength wire can be used as a mandrel. Mandrels also can be made with longitudinal grooves, to allow swaging of internally shaped tubes (Fig. 14.16c). For example, the rifling in gun barrels (internal spiral grooves to give gyroscopic effect to bullets) can be produced by swaging a tube over a mandrel with spiral grooves. Special machinery can swage gun barrels, and other parts, with starting diameters as large as 350 mm. ׉ 7cassandra://geAQWQiA3zj8z67VCIjEH2-bjif8lKVWmhyKSv26DV41`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://8XEKSc20FYT_UWuvyiQAlw1p1WIYj7TVtVBh3EBzXyI`׉ 7cassandra://SfQvungz-35nM3jr75tgCaP2PU5mU4cEA2VdXvnC6CwZ`s׉ 7cassandra://6h2PsseJtqgBPhxWAbiNXqkfeilI6HC91TT-UW9YxGE9`׉ 7cassandra://DSY_DLHjatYHKpixJDY0GfNic1w4xPq8bMYNJk8sHB0ya ͠^cZEbdט#u׉׉ 7cassandra://qVLp-iu_Qg-HQWeo4lBaqqWh7CgTl_nONAbFisNz_6M[`׉ 7cassandra://y5SuKQ0l1noElsnv8zcwVPeCEoQxeBlTY6L62_ITw44>`s׉ 7cassandra://5espSHpK1lWcanVtm5kRAxUwuRGRY88r0JjPc-1Cj-0`׉ 7cassandra://IOyOPum9Q5l07Cg4qhAiUrtOjxsYrfOHvhe8jRoZ0Cc͇J͠^cZEbe׉EForgeability of Metals; Forging Defects 401 Hammer Retainer Die Planetary rollers Driven Mandrel (a) (b) Cam Die Workpiece Backer Wedge Die 1. Open position 2. Swaging position (c) 3. Ejecting position (d) Figure 14.15: (a) Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a tubular workpiece by swaging. (c) A die-closing swaging machine, showing forming of a stepped shaft. (d) Typical parts made by swaging. Source: (d) Courtesy of Woodsage Holdings, LLC. 14.5 Forgeability of Metals; Forging Defects Forgeability is generally defined as the capability of a material to undergo deformation in forging without cracking. Various tests have been developed over the years to quantify forgeability; however, because of their complex nature, only two simple tests have had general acceptance: upsetting and hot twist. In the upsetting test, a solid, cylindrical specimen is upset between flat dies to the reduction in height at which cracks on the barreled surfaces begin to develop (see also Fig. 2.20d). The greater the deformation prior to cracking, the greater the forgeability of the metal. The second method is the hot-twist test, in which a round specimen is twisted continuously and in the same direction until it fails. This test is performed on a number of specimens and at different temperatures, and the number of complete turns that each specimen undergoes before failure at each temperature is plotted. The temperature at which the maximum number of turns occurs then becomes the forging temperature for maximum forgeability. This test has been found to be useful particularly for steels. The forgeability of various metals and alloys is given in Table 14.3, in decreasing order. More comprehensively, forgeability is rated on such considerations as (a) ductility and strength of the material, (b) forging temperature required, (c) frictional behavior between the die and workpiece, and (d) the quality of the forgings produced. These ratings should be regarded only as general guidelines. Typical Workpiece Part Ejector ׉ 7cassandra://6h2PsseJtqgBPhxWAbiNXqkfeilI6HC91TT-UW9YxGE9`cZ%Eb׉E402 Chapter 14 Metal-forging Processes and Equipment Tube Die Mandrel a (a) (b) (c) Figure 14.16: (a) Swaging of tubes without a mandrel; note the increase in wall thickness in the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the tube depends on the mandrel diameter. (c) Examples of cross sections of tubes produced by swaging on shaped mandrels. Rifling (internal spiral grooves) in small gun barrels can be made by this process. hot-forging temperature ranges for various metals and alloys are included in Table 14.3. For warm forging, temperatures range from 200◦ to 300◦C for aluminum alloys, and 550◦ to 750◦C for steels. Forging Defects. In addition to surface cracking, various other defects can develop during forging as a result of the material flow pattern in the die, as described in Section 14.6 regarding die design. For example, if there is an insufficient volume of material to completely fill the die cavity, the web may buckle and develop laps (Fig. 14.17a). Conversely, if the web is too thick, the excess material flows past the already formed portions of the forging and develop internal cracks (Fig. 14.17b). Table 14.3: Forgeability of Metals, in Decreasing Order. See also Table 15.1. Approximate range of hot-forging Metal or alloy Aluminum alloys Magnesium alloys Copper alloys Carbon- and low-alloy steels Martensitic stainless steels Austenitic stainless steels Titanium alloys Iron-based superalloys Cobalt-based superalloys Tantalum alloys Molybdenum alloys Nickel-based superalloys Tungsten alloys temperatures (◦C) 400–550 250–350 600–900 850–1150 1100–1250 1100–1250 700–950 1050–1180 1180–1250 1050–1350 1150–1350 1050–1200 1200–1300 ׉ 7cassandra://5espSHpK1lWcanVtm5kRAxUwuRGRY88r0JjPc-1Cj-0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Lzo68pRQNY511lpR9r0hjDWXTHyT-In3Fyc9O1kcv8YF`׉ 7cassandra://bGdcN3Wt5R86_rLtXiZmxH1CDNXOfW8oqIOcBBkLdsg[`s׉ 7cassandra://qWS2dlT3sD_g3zvwHcPoLwyh1P7bJahGh_fGtaMay4sr`׉ 7cassandra://sl-yabwTMlJlOAAEnsMOUcFsndllP8flRrRCkUceMSo;2.͠^cZEbhט#u׉׉ 7cassandra://N0lvnA7gBH0tFfzgp0IVGlHCmTudI1Oy0BOh9KCSwvgR `׉ 7cassandra://dHbHfY82wWlNctMJ6iOk0VrAKdOm7Y60ljE2uP9PwS0L`s׉ 7cassandra://0uyeQzya2u1UMIlWAau8vX7UxI6bp06nZWMeUlXkrds`׉ 7cassandra://ilrb59tZkwA82ooNhsUNt9Y7KojL2I6pLIUz5F5P_Xwͅ2͠^cZEbj׉E Die Design, Die Materials, and Lubrication 403 Die Die Rib Web Laps 1. Blocked forging 2. Begin finishing 3. Web buckles (a) Cracks 4. Laps in finished forging 1. Forging begins 2. Die cavities are being filled 3. Cracks develop in ribs (b) Figure 14.17: Examples of defects in forged parts. (a) Laps formed by web buckling during forging; web thickness should be increased to avoid this problem. (b) Internal defects caused by an oversized billet. Die cavities are filled prematurely, and the material at the center flows past the filled regions as the dies close. The various radii in the forging-die cavity can significantly influence the formation of defects. Internal defects also may develop because of (a) nonuniform deformation of the material in the die cavity, (b) temperature gradients developed throughout the workpiece during forging, and (c) microstructural changes caused by phase transformations. The grain-flow pattern of the material in forging also is important. The flow lines may reach a surface perpendicularly, as shown in Fig. 14.13. In this condition, known as end grains, the grain boundaries become directly exposed to the environment and can be attacked by it, developing a rough surface which also acts as stress raisers. Forging defects can cause fatigue failures, corrosion, and wear during the service life of the forging. The importance of inspecting forgings prior to their placement in service, particularly in critical applications is obvious. Inspection techniques for manufactured parts are described in Chapter 36. 14.6 Die Design, Die Materials, and Lubrication The design of forging dies requires considerations of (a) the shape and complexity of the workpiece, (b) forgeability, (c) strength and its sensitivity to deformation rate, (d) temperature, (e) frictional characteristics at the die–workpiece interfaces and (f) die distortion under the forging loads. The most important rule in die design is that the part will preferentially flow in the direction of least resistance. Workpiece intermediate shapes should be considered so that die cavities can be filled properly and without any defects. An example of the intermediate shapes for a connecting rod is given in Fig. 14.8a. With continuing advances in reliable simulation of all types of metalworking operations, software is widely available to help predict material flow in die cavities (see Fig. 14.18) and also predict final material microstructure and mechanical properties. The simulations incorporate various conditions, such as workpiece temperature, heat transfer to dies, frictional conditions at die–workpiece contact surfaces, and forging speed. Such software has now become essential in die design, especially for eliminating defects (see also Section 38.7). 4. Cracks propagate through ribs ׉ 7cassandra://qWS2dlT3sD_g3zvwHcPoLwyh1P7bJahGh_fGtaMay4sr`cZ%Eb׉E404 Chapter 14 Metal-forging Processes and Equipment Upper die Workpiece Effective Strain Lower die (a) (b) (c) (d) Figure 14.18: Deformation of a blank during forging as predicted by the software program DEFORM, based on the finite-element method of analysis. Source: Courtesy of Scientific Forming Technologies Corporation. Preshaping. The requirements for preshaping a workpiece are: (a) the material should not flow easily into the flash, as otherwise die filling will be incomplete, (b) the grain-flow pattern should be favorable for the strength and reliability of the part made, and (c) sliding at the die–workpiece interface should be minimized in order to reduce die wear. The selection of preshapes involves calculations of cross sectional areas at each location in the forging. Die Features. The terminology for forging dies is shown in Fig. 14.6d. For most forgings, the parting line is located at the largest cross section of the part. For simple symmetric shapes, the parting line is usually a single plane at the center of the forging; for more complex shapes, the line may not lie in a single plane. The dies are then designed in such a way that they properly engage while avoiding side thrust forces and maintaining die alignment during forging. After constraining lateral flow to ensure proper die filling, the flash material is allowed to flow into a gutter, so that the extra flash does not increase the forging load excessively. A general guideline for flash thickness is 3% of the maximum thickness (vertical dimension) of the forging. The length of the land is usually 2 to 5 times the flash thickness. Draft angles are essential in almost all forging dies in order to facilitate removal of the forging. Upon cooling, the forging shrinks both in radial and longitudinal directions, therefore internal draft angles (about 7◦ to 10◦) are made larger than external angles (about 3◦ to 5◦). ׉ 7cassandra://0uyeQzya2u1UMIlWAau8vX7UxI6bp06nZWMeUlXkrds`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://hHmWHCWx4Hh5P3hMRfaaJxEbH4qylgb0n8SXZ0TCTeo`׉ 7cassandra://esx64MHQQEhX8rcaYb9uiDGwg4cdU6_fRJVCDmMAxl0o`s׉ 7cassandra://X9kjR-Fdo1OuXEtOmn5-fkx_LPL76N78XqjcJFlNUmAT`׉ 7cassandra://YRuWVFeiQzRc775qj7uUBNgTWOn4q0UgJfDsNuOsVrIc͠^cZEbmט#u׉׉ 7cassandra://eLyjuGxNi6cniRkXYgDrWOCVHtAs1SIremumJBecgwkP`׉ 7cassandra://YkkZ-6zAk_Yn-zs3MgGxD3ETFKVkYKC-6AuGf9yCZVs^>`s׉ 7cassandra://dWeBpKtcv6HyuF9I0OZxGgpjw29iQYRRPf6E1_R4fo0>`׉ 7cassandra://YkVyQ-RuSwiATbyQ1SWs3O7hmYl9RifTACmLCkTjPfIV͠^cZEbn׉EDDie-manufacturing Methods and Die Failures 405 Selection of the proper corner and fillet radii is important to ensure smooth flow of the metal into the die cavity and for improving die life. Small radii generally are undesirable because of their adverse effects on metal flow and their tendency to cause rapid die wear (as a result of stress concentration and thermal cycling). Small fillet radii also can cause fatigue cracking of the dies. As a general rule, these radii should be as large as can be permitted by the design of the forging. As with the patterns used in casting (Section 12.2.1), allowances are provided in forging-die design when machining or grinding of the forging is necessary. Machining allowance should be provided at flanges, holes, and mating surfaces. Die Materials. General requirements for die materials are: • Strength and toughness, especially at elevated temperatures • Hardenability and ability to be hardened uniformly • Resistance to mechanical and thermal shock • Wear resistance, particularly abrasive wear, because of the presence of hard scale on the surfaces of hot forgings. Common die materials are tool and die steels, containing chromium, nickel, molybdenum, and vanadium (see Tables 5.7 and 5.8). Dies are made from die blocks, which themselves are forged from castings, and then machined and finished to the desired shape, dimensional accuracy, and surface finish. Lubrication. A wide variety of metalworking fluids are available for use in forging (Section 33.7). Lubricants greatly influence friction and wear, in turn affecting the forging forces required (see Eq. (14.1)), die life, and the manner in which the material flows into die cavities. Lubricants can also act as a thermal barrier between the hot workpiece and the relatively cool dies, thus slowing the rate of cooling of the workpiece and significantly improving metal flow. An additionally important function of the lubricant is to act as a parting agent, preventing the forging from sticking to the dies and to help release it from the die. 14.7 Die-manufacturing Methods and Die Failures Dies are an important factor in the overall economics of forging, as their cost and the lead time required to produce them can be extensive; some dies can take months to make and cost more than a million dollars. Equally important are the proper maintenance of dies and their modifications and repair. Several manufacturing methods, either singly or in combination, can be used to make dies for forging. These methods include casting, forging, machining, grinding, and by the advanced machining techniques described in Chapter 27. An important and continuing trend is the production of tools and dies by rapid tooling, using rapid prototyping techniques (Section 20.6). Producing a cavity in a die block is called die sinking. The process of hubbing (Section 14.4), either cold or hot, also may be used to make smaller dies with shallow cavities. Dies are subsequently heat treated, for higher hardness, toughness, and wear resistance (Chapter 33). If necessary, their surface profile and finish are further improved by finish grinding and polishing, either by hand or using programmable industrial robots (Section 37.6). The choice of a die-manufacturing method depends on die size and shape, and the particular operation in which the die is to be used, such as casting, forging, extrusion, powder metallurgy, or molding. As in all manufacturing operations, cost often dictates the process selected. Dies can be cast from steels, cast irons, and nonferrous alloys. The processes used for preparing them may range from sand casting (for large dies, weighing several tons) to shell molding (for small dies). Cast steels generally are preferred for large dies because of their strength, toughness, and wear resistance as well as the ease with which the steel composition, grain size, and other properties can be controlled and modified as necessary. Most commonly, dies are machined from forged die blocks using such processes as high-speed milling, turning, grinding, electrical discharge (including wire EDM), and electrochemical machining (see Part IV). ׉ 7cassandra://X9kjR-Fdo1OuXEtOmn5-fkx_LPL76N78XqjcJFlNUmAT`cZ%Eb׉E t406 Chapter 14 Metal-forging Processes and Equipment Such an operation is shown in Fig. I.11b for making molds for eyeglass frames. For high-strength and wearresistant die materials that are hard or are heat treated (and thus difficult to machine), processes such as hard machining and electrical and electrochemical machining are in common practice. An increasing trend is a die produced by additive manufacturing (see Section 20.6), although the number of parts made is lower than with conventionally produced tooling. Typically, a die is machined by milling on a computer-controlled machine tool, using various software packages (see Fig. I.11) that have the capability of optimizing the cutting-tool path. Thus, the best surface finish can be produced in the least possible machining time. Equally important is the setup for machining, because ideally dies should be machined in one setup, without having to remove them from their fixtures and reorient them for subsequent machining operations. After heat treating to achieve the desired mechanical properties, dies usually are subjected to finishing operations (Section 26.7), such as grinding, polishing, and chemical and electrical methods, for the desired surface finish and dimensional accuracy. Finishing includes laser surface treatments and coatings (Chapter 34) to improve die life. Laser beams also may be used for die repair and reconfiguration of the worn regions of dies (see also Fig. 33.12). Die Costs. Some qualitative ranges of tool and die costs are given throughout this book, such as in Table 12.6. Even small and relatively simple dies can cost hundreds of dollars. The cost of a set of dies for automotive body panels can be on the order of $2 million. On the other hand, because a large number of parts usually are made from one set of dies, die cost per piece made is generally a small portion of a part’s manufacturing cost (see also Section 40.10). The lead time required to produce dies also can have a significant impact on productivity and the overall manufacturing cost of parts made. Die Failures. Failure of dies generally results from one or more of the following causes: • Improper die design • Defective or improper selection of die material • Improper manufacturing, heat-treatment, and finishing operations • Overheating and heat checking (cracking caused by temperature cycling of dies) • Excessive die wear • Overloading (excessive force on the die) • Improper alignment of die components or segments • Improper handling or misuse of the die. Other Considerations. In order to withstand the forces involved, a die must have sufficiently large cross sections and clearances (to prevent jamming). Cooling channels can be machined into a die to help extract heat. Abrupt changes in cross section, sharp corners, radii, fillets, and a coarse surface finish (including grinding marks and their orientation on die surfaces) act as stress raisers, and thus die life. For improved strength and to reduce the tendency for cracking, dies may be made in segments and assembled into a complete die, with rings that prestress the dies. Proper handling, installation, assembly, and alignment of dies are essential. Overloading of tools and dies can cause premature failure. Acommon cause of damage to dies is the failure of the operator or of a programmable robot, to remove a formed part from the die before another blank is loaded into the die. ׉ 7cassandra://dWeBpKtcv6HyuF9I0OZxGgpjw29iQYRRPf6E1_R4fo0>`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://zamswlLcN-IdXx23ikJEGNnDh8SSFEMmdBHdD6x7taE`׉ 7cassandra://q-a_jbUQ1QZ2J-gufYK13J_7K-LfsKMEMZETL7_H0jwVo`s׉ 7cassandra://pNh9GJj467vzqPAaSdqYARa0WsMENO0jEjX7k9HfKZ0`׉ 7cassandra://2Jxly0enQQ6DvI7phkw1U9Wtj7ZVEb1bPoJ6rGQPH9M/O2͠^cZEbtט#u׉׉ 7cassandra://iULuN5znV_K_wr3Edv9at4eoSeLFk896RX_cXuTg7WE=`׉ 7cassandra://FJBtRczmVE9gTrw5KRYVMzJN280osMTExeCn1Nt5iK0tL`s׉ 7cassandra://9MFi5UGx1VHWAZgFwJdAg30_KxAz7poR0Vh-oKQXJQg^`׉ 7cassandra://yxOb83d2JDvh7xNaH5tZbqJateZ30fMeII7oiPEZ6CUj͠^cZEbu׉E Forging Machines 407 Table 14.4: Typical Speed Ranges of Forging Equipment Equipment m/s Hydraulic press Mechanical press Screw press Gravity drop hammer 0.06–0.30 0.06–1.5 0.6–1.2 3.6–4.8 Power drop hammer 3.0–9.0 Counterblow hammer 4.5–9.0 14.8 Forging Machines Various types of forging machines are available, with a wide range of capacities (tonnage), speeds, and speed–stroke characteristics (Table 14.4). Hydraulic Presses. These presses operate at constant speeds and are load limited (load restricted); the press stops if the load required exceeds its capacity. Large amounts of energy can be transmitted from the press to the workpiece by a constant load throughout the whole stroke, the speed of which can be controlled. Because forging in a hydraulic press takes longer than in other types of forging machines, the workpiece may cool rapidly unless the dies are heated (see isothermal forging, Section 14.4). Compared with mechanical presses, hydraulic presses are slower and involve higher initial costs, but they require less maintenance. A hydraulic press typically consists of a frame with two or four columns, pistons, cylinders (Fig. 14.19), rams, and hydraulic pumps driven by electric motors. The ram speed can be varied during the stroke. Press capacities range up to 125 MN (12,700 metric tons) for open-die forging, and up to around 730 MN (74,000 metric tons), although this is rare (450 MN (45,000 metric tons) is more common for high capacity hydraulic presses). The main landing-gear support beam for the Boeing 747 aircraft is forged in a 450-MN (45,000metric ton) hydraulic press (Fig. 14.19d), with the forging shown in the forefront. The is made of a titanium alloy and weighs approximately 1350 kg. Mechanical Presses. These presses are basically of the crank or of the eccentric type (Fig. 14.19a). The speed varies from a maximum at the center of the stroke to zero at the bottom of the stroke, thus are stroke limited. The energy in a mechanical press is generated by a large flywheel powered by an electric motor. A clutch engages the flywheel to an eccentric shaft; a connecting rod then translates the rotary motion into a reciprocating linear motion. A knuckle-joint mechanical press is shown in Fig. 14.19b. Because of the linkage design, very high forces can be applied (see also Fig. 11.19). Flywheel Fluid Friction drive Screw Ram Ram (a) Crank (b) Knuckle-joint Ram (c) Screw Ram (d) Hydraulic Figure 14.19: Schematic illustration of the principles of various forging machines. (a) Mechanical press with an eccentric drive; the eccentric shaft can be replaced by a crankshaft to give up-and-down motion to the ram. (b) Knuckle-joint press. (c) Screw press. (d) Hydraulic press. ׉ 7cassandra://pNh9GJj467vzqPAaSdqYARa0WsMENO0jEjX7k9HfKZ0`cZ%Eb׉E408 Chapter 14 Metal-forging Processes and Equipment The force available in a mechanical press depends on the stroke position, and it becomes extremely high at the end of the stroke. Thus, proper setup is essential to avoid breaking the dies or equipment components. Mechanical presses have high production rates, are easier to automate, and require less operator skill than do other types of machines. Press capacities generally range from 2.7 to 107 MN (270 to 10,800 metric tons). Mechanical presses are preferred for forging parts requiring high precision. Screw Presses. These presses (Fig. 14.19c) derive their energy from a flywheel, hence they are energy limited. The forging load is transmitted through a large vertical screw, and the ram comes to a stop when the flywheel energy has been dissipated. If the dies do not close, the operation is repeated until the forging is completed to its final shape. Screw presses are used for various open-die and closed-die forging operations. They are particularly suitable for small production quantities and for thin parts with high precision, such as turbine blades. Press capacities range from 1.4 to 280 MN (144 to 28,350 metric tons). Hammers. Hammers derive their energy from the potential energy of the ram, which is converted into kinetic energy, thus they are energy limited. Unlike hydraulic presses, hammers operate at high speeds, thus minimizing the cooling of a hot forging, thus allowing forging of complex shapes, particularly those with thin and deep recesses in dies. To complete the forging, several successive blows are usually made in the same die. Hammers are available in a variety of designs, and are the most versatile and the least expensive type of forging equipment. Drop Hammers. In power drop hammers, the ram’s downstroke is accelerated by steam, air, or hydraulic pressure. Ram weights range from 225 to 22,500 kg, with energy capacities reaching 1150 kJ. In the operation of gravity drop hammers, a process called drop forging, the energy is derived from the free-falling ram. The available energy is the product of the ram’s weight and the height of its drop. Ram weights range from 180 to 4500 kg, with energy capacities ranging up to 120 kJ. Counterblow Hammers. These hammers have two rams that simultaneously approach each other, horizontally or vertically, to forge the part. As in open-die forging operations, the workpiece may be rotated between blows to better shape the workpiece. Counterblow hammers operate at high speeds and transmit less vibration to their bases. Capacities range up to 1200 kJ. Servo Presses. Used for forging and stamping applications (Fig. 14.20), these presses utilize servo drives along with linkage mechanisms, as in mechanical, knuckle joint, or screw presses. There are no clutches or brakes; instead, the desired velocity profile is achieved through a servo motor controller. The servo drive allows considerable flexibility regarding speeds and stroke heights, thus simplifying set up and allows an optimized velocity profile for forging difficult materials or products. In addition, servo presses can produce parts with as little as 10% of the energy consumption of other presses, attributable mainly to their low energy costs when not producing parts (see Section 40.5). Servo presses can develop forces up to 25,000 kN (2500 metric tons); larger forces can be developed by hybrid machines that combine servo drives with energy storage in a flywheel. 14.9 Economics of Forging Several factors are involved in the cost of forgings, depending on the complexity of the forging and tool and die costs, which range from moderate to high. As in other manufacturing operations, these costs are spread out over the total number of parts forged with that particular die set. Thus, referring to Fig. 14.21, even though the cost of workpiece material per forging is constant, setup and tooling costs per piece decrease as the number of pieces forged increases. The ratio of the cost of the die material to the total cost of forging a part increases with the weight of forgings. Because dies must be made and forging operations must be performed regardless of the size of the forging, the cost of dies and of the forging operation relative to material cost is high for small parts. By contrast, die material costs are relatively low. ׉ 7cassandra://9MFi5UGx1VHWAZgFwJdAg30_KxAz7poR0Vh-oKQXJQg^`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://kcmS1-JHrSlUPXbrA8RqadoBdC0stJSLzLD5N6SEhLgA`׉ 7cassandra://sBWcm_3Ml0CjKMCcESSxxzTIMLoV-kisI8Uh79qZM38=C`s׉ 7cassandra://IkdpLM1jtfLUg1TLzE3LBSGNej3nMl7pd_CR_NcUnrU`׉ 7cassandra://aoeY2KLy8wyA3kyLDkhkNpUo2bUsMnK7Ju4fkZEuKuQ͓;"͠^cZEbwט#u׉׉ 7cassandra://t6snKpHxyAQYiIMvyuaNzMFVBzANpNn927eslqXAeto18`׉ 7cassandra://GlEnx0E-LoQvNNz11Ti9KLEkk24WXB1IMH7wRLmNO6E`I`s׉ 7cassandra://OwsfFUGiE96QsxrZlhWHZL_5H157wY3eI1m0GK5JCSs`׉ 7cassandra://FxunrVGYg4rSmAe3YynDTtTA32ezoMxl0sZMvdr5GeI}͠^cZEbx׉EEconomics of Forging 409 Capacitor bank Main gear Crankshaft Amplifier/ servo controller Connecting rod with ball connection Hydraulic overload protection Upper bolster plate Lower bolster plate Servomotor Pinion Drive shaft (a) (b) Figure 14.20: (a) Schematic illustration of a servo press, with the power source and transmission components highlighted. (b) An example of a servo press, with a 23,000 kN (2250 metric tons) capacity. Source: Courtesy of Aida Engineering, Inc. 1000 100 10 Setup cost 1 10 100 1000 Number of pieces Figure 14.21: Typical cost per piece in forging; note how the setup and the tooling costs per piece decrease as the number of pieces forged increases if all pieces use the same die. 10,000 Tooling cost Total cost per piece Material cost Relative cost per piece ׉ 7cassandra://IkdpLM1jtfLUg1TLzE3LBSGNej3nMl7pd_CR_NcUnrU`cZ%Eb׉E 410 Chapter 14 Metal-forging Processes and Equipment + 100 73 mm 16 mm 10 1 0.1 Sand casting 100 Permanentmold casting 1000 10,000 Number of pieces Figure 14.22: Relative unit costs of a small connecting rod made by various forging and casting processes. Note that, for large quantities, forging is more economical, and sand casting is the most economical process for fewer than about 20,000 pieces. The size of forgings also has some effect on cost. Sizes range from small forgings (utensils and small automotive components) to large ones (gears, crankshafts, and connecting rods for large engines). As the size of the forging increases, the share of material cost in the total cost also increases, but at a lower rate. This is because (a) the incremental increase in die cost for larger dies is relatively small, (b) the machinery and operations involved are essentially the same regardless of forging size, and (c) the labor involved per forging is not that much higher. The total cost involved is not influenced to any major extent by the type of materials forged. Because they have been reduced significantly by automated and computer-controlled operations, labor costs generally are moderate. Furthermore, die design and manufacturing are now performed by computer-aided design and manufacturing (Chapter 38), resulting in major savings. The cost of forging a part, compared to that of producing it by other processes, such as casting, powder metallurgy, machining, is an important consideration. For example, for shorter production runs and all other factors being the same, making a certain part by, say, expendable-mold casting may well be more economical than producing it by forging (Fig. 14.22). Recall that this particular casting method does not require expensive molds and tooling, whereas forging typically requires expensive dies. The competitive aspects of manufacturing and process selection are described in greater detail in Chapter 40. Case Study 14.2 Suspension Components for the Lotus Elise Automobile The Lotus Elise is a high-performance sports car, designed for superior ride and handling. The Lotus group investigated the use of steel forgings instead of extruded-aluminum suspension uprights in order to reduce cost and improve reliability and performance. Their development efforts consisted of two phases, shown in Fig. 14.23. The first phase involved the development of a forged-steel component that can be used on the existing Elise sports car; the second phase involved the production of a suspension upright for a new model. A new design was developed using an iterative process, with advanced software tools to reduce the number of components and to determine the optimum geometry. The material selected for the upright was an air-cooled forged steel, which gives uniform grain size and microstructure, and uniform high strength without the need for heat treatment. These materials also 100,000 Forging Investment casting Die casting Relative cost per piece ׉ 7cassandra://OwsfFUGiE96QsxrZlhWHZL_5H157wY3eI1m0GK5JCSs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://anuNJnmLkzkkwNDAZgAwZUYuoT_JQTeXr2iX76eS-XEs`׉ 7cassandra://Y1klT7d_VkcgCGZzMpbIV7qa8lBNcx40U37en637w9IB)`s׉ 7cassandra://RnrRsNSTrCTcJ5nTU5oXP0o0ZKxWQXcG_AjwWdEHC9c`׉ 7cassandra://a9BqVIRG01bPnwWQyWmGvDMZKWn1IYxyMvhscL17Wi4<͠^cZEbʂט#u׉׉ 7cassandra://wk2I8MLON5wul-KrDGP5mDBKGSaTyC2lYriWRhWyeug&`׉ 7cassandra://OoYJAnGUZfWk7K6FW5EUQu0QhVZTPM6XVEFo4ldX-kQEr`s׉ 7cassandra://N_GrqozEk-qY9ASdca7pyE7rd-Bqziqwexo4AHE7pMk&`׉ 7cassandra://TWPPUpYr0qaN_zFy0WWBWXRkT0hGSkHkElqcf5iG0i4F͠^cZEbʃ׉E6Economics of Forging 411 have approximately 20% higher fatigue strengths than traditional carbon steels, such as AISI 1548-HT used for similar applications. The revised designs are summarized in Table 14.5. As can be seen, the optimized new forging design (Fig. 14.23d) resulted in significant cost savings. Although it also resulted in a small weight increase, when compared to the aluminum-extrusion design, the weight penalty is recognized as quite small. Furthermore, the use of forged steel for such components is especially advantageous in fatigue-loading conditions, constantly encountered by suspension components. The new design also had certain performance advantages, in that the component stiffness is now higher, which registered as improved customer satisfaction and better “feel” during driving. Furthermore, the new design reduced the number of parts required, thus satisfying another fundamental principle in design. (a) (b) (c) (d) Figure 14.23: (a) The Lotus Elise Series 2 automobile, (b) illustration of the original design for the vertical suspension uprights, using an aluminum extrusion, (c) retrofit design, using a steel forging, (d) optimized steel forging design for new car models. Source: (a) Shutterstock/VanderWolf Images. Table 14.5: Comparison of Suspension Upright Designs for the Lotus Elise Automobile. Fig. 14.23 sketch (b) (c) (d) Material Aluminum extrusion, steel bracket, steel bushing, housing Forged steel Forged steel Application Original design Phase I Phase II Mass (kg) 2.105 Cost 85 2.685 (+28%) 27.7 (−67%) 2.493 (+18%) 30.8 (−64%) ׉ 7cassandra://RnrRsNSTrCTcJ5nTU5oXP0o0ZKxWQXcG_AjwWdEHC9c`cZ%Eb׉E412 Chapter 14 Metal-forging Processes and Equipment Summary • Forging denotes a family of processes in which deformation of the workpiece is carried out by compressive forces applied through a set of dies. The process is capable of producing a wide variety of structural parts, with favorable characteristics, such as higher strength, and improved toughness, dimensional accuracy, and reliability in service. • The forging operation can be carried out at room, warm, or high temperatures. Workpiece material behavior during deformation, friction, heat transfer, and material-flow characteristics in the die cavity are important considerations, as are the proper selection of die materials, lubricants, workpiece and die temperatures, forging speeds, and equipment. • Several defects can develop in a forging if the process is not designed or controlled properly. Computer-aided design and manufacturing techniques are used extensively in die design and manufacturing, preform design, predicting material flow, and avoiding the possibility of internal and external defects during forging. • A variety of forging machines is available, each with its own capabilities, characteristics, and costs. Forging operations are highly automated, using industrial robots and computer controls. • Swaging is a type of rotary forging in which a solid rod or a tube is reduced in diameter by the reciprocating radial movement of a set of two or four dies. The process is suitable for producing short or long lengths of bar or tubing, with various internal or external profiles. • Because die failure has a major economic impact on the operation, die design, material selection, and the specific production method are of critical importance. A variety of die materials and manufacturing methods is available, including advanced material-removal and finishing processes. Key Terms Barreling Closed-die forging Cogging Coining Edging End grain Flash Forgeability Forging Fullering Hammers Heading Hot-twist test Hubbing Impression-die forging Incremental forging Isothermal forging Net-shape forging Open-die forging Orbital forging Piercing Precision forging Presses Sizing Swaging Upsetting ׉ 7cassandra://N_GrqozEk-qY9ASdca7pyE7rd-Bqziqwexo4AHE7pMk&`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://LRLRtEq6dC7U2OQV4aSRMIUtRLMlX5kegFUgdlR994wu`׉ 7cassandra://XlZ-JT6FAH0Eq7lj_IXXj44rGHLTh6jZnu3evEx5HE0``s׉ 7cassandra://9ffbzhR2PsEPM0K01ZTgOpHikfDL9twy8LKduQ0d76o<`׉ 7cassandra://NwEajR8X6cn8cjLMK1pwAY2daQ144kdmZbvr0Iumcj8__͠^cZEbʅט#u׉׉ 7cassandra://3PT47jWHiOakIqF5_-zc5ceqCD6ezLjwC3WXPlxn9NMjx`׉ 7cassandra://rl5xYGTtlRGKQFOxtMEjaLtC4eQgs1iAyArW9kpBPkse`s׉ 7cassandra://L5g5uGkQPgSRPfFc6W-ex0ohlQjMK5whO5WDMnfcY5g`׉ 7cassandra://_CbCB6ZaUxaYHef7iOOovK1P2NSgupB2rCAMgiysSm8\͠^cZEbʆ׉E $Qualitative Problems 413 Bibliography Altan, T., Ngaile, G., and Shen, G. (eds.), Cold and Hot Forging: Fundamentals and Applications,ASM International, 2004. ASM Handbook, Vol. 14A: Metalworking: Bulk Forming, ASM International, 2005. ASM Handbook, Vol. 22A: Fundamentals of Modeling for Metals Processing, ASM International, 2009. ASM Handbook, Vol. 22B: Metals Process Simulation, ASM International, 2010. Boljanovic, V., Metal Shaping Processes, Industrial Press, 2009. Dieter, G.E., Kuhn, H.A., and Semiatin, S.L. (eds.), Handbook of Workability and Process Design,ASM International, 2003. Fundamentals of Tool Design, 6th ed., Society of Manufacturing Engineers, 2010. Hosford, W.F., and Caddell, R.M., Metal Forming: Mechanics and Metallurgy, 4th ed., Cambridge, 2011. Product Design Guide for Forging, Forging Industry Association, 1997. Review Questions 14.1. What is the difference between cold, warm, and hot forging? 14.2. Explain the difference between open-die and impression-die forging. 14.3. Explain the difference between fullering, edging, and blocking. 14.4. What is flash? What is its function? 14.5. Why is the intermediate shape of a part important in forging operations? 14.6. Describe the features of a typical forging die. 14.7. Explain what is meant by “load limited,” “energy limited,” and “stroke limited” as these terms pertain to forging machines. 14.8. What type of parts can be produced by rotary swaging? 14.9. Why is hubbing an attractive alternative to producing simple dies? 14.10. What is the difference between piercing and punching? 14.11. What is a hammer? What are the different kinds of hammers? 14.12. Why is there barreling in upsetting? 14.13. What are the advantages and disadvantages of isothermal forging? 14.14. Why are draft angles needed in forging dies? 14.15. Is a mandrel needed in swaging? Qualitative Problems 14.16. Describe and explain the factors that influence spread in cogging operations on square billets. 14.17. How can you tell whether a certain part is forged or cast? Explain the features that you would investigate. 14.18. Identify casting design rules, described in Section 12.2, that also can be applied to forging. 14.19. Describe the factors involved in precision forging. 14.20. Why is control of the volume of the blank important in closed-die forging? ׉ 7cassandra://9ffbzhR2PsEPM0K01ZTgOpHikfDL9twy8LKduQ0d76o<`cZ%Eb׉E 414 Chapter 14 Metal-forging Processes and Equipment 14.21. Why are there so many types of forging machines available? Describe the capabilities and limitations of each. 14.22. What are the advantages and limitations of cogging operations? Should cogging be performed hot or cold? Explain. 14.23. Describe your observations concerning Fig. 14.17. 14.24. What are the advantages and limitations of using die inserts? Give some examples. 14.25. Review Fig. 14.6d and explain why internal draft angles are larger than external draft angles. Is this also true for permanent-mold casting? 14.26. Comment on your observations regarding the grain-flow pattern in Fig. 14.13. 14.27. Review Fig. 14.13 and make a sketch that identifies the locations where the strain is highest. 14.28. Describe your observations concerning the control of the final tube thickness in Fig. 14.16. 14.29. By inspecting some forged products, such as hand tools, you will note that the lettering on them is raised rather than sunk. Offer an explanation as to why they are made that way. 14.30. Describe the difficulties involved in defining the term “forgeability” precisely. 14.31. Describe the advantages of servo presses for forging and stamping. 14.32. List the general recommendations you would make for forging materials with limited ductility. 14.33. Which would you recommend, hot forging and heat treating a workpiece, or cold forging it and relying upon strain hardening for strengthening? Explain your answer. Quantitative Problems 14.34. Take two solid, cylindrical specimens of equal diameter, but different heights, and compress them (frictionless) to the same percent reduction in height. Show that the final diameters will be the same. 14.35. Calculate the room-temperature forging force for a solid, cylindrical workpiece made of 5052-O aluminum that is 100 mm high and 125 mm in diameter and is to be reduced in height by 30%. Let the coefficient of friction be 0.15. 14.36. Using Eq. (14.2), estimate the forging force for the workpiece in Problem 14.35, assuming that it is a complex forging and that the projected area of the flash is 30% greater than the projected area of the forged workpiece. 14.37. To what thickness can a cylinder of 1020 steel that is 20 mm in diameter and 40 mm high be forged in a press that can generate 400 kN? 14.38. In Example 14.1, calculate the forging force, assuming that the material is 1100-O aluminum and that the coefficient of friction is 0.10. 14.39. Using Eq. (14.1), make a plot of the forging force, F, as a function of the radius, r, of the workpiece. Assume that the flow stress, σf of the material is constant. Remember that the volume of the material remains constant during forging; thus, as h decreases, r increases. 14.40. How would you go about calculating the punch force required in a hubbing operation, assuming that the material is mild steel and the projected area of the impression is 400 mm2? Explain clearly. (Hint: See Section 2.6 on hardness.) 14.41. A mechanical press is powered by a 25 kW motor and operates at 50 strokes per minute. It uses a flywheel, so that the crankshaft speed does not vary appreciably during the stroke. If the stroke is 160 mm, what is the maximum constant force that can be exerted over the entire stroke length? ׉ 7cassandra://L5g5uGkQPgSRPfFc6W-ex0ohlQjMK5whO5WDMnfcY5g`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://E5BGXzc9lEwV2iGfnYujg59EKyo6HFPIQVCCSv28PNwl|`׉ 7cassandra://Hbg_dFCuDzMeC_MTHCmSh0FMB3YorvjtpLe6Go3FAA0\`s׉ 7cassandra://UbtBARGhm6okHicvicmvztftJ6N2Mh9bSYlGI4Kz594`׉ 7cassandra://xG0KTV9DH_SZpQKh4XG8UwhKIRgjknwVVlkWfJP8YhM͐r͠^cZEbʈט#u׉׉ 7cassandra://MbpQjkStKVrpESOgaKSS0HQz16hpkDOFtTdvTXyBIXMJ`׉ 7cassandra://SJVMbOx5fclvFo3HHX7smn3NMyrYyw95NhDSuLA2fSA3`s׉ 7cassandra://hoWhxX14Vh2S23ZBmxMbtUEoGzXLEqehBdQ0g7J4Y_s`׉ 7cassandra://lQZm8oSHaUOM_5oUrrijZG8D5qXiJkr25aY39LxhsRM;͠^cZEbʉנcZEbʋ,>V9ׁHhttp://0.40.Thׁ ׁ Ј׉E *Synthesis, Design, and Projects 415 14.42. A solid cylindrical specimen, made of a perfectly plastic material, is being upset between flat dies with no friction. The process is being carried out by a falling weight, as in a drop hammer. The downward velocity of the hammer is at a maximum when it first contacts the workpiece and becomes zero when the hammer stops at a certain height of the specimen. Establish quantitative relationships between workpiece height and velocity, and make a qualitative sketch of the velocity profile of the hammer. (Hint: The loss in the kinetic energy of the hammer is the plastic work of deformation; thus, there is a direct relationship between workpiece height and velocity.) 14.43. Plot the force vs. reduction in height curve in open-die forging of a cylindrical, annealed Ti-6Al-4V specimen that is 10 mm high and 25 mm in diameter, up to a reduction of 50%, for the cases of (a) 20◦Cwith μ =0.2, and (b) a workpiece preheated to a temperature of 600◦Cwith μ =0.4. 14.44. Plot the force vs. reduction in height curve in open-die forging of a cylindrical, annealed Ti-6Al-4V specimen that is 10 mm high and 25 mm in diameter, up to a reduction of 50%, with μ =0.2,for the cases of (a) a hydraulic press with a speed of 0.1 m/s and (b) a mechanical press with a speed of 1 m/s. Assume the temperature is 800◦C for both cases. 14.45. Estimate the force required to upset a 6 mm diameter C74500 brass rivet in order to form a 12 mm diameter head. Assume that the coefficient of friction between the brass and the tool-steel die is 0.25 and that the head is 6 mm in thickness. Use Sy = 175 MPa. 14.46. A compressor blade is to be forged of Ti-6Al-4V at 900◦C, where K = 140 MPa and n =0.40.The volume of the compressor blade is 40,000 mm3, but the blank is oversized so that 20% of the blank volume will go into flash. In the finishing die, the projected area is 4000 mm2. Use a flash width of 5 mm, and recognize that the compressor blade is a simple shape, but has some complexity because of the thin sections and the detail at the mounting end. Estimate the required forging force if the largest strain in the finish forging is =0.25. 14.47. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 14.48. Devise an experimental method whereby you can measure only the force required for forging the flash in impression-die forging. 14.49. Assume that you represent the forging industry and that you are speaking with a representative of the casting industry. What would you tell that person about the merits of forging processes? 14.50. The figure below shows a round impression-die forging made from a cylindrical blank, as illustrated on the left. As described in this chapter, such parts are made in a sequence of forging operations. Suggest a sequence of intermediate forging steps to make the part on the right, and sketch the shape of the dies needed. ׉ 7cassandra://UbtBARGhm6okHicvicmvztftJ6N2Mh9bSYlGI4Kz594`cZ%Eb׉E416 Chapter 14 Metal-forging Processes and Equipment 14.51. In comparing forged parts with cast parts, we have noted that the same part may be made by either process. Comment on the pros and cons of each process, considering factors such as part size, shape complexity, design flexibility, mechanical properties developed, and performance in service. 14.52. From the data given in Table 14.3, obtain the approximate value of the yield strength of the materials listed at hot-forging temperatures. Plot a bar chart showing the maximum diameter of a hot-forged part produced on a press with a 54-metric ton capacity as a function of the material. 14.53. Review the sequence of operations in the production of the stepped pin shown in Fig. 14.14. If the conical-upsetting step is not performed, how would the final part be affected? 14.54. Using a flat piece of wood, perform simple cogging operations on pieces of clay and make observations regarding the spread of the pieces as a function of the original cross sections (for example, square or rectangular with different thickness-to-width ratios). 14.55. Discuss the possible environmental concerns regarding the operations described in this chapter. 14.56. Assume that in upsetting a solid cylindrical specimen between two flat dies with friction, the dies are rotated at opposite directions to each other. How, if at all, will the forging force change from that for nonrotating dies? (Hint: Note that the dies will now require torque, because of the change in the direction of frictional forces at the die–workpiece interfaces.) 14.57. List the advantages and disadvantages in using a lubricant in forging operations. ׉ 7cassandra://hoWhxX14Vh2S23ZBmxMbtUEoGzXLEqehBdQ0g7J4Y_s`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://2RiqCUKH2mWKWItPwc5VfiqyFWktUki3bw6p7aUrK_ApB`׉ 7cassandra://LwxM7Kc9mPHRrT0qFqQ8csGM3EJgI4jH1qH6AfRbi7E6`s׉ 7cassandra://cDZU5s9tr2fmLIMma_9gRVkUPmwDChv4xX4BUmPHjccE`׉ 7cassandra://kW9xcGCc6sFpklcbcOvfZdyROWbBUJtk0mQAexk9jdw7͠^cZEbʌט#u׉׉ 7cassandra://6pmI0Xp7OqUUjCJTCoyGY_lY6M0D9EKe6f-oiM5-NA4E`׉ 7cassandra://oHxW3-hGzZIw02p4gAiN0_o7mh22zUzKTF68y2i1EuM_`s׉ 7cassandra://Y8U53s2kvQS6JPaQgXwyYuTqOIbaFVhRWjYOuAJsK4Qa`׉ 7cassandra://ai5soCkqEzGTmJdcij-ZI1Kgou9spt5bJUaoOMxnNNokF͠^cZEbʍ׉ELChapter 15 Metal Extrusion and Drawing Processes and Equipment 15.1 Introduction 418 15.2 The Extrusion Process 419 15.3 Hot Extrusion 422 15.4 Cold Extrusion 425 15.5 Extrusion Defects 428 15.6 Design Considerations 430 15.7 Extrusion Equipment 431 15.8 The Drawing Process 433 15.9 Drawing Practice 434 15.10 Drawing Defects and Residual Stresses 436 15.11 Drawing Equipment 436 Example: 15.1 Calculation of Force in Hot Extrusion 421 Case Studies: 15.1 Manufacture of Aluminum Heat Sinks 425 15.2 Cold-extruded Part 427 • Extrusion and drawing involve pushing or pulling, respectively, a material through a die, for the purpose of modifying its cross section. • The chapter begins by describing the basic types of extrusion processes, and how the extrusion force can be calculated from material and processing parameters. • Hot and cold extrusion are then presented, including die design, and describing how cold extrusion is often performed in combination with forging to produce specific part shapes. • Extrusion practices and die designs to avoid common defects are also presented. 417 ׉ 7cassandra://cDZU5s9tr2fmLIMma_9gRVkUPmwDChv4xX4BUmPHjccE`cZ%Eb׉E $418 Chapter 15 Metal Extrusion and Drawing Processes and Equipment • Drawing of rod, wire, and tubing is in a similar manner, along with die design considerations. • Equipment characteristics for these processes are also described. Typical parts made by extrusion and drawing: Long pieces having a wide variety of constant cross sections, rods, shafts, bars for machinery and automotive power-train applications, aluminum ladders, collapsible tubes, wire for numerous electrical and mechanical applications and musical instruments. Alternative processes: Machining, powder metallurgy, shape rolling, roll forming, pultrusion, additive manufacturing, and continuous casting. 15.1 Introduction Extrusion and drawing have numerous applications in manufacturing continuous as well as discrete products from a wide variety of metals and alloys. In simple extrusion, a cylindrical billet is forced through a die (Fig. 15.1) in a manner similar to squeezing toothpaste from a tube. A wide variety of solid or hollow cross sections can be produced by extrusion, which basically are semifinished products. A characteristic of extrusion (from the Latin extrudere, meaning to force out) is that large deformations can take place without fracture, because the material is under high triaxial compressive stresses (see Section 2.2.8). Since the die geometry remains unchanged throughout the process, extruded products typically have a constant cross section along their length. Typical products made by extrusion are railings for sliding doors, window and door frames, tubing, aluminum ladder frames, and structural and architectural shapes. Extrusions can later be cut into desired lengths, which then become discrete parts, such as brackets, small gears, and coat hangers (Fig. 15.2). Commonly extruded materials are aluminum, copper, steel, magnesium, and lead; other metals and alloys also can be extruded, at various levels of difficulty. Each billet is extruded individually, thus extrusion is a batch or semicontinuous operation. The process can be economical for large as well as short production runs. Tool costs generally are low, particularly for producing simple, solid cross sections. Depending on the ductility of the material, the process can be carried out at room or at elevated temperatures. Extrusion at room temperature is often combined with forging operations, in which case it is called cold extrusion (see also Section 14.4), with numerous applications, such as fasteners and components for automobiles, bicycles, motorcycles, machinery, and transportation equipment. In drawing, developed between 1000 and 1500 A.D., the cross section of a solid rod, wire, or tubing is reduced or changed in shape by pulling it through a die. Drawn rods are used for shafts, spindles, and small pistons and as the raw material for fasteners such as rivets, bolts, and screws. In addition to round rods, various profiles also can be drawn. Container liner Billet Die Die backer Pressing stem (ram) Container (chamber) Extrusion Dummy block Figure 15.1: Schematic illustration of the direct extrusion process. ׉ 7cassandra://Y8U53s2kvQS6JPaQgXwyYuTqOIbaFVhRWjYOuAJsK4Qa`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://B3amaH6Y_mwJ8uqCZ22VFkUxt89LM6iwOpwkxJgYFGM`׉ 7cassandra://RbqDIgsy8ymt-vVpHHwxQKkjpmMaWiCUOMhHgE99EjMN6`s׉ 7cassandra://t46DdpGY5M7-BAc8GFIWzNJvIUCmZOU4Y76ch4X-5aY4`׉ 7cassandra://VjfvfBR-0yccc-wKaX3caZU_Z-bfQ3aj8YOOvWKaDEk-͠^cZEbʏט#u׉׉ 7cassandra://ic7bpSSpUczByDRx1T8A95v-g_5eGYYjucmNTPG13AE`׉ 7cassandra://duCrNER-4UXoiYX5ex1A8iTb0lX1aAXaRoSF2NdEDQ0S`s׉ 7cassandra://2kdva1H3mTC-ZoMqoprZFDX4KqHGNmy_N5S_oqAHDq4w`׉ 7cassandra://LlCp0hw5qz6wjADsDfkpyqhZ5zh60JDjKtwEHbejN0MF͠ͅ^cZEbʔ׉ELThe Extrusion Process 419 (a) (b) (c) (d) Figure 15.2: Extrusions and examples of products made by sectioning off extrusions. Source: Courtesy of Plymouth Engineered Shapes. The distinction between the terms rod and wire is somewhat arbitrary, with rod taken to be larger in cross section than wire. In industry, wire is generally defined as a rod that has been drawn through a die at least once, or whose diameter is sufficiently small so that it can be coiled. Wire drawing involves much smaller diameters than rod drawing, with sizes down to 0.01 mm for magnet wire; smaller diameters can be obtained for specialized applications. 15.2 The Extrusion Process There are three basic types of extrusion processes. In direct or forward extrusion, a billet is placed in a container (chamber) and forced through a die, as shown in Fig. 15.1. The die opening can be round or it may have various shapes, depending on the desired cross section. The function of the dummy block, shown in the figure, is to protect the tip of the pressing stem, particularly in hot extrusion. In indirect extrusion, also called reverse, inverted, or backward extrusion, the die moves toward the stationary billet (Fig. 15.3a). Indirect extrusion has the advantage of having no billet–container friction, since there is no relative motion; thus, it is used on materials with very high friction, such as high-strength steels. In hydrostatic extrusion (Fig. 15.3b), the billet is smaller in diameter than the container, which is filled with a fluid, and the pressure is transmitted to the fluid by a ram. The fluid pressure imparts triaxial compressive stresses on the billet, thus improving its formability (see Section 2.2.8). Furthermore, there is much less workpiece–container friction than in direct extrusion. A less common type of extrusion is lateral (or side) extrusion (Fig. 15.3c). ׉ 7cassandra://t46DdpGY5M7-BAc8GFIWzNJvIUCmZOU4Y76ch4X-5aY4`cZ%Eb׉E :420 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Billet Container liner Plate Tool stem Extrusion (a) Die Extrusion Backing plate Container Pressing stem Fluid Container Die backer (b) Container (c) Figure 15.3: Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral. In friction stir extrusion, the die or, the mandrel in the case of hollow parts, is rotated as the workpiece is being extruded. This has the effect of causing greater frictional heating and redundant work, leading to a hotter and softer workpiece. This process is especially useful for extruding high-strength alloys, but has also been demonstrated as a recycling approach for metal shavings or powder feedstocks. As can be seen in Fig. 15.4, the basic geometric variables in extrusion are the die angle, α,and the extrusion ratio, R (ratio of the cross-sectional area of the billet to that of the extruded part, Ao/Af); additional processing variables are the billet temperature, the speed at which the ram travels towards the die, and the type of lubricant used, if any. Extrusion Force. The force required for extrusion depends on (a) the strength of the billet material, (b) extrusion ratio, (c) friction between the billet, container, and die surfaces, and (d) processing variables. It has been shown that for a small die angle, α, the extrusion pressure can be approximated as p = Sy 1+  tanα μ   Rμ cot α − 1 ,  (15.1) where μ is the coefficient of friction, Sy is the yield strength of the billet material, and R is the extrusion ratio. The extrusion force can then be obtained by multiplying the pressure by the billet area, and can be simplified as F = Aok ln  Ao Af  , (15.2) where k is the extrusion constant, determined experimentally; thus k is a measure of the strength of the material being extruded and the frictional conditions. Figure 15.5 gives k for several metals and a range of extrusion temperatures. Chamber Billet Pressure a Ao Af Land Workpiece a Die Land Relief angle Extrusion Die holder Die Seals Dummy block Die Punch Figure 15.4: Process variables in direct extrusion. The die angle, reduction in cross section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure. The contour plot shows effective strain as obtained from a finite element simulation for the geometry shown. ׉ 7cassandra://2kdva1H3mTC-ZoMqoprZFDX4KqHGNmy_N5S_oqAHDq4w`cZ%EbācZ%EbÁ#בCט#u׉׉ 7cassandra://HvhO2_4VMBvWVqehOOW524a6LvMas4q46jDJtUrUGUs `׉ 7cassandra://PEdl2RxQ8k17od2U0VSXuX5sBX8qI2phslSDhnjhSb0J`s׉ 7cassandra://UDIhiYjzQdP4ALYiHk1FxEnKk4uNYeq-Ji6WKMHkADgO`׉ 7cassandra://_fc2T4aXC6jo9oCRtTIEUufeqlBq5N-d79kMBz_xqm8͂x͠^cZEbʘט#u׉׉ 7cassandra://1Bud61nlZwnOSUto8CIiRHsPqlgLfd9APrXJhD2Ak88 `׉ 7cassandra://_JGdX0Pfb8-FKKktXe-tLR2v9UfjnC_1C-j1cearHLg``s׉ 7cassandra://iqocXwiV_lhmHuVRjrCA7CRy95dHEcNVA20VW08umK0`׉ 7cassandra://qURZDSXQMGJTaKMBtk_LRrOkfS2JCVRCreOFaBq18-MͻN@͠^cZEbʚ׉EThe Extrusion Process 421 400 200 400 600 800 1000 1200 1400 Temperature (8C) Figure 15.5: Extrusion constant k for various metals at different temperatures, as determined experimentally. Source: After P. Loewenstein. Example 15.1 Calculation of Force in Hot Extrusion Given: A round billet made of 70–30 brass is extruded at a temperature of 675◦C. The billet diameter is 125 mm, and the diameter of the extrusion is 50 mm. Find: Calculate the extrusion force required. Solution: The extrusion force is calculated using Eq. (15.2), in which the extrusion constant, k, is obtained from Fig. 15.5. For 70–30 brass, k = 250 MPa at the given extrusion temperature. Thus, F = π(0.125/2)2(250 × 106)ln  π(0.050)2 =5.5 MN. π(125)2  Metal Flow in Extrusion. The metal flow pattern in extrusion, as in other forming processes, is important because of its influence on the quality and the final properties of the extruded product. The material flows longitudinally, much like an incompressible fluid flows in a channel; thus, extruded products have an elongated grain structure (preferred orientation, Section 1.6). Improper metal flow during extrusion can produce various defects in the extruded product, as described in Section 15.5. A common technique for investigating the flow pattern is to cut the round billet lengthwise in half and mark one face with a square grid pattern. The two halves are then placed together in the chamber and are extruded. Figure 15.6 shows typical flow patterns obtained by this technique, for the case of direct extrusion with square dies (90◦ die angle). The conditions under which these different flow patterns occur are described in the caption of Fig. 15.6. Note the dead-metal zone in Fig. 15.6b and c, where the metal at the corners essentially remains stationary. Processing Parameters. In practice, extrusion ratios usually range from about 10 to 100; they may be higher for special applications (such as 400 for softer nonferrous metals) or lower for less ductile materials. The ratio usually has to be at least 4 to deform the material plastically through the bulk of the workpiece. Extrusion constant, k (MPa) C p p o 1100 Aluminum r e Molybdenum Stainless steel 70–30 Brass Chromium Beryllium Cold-rolled steel ׉ 7cassandra://UDIhiYjzQdP4ALYiHk1FxEnKk4uNYeq-Ji6WKMHkADgO`cZ%Eb׉E 422 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Dead zone Dead zone (a) (b) (c) Figure 15.6: Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction or in indirect extrusion. (b) Pattern obtained with high friction at the billet–chamber interfaces. (c) Pattern obtained at high friction or with cooling of the outer regions of the billet in the chamber. This type of pattern, observed in metals whereby their strength increases rapidly with decreasing temperature, leads to a defect known as pipe,or extrusion defect. Extruded products are typically less than 7.5 m long, because of the difficulty in handling greater lengths; they can be as long as 30 m. Ram speeds range up to 0.5 m/s; generally, lower speeds are preferred for aluminum, magnesium, and copper, and higher for steels, titanium, and refractory alloys. Dimensional tolerances are usually in the range from ±0.25 to 2.5 mm; they increase with increasing cross sectional area. Because they have high ductility, aluminum, copper, and magnesium and their alloys, and steels and stainless steels, are extruded with relative ease into various cross sections. Metals such as titanium and refractory metals also can be extruded, but only with some difficulty and significant die wear. Most extruded products, particularly those with small cross sections, require subsequent straightening and twisting. This is typically done in a hydraulic stretcher equipped with jaws. The presence of a die angle causes a small portion at the end of the billet to remain in the chamber at the end of the ram stroke. This portion, called scrap or the butt end, is later removed by cutting it off. Alternatively, a graphite block or another billet may be placed in the chamber behind the previous extrusion. In coaxial extrusion, coaxial billets are extruded together, provided that the strength and ductility of the two metals are compatible. Stepped extrusions also are produced, by extruding the billet partially in one die and then in successively larger dies (see also cold extrusion, Section 15.4). Lateral extrusion (Fig. 15.3c) is used for the sheathing of wire and the coating of electric wire with plastic. 15.3 Hot Extrusion For metals and alloys that do not have sufficient ductility at room temperature, or in order to reduce the forces required, extrusion is carried out at elevated temperatures (Table 15.1). As in all other elevatedtemperature operations, hot extrusion has special requirements because of the high operating temperatures involved. For example, die wear can be excessive, and cooling of the hot billet’s surfaces (in the cooler container) and the die can result in highly nonuniform deformation of the billet, as shown in Fig. 15.6c. Thus, extrusion dies may be preheated, as is also done in hot-forging operations (Section 14.1). Because the billet is hot, it develops an oxide film, unless it is heated in an inert environment. Oxides can be abrasive (see Section 33.2), and can affect the flow pattern of the material. Their presence also results in an extruded product that may be unacceptable when good surface finish is required. To avoid forming of oxide films on the hot extruded product, the dummy block placed ahead of the ram (Fig. 15.1) is made a little smaller in diameter than the container. As a result, a thin shell (skull), consisting mainly of the outer oxidized layer of the billet, is left in the container; it is later removed from the chamber. ׉ 7cassandra://iqocXwiV_lhmHuVRjrCA7CRy95dHEcNVA20VW08umK0`cZ%EbƁcZ%EbŁ#בCט#u׉׉ 7cassandra://p8oiNptgqqRIgR1T6N8mISDtiOU9dFM0aNEUoA0oY60=`׉ 7cassandra://g2_GLB7Am-RYDs9b3hcCcKJ2MFlKxcDYkfFdWxKhT6QQ`s׉ 7cassandra://fNXiNEyPlj6kDIPL0wohtNrORcCUf3O1h3fq1wAnUpY`׉ 7cassandra://5uKqeogcHx1lCaWVwtMnzLS4jIEvqnqkgI181H5i_AY͸!D͠^cZEbʜט#u׉׉ 7cassandra://27Q3b4mNN441UOHB5qF2bqYlrNcBbhoM5nxH94cscFs`׉ 7cassandra://f9B1DVd_DaTWn4ioOV18Sh3WFEcYbtvOPC6zfIHeA3MX`s׉ 7cassandra://2-g7RTghbgK-KMyHMvDcqJmApsybFKyl-yx3_MmLKSYQ`׉ 7cassandra://ZOSE8zruLrDNEALFf09YtSXjd2geF9DpiJHK8j56UDk6͠^cZEbʠ׉E Hot Extrusion 423 Table 15.1: Typical Extrusion Temperature Ranges for Various Metals and Alloys (see also Table 14.3). Extrusion Material Lead Aluminum and its alloys Copper and its alloys Steels Refractory alloys temperature, ◦C 200–250 375–475 650–975 875–1300 975–2200 Die Design. Die design requires considerable experience, as can be appreciated by reviewing Fig. 15.7. Square dies, also called shear dies, are used in extruding nonferrous metals, especially aluminum. These dies develop dead-metal zones, which in turn form an effective die angle (see Fig. 15.6b and c) along which the material flows. These zones produce extrusions with bright finishes, because of the burnishing action (Section 16.2) that takes place as the material flows past the die; the workpiece also is initially oxide-free. Tubing can be extruded from a solid or hollow billet (Fig. 15.8).Wall thickness is usually limited to 1 mm for aluminum, 3 mm for carbon steels, and 5 mm for stainless steels. When solid billets are used, the ram is fitted with a mandrel that pierces a hole into the billet. Billets with a previously pierced hole also may be extruded in this manner. Because of friction and the severity of deformation, thin-walled extrusions are more difficult to produce than those with thick walls. Hollow cross sections (Fig. 15.9a) can be extruded by welding-chamber methods and a porthole die, spider die,or bridge die (Fig. 15.9b to d). During extrusion, the metal divides and flows around the supports for the internal mandrel into strands; this is a condition much like that of air or water flowing around an object and rejoining downstream. The strands being extruded then become rewelded, under the high pressure in the welding chamber, before exiting the die. The rewelded surfaces have good strength because they have not been exposed to the environment; otherwise, they would develop oxides on their surfaces, thereby inhibiting good welding. The welding-chamber process is suitable only for aluminum and some of its alloys, because they can develop a strong weld under high pressure, as described in Section 31.2. Lubricants cannot be used because they prevent rewelding of the metal surfaces within the die. Die Materials. Die materials for hot extrusion usually are hot-work die steels (Section 5.7). Coatings, such as partially stabilized zirconia (PSZ), may be applied to the die surfaces to extend their life. Dies made of Extrusion direction Extrusion direction Land 60° Land (a) (b) Figure 15.7: Typical extrusion–die configurations: (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for a T-shaped extrusion, made of hot-work die steel and used with molten glass as a lubricant. ׉ 7cassandra://fNXiNEyPlj6kDIPL0wohtNrORcCUf3O1h3fq1wAnUpY`cZ%Eb׉EN424 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Mandrel Ram Container Tube Ram Spider Billet Billet (a) (b) Figure 15.8: Extrusion of a seamless tube (a) using an internal mandrel that moves independently of the ram; an alternative arrangement has the mandrel integral with the ram, (b) using a spider die (see Fig. 15.9) to produce seamless tubing. PSZ (Section 8.2.2) also are used for hot extrusion of tubes and rods. However, they are not suitable for extruding complex shapes, because of the severe stress gradients that develop in the die, possibly leading to their premature failure. Lubrication. Lubrication is important in hot extrusion, because of its effects on (a) material flow during extrusion, (b) surface finish and integrity, (c) product quality, and (d) extrusion forces. Glass (Section 8.4) is an excellent lubricant for hot extrusion of steels, stainless steels, and high-temperature metals and alloys. In a process developed in the 1940s and known as the S´ejournet process, a circular glass or fiberglass pad is placed in the chamber at the die entrance. The hot billet conducts heat to the glass pad, whereupon a thin layer of glass melts, is entrained, and acts as a lubricant. Before the hot billet is directly placed in the chamber, its cylindrical surface is coated with a layer of powdered glass, to develop a thin glass lubricant layer at the billet–chamber interface. Spider Mandrel Mandrel Die Container Tube Die Die Welding chamber Die Spider Die Bridge (a) (b) Porthole die Inlet ports (c) Spider die (d) Bridge die Figure 15.9: (a) An extruded 6063-T6 aluminum-ladder lock for aluminum extension ladders. This part is 8 mm thick and is sawed from the extrusion (see Fig. 15.2). (b) through (d) Components of various dies for extruding intricate hollow shapes. Source: (b) through (d) after K. Laue and H. Stenger. ׉ 7cassandra://2-g7RTghbgK-KMyHMvDcqJmApsybFKyl-yx3_MmLKSYQ`cZ%EbȁcZ%Ebǁ#בCט#u׉׉ 7cassandra://0A6NtDRz9eRTvDIB-4APHzVz324U7SucY9A0XHAFYhg`׉ 7cassandra://-z_icF7qYapncJUeTgYUghCVcVA3xA9EXOacHrGQP4YSJ`s׉ 7cassandra://OoXRLPC0Nh4mKYYnJmNS6nvvUn9w8MdjurVjVeY4t30`׉ 7cassandra://meqJ_zAr0DkJ1QDlq17xCn8_k6bAssOeFSvCdA-fSXQ{Z ͠^cZEbʢט#u׉׉ 7cassandra://y16Gce70J0woOi7IyJgOZQTg6iz4qYqkyvtcU4Z0EDgb`׉ 7cassandra://7UK2zIKm5cOHdedEYPOLTWBg8FiOXqy31El5TEcYhZQO`s׉ 7cassandra://1L30ZhaO0BnKk30DFB9KeVAfBSLaraeD4pVbOUT5W9E`׉ 7cassandra://X3vfD7XmDXnZ2Lmm-cVpzmE_VKPGxVmaMToI3cGovMEͅ^͠^cZEbʣ׉ECold Extrusion 425 For metals that have a tendency to stick or even weld to the container and the die surfaces, the billet can be enclosed in a jacket, a thin-walled container made of a softer and lower strength metal, such as copper or mild steel; this procedure is called jacketing or canning. In addition to acting as a low-friction interface, the jacket prevents contamination of the billet by the environment. For billet materials that are toxic or radioactive, the jacket also prevents it from contaminating the environment. Case Study 15.1 Manufacture of Aluminum Heat Sinks Aluminum is used widely to transfer heat for both cooling and heating applications, because of its very high thermal conductivity. In fact, on a weight-to-cost basis, no other material conducts heat as efficiently as does aluminum. Hot extrusion of aluminum is preferred for heat-sink applications, such as those in the electronics industry. Fig. 15.10a shows an extruded heat sink, used for removing heat from a transformer on a printed circuit board. Heat sinks usually are designed with a large number of fins that maximize the surface area and assist in heat transfer to a cooler fluid flowing over them. The fins are very difficult and expensive to machine, forge, or roll form, but they can be made economically by hot extrusion, using dies made by electrical-discharge machining (Section 27.5). Fig. 15.10b shows a die and a typical hot-extruded heat sink cross section. The shapes shown also could be produced through a casting operation, but extrusion is preferred because there is no internal porosity in the part and its thermal conductivity is thus higher. 15.4 Cold Extrusion Developed in the 1940s, cold extrusion is a general term often denoting a combination of operations, such as a combination of direct and indirect extrusion and forging (Fig. 15.11). Cold extrusion is used widely for components in automobiles, motorcycles, bicycles, appliances, and in transportation and agricultural equipment. (a) (b) Figure 15.10: (a) Aluminum extrusion used as a heat sink for a printed circuit board, (b) extrusion die and extruded heat sinks. Source: Courtesy of Aluminum Extruders Council. ׉ 7cassandra://OoXRLPC0Nh4mKYYnJmNS6nvvUn9w8MdjurVjVeY4t30`cZ%Eb׉E 426 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Punch Punch Die Workpiece Punch (a) (b) Figure 15.11: Two examples of cold extrusion; arrows indicate the direction of metal flow during extrusion. The cold extrusion process uses slugs cut or sheared from cold-finished or hot-rolled bars, wire, or plates. Slugs that are less than about 40 mm in diameter are sheared (cropped), and, if necessary, their ends are squared off by processes such as upsetting, machining, or grinding. Larger diameter slugs are machined from bars into specific lengths. Cold-extruded parts weighing as much as 45 kg and having lengths of up to 2 m can be made, although most parts weigh much less. Powder-metal slugs (preforms) also may be cold extruded (Section 17.3.3). The force, F, in cold extrusion may be estimated from the formula F =1.7Aoσf, (15.3) where Ao is the cross-sectional area of the blank, σf is the average flow stress of the metal, and is the true strain that the piece undergoes, based on its original and final cross-sectional area. For example, assume that a round slug 10 mm in diameter and made of a metal with σf = 300 MPa that is reduced to a final diameter of 7 mm by cold extrusion. The force would be F =1.7(π)  0.0102 4   300 × 106   ln 10 7 Cold extrusion has the following advantages over hot extrusion: • Improved mechanical properties, resulting from work hardening, provided that the heat generated by plastic deformation and friction does not recrystallize the extruded metal. • Good control of dimensional tolerances, thus reducing the need for subsequent machining or finishing operations. • Improved surface finish, due partly to the absence of an oxide film and provided that lubrication is effective. • Production rates and costs are competitive with those of other methods of producing the same part. Some machines are capable of producing more than 2000 parts per hour. On the other hand, the stresses acting on the tooling in cold extrusion is very high (especially with steel and specialty-alloys), being on the order of the hardness of the workpiece material. The punch hardness usually ranges between 60 and 65 HRC, and the die hardness between 58 and 62 HRC. Punches are a critical component in cold extrusion, as they must possess not only sufficient strength but also high toughness and resistance to wear and fatigue failure. Lubrication is critical, especially with steels, because of the possibility 2 =28.6 kN. ׉ 7cassandra://1L30ZhaO0BnKk30DFB9KeVAfBSLaraeD4pVbOUT5W9E`cZ%EbʁcZ%EbɁ#בCט#u׉׉ 7cassandra://BdbKgF_xyJE3ab2gI2UccredwdQqDDx1F5kSVyBj3RI`׉ 7cassandra://e3sn2UYrwNoHvMl0VBqnTZJdsNtduEe8D-HEx5R_KxsQA`s׉ 7cassandra://n8fdor5Vi0X5MaFEryyPEQlSHlYrMhq4avHgAiklMqoH`׉ 7cassandra://2LFyyNhjuuKr2ynnz2iMZEiwf1lZv7Vv9bgFPq6U7X4_a͠^cZEbʥט#u׉׉ 7cassandra://R-T3IaxCU4ds_i1J9pCThrMxX_841V3mJ47MrAQIouM`׉ 7cassandra://xb8AYuGYXPNfySmhew6XUjLh504rnwOhrYUkrkjCblEF`s׉ 7cassandra://MNyKss_c-yImTE1Iw7j5GYmLh1gztdMjKm03nzsSTtw`׉ 7cassandra://ZqwEKiOWd7tSNV45X549o4IX8Pcq2ZiS5ywkUIa7Zgk]v͠^cZEbʨ׉E Cold Extrusion 427 Figure 15.12: Production steps for the metal portion of a cold-extruded spark plug. Source: Courtesy of National Machinery Company, LLC. of sticking (seizure) between the workpiece and the tooling. The most effective means of lubrication is the application of a phosphate-conversion coating over the workpiece surfaces, followed by a coating of soap or wax, as described in Section 34.10. Tooling design and the selection of appropriate tool and die materials are essential to the success of cold extrusion. Also important are the selection and control of the workpiece material with regard to its quality and the repeated accuracy of the slug dimensions and its surface condition. Case Study 15.2 Cold-extruded Part A typical cold-extruded part, similar to the metal component of an automotive spark plug, is shown in Fig. 15.12. First, a slug is sheared off (cropped) the end of a round rod (Fig. 15.12, left). It then is cold extruded (Fig. 15.12, middle) in an operation similar to those shown in Fig. 15.11, but with a blind hole. Then the material at its bottom is punched out, producing the small slug shown. Note the respective diameters of the slug and the hole at the bottom of the sectioned part. Investigating material flow during the deformation of the slug helps avoid defects and leads to improvements in punch and die design. The part usually is sectioned in the midplane, and then polished and etched to display the grain flow, as shown in Fig. 15.13 (see also Fig. 14.13). 15.4.1 Impact Extrusion Impact extrusion is similar to indirect extrusion, and the process often is included in the cold-extrusion category. The punch descends rapidly on the blank (slug), which is extruded backwards (Fig. 15.14). Because of volume constancy, the thickness of the tubular extruded region is a function of the clearance between the punch and the die cavity. Typical products made by this process are shown in Fig. 15.15a to c. Other examples are collapsible tubes, some cylindrical computer housings, light fixtures, automotive parts, and small pressure vessels. Most nonferrous metals can be impact extruded in vertical presses and at production rates as high as two parts per second. The maximum diameter of parts made is about 150 mm. The impact-extrusion process can produce thin-walled tubular sections, with thickness-to-diameter ratios as small low as 0.005. Consequently, the symmetry of the part and the concentricity of the punch and the blank are important. ׉ 7cassandra://n8fdor5Vi0X5MaFEryyPEQlSHlYrMhq4avHgAiklMqoH`cZ%Eb׉E428 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Figure 15.13: A cross section of the metal part in Fig. 15.12, showing the grain-flow pattern. Source: Courtesy of National Machinery Company, LLC. 15.4.2 Hydrostatic Extrusion In hydrostatic extrusion, the pressure required in the chamber is supplied via a piston and through an incompressible fluid medium surrounding the billet (Fig. 15.3b). Pressures are typically on the order of 1400 MPa. The high pressure in the chamber transmits some of the fluid to the die surfaces, where it significantly reduces friction. Hydrostatic extrusion is usually carried out at room temperature, typically using vegetable oils as the fluid. Brittle materials can be extruded successfully by this method, because the hydrostatic pressure, along with low friction and the use of small die angles and high extrusion ratios, increases the ductility of the material (Section 2.2.8). Hydrostatic extrusion has had limited industrial applications, mainly because of the complex nature of the tooling, the design of specialized equipment, and the long cycle times required, making the process uneconomical for most materials and applications. 15.5 Extrusion Defects Depending on workpiece material condition and process variables, extruded products can develop several types of defects that can affect significantly their strength and product quality. Some defects are visible to Punch Punch Stripper plate Blank Die (a) (b) Stripper plate Clearance Part Die (c) Figure 15.14: Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch. ׉ 7cassandra://MNyKss_c-yImTE1Iw7j5GYmLh1gztdMjKm03nzsSTtw`cZ%Eb́cZ%Ebˁ#בCט#u׉׉ 7cassandra://TXeY2CI9LpGhZS8-Q0D1ASIRwlnOtIckAcORzxMWt_c`׉ 7cassandra://PfKRFKWsPBciKn5WrS6TJihS0DXsw0b9WWYznUeaGUw`7`s׉ 7cassandra://YG39BrxRu2cKwp1iUV9ln-wlgYHxZSF0-1oKfZJOUBwh`׉ 7cassandra://KSJGyaBvf-VzooiKxJPYlrTuGXgVa9EhdLVqmUYwaqgͬ; L͠^cZEbʪט#u׉׉ 7cassandra://pnytHu-kJWJ-_0owOp31EQbdNrf04G49BcC5fudwrH8.`׉ 7cassandra://P3IXS46WRE1ouQYEVY8DHiEGln8t_w-UeGIPSy6RIS0] `s׉ 7cassandra://10uU8TNKBwSPxrxUB2GvLgA9aD03qqAOxAuYsQ0vedk`׉ 7cassandra://6o71SqpBFX2gkisi-HugPv2dYBZt7nvNjilrrQzkf1I͆T͠^cZEbʫ׉E Extrusion Defects 429 Before Punch Blank Die Knockout (ejector) (a) (b) (c) Figure 15.15: (a) Impact extrusion of a collapsible tube by the Hooker process. (b) and (c) Two examples of products made by impact extrusion. These parts also may be made by casting, forging, or machining. The choice of process depends on the materials involved, part dimensions and wall thickness, and the properties desired. Economic considerations also are important in final process selection. After Workpiece the naked eye, while others can be detected only by the techniques described in Section 36.10. There are three principal extrusion defects of importance: surface cracking, pipe, and internal cracking. Surface Cracking. If extrusion temperature, friction, and speed are too high, surface temperatures can rise significantly, which may cause surface cracking and tearing (fir-tree cracking or speed cracking). These cracks are intergranular (along the grain boundaries; see Fig. 2.27), and usually are caused by hot shortness (Section 1.5.2). Such defects occur especially in aluminum, magnesium, and zinc alloys; they can be avoided by lowering the billet temperature and the extrusion speed. Surface cracking may occur also at lower temperatures, attributed to periodic sticking of the extruded part along the die land. Because of its similarity in appearance to the surface of a bamboo stem, it is known as a bamboo defect. The explanation is that, when the product being extruded temporarily sticks to the die land (see Fig. 15.7), the extrusion pressure increases rapidly; shortly thereafter, it moves forward again, and the pressure is released. The cycle is repeated continually, producing periodic circumferential cracks on the surface. Pipe. The type of metal-flow pattern in extrusion shown in Fig. 15.6c tends to draw surface oxides and impurities toward the center of the billet, much like a funnel. This defect is known as pipe defect, tailpipe, or fishtailing; as much as one-third of the length of the extruded product may contain this type of defect, and has to be cut off as scrap. Piping can be minimized by modifying the flow pattern to be more uniform, such as by controlling friction and minimizing temperature gradients within the part. Another method is to machine the billet’s surface prior to extrusion (so that scale and surface impurities are removed) or by chemical etching of the surface oxides prior to extrusion. Internal Cracking. The center of the extruded product can develop cracks, variously called center cracking, center-burst, arrowhead fracture,or chevron cracking (Fig. 15.16a). Cracking has been attributed to a state of hydrostatic tensile stress that develops at the centerline in the deformation zone in the die (Fig. 15.16b); this condition is similar to the necked region in a tensile-test specimen (see Fig. 2.23). Such cracks also have been observed in tube extrusion and in tube spinning (see Fig. 16.49b and c), appearing on the inside surfaces of tubes. The tendency for center cracking (a) increases with increasing die angle, (b) increases with increasing amount of impurities in the material, and (c) decreases with increasing extrusion ratio and friction. ׉ 7cassandra://YG39BrxRu2cKwp1iUV9ln-wlgYHxZSF0-1oKfZJOUBwh`cZ%Eb׉E 430 Chapter 15 Metal Extrusion and Drawing Processes and Equipment Die Vo Rigid billet Central burst Plastic deformation zone (a) (b) Figure 15.16: (a) Chevron cracking (central burst) in extruded round steel bars. Unless the products are inspected, such internal defects may remain undetected and later cause total failure of the part in service. This defect can also develop in the drawing of rod, wire, and tubes. (b) Schematic illustration of rigid and plastic zones in extrusion. The tendency toward chevron cracking increases if the two plastic zones do not meet. Note that the plastic zone can be made larger either by decreasing the die angle, increasing the reduction in cross section, or both. Source: After B. Avitzur. 15.6 Design Considerations Extrusion of constant cross sections is often a more economical method of producing a part than by forging, casting, or machining. While there is considerable freedom in designing the cross sections, there are several general rules that should be followed to simplify production and reduce defects. Before laying out the cross section, the designer should consider the following: • Some guidelines for proper die design in extrusion are illustrated in Fig. 15.17. Note the (a) importance of symmetry of the cross section, (b) avoiding sharp corners, (c) maintaining uniform wall thickness, and (d) avoiding severe changes in die dimensions within the cross section. • Solid shapes are the easiest to extrude. When possible, the cross section should avoid hollow sections, although such sections can be extruded using porthole, bridge, or spider dies, as illustrated in Fig. 15.9. • If there is a critical dimension in a cross section, it should not be located at the end of a gap. Figure 15.17 shows the use of a metal web to decrease the tolerance on a critical dimension. Note that this design approach requires the extrusion of a hollow cross section; if the cross section is complex, it can be extruded in two sections, and then assembled using the geometries shown in Fig. 15.18. • Extrusions will usually develop some curvature, which may require straightening.Wide, thin sections can be difficult to straighten, hence the need for ribs as shown in Fig. 15.17. Impact extrusions should incorporate the following considerations: • They should be symmetrical about the punch. External and internal bosses can be used as long as they are in the part axis. • The maximum length-to-diameter ratio should not exceed 8 or so, to avoid punch failure. • For reverse extrusion, the outer radius can be small, but the inner radius should be as small as possible, and should preferably incorporate a chamfer. Rigid product Vf ׉ 7cassandra://10uU8TNKBwSPxrxUB2GvLgA9aD03qqAOxAuYsQ0vedk`cZ%Eb΁cZ%Eb́#בCט#u׉׉ 7cassandra://D9-8vEZNPE89iq4ibN93LA-iWjvTX1QL7JGPR9SE5Fw`׉ 7cassandra://wrOynhN7Tp4SCFfvfvpB_P1YP2zZZcvSzAvbt2uydOwB(`s׉ 7cassandra://wLuPIVmLkQuAqYQhHfl3QK6w-T-GDqRP0IwGrTe33n0`׉ 7cassandra://QbcdsyhRRc7KrskfJWhpD3c1OIf4Rz29KQtsPMY23tQͯR͠^cZEbʭט#u׉׉ 7cassandra://ZOEQXKGSkQ0OYr2vIjpl0Kq8vvxtCuUD4n-SeJQUxmgͭ`׉ 7cassandra://ApoXbBNmbshzffktwnDf74D1sVfxFFjOXE3Lz08e1GYFD`s׉ 7cassandra://OXjPcyypKb42tv9Y0ThqV2Q6NQpc0WaKsJmGlG3DsZE`׉ 7cassandra://iRLLKVOBYQSFGS-GXoAk1grBEJieXHAue6I950TtkPQ 2T). Note that for the ideal case, the neutral axis is at the center of the sheet thickness, k =0.5, and hence, Lb = α R+    T 2 . Rolling direction Cracks No cracks (16.3) a Bend radius, R Bend length, L Rolling direction (a) Elongated inclusions (stringers) (b) (c) Figure 16.17: (a) and (b) The effect of elongated inclusions (stringers) on cracking as a function of the direction of bending with respect to the original rolling direction of the sheet. (c) Cracks on the outer surface of an aluminum strip bent to an angle of 90◦. Note also the narrowing of the top surface in the bend area due to the Poisson effect. ׉ 7cassandra://42oqP--YdJCiOpZKUX9CP6OIH6WkS4r6Q-GeYrvc4GE`cZ%Eb׉Et460 Chapter 16 Sheet-metal Forming Processes and Equipment Table 16.3: Minimum Bend Radius for Various Metals at Room Temperature. Condition Material Aluminum alloys Beryllium copper Brass (low-leaded) Magnesium Steels Austenitic stainless Titanium Titanium alloys 0.5T 6T Low-carbon, low-alloy, and HSLA 0.5T 4T 0.7T 3T 2.6T 4T Minimum Bend Radius. The radius at which a crack first appears at the outer fibers of a sheet being bent is referred to as the minimum bend radius. It can be shown that the engineering strain on the outer and inner fibers of a sheet during bending is given by the expression 1 e = (2R/T)+1 . (16.4) Thus, as R/T decreases (i.e., as the ratio of the bend radius to the thickness becomes smaller), the tensile strain at the outer fiber increases, and the material eventually develops cracks (Fig. 16.17). The bend radius usually is expressed in terms of the thickness, such as 2T, 3T,and 4T (see Table 16.3). Thus, a 3T minimum bend radius indicates that the smallest radius to which the sheet can be bent, without cracking, is three times its thickness. It has been shown that there is an inverse relationship between bendability and the tensile reduction of the area, r, of the material (Fig. 16.18). The minimum bend radius, Rmin, is, approximately, Rmin = T  50 r − 1  . (16.5) Thus, for r =50, the minimum bend radius is zero; that is, the sheet can be folded over itself, called hemming (see Fig. 16.23), in much the same way as a piece of paper is folded. To increase the bendability of metals, their tensile reduction of area can be increased either by heating or by bending it in a high-pressure environment, which improves the ductility of the material (see hydrostatic stress, Section 2.2.8). 20 15 10 5 0 010 Soft Hard 0 0 0 6T 4T 2T 5T 13T R T 5 (60/r) 2 1 20 30 40 50 60 70 Tensile reduction of area (%) Figure 16.18: Relationship between Rmin/T and tensile reduction of area for sheet metals. Note that sheet metal with a 50% tensile reduction of area can be bent over itself in a process like the folding of a piece of paper without cracking. Source: After J. Datsko and C.T. Yang. Bend radius Thickness , Rmin T ׉ 7cassandra://S4frlXcJGn6d7p1C3mQgCPmxy5dg689th61Nmhw7FhA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://sDOV731s9y23sgcHkXm7B7jB825tjHx5IeVwqMaAlZoq`׉ 7cassandra://7DAZiElXyE4oGv56jO1nb4mbQCGy8nsWo-960JbMBzw\`s׉ 7cassandra://h7UvQdlIo0hkd6d-AqSthYFHiZiG93cQpKLcu_4ZtJg~`׉ 7cassandra://G_DjUk3c6Wx5oJEcpcI5c6e0iITTyPrTOZkF_OIvK-w}{͠^cZEbט#u׉׉ 7cassandra://ZNh3NtMudYO4_TgslMoSfWUJy_zvGqokgLHn_se0L34K`׉ 7cassandra://w3smJDcDBX4fxikS0fO1p13OM0nIef9VV727NPTPY2wG]`s׉ 7cassandra://A04VhMDx42V3ShFW66oy6YrxHQtDrxHUu3L3KR25uAg`׉ 7cassandra://xy40CEpx-XVIWwyLDp5rwSsWmJZ0DJetroElknBQKHk$͠^cZEb׉E Bending Sheets, Plates, and Tubes 461 Bendability also depends on the edge condition of the sheet. Since rough edges are areas of stress concentration, bendability decreases as edge roughness increases. Another significant factor is the amount, shape, and hardness of inclusions present in the sheet metal and the amount of cold working that the edges have undergone during shearing. Because of their pointed shape, inclusions in the form of stringers are more detrimental than globular-shaped inclusions (see also Fig. 2.23). The resistance to edge cracking during bending can be significantly increased by removing the cold-worked regions, by (a) shaving or machining the edges of the part (see Fig. 16.9) or (b) annealing the sheet to improve its ductility. Anisotropy of the sheet is another important factor in bendability. Cold rolling increases the anisotropy of the sheet by preferred orientation or by mechanical fibering due to the alignment of impurities, inclusions, and voids (Fig. 1.12). Prior to laying out or nesting the blanks (see Fig. 16.60) for subsequent bending or forming, caution should be exercised to cut, as much as possible, in the optimum direction from a rolled sheet. Springback. Because all materials have a finite modulus of elasticity, plastic deformation is always followed by some elastic recovery when the load is removed (Fig. 2.3). In bending, this recovery is called springback; it can easily be demonstrated by bending and then releasing a piece of sheet metal or wire. As noted in Fig. 16.19, the final bend angle of a sheet metal after springback is smaller than the angle to which the sheet was bent, and the final bend radius is larger than before springback. Springback can be calculated approximately in terms of the radii Ri and Rf (Fig. 16.19) as Ri Rf =4  RiSy ET 3 − 3  RiSy ET  +1. (16.6) Note from this formula that springback increases as the R/T ratio and the yield strength, Sy, of the material increase, and as the elastic modulus, E, decreases. In V-die bending (Figs. 16.20 and 16.21), it is possible for the material to also exhibit negative springback. This is a condition caused by the nature of the deformation occurring within the sheet metal just when the punch completes the bending operation at the end of the stroke. Negative springback does not occur in air bending, also called free bending (Fig. 16.22a), because of the absence of constraints that a V-die imposes on the bend area. Compensation for Springback. Springback in forming operations usually is compensated by overbending the part (Fig. 16.20a and b), although several trials may be necessary to obtain the desired results. Another method is to coin the bend area, by subjecting it to highly localized compressive stresses between the tip of the punch and the die surface (Fig. 16.20c and d); the technique is also called bottoming the punch. In another method, the part is subjected to stretch bending, in which the part is under external tension while being bent (see also stretch forming, Section 16.6). T After af ai Ri Rf Before Figure 16.19: Springback in bending. The part tends to recover elastically after bending, and its bend radius becomes larger. Under certain conditions, it is possible for the final bend angle to be smaller than the original angle (negative springback). ai af ׉ 7cassandra://h7UvQdlIo0hkd6d-AqSthYFHiZiG93cQpKLcu_4ZtJg~`cZ%Eb׉E462 Chapter 16 Sheet-metal Forming Processes and Equipment , 908 , 908 Wb Pcounter (a) (b) (c) (d) 908 908 Sheet Die Rocker 1. 2. (e) Figure 16.20: Methods of reducing or compensating for springback in bending operations. Bending Force. The bending force for sheets and plates can be estimated by assuming that the process is one of simple bending of a rectangular beam (as described in texts on mechanics of solids). Thus, the bending force is a function of the yield strength of the material, Sy, the length of the bend, L, the thickness of the sheet, T, and the die opening, W (see Fig. 16.21). Excluding friction, the maximum bending force, P,is P = kSyLT2 W , (16.7) where the factor k ranges from about 0.3 for a wiping die, to about 0.7 for a U-die, to about 1.3 for a Vdie (Fig. 16.21), and Sy is the yield strength of the material. For situations where the punch-tip radius and the sheet thickness are relatively small compared to the die opening, W, the maximum bending force is given by P = SutLT2 W where Sut is the ultimate tensile strength of the sheet. (16.8) 3. Punch Die W W (a) V-die (b) Wiping die Figure 16.21: Common die-bending operations showing the die-opening dimension,W, used in calculating bending forces. ׉ 7cassandra://A04VhMDx42V3ShFW66oy6YrxHQtDrxHUu3L3KR25uAg`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://YfAVpa4vXDjVOQDLLRbcfKsHZTMJTa0locq-_YklEus7`׉ 7cassandra://cwLtjreEShI6hw_fXDtqSxqSYJfC72jo0jkymCTjWVER`s׉ 7cassandra://dMoQwqgGqRl5d8fhFJ3ysFDe-UogP7cBV8A_Sia_yP0`׉ 7cassandra://IbsFnQ7isO2JQ-vXPdVecjd0XJjlpNPjSoEjt1NDtnc;f͠^cZEbט#u׉׉ 7cassandra://SMuMMrr5TV-K_X7vEQlWxpJrNRXBEsAdOr95ogpYWiM#`׉ 7cassandra://94cO1AC6AiSB83bYDfBqGHacGmwb0KCvmMVFyFxquhMMI`s׉ 7cassandra://TfLEHy4FCGvLDIN9Uyn46Jp-6UsHx_8EgAftGNiwQts`׉ 7cassandra://viavjL5LJAjh1FkDvo5nVkiXpFmoYtHzElaE8Ha3T9UTJ' ͠^cZEb׉E SMiscellaneous Bending and Related Forming Operations 463 Punch Die (a) Air bending (b) Bending in a four-slide machine Sheet Adjustable roll Driven rolls (c) Roll bending Polyurethane roll (d) Bending with a compliant roll Figure 16.22: Examples of various bending operations. The force in die bending varies throughout the bending cycle. It increases from zero to a maximum, and it may even decrease as the bend is being completed, but then it increases sharply as the punch reaches the bottom of its stroke (bottoming). However, in air bending (Fig. 16.22a) the force does not increase again after it begins to decrease, because the sheet is not subjected to any resistance in its movement downward. 16.6 Miscellaneous Bending and Related Forming Operations Press-brake Forming. Sheet metal or plate can easily be bent using simple fixtures in a press. Sheets or narrow strips that are 7 m or even wider usually are bent in a press brake (Fig. 16.23). The machine utilizes long dies, in a mechanical or hydraulic press, and is particularly suitable for small production runs. As can be seen in Fig. 16.23, the tooling is simple, the motions are only up and down, and the process is easily adaptable to a wide variety of part shapes. The operation can be easily automated for low-cost, high-production runs. Die materials for press brakes range from hardwood for low-strength materials and small-production runs, to carbides, for strong and abrasive sheet metals (such as carbon steel). For most applications, the dies are made of carbon steel or gray iron. Bending in a Four-slide Machine. Relatively short pieces can be bent on a machine such as the one shown in Fig. 16.22b. The lateral movements of the dies are controlled and synchronized with the vertical die movement. This process is typically used for making seamed tubing and conduits, bushings, fasteners, and various machinery components. Roll Bending. In this process (Fig. 16.22c), plates are bent using a set of rolls, where curvatures are controlled by adjusting the distance between the three rolls. Roll bending is used extensively for bending plates, applications as boilers, cylindrical pressure vessels and tanks, and curved structural members. Figure 16.22d shows the bending of a strip, with a compliant roll, made of polyurethane, which conforms to the shape of the strip as the hard upper roll presses upon it. ׉ 7cassandra://dMoQwqgGqRl5d8fhFJ3ysFDe-UogP7cBV8A_Sia_yP0`cZ%Eb׉E464 Chapter 16 Sheet-metal Forming Processes and Equipment Main gear Flywheel Channel forming (a) Joggling (b) Hemming (flattening) (c) Crown Main gear Connections Bed Ram Die holder Motor Clutch and brake unit Side housing Floor line Two-stage lock seam (d) Offset forming (e) (f) Figure 16.23: (a) through (e) Schematic illustrations of various bending operations in a press brake. (f) Schematic illustration of a press brake. Source: Courtesy of Verson Allsteel Company. Beading. In beading, the periphery of the sheet metal is bent in the cavity of a die (Fig. 16.24). The bead imparts stiffness to the part by increasing the moment of inertia of that section. Moreover, beads improve the appearance of parts and eliminate exposed sharp edges, which may be hazardous. Flanging. This is a process of bending the edges of sheet metals, usually to 90◦ (see also Section 16.7). In shrink flanging (Fig. 16.25a), the flange is subjected to compressive hoop stresses; if excessive, however, the stresses can cause the flange periphery to wrinkle. The wrinkling tendency increases with decreasing radius of curvature of the flange. In stretch flanging, the flange periphery is subjected to tensile stresses; if excessive, however, they can lead to cracking along the periphery of the flange. Die 1. Formed bead 2. (a) (b) (c) (d) Figure 16.24: (a) Bead forming with a single die. (b) through (d) Bead forming with two dies in a press brake. ׉ 7cassandra://TfLEHy4FCGvLDIN9Uyn46Jp-6UsHx_8EgAftGNiwQts`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://V5rTy8TLhe8Mpx4RzWjJP5mkLprL6yrvkSItDQD4rDI `׉ 7cassandra://c8y5wIt9zrcllyYvuGCcenag0KXG5fu0EaUrh6gnpxkS5`s׉ 7cassandra://QLF9HmJtLF_0x7rh70W4QGrM9CSDgzqNDkhjAe9afyM``׉ 7cassandra://7zKi0HPFgJ7kIxQP8kvMEnFDAOaSoeNwigft_WmldIg͒ ͠^cZEb ט#u׉׉ 7cassandra://Z7BBuMeTg0yI6fo-og-BF4dDNhcmhd-XQgWvaSUDogEh`׉ 7cassandra://mTAvZ4EkhYHHBTB-Tqh1SDCYkR-v0oc17v87P8kWjZoW`s׉ 7cassandra://jzgbRwV1Y5APc0SVKBy1W4Z-gAyyprzHgTBSl-tfD0w`׉ 7cassandra://QNUDozVt36MDI2cSVcJa3p39jt8JkRHlOzGIQsIrxkUf͠^cZ9Eb9׉E ^Miscellaneous Bending and Related Forming Operations 465 Straight flange Joggled flange Stretch flange Reverse flange Shrink flange Slug (a) Piercing punch (retracted) Stripper plate Sheet Die (c) (d) Figure 16.25: Various flanging operations. (a) Flanges on flat sheet. (b) Dimpling. (c) The piercing of sheet metal to form a flange. In this operation, a hole does not have to be pre-punched before the punch descends. Note, however, the rough edges along the circumference of the flange. (d) Flanging of a tube; note the thinning of the edges of the flange. Roll Forming. Also called contour-roll forming or cold-roll forming, this process is used for forming continuous lengths of sheet metal and for large production runs. As it passes through a set of driven rolls, the metal strip is bent in consecutive stages (Fig. 16.26). The roll-formed strip is then sheared into specific lengths and stacked. Typical roll-formed products are door and picture frames, panels, channels, gutters, siding, pipes, and tubing with lock seams (Section 32.5). The length of the part is limited only by the amount of sheet metal supplied to the rolls from a coiled stock. Sheet thickness typically ranges from about 0.125 to 20 mm. Forming speeds are generally below 1.5 m/s, although they can be much higher for specialized applications. In designing the rolls and their sequence, dimensional tolerances, springback, tearing, and buckling of the strip have to be considered. The rolls generally are made of carbon steel or gray iron; they may be chromium plated, to reduce wear of the rolls and for improved surface finish of the formed product. Lubricants may be used to reduce wear, improve surface finish, and to cool the rolls and the sheet being formed. Tube Bending and Forming. Bending and forming tubes and of other hollow sections requires special tooling because of the tendency for buckling and folding, as can be demonstrated by bending copper tubing or plastic soda straw. The oldest method of bending a tube or pipe is to first pack it with loose particles (commonly sand), and then bend it in a suitable fixture. The function of the loose filler is to prevent the tube from buckling inward; after bending, the sand is simply shaken out. Tubes also can be plugged with (b) Piercing punch Spring-loaded stripper Sheet Die block or die button Spring-loaded Pressure bushing After Before Flange Tube ׉ 7cassandra://QLF9HmJtLF_0x7rh70W4QGrM9CSDgzqNDkhjAe9afyM``cZ%Eb׉EW466 Chapter 16 Sheet-metal Forming Processes and Equipment (a) (b) Figure 16.26: (a) Schematic illustration of the roll-forming process. (b) Examples of roll-formed cross sections. Source: (b) Courtesy of Voestalpine Roll Forming Corporation. various flexible internal mandrels (Fig. 16.27), serving the same purpose as sand. Because of its lower tendency for buckling, a relatively thick tube can be bent safely without the use of fillers or plugs (see also tube hydroforming, Section 16.8). The beneficial effect of forming metals under high compressive stresses is demonstrated in Fig. 16.28 for bending a tube with relatively sharp corners. Note that, in this operation, the tube is subjected to longitudinal compressive stresses, which reduce the stresses in the outer fibers in the bend area, thus improving the bendability of the material (see also Section 2.2.8). Dimpling, Piercing, and Flaring. In dimpling (Fig. 16.25b), a hole is first punched and then expanded into a flange. Flanges may also be made by piercing using a shaped punch (Fig. 16.25c); tube ends can be flanged by a similar process (Fig. 16.25d). When the bend angle is less than 90◦, as in fittings with conical ends, Chuck Form block (fixed) Form block (rotating) Clamp Form block (fixed) Wiper shoe Plug Balls Laminated Cable Workpiece Chuck (a) Stretch bending Pressure bar (b) Draw bending Clamp (c) Compression bending (d) Mandrels for tube bending Figure 16.27: Methods of bending tubes. Internal mandrels or filling of tubes with particulate materials, such as sand, are often necessary to prevent collapse of the tubes during bending. Tubes also can be bent by a technique in which a stiff, helical tension spring is slipped over the tube. The clearance between the outer diameter of the tube and the inner diameter of the spring is small; thus, the tube cannot kink and the bend is uniform. ׉ 7cassandra://jzgbRwV1Y5APc0SVKBy1W4Z-gAyyprzHgTBSl-tfD0w`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://IgQJdsRr0gEAqLnCw721GdqQeHU5yrNA3TtahRL6fi4`׉ 7cassandra://adV_lzIPvOL4yvGhC3xYmwl57C8MnUjzYBHJXRzMkVQZ`s׉ 7cassandra://Y7YSzLxhVYSnTu2wevAMObf4pms0YU4e7SCWhrDHyXUv`׉ 7cassandra://SHrPH9kC3wZGWD0E2bDvVe9qFEf5E8-RvyyjILmAWbw͋+d͠^cZ9Eb>ט#u׉׉ 7cassandra://LRAOsiI2DVO5Ew7n0tk5HrRuncLG6vN6O09KydWeoTIj`׉ 7cassandra://LNHMclrtH6MmiUTvDNVQ5pRaUlTLaBGKDxXs52hJ1f8Ap`s׉ 7cassandra://efA_wkRy9iW7RvLFhTQJEhpN-2E_I3QFffWHlHAJywoB`׉ 7cassandra://GHMx0-DgBrfggKHxR6r7HJbHd7o8DCpP5uYXNAgnZA0~͠^cZ;Eb?׉E rMiscellaneous Bending and Related Forming Operations 467 Die Punch Tube Rubber or fluid Stops Die Punch (a) (b) Figure 16.28: A method of forming a tube with sharp angles, using an axial compressive force. Note that the tube is supported internally with rubber or fluid to avoid collapsing during forming. Source: After J.L. Remmerswaal and A. Verkaik. the process is called flaring. The condition of the edges (Fig. 16.3) is important in these operations, because stretching the material causes high tensile stresses along the periphery (tensile hoop stresses), which can lead to cracking and tearing of the flange. As the ratio of flange diameter to hole diameter increases, the strains increase proportionately. Depending on the roughness of the edge, there will therefore be a tendency for cracking along the outer periphery of the flange. To reduce this possibility, sheared or punched edges could be shaved off with a sharp tool (Fig. 16.9) to improve the surface finish of the edge. Hemming and Seaming. In the hemming process, also called flattening, the edge of the sheet is folded over itself (Fig. 16.23c). Hemming increases the stiffness of the part, improves its appearance, and eliminates sharp edges. Seaming involves joining two edges of sheet metal by hemming (Fig. 16.23d). Double seams are made by a similar process using specially shaped rolls for making watertight and airtight joints, such as those in food and beverage containers. Bulging. This process involves placing a tubular, conical, or curvilinear part into a split-female die, and then expanding the part, usually with a polyurethane plug (Fig. 16.29a). After forming, the punch is retracted, the plug returns to its original shape (by elastic recovery), and the formed part is removed by opening the split dies. Typical products made are coffee and water pitchers, beer barrels, and beads on oil drums. For parts with complex shapes, the plug is shaped, in order to be able to apply higher pressures at critical regions of the part. The major advantages of using polyurethane plugs is that they are highly resistant to abrasion and wear, and do not damage the surface finish of the part being formed (see also Section 16.8). Segmented Dies. These dies consist of individual segments that are placed inside the part to be formed, and expanded mechanically in a radial direction; the segments are then retracted to remove the formed part. These dies are relatively inexpensive, and they can be used for large production runs. Stretch Forming. In this process, the sheet metal is clamped along its edges and then stretched over a male die, called a form block or form punch. The die can move upward, downward, or sideways, depending on the particular design of the machine (Fig. 16.30). Stretch forming is used primarily to make aircraft wingskin panels, fuselages, and boat hulls. Aluminum skins for the Boeing 767 and 757 aircraft, for example, are ׉ 7cassandra://Y7YSzLxhVYSnTu2wevAMObf4pms0YU4e7SCWhrDHyXUv`cZ%Eb׉Em468 Chapter 16 Sheet-metal Forming Processes and Equipment Before After Fluid Ring Punch Knockout rod Rubber plug Die insert Two-piece die (hinged) (a) Figure 16.29: (a) The bulging of a tubular part with a flexible plug. Water pitchers can be made by this method. (b) Production of fittings for plumbing by expanding tubular blanks under internal pressure. The bottom of the piece is then punched out to produce a T. (c) Steps in manufacturing bellows. Sheet Tool Stretch gripper Turntable Hydraulic Table-mounted gripper (a) Crosshead Ram Stretching Upper tool Clamping fixture Sheet Lower tool Bed 1. 2. (b) Figure 16.30: Schematic illustration of a stretch-forming process. Aluminum skins for aircraft can be made by this method. Source: (a) Courtesy of Cyril Bath Co. 3. Workpiece (b) Fluid Die Fluid 1. Bulged tube (c) 2. Compressed tube stretching unit Adjustable slide ׉ 7cassandra://efA_wkRy9iW7RvLFhTQJEhpN-2E_I3QFffWHlHAJywoB`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://gZhOMC7FHmwD37GKFD_QLCFoTq9myazs_Q8bkICIqwEl`׉ 7cassandra://VUzOn_rQYGc1uHMogM3WK4PGk22U42NtrGs3eo8h0O8f`s׉ 7cassandra://XC3WCxihocu11tu24Y67Wh_F9W2yrCe-MdwC3xnQ89E`׉ 7cassandra://duB5BgEW7FZT1UXonJbL48KzgKoQGlj64P_y135T234p ͠^cZ;EbBט#u׉׉ 7cassandra://b5vfAJkeqLo0Q2AxC2Wl5Azv0rTKTq-EgKEABSOfVCk `׉ 7cassandra://wwHKbblRzrWw43ydm6wdNJgdDJ0VoNGD7dhhgH5-dvMO`s׉ 7cassandra://ZwJDi212kaPu1uvYsc0iKMOGjuRDjSXqzBfNtTBKIQs`׉ 7cassandra://ljHmz68FYJc2dMr9mxY7jxhYCDD_jgTWNBVfQdxttD4͠^cZEbNט#u׉׉ 7cassandra://wgXocfNHHzWdPcww2MLjIINHKDJziFq7xi_UfgNeH5s`׉ 7cassandra://gzMepZkBSI4ybtCTygT-_DYgqMImJHaXME0btCqluawrm`s׉ 7cassandra://nTZX_3XQbkf0Kj3hER4B3Sm0fwdMN8PISrmSJ5DcXUUx`׉ 7cassandra://izPJmpqUpI_1GGPVPeIZ_ldu1do84Dcyk2O-_jbqwhUk͠^cZ>EbO׉EDeep Drawing 475 Punch Sheet Die Cup 1. 2. 3. Figure 16.38: Stages in deep drawing without a blankholder, using a tractrix die profile. Drawing without a Blankholder. Deep drawing also may be carried out without a blankholder. The dies are specially contoured for this operation to prevent wrinkling; one example is shown in Fig. 16.38. The sheet metal must be sufficiently thick to prevent wrinkling. The following formula is a general guide: Do −Dp < 5T, (16.14) where T is the sheet thickness. Thus, the thicker the sheet, the larger the blank diameter, and the deeper the cup, without wrinkling. Embossing. This is an operation consisting of a shallow or moderate drawing, made with male and female matching shallow dies (Fig. 16.39). Embossing is widely used, principally for stiffening flat sheet-metal panels (thus increasing their moment of inertia) and for decorating, numbering, and lettering. Tooling and Equipment for Drawing. The most common tool and die materials for deep drawing are tool steels, cast irons, and carbides (Table 5.7). Die-manufacturing methods are described in detail in Section 14.7. Because of the generally axisymmetric shape of the punch and die components, such as for making cylindrical cans and containers, they can be made on computer-controlled machine tools (Section 25.2). The equipment for deep drawing is usually a double-action hydraulic press or a mechanical press, the latter being favored because of its higher operating speed. In a double-action hydraulic press, the punch and the blankholder are controlled independently. Punch speeds generally range between 0.1 and 0.3 m/s. Top view Side view Sheet Male die Female die Figure 16.39: An embossing operation with two dies. Letters, numbers, and designs on sheet-metal parts can be produced by this process. ׉ 7cassandra://Oh51sxyOU24RrXSC2MHUYBMjwnuE_ppwT1e_XK5iOTI`cZ%Eb׉Ev476 Chapter 16 Sheet-metal Forming Processes and Equipment Case Study 16.2 Manufacture of Food and Beverage Cans Can manufacturing is a major industry, with approximately 100 billion beverage cans and 30 billion food cans produced each year in the United States alone. These containers are strong and lightweight, typically weighing less than 14 g, and they are under an internal pressure of 620 kPa, reliably and without leakage of their contents. There are stringent requirements for their surface finish, since brightly decorated and shiny cans are preferred over dull-looking containers. Considering all of these features, metal cans are very inexpensive; can makers charge approximately $40 per 1000 cans, or about 4 cents per can. Food and beverage cans may be produced in several styles, the most common being two-piece and three-piece cans. A two-piece can consist of the body and the lid (Fig. 16.40a). The body is made of one piece, drawn and ironed, thus the industry refers to this style as D&I (drawn and ironed) cans. Threepiece cans are produced by attaching a lid and a bottom to a sheet-metal cylindrical body, which is typically made by forming a seam on a sheet metal blank. Drawn and ironed can bodies are produced from a number of aluminum alloys, but the most common is 3004-H19 (Section 6.2); electrolytic tin-plated ASTM A623 steel is also used for cans. Aluminum lids are made for both steel and aluminum cans, and are produced from 5182-H19 or 5182-H48 aluminum alloy. The lid has a demanding set of design requirements, as can be appreciated by reviewing Fig. 16.40b. Not only must the lid be scored easily (the curved grooves around the tab), but an integral rivet is formed and headed (Section 14.4) in the lid, to hold the tab in place. Aluminum alloy 5182 has the unique characteristics of having sufficient formability to enable forming of the integral rivet without cracking, and also has the ability to be scored. The lids basically are stamped from 5182 aluminum sheet; the pop-top is scored, and a plastic seal is then placed around the periphery of the lid. The polymer layer seals the can’s contents after the lid is seamed to the can body. The traditional method of making the can bodies is shown in Fig. 16.31. The process starts with 140-mm diameter blanks, produced from rolled sheet stock. The blanks are (a) deep drawn to a diameter of about 90 mm; (b) redrawn to the final diameter of around 65 mm; (c) ironed through two or three ironing rings, in one pass; and (d) domed, for shaping the can bottom. The deep-drawing and ironing operations are performed in a special press, typically producing cans at speeds over 400 strokes per minute. Following this series of operations, a number of additional processes take place. Necking of the can body is performed either by spinning (Section 16.9) or by die necking, which is a forming operation similar to that shown in Fig. 15.21a, where a thin-walled tubular part is pushed into the die, and then spin flanged. The reason for necking the can top is that the 5182 aluminum for the lid is relatively expensive; thus, by tapering the top of the can, a smaller volume of material is required. It should also be noted that the cost of a can often is calculated to millionths of one dollar, hence any design feature that reduces cost will be exploited by this competitive industry. Source: Courtesy of J.E. Wang, Texas A&M University. 16.8 Rubber Forming and Hydroforming The processes described in the preceding sections use dies that are made of solid materials, such as cast iron, steel, and carbides. In rubber forming, also known as the Guerin process, one of the die halves is made of a flexible material, typically a polyurethane membrane. Polyurethanes (Section 7.9) are used widely because of their abrasion resistance, fatigue life, and resistance to cutting or tearing. In bending and embossing of sheet metal by this process, the female die is replaced with a rubber pad (Fig. 16.41). Note that the outer surface of the sheet is now protected from damage or scratches, because it is not in direct contact with a hard metal surface during forming. Pressures in rubber forming are typically on the order of 10 MPa. ׉ 7cassandra://nTZX_3XQbkf0Kj3hER4B3Sm0fwdMN8PISrmSJ5DcXUUx`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://P-gRU6QZdztlZcPjeo395gId03fgDGZp38cfwMltDEw;^`׉ 7cassandra://B1i5zhkr3fSZyQ508rmBPD7O8nEP5_XXerb-NQ13ZRsL`s׉ 7cassandra://gbLxMbW0Q1iUiI5bS0gdtQJnD4oQqT32hUCtZ_-eJvQ`׉ 7cassandra://siDFWUQnvpKPiw2OB9wIc4s43MACRQ5_rRG2vK_jaQ4hqn͠^cZ@EbTט#u׉׉ 7cassandra://Q7JnrWj6hEcnnnRYrMbvElTQzhbpP9q-lr31fQP1J3Ii`׉ 7cassandra://JmwBKKjVA7N5j-JSa-fKMP1W8cs9PdF4w7qW7TfhES0FD`s׉ 7cassandra://NDN2mLF9_QxASFwoTqWKqIiGaNkVur8hRZ0kblRAVTs`׉ 7cassandra://uOWhus61P8f_IEQRx9bv3V_RRX5rJ5gK6bZz7PSverEDP͠^cZ@EbX׉ERubber Forming and Hydroforming 477 Scored region Integral rivet Pop-top cantilever (a) (b) Figure 16.40: (a) Aluminum beverage cans; note the smooth surface. (b) Detail of the can lid, showing the integral rivet and scored periphery for the pop-top. In the hydroform or fluid-forming process (Fig. 16.42), the pressure over the rubber membrane is controlled throughout the forming cycle, with a maximum pressure of up to 100 MPa. This method allows close control of the sheet during forming, and prevents its wrinkling or tearing. Deeper draws can be obtained as compared to conventional deep drawing, because the pressure around the rubber membrane forces the cup against the punch. As a result, the friction at the punch–cup interface increases, in turn reducing the longitudinal tensile stresses in the cup, thus delaying fracture. Control of frictional conditions in rubber forming, as well as in other sheet-forming operations, can be a critical factor in making successful parts. Using proper lubricants and their method of application are also important. In tube hydroforming (Fig. 16.43), metal tubing is shaped in a die, by pressurizing it internally by a fluid, usually water. This process can shape either simple tubes or various intricate hollow shapes (Fig. 16.43b). Parts made include automotive-exhaust and tubular structural components. Punch Blank Before Flexible Pad After (a) (b) (c) Figure 16.41: Examples of the bending and embossing of sheet metal with a metal punch and with a flexible pad serving as the female die. ׉ 7cassandra://gbLxMbW0Q1iUiI5bS0gdtQJnD4oQqT32hUCtZ_-eJvQ`cZ%Eb׉E478 Chapter 16 Sheet-metal Forming Processes and Equipment Pressure-control valve Rubber diaphragm Forming cavity (oil filled) Blank Punch 1. Draw ring Part 2. 3. 4. Figure 16.42: The hydroform (or fluid-forming) process. Note that, in contrast to the ordinary deep-drawing process, the pressure in the dome forces the cup walls against the punch. The cup travels with the punch; in this way, deep drawability is improved. Slide plate Centering Top die Seal punch Bottom die Bed plate Hydroformed part (a) Die-holder plate Horizontal cylinder Cylinder holder bracket Die-holder plate (b) Figure 16.43: (a) Schematic illustration of the tube-hydroforming process. (b) A vehicle body assembly module made up of seven hydroformed tubes. Source: Courtesy Vari-Form, Inc. When selected properly, rubber-forming and hydroforming processes have the advantages of (a) capability to form complex shapes, (b) forming parts with laminated sheets made of various materials and coatings, (c) flexibility and ease of operation, (d) avoiding damage to the surfaces of the sheet, (e) low die wear, and (f) low tooling cost. Case Study 16.3 Tube Hydroforming of an Automotive Radiator Closure The conventional assembly used to support an automotive radiator, or a radiator closure, is constructed through stamping of the components, then welding them together. To simplify the design and to achieve weight savings, a hydroformed assembly was designed, as shown in Fig. 16.44. Note that this design uses ׉ 7cassandra://NDN2mLF9_QxASFwoTqWKqIiGaNkVur8hRZ0kblRAVTs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://qhwBMLkua0ORJNUunVD1nQnzbmdmnXHznkyRk9sH6Z0j`׉ 7cassandra://iwK-aPvZPTaAYPXQpVrbahNlPpvpbT2W_w15sp2LRn4R`s׉ 7cassandra://4dg5EqxX4ASJPlutXSHOzRCkDdv9FgDXEgqBWbd_brAm`׉ 7cassandra://xhl99MggGjWwhx14KTIV8HtnnqUO55y_7WiyGoOrh_ESN͠^cZ@EbZט#u׉׉ 7cassandra://Kcxm36L91rpv-MJXWUR1KfBWf3ccSgb4ta8L3zNWEsw `׉ 7cassandra://hb-_8k-M-n0Q8L3vJIen71csOtU3aroGc8lKdyXn7XM8`s׉ 7cassandra://ZE1qloZgps-wFOsGTNC8CZ2rpYyPKFLvw_-FvxHZCDw`׉ 7cassandra://trgX4N_yna_aVj1Oa3QhYnTx_TuqkWLCjZXqghptiZgF͠^cZ@Eb[׉E -Rubber Forming and Hydroforming 479 varying cross sections, an important feature to reduce weight and provide surfaces to facilitate assembly and mounting of the radiator. A typical tube hydroforming processing sequence consists of the following steps: 1. Bending of tube to desired configuration 2. Tube hydroforming to achieve desired shape 3. Finishing operations, such as shearing of the ends and inspection 4. Assembly, including welding of components. The operations performed on one of the tube components of the closure is shown in Fig. 16.45. The tube, constructed of steel, with a 300 MPa yield strength, is first bent to shape (Fig. 16.27). The bent tube is then placed in a hydroforming press and the end caps are attached. Conventional hydroforming involves closing the die onto the tube, followed by internal pressurization to force the tube to the desired shape. Figure 16.46a shows a typical cross section. Note that as the tube is expanded, there is significant wall thinning, especially at the corners, because of friction at the tube–die interface. A sequence of pressures that optimize corner formation is thus followed, as shown in Fig. 16.46b. In this approach, a first pressure stage (prepressure stage) is applied as the die is closing, causing the tube to partially fill the die cavity and shape the cross-section’s corners. After the die is completely closed, the internal pressure is increased to lock-in the shape and provide the support needed for hole piercing. This sequence has the benefit of forming the sharp corners in the cross section by bending, as opposed to pure stretching as in conventional hydroforming. The final wall thickness is much more uniform, producing a more structurally sound component. Figure 16.47 shows a part being hydroformed. The assembly shown in Fig. 16.44 has 76 holes that are pierced inside the hydroforming die; the ends are then sheared to length. The 10 components in the hydroformed closure are then assembled through robotic gas-metal arc welding (Section 30.4.3), using threaded fasteners to aid in the part’s serviceability. Compared to the original stamped design, the hydroformed design has four fewer components, uses only 20 welds as opposed to 174 for the stamped design, and weighs 10.5 kg versus 14.1 kg. Furthermore, the stiffness of the enclosure and of the water cooling areas are both significantly increased. Source: Courtesy of B. Longhouse, Vari-Form, Inc. Figure 16.44: Hydroformed automotive radiator closure, which serves as a mounting frame for the radiator. Source: Courtesy of B. Longhouse, Vari-Form, Inc. ׉ 7cassandra://4dg5EqxX4ASJPlutXSHOzRCkDdv9FgDXEgqBWbd_brAm`cZ%Eb׉E_480 Chapter 16 Sheet-metal Forming Processes and Equipment 1 2 3 Figure 16.45: Sequence of operations in producing a tube-hydroformed component: (1) tube as cut to length; (2) after bending; (3) after hydroforming. Source: Courtesy of B. Longhouse, Vari-Form, Inc. Die Tube Fluid Expanded tube 1. Die is closed on tube. 2. Tube is expanded and takes the shape of the die cavity. (a) Conventional hydroforming Die pressure Fluid pressure Die closing force 1. Die is partially closed; low pressure forces tube to partially fill cavity 2. Die closing force and low pressure act to force tube into cavity; corner stresses are bending dominated to improve formability (b) Pressure sequence hydroforming Figure 16.46: Schematic illustration of expansion of a tube to a desired cross section through (a) conventional hydroforming and (b) pressure sequence hydroforming. ׉ 7cassandra://ZE1qloZgps-wFOsGTNC8CZ2rpYyPKFLvw_-FvxHZCDw`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://lmZUnEFEG_Epcj7riA2KdoqMrD64bf9iOrAj-pjHS-c"`׉ 7cassandra://UZsxWr_PiaA8ba29WmQCJHRptctT9eDkgT0SBxqgsHwC`s׉ 7cassandra://46XJvHNaq5xQU-PLJDfZNDZcOWl0EDEVHerrZpBVYMA4`׉ 7cassandra://1p2KPW3aeSmV0saJw_bkQKbRsHue3aTp-T025vLKImg?͠^cZGEbbט#u׉׉ 7cassandra://GmoyEo0ynM503RI5CGt1Bgv8GOiwU_ra0Llru83d51ciT`׉ 7cassandra://Pvn2YvxN1--esM4jXh3yBgvjl50b3Oo272bJHd_gOCwc`s׉ 7cassandra://GcKdejL2nvtI_cCJfkOdTqAACeKa6peqyR54gS-AScU `׉ 7cassandra://TbDVU-oa9HJw5NQWjQIgRVD5zZbzKaHTc1GAXr_klc4ͦ ͠^cZHEbc׉ESpinning 481 Figure 16.47: View of the tube-hydroforming press, with bent tube in place in the forming die. Source: Courtesy of B. Longhouse, Vari-Form, Inc. 16.9 Spinning Spinning is a process that involves forming of axisymmetric parts over a mandrel, using a variety of tools and rolls. It is a process similar to that of shaping clay on a potter’s wheel. Conventional Spinning. In this process, a circular blank of flat or preformed sheet metal is placed and held against a mandrel, and rotated while a rigid tool shapes the material over the mandrel (Fig. 16.48a). The tool may be activated either manually or, for higher production rates, through computer numerical control. The process typically involves a sequence of passes, requiring considerable skill. Conventional spinning Blank Mandrel Spindle Clamp Tool (a) (b) Figure 16.48: (a) Schematic illustration of the conventional spinning process. (b) Types of parts conventionally spun. All parts are axisymmetric. ׉ 7cassandra://46XJvHNaq5xQU-PLJDfZNDZcOWl0EDEVHerrZpBVYMA4`cZ%Eb׉E r482 Chapter 16 Sheet-metal Forming Processes and Equipment Forward to f t Mandrel f Workpiece Roller Cone (a) (b) (c) Figure 16.49: (a) Schematic illustration of the shear-spinning process for making conical parts. The mandrel can be shaped so that curvilinear parts can be spun. (b) and (c) Schematic illustrations of the tube-spinning process. is particularly suitable for making conical and curvilinear shapes (Fig. 16.48b), with part diameters that can range up to 6 m, which otherwise would be difficult or uneconomical to produce. Although most spinning takes place at room temperature, thick parts and metals with high strength or low ductility require spinning at elevated temperatures. Shear Spinning. Also called power spinning, flow turning, hydrospinning,and spin forging, this operation produces axisymmetric conical or curvilinear shapes, reducing the sheet’s thickness while maintaining its blank diameter (Fig. 16.49a). A single forming roll can be used, but two rolls are preferable to balance the radial forces acting on the mandrel. Typical parts made are rocket motor casings and missile nose cones. Parts up to 3 m in diameter can be formed by shear spinning. Shear spinning produces little material waste, and it can be completed in a relatively short time, in some cases in as little as a few seconds. Various shapes can be spun with fairly simple tooling, which generally is made of tool steel. The spinnability of a metal is generally defined as the maximum reduction in thickness to which a part can be subjected without fracture. Spinnability is found to be related to the tensile reduction of area of the material, just as is bendability (see Fig. 16.18). Thus, if a metal has a tensile reduction of area of 50% or higher, its thickness can be reduced by as much as 80% in one pass. For metals with low ductility, the operation is carried out at elevated temperatures, by heating the blank in a furnace and transferring it to the mandrel of the machine. Tube Spinning. In this process, the thickness of hollow, cylindrical blanks is reduced or shaped by spinning them on a round mandrel, using rolls (Fig. 16.49). The parts may be spun forward or backward. This operation is capable of producing a variety of external and internal profiles, using cylindrical blanks with constant wall thickness. The maximum reduction in thickness per pass is related to the tensile reduction in area of the material, as in shear spinning. Tube spinning can be used to make axisymmetric parts, such as rocket, missile, and jet-engine parts, pressure vessels, and car and truck wheels. Incremental Forming. Incremental forming, also called incremental sheet forming (ISF), is a term applied to a class of processes that are used to produce sheet-metal parts without dies. The simplest version is incremental stretch expanding (Fig. 16.50), wherein a blank is shaped by a rotating steel rod with a smooth hemispherical tip to produce axisymmetric parts. No special tooling or mandrel is required; the motion of the rod determines the final shape of the part, using one or more passes. Proper lubrication is essential. Blank a to Mandrel t f Roller Backward External Internal ׉ 7cassandra://GcKdejL2nvtI_cCJfkOdTqAACeKa6peqyR54gS-AScU `cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://4gqSTnlGOSw2jaFqA-az0ZK5oi_1Q9EkNApAb5-rtfs.`׉ 7cassandra://9XTy2Xf9jEWmYNpdVjsZ-p_ufedmpsXTDSQRrTZ1UMkb`s׉ 7cassandra://ZhfNAiwaef8veskyKg55SGMIQZo8zZWWFQJ6dRvYyKM`׉ 7cassandra://rT7_g2xAPo7cCYRIsIMB5du_efcvaYvFDhBYOwhD8DkASy.͠^cZIEbfט#u׉׉ 7cassandra://629O9iKCPxUqrzub5m8BAqvLuNLlHCr-7iRqwXlj3Ec`׉ 7cassandra://xTqr5J_y6QmIHrA5b_2MdsmVE57_jHJxSSfNx4oJ5RMY#`s׉ 7cassandra://aWuKayXpOEf9OR7yrI1dDpCa4r_o7ethM3mrhV_gPJU`׉ 7cassandra://YaCbGpLoeaiBHlW2GZsJ40CfpPvvEhHQh74kFp7scE4z͂͠^cZJEbg׉E Spinning 483 Clamp Blank Rotating tool (a) (b) Figure 16.50: (a) Illustration of an incremental-forming operation. Note that no mandrel is used and that the final part shape depends on the path of the rotating tool. (b) An automotive headlight reflector produced through CNC incremental forming. Note that the part does not have to be axisymmetric. Source: (b) Courtesy of J. Jeswiet, Queen’s University, Ontario. CNC incremental forming uses a computer numerical control machine tool (see Section 37.3), programmed to follow contours at different depths across the sheet surface. The blank is clamped and is stationary, and the forming tool rotates. Tool paths are calculated in a manner similar to machining (Part IV), using a CAD model of the desired shape as the starting point (see Fig. 20.3). Figure 16.50b depicts an example of a part produced by this method; note that the part does not have to be axisymmetric. The main advantages of CNC incremental forming are high flexibility in the shapes that can be produced and low tooling costs. This process has been used for rapid prototyping of sheet-metal parts (Chapter 20). The main drawbacks include low production rates and limitations on materials that can be shaped. Case Study 16.4 Computer-aided Incremental Sheet Forming A trend in modern automotive manufacture is the more frequent product changes and updates, as well as demand for customized parts and product personalization (see also Section 39.4). Accordingly, fast, highquality, low-cost prototyping and low volume production processes are needed. Additive manufacturing approaches (Chapter 20) are useful for many parts, but sheet metal sections in the sizes needed are difficult or impossible to produce. For thin sheet metal products, incremental sheet forming (ISF) is a new, promising alternative to additive manufacturing (Fig. 16.50). For small batch volume production (500 units per year, see Table 37.2), it offers lower manufacturing cost compared to conventional forming processes and also faster design time. During ISF, the sheet periphery is clamped in a blank holder. A generic tool, usually with a hemispherical end, is applied to locally deform the sheet with a predefined toolpath and the final desired geometry is achieved progressively. A typical spiral tool path is also shown in the figure. The die-less nature of SPIF combined with its universal tooling and high flexibility makes this approach highly favorable. A major advantage of ISF is that the process can be performed using standard CNC milling machines; for larger parts, robots have also been used to move the tool (Section 37.6). The final material properties are close to those produced by conventional forming, because ISF avoids melting and other metallurgical issues associated with additive manufacturing; it also cold works the sheet metal. In addition, the complex stress state developed during ISF suppresses necking, which significantly enhances the material formability as compared to conventional forming processes such as stamping and hydroforming. ׉ 7cassandra://ZhfNAiwaef8veskyKg55SGMIQZo8zZWWFQJ6dRvYyKM`cZ%Eb׉E -484 Chapter 16 Sheet-metal Forming Processes and Equipment Rapid design cycles require associated computer aided engineering models capable of predicting the part geometry (Section 38.4), especially since ISF has a fairly high failure rate unless tool paths are carefully planned. There are now commercial software packages for stamping operations that can predict final part geometry and thereby allow an engineer to accurately design the required stamping dies. Similar tools are under development for ISF to design the appropriate tool path. Modern models can predict part thickness (Fig. 16.51); current research efforts are underway to incorporate springback. Ford Motor Company has reported using ISF for producing body panels for prototype vehicle production. This technology offers the opportunity for evaluating design changes quickly, which can speed up the product development process. ISF is also being explored for aircraft frame construction where the annual volumes are consistent with the capability of this process. Boeing is leading a project being executed by the Manufacturing USA Lightweight Innovations for Tomorrow (LIFT) Institute to evaluate the potential of ISF. The target component, an airframe fuel cover, was shown to be economically competitive with the baseline hydroforming process up to volumes of around 500 units per year. Further advances are being made to enable faster tool speeds and larger spiral step sizes while still delivering the desired part geometry and mechanical properties, offering the potential for cost advantage at even higher production volumes. Source: Courtesy of A. Taub and M. Banu, University of Michigan. 16.10 Superplastic Forming The superplastic behavior of some metals (Section 2.2.7) within certain temperature ranges involves tensile elongations on the order of up to 2000%. Examples of such materials are zinc–aluminum and titanium alloys, with very fine grains, typically less than 10 to 15 μm (see Table 1.1). Superplastic alloys can be formed 2.0 0.4 0.8 1.2 1.6 0 010 Simulation Experiment Initial tool contact Depth Blank holder Thickness Final contact 20 30 Depth (mm) (a) 40 50 (b) Figure 16.51: Finite element model of single point incremental forming. (a) Validation of the thickness profile for a funnel shape; (b) Strain distribution of a 67◦ cone. Thickness (mm) ׉ 7cassandra://aWuKayXpOEf9OR7yrI1dDpCa4r_o7ethM3mrhV_gPJU`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://jZisuivp5zoJtZx6rVHXN3S5x-cuUXLWShqiuxuqSRo]`׉ 7cassandra://89A72ln-gF1l3rSooZDYP5mn7xkn4uAQa9KyBPcZMwgi`s׉ 7cassandra://f1uSfT6oGUg-Fi-8Czg180BnMeIFjDUOZ_13mehWww0`׉ 7cassandra://KRI_hhUhZyPzqy2PsPO_tPYn_r8wUxSE8hyrVDXzi6s^}͠^cZJEbjט#u׉׉ 7cassandra://2Fd_Hr7k3B0ylHEH6rRO3SOtg6UhNLw3CEbcdnhSk6Q Y`׉ 7cassandra://6dy2qLnb3QyXRu5WN3DbCaZ_AZOoqVSLaWu9ND-78xQY`s׉ 7cassandra://58g-f_oj9L0SFpjwRtVnbDhdB9rSSOD7yO8wA8YIR3o`׉ 7cassandra://d6pwIga8pjLV69DfzcmpVPReK064Lw8peGoMCCBR-MsK͠^cZKEbn׉EHot Stamping 485 into complex shapes by superplastic forming, a process that employs common metalworking techniques, as well as by polymer-processing techniques, such as thermoforming, vacuum forming, and blow molding (Chapter 19). The behavior of the material in superplastic forming is similar to that of bubble gum or hot glass, which, when blown, expands several times its original diameter before it bursts. Superplastic alloys, particularly Zn-22Al and Ti-6Al-4V, can also be formed by bulk-deformation processes, including closed-die forging, coining, hubbing, and extrusion (Chapters 14 and 15). Common die materials in superplastic forming are low-alloy steels, cast tool steels, ceramics, graphite, and plaster of paris. Their selection depends on the forming temperature and the strength of the superplastic alloy. The very high ductility and relatively low strength of superplastic alloys offer the following advantages: • Complex shapes can be formed from one piece, with fine detail, close tolerances, and elimination of secondary operations • Weight and material savings can be significant, because of the high formability of the materials • Little or no residual stresses are present in the formed parts • Because of the low strength of the material at forming temperatures, tooling can be made of materials that have lower strength than those in other metalworking processes, thus tooling costs are lower. On the other hand, superplastic forming has the following limitations: • The material must not be superplastic at service temperatures, as otherwise the part will undergo shape changes during its use • Because of the high strain-rate sensitivity of the superplastic material (Section 2.2.7), it must be formed at sufficiently low strain rates, typically 10−4 to 10−2/s. Forming times range anywhere from a few seconds to several hours; cycle times are thus much longer than those of conventional forming processes. Diffusion Bonding/Superplastic Forming. Fabricating complex sheet-metal structures by combining diffusion bonding with superplastic forming (DB/SPF) is an important manufacturing strategy, particularly in the aerospace industry. Typical structures produced are shown in Fig. 16.52, in which flat sheets are first diffusion bonded (Section 31.7) and then formed. In this process, selected locations of the sheets are first diffusion bonded while the rest of the interfaces remains unbonded, using a layer of material (stop-off)to prevent bonding. The structure is then expanded in a mold, thus taking the shape of the mold, typically by using pressurized neutral (argon) gas. These structures have high stiffness-to-weight ratios, because they are thin and, by design, have high section moduli; an important feature that makes this process particularly attractive in aerospace and aircraft applications. The DB/SPFS process improves productivity by eliminating mechanical fasteners and produces parts with good dimensional accuracy and low residual stresses. The technology is well advanced for titanium structures for aerospace applications. In addition to various aluminum alloys being developed using this technique, other metals for superplastic forming include various nickel alloys. 16.11 Hot Stamping Increasing fuel economy in automobiles has received considerable attention in recent years for both environmental and economic reasons. To achieve fuel economy without compromising performance or safety, manufacturers have increasingly applied advanced materials in automobiles. Die-cast magnesium or extruded aluminum components are examples, but these materials are not sufficiently stiff or as well suited as steel for occupant safety. Thus, there has been a recent trend to consider hot stamping of advanced high-strength steels. ׉ 7cassandra://f1uSfT6oGUg-Fi-8Czg180BnMeIFjDUOZ_13mehWww0`cZ%Eb׉EI486 Chapter 16 Sheet-metal Forming Processes and Equipment Clamping force Heating elements Argon Setup Press platen Sheet blank Die Stop-off (no bonding) Staggered stop-off Process Complete Pressurized argon Pressurized argon Part Made (a) (b) (c) Figure 16.52: Production of structures by superplastic forming and diffusion bonding of sheet metals. Such structures have high stiffness-to-weight ratio. The process involves heating the sheet, followed by pressurization of the working gas, usually argon. Cycle times are relatively long, allowing both superplastic forming and diffusion bonding in the same die. (a) Single sheet superplastic forming. (b) Two sheet superplastic forming with diffusion bonding. A stop-off, commonly yttrium for titanium structures, is placed where bonding is not desired, and the pressurized argon is introduced between the sheets to form them. (c) Three sheet superplastic forming with diffusion bonding. The inner and outer sheets conform to the die profile, while the middle sheet produces a structure with support struts. As described in Section 5.5.5, high-strength TRIP and TWIP steels are now available, with yield strengths and ultimate strengths exceeding 1300 MPa and 2000 MPa, respectively (Table 5.4). Conventional forming of these materials would be difficult or impossible, because of the high forces required and the excessive springback after forming. For these reasons, the sheet metal is preheated to above 900◦C (usually 1000◦-1200◦C) and hot stamped. To extend die life and to quench the material within the die, as described below, the tooling is maintained at a much lower temperature, typically 400◦–500◦C. Hot stamping allows exploitation of steel phases to facilitate forming and maximize part strength. Basically, the steel is maintained at elevated temperatures to form austenite (see Section 4.4), which has a ductile fcc structure at elevated temperatures. When shaped and brought into contact with the much cooler tooling, the steel is rapidly quenched, forming martensite, which is a very hard and strong but brittle form of steel (Section 4.7). ׉ 7cassandra://58g-f_oj9L0SFpjwRtVnbDhdB9rSSOD7yO8wA8YIR3o`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ZB7z-8xf5YBY6Rn0p5d1m4nvpTBSnl7Lib6b7t1iNaoX`׉ 7cassandra://WnfbSzP_gm0W68xI9fhqpG27oNlcDxRW1kCfkeNCIvIj`s׉ 7cassandra://iQTbw-dhYDi95i7MIV-kQ1BM4alacDZtoGP6c9cnDK8`׉ 7cassandra://sHg4b9OiKGLIVhNeY0h5BWzOH9HT0ZrjH4pLUmFJjXAc͠^cZLEbpט#u׉׉ 7cassandra://Cn_xa2mrEz6xXlu19GdpdDX8xDxXYuZDlkeazu-R3p8O8`׉ 7cassandra://OtPLqmHIhT4TzXPdBSd2T7cTSPWmSLcN4vcTi_HY8Z0c`s׉ 7cassandra://hA0e2M6Q8STAScOAQbphrrMRMw7dJ_UYH-hrKD10hBM`׉ 7cassandra://ENAYhnZxHmZKAssaGKSeFbNKitW4ehkRb6cv8UQljL4͔=͠^cZLEbq׉ESpecialized Forming Processes 487 A typical hot-stamping sequence involves the following steps: 1. The material is heated up to the austenization temperature, and allowed to dwell or soak for a sufficiently long time to ensure that quenching will be quick when it contacts the die, but not before. Three basic methods are used to heat blanks prior to stamping: roller hearth furnaces, induction heating, and resistive heating. The last two methods have the advantage of shorter soak times, but may not lead to uniform temperature distributions throughout the part. The soak time must be optimized in order to ensure proper quenching while minimizing the cycle time. 2. In order to avoid cooling of the part before shaping it, the blank must be transferred to the dies as quickly as possible. Forming must be performed quickly, before the beginning of transformation of austenite into martensite. 3. Once the part is formed, the dies remain closed while the part is quenched, which takes from 2 to 10 seconds, depending on sheet thickness, temperature of sheet and die, and workpiece material. The cooling rate must be higher than 27◦C/s to develop martensite. Forming is done with steel tools that have cooling channels incorporated in them, in order to maintain proper tooling temperature. A complete transformation into martensite results in the high strengths given in Table 5.4. It should be noted that quenching from austenite to martensite results in an increase in volume, thus influencing the residual stress distribution and workpiece distortion in forming. Pressurized hot gas (air or nitrogen) can also be used as a working media to form the material, similar to hydroforming (Section 16.8). This method improves formability and, with proper process control, allows for more uniform blank and tooling temperatures, and thus lower residual stresses and warping. Because the workpiece is hot and quenching must be done very rapidly, hot stamping is usually performed without a lubricant; also, shot blasting (Section 26.8) is often required after forming to remove scale from part surfaces. The steel may also be later coated with an aluminum-silicon layer, to prevent oxidation and eliminate the grit blasting step. In such a case, the coating requires a slightly longer soak time, in order to properly bond to the steel substrate. Hot stamping is not restricted to steels; magnesium alloys ZEK100, AZ31, and ZE10 are also attractive because of their light weight, but these materials have limited formability at room temperature. Therefore, they are stamped at temperatures up to 300◦C. Also, some advanced aluminum-alloy sheets are formed at elevated temperatures for improved ductility, and even develop superplastic behavior. Electrically Assisted Forming. A recent technology has involved the application of high current through a metallic workpiece during the forming operation, known as electrically assisted forming (EAF). This procedure has also been applied to bulk forming operations, and is believed to increase the formability of materials. The mechanisms involved may be associated with increased temperature, although it has been suggested that the heating is higher in the vicinity of dislocations (Section 1.4.1), and that the malleability associated with electron mobility is increased. 16.12 Specialized Forming Processes Although not as commonly used as the other processes described thus far, especially for high-rate and high-volume production, several other sheet-forming processes are used for specialized applications. Explosive Forming. By controlling their quantity and shape, explosives also are a source of energy for sheet-metal forming. In explosive forming, first utilized to shape metals in the early 1900s, the sheet blank is clamped over a die, and then the entire assembly is lowered into a tank filled with water (Fig. 16.53a). The air in the die cavity is evacuated, an explosive charge is placed at a certain height, and the charge is then detonated. ׉ 7cassandra://iQTbw-dhYDi95i7MIV-kQ1BM4alacDZtoGP6c9cnDK8`cZ%Eb׉E 488 Chapter 16 Sheet-metal Forming Processes and Equipment Explosive Standoff Water Water level Ground level Sheet Hold-down ring Die Vacuum line Tank (a) (b) Figure 16.53: (a) Schematic illustration of the explosive-forming process. (b) Illustration of the confined method of the explosive bulging of tubes. The explosive generates a shock wave, developing a pressure that is sufficient to form the blank. The peak pressure, p, generated in water is given by the expression p = K  3 √ a W R , (16.15) where p is in MPa, K is a constant that depends on the type of explosive, such as 21,600 for TNT (trinitrotoluene), W is the weight of the explosive in pounds, R is the distance of the explosive from the sheet-metal surface (called the standoff), in feet, and a is a constant, generally taken as 1.15. A variety of shapes can be formed by explosive forming, provided that the material is sufficiently ductile at the very high rates of deformation that is characteristic of this process (see Table 2.4). The process is versatile, as there is virtually no limit to the size of the sheet or the plate that can be formed. It is suitable particularly for low-quantity production runs of large parts, such as those used in aerospace applications. Steel plates 25-mm thick and 3.6 m in diameter have been formed by this method, as have tubes, with wall thicknesses as much as 25 mm. The explosive-forming method also can be used at a much smaller scale, as shown in Fig. 16.53b. In this case, a cartridge (canned explosive) is used as the source of energy. The process can be useful in bulging and expanding of thin-walled tubes for specialized applications. The mechanical properties of parts made by explosive forming have been found to be similar to those made by conventional forming techniques. Depending on the number of parts to be produced, dies for this method can be made of aluminum alloys, steel, ductile iron, zinc alloys, reinforced concrete, wood, plastics, or composite materials. Electromagnetically Assisted Forming. In electromagnetically assisted forming, also called magnetic-pulse forming, the energy stored in a capacitor bank is discharged rapidly through a magnetic coil. In a typical example, a ring-shaped coil is placed over a tubular workpiece; the workpiece is then collapsed by magnetic forces over the inner part (mandrel), thus making an integral assembly (Fig. 16.54). The mechanics of this process is based on the fact that a magnetic field produced by the coil (Fig. 16.54a), crosses the metal tube (an electrical conductor) and generates eddy currents in the tube. These currents in turn, produce their own magnetic field. The forces produced by the two magnetic fields oppose each other, repelling the coil and the tube from each other, and collapsing the tube over the inner piece. The higher the electrical conductivity of the workpiece, the higher are the magnetic forces. Note that it is not necessary for the workpiece material to have magnetic properties, but it must be electrically conducting. Under very high strain rates, the formability of the material is increased, dimensional accuracy is improved, and springback and wrinkling are reduced. The design of the magnetic coil is an important factor Cartridge Forming die Tube ׉ 7cassandra://hA0e2M6Q8STAScOAQbphrrMRMw7dJ_UYH-hrKD10hBM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://MGsLm-WA7YNl95ZNghyEK0f0CX_GpagFq9cW8iV3_9U`׉ 7cassandra://N1MEzDRCF6LmT3Kjxu0oMM3naO6ntE0X-th3ay27xTAM`s׉ 7cassandra://BdAVoLOB-XCuER_at_H9KonpJQGSQeUvJbMF1sLUVtU `׉ 7cassandra://NOgEniCc9oCiZz7EngKhkiIozd_soCSmdQ8pvo5rvt4͠^cZLEbsט#u׉׉ 7cassandra://Zk-m_s0xmzkQrovmDuZXcadaJtWQFTQVtAHoh0mbuXcE`׉ 7cassandra://MAhZOUeQaNp1dE4zIgR6wOnacPgh5lPneJ6CkJyStIkw`s׉ 7cassandra://XYCxR_iFY9lhCunz-rqCzb63qJm0iH7aACfUb1Gxhk0?`׉ 7cassandra://zIPnDZXX5eRs_8aqcUmxu8FqCYh7UNzaCaX_xpoMVCQl͠^cZMEbt׉ESpecialized Forming Processes 489 Before After forming Coil Mandrel Coil current Eddy current Tube CL (a) (b) Figure 16.54: (a) Schematic illustration of the magnetic-pulse-forming process used to form a tube over a plug. (b) Aluminum tube collapsed over a hexagonal plug by the magnetic-pulse-forming process. in the success of this operation. Flat magnetic coils can be made for use in such operations as embossing and shallow drawing of sheet metals. First used in the 1960s, electromagnetically assisted forming has been demonstrated to be particularly effective for aluminum alloys. Electromagnetically assisted forming has been applied to (a) collapsing thinwalled tubes over rods, cables, and plugs; (b) compression crimp sealing of automotive oil filter canisters; (c) specialized sheet-forming operations; (d) bulging and flaring operations; and (e) swaging end fittings onto torque tubes for the Boeing 777 aircraft. Peen Forming. As shown in Fig. 16.55, peen forming is used to produce curvatures on thin sheet metals by shot peening (Section 34.2) one surface of the sheet, subjecting the surface of the sheet to compressive stresses, which tend to laterally expand the surface layer (see also Section 2.11). Because the material below the peened surface remains rigid, the surface expansion causes the sheet to develop a curvature. Compressive surface residual stresses are also induced, improving fatigue strength. The shots are made of cast-iron or steel, and are discharged either from a rotating wheel or by an air blast from a nozzle. Peen forming is used by the aircraft industry to generate smooth and complex curvatures on aircraft wing skins. Cast-steel Traversing gantry machine Stationary workpiece Track Track Figure 16.55: Schematic illustration of a peen-forming machine to shape a large sheet-metal part, such as an aircraft-skin panel. Note that the sheet is stationary and the peening head travels along its length. Source: Metal Improvement Company. ׉ 7cassandra://BdAVoLOB-XCuER_at_H9KonpJQGSQeUvJbMF1sLUVtU `cZ%Eb ׉E490 Chapter 16 Sheet-metal Forming Processes and Equipment shot about 2.5 mm in diameter, traveling at speeds of 60 m/s, have been used to form wing panels 25 m long. For heavy structural sections, shot diameters as large as 6 mmmay be used. The peen-forming process also is used for straightening twisted or bent parts, including out-of-round rings to make them round. Laser Beam Forming. This process involves the application of laser beams as a localized heat source over specific regions of sheet metals. The steep thermal gradients developed through the thickness of the sheet produce thermal stresses, which are sufficiently high to cause localized plastic deformation of the sheet. With this method, a sheet can be bent permanently without using any dies. In laser-assisted forming, the laser acts as a localized heat source, reducing the strength of the sheet metal at specific locations, thus improving formability and increasing process flexibility. Applications include straightening, bending, embossing, and forming of complex tubular or flat components. Microforming. Microfilming is a more recent development and includes a family of processes that are used to produce very small metallic parts and components. Examples of miniaturized products include a wristwatch with an integrated digital camera and a multiple-gigabyte computer storage components. Typical parts made by microforming include springs, screws, small shafts for micromotors, and a variety of coldheaded, extruded, bent, embossed, coined, punched, or deep-drawn parts. Dimensions are typically in the submillimeter range, and part weights are on the order of milligrams. Electrohydraulic Forming. Also called underwater spark or electric-discharge forming, the source of energy is a spark between two electrodes connected to each other with a short, thin wire. The rapid discharge of the energy from a capacitor bank through the wire generates a shock wave in the water, similar to those developed in explosive forming. The pressure in the water medium is sufficiently high to form the part. The energy levels are lower than those in explosive forming, being typically a few kJ. Electrohydraulic forming is a batch process and can be used to produce various small parts. Case Study 16.5 Cymbal Manufacture Cymbals (Fig. 16.56a) are an essential percussion instrument for all forms of music. Modern drum-set cymbals cover a wide variety of sounds, from deep, dark, and warm to bright, high-pitched, and cutting. Some cymbals sound musical, while others are trashy. A wide variety of sizes, shapes, weights, hammerings, and surface finishes (Fig. 16.56b) is available to achieve the desired performance. Cymbals are produced from such metals as B20 bronze (80% Cu–20% Sn with a trace of silver), B8 bronze (92% Cu–8% Sn), nickel–silver alloy, and brass (see also Section 6.4). The manufacturing sequence for producing a bronze cymbal is shown in Fig. 16.57. The B20 metal is first cast into mushroom-shaped ingots. The ingot is then rolled successively up to 14 times, with water cooling the metal with each pass through the rolling mill. Special care is taken to roll the bronze at a different angle with each pass, to minimize anisotropy and develop an even, round shape. The as-rolled blanks are then reheated and stretch formed (pressed) into the cup or bell shape, which determines the cymbal’s overtones. The cymbals are then center drilled or punched, to make hang holes, and trimmed on a rotary shear to approximate final diameters. This operation is followed by another stretch-forming step, to achieve the characteristic Turkish dish form that controls the cymbal’s pitch. Automatic peen forming is done on machinery (Fig. 16.58a) and without using templates, since the cymbals have already been pressed into shape, but the peening pattern is controllable and uniform. The size and pattern of the peening operations depend on the desired response, such as tone, sound, response, and pitch of the cymbal. The cymbals are then hammered to impart a distinctive character to each instrument. Hammering can be done by hand, which involves placing the bronze blank on a steel anvil, where the cymbals then are struck manually by hand hammers. Several finishing operations are performed on the cymbals, which can involve merely cleaning and printing of identifying information, as some musicians prefer the natural surface appearance and sound of shaped, hot-rolled bronze. More commonly, the cymbals are turned on a lathe, and without using ׉ 7cassandra://XYCxR_iFY9lhCunz-rqCzb63qJm0iH7aACfUb1Gxhk0?`cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://TNB4KXESxJRqNkb3jJHVE1K6lOmTEoME4Oo03WGX8fE2~`׉ 7cassandra://JDMGGHaeUZfTD_evQL-199NEGqRKTfbnN-0mjzBxxHYY `s׉ 7cassandra://Ru-bI_bLRky0n7S7dYtsDXHeke5-DYom56KGKLVCDRYb`׉ 7cassandra://CnwCZtzLcFygioZGk_rM8CKhf26imlujIEdbTzaRV8Yw ͠^cZNEbyט#u׉׉ 7cassandra://9JJHI6-zSuSDBombih0qzp3W9G2OF1vQDPVZsITZbOM=`׉ 7cassandra://QoluN3wNk4m-9qWj2s5Dhbe4YGEoDtee0jXIiP1Jd2k1X`s׉ 7cassandra://MhMB8jukUf52lTCdyf8g06vQpS8ZPZF-Ey3qxj06XHsu`׉ 7cassandra://7znnw_Y9ewXUL1khO4JvtXsYlDRb6zLveY_DTk_8h0w /Ԁ͠^cZREb{׉E 1Manufacturing of Metal Honeycomb Structures 491 any machining fluid in order to remove the oxide surface and reduce the thickness of the cymbal for the desired weight and sound. As a result, the surface finish becomes lustrous and, in some cases, also develops a favorable microstructure. Some cymbals are polished to a glossy brilliant finish. In many cases, the surface indentations from peening persist after finishing; this is recognized as an essential performance feature of a cymbal, and it is also an aesthetic feature appreciated by musicians. Various surface finishes associated with modern cymbals are shown in Fig. 16.56b. Source: Courtesy of W. Blanchard, Sabian Ltd. 16.13 Manufacturing of Metal Honeycomb Structures A honeycomb structure basically consists of a core of honeycomb, or other corrugated shapes, bonded to two thin outer skins (Fig. 16.59). The most common example of such a structure is corrugated cardboard, which has a high stiffness-to-weight ratio and is used extensively in packaging for shipping consumer and industrial goods. Because of their light weight and high resistance to bending, metal honeycomb structures are used for aircraft and aerospace components, buildings, and transportation equipment. The chassis of the Koenigsegg, a Swedish sports car, for example, is made partly of aluminum honeycomb with an integrated fuel tank. Honeycomb structures also may be made of nonmetallic materials, such as polymers and a variety of composite materials. Honeycomb structures are made most commonly of 3000-series aluminum, but for specialized applications and corrosion resistance, they may also be made of titanium, stainless steels, and nickel alloys. Reinforced plastics, such as aramid-epoxy, also are used to make these structures. There are two basic methods of manufacturing honeycomb materials. In the expansion process, which is the more common (Fig. 16.59a), sheets are first cut from a coil, and an adhesive (Section 32.4) is applied at intervals (node lines) on their surfaces. The sheets are then stacked and cured in an oven, developing strong bonds at their adhesive surfaces. The block is then sliced to required dimensions and stretched to develop a honeycomb structure. In the corrugation process (Fig. 16.59b) the sheet first passes through a pair of specially designed rolls, thus making a corrugated sheet; it is then cut into specific lengths. An adhesive is then applied to the (a) (b) Figure 16.56: (a) Selected common cymbals. (b) Detailed view of different surface textures and finishes of cymbals. Source: Courtesy of W. Blanchard, Sabian Ltd. ׉ 7cassandra://Ru-bI_bLRky0n7S7dYtsDXHeke5-DYom56KGKLVCDRYb`cZ%Eb ׉E492 Chapter 16 Sheet-metal Forming Processes and Equipment 1. As cast 2. After rolling; multiple rolling–annealing cycles necessary 3. Stretch formed and trimmed 4. Hang hole punched 5. Stretch formed 6. Hammered 7. Lathe turned and polished Figure 16.57: Manufacturing sequence for the production of cymbals. Source: Courtesy of W. Blanchard, Sabian Ltd. Figure 16.58: Automated hammering of a cymbal on a peening machine. Source: Courtesy of W. Blanchard, Sabian Ltd. ׉ 7cassandra://MhMB8jukUf52lTCdyf8g06vQpS8ZPZF-Ey3qxj06XHsu`cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://U1tcC2is4TJOka64vsUCH0m0dc75sZNV258tHIiOREo)`׉ 7cassandra://ql-QRMdDyOKCFGbhQZL_vaIC8rCTy5g0WCk1krt-z9kR`s׉ 7cassandra://863136dcpNPJW8Fj2mpbAuirJatXQ9CLrEAqdpivayM`׉ 7cassandra://Q2eS9WduMNasCoNkS5Wh4wD52KQCaZFpruxgjXmp160m ͠^cZREb}ט#u׉׉ 7cassandra://S81gOYkxFvVXUeq6DgElhUhZmW3sDIflZLnWnN-qX1gN`׉ 7cassandra://qjQ6iRuGv19bJ4bLCwdRj0z1bi78HzUEwLHo8Qh1QjQ>`s׉ 7cassandra://ilbahfHTY1GVvjmNglKUlFrOaorxnulMjL1OJwWMkas`׉ 7cassandra://sMCRVJnqEulhqY9KMCYwVwHuURkh16Hv6sCWCdYSht0bA2͠^cZREb~׉EDesign Considerations in Sheet-metal Forming 493 Adhesive Slice Sheet Block Roll Expanded panel (a) Face sheet Adhesive impregnated scrim cloth (optional) (c) Figure 16.59: Methods of manufacturing honeycomb structures: (a) expansion process; (b) corrugation process; and (c) assembling a honeycomb structure into a laminate. node lines; the corrugated sheets are then stacked into a block, and the block is cured. Because the sheets are already preformed, no expansion stage is involved. The honeycomb is finally made into a sandwich structure (Fig. 16.59c), using face sheets that are joined by adhesives (or they are brazed; Section 32.2) to the top and bottom surfaces. 16.14 Design Considerations in Sheet-metal Forming The following design guidelines apply to sheet-metal forming operations, identifying the most significant design considerations. Blank Design. Material scrap is the primary concern in blanking operations (see also Table 40.4). Poorly designed parts will not nest properly, and there can be considerable scrap produced (Fig. 16.60). Bending. The main concerns in bending operations are fracture, wrinkling, and inability to properly make the bend. As shown in Fig. 16.61, a sheet-metal part with a flange will force the flange to undergo compression, which may cause buckling (see also flanging, Section 16.6). Buckling can be controlled with a relief notch, cut to limit the stresses developed in bending, or else a design modification as shown in Fig. 16.62 can be made. Right-angle bends have similar difficulties; relief notches can be used to avoid tearing as shown in the figure. Because the bend radius is a highly stressed area, all stress concentrations should be removed from the bend-radius location, such as holes near bends. It is advantageous to move the hole away from the bend area, but when this is not possible, a crescent slot or ear can be used (Fig. 16.63a). Similarly, in bending flanges, tabs and notches should be avoided, because their stress concentrations will greatly reduce formability. When tabs are necessary, large radii should be specified to reduce stress concentration (Fig. 16.63b). Expanded honeycomb core Face sheet (b) Roll Corrugating rolls Corrugated sheet Corrugated panel Corrugated block ׉ 7cassandra://863136dcpNPJW8Fj2mpbAuirJatXQ9CLrEAqdpivayM`cZ%Eb ׉EF494 Chapter 16 Sheet-metal Forming Processes and Equipment Poor 43.2 mm 13.2 mm 11.4 mm Better 39.6 mm Figure 16.60: Efficient nesting of parts for optimum material utilization in blanking. Source: Courtesy of Society of Manufacturing Engineers. If notches are necessary, it is important to orient them properly with respect to the grain direction of the sheet metal. As shown in Fig. 16.17, bends ideally should be perpendicular to the rolling direction of the sheet, or oblique if this is not possible, in order to avoid cracking. Bending with very small radii can be accomplished by scoring or embossing (Fig. 16.64), but this operation can cause fracture. Burrs should not be present in a bend allowance (see Fig. 16.2), because they are less ductile and can lead to crack initiation and propagation into the rest of the sheet. Poor Better Best Closed corner 3 3 material thickness (a) Relief notch Closed corner 3 3 material thickness Relief notch (b) Figure 16.61: Control of tearing and buckling of a flange in a right-angle bend. Source: Courtesy of Society of Manufacturing Engineers. ׉ 7cassandra://ilbahfHTY1GVvjmNglKUlFrOaorxnulMjL1OJwWMkas`cZ%EbcZ%Eb #בCט#u׉׉ 7cassandra://p6rLBOjr85Y5GuVAXNKpnH1F0d2Cc8a_gWjOlWiuA5o`׉ 7cassandra://rSXQ4A7oXkNsAdbKO8aZa6nuMB5NmtSTlUtxMinLzGc6`s׉ 7cassandra://TBdiF0nBr14AcXlsxckhAoLHr7bCFYhu_7K37oImKZE$`׉ 7cassandra://zZcBPwhyt4AA4PgCWxfGLib2gukbhAQ_7lxXZ5wowzo]H͠^cZSEb˄ט#u׉׉ 7cassandra://HqRspEYmlteL1fQzPf4o1kCn3iPB2o92UdrumKZrUCU`׉ 7cassandra://A5o8yEqLX8G6W5J1icvdcmn3r6x29xWBoVRXv6ItOXUd`s׉ 7cassandra://cB78HLlCpFRQbflmOxXiJkD3i4XbtOnZxoQ5r2Q7rqw`׉ 7cassandra://cuB0O12GALAik6NFzTmjwSwwQhrY2QNZRAi_KfaxPNQf͠^cZTEb˅׉EDesign Considerations in Sheet-metal Forming 495 Poor Tearing Good Notch Notch (a) Poor Good (b) (c) Figure 16.62: Application of notches to avoid tearing and wrinkling in right-angle bending operations. Source: Courtesy of Society of Manufacturing Engineers. Poor Good Poor Bend line Bend line x R x Better R (a) (b) Figure 16.63: Stress concentrations near bends. (a) Using a crescent or ear for a hole near a bend. (b) Reducing the severity of tab in a flange. Source: Courtesy of Society of Manufacturing Engineers. Before Sharp radius (a) Sharp radius (b) Figure 16.64: Application of (a) scoring or (b) embossing to obtain a sharp inner radius in bending. Unless properly designed, these features can lead to fracture. Source: Courtesy of Society of Manufacturing Engineers. After ׉ 7cassandra://TBdiF0nBr14AcXlsxckhAoLHr7bCFYhu_7K37oImKZE$`cZ%Eb׉E 496 Chapter 16 Sheet-metal Forming Processes and Equipment Roll Forming. The process should, in general, be designed so as to control springback. Also, it is not difficult to include perforating rolls in the forming line, so that periodic holes, notches, or embossed areas can be located on the roll-formed shape. Stamping and Progressive-die Operations. In progressive dies (Section 16.2.3), the cost of tooling and the number of stations are determined by the number and spacing of the features on a part. Thus, it is advantageous to keep the number of features to a minimum. Closely spaced features may provide insufficient clearance for punches, and may require two punches. Narrow cuts and protrusions may present difficulties in forming with a single punch and die set. Deep Drawing. After a cup is deep drawn, it invariably will spring back slightly towards its original shape. For this reason, designs requiring a vertical wall may be difficult to draw. Relief angles, at least 3◦ on each wall, make it easier to draw. Cups with sharp internal radii are difficult to draw, and deep cups will often require one or more subsequent ironing operations. 16.15 Equipment for Sheet-metal Forming For most general pressworking operations, the basic equipment consists of mechanical, hydraulic, pneumatic, or pneumatic–hydraulic presses; they are available with a wide variety of designs, features, capacities, and computer controls. Servo presses (Section 14.8) are now being used for forming sheet metals, because of their ability to vary speed and forces in a controlled manner during forming. Typical designs for press frames are shown in Fig. 16.65 (see also Figs. 14.19 and 16.23f). The proper design, stiffness, and construction of such equipment is essential to the efficient operation of the system, and for achieving high production rate, good dimensional accuracy, and high product quality. The traditional C-frame structure (Fig. 16.65a) has been used widely for ease of tool and workpiece accessibility; however, it is not as stiff as the box-type pillar (Fig. 16.65e) or the double-column frame structure (Fig. 16.65f). Accessibility to working areas in presses has become less important, due to advances in automation and in the use of industrial robots and computer controls. Press selection for sheet-metal forming operations depends on several factors: 1. Type of forming operation, size and shape of dies, and tooling required 2. Size and shape of parts 3. Length of stroke of the slide, number of strokes per minute, operating speed, and shut height (the distance from the top of the bed to the bottom of the slide, with the stroke down) 4. Number of slides: single-action presses have one reciprocating slide; double-action presses have two slides, reciprocating in the same direction; they typically are used for deep drawing, one slide for the punch and the other for the blankholder. Triple-action presses have three slides; they generally are used for reverse redrawing and other complicated forming operations. 5. Maximum force required (press capacity and tonnage rating) 6. Type and level of mechanical, hydraulic, and computer controls 7. Features for changing the dies; because the time required for die changes can be significant (as much as a few hours), they affect productivity, for which rapid die-changing systems have been developed. In a system called SMED (single-minute exchange of dies), die setups can be changed in less than 10 minutes, by using computer-controlled hydraulic or pneumatic systems. 8. Safety features. ׉ 7cassandra://cB78HLlCpFRQbflmOxXiJkD3i4XbtOnZxoQ5r2Q7rqw`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://fxDBS5kpeEmraGyOBxwBB6VJdzFonY91NwLTyFKVMnIn`׉ 7cassandra://GjIhkW5JolHFUQ5av8TrjWFyG313rJS00ukT1iI5gvsQ`s׉ 7cassandra://0BdxiacRNC7AJBDKnc8Uf4n72G3EdaoxVMXJPXc5VNA`׉ 7cassandra://BBRK38B4u7tvhBqucmXlmMdsLZQz-Q2sOVl5tJP8-WgE#͠^cZUEbˈט#u׉׉ 7cassandra://fa-eDpJIOyOfG0H8u2xTLgldRb4hkiTY5qS3pCqhUgo`׉ 7cassandra://rRIoaQuDsuy8wWO_Ro7-6h9Gne_tVkhokJVvdrxFlCw^`s׉ 7cassandra://i_c8F3UPkVIVZGn-kFi3geHOsdklqdPx_QhRY7YF0kg`׉ 7cassandra://gboW3xuJ_RcAVWaZTpvy6QBwYslnQNQWe7sPXMJadJUD2͠^cZUEbˊ׉EpEconomics of Sheet-forming Operations 497 Basic C-frame design (a) Wide design (b) Adjustable bed (c) Open-back inclinable (d) Pillar (e) Double column (f) (g) Figure 16.65: (a) through (f) Schematic illustrations of types of press frames for sheet-metal forming operations. Each type has its own characteristics of stiffness, capacity, and accessibility. (g) A large stamping press. Source: (g) Verson Allsteel Company. Because a press can be a major capital investment (see Table 40.6), its present and its future use for making a broad variety of parts and applications for a long period of time must be investigated. Versatility and multiple different uses are important factors in press selection, particularly for product modifications and for making new products to respond to continually changing market demands. 16.16 Economics of Sheet-forming Operations Sheet-metal forming involves economic considerations similar to those for the other metalworking operations. Sheet-forming operations are very versatile, and a number of different processes can be considered to produce the same part economically. The costs involved (Section 40.10) depend on the particular operations, such as tools, dies, molds, and equipment costs and labor. For small and simple parts, costs and lead times to make the dies are relatively low. On the other hand, for large-scale operations, such as stretch ׉ 7cassandra://0BdxiacRNC7AJBDKnc8Uf4n72G3EdaoxVMXJPXc5VNA`cZ%Eb׉E 498 Chapter 16 Sheet-metal Forming Processes and Equipment 2 3 4 5 6 7 8 1 0 1 0.19 m 0.3 m Drawing Spinning 0 2345 Number of parts (3 103) Figure 16.66: Cost comparison for manufacturing a round sheet-metal container either by conventional spinning or by deep drawing. Note that for small quantities, spinning is more economical. forming of aircraft panels and making boat hulls, these costs are very high. Moreover, because the volume of such products made is low, the cost per piece can be very high (see also Fig. 14.21). Deep drawing, for example, requires expensive dies and tooling, but a very large number of parts can be produced with the same setup, such as beverage cans, metal containers, and similar household products. These costs for other processes, such as punching, blanking, bending, and spinning, vary considerably. Equipment costs can vary widely, depending largely on the complexity of the forming operation, part loading and unloading features, part size and shape, and level of automation and computer controls involved. Automation, in turn, directly affects the labor and the skill level required; note that the higher the extent of automation, the lower the skill level required. Furthermore, sheet-metal parts generally require some finishing operations, one of the most common being deburring of the part edges, which generally is labor intensive. Even though significant advances have been made in automated deburring, it still requires costly computer-controlled equipment. As an example of the versatility of sheet-forming operations and the costs involved, recall that a cupshaped part can be made by (a) deep drawing, (b) spinning, (c) rubber forming, (d) explosive forming, impact extrusion, (e) casting, or (f) fabrication by assembling or welding together different pieces. Each of these methods involves different processes, and different costs. The part shown in Fig. 16.66, for example, can be made either by deep drawing or by conventional spinning, but the tooling costs for the two processes are significantly different. Deep-drawing dies have several components, and making them costs much more than the relatively simple mandrels and tools required in such a process as spinning; note also that surface finish, dimensional accuracy, and the properties of the two products will be different Consequently, the tooling cost per part in drawing will be high, especially if only a few parts are required. This part also can be shaped by deep drawing and in much shorter time than by spinning, even if the latter operation is automated and computer controlled; also, spinning generally requires more skilled labor. Considering these factors, the break-even point for this part is around 700 parts, and for quantities higher than that, deep drawing is more economical. Summary • Sheet-metal-forming processes are among the most versatile of all metalworking operations. They generally are used on workpieces having high ratios of surface area to thickness. Unlike bulk deformation processes, such as forging and extrusion, sheet-metal forming operations do not undergo much change in their thickness. • Several test methods have been developed for predicting the formability of sheet metals. Cost per part (relative) ׉ 7cassandra://i_c8F3UPkVIVZGn-kFi3geHOsdklqdPx_QhRY7YF0kg`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://3KieeS_9iuSFYT42V0GGki_Z8BCfkswFQyFEw7k8tPI$`׉ 7cassandra://cyPwR9J60dH13wSwQVIGpk25675VlpzeHQlrIQaz16cOk`s׉ 7cassandra://r2Qimdjta_H0paG0tN5kAr_v42XmudT_yJi8DilALh4 `׉ 7cassandra://VFobKugmmdKgMr8lJLbuMH4tx1mMKRlfUxghrfoKIgs>͠^cZUEbˌט#u׉׉ 7cassandra://mYltKlQM_CM6vb0U5apO9ocSCWGK48JyWnpIpkyiFwoğ`׉ 7cassandra://rGmGzUaURcWcUxsbt_mmexhtzJrO9PLkpXBPd9U7c2UgY`s׉ 7cassandra://iqOSkDmr-pWeZHvlzU3nxSOLbQKUUwbvKV2_dPxEbF8`׉ 7cassandra://8k6ztxZSRl1lz2XGxQynzDNXKe-mjUJaVftfiZzOtP4b͠^cZUEbˍ׉E9Key Terms 499 • Important material parameters are the quality of the sheared edge of the blank, the capability of the sheet to stretch uniformly, the material’s resistance to thinning, its normal and planar anisotropy, grain size, and for low-carbon steels, yield-point elongation. • The forces and energy required in forming processes are transmitted to the sheet through either solid tools and dies or by flexible rubber or polyurethane members, or by electrical, chemical, magnetic, and gaseous means. • Because sheet metals are thin, springback, buckling, wrinkling, and tearing are significant factors in shaping them. These tendencies can be reduced or eliminated by proper tool and die design, and minimizing the unsupported length of the sheet during processing. • Superplastic forming of diffusion-bonded sheets is an important process for making complex sheetmetal structures, particularly for applications where high stiffness-to-weight ratios are important. • For general sheet-forming operations, forming-limit diagrams are very useful, because they establish quantitative relationships among the major and minor principal strains, indicating safe regions of forming. Key Terms Beading Bendability Bend allowance Bending Blankholder Blanking Bulging Burnished surface Burr Clearance Compound dies Deburring Deep drawing Dent resistance Dimpling Drawbead Drawing Earing Electrohydraulic forming Embossing Explosive forming Fine blanking Flanging Formability Forming-limit diagram Hemming Honeycomb structures Hot stamping Hydroform process Incremental forming Ironing Laser forming Limiting drawing ratio L¨uder’s bands Magnetic-pulse forming Microforming Minimum bend radius Nesting Nibbling Normal anisotropy Peen forming Planar anisotropy Plastic anisotropy Press brake Progressive dies Punching Redrawing Roll forming ׉ 7cassandra://r2Qimdjta_H0paG0tN5kAr_v42XmudT_yJi8DilALh4 `cZ%Eb׉E 500 Chapter 16 Sheet-metal Forming Processes and Equipment Rubber forming Shaving Shearing Slitting Spinning Springback Bibliography Altan, T., and Tekkaya, E. (eds.), Sheet Metal Forming: Processes and Applications, ASM International, 2012. ASM Handbook, Vol. 14B: Metalworking: Sheet Forming, ASM International, 2006. Boljanovic, V., Sheet Metal Forming Process and Die Design, 2nd ed., Industrial Press, 2014. Davies, G., Materials for Automobile Bodies, 2nd ed., Butterworth-Heinemann, 2012. Fundamentals of Tool Design, 6th ed., Society of Manufacturing Engineers, 2010. Hosford, W.F., and Caddell, R.M., Metal Forming: Mechanics and Metallurgy, 4th ed., Cambridge, 2014. Hu, P., Ying, L., and He, B., Hot Stamping Advanced Manufacturing Technology of Lightweight Car Body, Elsevier, 2016. Jie, L.Y., Sheet Metal Bending Technology, Mechanical Industry, 2000. Kumar, S, and Hussein, H.M.A., AI Applications in Sheet Metal Forming, Springer, 2016. Marciniak, Z., Duncan, J.L., and Hu, S.J., Mechanics of Sheet Metal Forming, 2nd ed., ButterworthHeinemann, 2002. Rapien, B.L., Fundamentals of Press Brake Tooling, Hanser, 2010. Suchy, I., Handbook of Die Design, 2nd ed., McGraw-Hill, 2005. Tschaetch, H., Metal Forming Practise: Processes, Machines, Tools, Springer, 2006. Review Questions 16.1. How does sheet-metal forming differ from rolling, forging, and extrusion? 16.2. What causes burrs? How can they be reduced or eliminated? 16.3. Explain the difference between punching and blanking. 16.4. Describe the difference between compound, progressive, and transfer dies. 16.5. Describe the characteristics of sheet metals that are important in sheet-forming operations. Explain why they are important. 16.6. Describe the features of forming-limit diagrams (FLDs). 16.7. List the properties of materials that influence springback. Explain why and how they do so. 16.8. Give one specific application for each of the common bending operations described in this chapter. 16.9. Why do tubes buckle when bent? What is the effect of the tube thickness-to-diameter ratio? 16.10. Define normal anisotropy, and explain why it is important in determining the deep drawability of a material. 16.11. Describe earing and why it occurs. 16.12. What are the advantages of rubber forming? Which processes does it compete with? 16.13. Explain the difference between deep drawing and redrawing. 16.14. How is roll forming fundamentally different from rolling? Steel rule Stretch forming Superplastic forming Tailor-welded blanks Transfer dies ׉ 7cassandra://iqOSkDmr-pWeZHvlzU3nxSOLbQKUUwbvKV2_dPxEbF8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Xw6A1lkl_UXjxli4TgBfzLqQCu1dkDoFppB-D-ZyddY]`׉ 7cassandra://Ij0ByA8krf1cyr09EoVf8lwP4UnhkKK1iN0wW29rzDMke`s׉ 7cassandra://syl-pzx7QuQ12Bh_ocw1kq_oyhfOSuK5v4DXFLF3oNgM`׉ 7cassandra://bNa9LQh_uwpjkTzyNyNCoHrIrJ9Mdbk4P6JWn80owXoH͠^cZUEbˏט#u׉׉ 7cassandra://Sx88vslVmFKwYEVw9yRlv3JjfeF4fdf-TqD2fOBwFfwY`׉ 7cassandra://A9mf2d_IGJbuDoG5kvaa6su6JiiTUIveoyPkZ8wf6Xwe`s׉ 7cassandra://8dkvx-mKF4XAvkUmopNdEbLiiWIqxQTVqYxf8qBU4KM`׉ 7cassandra://vJywltLTTpITlk1aRsHR9SmWUxJ-woOLnMd70CKhL4Ua͠^cZUEbː׉E Qualitative Problems 501 16.15. What is nesting? What is its significance? 16.16. Describe the differences between compound, progressive, and transfer dies. 16.17. What is microforming? 16.18. Explain the advantages of superplastic forming. 16.19. What is hot stamping? For what materials is it used? 16.20. What is springback? What is negative springback? Qualitative Problems 16.21. Explain the differences that you have observed between products made of sheet metals and those made by casting and forging. 16.22. Take any three topics from Chapter 2, and, with specific examples for each, show their relevance to the topics covered in this chapter. 16.23. Do the same as for Problem 16.22, but for Chapter 3. 16.24. Identify the material and process variables that influence the punch force in shearing, and explain how each of them affects this force. 16.25. Explain why springback in bending depends on yield stress, elastic modulus, sheet thickness, and bend radius. 16.26. Explain why cupping tests may not predict well the formability of sheet metals in actual forming processes. 16.27. Identify the factors that influence the deep-drawing force, F, in Fig. 16.32b, and explain why they do so. 16.28. Why are the beads in Fig. 16.36b placed in those particular locations? 16.29. A general rule for dimensional relationships for successful drawing without a blankholder is given by Eq. (16.14). Explain what would happen if this limit were exceeded. 16.30. Section 16.2.1 stated that the punch stripping force is difficult to estimate because of the many factors involved. Make a list of these factors with brief explanations about why they would affect the stripping force. 16.31. Is it possible to have ironing take place in an ordinary deep-drawing operation? What is the most important factor? 16.32. Note the roughness of the periphery of the flanged hole in Fig. 16.25c, and comment on its possible effects when the part is used in a product. 16.33. What recommendations would you make in order to eliminate the cracking of the bent piece shown in Fig. 16.17c? Explain your reasons. 16.34. It has been stated that the quality of the sheared edges can influence the formability of sheet metals. Explain why. 16.35. Give several specific examples from this chapter in which friction is desirable and several in which it is not desirable. 16.36. As you can see, some of the operations described in this chapter produce considerable scrap. Describe your thoughts regarding the reuse, recycling, or disposal of this scrap. Consider its size, its shape, and its contamination by metalworking fluids during processing. 16.37. Through changes in clamping or die design, it is possible for a sheet metal to undergo a negative minor strain. Explain how this effect can be advantageous. ׉ 7cassandra://syl-pzx7QuQ12Bh_ocw1kq_oyhfOSuK5v4DXFLF3oNgM`cZ%Eb׉E 502 Chapter 16 Sheet-metal Forming Processes and Equipment 16.38. How would you produce the part shown in Fig. 16.43b other than by tube hydroforming? 16.39. It has been stated that the thicker the sheet metal, the higher is the curve in the forming-limit diagram. Explain why. 16.40. If a cupping test (see Fig. 16.13) were to be performed using a pressurized lubricant instead of a spherical die, would you expect the forming limit diagram to change? Why or why not? 16.41. What are the advantages of rubber forming? Which processes does it compete with? 16.42. Which of the processes described in this chapter use only one die? What are the advantages of using only one die? 16.43. It has been suggested that deep drawability can be increased by (a) heating the flange and/or (b) chilling the punch by some suitable means. Comment on how these methods could improve drawability. 16.44. Offer designs whereby the suggestions given in Problem 16.43 can be implemented. Would the required production rate affect your designs? Explain. Quantitative Problems 16.45. Calculate Ravg for a metal where the R values for the 0◦,45◦,and90◦ directions are 0.9, 1.6, and 1.9, respectively. What is the limiting drawing ratio (LDR) for this material? 16.46. Calculate the value of ΔR in Problem 16.45. Will any ears form when this material is deep drawn? Explain. 16.47. Estimate the limiting drawing ratio for the materials listed in Table 16.4. 16.48. Using Eq. (16.15) and the K value for TNT, plot the pressure as a function of weight (W)and R, respectively. Describe your observations. 16.49. Section 16.5 states that the k values in bend allowance depend on the relative magnitudes of R and T. Explain why this relationship exists. 16.50. For explosive forming, calculate the peak pressure in water for 0.11 kg of TNT at a standoff distance of 1.5 m. Comment on whether or not the magnitude of this pressure is sufficiently high to form sheet metals. 16.51. Measure the respective areas of the solid outlines in Fig. 16.14a, and compare them with the areas of the original circles. Calculate the final thicknesses of the sheets, assuming that the original sheet is 1-mm thick. 16.52. Plot Eq. (16.6) in terms of the elastic modulus, E, and the yield strength, Sy, of the material, and describe your observations. 16.53. What is the minimum bend radius for a 2.0-mm-thick sheet metal with a tensile reduction of area of 35%? Does the bend angle affect your answer? Explain. 16.54. Survey the technical literature and explain the mechanism by which negative springback can occur in V-die bending. Show that negative springback does not occur in air bending. 16.55. Using the data in Table 16.3 and referring to Eq. (16.5), calculate the tensile reduction of area for the materials and the conditions listed in the table. 16.56. What is the force required to punch a square hole 25 mm on each side in a 0.15-mm-thick 5052-O aluminum sheet by using flat dies? What would be your answer if beveled dies are used? 16.57. In Case Study 16.2, it was stated that the reason for reducing the tops of cans (necking) is to save material for making the lid. How much material will be saved if the lid diameter is reduced by 10%? By 20%? ׉ 7cassandra://8dkvx-mKF4XAvkUmopNdEbLiiWIqxQTVqYxf8qBU4KM`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://xmYfXkckjZ3ADD4iuU4LsFuk9Vg9ern8WgbCOdsU4c8`׉ 7cassandra://H4PFytXgfaLYzEXta1E57jzlFXbvLTh50e7TFmpI0jA=`s׉ 7cassandra://ari0WgTBDn2Y4uZv_drMZL_KaUYr9Wwr5KAjnvyCxNo`׉ 7cassandra://I-W3bRuM1fUmh2tJry67AB9BwJWL6lhO3jmvDGNplXI7FA͠^cZVEb˒ט#u׉׉ 7cassandra://RbzazaJcyJYjr_8FaWbkInbwRGxbZRCmv3vjVj5QbXg`׉ 7cassandra://7X0qclfzPke1gfIAVB6MBoFQpvrf7VGN4aEcEpRMsos\`s׉ 7cassandra://RH5rBASjKOT0R4pfOLh_p5LDCuTgkQDLMI8I3Ub8Z74`׉ 7cassandra://Bn5_63fyTO3zZNPNgbXtsgfotsvqTXjIKz27uPSjdak?T͠^cZVEb˕׉EQuantitative Problems 503 16.58. A cup is being drawn from a sheet metal that has a normal anisotropy of 3. Estimate the maximum ratio of cup height to cup diameter that can be drawn successfully in a single draw. Assume that the thickness of the sheet throughout the cup remains the same as the original blank thickness. 16.59. Estimate the percent scrap in producing round blanks if the clearance between blanks is one-tenth of the radius of the blank. Consider single and multiple-row blanking, as sketched below. 16.60. Estimate the maximum bending force required for a 3-mm thick and 300-mm wide Ti-6Al-4V titanium alloy, annealed and quenched at 25◦C, in a V-die with a width of 180 mm. 16.61. Plot the final bend radius as a function of initial bend radius in bending for (a) 5052-O aluminum; (b) 5052-H34 Aluminum; (c) C24000 brass; and (d) AISI 304 stainless steel. 16.62. The figure below shows a parabolic profile that will define the mandrel shape in a spinning operation. Determine the equation of the parabolic surface. If a spun part will be produced from a 15-mm thick blank, determine the minimum required blank diameter. 300 mm 100 mm 16.63. A straight bead is being formed on a 2-mm-thick aluminum sheet in a 20-mm-diameter die, as shown in the accompanying figure. Let Sy =90 MPa. Considering springback, calculate the outside diameter of the bead after it is formed and unloaded from the die. 20 R r ׉ 7cassandra://ari0WgTBDn2Y4uZv_drMZL_KaUYr9Wwr5KAjnvyCxNo`cZ%Eb׉E 504 Chapter 16 Sheet-metal Forming Processes and Equipment 16.64. Calculate and plot the springback in bending 1-mm thick sheet metal around radii from 0.25 to 250 mm for (a) 303 stainless steel; (b) 1100-O aluminum; (c) HK31A magnesium; (d) Ti-6Al-4V. 16.65. Circular blanks of 5052-O aluminum, with a diameter of 30 mm and a thickness of 4 mm are to be mass-produced as the starting material for a tube for a paintball gun. The available press has an 800 kN capacity and can take a maximum of 300 mm wide strip. Material utilization improves if more rows are cut from a strip. (a) Determine the force required to blank a single slug. (b) Determine the maximum number of slugs that can be blanked simultaneously by the press. (c) Determine the material utilization if the space around a blanked part needs to be the same as the thickness, or 4 mm. 16.66. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 16.67. Examine some of the products in your home or in an automobile that are made of sheet metal, and discuss the process or combination of processes by which you think they were made. 16.68. Consider several shapes to be blanked from a large sheet (such as oval, triangular, and L-shaped) by laser-beam cutting, and sketch a nesting layout to minimize scrap generation. 16.69. Give several specific product applications for (a) hemming and (b) seaming. 16.70. Many axisymmetric missile bodies are made by spinning. What other methods could you use if spinning processes were not available? 16.71. Give several structural designs and applications in which diffusion bonding and superplastic forming can be used jointly. Comment on whether this combination is capable of producing parts at high volume. 16.72. Metal cans are either two-piece (in which the bottom and sides are integral) or three-piece (in which the sides, the bottom, and the top are each separate pieces). For a three-piece can, should the vertical seam in the can body be (a) in the rolling direction, (b) normal to the rolling direction, or (c) oblique to the rolling direction? Prove your answer. 16.73. The design shown is proposed for a metal tray, the main body of which is made from cold-rolled sheet steel. Noting its features and that the sheet is bent in two different directions, comment on various manufacturing considerations. Include factors such as anisotropy of the rolled sheet, its surface texture, the bend directions, the nature of the sheared edges, and the way the handle is snapped in for assembly. 16.74. Suggest consumer-product designs that could utilize honeycomb structures. For example, an elevator can use a honeycomb laminate as a stiff and lightweight floor material. 16.75. Using a ball-peen hammer, strike the surface of aluminum sheets of various thicknesses until they develop a curvature. Describe your observations about the shapes produced. ׉ 7cassandra://RH5rBASjKOT0R4pfOLh_p5LDCuTgkQDLMI8I3Ub8Z74`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://WSQAkN4aGqIdwy0Kz2VJp1m0WOOqbQy1uXtMlpcH5uI!;`׉ 7cassandra://qw4oFtGxuFJiu8D6iOX6kAdQfdqujRj9Xl7pzUC82tAE`s׉ 7cassandra://g4drYYxrOZqRjN1LirhIUyVmUlMMluQNP0K2Q8gXXrk`׉ 7cassandra://YQaNIHJfEkKXmPWs9WFmS_b1Dux_dMAG0EqWQzfwHPw?͠^cZWEb˗ט#u׉׉ 7cassandra://c3tbsO5RSuPzE1HxORSQG_R4NmPB7Uj9vFyADsAIv84 `׉ 7cassandra://voQrpYy8mivN-aEpERtRKWmEE3LBGC20X_-3ZyicAJA?`s׉ 7cassandra://0a4mLj13VFaREoIKwlx1VFdx7fxlClXk7AOQfVl0uqI:`׉ 7cassandra://sFTV4dZVBik4aNRZXL2AA-KI8-xNg3Rx5WW1LOsB0488͠^cZXEb˘׉E Synthesis, Design, and Projects 505 16.76. Inspect a common paper punch and observe the shape of the punch tip. Compare it with those shown in Fig. 16.10 and comment on your observations. 16.77. Obtain an aluminum beverage can and slit it in half lengthwise with a pair of tin snips. Using a micrometer, measure the thickness of the can bottom and the wall. Estimate the thickness reductions in ironing and the diameter of the original blank. 16.78. In order to improve its ductility, a coil of sheet metal is placed in a furnace and annealed. However, it is observed that the sheet has a lower limiting drawing ratio than it had before being annealed. Explain the reasons for this behavior. 16.79. With automotive parts, it is often advantageous to have a part with tailored properties. For example, a pillar that provides structural support for the operator’s compartment may be strong but less ductile at the center, but more ductile and less strong where the pillar attaches to the remainder of the car structure. List ways of producing such tailored properties in hot stampings. 16.80. Give three examples of sheet metal parts that (a) can and (b) cannot be produced by incremental forming. 16.81. Conduct a literature search and obtain the equation for a tractrix curve, as used in Fig. 16.38. 16.82. On the basis of experiments, it has been suggested that concrete, either plain or reinforced, can be a suitable material for dies in sheet-metal forming operations. Describe your thoughts regarding this suggestion, considering die geometry and any other factors that may be relevant. 16.83. Investigate methods for determining optimum shapes of blanks for deep-drawing operations. Sketch the optimally shaped blanks for drawing rectangular cups, and optimize their layout on a large sheet of metal. 16.84. Design a box that will contain a 100-mm × 150-mm × 75-mm volume. The box should be produced from two pieces of sheet metal and require no tools or fasteners for assembly. 16.85. Repeat Problem 16.84, but design the box from a single piece of sheet metal. 16.86. Obtain a few pieces of cardboard and carefully cut the profiles to produce bends as shown in Fig. 16.61. Demonstrate that the designs labeled as “best” are actually the best designs. Comment on the difference in strain states between the designs. ׉ 7cassandra://g4drYYxrOZqRjN1LirhIUyVmUlMMluQNP0K2Q8gXXrk`cZ%Eb׉EChapter 17 Powder-metal Processes and Equipment 17.1 Introduction 507 17.2 Production of Metal Powders 508 17.3 Compaction of Metal Powders 514 17.4 Sintering 524 17.5 Secondary and Finishing Operations 526 17.6 Design Considerations 528 17.7 Economics of Powder Metallurgy 530 Case Studies: 17.1 Hot Isostatic Pressing of a Valve Lifter 520 17.2 Production of Tungsten Carbide for Tools and Dies 527 17.3 Powder Metallurgy Parts in a Snowblower 532 • This chapter describes the principles of powder metallurgy processes for producing net-shape parts from metal powders. • The chapter begins by reviewing methods of producing and blending of metal powders and investigates the shapes that powders will develop based on the particular process employed to make them. • Operations such as compaction to consolidate the powder into desired shapes and sintering to fuse the particles to achieve the required strength are then described in detail. • Additional processes particular to powder metallurgy products are then presented, and general design rules are reviewed. • The chapter ends with process capabilities and economics of powder metallurgy as compared with other competing manufacturing operations. Typical products made: Connecting rods, piston rings, gears, cams, bushings, bearings, cutting tools, surgical implants, magnets, metal filters, and surgical implants. Alternative processes: Casting, forging, additive manufacturing, and machining. 506 ׉ 7cassandra://0a4mLj13VFaREoIKwlx1VFdx7fxlClXk7AOQfVl0uqI:`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://3FiykbTx08leceFa4R2w2eNdEjj_EZQ5okHEcMnqjug{`׉ 7cassandra://kLAWui9EY8gXMq-CCGfNRMk9La_gRj0faF2G5Cg4tpES`s׉ 7cassandra://QBcEmdUE7MnH5crEQJAvIYdkjIWU2VMsvlr97FlC_G0 `׉ 7cassandra://zSPIgaVWdnVoiLefvjM6joEJfnM_Uz56wyR-LpQC4HA͂͠^cZXEb˚ט#u׉׉ 7cassandra://6JFEbesbQ_tPB1OpZhM9qBFLGIoHhxfi9-Ii-TxeW-cF`׉ 7cassandra://vLdWvXYoVP6Hca_c2My-yeMjnGUz-kxawiy-z3PnxlwC`s׉ 7cassandra://FYJ9DSLjRI83sJmPctT-b5Gn8VW--fbpXiKynbDEvDk(`׉ 7cassandra://ZBDLWWARcm-7G-TlDYbZZsXvRDsb5n9c2oWexlKWdqIv̺͠^cZXEb˛׉E oIntroduction 507 17.1 Introduction In the manufacturing processes described thus far, the raw materials have been metals and alloys that are either in a molten state (casting) or in solid form (metalworking). In powder metallurgy (PM), metal powders are compacted into desired and often complex shapes and then sintered (heated without melting) to form a solid piece. This process first was used in Egypt in about 3000 B.C. to make iron tools. One of its first modern uses was in the early 1900s to make the tungsten filaments for incandescent light bulbs. The availability of a wide range of metal powder compositions, the ability to produce parts to net dimensions (net-shape forming), and the unique mechanical properties that result from PM give this process its numerous attractive applications. A wide range of complex parts and components can be made by powder-metallurgy techniques (Fig. 17.1): balls for ballpoint pens, piston rings, connecting rods, brake pads, gears, cams, and bushings, tool steels, tungsten carbides, and cermets as tool and die materials, graphite brushes impregnated with copper for electric motors; magnetic materials; metal filters and oil-impregnated bearings with controlled porosity; metal foams; and surgical implants; as well as structural parts for aircraft, such as landing gear components, engine-mount supports, engine disks, impellers, and engine nacelle frames. Powder metallurgy has become competitive with processes such as casting, forging, and machining, particularly for relatively complex parts made of hard and high-strength alloys. Although most parts made typically weigh less than 2.5 kg, they can weigh as much as 50 kg. It has been shown that PM parts can be produced economically in quantities as small as 5000 per year, and as much as 100 million per year for vibrator weights for cell phones. (a) (b) (c) Figure 17.1: (a) Examples of typical parts made by powder-metallurgy (PM) processes. (b) Upper trip lever for a commercial irrigation sprinkler made by PM. This part is made of an unleaded brass alloy; it replaces a die-cast part with a 60% cost savings. (c) Main-bearing powder-metal caps for 3.8- and 3.1-liter General Motors automotive engines. Source: (a) and (b) Reproduced with permission from Success Stories on PM Parts, Metal Powder Industries Federation, Princeton, New Jersey, 1998. (c) Courtesy of the Metal Powder Industries Federation. ׉ 7cassandra://QBcEmdUE7MnH5crEQJAvIYdkjIWU2VMsvlr97FlC_G0 `cZ%Eb׉E508 Chapter 17 Powder-metal Processes and Equipment The most commonly used metals in PM are iron, copper, aluminum, tin, nickel, titanium, and refractory metals. For parts made of brass, bronze, steels, and stainless steels, prealloyed powders are used, where each powder particle itself is an alloy. The sources for metals are generally bulk metals and alloys, ores, salts, and various other compounds. 17.2 Production of Metal Powders The powder-metallurgy process basically consists of the following operations, in sequence (Fig. 17.2): 1. Powder production 2. Blending 3. Compaction 4. Sintering 5. Finishing. 17.2.1 Methods of Powder Production There are several methods of producing metal powders, and most powders can be produced by more than one method; the choice depends greatly on the requirements of the end product. The microstructure, bulk and surface properties, chemical purity, porosity, shape, and size distribution of the particles depend on the particular process used (Figs. 17.3 and 17.4). These characteristics are important because they significantly affect flow and permeability during compaction and in subsequent sintering operations. Particle sizes produced typically range from 0.1 to 1000 μm. Pressing Isostatic pressing Atomization Reduction Electrolytic deposition Carbonyls Comminution Mechanical alloying Metal powders Additives Lubricants Blending Hot compaction Isostatic pressing Explosive compaction Figure 17.2: Outline of processes and operations involved in producing powder-metallurgy parts. Pseudo-isostatic pressing Combustion synthesis Rolling Extrusion Injection molding Cold compaction Atmosphere Vacuum Sintering Secondary and finishing operations Coining Forging Machining Heat treating Impregnation Infiltration Plating ׉ 7cassandra://FYJ9DSLjRI83sJmPctT-b5Gn8VW--fbpXiKynbDEvDk(`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ihwEYTsEHQhdJOT_e29W3m8LwoYTdxmGsHHNhWFPHg8<`׉ 7cassandra://kXUf5qJsbs7C0vmF6OXjziwBoD1MOv1k_xTqmMPyN40It`s׉ 7cassandra://Kol4o4lliiq-ohWXJ6JIyxJ_IhACkl4qV9Qws8xzhKU`׉ 7cassandra://zQOQ64XeJQSh8qh9p-wCUzGnow350oh0NdESV6h1YgQ͸p͠^cZYEb˟ט#u׉׉ 7cassandra://7dSZFR146q5WuiDmTVNvpdwfUinr1l2gmX-AoN7336I9`׉ 7cassandra://GPAkppOYe_CK7AwixHKNxR6aovcy2-rTam6wstkjd74O`s׉ 7cassandra://FlmenvFEM6j4CmBh1xGHgXO1_XUi-K4vwa4Iyk6zca4`׉ 7cassandra://-ZpgcxnA6god6lCdYR-skPzr2dCOZnF3ubs43BNWzAwrͺZ͠^cZZEbˠ׉EProduction of Metal Powders 509 Acicular (chemical decomposition) Irregular rodlike (chemical decomposition, mechanical comminution) (a) One-dimensional Flake (mechanical comminution) Dendritic (electrolytic) (b) Two-dimensional Spherical (atomization, carbonyl (Fe), precipitation from a liquid) Irregular (atomization, chemical decomposition) Rounded (atomization, chemical decomposition) (c) Three-dimensional Figure 17.3: Particle shapes in metal powders and the processes by which they are produced. Iron powders are produced by many of these processes (see also Fig. 17.4). Porous (reduction of oxides) Angular (mechanical disintegration, carbonyl (Ni)) Atomization. Atomization involves a liquid-metal stream, produced by injecting molten metal through a small orifice, whereby the stream is broken up by jets of inert gas or air (Fig. 17.5a) or water (Fig. 17.5b), known as gas or water atomization, respectively. The size and shape of the particles formed depend on such factors as the temperature of the molten metal, rate of flow, nozzle size, and jet characteristics. The use of water results in a slurry of metal powder and liquid at the bottom of the atomization chamber. Although the powders must be dried before they can be used, the water allows for more rapid cooling of the particles, resulting in higher production rates. Gas atomization usually produces more spherical particles (see Fig. 17.3c). 10 mm 100 mm (a) (b) Figure 17.4: (a) Scanning-electron microscopy image of iron-powder particles made by atomization. (b) Nickel-based superalloy (Udimet 700) powder particles made by the rotating electrode process; see Fig. 17.5d. Source: After P.G. Nash, Illinois Institute of Technology, Chicago. ׉ 7cassandra://Kol4o4lliiq-ohWXJ6JIyxJ_IhACkl4qV9Qws8xzhKU`cZ%Eb׉E4510 Chapter 17 Powder-metal Processes and Equipment Ladle Molten metal Tundish Atomizing water spray Atomizing chamber Metal particles (a) Water atomization (b) Inert gas Molten metal Ladle Tundish Spindle Liquid metal Nonrotating Spinning disk (c) tungsten electrode Collection port (d) Figure 17.5: Methods of metal-powder production by atomization: (a) gas atomization; (b) water atomization; (c) centrifugal atomization with a spinning disk or cup; and (d) atomization with a rotating consumable electrode. In centrifugal atomization, the molten-metal stream drops onto a rapidly rotating disk or cup; the centrifugal forces then break up the stream and generate particles (Fig. 17.5c). In a variation of this method, a consumable electrode is rotated rapidly (at about 15,000 rev/min) in a helium-filled chamber (Fig. 17.5d); the centrifugal force then breaks up the molten tip of the electrode into metal particles. Reduction. The reduction of metal oxides (removing oxygen) uses gases such as hydrogen and carbon monoxide as reducing agents. By this means, very fine metallic oxides are reduced to the metallic state. The powders produced are spongy and porous, with uniformly sized spherical or angular shapes. Electrolytic Deposition. Electrolytic deposition utilizes aqueous solutions or fused salts. The powders produced are among the purest made. Carbonyls. Metal carbonyls, such as iron carbonyl [Fe(CO)5] and nickel carbonyl [Ni(CO)4], are formed by allowing iron or nickel to react with carbon monoxide. The reaction products are then decomposed to iron and nickel, and they turn into small, dense, and uniformly spherical particles, with high purity. Comminution. Mechanical comminution (pulverization) involves either crushing (Fig. 17.6), milling in a ball mill, or grinding brittle or less ductile metals into small particles. A ball mill (Fig. 17.6b) is a machine with a rotating hollow cylinder partly filled with steel or white cast-iron balls. The powder or particles placed Vacuum Rotating consumable electrode Dewatering Tundish High-pressure water manifold Atomization tank ׉ 7cassandra://FlmenvFEM6j4CmBh1xGHgXO1_XUi-K4vwa4Iyk6zca4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Bnf27YGncHUINjV9SsN4ypyukqazON0qIyb3wCG013cE`׉ 7cassandra://KZ4wZ6XgB2pY7aesgXUya0S2U9I-1g_SNLEru4Y7ctkR`s׉ 7cassandra://Qk2uZEtLqBIL3XMegcnKwLlL4Pburzs_WptrW1mBM186`׉ 7cassandra://8W1AgGBHa0UDuZm9VbMtyVrjjEe2sBBdO7ua9niH_0A ͠^cZpEb˴ט#u׉׉ 7cassandra://q9OCXMma74WxzEy1y9gEXHGDA-sUrRquPYhCcavO_LY`׉ 7cassandra://QUHosMjjNH73AymSiNTMIXcAhTQTLaR2Yjo7hjYY4NUkq`s׉ 7cassandra://DiS4PALr4w49yOOv-Kn-kDYqYOxfPmoMM0BVO6C9Lwsq`׉ 7cassandra://sXmNExY15MDtLCiNfJBgOYZaEnWHuIqNvf-0W7y_bb4n͠^cZpEb˵׉EXProduction of Metal Powders 511 Incoming material Milled powder Incoming material Hammers Crushed powder (a) (b) Balls (c) Figure 17.6: Methods of mechanical comminution to obtain fine particles: (a) roll crushing, (b) ball mill, and (c) hammer milling. into a ball mill are thus impacted by the balls as the cylinder is rotated, or its contents may be agitated. This action has two effects: (a) the particles are periodically fractured, resulting in smaller particles; and (b) the shape of the particles is affected, such that with brittle materials, the particles produced have angular shapes.With ductile metals, they are flaky and not particularly suitable for powder-metallurgy applications. Mechanical Alloying. In mechanical alloying, powders of two or more materials are mixed in a ball mill, as illustrated in Fig. 17.7. Under the impact of the hard balls, the powders fracture and bond together by diffusion, entrapping the second phase and forming alloy powders or composites. The dispersed phase can result in the strengthening of the particles or it can impart special electrical or magnetic properties to the powder. Miscellaneous Methods. Less commonly used methods for making powders are: • Precipitation from a chemical solution • Production of fine metal chips by machining • Vapor condensation. Nanopowders. More recent developments include the production of nanopowders of copper, aluminum, iron, titanium, and various other metals (see also nanomaterials in Section 8.8). When the material is subjected to large plastic deformation, by compression and shear and at stress levels of 5500 MPa during processing, the particle size is reduced and the material becomes pore free, thus possessing Dispersed particles Stainless steel ball Crushed powder Ni Milling liquid 1. Welded Ni flakes 2. 3. Figure 17.7: Mechanical alloying of nickel particles with dispersed smaller particles. As the nickel particles are flattened by the balls, the second, smaller phase is impressed into the nickel surface and eventually is dispersed throughout the particle due to successive flattening, fracture, and welding that occur. ׉ 7cassandra://Qk2uZEtLqBIL3XMegcnKwLlL4Pburzs_WptrW1mBM186`cZ%Eb׉E512 Chapter 17 Powder-metal Processes and Equipment enhanced properties. Because these powders are pyrophoric (they ignite spontaneously), or are contaminated readily when exposed to air, they are shipped as thick slurries under hexane gas (which itself is highly volatile and combustible). Microencapsulated Powders. In micro encapsulation, metal powders are coated completely with a binder. For electrical applications, such as magnetic components of ignition coils and in other pulsed AC and DC applications, the binder acts as an insulator, preventing electricity from flowing between the particles and thus reducing eddy-current losses. The powders are compacted by warm pressing, and are used with the binder still in place (see also powder-injection molding, Section 17.3.3). 17.2.2 Particle Size, Shape, and Distribution Particle size is generally controlled by screening, that is, passing the metal powder through screens (sieves)of various mesh sizes. The horizontal screens are stacked on top of each other, with the mesh size becoming finer as the powder flows downward through the screens. The larger the mesh size, the smaller is the opening in the screen. A mesh size of 30, for example, has an opening of 600 μm, size 100 has 150 μm, and size 400 has 38 μm. This method is similar to the numbering of abrasive grains; the larger the number, the smaller is the size of the abrasive particle (see Section 26.2). Several other methods also are available for particle-size analysis: 1. Sedimentation, which involves measuring the rate at which particles settle in a fluid. 2. Microscopic analysis, which may include the use of transmission and scanning-electron microscopy. 3. Light scattering, from a laser that illuminates a sample, consisting of particles suspended in a liquid medium. The particles cause the light to be scattered; a detector then digitizes the signals and computes the particle-size distribution. 4. Optical methods, such as particles blocking a beam of light, whereby they are sensed by a photocell. 5. Suspending particles in a liquid and detecting particle size and distribution by electrical sensors. Particle Shape. A major factor in processing characteristics, particle shape is described in terms of aspect ratio or shape factor. Aspect ratio is the ratio of the largest dimension to the smallest dimension of the particle, and ranges from unity (for a spherical particle) to 10 or higher (for flakelike or needlelike particles). Also called the shape index, shape factor (SF) is the ratio of the surface area of the particle to its volume. It is normalized by reference to a spherical particle of equivalent volume; thus, the shape factor for a flake is higher than that for a sphere. Size Distribution. The size distribution of particles is an important consideration because it affects the processing characteristics of the powder. The distribution of particle size is given in terms of a frequencydistribution plot (see Section 36.7), where the maximum is called the mode size. Other properties of metal powders that have an effect on their behavior in processing are (a) flow properties, when the powders are being filled into dies, (b) compressibility, when they are being compacted, and (c) density. 17.2.3 Blending Metal Powders Blending (mixing) powders is the next step in powder-metallurgy processing; it is carried out for the following purposes: • Powders of different metals can be blended in order to impart special physical and mechanical properties and characteristics to the product. Mixtures of metals can be produced by alloying the metal before producing its powder, or else the blends themselves can be produced. Proper mixing is essential to ensure the uniformity of mechanical properties throughout the part. ׉ 7cassandra://DiS4PALr4w49yOOv-Kn-kDYqYOxfPmoMM0BVO6C9Lwsq`cZ%Eb cZ%Eb#בCט#u׉׉ 7cassandra://aJT_5QgkVF4IdKhCky7x3xwc7XDqscvRMsp92qdDZgw `׉ 7cassandra://KxaKLanUflUqp8jmH6dhgxwMiW0fuKW-L48rOECsvoMT`s׉ 7cassandra://Lx2MLhEhpwZSj7_V55l1FOdwoTxrHUqj_JZpdDQ1DeU`׉ 7cassandra://p1B94GQCcymqgmtQxVvd34nDWy3BQVwSi0iVD-PAbeIP͠^cZqEb˸ט#u׉׉ 7cassandra://pny06M3LFqPHLz7gMB8dpolvBqYlBC1czZgP-bJsULUk`׉ 7cassandra://6IYNZymWMRxWr-cu5KwBgKvQsr3AqAfMop3F2Jey-kgC`s׉ 7cassandra://tQ-rVV71kUu8WWJujA67RBCcjLoUhps2kqT4o2U2VpU`׉ 7cassandra://EYcOAS3naARjoniLA19fqTsK07MAsXz1VtUDNnwsCsA#͠^cZqEb˽׉E pProduction of Metal Powders 513 • Even when made of a single metal, powders may vary significantly in their size and shape, hence they must be blended to ensure uniformity from part to part. An ideal mix is one in which all of the particles of each material, and of each size and morphology, are distributed uniformly. Blending powders of two size distributions can result in a higher compacted density. This can be visualized by considering a HCP structure (see Fig. 1.5) and then considering the same structure with small particles filling the gaps between large particles. However, this denser compact may not have the uniformity of makeup as a blend of equally sized powders. • Lubricants can be mixed with the powders to improve their flow characteristics. They reduce friction between the metal particles, improve flow of the powder mix into dies, and improve die life. Common lubricants are stearic acid or zinc stearate, in a proportion of from 0.25 to 5% by weight. • Other additives, such as binders (as in sand molds, Section 11.2.1), are used to impart sufficient green strength (see Section 17.3); additives also can be used to facilitate sintering. Powder mixing must be carried out under controlled conditions to avoid contamination and deterioration. Deterioration is caused by excessive mixing, which may alter the shape of the particles and work-harden them, thus making the subsequent compaction process more difficult. Powders can be mixed in air, in inert atmospheres (to avoid oxidation), or in liquids, which act as lubricants and make the mix more uniform. Several types of blending equipment are available (Fig. 17.8). These operations are now controlled by microprocessors to improve and maintain quality. Hazards. Because of their typically high surface area-to-volume ratio, metal powders can become explosive, particularly aluminum, magnesium, titanium, zirconium, and thorium. Great care must therefore be exercised, both during blending and in storage and handling. Precautions include (a) maintaining a humidity (a) (b) (c) (d) (e) Figure 17.8: Powder mixing and blending. (a) through (d) Some common bowl geometries for mixing or blending powders. (e) A mixer suitable for blending metal powders. Since metal powders are abrasive, mixers rely on the rotation or tumbling of enclosed geometries, as opposed to using aggressive agitators. Source: Courtesy of Kemutec Group, Inc. ׉ 7cassandra://Lx2MLhEhpwZSj7_V55l1FOdwoTxrHUqj_JZpdDQ1DeU`cZ%Eb!׉E514 Chapter 17 Powder-metal Processes and Equipment and grounding equipment and personnel to prevent static charge, (b) preventing sparks by using nonsparking tools, (c) avoiding friction as a source of heat, and (d) avoiding dust clouds and exposed ignition sources, such as open flames. 17.3 Compaction of Metal Powders Compaction,or pressing, is the step in which the blended powders are pressed into dies, as shown in Fig. 17.9. The purposes of compaction are to (a) impart the required shape, density, and particle-to-particle contact and (b) make the part sufficiently strong for further processing. The powder, or feedstock,isfedinto the die by a feed shoe, and the upper punch then descends into the die. The presses used are actuated either hydraulically or mechanically. The process generally is carried out at room temperature, although it can also be done at elevated temperatures for high melting-point metals. The stages in powder compaction are shown in Fig. 17.10. First, the powder is loosely packed, so there is significant porosity.With low applied pressure, the powder rearranges itself, filling the voids and producing a denser powder, but the stresses at contact points among the powders particles are still low. Continued compaction causes increased contact stress and thus plastic deformation of the powders, resulting in increased powder adhesion and compaction. Upper punch Powder Feed shoe Die Lower punch 1. 2. (a) Upper punch PM spur gear (green) Die Core rod Lower punch (b) Figure 17.9: (a) Compaction of metal powder to form a bushing. The pressed-powder part is called green compact. (b) Typical tool and die set for compacting a spur gear. Source: Courtesy of the Metal Powder Industries Federation. 3. Ejector 4. Compacted shape (green) ׉ 7cassandra://tQ-rVV71kUu8WWJujA67RBCcjLoUhps2kqT4o2U2VpU`cZ%Eb"cZ%Eb!#בCט#u׉׉ 7cassandra://4L-WsDg6QnEpzBHQnMZ6dw4_GPLyLhsYDcwRFgDVsG8?`׉ 7cassandra://_Ktzt2pn_zY-XMjpCDC6YEtXTCBGZyug0-DepbpKnUwe`s׉ 7cassandra://TPYavRuVltVJQmIgOGowhAjgT3Zt9fqbZHZN2tT4LQg`׉ 7cassandra://oMK3uu-zTJfKFBntmdReg0vg_XVdypWc3mhOCFKbN2w D͠^cZuEbט#u׉׉ 7cassandra://oIzmQRK0WOzRRfyKD6lhJXl5Jv0mvShTUNb9RFoP7Eku%`׉ 7cassandra://wUe7ErilsIITMsqu_kUdKcCCgMuEaBaeRbOgwlTE9TwM`s׉ 7cassandra://RWXF-O_9wfgMt_oHJoRNELVHHagAJUK3xY_DiC8zwJo `׉ 7cassandra://gZBkudglVXiCr3QRP-pZIAmm39surwzRvh3r4LgAx84ͱ)͠^cZuEb׉E bCompaction of Metal Powders 515 Bulk Homogeneous deformation compression Rearrangement Apparent density Compaction pressure Figure 17.10: Compaction of metal powders. At low compaction pressures, the powder rearranges without being deformed, leading to a high rate of density increase. Once the powders are more closely packed, plastic deformation occurs at their interfaces, leading to further density increases although at lower rates. At very high densities, the powder behaves like a bulk solid. The pressed powder is known as green compact, because the part has low strength, just as in green parts in slip casting (Section 18.2.1). These parts are very fragile and can easily crumble or become damaged—a situation that is exacerbated by poor pressing practices, such as rough handling or insufficient compaction. For higher green strength, the powder must be fed properly into the die cavity and sufficient pressure must be developed throughout the part. Density is relevant during three different stages in PM processing: (1) as loose powder, (2) as a green compact, and (3) after sintering. The particle shape, average size, and size distribution all affect the packed density of loose powder. An important factor in density is the size distribution of the particles. If all of the particles are of the same size, then there will always be some porosity when packed together. Theoretically, the porosity is at least 24% by volume. Observe, for example, a box filled with tennis balls; there are always open spaces between the individual balls. Introducing smaller particles into the powder mix will begin to fill the spaces between the larger powder particles, and thus result in a higher density of the compact. The density after compaction, called green density, depends primarily on the (a) compaction pressure; (b) powder composition; and (c) hardness of the powder (Fig. 17.11a). The higher the compacting pressure and the softer the powder, the higher is the green density. The density and its uniformity within a compact can be improved with the addition of a small quantity of admixed (blended-in) lubricant. The effect of particle shape on green density can best be understood by considering two powder grades with the same chemical composition and hardness: one with a spherical particle and the other with an irregular shape. The spherical grade will have a higher apparent density (fill density), but after compaction under higher pressure, compacts from both grades will have similar green densities. When comparing two similar powders that were pressed under some standard conditions, the powder that gives a higher green density is said to have a higher compressibility. The higher the density of the compacted part, the higher are its strength and elastic modulus (Fig. 17.11b). The reason is that with higher density, the compact has fewer and smaller pores in the same volume, and hence higher strength. Because of friction between (a) the metal particles in the powder and (b) the punch surfaces and die walls, the density within the part can vary considerably. This variation can be minimized by proper punch and die design and by control of friction. Thus, it may be necessary to use multiple punches, each with separate movements, in order to ensure that the density is more uniform throughout the part (Fig. 17.12). Recall a similar discussion regarding the compaction of sand in mold Density ׉ 7cassandra://TPYavRuVltVJQmIgOGowhAjgT3Zt9fqbZHZN2tT4LQg`cZ%Eb#׉E516 Chapter 17 Powder-metal Processes and Equipment 0 1 2 3 4 5 6 7 8 9 40 200 Density of iron Density of copper Apparent density Copper powder, coarse Copper powder, fine Iron powder, coarse Iron powder, fine 3.49 g/cm3 1.44 2.75 1.40 0 200 400 600 800 1000 1200 MPa (a) 8.0 8.2 8.4 8.6 8.8 Sintered density (g/cm3) (b) Figure 17.11: (a) Density of copper- and iron-powder compacts as a function of compacting pressure. Density greatly influences the mechanical and physical properties of PM parts. (b) Effect of density on tensile strength, elongation, and electrical conductivity of copper powder. Source: (a) After F.V. Lenel, (b) After the International Annealed Copper Standard (IACS) for electrical conductivity. making (see Fig. 11.7). On the other hand, in some compacted parts, such as gears and cams, density variations may be desirable. For example, densities can be increased in critical locations where high strength and wear resistance are important. Pressure distribution during compaction. As can be seen in Fig. 17.12, the pressure during compaction decays rapidly away from tooling surfaces. The pressure distribution along the length of the compact in a single action press can be determined to be px = poe−4µkx/D, (17.1) Pressure 700 MPa 600 500 400 300 L 200 100 C D/2 L L/D 5 1.66 (a) Single action C D/2 L 5.1 Density 5.5 5.4 5.3 5.2 5.15 5.0 4.9 4.8 C D/2 L (b) Dual action (c) Single action (d) Multi actio Figure 17.12: Density variation in compacting metal powders in various dies. Source: After P. Duwez and L. Zwell. 5.1 5.5 5.4 5.3 Density 5.25 5.5 5.4 5.3 5.2 20 25 100 150 30 35 100 95 90 85 80 g n o l E n o ti a C y ctivit d n o u T e e s li n n e r t s h t g Density (g/cm3) Tensile strength (MPa) Elongation (%) Electrical conductivity (% IACS) ׉ 7cassandra://RWXF-O_9wfgMt_oHJoRNELVHHagAJUK3xY_DiC8zwJo `cZ%Eb$cZ%Eb##בCט#u׉׉ 7cassandra://5GqsbtF1H9uCT9ZgsCaYgsFxSG_WI9qSZMPUuz-WiCU%`׉ 7cassandra://YE70-3vL7PKix4lIxKFTiA1G4cjnoyCUeP1v8T3GBesP `s׉ 7cassandra://CkYuH0--RHtT1DuDBAQM6SKl8O67inNUOLQVL9oGAAs`׉ 7cassandra://V1py7D_TyLFjko3kpkvWReTJ9xONxi-Da0i5ww7fk4ofD͠^cZvEbט#u׉׉ 7cassandra://GvlCD91LMeMZXr-Vtk-jyJO7Zgdkbd4knyuwa-j1kuYzK`׉ 7cassandra://uKC56bGkrSnXJubudOhOJJTyGhkaVnn4eJUTHmXwC1kL`s׉ 7cassandra://fbE2S8h9HoqRr5Fqcxi6t3V4eeY9QgUSpRARyhF5se45`׉ 7cassandra://7SuGNwPWC8woCSpZtuhqjovtVu4useIkmCJjBK0FYMM ,͠^cZxEb׉E Compaction of Metal Powders 517 Table 17.1: Compacting Pressures for Various Powders. Material Pressure (MPa) Metals Aluminum Brass Bronze Iron Tantalum Tungsten Other materials Aluminum oxide Carbon Cemented carbides Ferrites 70–275 400–700 200–275 350–800 70–140 70–140 110–140 140–165 140–400 110–165 where μ is the coefficient of friction between particles and the container wall, D is the compact diameter, and p is the pressure in the compacting direction, x. Note that the pressure on the bottom of the punch is po. Equation (17.1) also includes a variable to account for friction between particles, k, where σr = kpx and σr is the stress in the radial direction. If there is no friction between the particles, k =1, the powder behaves like a fluid, and thus σr = px, signifying a state of hydrostatic pressure. If there is very high friction, k =0, and the pressure will be low near the punch. It can be seen from Eq. (17.1) that the pressure within the compact decays as the coefficient of friction, the parameter k, and the length-to-diameter ratio increase. The pressure required for pressing metal powders typically ranges from 70 MPa for aluminum to 800 MPa for high-density iron parts (see Table 17.1). 17.3.1 Equipment Press capacities for powder metallurgy are generally around 1.8 to 2.7 MN (180 to 270 metric tons), although presses with much higher capacities are used for special applications. Most applications actually require less than 1 MN (90 metric tons). For small tonnage, crank- or eccentric-type mechanical presses are used; for higher capacities, toggle or knuckle-joint presses are employed (see Fig. 14.19b). Hydraulic presses (Fig. 17.13) with capacities as high as 45 MN (4500 metric tons) can be used for large parts. Press selection depends on part size and its configuration, density requirements, and production rate. However, the higher the pressing speed, the greater is the tendency for the press to trap air in the die cavity, and thus prevent proper compaction. 17.3.2 Isostatic Pressing Green compacts may subsequently be subjected to hydrostatic pressure in order to achieve more uniform compaction and density. Typical applications include automotive cylinder liners and high-quality parts, such as turbine shafts, oil pipeline component and pump manifolds, valves, and bearings. In cold isostatic pressing (CIP), the metal powder is placed in a flexible rubber mold (Fig. 17.14), typically made of neoprene rubber, urethane, or polyvinyl chloride (Section 7.9). The assembly is then pressurized hydrostatically in a chamber, usually using water. The most common pressure is 400 MPa, although pressures of up to 1000 MPa may be used. The ranges for CIP and other compacting methods in terms of the size and complexity of a part are shown in Fig. 17.15. ׉ 7cassandra://CkYuH0--RHtT1DuDBAQM6SKl8O67inNUOLQVL9oGAAs`cZ%Eb%׉EX518 Chapter 17 Powder-metal Processes and Equipment Figure 17.13: A 7.3-MN mechanical press for compacting metal powder. Source: Courtesy of Cincinnati Incorporated. In hot isostatic pressing (HIP), the container is typically made of a high-melting-point sheet metal, generally of mild or stainless steel, and the pressurizing medium is high-temperature inert gas or a vitreous (glasslike) fluid (Fig. 17.16). Typical pressures are as high as 100 MPa, although they can be three times higher with temperatures up to 1200◦C. The main advantage of HIP is its ability to produce compacts having almost 100% density, good metallurgical bonding of the particles, and good mechanical properties. The HIP process is used mainly to produce superalloy components for the aircraft and aerospace industries and in military, medical, and chemical applications. It also is used (a) to close internal porosity, especially for parts subjected to fatigue or wear environments; (b) to improve properties in superalloy and titanium-alloy castings for the aerospace industry; and (c) as a final densification step for tungsten-carbide cutting tools and PM tool steels (Chapter 22). Hot isostatic pressing is also applied to ceramics and additive manufactured parts where strength and wear resistance are important. Upper cover Cover Wire Pressure vessel Metal mandrel Fluid Pressure source (a) mesh basket Mold seal plate Rubber mold (bag) Powder Pressure vessel Rubber diaphragm Fluid Pressure source Lower inside cover (b) Figure 17.14: Schematic diagram of cold isostatic pressing. Pressure is applied isostatically inside a highpressure chamber. (a) The wet bag process to form a cup-shaped part; the powder is enclosed in a flexible container around a solid-core mandrel. (b) The dry bag process used to form a PM cylinder. Powder Lower outside cover Pressing rubber mold Forming rubber mold ׉ 7cassandra://fbE2S8h9HoqRr5Fqcxi6t3V4eeY9QgUSpRARyhF5se45`cZ%Eb&cZ%Eb%#בCט#u׉׉ 7cassandra://bPlLS7uw3xPyGtMxpx1Czlmw4ctMcPlNQDkiqK3YQ00A`׉ 7cassandra://RrX6o1nOglyBr-tRHMCgRDY3cPPuwabimBDU1fuFrGUB`s׉ 7cassandra://74Th7wjGi6IXAfLXB56d_uM47SvP1wo8kWa4auz4TYI`׉ 7cassandra://MG0-58o4IPQMd67D3LXFuWuM_k1kuOf_PJYnQ0Oki7YEͺ͠^cZyEbט#u׉׉ 7cassandra://bpbd2yG0QQTBCJiJO0p9aA3jmKjvwf7zDBRHC0lp9b8)b`׉ 7cassandra://Sgs6GfPp6bCfAoLg24m0BjjJJggK6kPO02YGwbrAqroV`s׉ 7cassandra://aMjsKQtFpoXfNEmSiz3Yci9kT7xkztxy0-5V_1FFT1s`׉ 7cassandra://UkrSOZKH24fZnoF7mAj3OKfteoDe6tQqDRLquG0ZVuI|W͠^cZyEb׉EGCompaction of Metal Powders 519 HIP 0.2 0.4 0.6 0 0 1 2 3 4 5 Relative shape complexity Figure 17.15: Capabilities, with respect to part size and shape complexity, available in various PM operations. PF = powder forging. Source: Courtesy of the Metal Powder Industries Federation. The main advantages of hot isostatic pressing over conventional PM are as follow: • Because of the uniformity of pressure from all directions, the absence of die-wall friction, and long processing times at elevated temperature (causing metal powders to creep; see Section 2.8), it produces fully-dense compacts of practically uniform grain structure and density, irrespective of part shape; thus the properties are isotropic. Parts with high length-to-diameter ratios have been produced, with very uniform density, strength, toughness, and good surface details. • HIP is capable of handling much larger parts than those in other compacting processes. On the other hand, HIP limitations are: – Dimensional tolerances are higher than those in other compacting methods. – Equipment costs are higher and production time is longer than those in other processes. • HIP is applicable only to relatively small production quantities, typically less than 10,000 parts per year. Gas inlet End cap Insulation Heating coils Workpiece High-pressure cylinder Part Pressure Temperature Time 6 CIP PF PM PIM End cap 1. Fill can 2. Vacuum bakeout 3. Hot isostatic press 4. Remove can Figure 17.16: Schematic illustration of hot isostatic pressing. The pressure and temperature variation versus time are shown in the diagram. Size (m) ׉ 7cassandra://74Th7wjGi6IXAfLXB56d_uM47SvP1wo8kWa4auz4TYI`cZ%Eb'׉E t520 Chapter 17 Powder-metal Processes and Equipment Figure 17.17: A valve lifter for heavy-duty diesel engines produced from a hot-isostatic-pressed carbide cap on a steel shaft. Source: Courtesy of the Metal Powder Industries Federation. Case Study 17.1 Hot Isostatic Pressing of a Valve Lifter An HIP-clad valve lifter, used in a full range of medium- to heavy-duty truck diesel engines, is shown in Fig. 17.17. The 0.2-kg valve lifter rides on the camshaft and opens and closes the engine valves. Consequently, it is desirable to have a tungsten-carbide (WC) face for wear resistance, and a steel shaft for fatigue resistance. Before the HIP valve lifter was developed, these parts were produced by furnace brazing (Section 32.2), but they resulted in occasional field failures and relatively high scrap rates. Because the required annual production of these parts is over 400,000, high scrap rates are particularly undesirable. The new part consists of a (a) 9% Co-bonded tungsten-carbide face made from pressed and sintered powder, (b) steel sheet-metal cap fitted over the WC disk, (c) copper-alloy foil interlayer, and (d) steel shaft. The steel cap is electron-beam welded to the steel shaft; then the assembly is hot isostatically pressed to provide a very strong bond. HIP takes place at 1010◦C and at a pressure of 100 MPa. The tungsten-carbide surface has a density of 14.52 to 14.72 g/cm3, a hardness of 90.8 ± 5 HRA, and a minimum transverse rupture strength of 2450 MPa. Secondary operations are limited to grinding the face to remove any protruding sheet-metal cap and to expose the wear-resistant tungsten-carbide face. The high reliability of the HIP bond greatly reduced scrap rates to under 0.2%. No field failures have been experienced in over four years of full production. Source: Courtesy of the Metal Powder Industries Federation. 17.3.3 Miscellaneous Compacting and Shaping Processes Powder-injection Molding. Also called metal-injection molding (MIM), in powder-injection molding (PIM), very fine metal powders (<10 μm) are blended with a 25 to 45% polymer or a wax-based binder. The mixture then undergoes a process similar to die casting (Section 11.4.5; see also injection molding ofplastics in Section 19.3), where it is injected into the mold at a temperature of 135◦ to 200◦C. Parts generally have sprues and runners, as with injected molded parts (Fig. 17.18), and hence they are carefully separated before additional processing. The molded green parts are placed in a low-temperature oven to burn off the plastic (debinding), or the binder is removed by solvent extraction. Often, a small amount of binder may be retained to provide ׉ 7cassandra://aMjsKQtFpoXfNEmSiz3Yci9kT7xkztxy0-5V_1FFT1s`cZ%Eb(cZ%Eb'#בCט#u׉׉ 7cassandra://ZIQ2ulMvouYAvQx9tzSgxoFIWx43tw4KLqqspk63svIB`׉ 7cassandra://c-FIlg-vc-G9iIvrnwglbtL1FxpcYVfJYu-x1owwJhsZ`s׉ 7cassandra://RA2JNyhttTdMo2tKDwq8PhL0P6hTVSun0C223h8si_4.`׉ 7cassandra://3dHUM1EVrKXQNyVdipx1O2r8r3Id0VqCUjHCjoTYxi4^ ͠^cZzEbט#u׉׉ 7cassandra://t-gpB6af7Qle_iUl5WG6ujDRWaMvRUKgmQpnHyEbxcMύ`׉ 7cassandra://2IGTLBbAkrdh5NMp46eiyuEH9lnRpIBWsQQV6sdXp1sC`s׉ 7cassandra://NjffHdfUlAK383krtd0upmhrPTNVeeKUaCSu35O6Xlo`׉ 7cassandra://Pjqr1c1xGBW2KEAzykAu-OvUK4LL0vUPhkAa_hZT5Mg`͠^cZ{Eb׉E sCompaction of Metal Powders 521 Figure 17.18: A single shot of metal injection molded components, with sprue, runners, and gates (see also Fig. 19.10). Source: Courtesy HARBEC, Inc. sufficient green strength for transfer of parts to a sintering furnace at temperatures as high as 1375◦C. Subsequent operations, such as hole tapping, metal infiltration, and heat treating, also may be performed as required. Generally, metals suitable for powder-injection molding are those that melt at temperatures above 1000◦C, such as carbon and stainless steels, tool steels, copper, bronze, and titanium. Typical parts made are components for small-caliber gun barrels, scope rings for rifles, door hinges, impellers for sprinkler systems, and surgical knives. The major advantages of powder-injection molding over conventional compaction are: • Complex shapes, with wall thicknesses as small as 5 mm, can be molded then removed easily from the dies. • Mechanical properties are nearly the same as those for wrought parts. • Dimensional tolerances are good. • High production rates can be achieved by using multicavity dies (see Figs. 11.20 and 19.10). • Parts produced by the PIM process compete well against small investment-cast parts and forgings, and complex machined parts. However, the PIM process does not compete well with zinc and aluminum die casting (Section 11.4.5), or with screw machining (Section 23.3.4). The major limitations of PIM are the high cost for small production runs and the need for fine metal powders. An example where the advantages of metal injection molding are apparent is in the production of lightduty gears, such as office equipment, where load and power is low. An inexpensive gear can be produced directly from metal injection molding, instead of first producing a blank, such as from casting or forging, followed by costly machining and finishing operations (Section 24.7). Avoiding the high machining costs thus results in significant savings; however, this approach may not be suitable for more demanding applications, such as automobile transmissions or in gear pumps. Forging. In powder forging (PF), the part produced from compaction and sintering serves as the preform in a hot-forging operation. The forged products are almost fully dense, and have good surface finish, good dimensional tolerances, and uniform and fine grain size. The superior properties obtained make powder forging particularly suitable for highly stressed parts, such as automotive connecting rods and jet-engine components. Rolling. In powder rolling, also called roll compaction, the metal powder is fed directly into the roll gap in a two-high rolling mill (Fig. 17.19), and is compacted into a continuous strip at speeds up to 0.5 m/s. The rolling operation can be carried out at room or elevated temperatures. Sheet metal for electrical and electronic components and for coins can be made by this process. ׉ 7cassandra://RA2JNyhttTdMo2tKDwq8PhL0P6hTVSun0C223h8si_4.`cZ%Eb)׉E$522 Chapter 17 Powder-metal Processes and Equipment Metal powder supply Direction baffles Hopper Powder Green sheet Shaping rolls Hot rolls Coiler Sintering furnace Cooling Figure 17.19: An illustration of powder rolling. Extrusion. Powders can be compacted by extrusion, where the powder is encased in a metal container and hot extruded. After sintering, the parts may be reheated and forged in a closed die (Section 14.3) to their final shape. Superalloy powders, for example, are hot extruded for enhanced properties. Pressureless Compaction. In this operation, the die is gravity filled with metal powder, and the powder is then sintered directly in the die. Because of the resulting low density, pressureless compaction is used principally for porous metal parts such as filters. Spray Deposition. This is a shape-generation process (Fig. 17.20), involving (a) an atomizer, (b) a spray chamber with an inert atmosphere, and (c) a mold for producing preforms. The mold may be made in various shapes, such as billets, tubes, disks, and cylinders. Induction-heated ladle Atomizer (nitrogen gas) Mandrel Tube Deposition chamber Powder recovery Figure 17.20: Spray deposition (Osprey process) in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe. Particle injector (optional) ׉ 7cassandra://NjffHdfUlAK383krtd0upmhrPTNVeeKUaCSu35O6Xlo`cZ%Eb*cZ%Eb)#בCט#u׉׉ 7cassandra://-0eo8VOrviml-jjwqeAXEJwI_OeF4YyoHzKU7bPUHiw`׉ 7cassandra://sHLqKTCZr1BO4AqXDGKK2lNyR0BjFRlLJJkuNFkHAswWP`s׉ 7cassandra://KqZX37v2BddExsPOairs2douY7RidDqJ4td6u2zm-aE`׉ 7cassandra://34lbEQTb3eVj7Iredv9gEqr-3h4NqtudyWAOorJypUw͙a j͠^cZ|Ebט#u׉׉ 7cassandra://3iPq5dlYMPguQ-uVKvRjW33O1mWUqZEbLH_PhQYUW-w`׉ 7cassandra://4Xhd8wlodxomKOm35_YPFDaoyJ0FNx-8fR8s8W_Xue8Z`s׉ 7cassandra://sJ9uzd_XTExAU5qaZHfdnmmDxs8BRY4svuo676ZmkZU#`׉ 7cassandra://u8E3bLsr54OPWQqam1JvvXpOlxB7LZnk6t8ksdi1gCYgT͠^cZ|Eb׉E Compaction of Metal Powders 523 Detonator Top plug (steel) Explosive Powder Steel tube Bottom plug (steel) (a) Top plug Compacted powder Detonation wave Expanding gases Shock wave Bottom plug (b) Figure 17.21: Schematic illustration of explosive compaction. (a) A tube filled with powder is surrounded by explosive media inside a container typically made of cardboard or wood. (b) After detonation, a compression wave follows the detonation wave, resulting in a compacted powder material. Although there are several variations of this process, the best known is the Osprey process, shown in Fig. 17.20. After the metal is atomized, it is deposited onto a cooled preform mold, typically made of copper or ceramic, where it solidifies. The metal particles bond together, developing a density that usually is above 99% of the solid-metal density. Spray-deposited forms may subsequently be subjected to additional shaping and consolidation processes, such as forging, rolling, or extrusion. The grain size of the part made is fine, and its mechanical properties are comparable to those of wrought products made of the same alloy. Dynamic and Explosive Compaction. Some metal powders that are difficult to compact with sufficiently high green strength can be compacted rapidly to near full density, using the setup shown in Fig. 17.21. The explosive drives a mass into green powder at high velocities, generating a shock wave that develops pressures up to 30 GPa. The shock wave traverses across the powder metal part at speeds up to 6 km/s. The powder is often preheated to prevent fracture during compaction. Combustion Synthesis. Combustion synthesis takes advantage of the highly combustible nature of metal powders, by placing a lightly compacted powder into a pressure vessel. An ignition source is then introduced, such as an arc from a tungsten electrode, igniting the powder. The explosion produces a shock wave that travels across the powder, developing heat and pressure that is sufficient for compaction. Pseudo-isostatic Pressing. In pseudo-isostatic pressing, a preform is preheated, surrounded by hot ceramic or graphite granules, and placed in a container. A mechanical press compacts the granules and the preform. Note that the granules are sufficiently large so that they cannot penetrate the pores of the PM part itself. Compaction is uniaxial, but because of the presence of the granules, the loading on the preform is multiaxial. This process has cycle times shorter than HIP, but because the pressure is not strictly hydrostatic, dimensional changes during compaction are not uniform. Powder Bed Processing. Some PM parts can be produced by powder bed processing, a class of additive manufacturing described in detail in Section 20.6. ׉ 7cassandra://KqZX37v2BddExsPOairs2douY7RidDqJ4td6u2zm-aE`cZ%Eb+׉E ]524 Chapter 17 Powder-metal Processes and Equipment 17.3.4 Punch and Die Materials The selection of punch and die materials for powder metallurgy depends on the abrasiveness of the powder metal and the number of parts to be produced. Most common die materials are air- or oil-hardening tool steels, such as D2 or D3, with a hardness range from 60 to 64 HRC (Table 5.8). Because of their higher hardness and wear resistance, tungsten-carbide dies are used for more severe applications. Punches generally are made of similar materials. Close control of die and punch dimensions is essential for die life and proper compaction. Too large a clearance between the punch and the die will allow the metal powder to penetrate the gap, where it will severely interfere with the operation and cause eccentricity in parts made. Diametral clearances generally are less than 25 μm. Die and punch surfaces must be lapped or polished, and in the direction of tool movements in the die, for improved die life and overall performance. 17.4 Sintering As described in Section 17.3, the green compact is brittle and its green strength is low. Sintering is the process whereby green compacts are heated, in a controlled-atmosphere furnace, to a temperature below the melting point of the metal, but sufficiently high to allow bonding (fusion) of the individual particles to impart strength to the part. The nature and strength of the bond between the particles involve the complex mechanisms of diffusion, plastic flow, evaporation of volatile materials in the compact, recrystallization, grain growth, and extent of pore shrinkage. The principal variables in sintering are temperature, time, and furnace atmosphere. Temperatures (Table 17.2) are generally within 70 to 90% of the melting point of the metal or alloy (see Table 3.1). Sintering times (Table 17.2) range from a minimum of about 10 minutes for iron and copper alloys to as much as eight hours for tungsten and tantalum. Continuous-sintering furnaces, used for most production, have three chambers: 1. Burn-off chamber, for volatilizing the lubricants in the green compact, so as to improve bond strength and prevent cracking. 2. High-temperature chamber, for sintering. 3. Cooling chamber. For optimum properties, proper control of the furnace atmosphere is essential for successful sintering. An oxygen-free atmosphere is necessary to control the carburization and decarburization of iron and iron-based compacts, and to prevent oxidation of the powders. A vacuum is generally used for sintering refractory-metal alloys and stainless steels. The gases most commonly used for sintering are hydrogen, dissociated or burned ammonia, partially combusted hydrocarbon gases, and nitrogen. Table 17.2: Sintering Temperature and Time for Various Metals. Material Copper, brass, and bronze Iron and iron–graphite Nickel Stainless steels Alnico alloys (for permanent magnets) Ferrites Tungsten carbide Molybdenum Tungsten Tantalum Temperature (◦C) Time (min) 10–45 760–900 1000–1150 1000–1150 1100–1290 1200–1300 1200–1500 1430–1500 2050 2350 2400 8–45 30–45 30–60 120–150 10–600 20–30 120 480 480 ׉ 7cassandra://sJ9uzd_XTExAU5qaZHfdnmmDxs8BRY4svuo676ZmkZU#`cZ%Eb,cZ%Eb+#בCט#u׉׉ 7cassandra://oHFCF2pXSKrk5zmHxzaSH8SLQx_uhALRNabTWpfHqB8l`׉ 7cassandra://Scpy_4vh0-QDx5wpKm1tUy80eS2d9UKnliFtX-CvGaoS`s׉ 7cassandra://9K-_USj4TZFwF7_6fVNuoPvArBX97Wo-4CEg9Pxys50k`׉ 7cassandra://jpk5EBcr8Zt53UXuTFVqLgyJArfllQlP8geRioWqXVQ e*͠^cZ}Ebט#u׉׉ 7cassandra://kWcm0f1vei7VPp7LkFlAoqohmzSwr0SFVIW-hV4hw54O3`׉ 7cassandra://DeQ-Q8JrBuhPxk9AE9W3X80raKwzqHrQjNuqkkcJUkYo`s׉ 7cassandra://IrT80pDZABEMKKzOUIVgtMQSuPfykyvEOVNszGX_0UI`׉ 7cassandra://_kV5M5YcrCIKJ_422Xn1kz_0bcOqWmuU8ZOnjELv2XQh͠^cZ}Eb׉ESintering 525 1. 1. R 2. Distance between particle centers decreased, particles bonded 3. (a) 3. (b) Figure 17.22: Schematic illustration of two mechanisms for sintering metal powders: (a) solid-state material transport; and (b) vapor-phase material transport. R is the particle radius, r is the neck radius, and ρ is the neck-profile radius. Sintering mechanisms depend on the composition of the metal particles, as well as on processing parameters. The mechanisms are diffusion, vapor-phase transport,and liquid-phase sintering. As the temperature increases, two adjacent powder particles begin to form a bond by a diffusion mechanism (solid-state bonding, Fig. 17.22a). As a result, the strength, density, ductility, and thermal and electrical conductivities of the compact increase. At the same time, the compact shrinks, thus allowances must be made for shrinkage, as are done in casting. A second sintering mechanism is vapor-phase transport (Fig. 17.22b). Because the material is heated to close to its melting temperature, metal atoms are released to the vapor phase from the particles. At convergent geometries (the interface of two particles), the melting temperature is locally higher, and the vapor phase resolidifies; thus, the interface grows and strengthens while each particle shrinks as a whole. If two adjacent particles are of different metals, alloying can take place at the interface of the two particles. If one of the particles has a lower melting point than the other, the particle will melt and will surround the particle that has not melted because of surface tension (Fig. 17.23). An example of this mechanism, known as liquid-phase sintering, is cobalt in tungsten-carbide tools and dies (see Section 22.4), and the parts made are denser and stronger. Base Additive (lower melting temperature) (a) (b) (c) Figure 17.23: Schematic illustration of liquid phase sintering using a mixture of two powders. (a) Green compact of a higher melting point base metal and lower temperature additive; (b) liquid melting, wetting and reprecipitation on surfaces; (c) fully sintered solid material. Neck formation by diffusion 2. r r Particles bonded, no shrinkage (center distances constant) Neck formation by vapor-phase material transport ׉ 7cassandra://9K-_USj4TZFwF7_6fVNuoPvArBX97Wo-4CEg9Pxys50k`cZ%Eb-׉Ex526 Chapter 17 Powder-metal Processes and Equipment In spark sintering, loose metal powders are placed in a graphite mold, then heated by electric current, subjected to a high-energy discharge and compacted, all in one step. Another technique is microwave sintering, which reduces sintering time and thereby prevents grain growth, which can adversely affect strength. Mechanical Properties. Depending on temperature, time, and processing history, different structures and porosities can be obtained in a sintered compact, thus affecting its properties. Porosity cannot be completely eliminated because (a) some voids remain after compaction and (b) gases evolve during sintering. Porosity may consist either of a network of interconnected pores or of closed holes. Generally, if the density of the part is less than 80% of its bulk density, the pores are interconnected. Although porosity reduces the strength of the PM product, it is an important characteristic for making metal filters and bearings, and to also allow for infiltration with liquid lubricants by surface tension. Typical mechanical properties for several sintered PM alloys are given in Table 17.3. The differences in mechanical properties of wrought versus PM metals are given in Table 17.4. To further evaluate the differences between the properties of PM, wrought, and cast metals and alloys, compare these tables with the ones given in Parts I and II. The effects of various manufacturing processes on the mechanical properties of a titanium alloy are shown in Table 17.5. Note that hot isostatic pressed (HIP) titanium has properties that are similar to those for cast and forged titanium. It should be noted, however, that unless they are precision forged, forgings generally require some additional machining or finishing operations whereas a PM component may not. 17.5 Secondary and Finishing Operations In order to further improve the properties of sintered PM products or to impart special characteristics, several additional operations may be carried out following sintering: 1. Coining and sizing are compacting operations, performed in presses under high pressure. The purposes of these operations are to further density and impart better dimensional accuracy to the sintered part and to improve its strength and surface finish. 2. Preformed and sintered alloy-powder compacts subsequently may be cold or hot forged or impact forged to the desired final shapes. The parts made have good surface finish and dimensional tolerances, and uniform and fine grain size. The superior properties obtained make this technology particularly suitable for such applications as highly stressed automotive and jet-engine components. 3. Powder-metal parts also may be subjected to other finishing operations, such as: • Machining, for producing various geometric features by milling, drilling, and tapping. • Grinding, for improving dimensional accuracy and surface finish. • Heat treating, for increasing hardness and strength. 4. The inherent porosity of PM components can be utilized as a design advantage by impregnating them with a fluid. Bearings and bushings that are lubricated internally with up to 30% oil by volume are made by immersing the sintered bearing in heated oil. In service, an interruption in lubricant supply or a temporary high load or speed will cause the bearing to increase in temperature. Since the liquid oil has a higher thermal expansion than the solid bearing, it will exude or percolate from the surface, leading to a self-lubricated condition. 5. Infiltration is a process whereby, for example, a slug of a lower-melting-point metal is placed in contact with the sintered part; the assembly is then heated to a temperature sufficiently high to melt the slug (see Fig. 20.15). The molten metal infiltrates the pores by capillary action, producing a relatively pore-free part having good density and strength. The most common application is the infiltration ׉ 7cassandra://IrT80pDZABEMKKzOUIVgtMQSuPfykyvEOVNszGX_0UI`cZ%Eb.cZ%Eb-#בCט#u׉׉ 7cassandra://wSDl8t2ra34NDhahvDcSbn4AWcIx-mjd_PR6Eii_Jz8`׉ 7cassandra://gOkL1mFmfx55NhBoE4cuazD-XY0md8vXdvO2PUbWx2E^*`s׉ 7cassandra://sqszboKM2dFE8KvNRwBNyNVG45RZeITi_nCa9Vkqxtk,`׉ 7cassandra://B6XiQwns_RhdmvKwAtx_0L-gzl4xU3lcrzefzpWx6l4qL͠^cZ~Ebט#u׉׉ 7cassandra://f_zl1gxskUFf-fXrHgP033EselmC5A-3ulqFiIfbfIA]`׉ 7cassandra://k5X_a_lzooq1gIax9XCD6ba71SRKw1HkqgSX1pyiU3IS`s׉ 7cassandra://z5XZ-nJLOpXX-PDHz4laUfSsqyBDAQD6ONa6ax4S4xY`׉ 7cassandra://48-6Zi7XDoEKzQs8DLjxQoNVaD8qtqU5KR29kudCagcp,h͠^cZEb׉E Secondary and Finishing Operations 527 Subsurface porosity 100 mm Densified surface Figure 17.24: Micrograph of a PM material surface after roll densification. Note the low porosity near the surface, increasing the material’s ability to support contact stresses and resist fatigue. Source: Courtesy of Capstan Atlantic Corp. of iron-based compacts by copper or bronze. The hardness and tensile strength of the part are thus improved and the pores are filled, preventing moisture penetration, which could cause corrosion. Moreover, because some porosity is essential for infiltration, the part may be sintered only partially, resulting in lower thermal warpage. 6. Electroplating (Section 34.9) can be applied to PM parts, but special care is required to ensure that the electrolytic fluid is thoroughly removed since it presents health hazards. Under certain conditions, electroplating also can seal a part and eliminate its permeability. 7. Densification,or roll densification, is similar to roller burnishing (Section 34.2), where a smalldiameter hard roll is pressed against a PM part, resulting in sufficiently high contact pressures to cause plastic deformation of its surface layers. Thus, instead of cold working the part, the effect is to cause an increase in density, or densification, of the part’s surface layers (Fig. 17.24). PM gears and bearing races are generally subjected to roll densification; the surface layer becomes more fatigue resistant and is better able to support higher contact stresses than untreated components. Case Study 17.2 Production of Tungsten Carbide for Tools and Dies Tungsten carbide is an important tool and die material, mainly because of its hardness, strength, and wear resistance over a wide range of temperatures (see Section 22.4.1); it is made by PM techniques. First, powders of tungsten and carbon are blended together in a ball mill or a rotating mixer. The mixture (typically 94% tungsten and 6% carbon, by weight) is heated to approximately 1500◦C in a vacuuminduction furnace; as a result, the tungsten is carburized, forming tungsten carbide in a fine powder form. A binding agent (usually cobalt) is then added to the tungsten carbide (together with an organic fluid, such as hexane), and the mixture is ball milled to produce a uniform and homogeneous mix. The process can take several hours, or even days. The mixture is then dried and consolidated, usually by cold compaction, at pressures in the range of 200 MPa. Finally, the compact is sintered in a hydrogen atmosphere or a vacuum furnace, at a temperature of 1350–1600◦C, depending on its composition. Powders may also be hot pressed at the sintering temperature, using graphite dies. At this temperature, the cobalt is in a liquid phase and acts as a binder for the carbide particles. During sintering, the tungsten carbide undergoes a linear shrinkage of about 16%, corresponding to a volume shrinkage of about 40%; thus, control of size and shape is important for producing tools with accurate dimensions. A combination of other carbides, such as titanium carbide and tantalum carbide, can likewise be produced, using mixtures made by the methods described in this example. ׉ 7cassandra://sqszboKM2dFE8KvNRwBNyNVG45RZeITi_nCa9Vkqxtk,`cZ%Eb/׉E 528 Chapter 17 Powder-metal Processes and Equipment Table 17.3: Mechanical Properties of Selected PM Materials. Ultimate Yield Material Ferrous F-0008-20 F-0008-35 F-0008-55HT strength (MPa) 170 260 — 240 450 tensile Elastic strength modulus (MPa) (GPa) 200 390 450 F-0008-85HT — 660 FC-0008-30 FC-0008-60 FC-0008-95 FN-0205-20 FN-0205-35 170 280 FN-0205-180HT FX-1005-40 FX-1005-110HT Stainless Steels SS-303N1-38 SS-304N1-30 SS-316N1-25 SS-316N2-38 CZ-1000-11 CZP-3002-14 CT-1000-13 Aluminum Alloys Ax 123-T1 Ax 123-T6 Ax 231-T6 Ax 231-T6 Ax 431-T6 Ax 431-T6 Titanium Alloys Ti-6Al-4V (HIP) Superalloys Stellite 19 310 260 230 310 Copper and Copper Alloys CZ-1000-9 70 80 110 110 200 390 200 310 270 440 917 — 340 240 520 720 280 480 1280 530 830 470 300 280 480 120 160 220 150 85 140 115 150 85 155 150 115 170 170 160 160 115 105 105 140 80 100 90 60 Hardness 35 HRB 70 HRB 22 HRC 35 HRC 50 HRB 84 HRB 43 HRC 44 HRB 78 HRB 78 HRB 82 HRB 38 HRC 70 HRB 61 HRB 59 HRB 65 HRB 65 HRH Elongation in 25 mm Density (g/cm3) (%) < 1 1 < 1 < 1 < 1 < 1 < 1 1 5 < 1 4 < 1 5 < 1 < 1 131 9 80 HRH 12 88 HRH 82 HRH 270 — 47 HRB 72 HRB 400 320 470 827 1035 — 220 — 55 HRB 77 HRB — 300 — 55 HRB 80 HRB — — — 17.6 Design Considerations Because of the unique properties of metal powders, their flow characteristics in the die, and the brittleness of green compacts, there are certain design principles that should be followed (Figs. 17.25 through 17.27): 1. The shape of the compact must be kept as simple and uniform as possible. Sharp changes in contour, thin sections, variations in thickness, and high length-to-diameter ratios should be avoided. 2. Provision must be made for ejection of the green compact from the die without damaging it. Holes or recesses should be parallel to the axis of punch travel. Chamfers should be provided to avoid damage to the edges during ejection. — — 3 < 1 1 < 1 5 2 — 49 HRC 16 4 5.8 7.0 6.3 7.1 5.8 7.2 7.1 7.4 7.3 Notes F-008 is often most cost effective. Copper added for strength, hardness, and wear resistance. 6.6 Good heat treated strength, 7.4 impact energy Copper infiltrated steel 7.3 Copper infiltrated steel 6.9 Good machinability 6.4 High corrosion resistance 6.4 Good general-purpose alloy 6.9 7.6 8.1 8.0 General purpose structural parts General purpose structural parts High strength structural parts 7.2 Common self-lubricated bearing material 2.7 2.7 General purpose structural parts 2.7 High wear resistance 2.7 2.8 High strength structural parts 2.8 13 < 1 Most common titanium alloy ׉ 7cassandra://z5XZ-nJLOpXX-PDHz4laUfSsqyBDAQD6ONa6ax4S4xY`cZ%Eb0cZ%Eb/#בCט#u׉׉ 7cassandra://9CQ55MVOrHEQThbT0JFrywFYEj5-YK63DxhgH6H_4QI&|`׉ 7cassandra://PfjvC5VRT8pUHCBO5i0-ckojp5EzkJZ8ps_U0ZwnBs8E`s׉ 7cassandra://qMgeLujL2On56uX_QrYwzu1EUMaufXZxGAKF2CarYw4K`׉ 7cassandra://HyeatG1Z8jI9n6mdDV9gaR7UzSAbfH5mGcb0uwh2kK8gz͠^cZEbט#u׉׉ 7cassandra://ROhefTrFt-bE1oNoF8ZJLdB8Y1Ag7dEoQQ61JS5M05g[`׉ 7cassandra://kmdlYWyKumfeQZNNKQCX4b9GGPPYGj4B0LZragH4LdUZ `s׉ 7cassandra://S4OlUZlPKUAbfqrTpjeXte9sPwJbIQaLjsO7iPcTnv8V`׉ 7cassandra://j5e7-Yn30I2T1pjFqmZRW_FrVj-8JDVekXQtFOXCEE4ͳE 2͠^cZEb׉EDesign Considerations 529 Table 17.4: Comparison of Mechanical Properties of SomeWrought and Equivalent PM Metals (as Sintered). Ultimate Relative densitya Metal Aluminum 2014-T6 6061-T6 Condition Wrought (W) PM W PM Copper, OFHCb W, annealed PM Brass, 260 Steel, 1025 W, annealed PM W, hot rolled PM Stainless steel, 303 W, annealed PM (%) 100 94 100 94 100 89 100 89 100 84 100 82 tensile strengtha (MPa) 480 330 310 250 235 160 300 255 590 235 620 360 processing, such as forging, isostatic pressing, and heat treatments. b OFHC = oxygen-free, high conductivity. 3. In order to increase tool and die life and reduce production costs, PM parts should be made with the widest acceptable dimensional tolerances, consistent with their intended applications. 4. Part walls generally should not be less than 1.5 mm thick; however, with special care, walls as thin as 0.34 mm can be pressed successfully on components as little as 1 mm in length. Walls with length-tothickness ratios greater than 8:1 are difficult to press, and density variations are virtually unavoidable. 5. Steps in parts can be produced if they are simple and their size doesn’t exceed 15% of the overall part length. Larger steps can be pressed, but they require more complex, multiple-motion tooling. 6. Letters and numbers can be pressed if they are oriented perpendicular to the direction of pressing, and these can be raised or recessed. Raised letters are more susceptible to damage in the green stage, and also may prevent stacking during sintering. Table 17.5: Mechanical Property Comparisons for Ti-6AL-4V Titanium Alloy. Ultimate Relative density Process Cast Cast and forged Blended elemental (P+S) Electron-beam melting (%) 100 100 98 Blended elemental (HIP) > 99 Prealloyed (HIP) 100 100 Yield strength (MPa) 840 875 786 805 880 910 tensile strength (MPa) 930 965 875 875 975 970 Elongation (%) 7 14 8 9 14 16 Reduction of area (%) 15 40 14 17 26 — ∗ P+S = pressed and sintered, HIP = hot isostatically pressed. Source: Courtesy of R.M. German and Stratasys, Inc. Elongation in 50 mm (%) 20 2 15 2 50 8 65 26 25 2 50 2 Notes: a The density and strength of PM materials greatly increase with further ׉ 7cassandra://qMgeLujL2On56uX_QrYwzu1EUMaufXZxGAKF2CarYw4K`cZ%Eb1׉E 530 Chapter 17 Powder-metal Processes and Equipment 7. Flanges or overhangs can be produced by providing a step in the die; however, long flanges can be broken during ejection, thus requiring more elaborate tooling. A long flange should incorporate a draft around the flange, a radius at the bottom edge, and a radius at the juncture of the flange and/or component body, in order to reduce stress concentrations and thus the likelihood of fracture. 8. A true radius cannot be pressed into the edge of a PM part because it would require the punch to be feathered (gently tapered) to a zero thickness, as shown in Fig. 17.26c. Chamfers or flats are preferred for pressing, and a 45◦ angle in a 0.25-mm flat is a common design practice. 9. Keys, keyways, and holes used for transmitting torques on gears and pulleys can be formed during powder compaction. Bosses (see Fig. 10.16) also can be produced, provided that proper drafts are provided, and their length is small compared to the overall component dimensions. 10. Notches and grooves can be made if they are oriented perpendicular to the powder pressing direction; circular grooves should not exceed a depth of 20% of the overall component, and rectangular grooves should not exceed 15%. 11. Parts produced by PIM have design constraints similar to those produced by injection molding of polymers (Section 19.3). With PIM, wall thicknesses should be uniform to minimize distortion during sintering. Also, molds should be designed with smooth shape transitions, to prevent powder accumulation and to allow uniform distribution of metal powder. 12. Dimensional tolerances of sintered PM parts are usually on the order of ±0.05 to 0.1 mm. Tolerances an be improved significantly with such subsequent operations as sizing, machining, and grinding. 17.7 Economics of Powder Metallurgy Because powder metallurgy can produce parts at net or near-net shapes, thus eliminating some secondary finishing and assembly operations, it has become increasingly competitive with casting, forging, and machining. On the other hand, the high initial cost of punches, dies, and equipment for PM processing means that production volume must be sufficiently high to warrant this expenditure. Although there are exceptions, the process generally is economical for quantities over 10,000 pieces. Step requires up to 128 taper to assist ejection 0.25–0.50-mm parallel surface to prevent punch jamming 28–38 taper to assist ejection 0.12–0.25-mm parallel surface to prevent powder capture in die Part Upper punch Die Maximum feasible taper is 158 when bottom compaction is employed 0.25–0.50-mm step to prevent powder capture in die Lower punch Figure 17.25: Die geometry and design features for powder-metal compaction. Source: Courtesy of the Metal Powder Industries Federation. ׉ 7cassandra://S4OlUZlPKUAbfqrTpjeXte9sPwJbIQaLjsO7iPcTnv8V`cZ%Eb2cZ%Eb1#בCט#u׉׉ 7cassandra://AlJTs_51lNHW-rIq_iszKvksf_j1uPud3fJcqCUk2ag_`׉ 7cassandra://faaqlJpyR-uY-qWqK6WehkibqBw3tuxOe40T_ubNulII `s׉ 7cassandra://AZcj0ZnVigpr9ua47iMcV83bsntyF16oL_T7MgOXMK8`׉ 7cassandra://QufBryNscXe_uWgmxjUgx4odR8mOMp1pfuxzhB_ZCRUϲ͠^cZEbט#u׉׉ 7cassandra://XYjWiGLISgoxa-UdmfL7wOlHVD12FRBOP7FWuVyyxBw `׉ 7cassandra://93XOztufzjtYXRF9KF2fWJdmZWxaMEVyrbES8itOr8sU`s׉ 7cassandra://IPEkMQMU_xF1lMx7GSxD04OkySdN7xFkBTFjAXPsavwN`׉ 7cassandra://DiFEPb9QjH1uH0-cDCfDy7LED-6caKjs5urprm32jFc͠^cZEb׉EEconomics of Powder Metallurgy 531 As in other processes, the cost of dies and tooling in powder metallurgy depends on part complexity and the method of processing the powders. Tooling costs for processes such as hot isostatic pressing and powder-injection molding are thus higher than more conventional powder processing. Because it is a near-net-shape manufacturing method, the cost of finishing operations in PM is low as compared to other processes. However, if there are certain features to the part, such as threaded holes, undercuts, and transverse cavities and holes, the finishing costs will increase. Consequently, following design guidelines in PM to minimize or avoid such additional operations can be more important in this process than in others. Poor Good Sharp radius Sharp radius Sharp radius Fillet radius Fillet radius Sharp radius Fillet radius Poor Fillet radius Good (a) (b) Upper punch Die Feather edge required on punch Workpiece (c) Acceptable Die Punch Max 30 Workpiece (e) Hole must be drilled (f) Figure 17.26: Examples of PM parts showing poor and good designs. Note that sharp radii and reentry corners should be avoided, and that threads and transverse holes have to be produced separately by additional machining operations. Source: Courtesy of the Metal Powder Industries Federation. Thread must be machined Best 0.25 mm Min. 308–458 (d) Acceptable with additional operations 0.25 mm Min. Flat Must be machined Can be molded ׉ 7cassandra://AZcj0ZnVigpr9ua47iMcV83bsntyF16oL_T7MgOXMK8`cZ%Eb3׉E532 Chapter 17 Powder-metal Processes and Equipment Poor Thin section Good Thicker flange Taper to assist ejection Radius for ease of ejection Radius to reduce likelihood of chipping r 0.2H (max) H 0.15H (max) H (a) (b) Figure 17.27: (a) Design features to use with unsupported flanges. (b) Design features for use with grooves. Source: Courtesy of Metal Powder Industries Federation. Table 17.6: Forged and PM Titanium Parts and Cost Savings. Weight (kg) Part F-14 Fuselage brace F-18 Engine mount support F-18 Arrestor hook support fitting F-14 Nacelle frame Forged billet PM Final part Cost savings (%) 1.1 2.8 7.7 79.4 143 2.5 25 82 0.8 0.5 12.9 24.2 50 20 25 50 Poor Good up to 128 r Equipment costs for conventional PM processing are somewhat similar to those for bulk deformation processing of metals, such as forging; however, the cost increases significantly when using methods such as HIP and PIM. Although the cost of materials has increased significantly (see Table 6.1), it has actually improved the economic viability of PM, since tooling and equipment costs are a smaller fraction of the total cost of production. Labor costs for PM are not as high as those in some other processes, primarily because the individual operations, such as powder blending, compaction, and sintering, are performed on highly automated equipment. The near-net-shape capability of PM significantly reduces or eliminates scrap. Weight comparisons for aircraft components, produced by forging and by PM processes, are shown in Table 17.6. Note that the PM parts are subjected to further machining processes; thus, the final parts weigh less than those made by either of the two processes alone. Case Study 17.3 Powder Metallurgy Parts in a Snowblower Some of the parts in the freewheeling steering system of a commercial snowblower are shown in Fig. 17.28. Among the 16 PM components, the sprocket is the largest, at around 140 mm in diameter. The final assembly incorporates a stamped steel frame, bronze and plastic bearings, and a wroughtsteel axle, to produce a highly functional and low-cost machine. Unique features compatible with PM manufacturing were incorporated into the design of these parts to enhance their functionality. ׉ 7cassandra://IPEkMQMU_xF1lMx7GSxD04OkySdN7xFkBTFjAXPsavwN`cZ%Eb4cZ%Eb3#בCט#u׉׉ 7cassandra://TJYplXPfS4Myo3YdSXxo0khICNz0wMkGLySI-zCPgNoA`׉ 7cassandra://TzElDzxdvfer6B48Rbt9ZI4n8a-oq8wb0MLyo-tQIEEJ`s׉ 7cassandra://VJhWqi8BcQJranb4T16A1uJOWAu0o2heyc7rV1IcRC8`׉ 7cassandra://R3UjEQ67V24UFzrkHlnqH_krze81D99mRKZ6zjJ2tWE[͠^cZEbט#u׉׉ 7cassandra://7Q7dXPLveqMyliQXOtLtvW6I4rePcvEPopIHhOXtb8g:m`׉ 7cassandra://_JYZsdZ5CrG0OXUcYtvDHGIlsidCn8GBU-_s5R6bnKgJ`s׉ 7cassandra://TQuVTZokj8FXtFmhr10N-h3aUbkAODKlYwMdmLB7JH0`׉ 7cassandra://ZO_0o5Jf7uRcXfNtPKqXd-4PX04bxVnMvAf-hd_U3nkJi͠^cZEb׉EEconomics of Powder Metallurgy 533 The PM components in the assembly range from single-level parts, with fixed features on punch faces and core rods, to intricate multilevel parts with complex die geometry, core rods, and transfer punches. These are unique features and they manage the powder for local density control. The clutch pawl, for example, is produced to a net-shape peripheral geometry that is not practical or economical with other manufacturing technologies. The material used is FLC4608-70 steel (a prealloyed powder of iron, with 1.9% Ni, 0.56% Mo, and 0.8% C mixed in with 2% Cu), with a tensile strength of 500 MPa and a density of 6.8 g/cm3. Part numbers are pressed into the face of the components, as a simple means of identifying them. Two of the components are made with especially close tolerances: The pawl latch gear has a 0.15-mm tolerance on the pitch diameter (PD), with 0.11 mm PD to ID run-out and 0.025 mm tolerance on the bore. The 32-tooth sprocket has a thin-walled 57.75 mm ID with a 0.05-mm tolerance. Both the pawl latch gear and the sprocket acquire a density of 6.7 g/cm3 and a tensile strength of 690 MPa. All components shown passed normal life-cycle testing and product-life testing, including shock loading by engaging the drive in reverse, while traveling at maximum forward speed down an incline. Clutch components, which were also subjected to salt-spray corrosion resistance, and proper operation in subzero temperatures, experienced no failures. No machining is required on these parts, as these are sufficiently net-shape components. The only additional operations, prior to final assembly, are vibratory deburring and honing of the 32-tooth sprocket, in order to produce a close-tolerance bore and surface finish. The clutch pawls, produced with sinter-hardened steel, are quenched in an atmosphere so that the porosity present can be filled with a lubricant, to provide lubricity at the interface of mating parts (see also Section 33.6). Source: Courtesy of the Metal Powder Industries Federation and Burgess-Norton Manufacturing Co. Figure 17.28: Powder metallurgy parts in a commercial snowblower. Courtesy of Metal Powder Industries Federation. ׉ 7cassandra://VJhWqi8BcQJranb4T16A1uJOWAu0o2heyc7rV1IcRC8`cZ%Eb5׉E534 Chapter 17 Powder-metal Processes and Equipment Summary • Powder metallurgy is a net-shape or near-net shape forming sequence consisting of metal powder production, blending, compaction in dies, and sintering in order to impart strength, hardness, and toughness. Although the size and the weight of PM products are limited, the process is capable of producing relatively complex parts economically, in net-shape form, to close dimensional tolerances, and from a wide variety of metal and alloy powders. • Secondary and finishing operations may be performed on PM parts to improve their dimensional accuracy, surface finish, mechanical and physical properties, and appearance. These operations include forging, heat treating, machining, grinding, plating, impregnation (as with oil), and infiltration (with lower melting-point metals). • Control of powder shape and quality, process variables, and sintering atmospheres are important considerations to ensure product quality. Density and mechanical and physical properties can be controlled by tooling design and by controlling the compacting pressure. • An important PM process is powder-injection molding, which involves mixing very fine metal powders with a polymer; the viscous mixture is then injected into molds to produce parts. • Sintering is a process whereby a porous metal powder compact or shape is heated to a high temperature in a controlled atmosphere. Sintering fuses the particles, but also leads to shrinkage and potentially warpage. • Design considerations for powder metallurgy include the shape of the part, the ability to eject the green compact from the die, and the dimensional tolerances that are acceptable for the particular application. • The PM process is suitable for medium- to high-volume production runs and for relatively small parts. Key Terms Atomization Blending Carbonyls Cold isostatic pressing Comminution Compaction Diffusion Electrolytic deposition Green compact Green strength Hot isostatic pressing Impregnation Infiltration Injection molding Mechanical alloying Metal injection molding Powder injection molding Powder metallurgy Pressing Pressureless compaction Reduction Screening Shape factor Sintering Spark sintering ׉ 7cassandra://TQuVTZokj8FXtFmhr10N-h3aUbkAODKlYwMdmLB7JH0`cZ%Eb6cZ%Eb5#בCט#u׉׉ 7cassandra://6HRAki_K2ZAOv3PUrTrjLAYlEXDCelFjA3oTQgBpsTs`׉ 7cassandra://fr20Yt7g8pMDXUhNHk3KXa1JeJI2VyAiJJ50e9aM7TQn`s׉ 7cassandra://SBNLeEh7_DNiOaXFp_ygl5NDhQxkBxlNpurUfHhOcas7`׉ 7cassandra://axW6aYU0uBnRYNoXNGSLVjvqmKHTgNai2dUse3q_CcAf_͠^cZEbט#u׉׉ 7cassandra://fkPqHq0NxDW7wQ6YVhlrZyF5uLRH7A5KAH_082lG8Fc`׉ 7cassandra://J55WfE_zXAgw2EPbRns5XEVv5vawfrSqG7274k7t7_wo`s׉ 7cassandra://Th8SPhI4qsefRGmh9eHGv_bzYXrQf2AAFZ6RQcy2pa4`׉ 7cassandra://Y9ZSIq9q8Y9SMPKiH1_sdwVemqx87Gn-gzIAhRPuiNMM[͠^cZEb׉E \Review Questions 535 Bibliography Allen, T., Powder Sampling and Particle Size Determination, Elsevier, 2003. ASM Handbook,Vol.7: Powder Metallurgy, ASM International, 2015. Brewin, P.R., and Coube, O., Modelling of Powder Die Compaction, Elsevier, 2007. Datta, B.K., Powder Metallurgy: An Advanced Technique of Processing Metals, 2nd ed., Prentice-Hall, 2014. El-Eskandarany, M.S., Mechanical Alloying, 2nd ed., William Andrew, 2015. Douvard, D. (ed.), Powder Metallurgy, ISTE Publishing, 2009. Fayed, M., and Otten, L. (eds.), Handbook of Powder Science and Technology, 2nd ed., Chapman & Hall, 1997. German, R.M., Sintering: From Empirical Observations to Scientific Principles, Butterworth-Heinemann, 2014. German, R.M., Powder Metallurgy and Particulate Materials Processing, Metal Powder Industry, 2006. Kang, S.-J., Sintering: Densification, Grain Growth and Microstructure, Butterworth-Heinemann, 2005. Klar, E., and Samal, P.K., Powder Metallurgy Stainless Steels: Processing, Microstructures and Properties, ASM International, 2008. Leander, F., and West, W.G., Fundamentals of Powder Metallurgy, Metal Powder Industries Federation, 2002. Neikov, O.D., et al. (eds.), Handbook of Non-Ferrous Metal Powders: Technologies and Applications, Elsevier, 2009. Powder Metallurgy Design Manual, 2nd ed., Metal Powder Industries Federation, 1995. Selesca, S.M., Selesca, M., and Danninger, S., Machinability of Powder Metallurgy Steels, Cambridge International Science, 2005. Upadhyaya, A., and Upadhyaya, G.S., Powder Metallurgy: Science, Materials, and Technology, Universities Press, 2012. Upadhyaya, G.S., Sintered Metallic and Ceramic Materials: Preparation, Properties and Applications, Wiley, 2000, Review Questions 17.1. Describe briefly the production steps involved in making powder-metallurgy parts. 17.2. Name the various methods of powder production and explain the types of powders produced. 17.3. Explain why metal powders may be blended. 17.4. Describe the methods used in metal powder compaction. 17.5. What is isostatic pressing? How is it different from pseudo-isostatic pressing? 17.6. What hazards are involved in PM processing? Explain their causes. 17.7. Describe what occurs to metal powders during sintering. 17.8. Describe the wet-bag and dry-bag techniques. 17.9. Why might secondary and finishing operations be performed on PM parts? 17.10. Explain the difference between impregnation and infiltration. Give some applications of each. 17.11. What is roll densification? Why is it done? 17.12. What is mechanical alloying? What are its advantages over the conventional alloying of metals? 17.13. What is the osprey process? ׉ 7cassandra://SBNLeEh7_DNiOaXFp_ygl5NDhQxkBxlNpurUfHhOcas7`cZ%Eb7׉E D536 Chapter 17 Powder-metal Processes and Equipment 17.14. What is screening of metal powders? Why is it done? 17.15. Why are protective atmospheres necessary in sintering? What would be the effects on the properties of PM parts if such atmospheres were not used? Qualitative Problems 17.16. Why is there density variation in the compacting of powders? How is it reduced? 17.17. What is the magnitude of the stresses and forces involved in powder compaction? 17.18. List the similarities and differences of forging and compacting metal powders. 17.19. Give some reasons that powder-injection molding is an important process. 17.20. How does the equipment used for powder compaction vary from those used in other metalworking operations in the preceding chapters? 17.21. Explain why the mechanical and physical properties depend on their density. 17.22. What are the effects of the different shapes and sizes of metal particles in PM processing? 17.23. Describe the relative advantages and limitations of cold and hot isostatic pressing. 17.24. How different, if any, are the requirements for punch and die materials in powder metallurgy from those for forging and extrusion operations? Explain. 17.25. The powder metallurgy process can be competitive with processes such as casting and forging. Explain why this is so. 17.26. What are the reasons for the shapes of the curves shown in Fig. 17.11 and for their relative positions on the charts? 17.27. Should green compacts be brought up to the sintering temperature slowly or rapidly? Explain your reasoning. 17.28. Because they undergo special processing, metal powders are more expensive than the same metals in bulk form, especially powders used in powder-injection molding. How is the additional cost justified in processing powder-metallurgy parts? 17.29. In Fig. 17.12c, it can be seen that the pressure is not uniform across the diameter of the compact at a particular distance from the punch. What is the reason for this variation? 17.30. Why do the compacting pressure and the sintering temperature depend on the type of powder metal? 17.31. What will be stronger: a blend of stainless steel and copper powder that is compacted and sintered, or a stainless steel powder that is compacted, sintered, and infiltrated by copper? Explain. 17.32. Name the various methods of powder production and sketch the morphology of powders produced. Quantitative Problems 17.33. Estimate the maximum tonnage required to compact a brass slug 150 mm in diameter. Would the height of the slug make any difference in your answer? Explain your reasoning. 17.34. Refer to Fig. 17.11a. What should be the volume of loose, fine iron powder in order to make a solid cylindrical compact 40 mm in diameter and 20 mm high? 17.35. Determine the shape factors for (a) a cylinder with a dimensional ratio of 1:1:1 and (b) a flake with a ratio of 1:10:10. ׉ 7cassandra://Th8SPhI4qsefRGmh9eHGv_bzYXrQf2AAFZ6RQcy2pa4`cZ%Eb8cZ%Eb7#בCט#u׉׉ 7cassandra://8ep1anzpiJi9FsTFUQPICLqfnvMlvRPlg0VjToaR8LM?`׉ 7cassandra://vnkM5Kivm4TFiN0m25wlTtErSIp0JwBWSBc9g01Qvm0V`s׉ 7cassandra://jsdVbkC1TcKJQypMAOazA0KbLVydIdHcSzjoWXXdHHg `׉ 7cassandra://npZLnhyxZR0g3AiKY74G0pBgexGv-Ui9ViX0BE1XXRA|J͠^cZEbט#u׉׉ 7cassandra://QRPzWXEALbVkyN6rFp642YF9psqhSW1VrdEax5DLiu4$`׉ 7cassandra://Rj8rgAPYGS8ovEazIUgoDY0NJeiBiDflWbfKE6sM0MkF`s׉ 7cassandra://HG_b6GRJaM-vs7kxGVmSQPzyo0DMqlJDvs1b3HzrpB4`׉ 7cassandra://D0e8t9J8JeYchg89AFqDURrthQl0xdgoNL3zgYiDprkW8͠^cZEb׉E BSynthesis, Design, and Projects 537 17.36. Estimate the number of particles in a 400-g sample of iron powder if the particle size is 40 μm. 17.37. Assume that the surface of a copper particle is covered by an oxide layer 0.15 mm in thickness. What is the volume (and the percentage of volume) occupied by this layer if the copper particle itself is 60 μm in diameter? 17.38. A coarse copper powder is compacted in a mechanical press at a pressure of 275 MPa. During sintering, the green part shrinks an additional 5%. What is the final density? 17.39. A gear is to be manufactured from iron powders. It is desired that it have a final density 90% that of cast iron, and it is known that the shrinkage in sintering will be approximately 5%. For a gear that is 60 mm in diameter and has a 25 mm hub, what is the required press force? 17.40. What volume of powder is needed to make the gear in Problem 17.39? 17.41. The axisymmetric part shown in the accompanying figure is to be produced from fine copper powder and is to have a tensile strength of 150 MPa. Determine the compacting pressure and the initial volume of powder needed. Dimensions in mm 25 10 12 20 25 17.42. The part considered in Problem 17.41 is to be compressed in a dual-action press. The density of loose powder is 40%, but it is desired to produce a green compact with 80% of full density. Specify the initial positions of the vertical features in the die. 17.43. Coarse iron powder is compacted into a cylinder with a 25 mm diameter and 40 mm height. The green part has a measured mass of 130 g. Calculate (a) the apparent density; (b) the percentage of the theoretical full density; (c) an estimate of the compacting pressure used. 17.44. The part in Problem 17.43 is to be hot isostatically pressed to full density. If shrinkage is the same in all directions, estimate the final part dimensions. 17.45. Fine iron powder is compressed into a cylinder (d=30 mm, h=20 mm), achieving 70% of theoretical density. It is to be hot forged to a height of 5 mm. What diameter should be planned in order to achieve a 95% final density? 17.46. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 17.47. Prepare an illustration similar to Fig. 13.1, showing the variety of PM manufacturing options. 17.48. Make sketches of PM products in which density variations (see Fig. 17.12) would be desirable. Explain why in terms of the functions of these parts. ׉ 7cassandra://jsdVbkC1TcKJQypMAOazA0KbLVydIdHcSzjoWXXdHHg `cZ%Eb9׉E538 Chapter 17 Powder-metal Processes and Equipment 17.49. Compare the design considerations for PM products with those for (a) casting and (b) forging. Describe your observations. 17.50. Are there applications in which you, as a manufacturing engineer, would not recommend a PM product? Explain. 17.51. Describe in detail other methods of manufacturing the parts shown in Fig. 17.1. 17.52. Using the Internet, locate suppliers of metal powders and compare the cost of the powder with the cost of ingots for five different materials. 17.53. Explain why powder-metal parts are commonly used for machine elements requiring good frictional and wear characteristics and for mass-produced parts. 17.54. It was stated that powder-injection molding competes well with investment casting and small forgings for various materials, but not with zinc and aluminum die castings. Explain why. 17.55. Describe how the information given in Fig. 17.15 would be helpful to you in designing PM parts. 17.56. It was stated that, in the process shown in Fig. 17.20, shapes produced are limited to axisymmetric parts. Do you think it would be possible to produce other shapes as well? Describe how you would modify the design of the setup to produce other shapes, and explain the difficulties that may be encountered. 17.57. It has been noted that PM gears are very common for low-cost office equipment such as the carriage mechanism of inkjet printers. Review the design requirements of these gears and list the advantages of PM manufacturing approaches for these gears. 17.58. The axisymmetric parts shown in the accompanying figure are to be produced through PM. Describe the design changes that you would recommend. 17.59. Assume you are working in technical sales. What applications currently using non-PM parts would you attempt to develop? What would you say to your potential customers during your sales visits? What kind of questions do you think they would ask? ׉ 7cassandra://HG_b6GRJaM-vs7kxGVmSQPzyo0DMqlJDvs1b3HzrpB4`cZ%Eb:cZ%Eb9#בCט#u׉׉ 7cassandra://6hv-Onn6FEY409nwW8HIyJo9-7W-EAhIRVl4EdI2NkQ?`׉ 7cassandra://XgtRJ5YEQwc6vB5uejENZ3IVFy5kgMzWZaxDWeSoyRwI`s׉ 7cassandra://mS-eNHLb0wpcemhjcTltiupqeUBJ3qRLthiyqWnmrDc `׉ 7cassandra://mfROve4slcXTYptHDgnAGDVVlwnRnTbF63fY0PTKH1o>͠^cZEbט#u׉׉ 7cassandra://-VGwRDBOkTxquCbZRlQGxf9ck-M5AABfV79MdxrAbJ04`׉ 7cassandra://g1bfHp34sXJEX87Ov_a_mE5jI5ykMEL3blAob99ydssR6`s׉ 7cassandra://-ucNx3NKPER9F7gyY4KFaLZJ-bunujCEJpZGzi68Rxo`׉ 7cassandra://UMjNGYnYjXhF29dM4h6DRMWEVcvQCOBuadWqrQlmsrYs͠^cZEb׉EChapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment 18.1 Introduction 499 18.2 Shaping Ceramics 499 18.3 Forming and Shaping of Glass 506 18.4 Techniques for Strengthening and Annealing Glass 510 18.5 Design Considerations for Ceramics and Glasses 512 18.6 Processing of Superconductors 513 Example: 18.1 Dimensional Changes During the Shaping of Ceramic Components 505 Case Study: 18.1 Production of High-temperature Superconducting Tapes 513 • This chapter presents the manufacturing processes for ceramics, glass, and superconductors. • It first describes the preparation of ceramic powders, followed by operations that produce discrete parts through the basic processes of casting, pressing, extrusion, and molding. • Drying and firing processes, followed by finishing operations for ceramics, are also discussed. • Glass manufacture involves production of continuous shapes, such as plate, tube, and bars, through drawing, rolling, or floating methods; the operations for discrete products typically consist of molding, blowing, or pressing. • The chapter ends with the processing of superconductors, which are produced mainly through the oxide-powder-in-tube process. Typical products made: Ceramics: electrical insulators, rotors for gas turbines, lightweight components for high-speed machines, ball and roller bearings, seals, furnace components, ovenware, and tiles. Glass: glazing, laminated glass, bulletproof glass, bulbs, lenses, bottles, glass fibers, rods, and tubing. Superconductors: MRI magnets. Alternative processes: Casting, forging, powder injection molding, blow molding, injection molding, additive manufacturing. 539 ׉ 7cassandra://mS-eNHLb0wpcemhjcTltiupqeUBJ3qRLthiyqWnmrDc `cZ%Eb;׉Eu540 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment 18.1 Introduction The properties and applications of ceramics and glasses are described in Chapter 8. These materials have important characteristics, such as high-temperature strength and hardness, low electrical and thermal conductivity, chemical inertness, and resistance to wear and corrosion. The wide range of applications for these materials include electrical insulators, ball bearings, cutting tools, floor tiles, and dishes. The processing methods employed for ceramics (Fig. 18.1) consist of (a) crushing the raw materials, (b) mixing/blending and shaping them by various means, (c) drying and firing, and (d) finishing operations, as needed, to impart the required dimensional tolerances and surface finish. For glasses, the processes involve (a) mixing and melting the raw materials in a furnace and (b) shaping them in molds using various techniques, depending on the shape and size of the part. Discrete products, such as bottles, and continuous products, such as flat glass, rods, tubing, and fibers, can be produced. Glasses can be strengthened by thermal or chemical means, as well as by laminating them with polymer sheets, as is done for automobile windshields and bulletproof glass. 18.2 Shaping Ceramics Several techniques are available for processing ceramics into useful products (Table 18.1), depending on the type of ceramic involved and their shapes. Production of some ceramic parts, such as pottery, ovenware, and floor tiles, generally does not involve the same level of control of materials and processes as do high-tech parts made of such structural ceramics as silicon nitride, aluminum oxide, and silicon carbide. Generally, the procedure involves the following steps (Fig. 18.2): 1. Crushing or otherwise processing the raw materials into very fine particles 2. Mixing them with additives to impart certain specific characteristics, and water for formability (a) (b) Figure 18.1: (a) Examples of typical glass parts. (b) Examples of ceramic parts. Source: (a) Courtesy of Commercial Optical Manufacturing, Inc. (b) Courtesy of Shutterstock/Xiao Zhou. ׉ 7cassandra://-ucNx3NKPER9F7gyY4KFaLZJ-bunujCEJpZGzi68Rxo`cZ%Eb<cZ%Eb;#בCט#u׉׉ 7cassandra://5RxKuKLH4HJtNhTJ0fZhBecuYMq61gMZCC3uiYdfR68V`׉ 7cassandra://mw3NBFOc18Vbp1WQh1xJRdEgXTqyjZ4884ImbAxxd1AE`s׉ 7cassandra://esf8fQtcceiG8YRYCUk6iH_7HC-JhcMaDSc54MUX7aA`׉ 7cassandra://SiGPxNLUGLogWsO7n1rLgCJzkwvMGIxuOcRDhsbHVYA{͠^cZEbט#u׉׉ 7cassandra://wgBU45l3hzDeKzrVYLsymX1sNCJsqkjdrRzubyCur8U`׉ 7cassandra://yJtGX-Hb_IBe_B8sGAez6TBQbdPyeH51OJkPG7E1STQ\`s׉ 7cassandra://xjDkOpLu2GNi3Q_jXLjZDWJJt5OE--v49b-5ckmzlHYb`׉ 7cassandra://8E-5LoP-Y_uMnawAdyvYOjoJ9wy5beT-bzMOambIIw0͞͠^cZEb ׉EShaping Ceramics 541 Process Slip casting Extrusion Dry pressing Wet pressing Hot pressing Isostatic pressing Jiggering Table 18.1: General Characteristics of Ceramics Processing. Advantages Limitations Large parts, complex shapes, low equipment cost Hollow shapes and small diameters, high production rate Close tolerances, high production rates (with automation) Complex shapes, high production rate Strong, high-density parts Uniform density distribution High production rate with automation, low tooling cost Injection molding Complex shapes, high production rate Low production rate, limited dimensional accuracy Parts have constant cross section, limited thickness Density variation in parts with high lengthto-diameter ratios, dies require abrasive-wear resistance, equipment can be costly Limited part size and dimensional accuracy, tooling costs can be high Protective atmospheres required, die life can be short Equipment can be costly Limited to axisymmetric parts, limited dimensional accuracy Tooling can be costly 3. Shaping, drying, and firing the material 4. Finishing, such as by machining, grinding, and glazing, which requires an additional firing step. The first step in processing ceramics is crushing, also called comminution or milling, of the raw materials. Crushing is generally done in a ball mill (see Fig. 17.6b), either dry or wet. Wet crushing is more effective, because it keeps the particles together and it also prevents the fine particles from contaminating the environment. The particles may then be sized by passing them through a sieve, followed by filtering and washing. The ground particles are then mixed with additives, the functions of which are one or more of the following: • Binder, for holding ceramic particles together. • Lubricant, to reduce internal friction between particles during molding and also to help remove the part from the mold. • Wetting agent, to improve mixing. Crushing Milling Raw materials Additives: binders, lubricants, wetting agents, plasticizers, deflocculent Forming, Shaping Slip casting Extrusion Pressing Injection molding Drying Green machining Firing, Sintering Finishing Machining Grinding Lapping Figure 18.2: Processing steps involved in making ceramic parts. ׉ 7cassandra://esf8fQtcceiG8YRYCUk6iH_7HC-JhcMaDSc54MUX7aA`cZ%Eb=׉E #542 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment • Plasticizer, to make the mix easier to shape. • Deflocculent, to make the ceramic–water suspension more uniform, by changing the electrical charges on the particles of clay, so that the particles repel rather than attract each other. Typical deflocculents are Na2CO3 and Na2SiO3, in amounts of less than 1%. The blended powder is then mixed with water to produce a pourable or formable mixture. The three basic shaping processes for ceramics are casting, plastic forming, and pressing. The parts made also may be subjected to additional processing, such as machining and grinding, for better control of their dimensions and surface finish. 18.2.1 Casting The most common casting process is slip casting, also called drain casting, as illustrated in Fig. 18.3. A slip consists of ceramic particles suspended in a liquid, generally water. The slip is poured into a porous mold and may consist of several components, as also done in other shaping processes. The slip must have sufficient fluidity and low viscosity for it to flow easily into the mold, much like the importance of fluidity of molten metals in casting operations, as described in Section 10.3. Pouring the slip must be done in a manner to avoid air entrapment, which can be significant during casting. After the mold has absorbed some of the water from the outer layers of the suspension, it is inverted and the remaining suspension is poured out. The product is now a hollow object, as in the slush casting of metals, described in Section 11.4.3. The top of the part is then trimmed (note the trimming knife in Fig. 18.3d), the mold is opened, and the part is removed. Large and complex parts, such as plumbing ware or art objects, can be made by slip casting. Although mold and equipment costs are low, dimensional control is poor and the production rate is low. In some applications, components of the product, such as handles for cups and pitchers, are made separately and then joined, using the slip as an adhesive. For solid-ceramic parts, the slip is supplied continuously into the mold to replenish the absorbed water, as otherwise the part will shrink. At this stage, the part is described as either a soft solid or semirigid. The higher the concentration of solids in the slip, the less water has to be removed. The part removed from the mold is referred to as a green part, as in powder metallurgy. Mold cavity Slip Slurry Plug (a) (b) (c) (d) Parting line (e) Figure 18.3: Sequence of operations in slip casting a ceramic part. (a) Mold is assembled and plug attached; some plugs incorporate draining features; (b) slurry, mixed from ceramic particles, binder and water, is poured into the mold; (c) the mold is inverted and the slurry is poured from the mold, leaving a thin coating over the mold cavity; (d) after an initial drying period, the slip is removed from the mold, and features such as parting lines and sprue lips are removed; (e) the slip is ready to be dried and fired in an oven, to develop strength and hardness. Trimming knife ׉ 7cassandra://xjDkOpLu2GNi3Q_jXLjZDWJJt5OE--v49b-5ckmzlHYb`cZ%Eb>cZ%Eb=#בCט#u׉׉ 7cassandra://Ulb6Oc69uXqvzzI-ymINup69idruvJw3UowciIjPmsMފ`׉ 7cassandra://7LlO_-OIdpuq4ICfHMZtut1eGp8Y5Z-uZIawubipu6wL`s׉ 7cassandra://t7MVIW8gMXarTYglwWCOa8fy_4MmK3YOVbwSjsOBdLg9`׉ 7cassandra://vtO5iO00NBmUs0qYo7yFWIE16r_ZBGz7S22SKtg52os<:\͠^cZEbט#u׉׉ 7cassandra://D02WXT6_oQpj8CvZCg6hiCYNyQK17JVSWDK8wXrgIb0r`׉ 7cassandra://c9OQUqMajyIQEnYiASYJVmABfZgxjSkb4Hvw2nOaJZA\`s׉ 7cassandra://AVRDWYkfM-ZZbI0_osxnwdcw4Vokjxfl_ETGMxTqN9g4`׉ 7cassandra://WXg54JIiy98dL6SXaCn86icL40EjufEhfqG5GDxr-04fX͠^cZEb׉EShaping Ceramics 543 Air (filtered) in Ceramic tape on carrier tape Drying chamber Exhaust out Slurry chamber and doctor blade Ceramic slurry supply tube Take-up spool Controller for take-up spool Carrier film Doctor blade Ceramic film Carrier film Figure 18.4: Production of ceramic sheets through the doctor-blade process. While the parts are still green, they may be machined to produce certain features or for better dimensional accuracy. Because of the delicate nature of the green compacts, however, machining is usually done manually using simple tools. For example, the flashing in a slip casting may be removed gently with a fine wire brush or any holes can be drilled in the mold. Doctor-blade and Other Processes. Thin sheets of ceramics, less than 1.5 mm thick, can be made by the doctor-blade process (Fig. 18.4). The slip is cast over a moving plastic belt, while its thickness is being controlled by a blade. Ceramic sheets also may be produced by such method as (a) rolling the slip between pairs of rolls and (b) casting the slip over a paper tape, which subsequently burns off during firing. 18.2.2 Plastic Forming Plastic forming, also called soft, wet,or hydroplastic forming, can be carried out by several methods, such as extrusion, injection molding, or jiggering (Fig. 18.5). Plastic forming tends to orient the layered structure of the clay along the direction of material flow, and thus tends to cause anisotropic behavior of the material, both in subsequent processing and in the final properties of the ceramic product. In extrusion, the clay mixture, containing 20% to 30% water, is forced through a die opening (see, for example, Fig. 19.3); the cross section of the extruded product is thus constant. There are limitations to wall thickness for hollow extrusions, because of the risk of fracture during firing (Section 18.2.4). Production rates are high and tooling costs are low. The green extrusion may be subjected to additional shaping operations. ׉ 7cassandra://t7MVIW8gMXarTYglwWCOa8fy_4MmK3YOVbwSjsOBdLg9`cZ%Eb?׉E 3544 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Water To vacuum Deairing chamber Clay slug Bat former Jigger tool Formed ware Extruder (a) Mold return (b) Figure 18.5: (a) Extruding and (b) jiggering operations. 18.2.3 Pressing Dry Pressing. This is a technique similar to powder-metal compaction, as described in Section 17.3. Dry pressing is used for relatively simple shapes, such as whiteware, refractories for furnaces, and abrasive products. The moisture content of the mixture is generally below 4%, although it may be as high as 12%. Organic and inorganic binders (such as stearic acid, wax, starch, and polyvinyl alcohol) are usually added to the mixture; these additives give strength and also act as lubricants to aide in compaction. Dry pressing has the same high production rates and close control of dimensional accuracy as does compaction in powder metallurgy. The pressing pressure ranges from 35 to 200 MPa. Density can vary significantly in dry-pressed ceramics, as in PM compaction (see Fig. 17.12), because of friction among the particles and at the mold walls. Density variations cause warping during firing, which is particularly severe for parts having high lengthto-diameter ratios, the recommended maximum ratio being 2:1. Several methods may be used to minimize density variations, including (a) proper design of tooling, (b) vibratory pressing and impact forming, particularly for nuclear-reactor fuel elements, and (c) isostatic pressing. Modern presses for dry pressing are highly automated. The dies, usually made of carbides or hardened steel, must have high wear resistance to withstand the abrasive ceramic particles. Wet Pressing. In wet pressing, the part is formed in a mold while under high pressure, in a hydraulic or mechanical press. Moisture content usually ranges from 10 to 15%. Production rates are high; however, (a) part size is limited, (b) dimensional control is difficult because of shrinkage during drying, and (c) tooling costs can be high. Wet pressing is generally used for making parts with intricate shapes, such as filters and electronic packaging. Isostatic Pressing. This process is used for ceramics in order to obtain a uniform density distribution throughout the part during compaction (see Section 17.3.2). The white insulators for automotive spark-plugs, for example, are made by this method and at room temperature. Silicon-nitride vanes for high-temperature applications (see Fig. 8.1) are made by hot isostatic pressing. Hot isostatic pressing (Section 17.3.2) also may be used, particularly to improve shape accuracy and the quality of high-technology ceramics, such as silicon carbide and silicon nitride. Glass-encapsulated HIP processing has been shown to be effective for this purpose. Hot isostatic pressing is usually necessary for fatigue or wear applications. ׉ 7cassandra://AVRDWYkfM-ZZbI0_osxnwdcw4Vokjxfl_ETGMxTqN9g4`cZ%Eb@cZ%Eb?#בCט#u׉׉ 7cassandra://pQ7lNzk5lLP-_rJIC7XZswpIKEMYm29Y0OjEN3uENdIn`׉ 7cassandra://H3IqrhigK24wcswJEWNtuM4INuQzxw8T5uejyuX_5oE_j`s׉ 7cassandra://MyKcH5vop6Jf_cuJvVx3lbydmf9pZFr7rb8EFU-cqWE`׉ 7cassandra://ms3TZhUQ1PxjueqUTIIgpuQHErZp5YYz1DQtjUP9jTw{`͠^cZEbט#u׉׉ 7cassandra://n7QMwI0q-Qeeg_MmFe1nTsgETaclOipe76aHc5-yedU`׉ 7cassandra://qJgqkQJcjBrOed20lTq0quIc9dVyaYuHYVpa6CpZcvgZ`s׉ 7cassandra://z8SJq-miRGeT7Gl52iAwCWVBfW4PBfMABAWH8NuEMaY `׉ 7cassandra://Vd-xIyNnA35IzfjQVy5_LtsgChu7spKbJ-lmwhgqOg8k͠^cZEb׉E ]Shaping Ceramics 545 Jiggering. As an example of jiggering, consider ceramic dinner plates that are made by a series of steps (Fig. 18.5). First, clay slugs are extruded and formed into a bat over a plaster mold; they are then jiggered on a rotating mold. Jiggering is a motion in which a clay bat is formed by means of templates or rollers; the part is then dried and fired. This process is confined to axisymmetric parts, and has limited dimensional accuracy. Injection Molding. Injection molding is used extensively for precision forming of ceramics in demanding applications such as for rocket-engine components. The raw material is first mixed with a binder, such as a thermoplastic polymer (polypropylene, low-density polyethylene, or ethylene vinyl acetate) or wax, and injection molded. The binder is usually removed by pyrolysis (inducing chemical changes by heat), and the part is then fired. The injection-molding process can produce thin sections, typically less than 10 to 15 mm thick, from most engineering ceramics such as alumina, zirconia, silicon nitride, silicon carbide, and sialon (see Chapter 8). Thicker sections require careful control of the materials used and of the processing parameters, in order to avoid such defects as internal voids and cracks. Hot Pressing. In this process, also called pressure sintering, pressure and heat are applied simultaneously, thereby reducing porosity in the part and making it denser and stronger. Graphite is commonly used as a punch and die material, and protective atmospheres usually are employed during the pressing step. 18.2.4 Drying and Firing The next step in ceramic processing is to dry and fire the part to give it the proper strength and hardness. Drying is a critical stage to reduce the tendency for the part to warp or to crack from variations in its moisture content and thickness. Control of atmospheric humidity and ambient temperature during drying is important in order to reduce warping and cracking. Loss of moisture during drying causes shrinkage of the part by as much as 20% from the original, moist size (Fig. 18.6). In a humid environment, the evaporation rate is low, and thus the moisture gradient across the thickness of the part is lower than that in a dry environment. The low moisture gradient prevents a large, uneven gradient in shrinkage from the surface to the interior, reducing the tendency for excessive warping or cracking. A ceramic part that has been shaped by any of the methods described thus far is in the green state. It can be machined in order to bring it closer to a near-net shape. Although the green part should be handled carefully, machining it is not particularly difficult, because of the relative softness of the materials. Firing, also called sintering, involves heating the part to an elevated temperature in a controlled environment; although some shrinkage occurs during firing, the ceramic part becomes stronger and harder. The Interparticle water Pore water (a) (b) Clay particles (c) Figure 18.6: Shrinkage of wet clay caused by the removal of water during drying. Shrinkage may be as much as 20% by volume. Source: After F.H. Norton. Dry ׉ 7cassandra://MyKcH5vop6Jf_cuJvVx3lbydmf9pZFr7rb8EFU-cqWE`cZ%EbA׉E X546 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment improvement in mechanical properties is due to (a) the development of strong bonds among the complex oxide particles in the ceramic body and (b) reduced porosity. Microwave sintering is also possible, and it can be significantly faster and less expensive than conventional sintering for larger production runs. Nanophase ceramics, described in Section 8.2.5, can be sintered at lower temperatures than those for conventional ceramics. They are easier to fabricate because they can be (a) compacted at room temperature to high densities, (b) hot pressed to attain theoretical density, and (c) formed into net-shaped parts without using any binders or sintering aids. 18.2.5 Finishing Operations Because firing causes dimensional changes, additional operations may be performed to (a) give the ceramic part its final shape, (b) remove any surface flaws, and (c) improve surface finish and dimensional accuracy. Although ceramics typically are hard and brittle, major advances have been made in producing machinable ceramics and grindable ceramics, thus enabling the production of ceramic components with high dimensional accuracy and good surface finish. An example is silicon carbide, which can be machined into final shapes from sintered blanks. The finishing processes employed can be one or more of the following operations, described in detail in various sections in Part IV: 1. Grinding, using a diamond wheel 2. Lapping and honing 3. Ultrasonic machining 4. Drilling, using a diamond-coated drill 5. Electrical-discharge machining 6. Laser-beam machining 7. Abrasive water-jet cutting 8. Tumbling, to remove sharp edges and grinding marks. Process selection is an important consideration because of the brittle nature of most ceramics and the additional costs involved in using some of these processes. The effect of the finishing operation on the final properties of the product also must be considered. For example, because of notch sensitivity (Section 2.9), the finer the finish of the part, the higher are its strength and load-carrying capacity, particularly its fatigue strength. Ceramic parts also can undergo static fatigue, as described for glass in Section 18.5. Glazing. To improve their appearance and strength and to make them impermeable, ceramic products often are coated with a glaze or enamel (Section 34.12), which forms a glassy coating after firing. Example 18.1 Dimensional Changes During the Shaping of Ceramic Components Given: A solid, cylindrical ceramic part is to be made, with a final length of L=20 mm. For this material, it has been established that linear shrinkages during drying and firing are 7% and 6%, respectively, based on the dried dimension, Ld. Find: Calculate (a) the initial length, Lo, of the part and (b) the dried porosity, Pd, if the porosity of the fired part, Pf,is3%. ׉ 7cassandra://z8SJq-miRGeT7Gl52iAwCWVBfW4PBfMABAWH8NuEMaY `cZ%EbBcZ%EbA#בCט#u׉׉ 7cassandra://KpSdgcjbAtIynxF40dSdwtSYzgFdhwQdPrBbiE5Z-uI`׉ 7cassandra://IHrZBn8L_IWaX6VdVbqBvNJ-ZWM5tQwe7aM8BRnZ4u8<:`s׉ 7cassandra://w6CI8Kom8yUvwPZlhSdPwlkySLkVkNC5A_3835hN6UEJ`׉ 7cassandra://oKGHlaMClTiFoXlNA3iwvPZx8eHlrGGqrXECzGXqvsAV͠^cZEbט#u׉׉ 7cassandra://ZT8qsoy5BQRbnp7G9cceuJmCtU11FFYbN7fViDXiTTc`׉ 7cassandra://fPHwQ2tleZzyvHEUduGoOksBNat7XsycrK3EExq3LQkN`s׉ 7cassandra://WbcABSgKZhQr3DFdkpwOnfHzN7JMfxlbRQVbK3M-HcQ`׉ 7cassandra://G624PVTC_uDs8TJIVBXzCricUWl7dvCAK3C2evChckE#|͠^cZEb׉EForming and Shaping of Glass 547 Solution: a) On the basis of the information given and noting that firing is preceded by drying, (Ld − L) Ld or L =(1 − 0.06)Ld. Hence, Ld = and Lo(1 + 0.07)Ld =(1.07)(21.28) = 22.77 mm. b) Since the final porosity is 3%, the actual volume, Va, of the solid material in the part is Va =(1 − 0.03)Vf =0.97Vf, where Vf is the volume of the part after firing. Because the linear shrinkage during firing is 6%, the dried volume, Vd of the part can be determined as Vd = Hence, Va Vd = 0.97 1.2 , or 81% Therefore, the porosity, Pd, of the dried part is 19%. (1 − 0.06)3 =1.2Vf. Vf 20 0.94 =21.28 mm =0.06, 18.3 Forming and Shaping of Glass Glass is processed basically by melting and shaping it, either in molds or by blowing. The shapes produced include flat sheets and plates, rods, tubing, glass fibers, and discrete products, such as bottles, lenses, automobile headlights, and cookware. Glass products may be as thick as those for large telescope mirrors, and as thin as those for holiday tree ornaments. The strength of glass can be improved by thermal and chemical treatments, inducing compressive surface residual stresses, or by laminating it with a thin sheet of tough plastic. Glass products generally can be categorized as follows: 1. Flat sheets or plates, ranging in thickness from about 0.8 to 10 mm, and used as window glass, glass doors, and tabletops. 2. Rods and tubing, used for neon lights, decorative artifacts, and for processing and handling chemicals. 3. Discrete products, such as bottles, vases, and eyeglasses. 4. Glass fibers, as reinforcements in composite materials (Section 9.2.1) and for use in fiber optics. ׉ 7cassandra://w6CI8Kom8yUvwPZlhSdPwlkySLkVkNC5A_3835hN6UEJ`cZ%EbC׉Ea548 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Molten tin Controlled atmosphere Furnace Rollers Furnace Float bath Lehr Figure 18.7: The float method of forming sheet glass. All forming and shaping processes begin with molten glass, at a temperature typically in the range of 1000◦ to 1200◦C, and has the appearance of a red-hot, viscous liquid. 18.3.1 Flat-sheet and Plate Glass Flat-sheet glass can be made by any of the following three methods from the molten state, with glass supplied from a melting furnace or tank: 1. In the float method (Fig. 18.7), molten glass from the furnace is fed into a long bath in which the glass, under a controlled atmosphere and at a temperature of 1150◦C, floats over a bath of molten tin. The glass, at a temperature of about 650◦C, then moves over rollers into another chamber (lehr) where it solidifies. Float glass has smooth (fire-polished) surfaces, thus further finishing operations, such as grinding or polishing, are not necessary; the width can be as much as 4 m. Both thin and plate glass are made by this process. 2. The drawing process for making flat glass sheets or plates involves passing the molten glass through a pair of rolls (Fig. 18.8a). The solidifying glass is squeezed between these two rolls, forming it into a flat sheet; it then moves forward over a set of smaller rolls. 3. In the rolling process (Fig. 18.8b), the molten glass is squeezed between powered rollers, thereby forming a sheet, with a surface that is somewhat rough. The surfaces of the glass may also be embossed with a pattern, using textured roller surfaces, thus the glass surface becomes a replica of the roll surface. 18.3.2 Tubing and Rods Glass tubing is basically produced by the process shown in Fig. 18.9. Molten glass is wrapped around a rotating (cylindrical or cone-shaped) hollow mandrel, and is then drawn out by a set of rolls. Air is blown Glass sheet Turning roll Forming roll Water-cooled shield Molten glass Burner (a) (b) Figure 18.8: (a) Drawing process for drawing sheet glass from a molten bath. (b) Rolling process. Molten glass Glass sheet Rollers ׉ 7cassandra://WbcABSgKZhQr3DFdkpwOnfHzN7JMfxlbRQVbK3M-HcQ`cZ%EbDcZ%EbC#בCט#u׉׉ 7cassandra://wDC0FGbW6v91oSL_DeMEDolyl4XCDIx8hMcWqThtoto`׉ 7cassandra://06lOBPDavYohnQnwwJHg8XQG-SdjeSXhO-EpGXh_PBM^`s׉ 7cassandra://rgbPKctTGxoU6NVZEm7LrEwGzPVUOELDlBJHU73UWB4`׉ 7cassandra://IMe7AQRlVSzOhSPDnNkwpXJ9yko_SyorPlInxl88I04͸͠^cZEbט#u׉׉ 7cassandra://Gi1dsZ_KcsJk5xHX0FxUg1gHZQretsvRBrZwn9ObMZs7`׉ 7cassandra://QZqCdVfZlRkz0rzMjAdmAacRjenEs6JFUnn_s0eW0-cHJ`s׉ 7cassandra://-7I2BtSADpVefxUYU6dNtNP6pWyGafOk3gfe-r5pERAg`׉ 7cassandra://6r25dxDwVTNNdP1tobm7tmtzeYs5SBcVjmLqIEAAGoA͠^cZEb ׉E Forming and Shaping of Glass 549 Air Molten glass Tube Mandrel Rollers Figure 18.9: Manufacturing process for glass tubing. Air is blown through the mandrel to keep the tube from collapsing. Glass tubes for fluorescent bulbs are made by this method. through the mandrel to prevent the glass tube from collapsing. The machines may be horizontal, vertical, or slanted downward. This method is also used in making glass tubes for fluorescent bulbs. An alternative method for making glass tubes is by extruding a strip of glass (with a thin rectangular cross section), which is then wrapped obliquely (at an angle) around a rotating mandrel. The molten glass strips bond together along their edges, forming a continuous tube; it is then drawn off the mandrel in a continuous manner. Glass rods are extruded or drawn directly from a molten bath, without the need for internal pressurization. 18.3.3 Discrete Glass Products Blowing. Hollow and thin-walled glass items, such as bottles, vases, and flasks, are made by blowing,aprocess that is similar to blow molding of thermoplastics (Section 19.4). The steps involved in the production of an ordinary glass bottle by the blowing process are shown in Fig. 18.10. Blown air expands a hollow gob of heated glass against the inner walls of the mold. The mold surfaces are usually coated with a parting agent, such as oil or emulsion, to prevent the glass from sticking to the mold surfaces. Blowing may be followed by a second blowing operation to finalize the product shape, called the blow and blow process. The surface finish of glass parts made by blowing is acceptable for most applications, such as bottles and jars. It is difficult to precisely control the wall thickness of the product, because of the lack of an inner mold, but the process is economical for high-rate production. Pressing. In the pressing method, a gob of molten glass is placed into a mold, and is pressed into a confined die cavity with the use of a plunger; the process is thus similar to closed-die forging (Section 14.3). The mold may be made in one piece, such as that shown in Fig. 18.11, or it may be a split mold (Fig. 18.12). After pressing, the solidifying glass acquires the shape of the mold-plunger cavity. Because of the confined environment, the product has a better dimensional accuracy than can be obtained with blowing. Pressing in one-piece molds cannot be used for (a) shapes of parts from which the plunger cannot be retracted or (b) thin-walled items, because of high forces needed to produce the walls, and distortion upon part removal. For example, split molds are used for bottles, whereas pressing can be combined with blowing for thin-walled items, in a process known as press and blow. The pressed part is subjected to air pressure (hence the term blow), which further expands the molten glass into the mold. Centrifugal Casting. Also known as spinning or rotocasting (Fig. 18.13), this process is similar to that used for casting metals (see Section 11.4.6), whereby the centrifugal force pushes the molten glass against the mold walls, where it begins to solidify. Typical products made are large lenses for research telescopes and architectural shapes. ׉ 7cassandra://rgbPKctTGxoU6NVZEm7LrEwGzPVUOELDlBJHU73UWB4`cZ%EbE׉E550 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Gob Blow head Air Blank mold Neck ring Tip Air 1. Gob falling into blank mold 2. Gob in blank mold 3. Blowdown in blank mold 4. Blowback in blank mold Baffle Air Tongs Blow mold Parison 5. Blank mold reversed 6. Parison hanging on neck ring, reheated during transfer 7. Parison in blow mold 8. Bottle blown, cooling 9. Finished bottle removed by tongs Figure 18.10: Steps in manufacturing an ordinary glass bottle. Source: After F.H. Norton. Sagging. Shallow dish-shaped or lightly embossed glass parts can be made by the sagging process. A sheet of hot glass is placed over a mold and heated, whereby the glass sags by its own weight and takes the shape of the mold. The process is similar to the thermoforming of thermoplastics (Section 19.6), but no pressure or vacuum is involved. Typical parts made are dishes, sunglass lenses, mirrors for telescopes, and lighting panels. Glass Ceramics Manufacture. Glass ceramics (trade names: Pyroceram, Corningware) contain large proportions of several oxides, as noted in Section 8.5. Their manufacture involves a combination of the methods used for ceramics and glasses. Glass ceramics are shaped into discrete products, such as dishes and baking pans, and then heat treated, whereby glass becomes devitrified (recrystallized). 1. Empty mold 2. Loaded mold 3. Glass pressed 4. Finished part Figure 18.11: Manufacturing a glass item by pressing molten glass into a mold. Source: Courtesy of Corning Glass Works. ׉ 7cassandra://-7I2BtSADpVefxUYU6dNtNP6pWyGafOk3gfe-r5pERAg`cZ%EbFcZ%EbE#בCט#u׉׉ 7cassandra://hjs-PrpUenFGzaKnn-EFrMiDrqQaRB_4hU7EoJO8BmY`׉ 7cassandra://kf_qHbA3m6Uzeden5yrjuUkeZUw_wWTiuh-SF99bKZcS`s׉ 7cassandra://n_ICg4cfHUp10KhrCtSXs0C_yLXR0-BsN4yslas_1-U`׉ 7cassandra://i8s8t_S3SeMWorOBAMnah9IXcEP87Rr8aA2Mz2gShwU"͠^cZEb"ט#u׉׉ 7cassandra://0rfBv-mF3nn5oGl0SH7IPxUJ9eQUc68fp2hmm1cIkAk`׉ 7cassandra://K94Dt-yndjrxjS0q6ZTOgEmZY4VTQJ0ZB7i63O0T-Tc]`s׉ 7cassandra://mceX_IYRAQiJYMdMS-0zDGgw5vI01nURvB0sLiBGXms`׉ 7cassandra://eouD1w9wHoUw357uhp0fRXhW0awyn85hsWoSrbA64h8͉͠^cZEb#׉E Techniques for Strengthening and Annealing Glass 551 Punch 1. Empty mold 2. Loaded mold 3. Glass pressed 4. Finished product Figure 18.12: Pressing molten glass into a split mold. Source: After E.B. Shand. 18.3.4 Glass Fibers Continuous glass fibers are drawn through multiple orifices (200 to 400 holes) in heated platinum plates, at speeds as high as 500 m/s. Fibers as small as 2 µm in diameter can be produced by this method. To protect their surfaces, fibers are subsequently coated with chemicals, known as sizing, which are mainly silane compounds in water, but other sizing blends are also used. Short or chopped fibers are produced by subjecting long fibers to compressed air or steam as they leave the orifice, being broken into very short pieces. Glass wool, which consists of short glass fibers, are used as a thermal insulating material and for acoustic insulation. They are made by a centrifugal spraying process, in which molten glass is ejected (spun)froma rotating head. The diameter of the fibers typically ranges from 20 to 30 µm. 18.4 Techniques for Strengthening and Annealing Glass Glass can be strengthened by several processes, and discrete glass products may be subjected to annealing and to other finishing operations to impart desired properties and surface characteristics. Thermal Tempering of Glass. In this process, also called physical tempering or chill tempering, the surfaces of hot glass are rapidly chilled by a blast of air (Fig. 18.14). As a result, the surfaces shrink and, at first, tensile stresses develop on the surfaces. The bulk of the glass then begins to cool, and because it contracts, the already solidified surfaces of the glass also are forced to contract. Consequently, compressive residual stresses develop on the surfaces, while the interior develops tensile stresses (see also Section 2.11). Compressive surface stresses improve the strength of the glass in the same way that they do in metals and other materials (see Section 2.11). The higher the coefficient of thermal expansion of the glass and the lower its thermal conductivity, the higher will be the level of residual stresses developed, and hence, the stronger the glass becomes. Thermal Mold (steel) Molten glass Figure 18.13: Centrifugal casting of glass. Source: Courtesy of Corning Glass Works. ׉ 7cassandra://n_ICg4cfHUp10KhrCtSXs0C_yLXR0-BsN4yslas_1-U`cZ%EbG׉E j552 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Step 1 Hot glass, no stresses Step 2 Thickness Surface cools rapidly, surface contracts, center adjusts, only minor stresses Compression Center cools, center contracts, surface is compressed, center in tension Tension Step 3 Residual stresses Figure 18.14: (a) Stages involved in inducing compressive surface residual stresses for improved strength. (b) Residual stresses in a tempered glass plate. Source: Courtesy of Corning Glass Works. tempering takes a relatively short time (minutes), and can be applied to most glasses. Because of the high amount of energy stored in residual stresses, tempered glass shatters into numerous pieces when broken. The broken pieces are not as sharp and as hazardous as those from a broken ordinary window glass, which has a sharp jagged fracture path. Chemical Tempering. In this process, the glass is heated in a bath of molten KNO3,K2SO4,orNaNO3, depending on its type. Ion exchanges then take place, with larger atoms replacing the smaller atoms on the surface of the glass; as a result, residual compressive stresses develop on the surface. Chemical tempering may be done at various temperatures. At low temperatures, part distortion is minimal, thus complex part shapes can be tempered. At elevated temperatures, there may be some distortion of the part, but the product can then be used at higher temperatures without loss of strength. The time required for chemical tempering is much longer than that for thermal tempering. Laminated Glass. Laminated glass consists of two or more pieces of flat glass with a thin sheet of tough plastic between each layer. As a result, when laminated glass cracks, the pieces are held together by the plastic sheet and it becomes far less hazardous. This is a phenomenon commonly observed in shattered automobile windshields. Flat glass for glazing windows and doors can be strengthened with wire netting with a hexagonal mesh, such as chicken wire, which is embedded in the glass during its production. When a hard object strikes its surface, the glass breaks but the pieces will be held together because of the embedded wire. This type of glass will have both toughness and flexibility (see also Section 2.2.4). Bulletproof Glass. Laminated glass has significant ballistic impact resistance, and it can prevent the full penetration of solid objects because of the presence of a tough thermoplastic polymer film in between the two layers of glass (see Section 7.3). Bulletproofglass, used in some automobiles, armored bank vehicles, and buildings, is a more challenging design, because of the very high speed and energy level of the bullet and the small size and the shape of the bullet tip. Although there are several designs, bulletproof glass basically consists of glass plates laminated with a polymer sheet. The capacity of a bulletproof glass to stop a bullet depends on (a) the type and thickness of the glass; (b) the size, shape, weight, and speed of the bullet; and (c) the properties and thickness of the polymer sheet. Polycarbonate is commonly used because of its high toughness and flexibility. Laminated ׉ 7cassandra://mceX_IYRAQiJYMdMS-0zDGgw5vI01nURvB0sLiBGXms`cZ%EbHcZ%EbG#בCט#u׉׉ 7cassandra://zRfzqqAIbSCyW2VwZK0eFpUh6kmpJCRPbcKcGRZNkdk``׉ 7cassandra://Dm68aqyzONxlxPcFPT5_0tpjTlip3Lwk7_3Xn2JFsE0i`s׉ 7cassandra://ShZHUoJ1xO5bH6keLu84WjCC-AF4khxX-m04Pvo-g1I`׉ 7cassandra://1WfkjUNHSEnBCwpjKSmz7ZZH5nzgeYw5z1E2aiN_YLQ_"͠^cZEb%ט#u׉׉ 7cassandra://qbpFeeIf94w53pZRXWFlmV6-xUSZ0-i5iCZr5LJKNBob`׉ 7cassandra://Ul2P3ULp-W1zNDc2SV1WurGoju8X8eUU5it5k9ZsCtQj`s׉ 7cassandra://vEHzG5nudChHMGITM2f4EL_7MEonAXQ9f1CuWPyRzRY`׉ 7cassandra://SAXLeaofyouCZpYdx56IabBfKTutsi5s8C8U8RG0bGYgX͠^cZEb&׉EDesign Considerations for Ceramics and Glasses 553 with thick glass, it can stop a bullet, although the glass itself develops a circular shattered region. In order to maintain the transparency of the bulletproof glass and minimize its distortion, the index of refraction of the glass and the polymer must be nearly identical. 18.4.1 Finishing Operations As in metal products, residual stresses can develop in glass products if they are not cooled at a sufficiently low rate. In order to ensure that the product is free of these stresses, it is annealed, by a process similar to the stress-relief annealing of metals (Section 4.11). The glass is first heated to a certain temperature, and then cooled slowly. Depending on the size, thickness, and type of the glass, annealing times may range from a few minutes to as long as 10 months, as in the case of a 600-mm mirror for a telescope in an observatory. In addition to annealing, glass products may be subjected to further operations, such as cutting, drilling, grinding, and polishing. Sharp edges and corners can be made smooth by (a) grinding, as can be seen in glass tops for desks and shelves, or (b) holding a torch against the periphery (fire polishing), which rounds the edges by localized softening of the glass and by surface tension. In all finishing operations on glass, as well as other brittle materials, care should be exercised to ensure that there is no surface damage, especially the presence of stress raisers such as rough surface finish and scratches. Because of its notch sensitivity, even just a single scratch on glass can cause premature failure, especially if the scratch is in a direction where the tensile stresses are a maximum. 18.5 Design Considerations for Ceramics and Glasses Ceramic and glass products require careful selection of composition, processing methods, finishing operations, and methods of assembly with other components. With such properties as poor tensile strength, sensitivity to internal and external defects, low impact toughness, and static fatigue, the consequences of part failure are always a significant factor in designing ceramic and glass products. On the other hand, these limitations must be balanced against such desirable and important material characteristics as hardness, scratch resistance, compressive strength at room and elevated temperatures, and a wide range of diverse physical properties. As noted in Section 8.3.1, ceramics and glasses undergo a phenomenon called static fatigue, whereby after a period of time they can suddenly and without any warning fracture under a static load. Although this phenomenon does not occur in a vacuum or in dry air, provisions must be made to prevent such failure. A general rule is that, in order for a glass item to withstand a certain load for 1000 hours or longer, the maximum stress that can be applied to it is about one-third of the maximum stress that it can withstand during the first second of loading. The control of processing parameters and of the quality and level of impurities in the raw materials are also important. Dimensional changes, warping, the possibility of cracking during processing, and service life are significant factors in selecting methods for shaping glass and ceramics. When a part made of such a material is a component of a larger assembly, its compatibility with other components is an important consideration. Particularly significant are the type of external forces and thermal expansion, such as in seals and windows with metal frames. Recall that Table 3.1 displayed a wide range of thermal expansion coefficients for various metallic and nonmetallic materials. Thus, when a plate glass fits too tightly within a metal window frame, factors such as temperature variations within the glass (sun shining on only a portion of a window) can cause thermal stresses so high that they may lead to cracking; this is a phenomenon often observed in some tall buildings. A common solution is placing rubber seals along the glass and the window frame, to allow for dimensional changes. ׉ 7cassandra://ShZHUoJ1xO5bH6keLu84WjCC-AF4khxX-m04Pvo-g1I`cZ%EbI׉Ei554 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment 18.6 Processing of Superconductors Although superconductors (Section 3.7) have major energy-saving potential in the generation, storage, and distribution of electrical power, their processing into useful shapes and sizes for practical applications has presented significant difficulties. The following are two basic types of superconductors: 1. Metals, called low-temperature superconductors (LTSC), include combinations of niobium, tin, and titanium. For example, niobium–tin alloys, cooled by liquid helium, constitute the superconducting magnet used in most magnetic resonance imaging (MRI) scanners for medical imaging. 2. Ceramics, called high-temperature superconductors (HTSC), include various copper oxides. Here, “high” means closer to ambient temperature, although the commercially important HTSCs maintain superconductivity above the boiling point of liquid nitrogen (−196◦C). Ceramic superconducting materials are available in powder form. The difficulty in manufacturing them is their (a) inherent brittleness and (b) anisotropy, making it difficult to align the grains in the proper direction to achieve high efficiency. The smaller the grain size, the more difficult it is to align the grains. The basic manufacturing process for superconductors consists of the following steps: 1. Preparing the powders, blending, and grinding them in a ball mill (see Fig. 17.6b) to a grain size of 0.5 to 10 µm. 2. Forming the powder into the desired shape. 3. Heat treating the product to enhance properties. The most common forming process is the oxide-powder-in-tube (OPIT) method. The powder is first packed into silver tubes (silver has the highest electrical conductivity of any metal; see Table 3.1), and sealed at both ends. The tubes are then shaped, by such processes as swaging, drawing, extrusion, isostatic pressing, and rolling. The final product may be wire, tape, coil, or in bulk form. Other methods of processing superconductors are (a) coating silver wire with superconducting material, (b) depositing superconductor films by laser ablation (removing material), (c) forming by the doctor-blade process (Section 18.2.1), (d) explosive cladding, and (e) chemical spraying. The shaped part subsequently may be heat treated to improve the grain alignment of the superconducting powder. Case Study 18.1 Production of High-temperature Superconducting Tapes Two bismuth-based oxides are preferred as superconducting ceramic materials for various commercial and military applications, such as electrical propulsion for ships and submarines, shallow-water and ground minesweeping systems, transmission cable generators, and superconducting magnetic energy storage (SMES). Different processing methods have been explored to produce wires and multifilament tapes. The powder-in-tube process (Fig. 18.15) has been used successfully to fabricate long lengths of bismuth-based wires and tapes, with specific desirable properties. The approach uses the following steps: 1. First, a composite billet is produced, using a silver casing and ceramic powder. The casing is made of an annealed high-purity silver, filled with the bismuth-ceramic powder in an inert atmosphere. A steel ram is used to compact the casing in several increments, up to a 30% relative density. In order to minimize density gradients, such as those shown in Fig. 17.12, about 1 g of powder is added to the billet for each stroke of the ram. Each billet is weighed and measured to verify the initial packing density. The billet ends are then sealed with a silver alloy, to avoid contamination during subsequent processing. ׉ 7cassandra://vEHzG5nudChHMGITM2f4EL_7MEonAXQ9f1CuWPyRzRY`cZ%EbJcZ%EbI#בCט#u׉׉ 7cassandra://Kqjcn9L5pYDTP74dbfnDm6vuSEX58XdjXHBGy4ApWkE0`׉ 7cassandra://kwnwvVdztlINncTsZoafDD_TVw_7U-TzSUvhGjoGrrIX`s׉ 7cassandra://_GPxkn0TV9sG2elkB41gsrbnOaPmlCPOO9q9v9CodJ4l`׉ 7cassandra://NLh5vEzexksx_H5YcriPsXl0TVGNOsD-5OZmkcqE3d4Û6d͠^cZEb(ט#u׉׉ 7cassandra://nXRSMvdy5Fm0LJNlY3Kvr4AePJlqWP_mPKSwUSWN6gw`׉ 7cassandra://Flr9GiLCxyqZYvpsNF-1_yjOlMVLAoNV2q-gwXDd6V8W`s׉ 7cassandra://Xfhb0sbIotRAPvZOZXQrkm4OKmjh172SwxJMVMqAY0w`׉ 7cassandra://UaiTxCFo7G5eXTxOgASpQyMCaP4Qw3cKOlhdVHY2y-YX͠^cZEb)׉E Summary 555 2. The billet is then extruded and drawn, to reduce its diameter and increase the powder density. Billets are drawn on a draw bench (see Fig. 15.24) down to a wire with a final diameter of 1.63 mm. It takes 12 passes, with a 20% reduction per pass, to perform this task. The dies have a semicone angle of 8◦, and the drawing speed is approximately 1.4 m/min. A semisoluble oil or zinc-stearate spray is used as a lubricant. 3. Following the drawing process, the wire is transformed progressively into tape in a single-stand rolling mill in two-high and four-high configurations. For the four-high case, the diameter of the backup rolls (which are the work rolls for the two-high configuration) is 213 mm, and the diameter of the work rolls is 63.5 mm. The final tape dimensions are 100 to 200 µm in thickness and 2 to 3 mm in width, with a ceramic core ranging from 40 to 80 µm in thickness and 1.0 to 1.5 mm in width. Source: Courtesy of S. Vaze and M. Pradheeradhi, Concurrent Technologies Corporation. Hopper Die Bi-2212 powder High-purity silver tube 1. Fill 2. Pack Cylindrical rolls Wire 3. Extrude and draw Strip 4. Roll Figure 18.15: Schematic illustration of the powder-in-tube process. Summary • Ceramic products are shaped by various casting, plastic forming, or pressing techniques; the parts are then dried and fired to impart strength and hardness. Finishing operations, such as machining and grinding, may be performed to give the part its final shape and dimensional accuracy, or to subject it to surface treatments. Because of their brittleness, ceramics are processed with due consideration of distortion and cracking. The control of raw-material quality and processing parameters also are important. • Glass products are made by several shaping processes, similar to those used for ceramics and plastics. They are available in a wide variety of forms, compositions, and mechanical, physical, and optical properties. Their strength can be improved by thermal and chemical treatments. • Continuous methods of glass processing are drawing, rolling, and floating. Discrete glass products can be manufactured by blowing, pressing, centrifugal casting, or sagging. The parts subsequently may be annealed to relieve residual stresses. • Design considerations for ceramics and glasses are guided by such factors as their low tensile strength and toughness, and their sensitivity to external and internal defects. Warping and cracking during production are also important considerations. • Manufacturing superconductors into useful consumer and industrial products can be challenging because of the anisotropy and brittleness of the materials involved. Although new processes are being developed, the basic process consists of packing the powder into a silver tube and forming it into desired shapes. ׉ 7cassandra://_GPxkn0TV9sG2elkB41gsrbnOaPmlCPOO9q9v9CodJ4l`cZ%EbK׉E556 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Key Terms Binder Blow and blow Blowing Bulletproof glass Centrifugal casting Chemical tempering Deflocculent Doctor-blade process Drawing Fire polishing Firing Float glass Gob High-temperature superconductors Hot pressing Injection molding Bibliography Advanced Ceramic Technologies & Products, Springer, 2012. Bansal, N.P., and Boccaccini, A.R., (eds.), Ceramics and Composites: Processing Methods, Wiley-American Ceramic Society, 2012. Bansal, N.P., and Lamon, J., Ceramic Matrix Composites, Wiley, 2014. Basu, B., and Balani, K., Advanced Structural Ceramics, Wiley-American Ceramic Society, 2011. Bengisu, M., Engineering Ceramics, Springer, 2010. Bourhis, E.L., Glass: Mechanics and Technology, 2nd ed., Wiley, 2014. Carter, C.B., and Norton, W.G., Ceramic Materials: Science and Engineering, 2nd ed., Springer, 2013. Holand, W., and Beall, G.H., Glass Ceramic Technology, 2nd ed., Wiley-American Ceramic Society, 2012. Marchi, J., Biocompatible Glasses, Springer, 2016. Rahaman, M.N., Ceramics Processing, 2nd ed., CRC Press, 2017. Richerson, D.W., Modern Ceramic Engineering: Properties, Processing, and Use in Design, 3rd ed., CRC Press, 2005. Riley, F.L., Structural Ceramics: Fundamentals and Case Studies, Cambridge, 2009. Shackelford, G.F., and Doremus, R.H., Ceramic and Glass Materials: Structure, Properties and Processing, Springer, 2008. Shelby, J.E., Introduction to Glass Science and Technology, 2nd ed., Royal Society of Chemistry, 2005. Review Questions 18.1. Outline the steps involved in processing (a) ceramics and (b) glasses. 18.2. List and describe the functions of additives in ceramics. 18.3. Describe the doctor-blade process. Jiggering Laminated glass Low-temperature superconductors Microwave sintering Oxide-powder-in-tube process Plastic forming Plasticizer Press and blow Pressing Sagging Slip Slip casting Static fatigue Tempered glass Thermal tempering Wetting agent ׉ 7cassandra://Xfhb0sbIotRAPvZOZXQrkm4OKmjh172SwxJMVMqAY0w`cZ%EbLcZ%EbK#בCט#u׉׉ 7cassandra://1t_aj6s05nmumvZz-AoOVhTHjvpARaABjAYavlC43rUl`׉ 7cassandra://vNf1ItdBl3x6zulQgkk5ivI_d9icw8GPZjUkYa_eu3w^@`s׉ 7cassandra://VKScpkY8ExePAx9R1VLgqcqujYohYWghI9sjTxxSadAT`׉ 7cassandra://nGEorAE3vjzA_pPGDsIsSIYseaOs-BHabdBBrLSuPLY?͠^cZEb+ט#u׉׉ 7cassandra://wJFwFu2pIu9Irs0ZbxGatWJ0lVDTf2rTcwR4jFYev2sJ`׉ 7cassandra://3uD-iV2VOvElxfmo-ZPTy2KhzOHD4VTQK1XP0TQYiMM``s׉ 7cassandra://luAIwZ9JQlPQknl5YtalYd4rvHjNLBwb56CXigT1cGE@`׉ 7cassandra://YALCUQxYk1BWw0WjLo2wR0GETJLGnPFIjl84m-c9cAIW͠^cZEb,׉E rQualitative Problems 557 18.4. Explain the advantages of isostatic pressing. 18.5. What is jiggering? What shapes does it produce? 18.6. Name the parameters that are important in drying ceramic products. 18.7. What types of finishing operations are used on ceramics? On glass? Why? 18.8. Describe the methods by which sheet glass is made. 18.9. What is float glass? 18.10. What is a gob? 18.11. How is glass tubing produced? 18.12. What is the difference between physical and chemical tempering of glass? 18.13. What is the structure of laminated glass? Bulletproof glass? 18.14. How are glass fibers made? What are their sizes? 18.15. Describe the processes of chemical and thermal tempering of glass. 18.16. What is the doctor-blade process? 18.17. Is diamond a ceramic? Explain. Qualitative Problems 18.18. Inspect various products; noting their shape, color, and transparency, identify those that are made of (a) ceramic, (b) glass, and (c) glass ceramics. 18.19. Describe the differences and similarities in processing metal powders vs. ceramics. 18.20. Which property of glasses allows them to be expanded to large dimensions by blowing? Can metals undergo such behavior? Explain. 18.21. Explain why ceramic parts may distort or warp during drying. What precautions should be taken to avoid this situation? 18.22. What properties should plastic sheets have to be used in laminated glass? Why? 18.23. It is stated that the higher the coefficient of thermal expansion of a glass and the lower its thermal conductivity, the higher the level of the residual stresses developed. Explain why. 18.24. Are any of the processes used for making discrete glass products similar to ones described in preceding chapters? Describe them. 18.25. Injection molding is a process that is used for powder metals, polymers, and ceramics. Explain why is this so. 18.26. Explain the phenomenon of static fatigue and how it affects the service life of a ceramic or glass component. 18.27. Describe and explain the differences in the manner in which each of the following would fracture when struck with a heavy piece of rock: (a) ordinary window glass, (b) tempered glass, and (c) laminated glass. 18.28. Is there any flash that develops in slip casting? How would you propose to remove such flash? 18.29. Explain the difficulties involved in making large ceramic components. What recommendations would you make to improve the process? ׉ 7cassandra://VKScpkY8ExePAx9R1VLgqcqujYohYWghI9sjTxxSadAT`cZ%EbM׉E [558 Chapter 18 Ceramics, Glasses, and Superconductors: Processing and Equipment Quantitative Problems 18.30. Using Example 18.1, calculate (a) the porosity of the dried part if the porosity of the fired part is to be 8% and (b) the initial length, Lo of the part if the linear shrinkages during drying and firing are 7% and 5%, respectively. 18.31. What would be the answers to Problem 18.30 if the quantities given were halved? 18.32. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 18.33. List some similarities and differences between the processes described in this chapter and those in (a) Part II on metal casting and (b) Part III on forming and shaping. 18.34. Consider some ceramic products with which you are familiar, and outline a sequence of processes that you think were used to manufacture them. 18.35. Make a survey of the technical literature, and describe the differences, if any, between the quality of glass fibers made for use in reinforced plastics and those made for use in fiber-optic communications. Comment on your observations. 18.36. How different, if any, are the design considerations for ceramics from those for other materials? Explain. 18.37. Visit a ceramics/pottery shop, and investigate the different techniques used for coloring and decorating a ceramic part. What are the methods of applying a metallic finish to the part? 18.38. Give examples of designs and applications in which static fatigue should be taken into account. 18.39. Construct a table that describes the approach for manufacturing plate from (a) metals; (b) thermoplastics; (c) ceramics; (d) powder metal; (e) glass. Include descriptions of process capabilities and shortcomings in your descriptions. 18.40. Pyrex cookware displays a unique phenomenon: it functions well for a large number of cycles and then shatters into many pieces. Investigate this phenomenon, list the probable causes, and discuss the manufacturing considerations that may alleviate or contribute to such failures. 18.41. It has been noted that the strength of brittle materials such as ceramics and glasses are very sensitive to surface defects such as scratches (notch sensitivity). Obtain some pieces of these materials, make scratches on them, and test them by carefully clamping in a vise and bending them. Comment on your observations. 18.42. Describe your thoughts on the processes that can be used to make (a) small ceramic statues, (b) whiteware for bathrooms, (c) common brick, (d) floor tile. 18.43. Perform a literature search, and make a list of automotive parts or components that are made of ceramics. Explain why they are made of ceramics. 18.44. Describe your thoughts on the processes that can be used to make (a) a small ceramic ball, (b) a small statue, (b) whiteware for bathrooms, (c) common brick, and (d) floor tile. 18.45. Describe any special design considerations in products that use ceramics with a near-zero coefficient of thermal expansion. ׉ 7cassandra://luAIwZ9JQlPQknl5YtalYd4rvHjNLBwb56CXigT1cGE@`cZ%EbNcZ%EbM#בCט#u׉׉ 7cassandra://93oGT0XJzRqucZ92koCjqgCXktb2S88Iw9il4F9tJqQv`׉ 7cassandra://mIv5DTzwb6At4t2qL3LUv8UPSKyFR95LkJYj1nQzwvQ5`s׉ 7cassandra://rk7r9BDsvLLb__kCBhw1VWVH-IkDfYZdsJ6EbSW5H24`׉ 7cassandra://jeoSyFir9NKNh2fRWRpLNeBvfuEeI45Q468xW6NVAeo'͠^cZEb.ט#u׉׉ 7cassandra://PviIXQUpk8v4itjcyIldQz3XbXiicQEbi-OeoGtfEsI`׉ 7cassandra://Zsko2ofSpO61F1EF5yWGY3PxRD2X3YX-WEIzRj1PfBoby`s׉ 7cassandra://AX99UUUhVIeer0xIFsWSqHuPDzLwCsN7Oww1TgzfV_I`׉ 7cassandra://Zg4bczqRvNuWRZZ4zwKxdxA-QJF85MKVKma4geHPucER͠^cZEb/׉EkChapter 19 Plastics and Composite Materials: Forming and Shaping 19.1 Introduction 560 19.2 Extrusion 562 19.3 Injection Molding 569 19.4 Blow Molding 574 19.5 Rotational Molding 574 19.6 Thermoforming 576 19.7 Compression Molding 577 19.8 Transfer Molding 578 19.9 Casting 579 19.10 Foam Molding 580 19.11 Cold Forming and Solid-phase Forming 581 19.12 Processing Elastomers 581 19.13 Processing Polymer-matrix Composites 582 19.14 Processing Metal-matrix and Ceramic-matrix Composites 590 19.15 Design Considerations 592 19.16 Economics of Processing Plastics and Composite Materials 594 Examples: 19.1 Blown Film 566 19.2 Force Required in Injection Molding 573 Case Studies: 19.1 Polymer Automotive-body Panels Shaped by Various Processes 588 19.2 Manufacturing of Head Protector R Tennis Racquets 588 19.3 Metal-matrix Composite Brake Rotors and Cylinder Liners 592 559 ׉ 7cassandra://rk7r9BDsvLLb__kCBhw1VWVH-IkDfYZdsJ6EbSW5H24`cZ%EbO׉E 560 Chapter 19 Plastics and Composite Materials: Forming and Shaping • This chapter describes the manufacturing processes for producing polymers and composite materials. • Extrusion is widely used to make rods, tubing, and also pellets as a base stock for producing plastic parts, sheet, and film. • The chapter describes several molding operations for producing discrete parts, including injectionand reaction-injection molding, transfer molding, rotational molding, and compression molding. • The processes associated with shaping composite materials are then described, including compression and vacuum molding, contact molding, pultrusion, and filament winding. • The chapter ends with a description of the characteristics of the machinery used, mold design principles, and economic considerations in polymer processing. Typical parts made: Extensive variety of consumer and industrial products with a range of colors and characteristics. Alternative processes: Casting, forming, additive manufacturing, powder metallurgy, and machining. 19.1 Introduction Processing of plastics and elastomers involves operations similar to those used for forming and shaping of metals described in preceding chapters. Processing of rubbers and elastomers began in the 1800s, with the discovery of vulcanization by C. Goodyear in 1839. Plastics began to be developed in the 1920s, and rapid progress in the 1940s and onward led to important advances in polymeric materials, design, and manufacturing to make numerous products in large quantities and low cost. In the 1970s, reinforced plastics began to be introduced, leading the way for rapid progress in the use of composite materials with unique properties and applications, as well as the associated challenges in producing them. As noted in Chapter 7, thermoplastics melt and thermosets cure at relatively low temperatures. Hence, unlike metals, they are relatively easy to handle, and requiring much less force and energy to process them. Plastics, in general, can be molded, cast, shaped, and machined into complex shapes, with relative ease and at high production rates (Table 19.1). Plastics can be joined by a variety of techniques (Section 32.6) and also can be coated using various techniques (Chapter 34). Plastics are shaped into discrete or continuous products, such as sheets, plates, rods, and tubing; they may then be shaped by secondary processes into products. The types and properties of polymers, and the complexity of the parts that can be produced, are greatly influenced by their manufacturing and processing characteristics. Plastics are usually shipped to manufacturing plants as pellets, granules, or powders, and are (a) softened or melted (for thermoplastics) just before shaping them and (b) cured and set (for thermosetting plastics). Liquid plastics that cure into solid form also are used, especially in making thermosets and reinforced-plastic parts. With increasing awareness of the environment, raw materials may consist of reground, chopped, or melted plastics, delivered from recycling centers or produced from natural sources. Following the outline shown in Fig. 19.1, this chapter describes the basic processes, operations, machinery, and economics of forming and shaping plastics. The processing techniques for reinforced plastics and metal-matrix and ceramic-matrix composites are also described. The chapter begins with melt-processing techniques, starting with extrusion, and continuing on to various molding processes. ׉ 7cassandra://AX99UUUhVIeer0xIFsWSqHuPDzLwCsN7Oww1TgzfV_I`cZ%EbPcZ%EbO#בCט#u׉׉ 7cassandra://uiFFWqb0Z8DyvvYdryc6zbSDMj6rRykNJ_KiDDz4RbE:`׉ 7cassandra://0dGlLd1qa-0BxNt92zIsACq8saSXjKuOlyatxLDEv0MK`s׉ 7cassandra://p5x43fE1SJcO-20HrO4fLrik89rG4G6nWhdcUrNdQsw`׉ 7cassandra://l4ontfVI4CERQYplpamSH12GUH8KNvWpbvrX-_IzExM͕vh͠^cZEb1ט#u׉׉ 7cassandra://FU_JSfvLjZoh5VM1wRWSG9A0jDTCBkvayb0pL_aw-tcv`׉ 7cassandra://aZppm0erfXgMguTEv34k2MqROR4jKadbDoKkZjsnmm0M<`s׉ 7cassandra://f9bZb3JfsvkoOvSniwnPJfVu05hInhBXsoYHQ47eJUM `׉ 7cassandra://WbO-0Agy5bA92w3zQuB0MvOL6Cy6ZF0uD856xr_jxIEH͠^cZEb5׉EIntroduction 561 Table 19.1: General Characteristics of Forming and Shaping Processes for Plastics and Composite Materials. Process Characteristics Extrusion Injection molding Structural foam molding Blow molding Rotational molding Thermoforming Compression molding Transfer molding Casting Continuous, uniformly solid or hollow, and complex cross sections; high production rates; relatively low tooling costs; wide tolerances Complex shapes of various sizes; thin walls; very high production rates; costly tooling; good dimensional accuracy Large parts with high stiffness-to-weight ratio; less expensive tooling than in injection molding; low production rates Hollow, thin-walled parts and bottles of various sizes; high production rates; relatively low tooling costs Large, hollow items of relatively simple shape; relatively low tooling costs; relatively low production rates Shallow or relatively deep cavities; low tooling costs; medium production rates Parts similar to impression-die forging; expensive tooling; medium production rates More complex parts than compression molding; higher production rates; high tooling costs; some scrap loss Simple or intricate shapes made with rigid or flexible low-cost molds; low production rates Processing of composite materials Long cycle times; expensive operation; tooling costs depend on process Extruded products Sheet TP, E TP, TS, E Pellets, Granules, Powder TP, TS TP, TS TS, TP, E TS, E TP, TS TP, TS Liquid, Paste TS Reactioninjection molding TP Extrusion Tube Injection molding Rotational molding Structural foam molding Compression molding Transfer molding Casting, Foam molding Fiber-reinforced sheet Vacuum-bag forming, Pressure-bag forming, Hand layup, Spray layup Fibers Filament winding Pultrusion Figure 19.1: Outline of forming and shaping processes for plastics, elastomers, and composite materials. (TP = Thermoplastic; TS = Thermoset; E = Elastomer.) Laminated sheet Blown Film Thermoforming Blow molding ׉ 7cassandra://p5x43fE1SJcO-20HrO4fLrik89rG4G6nWhdcUrNdQsw`cZ%EbQ׉Ed562 Chapter 19 Plastics and Composite Materials: Forming and Shaping Barrel liner Hopper Throat Barrel Thrust bearing Throat-cooling channel Gear reducer box Feed section Melt section Melt-pumping section Motor (a) Pitch H W D w Barrel (b) Figure 19.2: Common extrusion die geometries: (a) Schematic illustration of a typical screw extruder. (b) Geometry of an extruder screw metering or pumping section. Complex shapes can be extruded with relatively simple and inexpensive dies. 19.2 Extrusion In extrusion, which produces the largest volume of plastics made, raw materials in the form of thermoplastic pellets, granules, or powder, are placed into a hopper and fed into the barrel of a screw extruder. The barrel is equipped with a helical screw that builds up pressure in the barrel, blends the pellets, and conveys them down the barrel towards the extrusion die. The barrel heaters and the internal friction from the mechanical action of the screw heat the pellets, and liquefies them. Screws have three distinct sections: 1. Feed section: Conveys the material from the hopper into the central region of the barrel. 2. Melt section, also called compression or transition section: Where the heat, generated by the viscous shearing of the plastic pellets and by the external heaters around the barrel, cause melting to begin. 3. Metering or pumping section: Where additional shearing and melting take place, with pressure building up at the die entrance. The lengths of these individual sections can be changed to accommodate the melting characteristics of different types of plastics. A metal-wire filter screen is usually placed just before the die to filter out unmelted or congealed resin. The screen, which is replaced periodically, also causes back pressure in the Barrel Flight Barrel heater/cooler Thermocouples Wire filter screen Melt thermocouple Breaker plate Adapter Die Screw ׉ 7cassandra://f9bZb3JfsvkoOvSniwnPJfVu05hInhBXsoYHQ47eJUM `cZ%EbRcZ%EbQ#בCט#u׉׉ 7cassandra://8Vj7xAtxkCcQPd0ml0PS63tDs2WJ3-ZsBxbpG1MfKQU>`׉ 7cassandra://qAnepRz42JiNRzcJVV03yCz_xEYCNCMnppwZgJQYZk8P`s׉ 7cassandra://JX7YFDPz6NesN3ocqhS4-L4uRZ3RKJ4ov6UF6tbL27Q`׉ 7cassandra://0ffquSK9wWvWP0dS3xxJXnA2f3Dfle8cpLMhGm2kc04 ,b͠^cZEb7ט#u׉׉ 7cassandra://DGBD-LSU0wj1MjmUc16Ao_GwjALcxhM6nnWzSZUknnA`׉ 7cassandra://hQDRm49jdW_bC_iixq9EcLxg5vor9XAcCOwRtldPLN8t`s׉ 7cassandra://580p47EtTUY0Yiw9u16qhoAKzr8iWV-0hGcmVvY4swQ`׉ 7cassandra://BJH2UKrGa7S7rXouReFb8va6DULXLPo56g0RLWnQwHMo͠^cZEb8׉EExtrusion 563 Melt inlet Melt-distribution manifold Adjustable lip Die body Die bolt Melt-distribution manifold Die land Fixed lip Thermocouple well (a) Die shape Extruded product (b) Die shape Extruded product (c) Figure 19.3: Common extrusion die geometries: (a) coat-hanger die for extruding sheet; (b) round die for producing rods; and (c) nonuniform recovery of the part after it exits the die. barrel, which has to be overcome by the extruder screw. Between the screen and the die is a breaker plate, with several small holes in it, which helps improve mixing of the polymer prior to its entering the die. The extruded material is cooled, generally by exposing it to blowing air or by being passed through a water-filled channel (trough). Controlling the rate and uniformity of cooling is important in order to minimize product shrinkage and distortion. In addition to single-screw extruders, other types include twin (two parallel screws side by side) and multiple screws, for polymers that are difficult to extrude or require additional blending (see also reciprocating screw, Section 19.3). A typical helical screw metering section is shown in indicating the important parameters that affect the mechanics of polymer extrusion. At any point in time, the molten plastic is in the shape of a helical ribbon, with thickness H and width W; it is conveyed towards the extruder outlet by the rotating screw flights. The shape, pitch, and flight angle of the helical screw are important parameters, as they affect the flow of the polymer through the extruder. The ratio of the barrel length, L, to its diameter, D, is also important. In typical commercial extruders, the L/D ratio ranges from 5 to 30, and barrel diameters generally are in the range from 25 to 200 mm. Process Characteristics. Because there is a continuous supply of raw material from the hopper, long products, such as solid rods, sections, channels, sheet, tubing, pipe, and architectural components can be extruded continuously. Complex shapes with constant cross sections can be extruded with relatively inexpensive tooling. Some common die profiles are shown in Fig. 19.3b. Polymers usually undergo much greater Die shape Extruded product ׉ 7cassandra://JX7YFDPz6NesN3ocqhS4-L4uRZ3RKJ4ov6UF6tbL27Q`cZ%EbS׉E564 Chapter 19 Plastics and Composite Materials: Forming and Shaping and uneven shape recovery than is encountered in metal extrusion (Chapter 15). Because the polymer will swell after exiting the die, the openings shown in Fig. 19.3b are smaller than the extruded cross sections. After it has cooled, the extruded product may subsequently be drawn (sized) by a puller, then coiled or cut into desired lengths. The control of processing parameters, such as extruder-screw rotational speed, barrel-wall temperatures, die design, and rate of cooling of the extrudate are all important in order to ensure product quality and uniform dimensional accuracy. Defects observed in extruding plastics are similar to those in metal extrusion (Section 15.5). Die shape is important, as it can induce high stresses in the product, causing it to develop surface fractures, as also occur with metals. Other surface defects are bambooing and sharkskin effects, which are due to a combination of friction at the die–polymer interfaces, elastic recovery, and nonuniform deformation of the outer layers of the product with respect to its bulk during extrusion. Extruders generally are rated by the diameter, D, of the barrel and the length-to-diameter (L/D)ratio of the barrel. The cost of machinery is on the order of $300,000, including the cost for the equipment for downstream cooling and winding of the extruded product. 19.2.1 Miscellaneous Extrusion Processes There are several variations of the basic extrusion process for producing a number of different polymer products. Plastic Tubes and Piping. These products are produced in an extruder, using a spider die (Fig. 19.4a; see also Fig. 15.8 for details). For the production of reinforced tubing to withstand higher pressures, woven fiber or wire reinforcements also may be fed through specially designed dies; a typical product is a reinforced plastic water hose. Extrusion of tubes is also a first step for related processes, such as blown film production or extrusion blow molding (Section 19.4). Rigid Plastic Tubing. Extruded by a process in which the die is rotated, rigid plastic tubing causes the polymer to be sheared and biaxially oriented during extrusion. As a result, the tube has a higher crushing strength and a higher strength-to-weight ratio than conventionally extruded tubing. Coextrusion. Shown in Fig. 19.4b, coextrusion involves simultaneous extrusion of two or more polymers through a single die. The product cross section thus contains different polymers, each with its own characteristics and functions. Coextrusion produces such shapes as flat sheets, films, and tubes, and is used especially for food packaging where different layers of polymers have different functions. These include: (a) providing inertness for contact with food products, (b) serving as barriers to fluids such as water or oil, and (c) labeling of the product. Plastic-coated Electrical Wire. Electrical wire, cable, and strips are simultaneously extruded and coated with plastic. The metal wire is fed into the die opening at a controlled rate with the extruded plastic, depositing a uniform coating on the wire. To ensure proper insulation, extruded wires are checked continuously for their electrical resistance as they exit the die. At the same time, the wire is also marked with ink from a roller for identification purposes. Common plastic-coated wire paper clips, with different colors, also are made by this process. Polymer Sheets and Films. Generally, polymer sheet is considered to be thicker than 0.5 mm, and film is thinner than 0.5 mm. They are produced using a specially designed flat extrusion die (Fig. 19.3a). Also known as a coat-hanger die, it is designed to distribute the polymer melt evenly throughout the width of the die opening. The polymer is extruded by forcing it through the die, after which the extruded sheet is taken up by water-cooled rolls to cool the sheets, then by a pair of rubber-covered pull-off rolls. Thin Polymer Films. Common plastic bags and other thin polymer films are made from blown film, which itself is made from a thin-walled tube produced by an extruder. In this process, a tube is extruded vertically (Fig. 19.5), while it is continuously pulled upward and expanded into a balloon shape. Air is continuously ׉ 7cassandra://580p47EtTUY0Yiw9u16qhoAKzr8iWV-0hGcmVvY4swQ`cZ%EbTcZ%EbS#בCט#u׉׉ 7cassandra://LRb2P5HgtSjonhuVkQ52x_n7pN1a7FlvQ1Y-NrJJqBs`׉ 7cassandra://uv5ZnfrAlVE9DFnv8WVRSTkCOCoNtzcHD1_kaAz3IQgB@`s׉ 7cassandra://olkbsWmOOvbyX4MBiavzrfnjODtVJc66Phox3gNTlgk`׉ 7cassandra://v14G94Zkpcu8tYOqHg32UJi22pOaS1b9aSVG-Bg3ftAg׉EExtrusion 565 Breaker plate Extruder barrel Screen pack Melt flow direction Mandrel A v B Spider legs (3) Air channel Air in (a) Polymer melt A B Section B–B Spider legs (3) Section A–A Extruder 1 Plastic melt: two or more layers Parison Mandrel Extruder 2 (b) Figure 19.4: Extrusion of tubes. (a) Extrusion using a spider die (see also Fig. 15.8) and pressurized air. (b) Coextrusion for producing a parison for a bottle. blown through the center of the extrusion die until the specified film thickness is reached. Because of the molecular orientation of thermoplastics (Section 7.3), a frost line develops on the balloon, which reduces its transparency. The ratio of the blown diameter to the extruded tube diameter is called the blow ratio, which is usually between 1.5 and 5, but is about 3:1 in Fig. 19.5. Note that, as described in Section 2.2.7, the polymer must possess a high strain-rate sensitivity exponent, m, to be successfully blown in this process without tearing (see also Example 9.1). The balloon is usually cooled by air from a cooling ring, which also acts as a physical barrier to further diametral expansion of the balloon, thus controlling its major dimensions. The cooled tube is then continuously slit lengthwise, becoming film. It can also be pinched/welded and cut off, to produce a plastic bag. The width of the film produced after slitting can be on the order of 6 m or more. Plastic Films. Plastic films, especially polytetrafluoroethylene (PTFE; with the trade name of Teflon), can be produced by shaving the circumference of a solid round plastic billet using specially designed knives. The process is similar to producing veneer from a large block of round wood, in a process called skiving (see also Section 24.4). ׉ 7cassandra://olkbsWmOOvbyX4MBiavzrfnjODtVJc66Phox3gNTlgk`cZ%EbU׉E566 Chapter 19 Plastics and Composite Materials: Forming and Shaping Pinch rolls Wind-up Guide rolls Blown tube Mandrel Die Extruder Air Figure 19.5: Schematic illustration of the production of thin film and plastic bags from tube–first produced by an extruder and then blown by air. Pellets. Used as raw material for other plastic-processing methods described in this chapter, pellets are made by extrusion. A small-diameter, solid rod is extruded continuously, then chopped into short lengths, called pellets. With some modifications, extruders also can be used as simple melters for other shaping processes, such as injection molding and blow molding. Example 19.1 Blown Film Given: A typical plastic shopping bag made from blown film has a lateral dimension (width) of 400 mm. Find: (a) Determine the extrusion-die diameter. (b) These bags are relatively strong in use. How is this strength achieved? Solution: (a) The perimeter of the flat bag is (2)(400) = 800 mm. Since the original cross section of the film is round, the blown diameter should be πD = 800 thus D = 255 mm. Recall that in this process, a tube is expanded from 1.5 to 5 times the extrusion-die diameter. Taking the maximum value of 5, the die diameter is 255/5 = 51 mm. (b) After extrusion, the balloon is pulled upward by the pinch rolls. Thus, in addition to diametral stretching from the internal pressure and the attendant molecular orientation, the film is stretched and oriented in the longitudinal direction. The resulting biaxial orientation of the polymer molecules significantly improves the strength and toughness of the plastic bag. 19.2.2 Production of Polymer Reinforcing Fibers Polymer fibers have numerous important applications. In addition to their use as reinforcement in composite materials, they are used in a wide variety of consumer and industrial products, including clothing, carpeting, fabrics, rope, and packaging tape. ׉ 7cassandra://u0GtOJ1hjmc30ZzXIMwCbgLeG_gOwjsqZ-HD4VauU1U*`cZ%EbVcZ%EbU#בCט#u׉׉ 7cassandra://1hCKOk8H9JpX_fcrEXLofoHzobXofv4H1y2FYTl5AUYu`׉ 7cassandra://F7K626givBULUcoyj3CgH4xm-6U-UUi5qo35bF6zDWkU`s׉ 7cassandra://1LmkW9LjG49jHVb_eOm79ob24v6noMfkXVK0hIfzpeU`׉ 7cassandra://3BwrL1uq54zUPeW-2jDm08aXXamK7jcUxa_ixmtk7Dw͠^cZEbAט#u׉׉ 7cassandra://z1efewRK2SI3ujiHIMeADlrET9ByokHcrGOuhrtK-nw~`׉ 7cassandra://GTtEEDw9hYvtKOTEfWajMYbMnlMRTQx7XYTivnotxmIQv`s׉ 7cassandra://jtHSOFtdB_dVDuR9obEb9TSIkWJUOt1ZZyBKotnf5MQ`׉ 7cassandra://2HLg-_6nE4fAysJT8HH2YY4NNzuryW7C83-gyTzFFv46"͠^cZEbD׉E mExtrusion 567 Most synthetic fibers in reinforced plastics are polymers, extruded through one to several hundred holes in a spinneret, resembling a shower head, forming continuous filaments of semisolid polymer. If the polymer is a thermoplastic, it is first melted in an extruder (Section 19.2). Thermosetting polymers also can be formed into fibers, by first dissolving or chemically treating them so that they can be extruded. These operations are performed at high rates and with very high reliability. As the filaments emerge from the holes, the liquid polymer is first converted to a rubbery state, then it solidifies. This process of extrusion and solidification of continuous filaments is called spinning, a term also used for natural textile fibers, such as cotton or wool. There are four methods of spinning fibers: melt, wet, dry, and gel spinning. 1. In melt spinning (Fig. 19.6), the polymer melt is extruded through the spinneret, and then it solidifies directly by cooling. A typical spinneret for this operation is about 5 mm thick and has about 50 holes, about 0.25 mm in diameter. The fibers that emerge from the spinneret are cooled by forced-air convection; they are simultaneously pulled so that their final diameter becomes much smaller than the holes of the spinneret. Nylon, olefin, polyester, and PVC are produced in this manner. Melt-spun fibers also can be extruded from the spinneret in various other cross sections, such as trilobal (a triangle with curved sides), pentagonal, octagonal, and hollow shapes. Hollow fibers trap air, providing additional thermal insulation. 2. Wet spinning, the oldest process for fiber production, is used for polymers that have been dissolved in a solvent, by submerging the spinneret in a chemical bath. As the filaments emerge, they precipitate in the bath, producing a fiber which is then wound onto a bobbin (spool). The term “wet” refers to the use of a precipitating liquid bath, resulting in wet fibers; they require drying before they can be used. Acrylic, rayon, and aramid fibers are produced in this manner. 3. Dry spinning is used for thermosets dissolved by a fluid. Instead of precipitating the polymer by dilution, as in wet spinning, solidification is achieved by evaporating the solvent fluid in a stream Polymer chips Feed hopper Spinneret Cold air Melter/extruder Melt spinning Bobbin Stretching Twisting and winding Figure 19.6: The melt-spinning process for producing polymer fibers. The fibers are used in a variety of applications, including reinforcements for composite materials and fabrics for clothing. In the stretching box, the right roll rotates faster than the left roll. ׉ 7cassandra://1LmkW9LjG49jHVb_eOm79ob24v6noMfkXVK0hIfzpeU`cZ%EbW׉E )568 Chapter 19 Plastics and Composite Materials: Forming and Shaping Powder, pellets Hopper Heating zones Nozzle Mold Vent Piston (ram) Cooling zone Cylinder (barrel) Injection chamber Ejector pins Torpedo (spreader) (a) Sprue Molded part Vent Press (clamp) force Rotating and reciprocating screw (b) Figure 19.7: Schematic illustration of injection molding with (a) a plunger and (b) a reciprocating rotating screw. of air or inert gas. The filaments do not come in contact with a precipitating liquid, thus eliminating the need for drying. Acetate, triacetate, polyether-based elastane, and acrylic fibers are made by this process. 4. Gel spinning is a special process used to impart high strength or other properties to the fibers. Some polyethylene and aramid fibers are produced by gel spinning. The polymer is not melted completely, or dissolved in a liquid, but the molecules bond together at various points in liquid-crystal form. This operation produces strong interchain forces in the filaments produced, significantly increasing their tensile strength. Moreover, the liquid crystals are aligned along the fiber axis by the strain encountered during extrusion. Thus, the filaments emerge from the spinneret with a high degree of orientation relative to each other, further enhancing their strength. This process is also called dry–wet spinning, because the filaments first pass through air and are then cooled further in a liquid bath. An essential step in the production of most fibers is significant stretching to induce orientation of the polymer molecules in the fiber direction; the strain induced can be as high as 800%. Orientation is the main reason for the high strength of the fibers as compared with the polymer in bulk form. Stretching can be done while the polymer is still pliable (just after emerging from the spinneret) or it can be a cold-drawing operation. Graphite fibers are produced from polymer fibers by pyrolysis. In this operation, controlled heat is applied to the polymer fiber (typically polyacrylonitrile, PAN) to drive off all elements except the carbon; temperatures in the range of 1500◦ to 3000◦C are used. The fiber is under tension in order to impart a high degree of orientation in the resulting fiber structure (see also Section 9.2.1 on the properties of graphite fibers and other details). ׉ 7cassandra://jtHSOFtdB_dVDuR9obEb9TSIkWJUOt1ZZyBKotnf5MQ`cZ%EbXcZ%EbW#בCט#u׉׉ 7cassandra://eiMfRr1clbGlIHXw8VqqwhXLjeMELq1qRtH_81OV6s0=`׉ 7cassandra://wZ8qR72W--GJXkF0SvGrRMRHvPCm2udPzDslO0vVUMc_ `s׉ 7cassandra://1CB1H4IWpviAQZpfp7j5nkmEM4P8yyweSaqxtysGyos`׉ 7cassandra://4Bz91Fn9x9Giykv6ZGiIePxAUkyfEmHacK1wqPUgMtgͥ͠^cZEbFט#u׉׉ 7cassandra://-69n-SUpCMEQ8Lv0doIdWlW8WM0CAkKAS-fpBCE1TU8`׉ 7cassandra://7QFIb0lxZaf7gF3zOe0R39aEbEyvqolC028PLcOdt8IXg`s׉ 7cassandra://ASQEx7rEXuf8lb0enToJccXEI7lJfZCCFELKtSC0BmQ`׉ 7cassandra://72RXXJEtypPT7sQr75nXvOWKWEnS_VxKJlZfML7lW_ojs͠^cZEbG׉E Injection Molding 569 Rotating and reciprocating screw 1. Build up polymer in front of sprue bushing; pressure pushes the screw backwards. When sufficient polymer has built up, rotation stops. 2. When the mold is ready, the screw is pushed forward by a hydraulic cylinder, filling the sprue bushing, sprue, and mold cavity with polymer. The screw begins rotating again to build up more polymer. 3. After polymer is solidified/cured, the mold opens, and ejector pins remove the molded part. Figure 19.8: Sequence of operations in the injection molding of a part with a reciprocating screw. This process is used widely for numerous consumer and commercial products, such as toys, containers, knobs, and electrical equipment (see Fig. 19.9). 19.3 Injection Molding Injection molding is similar to hot-chamber die casting (Fig. 19.7 and Section 11.4.5). The pellets or granules are fed into the heated cylinder, and the melt is forced into the mold, either by a hydraulic plunger or by a rotating screw system similar to that of an extruder. As in plastic extrusion, the barrel (cylinder) is heated externally to promote melting of the polymer. In injection-molding machines, however, a far greater portion of the heat transferred to the polymer is due to frictional heating. Reciprocating or plasticating screw type injection molding machines (Fig. 19.7b) use the sequence of operations shown in Fig. 19.8. As the pressure builds up at the mold entrance, the rotating screw begins to move backwards under pressure to a predetermined distance. This movement controls the volume of material to be injected. The screw then stops rotating, and is pushed forward hydraulically, forcing the molten plastic into the mold cavity. The pressures developed typically range from 70 to 200 MPa. Several injection-molded products are shown in Fig. 19.9; others include cups, containers, housings, tool handles, knobs, toys, plumbing fixtures, and components for electrical and communicationsequipment. For thermoplastics, the molds are kept relatively cool, at about 90◦C. Thermoset parts are molded in heated molds, at about 200◦C, where polymerization and cross-linking take place (Section 7.2.1). Elastomers are also injection molded into discrete products using these processes. After the part has cooled sufficiently (for thermoplastics) or cured (for thermosets), the molds are opened, and the part is ejected. The molds are then closed and the process is repeated automatically. Because the polymer is molten when injected into the mold, complex shapes with good dimensional accuracy can be made. However, because of uneven cooling of the part in the mold, residual stresses will develop. Molds with moving and unscrewing mandrels also are used in injection molding, as they allow molding of parts with multiple cavities or internal and external threaded features. To accommodate part design, molds may have several components (Fig. 19.10), including runners (such as those used in metal-casting dies, Fig. 11.20), cores, cavities, cooling channels, inserts, knockout pins, and ejectors. ׉ 7cassandra://1CB1H4IWpviAQZpfp7j5nkmEM4P8yyweSaqxtysGyos`cZ%EbY׉E 570 Chapter 19 Plastics and Composite Materials: Forming and Shaping Figure 19.9: Typical products made by injection molding, including examples of insert molding. Source: After Rayco Mold and Mfg. LLC. There are three basic types of molds for injection molding: 1. Cold-runner, two-plate mold: This design is the simplest and most common, as shown in Fig. 19.11a. 2. Cold-runner, three-plate mold (Fig. 19.11b): The runner system is separated from the part when the mold is opened. 3. Hot-runner mold (Fig. 19.11c), also called runnerless mold: The molten plastic is kept hot in a heated runner plate. In cold-runner molds, the solidified plastic remaining in the channels connecting the mold cavity to the end of the barrel must be removed, usually by trimming. Later, this scrap can be chopped and recycled. In hot-runner molds, which are more expensive, there are no gates, runners, or sprues attached to the molded part. Cycle times are shorter, because only the molded part must be cooled and ejected. Multicomponent injection molding, also called co-injection or sandwich molding, allows forming of parts that have a combination of various colors and shapes. An example is molding of automobile rear-light covers, made of different materials and colors, such as red, amber, and white. For some parts, printed film also can be placed in the mold cavity, thus providing decoration or labeling during molding. Insert molding involves metallic components, such as screws, pins, and strips, that are placed in the mold cavity prior to injection of polymer, and become an integral part of the molded product (Fig. 19.9). The most common examples of such combinations are hand tools, where the handle is insert molded onto a metal component; other examples include electrical and automotive components. Overmolding. This is a process for making such products as hinge joints and ball-and-socket joints in one operation, without requiring postmolding assembly. Two different types of plastics are molded to ensure that no bonds will form between the molded halves of the joint, as otherwise their motion would be impeded. In ice-cold molding, the same type of plastic is used for both components of the joint. The operation is carried out in a conventional injection-molding machine and in one cycle. A two-cavity mold is used, with cooling inserts positioned in the area of contact between the first and second molded components of the joint. In this way, no bonds develop between the two pieces, and thus the two components have free movements, as in a hinge or a sliding mechanism. ׉ 7cassandra://ASQEx7rEXuf8lb0enToJccXEI7lJfZCCFELKtSC0BmQ`cZ%EbZcZ%EbY#בCט#u׉׉ 7cassandra://McyrB7EXkWG65jh1xZ5ctLw87w4Qxu1NMBztHWLmD64`׉ 7cassandra://9xMWz5jGcuBNZ4m0E3rgDehPHd8339bmVZlTyFMkepo=`s׉ 7cassandra://FThyfCsEEG1x5nc3wgYd0BFZgr_jjTbkyigJWXwjVgw`׉ 7cassandra://u7qK805B5bfJ0XOB_0kIJXqMyO-hpicDpz_MXOGq20Mj͠^cZEbIט#u׉׉ 7cassandra://5gdRIf6kNk-CeElxbz_TvEuQ5TVJvyfGWOh_u6DGRZM``׉ 7cassandra://3IseQxwYveUWLh6h2V3KO2OHH5bPQeA7fXo6hgq8JRQm(`s׉ 7cassandra://nMRqLjFbOVhZjaFw7hVAJD16sz0gDozCa5GOJXrNZ4Q`׉ 7cassandra://iVLsP5f3IB_dwcveGE0Xf81bVxIixp2Uy0lx3oAXXms`k͠^cZEbK׉EInjection Molding 571 Gate Cavity Sprue Part Gate Main runner Cold slug well Branch runner Cavity Main runner Sprue Guide pin (a) Branch runner Guide pin (b) Figure 19.10: Illustration of mold features for injection molding. (a) Two-plate mold with important features identified. (b) Schematic illustration of the features in a mold. Source: Courtesy of Tooling Molds West, Inc. Plate Gate Part Sprue bushing Sprue Part Parts Runner (a) Hot plate; Runner stays molten Plate Sprue bushing Parts (c) Figure 19.11: Types of molds used in injection molding: (a) two-plate mold; (b) three-plate mold; and (c) hot-runner mold. Ejector pins Plate (b) Ejector pins Sprue bushing Ejector pins Plate Plate Stripper plate Plate ׉ 7cassandra://FThyfCsEEG1x5nc3wgYd0BFZgr_jjTbkyigJWXwjVgw`cZ%Eb[׉E572 Chapter 19 Plastics and Composite Materials: Forming and Shaping Process Capabilities. Injection molding is a high-rate production process and permits good dimensional control. Although most parts generally weigh from about 100 to 600 g, they also can be much heavier. Typical cycle times range from 5 to 60 s, although they can be several minutes for thermosetting materials. Cycle times can be reduced by as much as 50% through the use of conformal cooling channels constructed in the mold (see Section 20.6.1 and Fig. 20.18). Injection molding is a versatile process and capable of producing complex shapes, with good dimensional accuracy. Mold design and control of material flow in the die cavities are important factors in the quality of the product, and in avoiding defects. Because of the basic similarities to metal casting regarding material flow and heat transfer, defects observed in injection molding are somewhat similar to those in casting. For example: • In Fig. 10.15g, the molten metal flows in from two opposite runners, and then meets in the middle of the mold cavity. Thus, a cold shut in casting is equivalent to weld lines in injection molding. • If the runner cross sections are too small, the polymer may solidify prematurely, thus preventing complete filling of the mold cavity. Solidification of the outer layers in thick sections can cause porosity or voids due to shrinkage, as in the metal parts shown in Fig. 12.2. • If, for some reason, the dies do not close completely or because of die wear, flash will form, in a manner similar to flash formation in impression-die forging (see Figs. 14.6 and 19.17c). • A defect known as sink marks or pull-in, similar to that shown in Fig. 19.30c, is observed in injectionmolded and cast parts. • Methods of avoiding defects include proper control of temperatures, pressures, and mold design modifications, using simulation software. Modeling techniques and simulation software continue to be developed for studying optimum gating systems, mold filling, mold cooling, and part distortion. Commercial software tools are available to expedite the design process for molding parts with good dimensional tolerances and other characteristics. The programs take into account such factors as injection pressure, temperature, heat transfer, and the condition of the polymers, and often work within a CAD software environment such as Autodesk and ProEngineer. Machines. Injection-molding machines are usually horizontal (Fig. 19.12); vertical machines are used for making small, close-tolerance parts and for insert molding. The clamping force on the dies is typically supplied by hydraulic means, although lighter and quieter electrically powered clamps also are used. Modern machines are equipped with microprocessors in a control panel and monitor all aspects of the molding operation. Injection-molding machines are rated according to the capacity of the mold and the clamping force. In most machines, this force ranges from 0.9 to 2.2 MN (90 to 225 metric tons). The largest machine in operation has a capacity of 75 MN (7500 metric tons), and it can produce parts weighing up to 40 kg. The cost of a 90-metric ton machine ranges from about $60,000 to about $90,000 and of a 270-metric ton machine from about $85,000 to about $250,000. Mold costs typically range from $20,000 to $200,000. Consequently, high-volume production is essential to justify such high expenditures. The molds generally are made of tool steels, beryllium–copper, or aluminum. They may have multiple cavities, so that more than one part can be molded in one cycle (see also Fig. 11.20). Mold life may be on the order of two million cycles for steel molds, but it can be only about 10,000 cycles for aluminum molds. Some rapid tooling approaches (Section 20.10) can produce metal, polymer, or hybrid molds, but they often have a life restricted to a few thousand parts. For low melting point metals, vulcanized rubber molds can be produced, although these are useful only for small batch production runs. ׉ 7cassandra://nMRqLjFbOVhZjaFw7hVAJD16sz0gDozCa5GOJXrNZ4Q`cZ%Eb\cZ%Eb[#בCט#u׉׉ 7cassandra://GGSA2I3cx6X3T6ex-KLQUxZnGpMC3QY7Xlz5fMGreEM|`׉ 7cassandra://XtLQbEk4Rse7ViymTlFPRKwoT2Ja2sUqPNb3Vca58GYU`s׉ 7cassandra://q1rawdtIS8pNDc0RN6pkCT1Gb8mCNytSRltyHHziwh4`׉ 7cassandra://Z5XrrSCmWAo9fPv8u18RdyqCkR7vmNdSgu5eSEuOna8͆ ͠^cZEbMט#u׉׉ 7cassandra://zhB8Hxzx79xrj7Dq95KB_IcfrDtKLC9Pdxrl5_6di04{`׉ 7cassandra://HxHNA6HzSV42FVyVB0YUTmCMnLweboD0smTAM24YidIT`s׉ 7cassandra://nWpreWH0megkHnYd432ykNgPT2DhiA-MPFjr61TDBlI`׉ 7cassandra://xVY5eC4YoYJ-j8uesarUx3tO0G8zQCz-J44D8tXXsAYj &͠^cZEbN׉EInjection Molding 573 Figure 19.12: A 2.2-MN (225-metric ton) injection-molding machine. The tonnage is the force applied to keep the dies closed during the injection of molten plastic into the mold cavities and hold it there until the parts are cool and stiff enough to be removed from the die. Source: Shutterstock/Value ho Example 19.2 Force Required in Injection Molding Given: A 2.25-MN injection-molding machine is to be used to make spur gears, 100 mm in diameter and 15 mm thick. The gears have a fine-tooth profile. Find: How many gears can be injection molded in one set of molds? Does the thickness of the gears affect the force? Solution: Because of the detail involved (fine gear teeth), assume that the pressure required in the mold cavity will be on the order of 100 MPa. The cross-sectional (projected) area of the gear is π(0.1)2/4=0.00785 m2. Assuming that the parting plane of the two halves of the mold is in the mid-plane of the gear, the force required is (100 × 106)(0.00785) = 785 kN. Since the capacity of the machine is 2.25 MN, the mold can accommodate (2250/785)=2.8 gears. Thus, there is some excess capacity, but this will make sure that the runners and spurs do not cause the molds to open. Therefore, the injection molding machine can be used to produce two gears per cycle. Because it does not influence the cross-sectional area of the gear, gear thickness does not directly affect the pressures involved and thus does not change the answer. 19.3.1 Reaction-injection Molding In the reaction-injection molding (RIM) process, a monomer (Section 7.2) and two or more reactive fluids are forced into a mixing chamber at high speed and a pressure of 10 to 20 MPa, and then into the mold cavity (Fig. 19.13). Chemical reactions rapidly take place in the mold cavity, where the polymer solidifies. Typical polymers used are polyurethane, nylon, and epoxy. Cycle times may range up to about 10 minutes, depending on the materials, part size, and shape complexity. Major applications of this process include automotive parts, such as bumpers, fenders, steering wheels, and instrument panels, thermal insulation for refrigerators and freezers, water skis, and stiffeners for structural components. Weight of parts made may range up to about 50 kg. Reinforcing fibers, such as ׉ 7cassandra://q1rawdtIS8pNDc0RN6pkCT1Gb8mCNytSRltyHHziwh4`cZ%Eb]׉E &574 Chapter 19 Plastics and Composite Materials: Forming and Shaping Heat exchanger Heat exchanger Pump Monomer 1 Stirrer Pump Mold Recirculation loop Figure 19.13: Schematic illustration of the reaction-injection molding process. Typical parts made are automotive-body panels, water skis, and thermal insulation for refrigerators and freezers. glass or graphite, also may be used to improve the strength and stiffness of the part. Depending on the number of parts to be made and part quality required, molds can be made of such materials as steel or aluminum. 19.4 Blow Molding Blow molding is a modified extrusion and injection-molding process. In extrusion blow molding,atube or preform, usually oriented so that it is vertical, is first extruded. It is then clamped into a mold with a cavity that is much larger than the tube diameter, and blown outward to fill the mold cavity (Fig. 19.14a). Depending on the material, the blow ratio may be as high as 7:1. Blowing usually is done with a hot-air blast, at a pressure ranging from 350 to 700 kPa. Plastic drums, with a volume as large as 2000 liters, can be made by this process. Typical die materials are steel, aluminum, and beryllium copper. In injection blow molding, a short tubular piece (parison) is injection molded (Fig. 19.14b) into cool dies. The dies are then opened, and the parison is transferred to a blow-molding die, using an indexing mechanism (Fig. 19.14c). Hot air is injected into the parison, expanding it to contact the walls of the mold cavity. Typical products made are beverage bottles made of polyethylene or polyetheretherketone (PEEK). A related process is stretch blow molding, in which the parison is simultaneously expanded and elongated, subjecting the polymer to biaxial stretching and enhancing its properties. Multilayer blow molding involves the use of coextruded tubes or parisons, and thus permitting the production of a multilayer structure (Fig. 19.4b). A typical example of a product is plastic packaging for food and beverages, having such characteristics as odor and permeation barrier, taste and aroma protection, scuff resistance, printable, and the ability to be filled with hot fluids. Other applications of this process include containers in the cosmetics and the pharmaceutical industries. 19.5 Rotational Molding Most thermoplastics and some thermosets can be shaped into large, hollow parts by rotational molding.A thin-walled metal mold is first made in two pieces (split-female mold), designed to be rotated about two Recirculation loop Mixing head Displacement cylinders Monomer 2 Stirrer ׉ 7cassandra://nWpreWH0megkHnYd432ykNgPT2DhiA-MPFjr61TDBlI`cZ%Eb^cZ%Eb]#בCט#u׉׉ 7cassandra://_jbzMRfbdA3LZdBpQDZ0Vv1Xtisl3dbvu2tv44-xbNkK`׉ 7cassandra://3a9HGQTZUaN2WsN7DLiChf15c0bTgcnNDI68QWeKA0A<`s׉ 7cassandra://1X7mRbOkouCjXu_ZEJeJXqFbEbfuLp39jxztf6IuDOc/`׉ 7cassandra://VPVYUs_OzZ-KsCgRVUa66Oz1kEZqYSw-QhEaqXu8LoA͠^cZEbSט#u׉׉ 7cassandra://QNmgEF1r62DINEz3YOVgRl1IRkM7_8zfs-alQP_lne0`׉ 7cassandra://sixR_2rkH_WDah9qwY93W0nwPyK8B_haw62YGfG1XJMUh`s׉ 7cassandra://QgTs6TmqLm_iv_OFILorLdihWKoSM91iJmuQDFDTsF0`׉ 7cassandra://_kreG85SVTUT299XjUKdkmASP_dZtNlmGUGlVfUXQGw:hͅz͠^cZEbU׉E0Rotational Molding 575 Extruder Extruded parison Knife Bottle mold Blown bottle Blow pin Mold closed and bottle blown (a) Blow pin removed Blow pin Injection-molding machine Parison mold Parison Cooling passages Parison transferred to blow mold (b) 2 Blown-mold station Blow-mold bottom plug Blown bottle Parison Blow-mold neck ring Transfer head Reciprocating-screw extruder 3 Stripper station Stripper plate Bottle Preform neck ring (c) Figure 19.14: Schematic illustrations of (a) the extrusion blow-molding process for making plastic beverage bottles; (b) the injection blow-molding process; and (c) a three-station injection blow-molding machine for making plastic bottles. Preform mold 1 Preform mold station Indexing direction Core-pin opening (Blown air passage) Blow mold Blown bottle Tail Heating passages ׉ 7cassandra://1X7mRbOkouCjXu_ZEJeJXqFbEbfuLp39jxztf6IuDOc/`cZ%Eb_׉E 576 Chapter 19 Plastics and Composite Materials: Forming and Shaping Pressurizing air inlet Outlet vent Primary axis Pressurizing fluid Mold Spindle Secondary axis Figure 19.15: The rotational molding (rotomolding or rotocasting) process. Trash cans, buckets, and plastic footballs can be made by this process. perpendicular axes (Fig. 19.15). For each production cycle, a premeasured quantity of powdered plastic material is placed inside the warm mold. The powder is from a polymerization process that precipitates a powder from a liquid. The mold is then heated, usually in a large oven, and rotated continuously about its two principal axes. The powder is tumbled against the mold, where the heat fuses the powder without melting it. For thermosetting parts, a chemical agent is added to the powder; cross-linking occurs after the part is formed in the mold. The machines are highly automated, with parts moved by an indexing mechanism, similar to that shown in Fig. 19.14c. A wide variety of parts are made by rotational molding, such as storage tanks, trash cans, boat hulls, buckets, housings, large hollow toys, carrying cases, and footballs. The outer surface finish of the part is a replica of the surface finish of the inside mold walls. Various metallic or plastic inserts also may be molded integrally into the parts. In addition to powders, liquid polymers (plastisols) can be used in rotational molding–PVC plastisols being the most common material. In this operation, called slush molding or slush casting, the mold is heated and rotated simultaneously. Due to the tumbling action, the polymer is forced against the inside walls of the mold, where it melts and coats the mold walls. The part is cooled while it is still rotating, and removed by opening the mold. Parts made are typically thin-walled products, such as boots, buckets for aerial cranes, and toys. Process Capabilities. Rotational molding can produce parts with complex, hollow shapes with wall thicknesses as small as 0.4 mm. Cycle times are longer than in other molding processes. Quality-control considerations usually involve accurate weight of the powder, proper rotational speed of the mold, and temperature–time relationships during the oven cycle. 19.6 Thermoforming Thermoforming is a process for forming thermoplastic sheets and films over a mold through the application of heat and pressure (Fig. 19.16). A sheet is first clamped and heated to the sag point (above the glasstransition temperature, Tg, of the polymer; Table 7.2), usually by radiant heating, and then forced against the ׉ 7cassandra://QgTs6TmqLm_iv_OFILorLdihWKoSM91iJmuQDFDTsF0`cZ%Eb`cZ%Eb_#בCט#u׉׉ 7cassandra://SQKw3LlXPxlhnrAhdxmYbMQaOMsXZc_U7TU7hp9P8k0G`׉ 7cassandra://NTIJ-hDSXqveEZdSeeaBbOKZWy1zhDlP8mXfz2YBbugk`s׉ 7cassandra://iypGYkeigC5yyczA4g6Mu_9qaKImUX_t2NbwJoAbYjo`׉ 7cassandra://H18jfIacrzGzCi1ITCCGJD_Q8zoKBaurAM0tzG-rK3gIB2R͠^cZEbWט#u׉׉ 7cassandra://Mi6XKzpRuj3P517bV41QP2fvStq62zjiO_MjlSXYQgg`׉ 7cassandra://l3lZUhogfro06Nkq-uwE9D3SPyj4RGwRA38ItswLNtsR`s׉ 7cassandra://4ZE9NCey0PJiAUZYLZyOq5gmKEezAuhIEnh9lnLoRiE`׉ 7cassandra://GoR1MF9YA_oKBHrZts5NGY0w_FN9qD7sbTlUo8U-5Qk͠^cZEbX׉E Compression Molding 577 Clamp Heater Plastic sheet Mold Vacuum line (a) Straight vacuum forming (b) Drape vacuum forming Mold Vacuum line Ram Clamp Plastic sheet Ring (c) Force above sheet (d) Plug and ring forming Figure 19.16: Various thermoforming processes for a thermoplastic sheet. These processes are commonly used in making advertising signs, cookie and candy trays, panels for shower stalls, and packaging. mold surfaces by applying a vacuum or air pressure. The sheets are available as a coiled strip or as blanks, with lengths and widths of various sizes. The mold is generally at room temperature, thus the shape produced becomes set upon contact with the mold. Because of the low strength of the materials shaped, the pressure difference caused by a vacuum is usually sufficient for forming. Thicker and more complex parts require air pressure, ranging from 100 to 2000 kPa, depending on the type of polymer and its thickness. Variations of the basic thermoforming process are shown in Fig. 19.16. Process Capabilities. Typical parts made by thermoforming are packaging, trays for cookies and candy, advertising signs, refrigerator liners, appliance housings, and panels for shower stalls. Parts with openings or holes cannot be formed by this process because the pressure difference cannot be maintained during forming. Because thermoforming is basically a combination of drawing and stretching operations, much like some sheet-metal forming processes, the material must exhibit high, uniform elongation, as otherwise it will neck and tear. Thermoplastics have high capacities for uniform elongation, by virtue of their high strain-rate sensitivity exponent, m (Section 2.2.7). Molds for thermoforming usually are made of aluminum, because high strength is not required and the machinability and thermal conductivity of aluminum is advantageous; thus, tooling is relatively inexpensive. The molds have small through-holes in order to aid vacuum forming. These holes typically are less than 0.5 mm in diameter, as otherwise they may leave circular marks on the parts being formed. Defects encountered in thermoforming include (a) tearing of the sheet during forming, (b) excessive nonuniform wall thickness, (c) improperly filled molds, (d) poor part definition, and (e) lack of surface details. 19.7 Compression Molding In compression molding, a preshaped charge of polymer, a premeasured volume of powder, or a viscous mixture of liquid-resin and filler material is placed directly into a heated mold cavity, which typically is around 200◦C but can be much higher. Forming is done under pressure from a plug or from the upper half of the die (Fig. 19.17), thus the process is somewhat similar to closed-die forging of metals (Section 14.3). Polymers also can be molded by cold or hot isostatic pressing (Section 17.3.2). Pressures range from about 10 to 150 MPa. As can be seen in Fig. 19.17, there is a flash that forms. Typical parts made are fittings, electrical and electronic components, washing-machine agitators, and housings. Fiber-reinforced parts, with chopped fibers, also are formed by this process. Compression molding is mainly used with thermosetting plastics, with the original material being in a partially polymerized state; thermoplastics and elastomers are also processed by compression molding. Curing times are in the range of 0.5 to 5 minutes, depending on the material and part thickness and its shape. The thicker the material, the longer the time required to cure. ׉ 7cassandra://iypGYkeigC5yyczA4g6Mu_9qaKImUX_t2NbwJoAbYjo`cZ%Eba׉E}578 Chapter 19 Plastics and Composite Materials: Forming and Shaping Heating elements Punch Open Charge Mold Knockout (ejector pin) Land Molded part (a) (b) (c) (d) Figure 19.17: Types of compression molding–a process similar to forging: (a) positive;(b) semipositive;and (c) flash, in which the flash is later trimmed off. (d) Die design for making a compression-molded part with external undercuts. Process Capabilities. Three types of compression molds are made: • Flash type, for shallow or flat parts • Positive type, for high-density parts • Semipositive type, for quality production. Undercuts in parts are not recommended; however, dies can be designed to open sideways (Fig. 19.17d) to allow removal of the molded part. In general, the complexity of parts produced is less than that from injection molding, but the dimensional control is better. Surface areas of compression-molded parts may range up to about 2.5 m2. Because of their relative simplicity, dies for compression molding generally are less costly than those for injection molding. Die materials typically are tool steels; they may be chrome plated or polished for improved surface finish of the molded part. 19.8 Transfer Molding Transfer molding is a further development of the compression molding process. The uncured thermosetting resin is placed in a heated transfer pot or chamber (Fig. 19.18), where it is heated, then injected into heated closed molds. Depending on the type of machine, a ram, plunger, or rotating-screw feeder forces the polymer to flow through the narrow channels into the mold cavity at pressures up to 300 MPa. The viscous flow generates considerable heat, raising the temperature and homogenizing the polymer; curing takes place by cross-linking. Because the resin is in a molten state as it enters the molds, the complexity of the parts made and their dimensional control approach those of injection molding. Process Capabilities. Typical parts made by transfer molding are electrical connectors, electronic components, rubber and silicone parts, and encapsulation of microelectronic devices. The process is especially Overlap Flash Closed Mold cavity Part Plug ׉ 7cassandra://4ZE9NCey0PJiAUZYLZyOq5gmKEezAuhIEnh9lnLoRiE`cZ%EbbcZ%Eba#בCט#u׉׉ 7cassandra://xX-1UfwIxur0x8uLiV_keAdQacibaoYGrxfNMEr-ZBM`׉ 7cassandra://zMtbhGQUGCEwvI4T-nJuirXE4t-r3D2GhTpaRbolFqEMe`s׉ 7cassandra://CA2dsHStsz4QGUOoozS2rH_0AGCkHRJYsi3GPSXBwYA`׉ 7cassandra://Ra_pFFb91GlnG0_GJJTRPNxmCRdYdM5f8kTjAa_k0T4͠k͠^cZEb[ט#u׉׉ 7cassandra://YW3xr6boDGAs2sL4dzCk15vUsdqTPKCuyTNXD8jH8-A`׉ 7cassandra://E6zoTMnSPzfk0eDgiOnZvuJur55v-kBiula0makwrSIs`s׉ 7cassandra://nus6k-N8kiVmYk9v6aawt0uWngRoVEqZRWOyz5i1Fpo`׉ 7cassandra://fS6vdCGJcHG52flw3UOe08Vzzmypyo0sBuNlL4BsnMQj͠^cZEb\׉ECasting 579 Sprue Transfer plunger Transfer pot and molding powder Knockout (ejector) pin Mold closed and cavities filled 1. 2. Mold open and molded parts ejected 3. Figure 19.18: Sequence of operations in transfer molding for thermosetting plastics. This process is suitable particularly for intricate parts with varying wall thickness. suitable for intricate shapes with varying wall thicknesses. The molds tend to be more expensive than those for compression molding, and some excess material is left in the channels of the mold during filling, which is later removed. 19.9 Casting Some thermoplastics, such as nylons and acrylics, and thermosetting plastics, such as epoxies, phenolics, polyurethanes, and polyester, can be cast into a variety of shapes, using either rigid or flexible molds (Fig. 19.19). Compared to other methods of processing plastics, casting is a slow but simple and inexpensive process. Also, the polymer must have sufficiently low viscosity in order to flow easily into the mold. Typical parts cast are gears (especially nylon), bearings, wheels, thick sheets, lenses, and components requiring resistance to abrasive wear (Section 33.5). In conventional casting of thermoplastics, a mixture of monomer, catalyst, and various additives (activators) is heated to above its melting point, Tm, and poured into the mold. The part is shaped after polymerization takes place at ambient pressure. Intricate shapes can be produced using flexible molds, which are Electrical leads Mold Liquid plastic Mold (a) (b) Electrical coil Housing or case (c) Figure 19.19: Schematic illustration of (a) casting, (b) potting, and (c) encapsulation processes for plastics and electrical assemblies, where the surrounding plastic serves as a dielectric. Coil Mold Punch Molded parts ׉ 7cassandra://CA2dsHStsz4QGUOoozS2rH_0AGCkHRJYsi3GPSXBwYA`cZ%Ebc׉E(580 Chapter 19 Plastics and Composite Materials: Forming and Shaping then peeled off and reused. As with metals, thermoplastics may be continuously cast (Section 5.4), with the polymer being carried over continuous stainless-steel belts and polymerized by external heat. Centrifugal Casting. This process is similar to centrifugal metal casting, described in Section 11.4.6, and is used with thermoplastics, thermosets, and reinforced plastics with short fibers. Potting and Encapsulation. In a variation of casting, potting and encapsulation involve casting the plastic material (typically a liquid resin, such as epoxy) around an electrical component, such as a transformer, to embed it in the plastic. Potting (Fig. 19.19b) is carried out in a housing or case, which becomes an integral part of the component. In encapsulation (Fig. 19.19c), the component is coated with a layer of the plastic, surrounding it completely and then solidifying. In both of these processes, the plastic material can serve as a dielectric (nonconductor); consequently, it must be free of moisture and porosity, which would require processing in a vacuum. Mold materials may be metal, glass, or various polymers. Small structural members, such as hooks and studs, may be encapsulated partially, by dipping them in a hot thermoplastic. A wide variety of polymer colors and hardnesses are available. These process are particularly important in the electrical and electronics industries. 19.10 Foam Molding Styrofoam cups, food containers, thermally insulating blocks, and shaped packaging materials, such as for shipping appliances, computers, and electronics, are made by foam molding, using expandable polystyrene beads as the raw material. The parts made have a cellular structure, wherein they may have open and interconnected porosity (for polymers with low viscosity) or have closed cells (for polymers with high viscosity). There are several techniques that are used in foam molding. In the basic operation, polystyrene beads, obtained by polymerization of styrene monomer, are placed in a mold with a blowing agent, typically pentane (a volatile hydrocarbon) or inert gas (nitrogen), and are exposed to heat, usually by steam. The beads expand, to as much as 50 times their original size, and take the shape of the mold cavity. The amount of expansion can be controlled by varying the temperature and time. Various other particles, including hollow glass beads or plastic spheres, may be added to impart specific structural characteristics to the foam produced. Polystyrene beads are available in three sizes: (a) small, for cups with a finished part density of about 50 kg/m3, (b) medium, for molded shapes, and (c) large, for molding insulating blocks, with a finished part density of about 15 to 30 kg/m3; they can all then be cut to size. The bead size selected also depends on the minimum wall thickness of the product; the thinner the part, the smaller is the size. Beads can be colored prior to expanding them, making a part integrally colored. Both thermoplastics and thermosets can be used for foam molding, but thermosets are in a liquid-processing form, and are thus in a condition similar to that of polymers in reaction-injection molding (Section 19.3.1). A common method of foam molding is using pre-expanded polystyrene beads, in which the beads are expanded partially by steam, hot air, hot water, or an oven, in an open-top chamber. The beads are then placed in a storage bin and allowed to stabilize for a period of 3 to 12 hours; they then can be molded into desired shapes. Structural Foam Molding. This process is used to make plastic parts that have a solid outer skin and a cellular core structure such as inexpensive furniture components, computer and business-machine housings, and moldings, thus replacing more expensive wood moldings. In this process, thermoplastics are first mixed with a blowing agent (usually an inert gas such as nitrogen), then injection molded into cold molds of desired shapes. The rapid cooling next to the cold mold surfaces produces a skin that is rigid, which can be as much as 2 mm thick; the core of the part is cellular in structure. The overall density of the part made can be as low as 40% of the density of the solid plastic. With a rigid skin and a less dense bulk, molded parts thus have a high stiffness-to-weight ratio (see also Fig. 3.2). ׉ 7cassandra://nus6k-N8kiVmYk9v6aawt0uWngRoVEqZRWOyz5i1Fpo`cZ%EbdcZ%Ebc#בCט#u׉׉ 7cassandra://8jbGuBwoi5KJTEfz9rjiKth1AjGxpJUJLI71FC1g9rU<`׉ 7cassandra://NoKqqxDypxu_BRQHvYP8thNvZtI-dXVxNKzCQxZZOuUl`s׉ 7cassandra://T-UsaPV2RrRsq4tHVl-YltSICRUehWhhXkf98NYjSQs`׉ 7cassandra://mCFFH1FMBULAcX8v30XkfLUYUMHzvZ6emFEtS2UPGZEa,͠^cZEb`ט#u׉׉ 7cassandra://EIbdnLUkSPbsLT3z37B2Zr_kzjDuDzsJhcR2UCIP8FQp`׉ 7cassandra://nEUY325h64uADRlcVr_TJX4Xy_d52VAlqGrOxtAt4EQ_n`s׉ 7cassandra://lJQjrYfIKrHqeStEmjJ4i_eSCF_M_f6X-CMYfwgX1Q8-`׉ 7cassandra://VW63AU2YrvPPFdlh0DtUJoWoEonCAuoHc4OHxcN7b1YͶ ͠^cZEba׉E5Processing Elastomers 581 Polyurethane Foam Processing. Such products as furniture cushions and insulating blocks are made by polyurethane foam processing. The operation starts with mixing two or more components; chemical reactions then take place after the mixture is (a) poured into molds of various shapes or (b) sprayed over surfaces with a spray gun, thus providing sound and thermal insulation. Several types of low-pressure and high-pressure machines are available, having computer controls to ensure proper mixing. The mixture solidifies into a cellular structure, the characteristics of which depend on the type and proportion of the components used. 19.11 Cold Forming and Solid-phase Forming Processes for cold working of metals, such as rolling, closed-die forging, coining, deep drawing, and rubber forming, described in Part III, also can be used to shape thermoplastics at room temperature (cold forming). Typical materials formed are polypropylene, polycarbonate, ABS, and rigid PVC. Important considerations are that (a) the polymer must be sufficiently ductile at room temperature, thus polystyrenes, acrylics, and thermosets cannot be formed, and (b) its deformation must be nonrecoverable, in order to minimize springback and creep of the shaped part. The advantages of cold forming over other methods of shaping plastics are: • Strength, toughness, and uniform elongation are increased • Polymers with high molecular weight (Section 7.2) can be used to make parts with superior properties • Cycle times generally are shorter than those in molding processes • Forming speeds are not affected by part thickness, because, unlike other processing methods, there is no heating or cooling involved. Solid-phase Forming. Also called solid-state forming, this process is carried out at a temperature 10◦ to 20◦C below the melting temperature of the plastic for a crystalline polymer. The shaping operation takes place while the polymer is still in a solid state. The main advantages of this process over cold forming are that forming forces and springback are lower. These processes are not used as widely as hot-processing methods and generally are restricted to special applications. 19.12 Processing Elastomers Recall from Section 7.9 that, in terms of its processing characteristics, a thermoplastic elastomer is a polymer; in terms of its function and performance, it is a rubber. The raw material is basically a compound of rubber with various additives and fillers. Additives include carbon black, an element that enhances elastomer properties such as tensile and fatigue strength, abrasion and tear resistance, ultraviolet protection, and resistance to chemicals. These materials are then mixed to break them down and to lower their viscosity; the mixture is subsequently vulcanized, using sulfur as the vulcanizing agent. This compound is then ready for such processes as calendering, extrusion, and molding, which may include placing fiber reinforcements. During final processing, the part becomes cross-linked, imparting the desirable elastic properties of rubber products, ranging from rubber boots to pneumatic tires. Thermoplastic elastomers are commonly shaped by extrusion or injection molding, extrusion being the more economical and faster process; they also can be shaped by blow molding or thermoforming. Thermoplastic polyurethane, for example, can be shaped by any of the conventional methods. It also can be blended with thermoplastic rubbers, polyvinyl chloride compounds, ABS, and nylon to impart specific properties. The temperatures for elastomer extrusion are typically in the range from 170◦ to 230◦C, and for molding are up to 60◦C. Dryness of the materials is important for product integrity. Reinforcements can be used in conjunction with extrusion, to impart greater strength. Examples of extruded elastomer products are tubing, ׉ 7cassandra://T-UsaPV2RrRsq4tHVl-YltSICRUehWhhXkf98NYjSQs`cZ%Ebe׉E 582 Chapter 19 Plastics and Composite Materials: Forming and Shaping Incoming material Figure 19.20: Schematic illustration of calendering. Sheets produced by this process subsequently are used in thermoforming. The process also is used in the production of various elastomer and rubber products. hoses, moldings, and inner tubes. Injection-molded elastomer products cover a broad range of applications, including components for automobiles and appliances. Rubber and some thermoplastic sheets are made by the calendering process (Fig. 19.20), wherein a warm mass of the compound is fed into a series of rolls, and masticated (compressed and kneaded into a pulp). Their thickness is typically 0.3 to 1 mm, but can be made less by stretching the material. The calendered rubber then may be molded into various products, such as tires and belts for machinery. Rubbers and thermoplastics also may be formed over both surfaces of a tape, paper, fabric, or plastics, thus making them permanently laminated. Roll surfaces may also be textured to produce a rubber sheet with various patterns and designs. Discrete rubber products, such as gloves, balloons, and swim caps, are made by repeatedly dipping or dip molding a solid metal form, such as in the shape of a hand for making gloves, into a liquid compound; the liquid that coats the mold takes the shape of the form. A typical compound is latex, a milk-like sap obtained from the inner bark of a tropical tree. The compound is then vulcanized (cross-linked), usually in steam, and then stripped from the form, becoming a discrete product. 19.13 Processing Polymer-matrix Composites As described in Chapter 9, polymer-matrix composites (PMC), also called reinforced plastics,are engineered materials, with unique mechanical properties, especially high strength-to-weight ratio, stiffness-to-weight ratio, fatigue strength, creep resistance, and directional properties. Because of their complex structure, however, reinforced plastics require special methods to shape them into consumer and industrial products (Fig. 19.21). Fabrication to ensure reliable properties in composite parts and structures can be challenging, particularly over the long range of their service life because of the presence of two or more different types of materials. The matrix and the reinforcing fibers in the composite have, by design, very different properties and characteristics; consequently, they have different responses to the methods of processing (Section 9.2). The several steps required for manufacturing reinforced plastics and the time and care involved make processing costs very high. This situation has necessitated the proper assessment and integration of design and manufacturing processes (concurrent engineering), in order to take advantage of the unique properties of these composites. The approach is to minimize manufacturing costs while maintaining long-range product integrity, reliability, and production rate. 19.13.1 Fiber Impregnation For good bonding between the reinforcing fibers and the polymer matrix, and to protect them during handling, fibers are surface treated by impregnation (sizing). When impregnation is carried out as a separate step, the resulting partially cured sheets are called by various terms, as described below. ׉ 7cassandra://lJQjrYfIKrHqeStEmjJ4i_eSCF_M_f6X-CMYfwgX1Q8-`cZ%EbfcZ%Ebe#בCט#u׉׉ 7cassandra://J6wckMAeMluzOpjrkzHs1wPcb8n9H2edN5AhjYkxaAI`׉ 7cassandra://-3jwHxuwMPuOEJi3kpJB7wlVjebDEBpHYhtPHw5tYnMN`s׉ 7cassandra://K9wgiZjjj2YjNEEBUYkZu6kbAA3MOWKwrbko3OeNga8Y`׉ 7cassandra://xKMeKBReEVs5jJ0_p8Iham5wKs5XrNoNzpip7L3V8rspf͠^cZEbeט#u׉׉ 7cassandra://LVplAizpWwF7NkDHVGI_32McXHm7y3gWyztdCZQVJcgZR`׉ 7cassandra://a9Um_CCj_zy0CRdBOHZwX6zmeUMyzpTlY8BNTEGE-IA``s׉ 7cassandra://OPou6XFZR78UnF8VD5ySJG9jsnFQ7joErl9FGgw--IA`׉ 7cassandra://AvnDmwlV2NxGhd4DlZyeLVCInlLxA9Cw2KyxbJVzwgAU$6͠^cZEbl׉EProcessing Polymer-matrix Composites 583 Figure 19.21: Reinforced-plastic components for a Honda motorcycle. The parts shown are front and rear forks, a rear swing arm, a wheel, and brake disks. Continuous strands Surface treatment Spools Resin Backing paper (a) (b) Figure 19.22: (a) Manufacturing process for polymer-matrix composite tape. (b) Boron-epoxy prepreg tape. These tapes are then used in making reinforced plastic parts and components with high strength-to-weight and stiffness-to-weight ratios, particularly important for aircraft and aerospace applications and sports equipment. Source: (a) After T.W. Chou, R.L. McCullough, and R.B. Pipes. (b) Courtesy of Avco Specialty Materials/Textron. Prepregs. In a typical procedure for making fiber-reinforced plastic prepregs (meaning pre-impregnated with resin), the continuous fibers are first aligned and subjected to a surface treatment to enhance their adhesion to the polymer matrix (Fig. 19.22a). They then are coated by dipping them in a resin bath and are made into a tape (Fig. 19.22b), typically in widths of 75 to 150 mm. Individual segments of prepreg tape are then cut and assembled into laminated structures, such as for the horizontal stabilizer for fighter aircraft. Typical composites made from prepregs are flat or corrugated architectural paneling, panels for construction and electrical insulation, and structural components of aircraft requiring good property retention over a period of time and under adverse conditions. These requirements include fatigue strength under hot or wet conditions, typically encountered by military aircraft. Because the process of laying prepreg tapes is a time-consuming and labor-intensive operation, highly automated computer-controlled tape-laying machines have been built for this purpose. Prepreg tapes are automatically cut from a reel and placed on a mold in the desired patterns, with much better dimensional control than can be achieved by hand. The layout patterns can be modified easily, quickly and with high reliability by computer control. ׉ 7cassandra://K9wgiZjjj2YjNEEBUYkZu6kbAA3MOWKwrbko3OeNga8Y`cZ%Ebg׉E 584 Chapter 19 Plastics and Composite Materials: Forming and Shaping Sheet-molding Compound. In making sheet-molding compound (SMC), continuous strands of reinforcing fibers are first chopped into short fibers (Fig. 19.23), and deposited in random orientations over a layer of resin paste. Generally, the paste is a polyester mixture, which may contain fillers, such as various mineral powders, and is carried on a polymer film, such as polyethylene. A second layer of resin paste is then deposited on top, and the sheet is pressed between rolls. The product is then shaped into rolls or is placed into containers, in several layers, and stored until it has undergone a maturation period and has reached the desired viscosity. The maturing process takes place under controlled conditions of temperature and humidity, and usually taking about one day. The molding compounds should be stored at a temperature sufficiently low to delay curing. They have a limited shelf life, usually about 30 days, and hence must be processed within this period. Alternatively, the resin and the fibers can be mixed together only at the time they are to be placed into the mold. Bulk-molding Compound. Bulk-molding compounds (BMC) are in the shape of billets (hence the term “bulk”), and generally are up to 50 mm in diameter. They are made in the same manner as SMCs and are extruded to produce a bulk form. When processed into products, BMCs have flow characteristics that are similar to those of dough, hence they also are called dough-molding compounds (DMC). Thick-molding Compound. Thick-molding compounds (TMC) combine the lower cost of BMCs with the higher strength of SMCs. They are generally injection molded, using chopped fibers of various lengths. One application is in electrical components because of the high dielectric strength of TMCs. 19.13.2 Molding of Reinforced Plastics There are several molding processes used for reinforced plastics. Compression Molding. The material is placed between two molds, and pressure is applied. The molds may be either at room temperature or heated to accelerate hardening of the part. The material may be a bulkmolding compound, powder, or it may be an uncured thermoset with a dough-like consistency. Generally, it is molded into the shape of a log, which subsequently is cut or sliced into the desired shape. Fiber lengths typically range from 3 to 50 mm, although longer fibers of 75 mm also may be used. Sheet-molding compounds also can be processed by compression molding; they are similar to bulkmolding compounds, except that the resin–fiber mixture is laid between plastic sheets to make a sandwich that can be handled easily. The sheets have to be removed before placing the SMC in the mold. Chopper Continuous strands Resin paste Carrier film Figure 19.23: Schematic illustration of the manufacturing process for producing fiber-reinforced plastic sheets. The sheet still is viscous at this stage and later can be shaped into various products. Source: After T.-W. Chou, R.L. McCullough, and R.B. Pipes. Resin paste Carrier film Compaction belt ׉ 7cassandra://OPou6XFZR78UnF8VD5ySJG9jsnFQ7joErl9FGgw--IA`cZ%EbhcZ%Ebg#בCט#u׉׉ 7cassandra://1AnFIZuqK7u2BPO4ua4VdMj6qmaoPZwUd4Y3NiLLZWY`׉ 7cassandra://hOSy928YYOLPbFoiqosERvnzZmPlvbUtfC8I_CA80iwj`s׉ 7cassandra://fxUfzWy9jE_oBhIfEg0O8ubL-ak5qqrZj4bWRwbxcJMs`׉ 7cassandra://yEj6W2H1Hc4GZwa3Y7ffpbLZr3kryjpxnOTMuDSRCLgͣ d͠^cZEbnט#u׉׉ 7cassandra://AM3tt2IdRxJ-ZKuh5VuP1YvSevc3NxPhCvXCZL9NK7A`׉ 7cassandra://_Y3G2fZzpevfoOa3M3actsY4-7g5byJIa0LgpOTsYdwV`s׉ 7cassandra://z7BTlbwv6Co8ksEn9ARY2nVPN7dl2NJgn4HuYU6puNc`׉ 7cassandra://Bf9cf5w91s_nX83YR98XGpRJn-3QwE5k_y0k7CSPlCIc͠^cZEbo׉EsProcessing Polymer-matrix Composites 585 Clamping bar Vacuum trap Atmospheric pressure Flexible bag Mold release Gel coat (a) Resin and glass Clamp Gasket Vacuum trap Mold Mold release Gel coat (b) Figure 19.24: Schematic illustration of (a) vacuum-bag forming and (b) pressure-bag forming. These processes are used in making discrete reinforced plastic parts. Source: After T.H. Meister. Vacuum-bag Molding. In this process (Fig. 19.24a), prepregs are laid in a mold to form the desired shape. The pressure required to shape the product and to ensure good bonding is applied by covering the layup with a plastic bag and creating a vacuum. Curing takes place at room temperature or in an oven. A variation of this process is pressure-bag molding (Fig. 19.24b). A flexible bag is placed over the resin and reinforcing fiber mixture, and pressure is then applied over the mold, at a range typically from 200 to 400 kPa. If higher heat and pressure are needed to make parts with higher density and fewer voids, the entire assembly is placed into an autoclave (a chamber under heat and pressure). Care should be exercised to maintain fiber orientation if specific directional properties are desired.With chopped-fiber materials, no specific orientation is intended. In order to prevent the resin from sticking to the vacuum bag, and also to facilitate removal of excess resin, several sheets of various materials, called release cloth or bleeder cloth, are placed on top of the prepreg sheets. Molds can be made of metal, usually aluminum, but more often they are made from the same resin (with reinforcement) as the material to be cured. This practice eliminates any difficulties caused by the difference in thermal expansion between the mold and the part. Contact Molding. Also referred to as open-mold processing, this is a series of processes that uses a single male or female mold, made of such materials as reinforced plastics, wood, metal, or plaster (Fig. 19.25). The operation is a wet method, in which the materials are applied in layers, and the reinforcement is impregnated with the resin at the time of molding. Contact molding is used in making laminated products,with high surface area–to-thickness ratios, hence the process is also called contact lamination. Typical examples of products made are backyard swimming pools, boat hulls, automotive-body panels, tub and shower units, and housings. The simplest method of contact molding is hand layup. The materials are first placed in proper order (resins and reinforcements), brushed with a liquid monomer, and shaped in the mold by hand using a roller (Fig. 19.25a). The squeezing action of the roller expels any trapped air bubbles while compacting the part. The reinforcements placed in the mold may consist of various shapes, including prepregs; their orientation in the final product can thus be controlled. In spray layup, molding is done by spraying the materials into the mold. As seen in Fig. 19.25b, both the resin and the chopped fibers are sprayed over the mold surfaces. Rolling the deposited materials to remove any porosity may be necessary, as is done in layup. Because the chopped fibers have random orientations, directional properties cannot be imparted in products made by spray layup. Note also that only the moldside surface of the formed part is smooth, because they have been in contact with the mold surfaces. Resin-transfer Molding. This process is based on transfer molding of polymers (Section 19.8). A resin is fist mixed with a catalyst, and is then forced by a piston-type, positive-displacement pump into the mold Resin and glass Metal or Flexible bag plastic mold Steam or hot water Air pressure 345 kPa ׉ 7cassandra://fxUfzWy9jE_oBhIfEg0O8ubL-ak5qqrZj4bWRwbxcJMs`cZ%Ebi׉E586 Chapter 19 Plastics and Composite Materials: Forming and Shaping Roving Roller Layup of resin and reinforcement Mold (a) Mold (b) Brush Chopped glass roving Spray Mold Resin Gantry crane Boat hull Mold (c) Figure 19.25: Manual methods of processing reinforced plastics: (a) hand layup and (b) spray layup. Note that, even though the process is slow, only one mold is required. The figures show a female mold, but male molds are used as well. These methods also are called open-mold processing. (c) A boat hull being made by these processes. Source: Courtesy of VEC Technology, LLC. cavity, which has been filled with a fiber reinforcement. The process is a viable alternative to hand layup, spray layup, or compression molding for low- or intermediate-volume production. Transfer/Injection Molding. This is an automated operation that combines the processes of compressionmolding, injection-molding, and transfer-molding. This combination has the good surface finish, dimensional stability, and mechanical properties obtained in compression molding; it also has the highautomation capability and low cost of injection molding and transfer molding. 19.13.3 Filament Winding, Pultrusion, and Pulforming Filament Winding. This is a process in which the resin and the fibers are combined at the time of curing in order to impart a composite structure (Fig. 19.26a). Axisymmetric parts, such as pipes and storage tanks, and even some nonsymmetric parts, are produced on a rotating mandrel. The reinforcing filament, tape, or roving is wrapped continuously around the form. The reinforcements are impregnated by passing them through a polymer bath. The products made by filament winding are very strong, because of their highly reinforced structure, and have high strength-to-weight ratios. Parts as large as 4.5 m in diameter and 20 m long have been made by this process. The process also has been used for strengthening cylindrical or spherical pressure vessels (Fig. 19.26b), made of such materials as aluminum and titanium, where the presence of a metal inner lining makes the part impermeable. Seven-axis computer-controlled machines can automatically dispense several unidirectional prepregs to make such nonsymmetric parts as aircraft engine ducts, fuselages, propellers, blades, and struts. ׉ 7cassandra://z7BTlbwv6Co8ksEn9ARY2nVPN7dl2NJgn4HuYU6puNc`cZ%EbjcZ%Ebi#בCט#u׉׉ 7cassandra://eXYB5WZqYP-iMbg1oqhw7zy436eoEYCjj2iN2CzWlGk`׉ 7cassandra://GZ_fLWXedergKxW-TuNPMzBWci0-2oMu1okWkY7Po4wV`s׉ 7cassandra://FIIQRZyj4__9KprOuKCEw4lCuuZk8KaOBXhwDOLRgl8Z`׉ 7cassandra://DRUqSkRYyUhOs1FH6pEW_iYv18iaosea86tsF595ywg*2T͠^cZEbtט#u׉׉ 7cassandra://bvM3ug9_gmYt9TwuVsjB3_qC-OW5LCZASh10GMQ_dHkF`׉ 7cassandra://Pi_BMUloI-KIftaeh5-DwPmxlcPshsZIPkyHQXv0mSEf`s׉ 7cassandra://Xn_71WPcSCuNjVObVcTSn3IvjexKT4orte-NgPasVyk`׉ 7cassandra://LpUPAysPuSBsshaE0ImB_uuG47Mv_fK_o5aFh37FbSIl͠^cZEbu׉EProcessing Polymer-matrix Composites 587 Continuous roving Traversing resin bath Rotating mandrel (a) (b) Figure 19.26: (a) Schematic illustration of the filament-winding process; (b) fiberglass being wound over aluminum liners for slide-raft inflation vessels for the Boeing 767 aircraft. Source: Courtesy of Brunswick Corporation. Pultrusion. Long parts with various uniform cross sections, such as rods, profiles, flat strips, and tubing, are made continuously by the pultrusion process; the sequence of operations is shown in Fig. 19.27. The reinforcements are typically glass roving or fabric, made of E type calcium aluminosilicate glass fiber (Section 9.2.1). It is continuously supplied through several bobbins. The bundle is first pulled through a thermosetting polymer bath (usually polyester), then through a preforming die, and finally through a heated steel die. The product is cured during its travel through the heated die, with a length of up to 1.5 m, and a speed that allows sufficient time for the polymer to set. Note that this is an operation similar to continuously baking bread or cookies, or making resin-bonded grinding wheels. The exiting material is then cut into desired lengths. Cross sections as large as 1.5 m × 0.3 m have been made by this process. Typical products made by pultrusion, which may contain up to about 75% reinforcing fiber, are golf clubs, ski poles, fishing poles, drive shafts, ladders, and handrails. Pulforming. Continuously reinforced products, other than those with constant cross-sectional profiles, are made by pulforming. After being pulled through the polymer bath, the composite is clamped between the two halves of a die and cured into a finished shape. Commonly made products are hammer handles reinforced by glass fibers and curved automotive leaf springs. Preforming die Heated die Saw Moving belt Pultrusion cut to length Infiltration tank Cured Puller pultrusion (a) (b) Figure 19.27: (a) Schematic illustration of the pultrusion process. (b) Examples of parts made by pultrusion. The major components of fiberglass ladders (used especially by electricians) are made by this process. They are available in different colors, but are heavier because of the presence of glass fibers. Source: Courtesy of Strongwell. Prepreg feed system ׉ 7cassandra://FIIQRZyj4__9KprOuKCEw4lCuuZk8KaOBXhwDOLRgl8Z`cZ%Ebk׉E d588 Chapter 19 Plastics and Composite Materials: Forming and Shaping Case Study 19.1 Polymer Automotive-body Panels Shaped by Various Processes Polymeric materials are commonly used for automobile bodies; this example outlines typical applications of polymers. Three commonly used and competing processing methods are: (a) injection-molding of thermoplastics and elastomers, (b) reaction-injection molding (RIM) of polyurea/polyurethanes, and (c) compression-molding of sheet-molding compound (SMC) with resin-transfer-molded polyester and vinylester. Typical examples of parts made are: • Body panels and other large exterior components made by injection molding • Front fenders and rear quarter panels made of polyphenylene-ether/nylon or thermoplastric polyester • Outer door panels made of polycarbonate/ABS • Fascias made of thermoplastic polyolefin. These materials are selected for design flexibility, impact strength and toughness, corrosion resistance, high durability, and low mass. Vertical panels and fascias are made in multicavity molds on large injection-molding machines; they are then assembled mechanically to a steel frame. Large exterior-body parts are made of reaction-injection molded polyurethane, although polyureas are important for body panels and bumpers. Thermoset fascias are made of reinforced RIM polyurethane and polyureas, because of their higher thermal stability, low-temperature toughness, and lower cycle times. Large horizontal exterior-body panels, such as hoods, roofs, and rear decks, are made of reinforced polyester or vinylester in the form of compression-molded sheet-molding compounds. Lower volume parts are made by resin-transfer molding. Environmental and recycling considerations in material and process selection for automobiles, as well as other products, continue to be important. For example, polyphenylene oxide is being replaced with polycarbonate, which is made out of 100% recycled or reclaimed materials. 19.13.4 Quality Considerations in Processing Reinforced Plastics The major quality considerations in the processes described thus far concern internal voids and gaps between successive layers of material. Volatile gases that develop during processing must be allowed to escape from the layup through the vacuum bag, in order to avoid porosity due to trapped gases. Microcracks may develop during improper curing or during the transportation and handling of parts. These defects can be detected using ultrasonic scanning and other techniques described in Section 36.10. Case Study 19.2 Manufacturing of Head Protector R Tennis Racquets Competitive tennis is a demanding sport; there is a continuing demand to produce exceptionally lightweight and stiff racquets to improve performance. A tennis racquet consists of a number of regions (Fig. 19.28), of which the sweet spot is of particular interest. When the tennis ball is struck at the sweet spot, the player has optimum control and power, and vibration is minimized. Several innovative racquet-head designs have been developed over the years to maximize the size of the sweet spot. A stiff composite material, with high-modulus graphite fibers in an epoxy matrix (Chapter 9), is used to make the racquet head. Orientation of the fibers varies in different locations of the racquet; the main tube for the racquet, for example, consists of carbon-epoxy prepreg, oriented at ±30◦ from layer to layer. ׉ 7cassandra://Xn_71WPcSCuNjVObVcTSn3IvjexKT4orte-NgPasVyk`cZ%EblcZ%Ebk#בCט#u׉׉ 7cassandra://otBIm6P3Xa-k0j4uyIHb3_HIrJZTscTbkvo8CSRLW6o `׉ 7cassandra://vyYbASd7xZbxGV2hOag804HmPtGRLhfziCUtR_VrTIQk`s׉ 7cassandra://QYQQ5fsE-kYXsWjMCI1K5JqRO5gX9_qD6fKijme-rYY`׉ 7cassandra://6NJub5LurONqJkorSXK9I-UD_cboncsDk9rT6UnqFtoRP͠^cZEbwט#u׉׉ 7cassandra://nw_az0h3_SQasDNfp6KmasSlZnnvDvLrJ5o65dnuYiE'q`׉ 7cassandra://WQUWCE5RmIhBeMZlw4e12yTPzbWUOr4gwgvpCVo1sYkBl`s׉ 7cassandra://BSL5IbxtUJdzqSJUSg7H3wjzrQlk-gtIQ1m4AzcRXRU`׉ 7cassandra://y1ZAhUtmTTpU7U1GQDqEJazPBVaFjoZaggOpPharcvo͂͠^cZEbx׉EProcessing Polymer-matrix Composites 589 The advantages in using such materials are obvious, in that stiff racquets allow higher forces to be applied to the ball. However, the use of these advanced materials also has led to an increased frequency of tennis elbow, a painful condition associated with tendons that anchor muscles to the bones at the elbow. The condition is due not only to the higher forces involved, but also to the associated greater vibration of the racquet encountered with every stroke, especially when balls are struck away from the sweet spot. An innovative design for a racquet, the Protector R (made by Head Sport AG) uses lead zirconate titanate (PZT) fibers, as an integral layer of the composite racquet frames. PZT is well known as a piezoelectric material (Section 3.7); that is, it produces an electric response when deformed. Modules of the fibers, called Intellifibers R, are integrated into the throat on all sides of the racket. The module consists of about 50 PZT fibers, each approximately 0.3 mm in diameter, sandwiched between two polyamide layers, with printed electrodes for generating the potential difference when the fibers are bent. During impact, the vibrations constantly excite the Intellifibers R, generating a very high voltage potential but at low current. The energy is stored, in real time, in coils on the printed circuit board (Chipsystem R) incorporated in the racquet handle, and released back to the Intellifibers R, in the optimal phase and waveform for the most efficient damping. The stored energy is sent back to the Intellifibers R in a phase that causes a mechanical force opposite to the vibration, thereby reducing it. The Chipsystem R is tuned to the first natural frequency of the racket, and it damps vibrations only within a range of its design frequency. Making a Protector R tennis racquet involves a number of steps: 1. A carbon-epoxy prepreg is first produced, as described in Section 19.13.1. 2. The prepreg is cut to the proper size and placed on a flat, heated bench to make the matrix material tackier, resulting in better adhesion to adjacent layers. 3. A polyamide sleeve (bladder) is then placed over a rod, and the prepreg is rolled over the sleeve. 4. When the bar is removed, the result is a tube of carbon-epoxy prepreg with a polyamide sleeve, that can be placed in a mold and internally pressurized to develop the desired cross section. The throat piece is molded separately by wrapping the prepreg around sand-filled polyamide preforms or expandable foam. Since there is no easy way to provide air pressure to the throat, the preform develops its own internal pressurization because of the expansion of air during exposure to elevated molding temperatures. If sand is used, it is removed through the holes drilled into the preform during the finishing operation. Prior to molding them, all components are assembled onto a template, and the final prepreg pieces are added to strategic areas. The main tube is bent around the template, and the ends are pressed together and wrapped with a prepreg layer, forming the handle. The PZT fibers are incorporated as the outer layer in the racquet in the throat area, and the printed electrodes are connected to the Chipsystem R.The racquet is then placed into the mold, internally pressurized, and allowed to cure. Note that this operation is essentially an internally pressurized, pressure-bag molding process (see Fig. 19.24b). Figure 19.29a shows a racquet as it appears directly after molding. The racquet then undergoes a number of finishing operations, including flash removal, drilling of holes to accommodate strings, and finishing of the handle, including wrapping it with a special grip material. A completed Head Protector R racquet is shown in Fig. 19.29b. This design has been found to reduce racquet vibrations by up to 50%, resulting in clinically proven reductions in tennis elbow, without any compromise in performance. Source: Courtesy of J. Kotze and R. Schwenger, Head Sport AG. ׉ 7cassandra://QYQQ5fsE-kYXsWjMCI1K5JqRO5gX9_qD6fKijme-rYY`cZ%Ebm׉E590 Chapter 19 Plastics and Composite Materials: Forming and Shaping Strings Beam Racquet head Sweet spot Shaft Open throat Grip Figure 19.28: A Head Protector R tennis racquet. Source: Courtesy of Head Sport AG. 19.14 Processing Metal-matrix and Ceramic-matrix Composites Metal-matrix composites can be made into near-net shaped parts by the following processes: • Liquid-phase processing basically consists of casting together the liquid-matrix material, such as aluminum or titanium, and the solid reinforcement, such as graphite, aluminum oxide, or silicon carbide by conventional casting processes or by pressure-infiltration casting. In the latter process, pressurized gas forces the liquid-metal matrix into a preform, usually shaped out of wire or sheet and made of reinforcing fibers. • Solid-phase processing utilizes powder-metallurgy techniques (Chapter 17), including cold and hot isostatic pressing. Proper mixing is important for homogeneous distribution of the fibers throughout the part. An example is the production of tungsten-carbide tools and dies, with cobalt as the matrix material. • Two-phase (liquid–solid) processing involves technologies that consist of rheocasting (Section 11.4.7) and the techniques of spray atomization and deposition. In the latter two processes, the reinforcing fibers are mixed with a matrix that contains both liquid and solid phases of the metal. In making complex metal-matrix composite parts with whisker or fiber reinforcement, die geometry and control of process variables are very important for ensuring the proper distribution and orientation of the fibers within the part. MMC parts made by PM techniques generally are heat treated for optimum properties. ׉ 7cassandra://BSL5IbxtUJdzqSJUSg7H3wjzrQlk-gtIQ1m4AzcRXRU`cZ%EbncZ%Ebm#בCט#u׉׉ 7cassandra://-KHnL6S4_wieRAXC65aQj2xsnug45xfrxwOxwjxkouMA\`׉ 7cassandra://FSiM3J-XAh9vePv_CQDTlwXqlDA0ygnDun2s8qyzTeI8'`s׉ 7cassandra://yN98edjSU07pjCfhvXckgNFiaBmLTfaj8CCqFYvFozA`׉ 7cassandra://QkRy9r8Y-ANV8SZzWvWfiud3y9l3Yt8HRyRxkYMNthMͨ/͠^cZEbzט#u׉׉ 7cassandra://-Nej1VZRTGOD8vRCJ-7b_kirxz5AIg5lyA18mAwCv7A$`׉ 7cassandra://60_Kmkkb1jPJmiW3NLNCLVKn5Xf4ou3NPStsOPLuA44f`s׉ 7cassandra://QppZio5udwjoiAybWWo2GRQHfq4eWQ7kY_BpVd_qf_wI`׉ 7cassandra://AElnHA168-TXqCK_K40I1Ehw4jXQRJcjb1hUviciQnk[I͠^cZEb{׉EProcessing Metal-matrix and Ceramic-matrix Composites 591 IntellifibersTM ChipsystemTM incorporated into handle Racquet head tube prepreg (a) IntellifibersTM Electrical connection to IntellifibersTM ChipsystemTM (b) Figure 19.29: (a) The composite Head Protector R racquet immediately after molding; (b) a completed Head Protector R racquet, highlighting the incorporation of piezoelectric Intellifibers R. Source: Courtesy of Head Sport AG. ׉ 7cassandra://yN98edjSU07pjCfhvXckgNFiaBmLTfaj8CCqFYvFozA`cZ%Ebo׉E 592 Chapter 19 Plastics and Composite Materials: Forming and Shaping Case Study 19.3 Metal-matrix Composite Brake Rotors and Cylinder Liners Some brake rotors are made of composites consisting of an aluminum-based matrix, reinforced with 20% silicon-carbide particles. First, the particles are stirred into molten aluminum alloys, and the mixture is cast into ingots. The ingots are then remelted and cast into shapes, by such processes as green-sand, bonded-sand, investment, permanent-mold, and squeeze casting. The rotors (a) are about one-half the weight of those made of gray cast iron, (b) conduct heat three times faster, (c) add stiffness and wearresistance characteristics of ceramics, and (d) reduce noise and vibration, because of internal damping in the rotors. To improve the wear- and heat resistance of cast-iron cylinder liners in aluminum engine blocks, aluminum-matrix liners are also available. The metal-matrix layer consists of 12% aluminum-oxide fiber and 9% graphite fiber, and has a thickness that ranges from 1.5 to 2.5 mm. 19.14.1 Processing Ceramic-matrix Composites Several processes, including such techniques as melt infiltration, controlled oxidation, and hot-press sintering, are used to make ceramic-matrix composites. • Slurry infiltration is the most common process for making ceramic-matrix composites. It involves the preparation of a fiber preform, which is first hot pressed and then impregnated with a combination of slurry (containing the matrix powder), a carrier liquid, and an organic binder. This process imparts high strength, toughness, and uniform structure, but the product has limited high-temperature properties. A further improvement on the process is reaction bonding or reaction sintering of the slurry. • Chemical-synthesis processes involve the sol-gel and the polymer-precursor techniques. In the solgel process,a sol (a colloidal fluid having the liquid as its continuous phase) that contains fibers is converted to a gel, which is then subjected to heat treatment to produce a ceramic-matrix composite. The polymer-precursor method is analogous to the process used in making ceramic fibers with aluminum oxide, silicon nitride, and silicon carbide. • In chemical-vapor infiltration, a porous fiber preform is infiltrated with the matrix phase, using the chemical vapor deposition technique (Section 34.6). The product has very good high-temperature properties, but the process is time consuming and costly. 19.15 Design Considerations Design considerations in forming and shaping plastics are similar to those for casting metals (Section 12.2). The selection of appropriate materials from an extensive list requires considerations of (a) service requirements, (b) possible long-range effects on properties and behavior, such as dimensional stability and wear, and (c) ultimate disposal of the product following its life cycle. Some of these issues are described in Sections I.4 and I.6 in the General Introduction, and Section 7.8. Outlined below are the general design guidelines for the production of plastic and composite-material parts: 1. The processes for plastics have inherent flexibility, thus a wide variety of part shapes and sizes can be produced. Complex parts, with internal and external features, can be produced with relative ease and at high production rates. Consequently, a process such as injection molding competes well with powder-injection molding and die casting. All are capable of producing complex shapes and having thin walls. ׉ 7cassandra://QppZio5udwjoiAybWWo2GRQHfq4eWQ7kY_BpVd_qf_wI`cZ%EbpcZ%Ebo#בCט#u׉׉ 7cassandra://hgMs_J7E93Ckmr_UZw_C9XmgxLcBnCQAt7--pUT8bgUW`׉ 7cassandra://H8P_do95ToLQDNvPze76FLXhAonalTlcGfn8TrsP5eA^`s׉ 7cassandra://94cHzXJTucPfEgEMV1E0UsZVOMIzhLfZvWvUUCBtLTw`׉ 7cassandra://c6p47Se40xLEas13gn3_no8R0RFz6zPgmk_OC77DB3A͠^cZEb}ט#u׉׉ 7cassandra://euR36VsFvPGEJZHwgNLs3RQE5GgriO6sTEqm4d0I2HIu`׉ 7cassandra://08CuKxYXrosY4_5UzV-61COfnu1LSrP2ZgqrINVY9ZYY`s׉ 7cassandra://Uyw2bn0DKcTn0os66Q1zJptWy31KRm9E9yI3ryQB2c4G`׉ 7cassandra://ajFbeLQ7jweCOkloShYZfnjzEvNc9QRVHcmEEoij_PEV̚͠^cZEb~׉E YDesign Considerations 593 2. In process substitutions, it is essential to consider the materials involved and that their characteristics may be very different, each having its own properties suitable to a particular method of production. 3. Compared with metals, plastics have much lower stiffness and strength; thus, section size, shape, and thickness must be selected accordingly. Depending on the application, a high section modulus can be achieved on the basis of design principles common to I-beams and tubes. 4. Large, flat surfaces can be stiffened by such simple means as specifying curvatures on parts. For example, observe the stiffness of very thin but gently curved slats in venetian blinds. Reinforcement with fibers or particles also are effective in achieving stiff and lightweight designs. 5. The overall part shape and thickness often determine the particular shaping or molding process to be selected. Even after a particular process is chosen, the design of parts and the dies should be appropriate for a particular shape generation (Fig. 19.30), dimensional control, and surface finish. 6. Because of low stiffness and thermal effects, dimensional tolerances, especially for thermoplastics, are not as small as in metalworking processes. For example, dimensional tolerances are much smaller in injection molding than they are in thermoforming. 7. As in casting metals and alloys, the control of material flow in the mold cavities is essential. The effects of molecular orientation during the processing of the polymer also must be considered, especially in extrusion, thermoforming, and blow molding. 8. Large variations in cross-sectional areas and section thicknesses, as well as abrupt changes in geometry, should be avoided. Note, for example, that the sink marks (pull-in) shown in the top piece in Fig. 19.30c are due to the fact that thick sections in a part solidify last. 9. Contractions in larger cross sections during cooling tend to cause porosity in plastic parts, as they do in metal casting (see Fig. 12.2), thus affecting product integrity and quality. By contrast, a lack of stiffness may make it more difficult to remove thin parts from molds after shaping them. 10. Low elastic moduli of plastics further requires that shapes be selected properly for improved stiffness of the component (Fig. 19.30b), particularly when saving material is an important factor. Note that these considerations are similar to those applicable to the design of metal castings and forgings, as is the need for drafts (typically less than 1 degree for polymers) to enable removal of the part from Original design Distortion Modified design Thick Pull-in (sink mark) Thin (a) (b) (c) Figure 19.30: Examples of design modifications to eliminate or minimize distortion in plastic parts: (a) suggested design changes to minimize distortion, (b) stiffening the bottoms of thin plastic containers by doming (a technique similar to the process used to shape the bottoms of aluminum beverage cans, see Fig. 16.31), and (c) design change in a rib to minimize pull-in (sink mark), caused by shrinkage during the cooling of thick sections in molded parts. ׉ 7cassandra://94cHzXJTucPfEgEMV1E0UsZVOMIzhLfZvWvUUCBtLTw`cZ%Ebq׉E 594 Chapter 19 Plastics and Composite Materials: Forming and Shaping molds and dies. Generally, the recommended part thickness ranges from about 1 mm for small parts to about 3 mm for large parts. 11. Physical properties, especially high coefficient of thermal expansion, are important factors. Improper part design can lead to uneven shrinking (Fig. 19.30a) and distortion (warping). Plastics can easily be molded around metallic parts and inserts; however, their interfacial strength and compatibility with metals, when so assembled, is an important consideration. 12. The properties of the final product depend on the original material and its processing history. For example, the cold working of polymers improves their strength and toughness. On the other hand, because of the nonuniformity of deformation, even in simple rolling, residual stresses develop in polymers, just as they do in metals. These stresses also can be due to thermal cycling of the part during processing. 13. However they are produced, the magnitude and direction of residual stresses are important factors, such as in stress cracking over time. Furthermore, these stresses can relax over a period of time and cause distortion of the part during its service life. 14. A major design advantage of reinforced plastics is the directional nature of the strength and stiffness of the composite (see, for example, Fig. 9.7). External forces applied to the part are transferred by the matrix to the fibers, which are much stronger and stiffer than the matrix. When all of the fibers are oriented in one direction, the resulting composite material is exceptionally strong in the fiber direction, but weak in the transverse direction. To achieve strength in two principal directions, individual unidirectional layers are laid at the controlled angles to each other, as is done in tape layup or filament winding. If strength in the third (thickness) direction is required, a different type of reinforcement, such as woven fiber, is used to form a sandwich structure. 19.16 Economics of Processing Plastics and Composite Materials General characteristics of processing of plastics and composite materials are given in Table 19.2. Note the wide range of equipment and tooling costs, and the economic production quantities. As described throughout this chapter, there is some relationship between equipment costs and tool and die costs. Table 19.2: Comparative Production Characteristics of Various Molding Methods. Equipment Molding method Extrusion Injection molding Rotational molding Blow molding Compression molding Transfer molding Thermoforming Casting Centrifugal casting Pultrusion Filament winding Spray layup and hand layup and tooling Production cost rate M–L VH M M H–M H M–L M–L H–M H–M H–M L–VL VH–H VH M–L H–M M M M–L M–L M–L H L L–VL VH = very high; H = high; M = medium; L = low; VL = very low. Economical production quantity VH VH M H H–M VH H–M L M–L H L L ׉ 7cassandra://Uyw2bn0DKcTn0os66Q1zJptWy31KRm9E9yI3ryQB2c4G`cZ%EbrcZ%Ebq#בCט#u׉׉ 7cassandra://8BDn2WgNaMeR8-7I7-j-KxDHyxBi-Iq6y_amUX8Ok8w*`׉ 7cassandra://IPhTu3E8qVu3Sa_ZFXZ6nVFk-IJb8S_mTIqNanuaNZ4\ `s׉ 7cassandra://2t382bylQr5YpyyNfYJBJzhzdJsggJPGuQrbUmGH9S0`׉ 7cassandra://hBCh5hl1p5VgQqHOqEneBWUhHJvO_qQaJk0qKfesYz8Fx͠^cZEb̀ט#u׉׉ 7cassandra://mavXuTegAF0QTQPcMFlKAAPWsKhSYPWDlCy7lBL_nhc:t`׉ 7cassandra://non_clp8M0D55XYAQL8f42716U7vA5P0kRKRJBQI12gKD`s׉ 7cassandra://NwwDl2BA50zEXPnizg9mEeZYusliLgnIQE-8WAPYG88`׉ 7cassandra://8NxOqLyHmt5ZKQsjeL_cufEvswJ8IWG_neyJV7XL3qYPT͠^cZEb́׉E Key Terms 595 The most expensive machines are for injection molding, followed by compression molding and transfer molding; tool and die costs also are high for these operations. Thus, in an operation like injection molding, the size of the die and the optimum number of cavities in the die for producing more and more parts in one cycle are important considerations, as they are in an operation like die casting. Larger dies may be considered in order to accommodate several cavities, with runners to each cavity, but at the expense of increasing die cost even further. On the other hand, more parts will be produced per machine cycle, thus the production rate will increase. A detailed analysis is thus required in order to determine the overall die size, the number of cavities in the die, and the machine capacity required to optimize the total operation, and to produce parts at minimum cost. Similar considerations also apply to all other processing methods described throughout this chapter. Summary • Thermoplastics can be shaped by a wide variety of processes, including extrusion, molding, casting, and thermoforming, as well as by some of the processes used in processing metals. The raw material usually is in the form of pellets, granules, and powders. • The high strain-rate sensitivity of thermoplastics allows extensive stretching in forming operations; thus, complex and deep parts can be produced easily. Thermosetting plastics generally are molded or cast, and they have better dimensional accuracy than forming thermoplastics. • Fiber-reinforced plastics are processed into structural components using liquid plastics, prepregs, and bulk- and sheet-molding compounds. Fabricating techniques include various molding methods, filament winding, pultrusion, and pulforming. The type and orientation of the fibers and the strength of the bond between fibers and matrix and between layers of materials are important considerations. • The design of plastic parts must take into account their low strength and stiffness, and such physical properties as high thermal expansion and generally low resistance to temperature. Inspection techniques have been developed to determine the integrity of plastic products. • Processing of metal-matrix and ceramic-matrix composites continues to undergo developments to ensure product integrity, reliability, and reduced costs. Metal-matrix composites are processed by liquid-phase, solid-phase, and two-phase processes. Ceramic-matrix composites can be processed by slurry infiltration, chemical synthesis, or chemical-vapor infiltration. • Relevant factors in the economics of the operations described include the costs of the machinery, level of controls, tooling, cycle times, and production rate and volume. Key Terms Blow molding Blow ratio Bulk-molding compound Calendering Casting Chemical synthesis Chemical-vapor infiltration Coat-hanger die Coextrusion Cold forming Compression molding Contact molding Encapsulation Extrusion Extrusion blow molding Filament winding Foam molding Hand layup ׉ 7cassandra://2t382bylQr5YpyyNfYJBJzhzdJsggJPGuQrbUmGH9S0`cZ%Ebs׉E 596 Chapter 19 Plastics and Composite Materials: Forming and Shaping Ice-cold forming Injection molding Insert molding Liquid-phase processing Masticated Melt spinning Open-die processing Overmolding Parison Pellets Plastisols Potting Prepregs Pulforming Pultrusion Reaction-injection molding Resin transfer molding Rotational molding Sheet-molding compound Sink marks Sizing Slurry infiltration Slush molding Solid-phase forming Solid-phase processing Spinneret Spinning Spray layup Structural foam molding Swell Thermoforming Thick-molding compound Transfer molding Two-phase processing Vacuum-bag molding Bibliography Advani, S.G., and Hsiao, K.-T. (eds.), Manufacturing Techniques for Polymer Matrix Materials, Woodhead Publishing, 2012. Baird, D.G., and Collias, D.I., Polymer Processing: Principles and Design, 2nd ed., Wiley, 2014. Boisse, P. (ed.), Advances in Composites in Manufacturing and Process Design, Woodhead Publishing, 2015. Brinson, G., Handbook of Advanced Composites and Polymer Manufacturing, Auris Reference Ltd., 2013. Campbell, F., Manufacturing Processes for Advanced Composites, Elsevier, 2004. Chanda, J.M., and Roy, S.K., Plastics Technology Handbook, 5th ed., CRC Press, 2017. Giles Jr., H.F., and Wagner, J.R., Jr., Extrusion, 2nd ed., Woodhead Publishing, 2013. Hopmann, C., Extrusion Dies for Plastics and Rubbers, 4th ed., Hanser, 2016. Johnson, P.S., Rubber Processing: An Introduction, Hanser, 2001. Kutz, M. (ed.), Applied Plastics Engineering Handbook: Processing and Materials, 2nd ed., William Andrew, 2016. Mallick, P.K., Processing of Polymer Matrix Composites, CRC Press, 2017. Mastro, P.F., Plastics Products Design, Wiley, 2016. Menning, G., Mold-Making Handbook, 3rd ed., Hanser, 2013. Osswald, T.A., Injection Molding Handbook, 2nd ed., Hanser, 2007. Rauwendaal, C., Polymer Extrusion, 5th ed., Hanser, 2014. Rosato, D., Plastics Technology Handbook, Momentum Press, 2010. Rotheiser, J., Joining of Plastics, 3rd ed., Hanser, 2009. Strong, A.B., Plastics: Materials and Processing, 3rd ed., Prentice Hall, 2005. ׉ 7cassandra://NwwDl2BA50zEXPnizg9mEeZYusliLgnIQE-8WAPYG88`cZ%EbtcZ%Ebs#בCט#u׉׉ 7cassandra://OLCFTGuhzrnbdoNSV13c22i_Corb9BefD4NGHdAHf30e`׉ 7cassandra://kcWysmYnWHYLM-Lv9XTQABmyEuBNUERyUoeFWKGpYLEV`s׉ 7cassandra://mO4aLgVDVbtXIslpzvDXHRZHdR25WjYQ4gWNmecLy2c`׉ 7cassandra://6rcw5IvTEMLWfmB_5CoUc6JBXh7Z-LsFBQtmXHys7WMP͠^cZEb̄ט#u׉׉ 7cassandra://nVIOkRCDqMn_7ozslvlwPZH28U5sfsjK_w9RTlbdCfEN`׉ 7cassandra://soem_Afhfh3FBSg-VREyk_pUeUw7xVIHiJde5vdfEVYt`s׉ 7cassandra://Imq7I_FsaTb8oC2ZZMKiRHWmoBvIq3i-9ouXMnYetsY\`׉ 7cassandra://tvu7xaZVZED_OGqc_dAe8HfuhlPX8BadhcYWu1xZh8kP͠^cZEb̅׉E JQualitative Problems 597 Strong, A.B., Fundamentals of Composites Manufacturing, 2nd ed., Society of Manufacturing Engineers, 2007. Tadmore, Z., and Gogos, C.G., Principles of Polymer Processing, 2nd ed., Wiley, 2006. Troughton, M.J., Handbook of Plastics Joining, 2nd ed., William Andrew, 2008. Wagner, J.R. (ed.), Handbook of Troubleshooting Plastics Processes, Wiley-Scriver, 2012. Review Questions 19.1. What are the forms of raw materials for processing plastics into products? 19.2. What is extrusion? What products are produced by polymer extrusion? 19.3. Describe the features of an extruder screw and their functions. 19.4. How are injection-molding machines rated? 19.5. What is (a) a parison, (b) a plastisol, and (c) a prepreg? 19.6. How is thin plastic film produced? 19.7. List several common products that can be made by thermoforming. 19.8. What similarities and differences are there between compression molding and closed-die forging? 19.9. Explain the difference between potting and encapsulation. 19.10. What is thermoforming? 19.11. Describe runner, gate, sprue and well. 19.12. Describe the advantages of cold-forming plastics over other plastic-processing methods. 19.13. What are the characteristics of filament-wound products? Explain why they are desirable. 19.14. Describe the methods that can be used to make tubular plastic products. 19.15. What is pultrusion? Pulforming? 19.16. How are plastic sheet and plastic film produced? 19.17. What process is used to make foam drinking cups? 19.18. If a polymer is in the form of a thin sheet, is it a thermoplastic or thermoset? Why? 19.19. How are polymer fibers made? Why are they much stronger than bulk forms of the polymer? 19.20. What are the advantages of coextrusion? 19.21. Explain how latex rubber gloves are made. Qualitative Problems 19.22. Describe the features of a screw extruder and its functions. 19.23. Explain why injection molding is capable of producing parts with complex shapes and fine detail. 19.24. Describe the advantages of applying the traditional metal-forming techniques, described in Chapters 13 through 16, to making (a) thermoplastic and (b) thermoset products. 19.25. Explain the reasons that some plastic-forming processes are more suitable for certain polymers than for others. Give examples. 19.26. Describe the problems involved in recycling products made from reinforced plastics. 19.27. Can thermosetting plastics be used in injection molding? Explain. 19.28. Inspect some plastic containers, such as those containing talcum powder, and note that the integral lettering on them is raised rather than depressed. Explain. ׉ 7cassandra://mO4aLgVDVbtXIslpzvDXHRZHdR25WjYQ4gWNmecLy2c`cZ%Ebu׉E =598 Chapter 19 Plastics and Composite Materials: Forming and Shaping 19.29. An injection-molded nylon gear is found to contain small pores. It is recommended that the material be dried before molding it. Explain why drying will solve this problem. 19.30. Explain why operations such as blow molding and film-bag making are performed vertically. 19.31. Comment on the principle of operation of the tape-laying machine. 19.32. Typical production rates are given in Table 19.2. Comment on your observations and explain why there is such a wide range. 19.33. What determines the cycle time for (a) injection molding, (b) thermoforming, and (c) compression molding? Explain. 19.34. Does the pull-in defect (sink marks) shown in Fig. 19.30c also occur in metal-forming and casting processes? Explain. 19.35. What determines the intervals at which the indexing head in Fig. 19.14c rotates from station to station? 19.36. Identify processes that would be suitable for small production runs on plastic parts, of, say, 100. 19.37. Identify processes that are capable of producing parts with the following fiber orientations in each: (a) uniaxial, (b) cross-ply, (c) in-plane random, and (d) three-dimensional random. 19.38. Inspect several electrical components, such as light switches, outlets, and circuit breakers, and describe the process or processes used in making them. 19.39. Inspect several similar products that are made of metals and plastics, such as a metal bucket and a plastic bucket of similar shape and size. Comment on their respective thicknesses, and explain the reasons for their differences, if any. 19.40. What are the advantages of using whiskers as a reinforcing material? 19.41. Construct a table that lists the main manufacturing processes described in this chapter. Indicate those that can be used for (a) thermoplastics; (b) thermosets; (c) composite materials. Quantitative Problems 19.42. Estimate the die-clamping force required for injection molding five identical 200 mm diameter disks in one die. Include the runners of appropriate length and diameter. 19.43. A 2-L plastic beverage bottle is made by blow molding a parison 125 mm long and with a diameter that is the same as that of the threaded neck of the bottle. Assuming uniform deformation during molding, estimate the wall thickness of the tubular portion of the parison. 19.44. Consider a Styrofoam R drinking cup. Measure the volume of the cup and its weight. From this information, estimate the percent increase in volume that the polystyrene beads have undergone. 19.45. In extrusion, what flight angle should be used on a screw so that a flight translates a distance equal to the barrel diameter with every revolution? 19.46. Consider the part in Problem 17.41. If this part is to be produced in injection molding, with four parts produced with each shot (see Fig. 19.10b), estimate the clamping force required. Sketch the layout of the part, sprue and runners. 19.47. Assume that you are asked to give a quiz to students on the contents of this chapter. Prepare five quantitative problems and five qualitative questions, and supply the answers. ׉ 7cassandra://Imq7I_FsaTb8oC2ZZMKiRHWmoBvIq3i-9ouXMnYetsY\`cZ%EbvcZ%Ebu#בCט#u׉׉ 7cassandra://xessueWPArXlpIPNYnacbT9uYcrPbteNbTdt660VlcUh`׉ 7cassandra://Qzo5XjJNzrDBEvdjXsYYN5OIlmYqPcoLBVpRubJr1e4iz`s׉ 7cassandra://zKSIcTW3IbFPxDouYlHDSlVsFR_h3yCkB-McjCNrQGg`׉ 7cassandra://iVQSN7QisWXNwddnSpYYiGRBw_XK-CB06cxb6Nqcu8wQ͠^cZEḃט#u׉׉ 7cassandra://p9fmNmVLWIPDTW7VDehBgyaQLWvMRhY1SpDayLfIR4w`׉ 7cassandra://qMijUWvkDNgyBfzihNoJ4lGozU2vSKrBREXBWQMLntA6`s׉ 7cassandra://3LoO63CyiaO3w90dZ2afP0tBNqCBHl6I7M_PdTiPtqAV`׉ 7cassandra://xgsXa9s4GTLUgJb7oGGCWdg7ueAq1iOhEm3N4EVflJc;U͠^cZEb̈׉E Synthesis, Design, and Projects 599 Synthesis, Design, and Projects 19.48. Make a survey of a variety of sports equipment, such as bicycles, tennis racquets, golf clubs, and baseball bats, and identify the components made of composite materials. Explain the reasons for and advantages of using composites for these specific applications. 19.49. Explain the design considerations involved in replacing a metal beverage can with one made completely of plastic. 19.50. Give examples of several parts suitable for insert molding. How would you manufacture these parts if insert molding were not available? 19.51. Give other examples of design modifications in addition to those shown in Fig. 19.30. 19.52. With specific examples, discuss the design issues involved in making products out of plastics vs. reinforced plastics. 19.53. Die swell in extrusion is radially uniform for circular cross-sections, but is not uniform for other cross-sections. Recognizing this fact, make a qualitative sketch of a die profile that will produce (a) square, (b) triangular, (c) elliptical, and (d) gear-shaped cross-sections of extruded polymer. 19.54. Inspect various plastic components in a typical automobile, and identify the processes that could have been used in making them. 19.55. Inspect several similar products that are made either from metals or from plastics, such as a metal bucket and a plastic bucket of similar shape and size. Comment on their respective shapes and thicknesses and explain the reasons for their differences. 19.56. Write a brief paper on how plastic coatings are applied to (a) electrical wiring, (b) sheet-metal panels, (c) wire baskets, racks, and similar structures, and (d) handles for electricians tools, such as wire cutters and pliers requiring electrical insulation. 19.57. It is well-known that plastic forks, spoons, and knives are not particularly rigid. What suggestions would you have to make them better? Describe processes that could be used for producing them. 19.58. Some plastic products have lids with integral hinges; that is, no other material or part is used at the junction of the two parts. Identify such products, and describe a method for making them. 19.59. Make a survey of the technical literature, and describe how different types of (a) pneumatic tires, (b) automotive hoses, and (c) garden hoses are manufactured. 19.60. Obtain a boxed kit for assembling a model car or airplane. Examine the injection-molded parts provided, and describe your thoughts on the layout of the molds to produce these parts. 19.61. In injection-molding operations, it is common practice to remove the part from its runner, place the runner in a shredder, and recycle the runner by producing pellets. List the concerns you may have in using such recycled pellets for products, as against “virgin” pellets. 19.62. An increasing environmental concern is the very long period required for the degradation of polymers in landfills. Noting the information given in Section 7.8 on biodegradable plastics, conduct a literature search on the trends and developments in the production of these plastics. 19.63. Examine some common and colorful plastic poker chips and give an opinion on how they were manufactured. 19.64. Obtain different styles of toothpaste tubes, carefully cut them across, and comment on your observations regarding (a) the type of materials used and (b) how the tubes were produced. 19.65. By incorporating small amounts of blowing agent, it is possible to manufacture polymer fibers with gas cores. List some applications for such fibers. ׉ 7cassandra://zKSIcTW3IbFPxDouYlHDSlVsFR_h3yCkB-McjCNrQGg`cZ%Ebw׉EChapter 20 Additive Manufacturing 20.1 Introduction 601 20.2 Additive Manufacturing Methodology 603 20.3 Extrusion-based Processes 606 20.4 Photopolymerization 608 20.5 Material Jetting 611 20.6 Powder Bed Processes 612 20.7 Laminated-object Manufacturing 617 20.8 Miscellaneous Processes 617 20.9 Emerging AM Applications 619 20.10 Direct Manufacturing and Rapid Tooling 619 20.11 Design for Additive Manufacturing 624 20.12 Additive Manufacturing Economics 626 Case Studies: 20.1 Functional Prototyping 602 20.2 Production of Athletic Shoes 610 20.3 Casting of Plumbing Fixtures 622 20.4 Implications of Powder Reuse 627 • This chapter describes the technologies associated with additive manufacturing (AM), sharing the characteristics of computer integration, production without the use of traditional tools and dies, and the ability to rapidly produce a single part or small batches of parts on demand. All have the basic characteristics of producing individual parts layer by layer. • Classes of processes used in additive manufacturing are reviewed, which include extrusionbased methods, photo polymerization, powder bed processes, sprayed powder approaches, and lamination-based methods. 600 ׉ 7cassandra://3LoO63CyiaO3w90dZ2afP0tBNqCBHl6I7M_PdTiPtqAV`cZ%EbxcZ%Ebw#בCט#u׉׉ 7cassandra://EgMG8ts9jCU9WJF1ZThFfgw3F86AMzMoMTU-dRO46zU'`׉ 7cassandra://-WQXcEolYEnuH0zni3pt541vfgrNK672OEShIh9NHi4P`s׉ 7cassandra://HmEWQeGpQ_X2hbQ0JC4MTYLy4n2W3nDn5t8hl1Slsf4`׉ 7cassandra://Uou6PE5WH7QzbhXNyGXo7h48Y7MjbavcktZb8mo_yKsg͠^cZEb̋ט#u׉׉ 7cassandra://k5gwh4QJ-E2OpSjBhFLrqEWzGBV16e88zqlIEW8uKK0]o`׉ 7cassandra://vfdxTJzKHK4M7YkBjKqyu9otuJij2crHZ2vhSiYC7V8]`s׉ 7cassandra://a11L1HI4Er5-QFaXbIGtrx7uGb-19B8OcWyO48K6Rbo`׉ 7cassandra://qzU7dyJzixPl-Vus9YbQ8Vec4Inz3HkxyVuPW68tj7oua ͠^cZEb̌׉EIntroduction 601 • The practice of applying additive manufacturing techniques to the production of tooling that can be used in other manufacturing processes is described. • The chapter closes with a summary of additive manufacturing design, opportunities, and economics. Typical parts made: A wide variety of metallic and nonmetallic parts for product design analysis, evaluation and finished products. Alternative processes: Machining, casting, molding, powder metallurgy, forging, and fabricating. 20.1 Introduction Making a prototype, the first full-scale model of a product, has traditionally involved flexible manufacturing processes, and often required several weeks or months. Prototypes can now be quickly produced through subtractive processes (basically involving computer-controlled machining operations, described in Chapters 21–25) or by virtual prototyping (involving advanced graphics and software). An important advance is additive manufacturing, by which a solid physical model of a part is made directly from a three-dimensional CAD drawing without the use of tools, and allowing for extremely complex geometries (Fig. 20.1). Additive manufacturing is a suite of processes using different approaches, including photo polymerization, robot controlled extrusion, selective sintering, etc. This chapter describes additive manufacturing, formerly called rapid prototyping, whereby parts are built in layers. Developments in additive manufacturing began in the mid-1980s. The advantages of this technology include: • Physical models of parts, produced from CAD data files, can be manufactured in a matter of minutes to hours, and thus allow the rapid evaluation of manufacturability and design effectiveness. In this way, additive manufacturing serves as an important tool in the product development process. • A wide variety of materials are available, ranging from compliant rubber-like polymers to stiff polymers, metals, and ceramics. (a) (b) (c) Figure 20.1: Examples of parts made by additive manufacturing processes: (a) a selection of parts from fused-deposition modeling; (b) full-color model of an anatomical model; and (c) a speaker cover produced by the CLIP process. Source: (a) and (b) Courtesy of Stratasys, Inc., (c) Courtesy of Carbon, Inc. ׉ 7cassandra://HmEWQeGpQ_X2hbQ0JC4MTYLy4n2W3nDn5t8hl1Slsf4`cZ%Eby׉E \602 Chapter 20 Additive Manufacturing • Additive manufacturing operations can be used in some applications to produce actual tooling for manufacturing operations (rapid tooling, see Section 20.10). Thus, one can make tooling in a matter of a few days. Case Study 20.1 Functional Prototyping Toys are examples of mass-produced items with universal appeal. Because some toys are actually complex, the function and benefits of a computer-aided design (CAD) cannot be ensured until prototypes have been made. Fig. 20.2 shows a CAD model and a rapid-prototyped version of a water squirt gun (Super Soaker Power Pack Back Pack R water gun), which was produced on a fused-deposition modeling machine. Each component was produced separately and assembled into the squirt gun; the prototype could actually hold and squirt water. The alternative would be to produce components on CNC milling machines or fabricate them in some fashion, but this can be done only at a much higher cost. By producing a prototype, interference issues and assembly problems can be assessed and, if necessary, corrected. Moreover, from an aesthetic standpoint, the elaborate decorations on such a toy can be more effectively evaluated from a prototype than from a CAD file. Also, they can be adjusted to improve the toy’s appeal. Each component, having its design verified, then has its associated tooling produced, with better certainty that the tooling, as ordered, will produce the parts desired. Additive manufacturing has now been transformed from a prototyping technology to a viable strategy for product production. In addition to traditional approaches to manufacturing, the use of additive manufacturing introduces opportunities, including the following: 1. A part produced from additive manufacturing can itself be used in subsequent manufacturing operations to produce the final parts. Also called direct prototyping, this approach can serve as an important manufacturing technology. 2. Mass customization can be achieved, where every part can be tailored to a particular user or an application. For example, it is possible to create prosthetic devices that are tailored to individual patients, based on scanned measurements of the person to produce an optimum fit and function. These prosthetics have the ability to allow bathing and are more comfortable, and have fewer complications than cast supports. (a) (b) Figure 20.2: Additive manufacturing of a Super Soaker R squirt gun. (a) original CAD description of a toy; (b) fully functional toy produced through fused-deposition modeling. Source: (b) Courtesy of Rapid Models and Prototypes, Inc., and Stratasys, Inc. ׉ 7cassandra://a11L1HI4Er5-QFaXbIGtrx7uGb-19B8OcWyO48K6Rbo`cZ%EbzcZ%Eby#בCט#u׉׉ 7cassandra://MOjktcLJreS93nbF6kZgl06nFLzHtc8nwPTWJZAyHD8eV`׉ 7cassandra://rLVC17pLjcHQMfPE3CW_45xvkhE_O067GAdvyj75pdca`s׉ 7cassandra://0BVCFPbpfZqmLWyl_MGDqVtTNmLovJdOBm-fruNmvDk`׉ 7cassandra://af_TXYmvt-H5V97KoaQNhcQHXHxm4F_3lohbj5dxVL8b̘͠^cZEb̎ט#u׉׉ 7cassandra://r4nwhrLj-jEiv_h9A5nUJQwz2ehv1gx9tRw3H7KCb1YU`׉ 7cassandra://V7bYX3tuaRPm5sK7mFGDNqD9chOZgS73AiaYK1lgWNkH`s׉ 7cassandra://qz-0xeWmuHDfXDyRJZBZ7qf9xVOOOi1KApM-nJfTcNU@`׉ 7cassandra://3Q1bYuu27t-a9jaapkqcvxU6lMSOZ-i7EltqJQvm-cE`p͠^cZEb̏׉E-Additive Manufacturing Methodology 603 Table 20.1: Characteristics of Additive Manufacturing Technologies. Supply Process Stereolithography CLIP Multijet/PolyJet Material jetting Fused-deposition modeling Binder jetting Selective laser sintering Selective Laser melting Electron-beam melting Laminated-object manufacturing Laser-engineered net shaping phase Layer creation technique Type of phase change Materials Liquid Liquid layer curing Photopolymerization Photopolymers (acrylates, epoxies, colorable resins, and filled resins) Liquid Liquid layer curing Liquid Droplet deposition Solid Extrusion of melted polymer Powder Binder-droplet deposition onto powder layer Powder Layer of powder Powder Layer of powder Powder Layer of powder Solid Deposition of sheet material Powder Injection of powder stream Photopolymerization Solidification No phase change Sintering Solidification Solidification No phase change Solidification Similar to stereolithography Liquid Liquid layer curing Photopolymerization similar to stereolithography Solidification Polymers and wax Thermoplastics such as ABS, polycarbonate, and polysulfone Ceramic, polymer, or metal powder; sand Polymer powder such as nylon Metal powders such as stainless steel, titanium, copper, and aluminum Titanium and titanium alloys, cobalt chrome Paper and polymers Titanium, stainless steel, aluminum 3. Widespread application of additive manufacturing allows distributed manufacturing,sothatparts can be produced anywhere and not only in factories. For example, prosthetics and braces can be produced at a hospital, avoiding time associated with orders and shipping. Thus, a child with a broken arm can be brought to a hospital and fixed with an optimum brace essentially as quickly as a typical conventional treatment. Almost all materials can be used through one or more additive manufacturing approaches, as outlined in Table 20.1. However, because their properties are more suitable for these unique operations, polymers are the most commonly used material today, followed by metals and ceramics (see Table 20.2); still, new processes are being introduced continually. The rest of this chapter serves as a general introduction to the most common additive manufacturing operations, describes their advantages and limitations, and explores the present and future applications of these processes. 20.2 Additive Manufacturing Methodology Additive manufacturing operations all build parts in layers, as summarized in Table 20.1. These processes use various physics to achieve a desired part, and a wide variety of materials can be used. All of the processes described in this section build parts layer by layer. In order to visualize the methodology employed, it is beneficial to think of the construction of a loaf of bread by stacking and bonding individual slices of bread on top of each other (hence the term additive). The main difference between the various additive processes lies in the method of producing the individual slices, which are typically 0.03 to 0.5 mm thick, although they can be thicker or thinner in some systems. All additive operations require dedicated software. Note as an example, the solid part shown in Fig. 20.3a. The first step is to develop a CAD file description of the part; the computer then constructs slices of the three-dimensional part (Fig. 20.3b). Each slice is analyzed separately, and a set of instructions is compiled in order to provide the AM machine with detailed information regarding the manufacture of the part. A trajectory often has to be planned in order to produce the slice. For example, Fig. 20.3d shows the ׉ 7cassandra://0BVCFPbpfZqmLWyl_MGDqVtTNmLovJdOBm-fruNmvDk`cZ%Eb{׉E 604 Chapter 20 Additive Manufacturing Table 20.2: Mechanical Properties of Selected Materials for Additive Manufacturing. Tensile Elastic Process Stereolithography Material Accura 60 Somos 9920 WaterClear Ultra WaterShed 11122 DMX-SL 100 PolyJet FC720 FC830 FC 930 Fused-deposition modeling Polycarbonate Ultem 9085 ABS-M30i strength modulus (MPa) 68 32 56 47.1–53.6 32 60.3 49.8 1.4 52 71.64 36 (GPa) 3.10 1.35–1.81 2.9 2.65–2.88 2.2–2.6 2.87 2.49 0.185 2.0 2.2 2.4 Elongation in 50 mm (%) 5 15–26 6–9 11–20 Characteristics Transparent; good general-purpose material for additive manufacturing Transparent amber; good chemical resistance; good fatigue properties; used for producing patterns in rubber molding Optically clear resin with ABS-like properties Optically clear with a slight green tinge; mechanical properties similar to those of ABS; used for rapid tooling 12–28 Opaque beige; good general-purpose material for additive manufacturing 20 20 218 3 5.9 Transparent amber; good impact strength, good paint adsorption and machinability White, blue, or black; good humidity resistance; suitable for general-purpose applications Semiopaque, gray, or black; highly flexible material used for prototyping of soft polymers or rubber White; high-strength polymer suitable for additive manufacturing and general use Opaque tan, high-strength FDM material, good flame, smoke and toxicity rating. 4 Available in multiple colors, most commonly white; a strong and durable material suitable for general use; biocompatible PC CLIP Rigid polyurethane Flexible polyurethane Epoxy Selective laser sintering Urethane methacrylate Aluminum AlSi12 alloy 17-4 stainless steel 316L stainless steel Titanium GR.5 WindForm XT Polyamide PA 3200GF SOMOS 201 ST-100c Electron-beam melting Ti-6Al-4V 68 45 29 88 46 1300 600 2.28 1.9 0.86 3.14 2 — 1100 77.85 45 – 305 970–1030 190 120 7.32 3.3 0.015 137 120 4.8 280 5.2 17 16 40 30 White; good combination of mechanical properties and heat resistance 100 Wide variety of colors Similar to rubber band 480 — 5.5 Common aluminum alloy for AM Properties are after heat treatment After stress relief Properties are after stress relief 2.6 6 110 10 Opaque black polymide and carbon; produces durable heat- and chemicalresistant parts; high wear resistance. White; glass-filled polyamide has increased stiffness and is suitable for higher temperature applications Multiple colors available; mimics mechanical properties of rubber Bronze-infiltrated steel powder 12–16 Can be heat treated by HIP to obtain up to 600 MPa fatigue strength ׉ 7cassandra://qz-0xeWmuHDfXDyRJZBZ7qf9xVOOOi1KApM-nJfTcNU@`cZ%Eb|cZ%Eb{#בCט#u׉׉ 7cassandra://FWOWrdH1bvsOD9rZmVuYMVLh6cKXBoUd9ufVCoN_MvUh`׉ 7cassandra://dbmQhj4k1xweTavm7EQUq3ecFXepv5TyxCsZigzckh8C`s׉ 7cassandra://na-IYCfHaabMNI3974zf16O1xflW3tLjLBGk3auQQ5s`׉ 7cassandra://FC4k-WlECIPWyQ50ITrWWQfceGz-9_oMCIiSZRtxWEct(͠^cZEb̑ט#u׉׉ 7cassandra://SSz9Dzgz8KT-SHfTy2aXGpAOk9OwVr74QCQNVP1oIlYs-`׉ 7cassandra://cLytlxCDA673VkH_tsp4ZzD_rj5gjn2HyaF2_FHkRrAl:`s׉ 7cassandra://f0gmOI2Ip0EGnZdJ_hca5Cwlch63oHIKRcm5csMR4IE`׉ 7cassandra://-xNrG1ddHbxXlNEbeCXrioKdEdqSuQPKO3xLdcDG9WM͔":͠^cZEb̒׉EbAdditive Manufacturing Methodology 605 (a) (b) Side View Model Model Support Support (c) (d) Figure 20.3: The computational steps in producing a stereolithography (STL) file. (a) Three-dimensional description of part. (b) The part is divided into slices; only 1 in 10 is shown. (c) Support material is planned. (d) A set of tool directions is determined to manufacture each slice. Also shown is the extruder path at section A–A from (c) for a fused-deposition-modeling operation. path of the extruder in one slice, using the fused-deposition-modeling operation (Section 20.3.1). Similar paths will also be planned for the traverse of a laser in a powder bed process (Section 20.6). Other processes, such as binder jetting or CLIPS, do not require a path to be generated, but still need a definition of the desired slice. Triangular tessellation of surfaces has become an industry standard and is widely used for the geometry definition (see Section 38.4.2). The production of a path requires operator input, both in the setup of the proper computer files and in the initiation of the production process. Following this stage, the machines generally operate unattended and produce a rough part after a few hours, or a few minutes for smaller parts. The part is then subjected to a series of finishing operations, such as sanding and painting, in order to complete the process. ׉ 7cassandra://na-IYCfHaabMNI3974zf16O1xflW3tLjLBGk3auQQ5s`cZ%Eb}׉E606 Chapter 20 Additive Manufacturing a Desired part (a) Gussets (b) Island (c) Ceiling within an arch (d) Ceiling (e) Figure 20.4: (a) A part with a protruding section that requires support material. (b)–(e) Common support structures used in additive manufacturing machines. Source: After P.F. Jacobs. The setup and finishing operations are very labor intensive and the production time is only a portion of the time required to make a functional prototype. In general, additive processes are much faster than subtractive processes for limited production runs, taking as little as a few minutes to a few hours to produce a part. Supports. Complex parts, such as that shown in Fig. 20.4a, may be difficult to build directly. For such processes as fused deposition modeling or stereolithography, a common difficulty is encountered once the part has been constructed up to height a. The next slice would require the filament to be placed at a location where no material exists to support it. The solution is to produce a support material separately from the modeling material, as shown in Fig. 20.4b. Note that the use of such structures allows all of the layers to be supported by the material directly beneath them. The support is made of a less dense, less strong, or a soluble material, so that it can be removed after the part is completed. 20.3 Extrusion-based Processes 20.3.1 Fused-deposition Modeling In the fused-deposition-modeling (FDM) or fused filament fabrication (FFF) process (Fig. 20.5), a gantry-robot controlled extruder head moves in two principal directions over a table, which can be raised and lowered as required. The extruder head is heated and extrudes a (usually thermoplastic) polymer filament through a small orifice at a constant rate. The head follows a predetermined path (see Fig. 20.3d); the extruded polymer bonds to the previously deposited layer. The initial layer is placed on a foam foundation or other base. When a layer is completed, the table is lowered so that the next layer can be superimposed over the previous one. When the part is finished, it can easily be removed. In the FDM process, the extruded layer’s thickness is typically 125–325 µm; this thickness limits the best achievable dimensional tolerance in the vertical direction. In the x-y plane, however, dimensional accuracy can be as fine as 0.025 mm, as long as a filament can be extruded into the feature. Close examination of an FDM-produced part will indicate that a stepped surface exists on oblique exterior planes. If the roughness of this surface is unacceptable, subsequent polishing or smoothing with a heated tool can be performed. Also, a coating can be applied, often in the form of a polishing wax. Unless care is taken in applying these finishing operations, the overall dimensional tolerances may be compromised. An extreme application of FDM is big area additive manufacturing (BAAM), which can produce parts as large as 6 m × 2.3 m × 1.8 m, with a positioning accuracy of 25 µm. The feedstock in this process is injection molding compound (pellets, sometimes with carbon fiber reinforcement) instead of a filament, so that material costs are significantly lower than in other additive manufacturing processes. Even so, the filament in FDM can be a low cost material, often around $20–$40 per kilogram of spooled filament. Upon expiration of the initial patents for fused deposition modeling, a large number of machines based on FDM have been developed. Some do-it-yourself machines are now freely available as plans that can be downloaded from the Internet. Alternatively, some very inexpensive desktop machines have been marketed, based on these crowd-sourced designs, such as the system shown in Fig. 20.6. ׉ 7cassandra://f0gmOI2Ip0EGnZdJ_hca5Cwlch63oHIKRcm5csMR4IE`cZ%Eb~cZ%Eb}#בCט#u׉׉ 7cassandra://uCsqWvVEGFp7vFFMerrfVED1qoMqQPTCrT4Ymp5FyzM7;`׉ 7cassandra://3si-9lUuc73UuRK9NU8OLdll52DQS7qUqDNoY2wXt1c:`s׉ 7cassandra://NECx6p6pfYyssyMdbMt4ky3YQeiuPCVR0UvBuhOvuBI`׉ 7cassandra://u88udsSPn0yIwpf4_3KUcZX3civ3WcmXDPSiS-VFdmo*͠^cZEb̔ט#u׉׉ 7cassandra://D-K3toNf2oMgP_-SWhhEULmhUJ9B8x2z_2-0tSvdJIc`׉ 7cassandra://PS6h4DLoGD2jcfuBtIAGSJz3dLGw8OZ2E-K6-Mse5KYc*`s׉ 7cassandra://Xr2Une6jgo3i7h9gYYcbY0nLLJ_IsD8X4Pc3sa_ziq8`׉ 7cassandra://o5Vp7jWHAxXlMWx4IO5BtrVX9moiqY05Eo2-scPKHQ0z͠^cZEb̖׉EqExtrusion-based Processes 607 Thermoplastic filament z y x Heated build head moves in x–y plane Table moves in z-direction Fixtureless foundation Filament supply (a) (b) Figure 20.5: (a) Schematic illustration of the fused-deposition-modeling process. (b) Removing a part from an F370, a popular fused-deposition-modeling machine. Source: Courtesy of Stratasys, Inc. Plastic model created in minutes Figure 20.6: Low-cost additive manufacturing machine. The F1000, based on digital light printing stereolithography (see Section 20.4.1). The maximum build space is 125 mm × 70 mm × 120 mm. Source: Courtesy of 3D Systems. ׉ 7cassandra://NECx6p6pfYyssyMdbMt4ky3YQeiuPCVR0UvBuhOvuBI`cZ%Eb׉E 608 Chapter 20 Additive Manufacturing (a) (b) Figure 20.7: Continuous fiber fabrication (CFF). (a) Motorcycle brake lever produced with CFF using continuous carbon fiber reinforcement. (b) The Mark X CFF machine. Source: Courtesy of Markforged, Inc. The general trend for FDM materials is that the higher strength polymers require a higher processing temperature. Thus, stronger materials are more difficult to process, and warpage will be a greater concern. One of the differences between desktop systems and industrial FDM machines is the ability to process materials with better mechanical properties in the latter. Acrylonitrile butadiene styrene (ABS) and polylactic acid (PLA) are commonly used for prototyping and in desktop machines; nylon, polycarbonate (PC), and polyetheretherketone (PEEK) are common for high-strength components. FDM materials are available in a wide variety of colors. A recent development with FDM is continuous fiber fabrication (CFF), where a first extruder prints nylon in the desired pattern. A second head extrudes a continuous carbon, kevlar, or fiberglass fiber inside the part (Fig. 20.7). Control software allows placement of fiber in locations and orientations desired. Metal parts can be produced through two main methods: • A plastic filament impregnated with metal powder can be used to produce the desired part. Once completed, the part is sintered to burn off the polymer and fuse the metal, as with powder injection molding (see Section 17.3.3). • A metal paste can be extruded. This is commonly combined with a second print head that deposited a thermoplastic, allowing for direct inclusion of conductors inside a polymer part. Low-cost machines have enabled the development of maker spaces, where individual designers (typically high school students) are given access to FDM equipment, sometimes for a nominal fee. Along with Internet-based services that accept CAD files, this trend has brought additive manufacturing capabilities to the general public. Moreover, because of the low cost and availability of these machines, researchers are now able to apply new and innovative materials to rapid prototyping machines. Recent novel approaches include printing of food or biological materials for making medical implants, printing of artificial organs (bioprinting), clothing, and shoes (Section 20.9). 20.4 Photopolymerization 20.4.1 Stereolithography A common additive manufacturing process, one that actually was developed prior to fused-deposition modeling, is stereolithography (STL), a term coined by Charles W. Hull in 1986. This process (Fig. 20.8) is based on the principle of curing (hardening) of a liquid photopolymer into a specific shape. A vat, containing a mechanism whereby a platform can be lowered and raised, is filled with a photocurable liquid-acrylate polymer. The liquid is a mixture of acrylic monomers, oligomers (polymer intermediates), and a photoinitiator (a compound that undergoes a reaction upon absorbing light). ׉ 7cassandra://Xr2Une6jgo3i7h9gYYcbY0nLLJ_IsD8X4Pc3sa_ziq8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://duGjeXtDGIYssE2L5vECC7KAZ7mI8XzVvEaLDSLj-Yc `׉ 7cassandra://GTaejNURL4E7slSPmqnsl3Zidnf3ZrNjmzuGnlS3v9Mhp`s׉ 7cassandra://ezyOPBYYcKxzKOjgNX9Us_QAJPwQOVtXUiGwcgYx0lY`׉ 7cassandra://pk2UASGggWim1xKjSu6PTE4OSfhJBG-OBeZw5lXd1uUsK͠^cZEb̚ט#u׉׉ 7cassandra://1egs3RguNAn6LtcDIF6pYJt98NfBY-fYwQqjwP0L9OA`׉ 7cassandra://HuAu6FLrQiH5v9jKN4nlUIul4_rVON_uRzrYE6CZIfgbw`s׉ 7cassandra://2HwhXynwUB6z2j_lXiOEueuscPbjBO9l8ENTCT7_ddQ`׉ 7cassandra://4vS7anlrvVivxxd1WlH6NVobV5nJoUpxpWWXAahSJ4U T͠^cZEb̛׉EZPhotopolymerization 609 Platform motion UV light source UV curable liquid Liquid surface Formed part c b a Platform Figure 20.8: Schematic illustration of the stereolithography process. At its highest position (depth a in Fig. 20.8), a shallow layer of liquid exists above the platform. A laser, generating an ultraviolet (UV) beam, is focused upon a selected surface area of the photopolymer, and then moved around in the x–y plane. The beam cures that portion of the photopolymer (say, a ring-shaped portion), and thereby producing a layer of solid body. The platform is then lowered sufficiently to cover the cured polymer with another layer of liquid polymer; the sequence is then repeated until level b in Fig. 20.8 is reached. A cylindrical part, with a constant wall thickness, has thus been generated. Note that the platform has now been lowered by a vertical distance ab. At level b,the x–y movements of the beam define a wider geometry, thus a flange-shaped portion is being produced over the previously formed segment. After the desired thickness of the liquid has been cured, the process is repeated, producing another cylindrical section between levels b and c. Note that the surrounding liquid polymer is still fluid (because it has not been exposed to the ultraviolet beam), and that the part has been produced from the bottom up in individual slices. The unused portion of the liquid polymer can be used again to make another part or another prototype. Note that the term stereolithography as used to describe this particular process comes from the observations that the movements are three dimensional (hence the word stereo) and the process is similar to lithography (see Section 28.7). Note also that, as in FDM, stereolithography will sometimes require a support material, depending on geometry. In stereolithography, this support often takes the form of porous structures. After its completion, the part is removed from the platform, blotted, and cleaned ultrasonically with an alcohol bath. The support structure is then removed, and the part is subjected to a final curing cycle in an oven. The smallest tolerance that can be achieved in stereolithography depends on the sharpness of the focus of the laser, typically being around 0.0125 mm. Oblique surfaces also can be produced, with high quality. Solid parts can be made by applying special laser-scanning patterns to speed up production. For example, by spacing the scan lines in stereolithography, volumes or pockets of uncured polymer can be formed within cured solid shells. When the part is later placed in a postprocessing oven, the pockets are cured and a solid part is produced. Similarly, parts that are to be investment cast (Section 11.3.2) will have a drainable honeycomb structure, which permits a significant fraction of the part to remain uncured. Total cycle times in stereolithography range from a few hours to one day, without requiring postprocessing steps, such as sanding and painting. Depending on their capacity, the cost of the machines is in the range from $100,000 to $400,000. The cost of the liquid polymer is on the order of $80 per liter. The maximum part size that can be produced is 0.5 m × 0.5 m × 0.6 m. The layer height in STL is 25–100 µm, depending on the machine, and use a laser spot size of 50–150 µm. Stereolithography has been used with highly focused lasers to produce parts with micrometer-sized features. The use of optics required to produce such features necessitates the use of thinner layers and lower volumetric cure rates. When used to fabricate micromechanical systems (Chapter 29), this process is called microstereolithography. Vat ׉ 7cassandra://ezyOPBYYcKxzKOjgNX9Us_QAJPwQOVtXUiGwcgYx0lY`cZ%Eb׉E 610 Chapter 20 Additive Manufacturing Build platform UV curable resin Cured voxel Oxygen permeable window Dead zone Projector Resin Oxygen permeable window Dead zone UV light Figure 20.9: The CLIP process. An array of micro mirrors directs light to cure a layer, but the photopolymer next to the oxygen-permeable window does not cure because oxygen is a curing inhibitor. The cured polymer can be pulled out of the liquid photopolymer, with liquid photopolymer flowing into the interface to replenish the build layer. Another form of stereolithography is mask projection stereolithography or direct light processing (DLP) with the advantage of much higher rate of part production. In this process, a DLP device made up of millions of microscopic mirrors is used to direct UV light from a lamp or light emitting diode to expose the entire layer at once. 20.4.2 Continuous Liquid Interphase Production The continuous liquid interphase production (CLIP) process is illustrated in Fig. 20.9. CLIP uses a special window that is transparent to light and is permeable to oxygen, much like a contact lens. By controlling the oxygen diffusing through the window, a “dead zone” is created in the resin pool just tens of microns thick where photopolymerization cannot occur, as oxygen acts as an inhibiter. This ensures that a liquid layer will persist adjacent to the optics, regardless of light exposure. The projector transmits light in the desired pattern into the resin pool from underneath, curing the polymer above the dead zone. The build plate pulls the printed physical object out of the vat, at a speed low enough so that the cured material maintains contact with the uncured liquid and new liquid flows into the curing zone. The CLIP process is continuous, but does require discretization of layers from a CAD file and exposure of layers. The light is projected an entire layer at a time, not in raster fashion as in selective laser sintering or conventional stereolithography. The layer is produced through digital light processing (DLP) hardware that is also common in projector systems and some televisions. A DLP device consists of an array of micromirrors, each of which can direct light towards the build chamber if activated; by activating selected mirrors, “pixels” in the build space are activated, curing the polymer into voxels. A typical voxel dimension is 75 µm. Parts produced through the CLIP process must undergo secondary operations, consisting of, at least, cleaning and either a secondary UV flood cure or thermal cure in an oven. A variety of polymeric material chemistries are now available, and CLIPs can achieve production rates two orders of magnitude higher than other additive manufacturing processes. Case Study 20.2 Production of Athletic Shoes CLIP represents a breakthrough additive manufacturing process in that it allows manufacture of parts at high quantities, and provides a strategy for mass production for certain parts (see Section 37.2.2). Carbon3D, the developer of CLIP, has also developed designs and software to produce a metamaterial (Section 6.16) that has similar mechanical properties as polymer foam, but is easier to clean (Fig. 20.10). Build direction ׉ 7cassandra://2HwhXynwUB6z2j_lXiOEueuscPbjBO9l8ENTCT7_ddQ`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HCKkHAo4ZVWUiANAzpvL6tlbYwVA6joj50Y9NX5nYogM&`׉ 7cassandra://EqRnw5ANxzqJvUUNQC_zk_-UEHhngF_XN8_sgRlWzzAU`s׉ 7cassandra://_D2LjEYr5L-GyW8U-_axUrBcy3jTjXVVYmE719t0I5I`׉ 7cassandra://u_jMEH2kTu3y2zqzDTjTrTWP_8nXp8g5BiW58dfkJj8j͠^cZEb̝ט#u׉׉ 7cassandra://aEGtdkFHNVodil-gRhBeuSIwly_6LLfd8jT4aU1UmE4 `׉ 7cassandra://F8iPBJL5mlF9FIRV5_X1-Fcvqq9v7bdnj6WpDpCJN5MRc`s׉ 7cassandra://hxXuUyFQuL6p67sFwLf7qrwzJaObvspkmvJiPKEKbyY`׉ 7cassandra://JfuRUzxp_3Tn_7q8KHDzT26wES03eXWsLsp0CtfD0dslA͠^cZEb̞׉E Material Jetting 611 By placing more material or changing the metamaterial design where a higher stiffness is desired, it is possible to produce a shoe insole with tuned stiffness, that can also be varied by location. Adidas, well known for its athletic products and materials, partnered with Carbon3D to produce high performance footware, called the Futurecraft 4D (Fig. 20.11). Adidas had collected and maintained athlete data, which was used for the development of a stiffness-tuned midsole for the new shoe design. Adidas and Carbon3D have developed a digitized footwear-component creation process that eliminates the need for traditional prototyping or molding. CLIP also allowed Adidas to create a monolithic midsole that addresses precise needs related to movement, cushioning, stability, and comfort. Further, over the course of product development, CLIP enabled Adidas to evaluate more than 50 design iterations, a substantial increase when compared with what is achievable with traditional injection molding in the same amount of time. Moreover, engineers from both companies collaborated closely and tested nearly 150 resin iterations. The final midsole material is made of a dual cure resin that generates a polyurethane upon thermal cure. It is a stiff elastomer printed in a lattice structure to develop a high-performance midsole that also offers excellent durability and is aesthetically pleasing. Adidas expects to produce 100,000 such annually. Source: Courtesy Steven Pollack, Carbon and Adidas. Figure 20.10: The use of a metamaterial shoe sole. Note that the pattern in the metamaterial changes with location to obtain a desired mechanical performance. Source: Courtesy of Carbon, Inc. 20.5 Material Jetting Material jetting (MJ) is a class of additive manufacturing processes that include drop on demand (DOD) and the Polyjet related processes, also known as multijet modeling (MJM). The main difference is that PolyJet uses photopolymer feedstocks; DOD uses thermoplastics or wax. In both cases, a low viscosity is needed to produce droplet jets, which may require preheating of the build material. The PolyJet process is a form of material jetting where print heads deposit a photopolymer on the build tray. Ultraviolet bulbs, alongside the jets, instantly cure and harden each layer, thus eliminating the need for any postmodeling curing that is required in stereolithography. PolyJet results in a smooth surface with layers as thin as 16 µm that can be handled immediately after the process is completed. Two different materials are used: the material for the actual model, and a gel-like resin for support, such as shown in Fig. 20.4. Each material is simultaneously jetted and cured, layer by layer. When completed, ׉ 7cassandra://_D2LjEYr5L-GyW8U-_axUrBcy3jTjXVVYmE719t0I5I`cZ%Eb׉E 612 Chapter 20 Additive Manufacturing Figure 20.11: The Adidas Futurecraft 4D shoe, using a CLIP-produced metamaterial sole. Source: Courtesy of Carbon, Inc. the support material is removed by soaking in an aqueous solution. Build sizes have an envelope of up to 500 mm× 400 mm× 200 mm. The PolyJet process has capabilities similar to those of stereolithography and uses similar resins (Table 20.2). The main advantages of this process are the capabilities of avoiding part cleanup and lengthy post-process curing operations and the much thinner layers produced, thus allowing for better resolution. In DOD, a stream of a material droplets are ejected through a small orifice and deposited on a surface (target), using an ink-jet type mechanism. A second print head deposits a support material that is soluble in water or related solvent. DOD is commonly used for producing investment casting patterns (see Section 11.3.2). DOD is considered the most accurate form of 3D printing because of the absence of thermal stresses and because it is mainly used for small parts. Generally, a tolerance of ±0.1 mm can be achieved. A recent innovation is the nano particle jetting (NPJ) process, which uses a suspension of nanoparticles in a liquid carrier as the liquid printed in material jetting. It was noted previously that particles smaller than around 20 µm are difficult to spread onto a build chamber because they can easily become airborne and interfere with optics and lasers. The liquid carrier prevents the entrainment of small particles into air, and therefore allows the incorporation of much smaller particles than other processes. The build chamber is heated sufficiently to evaporate the carrier and bond the nanoparticles. Once completed, the particles are sintered to create fully dense parts. The use of small particles allows printing of detailed features and the development of superior mechanical properties. 20.6 Powder Bed Processes Powder Bed Processes involve a number of approaches that utilize powder as the workpiece material, and where the powder is deposited layer-by-layer in a bed or build chamber. Several powder application systems are used, but they typically involve a counter-rotating roller or a wiping mechanism. The deposited powder has limited green strength but can serve as a support for complicated parts. Powder spreading is a critical step in these processes. Some of the considerations associated with powder spreading are as follows: ׉ 7cassandra://hxXuUyFQuL6p67sFwLf7qrwzJaObvspkmvJiPKEKbyY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://1SkCbkI12tM3vtZHVTkvviLvQbtjMCR5Hh5GzVVxBD8]`׉ 7cassandra://t6rgYW8DUONF7XnM2t2z9CfCe8RpageYCwgDLbmagx8XF`s׉ 7cassandra://u7zC4GIGiGnyJ_BZMRR13nudrtKSNXchhRzIuw2MT6A`׉ 7cassandra://HQYRVRz6Ob7YB-rfMvsGxd8wi8rpP8i5P5mZD1kvegoH͠^cZEb̠ט#u׉׉ 7cassandra://KF9PzXqmhznzIzLk1AWucgvAulX4A9t_MBjj0nckvnk`׉ 7cassandra://x2HMYQCZ4I_5kzViQzpdDSLRiQ_s783BPTLsnugENRor`s׉ 7cassandra://Pl3HY9GDcZSDMvQHA46iPl0_2sjYH0AoZnySxU3n2Ggn`׉ 7cassandra://tlJZfdtEK8bOI-gyX2KpYspGnsmvS5q9HGWIcuQbWMgo?͠^cZEb̡׉E Powder Bed Processes 613 1. The powder must be capable of spreading into thin layers. Because this can be compromised by moisture, polymer powders may need to be dried before they can be effectively spread. 2. Mean particle sizes of the powder are around the layer thickness, or slightly smaller; there is a range of powder sizes that can be used. Particles larger than the layer thickness will be pushed ahead of the wiper or the roller and are less likely to be part of a spread layer; particles that are too small are likely to become airborne and adhere to exposed surfaces. 3. The powder may be preheated to reduce laser or electron beam power required for melting. 4. Powder explosions or fire can result from static electric discharge; thus, safety protocols regarding equipment and worker grounding, oxygen-free shielding gases, and increased humidity must be carefully followed. 20.6.1 Selective Laser Sintering Selective Laser Sintering (SLS) is a process based on sintering (Section 17.4) of nonmetallic powders selectively into an individual object. Direct Metal Laser Sintering (DMLS) or Selective laser melting (SLM) is a related process used with metals; in both cases, material is always at least partially melted. The basic elements in this process are shown in Fig. 20.12. Note that the bottom of the processing chamber is equipped with two cylinders: 1. A powder-feed cylinder, which is raised incrementally to supply powder to the part-build cylinder. 2. A part-build cylinder, which is lowered incrementally as the part is being shaped. In the SLS process, a thin layer of powder is first deposited in the part-build chamber. Then a laser beam, guided by a process-control computer using instructions generated by the three-dimensional CAD program of the desired part, is focused on that layer, tracing and sintering a particular cross section into a solid mass. The powder in other areas remains loose, but this powder can support the sintered portion; with some processes, separate support structures may still be needed. Another layer of powder is then deposited, and the cycle is repeated continuously until the entire three-dimensional part has been produced. Galvanometers Sintering laser Laser Optics Process chamber Roller mechanism Process-control computer Motor Powderfeed cylinder Motor Figure 20.12: Schematic illustration of the selective-laser-sintering process. Part-build cylinder Environmentalcontrol unit ׉ 7cassandra://u7zC4GIGiGnyJ_BZMRR13nudrtKSNXchhRzIuw2MT6A`cZ%Eb׉E 614 Chapter 20 Additive Manufacturing The feed chamber contains the desired part supported by unfused powder, called the cake, which has very low green strength. The loose particles are shaken or brushed off, and the part is recovered. The part may not require further curing to develop strength. The supports are removed or machined, and the part is then post-processed as required. A variety of materials can be used in this process, including polymers (such as acrylonitrile butadiene styrene, polyvinyl chloride, nylon, polyester, polystyrene, and epoxy), wax, metals, and ceramics (with appropriate binders). Generally the feed and build chambers are preheated. A major concern is the thermal management in the build chamber, as is the control of the powder during spreading and lasing. Powder that becomes airborne in the build chamber could coat sensors or optics and compromise productivity. Modern SLS machines therefore use a constant flow of shielding gas that directs the powder away from sensitive machine elements. Shielding gases vary with the powder material; argon and nitrogen are common options. SLS parts are susceptible to shrinkage and warpage due to thermal stresses. Each layer that is built is produced on a previous layer; as the new layer cools, it shrinks, which can cause a part to curl upwards. In extreme cases, the part can collide with the wiper or roller depositing a new powder layer, necessitating a build to be aborted. Despite thermal effects, dimensional tolerances of ±0.1 mm can be achieved with welldesigned parts. Layer thickness range from around 30–100 µm depending on the material. Stainless steel and titanium alloys generally produce the best part fidelity because of their lower thermal conductivities. SLS has a number of advantages over other AM processes. The material is generally isotropic and accurate (although not as good as stereolithography or material jetting) with very good mechanical properties. SLS does not usually need support materials, except for metal parts that are bonded to a build plate by support material to prevent part curl. The main drawback is the high cost of machines; metal-capable SLS machines cost around $300,000 for low-end machines, and can cost almost $1 million. 20.6.2 Electron-beam Melting A process similar to selective laser sintering and electron-beam welding (Section 30.6), electron-beam melting (EBM) uses the energy source associated with an electron beam to melt titanium or cobalt-chrome powder to make metal prototypes. The workpiece is produced in a vacuum, making the part build size limited to around 200 × 200 × 180 mm. Electron-beam melting is up to 95% efficient from an energy standpoint, as compared with 10–20% efficiency for selective laser sintering. In EBM, the supply powder and the build chamber are heated to near the material’s melting point, significantly reducing the energy needed to melt the metal and also reducing thermal stresses. Because the build chamber is at an elevated temperature, the melted metal solidifies more slowly and results in more fully dense parts. A volume build rate of up to 60 cm3/hr can be obtained, with individual layer thicknesses of 0.050–0.200 mm. Parts may also be subjected to hot isostatic pressing (Section 17.3.2) to improve their fatigue strength. Although applied mainly to titanium and cobalt-chrome alloys to date, the process is being developed also for stainless steels, aluminum, and copper alloys. 20.6.3 Binder-jet Printing In the Binder-jet Printing (BJP) process, also known as Binder Jetting or Three-Dimensional Printing, a print head deposits an inorganic binder material onto a layer of sand, polymer, ceramic, or metallic powder, as shown in Fig. 20.13. A piston, supporting the powder bed, is lowered incrementally and with each step a layer is deposited and then fused by the binder. Binder-jet printing allows considerable flexibility in the choice of materials and binders used. Common powder materials are polymers (sometimes blended with fibers), metals, and foundry sand. Since multiple binder print heads can be incorporated into one machine, it is possible to produce full-color prototypes by having different color binders (Fig. 20.14). The effect is a three-dimensional analog to printing photographs using three ink colors (red, cyan, and blue) in an ink-jet printer. ׉ 7cassandra://Pl3HY9GDcZSDMvQHA46iPl0_2sjYH0AoZnySxU3n2Ggn`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://eBq4beQOdDm7fEWqolbM3TpdbE1YBrLyQ4y1c0mFUho/`׉ 7cassandra://WKZqsAAR5eA9yfJcwHbMmAwsVfd1Fi9PApLX1zHZF5QG`s׉ 7cassandra://rsUS6Boqo-W3jB6xeQ71s-P8Oo_Hdvk2EaMGAicf7CE`׉ 7cassandra://MOYW719ikzdk-fCeBTRqK--9mPJqQjA6BErVlZPeL4s͡ ͠^cZEb̤ט#u׉׉ 7cassandra://YJXZviY6PUoD2jAmZqwod9UlXTHt9ikEQnY89PhwwrYe`׉ 7cassandra://6GHQRFS3lw7XbKbuLUeLiMd3wkNE0Q0wgpbsQcxA9xQY`s׉ 7cassandra://U4Ukr8DoZ6DKYOlNMjlYQ_T9yvKhicUJBimilHqo9wY`׉ 7cassandra://p--wG8aUwAi7fV456JuovE13WdjajK0EhwYz0v3gtZci͠^cZEb̦׉E/Powder Bed Processes 615 Roller mechanism Powder Binder 1. Spread powder 2. Print layer 3. Piston movement 4. Intermediate stage 5. Last layer printed 6. Finished part Figure 20.13: Schematic illustration of the binder-jet printing process. (a) (b) Figure 20.14: Full color parts produced by binder jet printing. (a) A simple model of toy truck wheel; (b) a more detailed model of a human hand, with transparent and colored components. Source: Courtesy of Stratasys. A typical part produced by BJP from ceramic powder is a ceramic-casting shell (see Section 11.2.4), in which aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds are postprocessed in two steps: (a) curing at around 150◦C and (b) firing at 1000◦ to 1500◦C. Printing of sand molds is a common practice; molds can be produced with blind risers and with cores, thus avoiding the complicated assembly operations associated with copes and drags (see Section 11.2.1). The parts produced through the BJP process are somewhat porous, and thus may lack strength. Threedimensional printing of metal powders can also be combined with sintering and metal infiltration (see Section 17.4) to produce fully-dense parts, using the sequence shown in Fig. 20.15. Here, the part is produced as before by directing the binder onto powders. However, the build sequence is then followed by sintering in order to burn off the binder and partially fuse the metal powders, just as is done in powder injection molding (Section 17.3.3). Common metals used in 3DP are stainless steels, aluminum, and titanium. ׉ 7cassandra://rsUS6Boqo-W3jB6xeQ71s-P8Oo_Hdvk2EaMGAicf7CE`cZ%Eb׉E616 Chapter 20 Additive Manufacturing Binder deposition Infiltrating metal, permeates into PM part Microstructure detail Unfused powder Binder Metal powder Particles are loosely sintered; binder is burned off (a) (b) Infiltrated by lower-melting-point metal (c) Figure 20.15: Three-dimensional printing using (a) part-build, (b) sinter, and (c) infiltration steps to produce metal parts. Source: Courtesy of Kennametal Extrude Hone. The infiltrating materials typically are copper and bronze, which provide good heat-transfer capabilities as well as wear resistance. This approach represents an efficient strategy for rapid tooling (Section 20.10). Dimensional tolerances vary widely by machine manufacturer and feedstock in BJP. Sand molds and cores are commonly produced with layer thicknesses of 240–380 µm, but layers may be as low as 50 µm with some materials. A more recently developed process, known by its trade name of jet fusion, is based on BJP, but with a number of unique features. In jet fusion, • A powder of polymer is spread in a build chamber. • Binder is jetted onto the polymer to fuse the powder as desired in the layer. • A detailing agent is jetted adjacent to the regions where the binder had been applied. • The layer is then subjected to a heat source that cures the polymer containing binder. The function of the detailing agent needs some clarification. In any thermal curing approach, temperatures are difficult to control and can lead to poor part resolution. The detailing agent prevents curing, so that the boundary between the cured part and the unaffected polymer has very good definition with sharp and smooth edges. In addition, the jet fusion process uses an array of sensors to determine the temperature distribution in the build chamber. If the sensors determine an area of the bed has too high or too low of a temperature compared to the optimum, the intensity of the UV light over the build chamber is varied accordingly, leading to improved mechanical properties and dimensional accuracy. ׉ 7cassandra://U4Ukr8DoZ6DKYOlNMjlYQ_T9yvKhicUJBimilHqo9wY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://FygLfG-v_Z6G8gmMCUrVWkVvx5fXFtCW7e0EcUGcjO8)Y`׉ 7cassandra://HzrWr4pEJizJ_gdfaP38A5P1178KWF6f9p5bFe4k43cX`s׉ 7cassandra://iAKyGeGeDntHge43T8hnAqU6YiSZwb99Snm3pwl5fYc}`׉ 7cassandra://3qua03z43_W-b33NYmCvfEFB8FT40lsgwG-bJQS9dWw=͠ͅ^cZEb̨ט#u׉׉ 7cassandra://1yYTo59tS9javdpzSp7mW0EwkSCsH5GKsQNTelHE5uY=`׉ 7cassandra://XDN3UCBVr0eHILvP_O5FIyLU0CDPseEctW6SeCJvyGU\W`s׉ 7cassandra://8rI992NSbPzakTjetPNcNacfOc-oNUAkYt93yM3HRd8`׉ 7cassandra://T1NiWR707KUzwLhCWLBrR7RL_k6wK5pE84S9Q9JZQ74*Pg\͠^cZEb̩׉E ?Miscellaneous Processes 617 Laser Optics X–Y positioning device Layer outline and crosshatch Part block Take-up roll (a) Laminating roller Sheet material Platform Material supply roll (b) Figure 20.16: (a) Schematic illustration of the laminated-object-manufacturing process. (b) Turbine prototype made by LOM. Source: Courtesy of M. Feygin, Cubic Technologies, Inc. 20.7 Laminated-object Manufacturing Lamination involves laying down layers bonded adhesively to one another. Several variations of laminatedobject manufacturing (LOM) are now available. Producing parts by LOM systems can be elaborate, where the more advanced systems use layers of paper or plastic with a heat-activated glue on one side. The desired shapes are burned into the sheet with a laser, and the parts are built layer by layer (Fig. 20.16). On some systems, the excess material must be removed manually after the part is made; the removal is simplified by programming the laser to burn perforations in crisscrossed patterns. The resulting grid lines make the part appear as if it had been constructed from gridded paper, similar to graph paper. 20.8 Miscellaneous Processes 20.8.1 Laser-engineered Net Shaping Laser-engineered net shaping (LENS), also known as laser powder forming (LPF) involves the principle of using a laser beam to melt and deposit metal powder or wire, layer by layer, over a previously deposited layer (Fig. 20.17). The heat input and cooling have to be controlled precisely to develop a favorable microstructure. The deposition process is carried out inside an enclosed volume and in an argon environment, to avoid the adverse effects of oxidation, particularly on aluminum. It is suitable for a wide variety of metals and specialty alloys for the direct manufacturing of parts, including fully-dense tools and molds. The process can also be used for repairing thin and delicate components. There are other, similar processing methods, including controlled-metal buildup (CMB) and precision-metal deposition (PMD, a trade name). LENS has been found suitable for incorporation into hybrid machines that have both additive and subtractive (machining) manufacturing capabilities. The advantages are that complex shapes can be quickly produced without refixturing, with high dimensional tolerance and surface finish, and with little scrap. Usually, this operation involves the incorporation of a LENS deposition head in combination with a machining or turning center (see Section 25.2). This is a compelling combination, since LENS on its own does not maintain tight tolerances; ±1 mm is typical of the process limitations. ׉ 7cassandra://iAKyGeGeDntHge43T8hnAqU6YiSZwb99Snm3pwl5fYc}`cZ%Eb׉E F618 Chapter 20 Additive Manufacturing Laser Powder supply Focused laser beam Lens Powder supply tube Powder nozzle Converging powder streams Deposited material Build table Figure 20.17: Schematic illustration of the laser-engineered net shaping (LENS) process. 20.8.2 Friction Stir Modeling The friction stir modeling (FSM) process shares several similarities with friction stir welding (Section 31.4). In this process, powder is delivered to a build location by pushing it into a rotating tube. The friction between the powders and the substrate are sufficiently high to densify the powder and develop a solid material. Because the process is at solid state, there is no appreciable heat-affected zone. Friction stir modeling has been successfully applied to magnesium, aluminum, and titanium, and has the advantage of being able to change the deposited material during a build. For example, a lightweight aluminum part can possess an integral hardened surface for wear resistance. The equipment used for FSM involves conventional CNC milling machines (Section 24.2), modified to deliver the desired powder. Typical layer thickness is around 100 µm, and surface finish is generally poor, requiring subsequent machining to achieve smooth surfaces; a machining allowance is therefore essential. 20.8.3 Wire and Arc Additive Manufacturing As described in Section 30.4.3, gas metal arc welding and gas tungsten arc welding are commonly incorporated into robot welding systems; they involve material transfer from an electrode or filler material into a weld joint. The same approach can be used to deposit material in a controlled manner; a welding endeffector on a robot provides a platform for large volume, large deposition rate additive manufacturing. This arrangement is known as wire and arc additive manufacturing (WAAM), with a unique feature of the ability to produce designs that are not based on layers; the robot can follow any trajectory. 20.8.4 Hybrid Approaches Additive manufacturing has specific advantages in certain applications, but one of the drawbacks compared to machining is the inability to hold tight tolerances or to achieve a desired surface finish. One solution is to combine additive and subtractive processes in the same machine. To date, the most common hybrid approaches involve combining either laser engineered net shaping or selective laser sintering with a CNC machining center. With LENS, a part will generally be produced with a generous machining allowance, since the process can rarely hold tolerances better than a millimeter over 50 mm. The constructed part is then machined without refixturing. With SLS, the machining operations are performed after a layer or a group of layers is produced. Even though the build chamber can be disturbed by machining, fresh powder fills in machined areas while the wiper spreads the powder into smooth layers. ׉ 7cassandra://8rI992NSbPzakTjetPNcNacfOc-oNUAkYt93yM3HRd8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://dVCT_Li5Il4oSUIZgOTiqoNsIYbP0EgA-7DOV4QRLqUz`׉ 7cassandra://Yv-t9HbcXGITOCKxlG_QxdniZtQ0bSKlzsEPvmms8SEg|`s׉ 7cassandra://j8tTvoSKZOS5ttfdBqgoAyNFo2O6FjsfJSDxRfb1LbU`׉ 7cassandra://qz_tsigCgMRbppiu46cwZ6SChiNkAvqeGZwAzZNtgjQW͠^cZEb̫ט#u׉׉ 7cassandra://OBDMG4RiG7ffZKTWzCVyHTDJTvQnpjiY9AgxbWGV8C8`׉ 7cassandra://B__aeoFCYRB3GaaC1PQ-WDb7l6_RB7lA8IaLUP9vwdYO`s׉ 7cassandra://Fhl_evdQ0gOVcBLXKE7cXFJXEXs_Lq8hg5zUfnReVcE`׉ 7cassandra://gLqTJLi7eWiYdOKezAtGFwAbG2TXgApsgl8qH5UT75Ỳ͠^cZEb̬׉EDirect Manufacturing and Rapid Tooling 619 A wide range of capabilities have recently been developed into the machinery, including the ability to combine materials to produce composites or to tailor mechanical properties, the incorporation of sensors to detect defects during printing, and automated handling of completed parts. 20.9 Emerging AM Applications The low cost and high reliability of additive manufacturing has directly led to its widespread application in a number of areas that are far removed from industrial production. Bioprinting. The production of medical devices is well established and can also be done by additive manufacturing. An emerging area is bioprinting, involving the printing of living cells. Using processes related to fused-deposition modeling or binder-jet printing, cells are suspended in a liquid carrier or bioink, producing a construct. This approach allows printing of cells in desired structures and concentrations. While applications are emerging, bioprinting has the potential and aggressive goal of producing living functional tissue, such as organ transplants or tissue that can be used in drug studies. Current limitations are the cell survival during and after the printing process. Architectural applications. Processes that are based mainly on fused deposition modeling have been used to build buildings or various structures from extruded concrete. The approaches generally use tower robots that place concrete along the periphery of the desired structure. Trowels can be located near the extruder head to build near-vertical walls that match from layer to layer, a process variant called contour crafting. Alternative approaches use binder-jet printing with sand to produce structures. Permanent habitats on the moon or on Mars are now expected to be produced through additive manufacturing, using as much native soil as possible, given the cost of transport of materials. 20.10 Direct Manufacturing and Rapid Tooling While extremely beneficial as a demonstration and visualization tool, additive manufacturing processes also have been used to produce functional parts. There are two basic methodologies involved: 1. Direct production of engineering metals, ceramics, and polymer components or parts. 2. Production of tooling or patterns by additive manufacturing, for use in various manufacturing operations. Additive manufacturing operations can be also used to manufacture parts directly, referred to as direct manufacturing. This approach includes the case where a part involves a machining or grinding allowance or requires further finishing operations. Thus, the component is generated directly to a near-net shape, from a computer file containing part geometry. The main limitations to the widespread use of additive manufacturing for direct manufacturing, or rapid manufacturing, are as follow: • Raw-material costs are high, and the time required to produce each part is too long to be viable for large production runs. However, there are many applications in which production runs are sufficiently small to justify direct manufacturing through additive manufacturing technologies, or where the required material properties are attainable. • The long-term and consistent performance of rapidly manufactured parts (as compared with the more traditional methods of manufacturing them) should be considered, especially with respect to fatigue, wear, and life cycle. Much progress is being made to address and respond to these concerns in order to make rapid manufacturing a more competitive and viable option in manufacturing. ׉ 7cassandra://j8tTvoSKZOS5ttfdBqgoAyNFo2O6FjsfJSDxRfb1LbU`cZ%Eb׉E620 Chapter 20 Additive Manufacturing Several methods have been devised for the rapid production of tooling (RT) by means of additive manufacturing processes. The advantages to rapid tooling include the following: 1. The high cost of labor and the shortening supply of skilled patternmakers can be overcome. 2. There is a major reduction in lead time. 3. The integral use of CAD technologies allows the use of modular dies, with base-mold tooling (match plates) and specially fabricated inserts. 4. Chill- and cooling-channel placement in molds can be optimized more easily, leading to reduced cycle times. Conformal cooling is a strategy for producing cooling channels that are located in a way to maximize heat extraction from a mold or die, while preserving mechanical strength (Fig. 20.18). 5. Shrinkage due to solidification or to thermal contraction can be compensated for automatically, through software, to produce tooling of the proper size and, in turn, to produce the desired parts. Large flat parts should be oriented at an angle or vertically to minimize the cross-sectional area of each layer, thereby minimizing warpage. The main shortcoming of rapid tooling is the potentially reduced tool or pattern life, as compared to those obtained from machined tool and die materials, such as tool steels and tungsten carbides (Chapter 21). Temperature (8C) 270 240 (a) 210 180 150 (b) (c) (d) Figure 20.18: The benefit of conformal cooling in molds produced by additive manufacturing. The images on the left show conventional (machined or drilled) cooling channels, and those on the right show conformal cooling channels that can be produced in additive manufactured molds. The top images depict the channel layout; the bottom images the temperature distributions in the mold during production. Note that the temperature distribution is more uniform on the molds with conformal cooling, leading to less warpage and higher production rates. Source: Copyright image provided courtesy of Milacron product brand DME Company showing their TruCoolTM technology. ׉ 7cassandra://Fhl_evdQ0gOVcBLXKE7cXFJXEXs_Lq8hg5zUfnReVcE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Eqh2vGDg6-H_iGtS6QzFfSiZotIXwxVkU2s80xTeYEc!`׉ 7cassandra://hsQsDsByb_QQsmSm3DXFbQuCjophp-GyBrlR_y7cLpw]L`s׉ 7cassandra://WTccJovxRad-aWcDpwYGCBPthaWSsvpM4qAG6YS3lJA`׉ 7cassandra://TDJvKqKHEAcjIRBwgYHxlqpD0TmH1m-jHg2Epa6kLbwB)͆~͠^cZEb̴ט#u׉׉ 7cassandra://S6nbbkhRvHdxYzEjwfNjWIpUI0bmTJniK-1BWIWR4jg`׉ 7cassandra://Ve1gh-0Yhpdpm8gtiZeLEO8TGoFeHUltM2X_-QoZe6A{c`s׉ 7cassandra://w2On6xXZtjcMTu7IZZ6SPsKtUIIrzR6FKKa3w3TYctE`׉ 7cassandra://LidDJxjL-kizIpcQ9aSzvxlLqX6SgYAbpXCcgmhwD18qG͠^cZEb̵׉EXDirect Manufacturing and Rapid Tooling 621 1. Pattern creation 2. Tree assembly Crucible Heat Molten metal 3. Insert into flask 4. Fill with investment Grinding spatter Workpiece 5. Wax melt-out/burnout 6. Fill mold with metal 7. Cool 8. Finish Figure 20.19: Manufacturing steps for investment casting with rapid-prototyped wax parts as blanks. This method uses a flask for the investment, but a shell method also can be used. Source: Courtesy of 3D Systems, Inc. A number of strategies have developed to incorporate additive manufacturing into mold and die production. As an example, Fig. 20.19 shows an approach for investment casting. Here, the individual patterns are made in an AM operation (in this case, stereolithography), and then used as patterns in assembling a tree for investment casting (Fig. 11.14). Note that this approach will require a polymer that completely melts and burns from the ceramic mold; such polymers are available for all forms of polymer AM operations. Furthermore, as drawn in CAD programs, the parts are usually software modified to account for shrinkage, and it is the modified part that is produced in the additive manufacturing machinery. Binder-jet printing also can easily produce a ceramic-mold casting shell (Section 11.2.2) or a sand mold (Section 11.2.1), in which an aluminum-oxide or aluminum-silica powder is fused with a silica binder. The molds have to be postprocessed in two steps: curing at around 150◦C, and then firing at 1000◦–1500◦C. Another common application of rapid tooling is injection molding of polymers (Section 19.3), in which the mold or, more typically, a mold insert is manufactured by additive manufacturing. Molds for slip casting of ceramics (Section 18.2.1) also can be produced in this manner. To produce individual molds, AM processes are used directly, and the molds will be shaped with the desired permeability. For example, in fused-deposition modeling, this requirement mandates that the filaments be placed onto the individual slices, with a small gap between adjacent filaments; the filaments are then positioned at right angles in adjacent layers. ׉ 7cassandra://WTccJovxRad-aWcDpwYGCBPthaWSsvpM4qAG6YS3lJA`cZ%Eb׉E622 Chapter 20 Additive Manufacturing The advantage of rapid tooling is the capability to produce a mold or a mold insert that can be used to manufacture components without the time lag (typically several months) traditionally required for the procurement of tooling. Moreover, the design is simplified, because the designer needs to analyze only a CAD file of the desired part; software then produces the tool geometry and automatically compensates for shrinkage. In addition to the straightforward application of additive manufacturing technology to tool or pattern production, other rapid-tooling approaches, based on AM technologies, have been developed. Room-temperature vulcanizing (RTV) molding/urethane casting can be performed by preparing a pattern of a part by any AM operation, which is then used to produce an RTV mold. The pattern is first coated with a parting agent, and may or may not be modified to define mold parting lines. Liquid RTV rubber is then poured over the pattern, and cures (usually within a few hours) to produce mold halves. The mold is then used with liquid urethanes in injection molding or reaction-injection molding operations (Section 19.3). One main limitation of this approach is a lower mold life, because the polyurethane present in the mold causes progressive damage and the mold may be suitable only for as few as 25 parts. Epoxy and aluminum-filled epoxy molds also can be produced, but mold design requires special care. With room temperature vulcanizing (RTV) rubber, the flexibility of the mold allows it to be peeled off the cured part. With epoxy molds, their high stiffness precludes this method of part removal, and mold design is more complicated. Thus, for example, drafts are required, and undercuts and other design features that can be produced by RTV molding must be avoided. Acetal clear epoxy solid (ACES) injection molding, also known as direct AIM, refers to the use of additive manufacturing, usually stereolithography, to directly produce molds suitable for injection molding. The molds are shells, with an open end to allow filling with a material such as epoxy, aluminum-filled epoxy, or a low-melting-point metal. Depending on the polymer used, mold life may be as few as 10 parts, although hundred parts per mold are possible. Sprayed-metal tooling. In this process, shown in Fig. 20.20, a pattern is first created through AM. A metal spray operation (Section 34.5) then coats the pattern surface with a zinc-aluminum alloy. The metal coating is placed in a flask, and potted with an epoxy or an aluminum-filled epoxy material. In some applications, cooling lines can be incorporated into the mold before the epoxy is applied. The pattern is removed, and two such mold halves are used as in injection-molding operations. Mold life is highly dependent on the materials used and the temperatures involved, and can vary from a few to thousands of parts. Keltool process. In the Keltool process, an RTV rubber mold is first produced, based on a rapid-prototyped pattern, as described earlier. The mold is then filled with a mixture of powdered A6 tool steel (Section 5.7), tungsten carbide, and polymer binder, and is allowed to cure. The so-called green tool (green, as in ceramics and powder metallurgy) is fired to burn off the polymer and fuse the steel and the tungsten-carbide powders. The tool is then infiltrated with copper in a furnace to produce the final mold. The mold can subsequently be machined or polished to impart a superior surface finish and good dimensional tolerances. Keltool molds are limited in size to around 150 × 150 × 150 mm. Thus, a mold insert, suitable for high-volume molding operations, is made and installed. Depending on the material and processing conditions, mold life can range from 100,000 to 10 million parts. Case Study 20.3 Casting of Plumbing Fixtures A global manufacturer of plumbing fixtures and accessories for baths and kitchens used rapid tooling to transform its development process. One of the company’s major product lines is decorative water faucets, made from brass castings that are subsequently polished to achieve the desired surface finish. The ability to produce prototypes from brass is essential for quickly evaluating designs and identifying processing difficulties that may occur. A new faucet design was prepared in a CAD program; the finished product is shown in Fig. 20.21. As part of the product development cycle, it was decided to produce prototypes of the faucet to confirm the aesthetics of the design. Since such faucets are typically produced by sand casting, it was also essential ׉ 7cassandra://w2On6xXZtjcMTu7IZZ6SPsKtUIIrzR6FKKa3w3TYctE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://a0qcK7pa9-pdccT1slj7MGdwBVdJ-G_5iUrmYQAh_lUܜ`׉ 7cassandra://aPgxlGy_3mEPLemfu3S5hqNWTjKMBo1_j4iNYIuQdlkR`s׉ 7cassandra://BwTl5l4BUMKufnJxFIHUrG3D9metHZ77EtHkovFceIs`׉ 7cassandra://nfYn5Hx8s66-1evqjf7W2s1hnaDWR4mxCi6pHgDgRu4^(:͠^cZEb̷ט#u׉׉ 7cassandra://CVs4PfgQcqXl-Re8ZJD-24nTGFm2iVn7fy1pKD45E60\`׉ 7cassandra://uRrEUCGii0zGEVzOMFq-Q-3ByPxzpUx7x4dNC7Z8F_gA`s׉ 7cassandra://QougJEQbJ_pLSc1VSPukFS3OrvBJA1dHyZ3QeEY6zzoU`׉ 7cassandra://RKBEYXzw79YKZE30JoM9jBIvN4Kv7f3ajWgmZ1kEs68oQh͠^cZEb̸׉E pDirect Manufacturing and Rapid Tooling 623 to validate the design through a sand-casting operation, followed by polishing. This approach allowed evaluation of the cast parts in terms of porosity and various other casting defects, and also would identify processing difficulties that might arise in the finishing stages. A sand mold was first produced, as shown in Fig. 20.22. The mold material was a blend of foundry sand, plaster, and other additives that were combined to provide strong molds with good surface finish (see also Section 11.2.1). A binder was printed onto the sand mixture to produce the mold. The mold could be produced as one piece, with an integral core (see Figs. 11.3 and 11.6), but in practice, it is often desired to smoothen the core and assemble it later onto core prints. In addition, slender cores may become damaged, as support powder is being removed from the mold, especially for complex casting designs. Therefore, the core for this design is produced separately and then assembled into the two-part mold. Using 3D printing, the operation produced brass prototypes of the faucets in five days, which included the time required for mold design, printing, metal casting, and finishing. The actual print time of the mold was just under three hours, and the material cost was approximately $280. The production of pattern plates for sand casting is, in general, too expensive for producing prototypes, and would cost over $10,000 and add several months to the lead time. The incorporation of 3D printing into the design process thus provided new capabilities that confirmed the design aesthetics and function, as well as manufacturing robustness and reliability. Source: Courtesy of 3D Systems. Metal spray Aluminum powderfilled epoxy Alignment tabs Pattern Baseplate (a) Finished mold half Molded part Pattern Baseplate (d) Second mold half (e) Figure 20.20: Production of tooling for injection molding by the sprayed-metal tooling process: (a) A pattern and baseplate are prepared through a additive manufacturing operation; (b) a zinc–aluminum alloy is sprayed onto the pattern (see Section 34.5); (c) the coated baseplate and pattern assembly are placed together in a flask and backfilled with aluminum-impregnated epoxy; (d) after curing, the baseplate is removed from the finished mold; and (e) a second mold half suitable for injection molding is prepared. (b) (c) Coating Flask ׉ 7cassandra://BwTl5l4BUMKufnJxFIHUrG3D9metHZ77EtHkovFceIs`cZ%Eb׉Eo624 Chapter 20 Additive Manufacturing Figure 20.21: A new faucet design, produced by casting from rapid-prototyped sand molds. Source: Courtesy of 3D Systems. Figure 20.22: Sand molds produced through three-dimensional printing. Source: Courtesy of 3D Systems. 20.11 Design for Additive Manufacturing Additive manufacturing is attractive because designers are able to easily produce complex geometries. Often, this has been expressed as the notion that complexity is free, which has led to the development of design optimization software. This approach has led to, for example, minimum-weight parts given shape constraints and the loads applied (see Fig. 20.23), as well as the production of parts with inherent aesthetic aims. There are, however, limits to the shapes that can be produced by AM. Several design rules have been developed that are unique for additive manufacturing. Since machines are now available in a wide variety of capacities and capabilities, detailed design recommendations are manufacturer-specific. The following considerations are generic and are considered to be good design practice: 1. Additive manufacturing processes tend to warp the part, because of thermal stresses and shrinkage encountered during production. In general, the design guidelines for plastic parts, given in Section 19.15, are also applicable to parts produced through additive manufacturing. ׉ 7cassandra://QougJEQbJ_pLSc1VSPukFS3OrvBJA1dHyZ3QeEY6zzoU`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://lDryiM9zVs2oz57DKCPoif2f-K21NHgyPrs2STtGtm4XY`׉ 7cassandra://UlaxSYJlEjxaHvNBBu_52wWjcm3dBozewITXamnv8QU=`s׉ 7cassandra://wy_2594wqACNdmCxuD_gAr7sSYiey1tttwL1v8MZyao.`׉ 7cassandra://okn--Kb67T1zscp51R53itY6xNHBiOLy7DanX-f1S4AtL ͠^cZEb̺ט#u׉׉ 7cassandra://Zf6938SRe2ZQs3m5mLHpNRX86v_4GFqvn6abvCZNLjw2`׉ 7cassandra://9k8NCba-Bg_Wn33hWafrNaIzvMfLzlrqw7VrrmO5hZ4g`s׉ 7cassandra://fOuuEw21hDSN-hO7Cauf6KjAHUmb7eJu2HGp3CeZFdA&`׉ 7cassandra://N_08I6qQS_YB1G_aKUjTR95i1C8mKcOcL82MpaH8EfEb͠^cZEb̻׉EDesign for Additive Manufacturing 625 (a) (b) (c) Figure 20.23: Topology optimization to reduce the weight of a bracket. (a) Original bracket design; (b) predicted minimum-weight bracket from topology optimization software; and (c) final bracket, representing a 70% weight loss from the original design, as produced through selective laser sintering. Source: 3D Systems. 2. The dimensional tolerance standard used (see Section 35.8) should involve symmetric tolerances in order to be applied easily to additive manufacturing. 3. The tolerances within a plane can be much higher than those outside of a plane. Therefore, the part should be oriented to place the critical dimension in the plane of a build, not in its thickness direction. 4. Dimensional tolerances and surface finish depend on the particular machine, the material, and part size and its orientation. In stereolithography, tolerances of ±0.05 to 0.1 mm are achievable, or ±0.001 mm/mm for well-designed parts that do not warp excessively. Typical selective laser sintering of ׉ 7cassandra://wy_2594wqACNdmCxuD_gAr7sSYiey1tttwL1v8MZyao.`cZ%Eb׉E626 Chapter 20 Additive Manufacturing polymers yields tolerances of ±0.4 mm, or 0.1 mm/mm, whichever is greater. For metal selective laser systems, tolerances of 0.05–0.125 mm are generally achievable, with roughnesses in the range of 5–40 µm. For better tolerances, a machining allowance of 0.5–1 mm should be provided for postprocessing. 5. Steps can be noticeable in an inclined plane; generally, the use of flat planes or planes inclined at not less than 20◦ are producible without noticeable steps. 6. The same considerations as stated for powder injection molding are valid for binder jetting (see Section 17.6). 7. In selective laser sintering of polymers, it is recommended to have a clearance of 0.3–0.5 mm within the plane for surfaces that are not joined together; up to 0.6 mm is required in the build direction. 8. The thinnest wall that can be produced depends on the material and the aspect ratio; common ranges are 0.5–1.5 mm for polymers in selective laser sintering. In fused deposition modeling, it is generally recommended that a wall be at least four times wider than the thickness of the layer. 9. Recognizing that the powder in the build chamber may not be reusable, as well as to maximize production, it is beneficial to fill a build space with as many parts as possible, and nestable (Section 16.14) parts be used when possible. 10. To reduce costs, the height in the build direction should be low, and stackable parts should be used to increase the amount of powder that is fused in a build chamber. 11. Consideration must be given to the removal of the uncured photopolymer or powder when the parts being made are hollow. 12. Large parts are especially susceptible to warpage; it may be a good strategy to produce a part in components that can be assembled after printing, or else design parts to use as little mass as possible. 13. Plan the part to allow for powder or liquid photopolymer removal when appropriate. 14. Build time depends on the volume of the material that is to be fused in a process. It is therefore beneficial to model an object with solid surfaces, but supported by porous structures or struts, instead of a solid bulk. This approach produces designs that can be optimized to minimize weight by carefully designing the supporting structure. 15. Complexity is free. That is, there is no need to restrict designs to geometries that are easy to manufacture for casting, forging or machining operations. Corner radii, draft angles, accommodations for parting lines, etc., do not need to be included in AM part design.With binder jetting, color can be incorporated into designs easily. 20.12 Additive Manufacturing Economics As in all processes, design and manufacturing decisions are ultimately based on performance and cost, including the costs of equipment, tooling, and production. The final selection of a process or processes also depends greatly on production volume. High costs of equipment and tooling in plastics processing can be acceptable only if the production run is large, as is also the case in casting and forging. However, using additive manufacturing operations makes these processes economical for limited production runs by applying rapid tooling approaches (see Section 20.10), though the tools and molds have limited life. Additive manufacturing operations are suitable for prototypes and limited production runs, but they require expensive consumables, and thus are unsuitable for moderate to high production runs. This situation is complicated by the fact that some processes (such as selective laser sintering and electron beam melting) may require the unfused powder in the build chamber to be discarded. Thus, if only 10% of ׉ 7cassandra://fOuuEw21hDSN-hO7Cauf6KjAHUmb7eJu2HGp3CeZFdA&`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://o5IMMVGK-Xs502Ri66liW1E0Q-WGKNe0B8fEsCSr3pQDH`׉ 7cassandra://KkEtWHthJ1aFTEBy7XMtW0XWP13Eabt5T6mmC0lGFBkl=`s׉ 7cassandra://ClyFqy8k1rl1G8L7aeoepSuGSLWpog8IwvJUvjetolc)`׉ 7cassandra://BClLoUoCOXBSrZEJjB1xdoeBf9s_qtfLE164cJ0-1VM^͠^cZEb̾ט#u׉׉ 7cassandra://CnRFd_6ZWCzPudkdIGX8Op0gr6zWf4iaTXsHZJFNb1MV`׉ 7cassandra://9MPaN8rSs56saaw6aWj_oKU236-qyKNloeGFagfUUm4T`s׉ 7cassandra://eEsv2-Rz7bdKt8gk48EPyhU-q4PppzA2vvq8xO77Wgo6`׉ 7cassandra://OXjOB2erJ9TTBxu468wObwf_15ZFuXpa6Sk1m5zqy-s_̈͠^cZEb̿׉EAdditive Manufacturing Economics 627 the build volume is reused, the material cost for the part is ten times the nominal material cost. This is a significant concern; titanium (Ti-6Al-4V), for example, costs over $400/kg for the raw powder. The cost of a part produced by additive manufacturing can be generalized as Cp = Cm +Cs +Ct +Cf, (20.1) where Cm is the material cost, Cs is the setup cost, Ct is the cost of machinery or tooling per part, and Cf is the cost of finishing operations. The material cost is high compared to conventionally produced polymers (such as in injection molding) or metals (such as extrusions). However, Ct in conventional processing is generally much higher, since no tools are required in additive manufacturing. Finishing operations may or may not be necessary, so that Cf may often be ignored. When deciding if additive manufacturing is suitable for production, the cost compared to the conventional alternative has to be justified. Consider the case where a part is being considered for either injection molding or selective laser sintering. For low production runs, the high cost of tooling associated with injection molding dominates the part cost. However, for mass production, the tooling cost is amortized over many parts. The higher cost of the materials in additive manufacturing makes mass production a less economical option. Case Study 20.4 Implications of Powder Reuse Powder for additive manufacturing processes can be expensive; for example, $400 per kg of titanium powder is not unusual, and $100 per kg for high quality polymers is common. This high cost is especially important if a powder-bed process is used, as the volume fused in the build chamber may be as little as 10% of the total volume. A common concern is whether or not unfused powder can be reused, that is, taken from the build chamber and then placed into the feed chamber. Often, the unfused powder (or cake) is loosely adhering and has to be sieved or otherwise treated to break up clumps of powder. If powder is not reused, then the cost embedded into the material can be several times the powder cost, thus making AM uneconomical for almost all commercial applications. There are several strategies that can be applied in powder reuse: 1. The unfused powder can be taken as is from the build chamber, then sieved, examined, and placed in the feed chamber. Evaluations are generally associated with powder size distributions and the ability of the powder to flow or spread itself into a continuous and smooth layer. 2. The powder from the build chamber can be blended with virgin powder, often in a 1:1 mixture. Some of the concerns associated with powder reuse are: 1. Additive manufacturing takes place under a controlled atmosphere, generally argon or nitrogen, in order to prevent powder oxidation and also to control any fire or explosion hazards associated with powders. However, when the powder is removed from the build chamber and is sieved, it is exposed to air, and therefore has the potential of oxidation. 2. The additive manufacturing process is rather complex. Videos of selective laser sintering have shown that particles that have been exposed to laser energy jump off the powder layer, the melt pool is highly turbulent, and there is a contraction as the powder melts and then solidifies. 3. The size distribution of powders (see also Section 17.2.2) can change over time. 4. Careful examination of powder size distributions have found subtle changes. When the feed chamber piston moves upwards, and the wiper or cylinder moves across the build chamber to create a fresh powder layer, there is always a slight surplus of powder in order to ensure that the build chamber layer is fully developed. The larger particles are pushed by the wiper or cylinder into an ׉ 7cassandra://ClyFqy8k1rl1G8L7aeoepSuGSLWpog8IwvJUvjetolc)`cZ%Eb׉E 628 Chapter 20 Additive Manufacturing overflow trough, leaving the smaller particles in the build chamber layer. These smaller particles are consumed during additive manufacturing, but the overfeed trough is blended with the powder reintroduced into the feed chamber. The result is that the mean particle size tends to increase slightly as the powder is reused. 5. There is a major concern that the high temperatures associated with the melt pool could cause particles near the melt pool to fuse. However, sieving eliminates such particles from being introduced into the build chamber. 6. Selective laser sintering and electron beam melting involve preheating the build chamber, in order to have a more robust process and to reduce the power required in the laser. With selective laser sintering, this preheat is much lower than with electron beam melting; still, there is a concern that this preheat can alter the microstructure or the chemistry of the powder. Figure 20.24 shows the effect of reuse on ultimate tensile strength. Note that there is no noticeable reduction in mechanical properties associated with the first powder reuse for any of the metals considered. There is a drop in strength when a nylon powder is reused four times, but there is no further reduction through eight reuses. This is a main justification for the practice of blending virgin nylon powder with reclaimed powder from the build chamber. It should also be noted that the observations regarding Fig. 20.24 may not hold for all materials. It has been suggested that alloys that are especially sensitive to oxygen and water vapor (such as magnesium alloys) may undergo a degradation in mechanical properties associated with reuse, because of their exposure to humidity during reclamation and sieving. Regardless, the reuse of powders is now seen to be a plausible strategy for cost reduction in AM, and that it could greatly accelerate additive manufacturing application to actual production. Source: Courtesy of the National Center for Defense Manufacturing and Machining, America Makes, and the Air Force Research Laboratory. 1500 1000 500 0 Nylon 316L 17-4 PH Ti-6Al-4V Figure 20.24: Effect of powder reuse on mean ultimate tensile strength for nylon, 316L stainless steel, 17-4 precipitation hardening stainless steel and titanium alloy Ti-6Al-4V. Ultimate tensile strength (MPa) Virgin 1 Reuse 4 Reuses 8 Reuses Virgin 1 Reuse 4 Reuses 8 Reuses Virgin 1 Reuse 4 Reuses Virgin 1 Reuse 4 Reuses 8 Reuses ׉ 7cassandra://eEsv2-Rz7bdKt8gk48EPyhU-q4PppzA2vvq8xO77Wgo6`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://fpGBV4v4vpmmBeffpa6vHD7ps-R-7AnXNZlrtbdsLms`׉ 7cassandra://Tpqm1TODvli_eck4bl5ARbOfE7JaX2tKkC8VnMzourkS>`s׉ 7cassandra://JEX3sL7kKrpM3lr1eTblD6_qUYupMp-lfF4fVLSUWTY`׉ 7cassandra://cVWTKcY0jiS2JiQE1gbaMbP9GVVoVExxH9lxf18eO04G~͠^cZEbט#u׉׉ 7cassandra://oDlbCuI0Gx1zJdqUe3sKdnBo657D5qI4VO1krOtrNic|O`׉ 7cassandra://_DzmY1AXN2TChDRrpfZGDcEBqCJ2LvV-vSmZg20noWEa`s׉ 7cassandra://cqN3w2ZH2llQAaUX1CSsaQvy0IsSiglIQUqmzMQw25w`׉ 7cassandra://em6RwZG5h24Vp_3Cw7gbFGGqokQzUB5K7wOiAv-TLBk\͠^cZEb׉E 3Key Terms 629 Summary • Additive manufacturing techniques have made possible much faster product development times, and they are having a major effect on other manufacturing processes. When appropriate materials are used, additive manufacturing machinery can produce blanks for investment casting or similar processes, so that metallic parts can now be obtained quickly and inexpensively, even for lot sizes as small as one part. Such technologies also can be applied to producing molds for operations (such as injection molding, sand and shell mold casting, and forging), thereby significantly reducing the lead time between design and manufacture. • Additive manufacturing continues to grow into a valuable new manufacturing discipline. It is a useful technique for identifying and correcting design errors. Several techniques have been developed for producing parts through AM. • Fused-deposition modeling consists of a computer-controlled extruder, through which a polymer filament is deposited to produce a part slice by slice. • Stereolithography involves a computer-controlled laser-focusing system, that cures a liquid thermosetting polymer containing a photosensitive curing agent. • Multijet and PolyJet modeling use mechanisms similar to ink-jet printer heads to eject photopolymers to directly build prototypes. • Laminated-object manufacturing uses a laser beam or vinyl cutter to first cut the slices on paper or plastic sheets (laminations); then it applies an adhesive layer, if necessary, and finally stacks the sheets to produce the part. • Three-dimensional printing uses an ink-jet mechanism to deposit liquid droplets of the liquid binder onto polymer, metal, or ceramic powders. The related process of material jetting directly deposits the build material. Using multiple printheads, three-dimensional printing can also produce full-color prototypes. • Selective laser sintering uses a high-powered laser beam to sinter powders or coatings on the powders in a desired pattern. Selective laser sintering has been applied to polymers, sand, ceramics, and metals. • Electron-beam melting uses the power of an electron beam to melt powders and form fully-dense functional parts. Key Terms ACES Additive manufacturing Big area additive manufacturing Binder jet printing Bioprinting CLIP Continuous liquid interphase production Contour crafting Desktop machines Direct AIM Direct manufacturing Distributed manufacturing Direct prototyping Electron-beam melting Friction stir modeling Fused-deposition modeling Hybrid approaches JetFusion Keltool Laminated-object manufacturing ׉ 7cassandra://JEX3sL7kKrpM3lr1eTblD6_qUYupMp-lfF4fVLSUWTY`cZ%Eb׉E630 Chapter 20 Additive Manufacturing Laser-engineered net shaping Mask projection stereolithography Mass customization Material jetting Multijet modeling Photopolymer PolyJet Powder bed Prototype Bibliography Chua, C.K., and Leong, K.F., Additive manufacturing: Principles and Applications, 5th ed., World Scientific Publishing Co., 2017. Gebhardt, A., Understanding Rapid Prototyping, Rapid Tooling, Rapid Manufacturing, Hanser, 2011. Gibson, I., Rosen, D.W., and Stucker, B., Additive Manufacturing Technologies: 3D Printing, Rapid Prototyping, and Direct Digital Manufacturing, 2nd ed., Springer 2015. Kamrani, K., and Nasr, E.A. (eds.), Rapid Prototyping: Theory and Practice, Springer, 2005. Liou, F.W., Rapid Prototyping and Engineering Applications: AToolbox of Prototype Development,CRC Press, 2007. Milewski, J.O., Additive Manufacturing of Metals, Springer, 2017. Noorani, R.I., Rapid Prototyping: Principles and Applications, Wiley, 2006. Redwood, B., Sch¨offer, F., and Garrett, B., The 3D Printing Handbook, 3D Hubs, 2017. Wang, W., Stoll, H., and Conley, J.G., Rapid Tooling Guidelines for Sand Casting, Springer, 2010. Review Questions 20.1. What is the basic difference between additive manufacturing and rapid prototyping? 20.2. What is stereolithography? 20.3. What is virtual prototyping, and how does it differ from additive methods? 20.4. What is fused-deposition modeling? 20.5. Explain what is meant by rapid tooling. 20.6. Why are photopolymers essential for stereolithography? 20.7. Explain what each of the following means: (a) 3DP, (b) LOM, (c) STL, (d) SLS, (e) FDM, and (f) LENS. 20.8. What starting materials can be used in fused-deposition modeling and in three-dimensional printing? 20.9. What are the cleaning and finishing operations in additive manufacturing processes? Why are they necessary? 20.10. Which additive manufacturing technologies do not require a laser? 20.11. What are the advantages of electron beam melting? 20.12. What is the Keltool process? 20.13. What is CLIP? 20.14. What is unique about BAAM? Rapid tooling RTV molding/urethane casting Selective laser sintering Sprayed metal tooling Stereolithography Subtractive processes Three-dimensional printing Virtual prototyping Wire and arc additive manufacturing ׉ 7cassandra://cqN3w2ZH2llQAaUX1CSsaQvy0IsSiglIQUqmzMQw25w`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://yzeivGysRG2GbNIH3nmhmMztVxxLaWzjHaYVvhPoclU3`׉ 7cassandra://4FS7IJHDjtX5IhxSfoIrbSDfyuxpkT9njH2mn2emOIk]`s׉ 7cassandra://nE4QWDtHOaBihiab1vV4uqJUwtsZVAItfzWHaBx4REEa`׉ 7cassandra://3_eGNg-1GvBFqxi-OSZqHbeSiP7oWmnxOpEqeYRk5EYG͠^cZEbט#u׉׉ 7cassandra://hXotzztnfX_ileDKmVu5YK0B36bzqh4cAaQq1-DmXOoA`׉ 7cassandra://RuZYg50tTKEY-RgBVf4IoKHF3Q8gnCx9tqsLVMBsz44]`s׉ 7cassandra://C-207UxnVBLe1Z5dQt91wgHGWuV5rMXE9r0sXCDddL0`׉ 7cassandra://TObGUwfOn3Lw7ZuIku9fpIdBV5A6bzFlB0eoGkWPE8QLO͠^cZEb׉E Quantitative Problems 631 20.15. Why are supports needed with some parts? 20.16. Which materials can be processed with binder jet printing? 20.17. Which additive manufacturing operations can produce transparent workpieces? 20.18. Which additive manufacturing operations can produce multi-colored workpieces? Qualitative Problems 20.19. How can a mold for sand casting be produced using additive manufacturing techniques? Explain. 20.20. Examine a ceramic coffee cup and determine in which orientation you would choose to produce the part if you were using (a) fused-deposition manufacturing or (b) laminated-object manufacturing. 20.21. How would you rapidly manufacture tooling for injection molding? Explain any difficulties that may be encountered. 20.22. Explain the significance of rapid tooling in manufacturing. 20.23. List the processes described in this chapter that are best suited for the production of ceramic parts. Explain. 20.24. Few parts in commercial products today are directly manufactured through additive manufacturing operations. Explain. 20.25. Can rapid-prototyped parts be made of paper? Explain. 20.26. Careful analysis of a rapid-prototyped part indicates that it is made up of layers with a distinct filament outline visible on each layer. Is the material a thermoset or a thermoplastic? Explain. 20.27. Why are the metal parts in three-dimensional printing often infiltrated by another metal? 20.28. Make a list of the advantages and limitations of each of the additive manufacturing operations described in this chapter. 20.29. In making a prototype of a toy automobile, list the post-additive manufacturing finishing operations that you think would be necessary. Explain. 20.30. List approaches for quickly manufacturing tooling for injection molding. 20.31. What are the similarities and differences between stereolithography and CLIP? 20.32. List the additive manufacturing approaches that are suitable for metals. 20.33. Explain why part orientation in rapid prototyping is important. 20.34. Do you expect that materials produced from additive manufacturing will be isotropic? Explain. Quantitative Problems 20.35. Using an approximate cost of $1500 per liter for the liquid polymer, estimate the material cost of a rapid-prototyped rendering of a pen. 20.36. The extruder head in a fused-deposition modeling setup has a diameter of 1.27 mm and produces layers that are 0.28 mm thick. If the extruder head and polymer extrudate velocities are both 45 mm/s, estimate the production time for the generation of a 40-mm solid cube. Assume that there is a six-second delay between layers as the extruder head is moved over a wire brush for cleaning. 20.37. Using the data for Problem 20.36 and assuming that the porosity for the support material is 50%, calculate the production rate for making a 120-mm high cup with an outside diameter of 100 mm and a wall thickness of 5 mm. Consider the cases (a) with the closed end up and (b) with the closed end down. ׉ 7cassandra://nE4QWDtHOaBihiab1vV4uqJUwtsZVAItfzWHaBx4REEa`cZ%Eb׉E #632 Chapter 20 Additive Manufacturing 20.38. Inspect Table 20.2 and compare the numerical values given with those for metals and other materials, as can be found in Part I of this text. Comment on your observations. Synthesis, Design, and Projects 20.39. Additive manufacturing machines represent a large capital investment; consequently, many companies cannot justify the purchase of their own system. Thus, service companies that produce parts based on their customers’ drawings have become common. Conduct an informal survey of such service companies, identify the classes of additive manufacturing machines that they use, and determine the percentage use of each class. 20.40. One of the major advantages of stereolithography is that it can use transparent polymers, so that internal details of parts can readily be discerned. List and describe several parts in which this feature is valuable. 20.41. A manufacturing technique is being proposed that uses a variation of fused-deposition modeling in which there are two polymer filaments that are melted and mixed prior to being extruded to make the part. What advantages does this method have? 20.42. Identify the additive manufacturing processes described in this chapter that can be performed with materials available in your home or that you can purchase easily at low cost. Explain how you would go about it. Consider materials such as thin plywood, thick paper, glue, and butter, as well as the use of various tools and energy sources. 20.43. Design a machine that uses additive manufacturing technologies to produce ice sculptures. Describe its basic features, commenting on the effect of size and shape complexity on your design. 20.44. Because of relief of residual stresses during curing, long unsupported overhangs in parts made by stereolithography tend to curl. Suggest methods of controlling or eliminating this problem. 20.45. Describe methods that would allow the use of reinforced polymers to be used in additive manufacturing. 20.46. Conduct an Internet and literature study and write a two-page paper on developments of producing artificial organs through additive manufacturing related processes. 20.47. A current topic of research involves producing parts from additive manufacturing operations and then using them in experimental stress analysis, in order to infer the strength of final parts produced by means of conventional manufacturing operations. List your concerns with this approach, and outline means of addressing these concerns. 20.48. Outline the approach you would use to produce prototypes of metal gears from plastic. Assume the gears are 100 mm in diameter, 25 mm thick, and have 25 teeth. Explain how your preferred method of production would change if you needed to produce (a) one gear; (b) 100 per month; (c) 100 per day; (d) 100 per hour. 20.49. There is a great desire to increase the speed of additive manufacturing approaches. List three strategies for increasing the speed of a process, along with the advantages and disadvantages of each method. Write a one-page paper on the approach you think is best. ׉ 7cassandra://C-207UxnVBLe1Z5dQt91wgHGWuV5rMXE9r0sXCDddL0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://MmIBlYx6Sbvvp9cRgC_wESGSlelG2hGxwPekko2N1CM`׉ 7cassandra://h1jX9rA_vhSq696RcGIuCOynKVpuhRegQC2BRIem85A\`s׉ 7cassandra://9H6v35NSYowwu_g9tJ2reNRK-9R0oZuygl3iw-G8SvA`׉ 7cassandra://1JPs1wES6H8-s1hgwpxrPizo7CGyYhkGJMpEYDKIUmUb͠^cZEbט#u׉׉ 7cassandra://6pxEtfYIijoljT-B2ysedL9Bh9-L0S9hg-va_IWKkyEZ`׉ 7cassandra://Cvt9e-hhKnR_Ztx5Hi3T0Ms_5925pfEq4LzAmzyX-l8;`s׉ 7cassandra://qBo7CGckoDhcth6STaylACx1wj9-NtGDOTa3nwViZ48`׉ 7cassandra://43QICaROakptQcTgF6j-2K2g3sqcyqrDXoWK4B8xE4k9z͠^cZEb׉E PART IV Machining Processes and Machine Tools Parts made by the casting, forming, and shaping processes described in Parts II and III often require further operations before they are ready for use. Consider, for example, the following features and whether they could be produced by the processes described thus far: • Smooth and shiny surfaces, such as the bearing surfaces of the crankshaft shown in Fig. IV.1. • Small-diameter and deep holes in a part, such as the injector nozzle shown in Fig. IV.2. • Parts with sharp features, a threaded section, or specified close dimensional tolerances, such as the part shown in Fig. IV.3. • A threaded hole or holes on different surfaces of a part, for assembly with other components. • Complex geometries, often in hard or high-performance materials that cannot be easily or economically produced and in the quantities desired through the processes described earlier in the book (see Fig. 25.1). • Special surface finish and texture for functional purposes or for appearance. It soon will become clear that none of the processes described in the preceding chapters is capable of producing the specific characteristics outlined above, thus the parts will require further processing, generally referred to as secondary or finishing operations. Machining is a general term describing a group of processes that consist of the removal of material and modification of a workpiece surfaces after it has been made. The very wide variety of shapes produced by machining can be seen in an automobile, as shown in Fig. IV.4. In reviewing the contents of Parts II and III of this text, it will be recalled that some parts may indeed be produced to final shape (net shape) and in large quantities. However, machining processes may be preferable or even necessary for the following reasons: 1. Closer dimensional accuracy may be required than can be achieved by metalworking or casting processes alone. For example, the bearing surfaces in a crankshaft cannot be produced with good dimensional accuracy and surface finish through forging or sand casting alone. 2. Parts may require external and/or internal geometric features, such as sharp corners and internal threads, that cannot be produced by other processes. Figure IV.1: A forged crankshaft, highlighting the smooth and shiny machined and ground bearing surfaces. The shiny bearing surfaces cannot be made to their final dimensions and surface finish by any of the processes described in previous chapters. Source: Shutterstock/AleksandrN 633 ׉ 7cassandra://9H6v35NSYowwu_g9tJ2reNRK-9R0oZuygl3iw-G8SvA`cZ%Eb׉E634 Part IV Machining Processes and Machine Tools 3.5-mm dia. 8 holes, 0.17 mm 1.1-mm wall Figure IV.2: Cross section of a fuel-injection nozzle, showing a small hole made by the electrical-discharge machining process (Section 27.5). The material is heat-treated steel. 57 17 16.9 18.8 18.2 14 13.8 5 12.5 2.5 308 0.25 458 Chamfer 11.9 11.8 Figure IV.3: A machined and threaded part, showing various dimensions and tolerances; all dimensions are in mm. Note that some tolerances are only a few tenths of an mm. Master cylinder Keys Threaded fasteners Pistons Crankshaft Engine block Door locks Bolts and nuts Drilled and tapped holes Connecting rods, valve bodies Threaded holes, bolts Brake rotor Figure IV.4: Typical parts on an automobile that require machining operations to impart desirable shapes, surface characteristics, dimensions, and tolerances. 15.5 15.4 0.8 20.4 20.3 12.66 12.38 13 16.2 16.1 ׉ 7cassandra://qBo7CGckoDhcth6STaylACx1wj9-NtGDOTa3nwViZ48`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ezEtFsmevo6JDxpklZg9a7qpEdO7C9shLg_Faq87W2EԴ`׉ 7cassandra://WQOCcvzIKt7Z8zR9xrNytqewScj0zEL8tswIWlJuvGAhr`s׉ 7cassandra://lacPMk2ksx1_1qu8oeIZwMs6YxJ2NDKBhPLDhjvBBXA`׉ 7cassandra://LHlo7kz78tZE3r4StE11nCusSh-fQpbd_gcN0hjqiRIY ͠^cZEbט#u׉׉ 7cassandra://_qbN2TPI0teTrClZHiE-IXypYTN_u3_4yq5n3KRH6KEYz`׉ 7cassandra://XNdcpilTFyXckn5yg03kZwE18D5fdZMHmCruhLQ4g_Q0`s׉ 7cassandra://tUf4qGy241-T4-cirLwZfiSN7nvaaMjgBrrJdU9SyVg `׉ 7cassandra://zjTNiHYMQ8fS1LcKlX4nKVwFwEQ5ZMgI_P27tIESUto2͠^cZEb׉E`s׉ 7cassandra://f_EJx3pOcXEVi_JVzhVXBzkE9KnvzFxZGEKwUKuCvZg`׉ 7cassandra://crKl2SxZOBiuCZCNUmh3uPh0mTPvHn3AsaDanmqob2w͠^cZEbט#u׉׉ 7cassandra://__7T9nmm97gUVlcgzViQPRQsP27Cxm93_MD0-KvUkHg`׉ 7cassandra://0UzK6uzuOE_wfPlzVrX3jpTFjPUHNuJU2-xkqHgmUj4F`s׉ 7cassandra://MJdSyUcsSAVk8JhNkkSbBQbQxhLDbNVOnhmkK9kkAL8`׉ 7cassandra://-7tkQfbOWxFB12bcAxM-CbqqMUvQugAH8VCIk4PYSAQ$jL͠^cZEb׉EIntroduction 637 21.1 Introduction Machining processes remove material from the surfaces of a workpiece by producing chips. Some of the more common cutting processes, illustrated in Fig. 21.1 (see also Fig. I.6e), are: • Turning, in which the workpiece is rotated and a cutting tool removes a layer of material as the tool moves along its length, as shown in Fig. 21.1a. • Cutting off, in which the tool moves radially inward and separates a piece (on the right in Fig. 21.1b) from the blank. • Slab milling, in which a rotating cutting tool removes a layer of material from the surface of the workpiece (Fig. 21.1c). • End milling, in which a rotating cutter travels to a certain depth in the workpiece, producing a cavity (Fig. 21.1d). In the turning process, illustrated in greater detail in Fig. 21.2, the cutting tool is set at a certain depth of cut (mm), and travels to the left with a certain cutting speed as the workpiece rotates. The feed,or feed rate,is Tool Tool (a) Straight turning Cutter End mill (b) Cutting off (c) Slab milling (d) End milling Figure 21.1: Some examples of common machining operations. Feed (mm/rev) Depth of cut (mm) Tool Chip Tool holder Figure 21.2: Schematic illustration of the turning operation, showing various features. ׉ 7cassandra://f_EJx3pOcXEVi_JVzhVXBzkE9KnvzFxZGEKwUKuCvZg`cZ%Eb׉E638 Chapter 21 Fundamentals of Machining the distance the tool travels per revolution of the workpiece (mm/rev); this movement of the cutting tool produces a chip, which moves up the face of the tool. In order to analyze this basic machining process in greater detail, a two-dimensional model of it is presented in Fig. 21.3a. In this idealized model, a cutting tool moves to the left along the workpiece at a constant velocity, V, and a depth of cut, to. Ahead of the tool, a chip is produced by plastic deformation, shearing the material continuously along the shear plane. This phenomenon can easily be demonstrated by slowly scraping the surface of a stick of butter lengthwise with a sharp knife, and observing how a chip is being produced. Chocolate shavings, used as decorations on cakes and pastries, are produced in a similar manner. In comparing Figs. 21.2 and 21.3, note that the feed in turning is equivalent to to,and the depth of cut in turning is equivalent to the width of cut (the dimension perpendicular to the page). These dimensional tc Rough surface Chip 2 1 Shear plane a Workpiece to f V Tool Shiny surface Rake face Rake angle Flank face Relief or clearance angle Shear angle (a) tc Rough surface Chip Primary shear zone to Rough surface (b) Figure 21.3: Schematic illustration of a two-dimensional cutting process, also called orthogonal cutting: (a) Orthogonal cutting with a well-defined shear plane, also known as the M.E. Merchant model. Note that the tool shape, the depth of cut, to, and the cutting speed, V, are all independent variables. (b) Orthogonal cutting without a well-defined shear plane. 2 1 a V Rake face Tool Rake angle Flank face ׉ 7cassandra://MJdSyUcsSAVk8JhNkkSbBQbQxhLDbNVOnhmkK9kkAL8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://cibA2LTc7qjdcK3Y-VckVfEwKKd7WplfThNUOxVJ1xwp`׉ 7cassandra://N-r4FfDi16-01v8uHhe2fzkQnOv0-skZgPxDuBWj3ccfI`s׉ 7cassandra://JU_-sPvDkLltNmio6Qoas_yxBwAXJwEOGikPfmYe-gM`׉ 7cassandra://UKtvazpdC9t4vzbXL952v4-kUGoUebaM5ZOY9M1MXrEb4͠^cZEbט#u׉׉ 7cassandra://BZcraXGsZm8MLqnbsLnTi_zxqOjkI0bjr3NVBS69vz8e`׉ 7cassandra://FrfoTSZ8AEct3O84QSnrjEwQ9vSjfCKb4-TlAUTRFkcR`s׉ 7cassandra://BCsxWv4Bh-pSDOLoI966OPEGzTQTk-QY3t1c1o-AWWwm`׉ 7cassandra://J2le-bzplRIFtk8wKpJkGNoXhueYUoMYmuFwx3wepMYͷ͠^cZEb׉EMechanics of Cutting 639 relationships can be visualized by rotating Fig. 21.3 clockwise by 90◦. With this brief introduction as a background, the cutting process will now be described in greater detail. 21.2 Mechanics of Cutting The factors that influence the cutting operation are outlined in Table 21.1. In order to appreciate the contents of this table, consider the major independent variables in the basic cutting process: (a) tool material and coatings, if any; (b) tool shape, its surface finish and sharpness; (c) workpiece material and its processing history; (d) cutting speed, feed, and depth of cut; (e) cutting fluids, if any; (f) characteristics of the machine tool; and (g) the type of workholding device and fixturing. Dependent variables in machining are those that are influenced by changes made in the independent variables listed above. They include: (a) type of chip produced, (b) force and energy dissipated during cutting, (c) temperature rise in the workpiece, the tool, and the chip, (d) tool wear and failure, and (e) surface finish and surface integrity of the workpiece. The importance of establishing quantitative relationships among the independent and dependent variables in machining can best be appreciated by considering some typical questions to be posed: Which of the independent variables should be changed first and to what extent (a) if the surface finish of the workpiece being machined becomes unacceptable, (b) if the cutting tool wears rapidly and becomes dull, (c) if the workpiece becomes very hot, and (d) if the tool begins to vibrate and chatter. In order to understand these phenomena and respond to the questions posed, consider that the mechanics of chip formation have been studied extensively since the early 1940s. Several models, with varying degrees of complexity, have been proposed to describe the basic cutting process. More advanced machining models are being developed, especially computer simulation of the mechanics of the basic machining process. The simple model shown in Fig. 21.3a, and referred to as the M.E. Merchant model, developed in the early 1940s, is sufficient for the purposes of this introduction. This model is known as orthogonal cutting, because it is two-dimensional whereby the forces involved are perpendicular to each other. The cutting tool has a rake angle, α (positive as shown in the figure), and a relief or clearance angle. Microscopic examination of chips produced in actual operations reveal that they are produced by shearing (as modeled in Fig. 21.4a), a phenomenon similar to the movement of cards in a deck that is being deformed (see also Fig. 1.6). Shearing takes place within a shear zone (usually along a well-defined plane referred to as the shear plane) and at an angle φ (called the shear angle). Below the shear plane, the workpiece remains undeformed; above it, the chip (which is already formed) moves up the rake face of the tool. Parameter Cutting speed, depth of cut, feed, cutting fluids Tool angles Continuous chip Built-up edge chip Discontinuous chip Temperature rise Tool wear Machinability As above; influence on chip flow direction; resistance to tool wear and chipping Good surface finish; steady cutting forces; undesirable, especially in modern machine tools Poor surface finish and integrity; if thin and stable, edge can protect tool surfaces Desirable for ease of chip disposal; fluctuating cutting forces; can affect surface finish and cause vibration and chatter Influences tool life, particularly crater wear and dimensional accuracy of workpiece; may cause thermal damage to workpiece surface Influences surface finish and integrity, dimensional accuracy, temperature rise, forces and power Related to tool life, surface finish, forces and power, and type of chip produced Table 21.1: Factors Influencing Machining Operations. Influence and interrelationship Forces, power, temperature rise, tool life, type of chip, surface finish and integrity ׉ 7cassandra://JU_-sPvDkLltNmio6Qoas_yxBwAXJwEOGikPfmYe-gM`cZ%Eb׉E K640 Chapter 21 Fundamentals of Machining Rake angle, a Chip Tool d A Workpiece f B Shear plane (a) A C f O B (b) Figure 21.4: (a) Schematic illustration of the basic mechanism of chip formation by shearing. (b) Velocity diagram showing angular relationships among the three speeds in the cutting zone. In this figure, the dimension d is highly exaggerated in order to show the mechanism involved in chip formation; this dimension has been found to be only on the order of 10−2 to 10−3 mm. Some materials, notably cast irons machined at low speeds, do not undergo shearing along a welldefined plane, but instead within a shear zone, as shown in Fig. 21.3b. The shape and size of this zone is important in the machining operation, as will be described in Section 21.2.1. Cutting Ratio. It can be seen from Fig. 21.3a that the chip thickness, tc, can be determined from the depth of cut, to, the rake angle, α, and the shear angle, φ. The ratio of to/tc is known as the cutting ratio,or chip-thickness ratio, r. It is related to the two angles by the following relationships: tan φ = and r = to tc sin φ = cos (φ − α) . (21.2) Because the chip thickness is always greater than the depth of cut, the value of r is always less than unity. The reciprocal of r is known as the chip-compression ratio or chip-compression factor; it is a measure of how thick the chip has become as compared with the depth of cut. Thus, the chip-compression ratio always is greater than unity. As may be visualized by reviewing Fig. 21.3a, the depth of cut is also referred to as the undeformed chip thickness. The cutting ratio is an important and useful parameter for evaluating cutting conditions. Since the undeformed chip thickness, to, is easily specified as a machine setting, and is therefore known, the cutting ratio can be calculated by measuring the chip thickness, using a micrometer. With the rake angle also known for a particular cutting operation (since it is a function of the tool and workpiece geometries), Eq. (21.1) allows calculation of the shear angle. Although to is referred to as the depth of cut, note that in a machining process such as turning, shown in Fig. 21.2, this quantity is the feed or feed rate, expressed as the distance traveled per revolution of the workpiece. To visualize the situation, assume that the workpiece in Fig. 21.2 is a thin-walled tube, and that the width of cut is the same as the thickness of the tube. Then, by rotating Fig. 21.3 clockwise by 90◦,the figure now becomes similar to the view in Fig. 21.2. r cos α 1 − r sin α (21.1) (f 2 a) C Vc (f 2 a) a (908 2 a) V Vs f (908 2 f 1 a) ׉ 7cassandra://BCsxWv4Bh-pSDOLoI966OPEGzTQTk-QY3t1c1o-AWWwm`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://wvLFq3Cn_-m5oN4CE3CmicpVplOSWyV_2tx4T6S9W8g`׉ 7cassandra://UuBrNNaEUhGA-SZecw45_bO-x2EhlQsIImkkjWNFsFYT7`s׉ 7cassandra://Ltd_uFAujNKnBlFYdIhCNLmksKz4IsHsRkLzk4aTafc`׉ 7cassandra://UbwlLGk1Pjjoo0ktmqZHJClRuSG7eHvqj0vE-okmIjUm,͠^cZEbט#u׉׉ 7cassandra://pp6e3r1mCFoiYcoGxdQGfsb9HN---Pf7hSZOocOvo_c`׉ 7cassandra://U3n87GIlXK3heMNN0u_SZkiNFhB-mLm_qCMvU5TlSeQc`s׉ 7cassandra://VaK8mPpInb6jG8UZiEbvSVY4Dq32RvnlJUWH_cdbc34Q`׉ 7cassandra://KyLPSZvYoq0aoDdnVOxsfMWeecjGQHUCfCyxE_WB_eQe\͠^cZEb׉E Mechanics of Cutting 641 Shear Strain. Referring to Fig. 21.4a, it can be seen that the shear strain, γ, that the material undergoes can be expressed as γ = or γ =cot φ + tan (φ − α) . (21.3) Note that large shear strains are associated with (a) low shear angles and (b) with low or negative rake angles. Shear strains of 5 or higher have been observed in actual cutting operations. The material removed from the workpiece undergoes greater deformation during cutting than in forming and shaping processes, as is also seen in Table 2.4. Furthermore, deformation in machining generally takes place within a very narrow zone; in other words, the dimension d = OC in Fig. 21.4a is very small. Thus, the rate at which shearing takes place in machining is high. The nature and size of the deformation zone is further described in Section 21.3. The shear angle has a major significance in the mechanics of machining operations, as it influences force and power requirements, chip thickness, and temperature rise in machining. One of the earliest analyses was based on the assumption that the shear angle adjusts in order to minimize the cutting force, or that the shear plane is a plane of maximum shear stress. This analysis yields the expression φ =45◦ + α 2 − , β 2 (21.4) where β is the friction angle, and is related to the coefficient of friction, μ, at the tool–chip interface by the expression μ = tan β. Among several other shear-angle relationships that have been developed, another approximate but useful formula is φ =45◦ + α − β. (21.5) The coefficient of friction in metal cutting has been found to generally range from about 0.5 to 2 (see also Section 33.4), indicating that the chip undergoes considerable frictional resistance as it moves up the rake face of the tool. Experiments have shown that μ varies considerably along the tool–chip interface, because of large variations in contact pressure and temperature. Consequently, μ is also called the apparent mean coefficient offriction. Equation (21.4) indicates that (a) as the rake angle decreases or as the friction at the tool–chip interface increases, the shear angle decreases and the chip becomes thicker; (b) thicker chips indicate more energy dissipation, because the shear strain is higher, as can be noted from Eq. (21.2); and (c) because the work done during cutting is converted into heat, the temperature rise is also higher. Velocities in the Cutting Zone. Note in Fig. 21.3 that since the chip thickness is greater than the depth of cut, the velocity of the chip Vc has to be lower than the cutting speed V. Because mass continuity has to be maintained, Vto = Vctc or Vc = Vr. Hence, Vc = V sin φ cos (φ − α) Vs = cos (φ − α) to tc cos α = Vc V . A velocity diagram also can be constructed, as shown in Fig. 21.4b. From trigonometric relationships, V Vc = sin φ where Vs is the velocity at which shearing takes place in the shear plane. Note also that r = . (21.6) (21.7) AB OC = AO OC + OB OC , (21.8) ׉ 7cassandra://Ltd_uFAujNKnBlFYdIhCNLmksKz4IsHsRkLzk4aTafc`cZ%Eb׉E 642 Chapter 21 Fundamentals of Machining These relationships will be utilized later in Section 21.3, describing power requirements in machining operations. 21.2.1 Types of Chips Produced in Metal Cutting The types of metal chips commonly observed in practice and their photomicrographs are shown in Fig. 21.5. The four main types are: • Continuous • Built-up edge • Serrated or segmented • Discontinuous. Note that a chip has two surfaces: 1. A surface that has been in contact with the rake face of the tool and has a shiny and burnished appearance, caused by sliding as the chip moves up the tool face. 2. A surface that is the original surface of the workpiece; it has a rough, jagged appearance (as can be seen on the chips in Figs. 21.3 and 21.5) caused by the shearing mechanism shown in Fig. 21.4a. Continuous Chips. Continuous chips are generally formed with ductile materials, machined at high cutting speeds and/or at high rake angles (Fig. 21.5a). Deformation of the material takes place along a narrow shear zone, called the primary shear zone. Continuous chips may develop a secondary shear zone (Fig. 21.5b) because of high friction at the tool–chip interface; this zone becomes wider as friction increases. Deformation in continuous chips also may take place along a wide primary shear zone with curved boundaries (see Fig. 21.3b), unlike that shown in Fig. 21.5a. Note that the lower boundary of the deformation zone in Fig. 21.3b projects below the machined surface, subjecting it to distortion, as depicted by the distorted vertical lines within the machined subsurface. This situation generally occurs in machining soft metals, at low speeds, and low rake angles. It usually results in a poor surface finish and surface residual stresses, which may be detrimental to the properties of a machined part in its service life. Although they generally produce a good surface finish, continuous chips are not necessarily desirable as they tend to become tangled around the toolholder, the fixturing, and the workpiece. They also interfere with chip-disposal systems, described in Section 23.3.7. This situation can be alleviated using chip breakers (see below), as well as by changing processing parameters, such as cutting speed, feed, and depth of cut, or by using appropriate cutting fluids. Built-up Edge Chips. A built-up edge (BUE) consists of layers of material from the workpiece that gradually are deposited on the tool tip, hence the term built-up (Fig. 21.5c). As it grows larger, a BUE becomes unstable, and eventually breaks apart. A portion of the BUE material is carried away by the tool side or rake face of the chip; the rest is deposited randomly on the workpiece surface. Note that, in effect, a built-up edge changes the geometry of the cutting edge and dulls it, as can be seen in Fig. 21.6a. The cycle of BUE formation and destruction is repeated continuously during the cutting operation. Built-up edge is a major factor that adversely affects surface finish, as can be seen in Figs. 21.5c and 21.6b and c. On the other hand, a thin, stable BUE is generally regarded as desirable, because it reduces tool wear by protecting its rake face. Cold-worked metals have a lower tendency to form BUE than those in their annealed condition. Because of work hardening and deposition of successive layers of material, the BUE hardness is significantly higher than that of the workpiece (Fig. 21.6a). The tendency for BUE formation can be reduced by one or more of the following means: • Increase the cutting speed • Decrease the depth of cut ׉ 7cassandra://VaK8mPpInb6jG8UZiEbvSVY4Dq32RvnlJUWH_cdbc34Q`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://I54Q1W_C4IRpUcom_glS5chvFUQk3ZZLXnRzKTUHJncl`׉ 7cassandra://VbLiaCBgaeKEFAUypBQAfheolnKlJlg4YPtwfq7WPwYK`s׉ 7cassandra://hFWyHlmtoPRElegpqe-_1SPrI8GLgjNBUpRAAbyZuUI`׉ 7cassandra://q2AcWGiWq03-Jep-krQC9B50rrZikefaJhSK2LSJSRIX͠^cZEbט#u׉׉ 7cassandra://_ZyNHL73WYKEs1UF_5uYLq_3wYho4QCyYdnGN-7n1uor4`׉ 7cassandra://kg-HpURXZZ_R_kSAJjKYt3OpKP2wrVqeq8D-TMuVWQQK\`s׉ 7cassandra://gLxG0xjTaAbPvVceNaEQkqN-QGl02XwA6OCaER8FuocS`׉ 7cassandra://BdznBZlo9xHXjukUE7bfTDSjh6YQTO9MoHOXLLNXdF8\͠^cZEb׉E5Mechanics of Cutting 643 Secondary shear zones Tool Chip Primary shear zone Workpiece Chip Tool Primary shear zone BUE (a) (b) (c) Low shear strain High shear strain (d) (e) Figure 21.5: Basic types of chips produced in orthogonal metal cutting, their schematic representation, and photomicrographs of the cutting zone: (a) continuous chip, with narrow, straight, and primary shear zone; (b) continuous chip, with secondary shear zone at the chip–tool interface; (c) built-up edge; (d) segmented or nonhomogeneous chip; and (e) discontinuous chip. Source: After M.C. Shaw, P.K. Wright, and S. Kalpakjian. • Increase the rake angle • Use a sharper tool • Use a cutting tool that has lower chemical affinity for the workpiece material • Use an effective cutting fluid. Serrated Chips. Serrated chips, also called segmented or nonhomogeneous chips (Fig. 21.5d), are semicontinuous chips with large zones of low shear strain and small zones of high shear strain (called shear localization). These chips have a sawtooth-like appearance (not be confused with the illustration in Fig. 21.4a, in which the dimension d is highly exaggerated). Metals that have low thermal conductivity and strength that decreases sharply with temperature (called thermal softening) exhibit this behavior, and is most notably observed with titanium. ׉ 7cassandra://hFWyHlmtoPRElegpqe-_1SPrI8GLgjNBUpRAAbyZuUI`cZ%Eb׉E'644 Chapter 21 Fundamentals of Machining Chip 316 Built-up edge 474 661 588 565 492 588 656 466 704 587 704 297 409 544 503 372 329 289 286 604 432684 656 589 331 325 289 371 418 383 567 578 512 377 266 229 317 Workpiece 230 (a) (c) Figure 21.6: (a) Hardness distribution in a built-up edge in 3115 steel. Note that some regions within the built-up edge are as much as three times harder than the bulk metal being machined. (b) Surface finish produced in turning 5130 steel with a built-up edge. (c) Surface finish on 1018 steel in face milling. Magnifications: 15×. Source: (b) and (c) Courtesy of TechSolve, Inc. Discontinuous Chips. Discontinuous chips consist of segments, either firmly or loosely attached to each other (Fig. 21.5e). Discontinuous chips generally develop under the following conditions: • Brittle workpiece materials, because they do not have the capacity to undergo the high shear strains encountered in machining • Workpiece materials containing hard inclusions and impurities, or have structures such as the graphite flakes in gray cast iron (see Fig. 4.11a) • Very low or very high cutting speed, V • Largedepthofcut, d • Tools with low rake angle, α • Lack of an effective cutting fluid (Section 22.12) • Low stiffness of the toolholder or the machine tool, thus allowing vibration and chatter to occur (Section 25.4). Another factor in the formation of discontinuous chips is the magnitude of the compressive stresses on the shear plane. The maximum shear strain at fracture increases with increasing compressive stress. Because of the discontinuous nature of chip formation, cutting forces continually vary during machining. Consequently, the stiffness or rigidity of the cutting-tool holder, the workholding devices, and the machine tool and its condition (see Chapters 23 through 25) are significant factors in machining with serrated or discontinuous chips. If not sufficiently rigid, the machine tool may begin to vibrate and chatter, as 201 251 704 639 734770655 261 565 341 306 386 327 361 289 410 281 281 308 231 306 Hardness (HK) (b) ׉ 7cassandra://gLxG0xjTaAbPvVceNaEQkqN-QGl02XwA6OCaER8FuocS`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://kwnxeY9RO2usqBmQmXg66yevIHkjLseqbZb89YU0Cmc q`׉ 7cassandra://4MDT4wSgX57THKkUSWEPYnB7VBmBCoyX_IhckaNiqVki`s׉ 7cassandra://AH9LPOnwCHyCKSjG_gyyRh0m7yo-KkUZyWrJnO9Poc0`׉ 7cassandra://g3d6mTTf5n1rWMxt75TQr1Bo_DK4hIIIN6ePlWlb79QaJ͠^cZEbט#u׉׉ 7cassandra://2a3rylZAkFht5ticcRiOQij4JsrHiKKnRqJ3d6cVXk8_3`׉ 7cassandra://VAIwmmKkEZa1qM3w-3WT0111_q0bCf6V87fLstpCezA:`s׉ 7cassandra://atRE_Q2adT_zdpFI9CQSGFWmY1kFv6-inATmK_r3DIk1`׉ 7cassandra://Se7-KjvjAukmyMqTeHphuRNtQbFYJTYqcV1uvnIhGH0 6 ͠^cZEb׉E!Mechanics of Cutting 645 described in detail in Section 25.4. This condition, in turn, adversely affects the surface finish and dimensional accuracy of the machined part, and it may cause premature wear or damage to the cutting tool. Even the components of the machine tool may be damaged if the amplitude of the vibration is excessive. Chip Curl. In all cutting operations performed on metals and nonmetallic materials, chips develop a curvature (chip curl) as they leave the workpiece surface (Fig. 21.5). Among the factors affecting chip curl are: • The distribution of stresses in the primary and secondary shear zones • Thermal effects in the cutting zone • Work-hardening characteristics of the workpiece material • The geometry of the cutting tool • Process parameters • Cutting fluids. The first four items above are complex phenomena and beyond the scope of this text. As for the effects of process parameters: as the depth of cut decreases, the radius of curvature of the chip generally decreases (i.e., the chip becomes more curly). Also, cutting fluids can make chips become more curly, thus reducing the tool–chip contact area (see Fig. 21.7a) and concentrating the heat closer to the tip of the tool (Section 21.4). As a result, tool wear increases. Chip Breakers. As stated above, continuous and long chips are undesirable in machining operations because they tend to become severely entangled, interfere with the machining operation, and can also become a potential safety hazard. The usual procedure employed to avoid such a situation is to break the chip intermittently with special features on cutting tools, called chip-breakers, as shown in Fig. 21.7. The basic principles of a chip breaker on a tool’s rake face is to bend and break the chip periodically. Cutting tools and inserts (see Fig. 22.2) now have built-in chip-breaker features of various designs (Fig. 21.7). Chips also can be broken by changing the tool geometry to control chip flow, as in the turning operations shown in Fig. 21.8. Experience indicates that the ideal chip size to be broken is in the shape of either the letter C or the number 9, and fits within a 25-mm square space. Controlled Contact on Tools. Cutting tools can be designed such that the tool–chip contact length is deliberately reduced by recessing the rake face of the tool some distance away from its tip. The reduction in contact length then affects the chip-formation mechanics; primarily, it reduces the cutting forces and, thus, the energy and temperature in machining. Determining an optimum length is important, as too small a contact length would concentrate the heat at the tool tip, increasing tool wear. Machining Nonmetallic Materials. The mechanics of cutting metals are generally applicable to polymers as well as metals. A variety of chips are encountered in cutting thermoplastics (Section 7.3), depending on the type of polymer and process parameters, such as depth of cut, tool geometry, and cutting speed. Because they are brittle, thermosetting plastics (Section 7.4) and ceramics (Chapter 8) generally produce discontinuous chips. The characteristics of other machined materials are described in Section 21.7.3. 21.2.2 Oblique Cutting The majority of machining operations involve tool shapes that are three dimensional, whereby the cutting action is oblique. The basic difference between oblique and orthogonal cutting can be seen in Fig. 21.9a and c. In orthogonal cutting, the chip slides directly up the face of the tool and it becomes a spiral, whereas in oblique cutting, the chip becomes helical and leaves the workpiece surface at an angle i, called the inclination angle (Fig. 21.9b). Note the lateral direction of chip movement in oblique cutting is similar to the action of a snowplow blade, whereby the snow is thrown sideways as the plow travels straight forward. ׉ 7cassandra://AH9LPOnwCHyCKSjG_gyyRh0m7yo-KkUZyWrJnO9Poc0`cZ%Eb׉E646 Chapter 21 Fundamentals of Machining (a) (b) Chip breaker Without chip breaker With chip breaker Chip Tool Rake face of tool Clamp Chip breaker Tool Workpiece (c) Rake face (d) Radius Positive rake (e) Figure 21.7: (a) Machining aluminum using an insert without a chip breaker; note the long chips that can interfere with the tool and present a safety hazard. (b) Machining aluminum with a chip breaker. (c) Schematic illustration of the action of a chip breaker; note that the chip breaker decreases the radius of curvature of the chip and eventually breaks it. (d) Chip breaker clamped on the rake face of a cutting tool. (e) Grooves in cutting tools acting as chip breakers; the majority of cutting tools are now inserts with built-in chip-breaker features. Source: (a) and (b) Courtesy of Kennametal, Inc. Note in Fig. 21.9a that the chip moves up the rake face of the tool at an angle αc (called the chip flow angle), measured in the plane of the tool face. Angle αi is the normal rake angle, and is a basic geometric feature of the tool. It is the angle between line oz normal to the workpiece surface and line oa on the tool face. 08 rake ׉ 7cassandra://atRE_Q2adT_zdpFI9CQSGFWmY1kFv6-inATmK_r3DIk1`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://mLxnZJvee5tZR3eOhwicop4OewNy5zMGONijdaN0T28aa`׉ 7cassandra://t0JoZ7rytZk55oOEuMDbDD8xSh3uai8UmIBxoXPPYKUV`s׉ 7cassandra://7Gw38VCZyNsEPIXvZ0ruvRcunjGYF_VaUyvzxPYMNQU`׉ 7cassandra://DoIl8SbfRa58r3xhv5TYRhToKuHzqprVq8DRaomYu_wlCh͠^cZEbט#u׉׉ 7cassandra://WmhdqMGroSQYTNfC5PI6HTFVE4pgYNRBuiIUNOfD5NE"P`׉ 7cassandra://2Shi7RvSQ6Ue4dte9GFy6FxrlYNaIjHqbvNsvEIBYFYR`s׉ 7cassandra://ur79ExcJQF5HKwnff5IL-2ZKGvbkogp9YeWxQ7v3Vxw>`׉ 7cassandra://9BDRua44FCShg-jPu4L6RrDpP6zdlyTiu4sE1qTS5m0Ͳ͠^cZEbנcZEb}B79ׁHhttp://Vc.Thׁ ׁ Ј׉EMechanics of Cutting 647 Shank (a) (b) (c) (d) Figure 21.8: Chips produced in turning: (a) tightly curled chip; (b) chip hits workpiece and breaks; (c) continuous chip moving radially away from workpiece; and (d) chip hits tool shank and breaks off. Source: After G. Boothroyd. z Tool ac y i a o Chip o i Workpiece x (a) Workpiece (b) Chip (c) Figure 21.9: (a) Schematic illustration of cutting with an oblique tool; note the direction of chip movement. (b) Top view, showing the inclination angle, i. (c) Types of chips produced with tools at increasing inclination angles. In oblique cutting, the workpiece material approaches the cutting tool at a velocity V and leaves the surface (as a chip) with a velocity Vc.The effective rake angle, αe, is calculated in the plane of these two velocities. Assuming that the chip flow angle, αc, is equal to the inclination angle (an assumption that has been verified experimentally), the effective rake angle, αe,is αe =sin−1  sin2 i +cos2 i sin αn .  (21.9) Since both i and αn can be measured directly, the effective rake angle can now be calculated. Note that as i increases, the effective rake angle increases, the chip becomes thinner and longer and, as a consequence, the cutting force decreases. The influence of the inclination angle on chip shape is shown in Fig. 21.9c. A typical single-point turning tool, used on a lathe, is shown in Fig. 21.10a; note the various angles involved, each of which has to be selected properly for efficient cutting. Although these angles have traditionally been produced by grinding (Chapter 26), the majority of cutting tools are now widely available as inserts, as shown in Fig. 21.10b and described in detail in Chapter 22. Various three-dimensional cutting tools, including those for drilling, tapping, milling, planing, shaping, broaching, sawing, and filing, are described in greater detail in Chapters 23 and 24. Shaving and Skiving. Thin layers of material can be removed from straight or curved surfaces by a process similar to the use of a plane in shaving wood. Shaving is used particularly for improving the surface finish and dimensional accuracy of sheared sheet metals and punched holes, as shown in Fig. 16.9. Tool at Chip i 5 308 i 5 158 i 5 08 a Top view ׉ 7cassandra://7Gw38VCZyNsEPIXvZ0ruvRcunjGYF_VaUyvzxPYMNQU`cZ%Eb׉E 648 Chapter 21 Fundamentals of Machining Side-rake angle, 1 (SR) Axis End-cuttingedge angle (ECEA) Axis (a) (b) Figure 21.10: (a) Schematic illustration of a right-hand cutting tool. The various angles on these tools and their effects on machining are described in Section 23.2. Although these tools traditionally have been produced from solid-tool steel bars, they have been replaced largely with (b) inserts, typically made of carbides and other materials; they are available in a wide variety of shapes and sizes. A common application of shaving is in finishing gears, using a cutter that has the shape of the gear tooth (see Section 24.7). Parts that are long or have complicated shapes are shaved by skiving, using a specially shaped cutting tool that moves tangentially across the length of the workpiece shaved. 21.3 Cutting Forces and Power Studying the cutting forces and power involved in machining operations is important for the following reasons: • Data on cutting forces is essential so that 1. Machine tools can be designed to minimize distortion of their components, maintain the desired dimensional accuracy of the machined part, and help select appropriate toolholders and workholding devices. 2. The workpiece, the workholding devices, and the fixtures are capable of withstanding these forces without excessive distortion. • Power requirements must be known to enable the selection of a machine tool with sufficient capacity or to select process parameters that can be achieved by the machine selected. The forces acting in orthogonal cutting are shown in Fig. 21.11a. The cutting force, Fc, acts in the direction of the cutting speed, V, and supplies the energy required for cutting. The ratio of the cutting force to the cross-sectional area being cut (i.e., the product of width of cut and depth of cut) is referred to as the specific cutting force. The thrust force, Ft, acts in a direction normal to the cutting force. These two forces produce the resultant force, R, as can be seen from the force circle diagram shown in Fig. 21.11b. Note that the resultant force can be resolved into two components on the tool face: a friction force, F, along the tool–chip interface, and a normal force, N, perpendicular to it. Cutting edge Face Back-rake angle, 1 (BR) Nose radius Flank Side-relief angle Side-cutting-edge angle (SCEA) Clearance or end-relief angle Clamp screw Toolholder Insert Clamp Seat or shim Shank Axis ׉ 7cassandra://ur79ExcJQF5HKwnff5IL-2ZKGvbkogp9YeWxQ7v3Vxw>`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://0yPgjvOzy8fNak4vMnX_XxtR9Y6G3HBYZg_TJegA31g`׉ 7cassandra://Nvl9AHip4AJzlfv_RVRON-MOvk1Uq4lcALWAbRBrqFkH`s׉ 7cassandra://_wRIW-rKCeVUh-4B48JW82oa7YhCvoDnyPFC9YiScDw `׉ 7cassandra://VieKci7BE2bMsADzZLwTFV0wZnumuh2ZLoIepGO905Aͨ$ "͠^cZEbט#u׉׉ 7cassandra://ftTtwKFn149TOYHOLPydci3wh_k31QngrGHsC7rsaVQ`׉ 7cassandra://EnRQYjskmMhQp71cmljY_CVrz95C8B4p9b2MqL0rjBwPg`s׉ 7cassandra://qktSVuF2D6QLOxWwCftlUWVVyTNDoqwqikMVDY7x3ZU`׉ 7cassandra://TwM-hzazXaS2oP-OIJXgjVKGKwb_HHPca87zOob_DzIl ͠^cZEb׉ECutting Forces and Power 649 a a Tool Chip Fs Fn R f b Workpiece (a) N (b) Figure 21.11: (a) Forces acting in the cutting zone during two-dimensional cutting. Note that the resultant force, R, must be colinear to balance the forces. (b) Force circle to determine various forces acting in the cutting zone. It can also be shown that F = Rsin β and N = Rcos β. (21.11) Note that the resultant force is balanced by an equal and opposite force along the shear plane, and is resolved into a shear force, Fs,and a normal force, Fn. These forces can be expressed as Fs = Fc cos φ −Ft sin φ and Fn = Fc sin φ +Ft cos φ. (21.13) Because the area of the shear plane can be calculated by knowing the shear angle and the depth of cut, the shear and normal stresses in the shear plane can thus be determined. The ratio of F to N is the coefficient of friction at the tool–chip interface, μ, and the angle β is the friction angle (as in Fig. 21.11). The magnitude of μ can be determined as μ = F N = Ft +Fc tanα Fc −Ft tanα . (21.14) Although the magnitude of forces in actual cutting operations is generally on the order of a few hundred newtons, the local stresses in the cutting zone and the pressure on the cutting tool are very high because the contact areas are very small. For example, the tool–chip contact length (see Fig. 21.3) is typically on the order of 1 mm. Consequently, the tool tip is subjected to very high stresses, which lead to wear as well as chipping and fracture of the tool. Thrust Force. The thrust force in cutting is important because the toolholder, the work-holding devices, and the machine tool itself must be sufficiently stiff to support that force with minimal deflections. For example, if the thrust force is too high or if the machine tool is not sufficiently stiff, the tool will deflect away from the workpiece. This movement will, in turn, reduce the depth of cut, resulting in poor dimensional accuracy in the machined part. (21.12) (21.10) R b N F Ft Fc a Ft V Fc Fn Chip V Fs f b 2 a R F Workpiece Tool ׉ 7cassandra://_wRIW-rKCeVUh-4B48JW82oa7YhCvoDnyPFC9YiScDw `cZ%Eb׉E 650 Chapter 21 Fundamentals of Machining The effect of rake angle and friction angle on the magnitude and direction of thrust force can be determined by noting, from Fig. 21.11b, that Ft = Rsin (β − α) , or Ft = Fc tan (β − α) . (21.16) The magnitude of the cutting force, Fc, is always positive, as shown in Fig. 21.11, because it is this force that supplies the work required in cutting. However, the sign of the thrust force, Ft, can be either positive or negative, depending on the values of β and α. Note that when β> α, the sign of Ft is positive (downward), and when β< α, the sign is negative (upward). It is therefore possible to have an upward thrust force under the conditions of (a) high rake angles, (b) low friction at the tool–chip interface, or (c) both. A negative thrust force can have important implications in the design of machine tools and workholders and in the stability of the cutting process. Power. It can be seen from Fig. 21.11 that the power input in cutting is Power = FcV. (21.17) The power is dissipated mainly in the shear zone (due to the energy required to shear the material) and on the rake face of the tool (due to tool–chip interface friction). From Figs. 21.4b and 21.11, the power dissipated in the shear plane is Power for shearing = FsVs. Denoting the width of cut as w,the specific energy for shearing, us, is given by us = FsVs wtoV Similarly, the power dissipated in friction is Power for friction = FVc, and the specific energy for friction, uf,is uf = The total specific energy, ut,isthus ut = us + uf. (21.22) Because numerous factors are involved, reliable prediction of cutting forces and power still is based largely on experimental data, such as those given in Table 21.2. The wide range of values seen in the table can be attributed to differences in strength within each material group, and to other factors, such as friction, use of cutting fluids, the wide range in process parameters, and the sharpness of the tool tip. Dull tools require higher power and result in higher forces because the tip rubs against the machined surface and makes the deformation zone ahead of the tool larger. Measuring Cutting Forces and Power. Cutting forces can be measured using a force transducer (typically with quartz piezoelectric sensors), a dynamometer,ora load cell (with resistance-wire strain gages placed on octagonal rings) mounted on the cutting-tool holder. It is also possible to calculate the cutting force from the power consumption during cutting, using Eq. (21.4). It should be recognized that Eq. (21.4) represents the power in the machining process itself, and the machine tool will need additional power in order to overcome friction. Thus, to determine the cutting FVc wtoV = Fr wto . (21.21) (21.20) . (21.18) (21.19) (21.15) ׉ 7cassandra://qktSVuF2D6QLOxWwCftlUWVVyTNDoqwqikMVDY7x3ZU`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://HD6tgn0_PW4G1asSqj8oHt4CkSuEtXyuGsDv-Nyj_2w#`׉ 7cassandra://36SLzGtSSTmqcdOaZmb9SurDXfZGgaLlFAPKg8i7txs2`s׉ 7cassandra://CLAU0bvH__C5qbAnfTwhNYVgIiU2-S-ol3Ex46SKt2I]`׉ 7cassandra://fCwod4kgr4AQI_C8U3pYpGjfAIGPhURqK5h3ropyZVAtf͠^cZEbט#u׉׉ 7cassandra://Y3fzKwaTK-789DUllMHyv4JxMd-xmWAJxv-kmKdoB20`׉ 7cassandra://Z3EnGkdmLp17RTOlnI0ns8M69k5cTqwpVA1yZ6hOC88j`s׉ 7cassandra://2C_bHiT9asJpWaOLvmZ6EwNRWFV8yMeSPHzTUSR_UmE`׉ 7cassandra://VK3nU1Oq5NDSShlw5K6SFjAnMKxvQzUnub1_w2qzv7Q}2 ͠^cZEb׉EhCutting Forces and Power 651 Table 21.2: Approximate Range of Energy Requirements in Cutting Operations at the Drive Motor of the Machine Tool, Corrected for 80% Efficiency (for dull tools, multiply by 1.25). Material Aluminum alloys Cast irons Copper alloys High-temperature alloys Magnesium alloys Nickel alloys Refractory alloys Stainless steels Steels Titanium alloys Specific energy W-s/mm3 0.4–1 1.1–5.4 1.4–3.2 3.2–8 0.3–0.6 4.8–6.7 3–9 2–5 2–9 2–5 force from the measured machine power consumption, the mechanical efficiency of the machine tool must be known. The specific energy in cutting, such as that shown in Table 21.2, also can be used to estimate cutting forces. Example 21.1 Relative Energies in Cutting Given: In an orthogonal cutting operation, to =0.1 mm, V =2 m/s, α =10◦, and the width of cut is 5 mm. It is observed that tc =0.20 mm, Fc = 500 N, and Ft = 200 N. Find: Calculate the percentage of the total energy that goes into overcoming friction at the tool–chip interface. Solution: The percentage of the energy can be expressed as Friction energy Total energy where r = to tc 0.1 = 0.20 =0.50, F = Rsin β, Fc = Rcos (β − α) , and R = Thus, 500 = 538 cos (β − 10◦) , so β =32◦ and F = 538 sin 32◦ = 285 N. Hence, Percentage = (285)(0.5) 500 =0.28, or 28%.  F2 t +F2 c =  2002 + 5002 = 538 N. = FVc FcV = Fr Fc , ׉ 7cassandra://CLAU0bvH__C5qbAnfTwhNYVgIiU2-S-ol3Ex46SKt2I]`cZ%Eb׉EF652 Chapter 21 Fundamentals of Machining 21.4 Temperatures in Cutting As in all metalworking processes involving plastic deformation (Chapters 13 through 16), the energy dissipated in cutting is converted into heat which, in turn, raises the temperature in the cutting zone and the workpiece surface. Temperature rise is a major factor in machining because of its various adverse effects: • Excessive temperature lowers the strength, hardness, stiffness, and wear resistance of the cutting tool; tools may also soften and undergo plastic deformation, thus the altering tool shape. • Heat causes uneven dimensional changes in the part being machined, thus making it difficult to control its dimensional accuracy and tolerances. • An excessive temperature rise can induce thermal damage and metallurgical changes (Chapter 4) in the machined surface, adversely affecting properties. The main sources of heat in machining are: (a) work done in shearing in the primary shear zone, (b) energy dissipated as friction at the tool–chip interface, and (c) heat generated as the tool rubs against the machined surface, especially with dull or worn tools. Much effort has been expended in establishing relationships among temperature and various material and process variables in cutting. It can be shown that, in orthogonal cutting,the mean temperature, Tmean,inKis Tmean = 0.000665σf ρc  3 Vto K , (21.23) where σf is the flow stress (see Section 14.2), in MPa, ρc is the volumetric specific heat in kJ/m3·K, and K is the thermal diffusivity (ratio of thermal conductivity to volumetric specific heat) in m2/s. Because the material parameters in this equation also depend on temperature, it is important to use appropriate values that are applicable to the predicted temperature range. It can be seen from Eq. (21.23) that the mean cutting temperature increases with workpiece strength, cutting speed, and depth of cut, and decreases with increasing specific heat and thermal conductivity of the workpiece material. A simple expression for the mean temperature in turning on a lathe is given by Tmean ∝ Vafb, (21.24) where V is the cutting speed and f is the feed of the tool, as shown in Fig. 21.2. Approximate values of the exponents a and b are a =0.2 and b =0.125 for carbide tools and a =0.5 and b =0.375 for high-speed steel tools. Temperature Distribution. Because the sources of heat generation in machining are concentrated in the primary shear zone and at the tool–chip interface, it is to be expected that there will be severe temperature gradients within the cutting zone. A typical temperature distribution is shown in Fig. 21.12; note the presence of severe gradients, and that the maximum temperature is about halfway up the tool–chip interface. The temperatures typically developed in a turning operation on 52100 steel are shown in Fig. 21.13. The temperature distribution along the flank surface of the tool is shown in Fig. 21.13a for V =60, 90, and 170 m/min as a function of the distance from the tip of the tool. The distributions at the tool–chip interface for the same three cutting speeds are shown in Fig. 21.13b as a function of the fraction of the contact length. Thus, zero on the abscissa represents the tool tip, and 1.0 represents the end of the tool–chip contact length. Note from Eq. (21.23) that the temperature increases with cutting speed and that the highest temperature is almost 1100◦C. The presence of such high temperatures in machining can be verified simply by observing the dark-bluish color of the chips (caused by oxidation) typically produced at high cutting speeds. Chips can indeed become red hot, and thus create a safety hazard. From Eq. (21.24) and the values for the exponent a, it can be seen that the cutting speed, V,greatly influences temperature. The explanation is that, as speed increases, the time for heat dissipation decreases, ׉ 7cassandra://2C_bHiT9asJpWaOLvmZ6EwNRWFV8yMeSPHzTUSR_UmE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://qADeYbfuVcmfxCL3_lV6Ht1w4UKn612_UOmRz4c3KL8`׉ 7cassandra://kSEc2fvd1ONXc-_3h5KgtaYjWe8qLfJ1AiW-hojU5hY;`s׉ 7cassandra://BeuwpiiguadOsFcoDQjUdr-t4oB6_-NKUWqepy3UGlA`׉ 7cassandra://qgsOeYJ-rvV5pIAvb_Trw5EP637FjP0HmLLRxWhaLBQͨi͠^cZEbט#u׉׉ 7cassandra://FNpZmw6_Ltx_OhR-iAOh0whUR8NpGjNcfuJnIvkZ9JM`׉ 7cassandra://hWxOb8V4oMsHss_pdViQ0uUYkaG1hRNuuaUvPusXxhUZQ`s׉ 7cassandra://Z_ZepzoYY6Iy0is9CMc7egaqoYJIKvbyyAH4RqwT4XMv`׉ 7cassandra://Ew0jUA2OjfMkTh905w1xsyTagofKtVKF4L23vjfLDcÀ̆͠^cZEb׉EXTemperatures in Cutting 653 Chip Temperature (8C) Tool 130 80 30 Workpiece Figure 21.12: Typical temperature distribution in the cutting zone. Note the severe temperature gradients within the tool and the chip, and that the workpiece is relatively cool. 1100 Work material: AISI 52100 Annealed: 188 HB 700 600 Feed: 0.14 mm/rev . 500 400 0 0.5 mm (a) 1.0 1.5 500 300 0 0.2 0.4 0.6 0.8 1.0 Fraction of tool–chip contact length measured in the direction of chip flow (b) Figure 21.13: Temperatures developed in turning 52100 steel: (a) flank temperature distribution and (b) tool–chip interface temperature distribution. Source: After B.T. Chao and K.J. Trigger. and hence the temperature rises, eventually becoming almost an adiabatic process. The effect of speed can be simulated easily by rubbing hands together faster and faster. As can be seen in Fig. 21.14, the chip carries away most of the heat generated. In a typical machining operation, it has been estimated that 90% of the energy is removed by the chip, with the remainder taken by the tool and the workpiece. Note also that, as the cutting speed increases, a larger proportion of the total m Tool material: K3H carbide 900 700 500 s 450 2 8 . 600 1 5 1 500 600 600 650 . 700 650 400 380 V 360 s / 1 0 8 . 2 5 . Flank surface temperature (8C) Local temperature at tool–chip interface (8C) m / 1 5 . 0 ׉ 7cassandra://BeuwpiiguadOsFcoDQjUdr-t4oB6_-NKUWqepy3UGlA`cZ%Eb׉E `654 Chapter 21 Fundamentals of Machining Chip Cutting speed Figure 21.14: Proportion of the heat generated in cutting transferred to the tool, workpiece, and chip as a function of the cutting speed. Note that the chip removes most of the heat. heat generated is carried away by the chip, and less heat is transferred elsewhere. This is one reason for the continued trend of increasing machining speeds (see high-speed machining, Section 25.5). The other main benefit of higher cutting speeds is associated with the favorable economics in reducing machining time (see Section 25.8). Techniques for Measuring Temperature. Temperatures and their distribution in the cutting zone may be determined using thermocouples, embedded in the tool or the workpiece. The mean temperature can be determined using the thermal emf (electromotive force) at the tool–chip interface, which acts as a hot junction between two different materials (tool and chip). A third method is monitoring the infrared radiation from the cutting zone, using sensors; however, this technique indicates only surface temperatures, and its accuracy depends on the emissivity of the surfaces, which can be difficult to determine accurately. 21.5 Tool Life: Wear and Failure It can be noted from the previous sections that cutting tools are subjected to (a) high localized stresses at the tip of the tool, (b) high temperatures, especially along the rake face, (c) sliding of the chip at relatively high speeds along the rake face, and (d) sliding of the tool along the newly machined workpiece surface. These conditions induce tool wear, a major consideration in all machining operations (as are mold and die wear in casting and metalworking processes). Tool wear, in turn, adversely affects tool life, the quality of the machined surface, its dimensional accuracy, and, consequently, the economics of machining operations. Wear is a gradual process (see Section 33.5), much like the wear of the tip of an ordinary pencil. The rate of tool wear (that is, volume worn per unit time) depends on the workpiece material, tool material and its coatings, tool geometry, process parameters, cutting fluids, and characteristics of the machine tool. Tool wear and the resulting changes in tool geometry (Fig. 21.15) are generally classified as: flank wear, crater wear, nose wear, notching, plastic deformation, chipping,and gross fracture. 21.5.1 Flank Wear Flank wear occurs on the relief (flank) face of the tool, as shown in Fig. 21.15a, b, and e. It generally is attributed to (a) rubbing of the tool along the machined surface, thereby causing adhesive or abrasive wear and (b) high temperatures, adversely affecting tool-material properties. In a classic study by F.W. Taylor on the machining of steels conducted in the early 1890s, the following approximate relationship for tool life, known as the Taylor tool life equation, was established: VTn = C. (21.25) where V is the cutting speed, T is the time (in minutes) that it takes to develop a certain flank wear land (shown as VB in Fig. 21.15a), n is an exponent that depends on tool and workpiece materials and cutting W k o Tool p r i c e e Energy (%) ׉ 7cassandra://Z_ZepzoYY6Iy0is9CMc7egaqoYJIKvbyyAH4RqwT4XMv`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://DCuoxnUNCVFqzvFXPo_XEcFGMGAi2WSOvbBexQ6kfGw`׉ 7cassandra://gj0Eqh_nGrppTn_ul7_Wm0M70LodrSJupPyVfp0P4IkG`s׉ 7cassandra://BQrsBwPO-vA8y-8S7gjqSzyi4X8g0rPe0WAphP14ZQc`׉ 7cassandra://839sCvg9ffwO4afluRuP0DAprfRNdS9VZL2ihh8LkBwͳ͠^cZEbט#u׉׉ 7cassandra://VBe68Hi1SZmPVc47shPDvHdr_JHLWMajCC_YjHB5gic`׉ 7cassandra://lGZJfBANhotdH99pVVanSoFyG9MqVHTEfzu6P-0gnuELJ`s׉ 7cassandra://mmiAid-KufSUKkXtMynT6406nZTUCGN6C_2ZpfbDRGsF`׉ 7cassandra://zNmLHnatvvOcLpnu6idBC7qz37EPJTietk4zv57dSjw͎lt͠^cZEb׉ETool Life: Wear and Failure 655 Rake face Crater wear depth (KT) Flank wear Tool Flank face Rake face Nose R radius New tool Depth-of-cut line (a) Rake face Flank wear Flank face (b) Thermal cracking Rake face (d) (e) Figure 21.15: (a) Features of tool wear in a turning operation. The VB indicates average flank wear. (b)–(e) Examples of wear in cutting tools: (b) flank wear, (c) crater wear, (d) thermal cracking, and (e) flank wear and built-up edge. Source: (a) Terms and definitions reproduced with the permission of the International Organization for Standardization, ISO, copyright remains with ISO. (b)–(e) Courtesy of Kennametal Inc. conditions, and C is a constant. Each combination of workpiece and tool materials and each cutting condition have their own n and C values, both of which are determined experimentally, often based on surface finish requirements. Moreover, the Taylor equation is often applied even when flank wear is not the dominant wear mode (see Fig. 21.15), or if a different criterion (such as the machining power required) is used to define C and n. Generally, n depends on the tool material, as shown in Table 21.3, and C on the workpiece material. Note that the magnitude of C is the cutting speed at T =1 min. To appreciate the importance of the exponent n, Eq. (21.25) can be rewritten as T =  1/n C V , (21.26) where it can be seen that for a constant value of C, the smaller the value of n, the lower is the tool life. Cutting speed is the most important variable associated with tool life, followed by depth of cut and feed, f. For turning, Eq. (21.25) can be modified as VTndxfy = C, (21.27) Flank face BUE (c) Rake face Crater wear Flank face Flank wear Crater wear VBmax VB Flank face Depth-of-cut line ׉ 7cassandra://BQrsBwPO-vA8y-8S7gjqSzyi4X8g0rPe0WAphP14ZQc`cZ%Eb׉E b656 Chapter 21 Fundamentals of Machining Table 21.3: Ranges of n Values for the Taylor Equation [Eq. (21.25)] for Various Tool Materials. High-speed steels 0.08–0.2 Cast alloys Carbides Coated carbides Ceramics 0.1–0.15 0.2–0.5 0.4–0.6 0.5–0.7 where d is the depth of cut and f is the feed in mm/rev, as shown in Fig. 21.2. The exponents x and y must be determined experimentally for each cutting condition. Taking n =0.15, x =0.15,and y =0.6 as typical values encountered in machining practice, it can be seen that cutting speed, feed rate, and depth of cut are of decreasing importance. Equation (21.27) can be rewritten as T = C1/nV−1/nd−x/nf−y/n, or, using typical values for the exponents, as T ≈ C7V−7d−1f−4. For a constant tool life, the following observations can be made from Eq. (21.29): • If the feed or the depth of cut is increased, the cutting speed must be decreased, and vice versa. • Depending on the magnitude of the exponents, a reduction in speed can result in an increase in the volume of the material removed, because of the increased feed or depth of cut. Tool-life Curves. Tool-life curves are plots of experimental data, obtained from cutting tests for various materials and under different cutting conditions, such as cutting speed, feed, depth of cut, tool material and geometry, and cutting fluids. Note in Fig. 21.16, for example, that (a) tool life decreases rapidly as the cutting speed increases, (b) the condition of the workpiece material has a strong influence on tool life, and (c) there is a large difference in tool life for different microstructures of the workpiece material. Heat treatment of the workpiece is important, due largely to increasing workpiece hardness; for example, ferrite has a hardness of about 100 HB, pearlite 200 HB, and martensite 300 to 500 HB. Impurities 120 5 1 40 80 0 50 100 150 200 250 m/min Figure 21.16: Effect of workpiece hardness and microstructure on tool life in turning ductile cast iron. Note the rapid decrease in tool life (approaching zero) as the cutting speed increases. Tool materials have been developed that resist high temperatures, such as carbides, ceramics, and cubic boron nitride, as described in Chapter 22. 2 3 4 1 As cast 2 As cast 3 As cast 4 Annealed 5 Annealed Hardness (HB) 265 215 207 183 170 Ferrite Pearlite 20% 40 60 97 100 80% 60 40 3 — (21.29) (21.28) Tool life (min) ׉ 7cassandra://mmiAid-KufSUKkXtMynT6406nZTUCGN6C_2ZpfbDRGsF`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://rt9hdi2VyKMy-rlNxivmchSI8lLKtPv0ufrGt6aoqlob`׉ 7cassandra://XDyYOyoCFbTN394LqkHX4lDN536DKS0VgjoDbkOiRo0IL`s׉ 7cassandra://1-MB-_NdTl0MqdFVeogOPDo25UG2A4n7eSmkXOACULE`׉ 7cassandra://wqdgLfz-eDD8YrNZowsj7VusSmtuypt3s-us9VoDZnM{^͠^cZEbט#u׉׉ 7cassandra://bcvbbkriMwt6K2RM6OtPVWJPdDHRtxzjTzdnw5uv47Q`׉ 7cassandra://o72DMqLqj5ctbi093KGW4T_lp3jDCkWignXuMzyNFz8a`s׉ 7cassandra://AucT8LeKqSZAIMs1KTzadxy4n1idi9U1Y4pTlpTNhQU&`׉ 7cassandra://vcRQInqlb0MKGHvEty0eBrYW3ocZIvEKsp7XTYzbMawvT>͠^cZEb׉ETool Life: Wear and Failure 657 300 100 10 20 5 1 50 300 n 3000 Cutting speed (m/min) Figure 21.17: Tool-life curves for a variety of cutting-tool materials. The negative reciprocal of the slope of these curves is the exponent n in the Taylor tool-life equation [Eq. (21.25)], and C is the cutting speed at T =1 min, ranging from about 60 to 3000 m/min in this figure. and hard constituents in the material or on the workpiece surface, such as rust, scale, and slag, also are important factors, because their abrasive action (see Section 33.5) reduces tool life. The exponent n can be determined from tool-life curves (Fig. 21.17). Note that the smaller the value of n, the faster the tool life decreases with increasing cutting speed. Although tool-life curves are somewhat linear over a limited range of cutting speeds, they rarely are linear over a wide range. Moreover, n can indeed become negative at low cutting speeds, meaning that tool-life curves actually can reach a maximum and then curve downward. Caution should therefore be exercised in using tool-life equations beyond the range of cutting speeds to which they are applicable. Because temperature has a major influence on the physical and mechanical properties of materials (see Chapters 2 and 3), it is to be expected that temperature also strongly influences wear. Thus, as temperature increases, wear increases. Example 21.2 Increasing Tool Life by Reducing the Cutting Speed Given: Assume that for a given tool and workpiece combination, n =0.5 and C = 400. Find: Calculate the percentage increase in tool life when the cutting speed is reduced by 50%, using the Taylor equation [Eq. (21.25)] for tool life. Solution: Since n =0.5, the Taylor equation can be rewritten as VT0.5 = 400. Denote V1 as the initial speed and V2 as the reduced speed; thus, V2 =0.5V1. Because C is a constant at 400, 0.5V1 T2 = V1 T1.  Simplifying this equation, T2 T1 Thusthechangeintoollifeis T2 −T1 T1 =  T2 T1  −1=4−1=3, or that tool life is increased by 300%. Note that a reduction in cutting speed has resulted in a major increase in tool life. Note also that, for this problem, the magnitude of C is not relevant. 1 = 0.25 =4.  Tool life (min) High-speed steel Cast-cobalt alloy Carbide Ceramic ׉ 7cassandra://1-MB-_NdTl0MqdFVeogOPDo25UG2A4n7eSmkXOACULE`cZ%Eb׉E 658 Chapter 21 Fundamentals of Machining Table 21.4: Allowable Average Wear Land (see VB in Fig. 21.15a) for Cutting Tools in Various Machining Operations. Allowable wear land (mm) Operation Turning Face milling End milling Drilling Reaming High-speed steel tools 1.5 1.5 0.3 0.4 0.15 Carbide tools 0.4 0.4 0.3 0.4 0.15 Note: Allowable wear for ceramic tools is about 50% higher. Allowable notch wear (see Section 21.5.3), VBmax,isabout twice that for VB. Allowable Wear Land. Cutting tools need to be resharpened or replaced when (a) the surface finish of the machined workpiece begins to deteriorate, (b) cutting forces increase significantly, or (c) the temperature rises significantly. The allowable wear land, indicated as VB in Fig. 21.15a, is given in Table 21.4 for various machining conditions. For improved dimensional accuracy and surface finish, the allowable wear land may be smaller than the values given in the table. The recommended cutting speed for a high-speed steel tool (see Section 22.2) is generally the one that yields a tool life of 60 to 120 min, and for a carbide tool (Section 22.4), it is 30 to 60 min. Optimum Cutting Speed. Recall that as cutting speed increases, tool life decreases rapidly. On the other hand, if the cutting speed is low, tool life is longer, but the rate at which material is removed is also low. Thus, there is an optimum cutting speed, based on economic or production considerations, where the tool life is long and production speeds are reasonably high. This topic is described in greater detail in Section 25.8. Example 21.3 Effect of Cutting Speed on Material Removal The effect of cutting speed on the volume of metal removed between tool changes or resharpenings can be appreciated by analyzing Fig. 21.16. Assume that a material is being machined, in the as-cast condition, with a hardness of 265 HB. Note that when the cutting speed is 60 m/min, tool life is about 40 min. Therefore, the tool travels a distance of 60 m/min × 40 min = 2400 m before it has to be replaced. However, when the cutting speed is increased to 120 m/min, tool life is reduced to about 5 min, and thus the tool travels 120 m/min × 5 min = 600 m before it has to be replaced. Since the volume of material removed is directly proportional to the distance the tool has traveled, it can be seen that by decreasing the cutting speed, more material is removed between tool changes. Note, however, that the lower the cutting speed, the longer is the time required to machine a part, which has a significant economic impact on the operation (see Section 25.8). 21.5.2 Crater Wear Crater wear occurs on the rake face of the tool, as shown in Fig. 21.15a and c, and Fig. 21.18, which also illustrates various types of tool wear and failures. It can be seen that crater wear alters the tool–chip contact geometry. The most significant factors that influence crater wear are (a) the temperature at the tool– chip interface and (b) the chemical affinity of the tool and workpiece materials. Additionally, the factors influencing flank wear also may affect crater wear. Crater wear is generally attributed to a diffusion mechanism: the movement of atoms across the tool– chip interface. Because diffusion rate increases with increasing temperature, crater wear also increases as temperature increases. Note in Fig. 21.19, for example, how rapidly crater wear increases with temperature within a narrow range. Applying protective coatings to tools is an effective means of slowing ׉ 7cassandra://AucT8LeKqSZAIMs1KTzadxy4n1idi9U1Y4pTlpTNhQU&`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://nvSTFF9aYsA2o78Gs-9vAKm7dCQGTrDn5QN2MjoYdlkx`׉ 7cassandra://jsWNXAHkyu-HIMXWUlxKOlGD4mwU45wTCVuBhmEkovQF`s׉ 7cassandra://vyC71T-FXmdT612SSmFIPyuP6U9t8fyv6MM3x3UTRKQ`׉ 7cassandra://9kEU7mrVlYcUQNzLGd8tDHhckFLKQb6ddXOq-BSC9hE ͠^cZEbט#u׉׉ 7cassandra://_AwPfg5-Iy6ASf6zcR_0r0c0cZTrkPDE7FtzQa_sw7cI`׉ 7cassandra://QCWC9owfFhV-LrtwqmuHJSYsaaKuFwjas6b8b70ebFMY`s׉ 7cassandra://to9EsIp1YAqmxC6d3AOrGWmTDdNSJUEFNOiSRn76F3M`׉ 7cassandra://90_nqhxTwlacHnSf2UB7yfqep5kVaMw_e5kMhuNQQTwr̴͠^cZEb׉ETool Life: Wear and Failure 659 Thermal cracks in interrupted cutting Chamfer 2 4 1 Carbide 3 4 6 5 2 5 4 1 3 1 High-speed steel (a) Ceramic 2 1 Flank wear (wear land) 2 Crater wear 3 Primary groove or depth-of-cut line 4 Secondary groove (oxidation wear) 5 Outer-metal chip notch 6 Inner chip notch Chamfer 3 6 2 1 High-speed steel tool, thermal softening, and plastic flow (b) Figure 21.18: (a) Schematic illustrations of types of wear observed on various cutting tools. (b) Schematic illustrations of catastrophic tool failures. A wide range of parameters influence these wear and failure patterns. Source: Courtesy of V.C. Venkatesh. 5 2 4 3 1 Ceramic tool, chipping, and fracture 1 Flank wear 2 Crater wear 3 Failure face 4 Primary groove or depth-of-cut line 5 Outer-metal chip notch 6 Plastic flow around failure face 0.15 0.30 0 500 700 900 1100 Average tool–chip interface temperature (8C) Figure 21.19: Relationship between crater-wear rate and average tool–chip interface temperature: (1) highspeed steel, (2) C1 carbide, and (3) C5 carbide (see Table 22.5). Note how rapidly crater-wear rate increases with an incremental increase in temperature. Source: After B.T. Chao and K.J. Trigger. the diffusion process, and thus reducing crater wear. Typical tool coatings are titanium nitride, titanium carbide, diamondlike carbon, titanium carbonitride, and aluminum oxide, and are described in greater detail in Section 22.6. In comparing Figs. 21.12 and 21.15a, it can be seen that the location of the maximum depth of crater wear, KT, coincides with the location of the maximum temperature at the tool–chip interface. An actual cross section of this interface, for steel machined at high speeds, is shown in Fig. 21.20. Note that the wear pattern on the tool face coincides with its discoloration pattern, an indication of the presence of high temperatures. H i g h - c 5 s r a p r e di e d e e d s Crater-wear rate mm3/min et el C 1 C a c b b i ׉ 7cassandra://vyC71T-FXmdT612SSmFIPyuP6U9t8fyv6MM3x3UTRKQ`cZ%Eb׉E Q660 Chapter 21 Fundamentals of Machining Rake face Crater wear Chip Flank face Figure 21.20: Interface of a cutting tool (right) and chip (left) in machining plain-carbon steel. The discoloration of the tool indicates the presence of high temperatures. Compare this figure with the temperature profiles shown in Fig. 21.12. Source: After P.K. Wright. 21.5.3 Other Types of Wear, Chipping, and Fracture Nose wear (Fig. 21.15a) is the rounding of a sharp tool due to mechanical and thermal effects. It dulls the tool, affects the type of chip formation, and causes rubbing of the tool over the workpiece, raising temperature and inducing residual stresses on the machined surface. A related phenomenon is edge rounding,as shown in Fig. 21.15a. An increase in temperature is particularly important for high-speed steel tools, as can be appreciated from Fig. 22.1. Tools may also undergo plastic deformation, because of temperature rises in the cutting zone where temperatures can easily reach 1000◦C in machining steels, and can even be higher depending on the strength of the material machined. Notches or grooves that develop on cutting tools, as shown in Figs. 21.15a and 21.18, have been attributed to the fact that the region where they occur is the boundary where the chip is no longer in contact with the tool. Known as the depth-of-cut line (DOC), see Fig. 21.15a, this boundary oscillates, because of inherent variations in the cutting operation. If sufficiently deep, the groove can lead to gross chipping of the tool tip because of (a) its now reduced cross section and (b) the notch sensitivity of the tool material. Scale and oxide layers on a workpiece surface also contribute to notch wear, because these layers are hard and abrasive. Thus, light cuts should be avoided on such workpieces. In Fig. 21.3 for example, the depth of cut, to, should be greater than the thickness of the scale on the workpiece. In addition to being subjected to wear, cutting tools may also undergo chipping, where a small fragment from the cutting edge of the tool breaks away. This phenomenon, which typically occurs in brittle tool materials such as ceramics, is similar to chipping of the tip of a pencil if it is too sharp. The chipped fragments from the cutting tool may be very small (called microchipping or macrochipping, depending on its size), or they may be relatively large, in which case they are variously called gross chipping, gross fracture,and catastrophic failure (Fig. 21.18). Chipping also may occur in a region of the tool where there is a preexisting small crack or a defect during its production. Unlike wear, which is a gradual process, chipping is a sudden loss of the tool material, thus changing the tool’s shape. As can be expected, chipping has a major detrimental effect on surface finish, surface integrity, and the dimensional accuracy of the workpiece being machined. ׉ 7cassandra://to9EsIp1YAqmxC6d3AOrGWmTDdNSJUEFNOiSRn76F3M`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://JHfG7mj8DxfTPoot5natBWfnirufzsN4RcDBeJEJ-pA`׉ 7cassandra://kOj-fmabfS-VY_0f4priNleOjU4XxErZS6nQ_82sVJ4t`s׉ 7cassandra://lSXis-szZOwKIXKnovvjB-hElfK14NF_l8_4III7imgr`׉ 7cassandra://ctWqi5kz0Blb-kXtWCbJqYGjKbtxaBGTLKX1JW-lfvEh͠^cZEbט#u׉׉ 7cassandra://v9kewlmlZkUeqyhiAe57mgHSytNkCtptT1jcmXTwXQU¸`׉ 7cassandra://j52sU5jmXHl5cBvvRYMcDbkOxSMLdJJXnA-RAQK13DMQo`s׉ 7cassandra://zQ8dUR13e5DYEyt7C90AIruYBBLcZXJ8Vdt92Ad2lC4`׉ 7cassandra://TIRnRALrC8vLOBbWT-uv0RIbtQmrIlPZO2zEPXdvHuY͞10͠^cZEb׉EJSurface Finish and Integrity 661 Two main causes of chipping are: • Mechanical shock, such as impact due to interrupted cutting, as in turning a splined shaft on a lathe. • Thermal fatigue, due to cyclic temperature variations within the tool in interrupted cutting. Thermal cracks usually perpendicular to the cutting edge of the tool, as shown on the rake face of the carbide tool in Figs. 21.15d and 21.18a. Major variations in the composition or structure of the workpiece material also may cause chipping, due to differences in their thermal properties. Chipping can be reduced by selecting tool materials with high impact and thermal-shock resistance, as described in Chapter 22. High positive-rake angles can contribute to chipping because of the small included angle of the tool tip, as can be visualized from Fig. 21.3. Also, it is possible for the crater-wear region to progress slowly toward the tool tip, thus weakening the tip because of reduced volume of material. 21.5.4 Tool-condition Monitoring With rapid advances in computer-controlled machine tools and automated manufacturing, the reliable and repeatable performance of cutting tools is a major consideration. As described in Chapters 23 through 25, modern machine tools operate with little direct supervision by an operator. Moreover, they are typically enclosed, making it virtually impossible to closely monitor the machining operation and the condition of the cutting tool. It is thus essential to indirectly and continuously monitor the condition of the cutting tool. In machine tools, tool-condition monitoring systems are now integrated into computer numerical control and programmable logic controllers. Techniques for tool-condition monitoring typically fall into two general categories: direct and indirect. The direct method for observing the condition of a cutting tool involves optical measurements of wear, such as periodic observation of changes in the tool profile. This is a common technique, and is done using a toolmakers’ microscope. However, this method requires that the cutting operation be stopped for tool observation. Another direct method involves programming the tool to contact a sensor (touch probe)after each machining cycle; this approach allows the measurement of wear and/or the detection of broken tools. Indirect methods involve correlating the tool condition with parameters such as cutting forces, power, temperature rise, workpiece surface finish, vibration, and chatter. A common technique is acoustic emission (AE), which utilizes a piezoelectric transducer mounted on a toolholder. The transducer picks up acoustic emissions (typically above 100 kHz) which result from the stress waves generated during cutting. By analyzing the signals, tool wear and chipping can be monitored. This technique is effective particularly in precision-machining operations, where cutting forces are low (because of the small amounts of material removed). Another effective use of AE is in detecting the fracture of small carbide tools at high cutting speeds. A similar indirect technique consists of various sensors that are installed in the original machine tool, or are retrofitted on existing machines. The system continually monitors torque and forces during machining. The signals are analyzed and interpreted to differentiate between events: tool breakage, tool wear, a missing tool, overloading of the machine tool, or colliding with machine components. This system also can compensate automatically for tool wear, and thus improve the dimensional accuracy of the part being machined. The design of these systems must be such that they are (a) nonintrusive to the machining operation, (b) accurate and repeatable in signal detection, (c) resistant to abuse, (d) robust for the shop-floor environment (see Sections 36.5.1 and 40.7), and (e) cost effective. Sensors, including the use of infrared and fiber-optic techniques for temperature measurement during machining, are important components of the system. 21.6 Surface Finish and Integrity Surface finish influences not only the dimensional accuracy of machined parts but also their properties and performance in service. The term surface finish describes the geometric features of a surface (see Chapter 33); surface integrity pertains to properties, such as fatigue life and corrosion resistance, that are strongly influenced by the nature of the surface produced. ׉ 7cassandra://lSXis-szZOwKIXKnovvjB-hElfK14NF_l8_4III7imgr`cZ%Eb׉E0662 Chapter 21 Fundamentals of Machining (a) (b) Figure 21.21: Machined surfaces produced on steel (highly magnified), as observed with a scanningelectron microscope: (a) turned surface and (b) surface produced by shaping. Source: After J T. Black and S. Ramalingam. With its significant effect on changing the tool-tip profile, the built-up edge (see Fig. 21.6) has the greatest influence on surface finish. Fig. 21.12 shows the surfaces produced in two different cutting operations. Note the considerable damage to the surfaces from BUE and scuffing marks; they deviate from the straight grooves (tool marks) resulting from normal machining, as seen in Fig. 21.2. Ceramic and diamond cutting tools generally produce a better surface finish than other tools, largely because of their much lower tendency to form a BUE. A dull tool has a large radius along its cutting edges, as in the tip of a dull pencil or the edge of a knife. Figure 21.22 illustrates the relationship between the radius of the cutting edge and the depth of cut in orthogonal cutting. Note that at small depths of cut, the rake angle effectively can become negative, and the tool simply may ride over the workpiece surface, instead of cutting it and producing chips. This is a phenomenon similar to trying to scrape a thin layer from the surface of a stick of butter with a dull knife. Tool Workpiece Machined surface Figure 21.22: Schematic illustration of a dull tool with respect to the depth of cut in orthogonal machining (exaggerated). Note that the tool has a positive rake angle, but as the depth of cut decreases, the rake angle effectively can become negative. The tool then simply rides over the workpiece (without cutting) and burnishes its surface; this action raises the workpiece temperature and causes surface residual stresses. Increasing depth of cut ׉ 7cassandra://zQ8dUR13e5DYEyt7C90AIruYBBLcZXJ8Vdt92Ad2lC4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://X4s6pIlOkq4LEP8PmAuscbK9w0oPe7wCfWBbY6njoe49`׉ 7cassandra://5YSlY7eRoKucDCetl9MM25iURvfprIBodQWXgb9FlrAO`s׉ 7cassandra://6uWVuuLwsX2NGFutYw33CFMQo5D4mQaSwU2CKD9EAM4`׉ 7cassandra://eo_Ps07Qzibp23-sfvwxZUehrsvQu6RLMvip-awD-5w͘{ 0͠^cZEbט#u׉׉ 7cassandra://rJk7ddTtmM-vTLIuCRB2pFmRVP40iySuDZZOvKK_ByQ+`׉ 7cassandra://fHbaGzJn3xrZO5jLATsjl3Q_FY8ouXE_zsIyMBv_QlEiv`s׉ 7cassandra://s3xQvUN7QfnWmfIm3Ay30VqsCzBASLrwclzOLymcAaQz`׉ 7cassandra://YrifNLU1ubWMvDTllWHDjfdja6aAzePNgcrOZ4k9fV4b͠^cZEb׉E Surface Finish and Integrity 663 Roughness height, Rt Workpiece R Side-cuttingedge angle Tool End-cuttingedge angle Figure 21.23: Schematic illustration of feed marks on a surface being turned (exaggerated). If the tip radius of the tool (not to be confused with radius R in Fig. 21.15a) is large in relation to the depth of cut, the tool simply will rub over the machined surface. Rubbing will then generate heat and induce residual surface stresses, which in turn may cause surface damage, such as tearing and cracking. Consequently, the depth of cut should be greater than the radius on the cutting edge. In a turning operation, as in other cutting processes described in the rest of Part IV of this text, the tool leaves a spiral profile (called feed marks) on the machined surface as it moves across the workpiece (Figs. 21.2 and 21.23). It will be noted that the higher the feed, f, and the smaller the tool-nose radius, R, the more prominent the feed marks will be. It can be shown that the surface roughness, for such a case, is given by Rt = f2 8R , (21.30) where Rt is the roughness height, as described in Section 33.3. Although not significant in rough machining operations, feed marks are important in finish machining. Vibration and chatter are described in detail in Section 25.4. For now, it should be recognized that if the tool vibrates or chatters during machining, it will adversely affect the workpiece surface finish produced, for the reason that a vibrating tool periodically changes the dimensions of the cut and the surface properties. Chatter can cause chipping and premature failure of the more brittle cutting tools, such as ceramics and diamond. Factors influencing surface integrity are as follow: • Temperatures generated during cutting and possible metallurgical transformations • Surface residual stresses • Plastic deformation and strain hardening of the machined surfaces, tearing, and cracking. Each of these factors can have various adverse effects on the machined part. The difference between finish machining and rough machining should be emphasized. In finish machining, it is important to consider the surface finish to be produced, whereas in rough machining the main purpose is to remove a large amount of material at a high rate, regardless of the surface finish produced, since it will be greatly improved during finish machining. It is important to note that there be no subsurface damage resulting from rough machining that cannot later be removed during finish machining (see Fig. 21.21). Feed ׉ 7cassandra://6uWVuuLwsX2NGFutYw33CFMQo5D4mQaSwU2CKD9EAM4`cZ%Eb׉E664 Chapter 21 Fundamentals of Machining 21.7 Machinability The machinability of a material is usually defined in terms of four factors: 1. Surface finish and surface integrity of the machined part 2. Tool life 3. Force and power requirements 4. The level of difficulty in chip control after it is generated. Thus, for instance, good machinability indicates good surface finish and surface integrity, long tool life, and low force and power requirements. As for chip control, chips that are long, thin, stringy, and curled can severely interfere with the machining operation by becoming entangled in the cutting area of the machine tool (see Fig. 21.7). Because of the complex nature of machining operations, it is difficult to establish relationships that quantitatively define the machinability of a particular material. In practice, tool life and surface roughness generally are considered to be the most important factors in machinability. In subsequent chapters, several tables are presented in which, for various groups of materials, specific recommendations are given regarding such parameters as cutting speed, feed, depth of cut, cutting tool materials, tool shape, and type of cutting fluids. 21.7.1 Machinability of Ferrous Metals This section describes the machinability of steels, alloy steels, stainless steels, and cast irons. Steels. Carbon steels have a wide range of machinability, depending on their ductility and hardness. If a certain carbon steel is too ductile, chip formation can include built-up edge, leading to poor surface finish. If the steel is too hard, it can cause abrasive wear of the tool, because of the presence of carbides in the steel. The machinability of most steels is improved by cold working, which hardens the material and reduces the tendency for built-up edge formation. An important group of steels is free-machining steels, containing sulfur and phosphorus. Sulfur forms manganese-sulfide inclusions (second-phase particles, Section 4.2.3), which act as stress raisers in the primary shear zone. As a result, the chips produced break up easily and are small, thus improving machinability. The size, shape, distribution, and concentration of these inclusions significantly influence machinability. Elements such as tellurium and selenium, both of which are chemically similar to sulfur, act as inclusion modifiers in resulfurized steels. Phosphorus in steels has two major effects: (a) it strengthens the ferrite, causing increased hardness and resulting in better chip formation and surface finish, and (b) it increases hardness and thus causes the formation of short chips instead of continuous stringy ones, thereby improving machinability. In leaded steels, a high percentage of lead solidifies at the tips of manganese-sulfide inclusions. In nonresulfurized grades of steel, lead takes the form of dispersed fine particles. Lead is insoluble in iron, copper, and aluminum and their alloys, and because of its low shear strength, it acts as a solid lubricant (see Section 33.7) and is smeared over the tool–chip interface during machining. When the temperature rise is sufficiently high, such as at high cutting speeds and feeds, the lead melts directly in front of the tool, acting as a liquid lubricant. Lead also lowers the shear stress of the materials in the primary shear zone, thus reducing cutting forces and power consumption. Lead can be used with every grade of steel and its presence is identified by the letter L between the second and third numerals in steel identification (e.g., 10L45). In stainless steels, use of the letter L means low carbon, which improves their corrosion resistance. Because lead is a well-known toxin and a pollutant, there are continuing serious environmental concerns about its use in steels. There is a continuing trend toward eliminating the use of lead in steels (lead-free steels). Bismuth and tin are substitutes for lead in steels, but they are shown to be not as effective. ׉ 7cassandra://s3xQvUN7QfnWmfIm3Ay30VqsCzBASLrwclzOLymcAaQz`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://jokqWrBdp3J3uhfyp8ZyNBUTg2ZFuEOoDVuyN6oWf90I`׉ 7cassandra://O96D1Ty_teYMvBNfQiOtIsd9t00lWAVnXxyH1l_ERoYo[`s׉ 7cassandra://UKBoNyYYGjbm84DM3fXz5su75fxavj3A--3_CNHf6wg`׉ 7cassandra://_Txe0Ora1i8R9skp8ADWv7buR5X5-nLosBaGvFmvaYQi|͠^cZEbט#u׉׉ 7cassandra://3MUCGQr77QJ0IKi7f4-TZjT8hglOsAEQ4v2b7E5HGEY&`׉ 7cassandra://gXsQxKDzOkdxS7uLbXXPofXy9NA3aw8kJCGDk2rwlwki%`s׉ 7cassandra://-fwIfKxhWuk2ZkePJxDSWbUPlqvifbeP9lL5ObFb3JQ4`׉ 7cassandra://W4o9zaEeIGHB_8-jTEFOVVj8STZK5gZgWXJwyfNsJMoc͠^cZEb׉EMachinability 665 Calcium-deoxidized steels contain oxide flakes of calcium silicates (CaSO) that reduce the strength of the secondary shear zone, and decrease tool–chip interface friction and wear. Because temperature increase is reduced correspondingly, these steels produce less crater wear, especially at high cutting speeds. Alloy steels can have a wide variety of compositions and hardness, thus their machinability cannot be generalized. An important trend in machining these steels is hard turning, as described in detail in Section 25.6. Alloy steels at hardness levels of 45 to 65 HRC can be machined with polycrystalline cubic-boronnitride (cBN) cutting tools (see Section 22.7), producing good surface finish, integrity, and dimensional accuracy. Effects of Various Elements in Steels. The presence of aluminum and silicon is always harmful, because these elements combine with oxygen, forming aluminum oxide and silicates, which are hard and abrasive. As a result, tool wear increases and machinability is reduced. Carbon and manganese have various effects on the machinability of steels, depending on their composition. Plain low-carbon steels (less than 0.15% C) can produce poor surface finish by forming a built-up edge. Cast steels can be abrasive, although their machinability is similar to that of wrought steels. Tool and die steels are very difficult to machine, and usually require annealing prior to machining or using other machining techniques (see Chapter 27). Other alloying elements, such as nickel, chromium, molybdenum,and vanadium, that otherwise improve the properties of steels also generally reduce machinability. The effect of boron is negligible. Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel. Oxygen has been shown to have a strong effect on the aspect ratio of the manganese-sulfide inclusions (see also Section 2.10.1): The higher the oxygen content, the lower the aspect ratio, and the higher the machinability. It is important to also consider the possible detrimental effects of the alloying elements on the properties and strength of machined parts in service. At elevated temperatures, for example, lead causes embrittlement of steels (liquid-metal embrittlement and hot shortness; see Section 1.5.2), although it has no effect on mechanical properties at room temperature. Sulfur can reduce the hot workability of steels (see Section 14.5) severely, because of the formation of iron sulfide, unless sufficient manganese is present to prevent such formation. At room temperature, the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese-sulfide inclusions. Rephosphorized steels are significantly less ductile, and are produced solely for the purpose of improving their machinability. Stainless Steels. Austenitic (300 series) steels generally are difficult to machine. Chatter can be a problem, thus necessitating machine tools with high stiffness. Ferritic stainless steels (300 series) have good machinability. Martensitic (400 series) steels are abrasive, tend to form a built-up edge, and require tool materials with high hot hardness and crater-wear resistance. Precipitation-hardening stainless steels are strong and abrasive, thus requiring hard and abrasion-resistant tool materials. Cast Irons. Gray irons generally are machinable, although they can be abrasive, depending on composition, especially pearlite. Free carbides in castings reduce their machinability, and can also cause tool chipping or fracture. Nodular and malleable irons are machinable, using hard tool materials. 21.7.2 Machinability of Nonferrous Metals In alphabetic order: • Aluminum is generally very easy to machine, although the softer grades tend to form a built-up edge, resulting in poor surface finish. High speeds, high rake angles, and high relief angles are recommended. Wrought aluminum alloys with high silicon content and cast aluminum alloys are generally abrasive, hence they require harder tool materials. Dimensional tolerance control may be a problem in machining aluminum, because it has a high thermal expansion coefficient and a relatively low elastic modulus. ׉ 7cassandra://UKBoNyYYGjbm84DM3fXz5su75fxavj3A--3_CNHf6wg`cZ%Eb׉Es666 Chapter 21 Fundamentals of Machining • Beryllium is generally machinable; however, because the fine particles produced during machining are toxic, this requires a controlled environment. • Cobalt-based alloys are abrasive and highly work hardening; they require sharp, abrasion-resistant tool materials and low feeds and speeds. • Copper, in the wrought condition, can be difficult to machine, because of built-up edge formation. Cast copper alloys are easy to machine; brasses are easy to machine, especially with the addition of lead (leaded free-machining brass); note, however, the toxicity of lead and associated environmental concerns. New brasses and bronzes have been developed that are lead free; examples are bismuth-tin bronze, with 1 to 6% Bi and popular for bearing races; aluminum bronzes; and Envirobrass, used for drinking water pipes. Bronzes are more difficult to machine than brass. • Magnesium is very easy to machine, with good surface finish and long tool life; however, care should be exercised because of its high rate of oxidation (pyrophoric) and hence the danger of fire. • Molybdenum is ductile and work hardening; sharp tools are essential to prevent produce poor surface finish. • Nickel-based alloys and superalloys are work hardening, abrasive, and strong at high temperatures. Their machinability depends on their condition and improves with annealing. • Tantalum is very work hardening, ductile, and soft; it produces a poor surface finish, and tool wear is high. • Titanium and its alloys have very poor thermal conductivity (the lowest of all metals, see Table 3.2), thus causing a significant temperature rise and built-up edge. They are highly reactive and can be difficult to machine. • Tungsten is brittle, strong, and very abrasive; thus its machinability is low, although it improves greatly at elevated temperatures. • Zirconium has good machinability, but it requires a coolant-type cutting fluid (Section 22.12) because of the danger of explosion and fire. 21.7.3 Machinability of Miscellaneous Materials Thermoplastics generally have low thermal conductivity and low elastic modulus, and they are thermally softening. Consequently, machining them requires sharp tools with positive rake angles (to reduce cutting forces), large relief angles, small depths of cut and feed, relatively high speeds, and proper workholding devices to support of the workpiece. To keep the chips from becoming gummy and sticking to cutting tools, external cooling of the cutting zone may be necessary. Cooling can be achieved with a jet of air, a vapor mist, or using water-soluble oils. Thermosetting plastics are brittle and sensitive to thermal gradients during cutting; machining conditions generally are similar to those of thermoplastics. Polymer-matrix composites are very abrasive, because of the fibers present, hence they are difficult to machine. Fiber tearing, pullout, and edge delamination are significant problems, and can lead to severe reduction in the load-carrying capacity of machined components. Machining of these components requires careful handling and removal of debris, in order to avoid contact with and inhaling of the fibers. Metal-matrix and ceramic-matrix composites can be difficult to machine, depending on the properties of the matrix material and the type of reinforcing fibers. Graphite is abrasive; it requires sharp, hard, and abrasion-resistant tools. Ceramics have a steadily improved machinability, particularly with the development of machinable ceramics and nanoceramics (Section 8.2), and with the selection of appropriate processing parameters, such as ductile-regime cutting (described in Section 25.7). ׉ 7cassandra://-fwIfKxhWuk2ZkePJxDSWbUPlqvifbeP9lL5ObFb3JQ4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Ve3cY09Le52pYIeHJvEZ4WAvQUWFuxPGpxWPniIwEDwKG`׉ 7cassandra://L7mIU9kSVCnf_ja2_aDidkRPZ_Z_CiJTlkPDgZJLFZo[`s׉ 7cassandra://CoOuoYQqQurc5TA04BimiMRNzGD9NPlorgvzN811O4w`׉ 7cassandra://5asnjg8VjGatjTL9Wfbv-Kxc7PZ2s_YZsyQfbNEf69wTb͠^cZEbט#u׉׉ 7cassandra://MolJ5nnmSksPir5tTFE3esG9CMrCg58uU1zUjJfLYMgb`׉ 7cassandra://9jBLNT4fBkJ65_Xy8CZ_ta_EEs2xTu6kQSht2gVe1H4I|`s׉ 7cassandra://mkS0KmPgAHsaXr426un-LrQaSsQdFFtN8n8qYWpZ8k4&`׉ 7cassandra://vtpvGNiGrA7ZDx9zZIVnMQPHQYgkwPDRIDIXNJm0CvgMK͠^cZEb ׉E Summary 667 Wood is a complex material, with properties varying with its grain direction. Consequently, the type of chips and the surfaces produced also vary significantly, depending on the type of wood and its condition. Woodworking, which dates back to 3000 B.C., still remains largely an art. The basic requirements are generally sharp tools and high cutting speeds. 21.7.4 Thermally Assisted Machining Metals and alloys that are difficult to machine at room temperature can be machined more easily at elevated temperatures. In thermally assisted machining, also called hot machining, a source of heat (such as a torch, induction coil, electric current, laser-beam, electron-beam, or plasma arc) is focused onto an area just ahead of the cutting tool. First investigated in the early 1940s, this operation typically is carried out above the homologous temperature of T/Tm =0.5 (see Section 1.8, and Tables 1.2 and 3.1). Steels, for example, are hot machined above the temperature range of 650◦ to 750◦C. Although difficult and complicated to perform in production plants, the general advantages of hot machining are: (a) reduced cutting forces, (b) increased tool life, (c) higher material-removal rates, and (d) reduced tendency for vibration and chatter. Summary • Machining processes are often necessary to impart the desired dimensional accuracy, geometric features, and surface-finish characteristics to components, particularly those with complex shapes that cannot be produced economically using other shaping techniques. On the other hand, machining generally takes more time, wastes material, in the form of chips, and generally doesn’t affect the bulk properties of the workpiece; however, it may have adverse effects on surfaces produced. • Commonly observed chip types in machining are continuous, built-up edge, discontinuous, and serrated. Important process variables in machining are tool geometry and tool material; cutting conditions, such as speed, feed, and depth of cut; use of cutting fluids; and characteristics of the workpiece material and the machine tool. Parameters influenced by these variables are forces and power consumption, tool wear, surface finish and surface integrity, temperature rise, and dimensional accuracy of the workpiece. • Temperature rise in machining is an important phenomenon, since it can have adverse effects on tool life, as well as on the properties, dimensional accuracy, and surface integrity of the machined part. • Two principal types of tool wear are flank wear and crater wear. Tool wear depends on workpiece and tool material characteristics; cutting speed, feed, depth of cut, and cutting fluids; as well as the characteristics of the machine tool. Tool failure also may occur by notching, chipping, and gross fracture. • The surface finish of machined components can adversely affect product integrity. Important variables are the geometry and condition of the cutting tool, the type of chip produced, and process variables. • Machinability is generally defined in terms of surface finish, tool life, force and power requirements, and chip control. The machinability of specific materials depends on their composition, general properties, and microstructure. ׉ 7cassandra://CoOuoYQqQurc5TA04BimiMRNzGD9NPlorgvzN811O4w`cZ%Eb׉E668 Chapter 21 Fundamentals of Machining Key Terms Acoustic emission Allowable wear land Built-up edge Chip Chip breaker Chip curl Chipping of tool Clearance angle Continuous chip Crater wear Cutting force Cutting ratio Depth-of-cut line Diffusion Discontinuous chip Feed marks Flank wear Friction angle Hot machining Inclination angle Machinability Machine tool Machining Bibliography Ahmad, J., Machining of Polymer Composites, Springer, 2008. Altintas, Y., Machining Automation: Metal Cutting Mechanics, Machine Tool Vibrations, and CNC Design, 2nd. ed., Cambridge, 2012. Boothroyd, G., and Knight, W.A., Fundamentals of Metal Machining and Machine Tools, 3rd ed., CRC Press, 2006. Cormier, D., McGraw-Hill Machining and Metalworking Handbook, McGraw-Hill, 2006. Davim, J.P. (ed.), Machinability of Advanced Materials, Wiley, 2014. —, Machining of Complex Sculpted Surfaces, Springer, 2012. —, Machining of Composite Materials, Wiley, 2009. —, Machining of Hard Materials. Springer, 2011. —, Machining: Fundamentals and Recent Advances, Springer, 2010. —, Surface Integrity in Machining, Springer, 2010. Dudzinski, D., Molinari, A., and Schulz, H., (eds.), Metal Cutting and High-Speed Machining, Springer, 2002. Notch wear Oblique cutting Orthogonal cutting Primary shear zone Rake angle Relief angle Rephosphorized steel Resulfurized steel Secondary shear zone Serrated chip Shaving Shear angle Shear plane Skiving Specific energy Surface finish Surface integrity Taylor equation Thrust force Tool-condition monitoring Tool life Turning Wear land ׉ 7cassandra://mkS0KmPgAHsaXr426un-LrQaSsQdFFtN8n8qYWpZ8k4&`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://QO9F3EbVNn8vF2XGura8c1VLJ84Xm2CBjafm_cA-l6k`׉ 7cassandra://UbkKGx-REm5rjiyau_eFIlE470ZTxXO99TugFiRqeHge`s׉ 7cassandra://H9vlY8eX4a8o0TWEOmVVweVMhHFs7H1JJThbByP5v5k`׉ 7cassandra://Uu9Z0qlGzjLBuGUOMTAig3fplUWawIBwR1o8J2iIt6MQ͠^cZEb ט#u׉׉ 7cassandra://JRKvXvSpkd855rlwtSKwsH_rt7TG0L_87elQAYT3sAU`׉ 7cassandra://WWGPpoGnQnpPNmWFEpIPLhTYQhTVAKNDCyoZgB0dUn0g`s׉ 7cassandra://-Em0LCmbzv609XPYqvd6HrNHmt9q70L6wY3uucMQVSA9`׉ 7cassandra://rn5EJMfwkCu9sHFe1cupMiSX4Stk1SEr0VbJTtRkKA0L͠^cZEb ׉E Qualitative Problems 669 Erdel, B., High-Speed Machining, Society of Manufacturing Engineers, 2003. Jackson, M.J., and Morell, J., Machining with Nanomaterials, Springer, 2009. Liang, S. and Shih, A.J., Analysis of Machining and Machine Tools, Springer, 2016. Marinescu, I.D., Handbook of Advanced Ceramics Machining, CRC Press, 2006. Shaw, M.C., Metal Cutting Principles, 2nd ed., Oxford, 2005. Stephenson, D.A., and Agapiou, J.S., Metal Cutting: Theory and Practice, 2nd ed., CRC Press, 2005. Trent, E.M., and Wright, P.K., Metal Cutting, 4th ed., Butterworth-Heinemann, 2000. Tschatsch, H., Applied Machining Technology, Springer, 2009. Youssef, H.A. and El-Hofy, H., Machining Technology, Machine Tools and Operations, CRC Press, 2008. Review Questions 21.1. Explain why continuous chips are not necessarily desirable. 21.2. Name the factors that contribute to the formation of discontinuous chips. 21.3. What is the cutting ratio? Is it always greater than 1? Explain. 21.4. Explain the difference between positive and negative rake angles. What is the importance of the rake angle? 21.5. Explain how a dull tool can lead to negative rake angles. 21.6. Comment on the role and importance of the relief angle. 21.7. Explain the difference between discontinuous chips and segmented chips. 21.8. Why should we be interested in the magnitude of the thrust force in cutting? 21.9. What are the differences between orthogonal and oblique cutting? 21.10. What is a BUE? Why does it form? 21.11. Is there any advantage to having a built-up edge on a tool? Explain. 21.12. What is the function of chip breakers? Howdo they function? Do you need a chip breaker to eliminate continuous chips in oblique cutting? Explain. 21.13. Identify the forces involved in a cutting operation. Which of these forces contributes to the power required? 21.14. Explain the characteristics of different types of tool wear. 21.15. List the factors that contribute to poor surface finish in cutting. 21.16. Explain what is meant by the term machinability and what it involves. Why does titanium have poor machinability? 21.17. What is shaving? Qualitative Problems 21.18. List reasons that machining operations may be required, and provide an example for each reason. 21.19. Are the locations of maximum temperature and crater wear related? If so, explain why. 21.20. Is material ductility important for machinability? Explain. 21.21. Explain why studying the types of chips produced is important in understanding cutting operations. ׉ 7cassandra://H9vlY8eX4a8o0TWEOmVVweVMhHFs7H1JJThbByP5v5k`cZ%Eb׉E 670 Chapter 21 Fundamentals of Machining 21.22. Why do you think the maximum temperature in orthogonal cutting is located at about the middle of the tool–chip interface? (Hint: Note that the two sources of heat are (a) shearing in the primary shear plane and (b) friction at the tool–chip interface.) 21.23. Tool life can be almost infinite at low cutting speeds. Would you then recommend that all machining be done at low speeds? Explain. 21.24. Explain the consequences of allowing temperatures to rise to high levels in cutting. 21.25. The cutting force increases with the depth of cut and decreasing rake angle. Explain why. 21.26. Why is it not always advisable to increase the cutting speed in order to increase the production rate? 21.27. What are the consequences if a cutting tool chips? 21.28. What are the effects of performing a cutting operation with a dull tool? A very sharp tool? 21.29. To what factors do you attribute the difference in the specific energies in machining the materials shown in Table 21.2? Why is there a range of energies for each group of materials? 21.30. Explain why it is possible to remove more material between tool resharpenings by lowering the cutting speed. 21.31. Noting that the dimension d in Fig. 21.4a is very small, explain why the shear strain rate in metal cutting is so high. 21.32. Explain the significance of Eq. (21.9). 21.33. Comment on your observations regarding Figs. 21.12 and 21.13. 21.34. Describe the consequences of exceeding the allowable wear land (Table 21.4) for various cutting-tool materials. 21.35. Comment on your observations regarding the hardness variations shown in Fig. 21.6a. 21.36. Why does the temperature in cutting depend on the cutting speed, feed, and depth of cut? Explain in terms of the relevant process variables. 21.37. You will note that the values of a and b in Eq. (21.24) are higher for high-speed steels than for carbides. Whyisthisso? 21.38. As shown in Fig. 21.14, the percentage of the total cutting energy carried away by the chip increases with increasing cutting speed. Why? 21.39. Describe the effects that a dull tool can have on cutting operations. 21.40. Explain whether it is desirable to have a high or low (a) n value and (b) C value in the Taylor tool-life equation. 21.41. The Taylor tool-life equation is directly applicable to flank wear. Explain whether or not it can be used to model tool life if other forms of wear are dominant. 21.42. The tool-life curve for ceramic tools in Fig. 21.17 is to the right of those for other tool materials. Why? 21.43. Why are tool temperatures low at low cutting speeds and high at high cutting speeds? 21.44. Can high-speed machining be performed without the use of a cutting fluid? 21.45. Given your understanding of the basic metal-cutting process, what are the important physical and chemical properties of a cutting tool? 21.46. Explain why the power requirements in cutting depend on the cutting force but not the thrust force. 21.47. State whether or not the following statements are true, explaining your reasons: (a) For the same shear angle, there are two rake angles that give the same cutting ratio. (b) For the same depth of cut and rake angle, the type of cutting fluid used has no influence on chip thickness. (c) If the cutting speed, shear angle, and rake angle are known, the chip velocity can be calculated. (d) The chip becomes thinner as the rake angle increases. (e) The function of a chip breaker is to decrease the curvature of the chip. ׉ 7cassandra://-Em0LCmbzv609XPYqvd6HrNHmt9q70L6wY3uucMQVSA9`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://D1S3VegyUfMdlnxZK5heb1cZ8EcK0zVB6t6wf5BcAl8p`׉ 7cassandra://05kbNcxDmfgkvcQMdz1ZGOb1OjcR2d4WDgQQh6k7Aroe`s׉ 7cassandra://9kEZzqhG5a-VeIMSOSEOsoJcRyozCGZrhBVFvtprSG8`׉ 7cassandra://EfQrbYDeJ7VwoDvZCr4S_Be7SCXBIh4onbhl-Q8vN20kx͠^cZEbט#u׉׉ 7cassandra://DBsK6CWKIDizgI4BHeXmVs_wqBBb8manzHaDtv3ZngY`׉ 7cassandra://MJGVhv_zi6HYNIwv6ugwxOlCFRuYJ8DsXvsEbxDOTZo00`s׉ 7cassandra://u1J6-GY1Yt5DO58wAcaMZnVsz7DZ-C_mWLpwC91O-p0`׉ 7cassandra://1Hyug9GCMj_n8oKdVeRiLgIsAMTKWGZW4WWrPoeVJgEU  ͠^cZEb׉E oQuantitative Problems 671 Quantitative Problems 21.48. Let n =0.5 and C = 500 in the Taylor equation for tool wear. What is the percent increase in tool life if the cutting speed is reduced by (a) 40% and (b) 80%? 21.49. Assume that, in orthogonal cutting, the rake angle is 25◦ and the coefficient of friction is 0.25. Using Eq. (21.4), determine the percentage increase in chip thickness when the friction is doubled. 21.50. Derive Eq. (21.14). 21.51. Taking carbide as an example and using Eq. (21.24), determine how much the feed should be reduced in order to keep the mean temperature constant when the cutting speed is doubled. 21.52. Using trigonometric relationships, derive an expression for the ratio of shear energy to frictional energy in orthogonal cutting, in terms of angles α, β,and φ only. 21.53. An orthogonal cutting operation is being carried out under the following conditions: to =0.2 mm, tc =0.3 mm, width of cut =6 mm V =5 m/s rake angle =15◦, Fc = 650 N, and Ft = 250 N. Calculate the percentage of the total energy that is dissipated in the shear plane. 21.54. Explain how you would go about estimating the C and n values for the four tool materials shown in Fig. 21.17. 21.55. Derive Eqs. (21.1) and (21.3). 21.56. Assume that, in orthogonal cutting, the rake angle, α,is20◦ and the friction angle, β,is35◦ at the chip–tool interface. Determine the percentage change in chip thickness when the friction angle is 45◦. [Note: do not use Eq. (21.4) or Eq. (21.5)]. 21.57. Show that, for the same shear angle, there are two rake angles that give the same cutting ratio. 21.58. With appropriate diagrams, show how the use of a cutting fluid can change the magnitude of the thrust force, Ft, in Fig. 21.11. Consider both heat transfer and lubrication effects. 21.59. In a cutting operation using a −5◦ rake angle, the measured forces were Fc =1330 Nand Ft =740 N. When a cutting fluid was used, these forces were Fc = 1200 Nand Ft = 710 N. What is the change in the friction angle resulting from the use of a cutting fluid? 21.60. For a turning operation using a ceramic cutting tool, if the speed is increased by 50%, by what factor must the feed rate be modified to obtain a constant tool life? Use n =0.5 and y =0.6. 21.61. In Example 21.3, if the cutting speed V is doubled, will the answer be different? Explain. 21.62. Using Eq. (21.30), select an appropriate feed for R =1.5 mm and a desired roughness of 0.6 μm. How would you adjust this feed to allow for nose wear of the tool during extended cuts? Explain your reasoning. 21.63. With a carbide tool, the temperature in a cutting operation is measured as 650 K when the speed is 90 m/min and the feed is 0.05 mm/rev. What is the approximate temperature if the speed is doubled? What speed is required to lower the maximum cutting temperature to 480 K? 21.64. The following flank wear data were collected in a series of machining tests using C6 carbide tools on 1045 steel (HB=192). The feed rate was 0.38 mm/rev, and the width of cut was 0.75 mm. (a) Plot flank wear as a function of cutting time. Using a 0.38 mm wear land as the criterion of tool failure, determine the lives for the two cutting speeds. (b) Plot your results on log–log plot and determine the values of n and C in the Taylor tool life equation. (Assume a straight line relationship.) (c) Using these results, calculate the tool life for a cutting speed of 90 m/min. ׉ 7cassandra://9kEZzqhG5a-VeIMSOSEOsoJcRyozCGZrhBVFvtprSG8`cZ%Eb׉EQ672 Chapter 21 Fundamentals of Machining Cutting speed Cutting time Flank wear V (m/min) 120 (min) 0.5 2.0 4.0 8.0 180 16.0 24.0 54.0 0.5 2.0 4.0 8.0 13.0 14 240 300 0.5 2.0 4.0 5.0 0.5 1.0 1.8 2.0 (mm) 0.035 0.0575 0.075 0.1375 0.205 0.28 0.375 0.045 0.0875 0.15 0.25 0.3625 0.4 0.125 0.25 0.35 0.4 0.25 0.325 0.375 0.4 21.65. The following data are available from orthogonal cutting experiments. In both cases depth of cut (feed) to =0.13 mm, width of cut b =2.5 mm, rake angle α = −5◦, and cutting speed V =2 m/s. Workpiece material Aluminum Steel Chip thickness, tc (mm) 0.23 Cutting force, Fc (N) Thrust force, Ft (N) 430 280 0.58 890 800 Determine the shear angle φ, friction coefficient μ, shear stress τ andshearstrain γ on the shear plane, chip velocity Vc and shear velocity Vs, as well as energies uf, us and ut. 21.66. Estimate the cutting temperatures for the conditions of Problem 21.65 if the following properties apply: Workpiece material Aluminum Steel Cutting energy, u (N-mm/mm3) Thermal diffusivity, K (mm2/s) Volumetric specific heat, ρc (N/mm2◦C) 1320 97 2.6 2740 14 3.3 ׉ 7cassandra://u1J6-GY1Yt5DO58wAcaMZnVsz7DZ-C_mWLpwC91O-p0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://exyKcqgZfbgZ7DKyZt_CKnbNTaJcN28z7M8M88az3_sU`׉ 7cassandra://c7NravgYNa1z9wD8yNYJmv_8Z0zmy7Pq7ZNixBXd45Eb`s׉ 7cassandra://eP-mW35VkqsBuQUmKocUq-9y_mmcnRDEmpsfl2uSLVs`׉ 7cassandra://zWa4O1i8zkmZhieiAQa_QIkNBmKNYSFh-EuZR26P7GEO͠^cZEbט#u׉׉ 7cassandra://WR612fueISL7_6L2ekhxCQa7EzeOIgbx4M7QjslKgD0F`׉ 7cassandra://h8ogvOYkBHAmQkZe1jr2uhuYAOF7wIqhZjmqh1f-yD48`s׉ 7cassandra://H9A7wTUyqyv8aeqyTA7COo-8Bb__V6KfWYBPBLIMmbw3`׉ 7cassandra://WhxU7S_sfCGv0zF8ARzYyVYTNJ8hdUvxZCxJ76Q64Oo8͠^cZEb׉E Synthesis, Design, and Projects 673 21.67. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 21.68. Tool life is increased greatly when an effective means of cooling and lubrication is implemented. Design methods of delivering this fluid to the cutting zone and discuss the advantages and limitations of your design. 21.69. Design an experimental setup whereby orthogonal cutting can be simulated in a turning operation on a lathe. 21.70. Describe your thoughts on whether chips produced during machining can be used to make useful products. Give some examples of possible products, and comment on their characteristics and differences if the same products were made by other manufacturing processes. Which types of chips would be desirable for this purpose? 21.71. Recall that cutting tools can be designed so that the tool–chip contact length is reduced by recessing the rake face of the tool some distance away from its tip. Explain the possible advantages of such a tool. 21.72. Recall that the chip-formation mechanism also can be observed by scraping the surface of a stick of butter with a sharp knife. Using butter at different temperatures, including frozen butter, conduct such an experiment. Keep the depth of cut constant and hold the knife at different angles (to simulate the tool rake angle), including oblique scraping. Describe your observations regarding the type of chips produced. Also, comment on the force that your hand feels while scraping and whether you observe any chatter when the butter is very cold. 21.73. Experiments have shown that it is possible to produce thin, wide chips, such as 0.08 mm thick and 10 mm wide, which would be similar to the dimensions of a rolled sheet. Materials have been aluminum, magnesium, and stainless steel. A typical setup would be similar to orthogonal cutting, by machining the periphery of a solid round bar with a straight tool moving radially inward. Describe your thoughts regarding producing thin metal sheets by this method, taking into account the metal’s surface characteristics and properties. 21.74. Describe your thoughts regarding the recycling of chips produced during machining in a plant. Consider chips produced by dry cutting versus those produced by machining with a cutting fluid. 21.75. List products that can be directly produced from metal chips or shavings. 21.76. Obtain a wood planer and some wood specimens. Show that the chips produced depend on the direction of cut with respect to the wood grain. Explain why. 21.77. It has been noted that the chips from certain carbon steels are noticeably magnetic, even if the original workpiece is not. Research the reasons for this effect and write a one-page paper explaining the important mechanisms. 21.78. As we have seen, chips carry away the majority of the heat generated during machining. If chips did not have this capacity, what suggestions would you make in order to be able to carry out machining processes without excessive heat? Explain. 21.79. A common practice is to set the cutting speed so that the tool life is the same as a work shift, commonly eight hours. This allows the standard practice of changing a cutting tool at the start of each shift. List the advantages and disadvantages of following this practice. ׉ 7cassandra://eP-mW35VkqsBuQUmKocUq-9y_mmcnRDEmpsfl2uSLVs`cZ%Eb׉EChapter 22 Cutting-tool Materials and Cutting Fluids 22.1 Introduction 675 22.2 High-speed Steels 678 22.3 Cast-cobalt Alloys 678 22.4 Carbides 679 22.5 Coated Tools 682 22.6 Alumina-based Ceramics 686 22.7 Cubic Boron Nitride 687 22.8 Silicon-Nitride-based Ceramics 687 22.9 Diamond 688 22.10 Whisker-reinforced Materials and Nanomaterials 688 22.11 Tool Costs and Reconditioning of Tools 689 22.12 Cutting Fluids 689 Example: 22.1 Effects of Cutting Fluids on Machining 690 • Continuing the coverage of the fundamentals of machining in the preceding chapter, this chapter describes two essential elements in machining operations: cutting-tool materials and cutting fluids. • The chapter opens with a discussion of the types and characteristics of cutting-tool materials, including high-speed steels, carbides, ceramics, cubic boron nitride, diamond, and coated tools. • The types of cutting fluids in common use are then described, including their functions and how they affect machining. • Trends in near-dry and dry machining, and in methods for cutting fluid application, are also described, and their significance with respect to environmentally friendly machining operations is explained. 674 ׉ 7cassandra://H9A7wTUyqyv8aeqyTA7COo-8Bb__V6KfWYBPBLIMmbw3`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://GqI-9uPVs7wu5rRNh2qRlsaunAyfGw97PAVp1b4b5AsI`׉ 7cassandra://UndKsmvYIRRd1LJmUuHMzKCJsHES5l-OYDuJW8Hf2XwQ `s׉ 7cassandra://WUxg5lrO_skpODeAtJGqryzSX4F0SDT_55XV1BdAFMs`׉ 7cassandra://9lGDROnD6EQPxuk0Ie4CSVCsNW6XmPG9DYYT-Vnx4KE H͠^cZEbט#u׉׉ 7cassandra://H3xdvphy1Yikikqwags1ccm9072fhTbax22KkzaLzz0 _`׉ 7cassandra://wrI9VOHqHuU-5SyMmu9gn-W6n9nFkzCg7N4L1zAIiDkR`s׉ 7cassandra://s7yggEZ4tTxEfneE7H7IjDrKcZAkXqGsksEpnwqg3Wc+`׉ 7cassandra://pOO-s4Vxy551mtppLioIjqPwATA9jT1wpAA0yo1-UcIfT͠^cZEb׉E _Introduction 675 22.1 Introduction The selection of a cutting-tool material for a specific application is among the most important factors to consider in machining operations. This chapter describes the properties and performance characteristics of all major types of tool materials as a guide to tool selection. General guidelines and recommendations have been established by industry based on experience. More specific information on recommendations are presented beginning with Chapter 23. As described in the preceding chapter, a cutting tool is subjected to (a) high temperatures, (b) high forces and contact stresses, and (c) rubbing along the tool–chip interface and along the machined surface. Consequently, cutting-tool material must possess the following characteristics: • Hot hardness, so that the hardness, strength, and wear resistance of the tool can be maintained at the temperatures encountered in machining. Hot hardness ensures that the tool does not undergo any deformation, thus retaining its shape and sharpness. As shown in Fig. 22.1, tool-material hardness is a function of temperature. Note, for example, how rapidly carbon tool steels (Section 5.7) lose their hardness and how well ceramics (Chapter 8) maintain their hardness at high temperatures. • Toughness and impact strength (Section 2.9), so that forces on the tool encountered repeatedly in interrupted cutting operations (such as milling, Section 24.2) or forces due to vibration and chatter during machining do not chip or fracture the tool. • Thermal shock resistance, to withstand the rapid temperature cycling (Section 3.6), as encountered in interrupted cutting. • Wear resistance (Section 33.5), so that an acceptable tool life is maintained before tool replacement is necessary. • Chemical stability and inertness, with respect to the workpiece material, to avoid or minimize any adverse reactions, adhesion, and tool–chip diffusion that would contribute to tool wear. 55 60 65 70 75 80 85 90 95 100 300 500 Temperature (8C) Figure 22.1: The hardness of various cutting-tool materials as a function of temperature. The wide range in each group of materials is due to the variety of tool compositions and treatments available for that particular group. 700 20 25 30 35 40 45 50 55 60 65 70 Hardness (HRA) HRC Ceramics Carbides s a C High-speed steels e t t l o y a l s C obra n o t o l s l e s ׉ 7cassandra://WUxg5lrO_skpODeAtJGqryzSX4F0SDT_55XV1BdAFMs`cZ%Eb׉E 676 Chapter 22 Cutting-tool Materials and Cutting Fluids Table 22.1: General Characteristics of Tool Materials. Property Hardness High-speed Cast-cobalt steels alloys Compressive strength, MPa Impact strength, J Density, kg/m3 Carbides WC TiC Ceramics Cubic boron nitride Single-crystal diamond∗ 83–86 HRA 82–84 HRA 90–95 HRA 91–93 HRA 91–95 HRA 4000–5000 HK 7000–8000 HK 46–62 HRC 1800–2400 HK 1800–3200 HK 2000–3000 HK 4100–4500 1500–2300 Transverse rupture strength, MPa 2400–4800 1380–2050 1.35–8 Modulus of elasticity, GPa 200 8600 Volume of hard phase, % 7–15 1300 Thermal conductivity, W/m K 30–50 0.34–1.25 – 8000–8700 10–20 Melting or decomposition temperature, ◦C – – Coefficient of thermal expansion, × 10−6/◦C 12 – 4100–5850 1050–2600 0.34–1.35 520–690 10,000–15,000 70–90 1400 42–125 4–6.5 3100–3850 1380–1900 0.79–1.24 310–450 5500–5800 – 1400 17 7.5–9 2750–4500 345–950 < 0.1 310–410 4000–4500 100 2000 29 6–8.5 6900 700 < 0.5 850 3500 95 1300 13 4.8 *The values for polycrystalline diamond are generally lower, except for impact strength, which is higher. To respond to these demanding requirements, a variety of cutting-tool materials, with a wide range of mechanical, physical, and chemical properties, have been developed over the years (Table 22.1). The properties listed in the first column of the table are useful in determining desirable tool-material characteristics for a particular application. For example, • Hardness and strength are important with respect to the mechanical properties of the workpiece material being machined. • Impact strength facilitates making interrupted cuts, as in milling. • Melting temperature of the tool material is important as compared to the temperatures developed in machining. • Thermal conductivity and coefficient of thermal expansion indicate the resistance of the tool to thermal fatigue and shock. A particular tool material may not have all of the desired properties for a particular machining operation, a situation that can readily be noted from Table 22.2, by observing the opposite directions of the long horizontal trendlines. Note, for example, that (a) high-speed steels are tough but have limited hot hardness, and (b) ceramics have high resistance to temperature and wear but they are brittle and thus can easily chip. The operating characteristics of tool materials are shown in Table 22.3, listed in the order in which they were developed and implemented in industry. Many of these materials also are used for dies and molds in casting, forming, and shaping metallic and nonmetallic materials. 6900 1350 < 0.2 820–1050 3500 95 700 500–2000 1.5–4.8 ׉ 7cassandra://s7yggEZ4tTxEfneE7H7IjDrKcZAkXqGsksEpnwqg3Wc+`cZ%EbācZ%EbÁ#בCט#u׉׉ 7cassandra://w17nDUTR0VUTsM6D6SI5SLE1G0_lh4Z5l74gSAQ2HKI>`׉ 7cassandra://Fr4gBQn1xXzwMFDGAar3aPCOgHFK7Y7jEf3GGFZEoasQ`s׉ 7cassandra://UU17U8qGHAd2hkgz4QmjpMqsEDCwPj329uN3OowLMgAn`׉ 7cassandra://2MPGBFLtKzBg0AGApcd_0FC4evbJX-Ni2ViRHi3GnZQ\͠^cZEbט#u׉׉ 7cassandra://ZlpxrU6-dfH4aNVv9uwV5RHZBBJh782UlmlS7Xajyn4ذ`׉ 7cassandra://L72NErsHaeTJYEqAuhfBxpFs-yOp-QYZj3JiBeQzPLUcV`s׉ 7cassandra://iJB3uBwGlJp4qGJ43zIVDE8GdLUBYZwc3Fu2SYOLkE8`׉ 7cassandra://bY39utLYIJnBdzm6sIh4OJ2Ch9_TIjE1Vqc-IRAprNgb[͠^cZEb׉E Introduction 677 Table 22.2: General Characteristics of Cutting-tool Materials. These Materials Have a Wide Range of Compositions and Properties; Overlapping Characteristics Exist in Many Categories of Tool Materials. Polycrystalline cubic High-speed Cast-cobalt Uncoated steels alloys Hot hardness Toughness Impact strength Wear resistance Chipping resistance Cutting speed Thermal-shock resistance Tool material cost Depth of cut Depth of cut Depth of cut Processing method Processing method Processing method Light to heavy Wrought, cast, HIP* Processing method sintering Light to heavy Cast and HIP sintering Light to heavy Cold pressing and sintering Source: After R. Komanduri. ∗ Hot-isostatic pressing. ∗∗ Chemical-vapor deposition, physical-vapor deposition. Tool materials Table 22.3: General Operating Characteristics of Cutting-tool Materials. General characteristics Modes of tool wear or failure High-speed steels High toughness, resistance to fracture, wide range of roughing and finishing cuts, good for interrupted cuts Uncoated carbides High hardness over a wide range of temperatures, toughness, wear resistance, versatile, wide range of applications Coated carbides Ceramics Polycrystalline cubic boron nitride (cBN) Diamond Improved wear resistance over uncoated carbides, better frictional and thermal properties High hardness at elevated temperatures, high abrasive wear resistance High hot hardness, toughness, cutting-edge strength High hardness and toughness, abrasive wear resistance Limitations Flank wear, crater wear Low hot hardness, limited hardenability, and limited wear resistance Flank wear, crater wear Cannot be used at low speeds because of cold welding of chips and microchipping Flank wear, crater wear Cannot be used at low speeds because of cold welding of chips and microchipping Depth-of-cut line notching, microchipping, gross fracture Depth-of-cut line notching, chipping, oxidation, graphitization Chipping, oxidation, graphitization Low strength and low thermomechanical fatigue strength Low strength, and lower chemical stability than ceramics at higher temperature Low strength, and low chemical stability at higher temperatures Carbon steels are the oldest tool materials (Fig. 22.6), and have been used widely for drills, taps, broaches, and reamers. Low-alloy and medium-alloy steels were later developed for similar applications but with longer tool life. Although inexpensive and easily shaped and sharpened, these steels do not have Light to heavy CVD or PVD** Light to heavy or HIP sintering Light to heavy hightemperature sintering Very light for single-crystal diamond Cold pressing High-pressure, High-pressure, and sintering hightemperature sintering carbides Coated carbides Ceramics boron nitride Diamond ׉ 7cassandra://UU17U8qGHAd2hkgz4QmjpMqsEDCwPj329uN3OowLMgAn`cZ%Eb׉E2678 Chapter 22 Cutting-tool Materials and Cutting Fluids sufficient hot hardness and wear resistance for machining at high speeds, where the temperature rises rapidly. Their use is limited to very low speed cutting operations, particularly in woodworking. The following topics are described in this chapter: • Characteristics, applications, and limitations of cutting-tool materials, and their costs. • Applicable range of processing variables for optimal performance. • Types and characteristics of cutting fluids and their specific applications in machining. 22.2 High-speed Steels High-speed steel (HSS) tools are so named because they were developed to machine at speeds higher than was previously possible. Introduced in the early 1900s, these steels are the most highly alloyed of the tool steels (Section 5.7). They can be hardened to various depths, have good wear resistance, and are relatively inexpensive. Because of their toughness, hence high resistance to fracture, they are suitable especially for (a) high positive rake-angle tools (those with small included angles), (b) interrupted cuts, (c) machine tools with low stiffness thus subject to vibration and chatter, and (d) tools with complex geometries, such as drills, reamers, taps, and gear cutters. Their most important limitation is due to their lower hot hardness, whereby applicable cutting speeds are low as compared with those of carbide tools (see Fig. 22.1). There are two basic types of high-speed steels: molybdenum (M-series) and tungsten (T-series). The M-series contains up to about 10% Mo, with Cr, V, W, and Co as alloying elements. The T-series contains 12 to 18%W, with Cr, V, and Co as alloying elements. Carbides in these steels constitute about 10 to 20% by volume. The M-series generally has higher abrasion resistance than the T-series, undergoes less distortion during heat treating (Section 4.7), and is less expensive. Consequently, 95% of all high-speed steel tools are made of the M-series steels. These steels and their characteristics are listed in Table 5.8. High-speed steel tools are available in wrought (forged or rolled), cast, and powder-metallurgy (sintered) conditions; they are also available coated, for improved performance (Section 22.5). High-speed steel tools also may be subjected to surface treatments, such as case hardening for improved hardness and wear resistance (Section 4.10) or steam treatment at elevated temperatures to develop a hard, black oxide layer (bluing) for improved performance, including a lower tendency for built-up edge formation. The major alloying elements in HSS are chromium, vanadium, tungsten, cobalt, and molybdenum. Their role in cutting tools may be summarized as follows (see also Table 5.2): • Chromium improves toughness, wear resistance, and high-temperature strength. • Vanadium improves toughness, abrasion resistance, and hot hardness. • Tungsten and cobalt have similar effects, namely, improved strength and hot hardness. • Molybdenum improves wear resistance, toughness, and high-temperature strength and hardness. 22.3 Cast-cobalt Alloys Introduced in 1915, cast-cobalt alloys have the composition ranges of 38–53% Co, 30–33% Cr, and 10–20% W. Because of their high hardness, typically 58 to 64 HRC, they have good wear resistance and can maintain their hardness at elevated temperatures. They are not as tough as high-speed steels and are sensitive to impact forces; consequently, they are less suitable than high-speed steels for interrupted cutting operations. Commonly known as Stellite tools, they are cast and ground into relatively simple shapes. ׉ 7cassandra://iJB3uBwGlJp4qGJ43zIVDE8GdLUBYZwc3Fu2SYOLkE8`cZ%EbƁcZ%EbŁ#בCט#u׉׉ 7cassandra://KBc8f3yqtWgAlZFi1c3TB_5ShcZj77DrNIe-ZfchlMU`׉ 7cassandra://xgcATgyx9qU-tC5L-l8Eoe2mgySwsS8V99hPt2P9AhYP`s׉ 7cassandra://wpc60pPAI6W5P9mDCTOpBM9v1zqQs-g_Lsv46UkJIiUF`׉ 7cassandra://ql4G2yeBKEkD49XoOwtA8BxSp9q5STh08AbESIbjsa4u͠^cZEbט#u׉׉ 7cassandra://LZyroEJ_igo-YlFeV6lObY1bEWGvoN5jswf92LMbL-wR`׉ 7cassandra://ohoH6DUpQ-sTjmVz0sv-n8UdsgB5MEHhu1KDBmqHRDUcV`s׉ 7cassandra://8GFCIVYWODLqgVfo_YScsnhksK_vuP04LvGIKWZ-nDY`׉ 7cassandra://NoeqA5IG23UfNrVyYpTtyo0JdoQzba8igrAomlD3QE8͠^cZEb׉ECarbides 679 22.4 Carbides The two groups of tool materials just described possess the required toughness, impact strength, and thermal shock resistance, but they also have important limitations, particularly with respect to strength and hot hardness. Consequently, they cannot be used as effectively where high cutting speeds, hence high temperatures, are involved; such speeds often are necessary to improve productivity. To meet the challenge for increasingly higher cutting speeds, carbides, also known as cemented or sintered carbides, were introduced in the 1930s. Because of their high hardness over a wide range of temperatures (Fig. 22.1), high elastic modulus, high thermal conductivity, and low thermal expansion, carbides are among the most important, versatile, and cost-effective tool and die materials for a wide range of applications. The two major groups of carbides are tungsten carbide and titanium carbide. In order to differentiate them from the coated tools, described in Section 22.5, plain-carbide tools are referred to as uncoated carbides. 22.4.1 Tungsten Carbide Tungsten carbide (WC) typically consists of tungsten-carbide particles bonded together in a cobalt matrix. They are made using powder-metallurgy techniques (Chapter 17), hence the term sintered carbides or cemented carbides. Tungsten-carbide particles are first combined typically with cobalt, resulting in a composite material with a cobalt matrix surrounding the carbide particles. These particles, which are 1 to 5 µminsize, are then pressed and sintered into inserts (Section 22.4.3 and Fig. 22.2). Tungsten carbides frequently are also compounded with titanium carbide and niobium carbide to impart special properties to the material. The amount of cobalt present, ranging typically from 6 to 16%, significantly affects the properties of tungsten-carbide tools. As the cobalt content increases, the strength, hardness, and wear resistance decrease, Figure 22.2: Typical cutting tool inserts with various shapes and chip-breaker features: Round inserts also are available, as can be seen in Figs. 22.3c and 22.4. The holes in the inserts are standardized for interchangeability in toolholders. Source: Courtesy of Kennametal, Inc. ׉ 7cassandra://wpc60pPAI6W5P9mDCTOpBM9v1zqQs-g_Lsv46UkJIiUF`cZ%Eb׉E 680 Chapter 22 Cutting-tool Materials and Cutting Fluids while toughness increases because of the higher toughness of cobalt. Tungsten-carbide tools are generally used for cutting steels, cast irons, and abrasive nonferrous materials. Micrograin Carbides. Cutting tools also are made of submicron and ultra-fine-grained (micrograin) carbides, including tungsten carbide, titanium carbide, and tantalum carbide. Grain size is typically in the range from 0.2 to 0.8 µm. Compared with the traditional carbides, these tools are stronger, harder, and more wear resistant, thus improving productivity. In one application, microdrills, with diameters on the order of 100 µm, have been made from micrograin carbides, and used in the fabrication of microelectronic circuit boards (Section 28.13). Functionally Graded Carbides. In these tools, the composition of the carbide in the insert has a gradient through its near-surface depth, instead of being uniform as it is in common carbide inserts. The gradient has a smooth distribution of compositions and phases, with functions similar to those described as desirable properties of coatings on cutting tools. Graded mechanical properties eliminate stress concentrations and increase tool life; however, they are more expensive and cannot be justified for all applications. 22.4.2 Titanium Carbide Titanium carbide (TiC) has a nickel–molybdenum matrix; it has higher wear resistance than tungsten carbide but is not as tough. Titanium carbide is suitable for machining hard materials, mainly steels and cast irons, and for machining at speeds higher than those for tungsten carbide. 22.4.3 Inserts Tool changing operations can be time consuming, reducing productivity. The need for a more effective method led to the development of inserts, which are individual cutting tools with several cutting edges (Fig. 22.2). A square insert has eight cutting edges, and a triangular insert has six. Inserts are typically clamped on the toolholder, using a variety of locking mechanisms (Fig. 22.3). When one edge of the insert is worn, it is indexed (rotated in its holder) to make another edge available. A wide variety of other toolholders is available for specific applications, including those with quick insertion and removal features. The strength of the insert’s cutting edge depends on its shape; the smaller the included angle (see top of Fig. 22.4), the lower is the strength of the edge. In order to further improve edge strength and prevent chipping, insert edges are usually honed, chamfered, or made with a negative land (Fig. 22.5). Chip-breaker features (Fig. 21.7 and Section 21.2.1) on inserts are for the purposes of (a) controlling chip flow during machining, (b) eliminating long continuous chips, (c) reducing heat generated, and (d) reducing the tendency for vibration and chatter. Carbide inserts are available with a wide variety of complex Toolholder Clamp screw Insert Clamp Seat or shim (a) (b) (c) Figure 22.3: Methods of mounting inserts on toolholders: (a) clamping and (b) wing lockpins. (c) Examples of inserts mounted with threadless lockpins, which are secured with side screws. Source: Courtesy of Sandvik. Lockpin Insert Seat Shank ׉ 7cassandra://8GFCIVYWODLqgVfo_YScsnhksK_vuP04LvGIKWZ-nDY`cZ%EbȁcZ%Ebǁ#בCט#u׉׉ 7cassandra://bvKSnDsmTmfxUBbGyUjiUT-GoX6h-nRD9hawtXfWnMM`׉ 7cassandra://_LjLkdLpPXXS5vewhaugSjnmM_eaPHbNM6LroirVuk8H`s׉ 7cassandra://_kMoBXwZplXAOH9qvxfeMfuMTS15rCJw7S9OEUdSVNYL`׉ 7cassandra://NIpFQAKOvtxLR-3QnnxmjoYVAmbsqg_OxSPao_aXDh8͎IF͠^cZEb!ט#u׉׉ 7cassandra://jnGc97Kr9r2d5_WJNKpwOrL1MSYQRKTl29mCMa36mZQJ`׉ 7cassandra://Mk5eLGz1oSi75XZTX4vwlBjaDD2pH_tgkSB9CIU5rRAS`s׉ 7cassandra://z4Q7T3rRjsdcP1ceLhGHNRwRdniUNmsr4p86z1Q0VII`׉ 7cassandra://G3GZLel_Dpf4yLpDH85KlEuC0d9pqda3x_iLgpEalz0͊h͠^cZEb"׉E|Carbides 681 Increasing strength 1008 808 908 558 608 358 Increasing chipping and breaking Figure 22.4: Relative edge strength and tendency for chipping of inserts with various shapes. Strength refers to the cutting edge indicated by the included angles. Source: Courtesy of Kennametal Inc. Increasing edge strength Figure 22.5: Edge preparation for inserts to improve edge strength. Source: Courtesy of Kennametal Inc. chip-breaker features (Fig. 22.2). The selection of a particular chip-breaker feature depends on the feed and depth of cut of the operation, the workpiece material, the type of chip produced during machining, and whether it is a roughing or a finishing machining operation. Stiffness of the machine tool (Section 25.3) is of major importance in using carbide tools; light feeds, low speeds, and chatter are detrimental because they tend to damage the tool’s cutting edge. Light feeds, for example, concentrate the forces and temperature closer to the edge of the tool, increasing its tendency to chip off. 22.4.4 Classification of Carbides Carbide tool grades are classified using the letters P, M, K, N, S, and H (Tables 22.4 and 22.5) for a range of applications, including the traditional C grades used in the United States. Table 22.4: ISO Classification of Carbide Cutting Tools According to Use. Designation in increasing order of wear resistance and decreasing order of toughness in each category (in increments of 5) Symbol Workpiece Material P M K N S H Ferrous metals with long chips Stainless steels with long or short chips Cast iron with short chips Non-Ferrous metals High-temperature alloys Hardened Materials Color code Blue Yellow Red Green Orange Gray Uncoated P01, P05–P20 M10–M20 K05–K20 N10-20 S10-20 — Coated P20-P50 M20-M40 K05-K30 N05-N30 S20-S30 H10 Negative with land and hone Negative with land Negative honed Negative sharp Positive with hone Positive sharp ׉ 7cassandra://_kMoBXwZplXAOH9qvxfeMfuMTS15rCJw7S9OEUdSVNYL`cZ%Eb׉E 682 Chapter 22 Cutting-tool Materials and Cutting Fluids Table 22.5: Classification of Tungsten Carbides According to Selected Machining Applications. ANSI ISO standard K30–K40 K20 K10 K01 P30–P50 P20 P10 P01 classification number (grade) C1 C2 C3 C4 C5 C6 C7 C8 Materials to be machined Cast iron, nonferrous metals, and nonmetallic materials requiring abrasion resistance Steels requiring crater and deformation resistance Machining operation General purpose Light Type of carbide Roughing Wear-resistant grades; generally straight WC–Co with varying grain sizes finishing Precision finishing Roughing Crater-resistant grades; various WC–Co General purpose Light finishing Precision finishing Note: The ISO and ANSI comparisons are approximate. 22.5 Coated Tools The difficulty of machining newly developed materials efficiently and the need for improving their performance has led to important developments in coated tools. Compared to the tool materials themselves, coatings have such properties as: • Higher chemical inertness and resistance to wear and cracking • Higher hot hardness and impact resistance • Lower friction • Acting as a diffusion barrier between the tool and the chip. Coated tools can last 10 times more than those of uncoated tools, thus allowing for high cutting speeds and reducing both the time required for machining and production costs. As can be seen from Fig. 22.6, machining time has been reduced steadily by a factor of more than 100 since 1900, an improvement that has had a major impact on the economics of machining operations in manufacturing. This progress includes the continued improvements in the design and construction of modern machine tools and their computer controls (Chapter 25 and Part IX). 22.5.1 Coating Materials and Coating Methods Common coating materials are titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), and aluminum oxide (Al2O3). Typically in the thickness range from 2 to 15 µm, coatings are applied by two techniques, described in greater detail in Section 34.6: 1. Chemical-vapor deposition (CVD), including plasma-assisted chemical-vapor deposition 2. Physical-vapor deposition (PVD). compositions with TiC and/or TaC alloys Increasing feed rate Increasing cutting speed Characteristics of Cut Increasing cutting speed Carbide Increasing hardness and wear resistance Increasing strength and binder content Increasing hardness and wear resistance Increasing feed rate Increasing strength and binder content ׉ 7cassandra://z4Q7T3rRjsdcP1ceLhGHNRwRdniUNmsr4p86z1Q0VII`cZ%EbʁcZ%EbɁ#בCט#u׉׉ 7cassandra://1TBCAWiUU-UZHJr9jO3HKwwqNxCNJ1pJa--3caUiD5wF`׉ 7cassandra://_mxKF7ZBVX0TrUPXtj8zzq67D6KUTvxRrRq8IkgmF8UV`s׉ 7cassandra://h5XckhCSaNSrqUxDhm_2-NQwWXSMMfSb3fK0tmllk9A`׉ 7cassandra://I-tSMt6Kxxr-zOmwp8gdZayuTfY0gnicgfIYgj5b4tUt$͠^cZEb%ט#u׉׉ 7cassandra://J6t5_Uf33NQqyd-T14wyPCOnF-UZz-r1NUMwdXyWMHct`׉ 7cassandra://wc9IsiiiKboQBv6yK7iEY3sMKDH9V7gy3TQ7af7YERoL`s׉ 7cassandra://gRlhjSkf0T4eny0o-Mz5X0jaiZkXt0MVEQ8ZRrAje1k`׉ 7cassandra://eyJhnsuUk3pKT5jefBLQH55qGMruA76Tl9gY5VFVvvsz͠^cZEb&׉E Coated Tools 683 100 26 15 6 3 1.5 1 0.7 0.5 Carbon steel High-speed steel Cast cobalt-based alloys Cemented carbides Improved carbide grades First coated grades First double-coated grades First triple-coated grades Functionally graded triple-coated Optimized material combinations 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2010 Figure 22.6: Relative time required to machine with various cutting-tool materials, indicating the year the tool materials were first introduced. Note that machining time has been reduced by two orders of magnitude within a hundred years. Source: Courtesy of Sandvik. The CVD process is the most common method for carbide tools, with multiphase and ceramic coatings. However, the PVD-coated carbides with TiN coatings have higher cutting-edge strength, lower friction, and a lower tendency to form a built-up edge, and the coatings are smoother and more uniform in thickness, which is generally in the range from 2 to 4 µm. Another technology, used particularly for multiphase coatings, is medium-temperature chemical-vapor deposition (MTCVD), developed to machine ductile (nodular) iron and stainless steels and to provide higher resistance to crack propagation than CVD coatings provide. Coatings should have the following general characteristics: • High hardness at elevated temperatures, to resist wear. • Chemical stability and inertness to the workpiece material, in order to reduce wear. • Low thermal conductivity, to prevent temperature rise in the substrate. • Compatibility and good bonding, to prevent flaking or spalling from the substrate, which may be carbide or high-speed steel. • Little or no porosity, to maintain integrity and strength. Coating effectiveness is enhanced by the hardness, toughness, and thermal conductivity of the substrate. Honing (Section 26.7) of the cutting edges is an important procedure for maintaining coating strength, as otherwise the coating may peel off or chip at sharp edges and corners. Titanium-nitride. Titanium-nitride coatings have low friction, high hardness, good high temperature resistance, and good adhesion to the substrate. They greatly improve the life of high-speed steel and carbide tools, carbide tools, drill bits, and cutters. Titanium-nitride-coated tools (gold in color), perform well at higher cutting speeds and feeds, and flank wear is significantly lower than that of uncoated tools (Fig. 22.7). Flank surfaces can be reground after use, so long as regrinding that face does not remove the coating on the rake face of the tool. Coated tools do not perform as well at low cutting speeds, because the coating can be worn off by chip adhesion, thus the use of appropriate cutting fluids, to minimize adhesion, is important. Machining time (min) ׉ 7cassandra://h5XckhCSaNSrqUxDhm_2-NQwWXSMMfSb3fK0tmllk9A`cZ%Eb׉E 684 Chapter 22 Cutting-tool Materials and Cutting Fluids Rake face Tool Cutting direction TiN coated Uncoated Flank wear Figure 22.7: Schematic illustration of typical wear patterns on uncoated high-speed steel tools and titaniumnitride-coated tools. Note that flank wear is significantly lower for the coated tool. Titanium-carbide Coatings. Titanium-carbide coatings on tungsten-carbide inserts have high flank-wear resistance in machining abrasive materials. Ceramic Coatings. Because of their chemical inertness, low thermal conductivity, resistance to high temperature, and resistance to flank and crater wear, ceramics are good coating materials for cutting tools. The most commonly used ceramic coating is aluminum oxide (Al2O3). However, because they are very stable (e.g., not chemically reactive), oxide coatings generally do not bond well to the substrate. Multiphase Coatings. The desirable properties of coatings can be combined and optimized using multiphase coatings. Carbide tools are available with two or three layers of such coatings, and are particularly effective in machining cast irons and steels. For example, TiC can be deposited first over the substrate, followed by Al2O3, and then TiN; the first layer must bond well with the substrate, the outer layer should resist wear and have low thermal conductivity, and the intermediate layer should bond well and be compatible with both layers. Typical applications of multiple-coated tools are: • High-speed, continuous cutting: TiC/Al2O3 • Heavy-duty, continuous cutting: TiC/Al2O3/TiN • Light, interrupted cutting: TiC/TiC + TiN/TiN. Coatings can also be deposited in alternating multiphase layers; their thickness is on the order of 2 to 10 µm, this is thinner than regular multiphase coatings (Fig. 22.8). The reason for using thinner coatings is that coating hardness increases with decreasing grain size, a phenomenon similar to the increase in the strength of metals with decreasing grain size (see Section 1.5.1); thus, thinner layers are harder than thicker layers. A typical multiphase-coated carbide tool may consist of the following layers, starting from the top: 1. TiN: low friction 2. Al2O3: high thermal stability 3. TiCN: fiber reinforced, with a good balance of resistance to flank wear and crater wear, effective particularly for interrupted cutting 4. A thin carbide substrate: high fracture toughness 5. A thick carbide substrate: hard and resistant to plastic deformation at high temperatures. ׉ 7cassandra://gRlhjSkf0T4eny0o-Mz5X0jaiZkXt0MVEQ8ZRrAje1k`cZ%Eb́cZ%Ebˁ#בCט#u׉׉ 7cassandra://PE5-Ft-6SSyQCDkSU_1aUg_uQxNzZO3Qi8r7CsVf-MM@`׉ 7cassandra://q4a6H22zCyVjw-PPyvQEDE0sC_O7wC8CJxB_RrvwYY4T=`s׉ 7cassandra://aaTyvP5SyfGAqYddjQO0Q7DaTIpJg5nCf6MrV0cuPts`׉ 7cassandra://XzMAtlYHFDUdChhJSVhJ__TQZaH2TZczeBTi2sfrCP4̈́$<͠^cZEb)ט#u׉׉ 7cassandra://G7yWDJ_KDWkyxJ0K5IB_agUCQeoZioWlCuMs57EV8k0`׉ 7cassandra://WnUR6txqGeq_Mjacnef9CD2y6bwYBlUVbV4BVWriv4kO`s׉ 7cassandra://eHzjS7m7Menv2C38DQkzhcK0xHzT0m8-ZrA-e7dppukF`׉ 7cassandra://MbxSuCelpEkawZpZlj3MlgAkOy2FYUTWlC-ZUBHVlQ4t͠^cZEb*׉E Coated Tools 685 TiN TiC, N Al2O3 Al2O3 Al2O3 TiN TiN TiC, N Carbide substrate Figure 22.8: Multiphase coatings on a tungsten-carbide substrate. Three alternating layers of aluminum oxide are separated by very thin layers of titanium nitride. Inserts with as many as 13 layers of coatings have been made. Coating thicknesses are typically in the range from 2 to 10 µm. Source: Courtesy of Kennametal Inc. Diamond Coatings. The properties and applications of diamond, diamond coatings, and diamondlike carbon are described in Sections 8.7 and 34.13, and the use of these materials as cutting tools is given in Section 22.9. Polycrystalline diamond (PCD) is used widely as a coating material for tools, particularly on tungsten-carbide and silicon-nitride inserts. Diamond-coated tools are particularly effective in machining (a) nonferrous metals, (b) abrasive materials, such as aluminum alloys containing silicon, (c) fiber-reinforced and metal-matrix composite materials, and (d) graphite. As many as tenfold improvements in tool life have been obtained over the lives of other coated tools. Diamond-coated inserts have thin films deposited on substrates through PVD or CVD techniques (Section 34.6). Thick diamond films are produced by growing a large sheet of pure diamond, which is then laser cut to shape and brazed to a carbide insert. Multilayer nanocrystal diamond coatings have interlocking layers of diamond that give strength to the coating. As with all coatings, it is essential that the diamond film adheres well to the substrate and to minimize the difference in thermal expansion between the diamond and substrate materials selected (see Section 3.6). 22.5.2 Miscellaneous Coating Materials The hardness of some of the following coatings approaches that of cubic boron nitride (Fig. 2.15). 1. Titanium carbonitride (TiCN) and titanium-aluminum nitride (TiAlN) are effective in machining stainless steels. TiCN (deposited by physical-vapor deposition) is harder and tougher than TiN, and can be used on carbides and high-speed steel tools. TiAlN is effective in machining aerospace alloys. 2. Chromium-based coatings, such as chromium carbide (CrC), have been found to be effective in machining softer metals that have a tendency to adhere to the cutting tool, such as aluminum, copper, and titanium. Other coating materials include zirconium nitride (ZrN) and hafnium nitride (HfN). 3. Nanolayer coatings, such as carbide, boride, nitride, oxide, or some combination of these materials (see also Section 8.8). 4. Composite coatings, using a variety of materials. ׉ 7cassandra://aaTyvP5SyfGAqYddjQO0Q7DaTIpJg5nCf6MrV0cuPts`cZ%Eb׉E 686 Chapter 22 Cutting-tool Materials and Cutting Fluids 22.5.3 Ion Implantation In this process, ions are introduced into the surface of the tool, improving its surface properties without affecting the tool’s dimensions (Section 34.7). Nitrogen-ion implanted carbide tools have been used successfully on alloy steels and stainless steels. Xenon-ion implantation of tools is also under development. 22.6 Alumina-based Ceramics Ceramic tool materials, introduced in the early 1950s, consist primarily of fine-grained, high-purity aluminum oxide (Section 8.2). They are cold pressed into insert shapes under high pressure, then sintered at high temperature. The end product is referred to as white (cold-pressed) ceramics. Additions of titanium carbide and zirconium oxide help improve properties, such as toughness and thermal-shock resistance. Alumina-based ceramic tools have very high abrasion resistance and hot hardness (Fig. 22.9). Chemically, they are more stable than high-speed steels and carbides; they have less tendency to adhere to metals during machining, and thus a correspondingly lower tendency to form a built-up edge. Consequently, in machining cast irons and steels, good surface finish is obtained using ceramic tools. On the other hand, recall that ceramics generally lack toughness. Ceramic inserts are available in shapes similar to those for carbide inserts (Section 22.4.3). They are effective in high-speed, uninterrupted cutting operations. To reduce thermal shock, cutting should be performed either dry or with a copious amount of cutting fluid, applied in a steady stream (Section 22.12). Improper or intermittent application of the fluid can cause thermal shock, possibly leading to fracture of the tool. The shape of ceramic inserts and their setup are important. Negative rake angles (i.e., large included angles) generally are to be preferred to avoid chipping, due to the poor tensile strength of ceramics. Tool failure can be reduced by increasing the stiffness and damping capacity of machine tools, mountings, and workholding devices, thus reducing vibration and chatter. Single crystal diamond CVD diamond Polycrystalline diamond cBN Oxide ceramics Cermets Coated carbides Micrograin carbides Nitride ceramics Uncoated carbides High-speed steel Strength and toughness Figure 22.9: Ranges of mechanical properties for various groups of tool materials (see also Tables 22.1 through 22.5). Ideal cutting tool material Hot hardness and wear resistance ׉ 7cassandra://eHzjS7m7Menv2C38DQkzhcK0xHzT0m8-ZrA-e7dppukF`cZ%Eb΁cZ%Eb́#בCט#u׉׉ 7cassandra://SX9Zr3rNRKSzIE0OGeJzE1qyZg0NVlP9zMebZZW2_roUe`׉ 7cassandra://QAmdgpryaSUKu1_ln6aFehwR_bj5Pa-xuF4V2ufKHLEVg`s׉ 7cassandra://wGHeoHberzHM3Kzhe2IuBP7xsoUBC0TOTecMhwd7JoI`׉ 7cassandra://3yyCRvHUA7A4iUqnLVNtuGPZxgb_YT2BWkRWOmUVL6Mq]b͠^cZEb,ט#u׉׉ 7cassandra://WjLwbb8pNUak7e7sTeiyzQviU40liiRNMJ3fl24zKosV`׉ 7cassandra://JuJ4axbjphBGq4DaLViheeAqz-3auRqILqEIb0n_mQM``s׉ 7cassandra://t2HVeGSTJrKdxBBlYapW86rAhDL5ArmEy9lRckH2uT4Z`׉ 7cassandra://zuBFOLrVgXjJxEaiCKCv0zFq4B_p6m3L6WIReiYMSlA̓x ͠^cZEb-׉E Silicon-Nitride-based Ceramics 687 Cermets. Cermets (from the words ceramic and metal), first used in the early 1950s, consist of ceramic particles in a metallic matrix. They are referred to as black or hot-pressed ceramics (carboxides). A typical cermet consists of 70% aluminum oxide and 30% titanium carbide; other cermets contain molybdenum carbide, niobium carbide, and tantalum carbide. Although they have chemical stability and resistance to built-up edge formation, the brittleness and high cost of cermets have been a limitation to their wider use. Their performance is somewhere between that of ceramics and carbides, and has been particularly suitable for light roughing cuts and high-speed finishing cuts. Chip-breaker features are important for cermet inserts. 22.7 Cubic Boron Nitride Next to diamond, cubic boron nitride (cBN) is the hardest material available. Introduced in 1962 under the trade name Borazon, cubic boron nitride is made by bonding a 0.5 to 1 mm layer of polycrystalline cubic boron nitride to a carbide substrate, by sintering under high pressure and high temperature. While the carbide provides shock resistance, the cBN layer provides very high wear resistance and cutting-edge strength (Fig. 22.10). The thermochemical stability of cBN is a significant advantage; it can be used safely up to 1200◦C. At elevated temperatures, cBN maintains high chemically inertness to iron and nickel, hence there is no wear due to diffusion. Its resistance to oxidation is high, making it particularly suitable for machining hardened ferrous and high-temperature alloys (see hard machining, Section 25.6) and for high-speed machining operations (Section 25.5). cBN also is used as an abrasive; however, because these tools are brittle, the stiffness of the machine tool and the fixturing is important in order to avoid vibration and chatter. Furthermore, in order to avoid chipping and cracking due to thermal shock, machining generally should be performed dry, particularly in interrupted cutting operations. 22.8 Silicon-Nitride-based Ceramics Developed in the 1970s, silicon-nitride (SiN)-based ceramic materials consist of silicon nitride, with various additions of aluminum oxide, yttrium oxide, and titanium carbide. These tools have high toughness, hot hardness, and good thermal-shock resistance. An example of a SiN-based material is sialon, named after the elements silicon, aluminum, oxygen, and nitrogen. Sialon has higher thermal-shock resistance than silicon nitride; it is recommended for machining cast irons and nickel-based superalloys at intermediate cutting speeds. Because of their chemical affinity to iron at elevated temperatures, however, SiN-based tools are not suitable for machining steels. Tungsten-carbide insert Braze Polycrystalline cubic boron nitride or diamond layer Carbide substrate Figure 22.10: An insert of a polycrystalline cubic boron nitride or a diamond layer on tungsten carbide. ׉ 7cassandra://wGHeoHberzHM3Kzhe2IuBP7xsoUBC0TOTecMhwd7JoI`cZ%Eb׉E 6688 Chapter 22 Cutting-tool Materials and Cutting Fluids (a) (b) (c) Figure 22.11: Microphotographs of diamond compacts: (a) fine-grained diamond, with mean grain size around 2 µm; (b) medium grain, with mean grain size around 10 µm; (c) coarse grain, with grain size around 25 µm. Source: Courtesy of Kennametal, Inc. 22.9 Diamond Described in Section 8.7, diamond, as a cutting tool, has highly desirable properties such as low friction, high wear resistance, and the ability to maintain a sharp cutting edge. Diamond is used when good surface finish and dimensional accuracy are required, particularly when machining soft nonferrous alloys and abrasive nonmetallic and metallic materials, especially some aluminum–silicon alloys. Synthetic or industrial diamond is widely used because natural diamond has flaws, and thus its performance can be unpredictable. Although single-crystal diamond of various carats (1 carat=200 mg) can be used for special applications, they have been replaced largely by polycrystalline diamond (PCD) tools, called compacts (also used as dies for fine wire drawing; see Section 15.9). These diamond tools consist of very small synthetic crystals (Fig. 22.11), fused under a high-pressure, high-temperature process, to a thickness of about 0.5 to 1 mm, and bonded to a carbide substrate; this product is similar to cBN tools (Fig. 22.10). A unique feature is that the random orientation of the diamond crystals of the structure prevents the propagation of cracks, thus significantly improving its toughness (see also Section 2.10.2). Fine grains are used when a high cuttingedge quality and higher strength are required; coarse grains are preferred for increased abrasion resistance. Because diamond is brittle, tool shape and its sharpness are important. Low rake angles are generally used to provide a strong cutting edge, because of the larger included angles. Proper mounting and crystal orientation are important for optimum tool life. Wear may occur through microchipping (caused by thermal stresses and oxidation) and through transformation to carbon (caused by the heat generated during machining). Diamond tools can be used satisfactorily at almost any speed, but are most suitable for light, uninterrupted finishing cuts. To minimize tool fracture, the single-crystal diamond must be resharpened as soon as it becomes dull. Diamond is not recommended for machining plain-carbon steels or for titanium, nickel, and cobalt-based alloys, because of its strong chemical affinity to carbon at elevated temperatures (resulting in diffusion)). 22.10 Whisker-reinforced Materials and Nanomaterials To further improve the performance and wear resistance of cutting tools, continued progress is being made in developing new tool materials, with enhanced properties, such as: • High fracture toughness • Resistance to thermal shock ׉ 7cassandra://t2HVeGSTJrKdxBBlYapW86rAhDL5ArmEy9lRckH2uT4Z`cZ%EbЁcZ%Ebρ#בCט#u׉׉ 7cassandra://eqWkGuSDYHbqTUC1hiVBN6ZFgvqLPpQQwDGLt0Au8_E`׉ 7cassandra://G5-AS1cynEc6li5JyiQwG6a3V3wasX-InIg8Y87n5fka`s׉ 7cassandra://ApkruQXKyvgk4dglhNhNOuj0cIcuXGxduW4bFejS12Ad`׉ 7cassandra://5-1KsJnw7ip1c4UBQCHLrStZIz4Nv6baasGpx11W598a͠^cZEb0ט#u׉׉ 7cassandra://DZ6g4dlYKYlAV-kqOxGB4kcNarzTyo_RXLts9aH2Hfc9`׉ 7cassandra://NCqLTV5ceMB8XF1XCZ_YRgwm6q5YrZOnZEvDpFgE5Kcc`s׉ 7cassandra://czNjRRxhcm-su6_rlaFClEzxNPEihT5SFkeSOLv22bU`׉ 7cassandra://xwl2KpOjezm72VTVnnH7X7UkKWiUSLwcVr7Ti0x2-vYb͠^cZEb1׉E Cutting Fluids 689 • Cutting-edge strength • Creep resistance • Hot hardness. Whiskers are important as reinforcing fibers in composite tool materials. Examples of whiskerreinforced cutting tools include (a) silicon-nitride-based tools reinforced with silicon-carbide whiskers and (b) aluminum-oxide-based tools reinforced with 25% to 40% silicon-carbide whiskers, sometimes with the addition of zirconium oxide (ZrO2). Silicon-carbide whiskers are typically 5 to 100 µm long and 0.1 to 1 µm in diameter. The high reactivity of silicon carbide with ferrous metals, however, makes SiC-reinforced tools unsuitable for machining irons and steels. Nanomaterials are also becoming important in advanced cutting-tool materials (see Section 8.8). Suitable nanomaterials are carbides and ceramics. Often, they are applied as a thin coating, usually in an attempt to obtain a reasonable tool life without requiring a coolant (see dry machining, Section 22.12.1). 22.11 Tool Costs and Reconditioning of Tools Tool costs vary widely, depending on the tool material, size, shape, chip-breaker features, and quality. The approximate cost for a typical 12.5-mm insert is (a) $10 to $15 for uncoated carbides, (b) $10 to $25 for coated carbides, (c) $30 to $50 for ceramics, (d) $10 to $90 for diamond-coated carbides, (e) $75 to $225 for cubic boron nitride, and (f) $150 to $200 for a diamond-tipped insert. After reviewing the costs involved in machining and considering all of the aspects involved in the total operation, it can be seen that the cost of an individual insert is relatively insignificant. Tooling costs in machining have been estimated to be on the order of 2% to 4% of the manufacturing costs. This small amount is due to the fact that a single insert typically can perform a large amount of material removal before it is indexed to use all its cutting edges, and eventually recycled. Note from Section 21.5 that the expected tool life can be in the range of 30–60 minutes. Considering that a square insert, for example, has eight cutting edges, a tool can last a long time before it is removed from the machine tool and replaced. Cutting tools can be reconditioned by resharpening them, using tool and cutter grinders with special fixtures (Section 26.4). This operation may be carried out by hand or on computer-controlled grinders. Advanced methods of shaping cutting tools also are available, as described in Chapter 27. Reconditioning of coated tools is done by recoating them, usually in special facilities available for these purposes. It is important to ensure that reconditioned tools have the same geometric features as the original. Often, a decision has to be made whether further reconditioning of tools is economically viable. Recycling of tools is always a significant consideration, especially if they contain expensive and strategically important materials, such as tungsten and cobalt. 22.12 Cutting Fluids Cutting fluids are used in machining operations for the following purposes: • Reduce friction and wear, thus improving the tool life and surface finish of the workpiece. • Cool the cutting zone, thus improving tool life and reducing the temperature and thermal distortion of the workpiece. • Reduce cutting forces and energy consumption. • Flush away the chips from the cutting zone, preventing the chips from interfering with the cutting operation, particularly in drilling and tapping. • Protect the machined surface from environmental corrosion. ׉ 7cassandra://ApkruQXKyvgk4dglhNhNOuj0cIcuXGxduW4bFejS12Ad`cZ%Eb׉E 690 Chapter 22 Cutting-tool Materials and Cutting Fluids Depending on the type of machining operation, the cutting fluid required may be a coolant,a lubricant, or both. The effectiveness of fluids depends on several factors, such as the type of machining operation, tool and workpiece materials, cutting speed, and the method of application. Water is an excellent coolant, and can effectively reduce the high temperatures developed in the cutting zone. However, it is not an effective lubricant and it does not reduce friction, and can cause corrosion of workpieces and machine-tool components. The necessity for a cutting fluid depends on the severity of the particular machining operation, defined as (a) the level of temperatures and forces encountered and the ability of the tool materials to withstand them, (b) the tendency for built-up edge formation, (c) the ease with which chips produced can be removed from the cutting zone, and (d) how effectively the fluids can be supplied to the proper region at the tool–chip interface. The relative severities of specific machining processes, in increasing order of severity, are: sawing, turning, milling, drilling, gear cutting, thread cutting, tapping, and internal broaching. There are operations, however, in which the cooling action of cutting fluids can be detrimental. Cutting fluids may cause the chip to become more curly (see Fig. 21.9c), and thus concentrate the heat closer to the tool tip, reducing tool life. In interrupted cutting operations, such as milling with multiple-tooth cutters, cooling of the cutting zone leads to thermal cycling of the cutter teeth, which can cause thermal cracks by the mechanisms of thermal fatigue or thermal shock. Cutting-fluid Action. The basic mechanisms of lubrication in metalworking operations are described in Section 33.6. Studies have shown that the cutting fluid gains access to the tool–chip interface by seeping from the sides of the chip (perpendicular to the page in Figs. 21.11 and 21.12), through the capillary action of the interlocking network of surface asperities in the interface. Because of the small size of this capillary network, the cutting fluid should have a small molecular size and possess wetting (surface tension) characteristics. Grease, for example, cannot be an effective lubricant in machining, whereas low-molecular-weight oils suspended in water, known as emulsions,are very effective. Note also that in discontinuous machining operations, cutting fluids have more access to tool–chip-workpiece interfaces, although the tools become more susceptible to thermal shock. Example 22.1 Effects of Cutting Fluids on Machining Given: A machining operation is being carried out with a cutting fluid that is an effective lubricant. Find: Describe the changes in the cutting operation mechanics if the fluid supply is interrupted. Solution: Since the cutting fluid is a good lubricant, the following chain of events will take place after the fluid is shut off: 1. Friction at the tool–chip interface will increase. 2. The shear angle will decrease, in accordance with Eq. (21.3). 3. The shear strain will increase, as seen from Eq. (21.2). 4. The chip will become thicker. 5. A built-up edge is likely to form. As a result of these changes, the following events will occur: 1. The shear energy in the primary zone will increase. 2. The frictional energy in the secondary zone will increase. 3. The total energy will increase. ׉ 7cassandra://czNjRRxhcm-su6_rlaFClEzxNPEihT5SFkeSOLv22bU`cZ%EbҁcZ%Ebс#בCט#u׉׉ 7cassandra://J25FoHhDF2Otc1d8pKYb0YHQc92pPL6lifPsS36rf8k8K`׉ 7cassandra://VVCjG3aquqDYrv7DGmthUjs6Mr2nnk-7D9E_mskx2XYh`s׉ 7cassandra://IerxG_taw_k42DMltQy_3dozjfLc-dA8bMAqLbU68Wk`׉ 7cassandra://4l6VPx7hNytCc8J33nJwo1IKnZ2y-mCuk0lNZESEzco[!͠^cZEb3ט#u׉׉ 7cassandra://6bnvLtvBINQU9phJj2jQgAhOrnKF4DXuAGDGpqPGQ-k `׉ 7cassandra://gApAV0nYuG9WI00n1OdA5LD-Q-9W1DIZ5OhJmrBPkg0B_`s׉ 7cassandra://XImGieUobwpthVJu85jYLxsoJWfVV3oWm24BjO5zsrE`׉ 7cassandra://RL62knYfUCykxf3ypuJdUWQx9pPmnfmlO-isis70x_E `׉ 7cassandra://wM2eCQmM8WfwICskdZ8vzwBUUZv9AY0hCu-1NruWmKs*`s׉ 7cassandra://Nj7-mVI0ZkdAueFDDG_wVkoBhlGayv4EtQzlRx6zMPk`׉ 7cassandra://PrFB1fYepkrpJifBApIXo-msdq3Yvxb92gKp6HM1ohAT͠^cZEbV׉E^The Turning Process 709 Table 23.4: General Recommendations for Turning Operations. These recommendations are for guidance only, and are often exceeded in practice. (cont.) General-purpose starting conditions Depth of Feed, Workpiece material Stainless steel, austenitic Cutting tool Triple-coated carbide TiN-coated carbide Cermet High-temperature alloys, nickel based Ceramic-coated carbide TiN-coated carbide Al2O3 ceramic SiN ceramic Titanium alloys Aluminum alloys Free machining Polycrystalline cBN Uncoated carbide TiN-coated carbide Uncoated carbide TiN-coated carbide Cermet ” ” ” ” ” 1.0–3.8 ” 1.5–5.0 ” ” High silicon Polycrystalline diamond 0.1–4.0 Polycrystalline diamond ” ” ” ” ” ” 0.15 ” 0.45 ” ” 0.1–0.4 ” 45 30–55 260 215 150 35–60 30–60 490 550 490 760 530 ” ” ” ” ” 0.25–6.3 ” 0.25–8.8 ” ” ” ” ” ” ” ” ” 0.1–0.4 ” 0.08–0.62 ” ” ” ” 20–60 20–85 185–395 90–215 120–185 10–75 15–170 200–670 60–915 215–795 1000–5000 (3200–16,250 365–915 Uncoated carbide cut, mm mm/rev Cutting speed, 1.5–4.4 0.35 ” ” 2.5 ” 0.30 0.15 150 85–160 185–215 25–45 Depth of cut, ” ” 0.25–6.3 Range for roughing and finishing Feed, Cutting 0.5–12.7 0.08–0.75 ” ” 0.1–0.3 mm/rev speed, m/min 75–230 55–200 135–315 15–30 ׉ 7cassandra://L4gNLmzYYwpWbxufrK261nlUzlIz-D9tUIet4IdTSL4`cZ%Eb׉E#710 Chapter 23 Machining Processes: Turning and Hole Making Table 23.4: General Recommendations for Turning Operations. These recommendations are for guidance only, and are often exceeded in practice. (cont.) General-purpose starting conditions Depth of Feed, Workpiece material Copper alloys Cutting tool Uncoated carbide Ceramic-coated carbide Triple-coated carbide TiN-coated carbide Cermet Tungsten alloys Thermoplastics and thermosets Polycrystalline diamond Uncoated carbide TiN-coated carbide TiN-coated carbide Polycrystalline diamond Composites, graphite TiN-coated carbide reinforced Polycrystalline diamond cut, mm mm/rev Cutting speed, 1.5–5.0 0.25 ” ” ” ” ” 2.5 ” 1.2 ” 1.9 ” ” ” ” ” ” 0.2 ” 0.12 ” 0.2 ” 260 365 215 90–275 245–425 520 75 85 170 395 200 760 Source: Based on data from Kennametal Inc. Note: Cutting speeds for high-speed steel tools are about one-half those for uncoated carbides. Depth of cut, ” ” ” ” 0.05–2.0 Range for roughing and finishing Feed, Cutting 0.4–7.51 0.15–0.75 ” ” ” ” 0.03–0.3 0.25–5.0 0.12–0.45 ” ” 0.12–5.0 0.08–0.35 ” ” 0.12–6.3 0.12–1.5 ” ” mm/rev speed, m/min 105–535 215–670 90–305 45–455 200–610 400–1300 55–120 60–150 90–230 250–730 105–290 550–1310 ׉ 7cassandra://Nj7-mVI0ZkdAueFDDG_wVkoBhlGayv4EtQzlRx6zMPk`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://nedu0cKnLRPdl7uRpy3WpNlSHoNKu6h8QCsAvp4jLZwm`׉ 7cassandra://5v5IUpuQC4aYn-pVkaRD542KQOG-F2TL0ZHjbFQDTUc*`s׉ 7cassandra://rRXmxzVjlbgHCtnxnJ3KfyDHfCnZ7E4ZxH4PAGyYshc `׉ 7cassandra://1UKfRJGTyQ5CDcLoQTlGXpGFv301Y42VLq5SoC5AlK8k͠^cZEbXט#u׉׉ 7cassandra://fFCBiF32w67wf89NbA6_0YhaOKa2UqNj4c16SJLeIDAkx`׉ 7cassandra://QdtsOYyXHytMzh2ct0zznviMPXRC4D7y8q-_ZquEO4U:`s׉ 7cassandra://fLLV9QRcids6rIea_5IsLFhCs8R-eMp3WPO69-4_HqY`׉ 7cassandra://CsmiXA5yCczCS4dsIuaupVdgQUL3VdXMolR_JaVgEhs^J͠^cZEbZ׉EThe Turning Process 711 Convection holes (drilled) Brake disk/pad contact area (faced) Mounting holes (drilled) Hub (turned and faced) Figure 23.7: Brake disk, used in Case Study 23.1. Source: Courtesy of Kennametal, Inc. Table 23.5: Process Parameter Comparison for SiN and cBN Tools in Facing of a Brake Disk. Cutting tool material Parameter Cutting speed Tool life SiN Depthofcut,mm 1.5 Feed, mm/rev 0.5 700 40 Machine utility 82% cBN 2.0 0.4 1000 4200 94% Table 23.6: General Recommendations for Cutting Fluids for Machining (see also Section 33.7). Material Aluminum Beryllium Copper Magnesium Nickel Refractory metals Steels Titanium Zinc Zirconium Type of fluid D, MO, E, CSN MC,E,CSN D, E, CSN D, MO MC, E, CSN MC,E,EP Carbon and low-alloy D,MO,E,CSN, EP Stainless D, MO, E, CSN CSN, EP, MO C, MC, E, CSN D, E, CSN Note: CSN = chemical and synthetics; D = dry; E = emulsion; EP = extreme pressure; FO= fatty oil; and MO = mineral oil. ׉ 7cassandra://rRXmxzVjlbgHCtnxnJ3KfyDHfCnZ7E4ZxH4PAGyYshc `cZ%Eb׉E712 Chapter 23 Machining Processes: Turning and Hole Making Example 23.1 Material-removal Rate and Cutting Force in Turning Given: A 150-mm-long, 10-mm-diameter, 304 stainless-steel rod is being reduced in diameter to 8 mm by turning on a lathe. The spindle rotates at N = 400 rpm, and the tool is traveling at an axial speed of 200 mm/min. Find: Calculate the cutting speed, material-removal rate, cutting time, power dissipated, and cutting force. Solution: The cutting speed is the tangential speed of the workpiece. The maximum cutting speed is at the outer diameter, Do, and is obtained from the equation V = πDoN. Thus, V =(π)(0.010)(400) = 12.57 m/min. The cutting speed at the machined diameter is V =(π)(0.008)(400) = 10.05 m/min. From the information given, note that the depth of cut is d = 10 − 8 2 and the feed is f = 200 400 =0.5 mm/rev =0.0005 m/rev. According to Eq. (23.1), the material-removal rate is then MRR =(π)(9)(1)(0.5)(400) = 5655 mm3/min. The actual time to cut, according to Eq. (23.4), is t = 150 (0.5)(400) =1 mm =0.001 m. =0.75 min. The power required can be calculated by referring to Table 21.2 and taking an average value for stainless steel as 4.1 W-s/mm3. Therefore, the power dissipated is Power = (4.1)(5655) 60 = 386 W. The cutting force, Fc, is the tangential force exerted by the tool. Since power is the product of torque, T, and rotational speed in radians per unit time, T = Since T =(Fc)(Davg/2), Fc = (9.2)(2) (0.009) =2.0 kN. (386) (400)(2π/60) =9.2 Nm. ׉ 7cassandra://fLLV9QRcids6rIea_5IsLFhCs8R-eMp3WPO69-4_HqY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ibQcMpjYV54eBEIO-NgQfugOgVCNvvyWVDTQMRp_XG8H`׉ 7cassandra://vrp6-zFzPRmiYX3-fSXBGwhbp3fUjR_IrNQhsSHSN8cnh`s׉ 7cassandra://zVxeGtjCVawXB7N_4I23lVKOliK0Vc7jKqq3gte_mhU`׉ 7cassandra://tM7hF_e-lFivWihp5KsDjW7RZXqgxP9qTy9FnMErtZAg͠^cZEb^ט#u׉׉ 7cassandra://Ef6BE5r0pZflJbv1R_Agfg--fZR435W35oCq3QPDnso`׉ 7cassandra://GNbOEyUWZ58EhUtwiuR8ZnhWDfXaGiaI60dsaRFIm-wE`s׉ 7cassandra://ePUNRz3G5towNh75jGSKNEQEierk4U3oXZxAShufN5Q`׉ 7cassandra://3LnRdrfB2IYSMS1G09aLjE4IQ-G2_pz6weTsFt7HgEgb͠^cZEba׉ELathes and Lathe Operations 713 23.3 Lathes and Lathe Operations Lathes generally are considered to be the oldest machine tools. Although woodworking lathes originally were developed after 1000 B.C., metalworking lathes, with lead screws, were not built until the late 1700s. The most common lathe originally was called an engine lathe, because it was powered with overhead pulleys and belts from a nearby engine on the factory floor. Lathes became equipped with individual electric motors beginning in the late 19th century. The maximum spindle speed of lathes is typically around 4000 rpm, but may be only about 200 rpm for large lathes. For special applications, speeds may range to 10,000 rpm or even higher for very high-speed machining (see Section 25.5). The cost of lathes ranges from about $2,000 for bench types to over $100,000 for larger units. 23.3.1 Lathe Components Lathes are equipped with a variety of components and accessories, as shown in Fig. 23.2. Their basic features and functions are: Bed. The bed supports all major components of the lathe; it has a large mass and is built rigidly, usually from gray or nodular cast iron. The top portion of the bed has two ways, with various cross sections that are hardened and machined for wear resistance and good dimensional accuracy during turning. In a gap-bed lathe, a section of the bed in front of the headstock can be removed to accommodate workpieces with larger diameters (see also Section 25.3 on advanced materials for machine-tool structures). Carriage. The carriage, or carriage assembly, slides along the ways; it consists of an assembly of the cross-slide, tool post,and apron. The cutting tool is mounted on the tool post, usually with a compound rest that swivels for tool positioning and adjustments. The cross-slide moves radially in and out, controlling the radial position of the cutting tool in such operations as facing (see Fig. 23.1e). The apron is equipped with mechanisms for both manual and mechanized movement of the carriage and the cross-slide by means of the lead screw. Headstock. The headstock is fixed to the left side of the bed and is equipped with motors, pulleys, and V-belts, supplying power to a spindle at various rotational speeds, which can be set through manually controlled selectors or by electrical controls. Most headstocks are equipped with a set of gears, and some have various drives to provide a continuously variable range of speed to the spindle. Headstocks have a hollow spindle to which workholding devices (such as chucks and collets; see Section 23.3.2) are mounted; long bars or tubing can thus be fed through them for various turning operations. The dimensional accuracy of the spindle is important for precision in turning, particularly in high-speed machining. Preloaded tapered or ball bearings are typically used to rigidly support the spindle. Tailstock. The tailstock, which can slide along the ways and be clamped at any position, supports the right end of the workpiece. It is equipped with a center, which may be fixed (called dead center) or it may be free to rotate with the workpiece (live center). Drills and reamers (Sections 23.5 and 23.6) can be mounted on the tailstock quill (a hollow cylindrical piece with a tapered hole) to drill axial holes in the workpiece. Feed Rod and Lead Screw. The feed rod is powered by a set of gears through the headstock. It rotates during the lathe operation, and provides movement to the carriage and the cross-slide by means of gears, a friction clutch, and a keyway along the length of the rod. Closing a split nut around the lead screw engages the rod with the carriage; the split nut is also used for cutting threads accurately. Lathe Specifications. A lathe is generally specified by the following parameters: • Swing, the maximum diameter of the workpiece that can be accommodated (Table 23.7); it may be as much as 2 m. • Maximum distance between the headstock and tailstock centers. • Length of the bed. ׉ 7cassandra://zVxeGtjCVawXB7N_4I23lVKOliK0Vc7jKqq3gte_mhU`cZ%Eb׉E714 Chapter 23 Machining Processes: Turning and Hole Making Table 23.7: Typical Capacities and Maximum Workpiece Dimensions for Machine Tools. Maximum Maximum Machine tool Lathes (swing/length) Bench Engine Turret Automatic screw machines Boring machines (work diameter/length) Vertical spindle Horizontal spindle Drilling machines Bench and column (drill diameter) Radial (column to spindle distance) Numerical control (table travel) dimension (m) Power (kW) speed (rpm) 0.3/1 3/5 0.5/1.5 0.1/0.3 4/3 1.5/2 0.1 3 4 Note: Larger capacities are available for special applications. 23.3.2 Workholding Devices and Accessories Workholding devices are important, since they must hold the workpiece securely in place while machining. As shown in Fig. 23.3, one end of the workpiece is clamped to the lathe spindle either by a chuck, collet, face plate (see Fig. 23.8d), or a mandrel. A chuck is usually equipped with three or four jaws. Three-jaw chucks generally have a geared-scroll design which makes the jaws self-centering. They are used for round workpieces, such as bar stock, pipes, Hood Spindle nose cap Collet Split segments Spring collet Collet sleeve Headstock spindle sleeve (a) Spindle (c) Face plate Collet Workpiece Jaws (b) (d) Figure 23.8: (a) and (b) Schematic illustrations of a draw-in type of collet. The workpiece is placed in the collet hole, and the conical surfaces of the collet are forced inward by pulling it with a draw bar into the sleeve. (c) A push-out type of collet. (d) Workholding of a workpiece on a face plate. (mounted on spindle) Turned surface Workpiece Faced surface Workpiece < 1 70 60 20 200 70 10 – – 3000 12,000 6000 10,000 300 2000 12,000 – – ׉ 7cassandra://ePUNRz3G5towNh75jGSKNEQEierk4U3oXZxAShufN5Q`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://pdcRxqdn-etHo5_4vaT1H3xemJOrFHydBJ5SUBKCB4YE`׉ 7cassandra://-PjJQjcWdVYD31MT7WJ_PrEES6tE92dNshQHhIpDwPsgN`s׉ 7cassandra://vl1thHGM84Cw9fYxrHQeSWq1APEtqBkui49D3J4ms1s`׉ 7cassandra://enKkWBYfc1W_OYTY50zAsNh3UqYhxfRZHty-0HdRNTA͋3͠^cZEbcט#u׉׉ 7cassandra://J7_fdOObmaSLJG8qur70xipLtemqXKvIzV0vrI-Wz5s`׉ 7cassandra://8c2P95-7buU5rkTZhTeu5gMV_D0-fUtPz8lAc3YvS5csE`s׉ 7cassandra://5xvtcQtZ2-4Y_eHcGpJdlpy8OVgGmX1SlRaFLwKBz1M$`׉ 7cassandra://MZxBM0W4ueLELLgHpXtPLd0uHl1cpkPHozQGCf1B0UIi͠^cZEbd׉E Lathes and Lathe Operations 715 Mandrel Mandrel (tapered) Flat (a) Solid mandrel Workpiece Workpiece (b) Gang mandrel Workpiece (c) Cone mandrel Figure 23.9: Various types of mandrels to hold workpieces for turning; they are usually mounted between centers on a lathe. Note that in (a) both the cylindrical and the end faces of the workpiece can be machined, whereas in (b) and (c) only the cylindrical surfaces can be machined. and tubing, and typically can be centered to within 0.025 mm. Four-jaw chucks have jaws that can be moved and adjusted independently of each other; thus, they can be used for square, rectangular, or odd-shaped workpieces. The jaws in some types of chucks can be reversed to permit clamping of hollow workpieces, such as pipes and tubing, either on their outside or inside surfaces. Also available are jaws made of lowcarbon steel (called soft jaws) that can be machined into desired shapes. Because of their low strength and hardness, soft jaws also conform to small irregularities on workpieces, thus ensuring better clamping. Chucks can be power or manually actuated, using a chuck wrench. Power chucks, actuated either pneumatically or hydraulically, are used in automated equipment for high production rates, including loading of parts using industrial robots (Section 37.6). Also available are several types of power chucks, with lever- or wedge-type mechanisms for actuating the jaws. Chucks are available in various designs and sizes. Their selection depends on the type and speed of operation, workpiece size, production and dimensional accuracy requirements, and the jaw clamping forces required. By controlling the jaw forces, an operator can ensure that the part does not slip or distort in the chuck during machining. High spindle speeds can significantly reduce jaw forces due to centrifugal forces. A collet is basically a longitudinally split, tapered bushing. The workpiece, generally with a maximum diameter of 25 mm, is placed inside the collet, and the collet is pulled (draw-in collet; Fig. 23.8a and b) or pushed (push-out collet; Fig. 23.8c) mechanically into the spindle. The tapered surfaces shrink the segments of the collet radially, tightening them onto the workpiece. Collets are used for round or other shapes. An advantage to using a collet, rather than a three- or four-jaw chuck, is that the collet grips nearly the entire circumference of the part, making it well suited particularly for parts with small cross sections. Face plates are used for clamping irregularly shaped workpieces; they are round and have several slots and holes through which the workpiece is bolted or clamped (Fig. 23.8d). Mandrels (Fig. 23.9) are placed inside hollow or tubular workpieces, and are used to hold workpieces that require machining on both ends or on their cylindrical surfaces. Accessories. Several devices are used as accessories and attachments for lathes. Among these are the following: • Carriage and cross-slide stops, to stop the carriage at a predetermined distance along the bed. • Devices for turning parts having a variety of tapers. • Various attachments for milling, boring, drilling, thread cutting, gear-cutting, sawing, and grinding operations. 23.3.3 Lathe Operations In a typical turning operation, the workpiece is clamped by any one of the work-holding devices described previously. Long and slender parts must be supported by a steady rest placed on the bed, or by a follow Straight mandrel ׉ 7cassandra://vl1thHGM84Cw9fYxrHQeSWq1APEtqBkui49D3J4ms1s`cZ%Eb׉E716 Chapter 23 Machining Processes: Turning and Hole Making rest, to keep the part from deflecting excessively under the cutting forces. The rests usually are equipped with three adjustable fingers or rollers that support the workpiece while allowing it to rotate freely. Steady rests are clamped directly on the ways of the lathe (as in Fig. 23.2), whereas follow rests are clamped on the carriage and travel with it. The cutting tool is attached to the tool post, which is driven by the lead screw. The cutting tool removes material by traveling along the bed. A right-hand tool travels toward the headstock, and a left-hand tool travels toward the tailstock. Facing operations are done by moving the tool radially inward with the crossslide. Form tools are used to machine various shapes on solid, round workpieces (Fig. 23.1g), by moving the tool radially inward while the part is rotating. Form cutting is not suitable for deep and narrow grooves or sharp corners, because of vibration and chatter. As a general rule, (a) the formed length of the part should not be greater than about 2.5 times the minimum diameter of the part and (b) cutting fluids should be used. The stiffness of the machine tools and workholding devices also are important considerations. Boring involves machining inside hollow workpieces or enlarging a hole; it is similar to turning. Out-ofshape round holes also can be straightened by boring. Boring large workpieces is described in Section 23.4. Drilling (Section 23.5) can be performed on a lathe by mounting the drill bit in a chuck or in the tailstock quill. The workpiece is clamped in a workholder on the headstock, and the drill bit is advanced by rotating the handwheel of the tailstock. The concentricity of the hole can be improved by subsequently boring the drilled hole. For better dimensional accuracy and surface finish, drilled holes may later be reamed (Section 23.6) on lathes, in a manner similar to drilling. The cutting tools for parting, grooving,and thread cutting are specially shaped for their specific purpose or are available as inserts. Knurling is performed on a lathe, with hardened rolls (see Fig. 23.1l); the surface of the rolls is a replica of the profile to be generated. The rolls are pressed radially against the rotating workpiece while the tool moves axially along the part. 23.3.4 Types of Lathes Bench Lathes. As the name suggests, these lathes are placed on a workbench or a table; they have low power and are usually operated by hand feed. Toolroom bench lathes have higher precision, enabling the machining of parts to close dimensional tolerances. Special-purpose Lathes. These lathes are used for such applications as railroad wheels, gun barrels, and rolling-mill rolls. Tracer Lathes. These lathes have special attachments for turning parts with various contours. Also called a duplicating lathe or contouring lathe, the cutting tool follows a path that duplicates the contour of a template, similar to a pencil following the shape of a stencil. These machines have largely been replaced by numericalcontrol lathes and turning centers (Section 25.2), although tracer attachments are available for engine lathes. Automatic Lathes. Mechanisms have been developed that enable machining operations on a lathe to follow a certain prescribed sequence. In a fully automatic lathe, parts are fed and removed automatically; in semiautomatic machines, these functions are performed by the operator, although machining remains automatic. Automatic lathes, either with a horizontal or a vertical spindle, are suitable for medium- to high-volume production. Lathes without tailstocks are called chucking machines or chuckers. They are used for machining individual pieces with regular or irregular shapes, and are either single- or multiple-spindle types. In another type of an automatic lathe, the bar stock is fed periodically into the lathe, and after a part is machined, it is cut off from the end of the bar stock. Automatic Bar Machines. Also called automatic screw machines, these machines are designed for high-production-rate machining of screws and similar threaded parts; all operations are performed automatically, with tools attached to a special turret. After each part is machined to finished dimensions, the bar ׉ 7cassandra://5xvtcQtZ2-4Y_eHcGpJdlpy8OVgGmX1SlRaFLwKBz1M$`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://O1ADJ0YIgBpiZaEB-x1JcFdLdx3IlN2-HKFF3meM9HA`׉ 7cassandra://qHB7jrYE7qkYkjM3_NJVaRNXBJ1_8YDPzLVdnVZ01Wca~`s׉ 7cassandra://DVFWjN4EtThQGAeoAKTy3KAU3FNuTpuOarYExYua8EAj`׉ 7cassandra://MpHcrxVE0fjKuSjTsLMcuxi2SGfmO1j9nnQDVdZpsow NZ^͠^cZEboט#u׉׉ 7cassandra://IDcpPnz8DsgcJjpdWBbWvXL7rgtav53I4crwynTcGAM`׉ 7cassandra://bFZ5sBjMHJ0m8H8w06E2JUjXNSHBP-bLtq-Bpb5bN8Y\`s׉ 7cassandra://_oFbxT4flTuTrTQTd2rfcutjRU5CTERa7M7mLWRUmqg`׉ 7cassandra://inuZdj28lNsOqXw3HlDFOIq25NksneBOKGITDFAKC6U2;X͠^cZEbs׉E kLathes and Lathe Operations 717 Spindle speed selector Forward and reverse Stop rod Feed shaft Longitudinal feed lever Carriage handwheel Cross-slide handwheel Cross-feed lever Figure 23.10: Schematic illustration of the components of a turret lathe; note the two turrets: square and hexagonal (main). Hexagon (main) turret Turret stops Ram Feed selectors Turnstile (capstan wheel) Square turret stock is fed forward automatically through the hole in the spindle, and then the part cut off. Automatic bar machines may be equipped with single or multiple spindles; capacities typically range from 3- to 150-mm diameter bar stock. Single-spindle automatic bar machines are similar to turret lathes, and are equipped with various cam-operated mechanisms; they are capable of high-precision machining of small-diameter parts. There are two types of single-spindle machines. In Swiss-type automatics, the cylindrical surface of a solid-bar stock is machined, using a series of tools that move radially and in the same plane toward the workpiece. The bar stock is clamped close to the headstock spindle, to minimize deflections due to cutting forces. Multiple-spindle automatic bar machines typically have from four to eight spindles, arranged in a circle on a large drum, with each spindle carrying an individual workpiece. The cutting tools are arranged in various positions in the machine, and move in both axial and radial directions. Each part is machined in stages as it moves from one station to the next. Because all operations are carried out simultaneously, the cycle time per part is reduced. Turret Lathes. These machine tools are capable of performing multiple cutting operations, such as turning, boring, drilling, thread cutting, and facing (Fig. 23.10). Several cutting tools, usually as many as six, are mounted on the hexagonal main turret, which is rotated after each specific operation is completed. The lathe usually has a square turret on the cross-slide, and is equipped with as many as four cutting tools. The workpiece, generally long, round bar stock, is advanced a preset distance through the chuck. After the part is machined, it is cut off by a tool mounted on the square turret, which moves radially into the workpiece. The rod then is advanced the same preset distance, and the next part is machined. Turret lathes are versatile; the operations may be carried out automatically or by hand, using the capstan wheel. Once set up, these machines do not require highly skilled operators. Vertical turret lathes are more suitable for short, heavy workpieces, with diameters as large as 1.2 m. The turret lathe shown in Fig. 23.10 is known as a ram-type turret lathe, in which the ram slides in a separate base on the saddle. The short stroke of the turret slide limits this machine to relatively short workpieces and light cuts, in both small- and medium-quantity production. In another design, called the saddle type, the main turret is installed directly on the saddle, which slides along the bed; the length of the stroke is limited only by the length of the bed. This type of lathe is constructed more heavily, and is used for machining large workpieces. ׉ 7cassandra://DVFWjN4EtThQGAeoAKTy3KAU3FNuTpuOarYExYua8EAj`cZ%Eb׉ES718 Chapter 23 Machining Processes: Turning and Hole Making CNC unit Chuck Round turret for OD operations Drill Tool for turning or boring Reamer Individual motors Drill End turret for ID operations Tailstock (a) (b) Figure 23.11: (a) A computer-numerical-control lathe with two turrets. (b) A typical turret equipped with 10 tools, some of which are powered. Computer-controlled Lathes. The features of a computer-controlled lathe (CNC) are shown in Fig. 23.11a. These lathes generally have one or more turrets; each turret is equipped with a variety of tools and performs several machining operations on different surfaces of the workpiece (Fig. 23.11b), with diameters as much as 1 m. Computer-controlled lathes are designed to operate faster and with higher power, and are equipped with automatic tool changers (ATCs). Their operations are reliably repetitive, maintain dimensional accuracy, require less skilled labor, and are suitable for low- to medium-volume production. Example 23.2 Typical Parts Made on CNC Turning Machine Tools The capabilities of CNC turning-machine tools are illustrated in the machined parts shown in Fig. 23.12, indicating the workpiece material, the number of cutting tools used, and the machining times. These parts also can be made on manual or turret lathes, although not as effectively or consistently. Source: Courtesy of Monarch Machine Tool Company. Example 23.3 Machining of Complex Shapes Note in Example 23.2 that the parts are axisymmetric. The capabilities of CNC turning are further illustrated in Fig. 23.13, which shows three additional, more complex parts: a pump shaft, a crankshaft, and a tubular part with an internal rope thread. Descriptions of these parts are as follows; as in most operations, machining such parts consists of both roughing and finishing cuts: 1. Pump shaft (Fig. 23.13a). This part, as well as a wide variety of similar parts with external and internal features, including camshafts, was produced on a CNC lathe with two turrets. The lathe is similar in construction to the machine tool shown in Fig. 23.11a; each turret can hold as many Multitooth cutter ׉ 7cassandra://_oFbxT4flTuTrTQTd2rfcutjRU5CTERa7M7mLWRUmqg`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://rz0HH_ioYljyi5JErj_GmRo16T2JzFKLgOeCnjGdXhc`׉ 7cassandra://Vbx_CRKH7CswoYQDJ83Qisy2hKGFK1nIGLLYn3GctC8^ `s׉ 7cassandra://c5Pt0zg5HCf-PK6oXvk4ikpzeSXwUVmg3takSZ095dMb`׉ 7cassandra://QZX73FrbnhofylmYx8IKBJeIc6u3UI9J_zehE4z1bVAA!D͠^cZEbvט#u׉׉ 7cassandra://1RLkUYd-rM7c-pCCgBlIZAwMJQ5s6uyYdxFMKK0ggL8 d`׉ 7cassandra://JrxLc563xQUau48foPkaGeu7MHb5TE0tOUiVXZHjvYU0^`s׉ 7cassandra://dB7nc5pV2qv4AWCZ5w6wMEcwF9ConOeaKaVhXUxSREs`׉ 7cassandra://CDPY8IG0FXOJ2I-lDV0vH4blr9YaGouVQHKuKzCDGMc; ͠^cZEbw׉E %Lathes and Lathe Operations 719 as eight tools. To produce this particular shape, the upper turret is programmed in such a manner that its radial movement is synchronized with the shaft rotation (Fig. 23.13b). The spindle rotation is monitored directly, a processor performs a high-speed calculation, and the CNC then issues a command to the cam turret in terms of that angle. The machine has absoluteposition feedback, using a high-accuracy scale system. The CNC compares the actual value with the one commanded, then performs an automatic compensation, using a built-in learning function. The turret has a lightweight design for smooth operation, which also reduces inertial forces. The shaft can be made of aluminum or stainless steel. The machining parameters for aluminum are given in Table 23.8 (see Part (a) in the first column of the table). These parameters may be compared with the data given in Table 23.4, which has only a broad and approximate range as a guideline. The inserts were a K10 (C3) uncoated carbide, with a compacted polycrystalline diamond (see Fig. 22.10). The OD machining in the table shown refers to the two straight cylindrical ends of the part. The total machining time for an aluminum shaft was 24 min; for stainless steel, it was 55 min, because the cutting speed for stainless steel is considerably lower than that for aluminum. 2. Crankshaft (Fig. 23.13c). This part is made of ductile (nodular) cast iron; the machining parameters are shown in Part (b) of Table 23.8. The insert was K10 carbide. The machining time was 25 min, which is of the same order of magnitude as that for the pump shaft described above. 3. Tubular part with internal rope threads (Fig. 23.13d). This part, made of 304 stainless steel, was machined under the conditions given for Part (c) in Table 23.8. The starting blank was a straight tubular piece, similar to a bushing. The cutting tools were coated carbide and cermet. The boring bar was made of tungsten carbide, for increased stiffness, and, thus, improved dimensional accuracy and surface finish. For the threaded portion of the part, the dimensional accuracy was ±0.05 mm, with a surface finish of Ra =2.5 μm. Machining time for this part was 1.5 min, much shorter than those for the previous two parts. The reason is that (a) this part is shorter, (b) less material is removed, (c) it does not have the eccentricity features of the first two parts, so the radial movement of the cutting tool is not a function of the angular position of the part, and (d) the cutting speed is higher. Source: Based on technical literature supplied by Okuma Corp. 87.9 mm 67.4 mm 98.4 mm 85.7 mm 12 threads per cm 78.5 mm Material: Titanium alloy Number of tools: 7 Total machining time (two operations): 5.25 minutes (a) Housing base Material: 52100 alloy steel Number of tools: 4 Total machining time (two operations): 6.32 minutes (b) Inner bearing race Figure 23.12: Typical parts made on CNC lathes. Material: 1020 Carbon Steel Number of tools: 8 Total machining time (two operations): 5.41 minutes (c) Tube reducer 235.6 mm 50.8 mm 53.2 mm 23.8 mm ׉ 7cassandra://c5Pt0zg5HCf-PK6oXvk4ikpzeSXwUVmg3takSZ095dMb`cZ%Eb׉E720 Chapter 23 Machining Processes: Turning and Hole Making Lead 100 Cam turret 524 250 mm Turning turret (a) (b) Pitch: 12.7 30 4 50 160 mm (c) 75 mm (d) Figure 23.13: Examples of complex shapes that can be produced on a CNC lathe. Operation Table 23.8: Machining Summary for Example 23.3. Speed (rpm) Cutting speed Depth of cut Feed Parts a and b: Outer diameter (OD) Roughing Finishing Lead Roughing Finishing Part c: Eccentric shaft Roughing Finishing Part d: Internal thread Roughing Finishing 300 300 200 200 800 800 45 45 5–11 5–11 70 70 3 0.1 1.5 0.1 1.6 0.1 0.15 0.2 0.05 0.15 0.15 K10 0.15 Diamond compact K10 (C3) K10 (C3) Coated carbide Cermet 1150 1750 160 m/min 250 3 mm 0.3 mm/rev 0.2 0.15 Tool K10 (C3) K10 (C3) ׉ 7cassandra://dB7nc5pV2qv4AWCZ5w6wMEcwF9ConOeaKaVhXUxSREs`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://m_Mk1NsG1ykHtjOpjWBIYX9wP8DGaLDcG8-P340sQ_4`׉ 7cassandra://hRwtokL60BNsiuJWpaIfWUQFIxHmJDrLN8ZKVeUVGFoQ`s׉ 7cassandra://ByIe8OwM8aZ0NMyP1-kM2JJyFRj_L0L1dujXmupzl3o`׉ 7cassandra://_d6dGC_hpS4J53b6sAxZKMrCABchZLBIfhldA5lOv5McL͠^cZEbzט#u׉׉ 7cassandra://jEw23vImh2rmmvfqXV6w_vg-7SiJzYQHHTyoPO4ObQ8P`׉ 7cassandra://KiYllFvWnqBni0cQgZwtpgMv3bsEcRBYGv-eSPYXihgQ`s׉ 7cassandra://rz4jNrh6k1AvV1PLJOZzJKORxae4ZDeg0V4HwxcdJnkL`׉ 7cassandra://Bo2fLUfxCuMHzJ3VArSOH63HNumg4bk-HMR6PSXWNMUͅ\͠^cZEb|׉E Lathes and Lathe Operations 721 23.3.5 Turning-process Capabilities Relative production rates in turning, as well as in other machining operations described in the rest of this chapter and in Chapter 24, are shown in Table 23.8. These rates have an important bearing on productivity in machining operations. Note that there are major differences in production rates among the processes listed. The differences are due not only to the inherent characteristics of the processes and machine tools, but also to various factors, such as the setup times and the types and sizes of the workpieces to be machined. The ratings given in Table 23.9 are relative, and there can be significant variations in special applications. For example, heat-treated, high-carbon cast-steel rolls for rolling mills can be machined on special lathes, using cermet tools and at material-removal rates as high as 6000 cm3/min. Also called highremoval-rate machining, the process has at least two important requirements: (a) very high machine-tool rigidity, to avoid chatter and associated tool breakage, and (b) high power, of up to 450 kW. The surface finish (Fig. 23.14) and dimensional accuracy (Fig. 23.15) obtained in turning and related operations depend on several factors: the characteristics and condition of the machine tool, stiffness, vibration and chatter, processing parameters, tool geometry, tool wear, cutting fluids, machinability of the workpiece material, and, when applicable, operator skill. A wide range of surface finishes can be obtained, as shown in Fig. 23.14 (see also Fig. 33.5). 23.3.6 Design Considerations and Guidelines for Turning Operations Several considerations are important in designing parts to be machined economically by turning operations. Machining, in general, should be avoided whenever possible, because: 1. Machining can take considerable time, thus increasing production costs 2. Material and latent material energy are wasted (see Section 40.5), even though chips can be recycled 3. Economic considerations are paramount, and it may be more economical to produce a component through shaping operations. Table 23.9: Typical Production Rates for Various Machining Operations. Operation Rate Turning Engine lathe Tracer lathe Turret lathe Computer-controlled lathe Single-spindle chuckers Multiple-spindle chuckers Boring Drilling Milling Planing Gear cutting Broaching Sawing Very low to low Low to medium Low to medium Low to medium Medium to high High to very high Very low Low to medium Low to medium Very low Low to medium Medium to high Very low to low Note: Production rates indicated are relative: Very low is about 1ormorepartsper hour, medium is approximately 100 parts per hour, and very high is 1000 or more parts per hour. ׉ 7cassandra://ByIe8OwM8aZ0NMyP1-kM2JJyFRj_L0L1dujXmupzl3o`cZ%Eb׉Eg722 Chapter 23 Machining Processes: Turning and Hole Making Process Rough cutting Flame cutting Snagging (coarse grinding) Sawing Casting Sand casting Die casting Hot rolling Forging Extruding Cold rolling, drawing Roller burnishing Machining Planing, shaping Milling Broaching Reaming Turning, boring Drilling Advanced machining Chemical machining Electrical-discharge machining Electron-beam machining Laser machining Electrochemical machining Finishing processes Honing Barrel finishing Electrochemical grinding Grinding Electropolishing Polishing Lapping Superfinishing Figure 23.14: The range of surface roughnesses obtained in various processes; note the wide range within each group, especially in turning and boring. The following general design guidelines should be considered for machining: 1. Parts should be designed so that they can be fixtured and clamped easily into workholding devices. Thin, slender workpieces are difficult to support properly and must be able to withstand clamping and cutting forces (see also flexible fixturing, Section 37.8). 2. The dimensional accuracy and surface finish specified should be as wide as permissible, but the part must still be able to function properly. Permanent mold casting Investment casting Forming mm 50 25 12.5 6.3 3.2 1.6 0.8 0.40 0.20 0.10 0.05 0.025 0.012 Roughness (Ra) Average application Less frequent application ׉ 7cassandra://rz4jNrh6k1AvV1PLJOZzJKORxae4ZDeg0V4HwxcdJnkL`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://8UZuZnGOXDqbOFrX3b3OmglGdvIq4ggxRrp32wsEJgU`׉ 7cassandra://6Tbuiyue2zlT5TPKOcC44JR3MYjn6MVmhT_75BWyGxcB`s׉ 7cassandra://w6wVYELZbFENKkxq471RCvLqZh65eoiiwVLiHdQsv-8t`׉ 7cassandra://4wfx9FupM9Pq73NfG7JST_YQMGuPYO3Rbxzm_q_YQ_oQKT͠^cZEb~ט#u׉׉ 7cassandra://mzVEAEzglmpER-YiIamxhog_AHlqZOXlQ60x6ffdWSM`׉ 7cassandra://mjQ2VG3zl58v8Bq-zTeWCXx5-Uq8jn0NSY_37-kdFww`d`s׉ 7cassandra://PjGNQjxf8ODv0GpbgZ2d0GW9BMR-68N_SUe9F2Ms26o`׉ 7cassandra://kbjedPAGUpvXtLh7qm3hSFnQKSvHoMilumxL6U-mYEAiq$͠^cZEb׉ELathes and Lathe Operations 723 10 20 1 0.2 0.1 5 10 20 100 Diameter or length (mm) Figure 23.15: Range of dimensional tolerances in various machining processes as a function of workpiece size. Note that there is one order of magnitude difference between small and large workpieces. 3. Sharp corners, tapers, steps, and major dimensional variations in the part should be avoided. 4. Blanks to be machined should be as close to final part dimensions as possible, such as by near-netshape forming, so as to reduce production cycle time. 5. Parts should be designed so that cutting tools can travel directly across the workpiece without any obstruction. 6. Design features should use commercially available, standard cutting tools, inserts, and toolholders whenever practicable. 7. Workpiece materials should preferably be selected for their machinability (Section 21.7). Guidelines for Turning Operations. The following list outlines generally accepted guidelines for turning operations; see also Table 23.10 for probable causes of turning problems. 1. Minimize tool overhang 2. Support the workpiece rigidly 3. Use machine tools with high stiffness and high damping capacity 4. When tools begin to vibrate and chatter (Section 25.4), modify one or more of the process parameters, such as tool geometry, cutting speed, feed rate, depth of cut, and use of cutting fluid (see also adaptive control, Section 37.4, and stability lobes, Section 25.4). 250 Tolerance (mm) Drilling, rough turning, and boring (center lathe) Milling, planing, slotting, and finish turning (center lathe) Turret lathe (turning and boring) Bar and chuck automatics (turning and boring) Reaming and broaching Grinding Honing and lapping Fine turning and boring ׉ 7cassandra://w6wVYELZbFENKkxq471RCvLqZh65eoiiwVLiHdQsv-8t`cZ%Eb׉E 724 Chapter 23 Machining Processes: Turning and Hole Making Problem Tool breakage Excessive tool wear Rough surface finish Table 23.10: General Troubleshooting Guide for Turning Operations. Probable causes Tool material lacks toughness, improper tool angles, machine tool lacks stiffness, worn bearings and machine components, machining parameters too high Machining parameters too high, improper tool material, ineffective cutting fluid, improper tool angles Built-up edge on tool; feed too high; tool too sharp, chipped, or worn; vibration and chatter Dimensional variability Lack of stiffness of machine tool and work-holding devices, excessive temperature rise, tool wear Tool chatter Lack of stiffness of machine tool and work-holding devices, excessive tool overhang, machining parameters not set properly 23.3.7 Chip Collection Systems The chips produced in machining operations must be collected and disposed of properly. The volume of chips produced can be very high, particularly in ultra-high-speed machining and high-removal-rate machining operations. For example, in a drilling operation on steel during which only 15 cm3 of metal is removed, the loose bulk volume of the chips can, depending on chip type (see Section 21.2.1), be in the range of 600 to 12,000 cm3. Likewise, the milling of 15 cm3 of steel produces 450 to 700 cm3 of chips, while cast iron produces 105 to 225 cm3 of chips. Also called chip management, the operation involves collecting chips from their source in an efficient manner and removing them from the work area. Long and stringy chips are more difficult to collect than short chips (produced by using tools with chipbreaker features; see Figs. 21.7 and 22.2). The type of chip produced must therefore be an integral aspect of the chip-collecting system. Chips can be collected by any of the following methods: • Using gravity, dropping them directly onto a steel conveyor belt • Dragging the chips from a settling tank • Using augers with feed screws • Using magnetic conveyors, though for ferrous chips only • Employing vacuum methods of chip removal and collection. Modern machine tools are designed with automated chip-handling features. There may be a considerable amount of cutting fluid residue on the chips produced. The cutting fluid and sludge can be separated from chips using chip wringers (centrifuges). Chip-processing systems usually require considerable floor space in a plant, and can cost from $60,000 for small shops to over $1 million for large facilities. Collected chips may be recycled to reduce material and energy burden of machining (see Section 40.5). Prior to their removal from a manufacturing plant, the large volume of chips can be reduced to as little as one-fifth of their loose volume by compacting them into briquettes or by shredding them. Dry chips are more valuable for recycling, because of reduced environmental contamination. The method chosen for chip disposal depends on economics and on compliance with local, state, and federal regulations. 23.3.8 Thread Cutting Screw threads are typically on the outside or inside of a cylindrical piece, and they may be a (straight thread) or a (tapered thread). Machine screws, bolts, and nuts have straight threads, as do threaded rods for such applications as the lead screw in lathes and a wide variety of machinery components (Fig. 23.2). Tapered threads are commonly used for water or gas pipes and plumbing supplies. Threads may be right handed or left handed and can have various profiles. ׉ 7cassandra://PjGNQjxf8ODv0GpbgZ2d0GW9BMR-68N_SUe9F2Ms26o`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://CQY0ei-ZRBhzbfddpyFv7OBQjFcWsCiN5wUSG2u46Qo9`׉ 7cassandra://f_uvZigRFZUorJfetvHOPzBUgSJBwA7Z3D40OBiE5wUZ[`s׉ 7cassandra://pesHyIoe8hqJhwWlHl4SPlrclPDVvsClVH_SubChNGAv`׉ 7cassandra://iRAT5V0xpxYX3i0f0cevBlSbgZE-Ow-5mUlA6cOS8EA"͠^cZEb́ט#u׉׉ 7cassandra://Q3UHXm0QsRR3pTOYT1f806dHSRpBxhw-L23GP2DvM9kq`׉ 7cassandra://WasvYZOXv_hbMCzbEji2rHfKQA1QUS17M-KtUUbV5t0[`s׉ 7cassandra://7-lyDGoqIhPole-EmN3vDGIUJHQXouwx97plG0upkDA`׉ 7cassandra://KifOqrCYwSRM73eaRPA2ULEi8pGhA2_mhPZJn2HTjD8U3:4͠^cZEb͂׉E Lathes and Lathe Operations 725 Although threads traditionally have been machined, they are now increasingly made by thread rolling (Section 13.5). Rolled fastener threads constitute the largest quantity of external threaded parts produced. Threads can also be integrally cast or molded, but there are limitations to their dimensional accuracy, surface finish, and minimum dimensions. Threads can be machined either externally or internally, in a process called thread cutting or threading. External threads may also be cut with a die or by milling. Internal threads are typically produced by tapping, using a tap (Section 23.7). Threads may subsequently be ground, with high dimensional accuracy and surface finish, for such applications as power screw drives in machines. Screw-thread Cutting on a Lathe. A typical thread-cutting operation on a lathe is shown in Fig. 23.16a. The cutting tool, the shape of which depends on the type of thread to be cut, is mounted on a holder and moved along the length of the workpiece by the lathe’s lead screw. This movement is achieved by the engagement of a split nut, also called a halfnut, inside the apron of the lathe (see Fig. 23.2). The axial movement of the tool in relation to the workpiece rotation determines the lead of the screw thread (i.e., the axial distance moved in one complete revolution of the screw). For a fixed spindle speed, the slower the tool movement, the finer will be the thread. The cutting tool may be fed radially into the workpiece, thus cutting both sides of the thread at the same time, as in form cutting described earlier. However, this method usually produces a poor surface finish. Workpiece First cut (a) Finished thread Tool Radial Flank (b) Chuck Workpiece Tool Incremental (c) (d) Figure 23.16: (a) Cutting screw threads on a lathe with a single-point cutting tool. (b) Cutting screw threads with a single-point tool in several passes, normally utilized for large threads. The small arrows in the figures show the direction of feed, and the broken lines show the position of the cutting tool as time progresses. In radial cutting, the tool is fed directly into the workpiece. In flank cutting, the tool is fed into the piece along the right face of the thread. In incremental cutting, the tool is fed first directly into the piece at the center of the thread, then at its sides, and finally into the root. (c) A typical coated-carbide insert in the process of cutting screw threads on a round shaft. (d) Cutting internal screw threads with a carbide insert. Source: (c) Courtesy of Iscar Metals, Inc. ׉ 7cassandra://pesHyIoe8hqJhwWlHl4SPlrclPDVvsClVH_SubChNGAv`cZ%Eb׉E 726 Chapter 23 Machining Processes: Turning and Hole Making Die Workpiece Workpiece Circular chaser Cutting edge Cutting edge (a) (b) (c) Figure 23.17: (a) Straight chasers for cutting threads on a lathe. (b) Circular chasers. (c) A solid threading die; the screw adjusts the gap. A number of passes, in the sequence shown in Fig. 23.16b, generally are required to produce threads with good dimensional accuracy and surface finish. Figure 23.16c shows a carbide insert for screw-thread cutting (threading insert) for machining threads on a round shaft. Figure 23.16d shows an internal screwthread cutting process. Except for small production runs, thread cutting largely has been replaced by other methods, such as thread rolling, automatic screw machining, and using CNC lathes. The production rate in cutting screw threads can be increased with tools called die-head chasers (Fig. 23.17a and b), which typically have four cutters with multiple teeth and can be adjusted radially. After the threads are cut, the cutters open automatically (thus the alternative name self-opening die heads)by rotating them around their axes to allow the part to be removed. Solid-threading dies (Fig. 23.17c) also are available for cutting straight or tapered screw threads. Design Considerations for Screw Thread Machining. The design considerations to be taken into account to produce high-quality and economical screw threads are the following: • Rolled threads are generally preferable to cut threads; whenever practical, thread cutting should be avoided. • Designs should allow for the termination of threads before they reach a shoulder on the part. • Through-holes are preferable to blind holes when machining threads. The term blind hole refers to a hole that does not go through the thickness of the workpiece (see Fig. 23.1i). Internal threads in blind holes should have an unthreaded length at the bottom. • Shallow, blind tapped holes should be avoided. • Chamfers should be specified at the ends of threaded sections, to minimize finlike threads with burrs. • Threaded sections should not be interrupted with slots, holes, or other discontinuities. • Standard threading tooling and inserts should be used as much as possible. • Thin-walled parts should have sufficient thickness and strength to resist clamping and cutting forces. A common rule of thumb is that the minimum engagement length of a fastener should be 1.5 times its diameter. • Parts should be designed so that all cutting operations can be completed in one setup. ׉ 7cassandra://7-lyDGoqIhPole-EmN3vDGIUJHQXouwx97plG0upkDA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://RacDFOV15j1A60gTLabaARdR1-Oe5BgWUrjCyrXLucki `׉ 7cassandra://KeOALDUpl6XJmabFAyoCeVKSWdPD3HAihC16CEAYLqAT`s׉ 7cassandra://o5M7gjl7bCzm5Qx9tHrRoXahkivcv4UKPGlfDcVSpeA`׉ 7cassandra://JdtWU2xrlE-T8fhaTHGVnzy8yjPD-Z-rfc7QwYVl-qQ&͠^cZEb̈́ט#u׉׉ 7cassandra://-haJxE57bnsr-YbRzVR1-CoUItsjqeFl_41n37WNXcUr`׉ 7cassandra://71UuilokN3xbzsLF-yOTdx_6_KkkjgQG92prUzCIm80ol`s׉ 7cassandra://Zg7CfbsJKfuh64sNQM4CRvw_deIzeft2fCC-KT-ukgUv`׉ 7cassandra://mJPy02w7Kfb42I3Fc0zvJcqt5xixwg21WcnqEOS2hAso@͠^cZEbͅ׉E @Boring and Boring Machines 727 Steel or Insert carbide shank Coolant Tungsten-alloy disks (a) (b) Figure 23.18: (a) Schematic illustration of a steel boring bar with a carbide insert; note the passageway in the bar for cutting fluid application. (b) Schematic illustration of a boring bar with tungsten-alloy inertia disks, sealed within the bar to counteract vibration and chatter while boring. This system has been found to be effective for boring-bar length-to-diameter ratios of up to 6. 23.4 Boring and Boring Machines Boring enlarges a hole previously made by another process, or it produces circular internal profiles in hollow workpieces (Fig. 23.1h). The cutting tools are similar to those used in turning, and are mounted on a boring bar (Fig. 23.18a) in order to reach the full length of the bore. It is essential that the boring bar be sufficiently stiff to minimize tool deflection and vibrations, thus maintaining dimensional accuracy and surface finish. For this reason, a material with a high elastic modulus, such as tungsten carbide, is desirable. Boring bars have been designed and built with capabilities for damping vibration (Fig. 23.18b). Boring operations on relatively small workpieces can be carried out on lathes, whereas large workpieces are machined on boring mills. These machine tools are either horizontal or vertical, and are capable of performing such operations as turning, facing, grooving, and chamfering. In horizontal boring machines, the workpiece is mounted on a table that can move horizontally, in both the axial and radial directions. The cutting tool is mounted on a spindle that rotates in the headstock, and is capable of both vertical and longitudinal movements. Drills, reamers, taps, and milling cutters also can be mounted on the machine spindle. A vertical boring mill (Fig. 23.19) is similar to a lathe, has a vertical axis of workpiece rotation, and can accommodate workpieces as large as 2.5 m in diameter. The cutting tool is usually a single point, made of M2 or M3 high-speed steel, or P10 (C7) or P01 (C8) carbide. It is mounted on the tool head, which is capable of vertical movement (for boring and turning) and Cross-rail Tool head Workpiece Work table Bed Column Figure 23.19: Schematic illustration of a vertical boring mill. Such a machine can accommodate workpiece sizes as large as 2.5 m in diameter. ׉ 7cassandra://o5M7gjl7bCzm5Qx9tHrRoXahkivcv4UKPGlfDcVSpeA`cZ%Eb׉E728 Chapter 23 Machining Processes: Turning and Hole Making radial movement (for facing), guided by the cross-rail. The head can be swiveled to make conical (tapered) holes. Cutting speeds and feeds for boring are similar to those for turning (see Table 23.9). Boring machines are available with a variety of features. Machine capacities range up to 150 kW, and are available with CNC, allowing all movements of the machine to be programmed. Design Considerations for Boring. Guidelines for economical boring operations are similar to those for turning; additionally, the following factors should be considered: • Whenever possible, through holes rather than blind holes should be specified. • The greater the length-to-bore-diameter ratio, the more difficult it is to hold dimensions, due to the deflections of the boring bar under cutting forces and the greater tendency for vibration and chatter. • Interrupted internal surfaces, such as internal splines or radial holes that go through the thickness of the part, should be avoided. 23.5 Drilling, Drills, and Drilling Machines Hole making is among the most important operations in manufacturing, and drilling is a common holemaking process. The cost of hole making, for example, is among the highest machining costs in automotive engine manufacturing. Other basic processes for making holes are punching (Section 16.2) and a variety of advanced machining processes (Chapter 27). 23.5.1 Drills Drills typically have high length-to-diameter ratios (Fig. 23.20), hence they are capable of producing relatively deep holes. However, high ratios make drills somewhat flexible and prone to fracture or making inaccurate holes. Moreover, the chips produced can present significant difficulties in their disposal from the hole being drilled. Drills generally leave a burr on the bottom surface of the part upon their breakthrough, often necessitating subsequent deburring operations (Section 26.8). Also, because of its rotary motion, drilling produces holes with walls with circumferential marks; in contrast, punched holes have longitudinal marks (see Fig. 16.5a). This difference can be significant in terms of the hole’s fatigue properties (see Section 33.2). The diameter of a hole produced by drilling is slightly larger than the drill diameter (oversize), as one can note by observing that a drill can easily be removed from the hole it has just produced, assuming temperature effects are not present. Hole oversize depends on the quality of the drill, the equipment used, and on the machining practices employed. Depending on their thermal properties, some metals and nonmetallic materials expand significantly due to the heat produced during drilling, thus the final hole diameter could be smaller than the drill diameter after the part cools down. For better surface finish and dimensional accuracy, drilled holes may be subjected to subsequent reaming and honing. The capabilities of drilling and boring operations are shown in Table 23.11. Twist Drill. The most common drill is the conventional standard-point twist drill (Fig. 23.20a). The geometry of the drill point is such that the normal rake angle and velocity of the cutting edge vary with the distance from the center of the drill. The main features of this drill are, with typical ranges given in parentheses: (a) point angle (118◦ to 135◦), (b) lip-relief angle (7◦ to 15◦), (c) chisel-edge angle (125◦ to 135◦), and (d) helix angle (15◦ to 30◦). Two spiral grooves, called flutes, run the length of the drill, and the chips produced are guided upward through these grooves. The grooves also serve as passageways to enable the cutting fluid to reach the cutting edges. Some drills have internal longitudinal holes (see Fig. 23.23a), through which cutting fluids are forced, thus improving lubrication and cooling and washing away the chips. Drills are available with a chip-breaker feature on the cutting edges. This feature is important in automated machinery, where continuous removal of long chips without operator assistance is essential. ׉ 7cassandra://Zg7CfbsJKfuh64sNQM4CRvw_deIzeft2fCC-KT-ukgUv`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://m5ECm4Gthq2NILM7Un5PQabaHWp-cO_K8OOUcR4YPDQ6D`׉ 7cassandra://y5fByjwA6YJndlxwmAiaE8yOGTcogOjzbNFhebEOhk4C`s׉ 7cassandra://bbdZTiUJ_9qnTq6hZYrkC1HfkSQHf3O7GpgNPZFjPYc`׉ 7cassandra://MVR8S_jq9MTNAvS56_asoA93CJZan296pol4iGGVoiUO8͠^cZEb͈ט#u׉׉ 7cassandra://WOcc5yMEQ57V15CsCphBm-QDmXrO1-A0AY3PjrVQXEAtc`׉ 7cassandra://AiebreC6HRv2idlrQoQMX66UxgyX6bQrAiu_r06AnM4D`s׉ 7cassandra://wZiCu2pi14euk5dw1E-HK3phrC6AdKQs3lJ-N0eRmNQ`׉ 7cassandra://HutqTzNq8XOAeYLTfdF4aDY5bR3eKSzuhBdp8Jx9cxk$͠^cZEb͌׉EDrilling, Drills, and Drilling Machines 729 Chisel-edge drill Tang Taper shank Point angle Drill diameter Tang drive Flutes Helix angle Neck Shank diameter Straight shank Shank length Flute length Body Overall length (a) Crankshaft-point drill Margin Lip Lip-relief angle Body diameter clearance Chisel-edge angle Clearance diameter Web Chisel edge (b) Figure 23.20: Two common types of drills: (a) Chisel-edge drill. The function of the pair of margins is to provide a bearing surface for the drill against walls of the hole as it penetrates the workpiece. Drills with four margins (double-margin) are available for improved guidance and accuracy. Drills can have chip-breaker features. (b) Crankshaft drill. These drills have good centering ability, and because the chips tend to break up easily, crankshaft drills are suitable for producing deep holes. The various angles on a drill have been designed to produce accurate holes, minimize drilling forces and torque, and optimize drill life. Small changes in drill geometry can have a significant effect on a drill’s performance, particularly in the chisel-edge region, which accounts for about 50% of the thrust force in drilling. Too small a lip relief angle (Fig. 23.20a) increases the thrust force, generates excessive heat, and increases drill wear. By contrast, too large an angle can cause chipping or breaking of the cutting edge. Table 23.11: General Capabilities of Drilling and Boring Operations. Hole depth/diameter Cutting tool Twist drill Spade drill Gun drill Trepanning tool Boring tool Diameter range (mm) Typical Maximum 0.5–150 8 25–150 2–50 40–250 3–1200 30 100 10 5 50 100 300 100 8 Land ׉ 7cassandra://bbdZTiUJ_9qnTq6hZYrkC1HfkSQHf3O7GpgNPZFjPYc`cZ%Eb׉E730 Chapter 23 Machining Processes: Turning and Hole Making High-pressure coolant Figure 23.21: Various types of drills and drilling and reaming operations. Several other drill-point geometries have been developed to improve drill performance and increase the penetration rate required for high rate production. Special grinding techniques and equipment are used to produce these geometries. Other Types of Drills. Several types of drills are shown in Fig. 23.21. A step drill produces a hole with two or more different diameters. A core drill is used to make an existing hole larger. Counterboring and countersinking drills produce depressions on the workpiece surface to accommodate the heads of screws and bolts below the surface. A center drill is short and is used to produce a hole at one end of a round stock, so that it can be mounted between the centers of the headstock and the tailstock on a lathe (Fig. 23.2). A spot drill is used to start a hole at the desired location on a surface. Spade drills (Fig. 23.22a) have removable tips or bits, and are used to produce large-diameter deep holes. Because of the absence of flutes in the body of the drill, these drills have the advantages of higher stiffness, ease of grinding the cutting edges, and lower cost. A similar drill is the straight-flute drill (Fig. 23.22b). (a) (b) Carbide insert Braze Carbide inserts (c) (d) Figure 23.22: Various types of drills. (a) Spade drill; (b) straight-flute drill; (c) drill with indexable carbide inserts; (d) drill with brazed-carbide tip. Drill body (low-alloy steel) Drilling Core drilling Step drilling Counterboring Countersinking Reaming Center drilling Gun drilling ׉ 7cassandra://wZiCu2pi14euk5dw1E-HK3phrC6AdKQs3lJ-N0eRmNQ`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://S-30OPPzuesWMMrDCc6zDzHrUWy1gt1QyjCwz2hVc30`׉ 7cassandra://Spggp5KmmmS_zzRKJlVzXqA5xDOFvh3jslcXzMd9UHMJ`s׉ 7cassandra://y8byYWPywpiOFo0ljHr5TB4DyhQrpJ7RQGT8uhV_IxU`׉ 7cassandra://9_U02AaE3IPRg8M04_-mArGRlyQGZ8gk9-HYxPoUvWUͪ͠^cZEb͏ט#u׉׉ 7cassandra://BXzKI0ALGk9qpBBz1aJmMDRT2F2zeHihVVkS7CxPa8Q`׉ 7cassandra://tilHBud0UJz7eoOAHlMn4AD_1l-CAOfgPkHTMFD8GowR\`s׉ 7cassandra://jUNBRDY-pXejfnBJ2AFAB6xVqcIuCn6kKfhEm9fUBW8O`׉ 7cassandra://2qDwMsgXlUBU4RdFYgRxW9hRNe2j6qQGmndleVYTSZY |͠^cZEb͐נcZEb͒]B]9ׁHhttp://D.Thׁ ׁ Ј׉E Drilling, Drills, and Drilling Machines 731 Solid carbide and carbide-tipped drills (Fig. 23.22c and d) are made for drilling hard materials, such as cast irons, high-temperature metals, abrasive materials such as concrete and brick (called masonry drills), and composite materials containing abrasive fiber reinforcements, such as glass and graphite. Gun Drilling. Gun drilling is used for drilling deep holes; it requires a special drill, as shown in Fig. 23.23. The depth-to-diameter ratios of holes produced can be more than 300:1. Thrust force (the radial force that tends to deflect the drill sideways) is balanced by bearing pads on the drill that slide along the inside surface of the hole. Consequently, a gun drill is self-centering, an important feature in drilling straight, deep holes. A variation of this process is gun trepanning (described below), which uses a cutting tool similar to a gun drill, except that the tool has a central hole. Cutting speeds in gun drilling are usually high, and feeds are low, and tolerances are typically 0.025 mm. The cutting fluid is forced, under high pressure, through a longitudinal hole in the body of the drill (Fig. 23.22a). In addition to cooling and lubricating the workpiece, the fluid flushes out chips that otherwise would be trapped in the deep hole being drilled. Trepanning. In trepanning (from the Greek trypanon, meaning boring a hole). The cutting tool (Fig. 23.24a) produces a hole by removing a disk-shaped piece (core), usually from flat plates. A hole is thus produced without reducing all of the material to chips, as is the case in drilling. The trepanning process can be used to make disks up to 250 mm in diameter, from flat sheets, plates, or structural members such as I-beams. It also can be used to make circular grooves as seats for O-rings (similar to Fig. 23.1f). Trepanning can be carried out on lathes, drill presses, or other machine tools, using single-point or multipoint tools, as shown in Fig. 23.24b. Outer point angle Margin Flute Cutting-fluid passage Wear pads Inner point angle (a) Cutting-fluid feed line Rotating cutting-fluid transfer gland Spindle Drill Bed (b) Figure 23.23: (a) A gun drill, showing various features. (b) Schematic illustration of the gun-drilling operation. Feed carriage Workpiece Drill bushing ׉ 7cassandra://y8byYWPywpiOFo0ljHr5TB4DyhQrpJ7RQGT8uhV_IxU`cZ%Eb׉Ey732 Chapter 23 Machining Processes: Turning and Hole Making Shank Cutting tools Drill Tool Workpiece (a) (b) Figure 23.24: (a) Trepanning tool. (b) Trepanning with a drill-mounted single cutter. 23.5.2 Material-removal Rate in Drilling The material-removal rate (MRR) in drilling is the volume of material removed per unit time; thus for a drill with diameter D, the cross-sectional area of the drilled hole is πD2/4. The velocity of the drill perpendicular to the workpiece is the product of the feed, f (the distance the drill penetrates per unit revolution), and the rotational speed, N, where N = V/πD.Thus, MRR = 23.5.3 Thrust Force and Torque The thrust force in drilling acts perpendicular to the hole axis; if this force is excessive, it can cause the drill to bend or break. An excessive thrust force also can distort the workpiece itself, particularly if it does not have sufficient stiffness, as is the case for thin sheet-metal structures (Chapter 16). The thrust force depends on factors such as (a) the strength of the workpiece material, (b) feed, (c) rotational speed, (d) drill diameter, (e) drill geometry, and (f) cutting fluids. Forces typically range from a few newtons, for small drills, to as high as 100 kN, for drilling high-strength materials using large drills. Torque. The torque in drilling, essential for estimating the power requirement, is difficult to calculate because of the many factors involved. It can be estimated from the data given in Table 21.2 by noting that the power dissipated during drilling is the product of the torque and the rotational speed. It is also equal the product of specific energy and material removal rate. Torque in drilling can be as high as 4000 N-m. Example 23.4 Material-removal Rate and Torque in Drilling Given: A hole is being drilled in a block of magnesium alloy with a 10-mm drill bit at a feed of 0.2 mm/rev and with the spindle running at N = 800 rpm. Find: Calculate the material-removal rate and the torque on the drill. Solution: The material-removal rate is calculated from Eq. (23.4): MRR =  (π)(10)2 4  (0.2)(800) = 12, 570 mm3/min = 210 mm3/s.  πD2 4  fN. (23.4) Spindle ׉ 7cassandra://jUNBRDY-pXejfnBJ2AFAB6xVqcIuCn6kKfhEm9fUBW8O`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://IdTM8l9HYC44GOJu1y71HYGv_C6_oGYrc5eSjBW9JXQ@Q`׉ 7cassandra://_uWTVj569d_NWXR2A-kxuRUfLJh31N7BsIJN689dpswW`s׉ 7cassandra://CIEOYfvkkSjozCfMpxQItc7sRan-wvDHrsDD4tBw4xQK`׉ 7cassandra://lIwBQ4aLUTkwc5KVhlECwvN3lckofaiL5tMwK-O7ggwp4͠^cZEb͓ט#u׉׉ 7cassandra://0f_eVAwMZYWVr6E4FxKFMbW4PAx834AlVYST1XadCMAuk`׉ 7cassandra://cUqbDsxArUsmksp6EB5iO1_DvKe1X5JPv17cgtejttsa`s׉ 7cassandra://EgwxTO9ppbkEa_EV6YNOGOCEsUP5k78RwwEkz5vhChY`׉ 7cassandra://v67ZFao5vbaK4mj-2k5Z9r-DrgUw6f657lgy0Te04Xsrk͠^cZEb͔׉E Drilling, Drills, and Drilling Machines 733 Referring to Table 21.2, an average unit power of 0.5 W-s/mm3 is used for magnesium alloys. The power required is then Power = (210)(0.5) = 105 W. Power is the product of the torque on the drill and the rotational speed, which in this case is (800)(2π)60 = 83.8 radians per second. Noting that W = J/s and J = N-m, T = 105 83.8 23.5.4 Drill Materials and Sizes Drills are usually made of high-speed steels (M1, M7, and M10), solid carbides, or with carbide tips [typically made of K20 (C2) carbide] and brazed over a steel shank (Fig. 23.22c and d). Drills are commonly coated with titanium nitride or titanium carbonitride for increased wear resistance (see Section 22.5). Polycrystalline-diamond-coated drills are used for producing holes for fasteners in fiber-reinforced plastic structures; several thousand holes can be drilled with little damage to the workpiece. Although there are continued developments, standard twist-drill sizes consist basically of the following series: • Numerical: No. 97 (0.0059 in.) to No. 1 (0.228 in.) • Letter: A (0.234 in.) to Z (0.413 in.) • Fractional: Straight shank from and larger drills in larger increments. Taper shank from 1 1 64 to 11 1 16 -in. increments) • Millimeter: From 0.05 mm in increments of 0.01 mm. 23.5.5 Drilling Practice Drills and similar hole making tools usually are held in drill chucks, which may be tightened with keys or different mechanisms. Special chucks and collets, with various quick-change features that do not require stopping the spindle, are available for use on production machinery. Because it does not have a centering action, a drill tends to walk on the workpiece surface at the beginning of an operation, a problem particularly severe with small-diameter long drills that can bend and break. To start a hole properly, the drill bit should be guided, using fixtures such as a bushing, to keep it from excessively deflecting laterally. A small starting dimple or hole can be made with a punch or center drill or the drill point may be ground to an S shape (called helical or spiral point). This shape has a selfcentering characteristic, thus eliminating the need for center drilling, and produces accurate holes and with improved drill life. These factors are particularly important in automated production with CNC machines, in which the usual practice is to use a spot drill. To keep the drill more centered, the point angles of the spot drill and of the drill are matched. Drilling Recommendations. Recommended ranges for drilling speeds and feeds are given in Table 23.12. The speed is the surface speed of the drill at its periphery; thus, a 12.7-mm drill rotating at 300 rpm has a surface speed of V =  12.7 2 mm (300 rev/min)(2π rad/rev)   1 1000 m/mm =12 m/min.  4 (in 1 =1.25 N-m. 64 -in. increments) to 11 8 to 13 4 (in 1 2 in. (in 1 32 -in. increments), 64 increments) to 3.5 in. (in ׉ 7cassandra://CIEOYfvkkSjozCfMpxQItc7sRan-wvDHrsDD4tBw4xQK`cZ%Eb׉E734 Chapter 23 Machining Processes: Turning and Hole Making Table 23.12: General Recommendations for Speeds and Feeds in Drilling. Drill diameter Workpiece material Aluminum alloys Magnesium alloys Copper alloys Steels Stainless steels Titanium alloys Cast irons Thermoplastics Thermosets Surface speed m/min 30–120 45–120 15–60 20–30 10–20 6–20 20–60 30–60 20–60 Feed, mm/rev 0.30 0.30 0.25 0.30 0.18 0.15 0.30 0.13 0.10 Speed, rpm 1.5 mm 12.5 mm 1.5 mm 0.025 0.025 0.025 0.025 0.025 0.010 0.025 0.025 0.025 12.5 mm 6400–25,000 800–3000 1100–3000 9600–25,000 3200–12,000 400–1500 4300–6400 2100–4300 1300–4300 500–800 250–500 150–500 4300–12,000 500–1500 6400–12,000 800–1500 4300–12,000 500–1500 Note: As hole depth increases, speeds and feeds should be reduced. The selection of speeds and feeds also depends on the specific surface finish required. In drilling holes smaller than 1 mm, rotational speeds can range up to 30,000 rpm, depending on the workpiece material. The feed in drilling is the distance the drill travels into the workpiece per revolution. For example, Table 23.11 recommends that, for most workpiece materials, a 1.5 mm drill should have a feed of 0.025 mm/rev. If the speed column in the table indicates that the drill should rotate at, say, 2000 rpm, then the drill should travel into the workpiece at a linear speed of (0.025 mm/rev)(2000 rev/min) = 50 mm/min. Chip removal in drilling can be difficult, especially for deep holes in soft, ductile workpieces. Chips generally are removed by being forced up the flutes, and the force and the torque can become excessive if the flutes become loaded with chips. The drill should therefore be retracted periodically (called pecking), to remove chips that may have accumulated in the flutes; otherwise, the drill may break. A general guide to the probable causes of problems in drilling operations is given in Table 23.13. Drill Reconditioning. Drills are reconditioned by grinding them either manually or with special fixtures. Proper reconditioning of drills is important, particularly with automated manufacturing on CNC machines. Hand grinding is difficult, and requires considerable skill in order to produce symmetric cutting edges. Grinding on fixtures is accurate and is done on special computer-controlled grinders. Worn and dull coated drills also can be recoated in special facilities. Measuring Drill Life. Drill life, as well as tap life (see Section 23.7), is typically measured by the number of holes drilled before they become dull and have to be reconditioned or replaced. Drill life can be determined experimentally by first clamping a block of material on a suitable dynamometer or force transducer. Then, a number of holes are drilled while the torque or thrust force is monitored; drill life is the number of holes drilled until these two quantities begin to increase, indicating that the drill is becoming dull. Monitoring vibration and acoustic emissions (Section 21.5.4) also can be used to determine drill life. Problem Drill breakage Excessive drill wear Tapered hole Oversize hole Table 23.13: General Troubleshooting Guide for Drilling Operations. Probable causes Dull drill, drill seizing in hole because of chips clogging flutes, feed too high, lip relief angle too small Cutting speed too high, ineffective cutting fluid, rake angle too high, drill burned and strength lost when drill was sharpened Drill misaligned or bent, lips not equal, web not central Same as previous entry, machine spindle loose, chisel edge not central, side force on workpiece Poor hole surface finish Dull drill, ineffective cutting fluid, welding of workpiece material on drill margin, improperly ground drill, improper alignment ׉ 7cassandra://EgwxTO9ppbkEa_EV6YNOGOCEsUP5k78RwwEkz5vhChY`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://81KiLBNm8STt7inpYmKgBvJl4WkEScAtih95grYsF68Y`׉ 7cassandra://0X9EYpFBUJTaFyl-UfNBl1D7coWCaJ2eSuE7Mts3kQkR1`s׉ 7cassandra://3cYim6vrdn9q4BiMaooVg5XHhWDNLt7OehWznuAmJr4`׉ 7cassandra://SfKsLupa9ie4E5Rt3ZPDkJY1p6_DvE0rar7oTESuFzM9͠^cZEb͖ט#u׉׉ 7cassandra://cQJPb6B2l_VRm2TijJ8INwpKaU0oJNCFb-tj8kPqOG8`׉ 7cassandra://ykMPUI946pgtLkwwQn6Fq_-k923PQB5GUnRFI8m6fZQAd`s׉ 7cassandra://4TZTUlbAkVegyx4jxPIIMLxVUoUBkT3CNYpH83XYDEA`׉ 7cassandra://uw6zwvyAh_dbMk-EdMj73275Pl6xZTt8DoWLN7AnWdkz+ ͠^cZEb͗׉E IDrilling, Drills, and Drilling Machines 735 23.5.6 Drilling Machines The most common machine is the drill press, the major components of which are illustrated in Fig. 23.25a. The workpiece is placed on an adjustable table, either by clamping it directly into the slots and holes on the table or by using a vise, which itself is clamped to the table. The drill is lowered manually by a handwheel or by power feed at preset rates. Drill presses are usually designated by the largest workpiece diameter that can be accommodated on the table, and typically range from 150 to 1250 mm. In order to maintain proper cutting speeds at the cutting edges of drills, the spindle speed of the machines has to be adjustable for different drill sizes. Adjustments are made by means of pulleys, gearboxes, or variable-speed motors. The types of drilling machines range from simple bench-type drills, used to drill small holes, to large radial drills (Fig. 23.25b) that can accommodate large workpieces. The distance between the column and the spindle center can be as much as 3 m. The drill head of universal drilling machines can also be swiveled to drill holes at an angle. Modern drilling machines include numerically controlled three-axis machines, in which all operations are performed automatically and in their desired sequence, with the use of a turret (Fig. 23.26), which can hold several different drilling tools. Also used for boring and counterboring operations, drilling machines with multiple spindles (gang drilling) are used for high-production-rate operations, and are capable of drilling holes of varying sizes, depths, and locations in one cycle. Numerical-control turret drilling machines are also available. Workholding devices for drilling are essential to ensure that the workpiece is located and clamped properly to keep it from slipping or rotating during drilling. These devices are available in a variety of designs, with important features such three-point locating, for accuracy, and three-dimensional work holding, for secure fixturing (see also Section 37.8). Radial arm Fixed head (power head) Spindle Adjustable head Hand wheel Spindle Chuck Table Base (a) (b) Figure 23.25: (a) Schematic illustration of the components of a vertical drill press. (b) A radial drilling machine. Source: (b) Courtesy of Willis Machinery and Tools. Hand wheel Column Chuck Table ׉ 7cassandra://3cYim6vrdn9q4BiMaooVg5XHhWDNLt7OehWznuAmJr4`cZ%Eb׉E736 Chapter 23 Machining Processes: Turning and Hole Making Column Table Turret Figure 23.26: A three-axis computer-numerical-control drilling machine. The turret holds as many as eight different tools, such as drills, taps, and reamers. 23.5.7 Design Considerations for Drilling The basic design guidelines for drilling are: • Designs should allow holes to be drilled preferably on flat surfaces and perpendicular to the drill motion; otherwise, the drill tends to deflect and the hole will not be located accurately. The exit surfaces for the drill also should be flat. • Design of hole bottoms should match standard drill-point angles, whenever possible; thus, flat bottoms or odd shapes should be avoided. • When multiple holes are required, they should all have the same diameter, whenever practical, to avoid unnecessary tool changes. • Excessively deep holes should be avoided, and length-to-diameter ratios of three or less should be specified whenever possible, although a ratio of 8:1 ratio is feasible. • Through holes are preferred over blind holes. • If holes with large diameters are specified, the workpiece should have a preexisting hole, preferably made during fabrication of the part itself, such as by casting, powder metallurgy, or forming. • Dimples should be provided when preexisting holes are not practical to make, to reduce the tendency for the drill to walk. • Parts should be designed so that all drilling can be performed with a minimum of fixturing and without the need to reposition the workpiece. • Blind holes must be drilled deeper than subsequent reaming or tapping operations that may have to be performed, typically by an amount at least one-fourth of the hole diameter. ׉ 7cassandra://4TZTUlbAkVegyx4jxPIIMLxVUoUBkT3CNYpH83XYDEA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://kxkaFz2-IWu_n1OzUitV01YG9UH7bNgIjFbPFYDJIZE`׉ 7cassandra://hZg_6Bj_RrGKWU2BSqWI5cOg76HuNC4baSXwWVw6JLQG`s׉ 7cassandra://O331DAqnI5GDtlQJY88VkbXUgvKU1fv1L5Ju2wv0uAU&`׉ 7cassandra://N4ve_udDTe4SDVVI298Ir65_MS0GNC0y6TeWiHQCBdUͭE ͠^cZEb͙ט#u׉׉ 7cassandra://mIz_1LufY-vJRuI8a1BvKmSC_bd4X9G9QYO-gURBMtU@`׉ 7cassandra://bwQSi51MNOlSjwRErJGTru6_bDxwe5loX4qJCu8lm8gc`s׉ 7cassandra://qOyc-954F26tjd_lGEhhe5mpjStbl_mKeXE1IWXjlXA`׉ 7cassandra://iHhmLIrSWEEWDZkJidSwBPbLf0v3NOTIcxA6o2ljfvI)\͠^cZEb͚׉E#Reaming and Reamers 737 23.6 Reaming and Reamers Reaming is an operation used for (a) making an existing hole dimensionally more accurate than can be achieved by drilling alone and (b) improving its surface finish. The most accurate holes in workpieces generally are produced by the following sequence of operations: 1. Centering 2. Drilling 3. Boring 4. Reaming For even better accuracy and surface finish, holes may be burnished or internally ground and honed (Sections 26.4 and 26.7). A reamer (Fig. 23.27a) is a multiple-cutting-edge tool with straight or helically fluted edges that remove very little material. For soft metals, a reamer typically removes a minimum of 0.2 mm on the diameter of a drilled hole; for harder metals, about 0.13 mm is removed. Attempts to remove smaller layers can be detrimental, as the reamer may be damaged or the hole surface may become burnished (see also Fig. 21.22 as an analogy); in this case, honing would be preferred. In general, reamer speeds are one-half those of the same-size drill and three times the feed rate. Hand reamers are straight or have a tapered end in the first third of their length. Various machine reamers, also called chucking reamers because they are mounted in a chuck and machine-operated, are available in two types: (a) Rose reamers have cutting edges with wide margins and no relief (Fig. 23.27a). (b) Fluted reamers have small margins and relief, with a rake angle of about 5◦; they usually are used for light cuts. Shell reamers are hollow and mounted on an arbor, and are generally used for holes larger than 20 mm. Expansion reamers are adjustable for small variations in hole size; they also compensate for wear of the reamer’s cutting edges. Adjustable reamers (Fig. 23.27b) can be set for specific hole diameters, and are therefore versatile. Chamfer angle Chamfer length Chamfer relief Helix angle, – Primary relief angle (a) Locknut Blade Adjusting nut (b) Figure 23.27: (a) Terminology for a helical reamer. (b) Inserted-blade adjustable reamer. Body Setscrew Radial rake Margin width Land width ׉ 7cassandra://O331DAqnI5GDtlQJY88VkbXUgvKU1fv1L5Ju2wv0uAU&`cZ%Eb׉E 738 Chapter 23 Machining Processes: Turning and Hole Making Reamers may be held rigidly, as in a chuck, or they may float in their holding fixtures, to ensure alignment or to be piloted in guide bushings placed above and below the workpiece. A further development in reaming consists of the dreamer, a tool that combines drilling and reaming. The tip of the tool first produces a hole by drilling; the rest of the same tool then performs a reaming operation. A similar development involves drilling and tapping in one stroke, using a single tool. Reamers are typically made of high-speed steels (M1, M2, and M7) or solid carbides (K20, C2), or have carbide cutting edges. Reamer maintenance and reconditioning are important for hole accuracy and surface finish. 23.7 Tapping and Taps Internal threads can be produced by tapping,a tap being a chip-producing threading tool with multiple cutting teeth (Fig. 23.28a). Taps generally are available with two, three, or four flutes. The most common production tap is the two-flute spiral-point tap; it forces the chips into the hole so that the tap needs to be retracted only at the end of the cut. Three-fluted taps are stronger, because more material is available in the flute. Tap sizes range up to 100 mm; larger threads can be machined in a milling machine or a machining center (see Fig. 24.2f). Tapered taps are designed to reduce the torque required for the tapping of through holes. Bottoming taps are for tapping blind holes to their full depth. Collapsible taps are used in large-diameter holes; after tapping has been completed, the tap is collapsed mechanically and is removed from the hole without having to rotate them in the hole, as do regular taps. Chip removal can be a significant problem during tapping, because of the small clearances in the tap. If chips aren’t removed properly, the torque increases significantly and can break the tap. The use of a cutting fluid and the periodic reversal and removal of the tap from the hole are effective means of chip removal. For higher tapping productivity, drilling and tapping can be combined in a single operation (drapping)ina single tool. The tool has a drilling section at its tip, followed by a tapping section. Tapping may be done by hand or on machines, such as (a) drilling machines, (b) lathes, (c) automatic screw machines, and (d) vertical CNC milling machines, which combine the correct relative rotation and the longitudinal feed. Special tapping machines are available, with features for multiple tapping operations. Multiple-spindle tapping heads are used extensively, particularly in the automotive industry, where 30% to 40% of machining operations involve tapping holes. One simple method of automatic tapping of nuts is shown in Fig. 23.28b. Tap life can be determined with the same technique for measuring drill life. With proper lubrication, tap life may be as high as 10,000 holes. Taps usually are made of high-speed steels (M1, M2, M7, and M10). Productivity in tapping operations can be improved by high-speed tapping, with surface speeds as Chamfer angle Land Rake angle Cutting edge Heel (a) Flute Chamfer relief Tap Nut Hook angle (b) Figure 23.28: (a) Terminology for a tap. (b) Tapping of steel nuts in production. ׉ 7cassandra://qOyc-954F26tjd_lGEhhe5mpjStbl_mKeXE1IWXjlXA`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://QmG1_ldhXpRiKvFvPi7wOE24eQujKCGK0V4Z0XY9S_U`׉ 7cassandra://VjhSbaIp1RuKSG-qxIW5PRzmWd6ldXv604_KI5Lzyk4F`s׉ 7cassandra://hyyIV-yxpTI-tnaSyl822XMsRwf7_B8nzOOc-eHkqIU`׉ 7cassandra://Ep1wabfJsUCxcRlmILD4Nzz7aqQa7LNhpq9FbKh7F24ͭ2͠^cZEb͜ט#u׉׉ 7cassandra://dFjefwawrMZ7I-ZUo9rS6vcyZnMuaIVQ2543Nfit2LQ3x`׉ 7cassandra://xUeYEKaYiaw6ghLiuqmurM04s2ER-YN5JdI6_8cXH14e`s׉ 7cassandra://uYR60_yQn36ID4HMtm7tqqmtifSjDb0FplvS2CYq9J8`׉ 7cassandra://nEh5Jo45Skk9f-mBTnmAoWkRJite4GMIQ9ym74kOEb8M͠^cZEb͝׉EKTapping and Taps 739 high as 100 m/min. Self-reversing tapping systems are now in use with modern computer-controlled machine tools. Operating speeds can be as high as 5000 rpm, although actual cutting speeds in most applications are considerably lower. Cycle times typically are on the order of 1 to 2 seconds. Some tapping systems have capabilities for directing the cutting fluid to the cutting zone through the spindle and a hole in the tap, which also helps flush the chips out of the hole being tapped. Chipless tapping is a process of internal thread rolling using a forming tap (Section 13.5). Case Study 23.2 Bone Screw Retainer A cervical spine implant is shown in Fig. 23.29a. In the event that a patient requires cervical bone fusion at one or more vertebral levels, this implant can act as an internal stabilizer by decreasing the amount of motion in the region, and thereby help promote a successful fusion. The plate is affixed to the anterior aspect of the spine, using bone screws that go through the plate and into the bone. The undersurface of the plate has a very rough surface that helps hold the plate in place while the bone screws are being inserted. One concern with this type of implant is the possibility of the bone screws loosening with Plate Retainer Bone screw 10 mm Screw and retainer inserted in plate (a) 1.3 6 0.075 3X 6.68 2X 1208 608 R 5 0.25 S 10.7 3X R = 3.32 f 9.5 Note: Thread must start at point S to ensure that retainer interferes with bone screw. 2.5 10.040 20.0000 HEX (b) Figure 23.29: A cervical spine implant. (All dimensions in mm) 0.28 1.52 R 6 0.025 R 5 10.8 ׉ 7cassandra://hyyIV-yxpTI-tnaSyl822XMsRwf7_B8nzOOc-eHkqIU`cZ%Eb׉E740 Chapter 23 Machining Processes: Turning and Hole Making time, due to normal and repetitive loading from the patient. In extreme cases, this can result in a screw backing out, with the head of the screw no longer being flush with the plate, a condition that obviously is undesirable. The implant described here uses a retainer to prevent the bone screw from backing out away from the plate, as shown in the left half of Fig. 23.29b. The retainer has several design features that are essential for it to function correctly, and without complicating the surgical procedure. To ease its use in surgery, the plate is provided with the retainers already in place, with the circular notches aligned with the bone screw holes. This arrangement allows the surgeon to insert the bone screws without interference from the retainer. Once the screws are inserted, the surgeon turns the retainer a few degrees so that each screw head is then captured. In order to ensure the retainer’s proper orientation in the plate, the thread of its shank must start in the same axial location as point S in Fig. 23.29b. The manufacturing steps followed to produce this part are shown in Fig. 23.29b. First, a 12.7-mm diameter Ti-6Al-4V rod is placed in a CNC lathe and faced. Then the threaded area is turned to the diameter necessary to machine the threads. The thread is turned on the shank, but over a longer length than is ultimately required, because of difficulties in obtaining high-quality threads at the start of machining. The cap then is turned to the required diameter, and a 2.5-mm radius is machined on the underside of the head. The part is removed, inspected, and placed in another CNC lathe, where it is faced to the specified length. The spherical radius in the cap is then machined, the center hole is drilled, and the hex head is broached. The cap is removed and inspected, and if the desired length has not been achieved, the cap is lapped (Section 26.7) to the final dimension. At this point, the retainer is placed in a CNC milling machine, using a specially designed fixture that consists basically of a tapered and threaded hole. By carefully applying a predetermined torque on the retainer when placing it into the fixture, the starting location of the threads can be controlled accurately. Once the cap is located in the fixture, the three circular notches are machined as per the drawing. The retainer is then deburred by tumbling to remove all sharp corners, and the bottom is grit blasted to match that of the underside of the plate. Finally, the parts are anodized (Section 34.10) and passivated to obtain the desired biocompatibility. Source: Courtesy of J. Mankowski and B. Pyszka, Master Metal Engineering Inc., and C. Lyle and M. Handwerker, Wright Medical Technology, Inc. Summary • Machining processes that typically produce external and internal circular profiles are turning, boring, drilling, and tapping. Because of the three-dimensional nature of these operations, chip movement from the cutting zone and its control are important considerations. Chip removal can be a significant problem, especially in drilling and tapping, and can lead to tool breakage. • Optimization of each machining operation requires an understanding of the interrelationships among design parameters (such as part shape, dimensional accuracy, and surface finish) and process parameters (cutting speed, feed, and depth of cut), tool material and shape, the use of cutting fluids, and the sequence of operations to be performed. • The parts to be machined may have been produced by casting, forging, extrusion, or powder metallurgy. The closer the blank to be machined to the final shape desired (near-net shape), the fewer the number and extent of the subsequent machining processes required. ׉ 7cassandra://uYR60_yQn36ID4HMtm7tqqmtifSjDb0FplvS2CYq9J8`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://01Yz_S6y_lYYAYGNFXGSfXFxr4KZ5W7fMp-26sRUXTYU`׉ 7cassandra://30pYqDG24ysFumJ_VeWnnnEkqVyDEXW0r1awtzqvG5wBz`s׉ 7cassandra://hkUZIBFDzNgQXn5LFsLYPUGOEPjvPOyzRyMi-7tJB1A`׉ 7cassandra://SdfxsO4_ZwJkWdOuoJPG8GHRSG3loKhC0xmkejBvwJsU͠^cZEb͟ט#u׉׉ 7cassandra://T94xyHd0MCyzavXWyLth1TCMVsy3ZbPWKxoF1tmvqf0`׉ 7cassandra://xZ93NuYDUKR8A8sTYhhIp5-_40jlhWMB2RraKg-zz2kc`s׉ 7cassandra://FEiL9mSFKi_zNzrqDEXWJ04siuQD76Cgx4Ejmvd6Frsb`׉ 7cassandra://WnVBnrpOEPY_B-y3_iR6Wwe1Oy2bXfTjv3eIhT8SLH4B͠^cZEb͡׉EReview Questions 741 Key Terms Automatic bar machine Back rake angle Bed Boring Boring mill Carriage Chip management Chuck Collet Cutting-edge angle Drilling Drill life Drill press Engine lathe Face plate Facing Feed force Feed rod Finishing cuts Form tools Gun drilling Headstock Hole making Bibliography ASM Handbook, Vol. 16: Machining, ASM International, 1989. Byers, J.P. (ed.), Metalworking Fluids, 2nd ed., CRC Press, 2006. Hoffman, E.G., Jigs and Fixture Design, 5th ed., Industrial Press, 2003. Joshi, P.H., Machine Tools Handbook, McGraw-Hill, 2008. Krar, S.F., Gill, A.R., and Smid, P., Technology of Machine Tools, 7th ed., McGraw-Hill, 2010. Liang, S., and Shih, A.J., Analysis of Machining and Machine Tools, Springer, 2015. Lopez, L.N., and Lamikiz, A. (eds.), Machine Tools for High Performance Machining, Springer, 2009. Machinery’s Handbook, various editions, Industrial Press. Walsh, R.A., McGraw-Hill Machining and Metalworking Handbook, 3rd ed., McGraw-Hill, 2006. Review Questions 23.1. Describe the types of machining operations that can be performed on a lathe. 23.2. What is turning? What kind of chips are produced by turning? Knurling Lathes Lead screw Mandrel Material-removal rate Nose radius Parting Power chuck Rake angle Reamer Reaming Reconditioning Relief angle Roughing cuts Screw threads Side rake angle Tailstock Tapping Threading Trepanning Turning Turret lathe Twist drill ׉ 7cassandra://hkUZIBFDzNgQXn5LFsLYPUGOEPjvPOyzRyMi-7tJB1A`cZ%Eb׉E 742 Chapter 23 Machining Processes: Turning and Hole Making 23.3. What is the thrust force in turning? What is the cutting force? Which is used to calculate the power required? 23.4. What are the components of a lathe? 23.5. What is a tracer lathe? 23.6. Describe the operations that can be performed on a drill press. 23.7. Why were power chucks developed? 23.8. Explain why operations such as boring on a lathe and tapping are difficult. 23.9. What is an automatic bar machine? 23.10. Why are turret lathes typically equipped with more than one turret? 23.11. Describe the differences between boring a workpiece on a lathe and boring it on a horizontal boring mill. 23.12. How is drill life determined? 23.13. What is the difference between a conventional drill and a gun drill? 23.14. Why are reaming operations performed? 23.15. Explain the functions of the saddle on a lathe. 23.16. Describe the relative advantages of (a) self-opening and (b) solid-die heads for threading. 23.17. Explain how external threads are cut on a lathe. 23.18. What is the difference between a blind hole and a through hole? What is the significance of that difference? Qualitative Problems 23.19. Explain the reasoning behind the various design guidelines for turning. 23.20. Note that both the terms “tool strength” and “tool-material strength” have been used in the text. Do you think there is a difference between them? Explain. 23.21. List and explain the factors that contribute to poor surface finish in the processes described in this chapter. 23.22. List the advantages and disadvantages of turning or cold extruding a shaft. 23.23. Explain why the sequence of drilling, boring, and reaming produces a hole that is more accurate than drilling and reaming it only. 23.24. Why would machining operations be necessary even on net-shape or near-net-shape parts made by precision casting, forming, or powder-metallurgy products, as described in preceding chapters? Explain. 23.25. A highly oxidized and uneven round bar is being turned on a lathe. Would you recommend a small or a large depth of cut? Explain. 23.26. Describe the difficulties that may be encountered in clamping a workpiece made of a soft metal in a three-jaw chuck. 23.27. Does the force or torque in drilling change as the hole depth increases? Explain. 23.28. Drills usually have two flutes. Explain why. 23.29. Explain the similarities and differences in the design guidelines for turning and for boring. 23.30. Describe the advantages and applications of having a hollow spindle in the headstock of a lathe. ׉ 7cassandra://FEiL9mSFKi_zNzrqDEXWJ04siuQD76Cgx4Ejmvd6Frsb`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://Z9wSLQ62EpGDyadiCPhvM0yHNuuMxMHI4aRUqmbapLUZ`׉ 7cassandra://v-GseOOuszD6Z-47JlDMgFFmc_lc2spjl7Vk0kCv8Fod7`s׉ 7cassandra://D6R2L9t4zsJJ12sOD94kQA_xyuQOAMTD7SaqbODMZjU`׉ 7cassandra://BofldYXyckcvTipuehbIqR88CqjHxEogYsrrLlY3wVMU*͠^cZEbͣט#u׉׉ 7cassandra://M5aT9braykrFRrJXW3ws_RMwfXwOXLU5qVBiswMz5iA#`׉ 7cassandra://N2BnEyjgcrPZqpd47VbI-vg0tSN8EOTMplh4eNEskR8a`s׉ 7cassandra://9_qDlFpP0yruqunbl_tUa8qKwqF7UcBbwH7Y0Y5-zbYK`׉ 7cassandra://TqMH5EIrNWchVjOKYSzz1pYDTvTw2KU6qPFtxc2PKPMT͠^cZEbͤ׉E Quantitative Problems 743 23.31. Assume that you are asked to perform a boring operation on a large-diameter hollow workpiece. Would you use a horizontal or a vertical boring mill? Explain. 23.32. Explain the reasons for the major trend that has been observed in producing threads by thread rolling as opposed to thread cutting. What would be the differences, if any, in the types of threads produced and in their performance characteristics? 23.33. Describe your observations concerning the contents of Tables 23.2 and 23.4, and explain why those particular recommendations are made. 23.34. The footnote to Table 23.12 states that as the hole diameter increases, speeds and feeds in drilling should be reduced. Explain why. 23.35. In modern manufacturing, which types of metal chips would be undesirable and why? 23.36. Sketch the tooling marks you would expect if a part was (a) turned; (b) reduced in diameter with a straight form tool; (c) extruded. 23.37. What concerns would you have in turning a powder metal part, such as a shaft made from the Osprey process (see Fig. 17.20)? 23.38. The operational severity for reaming is much lower than that for tapping, even though they both are internal machining processes. Why? 23.39. Review Fig. 23.6, and comment on the factors involved in determining the height of the zones (cutting speed) for various tool materials. 23.40. Explain how gun drills remain centered during drilling. Why is there a hollow, longitudinal channel in a gun drill? 23.41. Comment on the magnitude of the wedge angle on the tool shown in Fig. 23.4. 23.42. If inserts are used in a drill bit (see Fig. 23.22), how important is the shank material? If so, what properties are important? Explain. 23.43. Refer to Fig. 23.11b, and in addition to the tools shown, describe other types of cutting tools that can be placed in toolholders to perform other machining operations. Quantitative Problems 23.44. Calculate the same quantities as in Example 23.1 for high-strength titanium alloy and at N = 700 rpm. 23.45. Estimate the machining time required to rough turn a 0.75-m-long annealed copper-alloy round bar from a 75-mm diameter to a 73-mm diameter, using a high-speed steel tool (see Table 23.4). Estimate the time required for an uncoated carbide tool. 23.46. A high-strength cast-iron bar 200 mm in diameter is being turned on a lathe at a depth of cut of d =1.25 mm. The lathe is equipped with a 12 kW electric motor and has a mechanical efficiency of 80%. The spindle speed is 500 rpm. Estimate the maximum feed that can be used before the lathe begins to stall. 23.47. A 7.5-mm-diameter drill is used on a drill press operating at 300 rpm. If the feed is 0.125 mm/rev, what is the MRR? What is the MRR if the drill diameter is doubled? 23.48. In Example 23.4, assume that the workpiece material is high-strength aluminum alloy and the spindle is running at N = 750 rpm. Estimate the torque required for this operation. 23.49. For the data in Problem 23.48, calculate the power required. 23.50. A 150-mm-diameter aluminum cylinder 250 mm in length is to have its diameter reduced to 115 mm. Using the typical machining conditions given in Table 23.4, estimate the machining time if a TiN-coated carbide tool is used. ׉ 7cassandra://D6R2L9t4zsJJ12sOD94kQA_xyuQOAMTD7SaqbODMZjU`cZ%Eb׉E E744 Chapter 23 Machining Processes: Turning and Hole Making 23.51. A lathe is set up to machine a taper on a bar stock 150-mm in diameter; the taper is 1 mm per 10 mm. A cut is made with an initial depth of cut of 4 mm at a feed rate of 0.300 mm/rev and at a spindle speed of 200 rpm. Calculated the average metal removal rate. 23.52. Assuming that the coefficient of friction is 0.25, calculate the maximum depth of cut for turning a hard aluminum alloy on a 15 kW lathe (with a mechanical efficiency of 80%) at a width of cut of 6 mm, rake angle of 0◦, and a cutting speed of 90 m/min. What is your estimate of the material’s shear strength? 23.53. A 75-mm-diameter gray cast iron cylindrical part is to be turned on a lathe at 500 rpm. The depth of cut is 6 mm and the feed is 0.5 mm/rev. What minimum horsepower is required for this operation? 23.54. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. Synthesis, Design, and Projects 23.55. Drill life could be greatly increased if an effective means of cooling and lubrication were developed. Design methods of delivering a cutting fluid to the cutting zone, and discuss the advantages and shortcomings of your design. 23.56. Would you consider the machining processes described in this chapter as net-shape processes, thus requiring no further processing? Near-net-shape processing? Explain with appropriate examples. 23.57. Would it be difficult to use the machining processes described in this chapter on various soft nonmetallic or rubberlike materials? Explain your thoughts, commenting on the role of the physical and mechanical properties of such materials with respect to the machining operation and any difficulties that may be encountered in producing the desired shapes and dimensional accuracies. 23.58. If a bolt breaks in a hole, it typically is removed by first drilling a hole in the bolt shank and then using a special tool to remove the bolt. Inspect such a tool and explain how it functions. 23.59. An important trend in machining operations is the increased use of flexible fixtures. Conduct a search on the Internet regarding these fixtures, and comment on their design and operation. 23.60. Review Fig. 23.8d, and explain if it would be possible to machine eccentric shafts, such as that shown in Fig. 23.13c, on the setup illustrated. What if the part is long compared with its cross section? Explain. 23.61. Boring bars can be designed with internal damping capabilities to reduce or eliminate vibration and chatter during machining (see Fig. 23.18). Referring to the technical literature, describe details of designs for such boring bars. 23.62. A large bolt is to be produced from extruded hexagonal bar stock by placing the hex stock into a chuck and machining the shank of the bolt by turning it on a lathe. List and explain the difficulties that may be involved in this operation. 23.63. Make a comprehensive table of the process capabilities of the machining operations described in this chapter. Using several columns, describe the machine tools involved, type of cutting tools and tool materials used, shapes of parts produced, typical maximum and minimum sizes, surface finish, dimensional tolerances, and production rates. ׉ 7cassandra://9_qDlFpP0yruqunbl_tUa8qKwqF7UcBbwH7Y0Y5-zbYK`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://tJLV7AB05MJVBRRFN08iRXjaN-leHBdAJf9VZfZHPsU`׉ 7cassandra://1tcqrxPz-Aj6WaxE1YpQ2sjPRxbSGQQbQiq6O1l7-UIA`s׉ 7cassandra://w12UErn5eWEP9mOUbztg4Y4qm0eOgwdjQv-QZQNnUSYT`׉ 7cassandra://4no1z-7Tw9XDac9frzQUI-I903Cyp6gzLHwd-_tfj6c:͠^cZEbͦט#u׉׉ 7cassandra://Q08BQE-4fQLg1WM0F-jCir6VJnTo3NqC1saP5Ia3BiY`׉ 7cassandra://GoAOFxT0T3z9gw1RaFKDyriigdZ9cVCJaU1h2Vlj7MIM `s׉ 7cassandra://4apMIHmdiZmFafe-aWpE1uTPASkVEqtM5RB0IQqfxCE`׉ 7cassandra://R0y3HQbplOluIk8h_B4lz5VNCKsbwDz7k7JlhojNV9o͠^cZEbͧ׉EChapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 24.1 Introduction 746 24.2 Milling and Milling Machines 746 24.3 Planing and Shaping 763 24.4 Broaching and Broaching Machines 763 24.5 Sawing 766 24.6 Filing 769 24.7 Gear Manufacturing by Machining 770 Examples: 24.1 Material-removal Rate, Power, Torque, and Cutting Time in Slab Milling 750 24.2 Material-removal Rate, Power Required, and Cutting Time in Face Milling 752 Case Studies: 24.1 Broaching Internal Splines 766 24.2 Ping Golf Putter 776 24.3 Machining of Aerospace Structures from Monolithic Extruded Aluminum 778 • This chapter begins with milling, one of the most versatile and important machining processes, in which a rotating cutter removes material while traveling along a specified path. • Described next are the processes of planing, shaping, and broaching, in which either the cutting tool or the workpiece travels along a straight path, producing flat or profiled machined surfaces. • Next described are sawing and filing, including tool design and machinery involved in these processes. • The chapter ends with descriptions of gear-manufacturing by machining, the special cutters used, the automated equipment involved, the quality and properties of the gears produced. 745 ׉ 7cassandra://w12UErn5eWEP9mOUbztg4Y4qm0eOgwdjQv-QZQNnUSYT`cZ%Eb ׉E746 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Typical parts made: Parts with complex external and internal features, splines, and gears. Alternative processes: Die casting, precision casting, precision forging, additive manufacturing, powder metallurgy, powder-injection molding, creep-feed grinding, electrical discharge machining, and fabrication. 24.1 Introduction In addition to producing parts with various external or internal round profiles, as described in Chapter 23, machining operations also can produce many other complex shapes (Fig. 24.1). Although processes such as die casting, precision forging, and powder metallurgy can produce parts with close tolerances and fine surface finish, it is often necessary to perform complex machining operations to respond to various design requirements and specifications. In this chapter, several important machining processes and machine tools capable of producing complex shapes, using single-point, multitooth, and profiled cutting tools, are described (see also Table 23.1). 24.2 Milling and Milling Machines Milling includes a number of highly versatile machining operations taking place in a variety of configurations (Fig. 24.2), with the use of a milling cutter,a multitooth tool that produces a number of chips in one revolution (Fig. 24.3). (a) (b) Stepped cavity (c) Drilled and tapped holes (d) (e) (f) Figure 24.1: Typical parts and shapes that can be produced with the machining processes described in this chapter. ׉ 7cassandra://4apMIHmdiZmFafe-aWpE1uTPASkVEqtM5RB0IQqfxCE`cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://Yf0OKlgg_TyEa943bn0hzCkcEwmJyiNd2Va8-9OXJbs&~`׉ 7cassandra://fBKTW4ZBeba7Fc0AWskHY8iDNGg7xh6Xw0eXJ6UmYyo:`s׉ 7cassandra://RmtxE1Xu8-n3uSM2eOOg5EOQO0QiASAgM5G_0G5pgHo`׉ 7cassandra://J6ttuJqTnifPVpyCJdEnsOIiF_0aO5i80X9cbdt3lEQ͇y͠^cZEbͩט#u׉׉ 7cassandra://7i2GrIg4c3uTDiv4SON2-H0dZHlsUjcQOxbU8HxmMVw `׉ 7cassandra://wUNqWCmE3nS6fDr9_QifZCMbv0ZurHPHCjy0eg6EgdcYT`s׉ 7cassandra://H1zU5XA7ljB_hQMRoOIhAjfXa_CXNQGsXjD_z7VXjm8`׉ 7cassandra://PZ5zIQKcVGaqgBw6pp4AOsSvDz0HTy6l9-Sl9BVF2LU4h͠^cZEbͪ׉EFMilling and Milling Machines 747 Face mill with inserts Square shoulder face mill Ball nose end mill Long edge milling cutter Ball nose end mill Roughing end mill Finishing end mill (a) Face milling Grooving cutter Slot milling cutter Slotting endmill (b) End or shoulder milling (c) Profile milling Tap Thread mill Grooving cutter (d) Slot milling (e) Slot and groove milling (f) Thread milling and tapping Figure 24.2: Some basic types of milling cutters and milling operations. (a) Face milling; (b) end or shoulder milling; (c) profile milling; (d) slot milling; (e) slot and groove milling; (f) thread milling and tapping. Source: Courtesy of Sandvik Coromant. Figure 24.3: Photograph of the cutting action of a milling cutter that uses a number of inserts to remove metal in the form of chips. Source: Courtesy of Sandvik Coromant. ׉ 7cassandra://RmtxE1Xu8-n3uSM2eOOg5EOQO0QiASAgM5G_0G5pgHo`cZ%Eb ׉E 748 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Cutter Arbor Figure 24.4: Schematic illustration of peripheral milling. 24.2.1 Peripheral Milling In peripheral milling, also called plain milling, the axis of cutter rotation is parallel to the workpiece surface, as shown in Fig. 24.4. The cutter body, which generally is made of high-speed steel (Section 22.2), has a number of teeth along its circumference; each tooth acts like a single-point cutting tool. When the cutter is longer than the width of the cut, the operation is called slab milling. Cutters for peripheral milling may have either straight or helical teeth, resulting in an orthogonal or oblique cutting action, respectively. Helical teeth generally are preferred over straight teeth, because each tooth is always partially engaged with the workpiece as the cutter rotates. Consequently, the cutting force and the torque on the cutter are lower, resulting in a smoother milling operation and reduced chatter. Conventional Milling and Climb Milling. Note in Fig. 24.5a that the cutter rotation can be either clockwise or counter-clockwise; this is significant in the milling operation. In conventional milling, also called up milling, the maximum chip thickness is at the end of the cut as the tooth leaves the workpiece surface. Thus, contaminants and scale (oxide layer) on the surface do not adversely affect tool life. This is the more common method of milling, where the cutting operation is smooth. However, the cutter teeth must be sharp, as otherwise the tooth will rub against the surface being milled and smear it for some distance before it begins to engage and cut. There may also be a tendency for the cutter to chatter (Section 25.4) and for the workpiece to be lifted upward, because of the cutter rotation direction. Proper clamping of the workpiece on the table of the machine is thus important. In climb milling, also called down milling, cutting starts at the surface of the workpiece where the chip is thickest. The advantage of this method is that the direction of rotation of the cutter will push the workpiece D Cutter tc N f v Workpiece Conventional milling (a) Climb milling (b) (c) Figure 24.5: (a) Schematic illustration of conventional milling and climb milling. (b) Slab-milling operation showing depth of cut, d; feed per tooth, f; chip depth of cut, tc, and workpiece speed, v. (c) Schematic illustration of cutter travel distance, lc, to reach full depth of cut. lc l d v Cutter d D ׉ 7cassandra://H1zU5XA7ljB_hQMRoOIhAjfXa_CXNQGsXjD_z7VXjm8`cZ%Eb cZ%Eb #בCט#u׉׉ 7cassandra://GrfG1Wr6TH98NcZlii5qg6UbHf26TSxcbzYkoz6goB8`׉ 7cassandra://iJ1GMELaqA6vThKQl0ySscn6XLEiy6xgz7P6QmWSTd4H`s׉ 7cassandra://xs0VE3zjD2vcwMnwPQ5sqXleatgV1i6N8W_wV3N2H4o`׉ 7cassandra://99BbUS6ZTTMety5RnETpPBpr6ADiQutUK_oi6NAXETI͗͠^cZEbͬט#u׉׉ 7cassandra://fwoEN7y_eE4GahWfWzRYjyseZ4cd415OOKSCPxFWK3U`׉ 7cassandra://mMORzRP-2h3X6EpM2xd2_eJa1pozJADx8obl8GxI4wIM2`s׉ 7cassandra://ViXhmmb1OyT1I8U9-tsvL7iTVszVgtWsVeWBP99Sks4`׉ 7cassandra://NWgTEs6-Dj_k3G-0icVEpVU56w9865TNVzeB4bEaI0ouV͠^cZEbͭנcZEbͯD̐9ׁHhttp://thedepthofcut.Asׁ ׁ Ј׉EOMilling and Milling Machines 749 downward, thus holding the workpiece in place, a factor particularly important for slender parts. Because of the resulting impact force when a tooth first engages the workpiece, however, this operation must have a rigid workholding setup, and gear backlash in the table feed mechanism must be eliminated. Climb milling is not suitable for machining workpieces having surface scale, such as metals that have been hot worked, forged, or cast. Scale is hard and abrasive, and thus causes excessive wear and damage to the cutter teeth, shortening their life. Milling Parameters. The cutting speed, V, in peripheral milling is the surface speed of the cutter, or V = πDN, (24.1) where D is the cutter diameter and N is the rotational speed of the cutter (Fig. 24.6). Note from Fig. 24.5b that the thickness of the chip in slab milling will vary along its length because of the relative longitudinal motion between the cutter and the workpiece. For a straight-tooth cutter, the approximate undeformed chip thickness (also called chip depth ofcut), tc, can be calculated from the equation tc =2f  d D , (24.2) where f is the feed per tooth of the cutter (the distance the workpiece travels per tooth of the cutter, in mm/tooth), and d is thedepthofcut.As tc becomes larger, the force on the cutter tooth will increase. Feed per tooth is determined from the equation f = v Nn , (24.3) where v is the linear speed (also called feed rate) of the workpiece and n is the number of teeth on the cutter periphery. lc Insert f v D Cutter w lc l d Cutter Machined surface f l v v w (a) (b) (c) (d) Figure 24.6: (a) Face-milling operation with cutter removed, showing the action of a single insert; (b) climb milling; (c) conventional milling; and (d) dimensions in face milling. The width of cut, w, is not necessarily the same as the cutter radius. ׉ 7cassandra://xs0VE3zjD2vcwMnwPQ5sqXleatgV1i6N8W_wV3N2H4o`cZ%Eb ׉E W750 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Table 24.1: Summary of Peripheral Milling Parameters and Formulas. N = Rotational speed of the milling cutter, rpm F = Feed, mm/tooth D = Cutter diameter, mm n = Number of teeth on cutter v = Linear speed of the workpiece or feed rate, mm/min V = Surface speed of cutter, m/min = DN f = Feed per tooth, mm/tooth = v/Nn l = Length of cut, mm t = Cutting time, s or min =(l + lc) /v, where lc = extent of the cutter’s first contact with the workpiece MRR = mm3/min = wdv, where w is the width of cut Torque = N-m = FcD/2 Power = kW = (Torque)(ω), where ω =2πN radians/min The cutting time, t, is given by the equation t = l + lc v , (24.4) where l is the length of the workpiece (Fig. 24.5c) and lc is the horizontal extent of the cutter’s first contact with the workpiece. Based on the assumption that lc l (although this generally is not the case), the material-removal rate (MRR) is MRR = lwd t = wdv, (24.5) where w is the width of the cut, which, in slab milling, is equal to the width of the workpiece. As stated in Section 23.2, the distance that the cutter travels in the noncutting cycle of the milling operation is an important economic consideration, and should be minimized by such means as faster travel of the machine tool components. The foregoing equations and the terminology used are summarized in Table 24.1. Although the power requirement in peripheral milling can be measured or calculated, the tangential, radial, and axial forces on the cutter (see also Fig. 23.5) are difficult to calculate. There are numerous variables involved, many of which pertain to the cutter geometry; these forces can be measured experimentally, while the torque on the cutter spindle (the product of the cutter radius and the tangential force) can be calculated from the power (see Example 24.1). The tangential force per tooth will depend on how many teeth are engaged at any moment during the cut. Example 24.1 Material-removal Rate, Power, Torque, and Cutting Time in Slab Milling Given: A slab-milling operation is being carried out on a 300 mm-long, 100-mm-wide annealed mildsteel block at a feed f =0.25 mm/tooth and a depth of cut d =3 mm. The cutter is D =50 mm in diameter, has 20 straight teeth, rotates at N = 100 rpm, and, by definition, is wider than the block to be machined. ׉ 7cassandra://ViXhmmb1OyT1I8U9-tsvL7iTVszVgtWsVeWBP99Sks4`cZ%EbcZ%Eb #בCט#u׉׉ 7cassandra://v_V24JH7KVhfuDhx2bxdEmGOFr8SjiS7j83DKSHxpdU#`׉ 7cassandra://8wMIKhYNc9-AOI1jkDLotl848ptFQmX2CxTVbM6Csh4WX`s׉ 7cassandra://ljqKvKPijePxLWh7SXsddd-OO8HG0RZv-45QElwEc5s`׉ 7cassandra://1krebd5WdX1mhsD3PAOSzSOa0Hf91wofhEdqIcSRyOsm͠^cZEbͰט#u׉׉ 7cassandra://i72ynplxOsVqnWmc1fuKcEL3jeYl_4GkMIRKQQqi3uEP`׉ 7cassandra://DH5sVxRizskvvT88LpaHkaBtL2Z489OtcqTz8AQZs1gXy`s׉ 7cassandra://G4833MdBh_y-kVkurDArdREXBe_ME2yUR1XYjMjtvpYv`׉ 7cassandra://S1-yeJ7-XdpQWC6OrsuYo-FMf6I4sdc2ZAJ6lB0U7MU͠^cZEbͱ׉E Milling and Milling Machines 751 Find: Calculate the material-removal rate, estimate the power and torque required for this operation, and calculate the cutting time. Solution: From the information given, the linear speed of the workpiece, v, can be calculated from Eq. (24.3): v = fNn =(0.25)(100)(20) = 500 mm/min. =0.00833 m/s. From Eq. (24.5), the material-removal rate is calculated to be MRR = (100) (3) (500) = 150, 000 mm3/min. = 2500 mm3/s. Since the workpiece is annealed mild steel, the unit power is estimated from Table 21.2 as 5.5 W-s/mm3. Therefore, the power required can be estimated as Power =(5.5)(2500) = 13.75 kW. Also, Torque = Power = Rotational Speed (100 rpm)(2π) (13.75 kW) =21.9 Nm. The cutting time is given by Eq. (24.4), in which the quantity lc can be shown, from simple geometric relationships and for D d, to be approximately equal to lc = Dd = √ Thus, the cutting time is t = 300 + 12.25 500 24.2.2 Face Milling In face milling, the cutter is mounted on a spindle having an axis of rotation perpendicular to the workpiece surface (Fig. 24.2a); it removes material in the manner shown in Fig. 24.6a. The cutter rotates at a rotational speed, N, and the workpiece moves along a straight path, at a linear speed, v. When the direction of cutter rotation is as shown in Fig. 24.6b, the operation is climb milling; when it is in the opposite direction (Fig. 24.6c), it is conventional milling. The cutting teeth, such as carbide inserts, are mounted on the cutter body, as shown in Fig. 24.7 (see also Fig. 22.3c). Because of the relative motion between the cutter tooth and the workpiece, face milling leaves feed marks on the machined surface (Fig. 24.8), similar to those left by turning operations as shown in Fig. 21.2. Note that the surface roughness of the workpiece depends on the corner geometry of the insert and the feed per tooth. The terminology for a face-milling cutter, as well as for various angles, is shown in Fig. 24.9. As can be seen from the side view of the insert in Fig. 24.10, the lead angle of the insert in face milling has a direct influence on the undeformed chip thickness, as it does in turning operations (see Fig. 23.3). As the lead angle (positive, as shown in Fig. 24.10b) increases, the undeformed chip thickness decreases, and the length of contact, and hence chip width, increases. Note, however, that the cross-sectional area of the undeformed chip remains constant. The lead angle also influences the forces in milling. It can be seen that as the lead angle decreases, there is a smaller vertical-force component (that is, the axial force on the cutter spindle). The lead angles for most face-milling cutters typically range from 0◦ to 45◦. A wide variety of milling cutters and inserts are available (Figs. 22.2 and 24.7). The cutter diameter should be chosen so that it will not interfere with fixtures, workholding devices, or other components in =0.625 min =37.5 s.  (50) (3) = 12.24 mm. ׉ 7cassandra://ljqKvKPijePxLWh7SXsddd-OO8HG0RZv-45QElwEc5s`cZ%Eb׉E D752 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Figure 24.7: A face-milling cutter with indexable inserts. Source: Courtesy of Ingersoll Cutting Tool Company. the setup. In a typical face-milling operation, the ratio of the cutter diameter, D, to the width of cut, w, should be no less than 3:2. The relationship of cutter diameter to insert angles, and their position relative to the surface to be milled are important, in that they will determine the angle at which an insert enters and exits the workpiece. Note in Fig. 24.6b for climb milling that, if the insert has zero axial and radial rake angles (see Fig. 24.9), the rake face of the insert engages the workpiece directly. As seen in Fig. 24.11a and b, however, the same insert may engage the workpiece at different angles, depending on the relative positions of the cutter and the workpiece width. Note in Fig. 24.11a that since the tip of the insert makes the first contact, there is a possibility for the cutting edge to chip off. In Fig. 24.11b, on the other hand, the first contacts (at entry, reentry, and the two exits) are at an angle and away from the tip of the insert. Consequently, there is a lower tendency for the insert to fail, because the forces on the insert vary more slowly. Note from Fig. 24.9 that the radial and axial rake angles also will have an effect on this operation. Figure 24.11c shows the exit angles for various cutter positions. Note in the first two examples that the insert exits the workpiece at an angle, thus causing the force on the insert to be reduced to zero at a slower rate (desirable for longer tool life) than in the third example, where the insert exits the workpiece abruptly. Example 24.2 Material-removal Rate, Power Required, and Cutting Time in Face Milling Given: Refer to Fig. 24.6 and assume that D = 150 mm, w =60 mm, l = 500 mm, d =3 mm, v =0.6 m/min, and N = 100 rpm. The cutter has 10 inserts, and the workpiece material is a high-strength aluminum alloy. Find: Calculate the material-removal rate, cutting time, and feed per tooth, and estimate the power required. Solution: First note that the cross section of the cut is wd = (60)(3) = 180 mm2. Then, noting that the workpiece speed, v,is 0.6 m/min = 600 mm/min, the material-removal rate (MRR) can be calculated as MRR = (180)(600) = 108, 000 mm3/min. ׉ 7cassandra://G4833MdBh_y-kVkurDArdREXBe_ME2yUR1XYjMjtvpYv`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://N2OX5e283g6-gRV0rpSEpg0zKdYBTZtqrQw0q54DzGY>`׉ 7cassandra://foz2g4PhMWWMC1Ss167LMqmbU5LDdVnOW0J3cf5vwrs>`s׉ 7cassandra://T1lfVYxABeDFmXSFGkv_e3tuLe3FXIEBmlcvPjlobDI`׉ 7cassandra://V7MU21awgoWB9Pl8LaJyPD0YemDt4nMwToZ8RzYk0DY͢͠^cZEbʹט#u׉׉ 7cassandra://ieJsEum8kFSu3M8UCJI_FfFGOojSMjYXQSFJ6Q_wp68}L`׉ 7cassandra://T3_2phB64jLjWE45_ItOTAnYUJG0E96dpKY4-jdxbV4>`s׉ 7cassandra://h_yiHVf2OLwdIrE2tr6FMAvtDwoJS-YwYgMR2M_ajbo3`׉ 7cassandra://ESYArlUuh_7FGHMTh1iK75uUJrd1OWYLJdMc4uPbZ9oͼT͠^cZEb͵׉E(Milling and Milling Machines 753 The cutting time is given by t = Note from Fig. 24.6 that, for this problem, lc = D t = 500 + 150 10 10 l +2lc v . 2 =75 mm. The cutting time is therefore =65 s =1.08 min. The feed per tooth can be obtained from Eq. (24.3), where N = 100 rpm =1.67 rev/s, and hence f = (1.67)(10) =0.6 mm/tooth. For this material, the unit power can be estimated from Table 21.2 to be 1.1 Ws/mm3. Thus, the power is Power =(1.1)(1800) = 1980 W =1.98 kW. Insert Facet width Small radius Feed marks (a) Corner radius (b) Corner flat Small radius Large radius Wiper (c) Wiper Feed Nose radius R Workpiece Cutter End cutting-edge angle Feed Insert R Feed, mm/tooth Workpiece (d) Figure 24.8: Schematic illustration of the effect of insert shape on feed marks on a face-milled surface: (a) small corner radius, (b) corner flat on insert, and (c) wiper, consisting of a small radius followed by a large radius, resulting in smoother feed marks. (d) Feed marks due to various insert shapes. Side cutting-edge angle or corner angle in face mills Workpiece ׉ 7cassandra://T1lfVYxABeDFmXSFGkv_e3tuLe3FXIEBmlcvPjlobDI`cZ%Eb׉E754 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing End cutting-edge angle Axial rake, 1 Corner angle Peripheral relief (radial relief) End relief (axial relief) Radial rake, 2 Figure 24.9: Terminology for a face-milling cutter. Insert Undeformed chip thickness, t Depth of cut, d f Feed per tooth, f (a) (b) Figure 24.10: The effect of the lead angle on the undeformed chip thickness, t in face milling. Note that as the lead angle increases, the chip thickness decreases, but the length of contact (i.e., chip width) increases. The edges of the insert must be sufficiently large to accommodate the contact length increase. Workpiece Exit Entry Reentry Exit Cutter Milled surface Desirable Cutter (a) (b) (c) Figure 24.11: (a) Relative position of the cutter and insert as they first engage the workpiece in face milling. (b) Insert positions towards the end of cut. (c) Examples of exit angles of the insert, showing desirable (positive or negative angle) and undesirable (zero angle) positions. In all figures, the cutter spindle is perpendicular to the page and rotates clockwise. Undesirable 2 1 Lead angle ׉ 7cassandra://h_yiHVf2OLwdIrE2tr6FMAvtDwoJS-YwYgMR2M_ajbo3`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://vGz04-aiiTPfPRiu6rkpErtp8OhDvIyWr7jb-DoG9Jk`׉ 7cassandra://8v_pydI-SZz7t5nbbL5Ni6inVfOt4DhPxrABSZ3wiiUc`s׉ 7cassandra://bj0ttHDkImoxjkrU7KbEBk3wsXtYReI1Sv5P2_vpeAw`׉ 7cassandra://L-s73s4d8HGyCDx3fSw3SQVKplOAeKKMKuJEshZjGIÍ͠^cZEbͷט#u׉׉ 7cassandra://5i9RzChxZwXUdF040LgiOGduZt8dNgUMKtDH03sVQJsi`׉ 7cassandra://HMOgge_dVG05ROy4Sa8HesqmmwKjl4XjPMXHwmyl5LA:`s׉ 7cassandra://Nl7GJrmi-ueqreixUE4asMABgNUQPdLusLkjfhIIqK0`׉ 7cassandra://qeSGlg9XaMbhZe2SjQo9atG5D1fDogSyMX5V8xL2ltIw ͠^cZEb͸׉E Milling and Milling Machines 755 24.2.3 End Milling End milling is an important and common machining operation, because of its versatility and capability to produce various profiles and curved surfaces. The cutter, called an end mill (Fig. 24.12), has either a straight shank (for small cutter sizes) or a tapered shank (for larger sizes), and is mounted into the spindle of the milling machine. End mills may be made of high-speed steels, solid carbide, or with coated or uncoated carbide inserts, similar to those for face milling. The cutter usually rotates on an axis perpendicular to the workpiece surface, but it can be tilted to conform to machine-tapered or curved surfaces. End mills are available with hemispherical ends (ball nose mills) for machining sculptured surfaces, such as in dies and molds. They can also be produced with a specific radius, profile, flat end, or with chamfer. Hollow end mills have internal cutting teeth, and are used to machine the cylindrical surfaces of solid, round workpieces. End milling can produce a variety of surfaces at any depth, such as curved, stepped, and pocketed (Fig. 24.2b). The cutter can remove material on both its end and on its cylindrical cutting edges, as can be seen in Fig. 24.2b. Vertical-spindle and horizontal-spindle milling machines (see Section 24.2.8), as well as machining centers (see Fig. 25.7), can all be used for end milling. The machines can be programmed such that the cutter can follow a complex set of paths so as to optimize the whole machining operation, for higher productivity and minimum cost. High-speed End Milling. High-speed end milling is an important process, with numerous applications such as the milling of large aluminum-alloy aerospace components and honeycomb structures (see also highspeed machining, Section 25.5). With spindle speeds up to 80,000 rpm, the machines must have high stiffness, usually requiring hydrostatic or air bearings and high-quality work holding devices. The spindles have a rotational accuracy of 10 μm; thus the surfaces produced have very high dimensional accuracy. At high rates of material removal, chip collection and disposal can be a significant problem, as described in Section 23.3.7. Machining cavities in dies (called die sinking, such as in forging or in sheet-metal forming) also is done by high-speed end milling, often using TiAlN-coated ball-nose end mills (Fig. 24.13). The machines generally have four-axis or five-axis movement capability (see, for example, Fig. 24.21), but machining centers (Section 25.2) can add more axes for more complex geometries. Such machines can accommodate dies as large as 3 m × 6 m and weighing 54 metric tons, costing over $2 million. The advantages of five-axis machines are that they (a) are capable of machining very complex shapes, in a single setup, (b) can use shorter tools, thus reducing the tendency for vibration and chatter, and (c) enable drilling of holes at various compound angles. Figure 24.12: A selection of end mills. The flute depth and helix angle are selected based on whether it is a roughing or finishing cut. Note the variety of geometries of the end of the mill; with the proper cutter, a radius, chamfer, or flat surface can be machined. Source: Courtesy of Kennametal, Inc. ׉ 7cassandra://bj0ttHDkImoxjkrU7KbEBk3wsXtYReI1Sv5P2_vpeAw`cZ%Eb׉E756 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Figure 24.13: Ball nose end mills. These cutters are can produce complex contours and are often used in machining dies and molds (see also Fig. 24.2d). Source: Courtesy of Dijet, Inc. 24.2.4 Other Milling Operations and Milling Cutters Several other milling operations and cutters are used to machine workpieces. In straddle milling,two or more cutters are mounted on an arbor, and used to simultaneously machine two parallel surfaces on workpieces (Fig. 24.14a). Form milling produces curved profiles, using cutters with specially shaped teeth (Fig. 24.14b); such cutters are also used for cutting gear teeth (Section 24.7). Slotting and slitting operations Arbor (a) Straddle milling (b) Form milling (c) Slotting (d) Slitting Figure 24.14: Cutters for (a) straddle milling, (b) form milling, (c) slotting, and (d) slitting with a milling cutter. ׉ 7cassandra://Nl7GJrmi-ueqreixUE4asMABgNUQPdLusLkjfhIIqK0`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://YyUL1eR84YY7pAAXS0gPPsIN2pGij_Z_u2JUaKYslesz`׉ 7cassandra://x9sG3wBn_MS0J-iyd5EJdk2jnTkQ7-F2SbB6YCDcpz0Y`s׉ 7cassandra://rgKsEMhWF4aRNAQVlmU7OmyyO0hDi0S1ZaMQXkEHCe0`׉ 7cassandra://4cWX5VUOp8B8aKHUS7SMn1Pw5utOBBJXaOZvQb-cReIΚ͠^cZEbͺט#u׉׉ 7cassandra://Q-ntdSfecNOtfWUn8_O3AkLQqKdqzRfiH-m92jtlsQUp`׉ 7cassandra://3rXaY1PgRXI53cWVM1WeIkGvuq5s9MbbU-lChafRc6cN`s׉ 7cassandra://FDGpUdwc0G2QegdzUGDq-liJRiAwENc9HKEUQB3_jWc:`׉ 7cassandra://FXkSWDq3bHNJgWhrFzsOAd7CHu2ws_8sXr2aEcNnW5QmJV͠^cZEbͻ׉E Milling and Milling Machines 757 are performed with circular cutters, as shown in Fig. 24.14c and d, respectively. The teeth may be staggered slightly, like those in a saw blade (Section 24.5), to provide clearance for the cutter width when machining deep slots. Slitting saws are relatively thin, usually less than 5 mm. T-slot cutters are used to mill T-slots, such as those in machine-tool worktables for clamping workpieces. As shown in Fig. 24.15a, a slot is first milled with an end mill, and then the cutter machines the complete profile of the T-slot, all in one pass. Key seat cutters are used to make semicylindrical (or Woodruff) key seats for shafts. Angle milling cutters, either single-angle or double-angle, are used to produce tapered surfaces with various angles. Shell mills (Fig. 24.15b) are hollow inside, and are mounted on a shank, thus allowing the same shank to be used for different-sized cutters. The uses of shell mills are similar to those for end mills. Milling with a single cutting tooth, mounted on a spindle, is known as fly cutting; it is generally used in simple face-milling and boring operations. The tool can be shaped as a single-point cutting tool, and can be placed in various radial positions on the spindle, in an arrangement similar to that shown in Fig. 23.24b. 24.2.5 Toolholders The stiffness of toolholders and cutters is important for surface quality and in reducing vibration and chatter during milling operations. Arbor cutters are mounted on an arbor (see Figs. 24.14 and 24.18a), for operations such as peripheral, face, straddle, and form milling. In shank-type cutters, the cutter and the shank are made in one piece, the most common examples being end mills. Small end mills have straight shanks, but larger ones have tapered shanks, for better mounting in the machine spindle in order to resist the high forces and torque involved during cutting. Cutters with straight shanks are mounted in collet chucks or in special end-mill holders; those with tapered shanks are mounted in tapered toolholders. 24.2.6 Milling Process Capabilities In addition to the various characteristics of the milling processes described thus far, milling process capabilities include such parameters as surface finish, dimensional tolerances, production rate, and cost considerations. Data on process capabilities are presented in Tables 23.1 and 23.9, Figs. 23.13 and 23.15, and in Chapter 40. The conventional ranges of cutting speeds and feeds for milling are given as guidelines in Table 24.2. Depending on the workpiece material, cutting-tool material and process parameters, cutting speeds are in the range of 30 to 3000 m/min. Feed per tooth typically ranges from about 0.1 mm to 0.5 mm, and depths of cut are usually 1 to 8 mm. For cutting-fluid recommendations, see Table 23.6. First cut (slotting) Second cut Workpiece T-slot (a) (b) Figure 24.15: (a) T-slot cutting with a milling cutter. (b) A shell mill. ׉ 7cassandra://rgKsEMhWF4aRNAQVlmU7OmyyO0hDi0S1ZaMQXkEHCe0`cZ%Eb׉E 758 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Table 24.2: General Recommendations for Milling Operations. Note that these values are for a particular machining geometry and are often exceeded in practice. Material Low-carbon and freemachining steels Alloy steels Soft Hard Cast iron, gray Soft Hard Cutting tool Uncoated carbide, coated carbide, cermets Uncoated, coated cermets Cermets, PcBN Uncoated, coated, cermets, SiN Cermets, SiN, PcBN Stainless steel, Austenitic Uncoated, coated, cermets High-temperature alloys Nickel based Titanium alloys Aluminum alloys Free machining High silicon Copper alloys Plastics Uncoated, coated, cermets, SiN, PcBN Uncoated, coated, cermets Uncoated, coated, PCD PCD Uncoated, coated, PCD PCD 0.13–0.15 0.13–0.23 0.13 0.13–0.23 Uncoated, coated, PCD 0.13–0.23 PCD 50–60 1200–1460 610 300–760 270–460 0.08–0.38 0.08–0.46 0.08–0.38 0.08–0.46 0.08–0.46 40–140 300–3000 370–910 90–1070 90–1370 Source: Based on data from Kennametal, Inc. Note: Depths of cut, d, usually are in the range of 1–8 mm. PcBN: polycrystalline cubic-boron nitride. PCD: polycrystalline diamond. See also Table 23.4 for range of cutting speeds within tool material groups. A general troubleshooting guide for milling operations is given in Table 24.3; the last four items in this table are illustrated in Figs. 24.16 and 24.17. Back striking involves double feed marks, which are made by the trailing edge of the cutter. Note from Table 24.3 that some recommendations (such as changing milling parameters or cutting tools) are easier to accomplish than others (such as changing tool angles, cutter geometry, and the stiffness of spindles and work holding devices). 24.2.7 Design and Operating Guidelines for Milling The guidelines for turning and boring, given in Sections 23.3.6 and 23.4, are also generally applicable to milling operations. Additional factors relevant to milling include the following: • Standard milling cutters should be used as much as possible, depending on part design features; costly special cutters should be avoided. 0.10–0.18 0.10–0.15 0.10–0.20 0.10–0.20 0.13–0.18 0.10–0.18 100–260 90–220 160–440 120–300 120–370 30–370 0.08–0.30 0.08–0.25 0.08–0.38 0.08–0.38 0.08–0.38 0.08–0.38 60–370 75–460 90–1370 90–460 90–500 30–550 General-purpose starting conditions Feed mm/tooth 0.13-0.20 Speed m/min 100-472 Range of conditions Feed mm/tooth Speed m/min 90-425 0.085-0.38 ׉ 7cassandra://FDGpUdwc0G2QegdzUGDq-liJRiAwENc9HKEUQB3_jWc:`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://UjmgiU2dN9hxjsLy1jq6MxbOuwPQUvEivBbA1DL7sjEW`׉ 7cassandra://ZoHrv3tGV-dhiE_MvmIDwSoSkmJpAy_eBu0DueEUAQII`s׉ 7cassandra://i9rBZCvD9nT5Ywfb7-GVSz-Jw2sGoWT4zVdO8slTzUgb`׉ 7cassandra://YPKpb2x7b7zy_7Fe8nDPimUzwCfl75tbLAjoqCoBMsIͰ r͠^cZEbͿט#u׉׉ 7cassandra://1-ukrYj_d7zfzHX6Crwh3gOPRocPnzh-rZufx0DMVu8x`׉ 7cassandra://73tDuzT8HzGqQsOyE-G13RGyWtFAZ44yToGFvZnZciMf`s׉ 7cassandra://4_6UWXuKOlOHVuZHMQsoMcsM3KvAQRYPNA7fe3i867g`׉ 7cassandra://hss2U-LX2Ojwk83U4477NI70Y4WB7XoKzUAl5KwI658c͠^cZEb׉ELMilling and Milling Machines 759 Problem Tool breakage Excessive tool wear Rough surface finish Tolerances too broad Table 24.3: General Troubleshooting Guide for Milling Operations. Probable causes Tool material lacks toughness, improper tool angles, machining parameters too high Machining parameters too high, improper tool material, improper tool angles, improper cutting fluid Feed per tooth too high, too few teeth on cutter, tool chipped or worn, built-up edge, vibration and chatter Lack of spindle and work holding device stiffness, excessive temperature rise, dull tool, chips clogging cutter Workpiece surface burnished Dull tool, depth of cut too low, radial relief angle too small Back striking Chatter marks Burr formation Breakout Dull cutting tools, tilt in cutter spindle, negative tool angles Insufficient stiffness of system; external vibrations; feed, depth of cut, and width of cut too large; select stable processing parameters Dull cutting edges or too much honing, incorrect angle of entry or exit, feed and depth of cut too high, incorrect insert shape Lead angle too low, incorrect cutting-edge geometry, incorrect angle of entry or exit, feed and depth of cut too high Milled surface Cutter (top view) Milled surface Chatter Back striking Direction of workpiece travel No back striking (a) (b) Figure 24.16: Machined surface features in face milling (see also Fig. 24.8). Cutter Insert Burr Small breakout (a) (b) (c) Figure 24.17: Edge defects in face milling: (a) burr formation along workpiece edge, (b) breakout along workpiece edge, and (c) how it can be avoided by increasing the lead angle (see also last row in Table 24.3). • Internal cavities and pockets with sharp corners should be avoided because of the difficulty of milling them, since cutting teeth or inserts have a finite edge radius. When possible, the corner radius should match the milling cutter geometry. • Bevels should be preferred over radii, because cutter and setup costs are higher for machining radii. If inner and outer mating surfaces have the same radius, then the transition between them has to Large breakout ׉ 7cassandra://i9rBZCvD9nT5Ywfb7-GVSz-Jw2sGoWT4zVdO8slTzUgb`cZ%Eb׉E 760 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing be very accurately machined and is very difficult. Specifying a chamfer for the inner surface ensures proper assembly can take place. • Although small milling cutters can be used for milling any surface, they are less rugged and more susceptible to chatter and to tool breakage than large cutters. • Workpieces should be sufficiently rigid to minimize deflections that may result from clamping and cutting forces. • Parts should be designed so that they can be clamped or held in fixtures during machining, and the fixturing should be designed to minimize the number of times that the part needs to be repositioned to complete the milling operation. Guidelines for avoiding vibration and chatter in milling are similar to those for turning; in addition, the following practices should be considered: • Cutters should be mounted as close to the spindle base as possible, in order to reduce tool deflections. • Toolholders and fixturing devices should be as rigid as possible, in order to avoid or minimize vibrations and chatter. • In cases where vibration and chatter may occur, tool shape and process conditions should be modified, including the use of cutters with fewer teeth or, whenever possible, with random tooth spacing (see Section 25.4). 24.2.8 Milling Machines Because they are capable of performing a wide variety of cutting operations, milling machines are among the most versatile of all machine tools. The first milling machine was built in 1820 by E. Whitney (1765– 1825); a wide selection of milling machines with numerous features is now available, the most common of which are described below. These machines have been and continue to be replaced with computer numericalcontrol (CNC) machines and machining centers. Modern machines are very versatile and capable of milling, drilling, boring, and tapping, with repeated and high accuracy (Fig. 24.20). Column-and-knee-type Machines. Used for general-purpose milling operations, column-and-knee-type machines have been the most common milling machines. The spindle on which the cutter is mounted may be horizontal (Fig. 24.18a), for peripheral milling, or vertical, for face and end milling, boring, and drilling operations (Fig. 24.18b). The basic components of these machines are: • Worktable: the workpiece is clamped on the worktable using T-slots; the table moves longitudinally relative to the saddle. • Saddle: supports the table and can move in the transverse direction. • Knee: supports the saddle and gives the table vertical movement so that the depth of cut can be adjusted and workpieces with various heights can be accommodated. • Overarm: used on horizontal machines; it is adjustable to accommodate different arbor lengths. • Head: contains the spindle and cutter holders. In vertical machines, the head may be fixed or it can be adjusted vertically; it can be swiveled in a vertical plane on the column for milling tapered surfaces. Plain milling machines have at least three axes of movement, with the motion usually imparted manually, either with a power screw actuator or by engaging powered actuators to the drive motor. In universal column-and-knee milling machines, the table can be swiveled on a horizontal plane. Complex shapes, ׉ 7cassandra://4_6UWXuKOlOHVuZHMQsoMcsM3KvAQRYPNA7fe3i867g`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://8EezTrMd1_OkCvEFKWrRGMghyNs5zyY9Qa9JLB0zeBg5`׉ 7cassandra://fs6pKG8Bwv_9WtyGAXMoEgGI8DNci9Foren1aKFkHH4R`s׉ 7cassandra://9fmjBPxBcWNUiLcmpzQR1C934TyQdnguT_TObaZh0FoB`׉ 7cassandra://8pZwWi3iKdhNJaV7JMiF83tclrqFEw6riRN0e_n3W44 ͠^cZEbט#u׉׉ 7cassandra://myYs1eZ3Tn1sq9fL4sKr_QtSm7uJ1aJKQeODT2FME-4Z`׉ 7cassandra://l_eIpFVilD6lv7Sd3GLIrKJ23Q1vostcrdTwJkPZhUc8-`s׉ 7cassandra://t17jNGk8vonAiBQcTKPG_Nf2r2TEhiYgRvdAFb3rCsQ`׉ 7cassandra://FdX_FhDoC7ICBC7X7LODUcmM_EfX20gNvPnCVvJB8Gs͞^͠^cZEb׉EMilling and Milling Machines 761 Head Overarm Column Arbor Column Workpiece Worktable Saddle Knee Base (a) Base (b) Figure 24.18: Schematic illustration of (a) a horizontal-spindle column-and-knee-type milling machine and (b) vertical-spindle column-and-knee-type milling machine. Source: After G. Boothroyd. such as helical grooves at various angles, can be machined to produce such parts as gears, drills, taps, and cutters. Bed-type Milling Machines. In bed-type machines, the worktable is mounted directly on the bed, which replaces the knee and moves only longitudinally (Fig. 24.19). Although not as versatile as other types, these machines have high stiffness and typically are used for high-production work. The spindles may be horizontal or vertical and of duplex or triplex types (with two or three spindles, respectively), for the simultaneous machining of two or three workpiece surfaces. Other Types of Milling Machines. Several other types of milling machines are available (see also machining centers, Section 25.2). Planer-type milling machines, which are similar to bed-type machines, are equipped with several heads and cutters to mill different surfaces. They are typically used for heavy workpieces and are more efficient than simple planers (Section 24.3) when used for similar purposes. Rotary-table machines are similar to vertical machines and are equipped with one or more heads for face-milling operations. Profile milling machines have five axes of movement (Fig. 24.21); note the three linear and two angular movements of the machine components. Workholding Devices and Accessories. The workpiece to be machined must be clamped securely to the worktable in order to resist cutting forces and prevent slipping during milling. Various fixtures and vises Spindle carrier Cutters Workpiece Cross-rail Spindle carrier Workpiece Saddle Knee Worktable Table Bed Figure 24.19: Schematic illustration of a bed-type milling machine. ׉ 7cassandra://9fmjBPxBcWNUiLcmpzQR1C934TyQdnguT_TObaZh0FoB`cZ%Eb׉E762 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Figure 24.20: A computer numerical-control (CNC) vertical-spindle milling machine. This is one of the most versatile machine tools. Source: Haas Automation, Inc. are generally used for this purpose (see also Section 37.8 on flexible fixturing). Mounted and clamped to the worktable using the T-slots seen in Fig. 24.18a and b, vises are used for small production runs on small parts, while fixtures are used for higher production runs, and can be automated by various mechanical and hydraulic means. Accessories for milling machines include various fixtures and attachments for the machine head and the worktable, designed to adapt them to different operations. The accessory that has been used most Column Workpiece Table Base Figure 24.21: Schematic illustration of a five-axis profile milling machine. Note that there are three principal linear and two angular movements of machine components. ׉ 7cassandra://t17jNGk8vonAiBQcTKPG_Nf2r2TEhiYgRvdAFb3rCsQ`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://hBVPLqQA04JOK2q6jVSD4fE2uCiL4_EJyToE9g_SnM0`׉ 7cassandra://o-CImWOh0g3_PoiC_PbKgkxet3ikag7aA57mQ0lmw5s^&`s׉ 7cassandra://9_jfF_YlTXqKDRO1gfB9iM9BHmme9PoKt-Xlx9QA-SQ`׉ 7cassandra://KcxyQqiAXu_zhggSEhkQdD5PdgIvN7o4byYUAr-30Vwq͠^cZEbט#u׉׉ 7cassandra://XkabY4JSiUuI2Q4sfjuM1FNobgHV4otkEzf39R-5j58dg`׉ 7cassandra://HsfiH8SxvPTnWjtYyptnLW7eoQVc1i08O8oTJTy1i9AKK`s׉ 7cassandra://hdGgx8akZ4b2FEgiYYVd6XNsktbC8r9mf5GVvteJouwj`׉ 7cassandra://IL5XsuWXS0kIp_ZCWJfjv61jqe5YkJ_7DSMnqsbUooM͠^cZEb׉E Broaching and Broaching Machines 763 Figure 24.22: Typical parts that can be made on a planer. commonly is the universal dividing (index) head. Manually operated, this fixture rotates (indexes) the workpiece to specified angles between individual machining steps. Typically, it has been used to mill parts with polygonal surfaces and to machine gear teeth. 24.3 Planing and Shaping Planing is a relatively simple machining operation by which flat surfaces, as well as cross sections with grooves and notches, can be made along the length of the workpiece (Fig. 24.22). Planing is usually done on workpieces as large as 25 m× 15 m, although 10 m is more typical. In a planer, also called a scalper when a layer is machined from a cast ingot, the workpiece is mounted on a table that travels back and forth along a straight path. A horizontal cross-rail, which moves vertically along the ways of the column, is equipped with one or more tool heads. The cutting tools are mounted on the heads, and machining is done along a straight path. In order to prevent the cutting edges from chipping when tools rub along a workpiece during the return stroke, tools are either tilted or lifted mechanically or hydraulically at each stroke. Because of the reciprocating motion of the workpiece, the noncutting time elapsed during the return stroke is significant. Consequently, these operations are neither efficient nor economical, except for low-quantity production, which is generally the case for large and long workpieces. The efficiency of the operation can be improved by equipping planers with toolholders and tools that cut in both directions of table travel. Also, because of the length of the workpiece, it is essential to equip cutting tools with chip breakers, as otherwise the chips produced can be very long, thus interfering with the machining operation and becoming a safety hazard. Shaping. Machining by shaping is basically the same as by planing, except that it is the tool and not the workpiece that travels; the workpieces are smaller, typically less than 1 m × 2 m of surface area. In a horizontal shaper, the cutting tool travels back and forth along a straight path. The tool is attached to the tool head, which is mounted on the ram; the ram has a reciprocating motion. In most machines, cutting is done during the forward movement of the ram (push cut); in others, it is done during the return stroke of the ram (draw cut). Vertical shapers (called slotters) are used to machine notches, keyways, and dies. Because of low production rates, only special-purpose shapers (such as gear shapers, Section 24.7.2) are in common use today. 24.4 Broaching and Broaching Machines Broaching is similar to shaping using a long, multiple-tooth cutter, and is used to machine internal and external surfaces, such as holes with circular, square, or irregular cross sections; keyways; the teeth of internal gears; multiple spline holes; and flat surfaces (Fig. 24.23). In a typical broach (Fig. 24.24a), the total depth ׉ 7cassandra://9_jfF_YlTXqKDRO1gfB9iM9BHmme9PoKt-Xlx9QA-SQ`cZ%Eb׉E764 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Operator controls (a) Broach Worktable Workpiece (b) (c) Figure 24.23: (a) Typical parts made by internal broaching. (b) Parts made by surface broaching. (c) Vertical broaching machine. Source: (a) and (b) Courtesy of General Broach and Engineering Company, (c) Courtesy of Ty Miles, Inc. Rake or hook angle Chip gullet Cut per tooth Workpiece (a) (b) Figure 24.24: (a) Cutting action of a broach, showing various features. (b) Terminology for a broach. of material removed in one stroke is the sum of the depths of cut of each tooth of the broach. A large broach can remove material as deep as 38 mm in one stroke. Broaching is an important production process, and can produce parts with good surface finish and dimensional accuracy. It competes favorably with other machining processes, such as boring, milling, shaping, and reaming, to produce similar shapes. Although broaches can be expensive, the cost is justified with high-quantity production runs. Broaches. The terminology for a typical broach is given in Fig. 24.24b. The rake (hook) angle depends on the material cut (as it does in turning and other cutting operations) and usually ranges from 0◦ to 20◦.The clearance angle is typically 1◦ to 4◦; finishing teeth have smaller angles. Too small a clearance angle causes rubbing of the teeth against the broached surface. The pitch of the teeth depends on factors such as the length of the workpiece (length of cut), tooth strength, and size and shape of chips. The tooth depth and pitch must be sufficiently large to accommodate the chips produced during broaching, particularly for long workpieces. At least two teeth should be in contact with the workpiece at all times. The following formula may be used to obtain the pitch for a broach to cut a surface of length l: Tooth depth Root radius Pitch Land Back-off or clearance angle ׉ 7cassandra://hdGgx8akZ4b2FEgiYYVd6XNsktbC8r9mf5GVvteJouwj`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://ySv0tGbFjTpQ6k9ePmpTUiD-CRC65tVgnCJKNmDwOBs[`׉ 7cassandra://eDBB6Yzu9qCDPcA0QR9EdkQfBO9VYSRNQmqFl5tXXKAV`s׉ 7cassandra://KBNyR6p6o3-aIHj13hvnYicNamRBhFbS0H5h4zsDf-U1`׉ 7cassandra://vsTzxdwXHJT07RGSpBXk4x5pnhRHVbe3YzymybGnBA8,h.͠^cZEbט#u׉׉ 7cassandra://pkwpgt205VFKOFzxtu1rCcHq7ESe-NxIg124uesBTc4 `׉ 7cassandra://pVyyS4o1Ea9B79eB6LM3Hwb5VMPJT6-HX_uz7WsVCRIb`s׉ 7cassandra://0xsO8s1QIvQklCBYVkuGX1VA2k0fglKyKTWly5LXqrE`׉ 7cassandra://eIKwU33Q-dquZJzsfP68k5Gv9Zpysi4OKyzi6II5riMcs͠^cZEbΑנcZEbЁ:<9ׁHhttp://B.C.Thׁ ׁ Ј׉E Broaching and Broaching Machines 765 Pitch = k √ l, (24.6) where k is a constant, equal to 1.76 when l is in mm. An average pitch for small broaches is in the range from 3.2 to 6.4 mm, and for large ones it is in the range from 12.7 to 25 mm. The depth of cut per tooth depends on the workpiece material and the surface finish required. It is usually in the range from 0.025 to 0.075 mm for medium-sized broaches, but can be larger than 0.25 mm for larger broaches. Broaches are available with various tooth profiles, including some with chip breakers (Fig. 24.25). The variety of surface broaches include slab (for cutting flat surfaces), slot, contour, dovetail, pot (for precision external shapes), and straddle. Internal broach types include hole (for close-tolerance holes, round shapes, and other shapes; Fig. 24.26), keyway, internal gear,and rifling (for gun barrels). Irregular internal shapes usually are broached by starting with a round hole drilled or bored in the workpiece. Turn Broaching. Turn broaching is a combination of shaving and skiving (removing a thin layer of material with a specially shaped cutting tool). The process is typically used for broaching the bearing surfaces of crankshafts and similar parts. The shaft is rotated between centers, and the broach, equipped with multiple carbide inserts, passes tangentially across the bearing surfaces and removes material. Straight as well as circular broaches can be used successfully in turn broaching, including simultaneously broaching a number of crankshafts. Broaching Machines. These machines are relatively simple in construction, as they have linear motions only; they usually are actuated hydraulically, although some are moved by crank, screw, or rack mechanisms. Several styles of machines are available, and sizes range from machines for making needle-like parts to those used for broaching gun barrels, including rifling (producing internal spiral grooves). Deep-slotted chip breakers Staggered simple chip breakers Staggered double-cut heavy chip breakers Staggered chip breakers Alternate breakers and smooth teeth Smooth finishing teeth (a) (b) Figure 24.25: Chip breaker features on (a) a flat broach and (b) a round broach. Semifinishing teeth Pull end Front pilot Roughening teeth Finishing teeth Rear pilot Follower diameter Root diameter Shank length Cutting teeth Overall length Figure 24.26: Terminology for a pull-type internal broach used for enlarging long holes. ׉ 7cassandra://KBNyR6p6o3-aIHj13hvnYicNamRBhFbS0H5h4zsDf-U1`cZ%Eb׉E 766 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Broaching machines, either pull or push type, are either horizontal or vertical. Push broaches usually are shorter, with lengths in the range of 150–350 mm. Pull broaches tend to straighten a hole, whereas a push broach permits the broach to follow any irregularity of the leader hole. The force required to pull or push a broach depends on the (a) strength of the workpiece material, (b) total periphery of the cut, (c) cutting speed, (d) tooth profile, and (e) type of cutting fluid used. The pulling force capacities of broaching machines are as high as 0.9 MN (90 metric tons). Process Parameters. Cutting speeds for broaching may range from 1.5 m/min for high-strength alloys to as much as 30 m/min for aluminum and magnesium alloys. The most common broach materials are M2 and M7 high-speed steels and carbide inserts. Smaller, high-speed steel blanks for making broaches can be produced by powder-metallurgy techniques (Chapter 17) for better control of quality. Cutting fluids generally are recommended, especially for internal broaching. Design Considerations. Broaching, as with other machining processes, requires that certain guidelines be followed in order to obtain economical and high-quality production. The major requirements are: • Blanks should be designed and prepared so that they can be securely clamped in broaching machines, and should have sufficient structural strength and stiffness to withstand the cutting forces during broaching. • Keyways, splines, gear teeth, etc., should all have standard sizes and shapes, so as to allow the use of common broaches. • Balanced cross sections are preferable to keep the broach from drifting laterally, thus maintaining close tolerances. • Radii are difficult to broach and chamfers are preferred; inverted or dovetail splines should be avoided. • Blind holes should be avoided whenever possible, but if necessary, there must be a relief at the end of theareatobebroached. Case Study 24.1 Broaching Internal Splines The part shown in Fig. 24.27 is made of nodular iron (65-45-15; Section 12.3.2), with internal splines, each 50 mm long. The splines have 19 involute teeth, with a pitch diameter of 63.52 mm. An M2 high-speed steel broach, with 63 teeth, a length of 1.448 m, and a diameter the same as the pitch diameter, was used to produce the splines. The cut per tooth was 0.116 mm. The production rate was 63 pieces per hour. The number of parts per grind was 400, with a total broach life of about 6000 parts. Source: ASM International. 24.5 Sawing Sawing is a common process dating back to around 1000 B.C.The blade or saw has a series of small teeth, each removing a small amount of material with each stroke of the saw. Sawing can be used for all materials and is capable of producing any shape (Fig. 24.28). It is an efficient material-removal process and can produce near-net shapes from blanks. The width of cut (kerf) in sawing usually is small, so that the process wastes relatively little material. Typical saw-tooth and saw-blade configurations are shown in Fig. 24.29, where tooth spacing is generally in the range from 0.08 to 1.25 teeth per mm. A wide variety of sizes, tooth forms, tooth ׉ 7cassandra://0xsO8s1QIvQklCBYVkuGX1VA2k0fglKyKTWly5LXqrE`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://_jCNvZR-n5Ktjla3vaY7nou01or9idD5nyNtg4rJ8RI[u`׉ 7cassandra://EmS-3LtYXy-PCDIzFeVAcYbeBmdqSEBrZuynvIbWXCgE.`s׉ 7cassandra://F1akUq5YiVva6Q9N7dJRQSwqZxIadrymz8iXrXdHKEo`׉ 7cassandra://jYAF6Ip6BA_pij9u5lTnlGYakceET2E4HQeFtddeNnE͚f#͠^cZEbט#u׉׉ 7cassandra://6oDGbxSqR7L6KwNbvV4OaInqdODy62Tz2Bo75JuyJuMy`׉ 7cassandra://pF5prt0P5uxSKlv4r6wQsWdLHVxpXLelsqsrvwUsu7oPR`s׉ 7cassandra://mO57V_IuDujfxSABKaqJVPuNo2QO9pgu8Em7PHq78Mo`׉ 7cassandra://gjEosXNwmT0dWJEOAJ6ceqPvYmUawbaIV93i0o_upMA͛ T͠^cZEb׉E%Sawing 767 203 mm 311 mm diameter Internal spline 50 mm Figure 24.27: Example of a part with internal splines that were produced by broaching. Saw (a) Ripping (b) Internal cuts (c) Angular cuts Ingot Wafer being sliced Diamond cutting edge Saw blade Feed (d) Contour cutting (e) Stack cutting (f) Wafer slicing Figure 24.28: Examples of various sawing operations. spacing, and blade thicknesses and widths are available. Saw blades generally are made from high-carbon and high-speed steels (M2 and M7); carbide or high-speed-steel tipped steel blades are used for sawing harder materials and at higher speeds (Fig. 24.30). The tooth set in a saw (Fig. 24.29b) is important in providing a sufficiently wide kerf for the blade to move freely in the workpiece without binding or excessive frictional resistance and thus reducing the heat generated. Elevated temperatures can have adverse effects on the material cut, especially for thermoplastics, which soften rapidly when temperature rises (see Fig. 7.11). The tooth set also allows the blade to track a path accurately, following the pattern to be cut without wandering. At least two or three teeth should always be engaged with the workpiece, in order to prevent snagging (catching of the saw tooth on the workpiece). This is the reason why thin materials, especially sheet metals, can be difficult to saw. The thinner the stock, the finer the saw teeth should be, and the greater the number of teeth per unit length of the saw. Cutting fluids are generally used to improve the quality of the cut and the life of the saw. ׉ 7cassandra://F1akUq5YiVva6Q9N7dJRQSwqZxIadrymz8iXrXdHKEo`cZ%Eb׉E `768 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing Back edge Tooth spacing Tooth face Tooth back (flank) Tooth back clearance angle (a) Gullet depth Tooth rake angle (positive) Wave tooth (b) Figure 24.29: (a) Terminology for saw teeth. (b) Types of tooth sets on saw teeth staggered to provide clearance for the saw blade to prevent binding during sawing. M2 HSS 64-66 HRC Electron-beam weld Flexible alloy–steel backing (a) Carbide insert (b) Figure 24.30: (a) High-speed-steel teeth welded onto a steel blade. (b) Carbide inserts brazed to blade teeth. Types of Saws. Hacksaws have straight blades and reciprocating motions. Developed in the 1650s, they generally are used to cut off bars, rods, and structural shapes; they may be manual or power operated. Because cutting takes place during only one of the strokes, hacksaws are not as efficient as band saws (see below). Power hacksaw blades are usually 1.2 to 2.5 mm thick, and up to 610 mm long. The rate of strokes ranges from 30 per minute for high-strength alloys to 180 per minute for carbon steels. The hacksaw frame in power hacksaws is weighted by various mechanisms, applying as much as 1.3 kN of force to the workpiece to improve the cutting rate. Hand hacksaw blades are thinner and shorter than power hacksaw blades, which have as many as 1.2 teeth per mm for sawing sheet metal and thin tubing. Circular saws, also called cold saws for cutting metals, generally are used for high-production-rate sawing, called cutting off. Cutting-off operations also can be carried out with thin, abrasive disks, as described in Section 26.4. Cold sawing is common in industry, particularly for cutting off large cross sections. These saws are available with a variety of tooth profiles and sizes. In modern machines, cutting off with circular saws produce relatively smooth surfaces, with good thickness control and dimensional accuracy because of the stiffness of the machines and of the saws. The inner-diameter-cutting saw, shown in Fig. 24.28f, is widely used to cut single-crystal silicon wafers in microelectronic devices (Section 28.4). Band saws have continuous, long, flexible blades. Vertical band saws are used for straight as well as contour cutting of flat sheets and other parts, supported on a horizontal table (Fig. 24.28d). Also available Raker tooth Width Tooth set Straight tooth ׉ 7cassandra://mO57V_IuDujfxSABKaqJVPuNo2QO9pgu8Em7PHq78Mo`cZ%Eb cZ%Eb#בCט#u׉׉ 7cassandra://lRAbm--BXiMHey9BgbCIhXyPR4ovfDKUUJUvtZGPxaw`׉ 7cassandra://hHI-aCFYyePIHx3Z0Hrt6t0Er3WTm4OLHYGQrJTmWto\E`s׉ 7cassandra://-bmz2R-0fgTKyGdhkcMQs0XRiJZjrSmdXibs4N5JPyE`׉ 7cassandra://jTRapfjpQvI4Di2ABHHCoLoXBo3EbSXGpGwQI1E-9NQ͢v͠^cZEbט#u׉׉ 7cassandra://rdNNHKkUWtrjUCJTSfWQB-gW0hfBX03URbMR-BxRBL0z`׉ 7cassandra://q3DvVq0QkD8nHcG-OI9X1FjLFTBiYdtLBXigFrdWPzIT`s׉ 7cassandra://pkbdYPXKiP3miKTV2fKIOTDtchKw13TBDNb0w2M8j3kH`׉ 7cassandra://Kf9IyQEI2f7FIEbk2tTZH3CO6zGisBN7VYzZTRUcnVw͈^͠^cZEb׉E Filing 769 are computer-controlled band saws, with the capability of guiding the contour path automatically. Power band saws have higher productivity than power hacksaws, because of their continuous cutting action. Using high-speed-steel blades, speeds for sawing high-strength alloys are up to about 60 m/min, and 120 m/min for carbon steels. Blades and high-strength wire can be coated with diamond powder (diamond-edged blades and diamond-wire saws), in which diamond particles act as cutting teeth (abrasive cutting); carbide particles also are used for this purpose. The blades and wires are suitable for sawing hard metallic, nonmetallic, and composite materials. Wire diameters range from 13 mm for use in rock cutting to 0.08 mm for precision cutting. Hard materials also can be sawed with thin, abrasive disks and with advanced machining processes (Chapter 27). Friction Sawing. In friction sawing, a mild-steel blade or disk rubs against the workpiece, at speeds of up to 7600 m/min. The frictional energy is converted into heat, which rapidly softens a narrow zone in the workpiece. The action of the blade, which can have teeth or notches for higher cutting efficiency, pulls and ejects the softened metal from the cutting zone. The heat generated produces a heat-affected zone (Section 30.9) on the cut surfaces; the workpiece properties along the cut edges can thus be affected adversely by this process. Because only a small portion of the blade is engaged with the workpiece at any time, the blade itself cools rapidly as it passes through the air. The friction-sawing process is suitable for hard ferrous metals and reinforced plastics, but not for nonferrous metals because of their tendency to stick to the blade. Friction sawing is commonly used to also remove flash from castings. Disks as large as 1.8 m in diameter are used to cut off large steel sections. 24.6 Filing Filing involves small-scale removal of material from a surface, corner, edge, or hole, including the removal of burrs (see Fig. 16.2). First developed around 1000 B.C., files are usually made of hardened steel, and are available in a variety of cross sections, such as flat, round, half-round, square, and triangular. They can have several tooth forms and coarseness grades. Although filing is usually done by hand, filing machines,with automatic features, are available for high production filing, with files reciprocating at up to 500 strokes/min. Band files consist of file segments, each about 75 mm long and riveted to a flexible steel band; they are used in a manner similar to band saws. Disk-type files also are available. Rotary files and burs (Fig. 24.31) are used for such applications as deburring, removing scale from surfaces, producing chamfers on parts, and removing small amounts of material in die making. These tools generally are conical, cylindrical, or spherical in shape, and have various tooth profiles. Their cutting action (similar to that of reamers, Section 23.6) removes small amounts of material at high rates. The rotational speed of burs ranges from 1500 rpm for cutting steels (using large burs) to as high as 45,000 rpm for magnesium (small burs). High-speed steel bur (a) Carbide bur (b) Rotary file (c) Figure 24.31: Types of burs used in burring operations. ׉ 7cassandra://-bmz2R-0fgTKyGdhkcMQs0XRiJZjrSmdXibs4N5JPyE`cZ%Eb!׉E 770 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 24.7 Gear Manufacturing by Machining Several processes for making gears or producing individual gear teeth were described in Parts II and III, involving such processes as casting, forging, extrusion, drawing, thread rolling, and powder metallurgy. Blanking of sheet metal also can be used for making thin gears, such as those used in mechanical watches, clocks, and similar mechanisms. Plastic gears can be made by casting (Chapter 11) or injection molding (Section 19.3). Gears may be as small as those used in watches or as large as 9 m in diameter for rotating mobile crane superstructures and mining equipment. The dimensional accuracy and surface finish required for gear teeth depend on their particular intended use. Poor gear-tooth quality contributes to inefficient energy transmission, increased vibration and noise, and adversely affects the gear’s friction and wear characteristics. Submarine gears, for example, have to be of extremely high quality so as to reduce noise levels. The standard nomenclature for an involute spur gear is shown in Fig. 24.32. Starting with a wrought or cast gear blank, there are two basic methods of making gear teeth: form cutting and generating. 24.7.1 Form Cutting In form cutting, the cutting tool is similar to a form-milling cutter made in the shape of the space between the gear teeth (Fig. 24.33a). The gear-tooth shape is reproduced by machining the gear blank around its periphery. The cutter travels axially along the length of the gear tooth, and at the appropriate depth to produce the gear-tooth profile. After each tooth is cut, the cutter is withdrawn, the gear blank is rotated (indexed), and the cutter proceeds to cut another tooth. This process continues in a cycle until all of the teeth are machined. Each cutter is designed to cut a range of numbers of teeth. The precision of a form-cut tooth profile depends on the cutter accuracy and on the machine and its stiffness. Because the cutter has a fixed geometry, form cutting can be used only to produce gear teeth that have a constant width, that is, on spur or helical gears but not on bevel gears. Internal gears and gear teeth Addendum Dedendum Clearance Fillet radius Dedendum circle Clearance circle Figure 24.32: Nomenclature for an involute spur gear. Face width Top land Face Flank Bottom land Outside circle Width of space Circular pitch Tooth thickness Pitch circle ׉ 7cassandra://pkbdYPXKiP3miKTV2fKIOTDtchKw13TBDNb0w2M8j3kH`cZ%Eb"cZ%Eb!#בCט#u׉׉ 7cassandra://dtkDq6Hi0rCUkDtjRpryV7PfXO03ZmahEx6POUW5FZA<`׉ 7cassandra://OmKMHBefyylXybuJponfAZzb5jlkBpVvk8Q28Wa-Mb0:`s׉ 7cassandra://7T24qtR5tvSIFkLmgkxiLoKYm9fp4F0GHJ_ULS7Tw2s `׉ 7cassandra://AYF0uedsQAy1vmCDQFKjIcDa9t__225Z7YZkweB2iOA49͠^cZEbט#u׉׉ 7cassandra://-IlUxc5sTDcR9KGyWVW7myxZ6VR6bD4VHSHTXPIY-jI`׉ 7cassandra://RPimEFQJwxe3-_Fc-9-WG_bOl3EmK8uuKWJGvRgKtyEs`s׉ 7cassandra://BzAVJASjr8Fx8DzpEZoYYymx4sWE23v0wl4DAg1dPmYf`׉ 7cassandra://BjoJOBnTOkzV3Ik3nF_wl9yIDQo9hRfvn4nwo8dnGdEd͠^cZEb׉EGear Manufacturing by Machining 771 Gear cutter Base circle Form cutter Pitch circle Gear blank Gear blank Pitch circle (a) (b) Axial movement (cutting direction) Cutter spindle Spacer Base circle Pinion-shaped cutter Gear blank Gear teeth Pinion cutter (c) Rack-shaped cutter Pinion-cutter rotation (d) Gear rotation Gear Gear blank (e) Figure 24.33: (a) Producing gear teeth on a blank by form cutting. (b) Schematic illustration of gear generating with a pinion-shaped gear cutter. (c) and (d) Gear generating in a gear shaper using a pinion-shaped cutter. Note that the cutter reciprocates vertically. (e) Gear generating with rack-shaped cutter. Source: (d) Courtesy of Steve Schmid, with permission from Schafer Gear Works. ׉ 7cassandra://7T24qtR5tvSIFkLmgkxiLoKYm9fp4F0GHJ_ULS7Tw2s `cZ%Eb#׉E*772 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing on straight surfaces, such as those in a rack and pinion, are form cut with a shaped cutter, on a machine similar to a shaper. Broaching can be used to machine gear teeth and is particularly suitable for producing internal teeth. The process is rapid and produces fine surface finish with high dimensional accuracy. However, because a different broach is required for each gear size and broaches are expensive, this method is suitable almost exclusively for high-quantity production. Although inefficient, form cutting also can be done on milling machines, with the cutter mounted on an arbor and the gear blank mounted in a dividing head. Gear teeth also may be cut on special machines, with a single-point cutting tool guided by a template, made in the shape of the gear-tooth profile. Because the template can be made much larger than the gear tooth itself, dimensional accuracy is improved. Form cutting is a relatively simple process and can be used for cutting gear teeth with various profiles. Nonetheless, it is a slow operation and some types of machines require skilled labor. Machines with semiautomatic features can be used economically for form cutting on a limited-production basis; generally, however, form cutting is suitable only for low-quantity production. 24.7.2 Gear Generating The cutting tool used in gear generating may be a pinion-shaped cutter, a rack-shaped straight cutter, or a hob. Gear-generating machines also can produce spiral-bevel and hypoid gears. Like most other machine tools, modern gear-generating machines are now computer controlled. Multiaxis computer-controlled machines are capable of generating several types and sizes of gears, using indexable milling cutters. 1. A pinion-shaped cutter can be considered as one of the two gears in a conjugate pair, with the other being the gear blank (Fig. 24.33b). This type of cutter is used on vertical gear shapers (Fig. 24.33c and d). The cutter has an axis parallel to that of the gear blank, and rotates slowly with the blank at the same pitch–circle velocity and in an axial-reciprocating motion. A train of gears provides the required relative motion between the cutter shaft and the gear-blank shaft. Cutting may take place at either the downstroke or the upstroke of the machine. Because the clearance required for the cutter travel is small, gear shaping is suitable for gears that are located close to obstructing surfaces, such as the flange in the gear blank shown in Fig. 24.33c and d. This process can be used for low-quantity as well as high-quantity production. 2. On a rack shaper, the generating tool is a segment ofa rack (Fig. 24.33e), which reciprocates parallel to the axis of the gear blank. Because it is not practical to have more than 6 to 12 teeth on a rack cutter, the cutter must be disengaged at suitable intervals, and returned to the starting point. The gear blank remains fixed during this operation. 3. A hob (Fig. 24.34) is basically a gear-cutting worm, or screw, made into a gear-generating tool by a series of longitudinal slots or gashes machined into it to form the cutting teeth. When hobbing a spur gear, the angle between the hob and gear-blank axes is 90◦ minus the lead angle at the hob threads. All motions in hobbing are rotary, and the hob and gear blank rotate continuously, much as two gears mesh, until all of the teeth are cut. Hobs are available with one, two, or three threads. For example, if the hob has a single thread and the gear is to have 40 teeth, the hob and the gear spindle must be geared together such that the hob makes 40 revolutions while the gear blank makes 1 revolution. Similarly, if a double-threaded hob is used, the hob would make 20 revolutions to the gear blank’s 1 revolution. In addition, the hob must be fed parallel to the gear axis, for a distance greater than the face width of the gear tooth (Fig. 24.32), in order to produce straight teeth on spur gears. The same hobs and machines can be used to cut helical gears, by tilting the axis of the hob spindle. Because it produces a variety of gears at high rates and with good dimensional accuracy, gear hobbing is used extensively in industry. Although the process is suitable also for low-quantity production, it is most economical for medium- to high-quantity production. ׉ 7cassandra://BzAVJASjr8Fx8DzpEZoYYymx4sWE23v0wl4DAg1dPmYf`cZ%Eb$cZ%Eb##בCט#u׉׉ 7cassandra://TSXpPSzpIPB-g0moj17ZrFRuGZ4kOjAhZmjd5JifZVwU`׉ 7cassandra://W7nWeQUnlSKJ9K60zUd45L2VAE1HTjz5c4K9UExSQNY9`s׉ 7cassandra://Dzka_bOjLOvgek_sHA6yGtgpxVjyyrp3-nf3_1kLPOA;`׉ 7cassandra://cVkcZsAV0ckb3cGchn3oTPWXaot--UaWDcD4T29VMtwX͠^cZEbט#u׉׉ 7cassandra://Cngx1bECOh0p6UzLL_TIsXlSDTE2lkEdC4Lqbi-mIWc`׉ 7cassandra://8-PL7yMnBAOswS85Ph_Zr6jXDTUgwK0R1hnc7rbR_YMp`s׉ 7cassandra://AMZ6MchCncpxcvuNcDPqoReJ-vGBaBdTIluThZ4qwiE`׉ 7cassandra://42UGDIfBqwU0RsGWq-9gqM8IttcTIhuckDVihUWx_kof͠^cZEb׉E.Gear Manufacturing by Machining 773 Top view Gear blank Hob (a) Hob Gear blank (b) Figure 24.34: (a) Hobs, used to machine gear teeth; (b) schematic illustration of gear cutting with a hob. Source: (a) Courtesy of Sandvik Coromant. 24.7.3 Cutting Bevel Gears Straight bevel gears generally are roughed out in one cut with a form cutter, on machines that index automatically; the gear is then finished on a gear generator. The cutters reciprocate across the face of the bevel gear, as does the tool on a shaper (Fig. 24.35a). The machines for spiral bevel gears operate essentially on the Gear blank Cutter Cutters Gear blank (a) (b) Figure 24.35: (a) Cutting a straight bevel-gear blank with two cutters. (b) Cutting a helical bevel gear. Source: Courtesy of Steve Schmid, with permission from Schafer Gear Works. ׉ 7cassandra://Dzka_bOjLOvgek_sHA6yGtgpxVjyyrp3-nf3_1kLPOA;`cZ%Eb%׉EE774 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing same principle, in which the spiral cutter is basically a face-milling cutter, with a number of straight-sided cutting blades protruding from its periphery (Fig. 24.35b). 24.7.4 Gear-finishing Processes As produced by any of the processes described, the surface finish and dimensional accuracy of gear teeth may still be insufficient for some applications. Several finishing processes are available to improve the surface quality of the gears, the choice being dictated by the method of gear manufacture, the desired performance, and whether the gears have been hardened by heat treatment. As described in Chapter 4, heat treating can cause distortion of parts; consequently, for a precise gear-tooth profile, heat-treated gears typically are subjected to finishing operations. Shaving. The gear-shaving process involves a cutter made in the exact shape of the finished tooth profile, which removes very small amounts of material from the surface of the gear teeth (see also Fig. 16.9). The cutter, which has a reciprocating motion, has teeth that are slotted or gashed at several points along its width, making the process similar to fine broaching. Shaving and burnishing (described next) can be performed only on gears with a hardness of 40 HRC or lower. Although the tooling is expensive and special machines are required, shaving is rapid and is the most commonly used process for gear finishing. It produces gear teeth with improved surface finish and good dimensional accuracy of the tooth profile. Shaved gears may subsequently be heat treated and then ground, for improved hardness, wear resistance, and a more accurate tooth profile. Burnishing. The surface finish of gear teeth also can be improved by burnishing. Introduced in the 1960s, burnishing is basically a surface plastic-deformation process (see Section 34.2), using a gear-shaped burnishing and specially hardened die, that subjects the tooth surfaces to a surface-rolling action, called gear rolling. The resulting cold working of the tooth surfaces not only improves the surface finish, but also induces compressive residual stresses on the surfaces of the gear teeth, thus improving their fatigue life. It has been shown, however, that burnishing does not significantly improve the dimensional accuracy of the gear tooth. With powder-metallurgy gears, burnishing leads to surface densification, with a significant improvement in performance (see Section 17.5). Grinding, Honing, and Lapping. For the highest dimensional accuracy in tooth spacing and form, and for superior surface finish, gear teeth may be ground, honed, and lapped (Chapter 26). Specially dressed grinding wheels are used for either forming or for generating gear-tooth surfaces. There are several types of grinders, with the single-index form grinder being the most commonly used. In form grinding,the shape of the grinding wheel is identical to that of the tooth spacing (Fig. 24.36a). In generating, the grinding wheel acts in a manner similar to a gear-generating cutter, described previously (Fig. 24.36b). The honing process is faster than grinding, and is used to improve surface finish. The honing tool is a plastic gear impregnated with fine abrasive particles. To further improve the finish, ground gear teeth are lapped, using abrasive compounds either with (a) a gear-shaped lapping tool made of cast iron or bronze, or (b) a pair of mating gears that are run together. Although production rates are lower and costs are higher, these finishing operations are particularly suitable for making hardened gears of very high quality, long life, and quiet operation. 24.7.5 Design Considerations and Economics of Gear Machining Design considerations for gear-cutting operations are summarized as follows: • Gears should preferably be machined prior to their assembly on shafts; wide gears are more difficult to machine than narrow ones. • Sufficient clearance should be provided between gear teeth and flanges, shoulders, and other features of the part, so that the cutting tool can machine without any interference. ׉ 7cassandra://AMZ6MchCncpxcvuNcDPqoReJ-vGBaBdTIluThZ4qwiE`cZ%Eb&cZ%Eb%#בCט#u׉׉ 7cassandra://aiQMVRUDoy0e5rB6uDzwE4nOCXmHdXvEWp9RzRGY39o`׉ 7cassandra://AEojCi_1G41SOjO71mfP05KifLd8gFX2sJ8xif4xoIYBW`s׉ 7cassandra://m5jxQsf_thlue9I68bGBPRV8JtGQqj1bL62250RE5doz`׉ 7cassandra://04ouU5tqOMAImSkcFQqHR2CousN2QJGgdiJohknlrGE͡M͠^cZEbט#u׉׉ 7cassandra://XUPdgMHpO36Bn-N_80pbnZIXypddZH0ku1ZLf2iCb7A`׉ 7cassandra://MrMphgr1yioKBbzo_vkjC_k5cY_rlQ3MrV6St9lh5WoTO`s׉ 7cassandra://oU1YNeaObhC4gOpeQ5fWq0GYP7TkNRIVRN0RfM_VmgQ`׉ 7cassandra://-LuKRUocLY7GgAx_mIbvY2TGojU-eyoynitRJ1rmgHEp;̨͠^cZEb׉EGear Manufacturing by Machining 775 Gear Grinding wheel Worm gear Single-ribbed grinding wheel (a) Grinding wheels Multi-ribbed grinding wheel Two grinding wheels Gear 158 or 208 position (b) Figure 24.36: Finishing gears by grinding: (a) form grinding with shaped grinding wheels; (b) grinding by generating, using two wheels. • Blank design is important for proper fixturing and to simplify cutting operations. Machining allowances must be provided in the blanks, and if machining is to be followed by finishing operations, the part must still be oversized after machining; that is, it must have a finishing allowance after being machined. • Spur gears are easier to machine than helical gears, which, in turn, are easier to machine than bevel gears or worm gears. • Dimensional tolerances and standardized gear shapes are specified by industry standards. A gear quality number should be selected so that the gear has as wide a tolerance range as possible, while still meeting performance requirements in service. Economics. As in all machining operations, the cost of gears increases rapidly with improved surface finish and gear quality. Figure 24.37 shows the relative manufacturing cost of gears as a function of quality, as specified by the American Gear Manufacturers Association (AGMA). The higher the number, the higher is the dimensional accuracy of the gear teeth. As noted in this figure, the manufacturing cost can vary by an order of magnitude, depending on dimensional tolerances. 08 position ׉ 7cassandra://m5jxQsf_thlue9I68bGBPRV8JtGQqj1bL62250RE5doz`cZ%Eb'׉E 776 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 125 100 10 Typical Dimensional Tolerance, mm 25 12.5 2.5 1.25 1 0.5 Special methods Production grinding Shaving Gear-shaper hobbing Cold forging Metal injection molding 4 6 810 12 14 16 AGMA quality index Figure 24.37: Gear manufacturing cost as a function of gear quality. Case Study 24.2 Ping Golf Putters In their efforts to develop high-end, top performing putters, engineers at Ping Golf, Inc., utilized advanced machining practices in their design and production processes for a new style of putter, called the Anser R series, shown in Fig. 24.38. In response to a unique set of design constraints, they had the task and goal of creating putters that would be practical for production quantities and also meet specific functional and aesthetic requirements. One of the initial decisions concerned the selection of a proper material for the putter to meet its functional requirements. Four types of stainless steel (303, 304, 416, and 17-4 precipitation hardening; see Section 5.6) were considered for various property requirements, including machinability, durability, and the sound or feel of the particular putter material, another requirement that is unique to golf equipment. Among the materials evaluated, 303 stainless steel was chosen because it is a free-machining steel (Section 21.7), indicating that in machining it produces smaller chips, lower power consumption, better surface finish, and improved tool life, thus allowing for increased machining speeds and higher productivity. The next step of the project involved determining the optimum blank shape and the sequence of operations to be performed during its production. For this case, engineers chose to develop a slightly oversized forged blank (Chapter 14). A forging was chosen because it provided a favorable internal grain structure as opposed to a casting, which could result in porosity and an inconsistent surface finish after machining. The blank incorporated a machining allowance, whereby dimensions were specified approximately 1.25–1.9 mm larger in all directions than that of the final part. The most challenging, and longest, task was developing the necessary programming and designing fixtures for each part of the putter. Beyond the common requirements of typical machined parts, including tight tolerances and repeatability, putters require an additional set of aesthetic specifications. In this case, both precise machining and the right overall appearance of the finished part were imperative. A machining technique known as surfacing or contouring (commonly used in making injection molds) was used to machine most of the finished geometry. Although this operation required additional machining, Relative cost ׉ 7cassandra://oU1YNeaObhC4gOpeQ5fWq0GYP7TkNRIVRN0RfM_VmgQ`cZ%Eb(cZ%Eb'#בCט#u׉׉ 7cassandra://A498ffBaovMnPfhSDXc9JFHa0noerd0AU99jh3ZW-Ng~`׉ 7cassandra://Wfj5Q51jR7y7eMj3gNdh8Dh0Hy03U0PpcR2MrQi7m20N~`s׉ 7cassandra://_41AqL-KGqKWrFeRd0j-T-nWNipIRNAHNQ3jC81-EmIu`׉ 7cassandra://SALH-8AZQ7tlZ9f0SERDXgymz9PtZtzvo9KNDDTiOTUz0͠^cZEbט#u׉׉ 7cassandra://EPx7-wVaOmfSBHdMmB5Z8ajpQt1GCnqJvhq-w3oIQg8-u`׉ 7cassandra://qFWdKZHsFLPzr170ZWDF6X_GQLfGeWcIp9Fqr6Qp61I\#`s׉ 7cassandra://1oiALVbPRwH1g50xB-b40Npwdc3yxgt7hRjFYejlxQo"`׉ 7cassandra://GoN6FxgorgPcTlIXcW1Z1jZfVkv_gynRSu4RCnzwuSsͪ_͠^cZEb׉EGear Manufacturing by Machining 777 (a) (b) (c) (d) Figure 24.38: (a) The Ping Anser R golf putter; (b) CAD model of rough machining of the putter outer surface; (c) rough machining on a vertical machining center; (d) machining of the lettering in a vertical machining center; the operation was paused to take the photo, as normally the cutting zone is flooded with a coolant. Source: Courtesy of Ping Golf, Inc., Phoenix, Arizona. it provided a superior finish on all surfaces and allowed machining of more complex geometries, thus adding value to the finished product. As for all high-volume machined parts, repeatability was essential. Each forged blank was designed with a protrusion across the face of the putter, allowing for the initial locating surfaces, for ease of fixturing. A short machining operation removed a small amount of material around the bar and produced three flat, square surfaces, as a reference location for the first primary machining operation. Each putter required six different operations in order to machine all of its surfaces, and each operation was designed to provide locating surfaces for the next step in the manufacturing process. Several operations were set up using a tombstone loading system (Section 37.8) on a horizontal-spindle CNC milling machine. This method allowed machine operators to load and unload parts while other parts were being machined, thus significantly increasing the efficiency of the operation. Modular fixturing and using tungsten-carbide cutting tools coated with TiAlN (Section 22.5) allowed for the quick changeover between right- and left-handed parts, as well as different putter models. After the initial locating operation was complete, the parts were transferred to a three-axis vertical machining center (see, for example, Fig. 25.7) to cut the putter cavity. Since the forged blanks were near net shape, the maximum radial depth of cut on most surfaces was 1.9 mm, but the axial depth of cut of 37.5 mm ׉ 7cassandra://_41AqL-KGqKWrFeRd0j-T-nWNipIRNAHNQ3jC81-EmIu`cZ%Eb)׉E 778 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing inside the cavity of the putter was the most demanding milling operation (see Fig. 24.38b and c). The putter has small inside radii with a comparatively long depth (7× the diameter or greater). A four-axis horizontal machining center (see, for example, Fig. 25.2) was used to reduce the number of setups in this operation. The rotary axis was used for creating the relatively complex geometry of the hosel (the socket for the shaft of the golf club). Since the hosel is relatively unsupported, chatter was the most complex challenge to overcome. Several iterations of spindle speeds were attempted in conjunction with upfront guidance from a simulation model. Modal analyses were conducted on the fixtured parts, in an attempt to identify and avoid the natural frequencies of the part or the fixture (see Section 25.4). The machines had spindle speeds ranging from 12,000 to 20,000 rpm, each having 22 kW. With the near-net-shape forging, the milling operations were designed to have low depths of cut, but high speed. After each machining operation was completed, a small amount of hand finishing was necessary in order to produce a superior surface appearance. The putters were then lightly shot blasted (with glass bead media, Section 34.2) for the purpose of achieving surface consistency. A black, nickel–chrome plating (Section 34.9) was then applied to all parts to enhance aesthetic appeal and protect the stainless steel from small dings and dents and from corrosion from specific chemicals that might be encountered on a golf course. Source: Courtesy of D. Jones and D. Petersen, Ping Golf, Inc. Case Study 24.3 Machining of Aerospace Structures from Monolithic Extruded Aluminum It is well known that the aircraft industry, and increasingly the automotive industry, places a premium on lightweight designs, using materials that reduce the weight of aircraft. It is also understood that aircraft components must be extremely reliable, as failure in service is potentially catastrophic. Figure 24.39a shows a typical component, in this case an avionics tray for an F/A-18 fighter aircraft. It is produced from a number of sheet metal pieces that are then assembled using a variety of fasteners (a) (b) Figure 24.39: Production of avionics tray by (a) riveting of multiple stamped parts, and (b) machining from a monolithic block of aluminum. ׉ 7cassandra://1oiALVbPRwH1g50xB-b40Npwdc3yxgt7hRjFYejlxQo"`cZ%Eb*cZ%Eb)#בCט#u׉׉ 7cassandra://pd8HjPRdU8wUYEXr8h1virBfO424HXTO9m83M2o5uOE`׉ 7cassandra://HoPCliqixVOiaDOspKhIpxgy3kyFY8BpXFzT-HhLjwc`S`s׉ 7cassandra://XEHMuRy03SAYEoLbWrBzEXcui_nl5qCXTTgnl-QAN0g}`׉ 7cassandra://N9jp8y6_l5BSBFBxIa3-5ckH8sYK9OCgHkkwTCKoVNg9-͠^cZEbט#u׉׉ 7cassandra://3y3gOLOKeH_X92614RsSuAcQx3NXdkz96zDl2DfsRHAڹ`׉ 7cassandra://w0FsYQNPcZBB2Bref9nBtLgA0L3wHXvAJSJeL5MavsYS`s׉ 7cassandra://uDWsNZ1cFgcn4-1_4nzFMKxtWxV4pmtAgkB9dDFuNOI`׉ 7cassandra://zPfZBLM1jgK8h2agJz_NpraeaF6QJL0diPxpmzeIO_EUL͠^cZEb׉E Gear Manufacturing by Machining 779 to form the component. This part is approximately 1000 mm x 500 mm x 100 mm. While quite elaborate shapes can be produced using this strategy, the design has a number of shortcomings: • Each of the multiple components requires its own set of tooling. The tooling cost must be amortized over limited production, so that each component has a very high cost. • The tooling must be stored when not in use and readily available when needed. • The sheet material is very reliable and has a repeatable strength. However, the use of assembly operations to join the sheets introduces a variety of failure modes. The fasteners pass through drilled holes which cause stress concentrations, and must be sealed. • It may be advisable from a lightweight or stiff design standpoint to have varying section thicknesses, but sheet metal assemblies are most easily built up from preforms with a constant metal thickness. • Assembled components have two layers of sheet metal, the weight of the fastener, and any shims and sealing material, so they are heavy. • The assembly operations are almost always manual (low volume limits the options for automation). • Tooling creates a tracking problem, and tooling has to be maintained, and can get lost. • To enable assembly, very tight tolerances have to be maintained, especially between the ribs and the mounting holes. It is not unusual to require tolerances of 0.125 mm for 2.5 m long parts. This is very challenging, and assemblies often require match drilling; parts then need to be dissembled, deburred, and then reassembled. An improved design strategy is shown in Fig. 24.39b, and demonstrates the approach of machining monolithic aluminum plate stock into functional components. To produce this component through machining, the following has to be noted: • Aluminum plate stock contains residual stresses. To prevent distortion of the part during and after machining, the residual stresses are reduced before machining, either by stretching or compression (Section 2.11). • A typical machining strategy of roughing to almost the finished dimension, and then finish machining does not work well for this kind of design. Thin parts are difficult to clamp (they deform under clamping forces), and they are difficult to machine (they deflect away from the tool or exhibit chatter). Instead, these parts are made using a “picture frame” strategy. The workpiece is the fixture, and the plate stock is simply clamped to the table. The part is machined layer by layer, using a relieved shank tool. This special tool prevents contact with the thin areas already machined. At each layer, the part is machined to the finished dimension, and then never machined again. The workpiece is cut where it is stiff, and the thin parts are left behind. As the part is almost finished, it is held in place with a few small tabs, which are removed last. • While this kind of part can be made with a conventional machine, the machining time may be long. It is more common to use a high-speed spindle. Solid carbide tools can withstand the melting temperature of aluminum without exhibiting a high wear rate. With aluminum components, and using the stability lobes approach discussed in Section 25.4, chatter can be avoided at even at very high material removal rates. ׉ 7cassandra://XEHMuRy03SAYEoLbWrBzEXcui_nl5qCXTTgnl-QAN0g}`cZ%Eb+׉E *780 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing The machined design is able to meet the stringent tolerances easily, and can achieve higher stiffness at lighter weight than the assembled component. This can be achieved, for example, by making high load sections thicker than the adjacent sections, eliminating the weight of the fasteners, and eliminating the weight of the double layers. These parts are significantly less expensive, even though 90% or more of the purchased plate stock is converted into chips. The parts are less expensive because: • The chips can be easily recycled. • The special tooling is eliminated. Only the part program must be stored. • The hand assembly operations are almost completely eliminated. • A significant portion of the cost is in the weight (in the fuel required to carry the part in during its service life), and the monolithic parts are lighter for the same function. A number of benefits were achieved with the machined components, as summarized in Table 24.4. Note the significant reduction in the number of parts in the structure and associated assembly time. The part cost was substantially reduced because of the ability to avoid tooling and assembly costs. Machining of monolithic aluminum components has become standard for high performance components, and has disseminated into a wide variety of applications including commercial aerospace, automotive, and personal electronics. Many laptop computer housings, for example, are machined monolithic aluminum. Source: Courtesy of K. Scott Smith, University of North Carolina at Charlotte. Table 24.4: Comparison of Stamped and Riveted Assembly vs. Monolithic Machined Components Depicted in Fig. 24.39. Stamped Number of parts Pan stock Weight (kg) Assembly time (hrs) Summary • A variety of complex shapes can be machined by the processes described in this chapter. Milling is one of the most common machining processes, because it is capable of economically producing a variety of shapes. • Although these processes are similar to turning, drilling, and boring, and involve similar cutting mechanics, tool materials, and cutting fluids, most of the processes described utilize multi-tooth tools and cutters at various axes with respect to the workpiece. • Machine tools used to produce complex shapes are now mostly computer controlled, having various dedicated features, and imparting much more flexibility in their application than traditional machine tools. 6 445 4.34 50 108 3.88 5.3 Percent and riveted Machined reduction 44 84 76 4.98 89 ׉ 7cassandra://uDWsNZ1cFgcn4-1_4nzFMKxtWxV4pmtAgkB9dDFuNOI`cZ%Eb,cZ%Eb+#בCט#u׉׉ 7cassandra://4yEx3rugjAxLS48wHtSB8gxeEhFWm8eNO0p_SsoSeM4:`׉ 7cassandra://zip4gBs_jy5U8bySzGR2S8rOcoRs9QGw6VgfEpCfl7YI`s׉ 7cassandra://KxX1DMr8eZ_PmHWBSlo9NgpeRrHaccMgtg_AFMPHdZ0`׉ 7cassandra://_UUS7AvH8HMoAFALAvegIYLb4I_9cpzUxLkgDXR8ACcS(͠^cZEbט#u׉׉ 7cassandra://zsbdbOU3RbcHPJvT5aFrrWz6dg-6Mbny99kDqyG3-sk `׉ 7cassandra://RwOud8WA6hVyZKyV2g7HvRl54oAIMLji6vvaZZ5hTfE_`s׉ 7cassandra://67OR5I6pWI7c7lsN2oQhBwZiMgU7-z_3Nf6wEgIHDjQ`׉ 7cassandra://nNEMmDJpUWPxa4d02ycxUdWsW8oV2OE6pHw6YDrYe2QJ5͠^cZEb׉ELReview Questions 781 • Broaching is a method of accurately enlarging a round hole or other profile in a workpiece. Sawing is the gradual removal of material by small teeth spaced on a saw, and is very versatile. Filing involves small-scale removal of material from a surface, especially the removal of burrs and sharp profiles. • In addition to being produced by various forming and shaping processes, gears also are produced by machining, either by form cutting or generating; the latter produces gears with better surface finish and higher dimensional accuracy. The quality of the gear-tooth profile is further improved by such process as shaving, burnishing, grinding, honing, and lapping. Key Terms Arbor Broaching Bur Burnishing Climb milling Die sinking End milling Face milling Filing Fly cutting Form cutting Friction sawing Gear generating High-speed milling Hob Honing Bibliography Brown, J., Advanced Machining Technology Handbook, McGraw-Hill, 1998. Davim, J.P. (ed.), Machining: Fundamentals and Recent Advances, Springer, 2010. Davis, J.R., (ed.), Gear Materials, Properties and Manufacture, ASM International, 2006. Joshi, P.H., Machine Tools Handbook, McGraw-Hill, 2008. Krar, S.F., Gill, A.R., and Smid, P., Technology of Machine Tools, 7th ed., McGraw-Hill, 2010. Lopez, L.N., and Lamikiz, A. (eds.), Machine Tools for High Performance Machining, Springer, 2009. Machinery’s Handbook, various editions, Industrial Press. Radzevich, S.P., Gear Cutting Tools: Science and Engineering, 2nd ed., CRC Press, 2017. —, Dudley’s Handbook of Practical Gear Design and Manufacture, 3rd ed., CRC Press, 2016. Stephenson, D.A., and Agapiou, J.S., Metal Cutting: Theory and Practice, 2nd ed., Marcel Dekker, 2006. Walsh, R.A., McGraw-Hill Machining and Metalworking Handbook, 3rd ed., McGraw-Hill, 2006. Review Questions 24.1. Explain why milling is such a versatile machining operation. 24.2. Describe a milling machine. How is it different from a drill press? Indexing Kerf Lapping Milling Planing Pull broach Push broach Rack shaper Sawing Shaping Shaving Slab milling Tooth set Turn broaching Workholding ׉ 7cassandra://KxX1DMr8eZ_PmHWBSlo9NgpeRrHaccMgtg_AFMPHdZ0`cZ%Eb-׉E 782 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 24.3. Describe the different types of cutters used in milling operations and give an application of each type. 24.4. Define the following: face milling, peripheral milling, shoulder milling, slot milling, thread milling. 24.5. Can threads be machined on a mill? Explain. 24.6. What is the difference between feed and feed per tooth? Can they ever be the same? 24.7. Explain the relative characteristics of climb milling and up milling. 24.8. Describe the geometric features of a broach and explain their functions. 24.9. What is a pull broach? A push broach? 24.10. Why is sawing a commonly used process? Why do some saw blades have staggered teeth? Explain. 24.11. What advantages do bed-type milling machines have over column-and-knee-type machines for production operations? 24.12. Explain why the axis of a hob is tilted with respect to the axis of the gear blank. 24.13. What is a shell mill? Why is it used? 24.14. Why is it difficult to saw thin sheet metals? 24.15. Of the processes depicted in Fig. 24.2, which is the most similar to hobbing? 24.16. Describe the tool motion during gear shaping. 24.17. When is filing necessary? Qualitative Problems 24.18. Would you consider the machining processes described in this chapter to be near-net or net-shape processing? Explain with appropriate examples. 24.19. Why is end milling such an important versatile process? Explain with examples. 24.20. List and explain factors that contribute to poor surface finish in the processes described in this chapter. 24.21. Are the feed marks left on the workpiece by a face-milling cutter true segments of a true circle? Explain with appropriate sketches. 24.22. Explain why broaching crankshaft bearings is an attractive alternative to other machining processes. 24.23. Several guidelines are presented in this chapter for various cutting operations. Discuss the reasoning behind these guidelines. 24.24. What are the advantages of helical teeth over straight teeth on cutters for slab milling? 24.25. Explain why hacksaws are not as productive as band saws. 24.26. What similarities and differences are there in slitting with a milling cutter and with a saw? 24.27. Why do machined gears have to be subjected to finishing operations? Which of the finishing processes are not suitable for hardened gear teeth? Why? 24.28. How would you reduce the surface roughness shown in Fig. 24.8? Explain. 24.29. Why are machines such as the one shown in Fig. 24.20 so useful? 24.30. Comment on your observations concerning the designs illustrated in Fig. 24.23b and on the usefulness of broaching operations. 24.31. Explain how contour cutting could be started in a band saw, as shown in Fig. 24.28d. 24.32. In Fig. 24.30a, high-speed steel cutting teeth are welded to a steel blade. Would you recommend that the whole blade be made of high-speed steel? Explain your reasons. ׉ 7cassandra://67OR5I6pWI7c7lsN2oQhBwZiMgU7-z_3Nf6wEgIHDjQ`cZ%Eb.cZ%Eb-#בCט#u׉׉ 7cassandra://wLVfgHQQN8kXDEU203ZDTtEMHGIzvGJgSXa7U3k9JNQ7`׉ 7cassandra://P1gb6rS8VQRz0v_aGkf0YGSgX5Eb_jXCOI9tnsQmlzsd`s׉ 7cassandra://jkx5r14Mxd7auFWQ-epCZf9ArZxR0fe_UfcIf6aa6Foa`׉ 7cassandra://_Xcb9vp4o0lHmPOZtLylAjJF0e5fQs4uhZdfAlI_H74U͠^cZEbט#u׉׉ 7cassandra://HqyI3mx-WQZ8bKXIYlAAYrDUA-fG9I8cuz2Ximg1qJ0c`׉ 7cassandra://-qMuYUWzdjl3KPPEBlK62rfIrlaC90sexAijUMP9ElcM`s׉ 7cassandra://ZOVM-grwGL38QeEk77fxcNpEWzWQ6Xu76vQ_foUk80oy`׉ 7cassandra://46GgjVpUXKmu2T9djmukBIPrSeizzzWIGOMDaPI6AWcN4͠^cZEb׉E Quantitative Problems 783 24.33. Describe the parts and conditions under which broaching would be the preferred method of machining. 24.34. With appropriate sketches, explain the differences between and similarities among shaving, broaching, and turn-broaching operations. 24.35. Explain the reason that it is difficult to use friction sawing on nonferrous metals. 24.36. Would you recommend broaching a keyway on a gear blank before or after machining the gear teeth? Why? Quantitative Problems 24.37. In milling operations, the total cutting time can be significantly influenced by (a) the magnitude of the noncutting distance, lc, shown in Figs. 24.5 and 24.6, and (b) the ratio of width of cut, w,tothe cutter diameter, D. Sketch several combinations of these parameters, give dimensions, select feeds and cutting speeds, etc., and determine the total cutting time. Comment on your observations. 24.38. A slab-milling operation is being performed at a specified cutting speed (surface speed of the cutter) and feed per tooth. Explain the procedure for determining the table speed required. 24.39. Show that the distance lc in slab milling is approximately equal to (see Fig. 24.5c). √ Dd for situations where D d 24.40. In Example 24.1, which of the quantities will be affected when the feed is increased to f =0.75 mm/tooth? 24.41. In Example 24.1, if the feed is increased to 0.75 mm/tooth, how are the chip depth of the cut and torque affected? 24.42. Estimate the time required to face mill a 250-mm-long, 25-mm-wide brass block with a 150-mmdiameter cutter with 10 high-speed steel inserts. 24.43. A 300-mm-long, 25-mm-thick plate is being cut on a band saw at 45 m/min. The saw has 0.5 teeth per 25 mm. If the feed per tooth is 0.075 mm, how long will it take to saw the plate along its length? 24.44. A single-thread hob is used to cut 40 teeth on a spur gear. The cutting speed is 35 m/min and the hob is 75 mm. in diameter. Calculate the rotational speed of the spur gear. 24.45. Assume that in the face-milling operation shown in Fig. 24.6 the workpiece dimensions are 100 mm by 250 mm. The cutter is 150 mm in diameter, has eight teeth, and rotates at 300 rpm. The depth of cut is 3 mm and the feed is 0.125 mm/tooth. Assume that the specific energy requirement for this material is and that only 75% of the cutter diameter is engaged during cutting. Calculate (a) the power required and (b) the material-removal rate. 24.46. A slab-milling operation will take place on a part 350 mm long and 45 mm wide. A helical cutter 75 mm in diameter with 10 teeth will be used. If the feed per tooth is 0.2 mm/tooth and the cutting speed is 0.8 m/s, find the machining time and metal-removal rate for removing 8 mm from the surface of the part. 24.47. Repeat Problem 24.46 if the same cutter diameter, number of inserts, and machining parameters are used, but the part is machined in a face milling operation. 24.48. A slab-milling operation is being carried out on a 0.75-m-long, 75-mm-wide high-strength-steel block at a feed of 0.3 mm/tooth and a depth of cut of 4 mm. The cutter has a diameter of 75 mm, has six straight cutting teeth, and rotates at 150 rpm. Calculate the material removal rate and the cutting time, and estimate the power required. 24.49. Explain whether the feed marks left on the workpiece by a face-milling cutter (as shown in Fig. 24.16a) are segments of true circles. Describe the parameters you consider in answering this question. ׉ 7cassandra://jkx5r14Mxd7auFWQ-epCZf9ArZxR0fe_UfcIf6aa6Foa`cZ%Eb/׉E 784 Chapter 24 Machining Processes: Milling, Broaching, Sawing, Filing, and Gear Manufacturing 24.50. In describing the broaching operations and the design of broaches, equations regarding feeds, speeds, and material-removal rates have not been given as was been done in turning and milling operations. Review Fig. 24.24 and develop such equations. Synthesis, Design, and Projects 24.51. The parts shown in Fig. 24.1 are to be machined from a rectangular blank. Suggest the machine tool(s) required, the fixtures needed, and the types and sequence of operations to be performed. Discuss your answer in terms of the workpiece material, such as aluminum versus stainless steel. 24.52. Would you prefer to machine the part in Fig. 24.1f from a preformed blank (near-net shape) rather than a rectangular blank? If so, how would you prepare such a blank? How would the number of parts required influence your answer? 24.53. If expanded honeycomb panels (see Section 16.13) were to be machined in a form-milling operation, what precautions would you take to keep the sheet metal from buckling due to tool forces? Think up as many solutions as you can. 24.54. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare two quantitative problems and supply the answers. 24.55. Suggest methods whereby milling cutters of various designs (including end mills) can incorporate carbide inserts. 24.56. Prepare a comprehensive table of the process capabilities of the machining processes described in this chapter. Using several columns, list the machines involved, types of tools and tool materials used, shapes of blanks and parts produced, typical maximum and minimum sizes, surface finish, dimensional tolerances, and production rates. 24.57. On the basis of the data developed in Problem 24.56, describe your thoughts regarding the procedure to be followed in determining what type of machine tool to select when machining a particular part. 24.58. Make a list of all the processes that can be used in manufacturing gears, including those described in Parts II and III of this text. For each process, describe the advantages, limitations, and quality of gears produced. 24.59. List the concerns you would have if you needed to economically pocket mill carbon-fiber-reinforced polymers or metal matrix composites with graphite fibers in an aluminum matrix. ׉ 7cassandra://ZOVM-grwGL38QeEk77fxcNpEWzWQ6Xu76vQ_foUk80oy`cZ%Eb0cZ%Eb/#בCט#u׉׉ 7cassandra://hj_QXNDEH8ZZUzju5jBF7UZMPhT29hsNeOLZgQt3n6o`׉ 7cassandra://t89CWzYQyudKMTMMkWWo2FpEm9ZvqOJbBGgDjMVuBeY>`s׉ 7cassandra://a9O8OIFTSyb6x2_wZqqtoLY20Hgvl2la43UFOK2HMOk`׉ 7cassandra://SwHk-PWMhQ_pswiIwkovYQQvvlDNMUmQPPI6J1GQKsw8[͠^cZEbט#u׉׉ 7cassandra://AvSv4llU5kZ30n-SbQ1ASelSpkcN9wugf_GGW5BBUP89`׉ 7cassandra://ZXUmKddcFCSq8EHZo-OLUXRbvCo0H7XeJwwvuedeB50V`s׉ 7cassandra://7l2mWhb_AngXQTKXHB8WPP-CRNknxPo7hut8oQedSZs`׉ 7cassandra://CgK5Qn1E9nmVC1DfXeEEAaRleh3013EnGh78vkof6es_ ͠^cZEb׉E'Chapter 25 Machining Centers, Machine-tool Structures, and Machining 25.1 Introduction 785 25.2 Machining Centers 786 25.3 Machine-tool Structures 793 25.4 Vibration and Chatter in Machining Operations 798 25.5 High-speed Machining 803 25.6 Hard Machining 804 25.7 Ultraprecision Machining 804 25.8 Machining Economics 805 Case Study: 25.1 Machining Outer Bearing Races on a Turning Center 793 • This chapter presents the characteristics, types, and advantages of machining centers. • Emphasis is placed on the importance of understanding the performance of machine tools and their modules and components, particularly with regard to their stiffness, vibration, chatter, and damping characteristics. • Presented next are the topics of high-speed machining, hard machining, and ultraprecision machining operations, all strongly tied to the economics of machining. • The chapter ends with a simple method of cost analysis for determining the conditions under which machining parameters can be selected, so that machining cost per piece or machining time per piece can be minimized. 25.1 Introduction The preceding four chapters described machining operations and machine tools, but have not emphasized the integration of advanced computer technology and the flexibility it allows in manufacturing operations. 785 ׉ 7cassandra://a9O8OIFTSyb6x2_wZqqtoLY20Hgvl2la43UFOK2HMOk`cZ%Eb1׉E 786 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Computers have dramatically improved the capabilities of machine tools, whereby they now have the capability of rapidly and repeatedly producing very complex part geometries economically. The programs controlling a machine tool can incorporate changes in machining conditions, compensate for tool wear, automatically change tools, and machine a workpiece without refixturing or having to transfer it to another machine tool. In addition to implementation of advanced computer technologies, vibration and chatter and their avoidance, high-speed machining, hard machining, and advanced analysis of machining economics are now highly developed and have revolutionized machining operations. 25.2 Machining Centers In describing individual machining processes and machine tools in the preceding chapters, it was noted that each machine, regardless of how highly it is automated, is designed to perform basically the same specific operation, such as turning, boring, drilling, milling, broaching, planing, or shaping. It was also shown that most parts manufactured by the methods described throughout this book require additional operations on their various surfaces before they are completed. Note, for example, that the parts shown in Fig. 25.1 have a variety of complex geometric features, and the surfaces on these parts require a different type of machining operation to meet a set of specific requirements concerning shapes, features, dimensional tolerances, and surface finish. Note also the following observations: • Some possibilities exist in net-shape or near-net shape production of these parts, depending on specific constraints on shapes, dimensional tolerances, detailed surface features, surface finish, and various mechanical and other properties to meet service requirements. Shaping processes that are candidates for such parts are precision casting, powder metallurgy, powder-injection molding, and precision forging. Even then, however, it is very likely that the parts will still require additional finishing operations for small-diameter deep holes, threaded holes, flat surfaces for sealing with gaskets, parts with very close dimensional tolerances, sharp corners and edges, and flat or curved surfaces, with different surface-finish requirements. Figure 25.1: Examples of parts that can be machined on machining centers, using processes such as turning, facing, milling, drilling, boring, reaming, and threading. Such parts ordinarily would require the use of a variety of machine tools. Forged motorcycle wheel, finish machined to tolerance and subsequently polished and coated. Source: Courtesy of R.C. Components. ׉ 7cassandra://7l2mWhb_AngXQTKXHB8WPP-CRNknxPo7hut8oQedSZs`cZ%Eb2cZ%Eb1#בCט#u׉׉ 7cassandra://QTGNqBPFqMGuTZ9pQh36UL1sd_0zYlKOHXIe5TPDgl4F`׉ 7cassandra://3g37T8uGQ8rgL4BkCIoncYYRTJ8oSccZL7QxHA0W0pQS`s׉ 7cassandra://XjjKhzUfdK1qCloHVnEskMMy4rJEIS3MwzKTZvtwG1E`׉ 7cassandra://nU4-Rxarn98y-4UlVQq3C8jUQI2NB_TNBtVGeTM2tNE͕͠^cZEbט#u׉׉ 7cassandra://_TZOCbgH5s6qYvescy7O5L45SobhDFjFBieFQUaFk3c`׉ 7cassandra://9Xa4ueNlxuLjVzOFsYOSALVySxZjlLas_rsm_ZEDv8Yh`s׉ 7cassandra://SlT6M83xAZJVhSW8DZMTQbPGGr0-dxaZvHlyziwKqo8`׉ 7cassandra://VMlQNlydFvvvstFJjPFsXEXhtWL_PnJIynlWfVWAaWwl͠^cZEb׉E Machining Centers 787 • If some machining is required or if it is shown to be more economical to finish machine these parts to their final shapes, it is obvious that none of the machine tools described in Chapters 23 and 24 could individually and completely produce the parts. Note also that traditionally, machining operations are performed by moving the workpiece from one machine tool to another until all of the required operations are completed. Machining Centers. The traditional method of machining parts by using different types of machine tools has been, and continues to be, a viable manufacturing method. This method can be highly automated to increase productivity, and in fact it is the principle behind transfer lines, also called dedicated manufacturing lines (DML) (see Section 37.2.4). Commonly used in high-volume or mass production, transfer lines consist of several specific (dedicated) machine tools, arranged in a logical and efficient sequence. The workpiece, such as an automotive engine block, is moved from one station to another, with a specific operation performed at each station; it is then transferred to the next station for another specific machining operations. There are situations, however, where transfer lines are not feasible or economical, particularly when the types of products to be processed change rapidly due to factors such as product demand or modifications in product shape or style. It is very costly and time-consuming to rearrange these machine tools to respond to the changed needs for the next and different production cycle. An important concept that addresses flexibility in manufacturing, developed in the late 1950s, is that of machining centers. A machining center (Fig. 25.2) is an advanced computer-controlled machine tool that is capable of performing a variety of machining operations on different surfaces and orientations of a workpiece, without having to remove it from its workholding device or fixture. The workpiece generally is stationary and the cutting tools rotate, as they do in such operations as milling, drilling, honing, and tapping. Whereas in Tool storage Tool-interchange arm Tools (cutters) Index table Traveling column Spindle Computer numerical-control panel Bed Figure 25.2: A horizontal-spindle machining center equipped with an automatic tool changer. Tool magazines can store up to 200 cutting tools of various functions and sizes. Source: Courtesy Haas Automation, Inc. ׉ 7cassandra://XjjKhzUfdK1qCloHVnEskMMy4rJEIS3MwzKTZvtwG1E`cZ%Eb3׉E 788 Chapter 25 Machining Centers, Machine-tool Structures, and Machining transfer lines or in traditional shops and factories the workpiece is brought to the machine, in machining centers, it is the machining operation that is brought to the workpiece. The development of machining centers is related closely to advances in automation and computer control of machine tools, the details of which are described in Chapter 37. Recall that, as an example of the advances in modern lathes, Fig. 23.10 illustrates a numerically controlled lathe or turning center,with two turrets, each carrying several cutting tools. Components of a Machining Center. The workpiece in a machining center is placed on a pallet,or module, which can be moved and swiveled (oriented) in various directions (Fig. 25.3). After a particular machining operation has been completed, another operation begins, which may require reindexing of the workpiece on its pallet. After all of the machining operations have been completed, the pallet automatically moves away with the finished part, and another pallet, carrying another workpiece or workpieces to be machined, is brought into position by an automatic pallet changer (Fig. 25.4). All movements are computer controlled, with pallet-changing cycle times on the order of only 10 to 30 seconds. Pallet stations are available with several pallets serving the one machining center. The machines also can be equipped with various automatic features, such as part loading and unloading devices. A machining center is equipped with a programmable automatic tool changer (ATC). Depending on the particular design, up to 100 cutting tools can be stored in a magazine, drum, or chain (tool storage). Auxiliary tool storage also is available on some special and large machining centers, raising the tool capacity to 200. The cutting tools are selected automatically for the shortest route to the machine spindle. The maximum dimensions that the cutting tools can reach around a workpiece in a machining center is called the work envelope, a term that was first used in connection with industrial robots (Section 37.6). The tool-exchange arm shown in Fig. 25.5 is a common design; it swings around to pick up a specific tool and places it in the spindle. Note that each tool has its own toolholder, thus making the transfer of cutting tools to the machine spindle highly efficient. Tools are identified by bar codes, QR codes, or coded tags attached directly to their toolholders. Tool-changing times are typically between 5 and 10 s, but may be up to 30 s for tools weighing up to 110 kg, and less than 1 s for small tools. Machining centers may be equipped with a tool-checking and/or part-checking station that feeds information to the machine control system, so that it can compensate for any variations in tool settings or tool wear. Touch probes (Fig. 25.6) can be installed into a toolholder to determine workpiece reference surfaces, for selection of tool settings and for online inspection of parts being machined. Note in Fig. 25.6 that several surfaces can be contacted (see also sensor technology, Section 37.7), and that their relative positions Spindle Pallet Traveling column Bed Figure 25.3: Schematic illustration of the principle of a five-axis machining center. The pallet, which supports and transfers the workpiece, has three axes of movement and can be swiveled around two axes (thus a total of five axes), allowing the machining of complex shapes, such as those shown in Fig. 25.1. Source: Courtesy of Toyoda Machinery. ׉ 7cassandra://SlT6M83xAZJVhSW8DZMTQbPGGr0-dxaZvHlyziwKqo8`cZ%Eb4cZ%Eb3#בCט#u׉׉ 7cassandra://c0JdS4xd691w_pxCq02ZEuRDXmZUtblQOlQgYMAsLSQ`׉ 7cassandra://q71xq1XqJG99vBK8vfZOhPGS7sGhsCczJGCs4ex7OCo>`s׉ 7cassandra://dasnCayMEKnN8PQqE9y9esOsSP__uh0PyIpiMwS-djo`׉ 7cassandra://f0S9LEEIbRU8MEWUscJ3aVCAwC8v07qeK7z79krnIBg{: D͠^cZEbט#u׉׉ 7cassandra://0H0DpVD0peDeX1I44CuRkkr7JvoX3YRyx7pyICkHDjQjk`׉ 7cassandra://oX8Eb47VVoF_px0cPwVZnhSJcwL4r1dDeb6qvoyT3hoL`s׉ 7cassandra://jl7ZnDT5edwumgIvLgkgeicMCH9C1UwK4a5iIGjbTR8F`׉ 7cassandra://NCovpmOql6YZnFqkAB0xpeIUE1T4Hjc17rUqQ87DILw ͠^cZEb׉EMachining Centers 789 Machining center Pallet Spindle Machining center Pallet carrier Pallet pool Pallets (a) Setup station Pallets Pallet pool line (b) Machining center Pallet pool Setup station (c) Figure 25.4: (a) Schematic illustration of the top view of a horizontal-spindle machining center, showing the pallet pool, setup station for a pallet, pallet carrier, and an active pallet in operation (shown directly below the spindle of the machine). (b) Schematic illustration of two machining centers, with a common pallet pool. (c) A pallet pool for a horizontal-spindle machining center. Various other pallet arrangements are possible in such systems. Source: (a) and (b) Courtesy of Hitachi Seiki Co., Ltd., (c) Courtesy of Haas Automation, Inc. are determined and stored in the database of the computer software. In more advanced machine tools, the data are then used to program tool paths (see, for example, Fig. 37.12) and to compensate for tool length, tool diameter, and for tool wear. Non-contact probes also can be used, and can measure dimensions, surface roughness, or temperature. 25.2.1 Types of Machining Centers There are various designs for machining centers. The two basic types are vertical spindle and horizontal spindle, although many machines are capable of operating along both axes. ׉ 7cassandra://dasnCayMEKnN8PQqE9y9esOsSP__uh0PyIpiMwS-djo`cZ%Eb5׉E3790 Chapter 25 Machining Centers, Machine-tool Structures, and Machining (a) (b) Figure 25.5: Swing-around tool changer on a horizontal-spindle machining center. (a) The tool-exchange arm is placing a toolholder, with a cutting tool, into the machine spindle. Note the axial and rotational movements of the arm. (b) The arm is returning to its home position. Note its rotation along a vertical axis after placing the tool, and the two degrees of freedom in its home position. Probe Y X (a) X–Y direction End mill or reamer Y Probe X (c) Diameter compensation (d) Tool-length offset Figure 25.6: Touch probes used in machining centers for determining workpiece and tool positions and surfaces relative to the machine table or column. Touch probe (a) determining the x-y (horizontal) position of a workpiece, (b) determining the height of a horizontal surface, (c) determining the planar position of the surface of a cutter (e.g., for cutter–diameter compensation), and (d) determining the length of a tool for tool-length offset. Vertical-spindle Machining Centers. Also called vertical machining centers (VMC), these machines are capable of performing various machining operations on parts with deep cavities, as in mold and die making (also called die sinking). A vertical-spindle machining center, which is similar to a vertical-spindle milling machine, is shown in Fig. 25.7. The tool magazine is on the left of the machine, and all operations and movements are directed and modified through the computer control panel, shown on the right. Because Probe (b) Z direction Z Z Drill ׉ 7cassandra://jl7ZnDT5edwumgIvLgkgeicMCH9C1UwK4a5iIGjbTR8F`cZ%Eb6cZ%Eb5#בCט#u׉׉ 7cassandra://fnjPx0oxvLiu5PO9oLsaITX1PaHw9q-tCd8vgmOkjvs`׉ 7cassandra://-_v1PR8sgUvT24pOLnDEyKfO9zyJ-ryy1ftSFPAZqNUP.`s׉ 7cassandra://xXwiHTzn6Xaz8CpkzLXF5HlI7BMp5xH8V5TbGlj4Efc`׉ 7cassandra://r3mvalAFp5SbLxBdQAoRNWjrLG7tkS9xMDAw0Rzfxd0͎f ͠^cZEbט#u׉׉ 7cassandra://ARgjkg6FUSsPDes3qKIVVHGgIkQjsfYwlQHgUjzmPnUh`׉ 7cassandra://RVppGa1z0XxTkk5-VrioymNQ2TbCxL_eIC8lZBQNMvsT`s׉ 7cassandra://Jly97lBLp9q-L7VVvG2K0WYDkQWg9dL3wBevmm2Ae6Q`׉ 7cassandra://VUYM8P1w4y6owu6KYunmy-no4pIepAb_ydzsg1hDlw4̓`̚͠^cZEb׉EDMachining Centers 791 Automatic tool changer Spindle Control panel Cutting tool Chip recovery system Worktable Bed Figure 25.7: A vertical-spindle machining center. The tool changer is on the left of the machine, and has a 40 tool magazine. Source: Courtesy of Haas Automation, Inc. the thrust forces in vertical machining are directed downward, such machines have high stiffness and hence able to produce parts with good dimensional accuracy. VMCs generally are less expensive than horizontal-spindle machines of similar capacity. Horizontal-spindle Machining Centers. Also called horizontal machining centers (HMC), these machines are suitable for large as well as tall workpieces that require machining on a number of surfaces. The pallet can be swiveled on different axes to various angular positions (see Fig. 25.3). Turning Centers. This is another category of horizontal-spindle machines; basically, they are computercontrolled lathes, with several features. A multi-spindle turning center is shown in Fig. 25.8. It is constructed with two horizontal spindles and two turrets, equipped with a variety of cutting tools used to perform several operations on a rotating workpiece. The turrets can be powered to allow for drilling or milling operations within the CNC turning center, and without the need to refixture the workpiece. For this reason, such machines are often referred to as CNC Mill-turn Centers. Universal Machining Centers. These machines are equipped with both vertical and horizontal spindles. They have a variety of features and are capable of simultaneously machining all surfaces of a workpiece, i.e., vertically, horizontally, and at a wide range of angles. 25.2.2 Characteristics and Capabilities of Machining Centers The major characteristics of machining centers are as follows: • Machining centers are capable of efficiently handling a wide variety of part sizes and shapes, economically, repetitively, and with high dimensional accuracy and tolerances on the order of ±0.0025 mm. • These machines are versatile and capable of quick changeover from one type of product to another. ׉ 7cassandra://xXwiHTzn6Xaz8CpkzLXF5HlI7BMp5xH8V5TbGlj4Efc`cZ%Eb7׉E792 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Turret head Control panel Spindle Figure 25.8: The Haas DS-30Y, a computer numerical-controlled turning center. The two spindle heads and tool-driven turret head make the machine very flexible in its machining capabilities. Source: Courtesy of Haas Automation, Inc • The time required for loading and unloading workpieces, changing tools, gaging of the part being machined, and troubleshooting is reduced. Because of the inherent flexibility in machining centers, the workpiece may not have to be refixtured during machining, referred to as the one and done approach. Productivity is improved, labor requirements (particularly skilled labor) are reduced, and production costs are minimized. • These machines can be equipped with tool-condition monitoring devices for the detection of tool breakage and wear, as well as with probes for tool-wear compensation and tool positioning. • In-process and postprocess gaging and inspection of machined workpieces are now features of machining centers. • These machines are highly automated and relatively compact, and have advanced control systems; one operator can attend to two or more machining centers at the same time, thus reducing labor costs. Because of the high productivity of machining centers, large amounts of chips are produced and must be collected and disposed of properly (see chip management, Section 23.3.7). Several system designs are available for chip collection, with one or more chain or spiral (screw) conveyors; they collect the chips along troughs in the machine and deliver them to a collecting point (Fig. 25.7). Machining centers are available in a wide variety of sizes and features; typical capacities range up to 75 kW. Maximum spindle speeds are usually in the range of 4000 to 8000 rpm, and some are as high as 75,000 rpm for special applications, using small-diameter cutters. Modern spindles can accelerate to a speed of 20,000 rpm in 1.5 s. Some pallets are capable of supporting workpieces weighing as much as 7000 kg, although even higher capacities are available for special applications. The cost of machining centers ranges from about $50,000 to $2 million and higher. ׉ 7cassandra://Jly97lBLp9q-L7VVvG2K0WYDkQWg9dL3wBevmm2Ae6Q`cZ%Eb8cZ%Eb7#בCט#u׉׉ 7cassandra://MidqGfB8zTywuzFJGYl8rfrgKjDwQ-PLVhlFgNgzB34Ů`׉ 7cassandra://n-02FGZgwFvokxm2ejc8Coa_9ZmUINwcqDJi54W-3r4\ `s׉ 7cassandra://apDbkX_EP8wqjNlSWuaOT9DvbmslzYRPdbVYkVJV4tAT`׉ 7cassandra://B89_2mfnLzWiTzrIh9NSZTN0k16O5lbbtSVRzFZaDLod7͠^cZ Ebט#u׉׉ 7cassandra://ThTiCH0Pl_9JXFm2_O3YzscWnJMcRx8GfsQjFQpnmkc`׉ 7cassandra://dnlM0UXENzts90Ht7py3Zy7hcukXSLa3ji5oH5I-6KYG`s׉ 7cassandra://XXaCnOoDLK6Tm4ZtDzXDNm24flWthUJuthNkd0BCeoc`׉ 7cassandra://SaTbxRJMGI7VQIfeOrimyQ04xQDHElfdXcd7u8YzTKE&.͠^cZ Eb׉E Machine-tool Structures 793 25.2.3 Selection of Machining Centers Machining centers generally require significant capital expenditure; to be cost effective, they may have to be operated for more than one shift per day. Consequently, there must be a sufficient and continued demand for parts to justify their purchase. Because of their inherent versatility, however, machining centers can be used to produce a wide range of products, particularly for mass customization or just-in-time manufacturing (Section 39.6). The selection of the type and size of machining centers depends on several factors, especially the following: • Type of products, their size, and shape complexity • Type of machining operations to be performed and the type and number of cutting tools required • Dimensional accuracy specified • Production rate required. Case Study 25.1 Machining Outer Bearing Races on a Turning Center Outer bearing races (Fig. 25.9) are machined on a turning center. The starting material is a hot-rolled 52100 steel tube, with 91 mm OD and 75.5 mm ID. The cutting speed is 95 m/min for all operations. All tools are carbide, including the cutoff tool (used in the last operation shown), which is 3.18 mm, instead of 4.76 mm for the high-speed steel cutoff tool that formerly was used. The amount of material saved by this change is significant, because the race width is small. The turning center was able to machine these races with repeatable tolerances of ±0.025 mm and at high speeds (see also Example 23.2). Source: Courtesy of McGill Manufacturing Company. 25.2.4 Reconfigurable Machines and Systems The need for the flexibility of manufacturing operations has led to the concept of reconfigurable machines, consisting of various modules. The term reconfigurable stems from the fact that, by using advanced computer hardware and reconfigurable controllers, and utilizing advances in information management technologies, the machine components can be arranged and rearranged into a number of configurations to meet specific production demands. Fig. 25.10 shows an example of how the basic machine-tool structure of a three-axis machining center can be reconfigured to become a modular machining center. With such flexibility, the machine can perform different operations while accommodating various workpiece sizes and part geometries. Another example is given in Fig. 25.11, where a five-axis (three linear and two rotational) machine can be reconfigured by assembling different modules. Reconfigurable machines have the aim of (a) improving the productivity and efficiency of manufacturing operations, (b) reducing lead time for production, and (c) providing a cost-effective and rapid response to market demands (see also Chapter 39). 25.3 Machine-tool Structures This section describes the materials and design aspects of machine-tool structures that are important in producing parts, with acceptable geometric features and dimensional and surface finish characteristics. ׉ 7cassandra://apDbkX_EP8wqjNlSWuaOT9DvbmslzYRPdbVYkVJV4tAT`cZ%Eb9׉E794 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Tube Form tool 1. Finish turning of outside diameter 2. Boring and grooving on outside diameter 3. Internal grooving with a radius-form tool Support bar Cut off tool Form tool 4. Finish boring of internal groove and rough boring of internal diameter 5. Internal grooving with form tool and chamfering Bearing race 6. Cutting off finished part; inclined bar picks up bearing race Figure 25.9: Machining of outer bearing races. 25.3.1 Materials The following is a list of the materials that commonly have been used for machine-tool structures. • Gray cast iron is the first material used in machine tool structures, and has the advantages of a good damping capacity and low cost, but has the limitation of being heavy. Most machine-tool structures are made of class 40 cast iron; some are made of class 50 (see Table 12.4). (a) (b) (c) Figure 25.10: Schematic illustration of a reconfigurable modular machining center capable of accommodating workpieces of different shapes and sizes and requiring different machining operations on their various surfaces. Source: After Y. Koren. ׉ 7cassandra://XXaCnOoDLK6Tm4ZtDzXDNm24flWthUJuthNkd0BCeoc`cZ%Eb:cZ%Eb9#בCט#u׉׉ 7cassandra://ByZf13F4MzoloC1jqOytmhQzlm_6m3WP6rMmOhRkmhg`׉ 7cassandra://n6R2G4sfjVk9o8ww9v_TLjGMUmpRMeMrUZsOAgQRGcYZD`s׉ 7cassandra://s6OqTT5nly1j_AjqK4ccqTIY5G2sMbtLVLV5UByGWrU`׉ 7cassandra://EM1O4cye88aQIf1WNRip0YxcqcvaoUnsmn83xLNzlyoIcZ%Eb=#בCט#u׉׉ 7cassandra://ZkxwJ6XW0L3IQz1rzzfVh_SoxN2myXK1IC1n4wHJXBU `׉ 7cassandra://zWIMGAfKRwRQfIRS6ZqpvKKuygFq03wt77o48IjfA9EWj`s׉ 7cassandra://RgaxgxqQoKBUbKBGIHTr2Kx-y-7Nb3eHg16pPNTtnBA`׉ 7cassandra://6J-tmj2sV4WEwClCS9uHv1pk_RI9JNcW7mZg0HeQUR8t;͠^cZ Eb ט#u׉׉ 7cassandra://9CpNKXMcUIdwPlbs3hfKkNE2TtJRhhhAL1CgOWujMJ8{`׉ 7cassandra://45IfxY0_LpeZWhn5S4LU54DwqWApL1Fq4BIltyT0fMgxb`s׉ 7cassandra://4H9O8X0wh--0KRiI-suqYza08q-s97opOQ4GhHByfew`׉ 7cassandra://oxoZocDTLa3cBs-tkNwC-xmf7cXGFC5Acb8HxhaBEQca͠^cZ Eb נcZ Eb̶9ׁHhttp://regenerative.Thׁ ׁ Ј׉E Vibration and Chatter in Machining Operations 799 (a) (b) Figure 25.13: Chatter marks in machining. (a) smooth surface, created using the stability lobe approach to avoid chatter; (b) milling chatter marks on the surface of an Al 7075 component. Source: Courtesy Scott Smith, University of North Carolina at Charlotte. • Poor surface finish, as shown in the right central region of Fig. 25.13 • Loss of dimensional accuracy of the workpiece • Premature wear, chipping, and failure of the cutting tool, a critical consideration with brittle tool materials, such as ceramics, some carbides, and diamond • Possible damage to the machine-tool components, from excessive vibration • Objectionable noise, particularly if it is of high frequency, such as the squeal heard when turning brass on a lathe. There are two basic types of vibration in machining: forced and self-excited. Forced Vibration. Forced vibration is generally caused by a periodic applied force that develops in the machine tool, such as from gear drives, imbalance of the machine-tool components, misalignment, and motors and pumps. In operations such as milling or turning of a splined shaft, or a shaft with a keyway or a radial hole, forced vibrations are caused by the periodic engagement of the cutting tool with the workpiece (see, for example, Fig. 24.9). The basic solution to forced vibration is to isolate or remove the forcing element. If, for example, the forcing frequency is at or near the natural frequency of a machine-tool component, one of these two frequencies may be raised or lowered. The amplitude of vibration can be reduced by increasing the stiffness or by damping the system. The cutting parameters generally do not appear to greatly influence the magnitude of forced vibrations; however, changing the cutting speed and the tool geometry can be helpful. It is also recognized that the source of vibrations also can be minimized by changing the configuration of the machine-tool components, as may be done when the driving forces are close to, or act through, the center of gravity of a particular component. This approach will reduce the bending moment on the component, thus reducing deflections and improving dimensional accuracy. Self-excited Vibration. Generally called chatter, self-excited vibration is caused by the interaction of the machining process with the structure of the machine tool. The vibrations usually have very high amplitude, and are audible. Chatter typically begins with a disturbance in the cutting zone, such as by (a) the type of ׉ 7cassandra://RgaxgxqQoKBUbKBGIHTr2Kx-y-7Nb3eHg16pPNTtnBA`cZ%Eb?׉E4800 Chapter 25 Machining Centers, Machine-tool Structures, and Machining chips produced, (b) inhomogeneities in the workpiece material or its surface condition, and (c) variations in the frictional conditions at the tool–chip interface, as influenced by cutting fluids and their effectiveness. The most important type of self-excited vibration is regenerative chatter, which is caused when a tool is cutting a surface that has a roughness or geometric disturbances developed from the previous cut (see Figs. 21.2 and 21.23). The depth of cut varies periodically, and the resulting variations in the cutting force subject the tool to vibrations. The process continues repeatedly, hence the term regenerative.Thistypeof vibration can easily be observed while driving a car over a rough road, the so-called washboard effect. Self-excited vibrations generally can be controlled by: • Increasing the stiffness, especially the dynamic stiffness, of the system. The system includes not only the tool, tool holder, machine frame, etc., but also the workpiece and how it is supported on the machine. • Damping the system. Dynamic stiffness is defined as the ratio of the applied-force amplitude to the vibration amplitude. For example, recall that in a trepanning operation (Fig. 23.24b), there are four machine components involved in the deflections that would cause vibrations: (a) spindle, (b) supporting arm for the cutting tool, (c) drill, and (d) cutting tool. Analysis and experience would suggest that, unless all of these machine components are sufficiently stiff, the trepanning operation will likely lead to chatter, beginning with the torsional vibration around the spindle axis and the twisting of the arm. Two similar examples are (a) long and slender drills, which may undergo torsional vibrations, and (b) cutting tools that are long or are not well supported, such as that shown schematically in Fig. 23.3. Factors Influencing Chatter. It has been observed that the tendency for chatter during machining is proportional to the cutting forces and the depth and width of the cut. Because the forces increase with strength (hence with hardness of the workpiece material), the tendency to chatter generally increases as hardness increases. Aluminum and magnesium alloys, for example, have a lower tendency to chatter than do martensitic and precipitation-hardening stainless steels, nickel alloys, and high-temperature and refractory alloys. Another important factor in chatter is the type of chip produced during machining. Continuous chips involve fairly steady cutting forces; such chips generally do not cause chatter. On the other hand, discontinuous chips and serrated chips (Fig. 21.5) may do so. These type of chips are produced periodically, and the resulting force variations during machining can thus cause chatter. Other factors that may contribute to chatter are using dull tools or cutters, lack of cutting fluids, and worn machine-tool ways and components. Damping. Damping is defined as the rate at which vibrations decay. This effect can be demonstrated on an automobile’s shock absorbers, by pushing down on the car’s front or rear end and observing how rapidly the vertical motion stops. Damping is a major factor in controlling machine-tool vibration and chatter; it consists of internal and external damping. 1. Internal damping results from the energy loss in materials during vibration. For example, composite materials have a higher damping capacity than gray cast iron (Fig. 25.14). The difference in the damping capacity of materials can easily be observed by striking them with a gavel and listening to the sound emitted. Try striking first a brass cymbal, then a piece of concrete, and then a piece of wood, and listen to the distinct variations in their sound. 2. Bolted joints in the structure of a machine tool also are a source of damping, their effectiveness depending on size, position, and the number of joints. Because friction dissipates energy, small relative movements along dry (unlubricated) joints increase damping. Because machine tools consist of a number of large and small components, assembled by various means, this type of damping is cumulative. Note in Fig. 25.15, for example, how overall damping increases as the number of components on a lathe and their contact areas increase. However, the overall stiffness of the machine tool will decrease as the number of joints increases. As described and illustrated in Fig. 23.18b, damping also can be accomplished by mechanical means, whereby energy is dissipated by the frictional resistance of the components within the structure of the boring bar. ׉ 7cassandra://4H9O8X0wh--0KRiI-suqYza08q-s97opOQ4GhHByfew`cZ%Eb@cZ%Eb?#בCט#u׉׉ 7cassandra://-R-_EBnkMvTjaC_4-J4VKud4mPTLX-g6Z4o-U93HS0sy`׉ 7cassandra://eVrjsjytYFBABydu227j7-QgegPEWUsf2OrgTG2BOtsL`s׉ 7cassandra://AE7MSZC2KLAEaZ216qMhIY6PctihctnkKh_DQ62Fz5I`׉ 7cassandra://LOrxn8hsv7lsX0O86cxWqk2wiQ0yKnbA8glJ15OcAdÀV͠^cZ Ebט#u׉׉ 7cassandra://iseZFJM0OMkN38gcBJ0UtBETpsxk7Fq1jjSWWvtgWJUB`׉ 7cassandra://Bol76bLwvrwtgyjgwNNzc0CUyDAewavCHpMjjVQaHe8I`s׉ 7cassandra://SJ17n85auyFEeiBKY8PBfmBTFyFrFDInuuG2eGiioMU`׉ 7cassandra://1C17pP1R4N27lTJVHhKQjVdSTCwUxlB4L6whpgjEd3wi#>͠^cZ Eb׉E_Vibration and Chatter in Machining Operations 801 12 8 4 0 24 28 212 216 220 Cast iron 0 10203040 Time (ms) (a) 12 8 4 0 24 28 212 216 220 Epoxy/graphite 0 10203040 Time (ms) (b) Figure 25.14: The relative damping capacity of (a) gray cast iron and (b) an epoxy–granite composite material. The vertical scale is the amplitude of vibration and the horizontal scale is time. 3. External damping is accomplished with external dampers, similar to shock absorbers on automobiles or machinery. Special vibration absorbers have been developed and installed on machine tools for this purpose. Also, the machines can be installed on specially prepared floors and foundations to isolate forced vibrations, such as those from nearby machinery on the same floor. Stability Lobes. Production demands now require the selection of process parameters that result in high material-removal rates without the risk of chatter. In recent years, stability lobes have been studied, an example of which is shown in Fig. 25.16. It has been noted that, for a given spindle in milling, chatter occurs at certain combinations of speed and tooth depth of cut. Avoiding these combinations results in chatter-free milling. Consider the tooth passing frequency, defines as the product of spindle speed and the number of teeth. The peak stable axial depths (green lines in Fig. 25.16, marked as stability lobes) occur at spindle speeds where the natural frequency of the machine is an integer multiple of the tooth passing frequency. The machine natural frequency can be either measured audibly or with sensors on the machine tool. Applying this concept allows the selection of processing parameters with very high speeds and material-removal rate, without a risk of chatter. Bed only Bed 1 carriage Bed 1 headstock Bed 1 carriage 1 headstock Complete machine Figure 25.15: The damping of vibrations as a function of the number of components on a lathe. Joints dissipate energy; the greater the number of joints, the higher is the damping capacity of the machine. Source: After J. Peters. Relative vibration amplitude (V) Increasing damping Relative vibration amplitude (V) ׉ 7cassandra://AE7MSZC2KLAEaZ216qMhIY6PctihctnkKh_DQ62Fz5I`cZ%EbA׉E7802 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Unstable Stable Stability lobes Spindle speed Figure 25.16: The milling stability lobe diagram, or stability map, showing the spindle speed against the limiting axial depth of cut to avoid chatter. Stable and unstable combinations are identified by the white and shaded regions. Source: After T. Schmitz, University of North Carolina at Charlotte. Guidelines for Reducing Vibration and Chatter. Chatter can be eliminated by maintaining operations in a stability lobe. To make lobes larger and allow greater material removal rates, a balance must be achieved between the increased stiffness of a machine tool and the desirability of increased damping, particularly in the construction of high-precision machine tools. Several guidelines from Chapters 23 and 24 that were given for reducing vibration and chatter in machining operations can be applied. These basic guidelines may be summarized as : • Minimize tool overhang • Improve the stiffness of work-holding devices and support workpieces rigidly • Modify tool and cutter geometry to minimize forces; specifically, changing the number of flutes is often advisable • Change process parameters, such as feed, depth of cut, and cutting fluids • Select a cutting speed so that a stable lobe is utilized • Increase the stiffness of the machine tool and its components, by improving their design, using larger cross sections and materials with a higher elastic modulus • Adjust the machining operation, to shift the cutting forces into the stiffer portions of the machine tool • Improve the damping capacity of the machine tool. These can be seen as adjusting either the critical frequency of the machine or the tooth passing frequency, thereby ensuring machining takes place within a stable lobe. Axial depth ׉ 7cassandra://SJ17n85auyFEeiBKY8PBfmBTFyFrFDInuuG2eGiioMU`cZ%EbBcZ%EbA#בCט#u׉׉ 7cassandra://GqHxMAFdRsX3lNfIbYibCUwYpbR_hpS-2O6SZWWufvs`׉ 7cassandra://PLK5-HqroaDHbEwmPDB6Go__fqY8B7GWCSFXk0RK3xcf,`s׉ 7cassandra://7YY0SJaeFkz28_KDMdoB3h-pKqfhq83rZCw7N6IyzrE`׉ 7cassandra://6Oa95AE29qENzKcddfubAVSIPFklhIkQtN0sd7exM4o\b͠^cZ Ebט#u׉׉ 7cassandra://pF7KygvhLqJ_zXZDx129aqHq9OYwi5rSbRu7Qnik0K0i`׉ 7cassandra://_XLeSHZTdag-CttmEQbZ9dE_R30a_NdGi-QW6Vk8QnQrZ`s׉ 7cassandra://29CMe9sn-h3hgPCyMcc_CWf-aTRzsUdnky3EstfCbjwu`׉ 7cassandra://EpSUkxYAFzJSjAcAmCtItAdRdcYFNLAT-TwDMknuJ8IhG͠^cZ Eb׉E~High-speed Machining 803 25.5 High-speed Machining With continuing demands for higher productivity and lower production costs, a continuing desire is increased cutting speed and material-removal rate in machining, particularly in the aerospace and automotive industries. The term high in high-speed machining (HSM) is somewhat relative; as a general guide, however, an approximate range of cutting speeds may be defined as follows: 1. High speed: 600 to 1800 m/min 2. Very high speed: 1800 to 18,000 m/min 3. Ultrahigh speed: Higher than 18,000 m/min. Spindle rotational speeds in machine tools now range up to 50,000 rpm, although the automotive industry generally has limited them to 15,000 rpm for better reliability and less downtime in case a failure occurs. The spindle power required in high-speed machining is generally on the order of 0.004 W/rpm, much less than in traditional machining (0.2 to 0.4 W/rpm). Feed rates in high-speed machining are now up to 1 m/s. Spindles for high speeds require high stiffness and accuracy, generally involving an integral electric motor. The armature is built onto the shaft, and the stator is placed in the wall of the spindle housing. The bearings may be rolling element or hydrostatic bearings; the latter is more desirable because it requires less space. Because of inertia during the acceleration and deceleration of machine components, the use of lightweight materials, including ceramics and composite materials, is an important consideration. The selection of appropriate cutting-tool materials is always a major consideration. On the basis of the discussions of tools and their selection in Chapter 22, and especially by reviewing Table 22.2, it is apparent that, depending on the workpiece material, multiphase coated carbides, ceramics, cubic-boron nitride, and diamond are all candidate tool materials for high-speed operations. It also is important to note that high-speed machining should be considered primarily for operations in which cutting time is a significant portion of the total time in the overall machining operation. As described in Chapter 40, noncutting time and various other factors are important considerations in the overall assessment of the benefits of high-speed machining. Studies have indicated that high-speed machining is economical for many applications. As successful examples, it has been implemented in machining (a) aluminum structural components for aircraft; (b) submarine propellers 6 m in diameter, made of a nickel–aluminum–bronze alloy, weighing 55,000 kg (45 metric tons); and (c) automotive engines, with 5 to 10 times the productivity of traditional machining. High-speed machining of complex three- and five-axis contours has been made possible by advances in CNC control technology, as described in this chapter and in Chapter 37. Another major factor in the adoption of high-speed machining has been the requirement to further improve dimensional tolerances. Note in Fig. 21.14 that as the cutting speed increases, a large percentage of the heat generated is removed by the chip. The tool and the workpiece remain closer to ambient temperature; this is beneficial because there is no significant thermal expansion, and thus warping, of the workpiece. Important considerations in high-speed machining are: 1. Spindle design, for stiffness, accuracy, and balance at very high rotational speeds 2. Fast feed drives 3. Inertia of the components of the machine tool 4. Selection of appropriate cutting tools 5. Processing parameters and their computer control 6. Work-holding devices that can withstand high centrifugal forces 7. Chip-removal systems that are effective at very high rates of material removal. ׉ 7cassandra://7YY0SJaeFkz28_KDMdoB3h-pKqfhq83rZCw7N6IyzrE`cZ%EbC׉E804 Chapter 25 Machining Centers, Machine-tool Structures, and Machining 25.6 Hard Machining It has been noted that as the hardness of the workpiece increases, its machinability decreases, and tool wear and fracture, surface finish, and surface integrity can become significant problems. However, it is still possible to machine hard metals and alloys by selecting an appropriate hard-tool material and using machine tools with high stiffness, power, and precision. An example is the finish machining of heat-treated steel (45 to 65 HRC) shafts, gears, pinions, and various automotive components, using polycrystalline cubic-boron nitride (PcBN), cermet, or ceramic cutting tools. Called hard machining or hard turning, it produces machined parts with good dimensional accuracy, surface finish (25 μm), and surface integrity. The important factors are the (a) available power, (b) static and dynamic stiffness of the machine tool and its spindle, and (c) workholding devices and fixturing. As described in Section 25.3, trends in the design and construction of modern machine tools, especially for hard machining, include the use of hydrostatic bearings for the spindles and slideways. The headstock and the slanted bed in the machines (see Fig. 23.11a) can be made of granite-epoxy composite materials, with unique properties, such as high stiffness-to-weight ratio, thermal stability, and good damping capacity. Cutting-tool selection and edge preparation also are important to avoid premature failure in hard machining. From technical, economic, and ecological considerations, hard turning has been found to compete successfully with grinding (Chapter 26). For instance, in some specific cases, hard turning has been shown to be three times faster than grinding, requiring fewer operations to finish the part, and utilizing five times less energy. A detailed comparative case of hard turning versus grinding is presented in Example 26.4. 25.7 Ultraprecision Machining Beginning in the 1960s, increasing demands have been made concerning precision manufacturing of components for computer, electronic, nuclear, and defense applications. Specific examples include optical mirrors and lenses, fiber optic connection components, computer memory disks, metrology equipment, and drums for photocopying machines. Surface-finish requirements are in the nanometer (10−9 m) range, and dimensional tolerances and shape accuracies are in the micrometer (μm) and submicrometer range. The trend toward ultraprecision manufacturing continues to grow. Modern ultraprecision machine tools, with advanced computer controls, can now position a cutting tool within an accuracy approaching 1 nm, as can be seen from Fig. 25.17. Note in this figure that higher precision is now being achieved by processes such as abrasive machining, ion-beam machining, and molecular manipulation. The cutting tool for ultraprecision machining applications is almost exclusively a single-crystal diamond, where the process is called diamond turning. The tool has a polished cutting edge, with a radius as small as a few nm. Wear of the diamond can be a significant problem; more recent advances include cryogenic diamond turning, in which the tooling system is cooled by liquid nitrogen, to a temperature of about –120◦C. The materials for ultraprecision machining include copper and aluminum alloys, silver, gold, electroless nickel, infrared materials, and plastics (acrylics). With depths of cut in the nm range, hard and brittle materials produce continuous chips, in a process called ductile-regime cutting (Section 26.3.4). Deeper cuts in brittle materials produce discontinuous chips. The machine tools for ultraprecision machining are built with very high precision and high stiffness of the machine, the spindle, and the workholding devices. The machines have components that are made of structural materials with low thermal expansion and good dimensional stability (see Section 25.3). They are located in a dust-free environment (clean rooms; Section 28.2), where the temperature is controlled to within a fraction of one degree. Vibrations from internal machine sources, as well as from external sources such as nearby machinery on the same floor, must be avoided. Laser metrology (Section 35.5) is used for feed and position control, ׉ 7cassandra://29CMe9sn-h3hgPCyMcc_CWf-aTRzsUdnky3EstfCbjwu`cZ%EbDcZ%EbC#בCט#u׉׉ 7cassandra://l5nlf75d-n5MZiTkz_NAJ_WdQSgsl4UtfGH-wHQ-80o3`׉ 7cassandra://RqpCY1VQXDWb-Ki2tCQthbaGOIczDisWDgR7HfOuwisV`s׉ 7cassandra://vSEVDYCs5SlAoH5un82yNSgsRzwLzLpxs4Ik9niSCXw`׉ 7cassandra://JNMQqvo11cqPwuYyMXU9RbayZ50fJDT7-bNtvPtfloAyh͠^cZ Ebט#u׉׉ 7cassandra://o2eoiRTmlHZUBE70xDhrPs14BnhAyUjfNWwRH25Ejes*w`׉ 7cassandra://_Fpmm4Nsz2BGWF0OkV20C4B6CHG4jqeY69kc4PGIuI8Y`s׉ 7cassandra://X3re0HkmcP3Rz22csv8J3VC-qMT2Z_4eyPS_8hjva0c&`׉ 7cassandra://Hv7DHs6u1JR2H4caF40fPpijlzx1EuC0VOToC5kwofkoH͠^cZ Eb׉E HMachining Economics 805 100 10 1 0.1 0.01 0.001 0.0001 Atomic lattice distance (1 nm) 1940 1960 1980 Turning and milling machines Grinding machines CNC machines Lapping, honing, boring and grinding machines Precision grinding and turning machines High-precision and ultraprecision machines Free abrasive machining Ion beam machining Molecular manipulation 2000 Figure 25.17: Improvements in machining accuracy over the years, using ultraprecision machining technologies. Source: After C.J. McKeown, N. Taniguchi, G. Byrne, D. Dornfeld, and B. Denkena. and the machines are equipped with highly advanced computer-control systems and with thermal and geometric error-compensating features. General Considerations for Precision Machining. There are several important factors in precision and ultraprecision machining and the machine tools, somewhat similar to those in high-speed machining: 1. Machine-tool design, construction, and assembly, including the spindle, must provide high stiffness, damping, and geometric accuracy 2. Motion control of the machine components, both linear and rotational 3. Thermal expansion of the machine tool, compensation for thermal expansion, control of the machinetool environment 4. Real-time performance and control of the machine tool and implementation of a tool-condition monitoring system. 25.8 Machining Economics Material and processing parameters relevant to efficient machining operations have been described in the preceding three chapters. In analyzing the economics of machining, several other factors also have to be considered. These factors include the costs involved in (a) machine tools, workholding devices and fixtures, and cutting tools; (b) labor and overhead associated with indirect costs; (c) the time required in setting up the machine for a particular operation; (d) material handling and movement, such as loading the blank and unloading the machined part; (e) gaging for dimensional accuracy and for surface finish; and (f) cutting and noncutting times. Actual machining time is an important consideration; recall also the discussion in Section 25.5 regarding the role of noncutting time in high-speed machining. Unless noncutting time is a significant portion of the floor-to-floor time, high-speed machining should not be considered. Economic analysis is based on the ability to achieve a desired outcome, such as tolerance and surface finish; as such, a machining process must be robust and under good control (see Section 36.5.1). Machining accuracy (mm) N U l t r a o r m a p r e c i s i l m a P r e o n m a c h i n c i s i o n m a c h i n i c h i n i n i n g n g g ׉ 7cassandra://vSEVDYCs5SlAoH5un82yNSgsRzwLzLpxs4Ik9niSCXw`cZ%EbE׉E 806 Chapter 25 Machining Centers, Machine-tool Structures, and Machining For example, if a milling cutter is mounted such that the exposed spindle length varies randomly with every tool change, then this alone could result in higher tolerances. The same analysis for different machine tools, where dynamic stiffness and damping may differ (see Section 25.3), the use of cutters with different numbers of inserts, or loss of ambient temperature control, all can result in variations that can significantly affect machining precision. Full-factorial design of experiments can characterize the machine-tool/workpiece/operator system, but this approach is complex and has its own limitations. The section below assumes that a process has been carefully designed to be robust, so that variations in these contributing factors can be ignored, and the effect of cutting speed on economics and productivity can be explored. Minimizing Machining Cost per Piece. As in all manufacturing processes and operations, the relevant parameters in machining can be selected and specified in such a manner that the machining cost per piece,as well as machining time per piece, is minimized. Several methods and approaches have been developed over the years to accomplish this goal, a task that has now become easier with increasing use of computers and user-friendly software. In order for the results of the methods used to be reliable, however, it is essential that input data be accurate and up to date. Described next is one of the simpler and more commonly used methods of analyzing machining costs in a turning operation. In machining a part by turning, the total machining cost per piece, Cp is given by Cp = Cm +Cs +Cl +Ct, where Cm = Machining cost Cs = Cost of setting up for machining, including mounting the cutter, setting up fixtures, and preparing the machine tool for the operation Cl = Cost of loading, unloading, and machine handling Ct = Tooling cost, often only about 5% of the total machining operation; consequently, using the least expensive tool is not necessarily the proper way for reducing machining costs. The machining cost is given by Cm = Tm(Lm +Bm), (25.2) where Tm is the machining time per piece, Lm is the labor cost of production personnel per hour, and Bm is the burden rate,or overhead charge, of the machine, including depreciation, maintenance, and indirect labor. The setup cost is a fixed figure in dollars per piece. The loading, unloading, and machine-handling cost is Cl = Tl(Lm +Bm), (25.3) where Tl is the time involved in loading and unloading the part, in changing speeds and feed rates, and making any other adjustments before machining. The tooling cost is Ct = 1 Ni [Tc (Lm +Bm)+Di]+ 1 Nf [Ti (Lm +Bm)], (25.4) where Ni is the number of parts machined per cutting tool insert, Nf is the number of parts that can be produced per insert edge, Tc is the time required to change the insert, Ti is the time required to index the insert, and Di is the depreciation of the insert, in dollars. The time required to machine one part is Tp = Tl +Tm + Tc Ni + Ti Nf , (25.5) (25.1) ׉ 7cassandra://X3re0HkmcP3Rz22csv8J3VC-qMT2Z_4eyPS_8hjva0c&`cZ%EbFcZ%EbE#בCט#u׉׉ 7cassandra://8rO8kE8PYbCStH5FC7-v9vdb2e_21FvDuBcMeIOJEo0}<`׉ 7cassandra://kp-lGC2es5N0utONjWr5PHlaG9dC1ofh9f6lRZRw9WI<)`s׉ 7cassandra://OGGyJgM4TQQv4TwkCnjj2tSIGuFiULX7D7JulhV8BsU `׉ 7cassandra://1GZKdmn56q6ibo-E4i1L0AykigOELJp4hfffSTeYV2gj8͠^cZ Ebט#u׉׉ 7cassandra://QTE_xyhlxQrrl240gLv2NoSDiwso-Qtks50djRHytZ4]k`׉ 7cassandra://1yJ6eF4_jr35y2fGmvmKdJokJUBdNn-Rmjq4nn70cV88j`s׉ 7cassandra://4CZ7uLBxcdRRToDoAbcLRZ51zPMd8TEY-Nc336SxTMc `׉ 7cassandra://ed8ZSqreWXsWF_DvnH3sZxlga72bC4Txlr58WEKUeuQzB͠^cZ Eb׉EMachining Economics 807 where Tm has to be calculated for each particular operation on the part. For example, in a turning operation the machining time (see Section 23.2) is given by L Tm = = fN πLD fV , (25.6) where L is the length of cut, f is the feed, N is the angular speed (rpm) of the workpiece, D is the workpiece diameter, and V is the cutting speed. (Note that appropriate units must be used in all these equations.) From Eq. (21.25) for tool life, T =  1/n C V , (25.7) where T is the time, in minutes, required to reach a flank wear of certain dimension, after which the tool has to be reground or changed. Note that the tool may have to be replaced due to other reasons as well, such as crater wear, built-up edge, or nose wear. This analysis is restricted to flank wear as the important tool-failure criterion, but could also be made more elaborate to include other variables. The number of pieces machined per insert edge follows from Eq. (25.7), as Nf = T Tm and the number of pieces per insert is given by Ni = mNf = Combining Eqs. (25.6) through (25.9) yields Ni = mfC1/n πLDV(1/n)−1 . (25.10) The cost per piece, Cp in Eq. (25.1), can now be defined in terms of several variables. To find the optimum cutting speed and the optimum tool life for minimum cost, Cp must be differentiated with respect to V and set to zero. Thus, ∂Cp ∂V The optimum cutting speed, Vo,is Vo =  and the optimum tool life, To,is To = 1 n − 1 =0. C (Lm +Bm)n n  1 m  1 n − 1  1 m [Tc (Lm +Bm)+Di]+ Ti (Lm +Bm) n (25.11) , mT Tm (25.8) . (25.9) (25.12) [Tc (Lm +Bm)+Di]+ Ti (Lm +Bm) Lm +Bm . (25.13) To determine the optimum cutting speed and the optimum tool life for maximum production, Tp must be differentiated with respect to V and set to zero. Thus, ∂Tp ∂V =0. (25.14) ׉ 7cassandra://OGGyJgM4TQQv4TwkCnjj2tSIGuFiULX7D7JulhV8BsU `cZ%EbG׉E-808 Chapter 25 Machining Centers, Machine-tool Structures, and Machining The optimum cutting speed then is Vo =  and the optimum tool life is To =  1 n − 1  Tc m +Ti  . (25.16) Qualitative plots of minimum cost per piece and minimum time per piece (hence the maximum production rate) are given in Fig. 25.18a and b. It should be noted that the cost of machining a part also depends on the surface finish required. The additional cost increases rapidly with finer surface finish, as shown in Fig. 26.35. The analysis above indicates the importance of (a) identifying all relevant parameters in a machining operation, (b) determining various cost factors, (c) obtaining relevant tool-life curves for the particular operation, and (d) properly measuring the various time intervals involved in the overall operation. The C 1 n − 1  Tc m +Ti n , (25.15) Total cost Machining cost Nonproductive cost Tool-change cost Tool cost Cutting speed (a) High-efficiency machining range Total time Machining time Nonproductive time Tool-changing time Cutting speed (b) Figure 25.18: Graphs showing (a) cost per piece and (b) time per piece in machining. Note the optimum speeds for both cost and time. The range between the two is known as the high-efficiency machining range. Time per piece Cost per piece ׉ 7cassandra://4CZ7uLBxcdRRToDoAbcLRZ51zPMd8TEY-Nc336SxTMc `cZ%EbHcZ%EbG#בCט#u׉׉ 7cassandra://UKs9oBkgSlH3DV73OSl5RSRfIIv3_8E1FAyzxCC4eSsn`׉ 7cassandra://JRGD4EeqvU2hm1OfwXapui02NvvHQR2UjR3X3Dldh3k^`s׉ 7cassandra://e2gBAzL_lPauzu_Omux9w0BAsxp1hniMkp7m2yc-kCwY`׉ 7cassandra://S5z6kGsbkd6DAHKlkdeSrMW5nZAosqAjYei5f8Cr-GwW͠^cZ Ebט#u׉׉ 7cassandra://Y13dDaEqGL-Mhgm6svQQ0OqzBTFHKY8tY9kmBv1H8PUͥ`׉ 7cassandra://s76I7flBPYmy7b8CA_A5WnMDKg5FFs8elNDctumImawV8`s׉ 7cassandra://pw_zMhQyHwym5LVs5bbxGP5bE5_ULX7pAM02QFHZh_YC`׉ 7cassandra://BiR_HmH8mN_cRt75c5I6FH5ispWkf3bJSug0FY3j36w]͠^cZ Eb׉E Key Terms 809 importance of obtaining accurate data is shown in Fig. 25.18. Note that small changes in cutting speed can have a significant effect on the minimum cost or minimum time per piece. The speeds and feeds recommended in Tables 23.4 and 24.2 generally lie in the high-efficiency machining range, which is between the speeds that yield the highest economy and highest production rate. For many applications, such as finish machining of surfaces on soft metal castings, the machining cost per piece is fairly insensitive to cutting speed within this range; that is, the curve in Fig. 25.18 is fairly flat. With difficult-to-machine materials, however, as are routinely encountered in the medical products and aerospace industries, the cost per piece is very sensitive to cutting speed. Consequently, greater care have to be taken to ensure that machining takes place at or near the desired speed. Moreover, it should also be recognized that the data given in Tables 23.4 and 24.2 are a summary for various tool and material grades; specific data is often available for machining particular alloys. Such an economic analysis can typically done for all manufacturing operations, and it can be a valuable tool for guiding process selection. For example, the cost per part in a sand-casting process to produce blanks, and in a machining operation to achieve final dimensional tolerances, can be calculated from an equation similar to Eq. (25.1), including also costs associated with sand casting, such as the cost of mold production and pattern depreciation. A similar calculation can be made on a processing approach that uses powder metallurgy (PM). Die and machinery costs will increase, but less machining is required because of PM’s ability to produce net-shape parts and with tighter tolerances, thereby reducing machining costs. A comparison of cost estimates can then help determine a processing strategy, as described in greater detail in Section 40.10. Summary • Because they are versatile and capable of performing a wide variety of machining operations on small and large workpieces of various shapes, machining centers have become among the most important machine tools. Their selection depends on such factors as part complexity, the number and type of machining operations to be performed, the number of cutting tools required, and the dimensional accuracy and production rate specified. • Vibration and chatter in machining operations are important considerations for workpiece dimensional accuracy, surface finish, and tool life. Stiffness and damping capacity of machine tools are major factors in controlling vibration and chatter. With proper selection of machining variables, operations can take place inside of stability lobes to avoid chatter. • The economics of machining operations depends on such factors as nonproductive costs, machining costs, tool-change costs, and tool costs. Optimum cutting speeds can be determined for both minimum machining time per piece and minimum machining cost per piece. Key Terms Automatic pallet changer Automatic tool changer Chatter Chip collection Damping Dynamic stiffness Forced vibration Hard machining Hexapods High-efficiency machining range High-speed machining Machining center Modular machining center Pallet Reconfigurable machines Regenerative chatter ׉ 7cassandra://e2gBAzL_lPauzu_Omux9w0BAsxp1hniMkp7m2yc-kCwY`cZ%EbI׉E 810 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Self-excited vibration Stability lobes Stiffness Tool-exchange arm Tool- and part-checking station Bibliography Cheng, K., Machining Dynamics: Fundamentals, Applications and Practices, Springer, 2010. Dashchenko, A.I. (ed.), Reconfigurable Manufacturing Systems and Transformable Factories, Springer, 2006. Davim, J.P., Machining of Hard Materials, Springer, 2011. —, Machining of Complex Sculptured Surfaces, Springer, 2012. Erdel, B., High-speed Machining, Society of Manufacturing Engineers, 2003. Gegg, B.C., Suh, C.S., and Luo, C.J., Machine Tool Vibrations and Cutting Dynamics, Springer, 2011. Ito, Y., Modular Design for Machine Tools, McGraw-Hill, 2008. —, Thermal Deformation in Machine Tools, McGraw-Hill, 2010. Joshi, P.H., Machine Tools Handbook, McGraw-Hill, 2008. Lopez de Lacalle, N., and Lamikiz, A. (eds.), Machine Tools for High Performance Machining,Springer, 2009. Mickelson, D., Hard Milling & High Speed Machining: Tools of Change, Industrial Press, 2006. Rivin, E.I., Stiffness and Damping in Mechanical Design, ASME Press, 2010. Schmitz, T.L., and Smith, K.S., Machining Dynamics: Frequency Response to Improved Productivity, Springer, 2008. Suh, C.S., and Liu, M.-K., Control of Cutting Vibration and Machining Instability, Wiley, 2013. Zhang, D., Parallel Robotic Machine Tools, Springer, 2010. Review Questions 25.1. Describe the distinctive features of machining centers and explain why these machines are so versatile. 25.2. Explain how the tooling system in a machining center functions. What are the typical tool-changing times? 25.3. Explain the trends in materials used for machine-tool structures. 25.4. Is there any difference between chatter and vibration? Explain. 25.5. What are the differences between forced and self-excited vibration? 25.6. Explain the importance of foundations for installing machine tools. 25.7. Explain why automated pallet changers and automatic tool changers are important parts of machining centers. 25.8. What types of materials are machine-tool bases typically made from? Why? 25.9. What is meant by the “modular” construction of machine tools? 25.10. What is a hexapod? What are its advantages? 25.11. What factors contribute to costs in machining operations? 25.12. What is the high-efficiency machining range? Why is it so called? 25.13. List the reasons that temperature is important in machining operations. Touch probes Turning center Ultraprecision machining Universal machining center Work envelope ׉ 7cassandra://pw_zMhQyHwym5LVs5bbxGP5bE5_ULX7pAM02QFHZh_YC`cZ%EbJcZ%EbI#בCט#u׉׉ 7cassandra://soEY9rJ9SK8dkLMF4oV55Zle-LoohUxbXH9AMD0L_eE`׉ 7cassandra://AVDcem9DmJHIZ1D9GwTVj7iVN1Swfi5-m7baKdcpVfQq@`s׉ 7cassandra://SDF4ulQtz7zsxoX-qlnnn5MFrO2K-WmDcv4m5_wibaIP`׉ 7cassandra://s1AoJ9Bt_ILtZiswAst7ngGl7b3Szv7OuJYRGlA7ujQF^͠^cZEb$ט#u׉׉ 7cassandra://6z72prUF3ds4s1ujTXjsxwnz7xOoaU9PiyaWy0sMUzA`׉ 7cassandra://O9QKTn3MsINwEbgbLjfF5lmL0hRLybrL715kXHGX3kMgI`s׉ 7cassandra://aSIn_r3lTAlTN7M-vjrGz4aRTVHONRPeuzvtERgk8T8`׉ 7cassandra://smlYAk7KsQwLqwM8PS6vBKsLe2IrrqsQhCEgJb_h5oMZ͠^cZEb%׉E Qualitative Problems 811 Qualitative Problems 25.14. Explain the technical and economic factors that led to the development of machining centers. 25.15. Spindle speeds in machining centers vary over a wide range. Explain why this is so, giving specific applications. 25.16. Explain the importance of stiffness and damping of machine tools. Describe how they are implemented. 25.17. Are there machining operations described in Chapters 23 and 24 that cannot be performed in machining and turning centers? Explain, with specific examples. 25.18. How important is the control of cutting-fluid temperature in operations performed in machining centers? Explain. 25.19. Review Fig. 25.10 on modular machining centers, and describe some workpieces and operations that would be suitable on such machines. 25.20. Review Fig. 25.15 and estimate the amount of damping you would expect in a hexapod. Is vibration a serious concern with hexapods? Explain. 25.21. Describe the adverse effects of vibration and chatter in machining operations. 25.22. Describe some specific situations in which thermal distortion of machine-tool components would be important. 25.23. Explain the differences in the functions of a turret and of a spindle in turning centers. 25.24. Explain how the pallet arrangements shown in Fig. 25.4a and b would be operated in using these machines on a shop floor. 25.25. Review the tool changer shown in Fig. 25.5. Are there any constraints on making their operations faster in order to reduce the tool changing time? Explain. 25.26. List the parameters that influence the temperature in metal cutting, and explain why and how they do so. 25.27. List and explain factors that contribute to poor surface finish in machining operations. 25.28. Can high-speed machining be performed without the use of cutting fluids? Explain. 25.29. In addition to the number of joints in a machine tool (see Fig. 25.15), what other factors influence the rate at which damping increases? Explain. 25.30. Describe types and sizes of workpieces that would not be suitable for machining on a machining center. Give specific examples. 25.31. Other than the fact that they each have a minimum, are the overall shapes and slopes of the total-cost and total-time curves in Fig. 25.18 important? Explain. 25.32. Explain the advantages and disadvantages of machine-tool frames made of gray-iron castings. 25.33. What are the advantages and disadvantages of (a) welded-steel frames, (b) bolted steel frames, and (c) adhesively bonded components of machine tools? Explain. 25.34. What would be the advantages and limitations of using concrete or polymer–concrete in machine tools? 25.35. Explain how you would go about reducing each of the cost factors in machining operations. What difficulties would you encounter in doing so? 25.36. Describe workpieces that would not be suitable for machining on a machining center. Give specific examples. 25.37. Give examples of forced vibration or self-excited vibration in general engineering practice. ׉ 7cassandra://SDF4ulQtz7zsxoX-qlnnn5MFrO2K-WmDcv4m5_wibaIP`cZ%EbK׉E'812 Chapter 25 Machining Centers, Machine-tool Structures, and Machining Quantitative Problems 25.38. A machining-center spindle and tool extend 200 mm from their machine-tool frame. Calculate the temperature change that can be tolerated in order to maintain a tolerance of 0.025 mm in machining. Assume that the spindle is made of steel. 25.39. Using the data given in the example, estimate the time required to manufacture the parts in Example 25.1 with conventional machining and with high-speed machining. 25.40. A machining-center spindle and tool extend 500 mm from its machine-tool frame. What temperature change can be tolerated to maintain a tolerance of 0.0025 mm in machining? A tolerance of 0.025 mm? Assume that the spindle is made of steel. 25.41. In the production of a machined valve, the labor rate is $30.00 per hour, and the general overhead rate is $25.00 per hour. The tool is a ceramic insert with four faces and costs $30.00, takes six minutes to change and one minute to index. Estimate the optimum cutting speed from a cost perspective. Use C = 100 for Vo in m/min. 25.42. Estimate the optimum cutting speed in Problem 25.41 for maximum production. 25.43. Develop an equation for optimum cutting speed in face milling using a cutter with inserts. 25.44. Develop an equation for optimum cutting speed in turning where the tool is a high speed steel tool that can be reground periodically. Synthesis, Design, and Projects 25.45. If you were the chief engineer in charge of the design of advanced machining and turning centers, what changes and improvements would you recommend on existing models? Explain. 25.46. Review the technical literature and outline the trends in the design of modern machine tools. Explain why there are those trends. 25.47. Make a list of components of machine tools that could be made of ceramics, and explain why ceramics would be suitable. 25.48. Survey the company literature from various machine-tool manufacturers, and prepare a comprehensive table indicating the capabilities, sizes, power, and costs of machining and turning centers. Comment on your observations. 25.49. The cost of machining and turning centers is considerably higher than for traditional machine tools. Since many operations performed by machining centers also can be done on conventional machines, how would you go about justifying the high cost of these centers? Explain with appropriate examples. 25.50. In your experience using tools or other devices, you may have come across situations in which where you experienced vibration and chatter. Describe your experience and explain how you would go about minimizing the vibration and chatter. 25.51. Describe your thoughts on whether or not it is feasible to include grinding operations (see Chapter 26) in machining centers. Explain the nature of any difficulties that may be encountered. 25.52. Is the accuracy and surface finish that can be achieved in a machining center a function of the number of inserts on a cutter? Explain. 25.53. The following experiment is designed to better demonstrate the effect of tool overhang on vibration and chatter:With a sharp tool, scrape the surface of a piece of soft metal by holding the tool with your arm fully outstretched. Repeat the experiment, this time holding the tool as close to the workpiece as possible. Describe your observations regarding the tendency for the tool to vibrate. Repeat the experiment with different types of metallic and nonmetallic materials. 25.54. Review the part in Fig. 25.1a and list the machining operations and machine tools you would recommend to produce this part. ׉ 7cassandra://aSIn_r3lTAlTN7M-vjrGz4aRTVHONRPeuzvtERgk8T8`cZ%EbLcZ%EbK#בCט#u׉׉ 7cassandra://RW4wC0UXev5JThYOmeVqdeTDbx9z3OH_XuQ0kZPJK7o8a`׉ 7cassandra://e2xxMHhPUXutDSS9GSetSVVDfkQykyZQq_mijzT1y-c5y`s׉ 7cassandra://6SDtXPhLMZICRYWp74iBN-cyxEWNSckDcHAMMOfDzjk`׉ 7cassandra://VJFPCZnjmpAYWJto_GcoO8p34U7DPtSp21CkAv7c02o9T͠^cZEb'ט#u׉׉ 7cassandra://nuVuJ0_cw--gfuvS8JPhbKrvqNgXo5NGHLmq2Qi7v58`׉ 7cassandra://-lbXkHj3OSmsb2R9Oed7v1nlUhD98rpDh4t2aSD3dRgR`s׉ 7cassandra://7x4b149O91SPxWRBK989zFmlD-Jpv97VU4EqEssdbDYU`׉ 7cassandra://WBcClSVQ-IpH6jW9ZRUMfkh3X8ZrgLWtDkbAxvUWSbI{f ͠^cZEb(׉EChapter 26 Abrasive Machining and Finishing Operations 26.1 Introduction 814 26.2 Abrasives and Bonded Abrasives 816 26.3 The Grinding Process 822 26.4 Grinding Operations and Machines 831 26.5 Design Considerations for Grinding 840 26.6 Ultrasonic Machining 840 26.7 Finishing Operations 841 26.8 Deburring Operations 846 26.9 Economics of Abrasive Machining and Finishing Operations 848 Examples: 26.1 Forces in Surface Grinding 825 26.2 Action of a Grinding Wheel 828 26.3 Cycle Patterns in Cylindrical Grinding 834 26.4 Grinding versus Hard Turning 837 26.5 Belt Grinding of Turbine Nozzle Vanes 842 Case Study: 26.1 Gear Grinding with Engineered Abrasives 849 • Abrasive machining is among the major finishing operations in the production of parts, and is important because of its capability to impart high dimensional accuracy and surface finish. • This chapter opens with a description of the grinding process, the mechanics of material removal, and the types of abrasives and bonds used in grinding wheels. 813 ׉ 7cassandra://6SDtXPhLMZICRYWp74iBN-cyxEWNSckDcHAMMOfDzjk`cZ%EbM׉E<814 Chapter 26 Abrasive Machining and Finishing Operations • Some abrasive machining operations, including polishing, buffing, honing, and sanding, require a bonded or coated abrasive; others, such as ultrasonic machining, lapping, abrasive flow machining, and electrochemical machining and grinding, have loose abrasives. • The fundamentals of all abrasive processes are described in detail, including their principles, applications, and design considerations. • The chapter concludes with a discussion of economic considerations for finishing operations. Typical parts made: Any part requiring high dimensional accuracy and surface finish, such as ball and roller bearings, piston rings, valves, cams, gears, and tools and dies. Alternative processes: Precision machining, electrical-discharge machining, electrochemical machining and grinding, and abrasive-jet machining. 26.1 Introduction There are numerous situations in manufacturing where the processes described thus far cannot produce the required dimensional accuracy or surface finish, or the workpiece material is too hard or too brittle to process. For example, consider the dimensional accuracy and fine surface finish required on ball bearings, pistons, valves, cylinders, cams, gears, molds and dies, and the precision components in instrumentation. One of the most common and economical methods for producing such demanding characteristics on parts is abrasive machining. An abrasive is a very small, hard particle having sharp edges and an irregular shape (Fig. 26.1). The simplest example is sand, which is capable of removing small amounts of material from a surface by scratching it, producing tiny chips. Familiar applications of abrasives are sandpaper or emery cloth, used to smoothen (a) (b) Figure 26.1: Typical abrasive grains; note the angular shape with sharp edges. (a) A single, 80-mesh Al2O3 grit in a freshly dressed grinding wheel; (b) an 80/100 mesh diamond grit. Diamond and cubic boron nitride grains can be manufactured in various geometries, including the “blocky” shape shown. Source: Courtesy of J. Badger. ׉ 7cassandra://7x4b149O91SPxWRBK989zFmlD-Jpv97VU4EqEssdbDYU`cZ%EbNcZ%EbM#בCט#u׉׉ 7cassandra://D3AdNryPs6Kk0au-11WqWB_uiylXhtk_HOclMFbOXBomw`׉ 7cassandra://Wpu1POyCNAfO7yn5vovOpZL7hBWCAYu2dcCga6Db9kE;`s׉ 7cassandra://ZGsTNJWCl9lMTFLxoIsntJNskpwHG857iGTjLXzlPPIl`׉ 7cassandra://b1VirfPPtW2RiPoDixpQfguSa-c5m2GhSvNZr7D4_XU}͠^cZEb,ט#u׉׉ 7cassandra://_vyUQ53tO6TwltfLXXnXUtzOIuFmEp5vCe8zyb3Ec7A`׉ 7cassandra://hcQb4AT6CL-sIk6jNpXADX8qeTkNhvzxiiO2nF1VlFQQ`s׉ 7cassandra://1Lrukou1xmxic9RtICd6bnHMPza-maadpn-TQgHKrLA`׉ 7cassandra://-rj77S8HA-GPNJ67-XgKaS3XUGw1qVP8EqIF5TTzI2Mi8͠^cZEb-׉EkIntroduction 815 Figure 26.2: A variety of bonded abrasives used in abrasive-machining processes. Source: Shutterstock/ Praethip Docekalova. surfaces and remove sharp corners, and grinding wheels, as shown in Figs. 26.2 and 26.3, to sharpen knives, tools, or to impart good dimensional accuracy and surface finish. Abrasives also are used to hone, lap, buff, and polish workpieces. With the use of computer-controlled machines, abrasive processes and equipment are now capable of producing a wide variety of part shapes, as can be seen in Fig. 26.3, and very fine dimensional accuracy and surface finishes, as shown in Figs. 23.14 and 33.5. For example, dimensional tolerances on parts can now be less than 1 μm and surface roughness can be as fine as 0.025 μm. Grinding wheel Workpiece (a) (b) Thin abrasive disks (c) Workpiece Grinding wheel (d) (e) Workpiece (f) (g) Figure 26.3: The types of workpieces and operations typical of grinding: (a) cylindrical surfaces, (b) conical surfaces, (c) fillets on a shaft, (d) helical profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and (g) internal grinding. ׉ 7cassandra://ZGsTNJWCl9lMTFLxoIsntJNskpwHG857iGTjLXzlPPIl`cZ%EbO׉E n816 Chapter 26 Abrasive Machining and Finishing Operations Because they are hard, abrasives also are used in finishing processes for heat-treated metals and alloys, and for very hard parts, in such applications as (a) finishing of ceramics and glasses, (b) cutting off lengths of bars, structural shapes, masonry, and concrete, (c) removing unwanted weld beads and spatter, (d) creating a very smooth and flat surface on silicon wafers to make integrated circuits, (e) polishing bearings and races, and (f) cleaning surfaces with jets of air or water containing abrasive particles. This chapter begins with a description of abrasive characteristics, along with their use in various abrasive material-removal processes. The mechanics of abrasive operations is first described. This knowledge is essential in establishing the interrelationships between the (a) workpiece material and process variables and (b) dimensional accuracy, surface finish, and surface integrity of the parts produced by abrasive machining. 26.2 Abrasives and Bonded Abrasives Abrasives that are used most commonly in abrasive-machining operations are: Conventional abrasives • Aluminum oxide (Al2O3) • Silicon carbide (SiC) Superabrasives • Cubic boron nitride (cBN) • Diamond As described in Chapter 8, these abrasives are much harder than conventional cutting-tool materials, as may be seen by comparing Tables 22.1 and 26.1 (see also Fig. 2.15). Cubic boron nitride and diamond are listed as superabrasives because they are the two hardest materials known. In addition to hardness, an important characteristic of abrasives is friability, defined as the ability of abrasive grains to fracture into smaller pieces. This property gives abrasives their self-sharpening characteristics, essential in maintaining their sharpness during use. High friability indicates low strength or low fracture resistance of the abrasive. Thus, a highly friable abrasive grain fragments more rapidly under grinding forces than one with low friability. Aluminum oxide, for example, has lower friability than silicon carbide and, correspondingly, lower tendency to fragment. The shape and size of the abrasive grain affect its friability. For example, blocky grains, which are analogous to a negative rake angle in single-point cutting tools (Fig. 21.3), are less friable than less blocky or platelike grains. Moreover, because the probability of defects decreases as grain size decreases, smaller grains are stronger and less friable than larger ones (a phenomenon known as size effect). Table 26.1: Ranges of Knoop Hardness for Various Materials and Abrasives. Common glass Flint, quartz Zirconium oxide Hardened steels Tungsten carbide Aluminum oxide Titanium nitride Titanium carbide Silicon carbide Boron carbide 350–500 800–1100 1000 700–1300 1800–2400 2000–3000 2000 1800–3200 2100–3000 2800 Cubic boron nitride 4000–5000 Diamond 7000–8000 ׉ 7cassandra://1Lrukou1xmxic9RtICd6bnHMPza-maadpn-TQgHKrLA`cZ%EbPcZ%EbO#בCט#u׉׉ 7cassandra://4UMB35s-ZjQt9jdRx6bz9SBuuNlMBjVGJrSr6PXgipI`׉ 7cassandra://6PclsNcR0KU1nnY7lVwKXTTx_wCjUc0mcROxXwgR1cge`s׉ 7cassandra://4Nqlyl2O-d52TcciUWN5u7jX0_Cz0JpqiyJrkL3zz7A1`׉ 7cassandra://5iHT7zUeQf8NR4HprEBmcdySoyrCKypsOD8WHwq9_YUd͠^cZEb/ט#u׉׉ 7cassandra://2COZLn5qc1bNntGCZ2RuCqgYwqHQ53iGw78anGyZl4I6``׉ 7cassandra://mwqgpuus69v98AJJ3T0MDyKsUvGSnXGnB6IUHXrueuka`s׉ 7cassandra://-ywutFGMUVFlhcPquHZPuoXOx5dYECyi4yTvU3VFK8Y{`׉ 7cassandra://yXctMxQVx5wv0Mft7wOm_MZqHvpMFBHNkWnt9g2wh1Aܶ͠^cZEb0׉EFAbrasives and Bonded Abrasives 817 Abrasive Types. The abrasives commonly found in nature are emery, corundum (alumina), quartz, garnet, and diamond. Because in their natural state abrasives generally contain impurities and possess nonuniform properties, their performance as an abrasive can be inconsistent and unreliable. Consequently, abrasives have been made synthetically for many years. • Aluminum oxide was first made in 1893, and is produced by fusing bauxite, iron filings, and coke. Fused aluminum oxides are categorized either as dark (less friable), white (very friable), or single crystal. • Seeded gel is the purest form of unfused aluminum oxide; it was first introduced in 1987. Also known as ceramic aluminum oxide, it has a grain size on the order of 0.2 μm (coarse human hair is about 200 μm), which is much smaller than other types of commonly used abrasive grains. The grains are sintered (heating without melting; Section 17.4) to become larger in size. Because they are harder than fused alumina and have relatively high friability, seeded gels maintain their sharpness and are especially effective for difficult-to-grind materials. • Silicon carbide was first discovered in 1891, and is made with silica sand and petroleum coke. Silicon carbides are classified as black (less friable) or green (more friable). They generally have higher friability than aluminum oxide; hence, they display greater tendency to fracture and thus remain sharp. • Cubic boron nitride was first developed in the 1970s; its properties and characteristics are described in Sections 8.2.3 and 22.7. • Diamond, also known as synthetic or industrial diamond, was first used as an abrasive in 1955; its properties and characteristics are described in Sections 8.7 and 22.9. Abrasive Grain Size. As used in manufacturing operations, abrasives generally are very small when compared to the size of cutting tools and inserts (Chapters 21 and 22). They have sharp edges, allowing removal of very small quantities of material from a workpiece surface, resulting in very fine surface finish and dimensional accuracy. The size of an abrasive grain is identified by a grit number, which is a function of sieve size; the smaller the grain size, the larger is the grit number. For example, grit number 10 is typically regarded as very coarse, 100 as fine, and 500 as very fine. Sandpaper and emery cloth also are identified in this manner, as can readily be observed by noting the grit number printed on the backs of abrasive papers or cloth. Compatibility of Abrasive and Workpiece Material. As in selecting cutting-tool materials for machining, the affinity of an abrasive grain to the workpiece material is an important consideration. The less the reactivity of the two materials, the less the wear and dulling of the grains during grinding, making the operation more efficient and causing less damage to the workpiece surface (see Section 26.3.1 for details). As an example of chemical affinity, diamond (a form of carbon, Section 8.7) cannot be used for grinding steels, since diamond dissolves in iron at the high temperatures encountered in grinding. Generally, the following recommendations are made with regard to selecting abrasives: • Aluminum oxide: Carbon steels, ferrous alloys, and alloy steels. • Silicon carbide: Nonferrous metals, cast irons, carbides, ceramics, glass, and marble. • Cubic boron nitride: Steels and cast irons above 50 HRC hardness and high-temperature alloys. • Diamond: Ceramics, carbides, and some hardened steels where the hardness of diamond is more significant than its reactivity with the carbon in steel. ׉ 7cassandra://4Nqlyl2O-d52TcciUWN5u7jX0_Cz0JpqiyJrkL3zz7A1`cZ%EbQ׉E 818 Chapter 26 Abrasive Machining and Finishing Operations Grinding wheel Bond Porosity Grain Bond fracture Microcracks Workpiece Attritious wear Wheel surface Grain fracture Figure 26.4: Schematic illustration of a physical model of a grinding wheel, showing its structure and its wear and fracture patterns. 26.2.1 Grinding Wheels Each abrasive grain typically removes only a tiny amount of material at a time; consequently, high materialremoval rates can only be achieved if a very large number of these grains remove material. This is often done by using bonded abrasives, typically in the form of a grinding wheel, in which the abrasive grains are distributed and oriented randomly. As shown schematically in Fig. 26.4, the abrasive grains in a grinding wheel are held together by a bonding material (Section 26.2.2), which acts as supporting posts or braces between the grains. In bonded abrasives, porosity is essential to provide clearance for the chips being produced; otherwise, the chips would severely interfere with the grinding operation. Porosity can be observed by looking at the surface of a grinding wheel with a magnifying glass or under a microscope. A very wide variety of types and sizes of abrasive wheels is now available. Some of the more commonly used types of grinding wheels made of conventional abrasives are shown in Fig. 26.5; superabrasive wheels are shown in Fig. 26.6. Note that, due to their high cost, only a small volume of superabrasive material is used on the periphery of these wheels; also, it is not necessary to have more abrasives because their wear is extremely small. Bonded abrasives are marked with a standardized system of letters and numbers, indicating the type of abrasive, grain size, grade, structure, and bond type. Figure 26.7 shows the marking system for aluminumoxide and silicon-carbide bonded abrasives. The marking system for diamond and cubic boron nitride bonded abrasives is shown in Fig. 26.8. The cost of grinding wheels depends on the type and size of the wheel. Small wheels [up to about 25 mm in diameter] cost approximately $2 to $15 for conventional abrasives, $30 to $100 for diamond, and $50 to $300 for cubic boron nitride wheels. For a large wheel of about 500 mm in diameter and 250 mm in width, the costs are $500 for conventional abrasives, $5000 to $8000 for diamond, and as high as $20,000 for cubic boron nitride. 26.2.2 Bond Types The common types of bonds used in bonded abrasives are: Vitrified. Also called ceramic bond, vitrified bonds (from the Latin vitrum for glass; Section 8.4) are the most common and widely used material. The raw materials consist of feldspar (a crystalline mineral) and clays. They are mixed with the abrasives, moistened, and molded under pressure into the shape of grinding wheels. These green wheels are then fired slowly, up to a temperature of about 1250◦C, to fuse the glass and develop structural strength. The wheels are then cooled slowly (to avoid temperature gradients within the ׉ 7cassandra://-ywutFGMUVFlhcPquHZPuoXOx5dYECyi4yTvU3VFK8Y{`cZ%EbRcZ%EbQ#בCט#u׉׉ 7cassandra://jBE_QMyawPOA_kciP8xy__2y3mtjGVMdxnohwWJ01ok`׉ 7cassandra://F9IvTQwGxfr2LeZIOvHLXkWSNBk_-XZZjDQA44H1ISM17`s׉ 7cassandra://FRf57TyV0dvCXPxqU93ck6rHBg8J4Lt-drP4gevBgos`׉ 7cassandra://9eOL14Aw4iPDe034HSsVilllDre0XLgV5oET8RuTs0c͠^cZ EbDט#u׉׉ 7cassandra://iEELeHVVeqcXyEINIkq1jD9yJhq6ShBfJM7HzxRmPjwb`׉ 7cassandra://HL1_j0fSRDv7qbSsT3YrReQ6QOgEaEuCEKmXWYaSsl4=c`s׉ 7cassandra://Vk_9TimJHq9ele6fKsY8_rxX0bDYaqM960SjH_ZaMBA/`׉ 7cassandra://GPSfs5RbSazz-iDaF9Uy6_JdIjOQiMSSCJqmY3Ovf1E$͠^cZ EbE׉EJAbrasives and Bonded Abrasives 819 Grinding face Grinding face (a) Type 1—straight (b) Type 2—cylinder Grinding face Grinding face (c) Type 6—straight cup (d) Type 11—flaring cup Grinding edge Grinding edge (e) Type 27—depressed center (f) Type 28—depressed center (g) Mounted Figure 26.5: Common types of grinding wheels made with conventional abrasives. Note that each wheel has a specific grinding face; grinding on other surfaces is improper and unsafe. Type 1A1 2A2 1A1RSS (a) (b) (c) 11A2 (d) DW (e) DWSE (f) Figure 26.6: Examples of superabrasive wheel configurations. The annular regions (rims) are grinding surfaces; the wheel itself (core) generally is made of metal or composites. The bonding materials for superabrasives are (a), (d), and (e) resinoid, metal, or vitrified; (b) metal; (c) vitrified; and (f) resinoid. ׉ 7cassandra://FRf57TyV0dvCXPxqU93ck6rHBg8J4Lt-drP4gevBgos`cZ%EbS׉E820 Chapter 26 Abrasive Machining and Finishing Operations Example: 51 Prefix Manufacturer’s symbol (indicating exact kind of abrasive) (use optional) – A Abrasive type Coarse 8 A Aluminum oxide C Silicon carbide 10 12 14 16 20 24 – 36 Abrasive grain size Medium Fine 30 36 46 54 60 70 80 90 100 120 150 180 – L Grade Very fine 220 240 280 320 400 500 600 – 5 Structure Dense 1 2 3 4 5 6 7 8 9 Open Soft A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Medium Grade scale Hard 10 11 12 13 14 15 16 etc. (Use optional) – V Bond type – 23 Manufacturer’s record Manufacturer’s private marking (to identify wheel) (use optional) B Resinoid BF Resinoid reinforced E Shellac O Oxychloride R Rubber RF Rubber reinforced S Silicate V Vitrified Figure 26.7: Standard marking system for aluminum-oxide and silicon-carbide bonded abrasives. Example:M– – Grade Prefix Abrasive type Manufacturer’s symbol (to indicate type of diamond) Diamond B Cubic boron nitride D Diamond 20 24 30 36 46 54 60 80 90 100 120 150 180 220 240 280 320 400 500 600 800 1000 A (soft) Z (hard) to 25 (low) 50 75 100 (high) concentration D 100 P 100 B Grit size 1/8 Bond B Resinoid M Metal V Vitrified Bond modification Diamond depth (mm) 1.58 4.76 6.35 Absence of depth symbol indicates all diamond A letter or numeral or combination (used here will indicate a variation from standard bond) Figure 26.8: Standard marking system for cubic boron nitride and diamond bonded abrasives. ׉ 7cassandra://Vk_9TimJHq9ele6fKsY8_rxX0bDYaqM960SjH_ZaMBA/`cZ%EbTcZ%EbS#בCט#u׉׉ 7cassandra://ZbCYdKTau_P9r7hL-rl8z0U7nxYT3Bx7axFoTPeunTMf`׉ 7cassandra://pcBtZcqR3jRN8Zt7I0B2_4HDQ6q8yGRNwRguvx7PUlEnc`s׉ 7cassandra://eWknLb1jS4YqZtYuHRrD67sL6vsHxw5_etpirVVM6Wc`׉ 7cassandra://GasaKbvtHSpyLGA6SCPTFBdz_FDPYZHljFNE1LUdnuM`|͠^cZ!EbGט#u׉׉ 7cassandra://liXTKc40AD_cyXAURha_13vfqYh_UvG86n3O2JA_gEs`׉ 7cassandra://F7n7H_2Liu0ycT7LD79jf4-eWScbPSYfwRCtMnXGDn4@`s׉ 7cassandra://-JmIUkWppx-uVIdHLGtpRM_x64N_hcpHQKNPhKXHziYB`׉ 7cassandra://_RC8dCnnUg76cJiOcxebIGTw85HXhNXtfm4AD7-7WWA͜͠^cZ!EbH׉EAbrasives and Bonded Abrasives 821 wheels and associated thermal cracking), finished to size, inspected for quality and dimensional accuracy, and tested for any defects. Wheels with vitrified bonds are strong, stiff, and resistant to oils, acids, and water; however, they are brittle and thus lack resistance to mechanical and thermal shock. To improve strength during their use, vitrified wheels also are made with steel-backing plates or cups, for better structural support of the bonded abrasives. The color of a grinding wheel can be modified by adding various elements during its manufacture, so that wheels can be color coded for use with specific workpiece materials, such as ferrous, nonferrous, and ceramic. Resinoid. Resinoid bonding materials are thermosetting resins, and are available in a wide range of compositions and properties (Sections 7.4 and 7.7). Because the bond is an organic compound, wheels with resinoid bonds also are called organic wheels. The process for making them consists basically of (a) mixing the abrasive with liquid or powdered phenolic resins and additives, (b) pressing the mixture into the shape of a grinding wheel, and (c) curing it at temperatures of about 175◦C to set the bond. In addition to pressing, injection molding also is used to make grinding wheels (see Sections 17.3 and 19.3). Because the elastic modulus of thermosetting resins is lower than that of glasses (see Table 2.2), resinoid wheels are more flexible than vitrified wheels. As a bonding material, polyimide (Section 7.7) also is used as a substitute for the phenolic resin; it is tougher and more resistant to higher temperatures. Reinforced Wheels. These wheels typically consist of one or more layers of fiberglass mats of various mesh sizes. The fiberglass in this laminate structure retards the disintegration of the wheel should the wheel break for some reason during its use. Large-diameter resinoid wheels can be further supported by using one or more internal rings, made of round steel bars inserted during molding of the wheel. Thermoplastic. In addition to thermosetting resins, thermoplastic plastics (Section 7.3) also are used in grinding wheels. Wheels are available with sol-gel abrasives bonded with thermoplastics. Rubber. The most flexible matrix used in abrasive wheels is rubber (Section 7.9). The manufacturing process consists of (a) mixing crude rubber, sulfur, and the abrasive grains together, (b) rolling the mixture into sheets, (c) cutting out disks of various diameters, and (d) heating the disks under pressure to vulcanize the rubber. Thin wheels (called cutoff blades) can be made in this manner, and are used like circular saws for cutting-off operations. Metal. Using powder-metallurgy techniques, the abrasive grains, usually diamond or cubic boron nitride, are bonded to the periphery of a metal wheel to depths of 6 mm or less (Fig. 26.6). Metal bonding is carried out under high pressure and temperature. The wheel itself (the core) may be made of aluminum, bronze, steel, ceramics, or composite materials, depending on such requirements as strength, stiffness, and dimensional stability. Superabrasive wheels may be layered, so that a single abrasive layer is plated or brazed to a metal wheel. Layered wheels are lower in cost, and are used for small production batches. 26.2.3 Wheel Grade and Structure The grade of a bonded abrasive is a measure of its bond strength, including both the type and the amount of bonding material in the wheel. Because strength and hardness are directly related (Section 2.6.2), the grade is also referred to as the hardness of a bonded abrasive. Thus, a hard wheel has a stronger bond and/or a larger amount of bonding material between the grains than does a soft wheel. The structure of a bonded abrasive is a measure of its porosity (the spacing between the grains, as shown in Fig. 26.4). The structure ranges from dense to open, as shown in Fig. 26.7. Recall that some porosity is essential to provide clearance for the chips, as otherwise they would interfere with the grinding operation. ׉ 7cassandra://eWknLb1jS4YqZtYuHRrD67sL6vsHxw5_etpirVVM6Wc`cZ%EbU׉EO822 Chapter 26 Abrasive Machining and Finishing Operations a Chip Chip f Workpiece (a) 10 mm (b) Figure 26.9: (a) Grinding chip being produced by a single abrasive grain; note the large negative rake angle of the grain. (b) Schematic illustration of chip formation by an abrasive grain with a wear flat; note the negative rake angle of the grain and the small shear angle. Source: (a) After M.E. Merchant. 26.3 The Grinding Process Grinding is a chip-removal process that uses an individual abrasive grain as the cutting tool (Fig. 26.9a). The major differences between the action of an abrasive grain and that of a single-point cutting tool can be summarized as: • Individual abrasive grains have irregular shapes (Fig. 26.1), and are spaced randomly along the periphery of the wheel (Fig. 26.10). • The average rake angle of the grains is highly negative, typically −60◦ or even less; consequently, grinding chips undergo much larger plastic deformation than they do in other machining processes (see Section 21.2). Abrasive grain f v Workpiece Grain V Wear flat Figure 26.10: The surface of a grinding wheel (A46-J8V), showing abrasive grains, wheel porosity, wear flats on grains, and metal chips from the workpiece adhering to the grains. Note the random distribution and shape of the abrasive grains. Magnification: 50×. Source: S. Kalpakjian. ׉ 7cassandra://-JmIUkWppx-uVIdHLGtpRM_x64N_hcpHQKNPhKXHziYB`cZ%EbVcZ%EbU#בCט#u׉׉ 7cassandra://4P3cJhQO2T1tD-kZQuTJcBc8tiWEsMz4kLRA1Z3xuj8L]`׉ 7cassandra://QdLP3qwmVm8aDEOHzAfRfEQH-EME_jBrM_SvnkqZfjsJo`s׉ 7cassandra://X34ktvjfmu-qgH5O-lgXV7dNF1lJVvoacnibH5_3ShE#`׉ 7cassandra://h_F3Nfl5w5jTlxpocyIXFT5vawt4gL8RctVHYA10NZY͡͠^cZ!EbJט#u׉׉ 7cassandra://BjbQTKoU0TgWDwAMkzkXSGDkaLEjuL6Of46UkVnPyNw`׉ 7cassandra://jiFN5j0gRgdnkgToh1CKmd3SqFIuUvcMdFYXrx0oDZgY`s׉ 7cassandra://VcOl4Qjv583voyGlvmSYYvHdAg_K_K__tONChqG1DREP`׉ 7cassandra://uj2HxIwYflu4O6SPIMefv_Yl_xqRH49VHZ8yLyOVNwUr ͠^cZ!EbK׉EThe Grinding Process 823 V D t d l v Workpiece Grinding wheel Grains Figure 26.11: Schematic illustration of the surface-grinding process, showing various process variables. The figure depicts conventional (up) grinding. • The radial positions of the grains over the peripheral surface of a wheel vary, and not all grains are active during grinding. • Surface speeds of grinding wheels (equivalent to cutting speeds) are very high, typically 20 to 30 m/s, and can be as high as 150 m/s in high-speed grinding, using specially designed wheels. The grinding process and its parameters can best be observed in a surface-grinding operation, shown schematically in Fig. 26.11. A straight grinding wheel (Fig. 26.5a), with a diameter of D, removes a layer of metal at a depth d (called wheel depth of cut). An individual grain on the periphery of the wheel moves at a tangential velocity of V, while the workpiece moves at a velocity of v. Each abrasive grain produces a small chip, which has an undeformed thickness (grain depth of cut), t,and an undeformed length, l. Typical chips from grinding operations are shown in Fig. 26.12; note that the chips, just as in machining, are thin and long. From geometric relationships, it can be shown that the undeformed chip length in surface grinding (Fig. 26.11) is approximated by the equation l = Dd √ and the undeformed chip thickness, t,by t =  4v     VCr   d D , (26.2) (26.1) (a) (b) (c) Figure 26.12: Typical chips, or swarf, from grinding operations. (a) Swarf from grinding a conventional HSS drill bit; (b) swarf of nickel-alloy workpiece using an Al2O3 wheel; (c) swarf of M2 high-speed steel using an Al2O3 wheel, showing a melted globule among the chips. Source: Courtesy of J. Badger, The Grinding Doc. ׉ 7cassandra://X34ktvjfmu-qgH5O-lgXV7dNF1lJVvoacnibH5_3ShE#`cZ%EbW׉E o824 Chapter 26 Abrasive Machining and Finishing Operations where C is the number of cutting points per unit area of the wheel periphery. Generally, C is in the range from 0.1 to 10 per mm2. The quantity r is the ratio of chip width to average undeformed chip thickness, and is estimated between 10 and 20. As an example, l and t can be taken as functions of process parameters. Consider the case where D = 200 mm, d =0.05 mm, v =30 m/min, and V = 1800 m/min. Using the preceding formulas gives l =   (200)(0.05) = 3.2 mm Assuming that C =2 per mm2 and that r =15 gives t = (4)(30) (1800)(2)(15)  0.05 200 =0.006 mm Because of plastic deformation during chip formation, the actual chip will be shorter and thicker than the values calculated (see Figs. 26.9 and 26.12). Note from this example that grinding chip dimensions typically are much smaller than those in metal-cutting operations. Grinding Forces. A knowledge of grinding forces is essential for • Estimating power requirements. • Designing grinding machines and workholding devices and fixtures. • Determining the deflections that the workpiece, as well as the grinding machine and its components, may undergo. Deflections adversely affect dimensional accuracy, and are especially critical in precision and ultraprecision grinding. Assuming that the cutting force on the grain is proportional to the cross-sectional area of the undeformed chip, it can be shown that the grain force (acting tangential to the wheel) is a function of process variables and is given as: Grain force ∝    d v V D (Sut). (26.3) Because of the small dimensions involved, forces in grinding are typically much smaller than those in the machining operations described in Chapters 23 and 24. Grinding forces should be kept low, in order to avoid distortion and to maintain high dimensional accuracy of the workpiece. Specific Energy. The energy dissipated in producing a grinding chip consists of the energy required for the following: • Plastic deformation in chip formation. • Plowing, as shown by the ridges formed in Fig. 26.13. • Friction, caused by rubbing of the abrasive grain along the workpiece surface. Note in Fig. 26.9b that, after some use, the grains along the periphery of the wheel develop a wear flat, a phenomenon similar to flank wear in cutting tools, shown in Fig. 21.15. The wear flat continuously rubs over the ground surface, dissipates energy (because of friction), and thus makes the grinding operation less efficient. The specific energy in grinding is defined as the energy per unit volume of material ground from the workpiece surface (Table 26.2). Note that the energy levels are much higher than those in machining operations (Table 21.2). This difference has been attributed to such factors as the presence of a wear flat, high negative rake angles of the abrasive grains (which require more energy; Section 21.3), and a possible ׉ 7cassandra://VcOl4Qjv583voyGlvmSYYvHdAg_K_K__tONChqG1DREP`cZ%EbXcZ%EbW#בCט#u׉׉ 7cassandra://X4fZBSrKmESfb2tf6OAzedtDFO_Ktw8povMqjRi4WCg`׉ 7cassandra://2f7YLfe0YP3L53cKTmtgTkz2pSOnXvkUgS8ErEJlMYUA`s׉ 7cassandra://F7AU23VTgOkt4PdmnTuoEWqiDOc6CW1LyTM-92dqqyg`׉ 7cassandra://V_FymPn6-xEzmlDse7OAWj694IC4w8hCi_43TLrGPzsͫ7 <͠^cZ"EbMט#u׉׉ 7cassandra://EF604BWYCypRf8i0lZd4w6tVB6qsOIpSdeol3rZT9gM\`׉ 7cassandra://yce4hg43kXZo6lp3508-CxbkSSaYk5ZJThc15wNiy3gaH`s׉ 7cassandra://fw3hK0wyx-eLQiuwqy92H71TDuBdUp-bU_SCHTAsZZU `׉ 7cassandra://xkdlDBVQJyppxFd4zHfJz_V0B9PbZvhQ0LWtIgwMfmMod ͠^cZ"EbNנcZ"EbP5|9ׁHhttp://mm3.Thׁ ׁ Ј׉EThe Grinding Process 825 Chip Ridges Workpiece Figure 26.13: Chip formation and plowing of the workpiece surface by an abrasive grain. Table 26.2: Approximate Specific-energy Requirements for Surface Grinding. Specific energy Workpiece material Aluminum Cast iron (class 40) Hardness 150 HB 215 HB Low-carbon steel (1020) 110 HB Titanium alloy Tool steel (T15) 300 HB 67 HRC W-s/mm3 7–27 12–60 14–68 16–55 18–82 contribution of the size effect, that is, the smaller the chip, the higher the specific energy required to produce it. Also, it has been observed that with effective lubrication, the specific energy in grinding can be reduced by a factor of four or more. Also, using engineered abrasives with controlled orientation, the specific energy can be reduced by a factor of three or more, as described in Case Study 26.1 at the end of the chapter. Example 26.1 Forces in Surface Grinding Given: Assume that a surface-grinding operation is being carried out on low-carbon steel, with a wheel of diameter D = 250 mm and rotating at N = 4000 rpm, and a width of cut of w =25 mm. The depth of cut is d =0.050 mm, and the feed rate of the workpiece, v, is 25 mm/s. Find: Calculate the grinding force (the force tangential to the wheel), Fc,andthe thrust force (the force normal to the workpiece surface), Fn, using specific-energy data. Solution: The material-removal rate (MRR) is determined as MRR = dwv =(0.05)(25)(25) = 31.25 mm3/min. The power consumed is given by Power =(u)(MRR), where u is the specific energy, which can be obtained from Table 26.2 (see also Section 21.3). For lowcarbon steel, it is estimated to be 110 W-s/mm3.Thus, Power = (110)(31.25) = 3440 W. Since power is defined as Power = Tω, Groove ׉ 7cassandra://F7AU23VTgOkt4PdmnTuoEWqiDOc6CW1LyTM-92dqqyg`cZ%EbY׉E /826 Chapter 26 Abrasive Machining and Finishing Operations where the torque T = FcD/2 and ω is the rotational speed of the wheel in radians per second (ω =2πN/60). It then follows that 3440 = (Fc)  0.25 2  (2π)(4000/60), so that Fc =65.7 N. The thrust force, Fn, can be calculated directly; however, it also can be estimated by noting from experimental data in the technical literature that it is about 30% higher than the cutting force, Fc. Consequently, Fn =(1.3)(65.7) = 85.4 N. Temperature. The temperature rise in grinding is an important consideration because of the following reasons: • It can adversely affect the surface properties of the workpiece, including metallurgical changes. • The temperature rise can cause residual stresses in the workpiece. • Temperature gradients in the workpiece cause distortions due to thermal expansion and contraction of the workpiece surface, thus making it difficult to control dimensional accuracy. The surface-temperature rise, ΔT, in grinding is related to process variables by the following expression: ΔT ∝ D1/4d3/4  1/2 V v . (26.4) Thus, temperature increases with increasing depth of cut, d, wheel diameter, D, and wheel speed, V,and decreases with increasing workpiece speed, v. Note from this equation that the depth of cut has the largest exponent; hence, it has the greatest influence on temperature. Although peak temperatures during grinding can reach 1600◦C, the time involved in producing a chip is on the order of microseconds; thus, the chip produced may or may not melt. Because the chips carry away much of the heat generated, as do chips formed in high-speed machining processes (Section 25.5), only a small fraction of the heat generated in grinding is conducted to the workpiece. If this was not the case, it would be very difficult to grind workpieces with sufficient dimensional accuracy and without causing any possible metallurgical changes to the workpiece. Sparks. The sparks produced when grinding metals are actually chips that glow due to the exothermic (heat producing) reaction of the hot chips with oxygen in the atmosphere. Sparks do not occur during grinding in an oxygen-free environment or when the workpiece material does not readily oxidize at elevated temperatures. The color, intensity, and shape of sparks depend on the composition of the metal being ground. Charts are available that, from the appearance of its sparks, help identify the type of metal being ground. If the heat generated due to exothermic reaction is sufficiently high, chips can melt, acquiring and solidifying into spherical shape because of surface tension (see Fig. 26.12c). Tempering. Excessive temperature rise in grinding can cause tempering and softening of the workpiece surface; processing variables must therefore be selected properly to avoid excessive temperature rise. Using grinding fluids (Section 26.4) is an effective means of controlling temperature. Burning. Excessive temperature rise during grinding may burn the workpiece surface. A burn is characterized by a bluish color on ground steel surfaces, an indication that high temperatures have caused oxidation of the workpiece. A burn can be detected by etching and metallurgical techniques; it may not be objectionable in itself, unless surface layers have undergone phase transformations (Chapter 4). For example, if ׉ 7cassandra://fw3hK0wyx-eLQiuwqy92H71TDuBdUp-bU_SCHTAsZZU `cZ%EbZcZ%EbY#בCט#u׉׉ 7cassandra://msAplNaotOogsrWbN5C99uupH_AZ9kg1uddUPwdCeRkz`׉ 7cassandra://BbYEdOWite52PB3iv1wSZX3b8UKjXfK8q7nfz3qGFCso=`s׉ 7cassandra://W8JA8ieA9TPW75G1PQ_0C4ZPDrS1Ivz-n6ECdnZKIJQ`׉ 7cassandra://WSFsw2y2DAqqhOiuDZ1LzlpGL3Q6eGtXEddVJqBARxoe͠^cZ#EbQט#u׉׉ 7cassandra://wSZ2vYLw8nH5fu2xN6O8w3hlUacrL3loVxzaEQPK0Sw`׉ 7cassandra://fE6XomS7LQiY_8ohGyyKPyv4wtoUS7ugKIqDnaQRy2Uf`s׉ 7cassandra://hSPn5FpVWzqxtjXA44Oh7TFjA6okKuyTC9lAYDKd1b8`׉ 7cassandra://w0kEFsWLgGtMZPwVC_v985D24o4LgcWZp2wGzjFKFrch0͠^cZ#EbRנcZ#EbUy+9ׁHhttp://V.Asׁ ׁ Ј׉EhThe Grinding Process 827 martensite forms in higher carbon steels from rapid cooling (called a metallurgical burn), it will adversely affect the surface properties of ground parts, and reduce surface ductility and toughness. Heat Checking. High temperatures in grinding may also develop cracks in the workpiece surface, known as heat checking. The cracks usually are perpendicular to the grinding direction, although under severe conditions, parallel cracks also may appear. As expected, such a surface lacks toughness and has low fatigue and corrosion resistance. Heat checking also occurs in dies during die casting (see Section 11.4.5). Residual Stresses. Temperature gradients within the workpiece during grinding are primarily responsible for the development of residual stresses. Grinding fluids and their method of application, as well as processing parameters such as depth of cut and speed, significantly influence the magnitude and type of residual stresses. Because of the adverse effect of tensile residual stresses on fatigue strength, processing variables should be selected accordingly. Residual stresses usually can be reduced by lowering wheel speed and increasing workpiece speed (called low-stress grinding or gentle grinding). Softer grade wheels, known as free-cutting grinding wheels, also may be used to reduce residual stresses. 26.3.1 Grinding-wheel Wear Similar to the wear on cutting tools, grinding-wheel wear is an important consideration, because it adversely affects the shape and dimensional accuracy of ground surfaces. Wear of grinding wheels is caused by three different mechanisms, as described below. Attritious Grain Wear. In attritious wear, which is similar to flank wear in cutting tools (see Fig. 21.15), the cutting edges of an originally sharp grain become dull and develop a wear flat (Fig. 26.9b). This type of wear involves both physical and chemical reactions, and is caused by the interaction of the grain material with the workpiece material. These complex reactions involve diffusion, chemical degradation or decomposition of the grain, fracture at a microscopic scale, plastic deformation, and melting. Attritious wear is low when the two materials are chemically inert with respect to each other, much like what has been observed with cutting tools (Section 22.1). The more inert the materials, the lower is the tendency for reaction and adhesion to occur between the grain and the workpiece. Thus, for example, because aluminum oxide is relatively inert with respect to iron, its rate of attritious wear when used to grind steels is much lower than that of silicon carbide and diamond. By contrast, silicon carbide can dissolve in iron, and hence it is not suitable for grinding steels. Cubic boron nitride has a higher inertness with respect to steels, and hence it is suitable as an abrasive. Grain Fracture. Because abrasive grains are brittle, their fracture characteristics in grinding are important. If the wear flat caused by attritious wear is excessive, the grain becomes dull and grinding becomes inefficient, and produces undesirably high temperatures. Ideally, a dull grain should fracture or fragment at a moderate rate, so that new sharp edges are produced continuously during grinding. This situation is equivalent to breaking a dull piece of chalk or a stone into two or more pieces in order to expose new sharp edges (see friability in Section 26.2). The selection of grain type and size for a particular application also depends on the attritious wear rate. A grain–workpiece material combination that has a high attritious wear and low grain friability dulls the grains and develops a large wear flat; grinding then becomes inefficient, and surface damage and burning are likely to occur. Bond Fracture. The strength of the bond (grade) is a significant parameter in grinding. If, for example, the bond is too strong, dull grains cannot be easily dislodged, preventing other sharp grains along the circumference of the wheel from contacting the workpiece. Conversely, if the bond is too weak, the grains are dislodged easily, and the wear rate of the wheel increases; maintaining dimensional accuracy then becomes difficult. ׉ 7cassandra://W8JA8ieA9TPW75G1PQ_0C4ZPDrS1Ivz-n6ECdnZKIJQ`cZ%Eb[׉E 828 Chapter 26 Abrasive Machining and Finishing Operations In general, softer bonds are recommended for harder materials, to reduce residual stresses and thermal damage to the workpiece. Hard-grade wheels are used for softer materials, for removing large amounts of material at high rates. 26.3.2 Grinding Ratio The grinding ratio, G, correlates the grinding wheel wear with the amount of workpiece material removed, and is defined as G = Volume of material removed Volume of wheel wear . (26.5) In practice, G varies widely, ranging from 2 to 200, and it may even be higher, depending on the type of wheel, workpiece material, grinding fluid, and processing parameters, such as the depth of cut and the speeds of the wheel and the workpiece. It has also been shown that effective grinding fluids can increase G by a factor of 10 or more, greatly improving wheel life. During grinding, a particular wheel may act soft (thus exhibiting high wear rate) or act hard (low wear rate), regardless of the wheel grade. Note, for example, that an ordinary pencil acts soft when writing on rough paper, but it acts hard when writing on soft paper, even though it is the same pencil. Acting hard or soft is a function of the force on the individual grain on the periphery of the wheel. The higher the force, the higher the tendency for the grains to fracture or to be dislodged from the wheel surface, and the higher the wheel wear and the lower the grinding ratio. Note from Eq. (26.3), that the grain force (a) increases with the strength of the workpiece material, work speed, and depth of cut and (b) decreases with increasing wheel speed and wheel diameter. Note also that attempting to obtain a high grinding ratio in practice (to extend wheel life) isn’t always desirable, because high ratios may indicate grain dulling and possible surface damage to the workpiece. A lower ratio may be acceptable when an overall technical and economic analysis justifies it. Example 26.2 Action of a Grinding Wheel Given: A surface-grinding operation is being carried out with the wheel running at a constant spindle speed. Assume that the depth of cut, d, remains constant and the wheel is dressed periodically (see Section 26.3.3). Find: Will the wheel act soft or hard as the wheel wears down over time? Solution: Referring to Eq. (26.3), note that the parameters that change over time in this operation are the wheel diameter, D, and the surface speed, V.As D becomes smaller, the relative grain force increases, thus the wheel acts softer. To accommodate the changes due to the wheel diameter reduction over time or to make provisions for using wheels of different diameters, some grinding machines are equipped with variable-speed spindle motors. 26.3.3 Dressing, Truing, and Shaping of Grinding Wheels Dressing is the process of (a) conditioning, that is, producing sharp new edges on worn grains on the grinding surface of a wheel and (b) truing, producing a true circle on a wheel that, for whatever reason, has become out of round. Dressing is necessary when excessive attritious wear dulls the wheel, called glazing (because of the shiny appearance of the wheel surface), or when the wheel becomes loaded (see below). For softer wheels, truing and dressing are done separately, but for harder wheels, such as cBN, both are done in one operation. Loading of a grinding wheel occurs when the porosities on the wheel surfaces (Fig. 26.10) become filled or clogged with chips from the workpiece. Loading can occur while grinding soft materials or from ׉ 7cassandra://hSPn5FpVWzqxtjXA44Oh7TFjA6okKuyTC9lAYDKd1b8`cZ%Eb\cZ%Eb[#בCט#u׉׉ 7cassandra://7dJg4QEMH-Y40c8LEfbatzqI-x4LBsYF17A8CdqpYhIm`׉ 7cassandra://EUE5gggnDBozqZNOZ-sCybhT55y6MtPmIjmedm-fIyYo`s׉ 7cassandra://yc5sdbjrVxt612LNZ3Ok37_RlwZ0BH_tTtbXRWgat6wK`׉ 7cassandra://Tc--N6fbH-x9K6ff82tDMmBFaI7eTEXksQKTRJntp_4a͠^cZ#EbVט#u׉׉ 7cassandra://23Vr3NQCHviVIFOW-HDAo8dMS2wxDvGYEWrp6pYK7wY`׉ 7cassandra://QuTFL_VJyf24Tb_BalJLSEFzh1Qwaru30yE_Wu6KZEsIT`s׉ 7cassandra://zU_oinaDRBLRAFlsFTJ7MbLA4x88x4gPbP8QC2K7ljo;`׉ 7cassandra://EOBPeUP1VIAR2l9bEDmP6b1fd-5HTntW-fMoatZCVLIWDP͠^cZ$EbW׉ERThe Grinding Process 829 improper selection of wheels or processing parameters. A loaded wheel grinds inefficiently and generates much frictional heat, resulting in surface damage and loss of dimensional accuracy of the workpiece. The techniques used to dress grinding wheels are the following: • A specially shaped diamond-point tool or diamond cluster is moved across the width of the grinding face of a rotating wheel, removing a very thin layer from the wheel surface with each pass. This method can be performed either dry or wet, depending on whether the wheel is to be used dry or wet (using grinding fluids), respectively. • Aset of star-shaped steel disks is pressed against the wheel. Material is removed from the wheel surface by crushing the grains. This method produces a coarse surface on the wheel and is used only for rough grinding operations on bench or pedestal grinders. • Abrasive sticks are used to dress grinding wheels, particularly softer wheels; however, this technique is not appropriate for precision grinding operations. • Dressing techniques for metal-bonded diamond wheels involve the use of electrical-discharge and electrochemical machining techniques (Chapter 27). These processes erode very thin layers of the metal bond, exposing new diamond cutting edges. • Dressing for form grinding involves crush dressing or crush forming, consisting of pressing a metal roll on the surface of the grinding wheel, which typically is a vitrified wheel. The roll, usually made of high-speed steel, tungsten carbide, or boron carbide, has a machined or ground profile on its periphery. Thus, it reproduces a replica of this profile on the surface of the grinding wheel being dressed (see Section 26.3.3). Dressing techniques and their frequency are important for quality control, because they affect grinding forces and workpiece surface finish. Computer-controlled grinders are equipped with automatic dressing features, which dress the wheel simultaneously as grinding progresses. The first contact of the dressing tool with the grinding wheel is very important, as it determines the nature of the new surface produced. This action is monitored precisely, by using piezoelectric or acoustic-emission sensors (Section 37.7). Vibration sensors, power monitors, and strain gages also are used in the dressing setup of high-precision grinding machines. For a typical aluminum-oxide wheel, the depth removed during dressing is on the order of 5 to 15 μm, but for a cBN wheel, it could be 2 to 10 μm. Modern dressing systems have a resolution as low as 0.25 to 1 μm. Grinding wheels can be shaped to the form to be ground on the workpiece (Section 26.4). The grinding face on the Type 1 straight wheel shown in Fig. 26.5a is cylindrical; thus, it produces a flat ground surface. The wheel surface also can be shaped into various forms by dressing it (Fig. 26.14a). Modern grinders are equipped with computer-controlled shaping features. Unless it already has the desired form, the diamond dressing tool traverses the wheel face automatically along a certain prescribed path (Fig. 26.14b), producing very accurate surfaces. Note in Fig. 26.14b that the axis of the diamond dressing tool remains normal to the grinding-wheel face at the point of contact. 26.3.4 Grindability of Materials and Wheel Selection The term grindability of materials, as in terms like machinability (Section 21.7) or forgeability (Section 14.5), is difficult to define precisely. It is a general indicator of how easy it is to grind a material, and includes such considerations as the quality of the surface produced, surface finish, surface integrity, wheel wear, grinding cycle time, and overall economics of the operation. Grindability of a material can be greatly enhanced by proper selection of processing parameters (Table 26.3), grinding wheels, grinding fluids, and by using the appropriate machine characteristics, fixturing methods, and workholding devices. Grinding practices are well established for a wide variety of metallic and nonmetallic materials, including newly developed composites. Specific recommendations for selecting wheels and appropriate process ׉ 7cassandra://yc5sdbjrVxt612LNZ3Ok37_RlwZ0BH_tTtbXRWgat6wK`cZ%Eb]׉E830 Chapter 26 Abrasive Machining and Finishing Operations Single-point dressing diamond for dressing forms up to 608 on both sides of the grinding wheel Grinding wheel Dressing tool Precision radius dresser for single- and twin-track bearing production Shaped diamond roll dressing for high-volume production Dressing wheel (a) Diamond dressing tool Grinding face Grinding wheel (b) Figure 26.14: (a) Types of grinding-wheel dressing. (b) Shaping the grinding face of a wheel by dressing it by computer control. Note that the diamond dressing tool is normal to the surface at the point of contact with the wheel. Source: Courtesy of Okuma Machinery Works, Ltd. parameters for metals can be found in various handbooks, manufacturers’ literature, and the references in the Bibliography of this chapter. Ductile-regime Grinding. It has been shown that with light passes and machine tools with high stiffness and damping capacity, it is possible to produce continuous chips and good surface finish in grinding of brittle materials, such as ceramics (Fig. 26.13), a process known as ductile-regime grinding. This regime produces fewer surface cracks and leads to better performance in fatigue and bearing applications. Ceramic chips, Table 26.3: Typical Ranges of Speeds and Feeds for Abrasive Processes. Grinding, Process variable conventional 10–60 0.01–0.05 Grinding, creep-feed 0.1–1 Polishing — Buffing Wheel speed (m/min) 1500–3000 1500–3000 1500–2400 1800–3500 Work speed (m/min) Feed (mm/pass) — 1–6 — — Silicon-carbide or diamond dressing wheel for dressing either diamond or cBN grinding wheels Dressing wheel Fixed-angle swiveling dresser to dress forms up to 908 on both sides of the 60 grinding wheel 8 Rotary dressing unit for dressing hard grinding wheels or for high-volume production ׉ 7cassandra://zU_oinaDRBLRAFlsFTJ7MbLA4x88x4gPbP8QC2K7ljo;`cZ%Eb^cZ%Eb]#בCט#u׉׉ 7cassandra://OAb8Mpqh2Kn80Hf6Z8gBcbrM0f07N0vZs3Ysgd0fPgQE`׉ 7cassandra://cAikMhzqI4RDhjc29U9tpBCkXwJNFsY3yhA1kYJgCBIb`s׉ 7cassandra://sIw3Eop_GqmKAzkvn4CME_Xwp5tjKGjTsLfcNMCxTkY`׉ 7cassandra://_5HJHbBA7nknFJutUT2_ut3nZXI66e9n3p48IxQNp3Qk_R͠^cZ)Ebcט#u׉׉ 7cassandra://Tz840DTsNpEFWZP8ylI06TfKPmwN7_Y3aKjFIZlcn88w`׉ 7cassandra://jbR1aP4ul7MI_0F5rspJVAXQ_et8ivczQ3QxFB7TUR0=`s׉ 7cassandra://CmmKxRMO7UEtDagYCFYUfcfgLZxIGAniEJEwSUdRyQc`׉ 7cassandra://MjQ3Vyd-nCGoPoiLJfhSOLljJADEbP1uvG5xuBsL-Fk>͠^cZ9Ebo׉EwGrinding Operations and Machines 831 typically 1 to 10 μm in size, are more difficult to remove from grinding fluids than metal chips, requiring the use of fine filters and special techniques. 26.4 Grinding Operations and Machines The selection of a grinding process and a machine tool for a particular application depends on the workpiece shape and features, size, ease of fixturing, and production rate required (Table 26.4). Modern grinding machines are computer controlled, and have such features as automatic workpiece loading and unloading, part clamping, and automatic dressing and wheel shaping. Grinders can be equipped with probes and gages, for determining the relative position of the wheel and workpiece surfaces (see also Fig. 25.6), as well as with tactile sensing features, whereby diamond dressing-tool breakage, for example, can be monitored readily during the dressing cycle. Surface Grinding. Surface grinding (Fig. 26.15) generally involves grinding flat surfaces. In this operation, a straight wheel is mounted on the horizontal spindle of the surface grinder. In traverse grinding, the table reciprocates longitudinally and is fed laterally (in the direction of the spindle axis) after each stroke. The workpiece is held on a magnetic chuck, attached to the worktable of the grinder (Fig. 26.16); nonmagnetic materials are held by vises, vacuum chucks, or other fixtures. The movement of the grinding wheel may be along the surface of the workpiece (traverse grinding, through-feed grinding, or cross-feeding), or the wheel may move radially into the workpiece (plunge grinding), as is the case when grinding a groove (Fig. 26.15b). Surface grinders make up the largest percentage of grinders used in industry, followed by bench grinders (typically with two wheels at each end of the spindle), cylindrical grinders, tool and cutter grinders, and internal grinders, as described below. In addition to the surface grinder shown in Fig. 26.16, other types include vertical spindle and rotary table (referred to as the Blanchard type, Fig. 26.15c). These configurations allow several pieces to be ground in one setup. Steel balls for ball bearings, for example, are ground in special setups and at high production rates (Fig. 26.17). Table 26.4: General Characteristics of Abrasive Machining Processes and Machines. Typical maximum dimensions, Process Surface grinding Cylindrical grinding Centerless Internal Honing Lapping Characteristics Flat surfaces on most materials; production rate depends on table size and level of automation; labor skill depends on part complexity; production rate is high on vertical-spindle rotarytable machines Round workpieces with stepped diameters; low production rate unless automated; low to medium labor skill Round and slender workpieces; high production rate; low to medium labor skill Holes in workpiece; low production rate; low to medium labor skill Holes in workpiece; low production rate; low labor skill Flat, cylindrical, or curved workpieces; high production rate; low labor skill Chemical mechanical polishing Flat surfaces, generally used for semiconductors for microelectronics or MEMS applications; moderate production rate; high labor skill. Abrasive flow machining Ultrasonic machining Used for debarring and finishing of complex geometries; low production rate; low labor skill Holes and cavities with various shapes; suitable for hard and brittle materials; medium labor skill *Larger capacities are available for special applications. length and diameter (m)* Reciprocating table L:6 Rotary table D:3 Workpiece D:0.8,rollgrinders D:1. universal grinders D:2.5 Workpiece D:0.8 Hole D:2 Spindle D:1.2 Table D:3.7 D:0.3 D:0.3 — ׉ 7cassandra://sIw3Eop_GqmKAzkvn4CME_Xwp5tjKGjTsLfcNMCxTkY`cZ%Eb_׉E1.8, 832 Chapter 26 Abrasive Machining and Finishing Operations Wheel Wheel Workpieces Wheel Worktable Workpiece Horizontal-spindle surface grinder: Traverse grinding (a) Workpiece Horizontal-spindle surface grinder: Plunge grinding (b) (c) Figure 26.15: Schematic illustrations of various surface-grinding operations. (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the workpiece. (c) A vertical-spindle rotary-table grinder (also known as the Blanchard type). Wheel guard Worktable Workpiece Saddle Feed Rotary table Wheel head Column Wheel Bed Figure 26.16: Schematic illustration of a horizontal-spindle surface grinder. Rough-ground balls Grinding wheel Guide for balls (a) Finish-ground balls (b) Figure 26.17: (a) Rough grinding of steel balls on a vertical-spindle grinder. The balls are guided by a special rotary fixture. (b) Finish grinding of balls in a multiple-groove fixture. The balls are ground to within 0.013 mm of their final size. ׉ 7cassandra://CmmKxRMO7UEtDagYCFYUfcfgLZxIGAniEJEwSUdRyQc`cZ%Eb`cZ%Eb_#בCט#u׉׉ 7cassandra://uQwggpLrq6_eTybWHH6WjGEymOofTYBVJH_ZcLAqGksO`׉ 7cassandra://qPA4EuOIRzaV7o3buORtcol1z8z_1TjCHP7Wr51EWHwF'`s׉ 7cassandra://jX0a-TJV46XfDybBDfVgvWGZTswzX64iKRtDwdtIf00D`׉ 7cassandra://ylF237JfKKO0lsuDKAtywV61uSfARIhzqPN6zwrxz5gN&͠^cZ;Ebvט#u׉׉ 7cassandra://x5NCXNM_OlRa10hen4oBEy78hxozH5YC3VV8LvfIlKk=`׉ 7cassandra://Eja25gafNpyW5HOUyC2NSG_jYeAMg0rYCL8jKAcIcKQI`s׉ 7cassandra://K8IOvH18ONQR1A5q2XwwoLTOZpiVdzGxG7r_LLfb32g`׉ 7cassandra://FHpU0MUUrFwqNraaH9De72eN4cjnWRziOhz3e30ebUQ͠^cZ;Ebx׉E Grinding Operations and Machines 833 Movements Grinding wheel (a) (c) (b) Figure 26.18: Examples of various cylindrical-grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding. Source: Courtesy of Okuma Machinery Works, Ltd. Cylindrical Grinding. In cylindrical grinding, also called center-type grinding (Fig. 26.18; see also Fig. 26.3), the external cylindrical surfaces and shoulders of workpieces, such as crankshaft bearings, spindles, pins, and bearing rings, are ground. The rotating cylindrical workpiece reciprocates laterally along its axis, to cover the whole width to be ground. In roll grinders, used for large and long workpieces such as rolls for rolling mills (see Fig. 13.1), the grinding wheel reciprocates. These machine tools are capable of grinding rolls as large as 1.8 m in diameter. The workpiece in cylindrical grinding is held between centers or in a chuck, or it is mounted on a faceplate in the headstock of the grinder. For straight cylindrical surfaces, the axes of rotation of the wheel and the workpiece are parallel, and each is driven by a separate motor and at different speeds. Long pieces with two or more diameters also can be ground on cylindrical grinders. As with form grinding and plunge grinding, the operation also can produce shapes in which the wheel is dressed to the workpiece form to be ground (Fig. 26.19). Cylindrical grinders are identified by the maximum diameter and length of the workpiece that can be ground. In universal grinders, both the workpiece and the wheel axes can be moved and swiveled around a horizontal plane, thus permitting grinding tapers and various shapes. Grinding wheel Workpiece Figure 26.19: Plunge grinding of a workpiece on a cylindrical grinder with the wheel dressed to a stepped shape. ׉ 7cassandra://jX0a-TJV46XfDybBDfVgvWGZTswzX64iKRtDwdtIf00D`cZ%Eba׉E834 Chapter 26 Abrasive Machining and Finishing Operations Grinding wheel Workpiece x Figure 26.20: Schematic illustration of grinding a noncylindrical part on a cylindrical grinder with computer controls to produce the shape. The part rotation and the distance x between centers are varied and synchronized to grind the particular workpiece shape. Grinding wheel (a) (b) Figure 26.21: Thread grinding by (a) traverse and (b) plunge grinding. With computer control, noncylindrical parts such as cams also can be ground. As illustrated in Fig. 26.20, the workpiece spindle speed is synchronized, such that the radial distance, x, between the workpiece and the wheel axes is continuously varied to grind a particular shape, such as the one shown in the figure. Thread grinding is done on cylindrical grinders, using specially dressed wheels matching the shape of the threads (Fig. 26.21, see also centerless grinding). Although expensive, threads produced by grinding are the most accurate of any manufacturing process, and have very fine surface finish. Typical applications requiring such threads include ballscrew mechanisms, used for precise movement of various machine components. The workpiece and wheel movements are synchronized to produce the pitch of the thread, usually in about six passes. Example 26.3 Cycle Patterns in Cylindrical Grinding As in most grinding operations, the grinding wheel typically makes several passes along a path, in order to produce the final geometry on the workpiece. Figure 26.22 illustrates the cycle patterns for producing various shapes on a multifunctional, computer-controlled precision grinder. The downward arrowheads with numbers in the figures indicate the beginning of the grinding cycle. Determination of the optimum and most economical pattern for minimum cycle time depends on the volume of material to be removed, the shape of the part, and the process parameters. All the patterns shown are automatically generated by the software in the computer controls of the grinder. Source: Courtesy of Toyoda Machinery. ׉ 7cassandra://K8IOvH18ONQR1A5q2XwwoLTOZpiVdzGxG7r_LLfb32g`cZ%EbbcZ%Eba#בCט#u׉׉ 7cassandra://5OqBW0dTYI8UmPjcGJ1jtvYkoYIYboe2ZScmvDeRc70W`׉ 7cassandra://RnQTsLO_YV0L9x4QnfPUSgbufKuHMO12iB_Tg1ifZaQK`s׉ 7cassandra://gfeCzecSJf6lOqNrUNYCxapWRJ4IEfxGo7DTF0WUkdo`׉ 7cassandra://3L43trIXGW0Bp9Y5KoSKCysT5LoaS1HlL24j-b_7-j8β(͠^cZ=Eb{ט#u׉׉ 7cassandra://ROKiqiCjymh6O0ggfYyf9-FEVSfUJOaiG0JNCeosC20`׉ 7cassandra://2eRyJ9LkceGb3I-YXv5Rjb_nlg9-hcjUmke_b1HJHW8D`s׉ 7cassandra://MkJUFyQA0jIemwXXQNvzK3V9ExQl_m4qF2W1mqWGZ5kO`׉ 7cassandra://6kxBlRHUUHDUxUCvhjzdHR0BGQFUsdLm-knayS9ajZU\͠^cZ>Eb~׉EGrinding Operations and Machines 835 1 Relative movement 2 Grinding wheel Workpiece 7 9 11 12 4 6 3 5 8 10 Figure 26.22: Cycle patterns for a CNC precision grinder. Workpiece Workpiece Wheel Wheel Workpiece Wheel (a) Traverse grinding (b) Plunge grinding (c) Profile grinding Figure 26.23: Schematic illustrations of internal grinding operations: (a) traverse grinding, (b) plunge grinding, and (c) profile grinding. Internal Grinding. In internal grinding (Fig. 26.23), a small wheel is used to grind the inside diameter of the part, such as in bushings and bearing races. The workpiece is held in a rotating chuck; the wheel rotates at 30,000 rpm or higher. Internal profiles also can be ground with profile-dressed wheels, that move radially into the workpiece. The headstock of internal grinders can be swiveled on a horizontal plane for grinding tapered holes. Centerless Grinding. Centerless grinding is a high-production process for grinding cylindrical surfaces. The workpiece is supported not by centers (hence the term centerless) or chucks, but by a blade, as shown in Fig. 26.24a and b. Typical parts ground are roller bearings, piston pins, engine valves, and camshafts; parts with diameters as small as 0.1 mm can be ground. Centerless grinders are capable of wheel surface speeds on the order of 10,000 m/min, typically using cubic boron nitride wheels. In through-feed grinding, the workpiece is supported on a work-rest blade and is ground continuously (hence the term through-feed) between two wheels (Fig. 26.24a). Grinding is done by the larger wheel, while the smaller wheel regulates the axial movement of the workpiece. The rubber-bonded regulating wheel is tilted and runs at a much slower surface speed of about one-twentieth of the grinding-wheel speed. ׉ 7cassandra://gfeCzecSJf6lOqNrUNYCxapWRJ4IEfxGo7DTF0WUkdo`cZ%Ebc׉E836 Chapter 26 Abrasive Machining and Finishing Operations Through-feed grinding Feed Grinding wheel a Workpiece Work-rest blade Regulating wheel End stop Regulating wheel (a) (b) Internal grinding Pressure roll Regulating wheel Plunge grinding Grinding wheel Grinder shaft Workpiece (revolves clockwise) Support roll (c) Figure 26.24: Schematic illustrations of centerless-grinding operations: (a) through-feed grinding, (b) plunge grinding, (c) and internal grinding. Parts with variable diameters, such as bolts, valve tappets, and multiple-diameter shafts, can be ground by a process called infeed or plunge grinding (Fig. 26.24b), an operation similar to plunge or form grinding on cylindrical grinders. Tapered pieces are centerless ground by end-feed grinding. Thread grinding can be done at high-production rates with centerless grinders, using specially dressed wheels. In internal centerless grinding, the workpiece is supported between three rolls and is ground internally; typical applications are sleeve-shaped parts and rings (Fig. 26.24c). Creep-feed Grinding. Although grinding traditionally has been associated with small rates of material removal (Table 26.3) and fine surface finishing operations, it can also be used for large-scale metal-removal operations. In creep-feed grinding, the wheel depth of cut, d, is as much as 6 mm and the workpiece speed is low (Fig. 26.25). The wheels are softer grade resin bonded and have an open structure (see Fig. 26.7), in order to keep workpiece temperatures low and to improve surface finish. The machine tools for creep-feed grinding have special features, such as power up to 225 kW, high stiffness (because of the high forces due to the large depth of material removed), high damping capacity, variable spindle and worktable speeds, and ample capacity for the grinding fluids required. They can continuously dress the wheel, using a diamond roll as the dressing tool. ׉ 7cassandra://MkJUFyQA0jIemwXXQNvzK3V9ExQl_m4qF2W1mqWGZ5kO`cZ%EbdcZ%Ebc#בCט#u׉׉ 7cassandra://nq6ovr4RbwGKgSSysapvgTxIFTpble5v04GpqCMn2O8-`׉ 7cassandra://LoyfOoEc_a-BTapm0ZgCTMLY2qJQl3regp_7BROZonAV/`s׉ 7cassandra://LsVk5Ks7QnOw0MJMqBL0xfz-eFOPClSSaJq0Cb3cE6k=`׉ 7cassandra://iEOmjci3x2kkRg-tF787DTPjHoJOfnTIC9wR7Zdl3-0͠^cZ>Eb΀ט#u׉׉ 7cassandra://GFDYl8SnAs68ycE1yofPnTa4b5jKJgsiBiTAfKSSzzI/`׉ 7cassandra://qCHLryl1HLIt9jEp3JdNntgMDV8zAusabUpze4Wwn48e`s׉ 7cassandra://BP7cq5wiws_FcHbGTx9hxnC2z5DTkW1ywQofuNhlpS8`׉ 7cassandra://3DcJUvruxxM9ARZHb_3nfE8udmcU7OchEet3ZLCDizU`͠^cZ?Eb΃׉E Grinding Operations and Machines 837 Grinding wheel Groove d 5 1 – 6 mm Low work speed, v Workpiece (a) (b) Figure 26.25: (a) Schematic illustration of the creep-feed grinding process; note the large wheel depth of cut, d. (b) A shaped groove produced on a flat surface by creep-feed grinding in one pass. Groove depth is typically on the order of a fewmm. This operation also can be performed by some of the processes described in Chapter 27. Creep-feed grinding can be competitive with other machining processes, such as milling, broaching, and planing. It is economical for such specific applications as shaped punches, key seats, twist-drill flutes, roots of turbine blades, and various complex superalloy parts. Because the wheel is dressed to the shape to be produced, the workpiece does not have to be shaped previously by milling, shaping, or broaching; nearnet-shape castings and forgings are therefore suitable for creep-feed grinding. Although a single grinding pass generally is sufficient, a second pass may be necessary for improved surface finish. Heavy Stock Removal by Grinding. Grinding can also be used for heavy stock removal by increasing process parameters, such as wheel depth of cut. This operation can be economical in certain specific applications and it can compete favorably with machining processes, particularly milling, turning, and broaching. In this operation, (a) surface finish is of secondary importance, (b) the dimensional tolerances are on the same order as those obtained by most machining processes, and (c) the grinding wheel or the belt can be utilized to its fullest capabilities, while minimizing grinding cost per piece. Heavy stock removal by grinding is also performed on welds, castings, and forgings to finish weld beads and remove flash. Example 26.4 Grinding versus Hard Turning In some specific applications, grinding and hard turning (Section 25.6) can be competitive. Hard turning continues to be increasingly competitive with grinding, and dimensional tolerances and surface finish are approaching those obtained by grinding. Consider the case of machining of heat-treated steels, with hardness above 45 HRC, using a single-point polycrystalline cubic boron nitride tool, versus grinding these steels. In comparing Tables 21.2 and 26.2, it will be noted that (a) turning requires much less energy than grinding; (b) thermal and other types of damage to the workpiece surface are less likely to occur in machining; (c) cutting fluids may not be necessary; and (d) lathes are less expensive than grinders. Moreover, finishing operations, including finish grinding, can be performed on the turned part while it is still chucked in the lathe. ׉ 7cassandra://LsVk5Ks7QnOw0MJMqBL0xfz-eFOPClSSaJq0Cb3cE6k=`cZ%Ebe׉E 838 Chapter 26 Abrasive Machining and Finishing Operations On the other hand, workholding devices for large and especially slender workpieces during hard turning can present significant problems, because cutting forces are higher than grinding forces. Furthermore, tool wear and its control can be a significant challenge as compared with the automatic dressing of grinding wheels. It is evident that the competitive positions of hard turning versus grinding must be evaluated individually for each application, in terms of product surface finish, integrity, quality, and overall economics. A number of grinders are used for various operations: • Universal tool and cutter grinders are used for grinding single-point or multipoint tools and cutters, including drills. They are equipped with special workholding devices for accurate positioning of the tools to be ground. A variety of CNC tool grinders is available, making the operation simple and fast and with consistent results. • Tool-post grinders are self-contained units, usually attached to the tool post of a lathe (see Fig. 23.2). The tool is mounted on the headstock and is ground by moving the tool post. These grinders are versatile, but it is essential for the lathe components to be protected from the abrasive debris. • Swing-frame grinders are typically used in foundries for grinding large castings. Rough grinding of castings is called snagging, and is usually done on floorstand grinders, using wheels as large as 0.9 m in diameter. • Portable grinders are used for such operations as grinding off weld beads and cutting off,using thin abrasive disks. They are driven either pneumatically, electrically, or with a flexible shaft connected to an electric motor or a gasoline engine. • Bench and pedestal grinders are used for routine grinding of tools and small parts. They usually are equipped with two grinding wheels, mounted on the two ends of the shaft of an electric motor; generally, one wheel is coarse for rough grinding and the other is fine for finish grinding. Grinding Fluids. The functions of grinding fluids are similar to those of cutting fluids (Section 22.12). Although grinding and other abrasive removal processes can be performed dry, the use of a fluid is important because it: • Reduces temperature rise in the workpiece • Improves part surface finish and dimensional accuracy • Improves the efficiency of the operation, by reducing wheel wear, reducing loading of the wheel, and lowering power consumption. Grinding fluids typically are water-based emulsions, for general grinding, and oils, for thread grinding, (Table 26.5). They may be applied as a stream (flood) or as mist (a mixture of fluid and air). Because of the high surface speeds involved, an airstream (air blanket) around the periphery of the grinding wheel may prevent the fluid from reaching the wheel–workpiece interface. Special nozzles that conform to the shape of the cutting surface of the grinding wheel have been designed whereby the grinding fluid is supplied under high pressure. There can be a significant rise in the temperature of water-based grinding fluids as they remove heat from the grinding zone, causing the workpiece to expand, thus making it difficult to control its dimensional accuracy. Acommon method to maintain a low workpiece temperature is to use refrigerating systems (chillers), through which the grinding fluid is circulated continuously and is maintained at about a constant ׉ 7cassandra://BP7cq5wiws_FcHbGTx9hxnC2z5DTkW1ywQofuNhlpS8`cZ%EbfcZ%Ebe#בCט#u׉׉ 7cassandra://yFxOnc2qCRAD4d4FZmXKe1HhQPCku4PRiiuYqc-xerA2`׉ 7cassandra://fbJcDy7ISsgheI-9GlLe7HxOLIPKSo6ARvFDDzi40jQT`s׉ 7cassandra://jG0OzEHCy21Qtxcuq1K92QJuOOv-gkv81ockzYiggBM`׉ 7cassandra://iqcOzs14yNFVCQjHLsmWFkV1eNpS96imorhaK7X1uPs͏͠^cZ?Eb΅ט#u׉׉ 7cassandra://O5rVOGgPY7kUkEJAMfb2LNnXIVeUuo6hDAKvxTfQo34_`׉ 7cassandra://_80oX43QV6HHEoVwsLcLmy6uIy8rZpbY6x7vtqXIf8Yjz`s׉ 7cassandra://_1EtXATWkSqz2zr1raNlBnfq5EiSh8kEJV8_-sltIN0`׉ 7cassandra://9zJPRzYrlDTeApQgY6mRja8tLY2f1F5eA73Yi2FhbsYcF͠^cZ?EbΆ׉E Grinding Operations and Machines 839 Table 26.5: General Recommendations for Grinding Fluids. Material Aluminum Copper Magnesium Nickel Refractory metals Steels Titanium Grinding fluid E, EP CSN, E, MO + FO D, MO CSN, EP EP CSN, E CSN, E D = dry; E = emulsion; EP = extreme pressure; CSN = chemicals and synthetics; MO = mineral oil; FO = fatty oil (see also Section 33.7). temperature. As described in Section 22.12, the biological and ecological aspects of disposal, treatment, and recycling of metalworking fluids are important considerations in their selection and use. The practices employed must comply with federal, state, and local laws and regulations. Grinding Chatter. Chatter is particularly important in grinding as a finishing operation, because it can adversely affect surface finish and wheel performance. Studying chatter marks on ground surfaces often can help identify their source, which may include: (a) bearings and spindles of the grinding machine, (b) nonuniformities in the grinding wheel, as manufactured, (c) uneven wheel wear, (d) improper dressing techniques, (e) grinding wheels that are not balanced properly, and (f) external sources, such as nearby machinery. The grinding operation itself can cause regenerative chatter, as it does in machining (Section 25.4). The important factors in controlling chatter in grinding are the stiffness of the machine tool, the stiffness of work-holding devices, and damping of the system. General guidelines include (a) using soft-grade grinding wheels, (b) dressing the wheel frequently, (c) changing dressing techniques, when necessary, (d) reducing the material-removal rate, and (e) supporting the workpiece rigidly. Safety in Grinding Operations. Because grinding wheels are brittle and rotate at high speeds, they can fracture. Certain procedures must be followed in their handling, storage, and use. Failure to follow these procedures, and the instructions and warnings printed on individual wheel labels, may result in serious injury or fatality. Grinding wheels should be stored properly and protected from environmental extremes, such as temperature or humidity. They should be inspected visually for cracks and damage prior to installing them. Vitrified wheels should be tested prior to their use by ringing them (supporting them at the hole, tapping them gently, and listening to the sound). A damaged wheel will have a flat ring to it, similar to that of a cracked dinner plate. Power supply Tool Workpiece (a) Transducer Abrasive slurry (b) Glass–graphite epoxy composite Glass 50 mm diameter Slots 0.64–1.5 mm Holes 0.4 mm in diameter (c) Figure 26.26: (a) Schematic illustration of the ultrasonic machining process. (b) and (c) Types of parts made by this process. Note the small size of the holes produced. 1.2 mm ׉ 7cassandra://jG0OzEHCy21Qtxcuq1K92QJuOOv-gkv81ockzYiggBM`cZ%Ebg׉E840 Chapter 26 Abrasive Machining and Finishing Operations Damage to a grinding wheel can severely reduce its bursting speed. Defined as the surface speed at which a freely rotating wheel bursts (explodes), the bursting speed (expressed in rpm) depends on the type of wheel, such as its bond, grade, and structure. In diamond and cBN wheels (Fig. 26.6), which are operated at high surface speeds, the type of the core material used in the wheel affects the bursting speed. Metal cores, for example, have the highest bursting speed, typically on the order of about 250 m/s. 26.5 Design Considerations for Grinding Design considerations for grinding are similar to those for machining, as described in various sections in Chapters 23 and 24. In addition, specific attention should be given in grinding to the following: • Parts should be designed so that they can be mounted securely, either in chucks, magnetic tables, or suitable fixtures and workholding devices. Thin, straight, or tubular workpieces may distort during grinding, requiring special attention. • If high dimensional accuracy is required, interrupted surfaces, such as holes and keyways, should be avoided, as they can cause vibrations and chatter. • Parts for cylindrical grinding should be balanced; long and slender designs should be avoided to minimize deflections. Fillets and corner radii should be as large as possible, or relief should be provided for them during previous machining in these regions. • In centerless grinding, short pieces may be difficult to grind accurately, because the blade may not support them sufficiently. In through-feed grinding, only the largest diameter on the parts can be ground. • The design of parts requiring accurate form grinding should be kept as simple as possible, to avoid frequent form dressing of the wheel. • Deep and small holes, and blind holes requiring internal grinding, should be avoided or they should include a relief. In general, part designs should have a minimum amount of material to be removed by grinding, except for creep-feed grinding. Moreover, in order to maintain good dimensional accuracy, designs preferably should allow for all grinding to be done without having to reposition the workpiece. 26.6 Ultrasonic Machining In ultrasonic machining (UM), material is removed from a surface by microchipping and erosion,withfineand loose abrasive grains in a water slurry (Fig. 26.26a). The tip of the tool (called a sonotrode) vibrates at a frequency of 20 kHz and an amplitude of 0.0125 to 0.075 mm. Vibration imparts high velocity to abrasive grains between the tool and the workpiece. The stress produced by the abrasive particles impacting the workpiece surface is high, because (a) the time of contact between the particle and the surface is on the order of only 10 to 100 μs and (b) the area of contact is very small. In brittle materials, impact stresses are sufficiently high to remove material from the workpiece surface. The abrasive grains are typically boron carbide, although aluminum oxide or silicon carbide grains are also used, with sizes ranging from grit number 100 for roughing to grit number 1000 (see Fig. 26.7) for finishing operations. The grains are carried in water slurry, with concentrations of 20 to 60% by volume; the slurry also carries the debris away from the cutting zone. Ultrasonic machining is best suited for materials that are hard and brittle, such as ceramics, carbides, precious stones, and hardened steels; two examples are shown in Fig. 26.26b. A special tool is required for each shape to be produced; thus it is also called a form tool. The tip of the tool, which is attached to a transducer through the toolholder, is usually made of mild steel. ׉ 7cassandra://_1EtXATWkSqz2zr1raNlBnfq5EiSh8kEJV8_-sltIN0`cZ%EbhcZ%Ebg#בCט#u׉׉ 7cassandra://afRXjUt7AESgm9Tj_86TNIAZjI6iUYbxXC_-vbJ7gBM`׉ 7cassandra://lEDNMdTRf6LVnco_MmQo9obIOurzlcC_nff5i6Ft43oa`s׉ 7cassandra://TWbNzCF4sZZ-tkg77ACA-BdWBEjz1IVXnMKVfo_46qA`׉ 7cassandra://HqsW1kLexq-cMqmAqvMKYtBO44J2rmm6JUBfcEYEEi4͔ ͠^cZ@EbΈט#u׉׉ 7cassandra://RZUjWr-dfziqOn4qbbpMcxz30grSv1xYTqLM6gRyeEU7`׉ 7cassandra://3ALJ2l1i_EdZ5Z4po3UCVEzpg5x2sYy4O664G8ShFGcP+`s׉ 7cassandra://HCIPiKk2QqwjS3wMKwcpfbpDkfiOq-VyD_q18zZdUoI`׉ 7cassandra://CN8bpQOHuQDpC0_13YtBOKM5aums2Z-XZbGieN36CP0l͠^cZAEbΌ׉E Finishing Operations 841 Rotary Ultrasonic Machining. In this process, the abrasive slurry is replaced by a tool with metal-bonded diamond abrasives, either impregnated or electroplated on the tool surface. The tool is vibrated ultrasonically and rotated at the same time while being pressed against the workpiece surface at a constant pressure. The process is similar to face-milling (Fig. 24.5), but with the inserts being replaced with abrasives. The chips produced are washed away by a coolant, pumped through the core of the rotating tool. Rotary ultrasonic machining (RUM) is particularly effective in producing deep holes in brittle materials and at high material-removal rates. Design Considerations for Ultrasonic Machining. The basic design guidelines for UM include the following: • Avoid demanding profiles, sharp corners, and radii, because they can be eroded by the flow of the abrasive slurry. • Holes produced will have some taper. • Because of the tendency of brittle materials to chip at the exit end of holes, the bottom of the parts should have a backup plate. 26.7 Finishing Operations Several other processes utilize fine abrasive grains and are used as a final finishing operation. Because these operations can significantly affect production time and product cost, they should be specified only after due consideration to their costs and benefits. Coated Abrasives. Common examples of coated abrasives are sandpaper and emery cloth; the majority is made of aluminum oxide, with silicon carbide and zirconia alumina making up the rest. Coated abrasives usually have a much more open structure than grinding wheels, and their grains are more pointed and aggressive. The grains are deposited electrostatically on flexible backing materials, such as paper or cloth. As shown in Fig. 26.27, the bonding material (matrix) typically is resin. It first is applied to the backing (called make coat); then the grains are bonded with a second layer (size coat). The grains have their long axes aligned perpendicular to the plane of the backing, thus improving their cutting action. Coated abrasives are available as sheets, belts, and disks. They are used extensively to finish flat or curved surfaces of metallic and nonmetallic parts, metallographic specimens, and also in woodworking. Belt Grinding. Coated abrasives also are used as belts for high-rate material removal with good surface finish. Belt grinding is an important production process, and in some cases competes well with, and is preferred to, conventional grinding operations. Belts with grit numbers ranging from 16 to 1500 (see Figs. 22.7 and 22.8) are available. Belt speeds are in the range of 700 to 1800 m/min. Machines for abrasive-belt operations require proper belt support and rigid construction to minimize vibrations. Conventional coated abrasives have randomly placed abrasives on their surface, and may consist of single or multiple layers of abrasives. An alternative surface is produced by microreplication, in which Abrasive grains Size coat Make coat Backing Figure 26.27: Schematic illustration of the structure of a coated abrasive. Sandpaper (developed in the 16th century) and emery cloth are common examples of coated abrasives. ׉ 7cassandra://TWbNzCF4sZZ-tkg77ACA-BdWBEjz1IVXnMKVfo_46qA`cZ%Ebi׉E 842 Chapter 26 Abrasive Machining and Finishing Operations Vane Root skirt Vane Abrasive belt Tip skirt (a) (b) Figure 26.28: Turbine nozzle vane considered in Example 26.5. abrasives in the shape of tiny aluminum-oxide pyramids are placed in a predetermined orderly arrangement on the belt surface. When used on stainless steels and superalloys, their performance is more consistent than conventional coated abrasives, and the temperature rise is lower. Typical applications include belt grinding of golf clubs, firearms, turbine blades, surgical implants, and various medical and dental instruments. Example 26.5 Belt Grinding of Turbine Nozzle Vanes The turbine nozzle vane shown in Fig. 26.28 was investment cast (Section 11.3.2) from a cobalt-based superalloy. To remove a thin diffusion layer from the root skirt and tip skirt sections of the vane, it was ground on a cloth-backed abrasive belt (60-grit aluminum oxide). The vanes were mounted on a fixture, and ground dry at a belt surface speed of 1800 m/min. The production rate was 93 seconds per piece. Each vane weighed 21.65 g before and 20.25 g after belt grinding, a reduction in weight of about 6.5%. Source: Courtesy of ASM International. Wire Brushing. In this process, also called power brushing, the workpiece is held against a circular wire brush that rotates at speeds from 1750 rpm for large wheels to 3500 rpm for small wheels. The tips of the wires produce longitudinal scratches on the workpiece surface. Performed under the proper conditions, wire brushing also may be considered as a very light material-removal process. Wire brushing is used to produce a fine or controlled surface texture. In addition to metal wires, polymeric wires (such as nylon; Section 7.6) embedded with abrasives can be used effectively (see also diamond wire saws; Section 24.5). Honing. Honing is an operation used primarily to improve the surface finish of holes made by such processes as boring, drilling, and internal grinding. The honing tool consists of a set of aluminum oxide or silicon-carbide bonded abrasive sticks, called stones (Fig. 26.29). They are mounted on a mandrel that rotates Spindle Stone Nonabrading bronze guide Figure 26.29: Schematic illustration of a honing tool used to improve the surface finish of bored or ground holes. ׉ 7cassandra://HCIPiKk2QqwjS3wMKwcpfbpDkfiOq-VyD_q18zZdUoI`cZ%EbjcZ%Ebi#בCט#u׉׉ 7cassandra://gfop5DDZfjMS9uDPwctvgBDTiG43hspa-F0GgMR5Ebg`׉ 7cassandra://B7uPG29oXzIpDF8BpubCN7_ZyJhIrA_eI45zSMHkZ8YQ`s׉ 7cassandra://HA0yRLs9-hGMkElVv7OWHLgx_u0C3Z33KpKxPm24cEsf`׉ 7cassandra://P8CEz-GLRag0YYwLg7ibxIvUKC-iM1_F4v63nk65ZWYbw{͠^cZEEbΐט#u׉׉ 7cassandra://Y7y6mXUU_VTRFuSegfe4BgaPG2wbnKPDflZsIZP3JtY `׉ 7cassandra://ABgXHFrXl0TqewyB3JA984Qam-e9Q5qXK6TDYlfylQE``s׉ 7cassandra://Lyofi57SucI3IlRTN7fZGGvkKMycbL7ra7BjkGL9C_YU`׉ 7cassandra://RY6VkfBLQtU7u7HlvkB43CZaFIaahsbedIMpBRLzn8IE ͠^cZFEbΑ׉EFinishing Operations 843 Oscillation (traverse if stone is shorter than workpiece) Stone Rotation Motor Holder Stone Workpiece Workpiece (a) Rolls (b) Figure 26.30: Schematic illustrations of the superfinishing process for a cylindrical part. (a) Cylindrical microhoning. (b) Centerless microhoning. at surface speeds of 45 to 90 m/min in the hole, applying a radial outward force on the hole surface. They can be adjusted radially for different hole sizes. The tool has a reciprocating axial motion, producing a crosshatched pattern on the hole surface. Oilor water-based honing fluids are used to flush away the debris and keep temperatures low. Honing is also done on external or flat surfaces, and to manually remove sharp edges on cutting tools and inserts. The quality of the surface finish produced by honing can be controlled by the type and size of the abrasive used, the pressure applied, and rotational speed. If not performed properly, honing can produce holes that are neither straight nor cylindrical, but rather in shapes that are bell mouthed, wavy, barrel shaped, or tapered. Superfinishing. In this process, the pressure applied is very light and the motion of the honing stone has short strokes. The motion is controlled so that the grains do not travel along the same path on the workpiece surface. Examples of external superfinishing of a round part are shown in Fig. 26.30. Lapping. This is an operation for finishing flat, cylindrical, or curved surfaces. Generally, the lap (Fig. 26.31a) is relatively soft and porous, and is made of such materials as cast iron, copper, leather, or cloth. The abrasive Lap position and pressure control Lap Workpiece Before Work-holding plate After (a) Guide r Workpieces (b) ail Machine pan Lower lap (c) Figure 26.31: (a) Schematic illustration of the lapping process. (b) Production lapping on flat surfaces. (c) Production lapping on cylindrical surfaces. Abrasives Upper lap Workpiece ׉ 7cassandra://HA0yRLs9-hGMkElVv7OWHLgx_u0C3Z33KpKxPm24cEsf`cZ%Ebk׉E 844 Chapter 26 Abrasive Machining and Finishing Operations particles either are embedded in the lap or may be carried in a slurry. Lapping of spherical objects and glass lenses is done with specially shaped laps. Running-in of mating gears can be done by lapping, as on hypoid gears for rear axles of automobiles. Depending on the type and hardness of the workpiece, lapping pressures range from 7 to 140 kPa. Dimensional tolerances on the order of ±0.0004 mm can be obtained in lapping by using fine abrasives (up to grit size 900), and the surface finish can be as smooth as 0.025 to 0.1 μm. Production lapping on flat or cylindrical parts is done on machines similar to those shown in Fig. 26.31b and c. Polishing. Polishing is a process that produces a smooth, lustrous surface. The basic mechanism involved in the polishing process is the softening and smearing of surface layers by frictional heating developed during polishing, as well as by some very fine-scale abrasive removal from a workpiece. The shiny appearance commonly observed on polished surfaces results from a smearing action. Polishing is done with disks or belts made of fabric, leather, or felt that typically are coated with fine powders of aluminum oxide or diamond. In double-sided polishing, pairs of pads are attached to the faces of platens that rotate in opposite directions. Parts with irregular shapes, sharp corners, deep recesses, and sharp projections can be difficult to polish. Chemical–mechanical Polishing. Chemical–mechanical polishing (CMP) is extremely important in semiconductor manufacturing (Chapter 28). This process, shown in Fig. 26.32, uses a suspension of abrasive particles in a water-base solution, with a chemistry selected to cause controlled corrosion. Workpiece surface changes are through combined actions of abrasion and corrosion; the result is an exceptionally fine finish, as well as a very flat part. For this reason, the process is often referred to as chemical–mechanical planarization (Section 28.4). A major application of this process is polishing of silicon wafers (Section 28.4), in which the primary function of CMP is to polish at the micrometer level without any lay (see Section 33.3). To remove material evenly and across the whole wafer surface, the wafer is held face down on a rotating carrier, and is pressed against a polishing pad attached to a rotating disk (Fig. 26.32). The angular velocities of the carrier and the pad are selected such that wear is uniform across the entire wafer surface. The velocities are adjusted such that there is a constant relative velocity between the carrier and the pad on the axis connecting their centers. The pad has grooves intended to uniformly supply slurry to all wafers; pad rotation ensures that a linear lay does not develop (see Section 33.3). Workpiece Workpiece carrier Abrasive slurry Polishing pad Workpiece (disk) Workpiece carrier 1 Polishing table Polishing table (a) Side view (b) Top view Figure 26.32: Schematic illustration of the chemical–mechanical polishing process. This process is used widely in making silicon wafers and integrated circuits and also is known as chemical–mechanical planarization. For other materials and applications, more carriers and more disks per carrier are possible. ׉ 7cassandra://Lyofi57SucI3IlRTN7fZGGvkKMycbL7ra7BjkGL9C_YU`cZ%EblcZ%Ebk#בCט#u׉׉ 7cassandra://5BahP8GLMyrR7koqgV8KX590eIHjlb70yUyw2NSbhTkӨ`׉ 7cassandra://XOuJRQRqVwjOJ0Ck3scPcj1Wc5Na_DLn8O2CyLEYEB0]\`s׉ 7cassandra://CmM7XQJGcFLqoVfJrExJkle1WSQ6Hw4Jk9ustBPdVlA1`׉ 7cassandra://Wla7AjOhZfjA2dK_Sud0pJDFBUlrSBrWaJduMM8p01IͲ͠^cZXEbΥט#u׉׉ 7cassandra://alOqLi3cQsUI-VuYRhRsG_Fvt7lc3ov-aHQlur969H40`׉ 7cassandra://4Wo4ieGEphsM04_8zoqLNDTQ5htNgLf1QnqeTPul-hMb`s׉ 7cassandra://8p-stN-Tn9dlCq1hR-CFeyWwNsFA7TRqm893axhcAR8`׉ 7cassandra://D8B59NHb0YdRX-Xe-0AXdpclXh9ny37xWAMMFCynN5k]͠^cZXEbΦ׉E HFinishing Operations 845 Specific abrasive- and solution-chemistry combinations have been developed for polishing copper, silicon, silicon dioxide, aluminum, tungsten, and other metals. For silicon dioxide or silicon polishing, for example, an alkaline slurry of colloidal silica (SiO2 particles in a KOH solution or in NH4OH) is fed continuously to the pad-wafer interface. Electropolishing. Mirrorlike finishes can be obtained on metal surfaces by electropolishing, a process that is the reverse of electroplating (Section 34.9). Because there is no mechanical contact with the workpiece, this process is particularly suitable for polishing irregular shapes as well. The electrolyte preferentially attacks the projections and the peaks on the workpiece, producing a smooth surface. Electropolishing is also used for deburring operations (Section 26.8). Polishing in Magnetic Fields. In this technique, abrasive slurries are supported with magnetic fields. There are two basic methods: 1. In the magnetic-float polishing of ceramic balls, illustrated schematically in Fig. 26.33a, a magnetic fluid (containing abrasive grains and extremely fine ferromagnetic particles in a carrier fluid such as water or kerosene) is filled in the chamber within a guide ring. The ceramic balls are located between a driveshaft and a float. The abrasive grains, the ceramic balls, and the float (made of a nonmagnetic material) are all suspended by magnetic forces. The balls are pressed against the rotating driveshaft and are polished by the abrasive action. The forces applied by the abrasive particles on the balls are extremely small and are controllable, and hence the polishing action is very fine. Because polishing times are much lower than those involved in other polishing methods, this process is highly economical and the surfaces produced have few, if any, significant defects. 2. In the magnetic-field-assisted polishing of ceramic rollers (Fig. 26.33), a ceramic or steel roller (as the workpiece) is clamped and rotated on a spindle. The magnetic poles are then oscillated, introducing a vibratory motion to the magnetic–abrasive conglomerate, an action that polishes the cylindrical roller surface. Bearing steels, with a hardness of 63 HRC, have been mirror finished in 30 s with this process. Buffing. This process is similar to polishing, with the exception that an even finer surface finish is obtained using very fine abrasives on soft disks, typically made of cloth. Driveshaft Guide ring S-pole Magnetic fluid and abrasive grains Ceramic balls (workpiece) Float Permanent magnets NN N NS S (a) SSS N N S Magnetic fluid (b) Figure 26.33: Schematic illustration of polishing of balls and rollers by magnetic fields. (a) Magnetic-float polishing of ceramic balls. (b) Magnetic-field-assisted polishing of rollers. Source: After R. Komanduri, M. Doc, and M. Fox. Workpiece N-pole ׉ 7cassandra://CmM7XQJGcFLqoVfJrExJkle1WSQ6Hw4Jk9ustBPdVlA1`cZ%Ebm׉E 846 Chapter 26 Abrasive Machining and Finishing Operations 26.8 Deburring Operations Burrs are thin ridges, usually triangular in shape, that develop along the edges of a workpiece from such operations as machining, shearing sheet metals (see Figs. 16.2 and 16.3), and trimming of forgings and castings. Burrs can be detected by such simple means as by direct touch or with a toothpick or cotton swab. Visual inspection of burrs includes the use of magnifiers and microscopes. Burrs can be detrimental: (a) They may interfere with the assembly of parts, and can cause jamming, misalignment of parts, and can cause short circuiting in electrical components. (b) Because they are usually sharp, they can be a safety hazard to personnel in handling parts. (c) Burrs may reduce the fatigue life of components. (d) Sheet metal may have lower bendability if the burr is on the tensile side (see Section 16.2). On the other hand, burrs can be useful on drilled or tapped thin components, such as tiny parts in mechanical watches and mechanisms, by providing additional thickness and thus improve the holding torque of screws. Several deburring processes are available. Their cost-effectiveness depends on such factors as the extent of deburring required, part complexity, burr location, the number of parts to be deburred, floor space available, labor costs, and safety and environmental considerations. Deburring operations include: 1. Manual deburring, using files and scrapers; however, it is estimated that manual deburring can contribute up to 10% of the cost of manufacturing a part 2. Mechanical deburring by various means 3. Wire brushing or using rotary nylon brushes consisting of filaments embedded with abrasives 4. Abrasive belts 5. Ultrasonic machining 6. Electropolishing 7. Electrochemical machining 8. Magnetic–abrasive finishing 9. Vibratory finishing 10. Shot blasting or abrasive blasting 11. Abrasive-flow machining, such as extruding a semisolid abrasive slurry over the edges of the part 12. Thermal energy, using lasers or plasma. The last four processes are described next; other processes are covered elsewhere in this book. Vibratory and Barrel Finishing. These processes are used to remove burrs from large numbers of relatively small parts. This is a batch-type operation, in which specially shaped abrasive pellets of nonmetallic or metallic media (stones or balls) are placed in a container, along with the parts to be deburred. The container is then vibrated or tumbled by various means. The impact of individual abrasives and metal particles removes the burrs and sharp edges from the parts. Depending on the application, this can be a dry or a wet process. Liquid compounds may be added for such purposes as adding corrosion resistance to the parts being deburred. When chemically active fluids and abrasives are used, this process becomes a form of chemical–mechanical polishing (Section 26.7). Shot Blasting. Also called grit blasting, this process involves abrasive particles (usually sand or specially engineered abrasives) propelled by a high-velocity jet of air, or by a rotating wheel, onto the surface of the part. Shot blasting is particularly useful in deburring metallic or nonmetallic materials, and in stripping, cleaning, and removing surface oxides. The surfaces have a matte finish, although surface damage can ׉ 7cassandra://8p-stN-Tn9dlCq1hR-CFeyWwNsFA7TRqm893axhcAR8`cZ%EbncZ%Ebm#בCט#u׉׉ 7cassandra://Ays11EKClKIyrsd5RZCYYPsOL_7QfQou-pn59FQkLKI `׉ 7cassandra://SEwcGWaCxd91Eod6GZwzs5q1FroCgmWxmcf4Rtt546Eb`s׉ 7cassandra://R6gSufyOILK5-ypb6Y84Q_FXuV4wf4S_7I6kJWvsKso`׉ 7cassandra://8dOMKk6jFozG8kt3YbLd6KwX-0OvnB1dncV3UEZNOv00͠^cZZEbάט#u׉׉ 7cassandra://N88gZvZULfgJOMEp4PgUsKslb-86AFyhA-QrKXvh57sV`׉ 7cassandra://CylriBLBun__8X-ixVnfjHlaWJYXZyFT9qEkVjbWeCIQL`s׉ 7cassandra://JFxK3YUkMQgQR3TvO7qoOLq_cqJcOmMGBjOirBpAFogB`׉ 7cassandra://tGMiPT7f4LDR3qRY5i5t8OuzDbX3UneNnLnEbVhgRLge ͠^cZZEbέ׉E %Deburring Operations 847 result if the process parameters are not controlled properly. Microabrasive blasting consists of small-scale polishing and etching, using very fine abrasives, on bench-type units. Abrasive-flow Machining. This process involves using abrasive grains, such as silicon carbide or diamond, mixed in a puttylike matrix, and then forced back and forth through the openings and passageways in the part. The movement of the abrasive matrix under pressure erodes away burrs and sharp corners, polishing the part. Abrasive-flow machining (AFM) is particularly suitable for workpieces with internal cavities, such as those produced by casting, that are inaccessible by other means. The pressure applied ranges from 0.7 to 22 MPa. External surfaces also can be deburred with this method, by containing the workpiece within a fixture that directs the abrasive media to the edges and the areas of the part to be deburred. The deburring of a turbine impeller by this process is illustrated in Fig. 26.34. In microabrasive-flow machining, the process mechanics are similar to those in abrasive-flow machining, but with much smaller abrasive media and using less viscous carriers. This technique allows the media to flow through very small holes, ranging from 50 μm to 750 μm diameter. Micro-AFM has been applied to the production of high-quality diesel-fuel injectors and other small nozzles, where a burr or rough surface finish could otherwise adversely affect the flow quality. Thermal Energy Deburring. This process consists of placing the part in a chamber, which is then injected with a mixture of natural gas and oxygen. When the mixture is ignited, a burst of heat is produced, at a temperature of about 3300◦C. The burrs are instantly heated and they melt while the temperature of the part only reaches about 150◦C. There are, however, drawbacks to this process: (a) Larger burrs tend to form beads after melting, (b) thin and slender parts may distort, and (c) the process does not polish or buff the surfaces, as occurs in other deburring processes. Robotic Deburring. Deburring and flash removal from finished products are being performed increasingly by programmable robots (Section 37.6). Using a force-feedback system for controlling the path and rate of burr removal, this method eliminates tedious and expensive manual labor, and it results in more consistent and repeatable deburring. In another application, the manual deburring of a double-helical gear for a helicopter gearbox was deburred in 150 min, whereas robotic deburring required 15 min. Before: Ra = 5–9 mm Workpiece Bottom machine plate Top machine plate Top tooling After: Ra = 0.4–0.7 mm Flow of abrasive media (a) Bottom tooling (b) Figure 26.34: (a) Schematic illustration of abrasive-flow machining to deburr a turbine impeller. The arrows indicate movement of the abrasive media. Note the special fixture, which is usually different for each part design. (b) Valve fittings subjected to abrasive-flow machining to eliminate burrs and improve surface quality. Source: Courtesy of Kennametal Extrude Hone. ׉ 7cassandra://R6gSufyOILK5-ypb6Y84Q_FXuV4wf4S_7I6kJWvsKso`cZ%Ebo׉E 848 Chapter 26 Abrasive Machining and Finishing Operations 26.9 Economics of Abrasive Machining and Finishing Operations Abrasive machining and finishing operations often are necessary, because forming, shaping, and machining processes alone do not achieve sufficiently high dimensional accuracy or surface finish. Abrasive processes may be used both as a finishing and as large-scale material-removal operation. For example, creep-feed grinding is an economical alternative to machining operations, such as milling or broaching, even though wheel wear is high. Much progress has been made in automating the equipment involved in these operations, including the use of computer controls, sensors, process optimization, and robotic handling of parts. Labor costs and production times have been reduced, even though such machinery generally requires major capital investment. Because they are additional operations, the processes described in this chapter can significantly affect product cost, especially since many of these processes are relatively slow. Moreover, as surface-finish requirements increase, more operations may be necessary, further increasing production costs, as clearly seen in Fig. 26.35. Note how rapidly the cost increases as surface finish is improved, by such additional processes as grinding and honing. The total cost of abrasive operations depends on several factors, such as part size, shape, surface finish, and dimensional accuracy required, as well as machine tools, tooling, fixturing, and labor involved. Whereas machinery costs can be high for grinding, the costs for machinery for finishing processes are rather low. Grinding-wheel costs are generally low, although they can go up to hundreds or even thousands of dollars, depending on their composition and size. The costs of finishing tools, such as those for honing and lapping, vary widely, and labor costs and operator skill depend greatly on how well the equipment is automated. If finishing is likely to be an important factor in manufacturing a part, the conceptual and original design stages should involve an analysis of the level of surface finish and the dimensional accuracy 300 400 200 100 Surface finish, Ra (mm) 0 50 10 5 Rough turn 1 0.4 Figure 26.35: Increase in the cost of machining and finishing a part as a function of the surface finish required. This is the main reason that the surface finish specified on parts should not be any finer than is necessary for the part to function properly. Machining cost (%) As-cast, sawed, etc. Semifinish turn Finish turn Grind Hone ׉ 7cassandra://JFxK3YUkMQgQR3TvO7qoOLq_cqJcOmMGBjOirBpAFogB`cZ%EbpcZ%Ebo#בCט#u׉׉ 7cassandra://tSFgbZXsIkmJ-_-0CVdm9UCKNcHwurG9E_2Sleillic8`׉ 7cassandra://1MQESfe1M6gDYzx16wcMRJynB3fn4p24We3Hk_hV0hsuS`s׉ 7cassandra://JPuxvQ6uaQ4sbzgh3HdxP2uiYnAFuqb3I8M1ae4CoPIE`׉ 7cassandra://2iAbRk9XyktgbpHf6qmiSXwoUVkL_-jA6bc6z-oWv9o_7͠^cZZEbίט#u׉׉ 7cassandra://2mphSXLGR5CVLDNffbhQJHaKiJLLRoonPg5-2fMAIzU<`׉ 7cassandra://psx8XJYkx6PaKEDPOB0nxmuKZsuq2BIKpFBZ1QXDmYAAD`s׉ 7cassandra://no-Sl7K_2-8IWSSmkSgxfqAc7xe9d4uLdyzLSzKm6Hk`׉ 7cassandra://qOn1XuZ7fFLX3f660KAaP-85V9TVkI8vfg7wVT96caof ͠^cZZEbΰ׉EEconomics of Abrasive Machining and Finishing Operations 849 required, and whether they can be relaxed. Furthermore, all processes that precede finishing operations should be analyzed for their capability to produce more acceptable surface characteristics. This can be accomplished through proper selection of tools, selection of processing parameters, metalworking fluids, and the characteristics of the machine tools, their level of automation, computer controls, and of the workholding devices involved. Case Study 26.1 Gear Grinding with Engineered Abrasives Grinding in the gear industry has usually focused on finishing steel gear profiles, diameters, and bores; the gears are generally made of hardened steel. Gear manufacturers have to consider grinding in context, weighing the performance and economic factors of grinding as compared to hard machining processes, such as hard hobbing and power skiving (Section 24.2.4), or to various honing or lapping technologies (Section 26.7). Grinding offers improved (a) surface quality, (b) dimensional accuracy, and (c) process reliability, with improvements in, for example, noise reduction and allowable contact stress. However, these are at the expense of reduced metal-removal rates, additional cost, and a somewhat higher environmental impact due mainly to the use of cutting fluid. In general, little attention has been paid to the potential of grinding the rough gear form prior to heat treatment; conventional gear cutting is preferred due to the inability of traditional abrasive technologies to provide the required metal-removal rates and power efficiencies to be competitive. Consider hobbing, where removal rates are on the order of 50–100 mm3/sec; specific power requirements are around 4 J/mm3. Hobbing is fast and energy efficient, due to the relatively small numbers of large chips produced. The disadvantages of a process like hobbing lie in the flexibility and reliability of the tooling. The Indiana Tool and Manufacturing Company (ITAMCO) product line involves unique gears, both in terms of size and geometry. For these applications, the cutting tools, although normally long-lived are costly, are custom to a given gear, and they require long lead times to make. Machining performance and quality also change as the tool wears. Grinding with conventional abrasives has historically been limited by low grinding ratios (Section 26.4), metal-removal rates, and high specific grinding energy requirements. Bonded cubic boron nitride superabrasive wheels also have been unable to achieve the high rates of material removal required, even at high wheel speeds; the wheels are also expensive. Moreover, like hobs, they are specific to a given gear tooth profile. Grinding soft steel also has been a problem due to wheel loading, unless accompanied by continuous dressing. CDCF (continuous dress creep feed grinding) was developed in the 1970s using high porosity wheels to take deep form cuts. It was found that by continuously dressing with a formed diamond roll dresser, to keep the abrasive grains clean and sharp, the specific grinding energy was reduced significantly. In combination with good coolant access through high wheel porosity, CDCF allowed an order of magnitude increase in stock removal rates. However, although this was very effective on tough-togrind metals, such as Inconel, the level of wheel wear from continuous dressing (typically 1μm/rev at 1000 rpm) resulted in uneconomic wear rates as compared to machining. Combined with the need for a specifically shaped diamond roll, again made CDCF impractical for rough grinding of gears. Recent advances in wheel technology, especially those related to dressable vitrified bonds utilizing engineered ceramic grains, such as 3M’s Cubitron II, make grinding feasible for these operations. Conventional wheel grains have random shapes, typically with roughly equiaxed grains that offers a random cutting edge. The Cubitron II has specially shaped ceramic grains that are more aggressive and are uniformly oriented on a grinding wheel or disk. Typical grains are shown in Fig. 26.36. Compared to conventional abrasives (Fig. 26.1), the oriented grains grind more efficiently, allowing the grinding wheel to operate at lower temperature and higher efficiency, and removing up to three times the removal rate of conventional wheels. ׉ 7cassandra://JPuxvQ6uaQ4sbzgh3HdxP2uiYnAFuqb3I8M1ae4CoPIE`cZ%Ebq׉E850 Chapter 26 Abrasive Machining and Finishing Operations As one of the largest open gear manufacturers in the world, ITAMCO has developed technologies that can achieve specific grinding energies and metal-removal rates that approach those of hobbing and shaping, but with a grinding ratio that allows deep form grinding without continuous dressing. This approach offers the possibility of grinding, in the soft state, on standard finish gear grinders. Dressing is done using a standard CNC contour diamond dress roll, instead of a specialty tool. As an example of processing capabilities, the large gear (Fig. 26.37) has a 25 mm diametrical pitch, 600 mm face width, 3 m diameter with a 9 degree helix angle, and weighing over 23,587 kg. ITAMCO was able to finish the gear to size in 120 hours in one workholding, using only two 3M Cubitron II grinding wheels, thus saving time and material. The immediate benefits of the approach described above are fast turnaround times, since there is no need to wait for specialty hob or shaper manufacture. Moreover, power requirements are significantly reduced, and more flexibility is achieved by using grinders that normally would be underpowered for demanding applications. Source: Courtesy of J. Neidig, ITAMCO. Figure 26.36: Cubitron II abrasives. The shaped particles are much more aggressive than conventional abrasives (Fig. 26.1). Source: Courtesy of J. Neidig, ITAMCO. Figure 26.37: Grinding of a 3-m diameter gear. ׉ 7cassandra://no-Sl7K_2-8IWSSmkSgxfqAc7xe9d4uLdyzLSzKm6Hk`cZ%EbrcZ%Ebq#בCט#u׉׉ 7cassandra://pW13sjf0StpGsDsWbvmUUHU7tTNZHgpH6eDb_yMhyUAn.`׉ 7cassandra://ukl9Q_qKShBe08bD-QeRz4VtV3jcE1Jop2qXjI8X8U4N`s׉ 7cassandra://pN5-oyTZU--wfG5Y0Fw_vGOhv0HxXoloFYac_Ml_VU4O`׉ 7cassandra://Tv4qwlQsozgUxNv0_guNVQnA736hTpA9ZkUMlzCgqwMD(͠^cZ[Ebβט#u׉׉ 7cassandra://hNR0Qs7iknlsv-WYZy2BExRBes1BqrAZkTT-q1teYXM[`׉ 7cassandra://Hpa9wNv5ECytD9O3-H81TyLqiHjq7kbg2dyC_rTlBFIWp`s׉ 7cassandra://6eDudUg_5oH2Avkh9N-IyesecitPdnbJxfPsxAYfPrg`׉ 7cassandra://Jv9MJ4DjsDyn__ZUOs9Xc-uLOSvvNCovkzUJoe1i_0wV͠^cZ[Ebγ׉E }Key Terms 851 Summary • Abrasive machining often is necessary and economical when workpiece hardness and strength are high, the materials are brittle, and surface finish and dimensional tolerance requirements are demanding. • Conventional abrasives consist of aluminum oxide and silicon carbide; superabrasives consist of cubic boron nitride and diamond. The friability of abrasive grains is an important factor in their performance, as are the shape and size of the grains. • Grinding wheels, also known as bonded abrasives (in contrast to loose abrasives), consist of a combination of abrasive grains and bonding agents. Important characteristics of wheels are type of abrasive grain and bond, grade, and hardness. Wheels may be reinforced to maintain their integrity, if and when a crack develops during their normal use. • Grinding wheel wear is an important consideration in the surface quality and integrity of the ground part. Dressing and truing of wheels are necessary operations, and are done by various techniques. • A variety of abrasive-machining processes and machinery is available for surface, external, and internal grinding. The process is also used for large-scale material-removal processes, such as creep-feed grinding, making it competitive with processes such as milling and turning. • The selection of abrasives and process variables, including grinding fluids, is important in obtaining the desired surface finish and dimensional accuracy; otherwise, damage to surfaces, such as burning, heat checking, detrimental residual stresses, and chatter may develop. • Several finishing operations are available for improving surface finish. Because they can significantly affect product cost, the appropriate selection and implementation of these operations is important. • Deburring may be necessary for some finished components. Commonly used methods are vibratory finishing, barrel finishing, and shot blasting, although thermal energy and other methods also are available. Key Terms Abrasive-flow machining Abrasives Aluminum oxide Attritious wear Barrel finishing Belt grinding Bonded abrasives Buffing Burning Burr Chatter marks Chemical-mechanical polishing Coated abrasives Creep-feed grinding Cubic boron nitride Deburring Diamond Dressing Ductile-regime grinding Electropolishing Engineered abrasive Finishing Free-cutting wheels Friability Glazing Grade Grain depth of cut Grain size ׉ 7cassandra://pN5-oyTZU--wfG5Y0Fw_vGOhv0HxXoloFYac_Ml_VU4O`cZ%Ebs׉E852 Chapter 26 Abrasive Machining and Finishing Operations Grindability Grinding Grinding ratio Grit number Hardness of wheel Heat checking Honing Lapping Loading Low-stress grinding Magnetic-field-assisted polishing Magnetic-float polishing Metallurgical burn Microabrasive-flow machining Microreplication Polishing Reinforced wheels Resinoid bond Robotic deburring Rotary ultrasonic machining Seeded gel Shot blasting Silicon carbide Snagging Sonotrode Sparks Specific energy Structure of wheel Superabrasives Superfinishing Tempering Truing Ultrasonic machining Vibratory finishing Vitrified bond Wear flat Wheel depth of cut Wire brushing Bibliography Astashev, V.K., and Babitsky, V.I., Ultrasonic Processes and Machines, Springer, 2007. Davim, J.P., (ed.), Nontraditional Machining Processes, Springer, 2013. Doi, T., Uhlmann, E., and Marinescu, I.D., Handbook of Ceramics Grinding and Polishing, 2nd ed., William Andrew, 2015. Gillespie, L.K., Deburring and Edge Finishing Handbook, Society of Manufacturing Engineers/American Society of Mechanical Engineers, 2000. —, Mass Finishing Handbook, Industrial Press, 2006. Grzesik, W., Advanced Machining Processes of Metallic Materials, 2nd. ed., Elsevier, 2016. Hwa, L.S., Chemical Mechanical Polishing in Silicon Processing, Academic Press, 1999. Jackson, M.J., and Davim, M.J., Machining with Abrasives, Springer, 2010. Kuchle, A., Manufacturing Processes 2: Grinding, Honing, Lapping, Springer, 2009. Malkin, S., and Guo, C., Grinding Technology, 2nd ed., Industrial Press, 2008. Marinescu, I.D., Hitchiner, M., Uhlmann, E., Rowe, W.B., and Inasaki, I., Handbook of Machining with Grinding Wheels, 2nd ed., CRC Press, 2016. Marinescu, I.D., and Rowe, W.B., Handbook of Abrasive Machining Processes, 2nd ed., William Andrew, 2012. Oliver, M.R., Chemical Mechanical Planarization of Semiconductor Materials, Springer, 2004. Rowe, W.B., Principles of Modern Grinding Technology, 2nd ed., William Andrew, 2013. ׉ 7cassandra://6eDudUg_5oH2Avkh9N-IyesecitPdnbJxfPsxAYfPrg`cZ%EbtcZ%Ebs#בCט#u׉׉ 7cassandra://7urfHwW5bjbH5940SH24Z_TzXuZVyipSP6kbpZmKCeI`׉ 7cassandra://rEm0EFVQKdMNpdDLlipY77UUGFSrkrGJfWZuWkxSVu4c`s׉ 7cassandra://UUZFig4yGJj5bemDfdwPlz6exJvYVC3xU7wMgDqAxq8`׉ 7cassandra://RcqpASRX6D4geNbvTJKyJ3UoavWNFQ_qFS4AnbzyaqwH͠^cZ[Ebζט#u׉׉ 7cassandra://CMIwXOCJHtQHCYOTuWQIi4ACEIZihh3zVPRKUhOBTwc`׉ 7cassandra://-s6eV4Q-ONb0ZRMAuA9D9aE4LCEIVOhhWVixrV1pQNs^`s׉ 7cassandra://r3Hpstg9T4s656lE05FMXOPfN1YpVMZK9rPCys-Tk0w `׉ 7cassandra://BVwY8hNiQNnhUMzFerSa9vbNmMVsVvIBbJH6PSWBkrk]͠^cZ[Ebη׉E Qualitative Problems 853 Review Questions 26.1. What is an abrasive? What are superabrasives? 26.2. What are the advantages of superabrasives over conventional abrasives? 26.3. How is the size of an abrasive grain related to its number? 26.4. Why are most abrasives made synthetically? 26.5. Describe the structure of a grinding wheel and its features. 26.6. Explain the characteristics of each type of bond used in bonded abrasives. 26.7. What causes grinding sparks in grinding? Is it useful to observe them? Explain. 26.8. Define metallurgical burn. 26.9. Define (a) friability, (b) wear flat, (c) grinding ratio, (d) truing, and (e) dressing. 26.10. What is creep-feed grinding and what are its advantages? 26.11. How is centerless grinding different from cylindrical grinding? 26.12. What are the differences between coated and bonded abrasives? 26.13. What is the purpose of the slurry in chemical mechanical polishing? Qualitative Problems 26.14. Explain why grinding operations may be necessary for components that have previously been machined. 26.15. Why is there such a wide variety of types, shapes, and sizes of grinding wheels? 26.16. Explain the reasons for the large difference between the specific energies involved in machining (Table 21.2) and in grinding (Table 26.2). 26.17. Explain the factors involved in selecting the appropriate type of abrasive for a particular grinding operation. 26.18. Explain how the grinding ratio, G, depends on the following factors: (a) type of grinding wheel; (b) workpiece hardness; (c) wheel depth of cut; (d) wheel and workpiece speeds; and (e) type of grinding fluid. 26.19. What are the consequences of allowing the temperature to rise during grinding? Explain. 26.20. Explain why speeds are much higher in grinding than in machining operations. 26.21. It was stated that ultrasonic machining is best suited for hard and brittle materials. Explain. 26.22. Explain why parts with irregular shapes, sharp corners, deep recesses, and sharp projections can be difficult to polish. 26.23. Describe your understanding of the role of friability of abrasive grains on grinding-wheel performance. 26.24. List the finishing operations commonly used in manufacturing operations. Why are they necessary? Explain why they should be minimized. 26.25. Referring to the preceding chapters on processing of materials, list the operations in which burrs can develop on workpieces. 26.26. Explain the reasons that so many deburring operations have been developed over the years. ׉ 7cassandra://UUZFig4yGJj5bemDfdwPlz6exJvYVC3xU7wMgDqAxq8`cZ%Ebu׉E 854 Chapter 26 Abrasive Machining and Finishing Operations 26.27. Outline the methods that are generally available for deburring parts. Discuss the advantages and limitations of each. 26.28. What precautions should you take when grinding with high precision? Comment on the machine, process parameters, grinding wheel, and grinding fluids. 26.29. Describe the factors involved in a grinding wheel acting “soft” or acting “hard.” Can the same grinding wheel act soft or hard? Explain. 26.30. What factors could contribute to chatter in grinding? Explain. 26.31. Generally, it is recommended that, in grinding hardened steels, the grinding wheel be of a relatively soft grade. Explain. 26.32. In Fig. 26.5, the proper grinding faces are indicated for each type of wheel. Explain why the other surfaces of the wheels should not be used for grinding and what the consequences may be in doing so. 26.33. Describe the effects of a wear flat on the overall grinding operation. 26.34. What difficulties, if any, could you encounter in grinding thermoplastics? Thermosets? Ceramics? 26.35. Observe the cycle patterns shown in Fig. 26.22 and comment on why they follow those particular patterns. 26.36. Which of the processes described in this chapter are suitable particularly for workpieces made of (a) ceramics, (b) thermoplastics, (c) thermosets, (d) diamond, and (e) annealed aluminum? Why? 26.37. Grinding can produce a very fine surface finish on a workpiece. Is this finish necessarily an indication of the quality of a part? Explain. 26.38. Jewelry applications require the grinding of diamonds into desired shapes. How is this done, since diamond is the hardest material known? 26.39. Why should we be interested in the magnitude of the thrust force in grinding? Explain. 26.40. List and explain factors that contribute to poor surface finish in the processes described in this chapter. Quantitative Problems 26.41. Calculate the chip dimensions in surface grinding for the following process variables: D = 250 mm, d =0.03 mm, v =0.20 m/s, V =30 m/s, C =1 per mm2,and r =20. 26.42. If the strength of the workpiece material is increased by 50%, what should be the percentage decrease in the wheel depth of cut, d, in order to maintain the same grain force, with all other variables being the same? 26.43. Assume that a surface-grinding operation is being carried out under the following conditions: D = 300 mm, d =0.15 mm, v =0.6 m/s, and V =60 m/s. These conditions are then changed to the following: D = 200 mm, d =0.15 mm, v =0.35 m/s, and V =30 m/s. What is the difference in the temperature rise from the initial condition? 26.44. Estimate the percent increase in the cost of the grinding operation if the specification for the surface finish of a part is changed from 6.4 to 0.8 μm. 26.45. Assume that the energy cost for grinding an aluminum part with a specific energy requirement of 8W-s/mm3 is $1.50 per piece. What would be the energy cost of carrying out the same operation if the workpiece material were T15 tool steel? ׉ 7cassandra://r3Hpstg9T4s656lE05FMXOPfN1YpVMZK9rPCys-Tk0w `cZ%EbvcZ%Ebu#בCט#u׉׉ 7cassandra://C-fu0ZepZTSPT2mF_Al7ADh8f2s4vBIyBMI63C5NPG8`׉ 7cassandra://slaf6OehER79NPkRAs5ZcQgMB5KO9mkoz-rOGs4nbc0h`s׉ 7cassandra://2t_CMtsnq4ZqPFRtAvo-cx2inwFBYskTA67WaVxeEr0`׉ 7cassandra://abB6ZTiy9pdRPvFKXG54ShpGZ4PNkGGW8m7qtu9VAeca~͠^cZ[Ebιט#u׉׉ 7cassandra://71qeWlHTia0xTh8pjJlYgv9DPJuYLdjrAhfaT9HWRhw`׉ 7cassandra://OynmboTCjIGmSR927wcTi8PamFAAKLRkxJMPANiZvB0$`s׉ 7cassandra://wyypD78xWG4FOTKAXat_ML_sVouPrjBIGfl3c_CS8UE `׉ 7cassandra://eHvrkWt_bFxN9BxCvcWhO_-Ra5QEDmAb_fvWwhixxP02͠^cZ[Ebκ׉ESynthesis, Design, and Projects 855 26.46. In describing grinding processes, we have not given the type of equations regarding feeds, speeds, material-removal rates, total grinding time, etc., as we did in the turning and milling operations discussed in Chapters 23 and 24. Study the quantitative relationships involved and develop such equations for grinding operations. 26.47. What would be the answers to Example 26.1 if the workpiece is high-strength titanium and the width of cut is w =20 mm? Give your answers in newtons. 26.48. It is known that heat checking occurs when grinding with a spindle speed of 4000 rpm, a wheel diameter of 250 mm, and a depth of cut of 0.0375 mm for a feed rate of 0.25 m/s. For this reason, the spindle speed should be kept at 3500 rpm. If a new, 200-mm-diameter wheel is used, what spindle speed can be employed before heat checking occurs? What spindle speed should be used to keep the same grinding temperatures as those encountered with the existing operating conditions? 26.49. A grinding operation is taking place with a 250-mm grinding wheel at a spindle rotational speed of 4000 rpm. The workpiece feed rate is 0.25 m/s, and the depth of cut is 0.050 mm. Contact thermometers record an approximate maximum temperature of 950◦C. If the workpiece is steel, what is the temperature if the spindle speed is increased to 5000 rpm? What if it is increased to 10,000 rpm? 26.50. Derive an expression for the angular velocity of the wafer shown in Fig. 26.31b as a function of the radius and angular velocity of the pad in chemical–mechanical polishing. 26.51. It is desired to grind a hard aerospace aluminum alloy. A depth of 0.080 mm is to be removed from a cylindrical section 250-mm long and with a 80-mm diameter. If each part is to be ground in not more than one minute, what is the approximate power requirement for the grinder? What if the material is changed to a hard titanium alloy? 26.52. A 150-mm diameter tool steel (u =60 W-s/mm3) work roll for a metal rolling operation is being ground by a 250-mm diameter, 75-mm wide, Type I grinding wheel. Estimate the chip dimensions if d =0.04 mm and C =5 grains per mm2. If the wheel rotates at N = 3000 rpm, estimate the cutting force if the work roll rotates at 1 rpm. Synthesis, Design, and Projects 26.53. With appropriate sketches, describe the principles of various fixturing methods and devices that can be used for the processes described in this chapter. 26.54. Describe the methods you would use to determine the number of active cutting points per unit surface area on the periphery of a straight (i.e., Type 1; see Fig. 26.5a) grinding wheel. What is the significance of this number? 26.55. Make a comprehensive table of the process capabilities of abrasive-machining operations. Using several columns, describe the features of the machines involved, the type of abrasive tools used, the shapes of blanks and parts produced, typical maximum and minimum sizes, surface finish, tolerances, and production rates. 26.56. Vitrified grinding wheels (also called ceramic wheels) use a glasslike bond to hold the abrasive grains together. Given your understanding of ceramic-part manufacture (as described in Chapter 18), list methods of producing vitrified wheels. 26.57. Conduct a literature search, and explain how observing the color, brightness, and shape of sparks produced in grinding can be a useful guide to identifying the type of material being ground and its condition. 26.58. Visit a large hardware store and inspect the grinding wheels that are on display. Make a note of the markings on the wheels and, on the basis of the marking system shown in Fig. 26.7, comment on your observations, including the most common types of wheels available in the store. ׉ 7cassandra://2t_CMtsnq4ZqPFRtAvo-cx2inwFBYskTA67WaVxeEr0`cZ%Ebw׉Ea856 Chapter 26 Abrasive Machining and Finishing Operations 26.59. Obtain a small grinding wheel or a piece of a large wheel. (a) Using a magnifier or a microscope, observe its surfaces and compare them with Fig. 26.9. (b) Rub the abrasive wheel by pressing it hard against a variety of flat metallic and nonmetallic materials. Describe your observations regarding the surfaces produced. 26.60. In reviewing the abrasive machining processes in this chapter, you will note that some use bonded abrasives while others involve loose abrasives. Make two separate lists for these processes and comment on your observations. 26.61. On the basis of the contents of this chapter, describe your thoughts on whether or not it would be possible to design and build a “grinding center” (see Chapter 25). Comment on any difficulties that may be encountered in such machines and operations. 26.62. Assume that you are an instructor covering the topics described in this chapter and you are giving a quiz on the numerical aspects to test the understanding of the students. Prepare three quantitative problems and supply the answers. ׉ 7cassandra://wyypD78xWG4FOTKAXat_ML_sVouPrjBIGfl3c_CS8UE `cZ%EbxcZ%Ebw#בCט#u׉׉ 7cassandra://vkdQVVcrccdxkWys5LMZN7oEKxXQp2OOyNtf8hoYh2k8`׉ 7cassandra://pinbVKpVPHG4JRAXbsTD6IPTCWECgQlKENn8fpEwvtQ;`s׉ 7cassandra://yqskNB1v6FemysSjiOJSa-VtScOQWObm4lEUlsYSWLs`׉ 7cassandra://Il7AYrfUgAAuHReVK8taKe0RFdSm_OuhK-szCsr3Unw9͠^cZ\Ebμט#u׉׉ 7cassandra://zm1IAySZfmmWFcn-ic5fzajpgtaoVzlBLZH4tOAuOcA `׉ 7cassandra://FF0WDiFNNFhBaRsGtYmbHEsXmvcoM4k8StcNkTCN760[}`s׉ 7cassandra://FuHreM4lc-4tPzi-SSm0rMEWKVhuPZsZGb9f4nqnsOM`׉ 7cassandra://6qhrkGPwkkUlv77djvRhwiTfyxDaCX0orQe_54SaVhÍd͠^cZ\Ebν׉EVChapter 27 Advanced Machining Processes 27.1 Introduction 858 27.2 Chemical Machining 859 27.3 Electrochemical Machining 862 27.4 Electrochemical Grinding 866 27.5 Electrical-discharge Machining 867 27.6 Laser-beam Machining 872 27.7 Electron-beam Machining 875 27.8 Water-jet Machining 876 27.9 Abrasive-jet Machining 878 27.10 Hybrid Machining Systems 878 27.11 Economics of Advanced Machining Processes 879 Example: 27.1 Combining Laser-beam Cutting and Punching of Sheet Metal 874 Case Studies: 27.1 Electrochemical Machining of a Biomedical Implant 865 27.2 Manufacturing of Small Satellites 879 • It is often necessary to machine or finish products made of very hard or strong materials where conventional machining and grinding strategies described thus far become impractical or uneconomical. This chapter describes advanced machining processes that are based on nonmechanical methods of material removal. • The chapter begins by examining chemical machining and photochemical blanking processes, in which material is removed through the corrosive action of a fluid. • Electrochemical machining and grinding are then described, where material is removed by the action of an electrical power source and ion transfer inside an electrolytic fluid. Electrical-discharge machining removes material by melting small portions of the workpiece by a spark. 857 ׉ 7cassandra://yqskNB1v6FemysSjiOJSa-VtScOQWObm4lEUlsYSWLs`cZ%Eby׉E858 Chapter 27 Advanced Machining Processes • Laser-beam and electron-beam machining processes, as well as water-jet and abrasive-jet machining operations, also are described, with examples of their unique applications. • The chapter ends with a review of trends in hybrid machining operations and the economics of advanced machining processes. Typical parts made: Skin panels for missiles and aircraft, turbine blades, nozzles, parts with complex cavities and small-diameter deep holes, dies, laser cutting of sheet metals, cutting of thick metallic and nonmetallic parts. Alternative methods: Abrasive machining, ultrasonic machining, and precision machining. 27.1 Introduction The machining processes described in the preceding chapters involve material removal by mechanical means of chip formation, abrasion, or microchipping. However, there are situations where mechanical methods are not satisfactory, economical, or even possible, for the following reasons: • The strength and hardness of the workpiece material are very high, typically above 400 HB (Fig. 2.15). • The material is too brittle to be machined without damage to the part, which typically is the case with highly heat-treated alloys, glass, ceramics, and powder-metallurgy parts. • The workpiece is too flexible or slender to withstand the forces involved in machining or grinding, or the parts are difficult to clamp in fixtures and workholding devices. • The part has a complex shape (Fig. 27.1), with such features as internal and external profiles or holes with high length-to-diameter ratios in very hard materials. • The part has special surface finish and dimensional tolerance requirements that cannot be obtained by other processes or are uneconomical to do so. • The temperature rise during processing and residual stresses developed in the workpiece are not acceptable. EDM electrode Turbine blade Holes Clamp (a) (b) Figure 27.1: Examples of parts made by advanced machining processes. (a) Samples of parts produced by water-jet cutting. (b) Turbine blade, produced by plunge electrical-discharge machining, in a fixture to produce forced air cooling channels also by electrical-discharge machining. Source: (a) Courtesy of OMAX Corporation; (b) Courtesy of HI-TEK Mfg., Inc. ׉ 7cassandra://FuHreM4lc-4tPzi-SSm0rMEWKVhuPZsZGb9f4nqnsOM`cZ%EbzcZ%Eby#בCט#u׉׉ 7cassandra://5aneOt3KaDpuVOfPO8OeB8Xj6Xah_SF-Fsxf8jt2rPA`׉ 7cassandra://0DAUKba7r7U-YCA0uU9sZf3dltl9H2mkwU0FHiR8oOwZq`s׉ 7cassandra://l7rLUdyw359V-i6s7ZyIM2wdWxW3JkteiVWtJfEgFeY`׉ 7cassandra://lxJEYTopzhar843mJy3cDV_0N7q7hySeixNIM0D1oe0ekT͠^cZ]Ebοט#u׉׉ 7cassandra://eKKK5xO5M2vI7U_aht89rGTd06QpQEc6TYdrJ6s6-rwT`׉ 7cassandra://PzDLv3f3W_PJsWjsmfd5HGINqtuMxRgcaSefZdGW_P8R8`s׉ 7cassandra://pv8at4afJTxNfTfUeTwu5LxsxXnndnoJLSY9KhHq65Qo`׉ 7cassandra://cAPtbj9WUcgKh3XR-ukGhfg0sCV8aGGLfLGuoWFuXK8 S"͠^cZ^Eb׉EmChemical Machining 859 Process Table 27.1: General Characteristics of Advanced Machining Processes. Characteristics Chemical machining (CM) Electrochemical machining (ECM) Shallow removal on large flat or curved surfaces; blanking of thin sheets; low tooling and equipment cost; suitable for low-production runs Complex shapes with deep cavities; highest rate of material removal among other nontraditional processes; expensive tooling and equipment; high power consumption; medium-to-high production quantity Electrochemical grinding (ECG) Cutting off and sharpening hard materials, such as tungsten-carbide tools; also used as a honing process; higher removal rate than grinding Electrical-discharge machining (EDM) Shaping and cutting complex parts made of hard materials; some surface damage may result; also used as a grinding and cutting process; expensive tooling and equipment Wire electrical-discharge machining Contour cutting of flat or curved surfaces; expensive equipment Laser-beam machining (LBM) Laser microjet Electron-beam machining (EBM) Water-jet machining (WJM) Cutting and hole making on thin materials; heataffected zone; does not require a vacuum; expensive equipment; consumes much energy Water-jet guided laser uses a 25–100 µm diameter stream to mill or cut; large depth of field; little thermal damage from laser machining Cutting and hole making on thin materials; very small holes and slots; heat-affected zone; requires a vacuum; expensive equipment Cutting all types of nonmetallic materials; suitable for contour cutting of flexible materials; no thermal damage; noisy Abrasive water-jet machining (AWJM) Single-layer or multilayer cutting of metallic and nonmetallic materials Abrasive-jet machining (AJM) Cutting, slotting, deburring, etching, and cleaning of metallic and nonmetallic materials; tends to round off sharp edges; can be hazardous Process parameters and typical material-removal rate or cutting speed 0.0025–0.1 mm/min. V: 5–25 D.C.; A: 1.5-8 A/mm2;2.5– 12 mm/min, depending on current density A: 1–3 A/mm2; typically 25 mm3/s per 1000 A V: 50–380; A: 0.1–500; typically 300 mm3/min Varies with material and thickness 0.50–7.5 m/min Varies with material; up to 20 mm in silicon, 2 mm in stainless steel; up to 300 mm/s in 50 µmthick silicon. 1–2 mm3/min Varies considerably with material Up to 7.5 m/min Varies considerably with material Beginning in the 1950s, these difficulties led to the development of chemical, electrical, laser, and high-energy beams as energy sources for removing material from metallic and nonmetallic workpieces (Table 27.1). Also called nontraditional or unconventional machining, these processes remove material not by producing chips, as in traditional machining and grinding, but by means such as chemical dissolution, etching, melting, evaporation, and hydrodynamic action, at times with the assistance of fine abrasive particles. A major advantage of these processes is that their efficiency is independent of workpiece hardness. When selected and applied properly, advanced machining processes offer major technical and economic advantages over more traditional methods. This chapter describes these processes, including their characteristics, typical applications, limitations, product quality, dimensional accuracy, surface finish, and economics. 27.2 Chemical Machining Chemical machining (CM) is based on the fact that chemicals attack and etch most materials, thereby removing small amounts of material from workpiece surfaces. TheCMprocess is carried out by chemical dissolution using reagents or etchants, such as acids and alkaline solutions. Developed in the 1950s, chemical machin׉ 7cassandra://l7rLUdyw359V-i6s7ZyIM2wdWxW3JkteiVWtJfEgFeY`cZ%Eb{׉E860 Chapter 27 Advanced Machining Processes 4 mm (before machining) 2 mm (after machining) Chemically machined area (a) Section (b) Figure 27.2: (a) Missile skin-panel section contoured by chemical milling to improve the stiffness-to-weight ratio of the part. (b) Weight reduction of space-launch vehicles by the chemical milling of aluminum-alloy plates. The plates are chemically milled after they have been formed into shape by a process such as roll forming or stretch forming. The design of the chemically machined rib patterns can readily be modified at minimal cost. ing is the oldest of the advanced machining processes, and has been used in engraving metals and stones, in deburring, and in the production of printed-circuit boards and microelectronic devices (Chapters 28 and 29). Chemical Milling. In chemical milling, shallow cavities are produced on plates, sheets, forgings, and extrusions, generally for overall reduction of weight, as can be seen in Fig. 27.2. The process has been used on a wide variety of metals, with depths of removal up to 12 mm. Selective attack by a chemical reagent on different areas of the workpiece surfaces is accomplished by removing layers of material from areas that are not masked (Fig. 27.3a). Material removal may also be done by partial immersion of the part in a reagent. The procedure for chemical milling consists of the following steps: 1. If the part to be machined has residual stresses left from prior processing, the stresses should first be relieved (Section 4.11) in order to prevent warping after chemical milling (see also Case Study 24.3). Agitator Workpiece Workpiece support Tank Chemical reagent (a) 3rd Maskant Undercut Depth Heating/cooling coils Workpiece (b) Figure 27.3: (a) Schematic illustration of the chemical-machining process; note that no forces or machine tools are involved in this process. (b) Stages in producing a profiled cavity by chemical machining; note the undercut. 2nd 1st Steps Material removed Edge of maskant ׉ 7cassandra://pv8at4afJTxNfTfUeTwu5LxsxXnndnoJLSY9KhHq65Qo`cZ%Eb|cZ%Eb{#בCט#u׉׉ 7cassandra://kLu9wNMuEe91OKtJQxZyLXRuWCU2GBTOb9zfqaVoadE`׉ 7cassandra://78hdkC4qp4WKm7y4Gfi7V8FP91p6c9A4CZZwOjVcD5Mk9`s׉ 7cassandra://OhvJjDdDTnlsuky10VJzTe73dOyZZSUXXY_INZsw-Xo`׉ 7cassandra://9NyNtUBBExvczABpO67CHO4yUzn8bYbXzAuLO-c3mMUSp͠^cZ^Ebט#u׉׉ 7cassandra://xbcgdbqr4EWTuk9WIhGTGCXXsiqY0Qga6tu4JWZiVgc`׉ 7cassandra://40HcPHt_ek4_nuIX3vscF2p_gUZXYNXwhYoO-j0vXTsaG`s׉ 7cassandra://tReOb70i3UC_jxjw9krsSVWOEppBzZkNY4JqwY3_dHQ`׉ 7cassandra://KLfP1_Ujx4Vq_yP0eRpzTlO8BVj48bMSdtetfqVKdX4`0͠^cZ^Eb׉E%Chemical Machining 861 2. The surfaces are thoroughly degreased and cleaned, to ensure both good adhesion of the masking material and maintaining uniform rate of material removal. Scale from prior heat treatment also should be removed. 3. The masking material (maskant) is applied, typically using tapes or paints, but also elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and polystyrene). It is essential that the maskant not react with the chemical reagent. 4. Areas in the maskant that will require etching are peeled off, using the scribe-and-peel technique. 5. The exposed surfaces are machined chemically, using etchants such as sodium hydroxide (for aluminum), solutions of hydrochloric and nitric acids (for steels), and iron chloride (for stainless steels). Temperature control and agitation (stirring) of the etchant during milling is important, in order to remove a uniform depth of material from the part surfaces. 6. The parts are then washed thoroughly with water, to prevent further reactions with or exposure to any remaining etchant residues. 7. The rest of the masking material is removed, and the part is cleaned and inspected. Note that although the maskant is unaffected by the reagent, it can easily be dissolved by a different and appropriate type of solvent, such as acetone or Piranha (see Table 28.3). 8. Additional finishing operations may be performed on the milled parts, such as abrasive flow machining (Section 26.9) or electroplating (Section 34.9). 9. This sequence of operations can be repeated to also produce stepped cavities and various contours on parts (Fig. 27.3b). Chemical milling is used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels (Fig. 27.2), and extruded parts for airframes. Tank capacities for reagents are as large as 3.7 m × 15 m. The process is also used to fabricate microelectronic devices, and is often referred to as wet etching, as described in Section 28.8.1. The ranges of surface finish and tolerances obtained by chemical machining and other machining processes are given in Fig. 33.5. Because of preferential etching and intergranular attack, some surface damage may result from chemical milling, adversely affecting surface properties. Chemical milling of welded and brazed structures also may result in uneven material removal, and castings may result in uneven surfaces, caused by porosity and property nonuniformities in the material. Chemical Blanking. This process is similar to blanking of sheet metals (Fig. 16.4). Typical applications are burr-free etching of printed circuit boards (Section 28.13), decorative panels, and thin sheet-metal stampings, as well as the production of complex or very small parts. Photochemical Blanking. Also called photoetching or photochemical machining, this is a modification of the chemical milling process. Material is removed, usually from flat thin sheet, by photographic techniques to first produce a mask, followed by chemical machining. Complex, burr-free shapes can be blanked on metal foil as thin as 0.0025 mm. This process is also used for etching, such as for electrical connectors or pattern plates for reflow or paste soldering (Section 32.3). The procedure in photochemical blanking consists of the following steps: 1. The design of the part to be blanked is prepared at a magnification of up to 100×. A photographic negative is then made and reduced to the size of the finished part, called artwork; note that the original (enlarged) drawing allows inherent design errors to be reduced in size by the amount of reduction, such as 100×, for the final artwork image. 2. The sheet blank is coated with a photosensitive material (photoresist, and often called emulsion), by dipping, spraying, spin casting, or roller coating; it is then dried in an oven. ׉ 7cassandra://OhvJjDdDTnlsuky10VJzTe73dOyZZSUXXY_INZsw-Xo`cZ%Eb}׉E L862 Chapter 27 Advanced Machining Processes 3. The negative is placed over the coated blank and is exposed to ultraviolet light, which hardens the exposed areas. 4. The blank is developed, dissolving the unexposed mask areas; it is then immersed into a bath of reagent (as in chemical milling), or is sprayed with the reagent, which etches away the exposed metal areas. 5. The masking material is removed, and the part is thoroughly washed with water, to remove all chemical residues. Handling of chemical reagents requires precautions and special safety considerations to protect the workers against exposure to both liquid chemicals and volatile chemicals. Furthermore, the disposal of chemical by-products from this process is a major drawback, although some by-products can be recycled. Although skilled labor is required, tooling costs are low, the process can be automated, and is economical for medium- to high-production volume. Photochemical blanking is capable of making very small parts in cases when traditional blanking dies (Section 16.2) are too difficult to make. The process is also effective for blanking fragile workpieces and materials; tolerances are on the order of 10% of the sheet thickness. Typical applications for photochemical blanking include fine metal screens, printed-circuit boards, electric-motor laminations, flat springs, and various components of miniaturized systems. Design Considerations for Chemical Machining. General design guidelines for chemical machining are: • Designs with sharp corners, deep and narrow cavities, severe tapers, folded seams, or porous part materials should be avoided, because the etchant continuously attacks all exposed surfaces. • Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop, as shown in Fig. 27.3 by the areas under the edges of the maskant. • To improve production rate, the bulk of the workpiece preferably should be first shaped by other and higher volume rate processes, such as machining, prior to chemical machining. • Because of size changes in the deposited mask pattern due to humidity and temperature, dimensional variations can occur. These variations can be minimized by properly selecting artwork media and by controlling both the environment in which the artwork is generated and in the production area in the plant. • Product designs are produced with computer-aided design systems (Chapter 38), and they can be translated into a useful format for etching machinery. 27.3 Electrochemical Machining Electrochemical machining (ECM) is basically the reverse of electroplating (Section 34.9). An electrolyte acts as the current carrier (Fig. 27.4), and the high flow rate of electrolyte in the tool-workpiece gap (typically 0.1 to 0.6 mm) washes metal ions away from the workpiece (anode) before they have a chance to plate onto the tool (cathode). Note that the cavity produced is the mating image of the tool shape. The tool, either in solid or tubular shape, is generally made of brass, copper, bronze, or stainless steel. The electrolyte is a highly conductive inorganic fluid, such as an aqueous solution of sodium nitrate, and is pumped through the passages in the tool at rates of 10 to 16 m/s. A DC supply in the range from 10 to 25 V maintains current densities, which, for most applications, are 20 to 200 A/cm2 of active machined surface. ׉ 7cassandra://tReOb70i3UC_jxjw9krsSVWOEppBzZkNY4JqwY3_dHQ`cZ%Eb~cZ%Eb}#בCט#u׉׉ 7cassandra://rud89YFMimwZcI0p0pD9_6yKLGhGD_uuOvxLkfJP4SI `׉ 7cassandra://f81V9P6a7BetdsvBqaO0qIqyk34h4-VIMxGLSjjyvBQb`s׉ 7cassandra://__V_VqkHq6qGfhETSOES5ER6ExyrR8I17zJIv6ITkvoD`׉ 7cassandra://CNXS6lIa0ZlO9I84yzZTTskvvumWasvIrYfHCCzC-38~Y͠^cZ_Ebט#u׉׉ 7cassandra://nyYWyj19JUTXhLAYqGiKFhzl2xAcAUQL48vystGyPnMdz`׉ 7cassandra://7UA58K-KBFFvihy_YWgjwyNguO9d6KiZiYqKF1I9noo?p`s׉ 7cassandra://vgU3yzt_m_iG97RkPnIdUsVe964DgWMTlKoHZd3HE38`׉ 7cassandra://fw7ODIe0YlVraVqdJ_aZIy_E-xVfN_p5dc126vzq5Zo͑͠^cZ`Eb׉E Electrochemical Machining 863 DC power supply Insulating coating Workpiece (1) (2) Pump for circulating electrolyte Tool Tank Electrolyte Figure 27.4: Schematic illustration of the electrochemical machining process. For a current efficiency of 100%, the material-removal rate (MRR) in electrochemical machining may be estimated from MRR = CI (27.1) where MRR is in mm3/min, I is the current in amperes, and C is a material constant with the unit of mm3/A-min. For pure metals, C depends on the valence: the higher the valence, the lower is the value of C. Machines having current capacities as high as 40,000 A and as low as 5 A are available. The penetration rate of the tool is proportional to the current density, and the material removal rate typically ranges between 1.5and 4mm3 per A-min. Because the metal-removal rate is a function only of the ion exchange rate, it is not affected by the strength, hardness, or toughness of the workpiece, a characteristic that is common to the processes described in this chapter. Process Capabilities. The basic concept of electrochemical machining developed rapidly beginning with the 1950s, whereupon it became an important manufacturing process. It is generally used to machine complex cavities and shapes in high-strength materials, particularly in the aerospace industry for the mass production of turbine blades, jet-engine parts, and nozzles (Fig. 27.5); other applications include the automotive (engines castings and gears) and medical industries. Electrochemical machining also is used for machining and finishing forging-die cavities (die sinking) and to produce small holes. Modifications of this process are used for turning, facing, milling, slotting, drilling, trepanning, and profiling operations, and in the production of continuous metal strips and webs. More recent applications of ECM include micromachining (Chapters 28 and 29) for the electronics industry. An advance in ECM is shaped-tube electrolytic machining (STEM), and used for producing small-diameter deep holes, as in turbine blades (Fig. 27.6). The electrolyte is acid-based, to ensure that the worn metal is dissolved and carried away by the solution. The tool is a titanium tube for corrosion resistance, coated with an electrically-insulating resin to restrict the electrolytic action to the front surface of the electrode. Holes as small as 0.5 mmcan be made, and at depth-to-diameter ratios as high as 300:1. Larger holes can be produced by electrolytic trepanning, as shown in Fig. 27.6b. The ECM process leaves a burr-free, bright surface; it can also be used as a deburring operation. The operation does not cause any thermal damage to the part, and the absence of tool forces prevents distortion, especially in thin, flexible parts. Furthermore, there is no tool wear, since only hydrogen is generated at the cathode, and the process is capable of producing complex shapes. However, the mechanical properties of components made by ECM should be compared with those of components made by other processes, to ensure that there has not been a significant compromise due to chemical reactions. Electrochemical-machining systems are available as numerically controlled machining centers, with capability of high production rates, high flexibility of operation, and the maintenance of fine dimensional tolerances. The ECM process also can be combined with electrical-discharge machining (EDM) on the same machine, called hybrid machining (see Section 27.10). ׉ 7cassandra://__V_VqkHq6qGfhETSOES5ER6ExyrR8I17zJIv6ITkvoD`cZ%Eb׉ET864 Chapter 27 Advanced Machining Processes 75 mm 140 mm Telescoping cover Insulating layer Feed 65 mm Forging Machined workpiece 14 holes 112 mm (b) (c) Figure 27.5: Typical parts made by electrochemical machining. (a) Turbine blade made of a nickel alloy of 360 HB; note the shape of the electrode on the right. (b) Thin slots on a 4340-steel roller-bearing cage. (c) Integral airfoils on a compressor disk. Electrode feed Electrode feed Insulating film Electrolyte flow Titanium electrode Insulating film Electrolyte flow Titanium electrode Electrolyte Copper electrode (a) 86 mm Electrode carrier Ram Workpiece Workpiece (a) (b) Figure 27.6: Shaped electrolytic machining operations. (a) Shaped-tube electrolytic machining, used to make small holes with aspect ratios as large as 300:1; (b) electrolytic trepanning, used for larger diameter holes. ׉ 7cassandra://vgU3yzt_m_iG97RkPnIdUsVe964DgWMTlKoHZd3HE38`cZ%Eb€cZ%Eb#בCט#u׉׉ 7cassandra://y5lOJQIKwfBAH-fqKSlqQlHh6dIGY6Ngxn0FCnWPkYo5`׉ 7cassandra://6sOn2QHRYkoP3FtigK5kq9xR8FG-2hOzepI0d-aoaS4i`s׉ 7cassandra://7ZiV9XRNl4umgMkoNTz61V8dnlExQTuVxyAwdsOZDBsZ`׉ 7cassandra://8KJySEIE1siTilw-L_oKrmJ9R04F0fokjZ_zV27C0LUb͠^cZ`Ebט#u׉׉ 7cassandra://5w4GKqZk-wIZwzmmbJENo5vPb53Hi7Jcjyl48gtRKLI`׉ 7cassandra://kbh93tAN73kfgfPFnJQSt46k_OC4mZxjoa8CMqQYtaY_`s׉ 7cassandra://8YxTD9Gokhne0cJpjLx4olKtgkRjxpN9yLO9EYdOUF8:`׉ 7cassandra://GvOzxN1AqCJBNvblTV8JswQD9KtXCY22mnUyJ-1fgAw͑uP͠^cZ`Eb׉EElectrochemical Machining 865 Design Considerations for Electrochemical Machining. The following are general design guidelines for electrochemical machining: • Part designs should make provision for a small taper for holes and cavities. • Sharp square corners or flat bottoms cannot be produced because of the tendency for the electrolyte to erode away sharp profiles. • Irregular cavities may not be produced to the desired shape and with acceptable dimensional accuracy, because of the difficulty in controlling electrolyte flow. 27.3.1 Pulsed Electrochemical Machining The pulsed electrochemical machining (PECM) process is a refinement of ECM; it uses very high current densities (on the order of 1 A/mm2), but the current is pulsed, rather than direct current. The purpose of pulsing is to eliminate the need for high electrolyte flow rates, which limit the usefulness of ECM in die and mold making. Investigations have shown that PECM improves fatigue life compared to ECM, and the process does not have a characteristic recast layer on die and mold surfaces. The tolerances obtained typically are in the range from 20 to 100 µm. Case Study 27.1 Electrochemical Machining of a Biomedical Implant A total knee-replacement system consists of a femoral and tibial implant, combined with an ultrahighmolecular-weight polyethylene (UHMWPE) insert (Fig. 27.7a). Polyethylene has superior wear resistance and low friction against the cobalt-chrome alloy femoral implant. The UHMWPE insert is compression molded (Section 19.7), and the metal implant is cast and ground on its external mating surfaces. Designers of implants, manufacturing engineers, and clinicians have long been concerned particularly with the contact surface in the cavity of the metal implant that mates with a protrusion on the polyethylene insert. As the knee articulates during its normal motions, the polyethylene implant slides against the metal part, becoming a potentially serious wear site (Section 33.5). This geometry is necessary to ensure lateral stability of the knee, preventing the knee from buckling sideways. To produce a smooth surface, grinding of the bearing surfaces of the metal implant, using both handheld and cam-mounted grinders, was a procedure that had been followed for many years. However, grinding produced marginal repeatability and part quality. The interior surfaces of this part are extremely difficult to access for grinding, and the cobalt–chrome alloy is difficult to grind consistently. Consequently, advanced machining processes, particularly electrochemical machining, were considered to be ideal candidates for this operation. As shown in Fig. 27.7b, the current procedure consists of placing the metal implant in a fixture to bring a tungsten electrode of the desired final contour in close proximity to the implant. The electrolyte is a sodium nitrate and water mixture, and it is pumped through the tool, filling the gap between the tool and the implant. A power source, typically 10 V and 225 A, is applied, causing local electrochemical machining of the high spots on the implant surface and producing a polished surface. The electrolyte flow rate can be controlled so as to maximize surface quality. If the rate is too low, defects appear on the machined surface as localized dimples, and if the flow rate is too high, machining times become longer, reducing production times. Typical machining times for this part are four to six minutes. The ECM process can be effective for micromachining as well (Section 29.2). Because of complete absence of tool wear, this process also can be used for making precision electronic components. Source: Courtesy of T. Hershberger and R. Redman, Zimmer Biomet, Inc. ׉ 7cassandra://7ZiV9XRNl4umgMkoNTz61V8dnlExQTuVxyAwdsOZDBsZ`cZ%Eb׉E 866 Chapter 27 Advanced Machining Processes Sodium–nitrate solution Copper–tungsten electrode Cobalt–chrome implant (a) (b) Figure 27.7: (a) Two total knee-replacement systems, showing metal implants (top pieces) with an ultrahighmolecular-weight polyethylene insert (bottom pieces). (b) Cross section of the ECM process, as applied to the metal implant. Source: Courtesy of Zimmer Biomet, Inc. Machines can perform a combination of both EDM and PECM, thus the need to move the tool and workpiece between the two processes is eliminated. If these operations occur on separate machines, it is difficult to maintain precise alignment when moving the workpiece from the EDM to the PECM operation. If misaligned significantly, all polishing will occur at a location where the gap is smallest, and passivation (Section 3.8) will occur where the gap is largest. Also, this process leaves metal residues suspended in the aqueous solution, which is harmful to the environment if disposed of without proper treatment. 27.4 Electrochemical Grinding Electrochemical grinding (ECG) combines electrochemical machining with conventional grinding. The equipment used is similar to a conventional grinder, with the exception that the wheel is now a rotating cathode, embedded with abrasive particles (Fig. 27.8a). The wheel is metal bonded with diamond or aluminum-oxide abrasives, and rotates at a surface speed from 1200 to 2000 m/min. The abrasives have two functions: (a) serve as insulators between the wheel and the workpiece and (b) mechanically remove electrolytic products from the working area. A flow of electrolyte solution, usually sodium nitrate, is provided for the electrochemical machining phase of the operation where current densities range from 1 to 3 A/mm2. The majority of metal removal in ECG is by electrolytic action, and, typically, less than 5% of the metal is removed by the abrasive action of the wheel; consequently, wheel wear is very low and the part remains cool. Finishing cuts usually are made by the grinding action, but only to produce a surface with good finish and dimensional accuracy. The ECG process is suitable for applications similar to those for milling, grinding, and sawing (Fig. 27.8b), but it is not adaptable to cavity sinking for die making. The process can be applied successfully to carbides and high-strength alloys, and it offers a distinct advantage over traditional diamond-wheel grinding of very hard materials, where wheel wear can be high. ECG machines are equipped with numerical controls, improving dimensional accuracy and repeatability, and increased productivity. Electrochemical honing combines the fine abrasive action of honing (Section 26.7) with electrochemical action. Although the equipment is costly, this process is as much as 5 times faster than conventional honing, and the tool lasts as much as 10 times longer. Electrochemical honing is used primarily for finishing internal cylindrical surfaces. ׉ 7cassandra://8YxTD9Gokhne0cJpjLx4olKtgkRjxpN9yLO9EYdOUF8:`cZ%Eb‚cZ%Eb#בCט#u׉׉ 7cassandra://dqQW6qUZVELp0bvHa1rEX5vVCt2YGAiyDOycM5PaPd0l4`׉ 7cassandra://Xk_w3dhnESrU39aGVGJH7OHiOfOSNfxaJEnVIEofzSsU`s׉ 7cassandra://7mJCEkgcQXunZR1sfSiapmye9Fah28uXqHr6lgRcLgc`׉ 7cassandra://oxMc3ZHRlvkMZAWG3vXHVXCyp-KF2bVqovckgi2ySmgͪ͠^cZaEbט#u׉׉ 7cassandra://k35DLI8IrOKEcAvPuT9Eg5FcP2mZ4IRVZLzyjVKYzlI `׉ 7cassandra://oslf7YbIqw9mFzHrf0TOV1goPBmcnZuQTdu7CNEnzvsTJ`s׉ 7cassandra://E_Q0rsoe2HeacKtbB3XtrGzwtsZN6o9s7G7ThMcwrPsA`׉ 7cassandra://eaL1t4GPmGqk9mBxgMUGyR-wGmGvwriLZ_6RjRs0UV8W ͠^cZaEb׉E \Electrical-discharge Machining 867 Electrolyte from pump Electrode (grinding wheel) Spindle Electrical connection 3.1 mm Insulating abrasive particles Workpiece Worktable (1) (a) (b) Figure 27.8: (a) Schematic illustration of the electrochemical-grinding process. (b) Thin slot produced on a round nickel-alloy (Inconel) tube by this process. Design Considerations for Electrochemical Grinding. In addition to the design considerations listed above for electrochemical machining, ECG requires two more: • Designs should avoid sharp inside radii. • If a surface is to be flat, it should be narrower than the width of the grinding wheel. 27.5 Electrical-discharge Machining The principle of electrical-discharge machining (EDM), also called electrodischarge or spark-erosion machining, is based on the erosion of metals by spark discharges. Recall that when two current-conducting wires are allowed to touch each other, an arc is produced. When the contact between the two wires is closely examined it will be noted that a small portion of the metal has been eroded away, leaving a small crater on the surface. Although this phenomenon has been known since the discovery of electricity, it was not until the 1940s that a machining process based on that principle was developed. The EDM process is one of the most important and widely used production technologies in manufacturing. Principle of Operation. The basic EDM system consists of a shaped tool (electrode) and the part, connected to a DC power supply and placed in a dielectric (electrically nonconducting) fluid, as shown in Fig. 27.9a. When the potential difference between the two is sufficiently high, the dielectric breaks down and a transient spark discharges through the fluid, removing a very small amount of metal from the workpiece surface. The capacitor discharge is repeated continuously, at rates between 200 and 500 kHz, with voltages usually ranging between 50 and 380 V and currents from 0.1 to 500 A. The volume of material removed per spark discharge is typically in the range from 10−6 to 10−4 mm3. The EDM process can be used on any material that is an electrical conductor. Two important physical properties determine the volume of metal removed per discharge: melting point and the latent heat of melting of the workpiece material. As these quantities increase, the rate of material removal decreases. The material-removal rate can be estimated from the empirical formula MRR =4× 104IT−1.23 w , (27.2) where MRR is in mm3/min, I is the current in amperes, and Tw is the melting point of the workpiece in ◦C. Insulating bushing DC (2) power supply 0.4 mm 0.5 mm ׉ 7cassandra://7mJCEkgcQXunZR1sfSiapmye9Fah28uXqHr6lgRcLgc`cZ%Ebƒ׉E868 Chapter 27 Advanced Machining Processes Rectifier Current control Servo control Movable electrode (1) Power supply Dielectric fluid Workpiece Tank Melted workpiece (a) 3.5 mm dia. Workpiece 8 holes, 0.17 mm 1.1-mm wall Heat-treated steel Electrode (b) (c) (d) Figure 27.9: (a) Schematic illustration of the electrical-discharge machining process; this is one of the most widely used machining processes, particularly for die-sinking applications. (b) Examples of cavities produced by EDM, using shaped electrodes. The two round parts (rear) are the set of dies used in extruding the aluminum piece shown in front (see also Fig. 15.9b). (c) A spiral cavity produced by EDM using a slowly rotating electrode similar to a screw thread. (d) Holes in a fuel-injection nozzle made by EDM; the material is heat-treated steel. Source: (b) Courtesy of AGIE USA, Ltd. The workpiece is fixtured within a tank containing the dielectric fluid, and its movements are controlled by numerically controlled systems. The gap between the tool and the workpiece (overcut) is critical; the downward feed of the tool is thus controlled by a servomechanism, automatically maintaining a constant gap. The frequency of discharge or the energy per discharge, the voltage, and the current are varied to control the removal rate. The rate and surface roughness produced increase with increasing current density and with decreasing frequency of sparks. Dielectric Fluids. The functions of the dielectric fluid are to: 1. Act as an insulator until the potential is sufficiently high 2. Provide a cooling medium 3. Act as a flushing medium and carry away the debris in the gap. EDM machines are equipped with a pump and filtering system for the dielectric fluid. The most common dielectric fluids are mineral oils, although kerosene and distilled or deionized water also are used in specialized applications. Although more expensive, low-viscosity transparent fluids that make cleaning easier are also available. Electrodes. Electrodes for EDM usually are made of graphite, although brass, copper, or copper–tungsten alloys also are used. The tools can be shaped by forming, casting, powder metallurgy, or by CNC machining (2) Worn electrode Spark ׉ 7cassandra://E_Q0rsoe2HeacKtbB3XtrGzwtsZN6o9s7G7ThMcwrPsA`cZ%Eb„cZ%Ebƒ#בCט#u׉׉ 7cassandra://X5c_k7IVhxEF_aq7u04J5RW737TAjzYyyN7O39xdT5U!`׉ 7cassandra://PTKTNg7wWBCmYZAWEK4EZ7_b3ln7O9QCWKggvsvVo2gi`s׉ 7cassandra://ghH_mG63885UNv4_XPDesyGq_lfc-nirusQa7m4uomc)`׉ 7cassandra://t1hI-XGnQIa3qX4EGGOgzkowdeDNwMPOzmPy40YFwWcf͠^cZbEbט#u׉׉ 7cassandra://ip8mEzx7YD79Ecg4-uIQYea3gPcBvxOMBOgS4qofbXw`׉ 7cassandra://rPRInXygse2u4HUGu-jyxRHQFM3mdxGdlnDSxG9eb0gI`s׉ 7cassandra://pGllTqn4Q-NUuh3boRlbOvMDO1GMV8LSGTL5wuEjEb07`׉ 7cassandra://oxE29pW-bwkrXWmybTI5REkL27adbnN5lBY31t9iLAo|[gX͠^cZdEb׉E>Electrical-discharge Machining 869 techniques. Tungsten-wire electrodes, as small as 0.1 mm in diameter, have been used to make holes with depth-to-hole diameter ratios of up to 400:1, a ratio that is much higher than those available by conventional methods (Table 23.11). The sparks in EDM also erode away the electrode, thus changing its geometry and adversely affecting the shape produced and its dimensional accuracy. Wear ratio is defined as the ratio of the volume of workpiece material removed to the volume of tool wear. It ranges from about 3:1 for metallic electrodes to as high as 100:1 for graphite electrodes. Tool wear is related to the melting points of the materials involved: the higher the melting point of the electrode, the lower is the wear rate; consequently, graphite electrodes have the highest wear resistance. Also, the higher the current, the higher is the wear. Tool wear can be minimized by reversing the polarity and using copper tools, a process called no-wear EDM. Care must be taken to control this process; it is, for instance, possible for the workpiece material to coat the electrode and thus change its shape. Process Capabilities. Electrical-discharge machining has numerous applications: dies for forging, extrusion, die casting, injection molding, and large sheet-metal automotive-body components (produced in die-sinking machining centers, with computer numerical control). Other applications include machining small-diameter deep holes, using tungsten wire as the electrode; narrow slots in parts; cooling holes in superalloy turbine blades; and various intricate shapes (see Figs. 27.9b and c). Stepped cavities also can be produced by controlling the relative movements of the workpiece in relation to the electrode. Blue Arc Machining. One variation of electrical discharge machining is the blue arc process, developed for roughing cuts of difficult-to-machine materials, especially nickel-based superalloys. The shape of bladed disks, called blisks, used in aircraft engines can be challenging to machine; the blue arc process removes most of the material for a rough shape, which then is finish machined through conventional CNC milling. This process uses an electrode and electrical discharge machining to remove material, also adding high pressure fluid flushing to remove chips from the cutting zone. Variations of this technique are available also for turning and grinding. Because of the molten and resolidified (recast) surface structure developed, high rates of material removal may produce a very rough surface finish, with poor surface integrity and low fatigue properties. Finishing cuts are therefore made at low removal rates, or the recast layer is subsequently removed by various finishing operations. Surface finish can be improved by oscillating the electrode in a planetary motion, at amplitudes of 10 to 100 µm. Design Considerations for EDM. The general design guidelines for electrical-discharge machining are the following: • Parts should be designed so that the required electrodes can be shaped economically. • Deep slots and narrow openings should be avoided. • For economic production, the surface finish specified should not be too fine. • In order to achieve high production rate, the bulk of material removal should be done by conventional processes, called roughing out. 27.5.1 Wire EDM An important variation of EDM is wire EDM or electrical-discharge wire cutting. This process is similar to contour cutting with a band saw (Fig. 24.28), in which a slowly moving wire travels along a prescribed path and cuts the workpiece by the EDM action. Figure 27.11a shows a thick plate being cut by this process, on a machine similar to that shown in Fig. 27.11b. Plates as thick as 300 mm, and punches, tools, and dies, made of hard metals, and intricate components for the electronics industry, can be cut by this process. ׉ 7cassandra://ghH_mG63885UNv4_XPDesyGq_lfc-nirusQa7m4uomc)`cZ%Eb…׉E 870 Chapter 27 Advanced Machining Processes Wire Dielectric supply Wire diameter Spark gap Workpiece Slot (kerf) Wire guides Reel Figure 27.10: Schematic illustration of the wire EDM process. As many as 50 hours of machining can be performed with one reel of wire, which is then discarded. The wire travels at a constant velocity in the range from 0.15 to 9 m/min, and a constant gap (kerf) is maintained during the cut. The cutting speed generally is given in terms of the cross-sectional area cut per unit time. Typical examples are 32,000 mm2/hr for 50-mm thick D2 tool steel and 80,000 mm2/hr for 150-mm thick aluminum. These removal rates indicate a linear cutting speed of 32,000/50 = 640 mm/hr = 10.7 mm/min and 80,000/150 = 533 mm/hr = 8.9 mm/min, respectively. The wire is usually made of brass, copper, tungsten, or molybdenum; zinc- or brass-coated, multicoated and steel-cored wires also are used. The wire diameter is typically about 0.30 mm for roughing cuts, and 0.20 mm for finishing cuts. The wire should have high electrical conductivity and tensile strength, as the tension on it is typically 60% of its tensile strength. It usually is used only once, as it is relatively inexpensive compared with the type of operation it performs. Multiaxis EDM wire-cutting machining centers are capable of producing three-dimensional shapes and are equipped with such features as: • Computer controls, for controlling the cutting path of the wire and its angle with respect to the workpiece plane • Multiheads, for cutting two parts at the same time • Controls for preventing wire breakage • Automatic self-threading capability, in case of wire breakage • Programmed machining strategies, to optimize the operation. Two-axis computer-controlled machines can produce cylindrical shapes, in a manner similar to a turning operation or cylindrical grinding. Modern wire EDM machines allow the control of the feed and take-up ends of the wire, in order to traverse independently in two principal directions, so that tapered parts also can be made. ׉ 7cassandra://pGllTqn4Q-NUuh3boRlbOvMDO1GMV8LSGTL5wuEjEb07`cZ%Eb†cZ%Eb…#בCט#u׉׉ 7cassandra://dnLZzlblqSeR-QAxIzP508MdeC4vWguoYM3H2khFOPE`׉ 7cassandra://C20fNy8BgIbO5Co7I_qLm5XqY_-2BZ0ellsrXq6NmbEJu`s׉ 7cassandra://VxGir1ry7GBladXiXG34_1vdKJizD_zy3OAR7EPLRR0h`׉ 7cassandra://YmD-z8l8KnTB4OjSv_1_dL8k3ewom43mJvnhD-t_2Y8i 4͠^cZeEbט#u׉׉ 7cassandra://-J8PCXL1_-nzjxq5KU1VzDCGJbfDA8WwgQOJwM6-LZE`׉ 7cassandra://C5iVlAObZGD4rWbXFNrDE9u_-O6qc-icEh4AqGNxaSYA;`s׉ 7cassandra://gGrNMvrxb3tt1lhJ_asRH2v1l2PdqHoaPlJTmHdukrI`׉ 7cassandra://3hGw7v9I5j0-_YeleTvvNSGTiDaAdwIpr4ITS0ObEsQ͓ :͠^cZeEb׉EWElectrical-discharge Machining 871 (a) Wire spool Wire (b) Figure 27.11: (a) Cutting a thick plate with wire EDM. (b) Acomputer-controlled wire EDM machine. Source: Courtesy of L. Love, Oak Ridge National Laboratory. 27.5.2 Electrical-discharge Grinding The grinding wheel in electrical-discharge grinding (EDG) is made of graphite or brass, and contains no abrasives. Material is removed from the workpiece surface by spark discharges between the wheel and the workpiece. Although this process is used primarily for grinding carbide tools and dies, it can also be used with fragile parts, such as surgical needles, thin-walled tubes, and honeycomb structures. The electrical discharges from the graphite wheel break up the oxide film on the workpiece, and is washed away by the flow of the electrolyte. The material-removal rate can be estimated from the equation MRR = KI, (27.3) where MRR is in mm3/min, I is the current in amperes, and K is a workpiece material factor in units of mm3/A-min; for example, K =4 for tungsten carbide, and K =16 for steel. In EDM sawing, a setup similar to a band or circular saw, but without any teeth, is used with the same electrical circuit as for EDM. Narrow cuts can be made in this way and at high rates of metal removal. Because the cutting forces are negligible, the process can also be used on thin and slender components. ׉ 7cassandra://VxGir1ry7GBladXiXG34_1vdKJizD_zy3OAR7EPLRR0h`cZ%Eb‡׉E872 Chapter 27 Advanced Machining Processes The EDG process can be combined with electrochemical grinding. Called electrochemical-discharge grinding (ECDG), the process uses a graphite wheel and intermittent spark discharges, from alternating current or pulsed direct current. ECDC also commonly uses a highly conductive electrolyte, instead of a dielectric fluid, and lower voltages. The process is faster than EDG, but power consumption is higher. 27.6 Laser-beam Machining In laser-beam machining (LBM), the source of energy is a laser (an acronym for light amplification by stimulated emission of radiation), which focuses optical energy on the workpiece surface (Fig. 27.12a). The highly focused, high-density energy source melts and evaporates portions of the workpiece in a controlled manner. This process, which does not require a vacuum, is used to machine a variety of metallic and nonmetallic materials. There are several types of lasers used in manufacturing operations (Table 27.2): 1. CO2 (pulsed or continuous wave) 2. Nd:YAG (neodymium: yttrium–aluminum–garnet) Flash lamp Laser crystal Reflective end Partially reflective end Lens Workpiece (a) Power supply 3.4-mm diameter 3.1 mm Rubber (b) 1 mm Plastic (c) (d) Figure 27.12: (a) Schematic illustration of the laser-beam machining process. (b) and (c) Examples of holes produced in nonmetallic parts by LBM. (d) Cutting sheet metal with a laser beam. Source: (d) Courtesy of SPI Lasers UK Ltd. ׉ 7cassandra://gGrNMvrxb3tt1lhJ_asRH2v1l2PdqHoaPlJTmHdukrI`cZ%EbˆcZ%Eb‡#בCט#u׉׉ 7cassandra://z6VgzxIgvJP0NUC1fbOaQG01maeYLObLVp2jAMzL8PM`׉ 7cassandra://quxEtoK42AxUW8nTiZxQcnIIgj3Y51_3RzDJhjZ1jQsM`s׉ 7cassandra://Pi7G4tBa66SOzLZtyBy9gntE0OTMep-hXjM2fDbPFBYp`׉ 7cassandra://B_rrC_hafCqoKk6mR4AooQecFqhR8PBDZWENpRedYN0rX͠^cZeEbט#u׉׉ 7cassandra://U2n2KwEHq0o8bz-qX3DiOLB00AAyCZOmX9Bq603tENg`׉ 7cassandra://91GUhCVnskccekDv3_aPIaUbO0TEIMpuoXDKN8zPkbon`s׉ 7cassandra://Dynj_daEUZHzCd4caIcLqjgcSKJxYqGGeCbOR4UQf4o`׉ 7cassandra://H9quH2KE1xev7a4SdBjFktDiCbBfbWCMMfNe9S2pQe0j͠^cZeEb׉E 4Laser-beam Machining 873 3. Nd:glass, ruby 4. Diode 5. Excimer, from the words excited and dimer, meaning two mers, or two molecules of the same chemical composition. Important physical parameters in LBM are the reflectivity and thermal conductivity of the workpiece surface and its specific heat and latent heats of melting and evaporation (Chapter 3). The lower these quantities, the more efficient is the process. The cutting depth may be expressed as t = CP vd , (27.4) where t is the depth, C is a constant for the process, P is the power input, v is the cutting speed, and d is the laser-spot diameter. Peak energy densities of laser beams are in the range from 5 to 200 kW/mm2. The surface produced by LBM is usually relatively rough and with a heat-affected zone (Section 30.9); the zone may have to be removed or heat treated for critical applications. Kerf width is an important consideration, as it is in other cutting processes, such as sawing, wire EDM, and electron-beam machining. In general, the smaller the kerf, the greater is the dimensional accuracy and material utilization, and the lower is the heat affected zone. Laser beams may be used in combination with a gas, such as an oxygen stream, called laser-beam torch, to increase energy absorption for cutting sheet metals. High-pressure, inert-gas assisted laser cutting is used for stainless steel and aluminum, leaving an oxide-free edge which can improve weldability of these metals. Gas streams (nitrogen or argon) also have the important function of blowing away molten and vaporized material from the workpiece surface. Process Capabilities. Laser-beam machining is widely used for hole making, trepanning, and cutting metals, nonmetallic materials, ceramics, and composite materials (Fig. 27.12b and c). The cleanliness of the operation has made laser-beam machining an attractive alternative to traditional machining methods. Holes as small as 0.005 mm, with depth-to-diameter ratios of 50:1, have been made with various materials, although a more practical minimum is 0.025 mm. Steel plates as thick as 32 mm also can be cut with laser beams. Table 27.2: General Applications of Lasers in Manufacturing. Application Laser type Cutting Metals Plastics Ceramics Drilling Metals Plastics Marking Metals Plastics Ceramics PCO2,CWCO2,Nd:YAG, ruby CWCO2 PCO2 PCO2, Nd:YAG, Nd:glass, ruby Excimer PCO2,Nd:YAG Excimer Excimer Surface treatment CWCO2 Welding Metals Plastics Lithography PCO2,CWCO2, Nd:YAG, Nd:glass, ruby, diode Diode, Nd:YAG Excimer Note: P = pulsed, CW= continuous wave, Nd:YAG = neodynmium: yttrium–aluminum–garnet. ׉ 7cassandra://Pi7G4tBa66SOzLZtyBy9gntE0OTMep-hXjM2fDbPFBYp`cZ%Eb‰׉E874 Chapter 27 Advanced Machining Processes Significant cost savings can be achieved by LBM, a process that competes with electrical-discharge machining. It is used increasingly in the electronics and automotive industries, and for composite materials. Two typical examples of laser machining are (a) the cooling holes in some vanes for the Boeing 747 jet engines, and (b) bleeder holes for fuel-pump covers and lubrication holes in transmission hubs. Lasers are also used for the following applications: • Welding (Section 30.7) • Small-scale and localized heat treating of metal and ceramic parts, to modify their surface mechanical and tribological properties • Laser forming and laser peen forming (Section 16.12) • Marking parts, such as letters, numbers, and codes; note that marking also can be done by (a) punches, pins, styluses, and scroll rolls, (b) stamping, and (c) etching. Although the equipment is more expensive than that used in other methods, laser marking and engraving has increasingly become common due to its accuracy, reproducibility, flexibility, ease of automation, and online application in manufacturing. The inherent flexibility of the laser-cutting process, including its fiber-optic beam delivery, simple fixturing, low setup times, availability of multi-kW machines, and two- and three-dimensional computercontrolled robotic laser-cutting systems are competitive and attractive features of laser-beam machining. Laser cutting of sheets, for example, successfully replace traditional punching processes (Chapter 16). Laser beams can be combined with other processes for improved overall efficiency (Section 27.10; see also Example 27.1. Example 27.1 Combining Laser-beam Cutting and Punching of Sheet Metal Laser cutting and punching processes have their respective advantages and limitations regarding both technical and economic aspects (see hybrid machining, Section 27.10). The advantages of laser-beam cutting generally are (a) flexibility of the operation, because hard tooling is not needed and there is no limitation to part size, (b) wide range of material thicknesses, (c) prototyping capability, and lot sizes that can be as low as one, (d) materials and composites that otherwise might be cut with difficulty, and (e) complex geometries that can easily be programmed. Drawbacks and advantages of punching include (a) large lot sizes that economically justify the purchase of tooling and equipment, (b) relatively simple shapes, (c) small range of part thicknesses, (c) fixed and limited punch geometries, even when using turrets, and (d) high production rate. The two processes cover different but complementary ranges. It is not difficult to visualize parts with some features that can be produced best by one process and other features that are best produced by the other process. Machines have been designed and built in such a manner that the processes and fixturing can be utilized jointly (hybrid machines) to their full extent, without interfering with each other’s operational boundaries. The purpose of combining them is to increase the overall efficiency and productivity of the manufacturing process, for parts that are within the capabilities of each of the two processes, similar to the concept of machining centers (Section 25.2). For example, turret-punch presses can be equipped with an integrated laser head; the machine can either punch or laser cut, but it cannot do both simultaneously. Several factors must be taken into account in such a combination of processes with respect to the characteristics of each operation: (a) ranges of sizes, thicknesses, and shapes to be produced, and how they are to be nested (see Fig. 16.59); (b) processing and setup times, including loading, fixturing, and unloading of parts; (c) programming for cutting; and (d) process capabilities of each method, including system dynamics, vibrations, and shock from mechanical punching that may disturb adjustments and alignments of the laser components. ׉ 7cassandra://Dynj_daEUZHzCd4caIcLqjgcSKJxYqGGeCbOR4UQf4o`cZ%EbŠcZ%Eb‰#בCט#u׉׉ 7cassandra://9ximY2XGwxTAtSAd8sNbyVc9hyaWfjS9cOpmLg9kBLcr_`׉ 7cassandra://V1RL1967zXRq_YYMKpo5T9A2uYL-QHz9Sa7jRzEzrosE`s׉ 7cassandra://J4QJVFDPP1TlGe8ehSvfX80nDj2W9kWjEwjXgAEplb4`׉ 7cassandra://bKclStUvmT59R3ltlwcd6eJlz4hs6Aa_X2IVkmqy1_k+MD͠^cZgEbט#u׉׉ 7cassandra://cv0z5UwoEJPqGqMJvnI4nvL8GJbGblDYPqInhN3tyLIO"`׉ 7cassandra://hAcyBmcFcDl3qajt_f8ogQcAvP2uNU1lJXe3OBdoLKcX`s׉ 7cassandra://1ttsWc6C8w0ZwQZ4Z7J1OX9UV0KYyOZb7_YF_EKVy9Aw`׉ 7cassandra://mlTN7JLjpY61ezvBL5f0SbvcIFc8Q30qou3wPqmKkLc͕t͠^cZgEb׉EVElectron-beam Machining 875 Design Considerations for LBM. General design guidelines for laser-beam machining are: • Sharp corners should be avoided, because they can be difficult to produce. • Deep cuts will produce tapered walls. • Dull and unpolished surfaces are preferable. • There can be adverse effects on the properties of the machined materials, caused by high local temperatures and the heat-affected zone. Laser microjet R. Laser microjet R, illustrated in Fig. 27.13, uses a low-pressure, laminar water stream to serve as a variable-length fiber-optic cable to direct the laser and deliver laser power at the bottom of the kerf. This has an advantage in that the laser focus is very deep, and cuts with large aspect ratios can be made. The water jet is produced by a sapphire or diamond nozzle, with an opening of 25 to 100 µm, and exerting a force of less than 0.1 N. In laser microjet R machining, material removal is due to the action of the laser, and the water provides cooling of the heat affected zone, and prevents weld splatter from attaching to the workpiece. The laser is typically a Nd:Yag laser, with micro- or nano-second pulse duration and power between 10 and 200 W. 27.7 Electron-beam Machining The energy source in electron-beam machining (EBM) is high-velocity electrons, striking the workpiece surface and generating heat (Fig. 27.14). The machines utilize voltages in the range from 150 to 200 kV, to accelerate the electrons to 50% to 80% of the speed of light (300,000 km/s). Applications of this process are similar to those of laser-beam machining, except that, unlike lasers, EBM requires a vacuum; consequently, it is used much less frequently than laser-beam machining. Laser Focusing lens Window Water chamber Nozzle Laser guided by water stream Workpiece Figure 27.13: Schematic illustration of the laser microjet R process. ׉ 7cassandra://J4QJVFDPP1TlGe8ehSvfX80nDj2W9kWjEwjXgAEplb4`cZ%Eb‹׉E v876 Chapter 27 Advanced Machining Processes High-voltage cable (30 kV, DC) Cathode grid Optical viewing system Viewing port Vacuum chamber Worktable Workpiece Highvacuum pump Figure 27.14: Schematic illustration of the electron-beam machining process. Unlike LBM, this process requires a vacuum, so the workpiece size is limited to the size of the vacuum chamber. Electron-beam machining can be used for very accurate cutting of a wide variety of metals, with surface finish that is better and the kerf is narrower than in other thermal cutting processes (see also Section 30.6 on electron-beam welding). However, the interaction of the electron beam with the workpiece surface produces hazardous X-rays, thus the equipment should be used only by highly trained personnel. Design Considerations for EBM. The guidelines for EBM generally are similar to those for LBM; additional considerations are: • Because vacuum chambers have limited capacity, individual parts or batches should closely match the size of the vacuum chamber. • If a part requires electron-beam machining on only a small portion of its volume, consideration should be given to making a number of smaller components, then assembling them. Plasma-arc Cutting. In plasma-arc cutting (PAC), plasma beams (ionized gas) are used to rapidly cut ferrous and nonferrous sheets and plates (Section 30.3). The temperatures generated in the torch are on the order of 9400◦C, when using oxygen as a plasma gas. Material-removal rates are thus much higher than those associated with the EDM and LBM processes. The process is rapid, kerf width is small, parts can be machined with good reproducibility, and the surface finish is good; parts as thick as 150 mm can be cut. Plasma-arc cutting is highly automated, using programmable controllers. 27.8 Water-jet Machining The principle of water-jet machining (WJM), also called hydrodynamic machining, is based on the force resulting from the momentum change of a stream of water. This force is sufficiently high to cut metallic and nonmetallic materials (Fig. 27.15). The water jet acts like a saw and cuts a narrow groove in the material (Fig. 27.15b; see also water-jet peening, Section 34.2). A wide variety of materials can be cut, including plastics, rubber, wood products, paper, fabrics, leather, insulating materials, brick, and composite materials (Fig. 27.15c). A pressure level of 400 MPa is generally used for efficient operation, although pressures as high as 1400 MPa are available. Jet-nozzle diameters typically range between 0.05 and 1 mm. The process also can be used for deburring operations. Valve Electron stream Anode Magnetic lens Deflection coils ׉ 7cassandra://1ttsWc6C8w0ZwQZ4Z7J1OX9UV0KYyOZb7_YF_EKVy9Aw`cZ%EbŒcZ%Eb‹#בCט#u׉׉ 7cassandra://vns1ZVoAO5yOCo3IKsG9CtTluA1ztHjzxQ1mvlg0if8`׉ 7cassandra://fxUHufkA5p0xl3ZU_gqGrVdQ5S4K1p1nCL-7-rg2Sj0BC`s׉ 7cassandra://l_mJpryn6vkAKNAEGmT_uOFJe5vrRSL-ySnNCzhnrGk`׉ 7cassandra://sy4lg_RPX1Kf0gYJV0n3IOwd1VoYFfeyVNaKAzSn5c4͛>͠^cZhEbט#u׉׉ 7cassandra://3B3vcUEFUhQCc23-5fmTGulyAj8LQM-ocyCgBYoiK1wk`׉ 7cassandra://oJo5ki7rZGXgiYcT-KqRlAwQs6ZiYgwWkCk5zZyFt4g]`s׉ 7cassandra://TEy3tCXACmSFi0-SS2qsXHtjjrEd0oPsN1k_Yoc4c4Q`׉ 7cassandra://_G1YvBWiVSurLvGymsHDjA0f5S6TpVxnQ9qH4YfGGOQ\&V͠^cZhEb׉EWater-jet Machining 877 Accumulator Controls Valve Fluid supply Mixer and filter Intensifier Pump (a) Control panel y-axis control x-axis control Hydraulic unit Sapphire nozzle Workpiece Water jet Drain Abrasive-jet head Collection tank (b) (c) Figure 27.15: (a) Schematic illustration of the water-jet machining process. (b) A computer-controlled waterjet cutting machine. (c) Examples of various nonmetallic parts produced by the water-jet cutting process. Source: Courtesy of OMAX Corporation. Depending on the materials, thickness can range up to 25 mm and higher. Vinyl and foam coverings for automobile dashboards, as well as some body panels, can be cut by multiple-axis, robot-guided water-jet equipment. Because it is an efficient and clean operation, as compared to most other cutting processes, it is also used in the food-processing industry for cutting and slicing food products. The advantages of WJM are: • Cuts can be started at any location without the need for predrilled holes. • No heat is produced. • No deflection of the rest of the workpiece takes place, thus making the process suitable for flexible materials. • Little wetting of the workpiece takes place. • The burr produced is minimal. • It is an environmentally safe manufacturing operation. ׉ 7cassandra://l_mJpryn6vkAKNAEGmT_uOFJe5vrRSL-ySnNCzhnrGk`cZ%Eb׉E 878 Chapter 27 Advanced Machining Processes AbrasiveWater-jet Machining. In abrasive water-jet machining (AWJM), the water jet contains abrasive particles, such as silicon carbide or aluminum oxide, which greatly increase the material-removal rate. In modern machines, the optimum level of abrasives in the jet stream is controlled automatically. Nozzles are typically of rubies, sapphires, and diamond (Fig. 27.15a). AWJM is particularly suitable for heat-sensitive materials that cannot be machined by processes in which heat is produced. Cutting speeds can be as high as 7.5 m/min for reinforced plastics, but are much lower for metals; consequently, the process may not be economical for applications requiring high production rates. Metallic, nonmetallic, and composite materials of various thicknesses, and in single layer or multilayers, can be cut. With multiple-axis and robot-controlled machines, complex three-dimensional parts can be machined economically to finish dimensions. 27.9 Abrasive-jet Machining In abrasive-jet machining (AJM), abrasive particles are propelled at the workpiece by a high-velocity jet of dry air, nitrogen, or carbon dioxide (Fig. 27.16). The particles impact the surface with a concentrated force (see also Section 26.6) that is sufficiently high to chip away materials. Typical applications are (a) cutting small holes, slots, and intricate patterns in very hard or brittle metallic and nonmetallic materials, (b) deburring or removing small flash from parts, (c) trimming and beveling of edges on parts, (d) removing oxides and other surface films, and (e) cleaning of parts with irregular surfaces. The gas pressure is on the order of 850 kPa; the abrasive-jet velocity can be as high as 300 m/s. Nozzles are usually made of tungsten carbide or sapphire, both of which have abrasive wear resistance. Abrasive particle size is in the range from 10 to 50 µm. Because the flow of the free abrasives tends to round off corners, designs for abrasive-jet machining should avoid sharp corners; also, holes made tend to be tapered, because abrasives preferentially wear the inlet side. There is some hazard involved in using this process, because of airborne particulates. 27.10 Hybrid Machining Systems Two or more individual machining processes can be combined into one system, thus taking advantage of the capabilities of each process. Examples of hybrid machining systems include combinations and integration of the following processes: Filters Powder supply and mixer Exhaust Pressure regulator Gas supply Vibrator Foot control valve (a) (b) Figure 27.16: (a) Schematic illustration of the abrasive-jet machining process. (b) Examples of parts made by abrasive-jet machining, produced in 50-mm thick 304 stainless steel. Source: Courtesy of OMAX Corporation. Hand holder Nozzle Hood Workpiece ׉ 7cassandra://TEy3tCXACmSFi0-SS2qsXHtjjrEd0oPsN1k_Yoc4c4Q`cZ%EbŽcZ%Eb#בCט#u׉׉ 7cassandra://JqfPWT_AAD6CWnHWZw7U3yRRKmby300bnT8KEB2_P3QbB`׉ 7cassandra://4z2hgmBQrRXO2BSENronehx5j5ILRksfPA6FEYEpnyUYP`s׉ 7cassandra://hXg0qW6giMlxsjdXbq2z0FjHoO3lu1nW41AmUfj_tQc`׉ 7cassandra://nexhTuc76fZ4jhDU6RDSokENNL6_mnImsbky4fSrkeAMt͠^cZhEbט#u׉׉ 7cassandra://QQWmo7FXJEjumvpB2OWUHYjIoGBHjMj3c-GrprA2wuI9`׉ 7cassandra://I3IJOYR3hMFeIWROUYR5g9-PFYusHQRHJcradlvuowoJ$`s׉ 7cassandra://BaBV2mkgTd2fdRdhgG5VJrB8_c0e2j3JQ-VifuO0Ms4`׉ 7cassandra://RgGeb8ellxVeSVgr2iNtGvWjPFMQeZ2CJcZ_iAY_wDIPD͠^cZhEb׉E Economics of Advanced Machining Processes 879 1. Abrasive machining and electrochemical machining 2. Abrasive machining and electrical discharge machining 3. Abrasive machining and electrochemical finishing 4. Water-jet cutting and wire EDM 5. High-speed milling, laser ablation, and abrasive blasting, as an example of three integrated processes 6. Machining and blasting 7. Electrochemical machining and electrical discharge machining (ECDM), also called electrochemical spark machining (ECSM) 8. Machining and forming processes, such as laser cutting and punching of sheet metal, described in Example 27.1 9. Combinations of various other forming, machining, and joining processes. The system is able to handle a variety of materials, including metals, ceramics, polymers, and composites. Implementation of these concepts and the development of appropriate machinery and control systems present significant challenges. Important considerations include such factors as: 1. The workpiece material and its manufacturing characteristics (see, for example, Table I.3 in the General Introduction). 2. Compatibility of processing parameters among the two or more processes to be integrated, such as speed, temperature, size, force, and energy. 3. Cycle times for each individual operation involved and their synchronization. 4. Safety considerations and possible adverse effects of the presence of various elements, such as abrasives, chips, chemicals, wear particles, and contaminants. 5. Consequence of a failure in one of the stages in the system, since the operation involves sequential processes. 27.11 Economics of Advanced Machining Processes Advanced machining processes have unique applications, and are important particularly for difficult-tomachine materials and for parts with complex internal and external features. The economic production run for a particular process depends on such factors as the (a) costs of tooling and equipment, (b) operating costs, (c) material-removal rate, (d) level of operator skill required, and (e) secondary and finishing operations that subsequently may be necessary. Case Study 27.2 Manufacturing of Small Satellites Satellites built in the early days of the Space Age (1960s) were very large, and those smaller than 1000 kg were very rare. Table 27.3 shows the classification of modern satellites by their mass. This case study describes the manufacture of propulsion systems for micro- and nanosatellites. There are several compelling reasons for reducing the size of satellites, none greater than the cost of putting the satellite into orbit. One of the main contributors to weight in a satellite is the propulsion system, essential for changing its orbit or correcting for any drift. Figure 27.18a shows the propulsion system ׉ 7cassandra://hXg0qW6giMlxsjdXbq2z0FjHoO3lu1nW41AmUfj_tQc`cZ%Eb׉E F880 Chapter 27 Advanced Machining Processes for a microsatellite, incorporating several cold-gas microthrusters, a propellant storage tank, filters, and temperature and pressure sensors. Selected components of the propulsion system are shown in Fig. 27.17b. Note that production of these miniature parts would be extremely difficult and costly if made through conventional forming, casting, or machining technologies. Moreover, connecting the plumbing for all these components would be very difficult, even with larger components, and almost impossible to perform inside a clean-room environment. An attractive alternative is the production of an integrated system, with fluid connections made internally through a photochemically etched and diffusion-bonded support, on which components are welded or fastened mechanically. Such a support is shown in Fig. 27.19, along with valve springs and filters that are made through a combination of photochemical blanking diffusion-bonding processes. Figure 27.19 depicts the manufacturing sequence involved. Titanium is commonly used for propulsion-system components, because it has a high strength-to-weight ratio, thus making possible lightweight designs. A mask is first prepared (Section 27.2), and the titanium is etched or blanked in a solution of hydrofluoric and nitric acid. Multiple layers of titanium are then diffusion bonded (Section 31.7), to produce internal features, such as flow channels. Such fully-integrated systems have resulted in making satellite propulsion systems that are less complex, more robust, and less massive than those in previous designs. Source: Courtesy of R. Hoppe, VACCO Industries, Inc. Table 27.3: Satellite Classification. Mass Group name Large satellite Medium satellite Minisatellite Small satellites Microsatellite Nanosatellite Picosatellite kg or g > 1000 kg 500–1000 100–500 10–100 1–10 0.1–1 Femtosatellite < 100 g In chemical machining, an inherently slow process, an important factor is the cost of reagents, maskants, and their disposal, together with the cost of cleaning the parts. In electrical-discharge machining, the cost of electrodes and the need to periodically replace them can be significant. The rate of material removal and the production rate can vary significantly, as can be seen in Table 27.1. The cost of tooling and equipment varies considerably, as does the operator skill required. The high capital investment for some machinery (Table 40.6), especially when equipped with robotic control, has to be justified in terms of the production runs and the feasibility of making the same part by other methods. ׉ 7cassandra://BaBV2mkgTd2fdRdhgG5VJrB8_c0e2j3JQ-VifuO0Ms4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://PfKRWQx4PvU7nGBsgyWvMJg3Dm68YVTpdOM0VEw280o`׉ 7cassandra://jRyZitb1htVNRJ59dcI6hwj1TfnqLJPnYIqLaLCgRPwMq`s׉ 7cassandra://1tdIOoQgjBSAfbWDs5O6jGnnEt0dZrWNtH1AXpotzcU`׉ 7cassandra://7IboOxHdOT_4ujSBeFHkhtYyErbawW3iFtuJJ0nXUkU|͠^cZiEbט#u׉׉ 7cassandra://e-FFg2IgD4Cghjj_sTQ46wdHhJHjDvQ2CZmaurLxs4g`׉ 7cassandra://xBwnBame0Jqh1fZvSBFMTtADulH85Ynj7fyRm-0NdNMI`s׉ 7cassandra://igfRq25pOCw6xWuw_r1xmq0EcmzajGn8taAn6ryz6Y4`׉ 7cassandra://lU6gT2NzJop9z0cTwl8pqOouAcjntWrd5OgjcewftZUͰ'͠^cZiEb׉E?Summary 881 (a) (b) Figure 27.17: Propulsion system for a small satellite. (a) Miniaturized system suitable for a micro- or nanosatellite, and (b) selected propulsion system components. Source: Courtesy of R. Hoppe, VACCO Industries, Inc. (a) (b) (c) Figure 27.18: Photochemically etched and blanked components for micro- and nanosatellites. (a) Mounting board incorporating fluid flow channels in an integrated package, (b) microscale valve spring placed next to a U.S. penny, and (c) fuel filter. Source: Courtesy of R. Hoppe, VACCO Industries, Inc. Summary • Advanced machining processes have unique capabilities, utilizing chemical, electrochemical, electrical, and high-energy-beam sources of energy. • Mechanical properties of the workpiece material are not significant, because these processes rely on mechanisms that do not involve strength, hardness, ductility, or toughness of the material; rather, they involve physical, chemical, and electrical properties. • Chemical and electrical methods of machining are particularly suitable for hard materials and complex part shapes. They do not exert forces on the workpiece, and therefore can be used for thin, slender, and flexible workpieces. However, their effects on surface integrity must be considered, as they can damage surfaces, reducing the fatigue life of the parts made. ׉ 7cassandra://1tdIOoQgjBSAfbWDs5O6jGnnEt0dZrWNtH1AXpotzcU`cZ%Eb‘׉Ez882 Chapter 27 Advanced Machining Processes (a) (b) (c) (d) (e) (f) Figure 27.19: Processing sequence for photochemical etching of microsatellite components: (a) clean the raw material, (b) coat with photosensitive material, (c) expose with photographic tool, (d) develop a resist image, (e) etch, and (f) remove the resist. Source: Courtesy of R. Hoppe, VACCO Industries, Inc. ׉ 7cassandra://igfRq25pOCw6xWuw_r1xmq0EcmzajGn8taAn6ryz6Y4`cZ%Eb’cZ%Eb‘#בCט#u׉׉ 7cassandra://N4NQJjH_eGzWBfYcToOhPW7PTEUxCRCQkJabj2puxDM'`׉ 7cassandra://0QizyOMUReFsB6ubqQLCsn0E46CGi-rdmzsyVm-OhlYY`s׉ 7cassandra://P_yPhKkgVwKhARNvChnbyp06f0OMWfQH_5ujn5wh_fo`׉ 7cassandra://P15pq_4yWerJf1k6Tm0nmdMTSQUAez2ga8MzPbYyt0AN͠^cZiEbט#u׉׉ 7cassandra://wP1wYpdCp4H1kE88TtW_Ua-g3m3EUWewiDbEjeCSPz0i`׉ 7cassandra://RPYEfsENHhpR07L7w14n-AGLmQjWPjbsusfefOPMT8sh`s׉ 7cassandra://jU1y5JJLzjspv9YZ7GlL1MFk226PXWdTQiO5cB9RYiQ`׉ 7cassandra://LQgle1onToB5jlTFKR22rzd7SRP-G-Q0GIpgDeaWU4oS͠^cZiEb׉EBibliography 883 • High-energy-beam machining processes basically utilize laser beams, electron beams, and plasma beams. They have important industrial applications, possess high flexibility of operation, with robotic controls, and are economically competitive with other processes. • Water-jet machining, abrasive water-jet machining, and abrasive-jet machining processes can be used for cutting as well as for deburring operations. Because they do not utilize hard tooling, they have an inherent flexibility of operation. • Hybrid machining processes offer possibilities for more efficient production of complex parts. Key Terms Abrasive-jet machining Abrasive water-jet machining Blue arc machining Chemical blanking Chemical machining Chemical milling Dielectric Die sinking Electrical-discharge grinding Electrical-discharge machining Electrochemical-discharge grinding Electrochemical grinding Electrochemical honing Electrochemical machining Electrode Electrolyte Electrolytic trepanning Electron-beam machining Etchant Bibliography Asibu, Jr., E.K., Principles of Laser Materials Processing, Wiley, 2009. Bass, M., Laser Material Processing, Elsevier, 2012. Dahotre, N.B., and Samant, A., Laser Machining of Advanced Materials, CRC Press, 2011. El-Hofy, H., Advanced Machining Processes: Nontraditional and Hybrid Machining Processes, McGrawHill, 2005. Grzesik, W., Advanced Machining Processes of Metallic Materials: Theory, Modelling and Applications, Elsevier, 2008. Kibria, G., Bhattacharyya, B., and Davim, J.P., Non-traditional Micromachining Processes, Springer, 2017. Ion, J.C., Laser Processing of Engineering Materials, Butterworth-Heinemann, 2011. Luo, X., and Qin, Y., Hybrid Machining, Academic Press, 2018. Hybrid machining Hydrodynamic machining Laser Laser-beam machining Laser microjet No-wear EDM Photochemical blanking Photochemical machining Photoetching Photoresist Plasma-arc cutting Plasma beams Pulsed electrochemical machining Reagent Shaped-tube electrolytic machining Undercut Water-jet machining Wire EDM ׉ 7cassandra://P_yPhKkgVwKhARNvChnbyp06f0OMWfQH_5ujn5wh_fo`cZ%Eb“׉E ,884 Chapter 27 Advanced Machining Processes Marinescou, I.D., Uhlmann, E., and Doi, T., Handbook of Lapping and Polishing, CRC Press, 2006. Schaaf, P., (ed.), Laser Processing of Materials, Springer, 2010. Schaeffer, R.D., Fundamentals of Laser Micromachining, CRC Press, 2012. Stafe, N., Marcu, A., and Puscas, N.N., Pulsed Laser Ablation of Solids, Springer, 2013. Steen, W.M., and Mazumder, J., Laser Material Processing, 4th ed., Springer, 2010. Review Questions 27.1. Describe the similarities and differences between chemical blanking and conventional blanking using dies. 27.2. Name the processes involved in chemical machining. Describe briefly their principles. 27.3. Explain the difference between chemical machining and electrochemical machining. 27.4. What is the underlying principle of electrochemical grinding? 27.5. Explain how the EDM process is capable of producing complex shapes. 27.6. What are the important features of the blue arc machining process? 27.7. What are the capabilities of wire EDM? Could this process be used to make tapered parts? Explain. 27.8. Explain why laser microjet has a large depth of field. 27.9. Describe the advantages of water-jet machining. 27.10. What is the difference between photochemical blanking and chemical blanking? 27.11. What is kerf? 27.12. What type of workpiece is not suitable for laser-beam machining? 27.13. What is an undercut? Why must it be considered in chemical machining? 27.14. Explain the principle of hybrid machining. Qualitative Problems 27.15. Give technical and economic reasons that the processes described in this chapter might be preferred over those described in the preceding chapters. 27.16. Why is the preshaping or premachining of parts sometimes desirable in the processes described in this chapter? 27.17. Why is the material removal rate in electrical-discharge machining a function of the melting point of the workpiece material? 27.18. Explain why the mechanical properties of workpiece materials are not significant in most of the processes described in this chapter. 27.19. List the processes that can produce shaped holes, that is, holes that are not circular. 27.20. List the advantages of laser microjet over conventional laser machining. 27.21. Why has electrical-discharge machining become so widely used in industry? 27.22. Describe the types of parts that are suitable for wire EDM. 27.23. Which of the advanced machining processes would cause thermal damage? What is the consequence of such damage to workpieces? 27.24. Which of the processes described in this chapter require a vacuum? Explain why? ׉ 7cassandra://jU1y5JJLzjspv9YZ7GlL1MFk226PXWdTQiO5cB9RYiQ`cZ%Eb”cZ%Eb“#בCט#u׉׉ 7cassandra://h-8f08LiwtR5vrfJ4-5W7cewU-4DYUZMX8NzYbYqQBk`׉ 7cassandra://fO6S8cA5RISru1fiu4SnD3FRNQ0qmT67ig1cz54RirM^`s׉ 7cassandra://kleUarzzflhW3G-VhJBADTVY7ECxP7fEof8j2ZAS2FQO`׉ 7cassandra://UEUd6YL_k5o0Z3IronZOsRTcue9H3cBgKTEmcmG3oykQ{͠^cZiEbט#u׉׉ 7cassandra://nj3ksoNi8ww5KcAXmsMUwHh0IsqHj3DppiBAPU89_2Y<`׉ 7cassandra://VrZv-zxKWW76b9zNsNNYisr4cPq1QojobTbtIwDMEe4Y`s׉ 7cassandra://D-aJS8mxiio3obiXzzG2QCaxPZ0FufxT9H3Tf0ZSPT4>`׉ 7cassandra://hRvopDijt17-SESC5o0xGoaObZpXr9xTCgG95LYx0CYL͠^cZjEb׉E Synthesis, Design, and Projects 885 27.25. Describe your thoughts regarding the laser-beam machining of nonmetallic materials. Give several possible applications, including their advantages compared with other processes. 27.26. Are deburring operations still necessary for some parts made by advanced machining processes? Explain and give several specific examples. 27.27. List and explain factors that contribute to a poor surface finish in the processes described in this chapter. 27.28. What is the purpose of the abrasives in electrochemical grinding? 27.29. Which of the processes described in this chapter are suitable for producing very small and deep holes? Explain. 27.30. Is kerf width important in wire EDM? Explain. 27.31. Why may different advanced machining processes affect the fatigue strength of materials to different degrees? 27.32. What are the functions of the fluid in EDM? Quantitative Problems 27.33. A 60-mm-deep hole, 30 mm in diameter, is being produced by electrochemical machining. A high production rate is more important than the quality of the machined surface. Estimate the maximum current and the time required to perform this operation. 27.34. If the operation in Problem 27.33 were performed on an electrical-discharge machine, what would be the estimated machining time? 27.35. A cutting-off operation is being performed with a laser beam. The workpiece being cut is 5 mm thick and 100 mm long. If the kerf width is 3 mm, estimate the time required to perform this operation. 27.36. A 20-mm-thick copper plate is being machined by wire EDM. The wire moves at a speed of 1.2 m/min and the kerf width is 1.6 mm. What is the required power? Note that it takes 1550 J to melt one gram of copper. Synthesis, Design, and Projects 27.37. It was stated that graphite is the preferred material for EDM tooling. Would graphite be useful in wire EDM? Explain. 27.38. Explain why it is difficult to produce sharp profiles and corners with some of the processes described in this chapter. 27.39. Make a list of the processes described in this chapter in which the following properties are relevant: (a) mechanical, (b) chemical, (c) thermal, and (d) electrical. Are there processes in which two or more of these properties are important? Explain. 27.40. Would the processes described in this chapter be difficult to perform on various nonmetallic or rubberlike materials? Explain your thoughts, commenting on the influence of various physical and mechanical properties of workpiece materials, part geometries, etc. 27.41. Describe the types of parts that would be suitable for hybrid machining. Consider one such part and make a preliminary sketch for a hybrid machine to produce that part. 27.42. Describe your thoughts as to whether the processes described in (a) Chapters 13 through 16, and (b) Chapters 23 and 24 can be suitable for a hybrid system of making parts. Give a preliminary sketch of a machine for the two groups of processes listed. ׉ 7cassandra://kleUarzzflhW3G-VhJBADTVY7ECxP7fEof8j2ZAS2FQO`cZ%Eb•׉E 5886 Chapter 27 Advanced Machining Processes 27.43. Make a list of machining processes that may be suitable for each of the following materials: (a) ceramics, (b) cast iron, (c) thermoplastics, (d) thermosets, (e) diamond, and (f) annealed copper. 27.44. At what stage is the abrasive in abrasive water-jet machining introduced into the water jet? Survey the available literature, and then prepare a schematic illustration of the equipment involved. 27.45. How would you manufacture a large-diameter, conical, round metal disk with a thickness that decreases from the center outward? Make appropriate sketches. 27.46. Describe the similarities and differences among the various design guidelines for the processes described in this chapter. 27.47. Describe any workpiece size limitations in advanced machining processes. Give examples. 27.48. Suggest several design applications for the types of parts shown in Fig. 27.4. 27.49. Based on the topics covered in Parts III and IV, make a comprehensive table of hole-making processes. Describe the advantages and limitations of each method, and comment on the quality and surface integrity of the holes produced. 27.50. Review Example 27.1 and explain the relevant parameters involved; then design a system whereby both processes can be used in combination to produce parts from sheet metal. 27.51. Marking surfaces with numbers and letters for part-identification purposes can be done with a variety of mechanical and nonmechanical methods. Based on the processes described throughout this book thus far, make a list of these methods, explaining their advantages, limitations, and typical applications. 27.52. Precision engineering is a term that is used to describe manufacturing high-quality parts with close dimensional tolerances and good surface finish. Based on their process capabilities, make a list of advanced machining processes with decreasing order of the quality of parts produced. Comment on your observations. 27.53. With appropriate sketches, describe the principles of various work-holding methods and workholding devices that can be used for the processes described in this chapter. 27.54. Make a table of the process capabilities of the advanced machining processes described in this chapter. Use several columns and describe the machines involved, the type of tools and tool materials used, the shapes of blanks and parts produced, the typical maximum and minimum sizes, surface finish, tolerances, and production rates. 27.55. One of the general concerns regarding advanced machining processes is that, in spite of their many advantages, they generally are slower than conventional machining operations. Conduct a survey of the speeds, machining times, and production rates involved, and prepare a table comparing their respective process capabilities. 27.56. It can be seen that several of the processes described in Part IV of this book can be employed, either singly or in combination, to make or finish dies for metalworking operations. Write a brief technical paper on these methods, describing their advantages, limitations, and typical applications. ׉ 7cassandra://D-aJS8mxiio3obiXzzG2QCaxPZ0FufxT9H3Tf0ZSPT4>`cZ%Eb–cZ%Eb•#בCט#u׉׉ 7cassandra://McyVDeWesLEVMuHotwp5OqUauoeYdtEsTI9OCzZp-H0wA`׉ 7cassandra://LIppjEqaIm6MwT38QpKogQe8iDMt17I8RMDBVBr-M6wa`s׉ 7cassandra://WwGs_HbI1_5R7HeMrp7Zk_ZdUPjtEKi7-Oa3p56uNRk`׉ 7cassandra://iuPTqD_cjhjWVmFjx9me4Nm0_m5epATYdhqqSJXt0qYt͠^cZjEbט#u׉׉ 7cassandra://Dh0OTCuiMib2V4F6JLD__ilObTGrcMsnJHJjmbVEf0o`׉ 7cassandra://LajKpI0lAzbLe8eOUi0hfgYBH65ZwwI1Ijqm0rxJIMEU`s׉ 7cassandra://XnXaimXzOpmJ9cGbzTjy-wobikF11pDKs8JZsIWyE6ME`׉ 7cassandra://4EhzspxsOTe1H-xZLvoDa5AhGEFZwAIRFPEFv6xhcPM͠^cZkEb׉E |PART VI Joining Processes and Equipment Some products, such as paper clips, nails, steel balls for bearings, screws, and bolts, are made of only one component; however, almost all products are assembled from components that have been made as individual parts. Even relatively simple products consist of at least two components, joined by various means. Note, for example: (a) the eraser at the end of an ordinary pencil is attached with a metal sleeve; (b) knives have wooden or plastic handles that are attached to the metal blade with fasteners; and (c) cooking pots and pans have metal, plastic, or wooden handles and knobs, attached to the pot by various methods. On a much larger scale, observe power tools, washing machines, motorcycles, ships, and airplanes, and how their numerous components are assembled and joined so that they not only can function reliably, but also are economical to produce. As shown in Table I.1 in the General Introduction, a rotary lawn mower has about 300 parts, a typical automobile has 15,000 components, and a Boeing 747-400 aircraft has more than 6 million parts. In contrast, a Boeing 787 Dreamliner has fewer parts because its composite fuselage eliminates a large number of fasteners. Joining is an all-inclusive term covering processes such as welding, brazing, soldering, adhesive bonding, and mechanical fastening. These processes are an essential and important aspect of manufacturing and assembly for one or more of the following reasons: 1. Even a relatively simple product may be impossible to manufacture as a single piece. Consider, for example, the tubular construction shown in Fig. VI.2a. Assume that each of the arms of this product is 5 m long, the tubes are 100 mm in diameter, and their wall thickness is 1 mm. After reviewing all of the manufacturing processes described in the preceding chapters, one would conclude that manufacturing this product in one piece would be impossible or uneconomical. 2. A product such as a cooking pot, with a handle, is easier and more economical to manufacture as assembly of individual components. 3. Products such as appliances, automobile engines, and hair dryers must be designed so as to be able to be easily taken apart for maintenance or for replacement of their worn or broken parts. Bonding of windshield to car body Fasteners Bolted engine assembly Soldered electrical circuitry Brazed joint for emission control Adhesively bonded fabric Spot-welded car body Mechanical fastening of body trim Welded pipes for exhaust system Seamed body components Seam-welded muffler Figure VI.1: Various parts in a typical automobile that are assembled by the processes described in Part VI. 968 ׉ 7cassandra://WwGs_HbI1_5R7HeMrp7Zk_ZdUPjtEKi7-Oa3p56uNRk`cZ%Eb—׉E fPart VI Joining Processes and Equipment 969 Carbide insert Braze Drill body (low-alloy steel) Tubes (a) (b) (c) Figure VI.2: Examples of parts utilizing joining processes. (a) A tubular part fabricated by joining individual components. This product cannot be manufactured in one piece by any of the methods described in the previous chapters if it consists of thin-walled, large-diameter, tubular-shaped long arms. (b) A drill bit with a carbide cutting insert brazed to a steel shank—an example of a part in which two materials need to be joined for performance reasons. (c) Spot welding of automobile bodies. Source: (c) Shutterstock/Jensen. 4. Different properties are often desirable for functional purposes of a product. For example, surfaces subjected to friction, wear, corrosion, or environmental attack generally require characteristics that are significantly different from those of the component’s bulk. Examples are: (a) masonry drills with carbide cutting tips, brazed to the shank of a drill (Fig. VI.2b); (b) automotive brake shoes attached to their support with rivets; and (c) grinding wheels bonded to a metal backing (Section 26.2). 5. Transporting the product in individual components and assembling them later may be easier and less costly than transporting the completed product. Note, for example, that metal or wood shelving, backyard grills, and large machinery are assembled after the components or subassemblies have been transported to their intended sites. Although there are different ways of categorizing the wide variety of available joining processes, they basically fall into the following three major categories (Figs. VI.3 and I.7f): • Welding • Adhesive bonding • Mechanical fastening Table VI.3 lists the general characteristics of various joining processes, and welding processes, in turn, are generally classified into three basic categories: • Fusion welding • Solid-state welding • Brazing and soldering As will be shown later, some types of welding processes can be classified into both the fusion and the solid-state categories. Fusion welding is defined as the melting together and coalescing of materials by means of heat, usually supplied by chemical or electrical means; filler metals may or may not be used. Fusion welding is composed of consumable- and nonconsumable-electrode arc welding and high-energy-beam welding processes. The welded ׉ 7cassandra://XnXaimXzOpmJ9cGbzTjy-wobikF11pDKs8JZsIWyE6ME`cZ%Eb˜cZ%Eb—#בCט#u׉׉ 7cassandra://jJwTJeS9sOZDQBL-lITD4_HPHUfu4BVIHMr066XukvA`׉ 7cassandra://MmHBONrgQZdyOU-0BJsF4b-pS14nhFNlme7PmXWvzroEf`s׉ 7cassandra://bZ0V6wTD53vY8_0csETUKu3kSGcT1WqSAH3AhBs74Tw,`׉ 7cassandra://ilmgheL0o6afZrMYDo9MIFwh6tIK5RIjToJR2nyW1Rw&,͠^cZkEbט#u׉׉ 7cassandra://8J0wUutZNuTmolaBBNt4Gj9bx-N7knTsw13tXhhA3nco`׉ 7cassandra://ITLgRu8UOCLppOyZgVdMq9q3yOQFwurdff5BgxhE6vk!`s׉ 7cassandra://Y6i0ShwbLOJghrg-ZWyItacts4ueqLvxh07OjAFQxNc `׉ 7cassandra://vWKAT3muPptTKLb6Zsi2tqrQwz4-6xVxMbQaFDNo9vwG̶͠^cZlEb׉E|970 Part VI Joining Processes and Equipment Joining processes and equipment The weld joint, quality, and testing (Chapter 30) Safety and environmental considerations Welding Adhesive bonding (Chapter 32) Fusion Brazing and soldering (Chapter 32) Chemical Oxyfuel gas Thermit Electrical Arc Resistance Electron beam Laser beam (Chapter 30) Electrical Resistance Solid state Mechanical fastening Fastening Seaming Crimping Stitching (Chapter 32) Chemical Diffusion Explosion (Chapter 31) Figure VI.3: Outline of topics described in Part VI. joint undergoes major metallurgical and physical changes, which, in turn, have a major influence on the properties and performance of the welded assembly. The terminology for some simple welded joints are illustrated in Fig. VI.4. In solid-state welding, joining takes place without fusion; consequently, there is no liquid (molten) phase in the joint. The basic processes in this category are diffusion bonding and cold, ultrasonic, friction, resistance, and explosion welding. Brazing uses filler metals and involves lower temperatures than in welding. Soldering uses filler metals (solders) and involves even lower temperatures. Mechanical Cold Friction Ultrasonic (a) Butt joint (b) Corner joint (c) T joint (d) Lap joint (e) Edge joint Figure VI.4: Examples of joints that can be made through the various joining processes described in Chapters 30 through 32. ׉ 7cassandra://bZ0V6wTD53vY8_0csETUKu3kSGcT1WqSAH3AhBs74Tw,`cZ%Ebš׉EPart VI Joining Processes and Equipment 971 Table VI.3: Comparison of Various Joining Methods. Characteristics Method Arc welding Resistance welding Brazing Bolts and nuts Rivets Seaming and crimping Adhesive bonding 1 2 3 1 3 1 2 2 2 1 2 2 2 1 3 1 1 3 3 2 3 3 1 2 1 3 1 3 3 1 1 1 1 1 1 3 1 3 2 3 1 1 3 1 2 3 1 1 1 3 1 2 1 3 1 1 2 3 2 3 3 2 Note: For cost, 1 is the lowest. Adhesive bonding has unique applications requiring strength, sealing, thermal and electrical insulating, vibration damping, and resistance to corrosion between dissimilar metals. Mechanical fastening involves methods of using various fasteners, such as bolts, nuts, and rivets. The joining of plastics can be accomplished by adhesive bonding, fusion by various external or internal heat sources, and mechanical fastening. 1 Strength Design Small parts Large parts Tolerances Reliability Ease of manufacture Ease of inspection Cost ׉ 7cassandra://Y6i0ShwbLOJghrg-ZWyItacts4ueqLvxh07OjAFQxNc `cZ%Eb›cZ%Ebš#בCט#u׉׉ 7cassandra://jDn6TNNF1nREpoka85WwYQ1msG0xKyUhJM1LtezHoT0`׉ 7cassandra://NmG6X93WqaEqIKeDJJ6Wv5ejd2rGtMsYW6DOgi526Wo>`s׉ 7cassandra://Go-F8Q3uYmUiV26ZGtnK-Q0rdEKgey-EBKGnJuxiCHA:`׉ 7cassandra://UEDgm5pSHHxdsc-j7vclMBuYlb39RluuLBr_bQJDBFs?͠^cZlEbט#u׉׉ 7cassandra://kRKP-m4xyycrJI3uITxOUUFAEhjHW7Nd8L764arvb0w+`׉ 7cassandra://cDDbvmiEWcPCLHKF2rO5WyiCtJkQUBbrAX28bq_4YRAV`s׉ 7cassandra://L5LfeazNYbzukUL2oAjt-0ssdJ6bp4t1dX9llUalFWgH`׉ 7cassandra://IjuBpxw9zs5ncJqhkUBtWsXEsCrvckB7WqIsW3ChGBgwB͠^cZlEb׉E Chapter 30 Fusion Welding Processes 30.1 Introduction 973 30.2 Oxyfuel–Gas Welding 973 30.3 Arc-welding Processes: Nonconsumable Electrode 977 30.4 Arc-welding Processes: Consumable Electrode 980 30.5 Electrodes for Arc Welding 985 30.6 Electron-beam Welding 987 30.7 Laser-beam Welding 988 30.8 Cutting 990 30.9 The Weld Joint, Quality, and Testing 991 30.10 Joint Design and Process Selection 1001 Examples: 30.1 Welding Speed for Different Materials 978 30.2 Laser Welding of Razor Blades 989 30.3 Weld Design Selection 1004 • This chapter describes fusion-welding processes, in which two pieces are joined together by applying heat, which melts and fuses the interface; the operation is sometimes assisted with a filler metal. • All fusion-welding processes are described in this chapter, beginning with oxyfuel–gas welding in which acetylene and oxygen provide the energy required for welding. • Various arc-welding processes are then described, in which electrical energy and consumable or nonconsumable electrodes are used to produce the weld; specific processes reviewed include shielded metal arc welding, flux-cored arc welding, gas tungsten-arc welding, submerged arc welding, and gas metal-arc welding. • Welding with high-energy beams is then described, in which electron beams or lasers provide highly focused heat sources. • The chapter concludes with a description of the nature of the weld joint, including weld quality, inspection, and testing procedures, along with weld design practices and process selection. 972 ׉ 7cassandra://Go-F8Q3uYmUiV26ZGtnK-Q0rdEKgey-EBKGnJuxiCHA:`cZ%Ebœ׉E Oxyfuel–Gas Welding 973 30.1 Introduction The welding processes described in this chapter involve the partial melting and fusion between two members to be joined. Fusion welding is defined as melting together and coalescing materials by means of heat. Filler metals, which are added to the weld area during welding, also may be used. Welds made without the use of filler metals are known as autogenous welds. This chapter covers the basic principles of each welding process; the equipment used; the relative advantages, limitations, and capabilities of the process; and the economic considerations affecting process selection (Table 30.1). The chapter continues with a description of the weld zone and the variety of discontinuities and defects that can exist in joints. Weldability of ferrous and nonferrous metals and alloys are then reviewed. The chapter concludes with design guidelines for welding, with several examples of good weld-design practices, and the economics of welding. 30.2 Oxyfuel–Gas Welding Oxyfuel–gas welding (OFW) is a general term used to describe any welding process that uses a fuel gas combined with oxygen to produce a flame, the source of the heat required to melt the metals at the joint. The most common gas-welding process uses acetylene, known as oxyacetylene–gas welding (OAW), typically used for structural metal fabrication and repair work. Developed in the early 1900s, OAW utilizes the heat generated by the combustion of acetylene gas (C2H2) in a mixture with oxygen. The primary combustion process, which occurs in the inner core of the flame (Fig. 30.1), involves the following reaction: C2H2 +O2 → 2CO+H2 + Heat. (30.1) This reaction dissociates the acetylene into carbon monoxide and hydrogen, and produces about one-third of the total heat generated in the flame. The secondary combustion process is 2CO+H2 +1.5O2 → 2CO2 +H2O+ Heat. Joining process Shielded metal arc Submerged arc Table 30.1: General Characteristics of Fusion-welding Processes. Skill level Welding Operation Manual Automatic Advantage Portable and flexible High deposition Gas tungsten arc Oxyfuel Manual or automatic Manual Most metals required High Low to medium Gas metal arc Semiautomatic Most metals Low to high or automatic Low to high Flux-cored arc Semiautomatic High deposition Low to high or automatic Portable and flexible Thermit Steels High Electron beam, Semiautomatic Most metals Medium to laser beam or automatic Manual high Low * 1 = highest; 5 = lowest position All Flat and horizontal All All All All All Flat and horizontal Current type AC, DC AC, DC DC AC, DC DC – – – Distortion* 1 to 2 1to (30.2) Typicalcostof equipment ($) Low (1500+) Medium (5000+) 2 to 3 Medium (5000+) 2to Medium (2000+) 1 to 3 Medium (2000+) 2to4 3 to 5 2to4 Low (500+) High (100,000–1 million) Low (500+) ׉ 7cassandra://L5LfeazNYbzukUL2oAjt-0ssdJ6bp4t1dX9llUalFWgH`cZ%EbcZ%Ebœ#בCט#u׉׉ 7cassandra://y9N1g39UsZj-6ih5t-tVqc-SIUouwyo3WY1RWVvwNcErP`׉ 7cassandra://ldigtAsE3nCaLmgnFJdwwt3cEyCDhD-3GJLsFZK4hi4\`s׉ 7cassandra://qWl4Z5jSwgjYZdPmxMcl3vCwy0K2_Iw1uAC_rYH1T4Yv`׉ 7cassandra://sE_-8AsyynmE6nWVlokkTCvfK29nbVBz7-3a_8Z1oxA>͠^cZlEbט#u׉׉ 7cassandra://FznbddKEdcNjwoE_UnFSETmQ4xdaj-lKc1IgJHQ3EWw%`׉ 7cassandra://Ux90vqyN4hcq9f6IHil8EzFAFovMP4jBYSyflTA__9oO`s׉ 7cassandra://1EuWy1vwTTTrHjt_6WBNvOZkj6658yeq-D8K_99FyXk`׉ 7cassandra://SQTpRCNI0kpYmYX0r80HxkRFsSMgTz75dP53q24L5Lo!͂ ͠^cZmEb׉E }974 Chapter 30 Fusion Welding Processes This reaction consists of the further burning of both the hydrogen and the carbon monoxide, producing about two-thirds of the total heat. The temperatures developed in the flame can reach 3300◦C. Note from Eq. (30.2) that the reaction also produces water vapor. Types of Flames. The proportion of acetylene and oxygen in the gas mixture is an important factor in oxyfuel–gas welding. At a ratio of 1:1 (i.e., when there is no excess oxygen), the flame is considered to be neutral (Fig. 30.1a). With higher oxygen supply, the flame oxidizes the metal (especially steels), and hence known as an oxidizing flame (Fig. 30.1b). Only in the welding of copper and copper-based alloys is an oxidizing flame desirable, because in those cases, a thin protective layer of slag (compounds of oxides) forms over the molten metal. If the oxygen is insufficient for full combustion, the flame is known as a reducing or carburizing flame (Fig. 30.1c). The temperature of a reducing flame is lower; hence, it is suitable for applications requiring low heat, such as in brazing and soldering (Chapter 32), and flame-hardening (Table 4.1). Other fuel gases, such as hydrogen and methylacetylene propadiene, also can be used in oxyfuel–gas welding. However, the temperatures developed by these gases are lower than those produced by acetylene. Therefore, they are used for welding metals with low melting points, such as lead, and parts that are small and thin. Filler Metals. Filler metals are used to supply additional metal to the weld zone, and are available as filler rods or wire (Fig. 30.1d) and may be bare or coated with flux. The purpose of the flux is to retard oxidation of the surfaces of the parts being welded by generating a gaseous shield around the weld zone. The flux also helps to dissolve and remove oxides and other substances from the weld zone, thus making the joint stronger. The slag developed (compounds of oxides, fluxes, and electrode-coating materials) protects the molten puddle of metal against oxidation as the weld cools. Welding Practice and Equipment. Oxyfuel–gas welding can be used with most ferrous and nonferrous metals and for almost any workpiece thickness; however, the relatively low heat input limits this process 21008C 12608C Outer envelope (small and narrow) Acetylene feather Inner cone 3040 to 33008C (a) Neutral flame Outer envelope Inner cone (pointed) (b) Oxidizing flame Bright luminous inner cone Blue envelope (c) Carburizing (reducing) flame Gas mixture Filler rod Molten weld metal Base metal (d) Figure 30.1: Three basic types of oxyacetylene flames used in oxyfuel–gas welding and cutting operations: (a) neutral flame; (b) oxidizing flame; (c) carburizing, or reducing, flame. The gas mixture in (a) is basically equal volumes of oxygen and acetylene. (d) The principle of the oxyfuel–gas welding process. Welding torch Flame Solidified weld metal ׉ 7cassandra://qWl4Z5jSwgjYZdPmxMcl3vCwy0K2_Iw1uAC_rYH1T4Yv`cZ%EbŸ׉EOxyfuel–Gas Welding 975 Valves Mixer Tip Enlarged view (a) Oxygen Acetylene Torch head Oxygen Union nut Mixer (b) Gas regulators Oxygen cylinder Combustiblegas cylinder (c) Figure 30.2: (a) General view of, and (b) cross section of, a torch used in oxyacetylene welding. The acetylene valve is opened first. The gas is lit with a spark lighter or a pilot light. Then the oxygen valve is opened and the flame is adjusted. (c) Basic equipment used in oxyfuel–gas welding. To ensure correct connections, all threads on acetylene fittings are left handed, whereas those for oxygen are right handed. Oxygen regulators usually are painted green and acetylene regulators red. to thicknesses of less than 6 mm. Small joints made by this process may consist of a single-weld bead; deepV groove joints are made in multiple passes. Cleaning the surface of each weld bead prior to depositing a second layer over it is important for joint strength and in avoiding defects (see Section 30.9). Wire brushes (hand or power) may be used for this purpose. The equipment for oxyfuel–gas welding basically consists of a welding torch, connected by hoses to high-pressure gas cylinders, equipped with pressure gages and regulators (Fig. 30.2). The use of safety equipment, such as goggles with shaded lenses, face shields, gloves, and protective clothing, is essential. Proper connection of the hoses to the cylinders also is an important factor in safety. The oxygen and acetylene cylinders have different threads, so that the hoses cannot be connected to the wrong cylinders. Although it can be mechanized, the operation is essentially manual, and therefore slow. It has, however, the advantages of being portable, versatile, and economical for simple and low-quantity work. Pressure-gas Welding. In this method, welding of two components starts with first heating the interface by means of a torch, using typically an oxyacetylene–gas mixture (Fig. 30.3a). After the interface begins to melt, the torch is withdrawn; a force is then applied to press the two components together and is maintained until the interface solidifies. Note in Fig. 30.3b the formation of a flash due to the upsetting of the joined ends of the two components. Welding tip Flame Tip Mixing chamber Gas control valves Hoses Welding torch ׉ 7cassandra://1EuWy1vwTTTrHjt_6WBNvOZkj6658yeq-D8K_99FyXk`cZ%Eb cZ%EbŸ#בCט#u׉׉ 7cassandra://cEr01n6MUPf5mdF94nra5Bspn3Z3CdZupNAhB4gyVR0`׉ 7cassandra://BdYcXeKyOMVTRo1-xDeN9XpGb1N9l_i1TO3w8ACVWPUD `s׉ 7cassandra://eXUd40PJkxcnpx1-pEHS_eEGW0e2yRARhA-yGk4ZHBg8`׉ 7cassandra://BnfoThcm7KR-kXCEi_mWTwdnwA2-ehfwv01PS1Kn1Bg0-͠^cZnEbט#u׉׉ 7cassandra://BPVmFVAajriSsN-bNCC79-1xzX-6I3HhQafqjO4DdGA5)`׉ 7cassandra://qoisoyVRc7k9LGHBIsE_lN50mgn2gvkGIWvP4GkQBpER2`s׉ 7cassandra://OcT-swmDuBLXpQSpcOPi3l2hgmbwgO-3_jthUrennzQ`׉ 7cassandra://9J2lfT7rDaB9qcD2E3RenEBMCcrcnRfsWXgqfffxAXs B>t͠^cZoEb נcZoEb xY9ׁHhttp://electrode.Anׁ ׁ Ј׉E976 Chapter 30 Fusion Welding Processes C2H2 1 O2 mixture Torch withdrawn Torch Flame heating of surfaces Upsetting force Clamp (a) (b) Figure 30.3: Schematic illustration of the pressure-gas welding process: (a) before and (b) after. Note the formation of a flash at the joint; later the flash can be trimmed off. Thermit Welding. Also known as thermite or exothermic welding, and developed in 1895, thermit welding involves mixing a metal powder with a metal oxide, then using a high-temperature ignition source to cause an oxidation-reduction reaction (Fig. 30.4). Acommon arrangement in this process is to use iron oxide (rust) powder in combination with aluminum powder; upon ignition by a magnesium fuse, the resulting chemical reaction forms aluminum oxide (Al2O3)andiron. Temperatures can reach up to 2500◦C, melting the iron, which subsequently flows into a pouring basin and then into a mold placed around the parts to be welded. The aluminum oxide floats to the slag basin because of its lower density. The features of a thermit welding mold are very similar to a casting mold (see Fig. 11.3). Note from Fig. 30.4 the presence of a heating port, a feature that allows insertion of an oxyacetylene torch to preheat the workpieces and prevent weld cracks (Section 30.9.1). Crucible Fe3O4 1 Al powder Slag basin Tapping pin Riser Gate Space for thermite weld Heating port Plug Workpiece Figure 30.4: Schematic illustration of thermite welding. ׉ 7cassandra://eXUd40PJkxcnpx1-pEHS_eEGW0e2yRARhA-yGk4ZHBg8`cZ%Eb¡׉EArc-welding Processes: Nonconsumable Electrode 977 Several combinations of powder and oxide can be used in thermit welding, although aluminum powder combined with iron oxide is the most common because of the widespread use of thermit welding for joining railroad rails. Some copper and magnesium oxides are often added to improve flammability. Other applications of thermit welding include welding of large-diameter copper conductors, using copper oxide, and field repair of large equipment, such as locomotive axle frames. 30.3 Arc-welding Processes: Nonconsumable Electrode In arc welding, developed in the mid-1800s, the heat required is from electrical energy. The process involves either a nonconsumable or a consumable electrode.An AC or a DC power supply produces an arc between the tip of the electrode and the workpiece to be welded. The arc generates temperatures of about 30,000◦C, higher than those developed in oxyfuel–gas welding. Nonconsumable-electrode welding processes typically use a tungsten electrode (Fig. 30.5). Because of the high temperatures involved, an externally supplied shielding gas is necessary in order to prevent oxidation of the weld zone. Typically, direct current is used, and, as described below, its polarity (the direction of current flow) is important. The selection of current levels depends on such factors as the type of electrode, the metals to be welded, and the depth and width of the weld zone. In straight polarity, also known as direct-current electrode negative (DCEN), the workpiece is positive (anode), and the electrode is negative (cathode). DCEN generally produces welds that are narrow and deep (Fig. 30.6a). In reverse polarity, also known as direct-current electrode positive (DCEP), the workpiece is negative and the electrode is positive. Weld penetration is less, and the weld zone is shallower and wider Travel Electrical conductor Tungsten electrode Gas passage Filler wire Molten weld metal (a) Cooling-water supply Torch Filler rod Drain Foot pedal (optional) Workpiece (b) Figure 30.5: (a) The gas tungsten-arc welding process, formerly known as TIG (for tungsten–inert gas) welding. (b) Equipment for gas tungsten-arc welding operations. Inert gas supply AC or DC welder Shielding gas Arc Solidified weld metal ׉ 7cassandra://OcT-swmDuBLXpQSpcOPi3l2hgmbwgO-3_jthUrennzQ`cZ%Eb¢cZ%Eb¡#בCט#u׉׉ 7cassandra://emuBl3mk42jQmSVOTRm7RFoyT29WHqEsaYGIfHvosEQ8`׉ 7cassandra://D6M85nEec06anFewvj4Z6GAo1-VTbku6uWPrOuP-4R0R`s׉ 7cassandra://URaKRQnqBJmQgp-HSamPKaAbD9tIF9F6cQBdsSevnqc`׉ 7cassandra://0E8PmpHVh5DTXulsSHon0qMrb47bPmPD9DP9M3G66Mc|,͠^cZoEb ט#u׉׉ 7cassandra://JfKC5X7-CA_PtZoobOUdtJpKpxSe_OzSELyNlepbOyQ`׉ 7cassandra://wNKBFyZ3WGUYs0_ee3maVyICFUBm5quIHXmdDozli9YX`s׉ 7cassandra://xSDTrWUgwuyhhlNBG69ljcF7NU_A2mb7ab7QR2T9nPA`׉ 7cassandra://yx8paSDkXYz4z8baRN_WkTyisJIdl0qWCUuw_W3L_28k@͠^cZpEb ׉E 978 Chapter 30 Fusion Welding Processes DC (1) (a) (b) DC (2) (c) Figure 30.6: The effect of polarity and current type on weld beads: (a) DC current with straight polarity; (b) DC current with reverse polarity; (c) AC current. (Fig. 30.6b); consequently, DCEP is preferred for sheet metals and for joints with wide gaps. In the AC current method, the arc pulsates rapidly. This method is suitable for welding thick sections and for using large-diameter electrodes at maximum currents (Fig. 30.6c). Heat Transfer in Arc Welding. The heat input in arc welding is given by the equation H l = e VI v , (30.3) where H is the heat input (J or BTU), l is the weld length, V is the voltage applied, I is the current (amperes), and v is the welding speed. The term e is the efficiency of the process, which varies from around 75% for shielded metal-arc welding to 90% for gas metal-arc and submerged-arc welding. The efficiency is an indication that not all of the available energy is beneficially used to melt the material; because the heat is conducted through the workpiece, some of it is dissipated by radiation and still more is lost by convection to the surrounding environment. The heat input given by Eq. (30.3) melts a volume of material (usually the electrode or filler metal), and can also be expressed as H = uVm = uAl, (30.4) where u is the specific energy required for melting, Vm is the volume of metal melted, l is the length of the weld bead, and A is the cross-section of the weld. Some typical values of u are given in Table 30.2. Equations (30.3) and (30.4) allow an expression of the welding speed as v = e VI uA . (30.5) Although these equations have been developed for arc welding, similar expressions can be obtained for other fusion-welding operations, while taking into account differences in weld geometry and process efficiency. Example 30.1 Welding Speed for Different Materials Given: Consider a welding operation being performed with V =20 volts, I = 200 A, and the crosssectional area of the weld bead of A =30 mm2. Find: Estimate the welding speed if the workpiece and electrode are made of (a) aluminum, (b) carbon steel, and (c) titanium. Assume an efficiency of 75%. Solution: From Table 30.2, the specific energy required for aluminum is u =2.9 J/mm3. Therefore, from Eq. (30.5), v = e VI uA =(0.75) (20)(200) (2.9)(30) =34.5 mm/s. Similarly, for carbon steel, u is estimated as 9.7 J/mm3 (average of extreme values in the table), and thus v =10.3 mm/s. For titanium, u =14.3 J/mm3,and thus v =7.0 mm/s. AC ׉ 7cassandra://URaKRQnqBJmQgp-HSamPKaAbD9tIF9F6cQBdsSevnqc`cZ%Eb£׉E Arc-welding Processes: Nonconsumable Electrode 979 Table 30.2: Approximate Specific Energies Required to Melt a Unit Volume of Commonly Welded Metals. Specific energy, u Material Aluminum and its alloys Cast irons Copper Bronze (90Cu–10Sn) Magnesium Nickel Steels Stainless steels Titanium J/mm3 2.9 7.8 6.1 4.2 2.9 9.8 9.1–10.3 9.3–9.6 14.3 Gas Tungsten-arc Welding. In gas tungsten-arc welding (GTAW), formerly known as TIG (for tungsten–inert gas) welding, the filler metal is supplied from a filler wire (Fig. 30.5a). Because the tungsten electrode is not consumed, a stable arc gap is maintained at a constant current level. The filler metals are similar to the metals to be welded, and flux is not used. The shielding gas is typically argon or helium, or a mixture of the two gases. Welding with GTAW may be done without filler metals, such as in welding close-fit joints. Depending on the metals to be joined, the power supply is either DC at 200 A or AC at 500 A (Fig. 30.5b). In general, AC is preferred for aluminum and magnesium, because the cleaning action of AC removes oxides and improves weld quality. Thorium or zirconium may be used in tungsten electrodes to improve their electron emission characteristics. The power supply ranges from 8 to 20 kW. Contamination of the tungsten electrode by the molten metal can be a significant problem, particularly in critical applications, because it can cause discontinuities in the weld; contact of the electrode with the molten-metal pool should be avoided. The GTAW process is used for a wide variety of metals and applications, particularly aluminum, magnesium, titanium, and refractory metals; it is especially suitable for thin metals. The cost of the inert gas makes this process more expensive than SMAW, but it provides welds of very high quality and good surface finish. GTAW is used in a variety of critical applications with a wide range of part thicknesses and shapes, and the equipment is portable. Plasma-arcWelding. Plasma is an ionized hot gas consisting of nearly equal numbers of electrons and ions. In plasma-arc welding (PAW), developed in the 1960s, a concentrated plasma arc is produced and directed toward the weld area. The arc is stable and reaches temperatures as high as 33,000◦C. The plasma is initiated between the tungsten electrode and the orifice by a low-current pilot arc. The plasma arc is concentrated, because it is forced through a small orifice. Operating currents usually are below 100 A, but they can be higher for special applications. When filler metal is used, it is fed into the arc, as is done in GTAW. Arc and weld-zone shielding is provided by means of an outer-shielding ring and the use of gases, such as argon, helium, or mixtures. There are two methods of plasma-arc welding: • In the transferred-arc method (Fig. 30.7a), the workpiece being welded is part of the electrical circuit. The arc transfers from the electrode to the workpiece, and hence the term transferred. • In the nontransferred method (Fig. 30.7b), the arc is between the electrode and the nozzle, and the heat is carried to the workpiece by the plasma gas. This thermal-transfer mechanism is similar to that for an oxyfuel flame (see Section 30.2). ׉ 7cassandra://xSDTrWUgwuyhhlNBG69ljcF7NU_A2mb7ab7QR2T9nPA`cZ%Eb¤cZ%Eb£#בCט#u׉׉ 7cassandra://IqLonneaaEJaa3lFEQuMAZl4a_3ioowvvQs8n2Abo_4`׉ 7cassandra://roKu745qbQoKFdM3OileZLM2V6qeugRK6_TDKndULU0V`s׉ 7cassandra://OezQeKiUkCdawQOOp9fDefJQOTAGVeAqznO5cq0zcSM`׉ 7cassandra://cnAk6JWJjQve0vqq3-E3lELd8p1Oc4_MWqt_yc6aZug͸͠^cZpEbט#u׉׉ 7cassandra://KxTpb1KgUVtxXtksVKBiJoL3ew_t6FjCE_pVnMXkF84`׉ 7cassandra://iOqQcf4zYeYHNL8gcq518UhC6XU03SCIgYLMky9qI_4Yt`s׉ 7cassandra://g7RnZckNLLTkomKdPQPd8PVa9Cq_JeGX8Fzvi-0BeIc`׉ 7cassandra://32shgcKLdmjUfuMRcrzWEo5dNMLJAKAsp_NsBnwiSBQ)*͠^cZqEb׉E Z980 Chapter 30 Fusion Welding Processes Tungsten electrode 2 Power supply + 1 (a) (b) Figure 30.7: Two types of plasma-arc welding processes: (a) transferred and (b) nontransferred. Deep and narrow welds can be made by these processes at high welding speeds. Compared with other arc-welding processes, plasma-arc welding has better arc stability, higher energy concentration, and less thermal distortion; the welds are deeper and narrower welds. Also, higher welding speeds, from 120 to 1000 mm/min, can be achieved. A variety of metals can be welded, with part thicknesses generally less than 6 mm. The high heat concentration can completely penetrate through the joint, with thicknesses as much as 20 mm for some titanium and aluminum alloys. Known as the keyhole technique, the force of the plasma arc displaces the molten metal and produces a hole at the leading edge of the weld pool. Plasma-arc welding (rather than the GTAW process) is often used for butt and lap joints, because of its higher energy concentration, better arc stability, and higher welding speeds. Safety considerations include protection against glare, spatter, and noise from the plasma arc. Atomic-hydrogen Welding. In atomic-hydrogen welding (AHW), an arc is generated between two tungsten electrodes within a shielding atmosphere of hydrogen gas. The gas normally is diatomic (H2); however, where the temperatures are over 6000◦C near the arc, the hydrogen breaks down into its atomic form, thus simultaneously absorbing a large amount of heat from the arc. When the gas strikes the relatively cold surface of the workpieces to be joined, it recombines into its diatomic form and rapidly releases the stored heat, reaching temperatures up to 4000◦C. Thus, it is one of the few joining processes that can be used for welding tungsten. The energy in AHW can be easily varied by changing the distance between the arc stream and the workpiece surface. Plasma gas Shielding gas 2 Power supply 30.4 Arc-welding Processes: Consumable Electrode There are several consumable-electrode arc-welding processes, as described below. 30.4.1 Shielded Metal-arc Welding Shielded metal-arc welding (SMAW) is one of the oldest, simplest, and most versatile joining processes; consequently, about 50% of all industrial and maintenance welding is done by this method. The electric arc is generated by touching the tip of a coated electrode against the workpiece, and withdrawing it quickly to a distance sufficient to maintain the arc (Fig. 30.8a). The electrodes are in the shapes of thin, long round rods that are held manually; hence, the process also is referred to as stick welding. ׉ 7cassandra://OezQeKiUkCdawQOOp9fDefJQOTAGVeAqznO5cq0zcSM`cZ%Eb¥׉E Arc-welding Processes: Consumable Electrode 981 Welding machine AC or DC power source and controls Work cable Arc Electrode Electrode holder Workpiece Base metal Electrode cable Weld metal Arc Figure 30.8: Schematic illustration of the shielded metal-arc welding process. About 50% of all large-scale industrial-welding operations use this process. The heat generated melts a portion of the electrode tip, its coating, and the base metal in the immediate arc area. The molten metal consists of a mixture of base metal (the workpiece), electrode metal, and substances from the coating on the electrode; this mixture becomes the weld when it solidifies. The electrode coating deoxidizes the weld area and provides a shielding gas to protect it from oxygen in the environment. A bare section at the end of the electrode is first clamped to one terminal of the power source, while the other terminal is connected to the workpiece being welded (Fig. 30.8b). The current, which may be DC or AC, usually ranges from 50 to 300 A. For sheet-metal welding, DC is preferred because of the steady arc it produces. Power requirements generally are less than 10 kW. The equipment consists of a power supply, cables, and an electrode holder. The SMAW process is commonly used in general construction, shipbuilding, pipelines, and for maintenance. It is especially useful for work in remote areas where a portable fuel-powered generator can be used as the power supply. This process is best suited for workpiece thicknesses of 3 to 19 mm, although this range can easily be extended by skilled operators using multiple-pass techniques (Fig. 30.9). The multiple-pass approach requires that slag be removed after each weld bead. Unless removed completely, the solidified slag can cause severe corrosion of the weld area, and thus lead to failure of the weld; also, it prevents fusion of weld layers, compromising weld strength. The slag can be removed by wire brushing or by chipping of the weld. Labor costs and material costs are both high. 30.4.2 Submerged-arc Welding In submerged-arc welding (SAW), the weld arc is shielded by a granular flux, which consists of lime, silica, manganese oxide, calcium fluoride, and other compounds. The flux is fed into the weld zone from a hopper by gravity flow through a nozzle (Fig. 30.10). The thick layer of flux completely covers the molten metal, and prevents spatter and sparks, and suppresses the intense ultraviolet radiation and fumes characteristic of the SMAW process. The flux also acts as a thermal insulator, by promoting deep penetration of heat into the workpiece. 7 5 4 6 3 1 2 8 Figure 30.9: A deep weld showing the buildup sequence of eight individual weld beads. Solidified slag Coating Electrode Shielding gas ׉ 7cassandra://g7RnZckNLLTkomKdPQPd8PVa9Cq_JeGX8Fzvi-0BeIc`cZ%Eb¦cZ%Eb¥#בCט#u׉׉ 7cassandra://Fr1ly9KSMUdm2PyuA869tTsKZ8yWTdSGthXVaqKA5Wk`׉ 7cassandra://45AIEgl7FwETuuS4QvU-qPiR4n7abudf5XwtNTUi3UMc`s׉ 7cassandra://Zavnb-M6rRicXuWoBbmsdzP38Om__Li8Z1ceUBbVw_Y`׉ 7cassandra://uvFDRKsjQwJtPwqr482Csv_6h0ljnfgN4xTbk2ndaUoImz͠^cZrEbט#u׉׉ 7cassandra://FFhKBqt2V85i7Op9ota0pkNfWU9wKsB3mqQ5xMfg7vA&`׉ 7cassandra://ZaeKmRnVFbSSSfgronD0wjACskkWN_FvDbbmwptQ6QsL`s׉ 7cassandra://IaG1qVO1U5hpEn2wX4zQ4735OAntDNfLO8A4mfkS3DIg`׉ 7cassandra://kMB7FN3S4p_7FwYJ0ch9NafPxcza5Gkw9jm6lejfgkQO7͠^cZrEb׉E 982 Chapter 30 Fusion Welding Processes Electrode-wire reel Flux hopper Voltage and current control Wire-feed motor Unfused-flux recovery tube Electrode cable Contact tube Workpiece Weld backing Ground Figure 30.10: Schematic illustration of the submerged-arc welding process and equipment. The unfused flux is recovered and reused. The consumable electrode is a coil of bare round wire, 1.5–10 mm in diameter, and is fed automatically through a tube (welding gun). Electric currents typically range from 300 to 2000 A, but multiple arc arrangements can be as high as 5000 A. The power supplies are usually connected to standard single- or three-phase power lines, with a primary rating up to 440 V. Because the flux is gravity fed, the SAW process is limited mostly to welds in a flat or horizontal position, and having a backup piece. Circular welds can be made on pipes and cylinders, provided that they can be rotated during welding. As Fig. 30.10 illustrates, the unfused flux can be recovered, treated, and reused; typically, 50–90% of the flux is recovered. The process is automated and is used to weld a variety of carbon and alloy steels and stainless-steel sheets or plates, at speeds as high as 5 m/min; occasionally it is also used for nickel-based alloys. The quality of the weld is very high, with good toughness, ductility, and uniformity of properties. The SAW process provides very high welding productivity, depositing 4–10 times the amount of weld metal per hour as the SMAW process. Typical applications include welding of thick plates for shipbuilding and pressure vessels. 30.4.3 Gas Metal-arc Welding In gas metal-arc welding (GMAW), developed in the 1950s and formerly called metal inert-gas (MIG) welding, the weld area is shielded by an inert atmosphere of argon, helium, carbon dioxide, or other gas mixtures (Fig. 30.11a). The consumable bare wire is automatically fed through a nozzle into the weld arc by a wirefeed drive motor (Fig. 30.11b). Multiple-weld layers also can be deposited at the joint. Deoxidizers usually are present in the electrode metal itself, in order to prevent oxidation of the molten-weld puddle. Metal can be transferred by three methods: 1. In spray transfer, small, molten droplets from the electrode are transferred to the weld area, at a rate of several hundred droplets per second. The transfer is spatter free and very stable. High DC currents and voltages and large-diameter electrodes are used with argon or an argon-rich gas mixture as the shielding gas. The average current required can be reduced by using a pulsed arc, superimposing high-amplitude pulses onto a steady low current. The process can be used in all welding positions. 2. In globular transfer, carbon-dioxide-rich gases are utilized, and the globules are propelled by the forces of the electric-arc transfer of the metal, resulting in considerable spatter. Welding currents are high, making it possible for deeper weld penetration; welding speeds are higher than in spray transfer. Heavier sections are commonly welded by this method. Voltage-pickup leads (optional) ׉ 7cassandra://Zavnb-M6rRicXuWoBbmsdzP38Om__Li8Z1ceUBbVw_Y`cZ%Eb§׉EArc-welding Processes: Consumable Electrode 983 Solid wire electrode Current conductor Travel Shielding gas Nozzle Arc Base metal (a) Feed control Control system Gas out Gun control Gun Workpiece Wire-feed drive motor Contactor control (b) Power supply Voltage control Welding machine Gas in Wire Shielding-gas source Shielding gas Wire guide and contact tube Molten weld metal Solidified weld metal Figure 30.11: (a) Schematic illustration of the gas metal-arc welding process, formerly known as MIG (for metal inert-gas) welding. (b) Basic equipment used in gas metal-arc welding operations. 3. In short circuiting, the metal is transferred as individual droplets (at a rate more than 50/s), as the electrode tip touches the molten weld metal and short circuits. Low currents and voltages are utilized with carbon-dioxide-rich gases, and electrodes made of small-diameter wire. Power requirement is about 2 kW. The temperatures generated in GMAWare relatively low; consequently, this method is suitable only for thin sheets and sections of less than 6 mm, as otherwise fusion may be incomplete. The operation, which is easy to perform, is commonly used for welding ferrous metals with thin sections. Pulsed-arc systems are used for thin ferrous and nonferrous metals. The GMAW process is suitable for welding most ferrous and nonferrous metals and is used extensively in the metal-fabrication industry. Because of the relative simplicity of the process, training of operators is easy. The process is versatile, rapid, and economical, and welding productivity is double that of the SMAW process. The process can easily be automated and lends itself readily to robotics and to flexible manufacturing systems (Chapters 37 and 39). 30.4.4 Flux-cored Arc Welding Flux-cored arc welding (FCAW), illustrated in Fig. 30.12, is similar to gas metal-arc welding, with the exception that the electrode is tubular and is filled with flux, and hence the term flux-cored. Cored electrodes ׉ 7cassandra://IaG1qVO1U5hpEn2wX4zQ4735OAntDNfLO8A4mfkS3DIg`cZ%Eb¨cZ%Eb§#בCט#u׉׉ 7cassandra://7xaDVMwvTu70e2x31vMxmRqFqTY56Oe7S2Jy-Vh0pBIk`׉ 7cassandra://_mfY07x00A5adKV0dRgUdiIbAgeVoBsh0fPEsI9BIbw\`s׉ 7cassandra://mpcmrh7cwEpsl6_25N7sBhcemz9s9XWycAxevIa95bMG`׉ 7cassandra://EcAYL5DjIJYlta_XA-OldoNWoLBiBhkO6Y4L77sd0lE#dN͠^cZsEbט#u׉׉ 7cassandra://YgEmfcyB411Ooic2U7r4Qbq1I2AkdrmEy-2MU5oAOxU`׉ 7cassandra://S8RlSflb90T6v3N-IAgmZ7vIDHA5jlIZOKA3KECCq9YT`s׉ 7cassandra://UqL7-w5fiLQKz6GftvkEp_gGiMmY5ja5g2HLZFNAtzMP`׉ 7cassandra://fjnGKvtZyBEMH9DXRSLm0Daok7ZXPz2_Sx1QPRdoUH0͕͠^cZsEb׉E 984 Chapter 30 Fusion Welding Processes Arc shield composed of vaporized and slag-forming compounds protects metal transfer through arc Insulated extension tip Current-carrying guide tube Solidified slag Molten slag Solidified weld metal Molten weld metal Figure 30.12: Schematic illustration of the flux-cored arc-welding process. This operation is similar to gas metal-arc welding, shown in Fig. 30.11. produce a more stable arc, improve the weld contour, and produce better mechanical properties of the joint. The flux in the electrodes is much more flexible than the brittle coating used on SMAW electrodes, as the tubular electrode can be made in long coiled lengths. The electrodes are usually 0.5–4 mm in diameter, and the power required is about 20 kW. Self-shielded cored electrodes also are available; they do not require any external shielding gas, because they contain emissive fluxes that shield the weld area against the surrounding atmosphere. Small-diameter electrodes have made welding of thinner materials not only possible but often preferable. Furthermore, small-diameter electrodes make it relatively easy to weld parts at various positions, and the flux chemistry permits the welding of many metals. The FCAW process combines the versatility of SMAW with the continuous and automatic electrodefeeding feature of GMAW. The process is economical and versatile; thus it is used for welding various types of joints, mainly on steels, stainless steels, and nickel alloys. The higher weld-metal deposition rate of this process, as compared with that of GMAW, has led to its use in joining sections with various thicknesses. Using tubular electrodes with very small diameters has extended the use of this process to workpieces with small cross-sections. A major advantage of FCAW is the ease with which specific weld-metal chemistries can be developed and used, by adding various alloying elements to the flux core. The process is easy to automate and is readily adaptable to flexible manufacturing systems and robotics. 30.4.5 Electrogas Welding Electrogas welding (EGW) is primarily used for welding the edges of parts, vertically and in one pass, with the parts placed edge to edge (butt joint). The process is classified as machine welding, because it requires special equipment (see Fig. 30.13). The weld metal is deposited into a weld cavity between the two parts to be joined. The space in between is enclosed by two water-cooled copper dams (shoes) to prevent the molten slag from running off; mechanical drives move the shoes upward. Circumferential welds, such as those on pipes, also are possible, provided that the workpiece can be rotated. Powdered metal, vapor- or gas-forming materials, deoxidizers and scavengers Arc Base metal Metal droplets covered with thin slag coating forming molten puddle ׉ 7cassandra://mpcmrh7cwEpsl6_25N7sBhcemz9s9XWycAxevIa95bMG`cZ%Eb©׉E 5Electrodes for Arc Welding 985 Electrode conduit Gas Oscillator Welding gun Welding wire Water out Water in Fixed shoe Primary shielding gas Water Gas Gas Water out Water in Supplementary shielding gas Moveable shoe Weld metal Figure 30.13: Schematic illustration of the electrogas-welding process. Single or multiple electrodes are fed through a conduit, and a continuous arc is maintained, using fluxcored electrodes at up to 750 A or solid electrodes at 400 A. Power requirements are about 20 kW. Shielding is provided by means of an inert gas, such as carbon dioxide, argon, or helium, depending on the type of material being welded. The gas may be supplied either from an external source or from a flux-cored electrode, or from both. Weld thickness ranges from 12 to 75 mm, on steels, titanium, and aluminum alloys. Typical applications include construction of bridges, pressure vessels, thick-walled and large-diameter pipes, storage tanks, and ships. The equipment is reliable and training for operators is relatively simple. 30.4.6 Electroslag Welding Electroslag welding (ESW) and its applications are similar to electrogas welding (Fig. 30.14), the main difference being that the arc is initiated between the electrode tip and the bottom of the part to be welded. Flux is added, which then melts by the heat of the arc. After the molten slag reaches the tip of the electrode, the arc is extinguished. Heat is produced continuously by the electrical resistance of the molten slag. Because the arc is extinguished, ESW is not strictly an arc-welding process. Single or multiple solid or flux-cored electrodes may be used. Electroslag welding is capable of welding plates with thicknesses ranging from 50 mm to more than 900 mm, and welding is done in one pass. The current required is on the order of 600 A at 40 to 50 V, although higher currents are used for thick plates; the travel speed of the weld is in the range of 12–36 mm/min. This process is used for large structural-steel sections, such as heavy machinery, bridges, oil rigs, ships, and nuclear-reactor vessels; weld quality is good. 30.5 Electrodes for Arc Welding Electrodes for consumable arc-welding processes are classified according to the following properties: • Strength of the deposited weld metal • Current (AC or DC) • Type of coating Gas box Welding wire Drive rolls ׉ 7cassandra://UqL7-w5fiLQKz6GftvkEp_gGiMmY5ja5g2HLZFNAtzMP`cZ%EbªcZ%Eb©#בCט#u׉׉ 7cassandra://AzCh166KH6vB9S8b5b6ITFrEg3SBieQVa4yEQC6SMJ0߬`׉ 7cassandra://Qeux20klaSrjer2e-orUEbnaWw7LrLFLMnFv5kE0CuwN`s׉ 7cassandra://aeE_85w3Qcn0zVzEadIWM5cHWc7XmlQQaPE8uPZJh3cf`׉ 7cassandra://RKKUUNX9db3tz-UjUkm-Pi89NReq96gmkvoEOeGU6g41.͠^cZtEbט#u׉׉ 7cassandra://9eLcawFdEBQkXzLN25Gn8bUcwwqVESIO-cIebHO8QfUIr`׉ 7cassandra://MWOYAg5To5BATSsv4_5h8jrSgYB3YHZqASlzoYeotH4R`s׉ 7cassandra://AiHVTZuYnTPFe6vfbhYah5PLT2if6OvvNnrpbYKsYpI`׉ 7cassandra://8YHjzfdWH_hzdaqTbDm-2rAU2aaYcjctfg5XNPRuiGI[L͠^cZtEb ׉ER986 Power source Chapter 30 Fusion Welding Processes Control panel Wire reel Wire-feed drive Electrode lead Oscillation (optional) Consumable guide tube Work Workpiece (ground) lead Water in Water out Figure 30.14: Equipment used for electroslag-welding operations. Electrodes are identified by numbers and letters (Table 30.3), or by color code if the numbers and letters are too small to imprint. Typical coated-electrode dimensions are in the range of 150–460 mm in length and 1.5–8 mm in diameter. Specifications for electrodes and filler metals, including dimensional tolerances, quality control procedures, and processes, are published by the American Welding Society (AWS) and the American National Standards Institute (ANSI). Some specifications are available in the Aerospace Materials Specifications (AMS) by the Society of Automotive Engineers (SAE). Electrodes are sold by weight and are available in a wide variety of sizes and specifications. Criteria for selection and recommendations for electrodes for a particular metal and its application can be found in suppliers’ literature and in various handbooks and references, as in the Bibliography of this chapter. Electrode Coatings. Electrodes are coated with claylike materials, which include silicate binders and powdered materials, including oxides, carbonates, fluorides, metal alloys, cotton cellulose, and wood flour. The coating is brittle and takes part in complex interactions during welding; it has the following basic functions: • Stabilize the arc • Generate gases to act as a shield against the surrounding atmosphere; the gases produced are carbon dioxide, water vapor, and small amounts of carbon monoxide and hydrogen • Control the rate at which the electrode melts • Act as a flux to protect the weld against the formation of oxides, nitrides, and other inclusions and, with the resulting slag, to protect the molten-weld pool • Add alloying elements to the weld zone to enhance the properties of the joint—among these elements are deoxidizers to prevent the weld from becoming brittle. Molten slag Molten weld pool Retaining shoe ׉ 7cassandra://aeE_85w3Qcn0zVzEadIWM5cHWc7XmlQQaPE8uPZJh3cf`cZ%Eb«׉E :Electron-beam Welding 987 Table 30.3: Designations for Mild-steel Coated Electrodes. The prefix “E” designates arc-welding electrode. The first two digits of four-digit numbers and the first three digits of five-digit numbers indicate minimum tensile strength: E60XX E70XX E110XX 413 MPa 482 MPa 758 MPa The next-to-last digit indicates position: EXX1X All positions EXX2X Flat position and horizontal fillets The last two digits together indicate the type of covering and the current to be used. The suffix (Example: EXXXX-A1) indicates the approximate alloy in the weld deposit: –A1 –B1 –B2 –B3 –B4 –B5 –C1 –C2 –C3 0.5% Mo 0.5% Cr, 0.5% Mo 1.25% Cr, 0.5% Mo 2.25% Cr, 1% Mo 2% Cr, 0.5% Mo 0.5% Cr, 1% Mo 2.5% Ni 3.25% Ni 1% Ni, 0.35% Mo, 0.15% Cr –D1 and D2 0.25–0.45% Mo, 1.75% Mn –G 0.5% min. Ni, 0.3% min. Cr, 0.2% min. Mo, 0.1% min. V, 1% min. Mn (only one element required) The deposited coating or slag must be removed following each pass in order to ensure a good weld. Bare electrodes and wires, typically made of stainless steels and aluminum alloys, also are available, and are used as filler metals in various welding operations. 30.6 Electron-beam Welding In electron-beam welding (EBW), developed in the 1960s, heat is generated by high-velocity, narrow-beam electrons. The kinetic energy of the electrons is converted into heat as they strike the workpiece to be welded. This process requires special equipment in order to focus the beam on the workpiece, typically in a vacuum. The higher the vacuum, the greater the depth the beam penetrates, and the greater becomes the depth-to-width ratio of the weld; thus, the methods are called EBW-HV (for high vacuum) and EBW-MV (for medium vacuum); some materials may also be welded by EBW-NV (for no vacuum). Almost any metal can be welded and workpiece thicknesses range from foil to plate. Capacities of electron guns range up to 100 kW; the intense energy also is capable of producing holes in the workpiece. Generally, no shielding gas, flux, or filler metal is required. The EBW process makes high-quality welds that are deep and narrow, and with small heat-affected zones (Section 30.9). Depth-to-width ratios are in the range of 10–30. The size of welds made are much smaller than those made by conventional processes. Using automation and servo controls, the processing parameters can be controlled accurately, at welding speeds as high as 12 m/min. Almost any metal can be welded with this process, in butt or lap configurations, and at thicknesses up to 150 mm. Distortion and shrinkage are minimal, and weld quality is good. Typical applications include aircraft, missile, nuclear, and electronic components, and gears and shafts for the automotive industry. EBW equipment generates X-rays; thus, proper monitoring and periodic maintenance of the equipment are essential. ׉ 7cassandra://AiHVTZuYnTPFe6vfbhYah5PLT2if6OvvNnrpbYKsYpI`cZ%Eb¬cZ%Eb«#בCט#u׉׉ 7cassandra://X5vKwigHjM2I6BQ0ysisZro16NUHQ2u05HjrUhbwHlE- `׉ 7cassandra://WLLnFMzkohW5tVM7SvZbjhkKlgK-LIBAIjVXIcJRnj0[`s׉ 7cassandra://8l3JdUmDoYAJJ1Qi_o_BvuSMEuwrlb32TkuVGdGOJ5g]`׉ 7cassandra://2U_IANhzobLJ1d64WBbFaUzScpXecRt_uCNib_I1MpAx͠^cZuEb#ט#u׉׉ 7cassandra://ckZf4f4fB5C4KOiTy6qtn94mmEhOII1qgEVqlFcxKeM_`׉ 7cassandra://32Ql7aenr2oFqg1Z_dDdJxjHZsPq2uvfqHuzsk7oIxc7`s׉ 7cassandra://soBn13DUeS6yVTfkkIWwXEeKMNdsxMvI8BFAxtP4IVAJ`׉ 7cassandra://vpkbfe2kj0l9JIm7X_fHFXs7fog8Vw8yb0RBidWAjIw+͠^cZuEb$׉E :988 Chapter 30 Fusion Welding Processes 30.7 Laser-beam Welding Laser-beam welding (LBW) utilizes a high-power laser beam as the source of heat to produce a fusion weld. Because it can be focused onto a very small area, the beam has high energy density and deep penetrating capability. The laser beam can be directed, shaped, and focused precisely, with laser spot diameters as low as 0.2 mm. LBW is suitable particularly for welding deep and narrow joints (Fig. 30.15) with depth-to-width ratios typically ranging from 4 to 10. Laser-beam welding has become very widespread and is now used by most industries. The laser beam may be pulsed (in milliseconds), with power levels up to 100 kW, for applications such as spot welding of thin materials. Continuous multi-kW laser systems are used for deep welds on thick sections. Laser beam welds have good quality, with minimal shrinkage or distortion. The welds have good strength and are generally ductile and free of porosity. The process can be automated and used on a variety of materials, with thicknesses up to 25 mm. As described in Section 16.2.2, tailor-welded sheet-metal blanks are joined principally by laser-beam welding, using robotics for precise control of the beam path. Typical metals and alloys welded include aluminum, titanium, ferrous metals, copper, superalloys, and the refractory metals. Welding speeds range from 2.5 m/min to as high as 80 m/min for thin metals. Because of the nature of the process, welding can be done in otherwise inaccessible locations. As in other and similar automated welding systems, the operator skill required is minimal. Safety is particularly important in laser-beam welding due to the extreme hazards to the eye and the skin; solid-state (YAG) lasers also are dangerous. While a filler wire can be used, laser-beam welding generally does not use a filler metal; instead, the laser melts the material which then solidifies to weld the components. In some arrangements (similar to laser-engineered net shaping, Section 20.8), powder can be blown in front of the laser, leading to deposition of metal. The major advantages of LBW over EBW are: • A vacuum is not required, and the beam can be transmitted through air. • Laser beams can be shaped, manipulated, and focused by means of fiber optics; hence the process can easily be automated. • The beams do not generate X-rays. • The quality of the weld is better than in EBW; there is less part distortion and the weld has less tendency for incomplete fusion, spatter, and porosity. Figure 30.15: Laser beam welding in progress. Source: Alamy/Warut Sintapanon. ׉ 7cassandra://8l3JdUmDoYAJJ1Qi_o_BvuSMEuwrlb32TkuVGdGOJ5g]`cZ%Eb­׉ELaser-beam Welding 989 Figure 30.16: Detail of razor cartridge, showing laser spot welds. Source: Shutterstock/All About Space. Example 30.2 Laser Welding of Razor Blades The Gillette Sensor R razor cartridge has two narrow, high-strength blades with 13 pinpoint welds, 11 of which can be seen as darker spots, about 0.5 mm in diameter, on each blade. The welds are made with an Nd:YAG laser, equipped with fiber-optic delivery. This equipment provides very flexible beam manipulation and can target exact locations along the length of the blade. With a set of these machines, production is at a rate of 3 million welds per hour, with consistent weld quality. Source: Courtesy of Lumonics Corporation, Industrial Products Division. Laser GMAW. Laser GMAW is an emerging hybrid welding technology that combines the narrow heataffected zone of laser welding with the high deposition rates of gas metal-arc welding. In this process, shown in Fig. 30.17, the laser is focused on the workpiece ahead of the GMAW arc, resulting in deep Focused laser beam Gas metal arc welding torch Travel Shielding gas Arc Workpiece Molten weld metal Solidified weld metal Figure 30.17: Schematic illustration of the LaserGMAWhybrid welding process. Source: Courtesy of Lincoln Electric. ׉ 7cassandra://soBn13DUeS6yVTfkkIWwXEeKMNdsxMvI8BFAxtP4IVAJ`cZ%Eb®cZ%Eb­#בCט#u׉׉ 7cassandra://ORFnQ9t01REoNfSP0CEfYgM9x624Zhgg2kRl26FQb4Iza`׉ 7cassandra://kweSLYIMbwlZJ84Q1lh13_qQ-iOld18O4WjM37ua9fIH`s׉ 7cassandra://mEbhXgJ_Kvx806cvHKG5V7fGA3J5i28bemdPU0bF6b8`׉ 7cassandra://lrK_xZg9PDXXnp3TX4YXjr694r7py4NhWF0IXvVUK8kͷr͠^cZvEb'ט#u׉׉ 7cassandra://na2RfKZFfFPVSNVhZauOpHGx72Cal6DUgWe8setiBREo`׉ 7cassandra://k9VbTP95ClrFQ_0xgI7yrWgq_EHt2Pb41X-5xOXmwS8Jl`s׉ 7cassandra://_EZ-jW_1vZN8buNcMXybpJgtz5FuVpAiPR6Qw-QQl2Ya`׉ 7cassandra://4PDpcZlxiHW3WIVK6jQ2xW_ZvVajp2o8BArrGNODrqI{t͠^cZvEb(׉E990 Chapter 30 Fusion Welding Processes penetration and allowing high travel speeds. In addition, the process can bridge gaps larger than in traditional laser welding, and the metallurgical quality of the weld is improved because of the presence of the shielding gas. 30.8 Cutting In addition to being cut by mechanical means, as described in Part IV, a material can be cut into various contours by using a heat source that melts and removes a narrow zone in the workpiece. The sources of heat can be torches, electric arcs, or lasers. Oxyfuel–gas Cutting. Oxyfuel–gas cutting (OFC) is similar to oxyfuel–gas welding (Section 30.2), but the heat source is now used to remove a narrow zone from a metal plate or sheet (Fig. 30.18a). OFC is suitable particularly for steels, where the basic reactions are Fe +O → FeO + Heat, 3Fe + 2O2 → Fe3O4 + Heat, and 4Fe + 3O2 → 2Fe2O3 + Heat. (30.8) The greater heat is generated by the second reaction, with temperatures rising to about 870◦C. However, because this temperature is not sufficiently high, the workpiece is first preheated with fuel gas, and then oxygen is introduced, as can be seen from the nozzle cross-section in Fig. 30.18a. The higher the carbon content of the steel, the higher is the required preheating temperature. Cutting takes place mainly by oxidation of the steel; some melting also takes place. Cast irons and steel castings also can be cut by this method. Cutting generates a kerf, similar to that produced in sawing with a saw blade or by wire electrical-discharge machining (see Fig. 27.12). Kerf width ranges from about 1.5 to 10 mm, with good control of dimensional tolerances. However, distortion caused by uneven temperature distribution can be a problem in OFC. Torch Oxygen Preheat flames (oxyacetylene) Plate Kerf Slag (iron and iron oxide) (a) Torch Workpiece Drag lines Thickness Drag (b) Figure 30.18: (a) Flame cutting of a steel plate with an oxyacetylene torch, and a cross-section of the torch nozzle. (b) Cross-section of a flame-cut plate, showing drag lines. (30.6) (30.7) ׉ 7cassandra://mEbhXgJ_Kvx806cvHKG5V7fGA3J5i28bemdPU0bF6b8`cZ%Eb¯׉E -The Weld Joint, Quality and Testing 991 The maximum thickness that can be cut by OFC depends mainly on the gases used. With oxyacetylene gas, for example, the maximum thickness is about 300 mm, whereas, with oxyhydrogen, it is about 600 mm. The flame leaves drag lines on the cut surface (Fig. 30.18b), resulting in a rougher surface than that produced by such processes as sawing and blanking that use mechanical cutting tools. Underwater cutting is done with specially designed torches that produce a blanket of compressed air between the flame and the surrounding water. Torches may be guided along specified paths either manually, mechanically, or automatically by machines, using programmable controllers and robots. Arc Cutting. Arc-cutting processes are based on the same principles as arc welding. A variety of materials can be cut at high speeds by arc cutting, although, as in welding, these processes also leave a heat-affected zone that has to be taken into account, particularly in critical applications. In air carbon-arc cutting (CAC-A), a carbon electrode is used and the molten metal is blown away by a high-velocity air jet. The process is used especially for gouging and scarfing (removal of metal from a surface). However, it is noisy, and the molten metal can be blown substantial distances and can cause safety hazards. Plasma-arc cutting (PAC) produces the highest temperatures, and is used for rapid cutting of nonferrous and stainless-steel plates. The productivity of this process is higher than that of oxyfuel–gas methods. PAC produces a good surface finish and with narrow kerfs. Electron beams and lasers also are used for very accurately cutting a wide variety of metals, as described in Sections 27.6 and 27.7. The surface finish is better than that of other thermal cutting processes, and the kerf is narrower. 30.9 The Weld Joint, Quality and Testing Three distinct zones can be identified in a typical weld joint, as shown in Fig. 30.19: 1. Base metal 2. Heat-affected zone 3. Weld metal Original structure Fusion zone (weld metal) Heat-affected zone Base metal Molten weld metal Melting point of base metal Temperature at which the base-metal microstructure is affected Original temperature of base metal Figure 30.19: Characteristics of a typical fusion-weld zone in oxyfuel–gas and arc welding. Temperature ׉ 7cassandra://_EZ-jW_1vZN8buNcMXybpJgtz5FuVpAiPR6Qw-QQl2Ya`cZ%Eb°cZ%Eb¯#בCט#u׉׉ 7cassandra://sBcE6RfN1Fj7re0UUbHnIjqHRhzKnT6sMZgsIibdFvo`׉ 7cassandra://arzODxtPdRPzcwqLiK1C3BN3VdOTD_RFh2di5AaDWlcY`s׉ 7cassandra://mbuTd17OSvpsZgqF25Hv1Dtw1LgVhjC3sOQTRIlDmF8`׉ 7cassandra://H4bgbe9Oj-RBbXCuH-jxyV3zLJ6H0LtNp47ckAXKRnc5͠^cZvEb*ט#u׉׉ 7cassandra://3il4j5T9wVi58U56GbplK5Ah7u4A3vbgv5xDyATJFscj`׉ 7cassandra://2GQK1AGKGU7n2SxiEss387nkuS-OMT8hZKXX1ilYNuA_K`s׉ 7cassandra://VBHCdbC_ZPFJyzMJ1StepDPaLhfJq-lwuuQoK6ak7RQ2`׉ 7cassandra://vpiroMzMPArPpBJsJww-cZIWrVKkl7HYQqMYvZh7w8I\͠^cZvEb+׉E 992 Chapter 30 Fusion Welding Processes The metallurgy and properties of the second and third zones depend strongly on the type of metals joined, the particular joining process, the filler metals used (if any), and welding process variables. Recall that a joint produced without using a filler metal is called autogenous; its weld zone is composed of the resolidified base metal. A joint made with a filler metal has a central zone, called the weld metal, and is composed of a mixture of the base and the filler metals. Solidification of the Weld Metal. After the application of heat and introducing the filler metal, if any, into the weld zone, the weld joint is allowed to cool to ambient temperature. The solidification process is similar to that in casting (Section 10.2); it begins with the formation of columnar (dendritic) grains, as shown in Fig. 10.3. These grains are relatively long and they form parallel to the heat flow. Because metals are much better thermal conductors than the surrounding air, the grains lie parallel to the plane of the two components being welded (Fig. 30.20a); in contrast, the grains in a shallow weld are as shown in Fig. 30.20b and c. Grain structure and grain size depend on the specific metal alloy, the welding process employed, and the type of filler metal. Because it begins with a molten state, the weld metal basically has a cast structure, and since it has cooled slowly, the grains are coarse. Consequently, this structure generally has low strength, toughness, and ductility; however, with proper selection of filler-metal composition or of heat treatments following welding, the mechanical properties of the joint can be improved. The resulting structure depends on the particular alloy, its composition, and the thermal cycling to which the joint is subjected. For example, cooling rates may be controlled and reduced by preheating the general weld area prior to welding it. Preheating is important, particularly for metals having high thermal conductivity, such as aluminum and copper (Table 3.2). Without preheating, the heat produced during welding dissipates rapidly through the rest of the parts being joined. Heat-affected Zone. The heat-affected zone (HAZ) is within the base metal itself. It has a microstructure different from that of the base metal prior to welding, because it has been temporarily subjected to elevated (a) (b) 1 mm 0.1 mm 0.43 mm 145 155 260 330 355 Hardness (HV) (d) Figure 30.20: Grain structure in (a) a deep weld and (b) a shallow weld. Note that the grains in the solidified weld metal are perpendicular to their interface with the base metal. (c) Weld bead on a cold-rolled nickel strip produced by a laser beam. (d) Microhardness (HV) profile across a weld bead. Melt zone (c) Heat-affected zone ׉ 7cassandra://mbuTd17OSvpsZgqF25Hv1Dtw1LgVhjC3sOQTRIlDmF8`cZ%Eb±׉E The Weld Joint, Quality and Testing 993 temperatures during welding. The portions of the base metal far away from the heat source do not undergo microstructural changes during welding, because of the much lower temperature to which they have been subjected. The properties and microstructure of HAZ depend on (a) the rate of heat input and cooling and (b) the temperature to which this zone was raised. In addition to metallurgical factors, such as original grain size, grain orientation, and degree of prior cold work, physical properties, such as the specific heat and thermal conductivity of the metals, also influence the size and characteristics of HAZ. The strength and hardness of HAZ (Fig. 30.20d) depend partly on how the original strength and hardness of the base metal was developed originally. As described in Chapters 2 and 4, they may have been developed by (a) cold working, (b) solid-solution strengthening, (c) precipitation hardening, or (d) heat treatments. The effects of these strengthening methods are complex. The heat applied during welding recrystallizes the elongated grains of the cold-worked base metal. Grains that are away from the weld metal will recrystallize into fine, equiaxed grains; grains close to the weld metal have been subjected to elevated temperatures for a longer time, and thus they will grow in size (grain growth, Section 1.7). This region will be softer and have lower strength; such a joint will be weakest at its HAZ. Joints made from dissimilar metals and for alloys strengthened by other methods, the effects of heat on HAZ are complex, and beyond the scope of this book. Details can be found in more advanced texts; see also the Bibliography at the end of this chapter. 30.9.1 Weld Quality As a result of a history of thermal cycling and its attendant microstructural changes, a welded joint may develop various discontinuities. Welding discontinuities also can be caused by an inadequate or careless application of welding techniques or poor operator training. The major discontinuities that affect weld quality are described below. Porosity. Porosity in welds may be caused by • Gases released during melting of the weld area but trapped during solidification • Chemical reactions during welding • Contaminants. Most welded joints have some porosity, generally in the shape of spheres or of elongated pockets (see also Section 10.6.1). The distribution of porosity in the weld zone may be random or the porosity may be concentrated in a certain region in the zone. Porosity in welds can be reduced by the following practices: • Proper selection of electrodes and filler metals • Improved welding techniques, such as preheating the weld area, or increasing the rate of heat input • Proper cleaning and prevention of contaminants from entering the weld zone • Reduced welding speeds, to allow time for gas to escape. Slag Inclusions. Slag inclusions are compounds, such as oxides, fluxes, and electrode-coating materials, that are trapped in the weld zone. If shielding gases are not effective during welding, contamination from the environment also may contribute to such inclusions. Welding conditions also are important: with control of processing parameters, the molten slag will float to the surface of the molten weld metal, and thus it will not become entrapped. ׉ 7cassandra://VBHCdbC_ZPFJyzMJ1StepDPaLhfJq-lwuuQoK6ak7RQ2`cZ%Eb²cZ%Eb±#בCט#u׉׉ 7cassandra://aVKGH0uGdJyQdNtxhlM9Y9SLeCa6XS1x_rwezqB604I `׉ 7cassandra://jTtYIE6xEh4L_FUKB88tJzbu0M6-6yy1PT-U2IGXTdMI`s׉ 7cassandra://t8n57Wwm8s6Tl44g5EcrKns7HuuA9B2Z19MHIS2oZ9k``׉ 7cassandra://UUb3X_z4kkp4ak6JZO_RIUiG6z12I7fototxIXn2wUM̈́͠^cZwEb-ט#u׉׉ 7cassandra://2tVlh5bAYSdty_rmMaM6Fsn4HW-mYIj5viQlZl-lhEoe`׉ 7cassandra://_2sKjLNX9RtNvFeOAj1Ef5lYqDQhVVekT6N585CTEtYJ`s׉ 7cassandra://okmU44-AwyJ8abBKGLuvPputKCPF6OG4Y9Wwt_khDOs!`׉ 7cassandra://59P2PsVWrqbUOS7BnEhD7nxQXV1HDeXWro5qClpMadw͠^cZwEb.׉E3994 Chapter 30 Fusion Welding Processes Slag inclusions can be prevented by implementing the following practice: • Cleaning the weld-bead surface with a wire brush (hand or power) or with a chipper before the next layer is deposited • Providing sufficient shielding gas • Redesigning the joint to permit sufficient space for proper manipulation of the puddle of molten weld metal. Incomplete Fusion. Incomplete fusion produces poor weld beads, such as those shown in Fig. 30.21. A better weld can be obtained by implementing the following practices: • Raising the temperature of the base metal • Cleaning the weld area prior to welding • Modifying the joint design • Changing the type of electrode • Providing sufficient shielding gas. Incomplete penetration occurs when the depth of the welded joint is insufficient. Penetration can be improved by: • Increasing the heat input • Reducing the travel speed during welding • Modifying the joint design • Ensuring that the surfaces to be joined fit together properly. Weld Profile. Weld profile is important not only because of its effects on the strength and appearance of the weld, but also because it can indicate incomplete fusion or the presence of slag inclusions in multiplelayer welds. • Underfilling results when the joint is not filled with the proper amount of weld metal (Fig. 30.22a). • Undercutting results from melting away of the base metal and the subsequent development of a groove in the shape of a sharp recess or notch (Fig. 30.22b). If it is deep or sharp, an undercut can act as a stress raiser, and thus reduce the fatigue strength of the joint and lead to premature failure. Weld Weld Base metal Incomplete fusion (a) Bridging Incomplete fusion Weld Incomplete fusion (b) (c) Figure 30.21: Examples of various discontinuities in fusion welds. ׉ 7cassandra://t8n57Wwm8s6Tl44g5EcrKns7HuuA9B2Z19MHIS2oZ9k``cZ%Eb³׉EThe Weld Joint, Quality and Testing 995 Underfill Inclusions Crack Base metal Incomplete penetration (a) Good weld Porosity Overlap (b) Undercut Lack of penetration (c) Figure 30.22: Examples of various defects in fusion welds. • Overlap is a surface discontinuity (Fig. 30.22b), usually caused by poor welding practice or by selection of improper materials. Figure 30.22c shows a weld that would be considered to be good. Cracks. Cracks may develop at various locations and directions in the weld area. Typical types of cracks are longitudinal, transverse, crater, underbead, and toe cracks (see Fig. 30.23). Cracks generally result from a combination of the following factors: • Temperature gradients, causing thermal stresses in the weld zone • Variations in the composition of the weld zone, causing different rates of contraction during cooling Toe crack Crater cracks Longitudinal crack Underbead crack Weld Weld Longitudinal crack Base metal (a) (b) Figure 30.23: Types of cracks developed in welded joints. The cracks are caused by thermal stresses, similar to the development of hot tears in castings, as shown in Fig. 10.14. Transverse crack Base metal Toe crack Weld Transverse crack Base metal ׉ 7cassandra://okmU44-AwyJ8abBKGLuvPputKCPF6OG4Y9Wwt_khDOs!`cZ%Eb´cZ%Eb³#בCט#u׉׉ 7cassandra://NgxBIC7M4umWCYibaz9PAq1o_dy24vTgcVlSX8YUtO4`׉ 7cassandra://qtSH0SsDqkEK-W-U2C1u9LHtEVudylI8SQIB-2rdxakKg`s׉ 7cassandra://mqY-u7UNJjlM1lgao4FQLPFbIv2RgbhriQSJbskxafI`׉ 7cassandra://2zkXg1aTgKgha8ZLpd0HlXx2NwRb2AfdISb3zwXxrmIT͠^cZwEb0ט#u׉׉ 7cassandra://EpIxghc6m08jvG95WvwXbIxFcFmMYDkj1yb_v5ZX2DkJ`׉ 7cassandra://fKe4rIFL5WcRa23JK3uTH4I9qpK09-SlpyWVj29a_6wJX`s׉ 7cassandra://2jNfhtzKR6gvda97WygAiCGl4DjJf91_hnoepCofWa8`׉ 7cassandra://4bh-7eU-X67KjIZjTSicM6bpSolCeAG-9iPfrdPYKRc͞ ͠^cZwEb1׉E ?996 Chapter 30 Fusion Welding Processes Figure 30.24: Crack in a weld bead. The two welded components were not allowed to contract freely after the weld was completed. • Embrittlement of grain boundaries (Section 1.5.2), caused by the segregation of such elements as sulfur to the grain boundaries and occurring when the solid–liquid boundary moves as the weld metal begins to solidify • Hydrogen embrittlement (Section 2.10.2) • Inability of the weld metal to contract during cooling (Fig. 30.24), a situation similar to hot tears that develop in castings (Fig. 10.14) and is related to excessive restraint of the workpiece during the welding operation. Cracks also are classified as hot cracks (developed while the joint is still at elevated temperatures) and cold cracks (after the weld metal has cooled). The basic crack-prevention measures in welding are: • Modify the joint design to minimize stresses developed from shrinkage during cooling • Change the parameters, procedures, and welding sequence • Preheat the components to be welded • Avoid rapid cooling of the welded joint. Lamellar Tears. In describing the anisotropy of plastically deformed metals in Section 1.5, it was stated that the workpiece is weaker when tested in its thickness direction because of the alignment of nonmetallic impurities and inclusions (stringers). This condition is observed particularly in rolled plates and structural shapes. In welding such components, lamellar tears may develop, because of shrinkage of the restrained components of the structure during cooling. Tears can be avoided by providing for shrinkage of the members or by modifying the joint design to make the weld bead penetrate the weaker component more deeply. Surface Damage. Some of the hot metal may spatter during welding and be deposited, as small droplets, on adjacent surfaces. In arc-welding processes, the electrode may inadvertently touch the parts being welded at places other than the weld zone, called arc strikes. The associated surface discontinuities may be objectionable for reasons of appearance or in subsequent use or assembly of the welded structure. If severe, these discontinuities may adversely affect the properties of the welded structure, particularly notch sensitive metals. Using proper welding techniques and procedures is important in avoiding surface damage. ׉ 7cassandra://mqY-u7UNJjlM1lgao4FQLPFbIv2RgbhriQSJbskxafI`cZ%Ebµ׉EvThe Weld Joint, Quality and Testing 997 Weld Weld Transverse shrinkage Angular distortion (a) Longitudinal shrinkage (b) Weld Neutral axis Weld (c) (d) Figure 30.25: Distortion of parts after welding. Distortion is caused by differential thermal expansion and contraction of different regions of the welded assembly. Residual Stresses. Because of localized heating and cooling during welding, the expansion and contraction of the weld area causes residual stresses (see also Section 2.11). Residual stresses can lead to the following defects: • Distortion, warping, and buckling of the welded parts (Fig. 30.25) • Stress-corrosion cracking (Section 2.10.2) • Additional distortion if a portion of the welded structure is subsequently removed, such as by machining, drilling, or sawing • Reduced fatigue life of the welded structure. The type and distribution of residual stresses developed in welds is best described by referring to Fig. 30.26a. When two plates are being welded, a long narrow zone is subjected to elevated temperatures, while the plates, as a whole, are essentially at ambient temperature. After the weld is completed and as time elapses, heat from the weld zone dissipates laterally into the plates, while the weld area begins to cool. The plates then begin to expand longitudinally, while the welded length begins to contract (Fig. 30.25). If the plate is not constrained, it will warp, as shown in Fig. 30.25a. If, however, the plate is not allowed to warp, it will develop residual stresses, which typically are distributed throughout the material (see stresses shown in Fig. 30.26b). Note that the magnitude of the compressive residual stresses in the Residual stress Compressive Tensile Base metal Weld (a) (b) Figure 30.26: Residual stresses developed in (a) a straight-butt joint; note that the residual stresses shown in (b) must be balanced internally (see also Fig. 2.30.) ׉ 7cassandra://2jNfhtzKR6gvda97WygAiCGl4DjJf91_hnoepCofWa8`cZ%Eb¶cZ%Ebµ#בCט#u׉׉ 7cassandra://efO1X1v7CpE4hG0EA58rsZS50eWMDEw_1Ev_GQUY9T8'`׉ 7cassandra://axc00DF-W5r71VXbM1yeLf7Xg4zFRHIObqhZiqysC9Ul`s׉ 7cassandra://7jlFsMvuN32EsHWB1qckJaC2iof4uquQfUU_n_jIV8Q{`׉ 7cassandra://4KumGP1k44FH9ZdtVwLMZV6BktuzIqgI-zrcoK5TpGQͳ͠^cZxEb3ט#u׉׉ 7cassandra://kQpaGwuv-Th2PcPPoXlkA13mHvGF40Fzgt3cIX338MI`׉ 7cassandra://32Nvrjhw6eSOujy7zegb3yr5Jo4VgdJ5XKsavtH3OaE``s׉ 7cassandra://sx3lO2u58k6McAFsVBqDI5ooNU5qSGV5VpVpD5tIeXo[`׉ 7cassandra://4-AQMT39UZal0LDLSCdKDvH6p9KW3B-CLeg09Dfv9gsb_͠^cZxEb4׉E998 Chapter 30 Fusion Welding Processes plates diminishes to zero at the top and bottom surfaces of the welded plate. Note also that because no external forces are acting on the welded plates, the tensile and compressive forces represented by these residual stresses must balance each other. The sequence of events leading to the distortion of a simple tubular welded structure are shown in Fig. 30.27. Prior to welding, the structure is stress free, as shown in Fig. 30.27a, and it may be sufficiently rigid; some fixturing may be present to support the structure as part of a larger assembly if necessary. During welding, the molten metal fills the gap between the surfaces to be joined and forms a weld bead. As the weld begins to solidify, both the weld bead and the surrounding material begin to cool down to room temperature. As they cool, they would contract but are constrained by the rest of the weldment; as a result, the part distorts (Fig. 30.27c) and residual stresses develop. The residual stresses produce the deformation shown in Fig. 30.27c and put the weld and the heat-affected zone into a state of residual tension, which is not desirable for fatigue performance. In general, HAZ is less fatigue resistant than the base metal. Because the residual stresses developed can be harmful, it is not unusual to stress relieve welds in highly stressed or fatigue-susceptible applications (see below). Recall that the weld itself may have porosity (see Fig. 30.22b), which also can act as a stress raiser and lead to fatigue crack growth. In complex welded structures, residual-stress distributions are three dimensional, and difficult to analyze. Note that the two plates shown in Fig. 30.26 were not restrained from movement; in other words, the plates were not an integral part of a larger structure. If, however, they were restrained, reaction stresses would develop, because the plates are not free to expand or contract, a situation that arises particularly in structures with high stiffness. Stress Relieving of Welds. Effects that residual stresses can cause, such as distortion, buckling, and cracking, can be reduced by preheating the base metal or the parts to be welded. Preheating reduces distortion by reducing the cooling rate following welding and the level of thermal stresses developed, by lowering the elastic modulus; this technique also reduces shrinkage and possible cracking of the joint. For optimum results, preheating temperatures and cooling rates must be controlled in order to maintain acceptable strength and toughness of welded structures. Workpieces may be heated in several ways, including (a) in a furnace, (b) electrically, either resistively or inductively, or (c) by radiant lamps or hot-air blast, especially for thin sections. The temperature and time required for stress relieving depend on the type of material and on the magnitude of the stresses developed. Other methods of stress relieving include peening, hammering, or surface rolling (Section 34.2) of the weldbead area. These techniques induce compressive residual stresses, which, in turn, lower or eliminate tensile residual stresses in the weld. For multilayer welds, the first and last layers should not be peened, to protect them against possible peening damage on the surface. Residual stresses also can be relieved or reduced by plastically deforming the structure itself by a small amount. This technique can be used in welded pressure vessels, by pressurizing the vessels internally, called Hot zone Metal frame Melt (pushed out) (expanded) No shape change Contraction Internal (residual) tensile stress Distortion (a) (b) (c) Figure 30.27: Distortion of a welded structure. Source: After J.A. Schey. ׉ 7cassandra://7jlFsMvuN32EsHWB1qckJaC2iof4uquQfUU_n_jIV8Q{`cZ%Eb·׉E The Weld Joint, Quality and Testing 999 proof stressing. In order to reduce the possibility of sudden fracture under high internal pressure, the weld must be made properly and must be free of notches and discontinuities, which can act as stress raisers. In addition to being preheated for stress relieving, welds may be heat treated by various techniques in order to modify other properties. These techniques include annealing, normalizing, quenching, and tempering of steels and solution treatment and aging of various alloys, as described in Chapter 4. 30.9.2 Weldability The weldability of a metal is generally defined as its capacity to be welded into a specific structure that has certain properties and characteristics and will satisfactorily meet service requirements.Weldability involves a large number of variables, and thus generalizations are difficult. Recall that material characteristics, such as alloying elements, impurities, inclusions, grain structure, and processing history, of both the base metal and the filler metal, are all important. For example, weldability of steels decreases with increasing carbon content, because of martensite formation (see Section 4.7) and thus reduces the strength of the weld. Coated steel sheets (Chapter 34) also present various challenges in welding, depending on the type and thickness of the coating. Because of the effects of melting and solidification and of the associated microstructural changes, a thorough consideration of the phase diagram and the response of the metal or alloy to sustained elevated temperatures is essential. Also influencing weldability are mechanical and physical properties: strength, toughness, ductility, notch sensitivity, elastic modulus, specific heat, melting point, thermal expansion, surface-tension characteristics of the molten metal, and corrosion resistance. Preparation of surfaces for welding is important, as are the nature and properties of surface-oxide films and of adsorbed gases (see also Section 33.2). The specific welding process employed significantly affects the temperatures developed and their distribution in the weld zone. Other factors that affect weldability are shielding gases, fluxes, moisture content of the coatings on electrodes, welding speed, welding position, cooling rate, and level of preheating, as well as such post-welding techniques as stress relieving and heat treating. Weldability of Ferrous Materials: • Plain-carbon steels: Generally excellent for low-carbon steels, fair to good for medium-carbon steels, and poor for high-carbon steels. • Low-alloy steels: Similar to medium-carbon steels. • High-alloy steels: Generally good under well-controlled conditions. • Stainless steels: Generally weldable by various processes. • Cast irons: Generally weldable, although their weldability varies greatly. Weldability of Nonferrous Materials: • Aluminum alloys: Weldable at a high rate of heat input; an inert shielding gas and lack of moisture are important. Aluminum alloys containing zinc or copper generally are considered unweldable. • Copper alloys: Depending on composition, generally weldable at a high rate of heat input; an inert shielding gas and lack of moisture are important. • Magnesium alloys: Weldable using a protective shielding gas and fluxes. • Nickel alloys: Similar to stainless steels; lack of sulfur is undesirable. • Titanium alloys: Weldable with proper use of shielding gases. ׉ 7cassandra://sx3lO2u58k6McAFsVBqDI5ooNU5qSGV5VpVpD5tIeXo[`cZ%Eb¸cZ%Eb·#בCט#u׉׉ 7cassandra://7L594zNV3GisSgm40MW0eZ156KS9pgYMIU0_8NgIxp4`׉ 7cassandra://Pe6JTxwO1Z3-UuSKETr6nYfmiJZVQrV19bBp_N738awX`s׉ 7cassandra://m9ai_4tBXjgNg7hwW43lGJTORcz33mG57WV0j9iJZ50`׉ 7cassandra://Wn9kxp6iRaOYigI0M5b1gajTgUV8EH_AvQAyPkDz6KQgt͠^cZxEb6ט#u׉׉ 7cassandra://TbzCHe7Wg7Cvq1UVXR_3EQvuQG2mci3EPHVOOv428-Iu`׉ 7cassandra://S-LQQ5TJhTnm1dXQ29TPxrNUwjIAZnshFqks779U7zQO`s׉ 7cassandra://v1Q0Ft5VUJtrs8SJ5s32ZSQ2GpCgv1Kgl-N2ZsULqzkE`׉ 7cassandra://_GwI8q4Nsu4h-do3KClWUXOX-dF5XJOipug8E6VAc2wͯP͠^cZyEb8׉E 1000 Chapter 30 Fusion Welding Processes • Tantalum: Similar to titanium. • Tungsten: Weldable under well-controlled conditions. • Molybdenum: Similar to tungsten. • Niobium (columbium): Good weldability. 30.9.3 Testing of Welds Several standardized tests and test procedures have been established and are available from organizations such as the American Society for Testing and Materials (ASTM), the American Welding Society (AWS), the American Society of Mechanical Engineers (ASME), the American Society of Civil Engineers (ASCE), and various federal agencies. Welded joints may be tested either destructively or nondestructively (see also Sections 36.10 and 36.11). Each technique has certain capabilities and limitations, as well as process parameter sensitivity, reliability, and requirements for special equipment and operator skill. Destructive Testing Techniques: • Tension test. Longitudinal and transverse tension tests are performed on specimens removed from actual welded joints and from the weld-metal area. Stress–strain curves are then developed, using the procedures described in Section 2.2. These curves indicate the yield strength, ultimate tensile strength, and ductility of the welded joint (elongation and reduction of area) in different locations and directions. • Tension-shear test. The specimens in the tension-shear test (Fig. 30.28a and b) are prepared to simulate conditions to which actual welded joints are subjected. The specimens are subjected to tension, so that the shear strength of the weld metal and the location of fracture can be determined. • Bend test. Several bend tests have been developed to determine the ductility and strength of welded joints. In one common test, the welded specimen is bent around a fixture (wraparound bend test, Fig. 30.28c). In another, the specimens are tested in three-point transverse bending (Fig. 30.28d; see also Fig. 2.11a). These tests help to determine the relative ductility and strength of welded joints. • Fracture toughness test. This test commonly utilizes impact testing techniques (Section 2.9). Charpy V-notch specimens are first prepared and tested for toughness. In the drop-weight test, the energy is supplied by a falling weight. • Creep and corrosion tests. Creep tests (Section 2.8) are essential in determining the behavior of welded joints and structures subjected to elevated temperatures. Welded joints also may be tested for their resistance to corrosion (Section 3.8); because of the difference in composition and microstructure, preferential corrosion may take place in the weld zone. Nondestructive Testing Techniques. Welded structures often have to be tested nondestructively (Section 36.10), particularly for critical applications in which weld failure can be catastrophic, such as in pressure vessels, load-bearing structural members, and power plants. Nondestructive testing techniques for welded joints generally consist of the following methods: • Visual • Radiographic (X-rays) • Magnetic-particle • Liquid-penetrant • Ultrasonic. ׉ 7cassandra://m9ai_4tBXjgNg7hwW43lGJTORcz33mG57WV0j9iJZ50`cZ%Eb¹׉EJoint Design and Process Selection 1001 Root bend Longitudinal tension shear Clamp Roller Weld Face bend Transverse tension shear Side bend (a) (b) (c) Figure 30.28: (a) Specimens for longitudinal tension-shear testing and for transfer tension-shear testing. (b) Wraparound bend-test method. (c) Three-point transverse bending of welded specimens. As an example of another nondestructive method, testing for hardness distribution (see Section 2.6 and Figs. 16.3 and 30.20) in the weld zone also would be a useful indicator of weld strength and microstructural changes. 30.10 Joint Design and Process Selection In describing individual welding processes, several examples were given regarding the types of welds and joints produced and their applications in various consumer and industrial products. Typical types of joints produced by welding, together with their terminology, are given in Fig. 30.29. Standardized symbols commonly used in engineering to describe the types of welds are shown in Fig. 30.30. These symbols identify the type of weld, groove design, weld size and length, welding process, sequence of operations, and various other essential information. General design guidelines for welding are given in Fig. 30.31. Various other types of joint design are given in Chapters 31 and 32. Important design guidelines are summarized below. • Product design should minimize the number of joints because, unless automated, welding can be time consuming and costly. • Weld locations should be selected so as to avoid excessive local stresses or stress concentrations as well as for better appearance. • Weld location should be selected so as not to interfere with any subsequent processing of the joined components or with their intended uses. • The need for edge preparation should be minimized or avoided. • Weld-bead size should be as small as possible while maintaining joint strength, in order to conserve weld metal and for better appearance. ׉ 7cassandra://v1Q0Ft5VUJtrs8SJ5s32ZSQ2GpCgv1Kgl-N2ZsULqzkE`cZ%EbºcZ%Eb¹#בCט#u׉׉ 7cassandra://ukyGhkQKxqKqLh20JtGBDtwbywMZAcuttfZ5hrg-jQQDE`׉ 7cassandra://VE9DRhMNrrIFmW47mZjyysclQx40Eeb7iQ1_toKrrZMF`s׉ 7cassandra://-Pt4m8vgnguYlYoCkTROIEfiZ3TDSit7dUgS8w8cRIQ`׉ 7cassandra://isPKtsE0Ktn8qCGc1JEM8rv_xrzi6MkF5ZncROIPGSQEe͠^cZzEb=ט#u׉׉ 7cassandra://1ySgHBnNScM67ZrtmO072hpi3NH5ojO3Uk937b1QOVk1`׉ 7cassandra://P_xijdSj9oBnow6BQcikqpRw17CsWWQHAXNliZrpEzYD`s׉ 7cassandra://z6cjXOVqepC20hu7j5OUKUFCTER9f1Owxq2vb8QnbFA`׉ 7cassandra://r06FGWUEINPnNJimyktSh1XLWGsSvE5CHPkSZh1qquoͰH͠^cZzEb?׉EK1002 Chapter 30 Fusion Welding Processes (a) Single square-groove weld (b) Single V-groove weld (c) Double V-groove weld (d) Single V-groove weld (with backing) (e) Single-flare bevel-groove weld (f) Single-flare V-groove weld (g) Double-flare bevel-groove weld (h) Double-flare V-groove weld (i) Butt joint (j) Corner joint (k) T joint (l) Lap joint (m) Edge joint Figure 30.29: Examples of welded joints and their terminology. Welding Process Selection. In addition to considering welding process characteristics, capabilities, and material considerations described thus far, selection of a weld joint and an appropriate process involve the following considerations (see also Chapters 31 and 32): • Configuration of the parts to be joined, joint design, thickness and size of components, and number of joints required • Methods used in making the components to be joined • Types of materials involved • Location, accessibility, and ease of joining • Weld application and service requirements, including type of loading, stresses generated, and environment • Effects of distortion, warping, appearance, discoloration, and service • Costs involved in edge preparation, joining, and post-processing, including machining, grinding, and finishing operations • Costs of equipment, materials, labor, and skills required of the whole operation. ׉ 7cassandra://-Pt4m8vgnguYlYoCkTROIEfiZ3TDSit7dUgS8w8cRIQ`cZ%Eb»׉EaJoint Design and Process Selection 1003 Bead Fillet Plug or slot Square V Bevel U J Basic arc- and gas-weld symbols Groove Basic resistance-weld symbols Spot Projection Seam Flash or upset Finish symbol Contour symbol Root opening, depth of filling for plug and slot welds Effective throat Depth of preparation or size in inches Reference line Specification, process or other reference Tail (omitted when reference is not used) Basic weld symbol or detail reference S(E) T A B Arrow connects reference line to arrow side of joint. Use break as at A or B to signify that arrow is pointing to the grooved member in bevel or J-grooved joints. Poor Load Figure 30.30: Standard identification and symbols for welds. Good Poor Load (a) Cut not square 908 (c) (d) Surface to be machined Burr (b) Deburred edge Good F A R L @ P Length of weld in inches Groove angle or included angle of countersink for plug welds Pitch (center-to-center spacing) of welds in inches Weld-all-around symbol Field weld symbol (e) (f) Figure 30.31: Some design guidelines for welds. Source: After J.G. Bralla. (Both sides) (Arrow side) (Other side) ׉ 7cassandra://z6cjXOVqepC20hu7j5OUKUFCTER9f1Owxq2vb8QnbFA`cZ%Eb¼cZ%Eb»#בCט#u׉׉ 7cassandra://xsnY4XwRybFxA6b1VdcEeT7X_GiIfUbdDWObDEfSYV8[#`׉ 7cassandra://ce2Na8T3r-EKHm1bWuQvjr2SDdEchMLIQ55YYlVwsAQK<`s׉ 7cassandra://ii7P9Bqp1IwgEfCOEWiJZ3MXDYSlFrAEx3XxvNHm2bwQ`׉ 7cassandra://Z050-rw6fXQBJP1JuZgFQffBBC9ikpTPi7vZZhkxag8͑͠^cZzEbBט#u׉׉ 7cassandra://yD0ogIwaJHK-jRrCFQku2P1g-Zkmby6hBfpL3CnOGug`׉ 7cassandra://zPgjpIvbajoxI0jdcBF-1tXbDs6YJvFy3duxPxTV0JUL`s׉ 7cassandra://6UjhtTCnC7v061ILv8XOb2TRowlXgZ47_3y0yg7j4NQ`׉ 7cassandra://KUyjaLtg2bXfdPpYiSYACGfnqsaOOCWjh8B_TIG57zUB͠^cZzEbC׉E1004 Chapter 30 Fusion Welding Processes Moment 5 M 3M Continuous weld Intermittent welds Welds (a) Weld Base metal (b) Single V-groove (c) Figure 30.32: Examples of weld designs used in Example 30.3. Example 30.3 Weld Design Selection Three different types of weld designs are shown in Fig. 30.32. The two vertical joints in Fig. 30.32a can be welded either externally or internally. Note that full-length external welding will take considerable time and will require more weld material than the alternative design, which consists of intermittent internal welds. Moreover, in the alternative method the appearance of the structure is improved and distortion is reduced. In Fig. 30.32b, it can be shown that the design on the right can carry three times the moment M of the one on the left. Note also that both designs require the same amount of weld metal and welding time. In Fig. 30.32c, the weld on the left requires about twice the amount of weld material than does the design on the right. Moreover, because more material must now be machined, the design on the left will require more time for edge preparation, and more base metal will be wasted. Summary • Oxyfuel–gas, arc, and high-energy-beam welding are among the most commonly used joining operations. Gas welding uses chemical energy to supply the necessary heat; arc and high-energy-beam welding use electrical energy. • In all the processes described, heat is used to bring the joint being welded to a liquid state. Shielding gases are used to protect the molten-weld pool and the weld area against oxidation. Filler metals may or may not be used in oxyfuel–gas and arc welding. • Selection of a welding process for a particular operation depends on the workpiece material, its thickness and size, its shape complexity, the type of joint required, the strength required, and the change in product appearance caused by welding. • A variety of welding equipment is available, now mostly computer and robot controlled, with programmable features. Double V-groove ׉ 7cassandra://ii7P9Bqp1IwgEfCOEWiJZ3MXDYSlFrAEx3XxvNHm2bwQ`cZ%Eb½׉EKey Terms 1005 • Cutting of metals also can be done by the processes based on oxyfuel–gas and arc welding. The highest temperatures for cutting are obtained by plasma-arc cutting. • The welded joint consists of solidified metal and a heat-affected zone; each has a wide variation in their microstructure and properties, depending on the metals joined and on the filler metals. The metallurgy of the welded joint is important in all welding processes, because it determines the strength, toughness, and quality of the joint. • Discontinuities, such as porosity, inclusions, incomplete welds, tears, surface damage, and cracks, can develop in the weld zone. Residual stresses and relieving them are important considerations. • Weldability of metals and alloys depends greatly on their composition, mechanical and physical properties, type of welding operation and process parameters employed, and the control of welding parameters. • General guidelines are available for the selection of suitable and economical methods for a particular welding application. Key Terms Arc cutting Arc welding Atomic-hydrogen welding Base metal Carburizing flame Coated electrode Consumable electrode Discontinuities Drag lines Electrode Electrogas welding Electron-beam welding Electroslag welding Filler metal Flux Flux-cored arc welding Fusion welding Gas metal-arc welding Gas tungsten-arc welding Heat-affected zone Inclusions Kerf Keyhole technique Laser-beam welding Laser GMAW welding Neutral flame Nonconsumable electrode Oxidizing flame Oxyfuel–gas cutting Oxyfuel–gas welding Plasma-arc welding Polarity Porosity Reducing flame Residual stresses Shielded metal-arc welding Slag Stick welding Submerged-arc welding Tears Thermit welding Weld profile Weld metal Weldability Welding gun ׉ 7cassandra://6UjhtTCnC7v061ILv8XOb2TRowlXgZ47_3y0yg7j4NQ`cZ%Eb¾cZ%Eb½#בCט#u׉׉ 7cassandra://tNjSZ8duF5zWbWVhCtgnH71fZ8x-tmLqzPG_CoPkBPk`׉ 7cassandra://d_B2U9hMMEkMAylD78l2Ts52P0Yr0y6GQ13SD3xPiP8f`s׉ 7cassandra://b7XK97elkaPqvTQ5SNZOgjBiqYJs0tiTkzXIdngs0a8`׉ 7cassandra://G8ze39H6_fx3X1RAHkB008xJ4OPm7SMKQ_mcOeTOK_Ee͠^cZzEbEט#u׉׉ 7cassandra://wlG8K0U45zm_ySivkgfCthXd8eTwF7ErQOrTR-rUqbILo`׉ 7cassandra://AfWLCnu9abyofMYPkYRIaD02qAjQ8wCNZEGObef68Mca3`s׉ 7cassandra://2wmQumi_6kqnAlW9VP_YuIYcZQYemUGRLXDDGoyBMG0E`׉ 7cassandra://O2hOi9pgt-0ci7RTcD5627qvkHNSydyuDZKaIl6ze_4Jz͠^cZzEbG׉E 1006 Chapter 30 Fusion Welding Processes Bibliography ASM Handbook,Vol.6: Welding, Brazing, and Soldering, ASM International, 1993. ASM Handbook,Vol.6A: Welding Fundamentals and Processes, ASM International, 2011. Bohnart, E,R., Welding: Principles and Practices, 5th ed., McGraw-Hill, 2017. Campbell, F.C., Joining, ASM International, 2011. Katayama, S., (ed.), Handbook of Laser Welding Technologies, Woodhead Publishing, 2013. Kou, S., Welding Metallurgy, 2nd ed., Wiley-Interscience, 2002. Lippold, J.C., Welding Metallurgy and Weldability, Wiley, 2014. Modern Welding Technology, 11th ed., Goodheart-Willcox, 2012. Phillips, D.H., Welding Engineering: An Introduction, Wiley, 2016. Welding Handbook, various volumes and editions, American Welding Society. Welding Inspection Handbook, 4th ed., American Welding Society, 2015. Weman, K., Welding Processes Handbook, 2nd ed., Woodhead Publishing, 2011. Yadav, K.S., Advanced Welding Technology, Rajsons Publishing, 2018. Review Questions 30.1. Describe fusion as it relates to welding operations. 30.2. Explain the features of neutralizing, reducing, and oxidizing flames. Why is a reducing flame so called? 30.3. What is stick welding? 30.4. Explain the basic principles of arc-welding processes. 30.5. Why is shielded metal-arc welding a commonly used process? Why is it also called stick welding? 30.6. What keeps the weld bead on a steel surface from oxidizing (rusting) during welding? 30.7. Describe the functions and characteristics of electrodes. What functions do coatings have? How are electrodes classified? 30.8. What are the similarities and differences between consumable and nonconsumable electrodes? 30.9. What properties are useful for a shielding gas? 30.10. What are the advantages to thermite welding? 30.11. Explain where the energy is obtained in thermite welding. 30.12. Explain how cutting takes place when an oxyfuel–gas torch is used. How is underwater cutting done? 30.13. What is the purpose of flux? Why is it not needed in gas tungsten-arc welding? 30.14. What is meant by weld quality? Discuss the factors that influence it. 30.15. How is weldability defined? 30.16. Why are welding electrodes generally coated? 30.17. Describe the common types of discontinuities in welded joints. 30.18. What types of destructive tests are performed on welded joints? 30.19. Explain why hydrogen welding can be used to weld tungsten without melting the tungsten electrode. 30.20. What materials can be welded by Laser SMAW hybrid welding? ׉ 7cassandra://b7XK97elkaPqvTQ5SNZOgjBiqYJs0tiTkzXIdngs0a8`cZ%Eb¿׉E uQualitative Problems 1007 Qualitative Problems 30.21. Explain the reasons that so many different welding processes have been developed over the years. 30.22. It has been noted that heat transfer in gas-metal arc welding is higher than in shielded-metal arc welding. Explain why this would be the case. Which process would lead to more heat-affected zone cracking in hardened steels? 30.23. Explain why some joints may have to be preheated prior to welding. 30.24. Describe the role of filler metals in welding. 30.25. List the processes that can be performed with two electrodes. What are the advantages in using two electrodes? 30.26. What is the effect of the thermal conductivity of the workpiece on kerf width in oxyfuel–gas cutting? Explain. 30.27. Describe the differences between oxyfuel–gas cutting of ferrous and of nonferrous alloys. Which properties are significant? 30.28. Could you use oxyfuel–gas cutting for a stack of sheet metals? (Note: For stack cutting, see Fig. 24.28e.) Explain. 30.29. What are the advantages of electron-beam and laser-beam welding compared with arc welding? 30.30. Describe the methods by which discontinuities in welding can be avoided. 30.31. Explain the significance of the stiffness of the components being welded on both weld quality and part shape. 30.32. Comment on the factors that influence the size of the two weld beads shown in Fig. 30.15. 30.33. Which of the processes described in this chapter are not portable? Can they be made so? Explain. 30.34. Thermit welding is commonly used for welding railroad rails. List the reasons that make thermit welding attractive for this application. Review your list and create a list of products that would be suitable for thermit welding, and then identify any difficulties you would expect in applying thermit welding to that application. 30.35. Describe your observations concerning the contents of Table 30.1. 30.36. What determines whether a certain welding process can be used for workpieces in horizontal, vertical, or upside-down positions or, for that matter, in any position (see Table 30.1)? Explain and give examples of appropriate applications. 30.37. Comment on the factors involved in electrode selection in arc-welding processes. 30.38. In Table 30.1, the column on the distortion of welded components is ordered from lowest distortion to highest. Explain why the degree of distortion varies among different welding processes. 30.39. Explain the significance of residual stresses in welded structures. 30.40. Rank the processes described in this chapter in terms of (a) cost and (b) weld quality. 30.41. Must the filler metal be made of the same composition as the base metal that is to be welded? Explain. 30.42. What is weld spatter? What are its sources? How can spatter be controlled? Explain. 30.43. Describe your observations concerning Fig. 30.20. 30.44. If the materials to be welded are preheated, is the likelihood for porosity increased or decreased? Explain. 30.45. Discuss the need for and role of fixtures in holding workpieces in the welding operations described in this chapter. 30.46. Why is the quality of welds produced by submerged arc welding very good? ׉ 7cassandra://2wmQumi_6kqnAlW9VP_YuIYcZQYemUGRLXDDGoyBMG0E`cZ%EbcZ%Eb¿#בCט#u׉׉ 7cassandra://5crqg8SG6O10cMnafafl00lOMJYBh4QsOzxsNY_GrQ0`׉ 7cassandra://3cC7qvS8A-BVGxRgWkBdRye69ER9sIrjN1E7ozcR6oYf8`s׉ 7cassandra://g5zOx5brsYUIa9AiJui_gUunFCPnuJD68TBoq_T7r1o`׉ 7cassandra://YJTDiW1AmIgTp42tx28xxaNY2iWD1-LQSJNGPAdZHp4V͠^cZ{EbIט#u׉׉ 7cassandra://pfX9M0P8zB7Rh7bOj0gKwD_Kmbsu8_Xsw7YT5WhVgV0`׉ 7cassandra://jZuRdOTlhSPwwpT7eyyZ6cSCuGOQhNUO73IdPX9viXI^`s׉ 7cassandra://z9XkLW29GzFcibEMhRmNsEYM0X9yaKQK_Ts4XyyZJZ4`׉ 7cassandra://WyNTP19O-_Cr58mjsMZ5rIK4k7IIFNkv6DRcM0nClcww͠^cZ{EbJ׉E c1008 Chapter 30 Fusion Welding Processes 30.47. Explain why the electroslag welding process is suitable for thick plates and heavy structural sections. 30.48. Explain why the grains in Fig. 30.20c grow in the particular directions shown. Quantitative Problems 30.49. Plot the hardness in Fig. 30.20d as a function of the distance from the top surface, and discuss your observations. 30.50. A welding operation will take place on carbon steel. The desired welding speed is around 20 mm/s. If an arc-welding power supply is used with a voltage of 12 V, what current is needed if the weld widthistobe5mm? 30.51. In Fig. 30.26b, assume that most of the top portion of the top piece is cut horizontally with a sharp saw. The residual stresses will now be disturbed and the part will change its shape, as was described in Section 2.11. For this case, how do you think the part will distort: curved downward or upward? Explain. (See also Fig. 2.30d.) 30.52. A welding operation takes place on an aluminum-alloy plate. A pipe 50 mm in diameter, with a 5 mm wall thickness and a 50 mm length, is butt welded onto an extruded L-section 150 mm by 150 mm by 5 mm thick, with a length of 1 m. If the weld zone in a gas tungsten arc welding process is approximately 10 mm wide, what would be the temperature increase of the entire structure due to the heat input from welding only? What if the process were an electron-beam welding operation, with a bead width of 2 mm? Assume that the electrode and aluminum alloy require 2.9 Joules to melt one cubic millimeter. 30.53. An arc welding operation is taking place on carbon steel. The desired welding speed is around 24 mm/sec. If the power supply is 12 V, what current is needed if the weld width is to be 6 mm? 30.54. In oxyacetylene, arc, and laser-beam cutting, the processes basically involve melting of the workpiece. If a 50 mm diameter hole is to be cut from a 250 mm diameter, 12 mm thick plate, plot the mean temperature rise in the blank as a function of kerf. Assume that one-half of the energy goes into the blank. 30.55. A submerged arc welding operations takes place on 10 mm thick stainless steel, producing a butt weld as shown in Fig. 30.29c. The weld geometry can be approximated as a trapezoid with 15 mm and 10 mm as the top and bottom dimensions, respectively. If the voltage provided is 40 V at 400 A, estimate the welding speed if a stainless steel filler wire is used. 30.56. 6061 aluminum plates with a 2.5 mm thickness are to be butt-welded by GMAW using a 1 mm diameter electrode. The applied voltage is 22 V, the current is 125A, and the arc travel speed is 16 mm/s. Calculate the power, the deposition rate of electrode material, and the required electrode feed rate. 30.57. Assume that you are asked to give a quiz to students on the contents of this chapter. Prepare three quantitative problems and three qualitative questions, and supply the answers. Synthesis, Design, and Projects 30.58. Comment on workpiece size and shape limitations for each of the processes described in this chapter. 30.59. Arc blow is a phenomenon where the magnetic field induced by the welding current passing through the electrode and workpiece in shielded metal arc welding interacts with the arc and causes severe weld splatter. Identify the variables that you feel are important in arc blow. When arc blow is a problem, would you recommend minimizing it by using AC or DC power? ׉ 7cassandra://g5zOx5brsYUIa9AiJui_gUunFCPnuJD68TBoq_T7r1o`cZ%Eb׉E Synthesis, Design, and Projects 1009 30.60. Review the types of welded joints shown in Fig. 30.29 and give an application for each. 30.61. Comment on the design guidelines given in various sections of this chapter. 30.62. The accompanying figure shows a metal sheave that consists of two matching pieces of hot-rolled, low-carbon-steel sheets. These two pieces can be joined either by spot welding or by V-groove welding. Discuss the advantages and limitations of each process for this application. 3.43 mm 61.91 mm (a) Spot weld (b) V-groove weld (c) 30.63. You are asked to inspect a welded structure for a critical engineering application. Describe the procedure that you would follow in order to determine the safety of the structure. 30.64. Discuss the need for, and the role of, work-holding devices in the welding operations described in this chapter. 30.65. Make a list of welding processes that are suitable for producing (a) butt joints, where the weld is in the form of a line or line segment, (b) spot welds, and (c) both butt joints and spot welds. Comment on your observations. 30.66. Explain the factors that contribute to the differences in properties across a welded joint. 30.67. Explain why preheating the components to be welded is effective in reducing the likelihood of developing cracks. 30.68. Review the poor and good joint designs shown in Fig. 30.31, and explain why they are labeled so. 30.69. In building large ships, there is a need to weld thick and large sections of steel together to form a hull. Consider each of the welding operations discussed in this chapter, and list the benefits and drawbacks of that particular joining operation for this application. 30.70. Inspect various parts and components in (a) an automobile, (b) a major appliance, and (c) kitchen utensils, and explain which, if any, of the processes described in this chapter has been used in joining them. 30.71. Comment on whether there are common factors that affect the weldability, castability, formability, and machinability of metals, as described in various chapters of this book. Explain with appropriate examples. 30.72. If you find a flaw in a welded joint during inspection, how would you go about determining whether or not the flaw is significant? 30.73. Lattice booms for cranes are constructed from extruded cross-sections (see Fig. 15.2) that are welded together. Any warpage that causes such a boom to deviate from straightness will severely reduce its lifting capacity. Conduct a literature search on the approaches used to minimize distortion due to welding and how to correct it, specifically in the construction of lattice booms. 30.74. A common practice in repairing expensive broken or worn parts (such as those that may occur when a fragment is broken from a forging) is to fill the area with layers of weld beads and then to machine the part back to its original dimensions. Make a list of the precautions that you would suggest to someone who uses this approach. ׉ 7cassandra://z9XkLW29GzFcibEMhRmNsEYM0X9yaKQK_Ts4XyyZJZ4`cZ%EbcZ%Eb#בCט#u׉׉ 7cassandra://mTq9CVWvWxdA01cOKWPNEzbueWqqKW7UKAWFTdZbOuQh`׉ 7cassandra://ReAnl-5C8BbN3sCSVXHxi7rc9KQRAmU8JMhqJKFMw2A!`s׉ 7cassandra://3Vu3GMvB2uyxFuYtO_Va9hoKTdvTvGfcd3BJ_7BALN8 `׉ 7cassandra://7Ks9h_gWzcEnxVFzQ2Mv1ZLm7LLo9LNiLT8vKGYJjLE0$͠^cZ{EbLט#u׉׉ 7cassandra://TZeDdOmnQjN4bjFtd2J5MRKoNl2DiGQLgVa44rvxN3gM`׉ 7cassandra://YLWoMV5hCDI3Ag99LwUkGxRMad_sVXdXSRn2mq9al9A7`s׉ 7cassandra://YluQG4jJubBtlYKKP4T6T-3TNhHV0NWYqS6rFrJfxloA`׉ 7cassandra://_kGXu9ZfusGpm65I2d0VlQwo7cUdgwm6CS49-zvqtSQ9͠^cZ{EbM׉E1010 Chapter 30 Fusion Welding Processes 30.75. Consider a butt joint that is to be welded. Sketch the weld shape you would expect for (a) SMAW; (b) laser welding; and (c) Laser-SMAW hybrid welding. Indicate the size and shape of the heat-affected zone you would expect. Comment on your observations. 30.76. Prepare a table listing the processes described in this chapter and providing, for each process, the range of welding speeds as a function of workpiece material and thickness. 30.77. Make an outline of the general guidelines for safety in welding operations described in this chapter. For each of the operations, prepare a poster which effectively and concisely gives specific instructions for safe practices in welding (or cutting). Review the various publications of the National Safety Council and other similar organizations. 30.78. Describe the reasons that fatigue failure generally occurs in the heat-affected zone of welds instead of through the weld bead itself. ׉ 7cassandra://3Vu3GMvB2uyxFuYtO_Va9hoKTdvTvGfcd3BJ_7BALN8 `cZ%Eb׉EhChapter 31 Solid-state Welding Processes 31.1 Introduction 1012 31.2 Cold Welding and Roll Bonding 1012 31.3 Ultrasonic Welding 1013 31.4 Friction Welding 1014 31.5 Resistance Welding 1016 31.6 Explosion Welding 1025 31.7 Diffusion Bonding 1025 31.8 Economics of Welding Operations 1028 Examples: 31.1 Roll Bonding of the U.S. Quarter 1013 31.2 Heat Generated in Spot Welding 1019 31.3 Resistance Welding vs. Laser-beam Welding in the Can-making Industry 1024 31.4 Diffusion-bonding Applications 1027 Case Study: 31.1 Linear Friction Welding of Blanes and Blisks in a Jet Engine 1029 • This chapter describes a family of joining processes in which the parts welded do not undergo a phase change; if heat is involved, it is generated internally. • The chapter begins with a description of cold welding, followed by ultrasonic welding and the friction-welding processes. • Resistance welding is then described, followed by explosion welding and diffusion bonding. These three processes have unique capabilities and applications, suitable for a wide variety of materials and can be automated for large-scale production. 1011 ׉ 7cassandra://YluQG4jJubBtlYKKP4T6T-3TNhHV0NWYqS6rFrJfxloA`cZ%EbācZ%EbÁ#בCט#u׉׉ 7cassandra://CIkYLJyllN0DpS-deoZmkNW0UY_wXV8h3jfGVu4lwm4`׉ 7cassandra://GHEo6C_rFWPhcVpDcSkqoFKLhJXrvpUmIjf1vXDfvyAb`s׉ 7cassandra://sQ4NyxJmRdkJQO17NwWhp5iuQfNzOBGdCQdQBfZZTWE`׉ 7cassandra://LwMTqT-UoRMQ36x7aaaDr4MTibbaQ7B2gcdA5Ytk2yQi7͠^cZ|EbOט#u׉׉ 7cassandra://AJvkY4Fk-nW-PD6j2dFi4GaQiarCj-HtuMn7ildo_bUk`׉ 7cassandra://GzAxyuFcWcrUVugP8NehXmKLFKdUERnN65jm3Y5J_j4d`s׉ 7cassandra://XZ2TXoifHB3i_Psd2l3NPcBrT6zbrpYaWcSbonZvRiQ `׉ 7cassandra://gnTi6efBqc28jUqIEof5bJUXqnZSUbvt8HGjAmTDkpk͙ ͠^cZ|EbP׉E i1012 Chapter 31 Solid-state Welding Processes • The chapter then examines the capabilities of diffusion bonding, and the joining processes that are combined with superplastic forming. • The final topic described concerns economic considerations in welding. 31.1 Introduction This chapter describes solid-state welding processes, in which joining takes place without fusion at the interface of the two parts being welded. Unlike the fusion-welding processes described in Chapter 30, in solid-state welding no liquid or molten phase is required for joining. The principle of solid-state welding is demonstrated best by the following example: If two clean metal surfaces are brought into close contact with each other under sufficient pressure, they form a bond and produce a joint. For a strong bond, it is essential that the interface be free of contaminants, such as oxide films, residues, metalworking fluids, and even adsorbed layers of gas. Solid-state bonding involves one or more of the following parameters: • Heat: Applying external heat increases diffusion (the transfer of atoms across an interface) and improves the strength of the weld between the two surfaces being joined, as occurs in diffusion bonding. Heat may be generated (a) internally, by friction, as utilized in friction welding; (b) through electricalresistance heating, as in resistance-welding, such as spot welding; and (c) externally, by induction heating (as in butt-welding of tubes). • Pressure: The higher the contact pressure, the stronger is the interface, as in roll bonding and explosion welding, where plastic deformation occurs. Pressure and heat may be combined, as in flash welding, stud welding,and resistance projection welding. • Relative interfacial movements: When sliding of the contacting surfaces, called faying surfaces, occurs (as in ultrasonic welding), even very small amplitudes will disturb the interface, breaking up any oxide films present, and generating new clean surfaces, thus improving weld strength. Most joining processes are now automated, with robotics, vision systems, sensors, and adaptive and computer controls (described in Part VIII). The reasons are to reduce costs (Section 31.8) and increase consistency of operation, reliability of weld quality, and higher productivity. 31.2 Cold Welding and Roll Bonding In cold welding (CW), pressure is applied to the workpieces through dies or rolls. Because of the plastic deformation involved, it is essential that at least one, but preferably both, of the mating parts be sufficiently ductile. Cold welding is usually performed on nonferrous metals or on soft iron with little, if any, carbon content. Prior to welding, the interface is first degreased, wire brushed, and wiped off to remove oxide. During joining of two dissimilar metals that are mutually soluble, brittle intermetallic compounds may form (Section 4.2.2); these will produce a weak and brittle joint. An example of weak bonding of aluminum and steel. The best bond strength is obtained with two similar materials. Roll Bonding. The pressure required for welding can be applied through a pair of rolls (Fig. 31.1), called roll bonding or roll welding (ROW). Developed in the 1960s, roll bonding is used for making U.S. coins (see Example 31.1). Surface preparation is important for good interfacial strength. The operation also can be carried out at elevated temperatures (hot roll bonding). ׉ 7cassandra://sQ4NyxJmRdkJQO17NwWhp5iuQfNzOBGdCQdQBfZZTWE`cZ%Eb׉E Ultrasonic Welding 1013 Cladding metal Base metal Rolls Figure 31.1: Schematic illustration of the roll-bonding, or cladding, process. Typical examples of this process are the cladding of (a) pure aluminum over precipitation-hardened aluminum-alloy sheet (Alclad, a trade name), which has a corrosion-resistant surface with a strong inner core, typically used in the aircraft industry; (b) stainless steel over mild steel, for corrosion resistance; and (c) copper over steel, for coaxial cables, with steel for strength. A common application of roll bonding is in making bimetallic strips, for thermostats and similar control units, using two thin layers of materials with different thermal-expansion coefficients (see Table 3.1). Bonding of only selected areas of the interface can be achieved by depositing a parting agent, such as graphite or ceramic, called stop-off(Section 31.7). Example 31.1 Roll Bonding of the U.S. Quarter The technique used for making composite U.S. quarters is the roll bonding of (a) two outer layers of 75% Cu–25% Ni (cupronickel), where each layer is 1.2 mm thick and (b) with an inner layer of pure copper 5.1 mm thick. For good bond strength, the faying surfaces are first cleaned chemically and wire brushed. Then the strips are rolled, first to a thickness of 2.29 mm, then down to a thickness of 1.36 mm. The strips thus undergo a total reduction in thickness of 82%. Because of the reduction in thickness, there is a major increase in the surface area between the layers. This extension in surface area under the high pressure applied by the rolls, combined with the solid solubility of nickel in copper (Section 4.2.1), produces a strong bond between the layers. 31.3 Ultrasonic Welding In ultrasonic welding (USW), the faying surfaces of the two components are subjected to a normal force and oscillating shearing (tangential) stresses. The shearing stresses are applied by the tip of a transducer (Fig. 31.2a), which is similar to that used for ultrasonic machining (see Fig. 26.26a). The frequency of oscillation is generally in the range of 10–75 kHz, although a lower or higher frequency also can be employed. Proper coupling between the transducer and the tip (called sonotrode, from the words sonic and electrode, also called the horn), is important for efficient operation. The shearing stresses cause plastic deformation at the interface of the two components, breaking up oxide films and contaminants, to allow for good contact and producing a strong solid-state bond. The temperature generated in the weld zone is usually one-third to one-half of the melting point (on the absolute scale) of the metals joined. Consequently, neither melting nor fusion takes place. In some situations, however, the temperature developed can be sufficiently high to cause metallurgical changes in the weld zone, thus affecting the strength of the bond. The ultrasonic-welding process is versatile and reliable, and it can be used with a wide variety of metallic and nonmetallic materials, including dissimilar metals (as in bimetallic strips). It is used extensively for joining plastics (Section 32.6), packaging with metal foils, and lap welding of sheet, foil, and thin wire and in automotive and consumer electronics industries. The welding tip can be replaced with rotating disks (Fig. 31.2b) for seam welding of structures in which one component is sheet, foil, or polymer-woven material (a process similar to resistance seam welding, Section 31.5.2). ׉ 7cassandra://XZ2TXoifHB3i_Psd2l3NPcBrT6zbrpYaWcSbonZvRiQ `cZ%EbƁcZ%EbŁ#בCט#u׉׉ 7cassandra://NOhy0iswQkRQt619fDG-jaDdIooUs6kLFloQ8xmYQ84`׉ 7cassandra://eh69kUelLYG5bGxh8QWL4XKOE-bDBqnP0v1YOwd4ka0^g`s׉ 7cassandra://J2t6iCfeTjs5S52oAgXKTE3_bl50uNYSWyFTGe9o0XI`׉ 7cassandra://OlzPIUfDcGlyDRZsXFVXklTmwcW9u-fNd_nKUuRqtLcR9>z͠^cZ}EbRט#u׉׉ 7cassandra://IBJugC0R7ousF6DHkHkzCg_LixjsVjRwzrAPVRQWrrMs`׉ 7cassandra://wTTA8dlB-3vY0HdAUq24aYazxbpdOWre37U432y-VL4OE`s׉ 7cassandra://hn53deSNjb7W_OgL_Lx7F7QwfiRC2P95sjTaRXEtWNc``׉ 7cassandra://RzAO64KiRVKuMa7whR4yaJLdvdHFyMZfEDm4NTjjJIIͼ<͠^cZ}EbV׉E 1014 Chapter 31 Solid-state Welding Processes Force Transducer Mass Transducer DC Coupling system Tip Workpiece Anvil (a) Direction of vibration polarization supply AC power supply Roller Workpieces Support (b) Figure 31.2: (a) Components of an ultrasonic-welding machine for making lap welds. The lateral vibrations of the tool tip cause plastic deformation and bonding at the interface of the workpieces. (b) Ultrasonic seam welding using a roller as the sonotrode. 31.4 Friction Welding In the joining processes described thus far, the energy required for welding is supplied from external sources, typically chemical, electrical, or ultrasonic energy. In friction welding (FRW), the heat required is generated through friction at the interface of the two components to be joined. Developed in the 1940s, one of the workpiece components in this process remains stationary while the other is placed in a chuck or collet, and rotated at a constant speed as high as 15 m/s. The two members are then brought into contact under an axial force (Fig. 31.3). After sufficient contact is established, the rotating member is brought to a quick stop (so that the weld is not destroyed by shearing) while the axial force is increased. Oxides and other contaminants at the interface are thus removed by the radially outward movement of the hot metal at the interface. The pressure at the interface and the resulting friction produce sufficient heat to develop a strong joint. The weld zone is usually confined to a narrow region; its size and shape depend on the (a) level of heat generated, (b) thermal conductivity of the materials, (c) mechanical properties of the materials at elevated temperatures, (d) rotational speed, and (e) the axial pressure applied (Fig. 31.4). Friction welding can be used to join a wide variety of materials, provided that one of the components has rotational symmetry. Solid or tubular parts can be joined, with good joint strength. Solid steel bars up to 100 mm in diameter and pipes up to 250 mm in outside diameter, have been friction welded successfully. Because of the combined heat and pressure, the interface in friction welding develops a flash by plastic deformation (upsetting) of the heated zone. If objectionable, it can easily be removed by machining or grinding. Friction-welding machines are fully automated, and the operator skill required is minimal, once individual cycle times for the complete operation are set properly. Inertia Friction Welding. This process is a modification of friction welding, although the two terms have been used interchangeably. In inertia friction welding, the energy required for frictional heating is supplied by a flywheel. It is first accelerated to the proper speed, the two members are brought into contact, and an axial force is then applied. As friction at the interface begins to slow the flywheel, the axial force is increased. Toolholder Roller Workpieces Support ׉ 7cassandra://J2t6iCfeTjs5S52oAgXKTE3_bl50uNYSWyFTGe9o0XI`cZ%Eb׉EFriction Welding 1015 Force 1. 2. Speed 3. Flash Force 4. Total upset length Time Figure 31.3: Sequence of operations in the friction-welding process: (1) The part on the left is rotated at high speed. (2) The part on the right is brought into contact with the part on the left under an axial force. (3) The axial force is increased, and the part on the left stops rotating; flash begins to form. (4) After a specified upset length or distance is achieved, the weld is completed. The upset length is the distance the two pieces move inward during welding after their initial contact; thus, the total length after welding is less than the sum of the lengths of the two pieces. The flash subsequently can be removed by machining or grinding. The weld is completed when the flywheel has come to a stop; the timing of this sequence is important for good weld quality. The rotating mass in inertia-friction-welding machines can be adjusted for applications requiring different levels of energy, depending on workpiece size and its properties. In one application, 10 mm diameter shafts are welded to automotive turbocharger impellers at a rate of one joint every 15 s. Linear Friction Welding. In a further development of friction welding, the interface of the two components to be joined is subjected to a linear reciprocating motion, as opposed to a rotary motion. Thus, in this process, the components do not have to be circular or tubular in cross-section. One part is moved across the face of the other part using a balanced reciprocating mechanism. The process is capable of welding square, rectangular, or round components, and made of metals or plastics. In one application, a rectangular titanium-alloy part was friction welded, at a linear frequency of 25 Hz with an amplitude of ±2 mm under a pressure of 100 MPa acting on a 240 mm2 interface. Rectangular cross-sections as large as 50 mm × 20 mm also have been welded successfully. Flash Force increased Beginning of flash (a) High pressure or low speed (b) Low pressure or high speed (c) Optimum Figure 31.4: Shape of the fusion zones in friction welding as a function of the axial force applied and the rotational speed. Speed, force, upset length Upset length ׉ 7cassandra://hn53deSNjb7W_OgL_Lx7F7QwfiRC2P95sjTaRXEtWNc``cZ%EbȁcZ%Ebǁ#בCט#u׉׉ 7cassandra://hQjv8arywke4rXzGa4M716heSHwqLeKy6bx2nj7e310`׉ 7cassandra://VdyxlsUUQNUaNE2sPqHJgwcFw8WozsJL2VTxjMdhlso_`s׉ 7cassandra://Nont0RQN0eT5ObgyhCNr_hv1_qDenWB-NIQNGWjtWP8`׉ 7cassandra://MAlViSB9rowZIEffkFeL1RakSh02j3xgvC-Dt_qEvmco!N͠^cZ~EbXט#u׉׉ 7cassandra://jjFENJFlm_ceQS7zqiFwIECXP97_jmcop8SPLxpXwz8k`׉ 7cassandra://fsFhoB5FAQTZ3cBwmMRTB2Rdp9xgc7tZPQA6E5oMkWcD)`s׉ 7cassandra://1g0tBiSa3qfKyFZ93jWMAqNkNgNiI4IZ9UdvYToZu_4+`׉ 7cassandra://nS5aNLekSp0FD5B-ZyudeeMYzYowg8L7KflLp7YZxtY"8͠^cZ~EbY׉E 11016 Chapter 31 Solid-state Welding Processes Thermomechanically affected zone (TMAZ) Stir zone (weld nugget) Advancing side Retreating side Heat-affected zone (HAZ) Shouldered nonconsumable tool Probe Weld (a) (b) Figure 31.5: The friction-stir-welding process. (a) Schematic illustration of friction-stir-welding. Aluminumalloy plates up to 75 mm thick have been welded by this process. (b) Multi-axis friction stir welding machine for large workpieces such as aircraft wing and fuselage structures. This machine can develop 67 kN axial forces and welding speeds up to 1.8 m/s. It is powered by a 15 kW spindle motor. Source: (b) Courtesy of Manufacturing Technology, Inc. Friction Stir Welding. In friction-stir-welding (FSW), developed in 1991, a third body (called a probe)is plunged into the joint, and it rubs against the two surfaces to be joined. The nonconsumable rotating probe is typically made of cubic boron nitride (Section 8.2.3), 5 to 6 mm in diameter and 5 mm high (Fig. 31.5). The contact pressure causes frictional heating, raising the temperature to 230◦C – 260◦C. The tip of the rotating probe forces mixing or stirring of the material in the joint. No shielding gas or surface cleaning is required. The thickness of the material can be as little as 1 mm and as much as 50 mm, welded in a single pass. Aluminum, magnesium, nickel, copper, steel, stainless steel, and titanium have been welded successfully; developments are taking place to extend FSW applications also to polymers and composite materials. The FSW process is being applied to aerospace, automotive, shipbuilding, and military vehicles, using sheets or plates. With developments in rotating-tool design, other possible applications include inducing microstructural changes, refining grain size in materials, and improving localized toughness in castings. The equipment can be a conventional, vertical-spindle milling machine (see Fig. 24.18b), and the process is relatively easy to implement. For special applications, dedicated machinery for friction stir welding is available (Fig. 31.5b). Welds produced by FSW have high quality, with minimal pores and uniform structure. Because the welds are produced with low heat input, there is low distortion and little microstructural changes. 31.5 Resistance Welding The category of resistance welding (RW) covers a number of processes in which the heat required for welding is produced by means of electrical resistance across the two components to be joined. These processes have major advantages, such as high-quality welds that do not require consumable electrodes, shielding gases, or flux, and can be produced at high rates. Resistance welding lends itself very well to automation, often using welding robots (see Section 37.6). The heat generated in resistance welding is given by the general expression H = I2Rt, (31.1) ׉ 7cassandra://Nont0RQN0eT5ObgyhCNr_hv1_qDenWB-NIQNGWjtWP8`cZ%Eb׉EResistance Welding 1017 where H is the heat generated in joules (watt-seconds), I is the current (in amperes), R is the resistance (in ohms), and t is the time of current flow (in seconds). Equation (31.1) is often modified so that it represents the actual heat energy available in the weld, by including a factor K, which compensates for the energy losses through conduction and radiation. This equation then becomes H = I2RtK, where it can be noted that the value of K is less than unity. The total resistance is the sum of the following (Fig. 31.6): 1. Resistances of the electrodes 2. Electrode–workpiece contact resistance 3. Resistances of the individual parts to be welded 4. Contact resistance between the faying surfaces of the two workpieces to be joined. The actual temperature rise in the joint depends on the specific heat and thermal conductivity of the metals to be joined. For example, metals such as aluminum and copper have high thermal conductivity (see Table 3.1); hence they require high heat concentrations. Similar and dissimilar metals can be joined by this process. The current may be as high as 100,000 A, although the voltage is typically only 0.5 to 10 V. The strength of the bond developed depends on surface roughness and on the cleanliness of the mating Electrodes Weld nugget Lap joint 1. Force applied 2. Current on 3. Current off, force on (a) Electrode Electrode tip Weld nugget Indentation Sheet separation 4. Force released (31.2) Heat-affected zone Electrode (b) Figure 31.6: (a) Sequence of events in resistance spot welding of a lap joint. (b) Cross-section of a spot weld, showing the weld nugget and the indentation of the electrode on sheet surfaces. This is one of the most commonly used processes in sheet-metal fabrication and in automotive metal-body assembly. ׉ 7cassandra://1g0tBiSa3qfKyFZ93jWMAqNkNgNiI4IZ9UdvYToZu_4+`cZ%EbʁcZ%EbɁ#בCט#u׉׉ 7cassandra://LTi8HHN0xh8Xp0kBfJi1xCmjH8JiseR09lE-81j74fER`׉ 7cassandra://FTN6k41DxK-hrqMOtpQfimIYjT934ozBuy5v5xZHwPkX{`s׉ 7cassandra://nAKgn4UiwHycxl9cif5yRmiHaks95ZH_gi7_TpTCKvU`׉ 7cassandra://UFCGGbk878Lh2er1TYoIbfOWXAHURAU01CUJxJOKSUw͍͠^cZEb[ט#u׉׉ 7cassandra://4g5BxHrK1Jg418p-4zG0Nc0mDf7uMgJhef5ZSYxA5Lk'`׉ 7cassandra://FtxmDHMnRPzc7ljHUWcJN_k_97kgzmHNrMu3Ps0sFkwB`s׉ 7cassandra://1n50LQmq7ZOheKYePsCwRBHzMffYTTlt4v5Cidid1oot`׉ 7cassandra://sc89Wsh7JRXtxJOx4doGcnImM_tvXqUwckesjx-3Jekލ@D͠^cZEb\נcZEb^)7I9ׁHhttp://mm3.Asׁ ׁ Ј׉E 1018 Chapter 31 Solid-state Welding Processes surfaces. Oil films, paint, and thick oxide layers should therefore be first removed, although the presence of uniform, thin layers of oxide and other contaminants is not as critical for bond strength. Developed in the early 1900s, resistance-welding processes require specialized machinery, now operated by programmable computer control. The machinery is generally not portable, and the process is suitable primarily for use in manufacturing plants and machine shops; operator skill required is minimal. 31.5.1 Resistance Spot Welding In resistance spot welding (RSW) for a lap joint, the tips of two opposing solid, cylindrical electrodes touch the joint of two sheet metals, and resistance heating produces a spot weld (Fig. 31.6a). In order to obtain astrongbondinthe weld nugget, pressure is applied until the current is turned off and the weld has solidified. Precise control and timing of the alternating current (AC) and of the pressure are essential for weld quality (see also high-frequency resistance welding, Section 31.5.3). The surfaces of a spot weld has a slightly discolored indentation; the weld nugget (Fig. 31.6b) may be up to 10 mm in diameter. Currents range from 3000 to 40,000 A, depending on the materials being welded and their thicknesses. For example, the current is typically 10,000 A for steels and 13,000 A for aluminum. Electrodes are typically made of copper alloys and must have sufficient electrical conductivity and hot strength to maintain their shape after repeated uses. The simplest and most commonly used resistance-welding process, spot welding may be performed by means of single or multiple pairs of electrodes (as many as a hundred or more); the required pressure is supplied through mechanical or pneumatic means. Rocker-arm-type spot-welding machines are typically used for smaller parts; press-type machines are used for larger workpieces. The shape and surface condition of the electrode tip and its accessibility are important factors. A variety of electrode shapes are used for areas that are difficult to reach (Fig. 31.7). Spot welding is used widely for fabricating sheet-metal parts; examples range from attaching handles to stainless-steel cookware (Fig. 31.8a), to spot-welding mufflers (Fig. 31.8b), and to large sheet-metal structures. Modern spot-welding equipment is computer controlled for optimum timing of current and pressure, and the spot-welding guns are manipulated by programmable robots (Fig. 31.8c). Testing Spot Welds. Spot-welded joints may be tested for weld-nugget strength by means of the following techniques (Fig. 31.9): • Tension-shear • Cross-tension • Twist • Peel Electrodes Workpiece Workpiece (a) (b) Figure 31.7: Two electrode designs for easy access to the components to be welded. ׉ 7cassandra://nAKgn4UiwHycxl9cif5yRmiHaks95ZH_gi7_TpTCKvU`cZ%Eb׉E!Resistance Welding 1019 Control panel (a) (b) 44 mm stroke; 200 mm maximum gap Electrodes Cooling lines (c) (d) Figure 31.8: Spot-welded (a) cookware and (b) muffler. (c) An automated spot-welding machine. The welding tip can move in three principal directions. Sheets as large as 2.2 × 0.55 m can be accommodated in this machine with proper workpiece supports. (d) A typical spot welding machine. Source: (c) and (d) Courtesy of Taylor-Winfield Technologies, Inc. The cross-tension and twist tests can indicate flaws, cracks, and porosity in the weld area. The peel test is commonly used for thin sheets. After the joint has been bent and peeled, the shape and size of the tornout weld nugget are evaluated. Because they are easy to perform and inexpensive, these tests are commonly used in fabricating facilities. Example 31.2 Heat Generated in Spot Welding Given: Assume that two 1 mm thick steel sheets are being spot-welded at a current of 5000 A and over a current flow time of 0.1 s by means of electrodes 5 mm in diameter. Find: Estimate the heat generated and its distribution in the weld zone if the effective resistance in the operation is 200 µΩ. Solution: From the information given, the weld-nugget volume can be estimated to be 30 mm3.Assume that the density for steel (Table 3.1) is 8000 kg/m3. Then the weld nugget has a mass of 0.24 g. The heat required to melt 1 g of steel is about 1400 J, so the heat required to melt the weld nugget is (1400)(1400)(0.24) = 336 J. The remaining heat (164 J) is dissipated into the metal surrounding the nugget. ׉ 7cassandra://1n50LQmq7ZOheKYePsCwRBHzMffYTTlt4v5Cidid1oot`cZ%Eb́cZ%Ebˁ#בCט#u׉׉ 7cassandra://QEDNnIveBFO4rELnC1mj_apSIca36WWKPspEkojDVR0`׉ 7cassandra://gGyH3TNQlCPUZCOfFOgtW9knxMkAwo9PVN3BduLaTzkRV`s׉ 7cassandra://TWndlY_eBPLo1bnM4l-1h38nE-RCIYKMHeU-S5sl1ys`׉ 7cassandra://KnfuZdvNJEyD6mMbXIoKaNyRqGv6px2LZYLRDWiUS8M͠^cZEb_ט#u׉׉ 7cassandra://USf_kbP4HondxwezUKM3wTexCpZvK1KLKFhf3ugsKGg`׉ 7cassandra://ZKS803UngXcrx6Yab-9RMaVBwS98PwVeLUYgsYFZT_YW `s׉ 7cassandra://1d5D2_v9qRQyDhbFDhcmsaf42zWNUe45-bgIG4Vcqzo`׉ 7cassandra://Cwjy7xeAv-sNE-0FsqMd8X6-8cvsG44Eiseeg-eDggcͥ͠^cZEb`׉Et1020 Chapter 31 Solid-state Welding Processes 31.5.2 Resistance Seam Welding Resistance seam welding (RSEW) is a modification of spot welding wherein the electrodes are replaced by rotating wheels or rollers (Fig. 31.10a). Using a continuous AC power supply, the electrically conducting rollers produce a spot weld whenever the current reaches a sufficiently high level in the AC cycle. The typical welding speed is 1.5 m/min for thin sheets. With sufficiently high frequency or slow traverse speed, spot welds actually overlap into a continuous seam and produce a joint that is liquid and gas tight (Fig. 31.10b). The RSEW process is used to make the longitudinal seam on steel cans for household products, mufflers, and gasoline tanks. In roll spot welding, the current to the rolls is applied intermittently, producing a series of spot welds at specified intervals along the length of the seam (Fig. 31.10c). In mash seam welding (Fig. 31.10d), the overlapping welds are about one to two times the sheet thickness; the welded seam thickness is about 90% of the original sheet thickness. This process is also used in producing tailor-welded sheet-metal blanks, which can be made by laser welding as well (Section 16.2.2). 31.5.3 High-frequency Resistance Welding High-frequency resistance welding (HFRW) is similar to seam welding, except that a high-frequency current of up to 450 kHz is employed. A typical application is the production of butt-welded tubing or pipe, where the current is conducted through two sliding contacts (Fig. 31.11a) to the edges of roll-formed tubes. The heated edges are then pressed together by passing the tube through a pair of squeeze rolls; flash that forms, if any, is then trimmed off. Structural sections, such as I-beams, can be fabricated by HFRW, by welding the webs and flanges. Spiral pipe and tubing, finned tubes for heat exchangers, and wheel rims also can be made by this technique. (a) 1. Raised nugget (b) Hole left in part Button diameter indicates quality (c) 2. 3. (d) Figure 31.9: Test methods for spot welds: (a) tension-shear test, (b) cross-tension test, (c) twist test, (d) peel test (see also Fig. 32.9). ׉ 7cassandra://TWndlY_eBPLo1bnM4l-1h38nE-RCIYKMHeU-S5sl1ys`cZ%Eb׉EResistance Welding 1021 Electrode wheels Electrode wheels Seam weld Sheet Weld nuggets Weld (a) (b) (c) (d) Figure 31.10: (a) Seam-welding process in which rotating rolls act as electrodes. (b) Overlapping spots in a seam weld. (c) Roll spot welds and (d) Mash seam welding. In another method, called high-frequency induction welding (HFIW), roll-formed tubes (Section 16.6) are subjected to high-frequency induction heating, as shown in Fig. 31.11b. 31.5.4 Resistance Projection Welding In resistance projection welding (RPW), high electrical resistance at the joint is developed by embossing one or more projections (dimples; see Fig. 16.39) on one of the surfaces to be welded (Fig. 31.12). The projections may be round or oval for design or strength purposes. High localized temperatures are generated at the projections, which are in contact with the flat mating part. Typically made of copper-based alloys, the electrodes are large and flat, and are water cooled to keep their temperature low. The weld nuggets are similar to those in spot welding; they are formed as the electrodes exert pressure to soften and compress and flatten the projections. Spot-welding equipment can be used for resistance projection welding by modifying the electrodes. Although embossing of the workpieces adds to production cost, the operation produces several welds in one pass and extends electrode life; moreover, it is capable of welding metals of different thicknesses, such as a sheet welded over a plate. Nuts and bolts also can be welded to sheets and plates by this process (Fig. 31.12c and d), with projections that may be produced either by machining or forging. Joining a network of rods and wires [such as in making metal baskets, grills (Fig. 31.12e), oven racks, and shopping carts] is considered resistance projection welding, because of the small contact area between crossing wires (grids). Contacts Vee Apex Tube travel (a) Current Squeeze roll (b) Figure 31.11: Two methods of high-frequency continuous butt welding of tubes. High-frequency coil Apex Current Squeeze roll ׉ 7cassandra://1d5D2_v9qRQyDhbFDhcmsaf42zWNUe45-bgIG4Vcqzo`cZ%Eb΁cZ%Eb́#בCט#u׉׉ 7cassandra://tIU4j1SMigFBi0WUPwp8uFygdtCAQAyeQwDgw82nG2gQ`׉ 7cassandra://cGZBwlgMBX1UXEh4308CbwGqzCjhKHIpazt4emB_nR8Y`s׉ 7cassandra://jN6i8Dt4KQlXthEVcP67sGKfE0iwLGCfiGFzBe8OlfY`׉ 7cassandra://ZiYSNbkrka9xGaaNLFleK766ub8eE8_Y1uCrGvFjKZ0͠^cZEbbט#u׉׉ 7cassandra://wdi0q1d3mmrAhn_194jub_qr1M6bvyBaMdHf9NS7Bcg`׉ 7cassandra://5SVEO1O7zLriScLyt7rHmMwCBBFUVwIffw-xnCf7y6I>Z`s׉ 7cassandra://Gubq7kTpA7FbQSCmMIfcGuTceGjwOhKg3jROqqklPpU|`׉ 7cassandra://cDROYZcbZMBT_bnSAtXBT1A4yRSmyRFiNtbdnIsEEJ0H{8R͠^cZEbc׉E <1022 Chapter 31 Solid-state Welding Processes Force Flat electrodes Sheet Workpiece Projections Force (a) Weld nugget Nut (c) (d) (e) Figure 31.12: (a) Schematic illustration of resistance projection welding. (b) A welded bracket. (c) and (d) Projection welding of nuts or threaded bosses and studs. (e) Resistance-projection-welded grills. 31.5.5 Flash Welding In flash welding (FW), also called flash butt welding, heat is generated rapidly from the arc as the ends of the two members begin to make contact, developing electrical resistance at the joint (Fig. 31.13a). After the proper temperature is reached and the interface begins to soften, an axial force is applied at a controlled rate, producing a weld by plastic deformation of the joint; joint quality is good. The mechanism involved is called hot upsetting (see Fig. 14.3); the term upset welding (UW) also is used for this process. Some molten metal is expelled from the joint as a shower of sparks during the process, thus the name flash welding. Because of the presence of an arc, the process can also be classified as arc welding. Impurities and contaminants are squeezed out during this operation, and a significant amount of material may be burned off during welding. The machines for flash welding usually are automated, with a variety of power supplies, ranging from 10 to 1500 kVA. The FW process is suitable for end-to-end or edge-to-edge joining of strips and sheets of similar or dissimilar metals, 0.2 to 25 mm thick and for end-joining bars 1 to 75 mm in diameter. Thin sections have a tendency to buckle under the axial force applied during welding. Rings made by forming, such as by the techniques shown in Fig. 16.22, can be flash butt welded. The process is also used to repair broken band-saw blades (Section 24.5), using fixtures mounted on the band-saw frame. The process can be automated for reproducible welding operations. Typical applications are the joining of pipe and of tubular shapes for metal furniture, doors, and windows. FW is also used for welding the ends of sheets or wire in continuously operating rolling mills (Chapter 13) and in the feeding of wiredrawing equipment (Section 15.11). Some design guidelines for mating surfaces in flash welding are shown in Fig. 31.13d and e; note the importance of having uniform cross-sections at the joint. 31.5.6 Stud Welding Stud welding (SW), also called stud arc welding, is similar to flash welding. The stud, which may be a threaded metal rod, hanger, or handle, serves as one of the electrodes while it is being joined to another component, Bolt Weld nugget (b) Weld nuggets ׉ 7cassandra://jN6i8Dt4KQlXthEVcP67sGKfE0iwLGCfiGFzBe8OlfY`cZ%Eb׉EDResistance Welding 1023 Weld Arc (a) (b) Poor (c) Good (d) (e) Figure 31.13: (a) Flash-welding process for end-to-end welding of solid rods or tubular parts. (b) and (c) Typical parts made by flash welding. (d) and (e) Some design guidelines for flash welding. usually a flat plate (Fig. 31.14). Polarity for aluminum is typically direct-current electrode positive (DCEP); for steels, it is direct-current electrode negative (DCEN). In order to concentrate the heat generated, and to prevent oxidation and retain the molten metal in the weld zone, a disposable ceramic ring (ferrule) is placed around the joint. The equipment for stud welding can be automated, with various controls for arcing and for applying pressure; portable stud-welding equipment is also available. Typical applications of stud welding include automobile bodies, electrical panels, shipbuilding, and in building construction. In capacitor-discharge stud welding,a DC arc is produced from a capacitor bank; no ferrule or flux is required, because the welding time is on the order of only 1 to 6 milliseconds. The choice between this Push Stud Ceramic ferrule Workpiece (base metal) 1. Arc 2. Molten weld metal 3. Weld 4. Figure 31.14: The sequence of operations in stud welding commonly used for welding bars, threaded rods, and various fasteners onto metal plates. Pull Push ׉ 7cassandra://Gubq7kTpA7FbQSCmMIfcGuTceGjwOhKg3jROqqklPpU|`cZ%EbЁcZ%Ebρ#בCט#u׉׉ 7cassandra://X7ENBj6eDRLAQ79toHlCe3X0wonF2-al1GaL0QfyS70_U`׉ 7cassandra://IgYXOhAzy8UyIHVzSEbBHOxbdYgFQuzcwyKL4qj3ZxkK`s׉ 7cassandra://pMaAbMteDbLZxlJKegKB7fCfYqvRSfMjCvZTSsQKbAc `׉ 7cassandra://Y4MkqQI6Hi6bvhMWMH0PuxzXwHblibyXT4VGSvTJCucxP͠^cZEbiט#u׉׉ 7cassandra://AcYwoi7y-ZLea9MNz5Rss5M9cS3pMB08vNVXj4HHKAs(`׉ 7cassandra://yNoJ47gJMB3fEoua02Y5OolyAqNS_CwLUnE6SUsTEY0Xl`s׉ 7cassandra://O0PaO3FpatCjIDhOiCH0X5Aytz9Mx4sMYot_4KiwVNg`׉ 7cassandra://4tivsxAqa6V7hltcRC1E9fA4PRoLpvaeUWt84FkMKVwo&͠^cZEbj׉E1024 Chapter 31 Solid-state Welding Processes Cross-section of conventional weld Cross-section of electron-beam or laser-beam weld bead Workpiece Figure 31.15: The relative sizes of the weld beads obtained by tungsten-arc and by electron-beam or laserbeam welding. process and the stud arc welding process depends on such factors as the metals to be joined, workpiece thickness and cross-section, stud diameter, and the shape of the joint. 31.5.7 Percussion Welding The resistance-welding processes described usually include an electrical transformer to meet the power requirements; alternatively, electrical energy for welding may be stored in a capacitor. Percussion welding (PEW) utilizes this technique, in which the power is discharged within 1 to 10 milliseconds, developing localized high heat at the joint. Percussion welding is useful where heating of the components adjacent to the joint is to be avoided, as, for example, in electronic assemblies and electrical wires. Example 31.3 Resistance Welding vs. Laser-beam Welding in the Can-making Industry The cylindrical bodies of cans for food and various household products have been resistance seam welded (with a lap joint up the side of the can) for many years. Beginning in the late 1980s, laser-beam welding technology was introduced into the can-making industry. The joints are welded by lasers, with the same productivity as in resistance welding but with the following advantages: • As opposed to the lap joints suitable for resistance welding, laser welding utilizes butt joints; some material is thus saved. Multiplied by the billions of cans made each year, this amount becomes a very significant saving. • Because laser welds have a very narrow zone (Fig. 31.15; see also Fig. 30.15), the unprinted area on the can surface (called printing margin) is greatly reduced. As a result, appearance and customer acceptance are improved. • The resistance lap-welded joint can be subject to corrosion by the contents of the can, which can be acidic, such as orange or tomato juice, thus changing their taste; a butt joint, made by laser-beam welding, eliminates the problem. Source: Courtesy of G.F. Benedict. ׉ 7cassandra://pMaAbMteDbLZxlJKegKB7fCfYqvRSfMjCvZTSsQKbAc `cZ%Eb׉E %Diffusion Bonding 1025 31.6 Explosion Welding In explosion welding (EXW), pressure is applied by detonating a layer of explosive, placed over one of the components being joined, called the flyer plate (Figs. 31.16a and b). The contact pressures developed are extremely high, and the kinetic energy of the plate striking the mating component causes a wavy interface. The impact mechanically interlocks the two surfaces (Figs. 31.16c and d), so that pressure welding by plastic deformation also takes place. The flyer plate is placed at an angle, and any oxide films present at the interface are broken up and propelled out from the interface. As a result, the bond strength from explosion welding is very high. The explosive may consist of a flexible plastic sheet, cord, or in granulated or liquid form, which is cast or pressed onto the flyer plate. The detonation speed is in the range from 2400 to 3600 m/s, depending on the type of explosive, thickness of the explosive layer, and packing density of the layer. There is a minimum denotation speed necessary for welding. Detonation is carried out with a standard commercial blasting cap. Explosive welding is suitable particularly for cladding a plate or a slab with a dissimilar metal. Plates as large as 6 m × 2 m have been clad explosively. They may then be rolled into thinner sections. Tubes and pipes can be joined to the holes in the header plates of boilers and heat exchangers by placing the explosive inside the tube; the explosion expands the tube. The process is inherently dangerous, thus it requires safe handling by well-trained and experienced personnel. 31.7 Diffusion Bonding Diffusion bonding,or diffusion welding (DFW), is a process in which the strength of the joint results primarily from diffusion (movement of atoms across an interface) and secondarily from plastic deformation of Detonator Explosive Clad metal (flyer) Constantinterface clearance gap Base plate (a) (b) Detonator Explosive a Buffer Clad metal Angular-interface clearance gap Base plate (c) (d) Figure 31.16: Schematic illustration of the explosion-welding process: (a) constant-interface clearance gap and (b) angular-interface clearance gap. (c) Cross-section of explosion-welded joint: titanium (top) and low-carbon steel (bottom). (d) Iron–nickel alloy (top) and low-carbon steel (bottom). ׉ 7cassandra://O0PaO3FpatCjIDhOiCH0X5Aytz9Mx4sMYot_4KiwVNg`cZ%EbӁcZ%Ebҁ#בCט#u׉׉ 7cassandra://WZ4FBkhISpKct534qC31ncFDQbJdL2OMY7U0kQmPDJUL`׉ 7cassandra://nz1vipw46LoAojZXPbrufPVnwzmnUs7HjU4nRNHTkBES`s׉ 7cassandra://KvgcHduvqotlIKmEHsv-_WYCD1MxwhTbA0nx6p0kt3I?`׉ 7cassandra://xTzN8ptFszr7KepdF9xB2F2gYrdeUgZt2c0QyQ1QvM4?n2͠^cZEbmט#u׉׉ 7cassandra://FyYpcIcdqNtiZ1plH_LWglC8UgKl-NYkKjG5iEk3HM4c`׉ 7cassandra://md1GiECaAjz2jxHxlscIqeikwl8tY79DBK7d-G-_FlkP`s׉ 7cassandra://VS25rT3QIKsLsuxbbxI9tmi5ji0Kwn4qO8rNf8gzvWk`׉ 7cassandra://EGPPixsHf6gWOHB8A3dSz6o5TTtfxs8alCi88b54pH8x*͠^cZEbn׉E 1026 Chapter 31 Solid-state Welding Processes the faying surfaces. The process requires temperatures of about 0.5Tm (where Tm is the melting point of the metal, on the absolute scale) in order to have a sufficiently high diffusion rate between the parts being joined (Sections 1.7 and 1.8). The interface in diffusion welding has essentially the same physical and mechanical properties as the base metal; its strength depends on (a) pressure, (b) temperature, (c) time of contact, and (d) cleanliness of the faying surfaces. These requirements can be relaxed by using a filler metal at the interface. Depending on the materials joined, brittle intermetallic compounds may form at the interface; they may be avoided by first electroplating (Section 34.9) the surfaces with suitable metal alloys. In diffusion bonding, pressure may be applied by dead weights, a press, differential gas pressure, or the thermal expansion of the parts. The parts usually are heated in a furnace or by electrical resistance; high-pressure autoclaves also are used for bonding complex parts. Although DFW was developed in the 1970s as a modern welding technology, the principle of diffusion bonding dates back centuries when goldsmiths bonded gold over copper to develop a product called filled gold. First, a thin layer of gold foil is placed over copper, and pressure is applied by a weight on top of the foil. The assembly is then placed in a furnace and left there until a strong bond is developed, hence the process is also called hot-pressure welding (HPW). Diffusion bonding generally is most suitable for joining dissimilar metals; it is also used for reactive metals (such as titanium, beryllium, zirconium, and refractory metal alloys) and for composite materials, such as metal-matrix composites (Section 9.5). Diffusion bonding is an important mechanism of sintering in powder metallurgy (Section 17.4). Because diffusion involves migration of the atoms across the joint, DFW is slower than other welding processes. Although diffusion welding is used for fabricating complex parts in low quantities, for aerospace, nuclear, and electronics industries, it has been automated to also make it suitable and economical for moderate-volume production. Unless highly automated, significant operator training and skill are required. Aft fuselage frames Outboard actuator fitting Inboard actuator fitting Nacelle frame Bulkhead Nacelle support beam Main landing-gear trunnion Figure 31.17: Aerospace diffusion bonding applications. ׉ 7cassandra://KvgcHduvqotlIKmEHsv-_WYCD1MxwhTbA0nx6p0kt3I?`cZ%Eb׉E]Diffusion Bonding 1027 Example 31.4 Diffusion-bonding Applications Diffusion bonding is especially suitable for such metals as titanium and the superalloys used in military aircraft. Design possibilities allow conservation of expensive strategic materials and reduction of manufacturing costs. The military aircraft illustrated in Fig. 31.17 has more than 100 diffusion-bonded parts, some of which are shown in the figure. Diffusion Bonding–Superplastic Forming. Sheet-metal structures can be fabricated by combining diffusion bonding with superplastic forming (see also Section 16.10). Typical structures in which flat sheets are diffusion bonded and then shaped are shown in Fig. 31.18. After diffusion bonding of selected locations on the sheets, the unbonded (stop-off) regions are expanded in a mold either by air or fluid pressure. The structures made are thin, with high stiffness-to-weight ratios; thus they are particularly useful in aircraft and aerospace applications. Diffusion bonding–superplastic forming improves productivity by eliminating the number of parts in a structure, mechanical fasteners, and reducing labor and manufacturing cost. It produces parts with good dimensional accuracy and low residual stresses. First developed in the 1970s, this technology is now well Stop-off Core sheet Bonding pressure Diffusion bonding Stop-off Stop-off Superplastic forming Gas pressure for forming Die Die Die Finished structure Die Figure 31.18: The sequence of operations in the fabrication of a structure by the diffusion bonding and superplastic forming of three originally flat sheets (see also Fig. 16.52). ׉ 7cassandra://VS25rT3QIKsLsuxbbxI9tmi5ji0Kwn4qO8rNf8gzvWk`cZ%EbՁcZ%Ebԁ#בCט#u׉׉ 7cassandra://fRv7i-NnFtV9gDeCZYCkEOzIx6TW_dmRCCmJaicJ-xMc`׉ 7cassandra://zXdlFTCKNw8_R2n3hi3flvZF6sAigw-mc8kFe7LkTIkNU`s׉ 7cassandra://qsqrsKD9hi6lsGNCMlRw0DXKrWkTN_tIxeRrvha4C5g`׉ 7cassandra://mu2WmjPrKX5NLG-zDKr35kGn3TPTMBeEFJbeP-SvfOI͟V͠^cZEbpט#u׉׉ 7cassandra://BkVCVg80v9yxaSifbcvGYZee4MvsFHgxCNyUIrxtvdoE`׉ 7cassandra://rtkD3C2FRa34IDBZvypr4FQyJr6jOhj5QCZ8C8_9sG8R)`s׉ 7cassandra://ByzflursUSZccpcWYUAKO97o1RP5TNuICwT5flB3QgM`׉ 7cassandra://w2dQMoiypwDCb9KhANG3T9fOX1C8do_92v_MUiFCesYw͠^cZEbq׉E81028 Chapter 31 Solid-state Welding Processes advanced for titanium structures, typically using Ti-6Al-4V and 7475-T6, and various alloys for aerospace applications. Other welding processes also can be used with post-welding superplastic forming of plates, notably friction welding and friction stir welding. 31.8 Economics of Welding Operations The characteristics, advantages, and limitations of the welding processes described thus far have included a brief introduction to welding costs. The relative costs of some selected processes are shown in Tables 30.1 and VI.1. As in all manufacturing operations, costs in welding and joining can vary widely, depending on such factors as equipment capacity, level of automation, labor skill required, weld quality, production rate, and preparation required, as well as on various other considerations specific to a specific joining operation. Welding and joining costs for some common operations (all described throughout Chapters 30 through 32) may be summarized as: • High: Brazing and fasteners (such as bolts and nuts), as they require hole-making operations and fastener costs. • Intermediate: Arc welding, riveting, adhesive bonding. • Low: Resistance welding, seaming, and crimping, as these operations are relatively simple to perform and to automate. Equipment costs for welding may be summarized as: • High ($100,000 to $200,000): Electron-beam and laser-beam welding. Figure 31.19: Illustration of a modern linear friction welding system for production of jet engine bladed disks (blisks). Source: After D. Adams, MTI Welding. ׉ 7cassandra://qsqrsKD9hi6lsGNCMlRw0DXKrWkTN_tIxeRrvha4C5g`cZ%Eb׉E Economics of Welding Operations 1029 Figure 31.20: Detailed view of blades attached to a compressor disk by linear friction welding. During welding, the parts encounter plastic deformation; the block shown is later removed by machining. Source: Courtesy of ACB - An Aries Alliance Company. • Intermediate ($5,000 to $50,000+): Spot, submerged arc, gas metal-arc, gas tungsten-arc, flux-cored arc, electrogas, electroslag, plasma arc, and ultrasonic welding. • Low ($500+): Shielded metal-arc and oxyfuel–gas welding. Labor costs in welding generally are higher than in other metalworking operations because of operator skill, welding time, and the preparations required. Much also depends on the level of automation of the equipment employed, including the wide use of robotics and computer controls, programmed to follow a prescribed path (seam tracking) during welding. It has been observed, for example, that in systems with robotic controls, the actual welding time reaches 80% of the total time, whereas in manual welding operations (Table 30.1), the actual time spent by the operator on welding is only about 30% of the total time. Labor costs may be summarized as: • High to intermediate: Oxyfuel–gas welding and shielded metal-arc welding. • High to low: Electron-beam and laser-beam welding and flux-cored arc welding. • Intermediate to low: Submerged-arc welding. Case Study 31.1 Linear Friction Welding of Blanes and Blisks in a Jet Engine. Titanium alloy Ti-6Al-4V bladed vanes (blanes) and bladed disks (blisks) are integral components of modern jet engines. Figure 31.19 shows a typical linear friction welding arrangement; Fig. 31.20 shows details of a typical blisk. Note that there are several blades mounted in close proximity to each other, and that very strict tolerances must be maintained for operating efficiency. Furthermore, the environment of a jet engine is very demanding; temperatures can easily exceed 1000◦C, and loadings are unsteady, so that fatigue failure is an important issue. Blanes and blisks traditionally required skilled machinists to attach them to a central hub using mechanical fasteners. This approach was time consuming and expensive, and product quality was difficult to control. Beginning with the 1990s, laser welding began to be used to fasten blades onto disks, with significant improvements in economics and performance; however, blade failures in the heat affected zone still occurred. ׉ 7cassandra://ByzflursUSZccpcWYUAKO97o1RP5TNuICwT5flB3QgM`cZ%EbׁcZ%Ebց#בCט#u׉׉ 7cassandra://vCkU0qe9M4RKfNugPHBrfQiP5M8Nbt0H9yUYmOuqQjc0`׉ 7cassandra://HuRPfLIiMB90TtkU5NHghHd9vL-wP29rIi7OTV31UTYe#`s׉ 7cassandra://NFtE-7KLuom2RmLPFALjTuhtCB2Nbitdl3bbWVLmzKcu`׉ 7cassandra://9oxCNWYomTXCZ7UZat844ikEuY9x8h4XdFLpDgKht3AH͠^cZEbsט#u׉׉ 7cassandra://e5wWh8huNras4y-qgUieBgIz7d0bVbnaw8vBflXhwPg8O`׉ 7cassandra://tdeD9oCQ9wELFZuNe8TH7t9wlO7FQIY4nC2Bq7ZfTs8J`s׉ 7cassandra://Q1iE1EWxT25iwvzC7xod8wCjm5oRI7EIIzhrUFTpftkE`׉ 7cassandra://Lt0VG2hejCjbl0je1IQZSSKiI9XftyzXFdK5-lmhKeY\͠^cZEbt׉E 1030 Chapter 31 Solid-state Welding Processes In 2001, linear friction welded (LFW) blanes and blisks began appearing in aerospace applications, and have seen steadily increasing use ever since. LFW involves reciprocating sliding motion under controlled pressure; the oscillation frequency is between 30 and 50 Hz, with an amplitude of 2.5 to 5.0 mm. As temperature increases, the load also increases, resulting in a pressure of around 100 MPa, sufficient to cause plastic deformation at the interface between the parts being joined. The deformation removes surface oxides and other defects from the joint. When the desired deformation is achieved, the relative motion between parts stops, resulting in a strong diffusion-based joint. Because the part cools fairly quickly, the joint is cold worked and has an advantageous microstructure for fatigue resistance. Linear friction welding has several advantages for this application: 1. The properties of the welded joints are superior to traditional fusion-based welded joints, since friction welding does not melt the parent material. Melting causes a major change in a material’s properties in the weld zone. The heat-affected zone (HAZ) of a friction-welded joint is narrow and fine grained, and with a smooth transition to the unaffected base material. 2. Complex geometries have a forged quality across the entire butt-welded area. 3. The welding process is very fast, 2 to 100 times faster than competing processes. Furthermore, it is possible to weld more than one blade at a time, thus reducing cycle times. 4. By welding the blades onto a disk, significant material savings can be achieved, as compared to designs involving machining from a single billet or block. 5. The process is energy efficient, because power requirements for LFW are as much as 20% lower than those for conventional welding. Also, the process is environmentally friendly since it requires no flux, filler metal, or shielding gases, and it does not emit smoke, fumes, or gases. 6. Being a steady-state process, LFW is extremely repeatable, and there is essentially no porosity, segregation, or slag inclusions in the weld area. As shown in Fig. 31.20, the blades are produced with a relatively large block, and the disks have a shoe or pad for the blades. Following welding, the block and the flash have to be removed by machining them, resulting in high-quality blisks required in modern aircraft engines. The blades are attached, with a higher fatigue-resistant weld and without a heat-affected zone; as a result, the blanes and blisks are more reliable. Source: D. Adams, Manufacturing Technology, Inc. Summary • In addition to the traditional joining processes of oxyfuel–gas and arc welding, several other joining processes that are based on producing a strong joint under pressure and/or heat also are available. • Surface preparation and cleanliness are important in some of these processes. Pressure is applied mechanically or by explosives. Heat may be supplied externally, by means of electrical resistance or furnaces, or it may be generated internally, as in friction welding. • Combining diffusion-bonding and superplastic-forming processes improves productivity and the capability to make complex parts economically. • As in all manufacturing operations, certain hazards are inherent in welding operations. Some of these relate to the machinery and equipment used, others to the nature of the process itself, as in explosion welding. Proper safety precautions must always be taken in work areas. ׉ 7cassandra://NFtE-7KLuom2RmLPFALjTuhtCB2Nbitdl3bbWVLmzKcu`cZ%Eb׉EReview Questions 1031 Key Terms Cold welding Diffusion bonding (welding) Explosion welding Faying surfaces Ferrule Filled gold Flash welding Flyer plate Friction stir welding Friction welding High-frequency resistance welding Horn Inertia friction welding Linear friction welding Percussion welding Bibliography ASM Handbook,Vol.6A: Welding Fundamentals and Processes, ASM International, 2011. Besharati., M.-K., and Asadi, P., Advances in Friction-Stir Welding and Processing, Woodhead Publishing, 2014. Bohnart, E.R., Welding: Principles and Practices, 5th ed., McGraw-Hill, 2017. Campbell, F.C., Joining, ASM International, 2011. Kou, S., Welding Metallurgy, 2nd ed., Wiley-Interscience, 2002. Lippold, J.C,, Welding Metallurgy and Weldability, Wiley, 2014. Lohwasser, D., and Chen, Z.W., bf Friction Stir Welding, Woodhead Publishing, 2010. Mishra, R.S., and De, P.S., Friction Stir Welding and Processing, Springer, 2014. Modern Welding Technology, 11th ed., Goodheart-Willcox, 2012. Phillips, D.H., Welding Engineering: An Introduction, Wiley, 2016. Welding Handbook, various volumes and editions, American Welding Society. Welding Inspection Handbook, 4th ed., American Welding Society, 2015. Weman, K., Welding Processes Handbook, 2nd ed., Woodhead Publishing, 2011. Yadav, K.S., Advanced Welding Technology, Rajsons Publishing, 2018. Zhand, H., and Senkara, J., Resistance Welding Fundamentals and Application, 2nd ed., CRC Press,2011. Review Questions 31.1. Explain what is meant by solid-state welding. 31.2. What is cold welding? Why is it so called? 31.3. What is (a) a ferrule, (b) filled gold, and (c) a flyer plate? 31.4. What are faying surfaces in solid-state welding processes? 31.5. What is the basic principle of (a) ultrasonic welding and (b) diffusion bonding? Resistance projection welding Resistance seam welding Resistance spot welding Resistance welding Roll bonding Roll spot welding Roll welding Seam welding Solid-state welding Sonotrode Stud welding Superplastic forming Transducer Ultrasonic welding Weld nugget ׉ 7cassandra://Q1iE1EWxT25iwvzC7xod8wCjm5oRI7EIIzhrUFTpftkE`cZ%EbفcZ%Eb؁#בCט#u׉׉ 7cassandra://qPzLCGoq6h65Ilh1l5cUmga5K6OcGpOBvtg_Hc0ILR8 `׉ 7cassandra://OvySXaChW8ra0iuDWwV5iW1QeuWEpI1wkJODJwJtdbAo`s׉ 7cassandra://40Y5_fo-tcFTO5u2AiDcZsYY4yleTkJc2_xWHR1iuCM,`׉ 7cassandra://uvyH9SUEXy6ZL-uRcPFSP9SxA0yOMa-kE_MpfE0EZSgI͠^cZEbvט#u׉׉ 7cassandra://LsDqRmjHjitwoSH6eu9sJ2--cLq0qq4LlLVE2e_e9ugN`׉ 7cassandra://ru3qxdLInU_jpdKJcpzq6KwqE_JXb_Fn6B2m60LqqEkc2`s׉ 7cassandra://SrOs80wVnnJMzR5ryUGVvqr62ONljH-7WqJXLYSA7_I`׉ 7cassandra://QVWfG3mC6L7o9ztxSL12um191K3KqVyBgkHrIkL5ZXcY3͠^cZEbx׉E N1032 Chapter 31 Solid-state Welding Processes 31.6. Explain how the heat is generated in the ultrasonic welding of (a) metals and (b) thermoplastics. 31.7. Describe the advantages and limitations of explosion welding. 31.8. Describe the principle of resistance-welding processes. 31.9. What materials would you recommend for resistance welding electrodes? 31.10. What type of products are suitable for stud welding? Why? 31.11. What is the advantage of linear friction welding over inertia friction welding? 31.12. What are the main forms of friction welding? 31.13. Which processes in this chapter are applicable to polymers? 31.14. Describe how high-frequency butt welding operates. 31.15. What is diffusion bonding? What materials is it typically used with? Qualitative Problems 31.16. Make a list of processes in this chapter, ranking them according to (a) the pressure achieved, (b) the maximum temperature, and (c) suitability for bonding dissimilar materials. 31.17. Make a list of standard abbreviations for welding processes. For example, cold welding is CW and roll welding is ROW. 31.18. Explain the reasons that the processes described in this chapter were developed. 31.19. Explain the similarities and differences between the joining processes described in this chapter and those described in Chapter 30. 31.20. Describe your observations concerning Fig. 31.16c and d. 31.21. Would you be concerned about the size of weld beads, such as those shown in Fig. 31.15? Explain. 31.22. What advantages does friction welding have over other methods described in this and in the preceding chapter? 31.23. What advantages do resistance welding processes have over others described in this chapter? 31.24. List the process parameters that you think will affect the weld strength of a friction weld, and explain why you think those parameters are important. 31.25. Describe the significance of faying surfaces. 31.26. Discuss the factors that influence the strength of (a) a diffusion-bonded and (b) a cold-welded component. 31.27. What are the sources of heat for the processes described in this chapter? 31.28. Can the roll-bonding process be applied to a variety of part configurations? Explain. 31.29. Why is diffusion bonding, when combined with the superplastic forming of sheet metals, an attractive fabrication process? Does it have any limitations? 31.30. List and explain the factors involved in the strength of weld beads. 31.31. Give some of the reasons that spot welding is used commonly in automotive bodies and in large appliances. 31.32. Explain the significance of the magnitude of the pressure applied through the electrodes during a spot-welding operation. 31.33. Give some applications for (a) flash welding, (b) stud welding, and (c) percussion welding. 31.34. Discuss the need for, and role of, work-holding devices in the welding operations described in this chapter. ׉ 7cassandra://40Y5_fo-tcFTO5u2AiDcZsYY4yleTkJc2_xWHR1iuCM,`cZ%Eb׉E Qualitative Problems 1033 31.35. Inspect Fig. 31.4, and explain why those particular fusion-zone shapes are developed as a function of pressure and speed. Comment on the influence of the material’s properties. 31.36. Could the process shown in Fig. 31.11 also be applicable to part shapes other than round? Explain, and give specific examples. 31.37. In spot-weld tests, what would be the reason for weld failure to occur at the locations shown in Fig. 31.9? 31.38. Can friction stir welding be used for powder metal parts? Explain. 31.39. Do any of the processes described in this chapter use a filler metal? Explain. 31.40. Which processes in this chapter are not affected by an oxide film? Explain. 31.41. Consider the situation where two round components are welded together. You suspect that the components were friction welded, with the flash removed by machining. How could you confirm or disprove your suspicion? 31.42. Is there any advantage in preheating the workpieces in friction welding? Explain. 31.43. Inspect the edges of a U.S. quarter, and comment on your observations. Is the cross-section, i.e., the thickness of individual layers, symmetrical? Explain. 31.44. What does the strength of a weld nugget in resistance spot welding depend on? 31.45. Which applications could be suitable for the roll spot welding process shown in Fig. 31.10? Give specific examples. Quantitative Problems 31.46. The energy required in ultrasonic welding is found to be related to the product of workpiece thickness and hardness. Explain why this relationship exists. 31.47. Two flat copper sheets (each 1.0 mm thick) are being spot welded by the use of a current of 7000 A and a current flow time of 0.3 s. The electrodes are 4 mm in diameter. Estimate the heat generated in the weld zone. Assume that the resistance is 200 µΩ. 31.48. Calculate the temperature rise in Problem 31.47, assuming that the heat generated is confined to the volume of material directly between the two round electrodes and the temperature is distributed uniformly. 31.49. Calculate the range of allowable currents in Problem 31.47 if the temperature should be between 0.7 and 0.8 times the melting temperature of copper. Repeat this problem for carbon steel. 31.50. A resistance projection welding machine is used to join two 1.5-mm-thick sheets with eight 6-mm-diameter spot welds produced simultaneously. If 12 seconds are needed for the welding operation, determine (a) the welding current (b) the required kVA if the applied voltage is 12 V, and (c) the electrical energy consumption for each weld. 31.51. The energy applied in friction welding is given by the formula E = IS2/C, where I is the moment of inertia of the flywheel, S is the spindle speed in rpm, andC is a constant of proportionality. (C = 5873 when the moment of inertia is given in kg-m2.) For a spindle speed of 600 rpm and an operation in which a steel tube with a 88 mm outside diameter and a 6 mm wall thickness is welded to a flat frame, what is the required moment of inertia of the flywheel if all of the energy is used to heat the weld zone, approximated as the material 6 mm deep and directly below the tube? Assume that 1.9 J is needed to melt the electrode. ׉ 7cassandra://SrOs80wVnnJMzR5ryUGVvqr62ONljH-7WqJXLYSA7_I`cZ%EbہcZ%Ebځ#בCט#u׉׉ 7cassandra://DDkkVKbJeRo1d8SUAP0171De2p_9d7ZRTsD5U7EL3ekJ`׉ 7cassandra://q72aEX9RrhsZfmWzLvaby8WMXFfYhuYDwWUz36-hXQch`s׉ 7cassandra://BKiqmgc7Be6dZGwbf3sjkg3rQ9ZlYO9JEVZykff-ti4`׉ 7cassandra://UnloKzxNifJr5utLi5kCuiyR3nxT7ak3V2huCPXzQSYQ͠^cZEb|ט#u׉׉ 7cassandra://IVTJL2_-PYyROHsbFSqg6oyB8ldr8AFtfv9BkxicoVE`׉ 7cassandra://fciQ32fKp8aMaZfX9zMv9MMDnEH2ZU5bAr3aGE910ho׉E Index 1299 electrochemical machining, 862 milling, 750 turning, 703–704 selection, 42, 61 substitution, 43, 1265–1266 Material-requirements planning, 1223 Matrix, 259 materials, 264 Mats, 264 Maximum production, 807 roughness height, 1070 Measurement standards, 1115 Measuring instrument characteristics, 1128 machines, 1125 Mechanical alloying, 511 assembly, 1053 cleaning, 1108 fastening, 971 fibering, 75, 108, 455, 461 plating, 1094 press, 407, 475 properties, 42, 62, 83 ceramics, 243 glass, 248 polymers, 212 powder metals, 526 stainless steels, 180 steels, 175, 177 of various materials, 86 sensors, 1196 shock, 661 surface treatments, 1092 Mechanization, 33, 1169 Medium batch, 1276 temperature CVD, 1099 Medium-carbon steel, 174 Melamine, 227 Melt spinning, 333, 567 Melting point, 124 various materials, 125–126 practice, 334 temperature, polymers, 220 Merchant model, 638 Mesoscale manufacturing, 960–961 Mesomanufacturing, 887, 960 Metal bond, 821 coating, 436 flow pattern, extrusion, 422 foams, 203 inert-gas welding, 982 paddings, 344 powder spraying, 1095 powders, 508 particle shape, 512 particle size, 512 production, 508 stitching, 1054 Metal-injection molding, 520 Metallic bond, 65 films, 1084 glasses, 203, 333 projections, 296 Metallization, integrated circuits, 918 Metallizing, 1095 Metallurgical burn, 827 transformation, 1069 Metal-matrix composite, 259, 269 brake calipers, 270 processing, 590 Metal-oxide semiconductor, 895 field effect transistor, 891 Metalworking fluids, 1081 Metamaterials, 204–205 Metastable, 145 Meter, 1115 Method of attributes, 1149 of variables, 1149 Metrology, 1115 Microabrasive blasting, 847 Microabrasive-flow machining, 847 Microalloyed steels, 176 Microchip, 1079 Microchipping, 840 Microcontact printing, 908 Microelectromechanical device, 939, 940 system, 889, 939, 940 Microelectronic device, 940 Microelectronics, 891 Microencapsulated powders, 512 Microforming, 490 Micrograin carbides, 680 Microhardness, 99 Microjet, 875 Micromachining, 940–941 bulk, 941 surface, 941 Micromanufacturing, 887 Micromechanical device, 889, 939, 940 Micrometer, 1117 depth gage, 1117 laser scan, 1126 Micromolding in capillaries, 908 Microporosity, 192 Microreplication, 841 Microscopes, 846 Microsegregation, 286 Microstereolithography, 609, 958 Microtransfer molding, 908 Microwave sintering, 526, 546 Microwelds, 1074 Mil standard rule, 1128 Mild wear, 1079 Mill, rolling, 376 Milling, 746 ceramics, 541 cutters, 748 forces, 750 machines, 760 torque, 750 MIMIC, 908 Mineral oils, 1083 Minimills, 383 Minimum bend radius, 460–461 cost, 807 quantity lubrication, 694 Miscible blends, 220 Misrun, 296 Mist, 691 Mixed lubrication, 1082 Mobile robot, 1191 Mode size, metal powders, 512 Modeling, 1217 casting, 290 Modified martempering, 158 Modifiers, glass, 247 Modular, 1175 construction, 793 fixturing, 1199 Module, 788, 793 Modulus of elasticity, 85 polymers, 212 various materials, 86 of resilience, 102 of rigidity, 96 of rupture, 97 Mohs hardness, 100 Moisture in ceramics, 545 effect on polymers, 223 ׉ 7cassandra://FdWKQlSJeLX6JDO56nGhuoZa1i3cp0j1Dim0eXe1f5Q`cZ%Eb?cZ%Eb>#בCט#u׉׉ 7cassandra://HaFmsQpKehfcJS37ghkrGG-1lBsqKD4SDaqXkbnvL5Il`׉ 7cassandra://BTf5qs8_zDSwJ-87gYU6HF3slVQnVXulID8Z1s9z9U8M`s׉ 7cassandra://8ztyN1iXdkYPRmAzOxCGwhFBh8Zfh0-uiS3xXI9W8m8`׉ 7cassandra://o4WPhdyjXr_HEo4y00x3fbYvrQb6jw6Fg-b4E8c8BP4<͠^cZEbAט#u׉׉ 7cassandra://TyTzLcLYYB1DrySuhCsOVdOq--p4ShnyKiJoNbYwAVsȹ`׉ 7cassandra://QPLbImNKPGYQuDpSO762aWhwonVpsRen-DsVlukWBdgKC`s׉ 7cassandra://uhm41CYfLaF3203FyKXNCAAjMPC8byYXmdBG1ahufPsY`׉ 7cassandra://EteAkLWPWJSBAwX3zsFA6Y-cLDVsQW5ospvpV09iXco5͠^cZEbB׉E 1300 Index Mold(s), 282, 287 ablation, 314 casting, 306 layout, casting, 348 wash, 290 Molecular beam epitaxy, 900 weight, polymers, 214 Molecule, 65 MolTun, 957 Molybdenum, 199–200 Molybdenum disulfide, 1084 Monel, 197 Monocode, 1227 Monomer, 211 Moore’s Law, 907 MOSFET, 891 Mounted wheels, 819 Move card, 1242 M-series high-speed steels, 678 MTConnect, 1245 Mullite, 241 Multiaxis EDM wire-cutting machining centers, 870 MultiClass coding, 1228 Multicomponent injection molding, 570 MultiJet modeling, 611 Multilayer blow molding, 574 X-ray lithography, 954 Multilayered board, 927 Multiphase coatings, 684 Multiple-pass welding, 981 Multiple-spindle automatic bar machines, 717 Multi-walled nanotubes, 251 Mushy state, 139 state processing, 330 zone, 284 N Nano particle jetting (NPJ), 612 Nanoalloyed steels, 178 Nanoceramics, 243 Nanocrystalline materials, 252–253 Nanogrinding, 953 Nanoimprint lithography, 962 Nanolayer coatings, 685 Nanolithography, 962 dip pen, 963 Nanomanufacturing, 887, 962 Nanomaterial cutting tools, 689 Nanomaterials, 252 health hazards, 253 Nanophase, 252 ceramics, 240, 243, 546 Nanopowders, 511 Nanoscale manufacturing, 939 Nanostructured materials, 252 Nanotubes, 250 Natural adhesives, 1046 aging, 154 language processing, 1247 rubber, 231 strain, 88 Naturally bonded sand, 308 Naval brass, 196 Near-dry machining, 694 Near-net shape forming, 389 manufacturing, 51 Necking, 85, 89, 108 sheet metals, 454 strain, 91 Negative springback, 461 Neodymium, 205 Nesting, 449 Net shape casting, 326 forming, 389, 507 manufacturing, 51, 280 Network polymers, 217 Neural networks, 55, 1147–1148 Neurons, 1148 Neutral axis, 459 flame, 974 point, 367 Nextel, 263 Nib, 435 Nibbling, 448–449 Nichrome, 197 Nickel and alloys, 196 Nickel-based superalloys, 197 Nimonic, 198 Niobium (columbium), 199 Nitinol, 203 Nitrides, 240, 242, 687 Nitriding, 155 Nitrogen-ion implantation, 686 No-bake mold, 309 Noble metals, 202 Nodular iron, 146, 355 Noise in experimental design, 1144 Nominal size, 1129 Nominal stress, 84 Nonconsumable electrode, 977 Nondestructive inspection, adhesives, 1049 testing, 1158 welded joints, 1000 Nonferrous metals, 189–190 casting, 351 Nonproductive labor, 58, 1275 Nonsynchronous system, assembly, 1202 Nontraditional machining, 859 Normal anisotropy, 470 distribution curve, 1150 force, cutting, 648 rake angle, 646 segregation, 286 Normalizing, 157 Nose radius, 648, 703 wear, 660 No-slip point, 367 NOT GO gage, 1122 Notch sensitivity, 106 wear, 660 Notched specimen, 106 Notching, 447 No-wear EDM, 869 Nucleating agent, 287 Nucleation, 71, 287 Nugget, weld, 1018 Numerical control, 53, 1177 computer, 1179 direct, 1179 distributed, 1179 programming for, 1183–1184 Numerically controlled robot, 1192 Nylons, 226 O Oblique cutting, 645 Offset, 85 Oils, 1083 cutting, 691 One and done, 792 One-piece pattern, 310 Online inspection, 1115 sensor systems, 1126 On-line inspection, 1162 Open riser, 309 Open-back inclinable press, 496 Open-die forging, 390 Open-hearth furnace, 167 Open-loop control, 1179 Open-mold processing, 585 ׉ 7cassandra://8ztyN1iXdkYPRmAzOxCGwhFBh8Zfh0-uiS3xXI9W8m8`cZ%Eb@׉E Index 1301 Operation (routing) sheets, 1222 Opitz, H., 1227 Optical character recognition, 1188 comparator, 1121 contour projectors, 1121 flat, 1119 properties, 130 ceramics, 245 glasses, 248 polymers, 218 Optical-interference microscope, 1072 Optimization, adaptive control, 1186 Optimum cutting speed, 658, 807 die angle, in drawing, 433 tool life, 808 Orange peel, 71, 76, 455 Orbital forging, 400 Order holon, 1241 Organic coatings, 1105–1106 wheels, 821 Orientation dependent etching, 912 polymers, 223 preferred, 74 Orthogonal arrays, 1144 Osprey process, 523 Out of control, 1154 Out of roundness, 1120 Outsourcing, 59, 1275 Over-aging, 154 Overarm, 760 Overbending, 461 Overcontrol, 1154–1155 Overcut, 868 Overhead, 58, 1275 Overlap, 995 Overmolding, 570 Oxalate coatings, 1085 Oxidation, 131, 900 dry, 900 wear, 1079 wet, 900 Oxide ceramics, 240–241 films, 202 Oxide-powder-in-tube process, 554 Oxidizing flame, 974 Oxyacetylene flames, 873 Oxyfuel-gas cutting, 990 welding, 973 Oxygen, embrittlement, 112 P Pack rolling, 373 Packaging, 1171 microelectronics, 920–921 Painting, 1107 Pallet, 788, 1239 changer, 788 Pancaking, 390 Paper clip, 72 Paperless design, 37, 1221 Parabolic interpolation, 1182 Parametric model, 1218 Parison, 574 Parsons, J. T., 1178 Part families, 1224 feeders, 1202 programming, 1184 Part size and dimensional accuracy, 48–50 Part-checking station, 788 Partially deoxidized steel, 170 Partially stabilized zirconia, 241 Particle size distribution, 512 Parting, 447 agent, 310, 405 line, 404 casting, 309, 347 forging, 404 Passivation, 131, 179, 1069 Pasty state, 139 Patenting, 434 Pattern recognition, 1192 taper, 310 Patternmaker’s shrinkage allowance, 346 Patterns, casting, 310 Pay-off reel, 371 Pearlite, 144, 147 Pearlitic gray cast iron, 146, 355 Pedestal grinder, 838 Pedestal-type fixtures, 1200 Peeling, adhesives, 1049 Peen forming, 489 plating, 1094 Peening, shot, 489, 1092 Pellets, 566 Pencil source, 906 Penetrants, liquid, 1158 Pennies, 44 Pepper mill, 52 Percussion welding, 1024 Perfection, 1244 Perforating, 447 Peripheral milling, 748 Permanent deformation, 67, 85 mold casting processes, 321 molds, 306 Permeability, sand, 308 Personal area networks, 1244–1245 Pewter, 202 Phase, 139 diagrams, 139–141 transformation, 147 Phase-change materials, 1200 Phases, 68 Phenolic, 224, 228 Phosphate coating, 1104 conversion coatings, 427, 1084–1085 Phosphor bronze, 195 Photochemical blanking, 861 machining, 861 Photoetching, 861 Photolithography, 901 Photomask, 901 Photonic integrated circuits, 959–960 Photopolymer, 609 Photoresist, 861, 901 Physical properties, 42, 123–124 ceramics, 245 glasses, 247–248 various materials, 125–126 tempering, 551 vapor deposition, 682, 1097 Physical-chemical etching, 915 Pianola, 1178 Pick-and-place robot, 1191 Pickling, 373 Piece-part production, 1174 Piercing, 398 sheet metal, 465 Piezoelectric effect, 129 Pig iron, 167 Pigments, polymers, 224 Pilger mill, 382 Pinch rolls, 170 Pin-grid array, 922 Pinion-shaped cutter, 772 ׉ 7cassandra://uhm41CYfLaF3203FyKXNCAAjMPC8byYXmdBG1ahufPsY`cZ%EbAcZ%Eb@#בCט#u׉׉ 7cassandra://n1WxnCITqCNOXiUicwp27B1uxX8f73nE_ETfYOuoEcQ7`׉ 7cassandra://qjiZoWZVrpXsxjQeowRyDaz20a_Q0Kwr3h1PnzpqVRoG`s׉ 7cassandra://0BdW8afE9f-V7JWVXBiMco75BhOAwSblMJN5ozjeCno`׉ 7cassandra://Khm6SENEvxgrnfvGFfFTRdAYkUV7S-J0E6FLYayk4rU/&͠^cZEbDט#u׉׉ 7cassandra://uEXEv6Hj2Rpdx2YKweD9QPqvqYYP7Iz0ejg-1BFUoJgu`׉ 7cassandra://h1uFAhPqf4cGb7i7B_7r24iZp2v78Y5VNIioXvPAmKQG`s׉ 7cassandra://WIzhZQ1sCUL2kJxXhrzUyYXQOpTGkHe-N3N9DFdNTew`׉ 7cassandra://4CP-3T_OC3trLaI4_uo-WORUEuij2fZOFX412gA2qTk6͠^cZEbE׉E 1302 Index Pinning, 142 Pipe defect, 429 extrusion, plastics, 564 Piranha, 913 Pit furnace, 159 Pitch in broaches, 764–765 splitting, 904 Pits, 1070 Pitting, 130, 1080 forging dies, 1080 Plain milling, 748 machines, 760 Planar anisotropy, 471–472 Planarization, 918, 953 Plane of twinning, 68 Planer, 763 Planer-type milling machines, 761 Planetary mill, 370 Planing, 763 Planning functions, 1214 Plant layout, 1187, 1225 Plasma, 1095 beams, 876 etching, 914 Plasma-arc cutting, 876, 991 welding, 979 Plasma-enhanced chemical vapor deposition, 899 Plaster-mold casting, 315 Plastic anisotropy, 470 Plasticating screw, 569 Plasticizers, 224 ceramics, 542 Plastic(s), 211 for coatings, 1106 conducting, 223 deformation, 67, 85, 660 polycrystalline metals, 73 forming, ceramics, 543 friction, 1075 joining, 1057 machining, 666 processing, 560 products, recommendations, 225 reinforced, 259–261 structure, 211 wear, 1080 Plastisols, 576 Plate glass, 548 metal, 367 Plating electroless, 1103 electroplating, 1101 mechanical, 1094 Platinum, 202 Playback robot, 1091–1092 Plowing, 1074 in grinding, 824 Plug gages, 1120 Plunge grinding, 831, 836 Pneumatic gages, 1122 Point angle, 728 cloud, 1124 defects, 68 Pointing, 434 Point-to-point control, 1181 robot, 1192 Poisson’s ratio, 87 ceramics, 244 polymers, 212 various materials, 86 Poka-yoke, 1205 Polarity, 977 Polishing, 844 Polyamides, 226 Polyblends, 220 Polycarbonates, 227 Polycodes, 1227 Polycrystalline cubic boron nitride, 687 diamond, 688 silicon, 940 Polycrystals, 73 Polyesters, 227 Polyethylenes, 227 Polygonization, 75 Polyimides, 227 Polyjet, 611 Polymer(s), 211, 212–214 concrete in machine tools, 795–796 fibers, 264 fibers production, 566 matrix composites, 259 precursor, 592 processing, 47 quenchants, 153 structure, 211 Polymeric films, 1084 Polymerization, 212–213 degree of, 214 Polymethylmethacrylate (PMMA), 217 Polymorphic transformation, 143 Polymorphism, 67 Polypropylenes, 227 Polystyrenes, 227 Polysulfones, 227 Polyurethane, 231 Polyurethane foam molding, 581 Polyvinyl chloride, 227 Population, 1149 Porcelain, 240 enamels, 1104–1105 Porosity, 108, 243 in casting, 297, 344 injection molding, 573 in welds, 993 Porous aluminum, 192 Porous ceramics, 243 Portable grinders, 838 Porthole die, 423 Position measurement, 1182–1183 Positional tolerancing, 1132 Positioning accuracy, 1182–1183 system, 1181 Postprocess inspection, 1115, 1162 Potting, 580 Pouring basin (cup), 287, 309 Pouring temperature, 292 Powder forging, 521 injection molding, 520 metallurgy, 507 equipment, 517 rolling, 521 Powder Bed Processes, 523, 605, 612–613 Powder-in-tube, 554 Power brushing, 842 chucks, 715, 1199 cutting, 650 drop hammer, 408 in milling, 750 spinning, 482 Power-head units, 1175 Prebaking, 901 Precious metals, 202 Precipitates, 153 Precipitation, 511 hardening, 153 stainless steels, 179 Precision, 1115, 1128 casting, 315 forging, 389, 395 machining, 796, 804 ׉ 7cassandra://0BdW8afE9f-V7JWVXBiMco75BhOAwSblMJN5ozjeCno`cZ%EbB׉E