Welding Metallurgy

Sindo Kou, PhD, is Professor and former Chair of the Department of Materials Science and Engineering at the University of Wisconsin. He graduated from MIT with a doctorate in metallurgy. He is a Fellow of American Welding Society and ASM International. He received the William Irrgang Memorial Award (2018), the Honorary Membership Award (2016), and the Comfort A. Adams Lecture Award (2012) from the American Welding Society (AWS); the Yoshiaki Arata Award (2017) from the International Institute of Welding (IIW); the Bruce Chalmers Award (2013) from The Minerals, Metals & Materials Society (TMS); the John Chipman Award (1980) from the Iron and Steel Society of AIME; and Chancellor's Award for Distinguished Teaching (1999) from the University of Wisconsin-Madison. His technical papers won the Warren F. Savage Memorial Award (4 times), Charles H. Jennings Memorial Award (4 times), William Spraragen Award (3 times), A.F. Davis Silver Medal Award, and James F. Lincoln Gold Medal of AWS; and the Magnesium Technology Best Paper Award of TMS.

Preface to Third Edition xxi

Part I Introduction 1

1 Welding Processes 3

1.1 Overview 3

1.1.1 Fusion Welding Processes 3

1.1.1.1 Power Density of Heat Source 4

1.1.1.2 Welding Processes and Materials 5

1.1.1.3 Types of Joints and Welding Positions 7

1.1.2 Solid-State Welding Processes 8

1.2 Gas Welding 8

1.2.1 The Process 8

1.2.2 Three Types of Flames 9

1.2.2.1 Neutral Flame 9

1.2.2.2 Reducing Flame 9

1.2.2.3 Oxidizing Flame 9

1.2.3 Advantages and Disadvantages 10

1.3 Arc Welding 10

1.3.1 Shielded Metal Arc Welding 10

1.3.1.1 Functions of Electrode Covering 10

1.3.1.2 Advantages and Disadvantages 11

1.3.2 Gas–Tungsten Arc Welding 11

1.3.2.1 The Process 11

1.3.2.2 Polarity 12

1.3.2.3 Electrodes 13

1.3.2.4 Shielding Gases 13

1.3.2.5 Advantages and Disadvantages 13

1.3.3 Plasma Arc Welding 14

1.3.3.1 The Process 14

1.3.3.2 Arc Initiation 14

1.3.3.3 Keyholing 15

1.3.3.4 Advantages and Disadvantages 15

1.3.4 Gas–Metal Arc Welding 16

1.3.4.1 The Process 16

1.3.4.2 Shielding Gases 16

1.3.4.3 Modes of Metal Transfer 17

1.3.4.4 Advantages and Disadvantages 18

1.3.5 Flux-Cored Arc Welding 18

1.3.5.1 The Process 18

1.3.6 Submerged Arc Welding 19

1.3.6.1 The Process 19

1.3.6.2 Advantages and Disadvantages 20

1.3.7 Electroslag Welding 20

1.3.7.1 The Process 20

1.3.7.2 Advantages and Disadvantages 21

1.4 High-Energy-Beam Welding 21

1.4.1 Electron Beam Welding 21

1.4.1.1 The Process 21

1.4.1.2 Advantages and Disadvantages 23

1.4.2 Laser Beam Welding 24

1.4.2.1 The Process 24

1.4.2.2 Reflectivity 24

1.4.2.3 Shielding Gas 25

1.4.2.4 Laser-Assisted Arc Welding 25

1.4.2.5 Advantages and Disadvantages 26

1.5 Resistance Spot Welding 26

1.6 Solid-State Welding 27

1.6.1 Friction Stir Welding 27

1.6.2 Friction Welding 29

1.6.3 Explosion and Magnetic-Pulse Welding 31

1.6.4 Diffusion Welding 31

Examples 32

References 33

Further Reading 34

Problems 35

2 Heat Flow in Welding 37

2.1 Heat Source 37

2.1.1 Heat Source Efficiency 37

2.1.1.1 Definition 37

2.1.1.2 Measurements 38

2.1.1.3 Heat Source Efficiencies in Various Welding Processes 41

2.1.2 Melting Efficiency 42

2.1.3 Power Density Distribution of Heat Source 43

2.1.3.1 Effect of Electrode Tip Angle 43

2.1.3.2 Measurements 43

2.2 Heat Flow During Welding 45

2.2.1 Response of Material to Welding Heat Source 45

2.2.2 Rosenthal’s Equations 45

2.2.2.1 Rosenthal’s Two-Dimensional Equation 46

2.2.2.2 Rosenthal’s Three-Dimensional Equation 47

2.2.2.3 Step-by-Step Application of Rosenthal’s Equations 48

2.2.3 Adams’ Equations 49

2.3 Effect of Welding Conditions 49

2.4 Computer Simulation 52

2.5 Weld Thermal Simulator 53

2.5.1 The Equipment 53

2.5.2 Applications 54

2.5.3 Limitations 54

Examples 54

References 57

Further Reading 59

Problems 59

3 Fluid Flow in Welding 61

3.1 Fluid Flow in Arcs 61

3.1.1 Sharp Electrode 61

3.1.2 Flat-End Electrode 63

3.2 Effect of Metal Vapor on Arcs 63

3.2.1 Gas−Tungsten Arc Welding 63

3.2.2 Gas−Metal Arc Welding 65

3.3 Arc Power- and Current-Density Distributions 68

3.4 Fluid Flow in Weld Pools 69

3.4.1 Driving Forces for Fluid Flow 69

3.4.2 Heiple’s Theory for Weld Pool Convection 71

3.4.3 Physical Simulation of Fluid Flow and Weld Penetration 72

3.4.4 Computer Simulation of Fluid Flow and Weld Penetration 75

3.5 Flow Oscillation and Ripple Formation 77

3.6 Active Flux GTAW 80

3.7 Resistance Spot Welding 81

Examples 84

References 85

Further Reading 88

Problems 88

4 Mass and Filler–Metal Transfer 91

4.1 Convective Mass Transfer in Weld Pools 91

4.2 Evaporation of Volatile Solutes 94

4.3 Filler-Metal Drop Explosion and Spatter 96

4.4 Spatter in GMAW of Magnesium 100

4.5 Diffusion Bonding 100

Examples 103

References 104

Problems 105

5 Chemical Reactions in Welding 107

5.1 Overview 107

5.1.1 Effect of Nitrogen, Oxygen, and Hydrogen 107

5.1.2 Protection Against Air 107

5.1.3 Evaluation of Weld Metal Properties 108

5.2 Gas–Metal Reactions 111

5.2.1 Thermodynamics of Reactions 111

5.2.2 Hydrogen 113

5.2.2.1 Magnesium 113

5.2.2.2 Aluminum 113

5.2.2.3 Titanium 116

5.2.2.4 Copper 116

5.2.2.5 Steels 116

5.2.3 Nitrogen 118

5.2.3.1 Steel 118

5.2.3.2 Titanium 121

5.2.4 Oxygen 121

5.2.4.1 Magnesium 121

5.2.4.2 Aluminum 121

5.2.4.3 Titanium 121

5.2.4.4 Steel 122

5.3 Slag–Metal Reactions 125

5.3.1 Thermochemical Reactions 125

5.3.1.1 Decomposition of Flux 125

5.3.1.2 Removal of S and P from Liquid Steel 126

5.3.2 Effect of Flux on Weld Metal Oxygen 127

5.3.3 Types of Fluxes, Basicity Index, and Weld Metal Properties 127

5.3.4 Basicity Index 128

5.3.5 Electrochemical Reactions 130

Examples 135

References 136

Further Reading 140

Problems 140

6 Residual Stresses, Distortion, and Fatigue 141

6.1 Residual Stresses 141

6.1.1 Development of Residual Stresses 141

6.1.1.1 Stresses Induced By Welding 141

6.1.1.2 Welding 141

6.1.2 Analysis of Residual Stresses 143

6.2 Distortion 145

6.2.1 Cause 145

6.2.2 Remedies 146

6.3 Fatigue 147

6.3.1 Mechanism 147

6.3.2 Fractography 147

6.3.3 S–N Curves 150

6.3.4 Effect of Joint Geometry 150

6.3.5 Effect of Stress Raisers 151

6.3.6 Effect of Corrosion 152

6.3.7 Remedies 152

6.3.7.1 Shot Peening 152

6.3.7.2 Reducing Stress Raisers 153

6.3.7.3 Laser Shock Peening 154

6.3.7.4 Use of Low–Transformation–Temperature Fillers 154

Examples 154

References 155

Further Reading 156

Problems 156

Part II The Fusion Zone 157

7 Introduction to Solidification 159

7.1 Solute Redistribution During Solidification 159

7.1.1 Directional Solidification 159

7.1.2 Equilibrium Segregation Coefficient k 159

7.1.3 Four Cases of Solute Redistribution 161

7.2 Constitutional Supercooling 166

7.3 Solidification Modes 168

7.4 Microsegregation Caused by Solute Redistribution 171

7.5 Secondary Dendrite Arm Spacing 174

7.6 Effect of Dendrite Tip Undercooling 177

7.7 Effect of Growth Rate 178

7.8 Solidification of Ternary Alloys 178

7.8.1 Liquidus Projection 178

7.8.2 Solidification Path 179

7.8.3 Ternary Magnesium Alloys 180

7.8.4 Ternary Fe-Cr-Ni Alloys 182

7.8.4.1 Fe-Cr-Ni Phase Diagram 182

7.8.4.2 Solidification Paths 185

7.8.4.3 Microstructure 186

Examples 189

References 191

Further Reading 193

Problems 193

8 Solidification Modes 195

8.1 Solidification Modes 195

8.1.1 Temperature Gradient and Growth Rate 195

8.1.2 Variations in Growth Mode Across Weld 197

8.2 Dendrite Spacing and Cell Spacing 200

8.3 Effect of Welding Parameters 201

8.3.1 Solidification Mode 201

8.3.2 Dendrite and Cell Spacing 202

8.4 Refining Microstructure Within Grains 203

8.4.1 Arc Oscillation 203

8.4.2 Arc Pulsation 205

Examples 205

References 206

Further Reading 207

Problems 207

9 Nucleation and Growth of Grains 209

9.1 Epitaxial Growth at the Fusion Line 209

9.2 Nonepitaxial Growth at the Fusion Line 212

9.2.1 Mismatching Crystal Structures 212

9.2.2 Nondendritic Equiaxed Grains 213

9.3 Growth of Columnar Grains 214

9.4 Effect of Welding Parameters on Columnar Grains 215

9.5 Control of Columnar Grains 218

9.6 Nucleation Mechanisms of Equiaxed Grains 219

9.6.1 Microstructure Around Pool Boundary 219

9.6.2 Dendrite Fragmentation 220

9.6.3 Grain Detachment 222

9.6.4 Heterogeneous Nucleation 222

9.6.5 Effect of Welding Parameters on Heterogeneous Nucleation 225

9.6.6 Surface Nucleation 228

9.7 Grain Refining 228

9.7.1 Inoculation 228

9.7.2 Weld Pool Stirring 229

9.7.2.1 Magnetic Weld Pool Stirring 229

9.7.2.2 Ultrasonic Weld Pool Stirring 229

9.7.3 Arc Pulsation 232

9.7.4 Arc Oscillation 232

9.8 Identifying Grain-Refining Mechanisms 233

9.8.1 Overlap Welding Procedure 233

9.8.2 Identifying the Grain-Refining Mechanism 235

9.8.3 Effect of Arc Oscillation on Dendrite Fragmentation 236

9.8.4 Effect of Arc Oscillation on Constitutional Supercooling 236

9.8.5 Effect of Composition on Grain Refining by Arc Oscillation 238

9.9 Grain-Boundary Migration 238

Examples 239

References 240

Further Reading 245

Problems 246

10 Microsegregation 247

10.1 Microsegregation in Welds 247

10.2 Effect of Travel Speed on Microsegregation 249

10.3 Effect of Primary Solidification Phase on Microsegregation 252

10.4 Effect of Maximum Solid Solubility on Microsegregation 253

Examples 259

References 261

Further Reading 262

Problems 262

11 Macrosegregation 263

11.1 Macrosegregation in the Fusion Zone 263

11.2 Quick Freezing of One Liquid Metal in Another 265

11.3 Macrosegregation in Dissimilar-Filler Welding 265

11.3.1 Bulk Weld-Metal Composition 265

11.3.2 Mechanism I 267

11.3.3 Mechanism II 270

11.4 Macrosegregation in Dissimilar-Metal Welding 279

11.4.1 Mechanism I 279

11.4.2 Mechanism II 283

11.5 Reduction of Macrosegregation 286

11.6 Macrosegregation in Multiple-Pass Welds 287

References 290

Further Reading 291

Problems 291

12 Some Alloy-Specific Microstructures and Properties 293

12.1 Austenitic Stainless Steels 293

12.1.1 Microstructure Evolution in Stainless Steels 293

12.1.2 Mechanisms of Ferrite Formation 294

12.1.3 Prediction of Ferrite Content 296

12.1.4 Effect of Cooling Rate 297

12.1.4.1 Changes in Solidification Mode 297

12.1.4.2 Dendrite Tip Undercooling 301

12.2 Low-Carbon, Low-Alloy Steels 301

12.2.1 Microstructure Development 301

12.2.2 Factors Affecting Microstructure 302

12.2.3 Weld Metal Toughness 306

12.3 Ultralow Carbon Bainitic Steels 306

12.4 Creep-Resistant Steels 308

12.5 Hardfacing of Steels 311

References 319

Further Reading 321

Problems 321

13 Solidification Cracking 323

13.1 Characteristics of Solidification Cracking 323

13.2 Theories of Solidification Cracking 323

13.2.1 Criterion for Cracking Proposed by Kou 327

13.2.2 Index for Crack Susceptibility Proposed by Kou 328

13.2.3 Previous Theories 330

13.3 Binary Alloys and Analytical Equations 331

13.4 Solidification Cracking Tests 334

13.4.1 Varestraint Test 334

13.4.2 Controlled Tensile Weldability Test 336

13.4.3 Transverse-Motion Weldability Test 337

13.4.4 Circular-Patch Test 341

13.4.5 Houldcroft Test 342

13.4.6 Cast-Pin Test 343

13.4.7 Ring-Casting Test 344

13.4.8 Other Tests 344

13.5 Solidification Cracking of Stainless Steels 345

13.5.1 Primary Solidification Phase 345

13.5.2 Mechanism of Crack Resistance 346

13.6 Factors Affecting Solidification Cracking 350

13.6.1 Primary Solidification Phase 350

13.6.2 Grain Size 350

13.6.3 Solidification Temperature Range 351

13.6.4 Back Diffusion 354

13.6.5 Dihedral Angle 355

13.6.6 Grain-Boundary Angle 359

13.6.7 Degree of Restraint 360

13.7 Reducing Solidification Cracking 360

13.7.1 Control of Weld Metal Composition 360

13.7.2 Control of Weld Microstructure 363

13.7.3 Control of Welding Conditions 365

13.7.4 Control of Weld Shape 366

Examples 367

References 370

Further Reading 376

Problems 376

14 Ductility-Dip Cracking 379

14.1 Characteristics of Ductility-Dip Cracking 379

14.2 Theories of Ductility-Dip Cracking 381

14.3 Test Methods 382

14.4 Ductility-Dip Cracking of Ni-Base Alloys 384

14.4.1 Grain-Boundary Sliding 384

14.4.2 Grain-Boundary Misorientation 386

14.4.3 Grain-Boundary Tortuosity and Precipitates 386

14.4.4 Grain Size 388

14.4.5 Factors Affecting Ductility-Dip Cracking 390

14.5 Ductility-Dip Cracking of Stainless Steels 390

Examples 392

References 394

Further Reading 396

Problems 396

Part III The Partially Melted Zone 399

15 Liquation in the Partially Melted Zone 401

15.1 Formation of the Partially Melted Zone 401

15.2 Liquation Mechanisms 403

15.2.1 Mechanism I: Alloy with Co > CSM 404

15.2.2 Mechanism II: Alloy with Co < CSM and no AxBy or Eutectic 405

15.2.3 Mechanism III: Alloy with Co < CSM and AxBy or Eutectic 405

15.2.4 Additional Mechanisms of Liquation 409

15.3 Directional Solidification of Liquated Material 411

15.4 Grain-Boundary Segregation 411

15.5 Loss of Strength and Ductility 413

15.6 Hydrogen Cracking 414

15.7 Effect of Heat Input 414

15.8 Effect of Arc Oscillation 415

Examples 416

References 417

Problems 418

16 Liquation Cracking 419

16.1 Liquation Cracking in Arc Welding 419

16.1.1 Crack Susceptibility Tests 421

16.1.1.1 Varestraint Testing 421

16.1.1.2 Circular-Patch Testing 422

16.1.1.3 Hot Ductility Testing 423

16.1.2 Mechanism of Liquation Cracking 423

16.1.3 Predicting Effect of Filler Metal on Crack Susceptibility 424

16.1.4 Factors Affecting Liquation Cracking 430

16.1.4.1 Filler Metal 430

16.1.4.2 Heat Source 430

16.1.4.3 Degree of Restraint 431

16.1.4.4 Base Metal 431

16.2 Liquation Cracking in Resistance Spot Welding 434

16.3 Liquation Cracking in Friction Stir Welding 434

16.4 Liquation Cracking in Dissimilar-Metal FSW 439

Examples 445

References 446

Problems 449

Part IV The Heat-Affected Zone 451

17 Introduction to Solid-State Transformations 453

17.1 Work-Hardened Materials 453

17.2 Heat-Treatable Al Alloys 455

17.3 Heat-Treatable Ni-Base Alloys 458

17.4 Steels 461

17.4.1 Fe-C Phase Diagram and CCT Diagrams 461

17.4.2 Carbon Steels 463

17.4.3 Dual-Phase Steels 470

17.5 Stainless Steels 471

17.5.1 Types of Stainless Steels 471

17.5.2 Sensitization of Unstabilized Grades 473

17.5.3 Sensitization of Stabilized Grades 473

17.5.4 σ-Phase Embrittlement 475

Examples 475

References 475

Problems 477

18 Heat-Affected-Zone Degradation of Mechanical Properties 479

18.1 Grain Coarsening 479

18.2 Recrystallization and Grain Growth 480

18.3 Overaging in Al Alloys 483

18.3.1 Al-Cu-Mg (2000-Series) Alloys 483

18.3.1.1 Microstructure and Strength 483

18.3.1.2 Effect of Welding Parameters or Process 488

18.3.2 Al-Mg-Si (6000-Series) Alloys 489

18.3.2.1 Microstructure and Strength 489

18.3.2.2 Effect of Welding Processes and Parameters 491

18.3.3 Al-Zn-Mg (7000-Series) Alloys 492

18.4 Dissolution of Precipitates in Ni-Base Alloys 494

18.5 Martensite Tempering in Dual-Phase Steels 498

Examples 500

References 500

Further Reading 502

Problems 502

19 Heat-Affected-Zone Cracking 505

19.1 Hydrogen Cracking in Steels 505

19.1.1 Cause 505

19.1.2 Appearance 506

19.1.3 Susceptibility Tests 507

19.1.4 Remedies 508

19.1.4.1 Preheating 508

19.1.4.2 Postweld Heating 509

19.1.4.3 Bead Tempering 509

19.1.4.4 Use of Low-H Processes and Consumables 509

19.1.4.5 Use of Lower-Strength Filler Metals 509

19.1.4.6 Use of Austenitic-Stainless-Steel Filler Metals 510

19.2 Stress-Relief Cracking in Steels 510

19.3 Lamellar Tearing in Steels 514

19.4 Type-IV Cracking in Grade 91 Steel 517

19.5 Strain-Age Cracking in Ni-Base Alloys 519

Examples 524

References 524

Further Reading 527

Problems 528

20 Heat-Affected-Zone Corrosion 529

20.1 Weld Decay of Stainless Steels 529

20.2 Weld Decay of Ni-Base Alloys 533

20.3 Knife-Line Attack of Stainless Steels 534

20.4 Sensitization of Ferritic Stainless-Steel Welds 536

20.5 Stress Corrosion Cracking of Austenitic Stainless Steels 537

20.6 Corrosion Fatigue of Al Welds 538

Examples 538

References 539

Further Reading 540

Problems 540

Part V Special Topics 541

21 Additive Manufacturing 543

21.1 Heat and Fluid Flow 543

21.2 Residual Stress and Distortion 545

21.3 Lack of Fusion and Gas Porosity 547

21.4 Grain Structure 550

21.5 Solidification Cracking 550

21.6 Liquation Cracking 553

21.7 Graded Transition Joints 558

21.8 Further Discussions 560

Examples 560

References 561

Further Reading 563

Problems 564

22 Dissimilar-Metal Joining 565

22.1 Introduction 565

22.2 Arc and Laser Joining 565

22.2.1 Al-to-Steel Arc Brazing 566

22.2.1.1 Effect of Lap Joint Gap 569

22.2.1.2 Effect of Heat Input 575

22.2.1.3 Effect of Ultrasonic Vibration 577

22.2.1.4 Effect of Preheating 578

22.2.1.5 Effect of Postweld Heat Treatment 578

22.2.1.6 Butt Joint 579

22.2.2 Al-to-Steel Laser Brazing 579

22.2.3 Al-to-Steel Laser Welding 580

22.2.4 Mg-to-Steel Brazing 582

22.2.5 Al-to-Mg Welding 583

22.3 Resistance Spot Welding 583

22.3.1 Al-to-Steel RSW 583

22.3.2 Mg-to-Steel RSW 586

22.3.3 Al-to-Mg RSW 588

22.4 Friction Stir Welding 589

22.4.1 Al-to-Cu FSSW 589

22.4.2 FSSW of Al to Galvanized Steel 592

22.4.3 Effect of Coating on Al-to-Steel FSSW 597

22.5 Other Solid-State Welding Processes 603

22.5.1 Friction Welding 603

22.5.2 Explosion Welding 606

22.5.3 Magnetic Pulse Welding 607

Examples 608

References 609

Further Reading 612

Problems 612

23 Welding of Magnesium Alloys 613

23.1 Spatter 613

23.1.1 Spatter in Mg GMAW 613

23.1.2 Mechanism of Spatter 614

23.1.3 Elimination of Spatter 614

23.1.4 Irregular Weld Shape and Its Elimination 617

23.2 Porosity 618

23.2.1 Porosity in Mg GMAW 618

23.2.2 Mechanisms of Porosity Formation and Elimination 620

23.2.3 Comparing Porosity in Al and Mg Welds 621

23.3 Internal Oxide Films 622

23.3.1 Mechanism 622

23.3.2 Remedies 624

23.4 High Crowns 625

23.4.1 Mechanism of High-Crown Formation 625

23.4.2 Reducing Crown Height 627

23.5 Grain Refining 628

23.5.1 Ultrasonic Weld Pool Stirring 628

23.5.2 Arc Pulsation 629

23.5.3 Arc Oscillation 629

23.6 Solidification Cracking 629

23.7 Liquation Cracking 629

23.7.1 A Simple Test for Crack Susceptibility 631

23.7.2 Effect of Filler Metals 634

23.7.3 Effect of Grain Size 636

23.8 Heat-Affected Zone Weakening 636

Examples 638

References 640

Further Reading 641

Problems 641

24 Welding of High-Entropy Alloys and Metal-Matrix Nanocomposites 643

24.1 High-Entropy Alloys 643

24.1.1 Solidification Microstructure 643

24.1.2 Weldability 644

24.2 Metal-Matrix Nanocomposites 646

24.2.1 Nanoparticles Increasing Weld Size 646

24.2.2 Nanoparticles Refining Microstructure 648

24.2.3 Nanoparticles Reducing Cracking During Solidification 650

24.2.4 Nanoparticles Allowing Friction Stir Welding 651

Examples 653

References 654

Further Reading 655

Problems 655

Appendix A: Analytical Equations for Susceptibility to Solidification Cracking 657

Index 659