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 xxiPart I Introduction 11 Welding Processes 31.1 Overview 31.1.1 Fusion Welding Processes 31.1.1.1 Power Density of Heat Source 41.1.1.2 Welding Processes and Materials 51.1.1.3 Types of Joints and Welding Positions 71.1.2 Solid-State Welding Processes 81.2 Gas Welding 81.2.1 The Process 81.2.2 Three Types of Flames 91.2.2.1 Neutral Flame 91.2.2.2 Reducing Flame 91.2.2.3 Oxidizing Flame 91.2.3 Advantages and Disadvantages 101.3 Arc Welding 101.3.1 Shielded Metal Arc Welding 101.3.1.1 Functions of Electrode Covering 101.3.1.2 Advantages and Disadvantages 111.3.2 Gas–Tungsten Arc Welding 111.3.2.1 The Process 111.3.2.2 Polarity 121.3.2.3 Electrodes 131.3.2.4 Shielding Gases 131.3.2.5 Advantages and Disadvantages 131.3.3 Plasma Arc Welding 141.3.3.1 The Process 141.3.3.2 Arc Initiation 141.3.3.3 Keyholing 151.3.3.4 Advantages and Disadvantages 151.3.4 Gas–Metal Arc Welding 161.3.4.1 The Process 161.3.4.2 Shielding Gases 161.3.4.3 Modes of Metal Transfer 171.3.4.4 Advantages and Disadvantages 181.3.5 Flux-Cored Arc Welding 181.3.5.1 The Process 181.3.6 Submerged Arc Welding 191.3.6.1 The Process 191.3.6.2 Advantages and Disadvantages 201.3.7 Electroslag Welding 201.3.7.1 The Process 201.3.7.2 Advantages and Disadvantages 211.4 High-Energy-Beam Welding 211.4.1 Electron Beam Welding 211.4.1.1 The Process 211.4.1.2 Advantages and Disadvantages 231.4.2 Laser Beam Welding 241.4.2.1 The Process 241.4.2.2 Reflectivity 241.4.2.3 Shielding Gas 251.4.2.4 Laser-Assisted Arc Welding 251.4.2.5 Advantages and Disadvantages 261.5 Resistance Spot Welding 261.6 Solid-State Welding 271.6.1 Friction Stir Welding 271.6.2 Friction Welding 291.6.3 Explosion and Magnetic-Pulse Welding 311.6.4 Diffusion Welding 31Examples 32References 33Further Reading 34Problems 352 Heat Flow in Welding 372.1 Heat Source 372.1.1 Heat Source Efficiency 372.1.1.1 Definition 372.1.1.2 Measurements 382.1.1.3 Heat Source Efficiencies in Various Welding Processes 412.1.2 Melting Efficiency 422.1.3 Power Density Distribution of Heat Source 432.1.3.1 Effect of Electrode Tip Angle 432.1.3.2 Measurements 432.2 Heat Flow During Welding 452.2.1 Response of Material to Welding Heat Source 452.2.2 Rosenthal’s Equations 452.2.2.1 Rosenthal’s Two-Dimensional Equation 462.2.2.2 Rosenthal’s Three-Dimensional Equation 472.2.2.3 Step-by-Step Application of Rosenthal’s Equations 482.2.3 Adams’ Equations 492.3 Effect of Welding Conditions 492.4 Computer Simulation 522.5 Weld Thermal Simulator 532.5.1 The Equipment 532.5.2 Applications 542.5.3 Limitations 54Examples 54References 57Further Reading 59Problems 593 Fluid Flow in Welding 613.1 Fluid Flow in Arcs 613.1.1 Sharp Electrode 613.1.2 Flat-End Electrode 633.2 Effect of Metal Vapor on Arcs 633.2.1 Gas−Tungsten Arc Welding 633.2.2 Gas−Metal Arc Welding 653.3 Arc Power- and Current-Density Distributions 683.4 Fluid Flow in Weld Pools 693.4.1 Driving Forces for Fluid Flow 693.4.2 Heiple’s Theory for Weld Pool Convection 713.4.3 Physical Simulation of Fluid Flow and Weld Penetration 723.4.4 Computer Simulation of Fluid Flow and Weld Penetration 753.5 Flow Oscillation and Ripple Formation 773.6 Active Flux GTAW 803.7 Resistance Spot Welding 81Examples 84References 85Further Reading 88Problems 884 Mass and Filler–Metal Transfer 914.1 Convective Mass Transfer in Weld Pools 914.2 Evaporation of Volatile Solutes 944.3 Filler-Metal Drop Explosion and Spatter 964.4 Spatter in GMAW of Magnesium 1004.5 Diffusion Bonding 100Examples 103References 104Problems 1055 Chemical Reactions in Welding 1075.1 Overview 1075.1.1 Effect of Nitrogen, Oxygen, and Hydrogen 1075.1.2 Protection Against Air 1075.1.3 Evaluation of Weld Metal Properties 1085.2 Gas–Metal Reactions 1115.2.1 Thermodynamics of Reactions 1115.2.2 Hydrogen 1135.2.2.1 Magnesium 1135.2.2.2 Aluminum 1135.2.2.3 Titanium 1165.2.2.4 Copper 1165.2.2.5 Steels 1165.2.3 Nitrogen 1185.2.3.1 Steel 1185.2.3.2 Titanium 1215.2.4 Oxygen 1215.2.4.1 Magnesium 1215.2.4.2 Aluminum 1215.2.4.3 Titanium 1215.2.4.4 Steel 1225.3 Slag–Metal Reactions 1255.3.1 Thermochemical Reactions 1255.3.1.1 Decomposition of Flux 1255.3.1.2 Removal of S and P from Liquid Steel 1265.3.2 Effect of Flux on Weld Metal Oxygen 1275.3.3 Types of Fluxes, Basicity Index, and Weld Metal Properties 1275.3.4 Basicity Index 1285.3.5 Electrochemical Reactions 130Examples 135References 136Further Reading 140Problems 1406 Residual Stresses, Distortion, and Fatigue 1416.1 Residual Stresses 1416.1.1 Development of Residual Stresses 1416.1.1.1 Stresses Induced By Welding 1416.1.1.2 Welding 1416.1.2 Analysis of Residual Stresses 1436.2 Distortion 1456.2.1 Cause 1456.2.2 Remedies 1466.3 Fatigue 1476.3.1 Mechanism 1476.3.2 Fractography 1476.3.3 S–N Curves 1506.3.4 Effect of Joint Geometry 1506.3.5 Effect of Stress Raisers 1516.3.6 Effect of Corrosion 1526.3.7 Remedies 1526.3.7.1 Shot Peening 1526.3.7.2 Reducing Stress Raisers 1536.3.7.3 Laser Shock Peening 1546.3.7.4 Use of Low–Transformation–Temperature Fillers 154Examples 154References 155Further Reading 156Problems 156Part II The Fusion Zone 1577 Introduction to Solidification 1597.1 Solute Redistribution During Solidification 1597.1.1 Directional Solidification 1597.1.2 Equilibrium Segregation Coefficient k 1597.1.3 Four Cases of Solute Redistribution 1617.2 Constitutional Supercooling 1667.3 Solidification Modes 1687.4 Microsegregation Caused by Solute Redistribution 1717.5 Secondary Dendrite Arm Spacing 1747.6 Effect of Dendrite Tip Undercooling 1777.7 Effect of Growth Rate 1787.8 Solidification of Ternary Alloys 1787.8.1 Liquidus Projection 1787.8.2 Solidification Path 1797.8.3 Ternary Magnesium Alloys 1807.8.4 Ternary Fe-Cr-Ni Alloys 1827.8.4.1 Fe-Cr-Ni Phase Diagram 1827.8.4.2 Solidification Paths 1857.8.4.3 Microstructure 186Examples 189References 191Further Reading 193Problems 1938 Solidification Modes 1958.1 Solidification Modes 1958.1.1 Temperature Gradient and Growth Rate 1958.1.2 Variations in Growth Mode Across Weld 1978.2 Dendrite Spacing and Cell Spacing 2008.3 Effect of Welding Parameters 2018.3.1 Solidification Mode 2018.3.2 Dendrite and Cell Spacing 2028.4 Refining Microstructure Within Grains 2038.4.1 Arc Oscillation 2038.4.2 Arc Pulsation 205Examples 205References 206Further Reading 207Problems 2079 Nucleation and Growth of Grains 2099.1 Epitaxial Growth at the Fusion Line 2099.2 Nonepitaxial Growth at the Fusion Line 2129.2.1 Mismatching Crystal Structures 2129.2.2 Nondendritic Equiaxed Grains 2139.3 Growth of Columnar Grains 2149.4 Effect of Welding Parameters on Columnar Grains 2159.5 Control of Columnar Grains 2189.6 Nucleation Mechanisms of Equiaxed Grains 2199.6.1 Microstructure Around Pool Boundary 2199.6.2 Dendrite Fragmentation 2209.6.3 Grain Detachment 2229.6.4 Heterogeneous Nucleation 2229.6.5 Effect of Welding Parameters on Heterogeneous Nucleation 2259.6.6 Surface Nucleation 2289.7 Grain Refining 2289.7.1 Inoculation 2289.7.2 Weld Pool Stirring 2299.7.2.1 Magnetic Weld Pool Stirring 2299.7.2.2 Ultrasonic Weld Pool Stirring 2299.7.3 Arc Pulsation 2329.7.4 Arc Oscillation 2329.8 Identifying Grain-Refining Mechanisms 2339.8.1 Overlap Welding Procedure 2339.8.2 Identifying the Grain-Refining Mechanism 2359.8.3 Effect of Arc Oscillation on Dendrite Fragmentation 2369.8.4 Effect of Arc Oscillation on Constitutional Supercooling 2369.8.5 Effect of Composition on Grain Refining by Arc Oscillation 2389.9 Grain-Boundary Migration 238Examples 239References 240Further Reading 245Problems 24610 Microsegregation 24710.1 Microsegregation in Welds 24710.2 Effect of Travel Speed on Microsegregation 24910.3 Effect of Primary Solidification Phase on Microsegregation 25210.4 Effect of Maximum Solid Solubility on Microsegregation 253Examples 259References 261Further Reading 262Problems 26211 Macrosegregation 26311.1 Macrosegregation in the Fusion Zone 26311.2 Quick Freezing of One Liquid Metal in Another 26511.3 Macrosegregation in Dissimilar-Filler Welding 26511.3.1 Bulk Weld-Metal Composition 26511.3.2 Mechanism I 26711.3.3 Mechanism II 27011.4 Macrosegregation in Dissimilar-Metal Welding 27911.4.1 Mechanism I 27911.4.2 Mechanism II 28311.5 Reduction of Macrosegregation 28611.6 Macrosegregation in Multiple-Pass Welds 287References 290Further Reading 291Problems 29112 Some Alloy-Specific Microstructures and Properties 29312.1 Austenitic Stainless Steels 29312.1.1 Microstructure Evolution in Stainless Steels 29312.1.2 Mechanisms of Ferrite Formation 29412.1.3 Prediction of Ferrite Content 29612.1.4 Effect of Cooling Rate 29712.1.4.1 Changes in Solidification Mode 29712.1.4.2 Dendrite Tip Undercooling 30112.2 Low-Carbon, Low-Alloy Steels 30112.2.1 Microstructure Development 30112.2.2 Factors Affecting Microstructure 30212.2.3 Weld Metal Toughness 30612.3 Ultralow Carbon Bainitic Steels 30612.4 Creep-Resistant Steels 30812.5 Hardfacing of Steels 311References 319Further Reading 321Problems 32113 Solidification Cracking 32313.1 Characteristics of Solidification Cracking 32313.2 Theories of Solidification Cracking 32313.2.1 Criterion for Cracking Proposed by Kou 32713.2.2 Index for Crack Susceptibility Proposed by Kou 32813.2.3 Previous Theories 33013.3 Binary Alloys and Analytical Equations 33113.4 Solidification Cracking Tests 33413.4.1 Varestraint Test 33413.4.2 Controlled Tensile Weldability Test 33613.4.3 Transverse-Motion Weldability Test 33713.4.4 Circular-Patch Test 34113.4.5 Houldcroft Test 34213.4.6 Cast-Pin Test 34313.4.7 Ring-Casting Test 34413.4.8 Other Tests 34413.5 Solidification Cracking of Stainless Steels 34513.5.1 Primary Solidification Phase 34513.5.2 Mechanism of Crack Resistance 34613.6 Factors Affecting Solidification Cracking 35013.6.1 Primary Solidification Phase 35013.6.2 Grain Size 35013.6.3 Solidification Temperature Range 35113.6.4 Back Diffusion 35413.6.5 Dihedral Angle 35513.6.6 Grain-Boundary Angle 35913.6.7 Degree of Restraint 36013.7 Reducing Solidification Cracking 36013.7.1 Control of Weld Metal Composition 36013.7.2 Control of Weld Microstructure 36313.7.3 Control of Welding Conditions 36513.7.4 Control of Weld Shape 366Examples 367References 370Further Reading 376Problems 37614 Ductility-Dip Cracking 37914.1 Characteristics of Ductility-Dip Cracking 37914.2 Theories of Ductility-Dip Cracking 38114.3 Test Methods 38214.4 Ductility-Dip Cracking of Ni-Base Alloys 38414.4.1 Grain-Boundary Sliding 38414.4.2 Grain-Boundary Misorientation 38614.4.3 Grain-Boundary Tortuosity and Precipitates 38614.4.4 Grain Size 38814.4.5 Factors Affecting Ductility-Dip Cracking 39014.5 Ductility-Dip Cracking of Stainless Steels 390Examples 392References 394Further Reading 396Problems 396Part III The Partially Melted Zone 39915 Liquation in the Partially Melted Zone 40115.1 Formation of the Partially Melted Zone 40115.2 Liquation Mechanisms 40315.2.1 Mechanism I: Alloy with Co > CSM 40415.2.2 Mechanism II: Alloy with Co < CSM and no AxBy or Eutectic 40515.2.3 Mechanism III: Alloy with Co < CSM and AxBy or Eutectic 40515.2.4 Additional Mechanisms of Liquation 40915.3 Directional Solidification of Liquated Material 41115.4 Grain-Boundary Segregation 41115.5 Loss of Strength and Ductility 41315.6 Hydrogen Cracking 41415.7 Effect of Heat Input 41415.8 Effect of Arc Oscillation 415Examples 416References 417Problems 41816 Liquation Cracking 41916.1 Liquation Cracking in Arc Welding 41916.1.1 Crack Susceptibility Tests 42116.1.1.1 Varestraint Testing 42116.1.1.2 Circular-Patch Testing 42216.1.1.3 Hot Ductility Testing 42316.1.2 Mechanism of Liquation Cracking 42316.1.3 Predicting Effect of Filler Metal on Crack Susceptibility 42416.1.4 Factors Affecting Liquation Cracking 43016.1.4.1 Filler Metal 43016.1.4.2 Heat Source 43016.1.4.3 Degree of Restraint 43116.1.4.4 Base Metal 43116.2 Liquation Cracking in Resistance Spot Welding 43416.3 Liquation Cracking in Friction Stir Welding 43416.4 Liquation Cracking in Dissimilar-Metal FSW 439Examples 445References 446Problems 449Part IV The Heat-Affected Zone 45117 Introduction to Solid-State Transformations 45317.1 Work-Hardened Materials 45317.2 Heat-Treatable Al Alloys 45517.3 Heat-Treatable Ni-Base Alloys 45817.4 Steels 46117.4.1 Fe-C Phase Diagram and CCT Diagrams 46117.4.2 Carbon Steels 46317.4.3 Dual-Phase Steels 47017.5 Stainless Steels 47117.5.1 Types of Stainless Steels 47117.5.2 Sensitization of Unstabilized Grades 47317.5.3 Sensitization of Stabilized Grades 47317.5.4 σ-Phase Embrittlement 475Examples 475References 475Problems 47718 Heat-Affected-Zone Degradation of Mechanical Properties 47918.1 Grain Coarsening 47918.2 Recrystallization and Grain Growth 48018.3 Overaging in Al Alloys 48318.3.1 Al-Cu-Mg (2000-Series) Alloys 48318.3.1.1 Microstructure and Strength 48318.3.1.2 Effect of Welding Parameters or Process 48818.3.2 Al-Mg-Si (6000-Series) Alloys 48918.3.2.1 Microstructure and Strength 48918.3.2.2 Effect of Welding Processes and Parameters 49118.3.3 Al-Zn-Mg (7000-Series) Alloys 49218.4 Dissolution of Precipitates in Ni-Base Alloys 49418.5 Martensite Tempering in Dual-Phase Steels 498Examples 500References 500Further Reading 502Problems 50219 Heat-Affected-Zone Cracking 50519.1 Hydrogen Cracking in Steels 50519.1.1 Cause 50519.1.2 Appearance 50619.1.3 Susceptibility Tests 50719.1.4 Remedies 50819.1.4.1 Preheating 50819.1.4.2 Postweld Heating 50919.1.4.3 Bead Tempering 50919.1.4.4 Use of Low-H Processes and Consumables 50919.1.4.5 Use of Lower-Strength Filler Metals 50919.1.4.6 Use of Austenitic-Stainless-Steel Filler Metals 51019.2 Stress-Relief Cracking in Steels 51019.3 Lamellar Tearing in Steels 51419.4 Type-IV Cracking in Grade 91 Steel 51719.5 Strain-Age Cracking in Ni-Base Alloys 519Examples 524References 524Further Reading 527Problems 52820 Heat-Affected-Zone Corrosion 52920.1 Weld Decay of Stainless Steels 52920.2 Weld Decay of Ni-Base Alloys 53320.3 Knife-Line Attack of Stainless Steels 53420.4 Sensitization of Ferritic Stainless-Steel Welds 53620.5 Stress Corrosion Cracking of Austenitic Stainless Steels 53720.6 Corrosion Fatigue of Al Welds 538Examples 538References 539Further Reading 540Problems 540Part V Special Topics 54121 Additive Manufacturing 54321.1 Heat and Fluid Flow 54321.2 Residual Stress and Distortion 54521.3 Lack of Fusion and Gas Porosity 54721.4 Grain Structure 55021.5 Solidification Cracking 55021.6 Liquation Cracking 55321.7 Graded Transition Joints 55821.8 Further Discussions 560Examples 560References 561Further Reading 563Problems 56422 Dissimilar-Metal Joining 56522.1 Introduction 56522.2 Arc and Laser Joining 56522.2.1 Al-to-Steel Arc Brazing 56622.2.1.1 Effect of Lap Joint Gap 56922.2.1.2 Effect of Heat Input 57522.2.1.3 Effect of Ultrasonic Vibration 57722.2.1.4 Effect of Preheating 57822.2.1.5 Effect of Postweld Heat Treatment 57822.2.1.6 Butt Joint 57922.2.2 Al-to-Steel Laser Brazing 57922.2.3 Al-to-Steel Laser Welding 58022.2.4 Mg-to-Steel Brazing 58222.2.5 Al-to-Mg Welding 58322.3 Resistance Spot Welding 58322.3.1 Al-to-Steel RSW 58322.3.2 Mg-to-Steel RSW 58622.3.3 Al-to-Mg RSW 58822.4 Friction Stir Welding 58922.4.1 Al-to-Cu FSSW 58922.4.2 FSSW of Al to Galvanized Steel 59222.4.3 Effect of Coating on Al-to-Steel FSSW 59722.5 Other Solid-State Welding Processes 60322.5.1 Friction Welding 60322.5.2 Explosion Welding 60622.5.3 Magnetic Pulse Welding 607Examples 608References 609Further Reading 612Problems 61223 Welding of Magnesium Alloys 61323.1 Spatter 61323.1.1 Spatter in Mg GMAW 61323.1.2 Mechanism of Spatter 61423.1.3 Elimination of Spatter 61423.1.4 Irregular Weld Shape and Its Elimination 61723.2 Porosity 61823.2.1 Porosity in Mg GMAW 61823.2.2 Mechanisms of Porosity Formation and Elimination 62023.2.3 Comparing Porosity in Al and Mg Welds 62123.3 Internal Oxide Films 62223.3.1 Mechanism 62223.3.2 Remedies 62423.4 High Crowns 62523.4.1 Mechanism of High-Crown Formation 62523.4.2 Reducing Crown Height 62723.5 Grain Refining 62823.5.1 Ultrasonic Weld Pool Stirring 62823.5.2 Arc Pulsation 62923.5.3 Arc Oscillation 62923.6 Solidification Cracking 62923.7 Liquation Cracking 62923.7.1 A Simple Test for Crack Susceptibility 63123.7.2 Effect of Filler Metals 63423.7.3 Effect of Grain Size 63623.8 Heat-Affected Zone Weakening 636Examples 638References 640Further Reading 641Problems 64124 Welding of High-Entropy Alloys and Metal-Matrix Nanocomposites 64324.1 High-Entropy Alloys 64324.1.1 Solidification Microstructure 64324.1.2 Weldability 64424.2 Metal-Matrix Nanocomposites 64624.2.1 Nanoparticles Increasing Weld Size 64624.2.2 Nanoparticles Refining Microstructure 64824.2.3 Nanoparticles Reducing Cracking During Solidification 65024.2.4 Nanoparticles Allowing Friction Stir Welding 651Examples 653References 654Further Reading 655Problems 655Appendix A: Analytical Equations for Susceptibility to Solidification Cracking 657Index 659