Welding Metallurgy

Welding Metallurgy

Third Edition

Sindo KouDepartment of Materials Science and EngineeringUniversity of WisconsinMadison, Wisconsin

 

This edition first published 2021© 2021 John Wiley & Sons, Inc.

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Library of Congress Cataloging‐in‐Publication DataNames: Kou, Sindo, author. Title: Welding metallurgy / by Sindo Kou, Professor Department of Materials

Science and Engineering University of Wisconsin, Madison, Wisconsin. Description: Third edition. | Hoboken, NJ : John Wiley & Sons, Inc., 2021.

| Includes bibliographical references and index. Identifiers: LCCN 2020004090 (print) | LCCN 2020004091 (ebook) | ISBN

9781119524816 (hardback) | ISBN 9781119524854 (adobe pdf) | ISBN 9781119524915 (epub)

Subjects: LCSH: Welding. | Metallurgy. | Alloys. Classification: LCC TS227 .K649 2021 (print) | LCC TS227 (ebook) | DDC

671.5/2–dc23 LC record available at https://lccn.loc.gov/2020004090LC ebook record available at https://lccn.loc.gov/2020004091

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To Tina, Nancy, and Katharine

vii

Preface to Third Edition xxi

Part I Introduction 1

1 Welding Processes 31.1 Overview 31.1.1 FusionWeldingProcesses 31.1.1.1 PowerDensityof HeatSource 41.1.1.2 WeldingProcessesand Materials 51.1.1.3 Typesof Jointsand WeldingPositions 71.1.2 Solid-StateWeldingProcesses 81.2 GasWelding 81.2.1 TheProcess 81.2.2 ThreeTypesof Flames 91.2.2.1 NeutralFlame 91.2.2.2 ReducingFlame 91.2.2.3 OxidizingFlame 91.2.3 Advantagesand Disadvantages 101.3 ArcWelding 101.3.1 ShieldedMetalArcWelding 101.3.1.1 Functionsof ElectrodeCovering 101.3.1.2 Advantagesand Disadvantages 111.3.2 Gas–TungstenArcWelding 111.3.2.1 TheProcess 111.3.2.2 Polarity 121.3.2.3 Electrodes 131.3.2.4 ShieldingGases 131.3.2.5 Advantagesand Disadvantages 131.3.3 PlasmaArcWelding 141.3.3.1 TheProcess 141.3.3.2 ArcInitiation 141.3.3.3 Keyholing 151.3.3.4 Advantagesand Disadvantages 15

Contents

Contentsviii

1.3.4 Gas–MetalArcWelding 161.3.4.1 TheProcess 161.3.4.2 ShieldingGases 161.3.4.3 Modesof MetalTransfer 171.3.4.4 Advantagesand Disadvantages 181.3.5 Flux-CoredArcWelding 181.3.5.1 TheProcess 181.3.6 SubmergedArcWelding 191.3.6.1 TheProcess 191.3.6.2 Advantagesand Disadvantages 201.3.7 ElectroslagWelding 201.3.7.1 TheProcess 201.3.7.2 Advantagesand Disadvantages 211.4 High-Energy-BeamWelding 211.4.1 ElectronBeamWelding 211.4.1.1 TheProcess 211.4.1.2 Advantagesand Disadvantages 231.4.2 LaserBeamWelding 241.4.2.1 TheProcess 241.4.2.2 Reflectivity 241.4.2.3 ShieldingGas 251.4.2.4 Laser-AssistedArcWelding 251.4.2.5 Advantagesand Disadvantages 261.5 ResistanceSpotWelding 261.6 Solid-StateWelding 271.6.1 FrictionStirWelding 271.6.2 FrictionWelding 291.6.3 Explosionand Magnetic-PulseWelding 311.6.4 DiffusionWelding 31 Examples 32 References 33 FurtherReading 34 Problems 35

2 HeatFlowin Welding 372.1 HeatSource 372.1.1 HeatSourceEfficiency 372.1.1.1 Definition 372.1.1.2 Measurements 382.1.1.3 HeatSourceEfficienciesin VariousWeldingProcesses 412.1.2 MeltingEfficiency 422.1.3 PowerDensityDistributionof HeatSource 432.1.3.1 Effectof ElectrodeTipAngle 432.1.3.2 Measurements 432.2 HeatFlowDuringWelding 452.2.1 Responseof Materialto WeldingHeatSource 45

Contents ix

2.2.2 Rosenthal’sEquations 452.2.2.1 Rosenthal’sTwo-DimensionalEquation 462.2.2.2 Rosenthal’sThree-DimensionalEquation 472.2.2.3 Step-by-StepApplicationof Rosenthal’sEquations 482.2.3 Adams’Equations 492.3 Effectof WeldingConditions 492.4 ComputerSimulation 522.5 WeldThermalSimulator 532.5.1 TheEquipment 532.5.2 Applications 542.5.3 Limitations 54 Examples 54 References 57 FurtherReading 59 Problems 59

3 FluidFlowin Welding 613.1 FluidFlowin Arcs 613.1.1 SharpElectrode 613.1.2 Flat-EndElectrode 633.2 Effectof MetalVaporon Arcs 633.2.1 Gas−TungstenArcWelding 633.2.2 Gas−MetalArcWelding 653.3 ArcPower-and Current-DensityDistributions 683.4 FluidFlowin WeldPools 693.4.1 DrivingForcesfor FluidFlow 693.4.2 Heiple’sTheoryfor WeldPoolConvection 713.4.3 PhysicalSimulationof FluidFlowand WeldPenetration 723.4.4 ComputerSimulationof FluidFlowand WeldPenetration 753.5 FlowOscillationand RippleFormation 773.6 ActiveFluxGTAW 803.7 ResistanceSpotWelding 81 Examples 84 References 85 FurtherReading 88 Problems 88

4 Massand Filler–Metal Transfer 914.1 ConvectiveMassTransferin WeldPools 914.2 Evaporationof VolatileSolutes 944.3 Filler-MetalDropExplosionand Spatter 964.4 Spatterin GMAWof Magnesium 1004.5 DiffusionBonding 100 Examples 103 References 104 Problems 105

Contentsx

5 ChemicalReactionsin Welding 1075.1 Overview 1075.1.1 Effectof Nitrogen,Oxygen,and Hydrogen 1075.1.2 ProtectionAgainstAir 1075.1.3 Evaluationof WeldMetalProperties 1085.2 Gas–MetalReactions 1115.2.1 Thermodynamicsof 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–MetalReactions 1255.3.1 ThermochemicalReactions 1255.3.1.1 Decompositionof Flux 1255.3.1.2 Removalof Sand Pfrom LiquidSteel 1265.3.2 Effectof Fluxon WeldMetalOxygen 1275.3.3 Typesof Fluxes,BasicityIndex,and WeldMetalProperties 1275.3.4 BasicityIndex 1285.3.5 ElectrochemicalReactions 130 Examples 135 References 136 FurtherReading 140 Problems 140

6 ResidualStresses,Distortion,and Fatigue 1416.1 ResidualStresses 1416.1.1 Developmentof ResidualStresses 1416.1.1.1 StressesInducedByWelding 1416.1.1.2 Welding 1416.1.2 Analysisof ResidualStresses 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–NCurves 150

Contents xi

6.3.4 Effectof JointGeometry 1506.3.5 Effectof StressRaisers 1516.3.6 Effectof Corrosion 1526.3.7 Remedies 1526.3.7.1 ShotPeening 1526.3.7.2 ReducingStressRaisers 1536.3.7.3 LaserShockPeening 1546.3.7.4 Useof Low–Transformation–TemperatureFillers 154 Examples 154 References 155 FurtherReading 156 Problems 156

PartII The FusionZone 157

7 Introductionto Solidification 1597.1 SoluteRedistributionDuringSolidification 1597.1.1 DirectionalSolidification 1597.1.2 EquilibriumSegregationCoefficient k 1597.1.3 FourCasesof SoluteRedistribution 1617.2 ConstitutionalSupercooling 1667.3 SolidificationModes 1687.4 MicrosegregationCausedbySoluteRedistribution 1717.5 SecondaryDendriteArmSpacing 1747.6 Effectof DendriteTipUndercooling 1777.7 Effectof GrowthRate 1787.8 Solidificationof TernaryAlloys 1787.8.1 LiquidusProjection 1787.8.2 SolidificationPath 1797.8.3 TernaryMagnesiumAlloys 1807.8.4 TernaryFe-Cr-NiAlloys 1827.8.4.1 Fe-Cr-NiPhaseDiagram 1827.8.4.2 SolidificationPaths 1857.8.4.3 Microstructure 186 Examples 189 References 191 FurtherReading 193 Problems 193

8 Solidification Modes 1958.1 SolidificationModes 1958.1.1 TemperatureGradientand GrowthRate 1958.1.2 Variationsin GrowthModeAcrossWeld 1978.2 DendriteSpacingand CellSpacing 2008.3 Effectof WeldingParameters 2018.3.1 SolidificationMode 201

Contentsxii

8.3.2 Dendriteand CellSpacing 2028.4 RefiningMicrostructureWithinGrains 2038.4.1 ArcOscillation 2038.4.2 ArcPulsation 205 Examples 205 References 206 FurtherReading 207 Problems 207

9 Nucleationand Growthof Grains 2099.1 EpitaxialGrowthatthe FusionLine 2099.2 NonepitaxialGrowthatthe FusionLine 2129.2.1 MismatchingCrystalStructures 2129.2.2 NondendriticEquiaxedGrains 2139.3 Growthof ColumnarGrains 2149.4 Effectof WeldingParameterson ColumnarGrains 2159.5 Controlof ColumnarGrains 2189.6 NucleationMechanismsof EquiaxedGrains 2199.6.1 MicrostructureAroundPoolBoundary 2199.6.2 DendriteFragmentation 2209.6.3 GrainDetachment 2229.6.4 HeterogeneousNucleation 2229.6.5 Effectof WeldingParameterson HeterogeneousNucleation 2259.6.6 SurfaceNucleation 2289.7 GrainRefining 2289.7.1 Inoculation 2289.7.2 WeldPoolStirring 2299.7.2.1 MagneticWeldPoolStirring 2299.7.2.2 UltrasonicWeldPoolStirring 2299.7.3 ArcPulsation 2329.7.4 ArcOscillation 2329.8 IdentifyingGrain-RefiningMechanisms 2339.8.1 OverlapWeldingProcedure 2339.8.2 Identifyingthe Grain-RefiningMechanism 2359.8.3 Effectof ArcOscillationon DendriteFragmentation 2369.8.4 Effectof ArcOscillationon ConstitutionalSupercooling 2369.8.5 Effectof Compositionon GrainRefiningbyArcOscillation 2389.9 Grain-BoundaryMigration 238 Examples 239 References 240 FurtherReading 245 Problems 246

10 Microsegregation 24710.1 MicrosegregationinWelds 24710.2 Effectof TravelSpeedon Microsegregation 24910.3 Effectof PrimarySolidificationPhaseon Microsegregation 252

Contents xiii

10.4 Effectof MaximumSolidSolubilityon Microsegregation 253 Examples 259 References 261 FurtherReading 262 Problems 262

11 Macrosegregation 26311.1 Macrosegregationin the FusionZone 26311.2 QuickFreezingof OneLiquidMetalin Another 26511.3 Macrosegregationin Dissimilar-FillerWelding 26511.3.1 BulkWeld-MetalComposition 26511.3.2 MechanismI 26711.3.3 MechanismII 27011.4 Macrosegregationin Dissimilar-MetalWelding 27911.4.1 MechanismI 27911.4.2 MechanismII 28311.5 Reductionof Macrosegregation 28611.6 Macrosegregationin Multiple-PassWelds 287 References 290 FurtherReading 291 Problems 291

12 SomeAlloy-SpecificMicrostructuresand Properties 29312.1 AusteniticStainlessSteels 29312.1.1 MicrostructureEvolutionin StainlessSteels 29312.1.2 Mechanismsof FerriteFormation 29412.1.3 Predictionof FerriteContent 29612.1.4 Effectof CoolingRate 29712.1.4.1 Changesin SolidificationMode 29712.1.4.2 DendriteTipUndercooling 30112.2 Low-Carbon,Low-AlloySteels 30112.2.1 MicrostructureDevelopment 30112.2.2 FactorsAffectingMicrostructure 30212.2.3 WeldMetalToughness 30612.3 UltralowCarbonBainiticSteels 30612.4 Creep-ResistantSteels 30812.5 HardfacingofSteels 311 References 319 FurtherReading 321 Problems 321

13 Solidification Cracking 32313.1 Characteristicsof SolidificationCracking 32313.2 Theoriesof SolidificationCracking 32313.2.1 Criterionfor CrackingProposedbyKou 32713.2.2 Indexfor CrackSusceptibilityProposedbyKou 32813.2.3 PreviousTheories 330

Contentsxiv

13.3 BinaryAlloysand AnalyticalEquations 33113.4 SolidificationCrackingTests 33413.4.1 VarestraintTest 33413.4.2 ControlledTensileWeldabilityTest 33613.4.3 Transverse-MotionWeldabilityTest 33713.4.4 Circular-PatchTest 34113.4.5 HouldcroftTest 34213.4.6 Cast-PinTest 34313.4.7 Ring-CastingTest 34413.4.8 OtherTests 34413.5 SolidificationCrackingof StainlessSteels 34513.5.1 PrimarySolidificationPhase 34513.5.2 Mechanismof CrackResistance 34613.6 FactorsAffectingSolidificationCracking 35013.6.1 PrimarySolidificationPhase 35013.6.2 GrainSize 35013.6.3 SolidificationTemperatureRange 35113.6.4 BackDiffusion 35413.6.5 DihedralAngle 35513.6.6 Grain-BoundaryAngle 35913.6.7 Degreeof Restraint 36013.7 ReducingSolidificationCracking 36013.7.1 Controlof WeldMetalComposition 36013.7.2 Controlof WeldMicrostructure 36313.7.3 Controlof WeldingConditions 36513.7.4 Controlof WeldShape 366 Examples 367 References 370 FurtherReading 376 Problems 376

14 Ductility-Dip Cracking 37914.1 Characteristicsof Ductility-DipCracking 37914.2 Theoriesof Ductility-DipCracking 38114.3 TestMethods 38214.4 Ductility-DipCrackingof Ni-BaseAlloys 38414.4.1 Grain-BoundarySliding 38414.4.2 Grain-BoundaryMisorientation 38614.4.3 Grain-BoundaryTortuosityand Precipitates 38614.4.4 GrainSize 38814.4.5 FactorsAffectingDuctility-DipCracking 39014.5 Ductility-DipCrackingof StainlessSteels 390 Examples 392 References 394 FurtherReading 396 Problems 396

Contents xv

PartIII The PartiallyMeltedZone 399

15 Liquationin the PartiallyMeltedZone 40115.1 Formationof the PartiallyMeltedZone 40115.2 LiquationMechanisms 40315.2.1 MechanismI:Alloywith Co>CSM 40415.2.2 MechanismII:Alloywith Co<CSMand noAxByorEutectic 40515.2.3 MechanismIII:Alloywith Co<CSMand AxByorEutectic 40515.2.4 AdditionalMechanismsof Liquation 40915.3 DirectionalSolidificationof LiquatedMaterial 41115.4 Grain-BoundarySegregation 41115.5 Lossof Strengthand Ductility 41315.6 HydrogenCracking 41415.7 Effectof HeatInput 41415.8 Effectof ArcOscillation 415 Examples 416 References 417 Problems 418

16 Liquation Cracking 41916.1 LiquationCrackingin ArcWelding 41916.1.1 CrackSusceptibilityTests 42116.1.1.1 VarestraintTesting 42116.1.1.2 Circular-PatchTesting 42216.1.1.3 HotDuctilityTesting 42316.1.2 Mechanismof LiquationCracking 42316.1.3 PredictingEffectof FillerMetalon CrackSusceptibility 42416.1.4 FactorsAffectingLiquationCracking 43016.1.4.1 FillerMetal 43016.1.4.2 HeatSource 43016.1.4.3 Degreeof Restraint 43116.1.4.4 BaseMetal 43116.2 LiquationCrackingin ResistanceSpotWelding 43416.3 LiquationCrackingin FrictionStirWelding 43416.4 LiquationCrackingin Dissimilar-MetalFSW 439 Examples 445 References 446 Problems 449

PartIV The Heat-AffectedZone 451

17 Introductionto Solid-StateTransformations 45317.1 Work-HardenedMaterials 45317.2 Heat-TreatableAlAlloys 45517.3 Heat-TreatableNi-BaseAlloys 458

Contentsxvi

17.4 Steels 46117.4.1 Fe-CPhaseDiagramand CCTDiagrams 46117.4.2 CarbonSteels 46317.4.3 Dual-PhaseSteels 47017.5 StainlessSteels 47117.5.1 Typesof StainlessSteels 47117.5.2 Sensitizationof UnstabilizedGrades 47317.5.3 Sensitizationof StabilizedGrades 47317.5.4 σ-PhaseEmbrittlement 475 Examples 475 References 475 Problems 477

18 Heat-Affected-ZoneDegradationof MechanicalProperties 47918.1 GrainCoarsening 47918.2 Recrystallizationand GrainGrowth 48018.3 Overagingin AlAlloys 48318.3.1 Al-Cu-Mg(2000-Series)Alloys 48318.3.1.1 Microstructureand Strength 48318.3.1.2 Effectof WeldingParametersorProcess 48818.3.2 Al-Mg-Si(6000-Series)Alloys 48918.3.2.1 Microstructureand Strength 48918.3.2.2 Effectof WeldingProcessesand Parameters 49118.3.3 Al-Zn-Mg(7000-Series)Alloys 49218.4 Dissolutionof Precipitatesin Ni-BaseAlloys 49418.5 MartensiteTemperingin Dual-PhaseSteels 498 Examples 500 References 500 FurtherReading 502 Problems 502

19 Heat-Affected-ZoneCracking 50519.1 HydrogenCrackingin Steels 50519.1.1 Cause 50519.1.2 Appearance 50619.1.3 SusceptibilityTests 50719.1.4 Remedies 50819.1.4.1 Preheating 50819.1.4.2 PostweldHeating 50919.1.4.3 BeadTempering 50919.1.4.4 Useof Low-HProcessesand Consumables 50919.1.4.5 Useof Lower-StrengthFillerMetals 50919.1.4.6 Useof Austenitic-Stainless-SteelFillerMetals 51019.2 Stress-ReliefCrackingin Steels 51019.3 LamellarTearingin Steels 514

Contents xvii

19.4 Type-IVCrackingin Grade91Steel 51719.5 Strain-AgeCrackingin Ni-BaseAlloys 519 Examples 524 References 524 FurtherReading 527 Problems 528

20 Heat-Affected-ZoneCorrosion 52920.1 WeldDecayof StainlessSteels 52920.2 WeldDecayof Ni-BaseAlloys 53320.3 Knife-LineAttackof StainlessSteels 53420.4 Sensitizationof FerriticStainless-SteelWelds 53620.5 StressCorrosionCrackingof AusteniticStainlessSteels 53720.6 CorrosionFatigueofAlWelds 538 Examples 538 References 539 FurtherReading 540 Problems 540

Part V Special Topics 541

21 Additive Manufacturing 54321.1 Heatand FluidFlow 54321.2 ResidualStressand Distortion 54521.3 Lackof Fusionand GasPorosity 54721.4 GrainStructure 55021.5 SolidificationCracking 55021.6 LiquationCracking 55321.7 GradedTransitionJoints 55821.8 FurtherDiscussions 560 Examples 560 References 561 FurtherReading 563 Problems 564

22 Dissimilar-Metal Joining 56522.1 Introduction 56522.2 Arcand LaserJoining 56522.2.1 Al-to-SteelArcBrazing 56622.2.1.1 Effectof LapJointGap 56922.2.1.2 Effectof HeatInput 57522.2.1.3 Effectof UltrasonicVibration 57722.2.1.4 Effectof Preheating 57822.2.1.5 Effectof PostweldHeatTreatment 57822.2.1.6 ButtJoint 579

Contentsxviii

22.2.2 Al-to-SteelLaserBrazing 57922.2.3 Al-to-SteelLaserWelding 58022.2.4 Mg-to-SteelBrazing 58222.2.5 Al-to-MgWelding 58322.3 ResistanceSpotWelding 58322.3.1 Al-to-SteelRSW 58322.3.2 Mg-to-SteelRSW 58622.3.3 Al-to-MgRSW 58822.4 FrictionStirWelding 58922.4.1 Al-to-CuFSSW 58922.4.2 FSSWof Alto GalvanizedSteel 59222.4.3 Effectof Coatingon Al-to-SteelFSSW 59722.5 OtherSolid-StateWeldingProcesses 60322.5.1 FrictionWelding 60322.5.2 ExplosionWelding 60622.5.3 MagneticPulseWelding 607 Examples 608 References 609 FurtherReading 612 Problems 612

23 Weldingof MagnesiumAlloys 61323.1 Spatter 61323.1.1 Spatterin MgGMAW 61323.1.2 Mechanismof Spatter 61423.1.3 Eliminationof Spatter 61423.1.4 IrregularWeldShapeand ItsElimination 61723.2 Porosity 61823.2.1 Porosityin MgGMAW 61823.2.2 Mechanismsof PorosityFormationand Elimination 62023.2.3 ComparingPorosityin Aland MgWelds 62123.3 InternalOxideFilms 62223.3.1 Mechanism 62223.3.2 Remedies 62423.4 HighCrowns 62523.4.1 Mechanismof High-CrownFormation 62523.4.2 ReducingCrownHeight 62723.5 GrainRefining 62823.5.1 UltrasonicWeldPoolStirring 62823.5.2 ArcPulsation 62923.5.3 ArcOscillation 62923.6 SolidificationCracking 62923.7 LiquationCracking 62923.7.1 ASimpleTestfor CrackSusceptibility 63123.7.2 Effectof FillerMetals 63423.7.3 Effectof GrainSize 636

Contents xix

23.8 Heat-AffectedZoneWeakening 636 Examples 638 References 640 FurtherReading 641 Problems 641

24 Weldingof High-EntropyAlloysand Metal-MatrixNanocomposites 64324.1 High-EntropyAlloys 64324.1.1 SolidificationMicrostructure 64324.1.2 Weldability 64424.2 Metal-MatrixNanocomposites 64624.2.1 NanoparticlesIncreasingWeldSize 64624.2.2 NanoparticlesRefiningMicrostructure 64824.2.3 NanoparticlesReducingCrackingDuringSolidification 65024.2.4 NanoparticlesAllowingFrictionStirWelding 651 Examples 653 References 654 FurtherReading 655 Problems 655

AppendixA:AnalyticalEquationsforSusceptibilitytoSolidificationCracking 657 Index 659

xxi

The 3rd edition of Welding Metallurgy includes updates and expands the 2nd edition that was published in 2003. About half of the 3rd edition has new content. It includes the significant new progress made in welding metal-lurgy since 2003. To help readers understand the subjects discussed, examples are provided in each chapter. To make it easier for readers to find cited articles or judge their relevance, the titles of the articles and the full names of the journals are provided.

In Part I, Introduction, Chapter 1 has been expanded, which also includes topics on resistance spot welding and solid-state welding (friction welding, friction stir welding, explosion welding, magnetic impulse welding, and dif-fusion welding). Chapter 3 includes the significant effect of metal vapor in the arc on weld penetration. It also includes the new progress made at UW-Madison on oscillatory Marangoni flow in the weld pool, weld-pool-sur-face deformation and oscillation, weld ripple formation, and how they are affected by the surface-active agent. Chapter 4 includes two new mechanisms proposed at UW-Madison for spatter in gas–metal arc welding of Al and Mg alloys.

Part II, The Fusion Zone, has been restructured and expanded to include four new chapters: Chapters 10, 11, 12, and 14. Ternary alloy solidification, which is often difficult for students to understand, has been explained with the liquidus projections and solidification paths of example alloys. Part II discusses more on the following new progress made at UW-Madison. A liquid-metal quenching technique to “freeze-in” and reveal the elevated temperature microstructure during welding is demonstrated, which is useful for understanding phase transformations, nuclea-tion mechanisms, microsegregation, diffusion, etc. Bending of growing columnar dendrites without breaking is shown to support thermal instead of mechanical, dendrite fragmentation. Identification of the grain refining mech-anism is demonstrated. A statistically significant measurement of microsegregation used in casting is applied to welding. Base-metal-like “beaches,” “peninsulas,” and “islands” surrounded by the weld metal, often found in dissimilar filler welding and dissimilar metal welding, is explained. A simple index is proposed to predict the solidi-fication cracking susceptibility of Al and Mg alloys and how effectively filler metals can reduce the susceptibility. A simple but improved new test for evaluating the solidification cracking susceptibility of various alloys and the filler metal effectiveness is demonstrated. New theories on the resistance of austenitic stainless steels to solidifica-tion cracking and ductility-dip cracking are presented.

Part III, The Partially Melted Zone, also discusses more on the new progress made at UW-Madison, including liquation (liquid formation) and liquation-induced cracking. A simple criterion is proposed to predict how filler metals can be selected in arc welding to eliminate liquation cracking. Evidence of liquation and liquation cracking in friction stir welding (FSW) is presented. In Al-to-Mg butt and lap FSW, the interesting and significant effect of the position of Al relative to Mg on the joint strength is explained. Interestingly, liquid droplets have been shown even though FSW is considered as solid-state welding.

Part IV, The Heat-Affected Zone (HAZ), has been reorganized into the following new chapters: Chapters 17, 18, 19, and 20.

Preface to Third Edition

Preface to Third Editionxxii

Part V, Special Topics, is a new chapter. It has been added to introduce some topics of high current interest, such as Chapters 21–24. Chapters 23 and 24, and several sections in Chapter 22 discuss heavily on the new progress at UW-Madison.

The author thanks God for giving him the opportunity and ability to do what he has done in welding metallurgy. He applied his training in transport phenomena (BS in Chemical Engineering at the National Taiwan University) and solidification (PhD in Metallurgy at the Massachusetts Institute of Technology under Professor Merton C. Flemings) to welding. His work was supported by funding from the National Science Foundation (NSF), NASA, the University of Wisconsin Foundation, the AWS Foundation, Hobart Brothers, General Motors, ALCOA, Howmet, CompuTherm, and other companies, which are greatly appreciated. The NSF support includes solidifi-cation cracking and ductility-dip cracking in stainless steel welds under grant number DMR1904503, solidifica-tion cracking of Al alloys under grant number DMR1500367, and many other topics throughout the book (under several NSF projects since 1980).

The author is grateful to CompuTherm, founded by Professor Y. Austin Chang of UW-Madison. Without their thermodynamic software, databases, and technical support, it would have been much more difficult for the author to develop his theories to predict liquation cracking, solidification cracking, and macrosegregation. He thanks the American Welding Society (AWS) for granting permission to use many figures and tables published in the Welding Journal and the Welding Handbook. Getting permissions is now costly for the author of a technical book. To keep the cost down, tens of figures from the work of many other researchers initially included in the manuscript had to be sadly removed.

The author acknowledges the numerous contributions to this book from his former students and associates at UW-Madison. The hard work of some of them has been recognized with the following technical paper awards, which they shared with the author: the Warren F. Savage Memorial Award (2012, 2009, 2008, and 2006), Charles H. Jennings Memorial Award (2014, 2010, 2002, and 2001), William Spraragen Award (2019, 2016, and 2007), A.F. Davis Silver Medal Award (2017), and James F. Lincoln Gold Medal (2016) of the American Welding Society, and the Magnesium Technology Best Paper Award (2017) of The Minerals, Metals & Materials Society (TMS).

Sindo KouMadison, Wisconsin, 2020

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Part I

Introduction

Welding Metallurgy, Third Edition. Sindo Kou.© 2021 John Wiley & Sons, Inc. Published 2021 by John Wiley & Sons, Inc.

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This chapter is intended to be a brief introduction to most fusion welding processes and some solid-state welding processes. The former includes gas welding, arc welding, laser-beam welding, electron-beam welding, and resist-ance spot welding (RSW). The latter includes friction stir welding (FSW), friction welding, explosion welding (EXW), magnetic pulse welding (MPW), and diffusion welding. The advantages and disadvantages of these processes are discussed.

1.1 Overview

1.1.1 Fusion Welding Processes

Fusion welding is a joining process that uses fusion of the base metal to make the weld. It is the most widely used joining process. Four major types of fusion welding processes will be discussed: gas welding, arc welding, high-energy beam welding, and resistance spot welding. These processes are listed as follows:

(a) Gas welding:Oxyacetylene welding (OAW)

(b) Arc welding:Shielded metal arc welding (SMAW)Gas−tungsten arc welding (GTAW)Plasma arc welding (PAW)Gas−metal arc welding (GMAW)Flux-cored arc welding (FCAW)Submerged arc welding (SAW)Electroslag welding (ESW)

(c) High-energy beam welding:Electron beam welding (EBW)Laser beam welding (LBW)

(d) Resistance spot welding:Resistance spot welding (RSW)

There is no arc in ESW except during initiation of the process. For convenience of discussion, however, it is grouped with arc welding processes.

1

Welding Processes

1 Welding Processes4

1.1.1.1  Power Density of Heat SourceIn fusion welding except for RSW, the power density is the power of the heat source divided by its cross-sectional area at the workpiece surface. Consider directing a 1.5-kW hair drier very closely to a 304 stainless steel sheet 0.25 mm thick. Obviously, the power spreads out over an area of roughly 50 mm diameter or greater, and the sheet just heats up gradually but will not melt. With GTAW at 1.5 kW, however, the arc can concentrate on a small area of about 5 mm diameter and can produce a weld pool. This example illustrates the importance of the power den-sity of the heat source in welding.

As shown schematically in Figure 1.1, the size of the heat source increases from high-energy beam welding to arc welding and to gas welding. The power density of the heat source and hence its ability to melt and weld deep decrease in the same order. As shown in Figure 1.2, as the power density of the heat source increases, the amount of heat absorbed by the workpiece before welding is completed decreases. A gas flame tends to heat up the workpiece so slowly that, before any melting occurs, a large amount of heat is already conducted away into the bulk workpiece. Excessive heating can damage the workpiece, weakening and distorting it. Contrarily, the same material heated by a

Figure 1.1 The size of the heat source and its effect on welding.

Figure 1.2 Heating of and hence damage to workpiece vs. power density of heat source.

1.1 ­OerOiee 5

sharply focused electron or laser beam can melt or even vaporize to form a deep keyhole instantaneously. This allows welding to be completed before much heat is conducted away into the bulk workpiece to cause any damage [1].

Therefore, the advantages of increasing the power density of the heat source include deeper weld penetration, higher welding speed, and better weld quality with less damage to the workpiece, as indicated in Figure  1.2. Figure 1.3 shows that the weld strength (of aluminum alloys) increases as the heat input per unit length of the weld per unit thickness of the workpiece decreases [2]. Figure 1.4a shows that the angular distortion of the work-piece is much smaller in EBW than in GTAW [2]. Unfortunately, as shown in Figure 1.4b, the costs of laser and EBW machines are very high [2]. This higher equipment cost is also shown in Figure 1.2.

1.1.1.2  Welding Processes and MaterialsTable 1.1 summarizes the fusion welding processes recommended for carbon steels, low-alloy steels, stainless steels, cast irons, nickel-base alloys, and aluminum alloys [3]. For one example, GMAW can be used for all the

Figure 1.3 Variation of weld strength with heat input per unit length of weld per unit thickness of workpiece. Source: Mendez and Eagar [2]. © ASM.

Productivity, cm/s

LBW and EBW machines make better welds and faster but are much more expensive

Flame

Arc

Laserelectron

beam

Productivity, inch of weld/s

Cap

ital e

quip

men

t, do

llars

Pow

er d

ensi

ty, W

/m2

0.1 1 10 100

0.04 0.4 4 40 400

103

105

107

103

105

107

(b)

t

EBW

GTA

W

Weld thickness t, mm0 20 40

2

4

6

8

0

(a)

α

Dis

tort

ion

angl

e α,

deg

ree

Figure 1.4 Comparisons between welding processes: (a) angular distortion; (b) capital equipment cost. Source: Mendez and Eagar [2]. © ASM.