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Weldability and Performance of GMAW Joints of Advanced High-

Strength Steels (AHSS)

Zhili Feng*, John Chang**, Cindy Jiang*** and Min Kuo****

* Oak Ridge National Laboratory** Ford Motor Company

*** AET Integration**** Mittal Steel

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Acknowledgements

• Research sponsored by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the Automotive Lightweighting Materials Program, under contract DE-AC05-00OR22725 with UT-Battelle, LLC

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Challenges in Welding of AHSS

• Higher carbon and alloying element contents make AHSS more sensitive to the welding thermal cycle, resulting greater variations of microstructures and properties of weld

• Microstructure and properties can highly depend on welding conditions and steel chemistry

• Welding practices developed for one types of AHSS may not apply to other types– Weld quality– Weld structural performance (static, fatigue, crash)

• There are wide range of grades and types of AHSS and they continue to evolve

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Objectives

• Fundamental understanding and predictive capability to quantify the effects of welding and service loading on the structural performance of welded AHSS auto-body parts

• Welding techniques and practices to improve structural performance of AHSS welded auto body components

• Develop design guidelines and weld performance data for rapid structure design and prototyping, CAE model for weld structure performance design

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Gas Metal Arc Welding

• Materials: mild steel, HSLA, DP, TRIP, martensitic, boron steel

• Gauge: 2mm• Filler metal: ER70S-3

– Under-matched for higher grade AHSS

• Baseline comparison of different AHSS welds– Consistent weld profile, consistent welding heat input level, but not

optimized for different AHSS– Lap joint– Static and fatigue– Microstructure characterization and correlation to mechanical

property

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AHSS Material Matrix Tested

• Group 2 (350 MPa ~500 MPa)– 2.0mm DR210 Bare

• Group 3 (500 MPa~ 800 MPa)– 2.0mm DP600 Bare– 2.0mm DP600 HDGI– 2.0mm HSLA590 Bare

• Group 4 (>800 MPa)– 2.0mm DP780 Bare– 1.5mm TRIP780 GA– 2.0mm DP980 Bare – 2.0mm M130 Bare, 2.0mm M220 Bare– 2.0mm Boron HT Bare, 2.0mm Boron UHT Bare

• From six different steel companies

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Microhardness Mapping:HAZ Softening in Hardened Boron Steel

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Minimal HAZ Softening in HSLA 590

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Hardened Boron Steel

DP980

HSLA590

Hv normalized to mean value of base metal

HAZ Softening is More Pronounced in Higher Strength Steels

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2.0mm DP980 uncoated-tensile tested

2.0 mm HSLA590 uncoated-tensile tested 2.0mm DP600 uncoated-tensile tested 2.0 mm DP780 uncoated-tensile tested

2.0 mm Boron non-heat treated uncoated-tensile tested

2.0 mm Boron heat treated uncoated-tensile tested

Failure Location, Static Tensile

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DP980

Static Tensile Failure Location Correlates to HAZ Softening Region

(DP980)

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Normalized Joint Strength = (weld strength of AHSS)/(weld strength of DR210)

Normalized Joint Strength (vs DR210)

352636 641 577 693 734 861

543721 714

924

395641 647 654 690

848 896 972 986

1593 1689100%

181% 182%164%

197%209%

245%

154%

205% 203%

263%

0

500

1000

1500

2000

2500

DR210 DP600HDGI, A

DP600HDGI, B

DP600 HSLA590 DP780 Boron, UHT M130 DP980 M220 Boron, HT

Base Metal

0%

50%

100%

150%

200%

250%

300%

Nor

mal

ized

Joi

nt S

tren

gth

Tensile Strength of Weld Actual Tensile Strength of Base Metal Normalized Joint Strength

Static Tensile Strength of AHSS Weld(Lap joint, t=2mm)

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352636 641 577 693 734 861

543721 714

924

395641 647 654 690

848 896 972 986

1593 1689

89%

99% 99%

88%

100%

87%

96%

56%

73%

45%

55%

0

500

1000

1500

2000

2500

DR210 DP600HDGI, A

DP600HDGI, B

DP600 HSLA590 DP780 Boron, UHT M130 DP980 M220 Boron, HT

Base Metal

0%

20%

40%

60%

80%

100%

120%

Join

t Effi

cien

cy

Tensile Strength of Weld Actual Tensile Strength of Base Metal Joint Efficiency

Joint efficiency = weld strength/BM strength

• The weld tensile strength of higher grade AHSS is lower than the base metal • The reduction is related to the softening in HAZ • Low heat input and/or fast cooling can be beneficial to minimize HAZ softening

Static Joint Efficiency vs BM Strength(Lap joint, t=2mm)

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2.0 mm DR210 uncoated-fatigue tested, 1,164,447 cycles at 1200/120 lbs

2.0 mm HSLA590 uncoated-fatigue tested, 749,637 cycles at 1200/120 lbs

2.0 mm DP780 uncoated-fatigue tested, 819,203 cycles at 1200/120 lbs

2.0 mm DP600 uncoated-fatigue tested, 177,810 cycles at 1200/120 lbs

2.0 mm DP980 uncoated-fatigue tested, 543,481 cycles at 1200/120 lbs

2.0 mm Boron heat treated uncoated-fatigue tested, 106,413 cycles at 1200/120 lbs

HAZ Softening Does Not Affect Fatigue Failure Location

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Cycles to Failure

Nom

inal

Str

ess

Ran

ge (

MPa

)

100000001000000100000100001000

300

200

100

Boron HTBoron UHTDP600 BareDP780HSLA590DP980DR210M130M220

DR210Boron HT

103 104 105 106 107

S-N Fatigue Data (Regression Plots) Baseline Welding Condition

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2.0 mm DP780 Bare, Improved2.0 mm DP780 Bare, Baseline

2.0 mm DP980 Bare, Baseline 2.0 mm DP980 Bare, Improved

Weld Profile Comparison: Baseline vs. Improvement

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Cycles to Failure

Nom

inal

Str

ess

Ran

ge (

MPa

)

100000001000000100000100001000

300

200

100

2.0mm DP780 Bare Baseline Welding Parameters2.0mm DP780 Bare Improved Welding Parameters

103 104 105 106 107

Fatigue Life Improvement by Welding

DP780: Over an Order of Magnitude at Low Stress Level

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Cycles to Failure

Nom

inal

Str

ess

Ran

ge (

MPa

)

100000001000000100000100001000

300

200

100

2.0mm DP980 Bare Baseline Welding Parameters2.0mm DP980 Bare Improved Welding Parameters

103 104 105 106 107

DP980 Fatigue LifeBaseline vs. Improved Welding Parameters

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Fatigue Life Comparison

Cycles to Failure

Nom

inal

Str

ess

Ran

ge (

MPa

)

100000001000000100000100001000

300

200

100

2.0mm HSLA080 Bare Improved Welding Parameters2.0mm DP780 Bare Improved Welding Parameters2.0mm DP980 Bare Improved Welding Parameters2.0mm Boron HT Bare Improved Welding Parameters

103 104 105 106 107

Baseline Range

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Cycles to Failure

Nom

inal

Str

ess

Ran

ge (

MPa

)

100000001000000100000100001000

300

200

100

DP600 Bare RegressionDP600 Bare Test DataHSLA 590 RegrssionHSLA 590 Test DataDP600 GI RegressionDP600 GI Test Data

Microstructure Effects of Microstructure on Fatigue Life

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Ntotal = Ni + Np

Ni: initiation lifeNp: propagation lifeΔS: nominal stress rangeKt: stress concentration factorKf: Fatigue notch factorSu: ultimate strength

Np =1C

da

ΔS πaF a( )( )ma0

ac∫

Ni =12

2 σ f' −σ m( )

K f ΔS

⎡

⎣ ⎢ ⎢

⎤

⎦ ⎥ ⎥

− 1b

Paris law for propagationCoffin-Manson Equation for initiation

b = −0.1667log 2.1+917Su

⎛

⎝ ⎜

⎞

⎠ ⎟

σ f' = 0.95Su + 370MPa

ap =1.187x105 /Su

K f =1+Kt −1

1+ap

ρ

Kt =1+ 0.5121θ 0.572 T

ρ⎛

⎝ ⎜

⎞

⎠ ⎟

0.469

After: P. Darcis et al, 2006F. Lawrence et al. 1995

θ ρ

T

Effects of Local Geometry and Material Strength on Fatigue Life

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Improvement in Kf

Mild (300MPa) DP7805% 151% 176%10% 223% 302%20% 463% 829%30% 906% 2098%40% 1688% 4955%50% 3014% 11032%

Improvement in Crack Initiation Life

AToe Angle: 45o

Toe Radius: 1.0mm

BToe Angle: 30o

Toe Radius: 2.5mm

Improvement from A to B for 2.0mm DP780 would improve Kf by 17% or fatigue life by 6.5 times

Improvement of Weld Profile May Particularly Benefit AHSS

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Process Model Structural Model

ExperimentsMicrostructuralModel

Welding ProcessWelding Process& Parameters& Parameters

ThermalThermalHistoryHistory

StructureStructure--PropertyPropertyRelationRelation

Weld PerformanceWeld PerformancePropertiesProperties

WeldmentWeldmentMicrostructureMicrostructure

• Choose appropriate individual models for each physical process, and integrate them•• Generally adopted for many welding processesGenerally adopted for many welding processes•• Many individual models are already availableMany individual models are already available

Integrated Model to Predict Welding Effects on AHSS Microstructure & Performance

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InputsInputs AnalysisAnalysis OutputsOutputs

TemperaturesTemperatures

ResidualResidualStresses & Stresses & DistortionsDistortions

Thermal AnalysisThermal AnalysisHeat Generation & FlowHeat Generation & Flow

Mechanical AnalysisMechanical AnalysisElasticityElasticityPlasticityPlasticity

Thermal Properties

Welding Parameters

Joint Configuration & Boundary Conditions

MechanicalProperties

Phase Phase Transformation Transformation

AnalysisAnalysis

Weld/HAZ Weld/HAZ Microstructure Microstructure & Properties& Properties

Composition &Initial Microstructure

Modeling Approach

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USER INPUT:Steel Chemistry

(C, Mn, Cr, Ni …)Welding Thermal Cycle

(FEM simulation)

PHASE TRANSFORMATION COMPUTATION ALGORITHMS

Transformation ThermodynamicsSystem equilibiria: Ts, A3, A1, Bs, Ms …Transformation driving force, ΔG

Transformation KineticsGrain growthAustenite formation on heatingDecomposition of austenite on cooling

– ferrite, bainite, martensite, etcFraction of phases

Property ModuleHv, σys

Hv in HAZ

Steel Microstructure Modeling based on Phase Transformation Theories

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Microstructure Modeling: Preliminary Results

Boron HT

DP980

• HAZ softening predicted• Weld metal under development

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Concluding Remarks

• Baseline static and fatigue properties are obtained for a wide range of AHSS

• Failure locations are different for static and fatigue loading– Different factors govern static strength are fatigue strength– Under-matched filler metal (ER70S-3) did not cause failure in the

weld metal • Fatigue Life

– Steel grade dependency observed– Not influenced by HAZ softening– Considerable fatigue life improvement can be achieved by

improving the welding conditions• Static Strength

– Extensive HAZ softening in higher grade AHSS welds can affect the static strength, as the welds fail in the soften region