Weldability and Performance of GMAW Joints of Advanced High- Strength Steels (AHSS)

Transcription

1 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

2 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

3 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

4 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

5 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

6 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

7 Microhardness Mapping: HAZ Softening in Hardened Boron Steel

8 Minimal HAZ Softening in HSLA 590

9 HAZ Softening is More Pronounced in Higher Strength Steels HSLA590 DP980 Hardened Boron Steel Hv normalized to mean value of base metal

10 Failure Location, Static Tensile 2.0 mm HSLA590 uncoated-tensile tested 2.0mm DP600 uncoated-tensile tested 2.0 mm DP780 uncoated-tensile tested 2.0mm DP980 uncoated-tensile tested 2.0 mm Boron non-heat treated uncoatedtensile tested 2.0 mm Boron heat treated uncoatedtensile tested

11 Static Tensile Failure Location Correlates to HAZ Softening Region (DP980) DP980

12 Static Tensile Strength of AHSS Weld (Lap joint, t=2mm) Normalized Joint Strength (vs DR210) % DR % 182% 164% 197% 209% DP600 HDGI, A DP600 HDGI, B 245% 154% 205% 203% DP600 HSLA590 DP780 Boron, UHT M130 DP980 M220 Boron, HT Base Metal % % 250% 200% 150% 100% 50% 0% Normalized Joint Strength Tensile Strength of Weld Actual Tensile Strength of Base Metal Normalized Joint Strength Normalized Joint Strength = (weld strength of AHSS)/(weld strength of DR210)

13 Static Joint Efficiency vs BM Strength (Lap joint, t=2mm) 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 % % 99% 99% 88% 100% 87% 96% 100% % 73% 45% % % 60% 40% Joint Efficiency DR DP600 HDGI, A DP600 HDGI, B DP600 HSLA590 DP780 Boron, UHT M130 DP980 M220 Boron, HT Base Metal % 0% Tensile Strength of Weld Actual Tensile Strength of Base Metal Joint Efficiency Joint efficiency = weld strength/bm strength

14 HAZ Softening Does Not Affect Fatigue Failure Location 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 DP600 uncoated-fatigue tested, 177,810 cycles at 1200/120 lbs 2.0 mm DP780 uncoated-fatigue tested, 819,203 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

15 S-N Fatigue Data (Regression Plots) Baseline Welding Condition Nominal Stress Range (MPa) Boron HT DR210 Boron HT Boron UHT DP600 Bare DP780 HSLA590 DP980 DR210 M130 M Cycles to Failure

16 Weld Profile Comparison: Baseline vs. Improvement 2.0 mm DP780 Bare, Baseline 2.0 mm DP780 Bare, Improved 2.0 mm DP980 Bare, Baseline 2.0 mm DP980 Bare, Improved

17 Fatigue Life Improvement by Welding DP780: Over an Order of Magnitude at Low Stress Level mm DP780 Bare Baseline W elding Parameters 2.0mm DP780 Bare Improv ed W elding Parameters Nominal Stress Range (MPa) Cycles to Failure

18 DP980 Fatigue Life Baseline vs. Improved Welding Parameters mm DP980 Bare Baseline Welding Parameters 2.0mm DP980 Bare Improved Welding Parameters Nominal Stress Range (MPa) Cycles to Failure

19 Fatigue Life Comparison Nominal Stress Range (MPa) mm HSLA080 Bare Improved Welding Parameters 2.0mm DP780 Bare Improved Welding Parameters 2.0mm DP980 Bare Improved Welding Parameters 2.0mm Boron HT Bare Improved Welding Parameters Baseline Range Cycles to Failure

20 Microstructure Effects of Microstructure on Fatigue Life Nominal Stress Range (MPa) DP600 Bare Regression DP600 Bare Test Data HSLA 590 Regrssion HSLA 590 Test Data DP600 GI Regression DP600 GI Test Data Cycles to Failure

21 Effects of Local Geometry and Material Strength on Fatigue Life N total = N i + N p N p = 1 C a c a 0 N i = 1 ' 2 σ f σ m 2 K f ΔS da ( ΔS πaf() a) m ( ) K f =1+ K t 1 1+ a p ρ 1 b T K t = θ ρ Paris law for propagation Coffin-Manson Equation for initiation b = log ' σ f = 0.95S u + 370MPa a p =1.187x10 5 /S u S u N i : initiation life N p : propagation life ΔS: nominal stress range K t : stress concentration factor K f : Fatigue notch factor Su: ultimate strength After: P. Darcis et al, 2006 F. Lawrence et al θ ρ T

22 Improvement of Weld Profile May Particularly Benefit AHSS A Toe Angle: 45 o Toe Radius: 1.0mm B Toe Angle: 30 o Toe Radius: 2.5mm Improvement in Kf Improvement in Crack Initiation Life Mild (300MPa) DP780 5% 151% 176% 10% 223% 302% 20% 463% 829% 30% 906% 2098% 40% 1688% 4955% 50% 3014% 11032% Improvement from A to B for 2.0mm DP780 would improve K f by 17% or fatigue life by 6.5 times

23 Integrated Model to Predict Welding Effects on AHSS Microstructure & Performance Welding Process & Parameters Weld Performance Properties Process Model Microstructural Model Structural Model Experiments Thermal History Weldment Microstructure Structure-Property Relation Choose appropriate individual models for each physical process, and integrate them Generally adopted for many welding processes Many individual models are already available

24 Modeling Approach Inputs Analysis Outputs Thermal Properties Welding Parameters Joint Configuration & Boundary Conditions Composition & Initial Microstructure Mechanical Properties Thermal Analysis Heat Generation & Flow Phase Transformation Analysis Mechanical Analysis Elasticity Plasticity Temperatures Weld/HAZ Microstructure & Properties Residual Stresses & Distortions

25 Steel Microstructure Modeling based on Phase Transformation Theories USER INPUT: Steel Chemistry (C, Mn, Cr, Ni ) Welding Thermal Cycle (FEM simulation) PHASE TRANSFORMATION COMPUTATION ALGORITHMS Transformation Thermodynamics System equilibiria: Ts, A 3, A 1, Bs, Ms Transformation driving force, ΔG Transformation Kinetics Grain growth Austenite formation on heating Decomposition of austenite on cooling ferrite, bainite, martensite, etc Fraction of phases Property Module Hv, σ ys Hv in HAZ

26 Microstructure Modeling: Preliminary Results HAZ softening predicted Weld metal under development Boron HT DP980

27 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