Cross Tension Strength Improvement for AHSS Using Post-Weld Heat Treatment

Transcription

1 Cross Tension Strength Improvement for AHSS Using Post-Weld Heat Treatment Stephen Kelley (Lead Engineer) Hassan Ghassemi-Armaki (Senior Research Engineer) ArcelorMittal Global R&D E. Chicago May 11, 2016

2 Strategy Select appropriate UHSS to take advantage of high strength and reduced thickness, achieving light weighting. Rapid cooling rate characteristic of RSW applied to UHSS composition guarantees a fully martensitic weld and supercritical heat-affected zone (HAZ). As-transformed martensite spot welds have high hardness but low cross tension strength, raising questions about weld response to a crash event. Can spot welds be modified in a way to improve crash response? Consider tempering the spot welds while still in the welder to improve toughness and impact resistance. 2

3 Weld and Temper Firing Pattern ST = squeeze time WT = weld time QT = quench time TT = temper time HT = hold time WC = weld current TC = temper current Weld Pulse - Melt and solidify Weld Quench - Cool and transform completely to martensite QT Temper Pulse - Soften martensitic weld and HAZ CURRENT FORCE WT WC TC HT ST TT 3

4 Application of SORPAS Software Use SORPAS software to determine spot weld time/temperature profile. Notch tip location will have slowest cooling rate during quench. To insure a fully martensitic spot weld, determine time for the notch tip to reach M F temperature; i.e., quench time. Nugget center Notch tip 4

5 Temperature ( C) Quenching - Formation of Martensite Predicted QT min 42 cy (@ 50 Hz) Notch tip Temperature M S M and M F Fully M Time (cycles) 42 cy (@ 50Hz) 50 cy (@ 60Hz) 5

6 Temperature ( C) Martensite Flash Temper Prediction Temperature Range for Noticeable Martensite Tempering Post-heat intensity (% Welding intensity) Temper Current Range 80 95% WC Hardness (Hv) A C1 limit Temperature Evolution Hardness Evolution 6

7 M1500 Example: Material and Welding Conditions Material 1.2 mm M1500 EG Mechanical properties: 1381 MPa YS, 1620 MPa UTS Dome electrode - 6 mm tip face dia. Electrode Force kn Weld time 16 cycles Minimum weld size 4.0 mm SORPAS Quench time - 50 cycles (@ 60 Hz) Temper currents - 80%, 87.5%, 95% AWS WC 7

8 Nugget Hardness Response to Temper Current In the as-welded condition, nugget hardness relatively flat and similar to BMH. As temper current increases from 80% to 87.5 % WC, see increasing nugget softening. At 95% WC, nugget hardness begins to recover, indicating reaustenitization & subsequent. transformation to untempered martensite. 8

9 Cross Tension Strength vs. Temper Current CTS increases with average weld current; i.e., with weld diameter Additional CTS improvement as temper pulse current increases Most consistent CTS increase in the range of 80% to 87.5% WC temper pulse MWS Minimum Weld Size AWS Avg Weld Size Exp Expulsion 9

10 Summary Temper Pulsing to Improve CTS Join the material using a suitable welding pulse Cool the fusion zone and HAZ by keeping the welded joint clamped between the water-cooled electrodes Employ quench time sufficient to fully transform the fusion zone and supercritical HAZ to Martensite ; i.e., to reach the M F temperature Apply a subsequent pulse of sufficient current and time to temper the Martensite without reaustenitizing the weld and forming fresh untempered Martensite (M M) 10

11 How Temper Pulse Can Improve the Crash Performance?

12 Studied Materials and Stackups 1.4mm PHS1500 was studied Totally, 4 stackups are compared: 1.4mm PHS1500 / 1.4mm PHS1500 (With Temper Pulse) 1.4mm PHS1500 / 1.4mm PHS1500 (Without Temper Pulse) 1.4mm PHS1500 / 1.4mm DP980 (Without Temper Pulse) 1.4mm DP980 / 1.4mm DP980 (Without Temper Pulse) Welding condition kept unchanged for all stackups. However, temper pulse cycles were added for first stackup 5-mm weld diameter considered for all stackups The optimized welding current was used for evaluation

13 How Crash Performance is Evaluated? Mechanics of Modeling Spot-welds in real components (e.g., B-Pillar, bumper ) are faced with a combination of loading modes. Spot-weld fails if the stress triple of the internal normal, bending and shear stresses is above the surface. Stress-Based Failure Model*: As long as temper pulse increases the 3D failure surface, crash performance improves Combined Tension-Shear & Cross-Tension; KSII (U-Shape Samples) Bending Stress; Coach-Peel Energy absorption and post-failure damage; Failure Mode *Seeger, Feucht, Frank, Haufe, and Keding, LS-DYNA Conf

14 2D Failure Surface for 1.4mm PHS1500 U-Shape KS-0 KS-30 KS % Increase in Failure Surface Axial Force Applied load Shear Force Without Temper Pulse: With Temper Pulse: 2D-failure surface increases more than twice Both shear and normal exponents increase, but normal force more.

15 2D-Failure Surface after Tempering vs HSS Stackup Homogenous 1.4mm DP980 (Without Temper Pulse) 1.4 mm PHS1500 / 1.4 mm DP980 (Without Temper Pulse) 2D-Failure surface increases significantly as compared to heterogeneous stackup and even HSS stackup

16 Coach-Peel Strength 65% Improvement Coach peel strength increases with temper pulse. Bending stress increases in 3D-failure surface.

17 Failure Mode and Post-Failure Damage FIF: PIF: PF: Loading Mode Fully Interfacial Failure Partial Interfacial Failure Plug Failure Without Temper Pulse With Temper Pulse Increase in area under curve after Max. Load Coach Peel FIF PF 2000% KSII-30 FIF PF 600% KSII-60 PIF PF 26% KSII-90 PIF PF 340% Coach Peel Failure mode changes from full or partial interfacial failure to plug failure. Improvement of post-failure damage is more pronounced for coach peel.

18 Conclusions Tempering of weld nugget microstructure by using temper pulse improves not only cross-tension strength, but also strength in other loading modes. 2D failure surface increases significantly. Resistance of weld to failure regarding to bending stress increases, too. Failure mode is improved and plug failure is achieved for most loading modes, which gives benefit of energy absorption. Finally, improvement of failure surface can be better than even heterogeneous and homogenous stackups by using HSS (e.g., DP980). 18

19 Questions?

20 #GDIS Presentations will be available May 16 at