INFLUENCES OF SECONDARY PULSE ON RESISTANCE SPOT WELDING OF HOT FORMING STEELS. Jeff Hou

Transcription

1 2014 International Symposium on Advances in Resistance Welding, Atlanta INFLUENCES OF SECONDARY PULSE ON RESISTANCE SPOT WELDING OF HOT FORMING STEELS Jeff Hou D.Saha, S. Nayak, N.Zhou, A.Gerlich (UofWaterloo) K.R.Chan, N.Scotchmer (Huys Industries)

2 OUTLINE Motivation Objectives Equipment and Material Specification Simulation Work (Swantec SORPAS) Experimental Work Summary Future Work

3 MOTIVATION - APPLICATION WINDSCREEN FRAME (UPRIGHT) B PILLER BUMPER BEAMS DOOR REINFORCMENTS Reference: ArcelorMittal

4 MOTIVATION USIBOR 1500P Physical & Mechanical Properties of USIBOR 1500P Coating: As Delivered Hot Stamped Aluminized YS (MPa) UTS (MPa) Coating Phases Al Si Al Si Fe AS RECEIVED MICROSTRUCTURE HOT STAMPED MICROSTRUCTURE Reference: ArcelorMittal

5 PROBLEM High base metal strength promotes interfacial failure mode for resistance spot welded components. For energy absorption during impact point of view, plug failure mode may be more beneficial.

6 OBJECTIVE To simulate a welding window and secondary pulsing schedules for welding. Explore the importance of intermission cooling time prior secondary pulsing. To investigate the resulting mechanical and metallurgical properties of the simulated welding schedules.

7 EQUIPMENT AND MATERIAL SPEC. Equipment: 144/180-kVA MFDC RSW 60kA Current 25kN Force 150kN Instron Tensile Tester Electrode: RWMA Female B Style CLASS II 6mm Diameter USIBOR 1500P Coupons: 1.5x60x140mm as per AWS D8.9:2012 Heat treated at 930 o C for 5-10mins and oil quenched (rate>50 o C/s)

8 SORPAS- SINGLE PULSE WELD WINDOW Weld Current (ka) Weld Force: 4.0 [kn] Minimum Required Nugget Diameter (mm): 4.90 Undersized nugget diameter Legend: Sufficient nugget diameter (No Expulsion) Nugget overgrowth (Risk of Expulsion) Electrode Melting Weld Time (Cycles) SORPAS Simulation

9 SORPAS SINGLE PULSE WELD Potential Welding Schedules 6kA 32 Cycles 7kA 20 Cycles 8kA 12 Cycles Weld Current (ka) Weld Force (kn) Process Time (Cycles) 0 6kA 7kA 8kA Force (kn)

10 SORPAS SECONDARY PULSE WELD 20 Cycle Cooling 60 Cycle Cooling Weld Current (ka) Weld Force (kn) Weld Current (ka) Weld Force (kn) Process Time (Cycles) Process Time (Cycles) 8kA Main 6kA 2nd 8kA 2nd 8kA Main 5kA 2nd 7kA 2nd 10kA 2nd Force (kn) 9kA 2nd Force (kn)

11 SORPAS 20 CYCLE COOLING Temperature ( C ) Process Time (ms) Main 6ka 8ka 10ka AC1 AC3 MS Melt MELT AC1 AC3 MS SORPAS Exp B FZ Thermal Profile

12 20 CYCLE COOLING NUGGET GROWTH 2 nd Pulse Initial Nugget Reheat Peak Temp Temp Legend 6kA 5.68mm 8kA 5.68mm 5.71mm 10kA 6.88mm SORPAS Exp A Nugget Growth

13 WELD PROPERTIES & FAILURE MODES 20 CYCLE COOLING Tensile Shear Strength (kn) kN Min Shear Strength AWS D8.9M Minimum Required Strength: 14kN (AWS D8.9:2012) Single Pulse Welds Single Pulse TSS(kN) Failure Mode 8kA 12 Cycle Interfacial Two Pulse Welds 2 nd Pulse TSS(kN) Failure Mode 6kA Interfacial 0 Process Parameters SPC 8kA TPC 6kA TPC 8kA TPC 10kA 8kA Interfacial 10kA Mix

14 20 CYC COOL- MICROSTRUCTURES 8kA Main Nugget 5.743mm 6kA TPC 5.740mm 50x Magnification Macro-structure

15 20 CYC COOL- MICROSTRUCTURES 8kA TPC 5.727mm 10kA TPC 6.797mm 50x Magnification Macro-structure

16 HARDNESS PROFILE 20 CYC COOL 600 FZ Boundary HAZ Boundary Vickers Hardness (HV500g) FZ HAZ BM Distance From Nugget Center (um) 8kA Main Nugget 6kA TPC 8kA TPC 10kA TPC FZ Boundary HAZ Boundary

17 SORPAS 60 CYCLE COOLING Temperature ( C ) Process Time (ms) Main 5ka 7kA 9kA AC1 AC3 MS Melt MELT AC1 AC3 MS SORPAS Exp B FZ Thermal Profile

18 60 CYCLE COOLING NUGGET GROWTH 2 nd Pulse Initial Nugget Reheat Peak Temp Temp Legend 5kA 5.68mm 7kA 5.68mm 5.68mm 9kA 5.69mm SORPAS Exp B Nugget Growth

19 WELD PROPERTIES & FAILURE MODES 60 CYCLE COOLING Tensile Shear Strength (kn) kN Min Shear Strength AWS D8.9M Minimum Required Strength: 14kN (AWS D8.9:2012) Single Pulse Welds Single Pulse TSS(kN) Failure Mode 8kA 12 Cycle Interfacial Two Pulse Welds 2 nd Pulse TSS(kN) Failure Mode 5kA Interfacial 0 Process Parameters SPC 8kA TPC 5kA TPC 7kA TPC 9kA 7kA Mix 9kA Mix

20 60 CYC COOL - MICROSTRUCTURES 8kA Main Nugget 5.743mm 5kA TPC 5.742mm 50x Magnification Macro-structure

21 60 CYC COOL - MICROSTRUCTURES 7kA TPC 5.970mm 9kA TPC 6.432mm 50x Magnification Macro-structure

22 HARDNESS PROFILE 60 CYC COOL FZ Boundary HAZ Boundary Vickers Hardness (HV 500g ) FZ HAZ BM Distance From Nugget Center (um) 8kA Main Nugget 5kA TPC 7kA TPC 9kA TPC FZ Boundary HAZ Boundary

23 SUMMARY Tensile Shear Strengths (TSS): No significant change in TSS unless nugget is overgrown (20 cycle cool) Extreme drop in TSS for 5kA secondary pulsing for (60 cycle cool). Failure Modes: Nugget overgrowth promotes plug failure (20 cycle cool). Recrystallization and nugget re-melt promote plug failure (60 cycle cool). Microstructure: Minimal microstructural differences for (20 cycle cool). Columnar feature in FZ disappears with 7kA secondary pulse. (60 cycle cool) Hardness Profiles: Prolonged heating creates broadening of HAZ (20 cycle cool). Tempering secondary pulse results in nugget softening (60 cycle cool).

24 FUTURE WORK Continuous optimization of welding operation. Investigation of failure initiation sites with different temper pulses partial pull tests. Explore effects of secondary pulsing on electrode durability.

25 Thank You! Any Questions? Acknowledgements