Improving Weldability of Press Hardened Steel through Combining Stepped Current Pulse and Magnetically Assisted Resistance Spot Welding Process

Abstract Press-hardened steel (PHS) with extremely high strength has wide applications in vehicle body manufacturing as an innovative lightweight material. However, the poor weldability of PHS results in poor weld toughness and a high risk of interfacial fracture, posing challenges to the resistance spot welding (RSW) process. Introducing an external magnetic field in the welding process to perform electromagnetic stirring (EMS), magnetically assisted RSW (MA-RSW) process has been proven an effective method to improve the weld toughness of high-strength steel, but it may increase the risk of expulsion. In this study, a new process called SPMA-RSW is developed to improve the weldability of PHS by combining MA-RSW and the stepped-current pulses (SP) technique, which can enlarge the weld lobe. Nugget appearance, microstructure, microhardness, and mechanical properties were systematically investigated by comparing traditional RSW, MA-RSW, SP-RSW, and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, finer the grain, and harden the nugget. This increased the lap-shear strength, energy absorption, and changed the fracture mode from brittle interfacial (IF) mode to ductile plug fracture (PF) mode at the same heat input. Then, a simple model was developed to reveal the mechanism of the effect of EMS on the fracture mode transition and was verified by experiment. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.