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 low weld toughness and a high risk of interfacial fracture, posing significant challenges to the resistance spot welding (RSW) process. Magnetically assisted RSW (MA-RSW) process, which introduces an external magnetic field in the welding process to perform electromagnetic stirring, has been shown to effectively improve the weld toughness for high-strength steel, although it may increase the risk of expulsion. To further improve the weldability of PHS, this study proposes a new process called stepped-current pulses magnetically assisted resistance spot welding (SPMA-RSW), which combines the MA-RSW with the stepped-current pulses (SP) method to enlarge the weld lobe. The study systematically investigates nugget appearance, microstructure, microhardness, and mechanical properties by comparing traditional RSW, MA-RSW and SPMA-RSW. The result showed that the SPMA-RSW process would significantly increase the nugget size, inhibit the shrinkage voids, refine the grain size and harden the nugget region of PHS welds, resulting in increased lap-shear strength and a transition from a brittle interfacial (IF) mode to a ductile button pullout fracture (BPF) mode at the same heat input. Specifically, the peak load and energy absorption were increased by 32.3% and 84.2%, respectively. This work can help improve the weld quality and thermal efficiency of the RSW process for PHS.

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