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Abstract
The failure mode of resistance spot welds (RSWs) (interfacial versus pullout) is a qualitative measure of RSW performance. Considering the adverse effect of interfacial failure mode on the vehicle crashworthiness, process parameters should be adjusted so that the pullout failure mode is guaranteed, ensuring reliability of spot welds during vehicle lifetime. In this paper, the microstructure and mechanical properties of dissimilar RSWs between HSLA420 and drawing quality specially killed low carbon steels are studied with particular attention to failure mode. Increasing the welding current alters the failure mode from interfacial failure mode to pullout mode due to the increase in fusion zone (FZ) size. The minimum required FZ size to ensure the pullout failure mode was estimated using an analytical model. The FZ size proved to be the most important controlling factor of the spot weld peak load and energy absorption.