The failure mechanism of resistance spot welded third-generation medium-Mn steel during shear-tension loading

Medium-Mn (M-Mn) steels have received considerable attention in the automotive industry due to their superior mechanical properties such as high strength to weight ratio. However, resistance spot welding of M-Mn steels is a problematic issue, mainly due to the occurrence of interfacial failure (IF) mode with low energy absorption capability during different loading conditions. Despite some efforts to understand the failure behavior of M-Mn steels, the mechanism that led to the IF mode is still poorly understood. The present study aims to gain insight into the failure mechanism of M-Mn spot weld during shear-tension loading. It was shown that while cracks initially propagated in the heat-affected zone, they eventually spread out towards the fusion zone (FZ) resulting in the occurrence of IF mode. Additionally, it was shown that crack propagation exhibits a competitive behavior, in which the fracture toughness of the weld region plays a critical role in determining the propagation path. Moreover, the present study proposed a novel method to enhance mechanical properties and failure behavior of resistance spot welded M-Mn steel based on the modification of the FZ composition/microstructure by using Ni-interlayer. In contrast to as-welded M-Mn steel with a fully martensitic microstructure in the FZ, the specimen with Ni-interlayer consisted of austenite islands embedded in an acicular ferrite matrix. This specific microstructure was sufficiently ductile and provided an in-situ strain hardening effect during loading which significantly enhanced the mechanical properties of the joint and transmitted the failure mode from IF to pull-out mode.