Microstructure and failure behaviour of resistance spot welded DP980 dual phase steel

In this research, microstructure and overload failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t^1/2 is not sufficient to ensure the pullout failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.