•This paper theoretically describes the failure modes and mechanism of RSW joints.•IF is mainly brittle fracture and PF is mainly plastic fracture.•The tensile stress leads to the IF and the shear stress leads to the PF.•The fracture is initiated from the DP780 sheet.Investigating the failure mode and failure mechanism of the dissimilar thickness dual phase sheets resistance spot welding joints was the objective of this study. Three distinct failure modes were observed during the tensile shear test: interfacial, partial interfacial and pullout failure. The results of the stress analysis of welded joints show that the tensile stress leads to the interfacial failure and the shear stress leads to the pullout failure. Due to more serious stress concentration and heat affected zone (HAZ) softening of DP780 side, the fracture is initiated from the DP780 steel.

•We examine changes of microstructure of dissimilar thickness DP600/DP780 joints.•The hardness profile of RSW joints can be predicted by the equation.•Failure modes, peak load and energy describes the mechanical properties of joints.•The nugget diameter is the key factor of transition between the failure modes.In this study, resistance spot welding (RSW) experiments were performed in order to evaluate the microstructure and mechanical properties of single-lap joints between DP780 and DP600. The results show that the weld joints consist of three regions including base metal (BM), heat affected zone (HAZ) and fusion zone (FZ). The grain size and martensite volume fractions increase in the order of BM, HAZ and FZ. The hardness in the FZ is significantly higher than hardness of base metals. Tensile properties of the joints were described in terms of the failure modes and static load-carrying capabilities. Two distinct failure modes were observed during the tensile shear test of the joints: interfacial failure (IF) and pullout failure (PF). The FZ size plays a dominate role in failure modes of the joints.