Failure mode of DP780 resistance spot welds in cross-tension loading

This paper aims at investigating the microstructure and failure mode of DP780 ferrite-martensite dual phase steel resistance spot welds between. Microstructural characterization, microhardness tests and the cross-tension shear tests were conducted. Results showed that the fusion zone microstructure exhibited mainly martensite with some Widmanstätten ferrite and bainite. A hardness reduction was observed in heat affected zone in respect to the base metal. Results showed that the mode of failure changed from interfacial fracture to pullout as the welding current was increased. Results indicate that there is a critical weld nugget size to ensure the pullout failure mode. Based on the failure mechanism of the spot welds in the cross-tension test, a simple analytical model is proposed to predict the minimum FZ size (DC) to ensure the pull-out failure mode. According to this mode, the sheet thickness, the hardness ratio of fusion zone to pull-out failure location (i.e. HAZ), the amount of shrinkage voids and porosity in fusion zone and the width of the HAZ are the main controlling factors for interfacial to pullout failure mode transition in cross-tension loading.