DP780 DUAL-PHASE-STEEL SPOT WELDS: CRITICAL FUSION-ZONE SIZE ENSURING THE PULL-OUT FAILURE MODE

This paper addresses the transition from the interfacial to the pull-out failure mode for DP780 dual-phase-steel resistance spot welds under tensile-shear and cross-tension loading conditions. It was studied whether industrial weld-size criteria can produce pull-out failure modes. Based on the failure mechanism of the spot welds in a cross-tension test, a simple analytical model is proposed to predict the critical FZ size to ensure the pull-out failure mode. According to the findings, the sheet thickness, the hardness ratio of the fusion zone to the sub-critical heat-affected zone, the amount of shrinkage voids in the fusion zone and the width of the HAZ are the main controlling factors for the transition from the interfacial to the pull-out failure mode in cross-tension loading. The fusion-zone size was proved to be the key factor controlling the cross-tension and tensile-shear strengths of DP780 resistance spot welds.