Dissimilar resistance spot welding of DP600 dual phase and AISI 1008 low carbon steels: correlation between weld microstructure and mechanical properties

The aim of this work is to investigate and analyse the microstructure and mechanical properties of dissimilar low carbon steel/dual phase steel (DP600) resistance spot welds. The failure modes of spot welds during the tensile-shear test were detailed by examination of the weld fracture surfaces. Relationships between the fracture path and the mechanical properties (peak load and energy absorption) were developed using the observed microstructures in the fusion and heat affected zones. It was found that the failure of DP600/low carbon steel is initiated from the stronger side (i.e. DP600 side). This was explained in terms of hardness profile, difference in tensile strength and workhardening behaviour of the base metals. A transition in the failure mode from interfacial failure mode to pullout failure mode was observed with increasing the fusion zone size caused by increasing the welding current. However, when expulsion occurred, the spot welds failed in the partial thickness-partial pullout mode, with reduced energy absorption and peak load, compared to those spot welds with the same or smaller nugget size, which failed in the pullout mode. This can be related to the low ductility of the location of the failure initiation (i.e. heat affected zone of DP600 side) in this mode.