Resistance Spot Welding of Dual-Phase Steels: Heat Affected Zone Softening and Tensile Properties

Dual-phase (DP) steel comprises a soft ferrite matrix dispersed with hard martensite phase to offer favourable combinations of high strength and good deformability, which has made DP steels a potential candidate for the fabrication of automobile body frame structures with the purpose of reducing fuel consumption through down-gauging and to improve the crashworthiness behavior. Resistance spot welding (RSW) is one of the most extensively used welding methods in automotive manufacturing and the favoured method for joining DP steels. However, the main challenge in employing DP steel in the automotive sector is to overcome the inconsistent failures encountered in the heat-affected zone (HAZ) and/or the fusion zone (nugget) in higher grades of DP steel. Failure at the HAZ of DP steel spot welds is related to softening phenomena which involves tempering of martensite as the temperature in this region reaches close to Ac1 line of the steel. In this study, tempering of martensite occurring at the sub-critical HAZ of resistance spot welded DP steels was evaluated and the degree of tempering been correlated to the softening. The performance of resistance spot welds was assessed through standardized testing methods. One of the major outcomes was that softening lowered the weld size at which transition from interfacial to pullout failure mode takes place along with increased load-bearing capacity and higher energy absorption. So, it is believed that softening benefits the lap-shear tensile joint performance of resistance spot welded DP steels by facilitating pullout failures through failure initiation at the sub-critical HAZ i.e. softened region.