Fatigue Life of Resistance Spot Welding on Dual-Phase Steels

The resistance spot welding (RSW) process was used to join dual-phase steels (DP590 and DP980). Mechanical properties of the welded joints were determined by microhardness measurements, tensile and high cycle fatigue tests. The hardness of different zones of the welded joints were represented by microhardness profiles. It was observed a hardness increment in the spot melting zone of around 150 HV0.5, with respect to the base materials (320 HV0.5 for DP980 and 200 HV0.5 for DP590). A soft region was detected in the heat affected zone of the DP980 material, whereas no soft zone was observed for the DP590 material. The shear strength for the welded joints was similar for both dual phase steels, however, a ductility increment for the DP590 steel was detected in comparison to the DP980. Wohler curves at a constant load ratio were obtained to determine the fatigue life of the welded joints. Failures of the joints were systematically initiated in the nugget zone. The fracture surfaces were analyzed by a scanning electron microscopy to elucidate the fracture mechanism of the welded joints.