Resistance Spot Welding Performance of CP780 Galvanized Complex-Phase Steel Plate

Resistance spot welding was conducted on CP780 galvanized complex-phase steel plate with 1.5 mm thickness. The welding current window, microstructure and mechanical properties of the steel plate were studied. The electrode life during the resistance spot welding was evaluated. The results show that the welding current window for the resistance spot welding of the test steel plate expanded from 6.0-8.0 kA. The microstructure of the weld under the maximum and minimum welding currents mainly consisted of lath martensite, and that of the heat affected zone consisted of martensite and some ferrite. The hardness of the nugget under the maximum and minimum welding currents was higher than that of the heat affected zone, and softening zones were found in the heat affected zone. The maximum shear force and positive tensile force of the spot weld joint at the maximum welding current were 21.80 kN and 10.56 kN, which were 51.8% and 38.0% higher than those at the minimum welding current, respectively. After continuous welding 2 000 points at the maximum welding current, the nugget diameter was still larger than the critical nugget diameter, indicating the electrode life exceeded 2 000 points.