Effects of Cover Sheets on the Nugget Growth and Fracture Behavior of Resistance Spot Welded Q&P980 Steel Joints

The resistance spot weldability of galvanized ultra-high-strength steels is not satisfied, the joints are prone to interfacial fracture and the weldable current range is narrow. To solve the problems, a novel method called resistance spot welding with double-sided cover sheets was introduced to weld a galvanized Q&P980 steel with the thickness of 1.2 mm. Two thin SPCC mild steel sheets were chosen as cover sheets and were placed symmetrically at both sides between the Q&P980 steels and the electrodes, then the RSW process was carried out. Compared with the traditional RSW method, the joints obtained by using the novel method achieved larger tensile shear strength and energy absorption, which increased by 26.9% and 52.6%, respectively. With increasing the welding current, the failure mode transferred from interfacial fracture to nugget pull-out fracture or base metal tearing fracture. By contrast, the joints always showed interfacial fracture without cover sheets. The improvement of the joint performance was mainly attributed to the enlargement of the nugget. With the help of finite element simulation, it was found that the cover sheets helped increase the contact area and reduced the current density during welding, which postponed the expulsion, and a larger area could be evenly heated. The application of the novel method can be easily extended to the resistance spot welding of other ultra-high-strength steels with various thicknesses.