Effect of cover sheet on dissimilar three-steel sheets resistance spot welding

The use of high-strength steels on car bodies has been increasing gradually for lightweight vehicle body. In general, the resistance spot weldability of high-strength steels is poor compared to that of low-carbon steels. In particular, it is difficult to form a robust nugget in dissimilar three-sheet resistance spot welding of thin low-carbon steel (SGACEN) and thick high-strength steels (DP980, CP1180) because of the resistance difference between base metals. A cover sheet was inserted between the electrode and SGACEN to compensate for the lack of resistance of the SGACEN side. The mechanical property of welds was characterized by suitable welding range and by tensile strength to investigate the effect of the cover sheet. It was found that the tensile shear strength increased and the suitable welding range was expanded when the cover sheet was inserted, in comparison with conventional welding without the cover sheet. To compare the difference of the nugget growth mechanism, cross sectional images of welds were observed. By using a cover sheet, a nugget significantly grew at the SGACEN side. The analysis of transition resistance, dynamic resistance, and welding power showed that the cover sheet induced additional resistance and Joule heat to the SGACEN side. Furthermore, macro and microstructure at the welded zone were analyzed.