Behaviour of resistance spot welded thermomechanically rolled high strength steel under tensile shear and cross-tension loads

Over the past decades, vehicle manufacturers have been striving to reduce vehicle weight to minimize energy consumption and emissions in line with key priorities of the environmental objectives of the European Union. These goals can be met by using high strength steels, but their application raises numerous challenges in welding. In general, the railway vehicle body consists of ribs-stiffened shell structure which bears the load together with chassis. By substituting mild steel with high strength steel in these structural elements, joined by resistance spot welding (RSW), significant weight reduction can be achieved. It is possible to use thermomechanically rolled high strength steels with relatively low carbon equivalent which can be ideal for the RSW of railway vehicles. In this paper, the results of tensile shear (TST) and cross-tension (CTS) tests of resistance spot-welded joints made of 3-mm thick S700M thermomechanically rolled high strength steel are presented. The main research scope was to develop a resistance welding technology including the determination of optimal welding parameters based on TST and CTS tests. The optimal technological parameters were nearly the same for the two loading circumstances; however, a bit shorter cycle was justified during the optimization for the cross-tension strength. The value of the tensile shear force exceeded the minimum value specified by the AWS recommendation D8.1M by more than 22% with an electrode indentation depth of 5.56% in average.