2گروه مهندسی مواد-دانشکده مواد-دانشگاه آزاد نجف آباد-اصفهان -ایران
This research investigates microstructure and mechanical properties of dissimilar resistance spot welded DP780/DP980 dual phase steels. In order to study the effect of welding current on mechanical performance, welding was performed on standard samples in the current range of 7 to 12 kA, with 0.5 kA steps. At welding currents lower than 7 kA low amount of melting led to very low strength of the joints and at currents higher than 12 kA, expulsion caused deterioration of mechanical properties. Microstructural studies showed that weld nugget was primarily comprised of martensite. The HAZ of both sides of the joint was comprised of three different microstructural zones; Upper Critical (UCHAZ), Inter Critical (ICHAZ) and Sub Critical (SCHAZ). Results of microhardness test showed that at both sides, softening (a decrease in hardness) occurred at SCHAZ. Tensile-shear test results showed that peak load and fracture energy increase before expulsion. Two different fracture modes of interfacial failure (IF) and pullout failure (PF) were observed in tensile-shear test. At welding currents lower than 10 kA the failure occurred in IF mode, while at higher welding currents PF was dominant. Weld nugget diameter at welding current of 10 kA; i.e. critical weld nugget diameter, was ~8.5 mm.