Mechanical and microstructural evaluation of resistance spot welded dissimilar TWIP/martensitic steel joints

AHSS grade dissimilar TWIP/martensitic steel joints were produced with resistance spot welding with different welding times and welding currents under 4-kN electrode force. The microstructures were examined with EDS, SEM, and XRD analyses. Fusion zone especially at high heat inputs exhibited considerable amount of shrinkage gaps and high Mn content–induced voids. (Fe, Mn) x C segregation behavior in heat-affected zone (HAZ) of TWIP was coincided. Segregation behavior adversely affected HAZ and limited tensile-shear force bearing capacity. Failure mode change from interfacial failure to nugget pullout failure mode was mainly controlled by the nugget size diameter. Tearing-type failure mode was not observed in all experiments. All nugget pullout separations were failed from the TWIP side due to segregation-induced intergranular gaps. Tensile-shear fracture surfaces were examined and M x C segregation–induced gaps were observed. The weld lobe was determined according to mechanical and geometrical calculations.