Second pulse current in resistance spot welded TRIP Steel - Effects on the microstructure and mechanical behavior

Resistance spot welds (RSW) in transformation-induced plasticity (TRIP) steel sheets were locally postweld heat treated by applying a second pulse of current in the welding schedule. The evolution of the fusion zone (FZ) microstruc-ture by optical and scanning electron microscopy, microhardness, and standardized quasi-static lap-shear tensile testing was investigated for all the pulse current conditions. The most important result of this study is that improved mechanical properties with desirable pullout failure mode is accomplished when the FZ mi-crostructure consists of a recrystallized structure of martensite, which was achieved in the medium level of the second pulse current postweld heat treatment. Even though a considerable reduction in FZ hardness was observed at the lowest value of the second pulse current due to the presence of plate-like ferrite and tempered martensite structure, the failure mode and load-bearing capacity of the weldment was not improved significantly. At higher current levels of the second pulse condition, remelting occurred with consequent formation of a new weld nugget comprising columnar structure but larger nugget size giving mixed failure modes. The fusion zone microstructures and microhardness results correlated well with the simulated welding thermal history of the postweld local heat treatments.