Microstructure and mechanical properties of resistance spot welded dual-phase steels with various silicon contents

Abstract

In this paper, silicon-containing dual-phase steels were produced by intercritical annealing heat treatment. The mechanical properties of heat treated steels were investigated and a correlation between microstructural evolutions and mechanical properties was established. It was found that an increase in silicon content leads to improve ductility and reduces the strength of steel. The resistance spot welding (RSW) process of dual-phase steels was performed under constant parameters and the only variable was the silicon element. The microstructure of different zones of steels after welding was studied and the results showed that the structure of the fusion zone (FZ) and the heat-affected zone (HAZ) were lath martensite. The structure of base metal (BM) also consisted of ferrite and martensite phases. The tensile-shear test was done to evaluate the mechanical behavior of spot welds. With increasing the amount of silicon from 0.34 to 1.51 wt%, the peak load was increased and no change was observed in fracture energy and the fracture toughness of the weld nugget. However, by a further increase in silicon content to 2.26 wt%, the peak load and also the fracture energy was reduced, which is caused by serious expulsion and extreme electrode indentation during welding. Also, the shrinkage cavity in the FZ of all specimens was observed which was due to the contraction of the weld pool during solidification. Macroscopic and microscopic observations were also performed after the tensile-shear test. It was concluded that fracture modes of interfacial fracture (IF), partial interfacial fracture (PIF), and pull-out fracture (PF) have occurred and then each fracture mode was discussed separately.