LAUSR

Resistance spot welding (RSW) is an autogenous welding process used to lap weld sheet metals. The coalescence obtained by melting and solidifying a small region by passing very high currents using copper electrodes is known as a nugget. The strength of the spot welds is entirely dependent on these nuggets, and any reduction in the size of it may result in the reduction in its load-carrying capacity. Cavity/void is a defect formed in RSW nugget that reduces its load-carrying area. Although cavities are formed in all grades of steels, it has a devastating effect in advanced high-strength steels (AHSS). However, it is yet to understand the exact mechanism of its formation and influence on mechanical properties of AHSS spots. In this context, the present paper is aimed at carrying out an extensive microstructural and computed tomography analysis of the cavities formed in the AHSS spots to understand its mechanism of formation. The results indicate that the cavities formed in AHSS had two distinctive regions: (1) an equiaxed dendritic region with smooth edges and surfaces, which occupied a larger portion of the void, and (2) a rough and irregular dendritic region with dendritic gaps. A thorough microstructural analysis revealed that the dendrites, which grew inwards in the smooth region of the cavity is nucleated from the cavity, indicates the presence of porosity in nugget prior to the start of solidification, while the dendrites of the rough region grew and the dendritic gaps between them indicates the role of thermal contraction in void formation towards the end of solidification.