Numerical and experimental investigation of current and electrode force effects on expulsion in resistance spot welding of galvanized and non-galvanized steel sheets

During the process of Resistance spot welding (RSW), some of the molten metal comes out of the interface of the two sheets, which causes contamination on the body of the cars. This named as expulsion and disrupts the staining process and reduces the safety of the workshop. In this study, by numerical and Experimental investigation, effect of welding current and electrode force on expulsion in RSW of galvanized and non-galvanized steel sheets the prediction of expulsion for both sheets was performed. Experimental results showed that with increasing the welding current, nugget diameter and weld strength increased continuously until expulsion occurrence. The resultant nugget in both types of sheets have almost similar diameter at the similar welding current however in the galvanized sheet, expulsion occurred at a larger nugget diameter. Increasing the electrode force before the occurrence of expulsion increased the nugget diameter, but then reduced the nugget diameter. The results of the investigation were found to be consistent with a modified numerical model. In this model, increase in the nugget diameter increased the force from within the nugget and accelerated the expulsion, while the electrode force was a hindrance to the expulsion. Zinc coated on galvanized steel sheets with low contact resistance and friction coefficient between zinc-zinc metal resulted in a good fit between two sheets during the welding process which according to the numerical model, causes expulsion to occur at a larger nugget diameter for galvanized steel sheets.