Resistance spot welding (RSW) is frequently accompanied by expulsion, an undesirable event taking place in manufacturing processes, including car body production. Many variables are involved in RSW, all of which can affect expulsion. This investigation aimed at developing an analytical model for predicting expulsion in RSW. Accordingly, based on force and moment balances, a criterion was established. It was proposed that expulsion took place when the molten metal ejected as a result of the pressure inside nugget. The model presented here considered that the pressure inside the nugget was mainly due to melting transformation and liquid thermal expansion. Hence, the model took into account the effects of nugget diameter, electrode force, base material yield stress, sheet thickness, and joint fit-up to predict the occurrence of expulsion. To assess the model, a number of experiments were done, where the effects of welding current, electrode force, sheet thickness, and galvanization coating of the steel were studied. Regarding the effects of electrode force and welding current, the prediction made by the analytical model was consistent with the results obtained from the experiments. However, it was observed that expulsion in the galvanized steel sheets occurred at larger nugget diameters. The macroscopic examination of the welded samples showed that, with galvanization the ability for a better electrode fit-up was increased, and according to the model this could result in thereby increasing the critical nugget size.
