Expulsion prediction in resistance spot welding

An expulsion model has been developed for resistance spot welding, based on consideration of the interaction between mechanical and metallurgical processes during welding. An expulsion criterion is proposed by comparing the electrode force with that from the liquid nugget; expulsion occurs when , the latter exceeds the former. An effective electrode force, instead of an applied/ nominal electrode force, is used in the criterion. This force can be calculated based on the applied electrode force and its offset from the liquid nugget force, which can be obtained through knowledge of the internal pressure and the dimensions of a nugget. Pressure components in a molten metal include those due to melting, liquid expansion, vapor pressure, and decomposition of surface agents, and are formulated by thermodynamic considerations. Experiments have been conducted to verify the model on an aluminum alloy (AA5754), and good agreement was achieved. The model can also be used to develop guidelines for electrode force selection.