Prediction and stabilization of initial resistance between electrodes for small-scale resistance spot welding

Quality of resistance spot-welded joints depends on Joule’s heat generation and, in turn, a current profile, dynamic resistance change, and its initial value. In the present work, the impact of electrode force and dome radius of hemispherical electrodes on initial resistance between electrodes was investigated. Two combinations of workpieces of zirconium alloy thickness of 0.25 + 0.25 mm and austenitic stainless steel thickness of 0.3 + 0.3 mm were used. The experimental results obtained were compared with calculated values using published equations and data on physical properties of these materials. After that, the possibility of stabilization resistance between electrodes by preheating current pulses was studied. The pulses had different algorithms of current rise: discrete and stepwise, as well as sharp and smooth with longer upslope. The results show impossibility of reliable prediction and absolute stabilization of initial values of resistance between electrodes. No clear relationship between these values and electrode force has been found. An increase in dome radius of hemispherical electrodes reduces mean resistance values for zirconium alloy but has no effect for stainless steel. Also, it does not affect dispersion of values for both materials. The rate of preheating current rise has no appreciable effect on stabilization of resistance between electrodes in all cases. Stepwise current rise significantly reduces dispersion of resistance values for zirconium alloy but has no effect for stainless steel. However, their dispersion significantly decreases after preheating in comparison with initial values.