This paper on "Temperature Gradient Development in Spot Welding Electrodes" was presented at the NAFEMS World Congress on Effective Engineering Analysis - 25-28 April 1999, Newport, Rhode Island, USA.

Summary

Previous computer simulations of electrode caps in electric resistance spot welding assumed a complete contact at the cap/workpiece interface. In reality, actual physical contact is made initially over a limited number of individual points. Furthermore the contact area depends on the welding pressure and the material deformation in the interface. Other limitations imposed upon earlier mathematic models include temperature-independent thermal material properties, and above all, no phase changes for the electrode and workpiece materials at elevated temperatures. Consideration of these important aspects was the purpose of this investigation.
For this paper, contact was assumed to occur initially in one place only, and growth of the contact area occurred during the spot weld cycle. Two modes of growth, linear and nonlinear, were considered. In the linear growth assumption, the contact radius is assumed to increase as a linear function of time. In the nonlinear growth assumption, the contact radius is assumed to be a function of the reciprocal of the experimentally determined electric resistance. The investigation suggested that, for the nonlinear contact growth case, maximum cap tip temperatures are produced within the initial period of 5.0ms of a total spot weld cycle of 165ms. Temperatures higher than the melting point of the Cu electrode are predicted at the cap tip surface. Because of the high temperature gradient within the cap, the temperature drops to less than 900^oK, lower than the melting point of the Al workpiece within 0.4mm of the surface. The maximum temperature difference of the models, with and without material phase changes, could be as much as 756^oK depending upon the size of the initial contact radius. By doubling the initial contact radius, the predicted maximum cap tip temperature, is reduced by approximately 50%. Thus the initial contact radius plays a very important role in terms of the cap tip temperature. The nonlinear growth contact model appears to be a more realistic assumption than the linear growth contact model.