A process developed to overcome pickup problems in the resistance welding of aluminum.

Aluminum is readily welded using the resistance spot welding process. However, because of the relatively low electrical resistance of aluminum, the current requirements for welding are two to three times the values required for welding a similar thickness of carbon steel. The high welding currents required for aluminum result in heating of the copper welding elec- trodes and “mushrooming” of the contact tips. There is also the problem of “pickup” of aluminum on the copper electrodes during the welding cycle. Both of these phenomena result in changes in current density, and therefore weld quality. Another problem is the oxide scale which forms on aluminum. In spite of careful cleaning methods prior to welding, the oxide layer forms on aluminum very quickly and can cause wide variations in the welding heat because of variations in the welding contact resistance.

One of the methods used to minimize these problems is to circulate a refrigerated coolant through the electrodes. A coolant temperature of -12°C (10°F) was found to reduce the softening and pickup problems to the point that the electrodes could be used to make as many as 2000 spot welds between electrode dressings (with a file or emery cloth). While this extended electrode service life by a factor of ten, the refrigerated coolant method has been partially replaced by the development of (1) copper alloy electrodes that do not deform plastically at the temperatures encountered, and (2) small replaceable electrode caps that resulted in discarding 14 g (1/2 oz) of copper rather than the previous 170 g (6 oz) when the tips were worn out.

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