CONTACT RESISTANCES IN SPOT-WELDING

The contact resistance of several aluminum alloys with different surfa ce conditions was measured as a function of the applied current and un der different applied loads. The magnitude of the contact resistance v aried over a wide range of values, depending upon load and surface con dition. Usually the contact resistance decreased with an increase in l oad, but if a surface lubricant was present, an increase in resistance was observed. Extensive plastic deformation occurred under the loadin g conditions imposed by the electrode tips. A cup and cone profile was found at the contact region of the faying surface after unloading. Un der slowly varying currents, similar to 1 A/s, electrical breakdown ef fects were observed when the potential across the surfaces was similar to 0.2 V. The nature of the change was ascribed to metallic conductio n and local fusion rather than oxide film breakdown. Under rapidly var ying currents, similar to 10(7) A/s, typical of a spot welding operati on, the contact resistance was found to decrease to similar to 20 mu O hm within the first quarter cycle of weld current, irrespective of the initial surface condition of the aluminum alloy. Continued weld cui r ent inputs caused a further decrease in the contact resistance to simi lar to 10 mu Ohm. It is concluded that the results of contact resistan ce tests may be influenced by the test procedure ii large cur rents ar e used that develop a significant potential difference, >0.2 V, across the interface.