ABSTRACT. The contact resistance of several aluminum alloys with different surface conditions was measured as a function of the applied current and under different applied loads. The magnitude of the contact resistance varied over a wide range of values, depending upon load and surface condition. Usually the contact resistance decreased with an increase in load, but if a surface lubricant was present, an increase in resistance was observed. Extensive plastic deformation occulred under the loading conditions imposed by the electrode tips. A cup and cone profile was found at the contact region of the faying surface after unloading. Under slowly varying currents, -1 A/s, electrical breakdown effects were observed when the potential across the surfaces was -0.2 V. The nature of the change was ascribed to metallic conduction and local fusion rather than oxide film breakdown. Under rapidly varying currents, -107 A/s, typical of a spot welding operation, the contact resistance was found to decrease to -20 [a~ within the first quarter cycle of weld current, irrespective of the initial surface condition of the aluminum alloy. Continued weld current inputs caused a further decrease in the contact resistance to -10 p~. It is concluded that the results of contact resistance tests may be influenced by the test procedure if large currents are used that develop a significant potential difference, >0.2 V, across the interface
