THE EFFECT OF SURFACE-ROUGHNESS, OXIDE FILM THICKNESS AND INTERFACIALSLIDING ON THE ELECTRICAL CONTACT RESISTANCE OF ALUMINUM

Contact resistance measurements were made on aluminium alloy sheet wit h a configuration relevant to electrical resistance spot welding. Prev ious work has shown that a small amount of sliding is required at the interface to break down contact resistance when the material has a thi n, insulating coating. Sliding on a macroscopic scale occurs at the el ectrode-sheet interface but not at the faying surface. For this reason , the contact resistance at the faying surface is high and its behavio ur with applied force is observed to be anomalous. However, local slid ing on a microscopic scale can occur at the faying surface, depending on the surface roughness. The effect of surface roughness and oxide fi lm thickness was investigated in the present work. In addition, the de gree of sliding required to break down contact resistance was quantifi ed in experiments in which relative rotation was induced at the faying surface. It is estimated that a sliding displacement of only about 10 mu m is required to produce a dramatic reduction in contact resistanc e. The results are interpreted in terms of Helm's constriction resista nce theory of microscopic spots of metal-to-metal contact within a mec hanical contact area largely insulated by the presence of the surface oxide films. (C) 1998 Elsevier Science S.A.