The effect of surface roughness, oxide film thickness and interfacial sliding on the electrical contact resistance of aluminium

Abstract

Contact resistance measurements were made on aluminium alloy sheet with a configuration relevant to electrical resistance spot welding. Previous work has shown that a small amount of sliding is required at the interface to break down contact resistance when the material has a thin, insulating coating. Sliding on a macroscopic scale occurs at the electrode-sheet interface but not at the faying surface. For this reason, the contact resistance at the faying surface is high and its behaviour with applied force is observed to be anomalous. However, local sliding on a microscopic scale can occur at the faying surface, depending on the surface roughness. The effect of surface roughness and oxide film thickness was investigated in the present work. In addition, the degree of sliding required to break down contact resistance was quantified in experiments in which relative rotation was induced at the faying surface. It is estimated that a sliding displacement of only about 10 μm is required to produce a dramatic reduction in contact resistance. The results are interpreted in terms of Holm's constriction resistance theory of microscopic spots of metal-to-metal contact within a mechanical contact area largely insulated by the presence of the surface oxide films.