Effects of Au plating on dynamic resistance during small-scale resistance spot welding of thin Ni sheets

The effects of Au plating on dynamic resistance during small-scale resistance spot welding of thin Ni sheets have been investigated. Compared to the small-scale resistance spot welding of bare Ni, the Au-plated material showed much lower initial static resistance, and the contribution of constriction resistance to overall dynamic resistance reduced to a negligible value very early in the welding sequence because of the low softening temperature and its lack of surface oxide film. For the same reason, the asperity heating and surface breakdown stages during resistance spot welding of bare Ni were not observed during resistance spot welding of Au-plated Ni. Furthermore, the partial surface melting stage was replaced by the solid-state bonding and brazing stages. Step increases in the sheet-to-sheet dynamic resistance curves of Au-plated Ni were shown to be due to the peripheral unzipping of a prior solid-state bond caused by uneven local thermal expansion. The electrode-to-electrode dynamic resistance curve when welding Au-plated material is not useful for monitoring the state of nugget formation.