| The mechanisms of heat generation and the resultant physical effects leading up to nugget development during small-scale resistance spot welding (SSRSW) of Ni sheets have been investigated. The primary investigative tool has been measurement of dynamic resistance during the joining process. This has been coupled with post-weld examination using tensile-shear testing, optical microscopy, scanning electron microscopy and energy dispersive x-ray spectroscopy.; For bare Ni sheets, the results showed no significant tendency to solid state bonding during the process: the first evidence of metallurgical bonding coincided with occurrence of incipient melting at the sheet-to-sheet interface. The dynamic resistance curve could be divided into the following stages, based on physical changes in the workpieces: asperity heating, surface breakdown, asperity softening, partial surface melting, nugget growth and expulsion. The sheet-to-sheet voltage drop required to initiate melting on bare Ni was very close to the value previously determined for softening/melting voltage for Ni. The static and dynamic resistance behaviour showed clear evidence of both constriction resistance, and film resistance associated with the oxide film on the material surfaces. It was found possible to separate these two components of faying surface contact resistance as a function of time and temperature during the process.; The application of the SSRSW process to Au-plated Ni sheet has also been studied. The presence of the Au plating caused a major alteration of the surface characteristics of the Ni sheets, and this led to a major alteration of the sequence of physical joining processes, involving solid-state bonding and brazing as well as fusion welding. In order to obtain a strong joint, fusion welding was no longer necessary for Au-plated Ni since brazed joints of Au-plated Ni made with high welding currents were stronger than the bare Ni joints even without a fusion nugget formed.; Compared to the 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 the lack of surface oxide film. (Abstract shortened by UMI.)... |
