Effects of electrical current on transport processes in resistance spot welding

The effects of alternating current (ac) and direct current (dc) on cooling rate, solute distribution and nugget shape after solidification, which are responsible for microstructure of the fusion zone, during resistance spot welding, are realistically and extensively investigated. The computer program developed by Wang and Wei is used to predict transport variables in workpieces and electrodes during heating, melting, cooling and freezing periods. The model accounts for electromagnetic force, heat generations at the electrode/workpiece interface and faying surface between workpieces, and dynamic electrical resistance including bulk resistance and contact resistances at the faying surface and electrode/workpiece interfaces, which are functions of hardness, temperature, electrode force and surface condition. The computed results show that in contrast to dc, using ac readily produces the nugget in an ellipse shape. Deficit and excess of solute content occur in a thin layer around the boundary and interior of the nugget respectively.