Modeling dynamic electrical resistance during resistance spot welding

Dynamic electrical resistance during resistance spot welding has been quant itatively modeled and analyzed in this work. A determination of dynamic res istance is necessary for predicting the transport processes and monitoring the weld quality during resistance spot welding. In this study, dynamic res istance is obtained by taking the sum of temperature-dependent bulk resista nce of the workpieces and contact resistances at the faying surface and ele ctrode-workpiece interface within an effective area corresponding to the el ectrode tip where welding current primarily flows. A contact resistance is composed of constrictions and film resistances, which are functions of hard ness, temperature, electrode force, and surface conditions. The temperature is determined from the previous study in predicting unsteady, axisymmetric mass, momentum, heat, species transport, and magnetic field intensity with a mushy-zone phase change in workpieces, and temperature and magnetic fiel ds in the electrodes of different geometries. The predicted nugget thicknes s and dynamic resistance versus time show quite good agreement with availab le experimental data. Excluding expulsion, the dynamic resistance curve can be divided into four stages. A rapid decrease of dynamic resistance in sta ge 1 is attributed to decreases in contact resistances at the faying surfac e and electrode-workpiece interface. In stage 2, the increase in dynamic re sistance results from the primary increase of bulk resistance in the workpi eces and an increase of the sum of contact resistances at the faying surfac e and electrode-workpieces interface. Dynamic resistance in stage 3 decreas es, because increasing rate of bulk resistance in the workpieces and contac t resistances decrease. In state 4 the decrease of dynamic resistance is ma inly due to the formation of the molten nugget at the faying surface. The m olten nugget is found to occur in stage 4 rather than stage 2 or 3 as quali tatively proposed in the literature. The effects of different parameters on the dynamic resistance curve are also presented.