Abstract
Resistance spot welding (RSW) of galvannealed and bare dual phase (DP600) steel is a challenging task as it involves the additional complexity of zinc coating layer. This study is focused on optimization of spot welding parameters, viz., welding current and time to achieve maximum load bearing capacity in tensile-shear and coach-peel configurations. Study revealed a minimum critical nugget diameter is needed above which favorable pull-out failure is observed. Critical nugget diameter for pull-out failure is determined to be 4.4 mm. Applicability of different empirical and metallurgical models for determination of critical nugget diameter were assessed. Transition from partial interfacial to pullout failure mode has been explained on the basis of combined effect of increased nugget diameter and micro-mechanism of fracture. It was observed that the there was an increase in nugget diameter and load bearing capacity of the spot welds with an increase in welding current as well as welding time. Best fit equations have been proposed for maximum load bearing capacity as a function of spot welding parameters and nugget diameter. Microstructural evolution and hardness variation across different welding zones have been presented in detail. Redistribution of zinc in different zones of welding has been evaluated using electron probe micro-analysis.
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