Resistance spot weld fatigue behavior and dislocation substructures in two different heats of AZ31 magnesium alloy

Fatigue life and dislocation substructure were evaluated in two groups of AZ31 Mg alloy resistance spot welds (heats SA and SB respectively, which had similar chemical compositions but different fusion zone microstructures). The results showed that the SA welds with a refined microstructure displayed a higher fatigue resistance than the SB welds when tested under conditions of higher cyclic load range causing interfacial failure across the fusion zone. TEM examinations revealed that typical dislocation configurations in the coarse-grained SB welds were parallel dislocation lines and parallelogram dislocation cells produced by basal slip, while elongated dislocation cells arising from basal and pyramidal multiple slips occurred in the SA welds. Twinning was observed to occur in both SA and SB welds, with more twins present in the SB welds. The strong slip incompatibilities between adjacent dendritic grains led to high local stress concentrations that activated twinning in the coarse-grained SB welds, while pyramidal slip together with twinning occurred in the fine-grained SA welds. This resulted in increased number and dispersion of slip systems which improved fatigue life in the SA welds.