Effect of an Al filler material on interfacial microstructure and mechanical properties of dissimilar friction stir welded Ti/Mg joint

Abstract

Dissimilar friction stir welding (FSW) of immiscible pure Ti/pure Mg using an Al filler material with a thickness of 0.05 mm was optimized through tuning welding speeds ranging from 50 mm min−1 to 200 mm min−1 at a constant tool rotation speed and a constant probe offset. With a lower welding speed, the average grain diameter in the Mg stir zone of the Ti/Mg joints became smaller even though the higher welding temperature and lower strain rate were achieved. This is because the Al concentration dissolved in the Mg matrix increased at the lower welding speed, leading to a reduction in the stacking fault energy thereby facilitating the occurrence of dynamic recrystallization rather than dynamic recovery during the FSW. The relatively high hardness value in the Mg stir zone of the Ti/Mg dissimilar joint fabricated at the lower welding speed was attributed not only by the grain refinement strengthening, but also by the solid solution strengthening due to the Al dissolving in the Mg matrix, while the increase in hardness was suppressed near the weld interface in the joint fabricated at the higher welding speed due to the decreased welding temperature and stirring effect in the Mg stir zone.