Softening behaviour of friction stir welded Al 6061-T6 and Mg AZ31B alloys

The influence of friction stir welding (FSW) on changes in microstructures has been studied extensively. In particular, the microstructural softening, which is manifested as a significant reduction in the hardness, tensile strength and residual stress, is one of the most significant issues. Here, we provide a brief review of the dominant softening mechanisms in FSW aluminium and magnesium alloys available in the literature. In particular, a direct comparison between a heat treatable Al 6061-T6 alloy and an AZ31B Mg alloy is presented to highlight key differences in the underlying mechanisms responsible for the apparently similar softening behaviour. The present study shows that the softening occurs in the wide region of the FSW Al 6061-T6 alloy due to the dissolution of the strengthening precipitates, while it happens mostly within the stir zone due to the localised texture variations in the FSW Mg AZ31B alloy. These softening behaviours are clearly represented in the residual stress profiles.