Influence of welding parameter on mechanical properties and fracture behavior of friction stir welded Al–Mg–Sc joints

8.1-mm thick Al–Mg–Sc plates were friction stir welded (FSW) at tool rotation rates of 400–800 rpm and welding speeds of 100–400 mm/min to investigate the effect of welding parameter on the microstructure, mechanical property and fracture behavior of joints. The lazy S and kissing bond, with quite different morphologies, were observed in the stir zone at 400 and 600 rpm, attributing to insufficient material flow. However, neither of them were detected at 800 rpm. Ultimate tensile strength of the joints was nearly equal to that of the base metal with the joint efficiency being 97–99%. Facture behavior of the joints changed with the welding parameter and it was strongly affected by the lazy S and kissing bond. For the joints at 400 and 600 rpm, fracture occurred partially along the lazy S at 100 mm/min or initiated at the kissing bond at 400 mm/min. However, the joints at 800 rpm exhibited a fracture independent of the lazy S or kissing bond. Furthermore, the present observations were compared with those made by other researchers and the reasons for insufficient material flow at lower rotation rates of 400 and 600 rpm were discussed.