Influence of post-welding heat-treatment on the monotonic and fatigue strength of 6082-T6 friction stir lap welds

Friction stir lap welds in thin sheets of AA6082-T6 aluminium alloy were performed at a constant tool rotation speed (600 rpm) and traverse speeds ranging from 250 to 1600 mm/min, using a non-threaded conical pin tool. As-welded and heat-treated samples were subject to tensile-shear and transverse-tensile testing in order to determine the joints strength and the weld materials mechanical properties, respectively. The presence of weld defects, as well as its location and severity, were evaluated by analysing the strain distribution in the welded samples, which was recorded using digital image correlation (DIC). The as-welded and heat-treated samples were also subject to tensile-shear fatigue testing. It was concluded that increasing the welding speed increases the incidence and severity of lap welding defects. In monotonic loading, the heat-treatment of the welds enabled to increase the yield strength efficiency of the joints free of defects to values close to 100%. In fatigue loading, the strength of the joints was decreased by heat-treating the welds. A FEM analysis was carried out in order to investigate this behaviour. It was concluded that the low strength of the weld material promotes a decrease in stress concentration at the non-welded tip of the joints, contributing to a better fatigue performance of the as-welded joints relative to the heat-treated ones.