Monotonic and Fatigue Response of Heat-Treated Friction Stir Welded Al 6061-T6 Joints: Testing and Characterization

The present study evaluates the influence of post-weld heat treatment on the fatigue strength of friction stir welded aluminum 6061-T6 joints. The solution-treatment artificial aging (STA) was applied to the friction stir welding samples prior to cyclic tests. Hardness distributions along the centerline of the welded specimens as well as the tensile properties and microstructural features were initially examined. The stress-controlled cyclic tests were then conducted at constant amplitude loading with the load ratio of R = 0.1. Experimental results revealed that tensile strength of STA heat-treated samples increased 73 % as compared to the as-welded (AW) joints. Furthermore, the STA heat-treated specimens experienced 28 % higher elongation than those of AW samples. A 3D finite element analysis was developed to (i) simulate residual stress distribution of the welded joints after heat treatment processing and (ii) analyze stress/strain components at weld toe induced as a result of applied loading cycles. The heat-affected zone with critical local stress/strain values was found to be the most vulnerable zone for cracking. Predicted fatigue lives by means of the Smith, Watson, and Topper model were found in close agreement with those experimentally obtained values.