Effect of post-weld heat treatment on the mechanical behavior and dislocation density of friction stir welded Al6061

Abstract

Al6061 is a heat-treatable aluminum alloy and an extremely versatile material used in settings where medium to high strength is required. To produce good joints using this alloy, a particular solid-state welding process called friction stir welding (FSW) is preferable instead of the conventional welding methods. In this study, the effects of T6 post-weld heat treatment (PWHT) on the mechanical properties, dislocation density, and microstructure of a friction stir welded (FSWed) Al6061 aluminum alloy were investigated. Results indicated that FSW had degraded the mechanical properties of Al6061. The welding efficiency decreased to 65% compared with the tensile strength of the base metal. After FSW, the samples demonstrated a serrated flow behavior known as the Portevin–Le Chatelier (PLC) effect. This effect was attributed to the nonuniform distribution of hard Mg2Si particles, coarsening and/or solutionizing of the strengthening elements in the stir and thermomechanically affected zones, and an over-aging effect in the heat-affected zone of the FSWed sample. However, the T6-PWHT performed on the FSWed sample diminished the PLC effect and concurrently improved the mechanical properties back to its original state. The PWHT also promoted precipitation hardening through a better distribution of Mg2Si particles that prevented grain growth and increased the dislocation density due to the applied strain, which subsequently improved the mechanical properties of the FSWed Al6061 alloy.