Microstructural evolution and mechanical properties of friction-stir-welded Al-Mg-Si joint

This study was conducted to investigate joint and mechanical properties and fatigue strength of a friction-stir-welded aluminum alloy (FSW material), using a platy extruded shape of JIS A6N01S-T5 alloy as a substrate. Furthermore, changes in mechanical properties of FSW material in a corrosive environment were examined. The substrate possessed a typical cubic crystallographic texture in a platy extruded shape of aluminum alloy. Approaching the stir zone from the substrate side, the texture greatly changed and a pair of symmetrical preferred orientations was formed. In the stir zone, the texture gradually changed along the outside of the probe of a rotating tool. FSW refined the microstructure in the stir zone through dynamic recrystallization. In the heat-affected zone (HAZ), reduction in hardness was marked because no refinement of the microstructure occurred and also heat input by FSW weakened precipitation hardening. As a result, tensile fracture of the FSW material occurred along the HAZ; however, tensile strength was maintained at 81% of the substrate. Even after immersion for a long term in salt water kept at a relatively high temperature, no harmful influence of FSW on mechanical properties was found, and rather tensile strength recovered to the level of the substrate by re-aging during immersion. Moreover, although fatigue cracks were initiated from the root of burrs formed by FSW, fatigue strength was maintained at 88% of the substrate.