Metallurgical factors affection Vickers hardness of friction stir welded joint of 7075 aluminum alloy

Vickers hardness of friction stir welded 7075-T6 and 7075-O aluminum alloy plates were investigated. During welding, the tool rotation and tool traverse speeds were 1500 rpm and 300 mm/min, respectively. In order to study the anisotropy in the welded joints, the joints were welded in directions parallel and right angle to the rolling direction. Defect-free joints were obtained under the above condition. A fine and equiaxed recrystallised microstructure with grain size of approximately 3 μm was formed in the stir zone of the joints. Anisotropy in welded joints along the welding directions was not detected in terms of the microstructural characteristics of the joints. After friction stir welding, the Vickers hardness of the stir zone of the T6 joints decreased, but that of the O joints increased. On the other hand, after natural aging for 5 months, the Vickers hardness of the stir zone of the T6 joints increased and closely reached the hardness level of the base metal, while the Vickers hardness of the stir zone of the O joints increased further. From the above-mentioned Vickers hardness distributions, no anisotropy was observed along the welding direction. The above-mentioned behavior in hardness of the joints was discussed briefly on the basis of the Hall-Petch relation as well as other strengthening mechanisms.