The Effects of Rotational Speed on Microstructure and Mechanical Properties of Friction Stir-Welded 7075-T6 Thin Sheet

In the present research, the effects of friction stir welding (FSW) on the microstructure of 7075-T6 aluminum alloy (AA) thin sheet were investigated. For this purpose, three different welding rotational speeds of 600, 1000 and 1600 rpm at the constant traverse speed of 50 mm/min were studied. It was observed that FSW of an AA7075 thin sheet leads to an irrecoverable drastic reduction in the tensile properties. However, it was proven that for a thin sheet, at different rotational speeds, totally very different mechanisms of microstructural evolution were present within the stir zone (SZ). Also, it was observed that the mean grain size and subgrain density of the SZ have a nonlinear relation with rotational speed. Electron backscattered diffraction analysis indicated that continuous and discontinuous dynamic recrystallization is present within SZ simultaneously as the mechanisms of grain refinement, but with different effects. It was confirmed that heat input of the welding process in FSW of a thin sheet has a negligible effect on the microstructure, although the plastic strain is the determinant parameter. Also, the relationship between the microstructural observations and mechanical properties of friction stir-welded AA7075-T6 is discussed.