Effect of tool rotational speed on residual stress, microstructure, and tensile properties of friction stir welded 6061-T6 aluminum alloy thick plate

In the present study, the effect of tool rotational speed on residual stress, microstructure, and tensile properties of friction stir welded AA 6061-T6 was investigated. AA 6061-T6 plates with a thickness of 16 mm were friction stir welded using three different tool rotational speeds of 500, 700, and 900 rpm and a constant welding speed of 120 mm/min. The results indicate that the longitudinal residual stress distribution is an M-shaped distribution in the transverse direction; in weld region, the tensile longitudinal stress in the advancing side is larger than that in the retreating side. With increasing rotation speed, the values of the tensile longitudinal residual stresses increased slightly, the peak tensile longitudinal stresses appear at the edge of the shoulder in the AS of the joints, and the maximum value of tensile longitudinal residual stress was 153 MPa with the rotation speed of 900 rpm which reaches up to 55 % of the yield strength of AA 6061-T6. Meanwhile, only the appearance of welded joint with 700 rpm is smooth. The grain size in WNZ grows up with increasing the rotation speed. The microhardness variation is determined by microstructure evolution, and the average microhardness decreases with increasing the rotation speed. The welded joint with the rotation speed of 700 rpm performs the maximum tensile strength of 236 MPa which is 76 % of the base material strength.