Effect of high rotational speed on temperature distribution, microstructure evolution, and mechanical properties of friction stir welded 6061-T6 thin plate joints

High rotational speed friction stir welding (FSW) was successfully employed to weld 6061-T6 aluminum alloy thin plates. The effect of high rotational speed and fast transverse speed on temperature distribution, microstructure evolution, and tensile properties of friction stir welded 6061-T6 joints was investigated in detail. The high rotational speed with fast transverse speed had a significant influence on the peak temperature in the nugget zone (NZ). Increasing the rotational speed and decreasing the transverse speed could obviously improve the peak temperature in the NZ, but exhibit little effect on the heat-affected zone under high rotational speed FSW. The NZ was characterized as a significant refinement of the equiaxed grain. The number of precipitates, subgrains, and low angle grain boundaries in the NZ of high rotational speed FSW joint increased significantly due to moderate heat input and strain rate. The weld zone was seriously softened at low rotational speed, whereas it was alleviated at high rotational speed and was affected slightly by rotational speed and transverse speed. The excellent mechanical properties of the friction stir welded 6061-T6 joints were obtained at a combination of high rotational speed and fast transverse speed. The maximum tensile strength reached 301.8 MPa, 85.8% of the base material.