Friction stir processing is an important method for acquiring ultrafine-grained materials. In this paper, 3mm ZK60 magnesium alloy sheet was carried for friction stir processing. The best processing parameters with a small grain size and maximum mechanical properties were obtained by comparing different rotation speeds and processing speeds. Fine recrystallized grains and high-angle grain boundaries were observed in stirring zone under different processing parameters. With increasing rotation speed, the grain size and high-angle grain boundary ratio increase; while with increasing processing speed, the grain size decrease, and the ratio of high-angle grain boundaries increase. When rotation speed and processing speed are 1400 r·min-1 and 100 mm·min-1, the processing plate have the largest ultimate tensile strength are 267.52 Mpa, that reached 84.62% of the base metals, and the yield strength, elongation and grain size are 166.97 Mpa, 15.32 % and 1.12 ± 1.64 µm, respectively. The processing plate has more excellent damping performance than rolled.

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Friction stir processing (FSP) is an important method for obtaining fine grains. To determine the effects of FSP and processing parameters on the microstructure and mechanical properties of rolled sheets, we performed single-pass FSP of a 2 mm thick 2524 aluminium alloy (AA2524) rolled sheet by comparing the combination of different processing parameters. The results show that lamellar grains (rolled state) are replaced by fine dynamic recrystallisation in the stir zone (SZ), and more Al2CuMg phases are precipitated simultaneously. As the rotation speed increases, the grain size and width of the pin stir zone (PSZ) increase, the microhardness first decreases and then increases; with the traverse speed increase, the grain size first decreases and then increases, and the width of the PSZ and microhardness decrease. The SZ has the smallest grain size, highest high-angle grain boundaries (HAGBs, with misorientation angles (θ) >15°) ratio, and largest ultimate tensile strength (UTS), when the rotation and traverse speed were 1000 r·min-1 and 125 mm·min-1, are 1.59 ± 0.82 µm, 0.91 and 451.23 ± 0.52 MPa, respectively, and the elongation to fracture is 13%. The UTS and elongation were only 95.12% and 98.48% of those of the base metals (BM), respectively, because of the significant decrease in the dislocation density. Fracture analysis revealed ductile fracture of the joint due to the large number of dimples and fine second-phase particles.

Friction stir spot welding (FSSW) is a clean, environmentally-friendly and cost-effective welding technology. To weld joints with improved mechanical properties, an FSSW experiment with a 2 mm-thick 2524 aluminum alloy sheet was performed to explore the influence of ambient cooling (AC), forced air cooling (FAC), waterflow cooling (WC), and an increasing rotational speed under WC, and to evaluate the welding method with regard to the resulting structural and tensile properties of the joint. The results showed that cooling-assisted welding reduced the width of the heat-affected zone (HAZ) and marginally increased the microhardness of the welding nugget zone (WNZ). The maximum tensile shear load (L) and effective width (W) values were 4673 N and 1958 μm at FAC, respectively, which were higher than the values of 4296 N and 1763 μm found with AC, respectively; in addition, the minimum values were 2946 N and 948 μm with WC, respectively. These results are not consistent with the idea that the joint strength can typically be improved with WC, because water absorbs a large amount of welding heat and reduces the plastic deformation capacity of the structure, thereby decreasing the W and L of the joint. Increasing the rotation speed of the welding tool can increase the heat input, which requires increasing the rotation speed along with WC. L and W reach their maximum values of 7652 N and 3320 μm, respectively, at 2500 r·min−1. As the rotation speed increases, L and W decrease. All joints underwent ductile fracturing, and the dispersion distribution of the second-phase particles at the bottom of the dimple exhibited good performance.