Effect of a cooling method on the structural and mechanical properties of friction stir spot welding with a 2524 aluminum alloy

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 μ m at FAC, respectively, which were higher than the values of 4296 N and 1763 μ m found with AC, respectively; in addition, the minimum values were 2946 N and 948 μ m 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 μ m, 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.