Effect of rotation speeds on microstructures and tensile properties of submerged friction stir processed AZ31 magnesium alloy

Submerged friction stir processing (SFSP) is a novel severe plastic deformation technique conducted underwater, which has great potential in preparation of fine-grained materials. In this study, AZ31 magnesium alloy is subjected to FSP in air (NFSP) and underwater at a fixed processing speed (60 mm min(-1)) and various rotation speeds (1100, 1300 and 1500 rev min(-1)), and the effect of rotation speeds on microstructures and tensile properties of the experimental materials is investigated. As the rotation speed increases, the grain size in the stir zone (SZ) increases. SFSP results in remarkable grain refinement compared with NFSP. The tensile properties of the NFSP and SFSP specimens decrease with the rotation speed increasing. Owing to much finer microstructure, SFSP results in improvement of tensile properties in comparison with NFSP, especially for the ductility. The fracture surfaces of the SFSP specimens exhibit much more uniform dimples and the dimples are much deeper.