Surface Grain Refinement of AZ91D Magnesium Alloy by Cold Spraying Shot Peening Process

In this paper, a new technology for surface grain refinement of AZ91D magnesium alloy induced by cold spraying shot peening (CS-SP) was investigated, and the influencing rule of the surface layer deformation and grain refinement process were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Meanwhile, the phase structure of surface layer in the AZ91D specimen before and after CS-SP treatment was analyzed using X-ray diffractometer (XRD). The microhardness distribution from the topmost surface to the matrix of the CS-SPed AZ91D specimen was tested by a microhardness tester and the wear performance of AZ91D alloy before and after CS-SP was studied using the sliding wear tester. Results showed that sub-micrometer even nanocrystallization surface layer of the AZ91D alloy was prepared by the CS-SP process and grain size of the topmost surface was about 50 nm estimated by XRD data and also observed by TEM. The microhardness of the topmost surface layer was increased up to 160Hv_0.025 above twice as that of the substrate. CS-SPed AZ91D alloy exhibited the lower friction coefficient and the higher wear resistance compared to that of untreated specimen and the wear mechanism was discussed.