Effects of shot peening parameters on gradient microstructure and mechanical properties of TRC AZ31

Microstructural variation along the depth of twin-roll cast sheet AZ31 Mg alloy was achieved by applying severe plastic deformation via intensified shot peening on the surface of the work-piece. Mechanical incompatibility between different layers from the ultra-fine crystallized structure on the peened surface to the un-deformed layers on the specimen interior introduces enhanced mechanical properties. AZ31 has shown distinct sensitivities to various parameters of the shot peening procedure such as pressure, shot size, and processing time. Thus, shot peening with shot sizes from less than 0.4 mm (conventional) to 3.18 mm for different processing times and under various pressures were performed to characterize their roles in controlling the microstructure and mechanical properties. Microstructural analysis revealed a direct relation between the thickness of the ultra-fine grained layer and severe shot peening parameters whereas each of them has a distinct effect on grain size. Electron backscatter diffraction data were used to define three distinct deformed layers wherein grain size and texture were changed during the process. Furthermore, microhardness tests demonstrated how pressure and shot size control fine grains at the surface. Tensile test results revealed that the best mechanical properties were obtained by maximum shot size and pressure at minimum processing time. Adding post-annealing treatment to the severely shot peened specimens enhanced ductility without a noticeable loss of ultimate strength. Consequently, this process has improved strength and ductility simultaneously.