Effect of shot peening on mechanical and fatigue properties of AZ31 magnesium alloy processed by multi-axial alternative forging

To improve the fatigue properties of AZ31 magnesium alloys, two technical approaches were used to reform the material structure. The first approach is grain refinement of the material structure by multi-axial alternative forging (MAF) to reduce anisotropy and to improve the strength. The second approach created an inclined recrystallized surface layer by shot peening to retard crack initiation on the surface layer and the propagation rate in the tissue layer of the fine-grained structure. The relationship between the microstructures and fatigue properties was examined. As a result, by combining MAF and heat treatment the endurance limit increased by 15%. Moreover, shot peening at medium temperatures formed a recrystallized submicrometer grain size microstructure in the surface layer and an inclined structure to a depth of over 100 μm below the surface, and the fatigue properties increased by about 30%. From measurement of the crack propagation rate, it was found that the improvement in fatigue properties led to the delay in crack initiation in the fine-grained surface layer and decreased the crack propagation rate in the residual strained layer. © 2012 The Japan Institute of Light Metals.