The effects of shot peening intensity and coverage on the surface properties and fatigue life of AZ31B magnesium alloy

Magnesium alloys are attractive materials in many sectors such as the automotive or the aerospace due to their impressive strength-to-weight ratio. However, the applications of magnesium alloys in these sectors demand high fatigue strength. Shot peening is widely used to improve the fatigue behaviour of metallic materials, but magnesium alloys are susceptible to overpeening due to their low hardness, which can hinder the improvement of fatigue behaviour. This research work aims to study the effects of fine particle shot peening treatments on the surface properties and fatigue life of a magnesium alloy AZ31B. These shot peening treatments induced twinning and grain refinement at the surface of the alloy, which is beneficial for the mechanical properties. A factorial design was developed in order to analyse the influence of Almen intensity and coverage on the roughness, wettability, hardness, residual stress and fatigue life of the alloy, and regression models were calculated for each response variable. Higher Almen intensities led to higher roughness values and more wettable surfaces, but coverage had no significant effect on these variables. Surface hardness and compressive residual stresses increased when using either high intensities or high coverages. Regression models were calculated and allowed to predict these experimental results with adjusted R 2 values higher than 0.90. Moreover, a significant regression model of fatigue life was obtained as a function of Almen intensity and coverage. The use of lower Almen intensities increased the range of coverage values that maximized the fatigue life of the alloy. The optimal shot peening parameters turned out to be an Almen intensity not higher than 0.05 mm A and coverage between 200% and 300%. At such conditions, fatigue life increased by a factor as high as 100 with respect to the non-treated alloy. • Shot peening was investigated on AZ31B alloy to improve fatigue behaviour. • Shot peening causes strain hardening by twinning and grain refinement. • Regression models (R 2 > 0.9) can accurately predict surface properties. • Shot peening can increase fatigue life of AZ31B alloy up to a factor of 100. • Optimal fatigue life occurs for intensity <0.05 mm A and coverage 200–300%.