Influence of Shot Peening on Fatigue Properties of Extruded Magnesium Alloy AZ80 and Its Anisotropy

The influence of shot peening on the fatigue strength in an extruded magnesium alloy AZ80 treated with T6 was investigated and its anisotropic behavior was discussed. The specimen was machined from different orientations in as-extruded rod. One was loaded in the transverse direction (T specimen). The other was loaded in the longitudinal direction (L specimen). The arc height value of shot peening was set to 0.15 and 0.40 mmN. The specimen surface became rough and hardness became high with an increase in the arc height value. Irrespective of specimen orientation, shot peening at low arc height value improved the fatigue strength. However, the shot peening at high arc height value did not improve the fatigue strength. The same tendency appeared in the case of T4-treated sample, suggesting that precipitation did not affect the fatigue strength degradation by hard shot peening. To investigate the influence of surface roughness formed by shot peening, fatigue tests were conducted using some specimens polished after shot peening. In the L specimen, the polishing after shot peening improves the fatigue life significantly. Therefore, the surface roughness formed by shot peening is one of the important factors in fatigue strength of shot-peened specimen. On the other hand, the fatigue life of T specimen decreased by polishing after shot peening. It was suggested that microstructural damage induced by shot peening decreases the fatigue strength in T specimen.