Effect of shot peening on the fatigue behaviour of cast magnesium A8

Shot peening is known to improve the fatigue performance of structural metallic materials. The improvement in fatigue life is derived primarily from compressive residual stresses that are introduced into the near-surface of the components and which hinder crack initiation and growth. Magnesium alloys are finding increasing use in automotive applications, but their relatively low strength means that they are highly susceptible to fatigue failures. This is particularly the case for cast alloys which may contain high levels of porosity. Shot peening may be of use, but the beneficial effect of the compressive stress may be offset by the surface damage associated with peening of a soft material. In this study the fatigue life of sand-cast A8 magnesium alloy has been investigated before and after a shot peening treatment to investigate whether shot peening is beneficial for a component with this combination of relatively low strength and relatively poor initial surface finish. Previous studies into the effect of shot peening on magnesium alloys have been limited to wrought alloys and there has been little work on the influence of shot peening on cast magnesium alloys. The residual stress before and after peening was determined by incremental hole drilling which shows that the peening process generated a compressive residual stress in the cast specimens. The fatigue results show that the fatigue life is significantly improved by the shot peening process, and there is also an improvement in the endurance limit. An increase in the surface roughness of the samples was found after peening but this was not found to be detrimental to the fatigue performance.