Improvement of the fatigue strength of aluminum alloy castings by shot peening and clarification of the improving factor.

The employment of aluminum alloy castings for automotive parts is increasing every year, and recently some of them require relatively high fatigue strength. Under these circumstances, the improvement of their fatigue strength is an important problem to be solved.One of the authors has found that it is very hard to improve the strength, especially the fatigue limit of aluminum alloy castings, either by T6 treatment or by changing their chemical compositions. Although numerous literatures show that shot peening increases the fatigue strength of many metallic materials, few studies deal with aluminum alloy castings.In this study, the effect of shot peening on the improvement of the fatigue strength of aluminum alloy castings was examined first. Secondly, the improving factor of the fatigue strength of the shot peened castings was identified through the other experiments with the upset specimens of the castings, and finally an evaluation method for the fatigue limit of shot peened aluminum alloy castings was proposed. The following conclusions were derived from the present study:(1) The fatigue strength of aluminum alloy castings was able to be improved with shot peening.(2) The improving factor is a compressive residual stress at the surface layer resulting from shot peening, and the fatigue limit is evaluated by dealing with the residual stress as the mean stress during cyclic loading.(3) Hardening at the surface layer resulting from shot peening does not contribute to the improvement of fatigue strength in aluminum alloy castings.