Simulating the residual stress in an A356 automotive wheel and its impact on fatigue life

Keeping the weight of unsprung rotating components low is critical for fuel efficiency in automobiles; therefore, cast aluminum alloys are the cur-rent material of choice for wheels. However, pores formed during solidification can combine with residual stresses and in-service loads to reduce the fatigue life of this safety critical part. In this study, a model of the residual stresses arising from the quench stage of a T6 heat treatment was developed. The resulting predictions were compared to residual strain measurements made on quenched wheels via a strain gage/sectioning technique. The predictions were shown to be sensitive to the alloy's flow stress behavior, yet no data were available for the temperature-dependent and strain-rate-dependent inelastic behavior of A356 in the as-solutionized condition. Measurements of this behavior were made using a GLEEBLE 3500, and the data were incorporated into the model, significantly improving the correlation between model and experiment. In order to determine the influence of residual stress upon the final fatigue performance of the wheel during service, the change in stress level due to machining was first calculated. The residual stress was then compounded together with a service stress to determine the local stress at all points in the wheel during idealized operation. Finally, the fatigue behavior was predicted using a unified initiation and propagation model based on this local stress and an idealized pore size.