Simulating the Residual Stress in an A356 Automotive Wheel and Its Impact on Fatigue Life
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Simulating the Residual Stress in an A356 Automotive Wheel and Its Impact on Fatigue Life P. LI, D.M. MAIJER, T.C. LINDLEY, and P.D. LEE Keeping the weight of unsprung rotating components low is critical for fuel efficiency in automobiles; therefore, cast aluminum alloys are the current 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. I.
INTRODUCTION
LIGHT weight wheels with good fatigue performance are required to reduce the fuel consumption of transport vehicles, especially as they are unsprung rotating components. Automotive wheels have a complex geometry and must fulfill multiple design criteria: light weight, high strength, good fatigue life, and visual aesthetics. Wheels cast in an A356 aluminum alloy and T6 heat treated fulfill these criteria. Such wheels are typically produced using low pressure die casting, followed by rough machining prior to a T6 heat treatment. Once heat treated, final machining is performed to satisfy dimensional requirements and a multistage paint process is employed to give the desired color and surface finish. Each of the processing steps (casting, heat treatment, and rough/final machining) can have a significant impact on the component’s in-service performance, particularly the fatigue life. The solidification behavior during the casting process defines the initial microstructure and defect distributions. Microsegregation, although partially homogenized during the heat treatment, can reduce the yield strength and thus diminish fatigue life. Pores formed at or in close proximity to the surface can act as crack initiators in the final component.[1,2,3] Rough machining following casting is perP. LI, Ph.D. Student, T.C. LINDLEY and P.D. LEE, Professors, are with the Department of Materials, Imperial College, London, SW7 2AZ, United Kingdom. Co
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