The Effect of Aging on the Relaxation of Residual Stress in Cast Aluminum
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INTRODUCTION
THE use of aluminum in automotive applications has increased significantly over the last decade. Currently, aluminum castings are used in place of numerous traditional iron and steel components to reduce weight and improve fuel economy in automobiles. In addition, the casting process also allows for the consolidation of several components into single casting, thereby reducing cost and providing value addition to the automobile. Part consolidations, demanding design requirements and high geometric tolerances, have resulted in very complex automotive castings like engine blocks and cylinder heads. To exceed the durability requirements for engine applications, the number of cast aluminum alloys that are being precipitation hardened has been steadily increasing. Precipitation hardening is a heat-treatment process which includes solution treatment, quenching, and an aging treatment.[1,2] The thermal gradients induced by rapid water quenching or water spray can lead to a substantial amount of residual stress, which, in certain cases, can result in permanent, measurable part distortion and residual internal stresses.[3–7] This situation is often the case in components such as engine blocks and cylinder
LARRY A. GODLEWSKI, Research Engineer, and XUMING SU, Technical Specialist, are with Ford Research and Innovation Center, Ford Motor Company, Dearborn, MI. Contact e-mail: [email protected] TRESA M. POLLOCK, formerly Thesis Advisor with NAE, University of Michigan, Ann Arbor, MI, is now Professor of Materials, with NAE, UC, Santa Barbara, CA. JOHN E. ALLISON, Professor of Materials, Thesis Advisor, is with NAE, University of Michigan. Manuscript submitted March 28, 2012. Article published online May 23, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
heads that undergo water-quenching operation during the course of their heat-treatment process. An example of this can be found in certain cast aluminum cylinder blocks that contain cast iron cylinder liners. Upon water quenching, residual stresses will arise because of two primary reasons. The first reason is attributed to dissimilar coefficients of thermal expansion between the aluminum and cast iron cylinder liner. Second, the residual stresses will arise because the complex structure of the block creates large thermal gradients within the block during quenching. If the residual stress is not properly relieved, then it can lead to distortion and/or cracking, resulting in reduced durability of the cast component. Therefore, a need exists to quantitatively understand how the heat-treatment process affects the residual stress at each step through the casting process, so that appropriate steps can be taken to minimize the amount of residual stress in a casting.[8–11]
A. Residual Stresses Residual stresses are very difficult to predict in a casting; an inverse modeling approach is being developed at Ford Research Laboratory to predict the amount of residual stresses present in a casting. This approach is outlined in Figure 1, which describes the required tools that are us
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