On the precipitation-hardening behavior of the Al-Mg-Si-Cu alloy AA6111
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THERE has been a resurgence of research interest in aluminum alloys in recent years, mainly due to the increasing demand for the utilization of lighter materials in the automotive industry. The heat-treatable, i.e., precipitation-hardening, aluminum alloys are of special interest for automobile outer panels, where high strength and dent resistance are required, and for bumpers, where good strength and shock absorption are needed. In both cases, good formability is also an important requirement. The heat-treatable 6000-series AlMg-Si(-Cu) alloys have been the material of choice for skin panels. The alloy AA6111, developed by Alcan in 1983,[1] has been chosen for skin-panel applications by North American auto manufacturers because it has a combination of good formability and high strengthening potential. Recently, the 6000-series alloys have also been considered for aerospace applications.[2,3,4] Despite the high strengthening potential of Al-MgSi(-Cu) alloys, processing factors such as the hold time between the solution treatment and the artificial aging processes, as well as the short duration of artificial aging, can seriously impede the strengthening process. The so-called paint-bake cycling (PBC) process, which is a thermal cycling process[5] designed to provide both paint curing and precipitation-hardening for the auto panels, involves a relatively short-duration artificial aging process, which may be simulated in the laboratory by aging for 30 minutes to 1 hour at 180 °C. It has been reported that natural aging prior to the S. ESMAEILI, formerly Ph.D. Student, Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4, is Industrial Research Fellow, Kingston Research and Development Centre, Alcan International Limited, Kingston, ON, Canada K7L 5L9. Contact e-mail: [email protected] X. WANG, Postdoctoral Fellow, is with the Department of Materials Science and Engineering, McMaster University, Hamilton, ON, Canada L8S 4L7. D.J. LLOYD, Principal Scientist, is with the Kingston Research and Development Centre, Alcan International Limited. W.J. POOLE, Associate Professor, is with the Department of Metals and Materials Engineering, University of British Columbia. Manuscript submitted September 25, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
PBC process has an adverse effect on the age-hardening response of Al-Mg-Si(-Cu) alloys, considerably limiting the final strength.[5,6] The adverse effect of natural aging on AlMg-Si(-Cu) alloys when aged to more advanced stages has also been reported.[7] The age-hardening response of the AA6111 alloy has been the subject of several studies in the past few years.[5,6,8–14] The areas of interest have included the natural aging response, the artificial aging response of the naturally aged material, as well as the influence of preaging on subsequent age hardening. All related studies have confirmed that natural aging does, indeed, have a deleterious effect on the artificial aging response of the alloy. This effe
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