Using Response Surface Methodology to Model the Age Hardening of AA6061
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A. Aging Behavior of 6000-Series Aluminum
HEAT-TREATABLE aluminum alloys, including AA6061, are widely used in the automobile industry due to the high ratio of strength to weight and the ability of age hardening. Because the AA6061 is among the heat-treatable aluminum alloys, it is essential to determine the optimal conditions to attain the maximum strength during its manufacturing process. Thus, with applying the proper thermomechanical treatments before final aging, one is able to improve significantly the final mechanical properties of age-hardenable materials. These have been done by Dehghani et al. in the case of ultra- and low-carbon steels.[1–4] Also, the authors recently modeled and optimized the paint baking of AA7075 using response surface methodology (RSM).[5] In the case of aluminum alloys, it is also reported that preaging before final aging can exhibit significant influence on the final aging response of Al6xxx alloys.[6–9] Another important factor affecting the aging response is the prestraining[10] or cold rolling.[7] The sheets used as car bodies/panels are normally stored for some time before they are finally stamped KAMRAN DEHGHANI, Associate Professor, and ATIYE NEKAHI, Master Student, are with the Department of Mining and Metallurgical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran. Contact e-mail: atiye.nekahi@ gmail.com Manuscript submitted November 26, 2009. Article published online August 26, 2010 3228—VOLUME 41A, DECEMBER 2010
followed by an final aging treatment (so-called paint baking) at the automobile plants. Hence, the preaging treatment applied in the present work can somehow simulate the natural aging occurred during the storage of aluminum products. As for the cold rolling, one practical approach to improve the mechanical properties of aluminum alloys is applying deformation before or after aging. This is because, in terms of industrial applications, some of the aluminum tempers such as T3, T8, T9, and T10 combine cold working with aging. These tempers are applied to products that are cold worked before or after they have been precipitation heat treated to improve their strength. In other words, applying cold working is unavoidable during the manufacturing processes of wrought aluminum alloys. For example, in processes such as extrusion, forging, and rolling, the final products are subjected to appreciable deformation so as to produce the desirable shapes. In addition, in many industrial processes, the cold work is applied to products for flattening or straightening. Thus, the cold rolling is employed here to simulate the aforementioned deformation steps involved in the manufacturing process of aluminum alloys. In general, deformation is associated with the formation of a large amount of microstructural defects such as dislocations, shear bans, and vacancies. The presence of these microstructural inhomogeneities can significantly affect the precipitation kinetics during the subsequent aging to improve the mechanical properties. In the present work, the a
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