Effect of Homogenization and Aging Treatment on Mechanical Properties and Stress-Corrosion Cracking of 7050 Alloys
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HIGH-STRENGTH 7000-series aluminum alloys have been widely used in aircraft structures because of their high strength/density ratio. This series of alloys provides high strength in the T6 temper but is susceptible to stress-corrosion cracking (SCC).[1,2] It is known that the resistance of SCC can be improved by overaging in the T73 temper. Unfortunately, however, as compared with the T6 temper, the strength is reduced by 10 to 15 pct. To satisfy the strength requirement in engineering applications, the size of the alloy parts must be enlarged. Therefore, it is essential to increase both the strength and the SCC resistance simultaneously in order to achieve more efficient and economic aircraft designs. Heat treatments for improving both the strength and the SCC resistance of 7000-series aluminum alloys have been studied. Cina[3,4] proposed a heat treatment known as retrogression and re-aging (RRA), in order to obtain an SCC resistance equivalent to that of the T73 temper combined with the same strength level of the T6. This RRA treatment is applied to the alloy in the T6 condition and involves a short period of heating (i.e., retrogression) at a specified temperature among approximately 200 C to 280 C, followed by a re-aging using BIN-LUNG OU, Associate Professor, is with the Department of Mechanical Engineering, National Central University, Chung-Li 320, Taiwan, Republic of China. JI-GANG YANG, Senior Specialist, is with the Chun-Shan Institute of Science and Technology, Lung-Tan, Taiwan, Republic of China. MON-YU WEI, Associate Professor, is with the Department of Automation Engineering, Ta Hwa Institute of Technology, Chiung-Lin, Hsin-Chu, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted June 27, 2005. Article published online July 14, 2007. 1760—VOLUME 38A, AUGUST 2007
conditions similar to those used for the original T6 aging. In the case of the research of Cina,[3,4] the time required to reach the minimum hardness on the retrogression stage was in the range of approximately 5 to 60 seconds and varied with the temperature of retrogression. These characteristics restrict the application of the treatment to thin sheets only. Afterward, Wallace et al.[5,6,7] developed a modified heat treatment using a lower retrogression temperature (160 C to 200 C) and a longer retrogression time (5 to 60 minutes), which allowed thicker components to also be treated, in comparison with the thin sheets investigated by Cina. Although the above RRA treatment can obtain a high SCC resistance similar to the T73 condition and a high strength similar to that of the T6 temper (i.e., without sacrificing their strength), the entire heat-treating time, including solution and aging, is actually much longer than that of conventional T6. The longer time may limit the use of such a treatment in industry. Therefore, developing a novel and practical process for improving both the strength and SCC resistance is important. Aging sequences of the 7000-series Al-Zn-Mg-(Cu) alloy[8] can be generalized as fo
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