Improved Resistance to Stress-Corrosion-Cracking Failures via Optimized Retrogression and Reaging of 7075-T6 Aluminum Sh
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INTRODUCTION
7XXX series aluminum alloys are lightweight materials that are widely used in the aerospace industry because of their superior specific strength. The main strengthening mechanism for these alloys is artificial aging commonly known as T6 temper. Although T6-tempered 7XXX series alloys are satisfactorily used in many engineering applications, they suffer from stress corrosion cracking (SCC), especially when working in environments containing chloride. Another aging route known as T73 temper has been developed to overcome this drawback. However, T73 temper is accompanied by a loss in strength of about 10 to 15 pct.[1] In 1974, Cina[2] proposed a new aging process called retrogression and reaging (RRA) to improve the SCC resistance of 7075 alloy without significant loss in strength when compared to the T6-temper state. To this date, several articles[3–8] have been published on the mechanical properties and corrosion resistance attained after the application of RRA treatment to 7XXX series alloys. These works revealed that SCC resistance close to T73 temper and strength similar to T6 temper could be obtained by optimizing the RRA parameters. Industrial entities such as Alcoa patented a heat-treatment procedure based on MURAT BAYDOGAN, Assistant Professor, and HUSEYIN CIMENOGLU and E. SABRI KAYALI, Professors, are with the Department of Metallurgy and Materials Engineering, Istanbul Technical University, 34469 Maslak—Istanbul, Turkey. JAHAN RASTY, Associate Professor, is with the Mechanical Engineering, Department, Texas Tech University, Lubbock, TX, 79409-1021. Contact e-mail: [email protected] Manuscript submitted July 17, 2006. Article published online August 7, 2008 2470—VOLUME 39A, OCTOBER 2008
RRA (designated as T77) in the 1980s for 7050, 7055, and 7150 alloys to be used in structural airframe applications requiring a combination of high strength and toughness with improved corrosion resistance.[9] The RRA is applied to T6-tempered 7XXX series alloys in two successive steps, namely, retrogression and reaging. In the first step (retrogression), the alloy is held at a temperature above the original aging temperature of the T6 temper and then water quenched. In the second step (reaging), it is reaged at the same temperature and time as that of T6 temper. The microstructure developed during retrogression has been extensively studied.[10–14] Dahn et al.[10] and Park and Ardell[11] explained the variation in the microstructure in three regions as a function of retrogression time. At short retrogression times (region I), main microstructural constituents of the T6-temper state, such as GP zones and small g¢ precipitates, dissolve. In region II, where retrogression time is longer, the matrix enriched in zinc and magnesium due to the dissolution encourages the formation of new g¢ precipitates, while existing g¢ precipitates grow and transform to g. On the other hand, g precipitates that mainly exist at grain boundaries are coarsened and become more spaced. It has been stated that the main microstructural change
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