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02159 UTILIZATION OF LOW TEMPERATURE RETROGRESSION AND RE-AGING (RRA) HEAT TREATMENTS TO IMPROVE STRENGTH/SCC PROPERTIES FOR THICK SECTION COMPONENTS OF ALUMINUM ALLOY 7075 FOR AGING AIRCRAFT...

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  • 02159 UTILIZATION OF LOW TEMPERATURE RETROGRESSION AND RE-AGING (RRA) HEAT TREATMENTS TO IMPROVE STRENGTH/SCC PROPERTIES FOR THICK SECTION COMPONENTS OF ALUMINUM ALLOY 7075 FOR AGING AIRCRAFT...
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Product Number: 51300-02159-SG
ISBN: 02159 2002 CP
Author: Charles P. Ferrer, Michelle Gaudett Koul, Brian J. Connolly, and Angela L. Moran
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Aluminum alloy 7075, a common material in the aerospace industry, is susceptible to intergranular stress-corrosion cracking (SCC) in the T6, or peak-aged temper. The susceptibility of this temper to SCC is alleviated through the use of the T73, or overaged temper that provides SCC resistance, but at a 10-15% strength loss compared to the T6 temper. Experimental test results by Cina and Ranish indicated that retrogress and re-aging (RRA) heat treatments with retrogression performed at 200-280°C for less than 10 minutes reduce the traditional trade-off between T6 strength and T73 SCC resistance. However, the short time heat treatment limited the applicability of RRA to thin sections of material. The primary goal of this effort was to determine if lower retrogression temperatures could be used in the RRA process to extend the applicability of this heat treatment to thick section aircraft components. Tensile, fatigue, fracture toughness, and hardness tests were conducted to characterize the mechanical properties of T6, T73, and various RRA tempers. Alternate immersion (AI) and doublecantilever beam (DCB) tests were conducted to evaluate the SCC resistance of the different tempers. Results verified that the T6 temper exhibits strengths that are ten to fifteen percent higher than the T73 temper, but has crack growth rates that are approximately ten times faster than T7. Furthermore, the T6 temper failed via environmental cracking much more quickly when subjected to 3.5% NaCl in the alternate immersion test. Improvements in properties were demonstrated using RRA heat treatments at lower temperatures and longer times than those previously investigated. In general, the various RRA tempers below 200°C produced strengths similar to that of T6 with lower crack growth rates. The RRA temper with retrogression at 160°C for 660 minutes produced the greatest improvement, with only a 4% reduction in strength below T6. Stress corrosion crack growth rates for the RRA were significantly lower than that of the T6 temper during the double-cantilever beam experiments and times to failure were substantially longer than T6 in the alternate immersion tests. Additionally, heat transfer analysis indicated the RRA tempers with lower retrogression temperatures and longer retrogression times are more viable for use in industry for thick section components.
Aluminum alloy 7075, a common material in the aerospace industry, is susceptible to intergranular stress-corrosion cracking (SCC) in the T6, or peak-aged temper. The susceptibility of this temper to SCC is alleviated through the use of the T73, or overaged temper that provides SCC resistance, but at a 10-15% strength loss compared to the T6 temper. Experimental test results by Cina and Ranish indicated that retrogress and re-aging (RRA) heat treatments with retrogression performed at 200-280°C for less than 10 minutes reduce the traditional trade-off between T6 strength and T73 SCC resistance. However, the short time heat treatment limited the applicability of RRA to thin sections of material. The primary goal of this effort was to determine if lower retrogression temperatures could be used in the RRA process to extend the applicability of this heat treatment to thick section aircraft components. Tensile, fatigue, fracture toughness, and hardness tests were conducted to characterize the mechanical properties of T6, T73, and various RRA tempers. Alternate immersion (AI) and doublecantilever beam (DCB) tests were conducted to evaluate the SCC resistance of the different tempers. Results verified that the T6 temper exhibits strengths that are ten to fifteen percent higher than the T73 temper, but has crack growth rates that are approximately ten times faster than T7. Furthermore, the T6 temper failed via environmental cracking much more quickly when subjected to 3.5% NaCl in the alternate immersion test. Improvements in properties were demonstrated using RRA heat treatments at lower temperatures and longer times than those previously investigated. In general, the various RRA tempers below 200°C produced strengths similar to that of T6 with lower crack growth rates. The RRA temper with retrogression at 160°C for 660 minutes produced the greatest improvement, with only a 4% reduction in strength below T6. Stress corrosion crack growth rates for the RRA were significantly lower than that of the T6 temper during the double-cantilever beam experiments and times to failure were substantially longer than T6 in the alternate immersion tests. Additionally, heat transfer analysis indicated the RRA tempers with lower retrogression temperatures and longer retrogression times are more viable for use in industry for thick section components.
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