Study On Heat Treatment Processes And Their Effects On Microstructures And Properties Of7449Aluminum Alloy

With high strength and low density, Al-Zn-Mg-Cu-Zr series alloys are widely used in aerospace field. Meanwhile, the optimal combination of high strength, fine toughness and brilliant corrosion resistance for this series of alloys was the ultimate goal that scientific researchers have been pursuing. In order to replace7150aluminum alloy, Alcan Company in France has explored7449aluminum alloy, which is one of the new type of Al-Zn-Mg-Cu-Zr alloy. Compared with7150-T7751alloy, the7449-T7951alloy has higher tensile strength and fracture toughness the equal level of stress corrosion resistance and exfoliation corrosion resistance, with more excellent fatigue crack propagation resistance. Therefore, it has been applied in latest civil aircraft made in Air Bus Company.Under the background of the reseach project about aluminum alloys used for big aerocraft making, by means of microscopic analysis method, including Optical microscope, Scanning electron microscope, Transmission electron microscope, and through Vikers hardness test, conductivity test, tensile test, slow strain rate tensile test and exfoliation corrosion experiment, the effects of the heat treatment processes on the microstructures and properties of7449aluminum alloy sheet were studied. The effects of solution treatment, T6peak aging, double aging treatment and retrogression and re-aging on mechanical properties, corrosibe nature and microstructures were studied systemly. The corresponding heat treatment processes were optimizated and main achieavements were obtained as follows. Meanwhile the results of this investigation have provided experimental reference criteria for the further research and application of7449aluminum alloy.The results illustrate that the optimal process of solution treatment is475℃/1h and the burnt temperature is490℃. The optimal process of the single aging treatment is120/24h. On this condition,7449aluminum alloy has higher tensile strength, yield strength and elongation. Their corresponding values are677.38MPa,556.59MPa,3%respectively.As concerned as double aging,120℃/6h+150℃/12h is the optimal process. Compared with single aging, the studied alloy’s tensile strength just decreases6%and the conductivity increases30%, and the stress corrosion resistance and exfoliation corrosion resistance have uprated obviously.When the time of retrogression is prolonged to30minutes, the practical RRA heat treatment process for the studied alloy has been explored. The optimal parameters of RRA process for the studied alloy is T6+190℃/30min+T6. Compared with the T6peak aging, alloy’s tensile strength has not decreased almostly and its conductivity has increased20%, but the increament of stress corrosion resistance and exfoliation corrosion resistance is higher.Among the above three heat treatment processes, the order of the stress corrosion sensitivity from low to high is:T6+190℃/30min+T6>Pre-aging+150℃/12h>T6, and exfoliation corrosion sensibility also abide by the identical rule.