| Ultra-high strength aluminum alloys7075and7050are widely used as structural materials in aeronautical industries due to their attractive comprehensive properties, such as low density, high strength and good formability. However, their poor stress corrosion resistance (SCC) restricts further development. In present, retrogression and re-ageing (RRA) treatment, which can improve both strength and SCC of Al-Zn-Mg-Cu series alloy, is a best method to solve this problem. The effect of RRA heat treatment on the microstructure, mechanical properties, and corrosion resistance of7075and7050alloys have been investigated by OM, SEM and XRD observation, tensile properties test, Vickers hardness, electrical conductivity and EXCO test. Moreover, the experimental results are discussed and the heat treatment of7075and7050ultra-high strength aluminum alloys is optimized.The results indicated that, when the retrogression temperature was190℃, the value of hardness decreased firstly, and then reached a minimum value. With the increase of retrogression time, the hardness increased to form a hardness peak and then decreased again at the retrogression curve. However, the RRA curve is different and has two stages:with retrogression time prolonging, the hardness increased to a peak first and then decreased monotonically. And the hardness carves of retrogression at190℃,200℃and210℃have the same characteristics. But when the regression temperature was200℃and210℃, in comparison with retrogression at190℃, the retrogression valley and peak promoted at the retrogression curve, and the hardness peak promoted at the RRA curve. There was similar characteristic between the hardness and tensile strength RRA curve. However, the variation of the elongation was quite different from the strength. In addition, the electrical conductivity value decreased monotonically with the increasing of the retrogression time and temperature. After pre-aging at120℃for24h, retrogression at200℃for7min, and re-aging at120℃for24h,7075alloy acquired tensile strength, yield strength, elongation, and conductivity,582MPa,514MPa,10.7%and32.8%IACS respectively, revealing superior comprehensive properties.RRA treatment performed on7075alloy also resulted in the lower ultimate tensile strength and yield strength level than the original peak-aged T6condition. In comparison with7075alloy, the strength change range of7050alloy is smaller after RRA treatments. And the ultimate tensile strength of7050alloy with RRA treatment (120℃/24h+200℃/7min+120℃/24h) was4.1%higher than that of the sample with T7451condition. Despite the loss in elongation capability, both7075and7050alloys after RRA treatments were less susceptibility to corrosion compared to that of peak-aged condition but more susceptibility compared to that of duplex duplex-aged condition, within the experimental conditions employed.The OM and SEM observation indicated that after RRA treatment, the matrix precipitations were mainly fine and dispersed η’ and a little η phases, while the equilibrium phases η on the grain boundaries coarsened obviously, distributing discontinuously and isolated. Hence, RRA treatment could improve the resistance to corrosion of the alloy without the big loss of strength. Moreover, the function R(T)=1010/1.5fOTe-12400/θ(t) dt could provide guidance in choosing suitable RRA treatment. |
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