Al - Zinc - Mg - Cu Alloy Quenching Sensitivity And Aging Precipitation Behavior Research

Age-hardenable Al-Zn-Mg-Cu alloys have been widely used as the structural materials in the aerospace industry due to its excellent performance. In recent years, thick-section materials which could be used for large-scale and integral aircraft components have been demanded in aerospace industry. It is urgently needed to develop Al-Zn-Mg-Cu alloys combined with high-strength, high-toughness, and low-quench sensitivity properties. Al-Zn-Mg-Cu alloys contain large amount of alloying elements, and when the cooling rate in the center layer of thick-section produces is slow, the super-saturated solid solution can decompose to form equilibrium phase particles. Consequently, lower overall performance is achieved after ageing. In order to improve the overall performance of thick-section produces, it is necessary to reduce quench sensitivity and conduct basic research in this area. In this work, the effects of elements content and recrystallization degree on the quench sensitivity, and age-precipitation behavior of Al-Zn-Mg-Cu alloys were studied by hardness testing, ambient temperature tensile testing, optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The main conclusions are as follows:(1) The effects of elements Zn, Mg and Cu content on the precipitation behavior during slow quenching and quench sensitivity were investigated. The result shows that the increasing of Cu and Mg content promotes the precipitation of coarse η (MgZn2) equilibrium phase particles within the recrystallized grains, also S and T phase in subgrain region, the degree of major alloying elements to increase the quenching sensitivity is:Cu>Mg>Zn. Reducing Cu content and improving Zn/Mg ratio appropriately could lower the quench sensitivity.(2) Comparision and analysis were made among alloys with different recrystallization degree after low rate quenching, and the effect of recrystallization degree on the quench sensitivity after ageing is revealed. The decreasing of recrystallization degree can reduce the quench sensitivity, but the recrystallised fraction decreased from89%to63%, the quench sensitivity only had little reduction, while dropped to16%, the quench sensitivity significantly reduced.(3) The influence of T6, T7and RRA treatments on the ageing precipitation behavior of the studied alloy after different cooling conditions were investigated. The intergranular corrosion resistance after different heat treatments was compared and analyzed. The decreased degree of the hardness of air quenched alloy to water quenched alloy after T6, T7and RRA treatments is25.14%,26.14%,26.29%respectively.