| Sustainable development is drawing more and more attention due to the energy dilemma. Higher requirement on the energy conservation, efficiency and cost in industrial production is proposed accordingly. As a kind of high strength aluminum alloy, 7000 series alloy is widely utilized in aeronautic, astronautic and transporting fields. The combination of the high strength, the good toughness and the high corrosion resistance is a hot topic of 7000 series alloy. To improve the comprehensive properties of 7000 series alloy and explore efficient heat-treatment technology, the microstructure evolution of the alloy during the hot working is investigated through metallographic tests, SEM and TEM. The property of the alloy and the precipitating behavior of the precipitates during the non-isothermal aging are systematically researched by hardness tests, conductivity tests, tensile tests, TEM and DSC. The homogenization process of the 7A60 alloy are researched by hardness tests, conductivity tests, SEM, TEM, DSC and XRD, which shows that the as-cast structure mainly contains-Al, Mg(Zn,Cu,Al)2 and a bit of iron-contained phases. Mg(Zn,Cu,Al)2 phase has Mg Zn2 structure, which indicates that the formation of Mg(Zn,Cu,Al)2 phase is caused by the solution of Al and Cu into Mg Zn2. After the homogenization, some residual Mg(Zn,Cu,Al)2 phases and a few iron-contained phases maintains in 7A60 alloy. In 7050 alloy, Mg(Zn,Cu,Al)2 phase transfers to S(Al2Cu Mg) phase and finally some S(Al2Cu Mg) and iron-contained phases are left. Zn is critical to the transfer from Mg(Zn,Cu,Al)2 phase to S(Al2Cu Mg) phase. The transformation stops when the content of Zn is higher than 8%.The solution of the alloy are researched by tensile tests, metallographic analysis and SEM, which indicates that with the rise of the solution temperature, the residual second phases gradually reduces and the tensile strength improves. Nevertheless, when the solution temperature gets higher than 480oC, obvious overburning, recrystallization and grain growth occur and the properties of the alloy rapidly decrease. With the prolonging of the solution time, the residual second phases decrease rapidly in the first 1 h and then reach to constant after 2 h. The effect of the solution time on the properties of the alloy is not obvious.The comprehensive properties of the 7000 series alloy are enhanced after heating aging. The strength and the conductivity after heating aging are higher than that after T6 treatment. After T6 treatment and heating aging(100-160oC 20oC/h), the intragranular precipitates mainly consist of tiny GP zones and phase. Stable phases distribute on the grain boundary. With the ending temperature rising and the heating rate slowing, the intragranular precipitates transfer from different-sized phases to coarse and phase. At the same time, the precipitates on the grain boundary rapidly grow and present chain distribution. The precipitation sequence during the heating aging is coordinate to that during the traditional isothermal aging, which is SSS GP zone ’. The precipitation process can be divided to nucleating, nucleating + growing, growing and coarsening.The comprehensive properties of the 7000 series alloy are also enhanced after cooling aging. Two precipitation sequences exist in the cooling aging: +(when the starting temperature is too high or the cooling rate is too slow) and + GP zone(when the starting temperature is relatively low or cooling rate is too fast). The precipitation process can be classified as fast-nucleating stage, growing stage(when the starting temperature is relatively low or cooling rate is too fast) or growing and coarsening stage(when the starting temperature is relatively high or the cooling rate is too slow), and re-nucleating and precipitating stage(when the starting temperature is relatively low or cooling rate is relativly fast).Simultaneously, at the beginning of the cooling aging, large amount of tiny stable strip-shaped phases form on the grain boundary. The phases continuously grow with the cooling aging proceeding. At the later period of the cooling aging, there is no big change for the intracrystalline precipitated phases during the 140-100oC interval. However, the precipitate distribution in the grain boundary transfers from continuous shape to necklace shape, which helps to improve the corrosion resistance of the alloy.Heating aging + cooling aging can remarkably improves the comprehensive properties of the alloys. The precipitates of 7050 alloy after heating aging and cooling aging mainly consist of phases. The phases increase with the max aging temperature rising and the temperature changing rate decreasing. The stable phases continuously distribute on the grain boundary. With the max aging temperature rising and the temperature changing rate decreasing, the precipitates on the grain boundary grow and their spacing increase. Also, the precipitation zone in the grain boundary turns to be wider.The mechanical properties and the grain boundary structure of the 7000 series alloy can be effectively improved after cooling aging + isothermal aging. The main precipitates of 7050 alloy are phase after cooling aging(180-120oC, 20 oC/h) and isothermal aging at 120 oC. During the isothermal aging, the size of the intracrystalline precipitated phases changes little with the prolonging of the aging time. However, the grain boundary structure can be improved. The stable phases on the grain boundary grow and their spacing increases. They present necklace shape and distribute discontinuously on the grain boundary, which improve the corrosion resistance.The non-isothermal aging technology not only promotes the comprehensive properties of 7000 series alloy effectively, but also shortens the aging time substantially, which is of great significance to the production efficiency, energy conservation and emission reduction. |
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