| A new Al-Zn-Mg-Cu-Zr alloy with high Zn content and ultra high strength was prepared by low frequency electro-magnetic casting processing. The tensile mechanical properties, microstructure and it's evolution of the studied alloy were investigated using hot working simulation and step by step heat treatment method, Differential Scanning Calorimetry analysis, hardness and electric conductivity measurement, room temperature and low strain tensile test, X-ray diffraction (XRD) , optical microscopy(OM), electronic microscopy analysis (SEM,TEM). The results were analysed and discussed theoretically. Several important conclusions can be summarized as follows:1. During homogenization, the variety of microstructure can be described in three aspects: the first, the dendrite structure and coarse equilibrium phases between dendrites in super saturation solution disappeared gradually with the increase of homogenization temperature and time ; the second, when the alloy is homogenized under the temperature of 400 ℃ , super-saturation solution decomposes and precipitates equilibrium η(MgZn2) phase. while the alloy is homogenized over the temperature of 400℃, the equilibrium phases re-dissolve into the solution matrix. the third, super-saturation solution decomposed and precipitated dispersed Al3Zr phases. The suitable homogenization treatment of the alloy is 450℃/24h.2. After 450℃/24h homogenization, study on hot working simulation of studied alloy shows that at the same deform temperature, with the increase of strain rate, flow stress increases rapidly first, after reaching peak stress, flow stress decreases gradually, going to a stable value of strain rate. At the same deformation temperature, the flow stress increases with the increase of strain rate. Using linar retrogression method,four characteristic constants were obtained as follows: n=8.76, a =0.016mm2/N, A=4.3×1012S-1, Q=139.4KJ/mol.Under deformation condition, the relation of flow stress, strain rate and deformation temperature can be expressed as follows:ε=4.3×1012[sinh(0.016σ)]8.76exp(-139400/RT) σ=61.71n{[Z/4.3×1012]1/8.76+[(Z/4.3×1012)2/8.76+1]1/2} 3.When the hot working temperature is under 400℃,the hot deformationstructure is mainly sub-grain structure and the character of which was determined by the value of Z. As deformation temperature increases and strain rate decreases, value of Z decreases, the size of sub-grains increases. The relation between the reciprocal of subgrain size and lnZ meets the linar relation. When deformation temperature is over 400°C and lnZ< 18.01, dynamic recrystallize occurred in the alloy. The nucleation mechanism of recrystallizing process is grainboundery protrude, subgrain coalescence and subgrain growth. The favorable hot processing temperature range is 350°C 400°C.4. Two-step solution treatment can decrease the remain eutectic structure effectively after hot working and increase the super-saturation degree of solution matrix, and then enhance the effect of final ageing strengthening. The suitable two-step solution treatment is solution at 450 °C for 2h and then 470 °C for lh. Compared with 460°C/lh single solution treatment, at the same aging condition, two-step solution treatment can let the tensile strength and yield strength increase by 25 - 30 MPa, while elongation still maintain to 9%.5. Single ageing treatment was performed after solution treatment. During single ageing, studied alloy shows remarkable ageing hardening characteristic and high resistance to over ageing. The peak ageing occurrs at 120°C for 24h, at this condition, tensile strength , yield strength ,elongation and electrical conductivity is 730MPa, 705 MPa ,8.8% and 29.8%IACS, respectively.6.Using a Back-Error propagation artificial neural network(ANN),the non-linear relationship between parameters of ageing treatment processes and properties such as hardness and conductivity of alloy was analyzed. A basic repository on the domain knowledge of ageing treatment processes is established via sufficient data mining by the network. The prediction results of hardness and conductivity showed good agreement with the experiment data. The model provided a new road for the prediction and control of the ageing properties of aluminum alloys.7. Two-step ageing was performed after solution treatment. Suitable two-step ageing is ageing at 160°C for lh followed by 100°C for 4h. At this condition, the tensile strength and yield strength and elongation of studied alloy is 718 MPa, 690MPa and 8.6% respectively, electrical conductivity is 32.2%IACS. Compared with T6 temper, two-step ageing can increase electrical conductivity and SCCR (Resistance to stress corrosion crack) at the precondition of not decreasing strength. And more than, elongation decreases little, heat treatment time isshortened greatly.8. Three-step ageing was performed after solution treatment, during RRA treatment (Retrogression and Re-ageing), Retrogression temperature and time have greatly influence on the properties of studied alloy. Suitable RRA process is pre-ageing at 120 °C for 24h, retrogression at 180 °C for 60min,and then re-ageing at 120°C for 24 h. At this condition, the tensile strength, yield strength, elongation and electric conductivity is 721MPa, 700MPa, 8.1% and 35.0%IACS, respectively. Compared with single peak ageing and two-step ageing, the composite property of studied alloy is better. Especially, electric conductivity increases greatly which reflects the distinct improvement of SCCR. The basic properties of studied alloy achieve the target of 863 project.9. During RRA treatment, the property changes of the alloy during RRA are closely related to the microstructural evolution. In the early stage of retrogression, the re-dissolution of GP zone and A ' leads to the decrease of hardness, while precipitation of t\ ' and n makes hardness increase again to a peak value. Then, the transition of n ' -*? n. and coarsening of n lead to the decrease of hardness. Precipitation of n ' after reageing increases the hardness, strength and electric conductivity of the studied alloy. At the same time, During RRA, continuously distributed n equilibrium phase on the grainboundery grow up and change to be discontinuous, this grain-boundery structure is similar to that resulted from two step aging. After RRA treatment, the microstructure of studied alloy combined the advantages of both single peak ageing and over-ageing, leading to ultra high strength and nice SCCR.
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