| The mechanical properties, microstructures and it's variation of the ultra-high strength AA7085 aluminum alloy at defferent heat treatment conditions were investigated using hardness analysis, room temperature tensile test, electrical conductivity measurement, optical microscopy(OM), X-raydiffraction(XRD), electronic microscopic analysis (SEM, TEM). The heat-treating parameters were optimized. By analyzing the experimental results theoretically, several conclusions can be summarized as follows:1. The AA7085 alloy should be homogenizated treatment due to its serious dendritic segregation. In the process of homogenization treatment, the hardness and electrical conductivity of the alloy changed regularly while the dendritic and non equilibrium eutectic structures in the cast alloy were disappeared gradually with the increasd of homogenization temperature and time. The optimal homogenization process is heating under 430℃for 12h and then 467℃for 24h.2. After homogenization treatment, the extruded bars are taken subject to strengthening solution treatments. It showed that two step solution treatments can decrease and even eliminate the eutectic phase which were residued in the process of heat treatments effectively, and increase the super-saturation degree of matrix after hardening so as to make the solution temperature surpass the multi-phase eutectic temperatures and enhance final ageing strengthening effect. The optimal two step solution process is soluting at 450℃for 2h and then 480℃for 0.5h.3. Single ageing treatment was performed after the solution treatment of 450℃for 2h and then 480℃for 0.5h. The alloy shows relatively strong ageing hardening characteristic and high resistance to over ageing. The appropriate peak ageing (T6) treatment which is heating at 120℃for 24h. And the tensile strength, yield strength, elongation and electrical conductivity areσb=659.1AMPa,σ0.2=637.8MPaδ=11.6%,γ=33.4%IACS, respectively.4. After single aging treatment, the intragranular structures are GP zones andη' phases which are fine and dispered, and there are continuously distributed precipitation in grain boundary. The grain boundary has narrow PFZ under the peak ageing which are gradually broadened with the increasing of the aging time. Therefore, the comprehensive performance of alloy has a relationship with the type, distribution and size of precipitates.5. After strengthening solution, the effects of two-step treatments on the microstructure and performance of alloy are also investigated. The suitable two-step ageing is ageing at 120℃for 12h followed by 160℃for 4h. And the tensile strength, yield strength, elongation and electrical conductivity of alloy are 648.6MPa,627.2MPa, 13.1% and 37.8%IACS respectively. Compared with T6 treatment, the strength of alloy is not affected, and the elongation increased a little whlie electrical conductivity increased 4.4% IACS. The result shows that the stress corrosion resistence is improved.6. After two-step ageing, the intragranular structure were fine and dispered GP zones andη' phases, which were similar to that of T6 treatment. However, there were continuously distributed precipitations in the grain boundary accompanied with the narrow PFZ. The results show that the precipitates become coarser, and the PFZ are gradually broadened with the increasing of the two-step aging time. Therefore, the alloy strength is slightly lower than that of the peak aging, but the stress corrosion resistance is improved. |
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