The Influence Of Heat Treatment On The Microstructure And Properties Of 7055 Alloy

The mechanical properties, electrical properties, microstructure and it's evolution rules of the ultra high strength 7055 aluminum alloy with high Zn content at defferent heat-treatment condition were investigated using room temperature tensile test, hardness and electric conductivity measurement, X-ray diffraction (XRD), optical microscopy (OM), electronic microscopy analysis (SEM, TEM), and the heat-treatment regimes were optimized. The results were analyzed and discussed theoretically. Several conclusions can be summarized as follows:1. The favorable solution-single-ageing treatment of 7055 alloy is 480℃/60min solution, quenching in water at room temperature, then ageing at 120℃ for 24h. At this condition, tensile strength, yield strength, elongation, hardness and electrical conductivity of the alloy is 585MPa, 533 MPa, 10.2%, 177 and 29.0%IACS, respectively.2. During single ageing, 7055 alloy with high Zn and Mg content shows remarkable ageing hardening characteristic and high resistance to over-ageing. Ageing at 120℃ for just 4h, the hardness and strength of the alloy are close to peak value, and they maintain the peak value in the process of ageing. During this process, η′(MgZn₂) phase precipitates from the supersaturated solid solution, and the precipitation increases with the lengthening of ageing time.3. At the beginning of ageing, GP zone was formed with the decomposition of the supersaturated solid solution, electron scattering increased, the electrical conductivity decreased. After that, GP zone transformed to η′(MgZn₂) phase, solid solubility of the matrix decreased, and the electrical conductivity increased. The higher the ageing temperature, the longer the time, the more the precipitation, and the higher the electrical conductivity increased.4. The suitable two-step ageing treatment of the alloy is ageing at 160℃ for 20h followed by 120℃ for 8h. At this condition, the tensile strength, yield strength, elongation, hardness and electric conductivity of the studied alloy is 529MPa, 486MPa, 9.4%, 163 and 40.1%IACS, respectively. Compared with T6 state, two-step ageing can increase electrical conductivity and SCCR (Resistance tostress corrosion crack) at the precondition of a little decreasing of strength.5. After two-step ageing, the structure in the grain distributed with tiny particles of GP zone and rf phase, while at the grain boundary, the precipitations were make-and-break and PFZ was narrow. So the strength was lower than the peak value, while SCCR was improved.6. During RRA treatment (Retrogression and Re-ageing), Retrogression temperature and time have great influence on the properties of the studied alloy. Suitable RRA process is pre-ageing at 120°C for 24h, retrogression at 170°C for 1.5h, and then re-ageing at 120°C for 24 h. At this condition, the tensile strength, yield strength, elongation, hardness and electric conductivity is 567MPa, 537MPa, 8.9%, 172 and 39.4%IACS, respectively. Compared with T6 state, the strength was almost the same, while the electric conductivity and SCCR increased greatly, the composite property of the studied alloy was improved. The properties of the studied alloy achieved the target of the project.7. At the early stage of retrogression, the re-dissolution of GP zone and rj leads to the decrease of strength and hardness, at the middle stage, precipitation of rj' and rj makes hardness increase again to a peak value. Then, the transition of rf-*rj and coarsening of rj lead to the decrease of strength and hardness.8. After RRA treatment, tiny rj' phase particles distributed in the grain, with a little t] phase and GP zone. Compared with T6, the precipitations after RRA treatment were coarser and more stable, while tj precipitates distributed discontinuously at the grain boundary, with wider PFZ, which was similar to those of over ageing. This microstructure of the studied alloy combined the advantages of both T6 and T7 treatment, leading to ultra high strength and nice SCCR.