The effect of homogenization and aging treatments on the strength and the stress-corrosioncracking (SCC) resistance of the 7050 aluminum alloy has been investigated and compared with those of the same-series 7075 alloy. The recrystallized structure and the quench sensitivity are found to be significantly affected by the dispersoid distribution, depending on the homogenization conditions. The finest and densest dispersoid distribution, generated by the stephomogenization (Step-H) treatment, can effectively inhibit recrystallization to obtain the smallest fraction of recrystallized structure. Such a characteristic lowers considerably the quench sensitivity of the 7050 alloy, but it produces the reverse in the 7075 alloy. For the 7050 alloy,Step-H always exhibits the highest strength among all the aging conditions, and the proposed step-quench and aging (SQA) treatment is confirmed to achieve an optimum strength and coarsened and wide-spaced grain-boundary precipitates (GBP), which have been found to improve the resistance of the SCC by the slow-strain-rate test (SSRT). Therefore, the attainment of both optimum strength and SCC resistance is possible for the 7050 alloy via the Step-H and SQA treatment. However, such treatment is not applicable to the 7075 alloy because of its inborn high quench sensitivity.

The influence of welding thermal cycle peak temperatures and post-weld heat treatments on the microstructures and mechanical properties of the heat affected zone (HAZ) for 2024-T3 aluminium alloy have been investigated by Gleeble HAZ simulation. Differential scanning calorimetry (DSC) in conjunction with transmission electron microscopy (TEM) is used to characterise the HAZ microstructures. The welded HAZ in the region of peak temperature 414°C has the lowest hardness after natural aged temper, which is primarily due to the precipitation and coarsening of stable S phases. When the peak temperature of welded HAZ is larger than 414°C, the hardness of HAZ increasing with an increasing peak temperature can be seen, which is due to higher peak temperature thermal cycles treatment inducing the dissolution of precipitations in the matrix, and, after the natural aging treatment, Guinier-Preston (GPB) and GPB2 zones precipitating out in the matrix again can be seen. Post-weld T81 artificial aging (PWAA-T81) heat treatment has no effect on improving the HAZ hardness; the HAZ hardness of the 2024-T3 alloy obtained by PWAA-T81 is less than that obtained by natural aging, and its lowest hardness is shifted to the region of peak temperature, which is 452°C, because overaging induces coarse and sparse amounts of stable S phase.

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The influence of matrix and grain boundary microstructural characteristics on the mechanical properties and resistance to stress corrosion cracking (SCC) of the 7050 aluminum alloy was investigated. The proposed step‐quench and aging (SQA) treatment can effectively improve the SCC resistance and attain optimum strength by controlling the microstructures, i.e., the coarse and widely‐spaced grain boundary precipitates (GBPs), and a matrix of GP zones with η′ phases. The highest strength of the 7050 alloy is obtained by using duplex aging at 120°C for 6 h and then 168°C for 3 h in the T6 temper. When the duplex aging is used in the SQA treatment, not only the SCC resistance and yield strength are significantly improved, but also the aging time is much reduced simultaneously, as compared to those obtained by the conventional T73 treatment. The process has good potential to be applied in industry. However, the SQA treatment is not applicable to the 7075 alloy because of its high quench sensitivity.