Effect Of Solution Treatment On Microstructure And Properties Of 7A04 Aluminum Alloy

7A04 aluminum alloy has been widely used in aerospace, transportation and other fields for its excellent properties, such as high strength, high hardness, corrosion resistance, and good processability. But 7A04 aluminum alloy is easy to produce stress corrosion crack, that seriously affects its application.The 7A04 aluminum alloy was researched in this paper. The microstructure, chemical composition and phases of the alloy were analyzed by scanning electron microscope with energy dispersive spectroscopy. The electrical conductivity was tested by conductivity meter. Micro-hardness was tested by vicker micro hardness meter. Tensile test and slow strain rate testing were experimented by electronic universal testing machines and low strain stress corrosion testing machine. The conclusions are as follows:(1) The tensile strength and hardness of 7A04 aluminum alloy after heat treatment is twice as much as 7A04-H112 aluminum alloy, because there are almost no strengthening phase MgZn2 in 7A04-H112 aluminum alloy, but it appears in the alloy after heat treatment. The second phase of 7A04-H112 is dispersed distributed and become the core of dimple, which makes it that has higher toughness than 7A04 after heat treatment.(2) Slow strain rate testing experiment in 3.5% NaCl solution, the strength and elongation of 7A04-H112 and 7A04 aluminum alloy after heat treatment are not much different from those of the samples in air, peak rate of decrease of strength is 2.4% and peak rate of decrease of elongation is 14.6%, which means the SCC sensitivity in 3.5% NaCl solution is low. Compared with air, in (3.5% NaCl+0.3% H2O2) solution, the strength and the elongation of 7A04-H112 and 7A04 after heat treatment decrease 4%～9% and 20%～45%, which means the SCC sensitivity in (3.5% NaCl+0.3% H2O2) solution is higher than in 3.5% NaCl solution, and H has a great influence on them, hydrogen-induced cracking is obvious. The change of SCC sensitivity when the temperature of (3.5% NaCl+0.3% H2O2) solution was changed are less than 5%, which means the effect of temperature on the stress corrosion resistance of the 7A04 after heat treatment is very small.(3) By means of SEM, it can be found that there are a lot of dimples in the fractures of all samples tested in air medium, which means ductility fracture is the major fracture mode. In 3.5% NaCl solution, there are some secondary crack, cleavage fracture, and a lot dimples in the fracture, it means there are some brittle fracture, but ductility fracture is the major fracture mode. In (3.5% NaCl+0.3% H2O2) solution, there are more secondary crack and cleavage fracture, and the number of dimple is decreases, unlike the earlier example, the fracture mode includes brittle fracture and ductility fracture.(4) By calculating the susceptive index of the stress corrosion(ISSRT), the optimized draft of solution treatment is 470℃,70 minutes. In 3.5% NaCl solution, the SCC sensitivity of the alloy after solution treatment is half as much as that of 7A04-H112, which implys the solution treatment improve stress corrosion resistance of 7A04 in 3.5% NaCl solution.