The results show that the strength of 7075 rheo squeeze casting parts can be greatly increased by aging treatment. The mechanism of aging hardening is to heat and hold the supersaturated solid solution which is more dissolved than equilibrium state after solid solution treatment of alloy parts, precipitate excess alloy phase from supersaturated solid solution, and try to adjust the state of second phase dispersion precipitation to achieve the highest strength. The reason is that the formation rate of G.P. zone of Al Zn mg Cu alloy is slow, the natural aging process often lasts several months to reach the stable stage, and compared with artificial aging, the stress corrosion resistance of superhard aluminum alloy is also poor, so the superhard aluminum alloy is only treated by artificial aging. The artificial aging system of superhard aluminum alloy can be divided into single-stage aging and graded aging. In the actual production, the single-stage aging process is relatively simple, the production efficiency is high, and high strength and other mechanical properties can be obtained. Therefore, only the single-stage aging system of 7075 alloy was studied.
The aging time was set as 16h, and the aging temperature was 120 ℃, 130 ℃, 140 ℃, 150 ℃ and 160 ℃, respectively.
The microstructure of 7075 rheo squeeze casting parts at different aging temperatures is shown in Figure 1. The hardness values of 7075 rheo squeeze casting parts at different aging temperatures are counted in Table 1, and the corresponding data are shown in Fig. 2. The variation curve of hardness values with aging temperature of 7075 rheo squeeze casting parts after solution aging treatment is shown.
It can be seen from the metallographic structure analysis of samples aged at different temperatures (Fig. 1), FIG. (a) and Fig. (b) that the precipitated phase particles are more and coarser when aged at 120 ℃ and 130 ℃ for 16h, and more precipitated phase particles are precipitated at 120 ℃. With the increase of temperature, the precipitation phase transformation is fine and the dispersion distribution is more uniform at 140 ℃, and the average grain size is about 35 μ M. When the aging temperature reaches 150 ℃, the grain size increases and the average grain size reaches about 58 μ M. the precipitated phase appears in the grain as a reticular film, which is easy to cause cracks on the phase interface and damage the strength and properties. However, when the temperature is further increased to 160 ℃, the precipitates are almost continuous reticular distribution, which will have a serious impact on the properties. Therefore, the 16 h aging process above 150 ℃ is over aging and cannot be used. The Brinell hardness of the samples was measured by electronic aging test (hbe-15 × 15m) for three times. The results show that with the increase of aging temperature, the hardness of the sample increases gradually. When the temperature is 140 ℃, the average best hardness value of the sample reaches 183hbs 10 / 1500kgf / 30s.
Based on the above analysis, 140 ℃ is the best aging temperature.