A number of heat-treated samples with the size of 15 × 15 × 15mm were selected from the successful parts. The steel seal was numbered according to the set aging holding time of 12, 14, 16, 18, 20, 22, 24 and 26h, and the solution treatment process of 475 ℃ for 2 hours was adopted, followed by aging treatment (temperature 140 ℃). The samples were taken out according to the holding time and naturally cooled to room temperature. After grinding, polishing and corrosion, the metallographic structure was observed under the electron microscope. After the metallographic structure of all samples was observed, the Brinell hardness test was carried out. The microstructure of 7075 alloy rheo squeeze casting parts under different aging time is shown in Fig. 1, and the variation curve of hardness value of 7075 alloy rheo squeeze casting parts with aging time is shown in Fig. 2.
From the metallographic structure analysis of the samples aged at different time, it can be seen from Fig. 1 (a) and Fig. 1 (b) that the grain boundary is fuzzy, the solid solution phase is not fully dissolved, and the precipitated phase particles are relatively small when aged at 140 ℃ for 12h and 14h. With the increase of time, the second phase is fully dissolved at 16h as shown in Fig. 1 (c), and the precipitation phase transformation is fine and uniformly dispersed. The average grain size is about 35 μ M. When the aging time reaches 18 h and above, the grain size increases, and the precipitated phase particles are relatively large. When the aging time is 22 h and 24 h, see Fig. 1 (f) and Fig. 1 (g), the microstructure of the sample also finds that there are obvious holes near the grain boundary, and the generation of holes is easy to produce cracks on the interface, which makes the macro strength and plasticity and other mechanical properties adversely affected. Therefore, the aging process of more than 18 hours is over aging, so it is not recommended to use it. After aging treatment of 15 × 15 × 15mm sample with different aging time (temperature 140 ℃), the surface of the sample is polished. The Brinell hardness test is carried out with hbe-3000a electronic Brinell hardness tester, and the average value of three measurements is taken, as shown in Figure 2. The results show that with the increase of aging holding time, the hardness of the sample increases gradually. When the holding time is 16h, the average best hardness value of the sample reaches 183hbs 10 / 1500kgf / 30s.
Based on the above analysis, the best aging time is 16h.