Mechanical properties and microstructure of rheo SQUEEZE CASTING 7075 alloy after heat treatment

According to the above research and analysis, it is concluded that the best T6 heat treatment process for rheo SQUEEZE CASTING 7075 alloy parts is 475 ℃ solution treatment, holding for 2h, room temperature water quenching, aging temperature for 140 ℃, holding for 16h, air cooling. According to this process, the center part of the successful wheel shaped part is processed into a 100 × 100 mm square, and six round rods with a diameter of 15 × 100 mm are cut from a, B, C, D, e and F in the middle part as shown in Fig. 4.3. The round bar was treated by T6 heat treatment according to the above process parameters. After heat treatment, it was turned into standard sample for tensile test. After tensile test, the fracture surface was analyzed by SEM and EDS.

(a) Optical and SEM metallographic photos (b) Grain size and corresponding quantity distribution

After T6 heat treatment, the mechanical properties of 7075 rheo squeeze casting alloy parts are greatly improved, the average tensile strength is 520mpa, the average yield strength is 467mpa, and the average elongation is 9.05%. The maximum tensile strength is 552mpa, the maximum yield strength is 500MPa, and the maximum elongation is 13.77%. The best performance index reaches the forging level. After T6 heat treatment according to the optimized parameters, the microstructure and grain size distribution of the parts are shown in Fig. 1. The grains are fine and the precipitates are evenly distributed.

(a) TEM image of grain (b) Enlarged TEM images of precipitation phase η′

The precipitation order of 7075 alloy during aging treatment is: α (supersaturated solid solution) → coherent G.P. zone → semi coherent η′ phase (metastable mgzn2) → incoherent η phase (equilibrium mgzn2). The strength and hardness of 7075 aluminum alloy are mainly determined by G.P. zone, η′ phase and η phase. Through TEM and TEM observation, it is found that the precipitates are uniformly distributed and achieve a better strengthening effect. The morphology is shown in Fig. 2.

(a) Fracture dimple (b) Shallow dimple

The comprehensive mechanical properties of 7075 alloy wheel parts in as cast and T6 states by rheo squeeze casting were studied and analyzed. It was found that the plasticity of as cast parts was excellent. After T6 heat treatment, the strength of the parts was significantly improved, but the plasticity decreased significantly. The microstructure of fracture surface of tensile specimen after T6 heat treatment was observed. It was found that the dimples of fracture surface with good plasticity were obvious, as shown in Fig. 3 (a). At the same time, it was also found that there were most dense shallow dimples, as shown in Fig. 3 (b). The relationship between the elongation and microstructure of T6 sample needs further study.