The heat treatment system of 7075 aluminum alloy wheel shaped parts formed by rheo squeeze casting was analyzed and tested. Through DSC analysis of the parts, different solution temperature ranges were determined. Through a large number of samples, a variety of heat treatment schemes with different solution temperature, solution time, aging temperature and aging holding time were explored. The microstructure and hardness of the products after heat treatment were analyzed and compared, and the best T6 single-stage aging heat treatment system was obtained. The mechanical properties and microstructure of 7075 alloy after heat treatment were studied, and the strengthening mechanism of heat treatment was analyzed
(1) Under the experimental conditions, the optimum T6 single-stage aging process specification of 7075 alloy semi-solid rheo squeezeis as follows: solution treatment at 475 ℃, holding for 2 h, water quenching at room temperature, aging temperature at 140 ℃, holding for 16 h, air cooling.
(2) According to the optimized T6 process, the mechanical properties of 7075 rheo squeeze casting alloy parts are significantly 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.
(3) Through TEM and TEM observation, it is found that the precipitates are uniformly distributed and achieve a good strengthening effect. The microstructure of 7075 alloy by rheo squeeze casting was studied. It was found that after solution treatment, there were various holes left by eutectic phase with low melting point near the grain boundary. After T6 treatment, there were also various primary melting holes near the grain boundary. These holes will seriously affect the comprehensive mechanical properties of the parts. The tensile fracture of heat treatment is composed of large dimples, flat areas, dense shallow dimples and tearing ridges. The micro characteristics of tensile fracture show that the tensile fracture mechanism of the formation of tearing band is the aggregation and expansion of pores. In the tensile fracture, the pores exist in the form of large dimples, and the tearing cracks originate from the pores, and then with the increasing of tensile stress, the pores grow The gap and the hole gather and expand each other to form the tearing ridge until the fracture. The equivalent stress field model of the hole under tensile stress is established by ANSYS software, and the simulation results are consistent with the above analysis. Therefore, it can be inferred that the significant decrease of elongation after T6 heat treatment is mainly caused by the existence of primary melting holes.
(4) The results show that there are more spherulites and finer grains in 7075 alloy after rheo squeeze casting, which increase the stress of dislocation movement. In the process of solution treatment, due to the high solid solubility of Cu and Mg elements, there are a lot of dislocations, vacancies and other defects in the matrix, which will effectively accelerate the diffusion and dissolution speed of alloy elements, especially copper, and shorten the solution time significantly, thus improving the production efficiency.