Research On Heat Treatment Of 7050 Alloy Cast By Electromagnetic Continuous Casting Technology

In recent years,7xxx high strength aluminum alloys are widely used in aerospace and military industry, which puts forward higher requirements for further improve the alloys' performance. Heat treatment is an important method for improving the performance of 7xxx aluminum alloys. Homogenization is absolutely necessary to electromagnetic casting 7050 aluminum alloy. The existing homogenization processes are either time-consuming or too complicated. It has been proved that two-stage homogenization is effective for high alloying 7xxx alloys which have complex changes of second phase, so a two-stage homogenization of 7050 alloy cast by electromagnetic continuous casting technology is studied.Microstructure and composition of 7050 alloy cast by electromagnetic continuous casting technology are distributed unevenly while there is serious dendritic segregation, moreover, non-equilibrium eutectic phases are distributed as continuous network. In this paper, a proper two-stage homogenization process is found out:with the furnace heated to 460℃and holding for 24h, then with the furnace heated to 475℃and holding for 2h. After two-stage homogenization, the microstructure and composition are more uniform, and the residual eutectic is further reduced. With two-stage homogenization, the performance of the alloy is significantly improved:strength up to 530.11 MPa, elongation up to 12.9% and hardness up to HBW161.There are different arguments about final thermomechanical treatment (FTMT) on the role of 7xxx aluminum alloys. Some people think that high-strength 7xxx aluminum alloys' toughness, fatigue resistance and stress corrosion cracking resistance can be improved by FTMT process. Some others believe that FTMT can improve the strength, but has little effect on improving the fracture toughness and fatigue crack propagation resistance. In this paper, with the method of using optical microscopy(OM), scanning electron microscopy(SEM), mechanical tensile test, hardness test and conductivity test, the effects of final thermomechanical treatment FTMT on the microstructure and properties of 7050 alloy are studied.The electromagnetic casting 7050 alloy after FTMT is deformed by the deformation process after quenching. Through the combination of plastic deformation and heat treatment, work-hardening and precipitation-hardening can be fully used to get higher strength as well as good elongation, toughness and resistance to stress corrosion. In this paper, a proper FTMT process is found out:pre-ageing 105℃/8h+cold rolling 20% +final ageing 140℃/14h. With this FTMT process, the alloy's performance increased significantly compared with that with two-stage aging heat treatment:tensile strength 631.28MPa, yield strength 611.80 MPa, elongation rate 14.68%, hardness HV271.0, conductivity 32.1% IACS, and the fracture mechanism changes from intergranular dimple to shearing and transgranular mixed fracture mechanism.