Study On Microstructure And Properties Of Superhigh Strength Large-scale 7055 Aluminum Alloy By Spray Forming

In this paper, the effect of solution and aging treatment on the microstructure and properties and the SCC of 7055 alloys processed by spray forming,were studied by using such research means as optical microscopy, X-ray diffraction, SEM, TEM and EDS, and through different performance tests such as room temperature tensile test, conductivity test and SSRT etc,which could provide scientific basis for optimization on the parameters of spray forming , heat treatment process , SCC , to improve the mechanical properties of the aluminum alloys. The main results are showed as follows:(1)An industrial large-scale 7055 Aluminum alloy ingots with a size ofФ500 mm×1600 mm was successfully prepared by automatic control of reciprocating spray forming technology. Diffent heat treatments are researched after the hot-extruded.(2) The optimum single solution process of 7055 Aluminum alloy is: 475℃/3h . Through the analysis of Orthogonal experiments, the optimum two-stage solution process is: 450℃/2h +475℃/2h. The alloy has excellent comprehensive properties after the this treatment. The tensile strength, the yield strength, the extensibility are 710 MPa, 654MPa, 11.2 % respetively.After the two-stage solution process,the alloy has higher strength and extensibility.(3) The spray-deposited 7055 Aluminum alloy had the highest strength after the hot-extruded, double-stage solution and the single-stage aging (120℃/24h) . The tensile strength, the yield strength, the hardness of HRB, the extensibility and the conductivity are 710 MPa, 654 MPa, 11.2%,96and 30% IACS respectively.The optimum two-stage aging process through the analysis of Orthogonal experiments is: 120℃/8h +170℃/8h. The alloy has excellent comprehensive properties after the two-stage aging treatment. The tensile strength, the yield strength, the hardness of HRB, the extensibility and the conductivity are 659 MPa, 630 MPa, 95, 11.7% and 39% IACS respectively. Compared with the single-stage aging treatment, the conductivity increased by 30%, stress corrosion resistance improved significantly.(4) After single-stage aging treatments, the TEM analysis indicates that the discontinuous and continuous phasesη(MgZn2) precipitated in the triangular grain boundary and the tiny disperse GP area and transitional phasesη' (MgZn2) existed in matrix. However, after doule-stage aging treatments, the phases which in matrix growes and the phases in grain boundary become uncontinuous. With the aging temperature and aging time of the duplex aging increasing, the alloy strength decreases, plasticity increases and the conductivity increases.(5)The experiment of SSRT of 7055 Aluminum alloy indicates that the alloy has the worst stress corrosion resistance with the rate of 0.5x10-6 S-1,and the tensile strength and the plasticity decrease.It is also showed that the stress corrosion resistance of alloy in the air is better than in the 3.5% NaCl Solution. The analysis of fracture surface of 7055 alloy after SSRT testing indicates that the fracture is fracture toughness in dry Air and the fracture is mixed fracture of intergranular and transgranular in the 3.5% NaCl Solution. The stress corrosion resistance of 7055 Aluminum alloy needs further exploration and study.