Study On Microstructure And Mechanical Properties Of 7055 Aluminum Alloys By Spray Forming

In this study, the microstructures of as-deposited, as-extruded and heat treated high-strength 7055 Al alloy prepared by spray forming process were analyzed by using optical microscopy(OM), (XRD), (SEM), (TEM), (EDS), and evolution on the microstructures of high-strength 7055 Al alloy under various conditions was summarized, which could provide scientific basis for optimization on the parameters of spray forming ,hot deformation and heat treatment process , to improve the mechanical properties of the aluminum alloys. The results showed as follows:A large-scale 7055 Al alloy billet by spray forming with a size of ^Φ260×1600 mm was successfully prepared by fully automatic controlled reciprocating spray forming technology. The surface of the billet was good and it's satisfiable to turn 2³mm thickness. The microstructures of as-deposited billet are composed of some equixed grains with no distinct difference ,and the sizes of grain are between 30μm and 50μm. The billet has no macro-segregation structure and its density gets up to 98.2% . The second-phases in the microstructures of as-deposited billet are composed of reticular phases of grain boundary and transgranular needle-like phases and transgranular near-spherical phases and a few blocky phases . There were lots of Cu and Al in triangular grain boundary and the MgZn₂ dissolved the Cu and Al elements, and formed the Mg(Zn,Cu,Al)2 phase and the transgranular precipitated phases were MgZn₂ . The content of Al2CuMg is too little to determine .After the heat treatment, the grain boundary second-phases of as-deposited materials reduced, even vanished and the grain boundary stricts and the grains were unified closer and the transgranular needle-like phases vanished and the transgranular mass was distributing the nanometer level precipitated phases. The result of EDS analysis indicates that the content of Mg, Cu and Zn from original 12.97%, 21.60% and 30.99% drop to 2.18%,2.18% and 11.05% in second-phases of triangular grain boundary. After the extrusion, the primitive grain boundary of as-deposited material vanished and the reticular phases was disrupted and the grain broke up. The microstructure was more compact and more tiny crowded second-phases precipitated. As the extrusion ratio increases, the distortion degree of grains is bigger and the breaking of grain is fiercer and the quantity of grain boundary are more and then precipitated phases are more during heat treatment, thus strength increases correspondingly, but when the extrusion ratio increases to certain value, the ability of dispersion strengthening and fine grain strengthening achieves the apex and the mechanical properties no longer increase. The TEM analysis indicates that the grain size of T6 material with an extrusion ratio of 6 is about 5μm, the tiny subgrain formed near the triangular grain boundary. As well as the discontinuous and continuous precipitated phasesη(MgZn₂) precipitated in the triangular grain boundary and the tiny disperse GP area and transitional phasesη' (MgZn₂) existed in matrix.The longitudinal and transverse tensile strength of as-deposited materials of 7055 aluminum alloys were 222 MPa and 252 MPa respectively . And the ultimate tensile strength of as-deposited materials after T6 heat treatment got up to 500⁵43 MPa , which indicated that there were few defects in the as-deposited billets produced by fully automatic controlled reciprocating spray forming technology. After deformation with small extrusion ratio, the billets could be fully densified. As well as the ultimate tensile strength of the large-scale materials at T6 state was increased to 745 MPa , and the elongation was 12.8%.