Study On Anisotropy Of7449Aluminum Alloy Plates

Because of the high specific strength and hardness, fine corrosion resistance and toughness,7XXX alloys are widely used in aerospace and civil industries. As the requirements to performance of materials used in aerospace field increasing, the materials with combination of high strength and good stress corrosion crack resistance are desired. Being precipitation hardening alloys, the properties of this serious alloy strongly depend on the solution and ageing process. The differences of the micro structure, second phase particles and textures due to different forming and heat treatment processes will result in anisotropy of tensile properties and the fracture toughness in different orientations. How to well control and make use of mechanical properties in different orientations is one of the important aspects in studies of such kind of alloy.Plates in T7951and rolled sheets of7449aluminum alloy were used for investigation subjects in this paper. With the help of Vikers hardness, conductivity, tensile and fracture toughness test methods, and through microscopic analysis approaches, including Optical microscope (OM), Scanning electron microscope (SEM), Transmission electron microscopy (TEM) and X-ray diffraction (XRD) texture analysis techniques, tensile properties and fracture toughness in different position along the thickness direction and different orientations in the same position of the studied alloy were tested and analyzed. Anistropy of tensile properties of rolled sheets being single aged, double aged, retrogression and re-ageing (RRA) treated was also studied. The influence factors and mechanics of the inhomogeneity and anisotropy of the studied alloy plates were discussed. The main results are as follows:Microstructure and mechanical properties of7449-T7951Al alloy plates are inhomogeneous in the different layers along the thickness direction, that is, from the surface to center layer, the tensile strength fall down at first, and then arise, and there is40.6Mpa between the max and the min tensile strength. It also shows the plane anisotropies in the same layer, the largest IPA was7.3%, which appeared in the center layer.During the aging process, the main precipitation sequence of7449Al alloy sheets is: a (supersaturated solid solution)→coherent GP zone→half-coherent phase η’(semi-stable state MgZn2)→incoherent phase η(stable state MgZn2). The plane anisotropies of7449alloy sheets after different aging treatments have approximately similar regularity:the lower strength appears in the30°or45°direction along with rolling direction. The assessment of IPA is different, and the lowest IPA of mechanical properties is under Retrogression and Re-aging heat treatment, which is5.03%. Has been RRA heat treatment, the sheets not only own the high strength and corrosion resistance, but also have the best homogeneity of the properties of alloy sheets along the different direction.The main texture types of7449alloy are Brass texture and Goss texture. The schmid factor was used to analyze the impact of textures on yield strength anisotropies, and the,result illustrates that the main reason of yield strength anisotropies of alloy plate is the textures.The effects of the distribution of directional second-phases and microstructure such as fiber structure are also the important factors for the anisotropy of7449Al alloy.