Defect Mechanism And Control Of Microstructure And Mechanical Properties Of 7A09 Aluminum Alloy Disk Forging With Complex Shape

Aluminum alloy disk forgings have been widely used in aircraft domains. In recently years, the parts are designed mostly as integral structures with complex shape in order to lighten the weight of aircraft. Moreover, there are higher and higher requirements for the precision, the microstructures and properties of the forgings, and as for some important structrural parts which bear loading, it is also demanded that the flow lines distribute along geometrical shapes of the forgings. As for aluminum alloy complex shape forgings, the defects such as underfilling and folding are very likely to happen because of the long flow distance of some local metal, the complexity and controlling difficulty of metal deformation and flow. Especially for some key stressed disk forgings, the defects such as flow line outcrop, flow line vortex and fibre breaking are very likely to occur because the required radial or circumstantial flow lines result from the original bars with the axial flow lines. Furthermore, the metal deformation of each location is inhomogeneous because of the complex of the forgings and the influence of billet as well as the deformation scheme, which will make it difficult to forecast the forged microstructures and properties. It has not beeen studied profoundly on the forming mechanics of the defects, the microstructures and mechanical properties control of this kind of forgings at present, and it has been no systemic theories to guide so far. Therefore, in this dissertation, the defect mechanism of 7A09 aluminum alloy disk forging with complex shape were studied profoundly and systemically by combining FEM simulation with experiment, and the influence laws of forming scheme and heat treatment on microsturctures and properties of the forgings were disclosed.The hot deformation behavior of 7A09 aluminum alloy was investigated by means of thermal simulating experiments, and constitutive equation of flow stress of the alloy at high temperatures was obtained. What's more, the hot processing maps of 7A09 aluminum alloy were drawn, and the optimal process parameters of isothermal forming of the alloy were established by combining the processing maps with deformed microstructures.The forming scheme and dies of isothermal precision forming for disk forging with complex shape were designed by means of digital technology. The integrated designing and manufacturing of dies was realized, which enchanced the design quality and manufacturing precision of dies, shortened the design and manufacturing period of dies, and reduced the costs. By combining FEM simulation with experiments, the cavity filling laws of forming disk forgings with complex shape were analyzed, the influences of billet schemes on forming qualities were studied, the occuring mechanisms of the defects arising in isothermal forming, such as underfilling, folding, coarse grain, flow line outcrop, flow line vortex, fibre breaking and rib breaking, were revealed, and the valid measures to control the forging defects of disk forgings with complex shape were put forward. What's more, 7A09 aluminum alloy disk forgings with complex shape were obtained by optimal process, which are acceptable on not only size precision, but also surface qulity and interior flow lines.The analytical methods such as optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to study the influence of forming process on microstructures and mechanical properties of 7A09 aluminum alloy disk forging with complex shape through a lot of experiments. The experimental results show that the final microstructures and mechanical properties of the forgings were significantly influenced by deformation extent as well as friction condition during preforming, and by billet thickness and fire times during die forging. Thereby more excellent microstructures and mechanical properties can be gained only by considering these factors in determining the process schemes of isothermal forming.The influence laws of heat treatment on microstructures and mechanical properties of forgings formed by multi-fires isothermal forging were studied. The results show that proper combination properties can be obtained by adopting conventional overageing treatment for 7A09 aluminum alloy disk forgings with complex shape. By reasonable two-stage ageing or retrogression and reageing treatment, better strength, hardness and stress corrosion resistance can be gained under the condition of good plasticity.