Investigation Of Grain Refinement And Superplasticity Of2A97Al-Li Alloy

Abstract:Al-Li alloy plates have important application in the fields of aviation and aerospace due to their merits, such as low density, high specific stiffness, excellent corrosion resistance and good performance at low temperature. However, Al-Li alloy plates exhibit lower ductility at room temperature, which results in the premature fracture during deformation. In addition, they also show severe anisotropy in the mechanical process. However, superplastic deformation can solve the problems mentioned above. Their superplastic deformation requires the material to have fine grains.2A97alloy belongs to the third generation of Al-Li alloys produced by Southwest Aluminum (Group) Co., Ltd. In the thesis, the experiments are carried out on the hot-rolled plates of2A97Al-Li alloy, which is from Southwest Aluminum (Group) Co., Ltd. By means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), XRD and high temperature tensile test, the effects of different aging treatments and intermediate annealing temperatures during preparing process on grain refinement and superplasticity of2A97Al-Li alloy plates have been investigated. Main conclusions are listed as follows:(1)An appropriate overaging approach during the thermo-mechanical processing was suggested. The second phase particles, which size were larger than0.8μm, were able to be obtained at the aging temperature from300℃to400℃.And the particles, with the sizes of0.8μm～2.0μm, for stimulating nucleation through the overaging treatment of400℃/48h was obtained with maximum volume fraction of7.60%, and the grains were refined through severe deformation and recrystallized annealing.(2)The optimum superplastic conditions, temperatures and strain rates were determined. For the samples aged at400℃for48h, forming temperature range of480℃～490℃and strain rate range of1×10-3s-1～2.5×10-3s-1,by which the elongations reached600%～850%The optimum superplastic conditions were490℃and the strain rate2×10-3s-1.Through those conditons, the maximum elongation was up to850%. (3)The influences of different intermediate annealing temperatures on the microstructures were demenstrated. Through severe deformation of rolling to88%after overaging at400℃for48h, the plates with visible cracking were rolled to92%after annealing respectively with the intermediate annealing treatment of240℃/1h、300℃/1h、400℃/1h. The results showed that the structure of the plates annealed at240℃for h retained lots of dislocations, and the degree of recovery is increased gradually with increasing of the intermediate temperature. Through the investigation of the superplastic deformation at different intermediate annealing temperatures, it can be concluded that the samples treated at240℃for an hour have better superplasticity, and the maximum elongtation of the plates is up to650%.