Dependence of Mechanical Properties and Microstructures of 2195 Al-Li Alloy on Cold and Hot Deformation Process

Al-Li alloys are suitable for application in aerospace, due to their low density, high specific strength and stiffness and low fatigue crack growth rate. Among them, 2195 Al-Li alloy is the main structural materials of rocket cryogenic fuel tank. It is usually used as plate, and there are few reports on the properties of extruded profiles. In addition, the properties and structures of Al-Li alloys are highly dependent on the proccessing. To widen the application of 2195 Al-Li alloy, the mechanical properties and structures of its extruded profiles were investigated. Meanwhile, the mechanical properties and structures of its cold-rolled thin sheet were investigated, and the influence mechanism of processing was clarified. The 2195 Al-Li alloy ingot with chemical composition of Al-4.11Cu-1.03Li-0.4Mg -0.4Ag-0.12Zr (%, mass fraction) was prepared through melting and casting. After homogenization annealing, it was extruded to a plate with 5 mm thickness at 460℃. A part of the extruded plate was annealed and then rolled to a sheet with a thickness of 2 mm through cold-rolling. For simplification, the extruded plate with 5 mm thickness was denoted as E-plate, and the cold-rolled sheet with 2 mm thickness was referred to as CR-sheet. After solutionization at 505℃ for 60 min, T8 aging at 148℃ for 38 h following 3.8% pre-stretch was applied to the E-plate and CR-sheet. The mechanical properties of the T8 aged E-plate and CR-sheet were measured through an MTS 858 test machine. The grain structures were investigated through optical microscope (OM). Their textures were measured by an X-ray diffractometer (XRD, Brucker D8 Discovery). Meanwhile, the aging precipitates in the T8 aged samples were observed through a Tecnai G2 20 transmission electron microscopy (TEM). The results were as follows: (1) After the same T8 aging, the average tensile strength and yield strength of the T8 aged E-plate were about 591 MPa and 555 MPa, respectively, which were 45-55 MPa higher than those of the T8 aged CR-sheet. Their elongation is almost equivalent in the range of 11%-12%. (2) The recrystallization degree in the solutionized E-plate and CR-sheet was different. The CR-sheet was highly recrystallized during the solutionization process due to its high energy storage. The energy storage was much low in the E-plate, the solutionized E-plate therefore was not recrystallized and composed of fiber-like grains. (3) The texture component of the solutionized 2195 Al-Li alloy with different deformation modes was different. Corresponding to un-recrystallized grain structures, deformation textures (Brass and S textures) with a high volume fraction of about 66% and cube textures with volume fraction of about 18% existed in the solutionizaed E-plate. However, the volume fraction of deformation textures (Brass and S textures) was decreased to about 14%, and a large number of R-oriented textures (R-Cube and R textures, volume fraction of about 21%) appeared in the solutionized CR-sheet, which corresponded to high recrystallization of the solutionized CR-sheet. (4) The main aging precipitates were T1 (Al2CuLi) and θ' (Al2Cu) in both the T8 aged E-plate and T8 aged CR-sheet, but their distribution was different. It was interesting that the average number density of T1 and θ' precipitates in the TEM foil of the T8 aged E-plate was lower, but their average size was larger than that of the T8 aged CR-sheet. In addition, the T1 precipitates were distributed evenly in the T8 aged Cr-sheet, but unevenly in the T8 aged E-plate. The deformation process impacted the mechanical properties and structures of 2195 Al-Li alloy. The extrusion at high temperature was beneficial to retain deformation texture and reduce recrystallization degree in the solutionized state, and therefore enhanced the mechanical properties of the T8 aged state. However, the cold-rolling increased the recrystallization degree and decreased deformation textures in the solutionized state, and therefore lowered the mechanical properits of the T8 aged state. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.