Research On Deformation Behavior And Formability Of Ti-22Al-25Nb Alloy Sheet At Elevated Temperatures

With rapid development of aerospace industry, the requirements of a variety of aircrafts for material performance are higher and higher, especially for aircraft structural materials, requiring further loss in weight and better high-temperature service performance. Ti2AlNb alloy is one of the most potential aerospace engine materials used at650~750°C, due to its excellent physical and mechanical properties. In terms of the intrinsic brittleness of Ti2AlNb alloy as an intermetallic compound, it has to go through a series of thermal processing from ingots to parts, such as isothermal forging, hot-extrusion, hot-rolling, hot forming, heat treatment, and so on. In this paper, the deformation behavior and formability of Ti-22Al-25Nb (at.%) alloy at elevated temperatures as well as the mechanical properties of it with various microstructures were systematically studied in order to provide theoretical guidance for thermal processing of this alloy.To achieve the excellent performance of the alloy sheet, the static phase transformation under different heat treatments and the mechanical properties with various microstructures were studied. The strengthening effect of the O-phase lamellar was found to be closely related to its size and content, leading to the first decrease and then increase in the hardness and yield strength of the alloy with aging temperature increasing, and the strengthening effect of the O-phase lamellar at high-temperature is better than that at room temperature. The surface and fracture of tensile samples were observed with SEM method. Cracks were found to initiate easily inside as well as at the boundaries of coarse and equiaxed O or α2-phase, however, their further expansion can be inhibited by B2-phase. Slip lines can transmit between lamellar O-phase and B2-phase without stress concentration at the phase boundaries. If the B2-phase grains were too coarse, stress concentration at grain boundaries and the deformation incompatibility would be enormously enhanced, leading to premature brittle fracture. Therefore, the basic principles of microstructure control during thermal processing were: B2-phase grains should be refined as small as possible; equiaxed α2or O-phase should be reduced as minimal as possible; a good match of strength and ductility of the alloy can be realized through controlling the size and content of lamellar O-phase.To provide parameters for process formulation and numerical simulation of the hot forming, the mechanical behavior of deformation at various temperatures and strain rates were investigated using uniaxial tensile tests. The effect of deformation conditions on the flow stress and strain rate sensitivity was clarified. With deformation temperature increasing and strain rate decreasing, the yield or peak stress and the strain softening effect decreased, while the stable deformation stage and the strain rate sensitivity increased. In order to give a quantitative description, various high temperature deformation constitutive models were established and their suitabilities were also discussed. Arrhenius model and Backoften model were suitable for deformation with small strain rate, while the improved Johnson-Cook model was suitable for deformation with large strain rate. The effect of strain on the flow stress can be described through introducing a softening factor exp(sε) into the constitutive equation.In order to reveal the physical mechanism of high temperature deformation of this alloy, the microstructure evolution was studied using SEM, TEM and EBSD methods. During deformation at850~950°C (B2+O phase field), O-phase dynamically precipitated, then coarsened and spheroidized with Nb element depleted. The deformation at950°C promoted the transformation from O-phase to α2-phase. During deformation at970°C, the microstructure stayed relatively stable. The deformation mechanism of B2-matrix was dominanted by dislocation slip and that of α2-phase was dominanted by grain boundary sliding and accommodated by dislocation slip. The hardening mechanism was strain hardening coupled with strain rate hardening. The softening mechanism is dynamic recovery, which offseted the strain hardening effect.To study the formability of the alloy sheet in dual tensile stress state, the free bulging process at930°C and970°C with constant pressure was investigated through theoretical analysis, numerical simulation and experiments. Changes in the shell contour and thickness distribution were clarified. With the shell height increasing, the curvature radius of the shell top tended to get smaller. For the shell with the same height, the curvature radius got smaller and the ellipsoid degree got larger with the bulging temperature decreasing (m value decreasing). Thickness decreased gradually from the bottom to the top of the shell and the uniformity got worse when the bulging temperature got lower. The microstructure and properties were compared before and after forming. When bulging at930°C, a lot of O-phase precipitated, coarsened and spheroidized, resulting in decrease in hardness and formation of cavities. When bulging at970°C, microstructure and properties distributed homogeneously. O-phase precipitated in lamellar shape during cooling from the forming temperature, making the strength of the parts enhanced. Using induction coils to heat the overall sheet and molds, a gradient temperature field abide with the Gaussian distribution formed. When forming in this temperature field, the thickness distribution can be significantly improved, the ellipsoidal degree of the shell can be weakened, the heating rate can be accelerated and consequently the homogeneous and quick plastic forming can be realized. However, if the bulging pressure was too large, where it contacted with the fillet, the inner and outer sides of the sheet would be subjected to shear deformation, leading to excessive thinning and shear cracking.