Hot Deformation Behavior And Diffusion Bondability Of Ti-Al System Alloy

It is a trend to develop the light-weight structure of light-weight hightemperature materials with the demonds of lighter weight and higher service temperature.Ti-Al system alloy has been considered as one of the promising candidates for light-weight high-temperature applications in aviation and aerospace industris,because of its excellent properties,such as low density,high melting point,high specific strength,sufficient oxidation and corrosion resistance,and good creep resistance at elevated temperature.While its low plasticity,ductility and poor workability at room temperature make it necessary to deform at elevated temperature for Ti-Al system alloy.The traditional light-weight structure is multi-layer structure,and the forming process of multi-layer structure is the combining process between forming and joining technology.Diffusion bonding is an effective way to avoid welding defects during joining process and can achieve sound bonding of Ti-Al system alloy.So the high temperature deformation behavior and diffusion bonding of three kinds of Ti-Al system alloys with different service temperatures were investigated,and the multi-layer structures of Ti-Al system alloys were successfully fabricated.The high temperature deformation behavior and microstructure evolution of hotextrusion Ti-45Al-8Nb alloy at relatively high strain rates(≥10-3s-1)have been investigated,then the effects of deformation parameters on the mechanical behavior and microstructure are analyzed and the constitutive model is established.Moreover,The high temperature flow behavior of as-extruded Ti-47.5Al-Cr-V alloy has been investigated at the temperature between 1100℃ and 1250℃ by hot compression tests and the constitutive equation is also established.Based on the dynamic material model(DMM)and instability criteria(Prasad),the processing maps of this alloy at different true strains are constructed and the instable region are the high strain rate region(1s-1).Connected with the microstructure,the optimum hot working window is the region at 1150-1200℃/0.001-0.03s-1,in which the dynamic recrystallization takes place sufficiently.The vacuum diffusion bonding of as-extruded Ti-47.5Al-Cr-V alloy is carried out at the temperature of 1000℃-1100℃ for 1h-3h under the pressure of 10MPa-30 MPa.The shear strength of bonding joints is measured and the microstructure of bonding interface is analyzed.The appropriate bonding parameters were to be 1050-1100℃ at temperature,20-30 MPa at bonding pressure and 2-3h at bonding time.Based on the bending property of Ti-47.5Al-Cr-V alloy at high temperatures and the plasticity of Ti-45Al-8Nb alloy treated as an reference,the hot forming experiment of corrugated core sheet is carried out at 1050℃,then the diffusion bonding between core sheet and two face sheets is conducted at the condition of 970℃/30MPa/2h and the three-layer corrugated structure is fabricated.And the maximum compressive stress of the corrugated structure is about 15.7MPa.The superplasticity of Ti-22Al-27 Nb alloy is investigated and the uni-axial tensile experiment shows that the tensile elongation can reach up to 236% at the temperature of 970℃ with the strain rate of 3×10-4s-1.Based on its superplasticity,the finite element analysis(FEA)of forming process for two types of four-layer structure is carried out and the results show that the forming of honeycomb structure is flexible for Ti-22Al-27 Nb alloy with limited superplasticity.Then the effect of bonding area’ width on forming quality is researched by finite element analysis and the smaller bonding width will make it easier to form the sunk part at face sheets.Finally,the appropriate structural parameters of honeycomb structure are established.The diffusion bonding property of Ti-22Al-27 Nb alloy is carried out at the temperature of 950℃ and 970℃.Combining the microstructure of bonding interface with shear strength,the diffusion bonding parameter for Ti-22Al-27 Nb alloy is: 970℃/10MPa/2h.The honeycomb structure of Ti2 AlNb based alloy is fabricated by superplastic forming/diffusion bonding technology,then the microstructure and mechanical properties of material after forming process are investigated.The thickness distribution of the honeycomb structure is uniform and consistent with that of the FEA result.The presence of α2 phase takes place after forming,and it leads to the reduction of the tensile strength and elongation of material at 20-800℃ and the reduction is about 14-30% and 20-50%,respectively.The maximum compressive strength of this structure is about 7.7MPa,and the main deformation behavior is the severe buckle of core sheets and the deflection of diffusion areas between two core sheets.