Study On Solid Diffusion Bonding Of Ti₂AlNb Alloy

Ti2AlNb alloy has high specific strength, oxidation resistance, nonmagnetic, fracture toughness and other advantages, it can be used in 600～800℃ for a long time, more than 1000℃ for short time, has become the most potential new type of lightweight high temperature structural materials of aeronautics and astronautics. Welding technology is a key link in the process of this kind of alloy practical process. In this paper, using vacuum diffusion bonding method, Ti2AlNb alloy with the same materials and Ti2AlNb/TA15 dissimilar materials directly without the middle tier solid diffusion bonding process were studied.The structures and performances of the bonding interfaces of O phase alloy with the same materials were analyzed by scanning electron microscope and tensile test. The results indicate that, when temperature is 950℃, the tensile strength is the largest, and the tensile fracture is composed of a lot of toughening nests, which has typical ductile fracture characteristics. Increasing connection pressure is beneficial to the metallurgical bonding interface, but in reality, low pressure is expected. The depth of diffusion layer is determined by heat preservation time, when the time is 120min, the depth is about 2μm, the junction surface achieves good metallurgical bonding. The better parameters were determined by experiment, which are T=950℃, pressures are 10～15MPa, time t=120min.Using SEM, EDS, XRD and other analysis methods, Ti2AlNb/TA15 interface combination state and organizational structure under different process parameters were studied. Experimental results show that, with increasing heating temperature and prolong holding time, interface diffusion layer depth increases. And the influence of temperature on the diffusion layer is greater than the pressure connection. The element redistribution in the diffusion layer resulted in the diffusion layer organization differs from the parent metal. XRD results show that the microstructure at Ti2AlNb fracture with a2 phase increased, B2 and O phase reduction, TA15 fracture α phase content reduce, the β phase content increased, and there is also part of B2, a2 and O phase. When the temperature of 940℃, the heat preservation time of 150min, connect pressure of 10MPa, good diffusion bonding joint can be obtained, The depth of diffusion layer up to 30μm. The finite element analysis results of residual stress distribution of Ti2AlNb/TA15 diffusion joint show that radial residual stress of the joint is Ti2AlNb side by tensile stress, the maximum value is 105.1MPa, TA15 side by compressive stress, the maximum value is 99.97MPa. The axial residual stress mainly distributed on the edge of the joint area, maximum tensile stress is 55.84MPa, the maximum compressive stress is 88.7MPa.Using molecular dynamics method, O phase and B2 phase calculation mode of diffusion behavior were built. The diffusion behavior of O phase and B2 phase alloy in different temperature and different pressure conditions were simulated. Results show that, under the condition of the micro, diffusion coefficient increases with temperature rise, and decreased with the increase of the pressure. Through the Arrhenius relationship between diffusion coefficient and temperature, calculate the self-diffusion B2, O and B2 diffusion activation energy, is respectively 0.67eV and 0.77eV.