Researchs On Microstructure Evolution And Thermal Stability Of The Welding Interface Of TC11/Ti-Al-Nb

As the temperature gradient and stress gradient for high pressure compressor disks in high-performance engines is large, the conventional single alloy disks are difficult to meet the properties requirement. Joining the intermetallic compounds of excellent high-temperature performance with titanium alloys of excellence low-temperature to replace the high pressure compressor disk for superalloys can reduce the weight of engine effectively and increase the thrust-weight ratio. Because the chemical compositions and properties of joint for dual-alloy are different from the base alloys, exploring the deformation parameter to improve the stability for microstructure and property is a critical process. As the high-pressure compressor disks service environments is severe and contact with the high temperature and pressure gas, so the stability of microstructures for joints is crucial. In this paper, the deformation parameters were optimized based on the isothermal hot compression test of TC11/Ti-Al-Nb dual-alloy joints. At the same time, combined isothermal forging experiments and thermal stability experiments, microstructure evolution for welding interface under thermal and mechanical coupling were studied. The influence of microstructure types on the mechanical property was revealed and the fracture mechanisms were deeply analyzed. Meanwhile, the quantitative relationships between microstructure parameters and tensile properties were established. The main research work was as follows:The flow stress of TC11/Ti-22Al-25 Nb dual-alloy during high temperature deformation was very sensitive to the temperature and strain rate, and the flow stress decreased with the increase in temperature and decrease in strain rate. Discontinuous yielding characteristic curve appeared at the high temperatures and large strain rate. A sin-hyperbolic Arrhenius constitutive equation could accurately simulate the deformed behavior of dual-alloy.The processing maps of TC11/Ti-22Al-25 Nb dual-alloy were constructed based on the dynamic material model and Prasad instability criterion to optimize the thermal processing parameters. The hot working process can be carried out safely in the domain with the strain rate of 0.001-0.6s-1 and the temperature of 900-1060℃. When strain rates above 0.6s-1, flow instability appeared which characterized by adiabatic shear bands, localized plastic flow and microcracks in the interface of weld and base alloy. With the increase in the degree of deformation, plastic instability regions that in the low temperature and high strain rate, high temperature and high strain rate were extended to the regions for low strain rate and intermediate deformation temperature.The influence of alloy elements on the microstructures and mechanical properties of TC11/Ti-Al-Nb dual-alloy have been studied. It can be found that the α2 phases precipitated in the fusion zone decreased as the element content of Nb increased. Through the comparison of mechanical properties, it can be found that the properties of TC11/Ti-22Al-27 Nb dual-alloy were well. The changes of microhardness and elastic modulus in interface were caused by the microstructures evolution. Microhardness in heat affected zone of TC11 alloy decreased as the martensite α’ decomposed, while microhardness in fusion zone and heat affected zone of Ti-22Al-27 Nb alloy increased as the secondary phases precipitated. The distribution of elastic modulus in weld area showed a “U” shape while the fusion zone had a lower value. Meanwhile, the elastic modulus of the weld area can be improved by isothermal forging and heat treatment.The microstructures and properties of TC11/Ti-22Al-25 Nb dual-alloy were influenced by the hot working process. In the welding condition, the fusion zone was composed of β phase. When deformed at 940℃, grain boundaries consisted by intermittent α/α2 phases, equiaxed α2 phase and fine O phase distributed in grains. As the deformation temperatures rose to 980℃ and 1020℃, equiaxed α2 phase disappeared and only fine O phase distributed in β matrix. By the room-temperature tensile properties, it found that tensile strength can be improved through the isothermal deformation. As deformation degree increased, the thickness of O phase in fusion zone increased resulting in the room-temperature tensile strength decreased. In comparison, the O phase laths increased as the strain rate increased resulting in the room-temperature tensile strength increased duo to the higher dispersion strengthening contributions. The specimens were fractured in TC11 alloy side when tensile tests at 300℃ and 500℃.The thermal stability mechanism for TC11/Ti-22Al-25 Nb dual-alloy was obtained during the thermal exposure. Sheet-like α phase in TC11 alloy was stability during exposure temperature of 500℃. While exposure temperature rose up to 600℃ amd 700℃, sheet-like α phase got thickened and brittle α2 phase and silicides precipitated in TC11 alloy. Ti-22Al-25 Nb alloy was stability as the thermal exposure temperature below 700℃. While temperature reached 700℃, the B2 phase became to decompose and transformed to O phase and disordered structure β phase. In fusion zone, the thickness of O phase and grain boundary increased gradually as the exposure temperature rose and exposure time increased; when thermal exposure at 700℃, the thickness of O phases increased rapidly. The tensile strength and plasticity properties reduced as the exposure time increased. The dual-alloy joints were all fractured in the fusion zone after thermal exposure at 500℃. By comparison, joints were all fractured in the TC11 alloy side after thermal exposure at 600℃ and 700℃. The concentration differences of elements on both sides of the interface became slow as the thermal exposure temperature increased.The mechanical properties change for dual-alloy joints was influenced by the microstructures evolution. The mechanism of mechanical properties improved for joints was the effect of precipitation strength of the second phases. Meanwhile, as the distribution of second phases is nonuniform resulting in the plasticity properties reduced. Sheet-like α phase and coarse grain boundary α2 phase were harmful to the properties. During the thermal exposure, the coarsening of sheet-like α phase and O phase was harmful to the plasticity properties. As the thermal exposure temperature rose, α2 phase was precipitated in grain boundaries and grew up resulting in the brittleness of grain boundaries increased. Meanwhile, the O phases coarsened during the thermal exposure. Combined action of α2 phase and O phases, the impact toughness of joints reduced during the thermal exposure process.Calculating the growth kinetic parameters of O phase and α phase, it can be found that the growth exponent increased as the aging temperature increased while the activation energy changed slightly as the holding time increased. Constructing the relationship between the thickness of lamellar O phase and α phase and tensile perperties, the yield strength and elongation agreed well with the Hall-Petch relationship via varying the thickness of phases.