Research On Brazing Process And Mechanism Of TiAl Alloy To Si₃N₄Ceramic

Due to the excellent mechanical properties and good wave penetratingperformance, Si₃N₄ceramics are preferred as the next generation materials forradomes, which need be connected with metallic materials in the assembly andservice process. TiAl alloys possess high specific strength, high specific stiffness,good oxidation resistance, as well as excellent high-temperature mechanicalproperties. The components made by joining the two materials could be applied tothe fields of aerospace and weaponry, which can not only reduce weight but alsoincrease service temperature. The objective of this study is to achieve the joiningof TiAl alloys and Si₃N₄ceramics. In the present study, appropriate brazing alloysystems were determined by experiments. The interactions between molten brazingalloys and parent materials as well as the brazing mechanism was investigatedbased on the characterization of interfacial microstructure. In addition, accordingto the composite reinforcement theory, AgCuTi+nano-Si₃N₄composite brazingalloys were developed and used to achieve the brazing of the two materials. Thedispersion mechanism of fine compounds in brazing seams was discussed and thedistribution of residual stress in joints was analyzed by finite element methodsimulation and X-ray analysis.TiNiNb and TiNiV eutectic brazing alloys were developed and reliablejoining of TiAl alloys was realized with the two brazing alloys. The typcialinterfacial microstructure of TiAl joints brazed using TiNiNb and TiNiV brazingalloys were TiAl/B2+γ+O-Ti2AlNb/γ+τ3-Al3（Ti,Nb）2Ni+O-Ti2AlNb and TiAl/B2+γ/τ3-Al3NiTi2+B2+γ/B2+τ3-Al3NiTi2resepectively, which were determined bySEM, EDS, TEM and XRD. The highest shear strength at room temperature andhigh temperature （600°C） reached308MPa and172MPa when joints were brazedusing TiNiNb brazing alloy at1200°C and1220°C resepectively. Metallurgicalreactions occurred when the two eutectic brazing alloys were used to braze Si₃N₄ceramics to themselves and to TiAl alloys, which resulted in a continuous TiNlayer was formed adjacent to Si₃N₄side and a brittle Ni2Ti layer in the brazingseam. However, cracks always formed in these brittle reaction layers because oflarge residual stress in joints, which resulted in the failure of brazed joints eventhough Nb foil was introduced as a stress-release interlayer.Brazing of TiAl alloys and Si₃N₄ceramics was achieved using AgCu eutucticbrazing alloy. The interfacial microstructure was TiAl/B2/AlCuTi/AlCu₂Ti/Ag（s,s）+AlCu₂Ti/TiN+Ti₅Si₃+AlCu₂Ti/Si₃N₄. During brazing, active element Ti reactedwith Si₃N₄to form a continuous TiN+Ti₅Si₃layer and a part of Cu reacted withTiAl to form AlCuTi and AlCu₂Ti reaction layers. V reacted with Al to form fine dispersed Al3V particles, as the nucleations of AlCu₂Ti phase, resulting in thedispersion of granular AlCu₂Ti compounds in brazing seam. In addition, thekinetic equation of TiN layer was obtained based on the experimental datas. As theincrease of brazing temperature or brazing time, the thickness of reaction layersincreased and granular AlCu₂Ti compounds aggregated, even some microcrackswere formed in brazing seams. The optimal process parameter was860°C/5min.AgCuTi+nano-Si₃N₄composite brazing alloys （AgCuTic） were developed andused to braze TiAl alloys and Si₃N₄ceramics in order to reduce residual stress injoints and improve joining properties. The typical interfacial microstructure ofTiAl/AgCuTic/Si₃N₄joint was TiAl/AlCu₂Ti/Al₄Cu₉+TiN+Ti₅Si₃+Ag（s,s）/TiN+Ti₅Si₃/Si₃N₄. During brazing, nano-TiN and nano-Ti₅Si₃compounds were formedby the reaction of Ti and nano-Si₃N₄, which acted as nucleations of Al₄Cu₉compounds resulting in the formation of Ag based composite structure in brazingseams. The investigation shows that the dissolution and diffusion of element Tiwere the key foctors in the formation of composite brazing seams and thedissolution equation of TiAl was obtained. In addition, reinforcement content,brazing temperature, as well as brazing time had a significant impact on theinterfacial microstructure and joining properties. The highest shear strength atroom temperature and high temperature （400°C） were115MPa and156MParespectively when brazed at880°C for5min using AgCuTic with3wt.%nano-Si₃N₄addition, which was2times as high as that of joints brazed usingAgCuTi brazing alloy.The effects of composite brazing alloy on improving joining properties wereinvestigated and the results show that brazing seams were strengthened by loadbearing effect, dislocation strengthening and Orowan strengthening. Furthermore,the residual stress in joint was reduced due to the reduction of the CTE coefficientof brazing seams. FEM simulation results show that residual stress distributionform unchanged but both the stress area and stress peaks were reduced whencomposite brazing alloys were utilized. X-ray stress analysis show thatcompressive stress peak on the surface of Si₃N₄ceramics was reduced by70MPawhen AgCuTic with3wt.%nano-Si₃N₄addition was used in this research, whichwas coincide with the simulation results.