| Porous Si3N4 ceramic is one of the missile radome shell material.It possesses low permittivity,thermal shock resistance and antioxidation.In the process of the radome assembled and used,the ceramic shell material has to be effectively connected with the metal ring.Invar alloys are often used as metal ring materials due to their extremely low coefficient of thermal expansion at room temperature.It is difficult to obtain a joint which satisfies the high-temperature performance,like the problem of the brazing connection using Ag-Cu-Ti solder,and in order to overcome the drawback of the low strength and easy aging of the ordinary adhesive joint.In this paper,Cu-Ti active brazing filler was used to realize the high temperature brazing connection of porous Si3N4 ceramics and Invar alloy.The influence of brazing temperature and holding time on the microstructure and properties of the joint was studied by theoretical analysis.Based on the problems in the actual brazing process of Cu-Ti brazing filler metal,the brazing filler metal system was redesigned by adding the middle layer,which greatly controlled the joint structure and improved the mechanical properties of the joint.The porous Si3N4 ceramic was brazed to Invar alloy using Cu80Ti20 brazing filler.The typical microstructure of porous Si3N4/Invar joint was as follows: The porous Si3N4 ceramics / Ti N + Ti5Si3 + Fe2 Ti / Cu(s,s)+ Fe2 Ti + Ni Ti / Invar alloy.The influence of brazing temperature and holding time on the microstructure and properties of the joint was studied.With the increase of brazing temperature or the holding time,the thickness of the interface layer in the interface increased,the number of intermetallic compounds increased,and the shear strength first increasde and then decreased.When the brazing temperature is too low or the holding time is too short,interface reaction of the ceramic side is not sufficient,and failed to achieve a reliable connection joints.In the scope of this study,The maximum joint strength(20.1MPa)was obtained while the porous Si3N4 ceramic and Invar alloy were brazed at 1050? for 15 min.The porous Si3N4 ceramic was brazed to Invar alloy by using the Cu-Ti / Nb /Ni-Si-B brazing filler which is a multi-layer composite manner.The typical structure was: porous Si3N4 ceramic / interfacial reaction layer(Ti N + Cu Solid solution)/weld(Cu-based solid solution + Nb5Si3 + Nb3 Si + Ti N)/ Nb / weld(Ni6Nb7 + Ni3Nb+ Ni-based solid solution + Ni2Si)/ Invar alloy.The effects of Cu-Ti / Nb-Ni-Si-B multilayered brazing on the microstructure and properties of the interface were investigated.The results showed that the joint shear strength of Cu-Ti / Nb-Ni-Si-B brazing filler metal was higher than that of Cu-Ti When the brazing temperature is1080 ? and the holding temperature is 15 min,the maximum shear strength of the joint is 46.1MPa,which is twice as high as that of Cu-Ti solder.The connection mechanism of porous Si3N4 / Nb side in brazing porous Si3N4 /Invar by using Cu-Ti / Nb-Ni-Si-B brazing filler metal was investigated.Cu-Ti solder is melted and,Ti atoms and Cu atoms penetrate into the porous Si3N4,in which the active element Ti reacts with Si3N4 to form Ti N phase and is wrapped around the Si3N4 particles,causing the peeling and disappearing of the ceramic base material.A large amount of Si atoms was released,reacts to Nb atoms,which with the rapid diffusion from The Nb at the interface of the reaction layer,form the Nb-Si compound phase.At the same time,the N atoms decomposed by the porous Si3N4 ceramic will diffuse into the liquid brazing filler metal and react with the Ti atom to form the Ti N phase which is black particles due to the high reaction temperature.The connection mechanism of Nb / Invar side in the joint was investigated.During the brazing heating process,with the melting of the brazing filler metal,a small amount of Nb atoms at the surface of the Nb foil diffuses into the liquid brazing material and reacts with a large amount of Ni atoms to form Ni3 Nb phase.That is the diffusion of Ni atoms in the brazing filler metal and the diffusion of Nb atoms into the Nb foil interface in the Nb foil.In this dynamic equilibrium process,the concentration of Ni element decreases gradually along the direction of the liquid solder to the Nb foil The concentration of Nb and the Nb atoms gradually increase,and finally,during the brazing cooling process,the multilayer compound reaction layers were formed at the interface of Nb / brazing filler metal,and the Ni solid solution and the silicon compound are solidified in the weld. |
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