Preparation Of Low Thermal Expansion Sc₂W₃O₁₂ Composite Brazing Filler And Brazing Mechanism Of C_f/SiC Composite And GH3536

C_f/SiC composites have the advantages of high strength,low density,ablative resistance and high temperature resistance,show huge potential in the application of aerospace engine nozzle materials.By using C_f/SiC composite material as nozzle material,not only can reduce the overall mass of the engine,but also can improve the combustion temperature of the engine,to greatly improve the thrust-weight ratio of space engine.This topic aims at the connection requirements of C_f/SiC composite nozzle and GH3536 thrust chamber of the new generation"Kuaizhou"series 2000N attitude control engine.In the face of huge residual stress and interface reaction control problems in large size brazed joint,this paper presents a method to prepare low expansion Sc₂W₃O₁₂ composite filler to improve the joining strength between C_f/SiC composites and GH3536.The complex interfacial reaction mechanism between the negative expansion material and the active filler metal was clarified by combining theoretical simulation and experiment,and the mechanism of carbon interfacial barrier layer alleviating the interfacial reaction and the residual stress relieving mechanism of brazed joint was revealed.On this basis,the Sc₂W₃O₁₂network composite interlayer was prepared by three methods.By taking advantage of the characteristics of the negative expansion material and the advantages of the network structure,the brazing property was adjusted to realize the gradient transition of the joint property to relieve the stress of the joint.The reliable connection between C_f/SiC composites and GH3536 was realized.The negative expansion Sc₂W₃O₁₂ material was designed and fabricated,and the wetting behavior of Ag Cu Ti filler metal on the surface of Sc₂W₃O₁₂ material was revealed.The wetting experiment results show that the wetting angles of the filler metal are 48^o and 15^o when the filler metal is maintained for 10 min and 60 min.The typicalstructureofthewettinginterfaceis Ag（s,s）+Cu（s,s）/Cu₃Ti₃O+Ti₃O₅/Sc₂W₃O₁₂+Cu_0.4W_0.6.Based on first principles,the initial interface and post-reaction interface bonding properties of the filler metal on the surface of Sc₂W₃O₁₂ were calculated.It is found that the initial interfacial separation work of Sc₂W₃O₁₂/Ti interface is-3.42 J/m²,and when Ti reacts with Sc₂W₃O₁₂ to form Ti/Ti₃O₅ interface,the interfacial separation work is 1.30 J/m².The calculation results show that the Ti/Ti₃O₅ interface is more conducive to improving the interface bonding strength than the Sc₂W₃O₁₂/Ti interface.The wetting mechanism between Sc₂W₃O₁₂ and Ag Cu Ti filler metal is determined to be interfacial reaction driven wetting through the combination of experiment and theory.The method of preparing interfacial barrier layer on the surface of Sc₂W₃O₁₂material to improve the retention rate of negative expansion material was proposed,and the mechanism of interfacial barrier layer was studied.The wettability and spreading wetting ability of Ag Cu Ti filler metal on the surface of the negative expansion material are greatly improved by the introduction of carbon interface barrier,which makes the negative expansion material and the filler metal form a stable interface in a shorter time.The first principles calculation shows that the adsorption energy of Ti atom on the Ti C surface is higher than that on Ti₃O₅.The Ti C surface promotes the adsorption of Ti atom and the spreading wetting ability of the metal filler.The thermodynamic calculation results show that the Ti atom diffuses from the filler metal to the Ti C surface under the drive of chemical potential.The Ti C layer effectively avoids the direct contact between the filler metal and Sc₂W₃O₁₂ material,and the diffusion distance of Ti atom is increased.The brazing test showed that the thickness of the interface reaction layer between the negative expansion material and the filler metal decreased from 1.3μm to 0.12μm,and the retention rate of the negative expansion material increased from 65.8%to 97.6%.The thermal expansion coefficient of the composite filler is reduced from 17.1×10^-6 K^-1 to 13.9×10^-6 K^-1 by18.7%.The maximum shear strength of brazed joints can reach 77 MPa,which is 3.2times that of the direct brazed joint.The mechanism of Sc₂W₃O₁₂ on the residual stress of brazed joint was revealed by finite element simulation,and the effects of yield strength,thermal expansion coefficient and elastic modulus of brazing filler on the residual stress of brazed joint were clarified.The results show that the residual stress of brazed joints is mainly concentrated at the side interface of C_f/SiC composites.After the negative expansion material was introduced,the side residual stress of C_f/SiC composites was effectively alleviated,and the Mises stress at the interface was reduced from 445 MPa to 374MPa,and the compressive stress at the interface was reduced from 337 MPa to 295MPa.In addition,by predicting the properties of the brazing seam,the results show that reducing the yield strength of the brazing seam properly,reducing the thermal expansion coefficient and elastic modulus of the brazing seam is beneficial to relieve the residual stress of the brazed joint,which provides guidance for the design of the negative expansion network interlayer.Based on the above research,the preparation method of Sc₂W₃O₁₂ network composite interlayer was developed,and the stress relieving mechanism of the network structure on the joint and the strengthening mechanism of the brazed joint were expounded.The Sc₂W₃O₁₂ network interlayer with high content of Sc₂W₃O₁₂was prepared by three methods,which solved the problem that the reinforcement with high content of Sc₂W₃O₁₂ is easy to aggregate.The Sc₂W₃O₁₂-Cu_f network interlayer prepared by the metal foam template method can increase the content of the negative expansion material to 20 vol.%,and the negative expansion material is evenly distributed in the brazing seam through the three-dimensional network structure of copper foam,which greatly alleviates the residual stress of the joint,and the maximum shear strength of the joint can reach 96 MPa.The Sc₂W₃O₁₂ network interlayer with continuous structure was prepared by using polyurethane as template by organic foam template method.Compared with Ag Cu Ti filler,the thermal expansion coefficient of the composite filler prepared by Sc₂W₃O₁₂ network interlayer was reduced by 29%from 16.6×10^-6 K^-1 to 11.9×10^-6 K^-1,greatly reducing the thermal expansion coefficient of the brazing seam.The residual stress of brazed joint can be effectively alleviated,and the shear strength of brazed joint can reach130 MPa.Finally,a Sc₂W₃O₁₂ dispersive interlayer was prepared based on the design idea of metal composite material.The content of Sc₂W₃O₁₂ material was increased to40 vol.%,and the thermal expansion coefficient of the composite filler metal was reduced to 10.9×10^-6 K^-1,which was 34%lower than that of the Ag Cu Ti filler metal.The shear strength of brazed joints with dispersive negative expansion interlayer can reach 141 MPa,which is 5.9 times higher than that with Ag Cu Ti filler metal and 1.8times higher than that with Sc₂W₃O₁₂ particle reinforcement.