| Carbon fiber reinforced silicon carbide?Cf/SiC?composie is a new type of thermal structure ceramic matrix composite,which exhibits an excellent combination of mechanical properties,such as high temperature resistance,wear-resisting,excellent oxidation resistance and low density.It can be used to manufacture rocket engine combustion chamber instead of high temperature alloys.It can improve the working temperature of the combustion chamber,and greatly reduce the weight of the engine,so as to improve the thrust-weight ratio of rocket engine.In the application of the rocket engine,Cf/SiC composite needs to be joined to itself or Nb.In the present study,a suitable high temperature filler alloy was designed and fabricated to join Cf/SiC composite and Nb and Cf/SiC composite itself.The microstructure and properties of the joints were investigated,and the content and thickness of the filler alloy and brazing parameters were explored.Also,the finite element simulation software was used to analyze the influence factors of the joint performance.The residual stress of the joints after cooling with different yield strength,thermal expansion coefficient and elastic modulus of the filler alloy was investigated using Abaqus finite element analysis software.The results showed that the lower the yield strength and thermal expansion coefficient was,the smaller the residual stress of the joint was.While the elastic modulus of the filler alloy only works when no plastic deformation took place in filler alloy.According to the simulation results and the joint service temperature,TiCoNb high temperature filler alloy was designed and the microstructure and thermal physical properties of the filler alloy were investigated.The results showed that Ti35Co35Nb30 alloy was made up of TiCo and Nb?s,s?eutectic.It started to melt at 1260?,and the thermal expansion coefficient changed between 8-11×10-6?-1.In addition,the yield strength and fracture strength of the filler alloy were 275 MPa and 600 MPa,respectively.Then,the Ti35Co35Nb30 filler alloy,TiCo eutectic alloy and Ag-Cu-Ti alloy were introduced into joining Cf/SiC composite and Nb,and the shear strength of the joints were 178 MPa,72MPa ? 112 MPa,respectively.As TiCoNb filler alloy was used to join Cf/SiC composite and Nb,the Nb content in the filler alloy did not affect the microstructure and properties of the joints.However,the thickness of the filler alloy could influence the thickness of the brazing seam,and the best thickness of the filler alloy was 550?m.Then the Ti42.5Co42.5Nb15 alloy with the thickness of 550?m was introduced into joining Cf/SiC composite and Nb.While rising the brazing remperature or prolonging the holding time,the reaction between filler alloy and Cf/SiC composite became more violent,the thickness of the reaction layer became thicker,CoNb4Si brittle phase was increased,the width of the brazing seam was decreased.The shear strength of the joint firstly increased and then decreased.The optimum brazing parameters were 1320? for 10 min,and the joint obtained a maximum shear strength of 187 MPa.The typical microstructure of Cf/SiC-Nb joint was Cf/SiC composite/?Ti,Nb?C /?Ti,Nb?2Co/TiCo+Nb?s,s?+CoNb4Si/Nb.The high temperature shear strength of the joint reached 202 MPa and 124 MPa at 800? and 1000?,respectively.And the joint strength also remained above 150 MPa after 20 times thermal shock test.The shear strength of the Cf/SiC-Cf/SiC joint firstly increased and then decreased with increasing the thickness of the Ti35Co35Nb30 filler alloy,the optimum thickness of the filler alloy was 800?m with the highest shear strength of 34.4MPa.The microstructure of the joint was investigated.The results showed that brazing seam mainly consist of TiCo intermetallic compound,CoNb4Si and?Ti,Nb?2Co compound and TiCo and Nb?s,s?eutectic.Inorder to improve the microstructure of the joint,Nb foil was introduced into the filler alloy.With elevating brazing temperature or prolonging holding time,the Nb foil thickness thinning and disappeared gradually,TiCo and Nb?s,s?eutectic and CoNb4Si compound increased,and massive TiCo intermetallic compounds disappeared gradually.The highest shear strength with 66 MPa was received while the joint brazed at 1300? for 15 min.The Abaqus finite element simulation software was used to establish the finite element model of the Cf/SiC-Nb and Cf/SiC-Cf/SiC joint.The cooling process and shear test process were simulated.Results showed that the maximum stress occurred in the vicinity of the Cf/SiC-filler alloy interface.Conparison of simulation results on different joint microstructure,structure and material system,it could be found that the stress would be reduced and joint strength increased by increasing the reaction layer,thickness of brazing seam,plastic of joint microstructure.It also verified that the exist of soldering angle and pinning structure in the joint was benefical to the joint property.Lastly,the simulation results explained why the shear strength of the Cf/SiC-Nb joint was higher than Cf/SiC-Cf/SiC joint. |
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