Process And Joining Mechanism Of Brazing SiC Ceramics With Ag-Cu-Ti Composite Filler

As an important member of structural ceramics,silicon carbide ceramics have wide application prospects in industry because of its excellent performance.However,it is difficult to directly obtain large size complex structural parts due to the limitation of preparation and processing,which can only be achieved through the connection of materials.Brazing is the most common bonding technology in the application of ceramics.However,large residual stress was produced due to the obvious difference of thermal expansion coefficient and elastic modulus between metal filler and SiC ceramics,the joint was unreliable and even cracked.From the point of view of adjusting the coefficient of thermal expansion of Ag-Cu-Ti filler,seeking the strengthening phase which can reduce the coefficient of thermal expansion and increase the strength of joint,and determined the appropriate parameters of brazing process.The microstructure of brazing joint and the formation mechanism of interfacial reaction layer were studied by means of SEM,EDS,XRD and TEM.The shear strength of joint was tested by material testing machine.The results showed that,when brazing SiC ceramics with Ag-Cu-Ti filler,the brazing seam is mainly composed of Ag-based solid solution and Cu rich phase,and the interface reaction layer is composed of Ti₅Si₃+TiC reaction layer.The effects of brazing temperature,holding time,volume fraction of reinforcing phase on the microstructure and mechanical properties of the joints were investigated.The results showed that the phases composition of brazing seam would not be changed by adding a small amount of SiC,TiC,W and Cr₃C₂ particles into the Ag-Cu-Ti composite filler.The reinforced phases will be diffusely distributed in the brazing seam.When the proper amount?5%¹0%?of second phase particles were added,the interlayer microstructure can be refined by the promoted of heterogeneous nucleation of the solid solution.At the same time,the residual stress of the joint can be alleviated due to the reduced of thermal expansion coefficient of the composite filler,thus the bonding strength of the joint can be improved.The clustering and particle-to-particle contact behavior was observed in the joint with high amount of second phase particles?>15%?,which can lead negatively affect on the connection of the joint due to the reduction of capillary flow of composite filler.The filler has not been fully melted when the brazing temperature was too low?787??or holding time was too short?5 min?.No continuous dense reaction layer can be formed and the intermediate layer have defects such as porosity and porosity.The element diffused violently when the brazing temperature was too high?827⁸47??.The increase of Ti₅Si₃ brittleness phases caused the decrease of strength.The reaction layer was thicker when the holding time was longer?>20 min?,which lead to the increase of lattice mismatch force and caused the decrease of strength.Every phase of the interface was fully diffused and the reaction layer was continuous and uniform,the best joint was obtained at proper brazing temperature and holding time.The maximum shear strength?212.83 MPa?was obtained when the volume fraction of Cr₃C₂ particles was 10%,the brazing temperature was 807?and holding time was10 min.The increase of the shear strength was 103.36 MPa and almost 170%higher than the maximum shear strength?78.58 MPa?brazed by pure Ag-Cu-Ti filler alloy.Compared with the Ag-Cu-Ti-xSiC increased to 103.36 MPa,Ag-Cu-Ti-xW increased to 95.05 MPa and Ag-Cu-Ti-xTiC increased to 115.09 MPa,it can be seen that the addition of Cr₃C₂ particles has a more significant effect on the adjustment of composite filler.The joint was subjected to thermal exposure at 500?.With the prolongation of the thermal exposure time,the microstructure of the brazing seam appeared porosity and even the abnormal grain growth occurred,which caused the decrease of the strength.