Process And Mechanism For Brazing ZrB₂-SiC Composite Ceramic To Nb Alloy With Zr-Ni Filler

Ultra-high temperature ceramics?UHTCs?have attracted much attention due to their high melting point,high hardness and stable high-temperature chemical properties.As a representative of ultra-high temperature materials,ZrB₂-SiC has been widely studied by scholars for its unique high-temperature stability,high conductivity and thermal conductivity,high strength and high corrosion resistance.However,due to the poor plasticity and intrinsic brittleness of ceramics,the application range of ceramics becomes smaller.The strong plasticity of metals can make up for the defects of ceramic applications.Therefore,in this experiment,Zr-Ni filler was used to braze ZrB₂-SiC composite ceramics and Nb superalloy,the influence of brazing temperature,the thickness of brazing filler and holding time on the joint microstructures and mechanical properties were investigated,the connection mechanism was analyzed,and the residual stress in the joint was analyzed and controlled.ZrB₂-SiC composite ceramics and Nb alloy were brazed with Zr-Ni filler.The results show that the typical microstructures of the joint is ZrB₂-SiC/Zr₂Si+Nb C+ZrC +ZrB₂/Zr₂Ni+?Zr,Nb?₂Ni/?Zr,Nb?/Nb.The interface reaction layer is formed near the ceramic side,and the diffusion layer is formed near the Nb side.The width of the weld increases with the increase of the brazing temperature and holding time,and the amount of Nb fusion diffusion increases and gradually thickens.With the increase of brazing temperature,holding time and filler layer thickness,the mechanical properties of ZrB₂-SiC/Nb brazed joints all increased first and then decreased.When the brazing temperature was 1090?,the holding time was 20 min and the thickness of filler was 400?m,the shear resistance of the joints was the highest,which was 95 MPa.The connection mechanism of ZrB₂-SiC/Nb brazed welding joint is that when the temperature reaches the melting point of the filler metal,the filler metal starts to melt,the Zr in the filler metal reacts with the SiC in the base material to form Zr₂ Si and ZrC,and the Nb element diffuses to the ceramic side,and after cooling and solidification,the reaction layer of Nb C and ZrC is formed on the ceramic side.A diffusion layer?Zr,Nb?is formed on the metal side.In the middle layer of the weld,?Zr,Nb?₂Ni is generated,and finally a stable joint is formed.The Abaqus finite element simulation software was used to model the ZrB₂-SiC/Nb connection system,and the stress analysis and control of the cooled joint were simulated.With the increase of the filler thickness,the stress on the ceramic side will decrease gradually,and the maximum stress on the edge will decrease from 298 MPa of 50?m to 283 MPa of 70?m.but when the thickness of solder layer increases to a certain extent,the influence of the variation of the solder layer thickness on the stress on the ceramic side will be reduced by only 2MPa.The stress on the ceramic side decreases with the increase of the thickness of the reaction layer,and the maximum stress on the edge side decreased from 296 MPa of 10?m of the reaction layer to 275 MPa of 40?m.In order to further control the residual stress of the joint,the low expansion middle layer Mo was designed to adjust the residual stress of the ZrB₂-SiC/Nb joint,and the influence of Mo middle layer on the residual stress of the ZrB₂-SiC/Nb brazed joint was analyzed.The Mo middle layer not only reduced the overall stress level of the ceramic side,but also changed the distribution state of the stress on the ceramic side.When the thickness of Mo intermediate layer increased to 200?m,the maximum residual stress in the ceramic decreased from 283 MPa to 252 MPa,and the maximum stress level decreased by 7%.