Brazing Mechanism And Performance Optimization Of The Joint Of Al₂O₃ Ceramic And Nb

Al₂O₃ ceramic has superior high-temperature properties as well as excellent wear resistance,corrosion resistance and insulation.As a refractory metal,niobium has good thermal properties,processability and corrosion resistance.Joining Al₂O₃ceramic and Nb can meet the requirements of high temperature strength and corrosion resistance of complex structures.However,the relief of residual stress is an important challenge in the ceramic/metal brazed joint.In this paper,two forms of filler,the Mo particle-reinforced composite filler and composite interlayer with Cu were used to braze Al₂O₃ ceramic and Nb to reduce residual stress and improve joint strength.In this paper,Al₂O₃ ceramic was brazed to Nb with Ag-Cu-Ti filler and Mo particle reinforced Ag-Cu-Ti composite filler.SEM,EDS,XRD,TEM and universal testing machine were used to investigate the effect of Mo particle on the interfacial microstructure,residual stress and shear strength of the joints.The results show that the typical interfacial structure of Al₂O₃/Nb joint brazed with Ag-Cu-Ti filler at 900°C for 10min was Al₂O₃/Ti₃（Cu,Al）₃O/TiCu+Ti₂Cu₃+TiCu₄+Ag（s,s）+Cu（s,s）/Nb.When using composite filler with Mo particles,fine dispersed Ti-Cu compounds replaced the original blocky Ti-Cu compounds.When the Mo content was 8wt.%,the residual stress of the joint was-245MPa,which was 28%lower than that of the joint brazed with Ag-Cu-Ti filler.At the same time,the strength of the joint reached 203MPa,which was 33%higher than that of the joint brazed with Ag-Cu-Ti filler.Ag-Cu-Ti/Cu/Ag-Cu-Ti composite interlayer was used to braze Al₂O₃ ceramic and Nb.It is found that the interfacial structure changed to the structure of Ag-Cu-Ag based solid solution,and the residual Cu layer existed in the center of the brazing seam.The effect of Cu interlayer on residual stress distribution and magnitude of brazed joint was studied by FEM simulation of real brazing seam structure,and the strengthening mechanism of Cu interlayer on brazed joint was explored.The results show that the peak value of residual stress calculated by FEM decreased from 384MPa to 119 MPa when the thickness of Cu foil in the composite interlayer was 150μm.The reliability of FEM calculation was confirmed by the residual stress comparison between X-ray test value and FEM calculation value.It is proved that the ceramic strain energy is also a kind of physical quantity,which can characterize the properties of brazed joints.The properties of brazed joints with different material systems were unified by using the strain energy extracted by FEM.The effect of material properties on the ceramic strain energy was explored through two forms of ceramic/metal joints without interlayer and single interlayer.Finally,the relationship between material properties and ceramic strain energy was quantitatively expressed by mathematical formula.This would play a guiding role in the design of the experimental scheme.