| Hexagonal BN ceramic is characterized by excellent heat resistance,dielectric property,chemical corrosion resistance and so on,which is widely used in machinery,metallurgy,aerospace and other fields.However,due to its lower strength and hardness than other ceramics,the application scope of BN is limited.Titanium alloys,especially TC4,has superior comprehensive performance and occupies an important position in many industrial areas.The reliable joining of BN and titanium alloy can complement each other’s advantages and expand the application range of these materials.However,the elastic modulus and coefficient of thermal expansion between BN and TC4 show great differences.The residual stress in BN/TC4 joint is considered as a obvious problem restricting the quality of the reliable joining of BN to TC4.In this work,based on the brazing of BN to TC4,the distribution of residual stress in the joint was analyzed and the residual stress was regulated.Vacuum brazing of BN/TC4 was realized by using Ag-Cu intermediate filler metal.The influence of different brazing temperatures on the interfacial micstrutures of BN/TC4 joint was investigated.An obvious Ti-Cu reaction layer was observed at the ceramic interface,which promoted the wetting and spreading of the molten filler metal on the BN surface.TC4 interface was composed of multiple diffusion reaction layers,and the change of these diffusion reaction layers of a typical joint from brazed seam to base material was Cu4Ti+Cu Ti → Cu Ti →Cu Ti+Cu Ti3→Ti+Cu Ti3.With the consideration of actual brazing conditions,the diffusion behavior of the interfacial elements and the form ation mechanism of the reaction layer were analyzed by combining the binary and ternary thermodynamic calculation.Besides,the joint mechanical property and failure modes were analyzed.The residual stress of BN/TC4 joint was predicted by finite element modeling numerical simulation.The residual stress of the joint was mainly concentrated in the area near the interface,and the stress on the ceramic side was obviously larger.S11 and S22 at BN interface were compressive stresses,generally higher than 200 MPa.The S33 stress presented compressive stress with the value no more than 50 MPa on the BN interface,but it mutated to tensile stress with the value of around 100 MPa at the edge.The regulation principle of residual stress in BN/TC4 joint was determined.The residual stress in BN/TC4 joint was not significantly affected by brazing cooling rate.The temperature of 400 ℃ and its vicinity was a relatively dangerous temperature range for the using of BN/TC4 joint due to the tensile stress change of S11 and S22 at BN interface.The interface design of BN/TC4 joint was conducted using finite element modeling numerical simulation,and the influence of different intermediate layer thickness on residual stress was simulated.It was found that the residual stress of the joint was alleviated with the increase of Ag-Cu intermediate layer thickness.Groove arrays with different patterns,intervals and depths were designed on the BN surface to obtain non-straight surface.The influence of different groove parameters on residual stress was analyzed.Triangular groove surfaces with larger depth and adequate distance showed the best alleviating effect on residual stress.Combined with the design of the the intermediate layer thickness and the design of the non-flat surface on the BN side,the optimal processing scheme to regulate the residual stress of BN/TC4 brazed joint was obtained. |
