Research On The Exploration Of Mechanism Of Pressure Brazing And Its Several Applications In The Regulation Of Dissimilar Interface Structure

With the development of science and technology,the service environment for various high-end equipment is becoming increasingly complex and demanding.Parts made of single materials are unable to satisfy these service needs.Dissimilar materials can leverage the advantages of different materials and obtain comprehensive performance that a single material cannot achieve,and have been widely used in various fields.However,due to significant physical and chemical performance mismatches between different materials,the welding of dissimilar materials is relatively difficult.In order to meet the growing demand for welding dissimilar materials,researchers have focused on both the development of new technologies and the potential of existing technologies.Pressure brazing is a common process in connection of dissimilar materials.This article focuses on the application of pressure brazing in the welding of dissimilar materials.Through experimental design,three potential mechanisms of action are uncovered and applied to welding of heterogeneous material parts,with the aim of providing new ideas for regulating the interface structure and relieving residual stress in heterogeneous materials.The main research results of this paper are as follows:Using AgCu28 and TiZrCuNi as filler,with oxygen-free copper as a soft interlayer,LSI C/SiC and Q235 were brazed with or without pressure.Combined with experimental results and finite element calculations,it was found that the yield strength of the brazing seam between C/SiC and Cu was significantly increased due to the free Si dissolved into the liquid filler during brazing at 860℃ for 15 min.This interface structure of brazing joint increases the distance between the Cu interlayer and C/SiC,and raises the Critical value for stress transmission to the Cu interlayer,which is not conducive for the Cu interlayer to reduce residual stress in the C/SiC-Cu-Q235 joint through its own plastic deformation.The shear strength of the C/SiC-Cu-Q235 brazing joints was 25.5 MPa.Therefore,this article uses a pressure brazing technology based on the diffusion reaction-extrusion composite mechanism,which uses the Cu interlayer and free Si in C/SiC to generate Cu-Si liquid phase by eutectic reaction at 950℃ for 15 min under 5 MPa,and extrude it out of the brazing seam.On the one hand,it can consume the free Si in the C/SiC near the interface;on the other hand,the Cu-Si liquid phase dissolves the remaining active TiZrCuNi brazing filler and carries it out of the brazing seam.Finally,the high-hardness brazing seam between C/SiC and the Cu interlayer was greatly reduced,and the low yield strength Cu(s,s)is replaced.This interface microstructure reduces the distance between the Cu interlayer and C/SiC,lowers the yield strength of brazing seam between C/SiC and the Cu interlayer,and is conducive to enabling the Cu interlayer to fully exert its ability to release residual stress.As a result,the shear strength of the C/SiC-Cu-Q235 joint is increased to 29.7 MPa.Ag-Ti filler was used to connect AMB Cu-metalized AlN substrates under different auxiliary pressures.Based on the experimental results and finite element calculation,it was found that the Cu-metalized AIN substrates had the problems of low welding rate and high residual stress when no auxiliary pressure was used.Firstly,this article uses a pressure brazing process based on the micro-liquid-phase-interdiffusion mechanism.Through the optimization of the amount of Ag element and the auxiliary pressure,under the experimental conditions of a mg/cm2 amount of Ag element(2.5<a<3.0)and 2 MPa,a near 100%welding rate is achieved for Cu-metalized AlN substrates.At the same time,due to the minimal amount of liquid phase generated during the pressure brazing,the brazing seam between AlN/Cu interface was solidified by the diffusion of Cu foil and micro-liquid AgCuTi phase during the holding stage at 830℃ for 30 min.Then,by optimizing the amount of Ti element and the brazing temperature,the growth state of TiN interface reaction is controlled.When a TiN reaction layer with an average thickness of about 0.7μm is generated on the AlN interface,the bonding strength of the AlN/Cu interface reaches its highest level.This interface structure reduces the distance between the AlN substrate and Cu foil,improves the plastic deformation ability of the AlN/Cu interface organization,and is beneficial to reducing the residual stress of the Cu-metalized AlN substrates and improving its thermal cycling reliability.The peeling strength of the AlN-covered copper clad laminate is as high as 34.6 N/mm,and its-55℃-150℃ thermal cycling life is up to 1000 cycles.The welding difficulty of dissimilar materials is not only affected by the physical and chemical property mismatch of the materials,but also by the joint structure.The feasibility of manufacturing high-temperature resistant steel-copper channel structure is explored by preparing 316SS-QCr0.5 joints using brazing,diffusion bonding,and pressure brazing.According to the analysis of the joint interface structure and mechanical testing,research has found that the heat resistance of the 316SS-QCr0.5 brazing joints with AgCu28 filler is poor,with a high temperature strength of 31.0 MPa at 600℃.The deformation rate of the Ni-plated 316SS-QCr0.5 diffusion bonding joint is as high as 6.4%.Both brazing and difficult bonding are difficult too meet the requirements of high temperature resistant steel-copper channel structures.When the BNi2 filler was used to braze 316SS-QCr0.5 joints,a large amount of Cr2B brittle phase was formed in the brazing seam due to the low diffusion reaction rate of B element in QCr0.5.By introducing lower auxiliary pressure in the brazing process,excess liquid BNi2 filler can be extruded out of the brazing seam,and promote the solid-liquid mutual diffusion between the base metal and liquid filler,thereby reducing the Cr2B brittle phase in the brazing seam significantly.Consequently,the tensile strength of the 316SSQCr0.5 pressure welded joint at 600℃ is 84.0 MPa,and the deformation rate is only 1.6%.Using this pressure brazing process,high-pressure resistant 316SS-QCr0.5 channel structures were successfully manufactured.In summary,this paper successfully applied three pressure brazing techniques based on different mechanisms to the welding of dissimilar materials,providing new ideas and reference examples for interface organization control and residual stress relief of heterogeneous material joints.