Research On Brazing Process And Mechanism Of ZrC-SiC Ceramic And TC4 Titanium Alloy

The rapid development of aerospace technology calls for high-temperature structural materials with improved performance,which highly promotes the development and application of ultra-high temperature resistant ceramic materials at present.Among them,Zr C-Si C ceramic?referred to as ZS ceramic for short?is such a new type of ultra-high temperature resistant ceramic with extremely great resistance to heat and oxidation,which can thus be used as a new generation of attitude control engine nozzle component manufacturing material.Moreover,TC4 titanium alloy,a well-known material commonly used in aerospace,is characterized by low density,high specific strength,excellent corrosion resistance,and fatigue resistance.Therefore,the reliable connection between ZS ceramics and TC4 titanium alloy is the basis for practical application in the nozzle component.It can not only meet the high-temperature service requirements,but also ensure light weight as much as possible.In the ceramic/metal brazing systems,ceramics material generally faces with problem of surface inertness,while TC4 titanium alloys are prone to excessive dissolution and thus lead to a sharp decline in the performance of the base material.Therefore,in this study,the Cu-Zr alloy system is used to perform brazing on ZS ceramics and TC4 titanium alloy,revealing the process of joint formation.Moreover,the composition of the brazing filler metal is optimized for the problems in the brazing process,and the graphene reinforced composite brazing filler metal is further employed to improve the quality of the brazed joint and realize the high-quality connection of the ZS ceramic and the TC4 titanium alloy.Cu-54 Zr alloy is used to braze the connection between ZS ceramic and TC4 titanium alloy.The typical interface structure of the joint is ZS ceramic/Ti C/?Ti,Zr?₂Cu+?Zr,Ti?₂Cu+Ti?s,s?+?Ti,Zr?₂Cu/Ti?s,s?+?Ti,Zr?₂Cu/TC4 titanium alloy.With the increase of the brazing temperature and holding time,the thickness of the Ti C reaction layer on the ZS ceramic side and the Ti?s,s?diffusion layer on the TC4 side increase,and the volume fraction of Ti?s,s?+?Ti,Zr?₂Cu in the brazing seam gradually increases,the shear strength of the brazed joint firstly increases and then decreases.When the brazing temperature reaches 990°C and the holding time is set as 10 min,the TC4 base material dissolves moderately,the Ti C reaction layer is continuous,and its thickness is uniform,the joint can achieve the maximum shear strength of 41 MPa.Furthermore,with the aid of thermodynamic analysis,the joint interface evolution mechanisms during the brazing process can be summarized into four stages: solder melting,element interdiffusion,interface reaction,and structure stabilization.However,the fracture positions of ZS ceramic/Cu-54Zr/TC4 titanium alloy brazing joints are all ZS ceramic base materials,and the fracture surface is arched,indicating that the large residual stress at this time is the main reason for the failure of the brazing joints.In order to alleviate the residual stress of the brazed joint,the composition of the brazing material is optimized,and Cu-12 Zr is used to braze the connection between the ZS ceramic and the TC4 titanium alloy.At the same time,graphene is introduced into the brazing filler metal as a reinforcing phase,PECVD was used for the in-situ growth of vertical few layers of graphene?VFG?on the surface of the Cu powder,and VFG/Cu-12 Zr composite filler was prepared by mixing with ZrH₂ powder.The optimal process parameters of graphene growth were determined as the dispersant dosage of 2 wt.%,the growth temperature of 750°C,the holding time of 60 min,the gas pressure of 600 Pa,the RF power of 200 W,and the gas flux with the ratio of Ar:CH4=80:20.The prepared VFG is evenly and densely distributed on the surface of the Cu powder,and has good morphology.Then,the VFG/Cu-12 Zr composite filler was employed to braze the ZS ceramic and TC4,so as to achieve an typical interface structure of ZS ceramic/Ti C/Ti C+Ti₂Cu+?Ti,Zr?Cu/Ti?s,s?+Ti₂Cu/TC4 titanium alloy.When the brazing temperature reaches 990°C and the holding time is set as 10 min,the average shear strength of the joint at room temperature is 91 MPa,which is 122% higher than the Cu-54 Zr filler brazed joint.The quality and reliability of joints have been significantly improved.By comparing the interface structure and performance of VFG/Cu-12 Zr and Cu-12 Zr brazed ZS ceramic and TC4 titanium alloy joints,the effect of adding VFG on the brazed joints was explored.And it was found that the brazed joint interface hard and brittle phase of VFG/Cu-12 Zr composite filler Ti₂Cu is reduced and the?Ti,Zr?Cu phase with better toughness and plasticity increases.In addition the intermittent Ti C phase appears on the ZS ceramic side,and the proportion of the thickness of the Ti?s,s?diffusion layer on the side of the TC4 titanium alloy in the crack is reduced.And the average shear strength of the corresponding joint increases by about 20%.Through thermodynamic analysis of interface reaction products and theoretical calculation of joint residual stress,the VFG in the filler can react with the Ti element dissolved in the TC4 base material to adjust the interface structure and reduce the proportion of brittle and hard phase Ti₂Cu in the solder joint;At the same time,the addition of VFG reduces the coefficient of linear expansion of the brazing filler metal,which reduces the residual stress of the joint by 11% and further improves the joint performance.