Research On Processing And Mechanism Of Brazing C/C Composites With Graphene Sponge

C/C composites have excellent high-temperature properties and good ablation resistance.At the same time,they also have the advantages of low density and low thermal expansion coefficient.They are often used as structural materials for rocket engine nozzles.Therefore,achieving its own reliable connection has important application value.When brazing C/C composites,the significant physical and mechanical properties difference between the parent metal and the brazing filler metal are likely to generate large residual thermal stress in the brazed joints and reduce the mechanical properties of the joints.In this paper,the graphene sponge(GAI)was used as the intermediate layer,and Ti C reinforcement phase was introduced through in-situ reaction of Ti in the GAI and Ag Cu Ti solder,and the linear expansion coefficient of the joint was adjusted to achieve a high quality connection of the C/C composite.Brazing of C/C composites with Ag Cu Ti solder and GAI composites can effectively reduce interface hole defects and achieve high quality connections.At the brazing temperature of 860°C and holding time of 20 min,the typical interface structure of the joint can be expressed as: C/C/Ti C+Cu Ti+Cu2Ti+Cu Ti/Ag(s,s)+Cu(s,s)+Ti C/Cu Ti+Cu2 Ti+Cu Ti+Ti C/C/C.The effect of GAI thickness on the interfacial microstructure was explored,and the best weldability was obtained when the thickness was 3mm.The effects of brazing temperature and holding time on the interfacial microstructure and mechanical properties of the joints were investigated.The results show that the volume fraction of Cu2 Ti compounds in the brazing joints gradually decreases with the increase of brazing temperature or the holding time.GAI and The Ti-reaction produces more and more stripelike structures.The effect of brazing temperature on mechanical properties of joints is more significant than that of holding time.At the brazing temperature of 890°C and holding time of 20 min,the joint shear strength at room temperature reaches the highest value of 55 MPa,which is 57% higher than the 35 MPa of the direct brazed joint.At 860°C for 20 minutes,there is a certain amount of residual graphene after reaction,which is represented by the interface structure of the solder matrix/Ti C reaction layer/residual graphene.Among them,the Ti C reaction layer is composed of fine-grained Ti C and coarse-grained Ti C,and the Ti C reaction layer/residual graphene interface is tightly bonded and no defects are generated.At the graphene side close to the interface,the insertion of Ti C particles and the transition of graphene to amorphized carbon occur.At the graphene side close to the interface,the insertion of Ti C particles and the transition of graphene to amorphized carbon occur.The increase of temperature will cause the grain boundary in the nanocrystalline Ti C reaction layer to gradually become unstable,which will cause Ag Cu solder to infiltrate into it,and the final grain will break up and become discrete Ti C nanoparticles.The DIC-based in-situ tensile test was performed on the braze joints before and after the addition of GAI to analyze the local strain distribution during the stretching process.The addition of GAI can improve the local strain distribution of the joint to some extent,relieve the residual thermal stress problem and improve the compressive strength of the joint.However,the tensile strength of the joint is reduced and brittle fracture is easy to occur.By calculating the residual stress value of the joint through the mechanical model theory,it was found that the introduction of GAI can effectively relieve the residual stress in the C/C parent metal.The finite element method was used to simulate the influence of GAI on the residual stress distribution of the joints.It was found that the in-situ generated Ti C phase of GAI can significantly reduce the stress concentration of the joints and reduce the stress peak of the C/C side parent metal,thereby improving the mechanical properties of the joints.