GNPs Reinforced Composite Filler Alloy Design&brazing Process And Mechansim Of C_f/SiC-GH99

Carbon fiber reinforced silicon carbide ceramic matrix composite（C_f/Si C）is a high-temperature structural material with excellent performance.It has been widely used in high-performance aerospace engine combustion chambers,nozzles and high-speed aircraft heat-resistant structures that require harsh working environments.However,the complicated preparation process and the poor machinability of C_f/Si C making it difficult to manufacture complex structures.By realizing the reliable connection between C_f/Si C and superalloys,large-sized complex components can be manufactured,the weight of structure can be lightened and their excellent performance can be better utilized as well.To achieve a reliable connection between C_f/Si C and GH99,the nanosecond laser was utilized to process a blind hole structure on the C_f/Si C surface to be brazed and the brazed joint with excellent mechanical properties was obtained by using a self-designed filler alloy.First,the stress distribution of the joint was simulated and calculated by Abaqus finite element software,and the results showed that the main reason of ineffective connection of joint was the severe stress concentration in the C_f/Si C interface.The introduction of the blind hole structure into the C_f/Si C surface to be brazed will significantly change the distribution of interface stress,which effectively relieve the stress concentration.The blind hole structure processing on the C_f/Si C was realized by nanosecond laser,and the shear strength of the joint was improved evidently.The G-Ti Ni Cu（53Ti-23.5Ni-23.5Cu,at.%）composite brazing filler alloy with different graphene nanosheets（GNPs）content（0wt.%,0.3wt.%,0.6wt.%,0.9wt.%,1.2wt.%）was designed and prepared.The microstructure of filler alloy is composed of island-shaped Ti₂（Ni,Cu）compounds distributed on large-scale Ti（Ni,Cu）compounds.GNPs react with Ti to form dispersed Ti C_x particles during the preparation process of the filler alloy.With GNPs content increases,the amount of Ti₂（Ni,Cu）compounds decreases,and the fluidity of the composite filler deteriorates.By using Ti Ni Cu filler alloy and 150μm thick Nb layer,the typical interfacial microstructure of the joint obtained under the conditions of brazing temperature of1010°C and holding time of 15min is:C_f/Si C/（Ti,Nb）C+（Ti,Nb）₅Si₃/Ti（s,s）/（Ti,Nb）₂（Ni,-Cu）+Ti Ni Cu-β（Nb,Ti）eutectic/Nb/Ti Ni/Cr rich-Ti Ni/Ti rich-Ni,Cr（s,s）-/Ni,Cr（s,s）/GH-99.The strength of the joint firstly increases and then decreases with the increases of GNPs content in filler alloy.With high content of GNPs is 1.2wt.%in filler alloy,the connection between C_f/Si C and GH99 cannot be achieved;With the increases of brazing temperature,the amount of dissolved Nb in the intermediate layer increases,and GH99dissolves into the filler alloy after the Nb layer is penetrated,the joint strength is significantly reducing as well.When the holding time is kept for 5 minutes,the blind holes are not fully filled by incomplete melted filler alloy.When the holding time is 15 minutes,the blind holes are completely filled by the filler alloy with good fluidity,no voids can be detected in the interface,and the highest joint strength was achieved.Further extend the holding time,the filler alloy outflow seriously,voids appear in the interface,and the joint strength begins to decrease;The thickness of Nb layer does not affect the interfacial microstructure of C_f/Si C side significantly,and the joint shear strength increases.Using Ti Ni Cu filler alloy under the best process parameters of 200μm thick Nb layer,brazing temperature of1010°C,and holding time of 15min,the average shear strength of joint at room temperature is 64.2MPa,the highest reaches 65.8MPa.The average strength of the joint at 800°C is 53MP,reaches 82.6%of the strength at room temperature.A small amount of GNPs does not change the composition and microstructure of the reaction layer on C_f/Si C side.After the introduction of GNPs into the filler alloy,Ti（Nb）C particles surrounded by（Ti,Nb）₂（Ni,Cu）compounds appear at the joint interface;When the GNPs content is 0.3wt.%,the average shear strength at room temperature is 84.5MPa,31.6%higher than the joint brazed by Ti Ni Cu filler,the highest strength at room temperature reaches 106MPa.The high temperature shear strength of the joint is 76.4MPa at 800°C,reaches 90.2%of the room temperature strength.GNPs in the interface,do not affect the fracture morphology and characteristics of the joint.The fracture area of the joint can be divided into the fracture zones of filler column in the blind hole and the straight interface.The filler in the blind hole shows brittle transcrystalline fracture,and the fracture zone of straight interface is accompanied by Cf Pull off and pull out.The existence of the filler column extends the crack propagation path and forms a pinning effect at the interface.Both of them increase the strength of the joint.At a high temperature of 800°C,the filler softens,and obvious plasticity occur before the filler column breaks.The formation of the joint includes five processes:the contact reaction between GH99 and Ti foil,the formation of the eutectic liquid phase of the brazing joint,the reaction of Ti C_x and the eutectic liquid phase,the filling of the liquid filler into the blind hole and the C_f/Si C interface reaction.