Study On Process And Mechanism Of Brazing Surface Alloying C/SiC Composite And Nb By Carbon Fiber Woven Cloth

C/SiC composites have high strength,high hardness and wear resistance,as well as excellent high temperature and oxidation resistance.C/Si C has better thouhness and lower density than traditional Si C because of the low thermal expansion coefficient and absorbing energy by pulling out and deflecting of reinforced fibers in fracture process,which widely used in the aerospace fiels.C/Si C is often connected to the high-temperature metal Nb to obtain better material properties,but due to the large difference in physical properties between the base material and the brazing filler metal,large residual stress often occurs after welding and The reaction layer on the side of the composite material is weak.In this regard,this subject takes C/Si C surface alloying,to adjusts the thermal expansion coefficient through the internal infiltration layer to reduce the residual stress,optimizing the structure of the infiltration coating layer,the surface W reinforced network C fiber cloth intermediate layer was used to optimize the quality of the joints and ultimately obtain high-quality brazed joints.In order to solve the problem of excessive residual stress caused by the large difference in the properties of base metal,The C/Si C surface was alloyed with Ni-CrSi alloy,and the wettability of the brazing filler metal on the C/Si C surface increases with the increase of Cr element content.Ni element infiltrates into Ni2 Si phase layer,which plays a role of gradient transition of thermal expansion coefficient.The surface infiltration layer is mainly composed of binary and ternary compounds produced by infiltration reaction of Ni,Cr and Si,which has a certain brittleness and is continuously distributed.Through the finite element stress simulation,it was found that the residual stress distribution state is optimal when the thickness of the Ni2 Si phase layer in C/Si C was 75 ?m.The thickness of the infiltrated layer can be adjusted by changing the infiltrating parameters.As the infiltrating temperature,brazing material quality and holding time increasing,the thicknessi of nfiltrating layer increased,and the shear strength first increased and then decreased.The thickness of the infiltrated layer is 77?m under the optimal plating parameters,which was basically consistent with the simulation results.The corresponding shear strength is 115.2MPa,which is 35% higher than that of the unalloyed C/Si C-Nb joint.In order to solve the problem of continuous distribution of brittle phases in the surface infiltrated layer,this subject takes the W and WC particles as wild phase in the infiltrated alloy,and the Ni4 W generated by the reaction can break the continuously distributed brittle phase.When the particle size is large,the effect of breaking the continuous brittle phase is not obvious,and when the size is small,the brittle phase can be distributed intermittently.Through the optimization of the structure of the surface infiltration coating,the linear expansion coefficient of the C/Si C surface infiltration coating is alleviated,and the optimal shear strength of the joint obtained by adding W and WC particles is 126.4MPa and 121.8MPa respectively,which has been improved during optimization.In order to further reduce the coefficient of thermal expansion of weld,Based on the principle that network C fiber cloth and W can effectively reduce the linear expansion coefficient,WO3 reinforced network C fiber cloth was prepared by hydrothermal reaction,and then W-wrapped network C fiber cloth was obtained by reduction.By controlling the parameterss of hydrothermal reaction can adjust the surface morphology of WO3,when the concentration of the H2WO4 precursor solution and(NH4)2SO4 is too small or too large,defects such as incomplete WO3 encapsulation and WO3 agglomeration occur.The presence of W can fill the voids inside the network C fiber cloth and form a W-rich region near the C fiber.As the carrier of W,the network C fiber cloth doesn't shift in the weld.and because it consumes a part of Ti in the filler alloy,which makes the reaction layer on the C/Si C side interface become thin.Finally,when the interlayer of W-reinforced network C fiber cloth is used for auxiliary brazing,the strength can reach 131.8MPa,which is 54% higher than that when no intermediate layer is added.