| In this dissertation,to solve the problems of joining thermal stress and high-temperature resistance of the Cf/SiC composite/stainless steel joints,based on the reaction of CuTi15+C+Ni→TiC+(Cu,Ni)ss,a joining technology named reaction-composite diffusion brazing which combined reaction-composite brazing and partial transient liquid phase bonding(PTLP)was developed.To solve the problem of interface cracks on the stainless steel side,which were caused by the high-brittleness TiFe compound,the surface of 304 stainless steel was first electroplated with nickel before brazing,and then mixed composie material fillers of CuTi15 alloy powders,C powders(diamond powders)and Ni powders were used to join Cf/SiC composite and stainless steel(nickel-plated)with the reaction-composite diffusion brazing process.The joint microstructure and the effects of joining parameters,such as the thickness of nickel coating,materials parameters and process parameters,on the joint microstructure were analyzed.Results showed that,the typical microstructure of the joint was Cf/SiC composite/TiC+Ti5Si3+Ti3SiC2+Ni16Ti6Si7(interface on the Cf/SiC composite side)/(Cu,Ni)ss+TiC+C+Ti(NixCu1-x)2(joining layer)/Ti(NixCu1-x)2+TiNi+(Cu,Ni)ss+(Ni)ss+(Fe,Ni)ss(interface on the stainless steel side)/304.During the brazing process,the nickel plating layer not only effectively prevented the formation of brittle TiFe compounds,but also diffused Ni elements into the joining layer,which eliminated interface cracks on the stainless steel side and increased the high-temperature resistance of the joining layer;C powders consumed the melting point depressant element Ti,and formed TiC phase around C particles(reaction-composite brazing).Ni powders difused element Ni into the joining layer,and reacted with the matrix of the joining layer to form Ti(NixCu1-x)2 phase.With the increase of brazing temperature and the extension of holding time,Ti atoms were gradually consumed by C particles,and Ti(NixCu1-x)2 phase was gradually decomposed according to the reaction of Ti(NixCu1-x)2+C→(Cu,Ni)ss+TiC in the joinig layer.However,excessive process parameters lead to the formation of thicker reaction layer at the interface of Cf/SiC composite side,which is unfavorable to the joint mechanical properties.The interfacial reaction behavior on the Cf/SiC composite side and the isothermal solidification behavior of the joining layer were studied,and the formation mechanism of the joint was also revealed.Results showed that the growth of the reaction layer followed the parabolic law controlled by diffusion.The addition of C powders and Ni powders weakend the fluidity of the liquid phase,resulting in the decrease of the growth rate of the interface reaction layer;the increase of brazing temperature not only increased the diffusion coefficient of reaction layer,but also increased the thickness of reaction layer during heating and cooling process,which is conducive to the growth of interface reaction layer.Due to the reaction between C powders and Ti element and the isothermal diffusion between Ni powders and the joining layer matrix,the joining layer isothermally solidified in a short time,and the reaction behavior between C powders and Ti element was the main controlling factor for the isothermal solidification of the joining layer.The mechanical properties and high-temperature resistance of the joint were evaluated,the influence of the joining parameters on the performances of the joint were analyzed,and the optimal joining parameters were finally determined.Results showed that the increase of the addition of C powders improved the mechanical properties and high-temperature resistance of the joint at the same time,but the addition of Ni powders only improved high-temperature resistance of the joint.The increase of brazing temperature and the extension of holding time are conducive to the improvement of heat resistance of the joint,but high strength joints can be obtained only under appropriate process parameters.The optimal joining parameters of the joint were C/Ti=2/3,Ni/Cu=3/27,T=990℃ and t=90min.Under the optical parameters,the compatibility of mechanical properties and high-temperature resistance of the joint could be controlled.The corresponding shear strength of the joint at room temperature was 197.6MPa,and the initial melting temperature of the joining layer was 1057℃,which was significantly higher than the brazing temperature and the melting point of CuTi 15 alloy.Obviously,the joints were realized "low temperature joining and high temperature service".The finite element model of composite joining layer with random distribution particles was established,and the stress distribution characteristics of joinig layer with TiC particles and the influence of joining parameters on the thermal stress of the joint were simulated and analyzed.Simulation results showed that the Mises equivalent stress,transverse(S11)and longitudinal(S22)normal stress of the joint of CdSiC composite and stainless steel were mainly symmetrically distributed in the joining layer and the substrates near the joining layer.The introduction of reinforcements in the jointing layer changed the value of the thermal stress in the joint and the stress distribution in the joining layer,but did not change the stress distribution characteristics in the substrates.The particle reinforcements reduced the damage of thermal stress to the mechanical properties of the joint through the following mechanisms:first,they reduced the tensile stress peak near the interface of Cf/SiC composite/joining layer and increased the internal compressive stress of Cf/SiC composite;second,the continuous S11 tensile stress at the interface of Cf/SiC composit/joining layer was transformed to alternating distribution of tensile stress and compressive stress;third,a large amount of compressive stress was introduced into the joining layer,which not only reduces the proportion of tensile stress in the joining layer,but also disperses the continuously distributed tensile stress. |
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