Technology And Mechanism Study On Brazing ZSC Composite To GH99Nickel-base Superalloy

ZSC composite possesses a combination of several excellent properties, such asextremely high melting temperatures, high strength, good thermal and electricalconductivities, and high temperature oxidation resistance. Joining ZSC composite toGH99Ni-based supperalloy can be potentially applied to the space vehicle thermalprotection systems, such as nose cone and wing leading edge. However, during thebrazing process, there was an intense interaction between ZSC composite and GH99,which weakened the microstructure and mechanical properties of the joint drastically. Inorder to control the interfacial reaction, NiCrNb brazing filler and HEA-0.8Ti highentropy alloy brazing filler were fabricated as the interlayer to join ZSC/GH99in thispaper. The purpose was to study the influence of brazing parameters on themicrostructure and mechanical properties of joints, and optimize the brazing process;reveal thermodynamic conditions for inhibiting the reaction between Ni and ZSC. Onaccount of the high mismatch of thermal expansion conefficient between ZSC and GH99,reticulated composite interlayer structure was designed to relieve the residual stress.When ZSC/GH99was joined at the optimized brazing parameter of T=1190℃/t=20min with NiCrNb as the interlayer, active elements Nb and Cr clustered at the ZSCcomposite and the harmful reaction between Ni and ZSC was inhibited via thecorresponding interface reaction. The microstructure of the joint mainly consisted ofNi-based solid solution, and the shear strength of the joint was48MPa. With the changeof brazing parameters, the interface microstructure of the joints varied, when the brazingtemperature is1220℃and holding time is20min, composition of the brazing filler wasdamaged by excessive dissolution of the metal substrate. The activity of Cr and Nbdecreased, and consequently Ni and ZSC composite reacted severely, causing adeterioration of microstructure and mechanical properties of the joints.Using HEA-0.8Ti composite foils as the interlayer, typical interface structure is:ZSC composite/TiB/CrB+Cr₂B+Cr5B₃B/FCC+TiC/FCC/GH99. Due to the high entropyeffect and diffusion delayed effect, when brazing temperature was above the soldermelting point20⁸0℃, the microstructure and mechanical properties of joints werestable. When the brazng temperature is1180℃, holdng time is60min, the maxium shearstrength of the joints at room temperature and800℃were71MPa and48MParespectively.The reticulated composite interlayer structure was used to braze ZSC/GH99auxiliarily, By changing the type of metal mesh and components of the filling phase,good match of the expansion coefficient and the ability of elastic and plastic deformationcan be achieved, moreover, the brazing area of the joints can be controlled in advance. By combining Miedema model and Zhou Guozhi’s geometric model, activitycoefficient was caculated. Using Dictra software, the dissolution process was simulated.The results showed that, according to the caculated activity of active elements indifferent solders, the interfacial reaction at ZSC composite can be effectively controlledin advance. And by the simulation of dissolution process, the solder stability can beevaluated and the brazing parameters can be optimized.