Research On Process And Mechanism Of Brazing SiC Ceramic And GH99 With GNPs Reinforced Composite Filler

SiC-GH99 superalloy(GH99)composite components have been widely utilized in several fields sucha as aerospace.However,high residual stress ouucrred at the brazed joint due to the diversity in coefficient of thermal expansion(CTE)between SiC ceramic and GH99,thus reduced the quality of brazing.From the perspective of adjustment of CTE and reinforcement of nano-particles,Graphene nanoplatelets(GNPs)reinforcened Ag Cu Ti composite(Ag Cu Ti G)filler was prepared in this study,the in situ synthesized Ti C nano-particles could adjusted the CTE of the brazing seam.The Ag Cu Ti G filler was applied to the brazing of SiC/SiC and SiC/GH99,and the reliable joining of SiC/SiC and SiC/GH99 was realized.According to the analysis of interfacial microstructure and mechanical properties of the brazed joint,the strengthening mechanism of GNPs during the brazing process was studied.The interfacial microstructure of SiC/SiC joint brazed with Ag Cu Ti G filler was SiC/Ti C+Ti5Si3/Ag(s,s)+Cu(s,s)+GNPs+Ti Cu+Ti C/Ti C+Ti5Si3/SiC.The GNPs reacted with element Ti from the brazing alloy,forming Ti C nano-particles in situ during the brazing process,and the microstructure of brazing seam was refined effectively.The in situ reaction of Ti and GNPs was conducted sufficiently with the increase of brazing temperature or prolonging of holding time,reducing the CTE of the microstructure of brazing seam and relieving the residual stress of brazed joint.Thus the shear strength of brazed joint was improved effectively.The maximum shear strength was 41.9MPa when the joint was brazed at 950°C for 10 min with Ag Cu Ti-1wt.%GNPs composite filler,which was 163.5% higher than that of the joint brazed using Ag Cu Ti filler.The brazing of metallized SiC ceramic and GH99 was realized using composite filler.The interfacial microstructure of SiC ceramic/GH99 brazed joint was SiC ceramic/Ti C+ Ti5Si3+Ti2Ni/Ag Cu eutectic structure+Ti2(Ni,Cu)+GNPs+Ti C/Cu(s,s)+Ti Cu Ni2 + Ag(s,s)+Cr(s,s)/GH99.During the brazing process,Ti2(Ni,Cu)compounds were refined and uniformly distributed in the brazing seam resulted from the existence of Ti C nanoparticles,forming second phase particles strengthened Ag-based composite brazing seam.In addition,GNPs was effective in retarding the solute atoms diffusion during brazing process.GNPs impeded the precipitation of Ti Cu Ni2 brittle compounds at GH99 side thus improved the plasticity of the brazed joint.The maximum shear strength was 26.4MPa when the joint was brazed at 860°C for 5min with Ag Cu Ti-1wt.%GNPs composite filler,which was 151.4% higher than that of the joint brazed using Ag Cu Ti filler.The effect of composite filler components on the mechanical proeprties of the brazed joint was studied by changing the content of GNPs.The second phase particles uniformly distributed in the Ag-based composite brazing seam strengthened the Ag-based substrate through load transfer,dislocation strengthening and Orowan strengthening mechanisms,improving the performance of brazing seam.Furthermore,self-adjusting CTE and elasticity modulus reduced the mismcatch of the physical properties of the brazed joint.Composite brazing seam relieved the residual stress of the brazed joint and improved the shear strength of the brazed joint.