Research on Brazing Process and Mechanism of ZrC-SiC Ceramic and TC4 Titanium Alloy

Author:Shi Jun Miao

Supervisor:feng ji cai zhang li xia


Degree Year:2018





Now the heat-resistance temperature of the nozzle materials in gesture control engine is low,limiting the application of the nozzle at high temperature significantly.Recently,developing the ultra high temperature ceramic nozzle has attracted extensive attention.ZrC-SiC is a novel composite ultra high temperature ceramic,the addition of ZrC and SiC makes the heat-resistance temperature and resistance to oxidation of ZrC-SiC high.Therefore,ZrC-SiC is a potential candidate for the next generation nozzle material.In the practical application of nozzle structure,the nozzle needs to join to TC4 transition ring.We plan to join the two materials by brazing method.However,two problems should be solved in the ceramic and metal brazing system.Firstly,the ceramic is too inert to react with common metals.Secondly,the coefficient of thermal expansion(CTE)mismatch between the ceramic and metal induces residual stress in the brazed joint,deteriorating the joint strength significantly.To overcome the difficulties,our study focuses on two aspects.Firstly,the ZrC-SiC and TC4 were brazed using Ti contained active brazing filler.Through optimizing the composition of the brazing filler and the brazing parameters,the joints with high strength could be obtained and the brazing mechanism was also revealed.Secondly,laser cladding functionally graded material(FGM)layers were designed to relieve the residual stress in the joint,and the strengthening mechanism of the FGM layers was also analyzed.The ZrC-SiC and TC4 were brazed using AgCuTi filler to investigate the weldability of the brazing system.The reaction of the Ti and ZrC-SiC was the key to the brazing process,and the formation of the phases with high ductility in the brazing seam was beneficial to the joint strength.In order to improve the high temperature strength of the joints,the ZrC-SiC and TC4 were brazed using TiCuNi filler.During the brazing process,the braze filler reacted with the ZrC-SiC to form TiC layer,and then the brazing was achieved.Meanwhile,IMCs zone,hypoeutectic zone and hypereutectoid zone formed in the brazing seam.When ZrC-SiC and TC4were brazed using 70wt.%Ti content braze filler at 970℃for 10min,the maximum shear strength of the joint reached 43MPa,and the crack occurred in the ZrC-SiC substrate near the brazing seam.The activity of Ti in the braze filler was calculated.Through comparing the Ti chemical potential of the solid and liquid interface,the interfacial reaction mechanism was analyzed.The Ti chemical potential of braze filler was higher than that of TiC reaction layer.The TiC layer released C atom into the braze fill er,providing C source for the growth of TiC.The formation of the brazed joint contained four steps.Firstly,the braze filler melted and reacted with the ZrC-SiC to form TiC layer and a little(Ti,Zr)5Si3,isothermal solidification occurred adjacent to the TC4.Secondly,during the cooling process,the hypoeutectic zone formed in the brazing seam.Thirdly,when the temperature decreased to about 900℃,the residual liquid braze filler solidified to IMCs zone.Finally,the hypereutectoid zone formed in the isothermal solidification zone during the further cooling process.The residual stress due to the cooling of the joint and the stress during the shear test process were simulated to design the stress relieving FGM layers.The CTE of the ZrC-SiC,brazing seam,FGM layers and TC4 were different,then the shrinks of the different materials were different.In the ZrC-SiC,compressive stress parallel to the brazing interface formed.Tensile stress perpendicular to the brazing seam formed in the ZrC-SiC edges.The addition of the FGM layers made the residual stress along different directions decrease,and the maximum principal stress peak in the ZrC-SiC edges decreased from 564MPa to 452MPa.During the shear test,the plus of the residual stress and stress induced by the shear force produced stress concentration in the ZrC-SiC edges near the brazing seam,which was responsible for the crack of the joint.According to the ZrC-SiC/TC4 model,the fracture stress of the ZrC-SiC was near 648MPa,then the shear strength of the ZrC-SiC and TC4joint increased from 43MPa to 81MPa due to the FGM layers addition according to the simulation results.Two SiCp reinforced FGM layers were fabricated on the TC4 surface by laser cladding technology.Through optimizing the cladding parameters,the volume fractions of SiCp in the FGM layers were 20%and 39%.The interfacial reaction mechanism of SiC and TC4 during the laser cladding process was revealed.SiCp would dissolve into Si and C when it contacted the liquid TC4.The Si and C in the liquid TC4 reacted with Ti to form TiC and Ti5Si3 respectively.The TiC and Ti5Si3promoted the hardness of the TC4 matrix of the FGM layers to increase.From the TC4 substrate to the FGM layers,the hardness was 400HV,600HV and 800HV respectively.The TC4 coated with the FGM layers(TC4-FGM)and ZrC-SiC were brazed using TiCuNi filler.The interfacial reaction phases and the joint microstructure were analyzed.The effect of the brazing parameters was also investigated.The brazing parameters controlled the joint property through affecting the Ti2(Ni,Cu)thickness and reaction of the braze filler and FGM layers.The comparison of the microstructure of the joint with and without the FGM layers showed that the fabrication of the FGM layers mainly affected the thickness of reaction layer adjacent to the ZrC-SiC and the formation of the hypereutectoid zone.Moreover,the effect of the Layer II thickness and the SiC content in Layer in the joint strength was investigated.When the ZrC-SiC and TC4-FGM were brazed at 970℃for 10min,the maximum shear strength of the joint was 91MPa.The elastic model calculation results showed that compressive stress existed in the ZrC-SiC near the brazing seam,while tensile stress formed in TC4.The addition of the FGM layers reduced the maximum stress in the ZrC-SiC from 499MPa to444MPa.The increase of the Layer II thickness and the SiCp content in Layer II could make the residual stress decrease.The change of the residual stress corresponded to the joint strength change.In addition,the FGM layers also reduced the production of Ti2(Ni,Cu)adjacent to the ZrC-SiC,which was also beneficial to relieving the residual stress in the joint.