Research On Brazing Mechanism Between Nb And C_f/SiC Composites Modified With Surface Structure

C_f/Si C composites possess the high specific strength,outstanding high temperature strength and good thermal-shock property,which have been widely applied in aerospace and other fields.Due to the poor workability,C_f/Si C composites should be joined with metals to form heterogeneous components,meeting the practical requirements.This thesis aims at the high joining quality of the large size C_f/Si C-Nb joint designed for"Kuaizhou"series 2000N attitude control engine.However,the increased joint size induces problems of the interface structure,stress and quality control.In order to achieve the high strength joining and high temperature service,the surface structure of C_f/Si C composites is improved by the wire electrical discharge process,the thermal corrosion and Mo modification.It tends to improve the microstructure and increase the joining area.Based on the above modification,the low expansion composite high temperature brazing alloy is adopted to braze the joint reinforced with the Mo modified carbon cloth.The residual stress is decreased further and improves the high temperature mechanical properties of the joint to realize the reliable joining of C_f/Si C-Nb with the large size.Firstly,the patterned surface structure of C_f/Si C composites is prepared by wire cutting process,tending to increase the joining area and improve the microstructure to modify the fracture path.Effect of groove structure parameters on the shear strength and fracture is studied.It is found that the crack propagated into the groove structure to hinder the crack of the reaction layer,which can greatly improve the joint strength.The fracture path can be improved with the depth-to-width ratio of grooves greater than 2/3.When the groove ratio is about60%,it can ensure the uniform bearing of the brazed joint.Obtuse-angled grooves can provide a certain degree of interlocking to mitigate the torsion effect.Based on the above discussion,branch reinforced groove structure is prepared to enhance the interlocking effect.The groove structure reinforced with branches can increase the shear strength of C_f/Si C-Nb joints to 144MPa.In order to further strengthen the interface bonding of C_f/Si C composites to alloy and relieve the residual stress at the interface,C_f/Si C composites are treated with the thermal corrosion to prepare the microscopic structure.The Si C matrix can be selectively removed without etching carbon fibers to form the carbon fiber reinforced alloy transition area,which can replace the reaction layer to eliminate the weakest area in the joint and hinder the crack propagation.The required depth can be prepared with the appropriate parameters accurately.Moreover,the transition area reduces the mismatch of the coefficient of thermal expansion between C_f/Si C composites and brazing alloy,which can fully relieve the residual stress in the joint.The shear strength of joint reinforced with the corrosion depth of 100μm can reach152MPa.After the surface structure is improved at macro and micro level,the residual stress needs to be further relieved in the fiber reinforced transition area and the brazing seam.The modulation of two domains can be accomplished simultaneously with the method of Mo nanoparticle modified carbon fibers.The deposition and reduction processes are studied,which can control the Mo content in carbon fibers.When the fiber reinforced transition region is regulated,Mo nanoparticles are evenly distributed around the carbon fiber,which can reduce the difference in the coefficient of thermal expansion between the carbon fiber and the brazing filler metal to relieve the high residual stress state of carbon fibers.And Mo nanoparticles can improve the path of crack propagation to increase the pull-out of carbon fibers.The reinforcing effect on the joint can reach nearly 1 time.Mo nanoparticle modified carbon cloth is prepared to modify the brazing seam,which can fully reduce the coefficient of thermal expansion of the brazing seam.The residual stress can be relieved effectively,and the shear strength of the joint can be increased by75%.Ti-Fe24-Nb19 high temperature brazing alloy is designed and prepared to improve the high temperature service performance of the joint.In order to cope with the increase of residual stress caused by the growth of brazing temperature,Zr₂P₂WO₁₂ negative expansion particle is adopted to reinforce brazing alloy.When the content of Zr₂P₂WO₁₂ is 8vol%,the coefficient of thermal expansion of the composite brazing alloy is close to that of Nb.The shear strength of the joint at room temperature and 800℃is increased to 157MPa and 133MPa,respectively.Finally,the surface structure design and surface modification of C_f/Si C composites are integrated,and high temperature brazing alloy is used for brazing Nb to achieve the high quality joining.The shear strength at the room temperature and 800℃of the joint can be increased to 165MPa and 147MPa,respectively.