Research On Pressure-less Active Brazing Of SiC Fiber Composite Clad Tube And Kovar Alloy

SiC_f/SiC-based composite is considered an ideal material for nuclear fuel cladding that can replace zirconium alloys.However,sealing technology with terminals is a key challenge that limits its application in pressurized water reactors.Compared to silicon carbide ceramics,the microstructure of the surface of the SiC_f/SiC composite tube is different and directional,and no pressure can be applied during the sealing process.This affects the morphological evolution of the brazed interface and the fracture mechanism of the joint.Therefore,the 4J29 Kovar alloy,which has a similar thermal expansion coefficient to SiC,is chosen as the metallic joining material in this work.The reliable joint between fiber-reinforced SiC composite tubes and Kovar alloy was made with two kinds of brazing alloys,Ag-Cu-Ti and Ag-Cu-Ti-In.The effects of process parameters,such as brazing temperature and holding time,on the microstructure and mechanical properties of the brazed joints were investigated.The mechanism of interfacial bonding between the SiC fiber-reinforced composite tubes and Kovar alloy was discussed,and the fracture mechanism of the joints was analyzed.In addition,the residual stresses of the joints after welding were simulated using Abaqus finite element software,which provides ideas and guidance to improve the performance of SiC fiber-reinforced composite tubes and metal welded joints.The main results are as follows:Compared with traditional pressure brazing,the presence of the active element Ti in the brazing material enables effective wetting and filling of the joint under non-pressurized conditions.The Ag-Cu-Ti brazing alloy has shown effective interfacial reactions with SiC fibers.At the brazing temperature of 880℃,there are no significant defects such as cracks and pores in the brazed joint.The typical microstructure of the joint is SiC/Ti C+Ni₂Si+Fe₂Si+Cu（s,s）+Ag（s,s）+Fe₂Ti/Kovar alloy.Raising the brazing temperature or prolonging the holding time will lead to an increase in the thickness of the ceramic interfacial reaction layer and the amount of brittle compounds in the brazed joint.The room temperature shear strength of the joint initially increases and then decreases.At a holding time of 30 minutes,the room temperature shear strength of the joint reaches its maximum value of 41.2 MPa.During the welding process,Fe and Ni elements in the Kovar alloy diffuse and dissolve into the weld seam,reacting with Si atoms at the SiC interface to form Ni₂Si and Fe₂Si,as indicated by the microscopic characterization and thermodynamic analysis of the interface.The active element Ti in the brazing material diffuses to both sides of the weld seam and reacts with C on the side of SiC ceramic to form a Ti C layer,while also reacting with Fe on the side of Kovar alloy to form Fe₂Ti compound.The product extends in a continuous chain-like form towards the center of the weld seam.The morphology analysis of the shear fracture surface reveals that the joint fracture mainly occurs in the region where the brittle phase of Ni₂Si+Fe₂Si is located near the SiC fiber matrix side,exhibiting obvious shear fracture characteristics.In order to further reduce the brazing temperature,prevent excessive dissolution of Fe and Ni elements in Kovar alloy into the weld seam,and reduce the production of brittle intermetallic compounds in the interface reaction layer,a low-melting-point Ag-Cu-Ti-In brazing material was used to braze SiC fiber tube/Kovar alloy,which successfully prevented the production of Ni₂Si and Fe₂Si brittle compounds in the joint.At a brazing temperature of 780℃and a holding time of 30 minutes,the interface structure of the brazed joint is good,and the organization consists of SiC/Ti C+Ti₅Si₃+Ag（s,s）+Cu（s,s）+Ti Ni+（Ag,In）（s,s）/Kovar alloy.According to the results of elemental surface scanning,the content of Fe and Ni elements on the ceramic side interface reaction layer has significantly decreased,which effectively improves the strength of the joint.Under these process parameters,the room temperature shear strength of the brazed joint reached 86.1 MPa,which is twice the maximum shear strength obtained using Ag-Cu-Ti brazing material.Fracture mechanism analysis shows that reducing the presence of brittle interfacial metallic compounds can effectively enhance the bonding strength of the interface.The fracture of the joint mainly occurs near the interior of the substrate adjacent to the interface,with the main form being the pull out of SiC fibers.Due to the directionality of SiC fiber composite materials,cracks exhibit significant deflection and branching during extension,which is an important reason for the increased bonding strength.Abaqus software was used to simulate the residual stresses in SiC fiber-reinforced Kovar alloy joints after welding.Various factors,such as the type of brazing material,the width of the weld seam,and the surface of the base material was metallized,were studied to investigate their effects on residual stresses.The results showed that both the type of brazing material and the metallization treatment of the ceramic substrate surface significantly affected the peak residual stress of the joint,but had little impact on the distribution trend of residual stress.Compared to Ag-Cu-Ti brazing alloy,the joint brazed with Ag-Cu-Ti-In brazing alloy has smaller residual stresses,which may be one of the important reasons why the shear strength of the joint brazed with the latter is significantly higher than that of the former.In the future,it is feasible to improve the performance of joints by increasing the plasticity of the brazing material or reducing its coefficient of thermal expansion through the addition of hard particles.