Research On Brazed Process Of SiC Ceramic And Kovar Alloy

SiC ceramic is an ideal cladding substitute material in accident-resistant fuel.SiC ceramics have superior high-temperature strength,radiation resistance and oxidation resistance.However,the hardness of SiC ceramics is so great that it is not easy to prepare complex structural parts.Kovar alloy is a constant expansion alloy with good structure stability and workability.At room temperature,the thermal expansion coefficient of Kovar alloy is close to that of SiC ceramics.SiC/Kovar heterogeneous material bonding structure has great application value in nuclear industry.The effective connection of SiC and Kovar is a technical problem.This paper uses Ag-Cu-Ti solder to achieve a reliable connection between SiC ceramic and Kovar alloy by brazed.The thermodynamic analysis and formation mechanism of the phases that may exist in the weld were studied.SiC ceramics and Kovar alloy was brazed with Ag-27.4Cu-2Ti solder.The brazed process has an important influence on the microstructure and comprehensive performance of the weld.With the increasing of the brazing temperature,the shear strength of the joint would decreases.With the increasing of the holding time,the shear strength of the joint would increases first and then decreases.The maximum joint strength（20MPa）was obtained while the SiC ceramic and Kovar alloy were brazed with Ag-Cu-Ti solder at 810℃for 15min.The typical microstructure of SiC/Kovar joint was as follows:SiC ceramic/Ni₂Si+Fe₃Si/Ti C+Cu（s,s）+Ag（s,s）/Fe₃Si+Fe₂Ti/Kovar alloy.Based on the reduction of the strain energy U_e,cof the ceramic matrix and the dissolution mechanism of the base material,the Ag-Cu-Ti/Cu/Ag-Cu composite intermediate layer solder was designed to braze SiC ceramics and Kovar alloys.The results demonstrated that the addition of the composite intermediate layer suppresses the dissolution of the base material and the content of brittle compounds in the weld.With the increasing of the brazing temperature,the shear strength of the joint would decreases.With the increasing of the holding time,the shear strength of the joint would increases first and then decreases.The maximum joint strength33MPa was obtained while the SiC ceramic and Kovar alloy were brazed with Ag-Cu-Ti solder at 810℃for 15min.The shear strength of brazed joints was 1.65 times higher than the joints brazed with a single Ag-Cu-Ti solder.The typical microstructure of SiC/Kovar joint was as follows:SiC ceramic/Ti C/Ag（s,s）+Cu（s,s）/Fe₂Ti+Fe₃Si/Kovar alloy.In the brazing process,Ag-Cu foil melts before Ag-Cu-Ti solder and Cu foil can only be dissolved.Ti atoms would diffuse towards the interface of the base metal on both sides.Ti C was formed by bonding with C atoms at the interface of SiC ceramics.Fe₂Ti was formed by bonding with Fe atoms at the interface of Kovar alloy.The SiC base material was dissolved.Under the action of concentration difference dynamic,Si atom diffused through Cu interlayer to the surface of Kovar alloy to form Fe₃Si.Fe₃Si and Fe₂Ti existed together in chains at the Kovar interface.During the brazed cooling process,Cu precipitates in the form of solid solution.ABAQUS was used to simulate the size and distribution of residual stress in SiC/Kovar joints brazed with two kinds of solders.The results demonstrated that the residual stresses of the two types of brazed joints were distributed in an arch shape.The peak stress was located near the seam of the ceramic base material edge.The residual stress arch of the brazed joint of the composite middle layer was relatively gentle.The peak stress of brazed joints was 171MPa and 59%lower than the joints brazed with a single Ag-Cu-Ti solder.