Stress Simulation And Technical Research On Active Brazing Of MgAl₂O₄ Transparent Ceramic To Kovar Alloy

MgAl₂O₄ transparent ceramics are widely used in infrared photoelectric systems,transparent armor,and high-temperature observation windows due to its high strength,high transmittance,and optical isotropy.However,the shortcomings of ceramics such as high brittleness and poor workability make it difficult to be processed into large-sized components with complicated shapes.Therefore,in practice,it is often necessary to connect with metals to form composite components with excellent performance of both metals and ceramics.In view of the practical engineering application,especially the urgent need of military infrared photoelectric systems,this thesis selected a suitable metal and MgAl₂O₄ transparent ceramic for active brazing connection based on the angle of joint stress,and studied the influence of different process parameters on joint structure and performance.What’s more,the formation mechanism of joint structure is explained,and a variety of ways to relieve joint stress are designed to provide guidance for engineering applications.In consideration of the fact that there are no reports about connection between MgAl₂O₄ transparent ceramic and metal,Ag-Cu-Ti was selected as the brazing filler metals according to the use requirements of infrared photoelectric systems.And the finite element simulation was used to simulate the joint stress of MgAl₂O₄ transparent ceramic/Ag-Cu-Ti/different metals.The results showed that Kovar alloy is an ideal metal material for the connection with MgAl₂O₄ transparent ceramics in terms of economy,practicability and rationality of stress distribution.The typical joint structure of MgAl₂O₄ transparent ceramic/Ag-Cu-Ti/Kovar alloy was MgAl₂O₄ transparent ceramic/TiO/Ti_xCu_y/TiFe₂+TiNi₃/Ag-Cu eutectic/Ti Fe2/Kovar alloy.The Ti element preferentially reacted with the Fe and Ni elements diffused on the metal side during the brazing process.When the reaction with the metal side progressed to a certain extent,the Ti element and the O element diffused on the ceramic side generated a TiO interface reaction layer,the existence of Ti O reaction layer can effectively improve the quality of joint connection.When the temperature is too high or the holding time is too long,a mass of Fe and Ni elements in the brazing seam generated a large amount of intermetallic compounds which consisted of TiFe₂ and TiNi₃,which would affect the performance of the joint.The results of finite element analysis showed that the ceramic edge near the interface was the weak point of the shear test specimen joint.Increasing the thickness of the brazing filler metal could alleviate the stress concentration of the joint,which was consistent with the experimental results.By designing the joint structure and increasing the thickness of the brazing material layer,the joint stress of the engineering parts could be effectively regulated while ensuring the joint bearing capacity.When using a cover joint with the Kovar alloy thickness reduced to 2mm and the active braze alloy thickness increased to 150μm,the peak value of the main stress affecting the joint was reduced to below 50 MPa,and the stress concentration basically disappeared.