| Ceramic-metal sealing is a key technology in the manufacturing of traveling wave tube.The manufacturing of ultra-high frequency,high-power and long-life traveling wave tube sets higher requirements for the stability of ceramic-metal sealing process and reliability in service.From previous research results,we can conclude that indirect brazing ceramic-metal sealing structure has the characteristics of multi-interface and multi-material.The difference of thermal expansion coefficient of materials will lead to the existence of large residual stress in the structure.The interaction between stress and material,interface and defects during thermal cycle will have great impacts on the stability of manufacturing process and service reliability of the sealing structure.Therefore,based on the analysis of the microstructure of 95%Al2O3/4J33 joints brazed by molybdenum-manganese method,the distribution of residual stress in ceramic-metal seals is studied,and the stress field and failure law of the sealing structure under thermal shock and space thermal cycle are discussed,which lays a foundation for the reliability of ceramic-metal seals.The microstructures of 95% Al2O3/4J33 joint indirectly brazed by molybdenummanganese method with AgCu eutectic as filler metal are analyzed.The composition of the joint is determined to be 4J33/Ni diffusion layer/Ni dissolution layer/Ag-Cu layer/Ni.Shrinkage voids were found in the filler metal,which may expand into crack defects during thermal shock and thermal cycle.The three-point bend test shows that the metallized layer/solder interface is the weak interface of the joint.The welding residual stress distribution of standard part and risk part of 95% Al2O3/4J33 sealing joints is studied.The results show that thinner metallized layer(< 40 micron)can achieve higher joint strength,and excessively thin solder layer(< 30 micron)tends to cause the defect of the solder to expand into voids and cracks due to accumulation of plastic deformation.The stress field and failure law of 95% Al2O3/4J33 sealing structure under thermal shock and space thermal cycle are studied.The experiment result shows that the structure of the joint does not change after 10 thermal shocks from 750℃ to 25℃ for standard parts,but the size of shrinkage porosity in the filler metal obviously increases,and the mechanical properties of the joint decrease with the increase of thermal shocks.The results of thermal shock simulation show that thermal shock can increase the interfacial tension stress of metallized layer/solder layer,promote the expansion of original void defects,which results in the decrease in interface strength.During the service of travelling-wave tube,it will undergo space thermal cycle(-30℃~150℃).The experiment result shows that no damage is found in the standard parts after 1500 thermal cycles,and the mechanical properties remain unchanged.The simulation results show that the joint cannot reach the fatigue damage based on the energy criterion under space thermal cycle,and the shrinkage porosity in the original joint has no effect on the joint’s thermal cycle life,but when the cycling temperature changes to-30℃~250℃,the plastic strain of the filler metal accumulates with the increase of thermal cycle numbers,and ductile damage occurs at 1200 thermal cycles. |
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