Research On Brazing Process And :Mechanism Of TC4 Alloy To Si₃N₄ Ceramic

Advanced ceramic have been widely used in aerospace field, weaponry field and other advanced technology field for their outstanding properties in all round. Among these advanced ceramics, Si3N4 ceramics are preferred as the next generation materials for radomes due to the excellent mechanical properties and good wave penetrating performance, which need to be connected with metallic materials in the assembly and service process. Nowadays, TC4 alloys which possess high specific strength, high specific stiffness, good oxidation resistance, as well as excellent high-temperature mechanical properties are main holder material in application of radomes. Realizing the reliable and efficient connection of TC4 alloy and Si3N4 ceramics is an urgent problem to be solved for the great differences of physical properties and mechanical properties between them. The objective of this study is to achieve the joining of TC4 alloys and Si3N4 ceramics. The AgCu-based composite filler reinforced by nano-Si3N4 particles was prepared according to the strengthen theory of composite material. The brazing of TC4 alloy and Si3N4 ceramic was achieved by AgCu-based composite filler in this study. The reactions between filler and substrates were unriddled by analyzing the interfacial microstructure of TC4/AgCuc/Si3N4 brazed joint. The mechanism of brazing TC4/Si3N4 and effect of brazing temperature and holding time on interfacial microstructure and mechanical properties were discussed. What’s more the distribution of residual stress in joints was analyzed by finite element method simulation.AgCu+nano-Si3N4+Ti composite brazing alloys (AgCuc) were developed and used to braze TC4 alloys and Si3N4 ceramics in order to improving the joining properties. The AgCu-based composite filler reinforced by nano-Si3N4 particles was prepared by mechanical ball milling. The composite filler was adopted for releasing the residual stress caused by different thermal expansion coefficient between two substrates. Brazing of TC4 alloys and Si3N4 ceramics was achieved by using AgCu-based composite filler. The typical interfacial microstructure of TC4/ AgCuc/Si3N4 joint was TC4/Ti-Cu IMC/Ag(s, s) + Ti-Cu IMC + TiNP + Ti5Si3P/ TiN+Ti5Si3/Si3N4 which was detected by means of scanning electron microscopy, energy disperse spectroscopy, transmission electron microscope and so on. The brazing mechanism of the TC4/AgCuc/Si3N4 brazed joints can be summarized as follow. The dissolution and diffusion between TC4 alloy and composite filler provide enough element Ti in braze to forming the compounds. During brazing process, nano-TiN particles and nano-Ti5Si3 particles were formed by the reactions between Ti and nano-Si3N4 particles which acted as nucleations of Ti-Cu intermetallic compounds resulting in the formation of Ag based composite structure in brazing seams. The investigation shows that the dissolution and diffusion of element Ti were the key factors in the formation of composite brazing seam and the dissolution equation of TC4 was obtained. In addition, brazing temperature and brazing time had significant impact on the interfacial microstructure and joining properties. The following conclusions were reached by analysis the interfacial microstructure and mechanical property of the joint brazed in different brazing process. A stable joint can’t be achieved with thin reaction layer between substrate and composite filler when the brazing temperature was excessive low or the holding time was excessive short. Similarly, when the brazing temperature was too high or the holding time was too long, the over growth of reaction layers and compounds in the brazed joints was increased the brittleness of the joints. The composite like interfacial microstructure was only appeared when joints were brazed at 880℃ for 10min. The highest shear strength at room temperature was 78.4MPa when brazed at 880℃ for 10min using AgCuc filler, which was 50% higher than that of joints brazed using AgCu brazing alloy.The reason for the good performance of AgCuc applied in TC4/Si3N4 joints is the dispersion nano-particles in braze playing a role of dispersion strengthening in the interfacial microstructure according to the discussion of brazing mechanism. Furthermore, the residual stress in joint was reduced due to the reduction of the CET coefficient of brazing seams. FEM simulation results show that residual stress distribution form unchanged but alloys were utilized, and the compressive stress peak on the surface of Si3N4ceramics was reduced by nano-Si3N4 particles addition.