Study On Brazing Performance And Properties Of Brazed Joint Of Metastable Cu Brazing Filler Metal

Since only a small amount of Sn can be added into Cu-based filler metal without badly deteriorating its forming property, in this study a new preparation method of Cu-based filler metal with high tin content, low melting point by hot-dipping Sn coating on Cu brazing filler is introduced. The tin content is controlled by tin scraping mode. Effect of tin coating on the brazing performance and brazed joints of Cu brazing filler is studied. Then the brazing performance and brazed joints are analysed comparatively between metastable alloy and traditional alloy. Diffusion treatment is used to optimize the performance of metastable brazing filler metal finally. Effect of diffusion treatment on the brazing performance and brazed joints of metastable brazing filler metal is studied.The optimum process of hot dipping tin on the surface of BCu68 Zn alloy has been obtained by hot dipping test. Plating assistant agent is Zn Cl2(120~160 g/L)and NH4Cl(120~160 g/L), plating temperature is 70~80℃, plating time is 5 min. The suitable dipping temperature is 250~280℃, the suitable dipping time is 10~20s.Effect of tin content on melting properties of metastable brazing filler metal,traditional brazing filler metal and metastable brazing filler metal after diffusion treatment are studied by differential thermal analysis. Absorption peak of the metastable brazing filler metal shifted to the left with increasing tin content. The liquidus temperature and solidus temperature decrease with increasing tin content. The temperature interval of solid-liquid phase line decreases with increasing tin content. DSC curves of metastable brazing filler metal have two endothermic peaks, but DSC curves of traditional brazing filler metal have only one. The melting temperature of metastable brazing filler metal and traditional brazing filler metal are basically same. The temperature interval of solid-liquid phase line is smaller for traditional brazing filler metal. The height of the first endothermic peak for the metastable brazing filler metal decreases with the increase of diffusion time. The peak temperature of the second endothermic peak for metastable brazing filler metal is almost the same with the increase of diffusion time. Solidus temperature slightly increases while the liquidus temperature decreases with the increase of diffusion time. The temperature interval of solid-liquid phase line decreases with the increase of diffusion time.Effect of tin content on wettability of metastable brazing filler metal, traditional brazing filler metal and metastable brazing filler metal after diffusion treatment are studied by the wettability test. The wettability of metastable brazing filler metal increases with the increase of tin content, the spreading area of matrix brazing filler metal is 229.83mm2, the spreading area can reach 299.52mm2 when tin content is 6.4%. The spreading area of traditional brazing filler metal increases with the increase of tin content, the spreading area of traditional brazing filler metal is slightly larger than the metastable brazing filler metal. The spreading area of metastable brazing filler metal increases with the increase of diffusion time.The organization,composition, fracture and strength of brazed joint for metastable brazing filler metal, traditional brazing filler metal and metastable brazing filler metal after diffusion treatment is analysed using scanning electron microscope, energy spectrometer, optical microscope, mechanical property test machine. The brazed joint microstructure of BCu68 Zn brazing filler metal is single-phase α solid solution. β phase with poor plasticity and γ phase with poorer plasticity appear in the microstructure of brazed joints with the increase of tin content for the metastable brazing filler metal. The tensile strength of brazed joint for the metastable brazing filler metal increases first and then decreases with the increase of tin content. The tensile strength reaches 344.53 MPa when the tin content is 1.6%, then it decreases gradually. The tensile strength of brazed joint for the traditional brazing filler metal also increases first and then decreases with the increase of tin content, but the tensile strength of brazed joint for the traditional brazing filler metal is slightly higher than the metastable brazing filler metal with the same tin content. The tensile strength of brazed joint for the metastable brazing filler metal increases with the increase of diffusion time. The fracture of BCu68 Zn brazing filler metal joint is ductile, but fracture of brazed joint for the metastable brazing filler metal gradually changes from toughness to brittleness.