Research On Influence Of Impurity And Representation Of Cleanliness For Ag-based Filler Metals

In recent years, quality requirements of brazing filler metals are increasing with research deeply and applications development. However, effect of trace impurity element during production and application was once ignored. Harmful impurity elements will seriously affect the solder wettability, melting properties, toughness and so on, and it has become a major restricting factor to improve performance of Ag-based filler metals. Although many scholars have paid more attention on these issues, and related research has also been gradually carried out, impact mechanism of the impurity elements calcium, carbon, sulfur, oxygen on Ag-based filler metals and representation method of cleanliness have not been yet researched and reported systematically.Supported by the national 973 project "Research on clean steel process simulation and interface structure and characterization representation", existence form and impact mechanism of calcium, carbon, sulfur, oxygen for Ag-based filler metals will have been researched systematically by optical microscope (OM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), etc., and the representation method of cleanliness will be set up preliminarily.The results showed that ternary eutectic point E of BAg45CuZn geared to Ag because of Ca addition. With the increment of Ca content, grain size of Cu solid solution and Ag solid solution expanded and the content of Ag solid solution decreased. Eutectic structure increased while blade pitgh decreased. Ca intensive dispersaled of phase interface of Ag solid solution and Cu solid solution, and the increment of the Ca resulted in the formations of high brittle and orthorhombic lattice intermetallic compounds a-CaCu. Because of Ca is a kind of active chemical element, which could react with oxygen to form stopping-off agent and hamper the fluidity of brazing filler metals. By way of increasing the surface tension between base metal and filler metals, can decrease the brazing property of filler metals.Sulpher had serious perniciousness, and the sulphuration products of high melting temperature mainly consisted of Cu2S, ZnS, Ag2S which distributed in the surface of filler metals. With the curing time expanded, central structure of filler metals varied little, but the sulpher diffused into the interior and content of which in Ag solid solution more than Cu solid solution. The fracture of tensile sample evolved from ductile fracture to brittle fracture with the increment of sulpher, and much granular sulphide which cut apart the matrix existed in the fracture. The sulphide improved the viscidity of filler metals and resulted in interfacial tension enlargement to form incomplete penetration and decreased the joint property during the process of brazing.Carbon-containing layer was formed in the surface of brazing filler metals by rolling oil, and the thickness varied from 0.5μm to 7μm according to the different rolling frequency. The major existence forms of carbon were free carbon molecules, ZnC8, C2CdO4. Part of free carbon molecules and carbide had no enough time to float to the surface to be wrapped inside, and portion pushed to the forefront of spreading to form a pocket which set back the fluidity around the filler metals, still a tiny percentage of free carbon formed some carbide easily with Cr, Ni, Mn, Fe, etc. in steel.Existence form of oxygen was oxide mainly, adsorption and solution little. Powder particles adhered and some appeared cracks on the surface with the increment of oxygen content. The solid line temperature and apparent activation energy of powder filler metals increased with the increment of oxygen content while the wettability and tensile strength decreased. The surface formation of powdered filler metals was comprised by adsorption layer of free water, polluted layer of hydrocarbon and oxides layer. The inclusion in joints was crack source of fatigue test which was the main reason why the brazed property decreased.The concept of "clean filler metals" was brought in Ag-based filler metals. Flow rate evaluation method made the evaluation criterion of brazing process a substantial progress from traditional quality-area to quality-area-time. The internal reason why the filler metals with same composition and microstructure had different property was revealed. The representation system of clean filler metals was built preliminarily by "process entropy" and "property entropy".