Influence Of Indium And Stannum On Microstructure And Properties Of BAg20CuZn Filler Metal

Silver based brazing filler metals containing cadmium have been widely used inaerospace, electric appliance, spectacle frame and household appliances industry dueto its favourable processing property. However, cadmium is a type ofnon-biodegradable toxic element and easily causes environmental pollution. Thepractical application of silver based brazing filler metals containing cadmium hasconfronted grand challenge. A few kinds of silver based brazing filler metalscadmium-free were developed by various countries all over the world. But the silverbased brazing filler metals still dissatisfied people in mechanical properties,processing technology, production cost and so on. Consequently, the further study ofneotype environmental cadmium-free silver filler metals have important significancefor its application in manufacturing.Based on BAg20CuZn filler metals, several kinds of neotype alloys weredesigned by adding indium and stannum. Effects of indium and stannum onmicrostructures, melting properties, wettability of silver brazing filler metals andmicrostructures and mechanical properties of the brazing joints were studied byoptical microscope, scanning electron microscopy, energy-dispersive X-rayspectroscopy, X-ray diffraction, differential scanning calorimetry thermoanalysis andmicrohardness and tensile strength test, so as to optimize the neotype cadmium-freesilver brazing filler metals with superior comprehensive performance. These canprovide certain reference for cadmium-free silver based filler metals further researchand application.The microstructures of silver based brazing filler metals and its correspondingbrazed butt joints without indium include Cu-Zn compounds based solid solution,silver based solid solution and AgZn compounds. The microstructures of silver basedbrazing filler metals its corresponding brazed butt joints within0.5wt.%~3.0wt.%indium contain Cu-Zn compounds based solid solution, silver based solid solutionand Cu7In3compounds. Cu-Zn compounds based solid solution in filler metals andbrazed joints reduced gradually with the increasing of indium. Cu-Zn compounds andCu7In3appeare instead of Cu-Zn compounds based solid solution. When indium is 3.0wt.%, the distribution of silver based solid solution changed prominently in thefiller metals. Melting temperature intervals of filler metals are shortened andwettability are enhanced after adding indium to BAg20CuZn filler metals.Microhardness, tensile strength and toughness of filler metals containing indium areincreased. The distribution of silver based solid solution became more homogeneousand dispersive. Moreover, spreading area of filler metals on the stainless steel sheetreached maximum in the filler metal with1.5wt.%indium. The better toughness andhighest microhardness and tensile strength of brazed butt joints were obtained when1.5wt.%indium was added to the BAg20CuZn filler metal.Adding different stannum content and1.5wt.%indium to BAg20CuZn fillermetals, the microstructures of filler metals and its corresponding brazed butt jointsmainly include silver based solid solution, Cu-Zn compounds, Cu10Sn3and a smallamount of Cu9In compounds. The volume fractions of Cu-Zn compounds decreasedincreasingly and formed many brittle Cu10Sn3compounds which result in brittlenessincreased and lowering the processing performance of filler metals. The meltingtemperature interval of filler metals became shorter and spreading wetting changedbetter with1.5wt.%indium and1.5wt.%stannum. The better toughness and highestmicrohardness and tensile strength of brazed butt joints were also obtained when1.5wt.%indium and1.5wt.%stannum were added to the BAg20CuZn filler metal.In conclusion, on the base of BAg20CuZn filler metal, the optimized chemicalcomposition of neotype cadmium-free silver brazing filler metals contains1.5wt.%indium and1.5wt.%stannum. Microstructure of silver based brazing filler metals iscomposed of silver based solid solution, Cu-Zn compounds, Cu10Sn3compounds anda lot of Cu9In compounds. Melting temperature interval is39.8℃and spreadingwetting is predominant. Microhardness and tensile strength of brazed butt joints are216.6HV0.2,511.924MPa respectively, and toughness is preferable.