Investigation On Microstructure And Properties Of Low Silver-Containing Cadmium-Free Intermediate Temperature Brazing Filler Metals

The cadmium-containing silver-based brazing filler metal with low melting temperature and good technologic property is applied extensively to the bonding of mild steel, stainless steel, dissimilar metal, copper and copper alloy in the industry field. The obtained bonding joints have good properties. But cadmium element is toxic, and harmful to human health if contacting it in a long time. In this paper, low silver-containing cadmium-free intermediate temperature brazing filler metals were investigated. Effect of alloy element on melting property, microstructure, wettability of brazing filler metals and joint mechanical properties was researched by changing the content of alloy element on the AgCuZn alloy system. And joint microstructure was observed and analyzed simultaneously.Effect of alloy element on the melting point and melting temperature range was studied by DTA analysis. The result shows that melting temperature descends with the increase of Sn or P content, but ascends with the increasing of Ni content. The addition of Ni and P in the brazing filler metals can reduce the melting temperature range, and improve the alloy fluidity. But the excessive addition of Sn will increase the temperature range, and lead to a bad influence on the alloy fluidity.The experimental brazing filler metals and joint interface were analyzed by SEM, EDAX and XRD. The result shows that AgCuZn experimental brazing filler metals mainly consist of Cu-rich phase, CuZn compound, Cu5.6Sn compound, Cu40.5Sn11 compound and Ag-rich phase. Phosphorus exists as Cu3P compound in the brazing filler metals. Addition of La can make microstructure homogeneous, and restrain the growth of IMC. Dendrite that forms in the alloy microstructure will be broken into pieces after the addition of Ni. Interfacial microstructure of copper-joint is good, and brazing seam consists of diffusion layer and remainder of brazing filler metal. Iron-base solid solution layer and intergranular penetrating structure exist in the stainless steel brazing seam.The result of wettability experiment for AgCuZn brazing filler metals shows that wettability for copper is better than wettability for stainless steel. Wettability of the experimental brazing filler metals is similar to BAg30CuZnSn, but not better than BAg40CuZnSn. Alloy element Sn and P can improve wettability. When the La content is more than 0.5%, the wettability of experimental brazing filler metals will descend.Joint mechanical properties were analyzed. The result shows that the shearing strength of stainless steel joint of the experimental brazing filler metals is about 90MPa, the shearing strength of copper joint can reach to 110MPa.