Microstructure and properties of continuous casting Ag–28Cu–8Sn alloy fabricated by dieless drawing

Ag–28Cu–8Sn (wt%) alloy is a widely used brittle silver-based brazing filler metal. The wire of brazing filler metal was prepared by continuous casting process and dieless drawing technology. The phase structure was measured by X-ray diffraction (XRD), and the microstructure of wetting interface, cast states, processing states and fracture morphologies were characterized by the optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The electrical conductivity, hardness, tensile strength and elongation rate were tested as well. Furthermore, the solid–liquid phase temperature was measured by a differential scanning calorimeter (DSC), and the wettability of brazing filler metal was tested by spreading method. The outcomes obtained show that the as-cast microstructure is a typical three-zone structure, including region of surface fine grain, zone of columnar grain and region of center equiaxed crystal. Ag–28Cu–8Sn alloy is mainly composed of Ag-rich α-phase, Cu-rich β-phase and intermediate compounds. Grain refinement appears in the cross section, as for grains of the longitudinal section, the shape is changed from ribbon to fiber to form a silk texture. The strength and hardness improve with the increase in the true strain, while the conductivity and elongation are reduced. Furthermore, the solid-phase temperature is 605.9 °C, and the liquid-phase temperature is 725.1 °C. The spreading area of Ag–28Cu–8Sn brazing filler metal is 174 mm², and the metallurgical bonding occurs between Ag–28Cu–8Sn brazing filler metal and Cu matrix. In addition, compared with cold drawing process, there are not any microcracks at the fracture morphology for the alloy fabricated by dieless drawing. The dieless drawing process overcomes some processing defects of traditional cold drawing, and the processing performance of Ag–28Cu–8Sn alloy is improved.