Influence of microalloying with B on the microstructure and properties of brazed joints with Ag-Cu-Zn-Sn filler metal

Processing and formation of Ag-Cu-Zn-Sn filler metals with high Sn content are difficult owing to their high brittleness. A composite flux-cored silver (Ag) filler metal of a CuSn + XB alloy powder was designed using the principle of in situ synthesis. Copper-Copper (Cu-Cu) joints were obtained using a novel Ag-based filler metal (at B contents of 0, 2, and 3%). The microstructure and evolution of the mechanical properties of the Cu-Cu brazed joints were studied by field emission scanning electron microscopy, electron probe microscope, electron backscattering diffraction, tensile testing, and nanoindentation tester. The brazed joints mainly consisted of Ag-based solid solution (Ag(s.s)) and Cu-based solid solution (Cu(s.s)). Due to the addition of 3% B, (Ag + Cu) eutectic network structures were formed in the brazing seam. At the same time, the grain size, texture strength, misorientation distribution, and deformation of Ag(s.s) and Cu(s.s) in the brazing seam changed significantly. The overall microhardness of the brazing seam significantly increased as the B content increased in the filler metals. The tensile strength of the joints followed a parabola. At the B content of 2%, the ultimate tensile strength of the Cu-Cu joints was 248.0 MPa. The brazed joints featured intergranular, dimple, and cleavage fractures.