Evolution behavior of microstructure and properties of Cu-20Sn-15Ti filler metal regulated by Si

To improve the microstructure and properties of Cu-Sn-Ti brazing alloy through component control, the effect of Si on the microstructure and properties of Cu-20Sn-15Ti brazing filler metals was studied using scanning electron microscopy, X-ray diffraction, and EDS energy spectrum analysis. The results show that the microstructure of Cu-20Sn-15Ti brazing filler metal is composed of large-sized polygonal CuSn₃Ti₅ phase, eutectic structure and α-Cu phase. A small amount of Si (≤ 2.0wt%) refines the polygonal-shaped CuSn₃Ti₅ phase in the brazing filler metal and generates small-sized Si₃Ti₅ phase. In contrast, a larger amount of Si (≥ 3.0wt%) differentiates the polygonal-shape CuSn₃Ti₅ phase, reduces the proportion of eutectic structure, and increases the content and size of the Si₃Ti₅ phase. When the Si content increases to 5.0wt%, the filler metal no longer generates polygonal-shaped CuSn₃Ti₅ phase and eutectic structure. Instead, Ti primarily forms Ti₅Si₃ phase, and the microstructure mainly consists of Ti₅Si₃ phase, α- Cu phase, Cu₄₁Sn₁₁ phase, and a small amount of strip-shaped CuSn₃Ti₅ phase. Compared with Cu and Sn, Si has a stronger chemical affinity with Ti and preferentially reacts with Ti to form Ti₅Si₃ phase. The three-dimensional structure of Ti₅Si₃ phase is prismatic and exhibits agglomerated growth characteristics. The coarse strip Ti₅Si₃ phase has central or lateral pore defects, which are mainly related to the growth mechanism. As the Si content increases, the shear strength of the filler metals shows a trend of “increasing-decreasing-increasing”, and the fracture morphology transitions from a mixed morphology of quasi-cleavage fracture and cleavage fracture to cleavage fracture. The CuSn₃Ti₅ phase is prone to breakage and cracking, becoming the source of cracking. The presence of coarse CuSn₃Ti₅ phase in different states can deteriorate the shear strength of the brazing filler metals to some extent.