Effect of Bismuth Addition on Microstructure and Properties of Sn-6.5Zn Lead-Free Brazing Alloy

In this study, a Sn-6.5Zn-xBi brazing alloy was prepared using a melting coating agent. The effects of different Bi additions on the microstructure, melting properties, oxidation resistance, wettability, and corrosion resistance of the Sn-6.5Zn lead-free solder were investigated. The results indicate that the addition of a small amount of Bi into the intermediate alloy leads to a gradual transformation of the β-Sn phase in the microstructure from a regular oval island shape to an irregular shape, eventually losing its defined form. The inclusion of Bi reduces the melting point of the alloy, but it can also result in an increased melting range. At a Bi content of 3.0 wt.%, the alloy exhibits a melting point and melting range of 201.7℃ and 16.5℃, respectively, demonstrating relatively excellent thermal properties.

Within the range of Bi addition, the number of acicular Zn phases gradually increases. Moreover, when the Bi content ranges from 0.0 to 3.0 wt.%, the size of the acicular Zn phase initially increases and then decreases. The addition of trace amounts of bismuth enhances the oxidation resistance and wettability of the filler metal alloy, as Bi functions as an active element and exists in the alloy as a solid solution phase. Notably, at a Bi content of 1.0 wt.%, the brazing alloy exhibits the best oxidation resistance, while at a Bi content of 3.0 wt.%, the solder demonstrates optimal wettability and corrosion resistance.