Microstructure and properties of Zn-Cu-Sn-Bi high-temperature solders

In order to develop high temperature solders for brazing Cu and steel,a series of Zn-2Cu-Sn-Bi (wt-%) solders have been investigated. Through differential scanning calorimeter (DSC) analysis, the thermal properties of Zn-Sn-Cu-Bi solders were measured. The results showed that the solidus and liquidus temperatures of Zn-2Cu-Sn-Bi solders are 386.00-391.80 ℃ and 398.64-401.50 ℃, respectively, and the under-cooling temperatures are between 9.37 ℃ and 12.64 ℃. The wettability tests showed that the increase of Sn in Zn-2Cu-Sn-2Bi solders resulted in the increase of wetting area. The average wetting area was a of 206 mm², and the smallest contact angle was of 5°. The metallographic observations indicated that Zn-2Cu-Sn-Bi solder consisted of η-Zn matrix, dendrite β-Sn crystals, Bi phaseand ε-CuZn₅ intermetallic compounds. The Zn-Cu compounds were formed in the joints. The interfacial layer was about 80 μm thick, with one scallop-shaped ε-CuZn₄/CuZn₅ layer, and two flat layers β-CuZn and γ-Cu₅Zn₈.