Wetting Behavior and Interfacial Characteristics of High Temperature Melts Under Microgravity

The interfacial characteristics and microstructure of solder/substrate is an important indicator for welding performance. Due to weak gravity-induced convection under microgravity condition, Marangoni convection effect becomes more obvious, which significantly influences the interfacial characteristics and solidification microstructure of alloy by controlling bubble behavior and mass transfer in the melt. To obtain strong Marangoni convection, Sn-3.5Ag/Sn-17Bi-0.5Cu and Sn-3.5Ag/Sn-10Sb alloys with different surface tension are constructed. The alloys are solidified on Cu ring substrate under space microgravity condition (SJ-10 satellite) to study the reaction mechanism of melt/substrate interface and microstructure evolution of alloys based on the analysis of the structure of intermetallic compounds and element distribution in solidified alloy. The corresponding wetting experiment of Sn-3.5Ag and Sn-17Bi-0.5Cu or Sn-10Sb on the Cu ring ground is done under ground. The results indicate that, under microgravity condition, a large number of fine pores appear in the alloys, and a lot of scalloped or small rod-like crystals are formed at alloy/substrate interface. Many big rod-like crystals are also formed in the bulk of alloy. The elementary analysis results show that, after Sn-3.5Ag/Sn-17Bi-0.5Cu alloy being melted, Ag is evenly diffused and distributed in the bulk phase, Bi is also evenly diffused and block aggregation is formed, which is mainly clustered at the outer edge of interface layer at the bulk phase side. There are three layers of intermetallic compound at the interface of alloy and substrate, that is, the thin Cu41Sn11 transition layer near the substrate side, the thick Cu3Sn layer in the middle, and Cu6Sn5 layer near the solder. Two layers of substances are formed on the ground. That is, the thin Cu3Sn transition layer near the substrate side and the Cu6Sn5 layer near the solder. Similarly, after Sn-3.5Ag/Sn-Sb alloy being melted, interface reaction happens on the melt/substrate interface. And three layers of substances are also formed: the thin Cu41Sn11 layer near the substrate side, the Cu3Sn in the middle side and the Cu6Sn5 layer near the solder side. The Sb element is more enriched in the alloy Sn in the other area, but in the internal rod-like crystal of bulk phase, Sn is not detected.