Marangoni-Convection-Driven Bubble Behavior and Microstructural Evolution of Sn-3.5Ag/Sn-17Bi-0.5Cu (Wt Pct) Alloy Solidified on Cu Substrate Under Space Microgravity Condition

Marangoni convection significantly affects the solidification structure as it controls the bubble behavior and mass transfer in the melt. Sn-3.5Ag/Sn-17Bi-0.5Cu (wt pct) alloy with different surface tension gradients was fabricated and solidified on a Cu ring substrate under space microgravity condition (SJ-10 satellite) to study the Marangoni convection formation mechanism. The pore and element distributions in the solidified alloy and surface tension gradient in the melt were analyzed. The differences between the microstructures of alloys solidified under microgravity and normal gravity conditions were also investigated. The surface tension gradient induced by Bi concentration difference resulted in the formation of Marangoni convection from the right to left of the melt under the microgravity condition. In the left (Bi-scarce) part of the melt, Marangoni convection induced by the Cu concentration difference flowed from outside to inside. Driven by bubble-agitation convection, Cu mainly migrated from the substrate to the right part of the melt. Therefore, dendrite-like Cu_xSn_y was distributed along a gradient. Under the normal gravity condition, significant gravity-induced convection resulted in an even distribution of Bi and Cu, which decreased the contact angle and reduced the surface tension, thus promoting nucleation of the alloy. Therefore, fine dendrite-like Cu_xSn_y with larger number density were uniformly distributed in the melt.