Marangoni-Convection-Driven Bubble Behavior and Microstructural Evolution of Sn-3.5Ag/Sn-17Bi-0.5Cu (Wt Pct) Alloy Solid
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e development of space technology, microgravity solidification for preparing materials with extraordinary microstructures is attracting increasingly more attention.[1,2] The Marangoni convective effect results in significant convection during the alloy solidification process, especially under space microgravity condition, where gravity-induced convection is ignored.[3–6] The heat and mass transfer as well as the bubble behavior in the alloy melt are determined by Marangoni convection, which affects the microstructure and elemental distribution (i.e., phase distribution) of the alloy.[7–11] In addition, the solidification microstructure of the material plays a key role in its practical application. There is thus an urgent need to obtain a better understanding of the effects of Marangoni
ZHANGFU YUAN, RONGYUE WANG, and XIANGTAO YU are with the Collaborative Innovation Center for Steel Technology, University of Science and Technology Beijing, Beijing 100083, China. Contact e-mail: [email protected] Manuscript submitted November 21, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
convection on alloy solidification microstructures under space microgravity condition. Generally, the formation of Marangoni convection is attributed to the existence of a surface tension gradient which is mainly caused by a concentration or temperature gradient.[12–14] Under space microgravity condition, the heat convection induced by gravity is extremely weak and the thermal conductivity is low because of the lack of a heat-transfer medium (in vacuum condition).[15] Moreover, the constant-temperature zone of a furnace is large for an experimental alloy. It is thus difficult to form a temperature gradient in the alloy melt. Consequently, the concentration gradient, instead of the temperature gradient, is the main factor affecting the Marangoni convection. Pb-free, Sn-based alloys are one of the most promising brazing materials.[16–20] However, the observation of Marangoni convection in an alloy melt under microgravity condition remains challenging. In our previous work,[21] it was confirmed that Bi is an effective surfactant that can significantly reduce the surface tension of Sn-based alloys. If a Sn-3.5Ag (wt pct) with large surface tension and a Sn-17Bi-0.5Cu (wt pct) with low surface tension were combined with a Cu ring, a surface tension gradient would be formed over a large
range in the Sn-3.5Ag/Sn-17Bi-0.5Cu (wt pct) alloy melt, and obvious Marangoni convection could be observed. In this work, Sn-3.5Ag/Sn-17Bi-0.5Cu (wt pct) alloy was solidified on a Cu ring substrate under space microgravity condition (SJ-10 satellite). The pore and element distributions in the solidified alloy were investigated, and the effects of the element concentration on the surface tension in the alloy melt were studied, allowing the formation mechanism of Marangoni convection to be analyzed. The microstructural evolution mechanism of the Sn-based alloy was also investigated by comparing the microstructures of the alloy solidified under both normal gravity and space microgravi
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