Interface Shape and Melt Flow Near the Solid-Liquid Interface During Horizontal Unidirectional Solidification

Generally, unindirectional solidification experiments of transparent alloys are conducted using thin film samples sandwiched between two glass slides with a small channel height[1]. In such systems, natural convection and melt flow can be assumed negligible and the solification process is diffusive in nature [2,3]; these physical realities allow fundamental simplifications in theoretical/numerical modeling, without losing physical significance. However, natural convection and melt flow do exist in all actual solidification processes and have a significant effect on interface morphology and microstructure formation and development. Recognizing the latter, a great deal of effort went in recent years towards the investigation of the effect of melt flow on interface dynamics and morphology [3–5]. The objective of this paper is to study the natural convection and melt flow near the solid-liquid interface during horizontal unidirectional solidification. In particular, the authors are interested in the melt flow and solid-liquid interface under various channel heights (H) and temperature differences across the hot and cold ends (ΔT) of the samples. A horizontal unindirectional solidification experimental system was constructed. The samples used here are rectangular ampoules made of borosilicate glass that is 3.2 mm thick (on bottom and top sides) and 2.3 mm thick (on the vertical sides). The channel formed in the sample is 75 mm long and 50 mm wide. Three ampoules with channel heights of 1, 3.2 and 5 mm, respectively, are used in these experiments. The ampoules are filled with succinonitrile (99% pure) seeded with polyamide tracer particles (5 μm in diameter, density ρ=1030 kg/m³); the latter are used to resolve and visualize the fluid velocity in the melt. Surface temperatures of the sample on the hot end and cold end are measured with J-type thermocouples. The unidirectional solifification setup is mounted on a microscope stage so that the interface can be observed from the top with the regular microscope. A long distance microscope (LDM) affixed either to a photo- or video- camera is used to observe the vertical shape of the interface, as well as to qualitatively and quantitatively assess the flow. During experiments, the sample is allowed to reach both thermally and flow-wise a steady state situation. The heating and cooling systems are adjusted to make the solid-liquid interface stay at the center of the gap between the heating and cooling chambers for case of observation. The density of polymide particle being close to that of succinonitrile melt allows an almost neutrally buoyant behavior of the tracing particles and thus minimizes the error in flow velocity calculations as well as enhances confidence in the observed qualitative flow patterns. With the help of proprietary computer software, the flow velocity is obtained by evaluating the difference in successive two images of the same particle at time intervals consistent with the sampling speed of video-camera (0.033 sec).