On the Coupling Mechanism of Equiaxed Crystal Generation with the Liquid Flow Driven by Natural Convection During Solidification

Abstract

The influence of the melt flow on the solidification structure is bilateral. The flow plays an important role in the solidification pattern, via the heat transfer, grain distribution, and segregations. On the other hand, the crystal structure, columnar or equiaxed, impacts the flow, via the thermosolutal convection, the drag force applied by the crystals on the melt flow, etc. As the aim of this research was to further explore the solidification-flow interaction, experiments were conducted in a cast cell (95 * 95 * 30 mm 3 ), in which an ammonium chloride-water solution (between 27 and 31 wt pct NH 4 Cl) was observed as it solidified. The kinetic energy (KE) of the flow and the average flow velocity were calculated throughout the process. Measurements of the volume extension of the mush in the cell and the velocity of the solid front were also taken during the solidification experiment. During the mainly columnar experiments (8 cm liquid height) the flow KE continuously decreased over time. However, during the later series of experiments at higher liquid height (9.5 cm), the flow KE evolution presented a strong peak shortly after the start of solidification. This increase in the total flow KE correlated with the presence of falling equiaxed crystals. Generally, a clear correlation between the strength of the flow and the occurrence of equiaxed crystals was evident. The analysis of the results strongly suggests a fragmentation origin of equiaxed crystals appearing in the melt. The transition from purely columnar growth to a strongly equiaxed rain (CET) was found to be triggered by (a) the magnitude of the coupling between the flow intensity driven by the equiaxed crystals, and (b) the release and transport of the fragments by the same flow recirculating within the mushy zone. Graphical AbstractMIHAELA STEFAN-KHARICHA, ABDELLAH KHARICHA, MENGHUAI WU, and ANDREAS LUDWIG are with the

Simultaneous optical measurement of temperature and velocity fields in solidifying liquids

“…In accordance with Stefan- Kharicha et al (2018b), we assume that after that time, only smaller crystal fragments whose size is below the camera resolution were further detached from the upper columnar front in larger quantities. These then stayed suspended in the convectional flow of the increasingly supercooled fluid ( T sup ≈ 1.8 … 4.2 K at 16.5 min ) where they grew into visible equiaxed crystals.…”

Review of Ammonium Chloride–Water Solution Properties

“…Recently a series of experiments were performed on the solidification of a hypereutectic ammonium chloride alloy − in a cast cell cooled homogeneously from the three walls. Heterogeneous solidification occurred simultaneously along verticals and horizontal walls where a columnar mushy zone developed.…”

“…Heterogeneous solidification occurred simultaneously along verticals and horizontal walls where a columnar mushy zone developed. The occurrence of equiaxed crystals was sometimes observed, and a mechanism was proposed to explain the equiaxed crystal’s origin and growth in an undercooled melt for which convection is important . With the PIV (particle image velocimetry) technique the investigation of the flow was possible in the same time as the NH 4 Cl columnar/equiaxed growth.…”

“…The occurrence of equiaxed crystals was sometimes observed, and a mechanism was proposed to explain the equiaxed crystal's origin and growth in an undercooled melt for which convection is important. 23 With the PIV (particle image velocimetry) technique the investigation of the flow was possible in the same time as the NH 4 Cl columnar/equiaxed growth. Such experimental work constitutes a valuable benchmark for validating numerical models.…”