Observation of flow regimes and transitions during a columnar solidification experiment

Stefan-Kharicha, Mihaela; Kharicha, Abdellah; Wu, Menghuai; Ludwig, Andreas

doi:10.1088/0169-5983/46/4/041424

Cited by 12 publications

(9 citation statements)

References 26 publications

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“…In both stages, the main flow characteristics in the bulk melt were observed as being turbulent/chaotic but downward, so that in conclusion it can be stated that the interdendritic flow in the columnar mushy zone along the vertical walls was upwards. [8,15] In contrast to the geometrical constrains for the small containment used in References 8 and 15 the present larger containment allows a relatively free development of solutal buoyancy flow-especially for the segregated interdendritic melt rising along the lateral walls. It is believed that this interdendritic flow results in an increase of the local microsegregation level in the upper part of the containment.…”

Section: Discussionmentioning

confidence: 90%

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Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

Ludwig

Stefan-Kharicha

Kharicha

et al. 2017

Metall Mater Trans A

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It is well known that the growth and motion of equiaxed crystals govern important microstructural features, especially in larger castings such as heavy ingots. To determine the origin of the equiaxed crystals, heterogeneous nucleation, and/or fragmentation of dendrite arms from columnar regions are often discussed. In the present study, we demonstrate that under certain conditions relatively large areas of mushy regions slide downward and form spectacular crystal avalanches. These avalanches crumble into thousands of dendritic fragments, whereby the larger fragments immediately sediment and the smaller proceed to behave as equiaxed crystals. Traces of such crystal avalanches can be seen by conspicuous equiaxed layers in the lower part of the casting. From the arguments in the discussion, it is believed that such a phenomenon may occur in alloys which reveal an upward solutal buoyancy in the interdendritic mush. This would include certain steels and other alloys such as Cu-Al, Pb-Sn, or Ni-Al-alloys. Moreover, the occurrence of crystal avalanches contribute to the formation of V-segregations.

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Section: Discussionmentioning

confidence: 90%

“…[8,14,15] Before solidification starts thermal buoyancy results in a downward flow along the cold lateral walls and a rising flow at the cell center. Such a flow pattern is also expected to occur in the present larger containment (0.6 9 0.4 9 0.06 m 3 ).…”

Section: Discussionmentioning

confidence: 99%

Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

Ludwig

Stefan-Kharicha

Kharicha

et al. 2017

Metall Mater Trans A

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“…[ [20][21][22] Once the velocities are extracted, the flow kinetic energy (KE) can be calculated as follows:…”

Section: Methodsmentioning

confidence: 99%

“…[20,22] However, to gain a better understanding, a short overview of flow regimes found in currents experiments is in Figure 3. The flow vector maps corresponding to these regimes for a typical equiaxed experiment (shown in Figure 2, red curve), performed at 29.6 wt pct NH 4 Cl in the 3-cm-thick cell at a liquid height of 9.5 cm, are presented: (a) thermal symmetric (TH) flow regime, when only the thermal convection plays a role; (b) turbulent solutal flow (TU) regime; (c) equiaxed-driven flow (EQ) regime when a lot of equiaxed occurred (this flow regime does not exist during experiment with mainly columnar solidification); and (d) a meandering flow (MF) regime when the double diffusion becomes the main event.…”

Section: Flow Regimes During Solidificationmentioning

confidence: 99%

“…In previous works, [20][21][22] the authors have presented in detail the flow behavior and characteristics during a mainly columnar solidification of an NH 4 Cl alloy in a thin (1 cm thickness) cast cell at liquid height of 8 cm. However, few equiaxed crystals occurred during the turbulent flow regime, in the region closed to the lateral mushy zone.…”

Section: Introductionmentioning

confidence: 99%

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

Stefan-Kharicha

Kharicha

et al. 2018

Metall Mater Trans A

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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

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The Effect of Static Magnetic Field on the Channel Formation during Directional Solidification of Aqueous Ammonium Chloride Solution

Ding

Tao

Ren

et al. 2018

Crystal Research and Technology

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This study investigates the effect of magnetic field on the channel formation by using a 27 wt.% aqueous ammonium chloride solution at an initial base temperature of 246 K (−27°C) under magnetic fields of 0.03T, 0.05T, and 0.1T. The application of a magnetic field suppresses channel formation, as reflected by the delayed occurrence, the decreased number and density of channels, and the increased areas with no channels and mushy-zone height necessary to form channels. The suppression of channel formation is caused by the magnetohydrodynamic (MHD) effect on convection during directional solidification. The higher temperature gradient in the mushy zone under an applied magnetic field further confirms the mechanism. The present investigation would help to provide useful predictions for the influence of magnetic field on channel formation during the directional solidification of alloys.

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Observation of flow regimes and transitions during a columnar solidification experiment

Cited by 12 publications

References 26 publications

Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

Massive Formation of Equiaxed Crystals by Avalanches of Mushy Zone Segments

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

The Effect of Static Magnetic Field on the Channel Formation during Directional Solidification of Aqueous Ammonium Chloride Solution

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