Controlling Solute Channel Formation using Magnetic Fields

Motivation

Solidification processes are affected by natural convection as soon as instable density stratifications arise in the melt from local variations of the temperature and/or the concentration field. These variations in temperature and composition directly influence local density, hence buoyancy-driven fluid flow may occur. During metallic alloy solidification and magma flows, preferential flow channels or chimneys are frequently observed (also termed a solute finger/channel, or channel segregation), once solidified, these regions become defects, namely ‘freckles’. During the solidification of metallic alloys, these channels are usually formed in the interdendritic regions, and because they are solute-rich, they solidify during the final stages of solidification, and can have quite different mechanical properties than the bulk component. The academic and industrial communities have a strong interest in developing methods for solute channel control, including by introducing external forces to disrupt the natural convection.

Goal

A joint theoretical and experimental program between research groups at the HZDR, the University of Greenwich and University College London using synchrotron radiation was designed to understand how the application of a magnetic field can be used to control and ultimately mitigate freckle defects. Such defects are common in the casting of Ni-based superalloys for turbine blades. The synchrotron experiments use the model alloy system Ga-In, which has similar properties to superalloys but is liquid at room temperature and therefore easier to handle [1].

Synchrotron experiments

Applying an external magnetic field interacts with thermoelectric currents at solid/liquid interfaces generating additional flow fields. This thermoelectric (TE) magnetohydrodynamic (TEMHD) effect can impact on solute channel formation, via a mechanism recently drawing increasing attention. To investigate this phenomenon, we combined in situ synchrotron X-ray imaging and Parallel Cellular Automata Lattice Boltzmann method-based numerical simulations [2-4] to study the characteristics of flow and solute transport under TEMHD. The solidification experiments were performed at the I12 beamline at Diamond Light Source (DLS) [5].

When applying a transverse magnetic field, an off-centre solute channel was formed as a result of microscopic TEMHD flow. Both experimental and simulation results show that plumes migrate to two points during the early stage of solidification, our simulation demonstrates that a macroscopic TEMHD flow is responsible for this plume lateral migration.

We demonstrated that the macroscopic TEMHD flow ahead of the solidification front, along with the microscopic TEMHD flow arising within the mushy zone are the primary mechanisms controlling melt flow. Two thermoelectric regimes were revealed, each with distinctive mechanisms that control flow.

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Selected publications:

[1] Shevchenko, N.; Roshchupkina, O.; Sokolova, O.; Eckert, S.,
The effect of natural and forced melt convection on dendritic solidification in Ga-In alloys
Journal of Crystal Growth 417(2015), 1-8

[2] A. Kao, N. Shevchenko, M. Alexandrakis, I. Krastins, S. Eckert, and K. Pericleous
Thermal dependence of large-scale freckle defect formation
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 377, no. 2143, 2019

[3] A. Kao, I. Krastins, M. Alexandrakis, N. Shevchenko, S. Eckert, and K. Pericleous
A Parallel Cellular Automata Lattice Boltzmann Method for Convection-Driven Solidification
JOM, vol. 71, no. 1, 2019

[4] A. Kao, N. Shevchenko, S. He, P.D. Lee, S. Eckert, K. Pericleous
Magnetic effects on microstructure and solute plume dynamics of directionally solidifying Ga-In alloy
Jom 72 (2020) 3645–3651

[5] X. Fan, N. Shevchenko et al.
Controlling solute channel formation using magnetic fields
Acta Mater, vol. 256, p. 119107, Sep. 2023