Study on the Effect of Melt Convection on Phase Separation Structures in Undercooled CuCo Alloys Using an Electromagnetic Levitator Superimposed with a Static Magnetic Field

Effect of Static Magnetic Field on Recalescence and Surface Velocity Field in Electromagnetically Levitated Molten CuCo Droplet in Undercooled State

“…Moreover, on the basis of both the above observations and numerical simulations of MHD convection in the same system as that used for the observations, they proposed a critical Reynolds number of 600, at which the laminar-turbulent transition of MHD convection occurs in electromagnetically levitated molten metal droplets. Recently, Lee et al [13] have performed numerical simulations on laminar and turbulent MHD convection in electromagnetically levitated droplets of metallic alloys and investigated whether the flow of metallic alloys under operational conditions in space experiments on multiphase solidification phenomena is laminar or turbulent, with consideration of the critical Reynolds number proposed by Hyers et al [12] The aim of this work is to investigate the hypothesis in our previous work, [11] namely, that the marked change in the phase separation structures in an undercooled molten CuCo droplet at a static magnetic field of approximately 1.5 T is due to a convective transition from turbulent flow to laminar flow in an electromagnetically levitated droplet, by carrying out procedures other than periodic laser heating. Therefore, we directly observed the recalescence events of phase-separated Co-rich phases in electromagnetically levitated molten CuCo droplets in an undercooled state under several static magnetic fields using a high-speed camera.…”

“…[11] It was revealed that, for a relatively small magnetic field, dispersed structures with relatively fine Co-rich spheres distributed in the matrix of the Cu-rich phase were observed, while a few large, coalesced Co-rich phases appeared in the Cu-rich matrix above a magnetic field of approximately 1.5 T. Moreover, we speculated that such a marked change in the phase separation structures at approximately 1.5 T is due to a convective transition from turbulent flow to laminar flow in the molten CuCo droplet. This hypothesis was based on the results of periodic laser heating, in which the time variation of the temperature in the lower part of the electromagnetically levitated droplet was measured while the upper part of the droplet was periodically heated.…”

“…EXPERIMENT Figure 1 shows a schematic diagram of the electromagnetic levitator used to observe the recalescence events of undercooled molten CuCo droplets during cooling and to measure the surface velocities of the droplets. This levitator is the same as that used in our previous work, [11] except that a high-speed camera (FASTCAM 4A, Photron Ltd.) together with a mirror was set above the chamber for direct observation of the electromagnetically levitated molten CuCo droplet, instead of a laser in our previous work. [11] An RF coil system with seven turns, i.e., five turns for the electromagnetic levitation of the droplet and two opposite turns for the stabilization of the droplet, was installed inside the superconducting magnet (JMTD-10T120 SSFX, Japan Superconductor Technology Inc.), where the frequency of the RF current was 280 kHz.…”

“…Then, the solidified samples were levitated and remelted by the electromagnetic levitator in Figure 1 so that the compositions in the samples could be homogenized by utilizing MHD convective mixing, similarly to in our previous work. [11] In the experiment, a CuCo solid sample was levitated and melted in an Ar atmosphere with 5 pct H 2 . After maintaining the sample at a constant temperature above the liquidus temperature of CuCo for 300 seconds to make the composition in the sample completely uniform by convective mixing in the absence of a static magnetic field, the levitated droplet was cooled by blowing He gas from the gas nozzle in Figure 1 under a given static magnetic field between 0 and 4 T. The sample temperature was measured using a single-color optical pyrometer (spectral range of 1450 to 1800 nm, Infratherm IGA 140; IMAPC Infrared GmbH, Frankfurt).…”

“…[11] Here, each solidified sample was cut into two pieces and the structures in the two parallel cross sections were observed after polishing by a polishing machine (ML-150P, Maruto Instrument Co., Ltd.).…”