Revealing the mechanisms by which magneto-hydrodynamics disrupts solidification microstructures

Abstract

A key technique for controlling solidification microstructures is magneto-hydrodynamics (MHD), resulting from imposing a magnetic field to solidifying metals and alloys. Applications range from bulk stirring to flow control and turbulence damping via the induced Lorentz force. Over the past two decades the Lorentz force caused by the interaction of thermoelectric currents and the magnetic field, a MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD), was also shown to drive inter-dendritic flow altering microstructural evolution. In this contribution, high-speed synchrotron X-ray tomography and computational simulation are coupled to reveal the evolution, dynamics and mechanisms of solidification within a magnetic field, resolving the complex interplay and competing flow effects arising from Lorentz forces of different origins. The study enabled us to reveal the mechanisms disrupting the traditional columnar dendritic solidification microstructure, ranging from an Archimedes screw-like structure, to one with a highly refined dendritic primary array. We also demonstrate that alloy composition can be tailored to increase or decrease the influence of MHD depending on the Seebeck coefficient and relative density of the primary phase and interdendritic liquid. This work paves the way towards novel computational and experimental methods of exploiting and optimising the application of MHD in solidification processes, together with the calculated design of novel alloys that utilise these forces.

Effect of longitudinal magnetic field on grain growth of hollow stud welded joint

“…The electromagnetic damping phenomenon would occur when the magnetic eld current was further increased [18]. The molten pool convection would be inhibited as the electromagnetic damping played a dominant role in the magnetic eld [19,20]. At the same time, heat conduction was inhibited, which reduced the undercooling degree of molten pool metal and increased the growth tendency of proeutectoid ferrite and grain coarsening, as shown in Fig.…”

Review on Eutectic-Type Alloys Solidified under Static Magnetic Field

“…As shown in Figure 2c, an electromagnet which can generate a magnetic field of 1 T between two iron cores can be used for in situ observation process. The SMF can act on the solid-liquid interface to control the microstructure during directional solidification [81]. Encapsulation of the sample is crucial for in situ observation of the solidification process as illustrated in Figure 2d.…”

“…In the 3D imaging field, Koe et al [107] developed a novel electromagnetic apparatus for in situ synchrotron X-ray imaging to study the separation of phases in metal solidification, showing brief results for Al-20wt.%Si. The first 4D (3D plus time) experiment under an SMF was conducted by Cai et al in an Al-15wt.%Cu alloy under a 0.5 T SMF [81]. This study coupled high-speed synchrotron X-ray tomography and high-performance computational simulation to reveal the evolution, dynamics, and mechanisms of solidification, unraveling the complex interplay and competing flow effects arising from the Lorentz forces of different origins.…”

Thermoelectric Magnetohydrodynamic Control of Melt Pool Flow During Laser Directed Energy Deposition Additive Manufacturing

“…TECs interact with an external magnetic field, generating a Lorentz force that drives a new flow, known as thermoelectric magnetohydrodynamic (TEMHD) flow. TEMHD flow has been proven useful to control solute segregation [30][31][32][33] and refine dendrite arm spacing [34,35] in traditional casting and realising self-stirring for liquid lithium [36,37].…”