Thermo-Electric-Magnetic Hydrodynamics in Solidification: In Situ Observations and Theory

Abstract

Solidification of liquid metals contains all the ingredients for the development of the thermo-electric (TE) effect, namely liquid-solid interface and temperature gradients. The combination of TE currents with a superimposed magnetic field gives rise to thermo-electromagnetic (TEM) volume forces acting on both liquid and solid. This results in the generation of fluid flows, which considerably modifies the morphology of the solidification front as well as that of the mushy zone. TEM forces also act on the solid and cause both fragmentation of dendrite branches and a movement of equiaxed grains in suspension. These phenomena have already been unveiled by post-mortem analysis of samples, but they can be analyzed in more detail by using x-ray in situ and real-time observations. Here, we present conclusive evidence of all the aforementioned effects thanks to in situ observations of Al-Cu alloy solidification under static magnetic field.

Thermoelectric magnetohydrodynamic control of melt pool dynamics and microstructure evolution in additive manufacturing

“…In the presence of an applied magnetic field, the TE currents interact generating a Lorentz force that drives flow, convectively transporting both heat and mass. Significant changes to solidification processes due to TEMHD have been observed experimentally and predicted theoretically in situations ranging from the low thermal gradient and slow solidification typical of directional solidification processes [28][29][30] to the high thermal and rapid solidification process of high undercooled growth [31,32]. In terms of solidification conditions, AM thermal gradients are high with moderate solidification velocities compared to high undercooled growth, and, therefore, TEMHD effects could be quite significant.…”

Columnar to Equiaxed Transition during Directionally Solidifying GCr18Mo Steel Affected by Thermoelectric Magnetic Force under an Axial Static Magnetic Field

“…33) In addition, we have found that TEMF also cause a movement of equiaxed grains in suspension by using x-ray in situ and real-time observations under static magnetic field. 34,35) To gain more insight, the solidification microstructures and the maximum value of computed TEMC and the von Mises stress at various growth rates and magnetic field intensities under the temperature gradient of 104 K/cm are plotted in Fig. 8.…”

Revealing the mechanisms by which magneto-hydrodynamics disrupts solidification microstructures

“…) and it is well known that applying a static magnetic field changes the melt flow leading to electromagnetic damping [8] or alternatively, a travelling field (including a rotating magnetic field) leads to electromagnetic stirring (EMS) [9,10]. A static magnetic field has also been seen to drive flow through its interaction with inherent thermoelectric currents [11][12][13][14], generated in solidifying alloys due to spatial variations in temperature and Seebeck coefficient [11,[15][16][17]. This thermoelectric Lorentz force ( #! )…”

“…This thermoelectric Lorentz force ( #! ) produces flow in the mushy zone between growing dendrites, an MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD) [11,13,15,18]. !"…”

“…Hence, it is very difficult to determine the exact mechanisms attributable to the application of magnetic fields on microstructural changes. In situ radiographic studies have proved more useful in this respect, demonstrating the effects of magnetic field-induced convection on solidification [13,26,27]. Recent advances in highspeed X-ray tomography enable time-resolved 3D observation of alloy solidification (known as 4D imaging -3D space and time) [28][29][30][31], providing a useful tool to study solidification under magnetic fields.…”