Abstract
In the undercooled solidification of pure metals, the dendrite tip velocity has been shown experimentally to have a strong dependence on the intensity of an external magnetic field, exhibiting several maxima and minima. In the experiments conducted in China, the undercooled solidification dynamics of pure Ni was studied using the glass fluxing method. Visual recordings of the progress of solidification are compared at different static fields up to 6 T. The introduction of microscopic convective transport through thermoelectric magnetohydrodynamics is a promising explanation for the observed changes of tip velocities. To address this problem, a purpose-built numerical code was used to solve the coupled equations representing the magnetohydrodynamic, thermal and solidification mechanisms. The underlying phenomena can be attributed to two competing flow fields, which were generated by orthogonal components of the magnetic field, parallel and transverse to the direction of growth. Their effects are either intensified or damped out with increasing magnetic field intensity, leading to the observed behaviour of the tip velocity. The results obtained reflect well the experimental findings.This article is part of the theme issue ‘From atomistic interfaces to dendritic patterns’.
Highlights
The study of free undercooled growth of pure materials is key to understanding the fundamentals of solidification
The numerical results present a parametric study of magnetic field intensity that captures the time evolution of the tip until the equilibrium velocity is found
The results show that there are three magnetic field intensity regimes: a low magnetic field of 0–2.7 T, where the tip velocity decreases; a moderate magnetic field of 2.7–6.7 T, where the tip velocity recovers; and a high magnetic field greater than 6.7 T, where the tip velocity decreases and plateaus
Summary
The study of free undercooled growth of pure materials is key to understanding the fundamentals of solidification. With the AC field switched off, residual bulk flow velocities can persist past nucleation [9] These velocities and the resulting convective heat transport in the sample are assumed to be a primary reason for the mismatch between existing theory and experiments [4]. The Alexandrov and Galenko (AG) theory [10,11] was developed to account for dendritic growth with convection It is an extension of the LKT theory and assumes a component of the fluid velocity is incident to the growing tip. TEMHD is strongly dependent on the orientation of the magnetic field relative to the growth direction This is highlighted, where a magnetic field oriented along the direction of growth interacts with the radial component of the current emanating from the tip, forming a force that drives a rotational flow around the tip. The mesh moves when the solidification front reaches a layer of cells half way up the domain
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