Vacuum Arc Remelting (VAR) is employed to produce homogeneous ingots with a
controlled, fine, microstructure. It is applied to reactive and segregation prone alloys
where convection can influence microstructure and defect formation. In this study, a
microscopic solidification model was extended to incorporate both forced and natural
convection. The Navier-Stokes equations were solved for liquid and mushy zones using a
modified projection method. The energy conservation and solute diffusion equations
were solved via a combined stochastic nucleation approach along with a finite difference
solution to simulate dendritic growth. This microscopic model was coupled to a 3D
transient VAR model which was developed by using a multi-physics modelling software
package, PHYSICA. The multiscale model enables simulations covering the range from
dendrites (in microns) to the complete process (in meters). These numerical models were
used to investigate: (i) the formation of dendritic microstructures under natural and forced
convections; (ii) initiation of solute channels (freckles) in directional solidification in
terms of interdendritic thermosolutal convection; and (iii) the macroscopic physical
dynamics in VAR and their influence on freckle formation.
2D and 3D dendritic microstructure were simulated by taking into account both solutal
and thermal diffusion for both constrained and unconstrained growth using the
solidification model. For unconstrained equiaxed dendritic growth, forced convection
was found to enhance dendritic growth in the upstream region while retarding
downstream growth. In terms of dimensionality, dendritic growth in 3D is faster than 2D
and convection promotes the coarsening of perpendicular arms and side branching in 3D.
For constrained columnar dendritic growth, downward interdendritic convection is
stopped by primary dendritic arms in 2D; this was not the case in 3D. Consequently, 3D
simulations must be used when studying thermosolutal convection during solidification,
since 2D simulations lead to inappropriate results. The microscopic model was also used
to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation
during directional solidification at a microstructural level for the first time. These
simulations show that the local remelting due to high solute concentrations and
continuous upward convection of segregated liquid result in the formation of sustained
open solute channels. High initial Sn compositions, low casting speeds and low
temperature gradients, all promote the initiation of these solute channels and hence
freckles.
to study the initiation of freckles for Pb-Sn alloys, predicting solute channel formation
during directional solidification at a microstructural level for the first time. These
simulations show that the local remelting due to high solute concentrations and
continuous upward convection of segregated liquid result in the formation of sustained
open solute channels. High initial Sn compositions, low casting speeds and low
temperature gradients, all promote the initiation of these solute channels and hence
freckles
