XXI ICTAM, 15-21 August 2004, Warsaw, Poland
The predicted and measured temperature curves are in good agreement. Figure 1 presents the spatial distributions of the
volume fraction of liquid, the temperature and the velocity field 60 sec after the start of the solidification at the bottom
of the container. It can be seen that fluid motion caused by the RMF at such small Taylor number has no influence on
the shape of the mushy zone front due to the high cooling rate. The arising secondary flow in the radial-meridional plane
was found to have a significant influence on the mass and heat transfer at the solidification front. Time-dependent
calculations revealed that this secondary flow structure is strongly influenced by the advancement of the solidification
front. The resulting permanent change of the aspect ratio of the fluid volume prevents the development of the double-
vortex structure being well known for the isothermal case. These findings were confirmed by fluid velocity
measurements during solidification.
Experimental results
The Pb-Sn alloys were solidified directionally from the bottom in a cylindrical stainless steel mold having an internal
diameter of 50 mm, a height of 100 mm and a wall thickness of 3 mm. The side walls were covered by a 5 mm heat
insulation to prevent a radial heat transfer from the mold. The filling height for each charge was 60 mm. During
solidification continuous temperature measurements were performed by means of a set of six thermocouples installed in
the center of the mold at vertical positions of 1.5, 11.5, 22, 32, 41.5 and 51.5 mm measured from the base of the ingot.
The ultrasound Doppler velocimetry (UDV) was used to measure the velocity inside the liquid phase as well as the
position of the solidification front. Selected transverse and longitudinal sections of the solidified specimens were
polished and etched for microscopic examination.
The RMF drives a convection in the bulk liquid showing a distinct influence on the solidification parameters and the
properties of the microstructure of the ingots. The fluid flow increases the heat transfer rate and decreases the
temperature gradients in the melt. Such small temperature gradients reduce the probability of remelting of solid
fragments ahead of the solidification front required for the promotion of the CET. Moreover, the temperature
measurements also revealed the occurrence of strong temperature fluctuations arising from the turbulent flow in the
vicinity of the solidification front. These fluctuations may promote a localized remelting of dendrites and thus the
creation of solid fragments. Velocity measurements have been obtained in the melt during solidification with applied
RMF by means of UDV. If the temperature falls below the liquidus isotherm, a strong decrease of the velocity amplitude
was observed indicating a rapid increase of the fluid viscosity due to an increase of the solid fraction. The position
where the velocity becomes zero can be approximately identified with the position of the solidification front as long as a
columnar front grows.
If the ingot is solidified under the influence of forced bulk flow induced by RMF a CET can be observed. As shown in
Figure 2 the CET position is affected by the convection. An enhancement of the magnetic Taylor number Ta results in a
reduction of the length of the columnar region. The distribution of the eutectic in the microstructure is not homogeneous.
In the equiaxed domain an up to 10% higher eutectic content is observed as compared to the columnar region. The
highest concentration of eutectic is found directly at the CET position (Figure 3).