Thixoforming : Semi-solid Metal Processing

of (a) an isothermal and (b) a non-isothermal filling experiment at three different
times.
Under isothermal conditions, the surface of the flow front is relatively smooth. At
the point where the flow direction changes from vertical to horizontal, the material
temporarily detaches from the die walls (Figure 6.41a). In the non-isothermal filling
process, the material forms a less smooth surface (Figure 6.41b). At the beginning,
the semi-solid alloy flows through the vertical part of the die as a thin jet with almost
no contact with the die walls. It reaches the upper wall of the die earlier than in the
isothermal case. The contour of the flow front leads to the conclusion that the solid
fraction in this area is rather low. Starting at the inflow to the vertical duct, the hot
material solidifies at the colder walls, which causes a reduction in the free crosssection.
Hence the flow velocity rises and a jet is formed. Additionally, the pressure
drop increases, which leads to a higher degree of segregation.
The observed results were compared with numerical simulations. Figure 6.42
shows the simulated solid fraction and the flow front in the lower vertical duct for
both isothermal and non-isothermal cases. The difference in the shape of the flow
front for isothermal and non-isothermal simulation is comparable to the experimental
results. It can also be seen that solidification occurs in contact with the cold
walls and that the thickness of the solidified layer increases during the die filling.
Figure 6.43 shows the influence of the solidification on the velocity field in this
region. Due to the small free cross-section, the degree of segregation is considerably
large. In the simulation, material with a low solid content flows through the vertical
duct and fills the horizontal part. This corresponds to the measured low solid
content in the horizontal duct. With the optimized set of model parameters, the
simulated flow front and the predicted segregation agree qualitatively with experimental
data.
Figure 6.42 Numerical results: (a) isothermal simulation and (b)
non-isothermal simulation with solidification effects near the die
walls. The dotted lines indicate the growth of the solid layer
thickness during the filling.
6.2 Numerical Modelling of Flow Behaviourj207