Thixoforming : Semi-solid Metal Processing

Figure 6.39 Isothermal experiment, one- and two-phase
isothermal simulation: pressure comparison.
6.2 Numerical Modelling of Flow Behaviourj205
fraction of 52% and material and die temperature of 192 C. One can see that the
calculated shapes of the flow front at different filling velocities are in good agreement
with the observed flow fronts (Figure 6.38). Problems arise, however, in the case of
one-phase simulation, where using one set of material parameters it was not possible
to achieve satisfactory results for different piston velocities. The initial set of
parameters gave reasonable results only for one piston velocity, and for other
velocities there was no agreement between the experimental and numerical results
and the parameters have to be readjusted.
Figure 6.39 shows a comparison of the calculated and measured pressure.
Although it was possible to achieve good agreement between the one-phase simulation
and the observed flow front, this simulation gives no reliable results for pressure.
In contrast to the experiment, the simulation yields no further increase in pressure
during filling of the horizontal part but rather a nearly constant pressure. This is in
accordance with the theoretical assumption for free jet flows but not with the
experimental results. The wrong calculation of the pressure during the filling of
the horizontal duct is caused by the disregard of the segregation.
The two-phase simulation of this filling process shows good agreement of the
measured and calculated pressure course (Figure 6.39). The pressure course increases
until the end of the filling process because segregation effects were taken into
account. Segregation first occurs when the material enters the vertical duct. Then the
solid content of the flow front is rather low and no significant segregation effects in
the flow front appear while it moves through the die. Hence the solid fraction of the
material increases while flowing through the vertical duct, which results in higher
viscosity values and thus in a continuous increase in pressure (Figure 6.39).
The analysis of the solid fraction distribution showed that significant segregation
occurs at high initial solid fraction (Figure 6.40). At the beginning of the filling
process, the liquid phase is squeezed out of the billet like water out of a sponge. Hence