Thixoforming : Semi-solid Metal Processing

202j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.35 Experiment versus one-phase simulation:
comparison of the experimentally obtained and simulated flow
front for two piston velocities, (a) v 1 ¼ 250 mm s 1 and (b)
v2 ¼ 500 mm s 1 .
in the terms of contour of the front, and the transition between laminar and turbulent
filling can be represented reasonably well.
Although the developed model should be suitable for any globular semi-solid
material, the disadvantage of this approach is that the parameters have to be adjusted
for each different filling case. In the present simulation, the parameters for the model
were adjusted empirically during the simulation study. The simulation gives reasonable
results for flow front, but reasonable results for the pressure cannot be obtained.
Another disadvantage when using a single fluid simulation is the inability to simulate
structural changes within material caused by segregation and solidification. Therefore,
two-phase simulation is necessary.
6.2.4.2 Two-phase, Finite Element Based on PETERA
As a two-phase numerical example, simulation of the simple T-shaped die filling
experiment is presented. The specially designed die filling experiment (Figure 6.36)
permits the continuous observation of the flow pattern. The pressure is measured
with a pressure sensor, which is placed near the inlet of the vertical bar. The die is
placed inside an oven, which guarantees isothermal conditions in the die and allows
the adjustment of different die temperatures. Figure 6.36 also shows the container,
carrying the metallic alloy, which is connected to the lower side of the die.
The experiments were performed at different filling velocities and die temperatures.
After each experiment, a chemical analysis of the spatial distribution of the lead
content in the produced part was made. With this lead content and the two-phase
diagram for tin–lead alloys, the local solid fraction is determined.
Before being able to simulate the filling process, parameter determination has to
be carried out.