Thixoforming : Semi-solid Metal Processing

solidification could be realized by increasing the press velocity. Shell formation, the
origin of which has not yet been clarified, should be prevented.
11.5.6
Summary and Discussion
11.5 Non-isothermal Thixoextrusionj439
Figure 11.32 Extrusion simulation with integrated active cooling in the forming die.
The thixoextrusion experiments were carried out with a modular tool where all
relevant process parameters can be modified. With this extrusion tool, the forming
die, the discard and the dummy plate can be removed by using the opening
mechanism integrated in the tool. For the first experiments, the parameters press
velocity, extrusion channel length, diameter and geometry were modified using a
constant liquid fraction of 40%, which did not result in extruded bars but rather in
extruded clews. A reduction in the liquid fraction by changing the other parameters in
the same range results in massive shell formation or even blocking of the extrusion
channel. Hence a liquid fraction of 10% is not suitable for these experimental
procedures. The massive shell formation allows the formation of very thin bars due to
a proper press velocity/heat transfer ratio. The investigated PVD coatings simplified
cleaning of the forming dies but had no significant influence on the bar extrusion. In
comparison with the conventional extrusion process, the average extrusion force
required is lower. For thixoextrusion processes, the resulting press force increases
towards the end of the process because the material (surface area of the billet) cools
and solidifies during the process.
For a first temperature development estimation of material located in the extrusion
channel, a simulation under adiabatic and ideal contact conditions was performed in
stationary conditions. These simulations revealed that the cooling behaviour of the
material in the extrusion channel is independent of the die material.
With the forging press and the designed extrusion tool, the dwell time of the semisolid
material required in the extrusion channel could not be realized. To achieve