Thixoforming : Semi-solid Metal Processing

440j 11 Thixoextrusion
solidification of the material, experiments using the cooling tube of the isothermal
thixoextrusion processes were performed. Using a liquid fraction of 40% and a
cooling tube after the tool resulted in successful bar extrusion. The variations of
process parameters showed almost the same results. The maximum length of
the extruded bars was limited to 460 mm due to the increasing weight of the bar
while the resistance of the semi-solid material remained constant. When the gravity
force increased due to increasing weight, the bars dropped down. The extruded bars
were characterized by a reduced diameter compared with the extrusion channel
diameter because of shell formation, which could not be prevented. Independently of
the chosen process parameters, which have no significant influence on the solidification
of the semi-solid material, the solidification was realized only by the cooling
tube. Regarding the microstructure analysis, the thixoextrusion experiments performed
show a process parameter- independent behaviour. For the chosen process
conditions, the extruded profiles show a similar homogeneous globulitic microstructure.
Concerning Vickers hardness, the eutectic shows doubled the values for the
primary grains.
Further simulations of the thixoextrusion process permit the complete solidification
using low press velocities of 0.5 mm s 1 . Active cooling systems integrated in the
forming die allow an increase in the press velocity to 3 mm s 1 . Shell formation, the
origin of which has not yet been clarified, has to be prevented for these conditions.
The simulation results show that a tool combination where shell formation is
prevented and complete solidification is realized with an active cooling system
achieved simultaneously would be a solution for successful applications.
11.6
Conclusion
Ensuring the solidification of the semi-solid material after forming the material is the
most important challenge of semi-solid extrusion processes. This solidification can
be realized in or after the forming die, which results in two different tool concepts as
presented previously: the isothermal and the non-isothermal tool concepts. Both
concepts require specific tool setups and process strategies. The general feasibility of
both concepts has been shown but open challenges still remain.
With the tool for the isothermal semi-solid extrusion process, forming of the semisolid
material is possible but handling of the bars after the forming die has not yet
been solved. The extruded bars drip and limit the bar length in the experiments.
Similarly to other complex, but established, metal forming processes, such as
continuous casting and thin strip casting, accurate solidification control or bar
suspension has to be integrated in the tool.
The non-isothermal extrusion tool setup has not so far achieved the goal of
complete solidification of the material in the extrusion channel. Either the press
velocity would have to be extremely low or the extrusion channel length extremely
long to ensure complete solidification. In addition, the material which comes in
contact with the extrusion channel began to stick to the wall, forming a non-extruded