Thixoforming : Semi-solid Metal Processing

416j 11 Thixoextrusion
parameter concepts for reproducible long bar extrusion were presented. The first
investigation of this aspect is represented by recent work on an appropriate determination
of the solidification stage by applying numerous different die concepts and
cooling devices [9, 10].
11.3
Tool Concepts
As described above, the low strength of semi-solid metals may cause defects if the
extruded bar leaves the die in a semi-solid condition. On the other hand, the
advantage of low forces can only be achieved if the material passes the deformation
zone in a semi-solid condition. Two opposite concepts, which, however, can be
combined to some extent, attempt to solve this contradiction.
11.3.1
Isothermal Tool Concept
Since the extrusion process takes some time, this favours the use of a billet container
which is heated to the semi-solid temperature so that the billet condition remains the
same during the process (isothermal tool concept for steel). This can only be achieved
using ceramic containers, which will not be able to withstand high tensile stresses
caused by high extrusion pressures, which might be desired, for example, to avoid
solidification porosity, or could be caused by any problem within the process. Due to
the forming process in the semi-solid state and the temperature of the tool, the
solidification has to take place after the isothermal part of the forming die. This could
either be a non-isothermal elongation of the forming die or strong active cooling
directly after leaving the die, such as performed in continuous casting of aluminium,
where the mould length is very short. In the latter case, a mechanism to carry the
weight of the extruded bar is required. One possible solution is guide rollers as shown
in Figure 11.2.
The first challenge of this concept is the design of the isothermal die setup,
including the heating device. Further, the cooling of the bar and the guiding system,
which must be carefully coordinated with the extrusion punch, has to be
implemented.
11.3.2
Non-isothermal Tool Concept
The other alternative is to accept that the billet in the container forms a solid shell
during extrusion and to extrude deliberately only the inner portion of the billet, thus
ensuring that any oxides which might have formed on the surface will not be extruded
(non-isothermal tool concept for steel). In this case, a strong metallic container can be
used to apply high pressure and allow complete solidification within the extrusion
die. This requires that the extrusion channel length is chosen long enough to provide