Thixoforming : Semi-solid Metal Processing

418j 11 Thixoextrusion
Both concepts, which could also be combined to a certain extent, have advantages
and disadvantages and will therefore be evaluated in more detail in the following
sections.
11.4
Isothermal Thixoextrusion
11.4.1
Experimental Strategy, Tools and Process Parameters
The tool design and experimental setup used for isothermal thixoextrusion of steel
are based on the ceramic tool concept introduced in Chapter 8. In order to determine
the principal operating parameters of this novel tool setup, small-scale extrusion tests
were performed on an INSTRON universal testing machine, allowing precise control
of process loads and extrusion velocity. Subsequently, the tool design was adapted and
scaled up to extrusion tests on the SMS Meer open die forging press.
The laboratory tool setup is shown in Figure 11.4. For these first laboratory
experiments, the steel billets were manufactured from rods of 30 mm diameter,
while the punch diameter was set to 28 mm. This was due to the billets being
preheated in air, inducing the formation of an oxide layer on the billet surface to
facilitate handling of the semi-solid billets. The semi-solid slurry was squeezed out of
this oxide shell by a smaller punch, leaving the scale in the container.
The specific characteristic of this tool is the die geometry, consisting of an axialsymmetric
shamrock-like cross-section (Figure 11.5), exhibiting a high ratio of
circumference to cross-sectional area (Table 11.1). The tool preheating strategy was
adapted to the work alloy. As discussed in Section 11.3, two temperature zones may be
distinguished in isothermal extrusion: (i) the forming zone, in which the material
Figure 11.4 Tool geometry as applied in laboratory-scale isothermal thixoextrusion tests.