Thixoforming : Semi-solid Metal Processing

312j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Third, the shot weight is greater than the weight of the part because of the material
of the biscuit, gating system and the overflows. The use of multiple moulds is as usual
for thixocasting as for the high-pressure die casting (HPDC) process.
Fourth, the semi-solid metal (SSM) casting processes seem very useful for filigree
and complex geometries. Complexity includes undercuts formed by core pullers and
also long flow lengths enabled by a high shot velocity and thick- and thin-walled
sections in one part.
9.2
SSM Casting Processes
The SSM casting processes are usually divided into three main types: thixocasting,
rheocasting and thixomoulding (Figures 9.1 and 9.2).
The thixocasting process is usually divided into three steps: pre-material billet
production, reheating into the semi-solid state and forming.
Depending on the alloy, several methods for producing a pre-material with a
globular microstructure have been developed. During reheating of the pre-material
billets, very close control of the process is needed to ensure the correct solid/liquid
fraction and a homogeneous temperature distribution of the billet. After reheating,
the semi-solid billet is placed in the modified shot chamber of an HPDC machine or a
squeeze casting machine and formed [5, 6].
In contrast to thixocasting, the rheocasting processes deal with integration of the
pre-material production and the subsequent preparation of a semi-solid slurry with
thixotropic properties [7]. An important development in the field of rheocasting was
the so-called new rheocasting introduced by UBE, Japan. With this technology, the
generation of a globular microstructure is not caused by agitation during solidification
but directly through forced nucleation by cooling into the semi-solid state. Melt
just above its liquidus temperature is poured into a permanent steel mould, of a
defined geometry, where it spontaneously cools into the semi-solid state. At the
same time, nucleation of the material takes place according to the heterogeneous
nucleation theory and through controlled cooling the desired globular microstructure
is produced.
Figure 9.1 Layout of a thixocasting process.