Thixoforming : Semi-solid Metal Processing

320j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
decides whether the reheating matches the chosen tool and process window. Third,
the warming must be controlled sufficiently exactly that the homogeneity of the liquid
phase varies by only a few percent about the bolt length and the bolt crosscut. This
homogeneity can be quantified with axial and radial temperature measurements.
Since the heat conductivity of the steels is lower than that of aluminium, more time
must be allowed for the temperature compensation. Furthermore, the radiation
losses have to be considered and without countermeasures lead to a solid billet skin of
some millimetres thickness, with which the oxide restraint system of the Deutsche
Gesellschaft f€ur Materialforschung (DGM) would be rapidly overtaxed. Fourth, good
reproducibility is imperative for constant, high component quality. Close control and
regulation, high-quality components and correctly selected materials in the direct
surroundings of the billet are prerequisites. The avoidance of oxidation is an essential
point for steel billets since no closed oxide film develops as on a semi-solid
aluminium billet and the oxidation would steadily and rapidly progress. Inductive
heating best fulfils the qualification profile for many materials and for this reason is
frequently used [34].
The heating device from Elotherm (Figure 9.8) works with frequencies between
750 and 950 Hz according to the oscillating circuit principle. The plant consists of a
water-cooled, cylindrical coil with 29 meanders embedded in an isolating mass.
The oscillating circuit parameters are included after every constructive change in
the form of a calibrating measurement in neutral to determine the dependence on the
output power. During the subsequent heating the computer accesses this file.
The method of measurement of the working energy was implemented by
Elotherm and integrates the real energy entry with consideration of the oscillating
circuit loss resistance RV0. The energy entry is determined in discrete timing steps
with a reference specification, the so-called energy set point curve that has been
adapted previously to the material and its geometry.
Figure 9.8 Single inductive reheating unit of Elotherm. The
horizontal layout of the billet tray allows wide variations of the
liquid fraction.