Thixoforming : Semi-solid Metal Processing

322j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.10 Detail of the transition in the hold step: Temperature
compensation in the 100Cr6 billet by heat conduction. The
overheating of the outer shells (T1, T4) delivers energy into the
bolt inside (T2, T3). The liquid state contents adjust strongly since
the difference in temperature is 9 K at the end of heating.
In an improved solution, complete encapsulation of the billet in a tight-fitting
high-temperature isolation unit was applied. In addition, the reheated billet should
not be removed from the encapsulation shell. This was achieved by keeping the semisolid
billet in the heating and transportation container and driving it out by the shot
piston itself [shot-through cartridge (STC)].
To be able to use the same moulds as built for Al alloys, the billet diameter was
increased to 76 mm, which made possible a maximum shot weight of 6.05 kg at a
length of 170 mm.
For isolation, vacuum-formed parts have been produced consisting of 48% Al2O3
and 52% SiO 2, which provide a sufficient supporting effect for the more than 5 kg
weight of the billet, a good isolation behaviour and a short time resistance for
temperatures over 1490 C (Figure 9.12). At the density of 320 kg m 3 used here, the
piston needs 115 N to compress the material from the diameter 76 mm to the
shooting piston diameter of 85 mm radially.
Figure 9.11 Schematic drawing of the billet tray including cover
and billet (a) and manufactured ceramic insulation inserts in the
initial state (b).