Thixoforming : Semi-solid Metal Processing

324j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.13 B€uhler H-630 SCreal-time controlled HPDC machine,
heating/cooling and vacuum units.
simulation software MAGMAsoft. The pictures in Figure 9.14 show the heat
distribution inside the heavy loaded shot chamber and the die during the solidification
of the suspension 15 s after the mould filling for the first and the tenth cycles of a
serial production (cycle time 500 s). The surface near regions of the die tools, and
also the shot chamber, reach temperatures above 700 C, causing a dramatic loss of
hardness. Therefore, the solidification time should be reduced to a minimum to
avoid massive heating through the die tools and thereby a dramatic loss of surface
hardness.
In consideration of the numerical results, the new shot chamber is constructed as a
multipart system with separate pressure and charging sections connected by a
clamping ring (Figure 9.15). Multiple electric heating cartridges, giving a maximum
temperature of about 600 C, individually temper both parts. Due to the clamping and
the selective heating, both radial and axial thermal stresses that occur can be
effectively reduced. The device allows a fast and easy tool change in case of any
malfunction. Moreover, the chamber can be switched from thixocasting to fully liquid
casting within a few working steps.
In order to protect the chamber from massive abrasive wear, a special coating is
applied before every casting operation.
Figure 9.14 Simulated heat expansion (heat flow
1000 W m 2 K 1 ) through the shot chamber and the die 15 s after
the mould filling for up to nine consecutive cycles at a cycle time of
about 500 s.