Thixoforming : Semi-solid Metal Processing

344j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.34 Cooling channel process; automated pouring over the channel.
ceramic mould, but obviously insufficient for globular grain growth (Figure 9.35).
Due to the flow, the seed crystals disperse homogeneously in the container. In the
wake of the flow over the cooling channel, the grain density increases and a fine and
globular microstructure will be obtained.
The second process step is the cooling of the semi-solid suspension to the process
temperature. For slow, decelerated cooling, a ceramic mould was chosen.
As mentioned previously, the dominant factors to achieve a fine and globular
microstructure are the grain density and the cooling rate. The effects of these factors
are considered in the following. Table 9.3 gives an overview of the modifiable
parameters in the cooling channel process.
9.4.1.1 Parameters: Seed Crystal Multiplication on the Cooling Channel
Using experimentally determined parameters, molten, slightly superheated metal
was poured over the inclined (5 ) and tempered (120 C) steel channel (Figure 9.34).
The tempering of the channel is caused by constant process conditions and the
minimization of the remaining metal upon the channel (metal loss).
With an automatic robot, high process stability is achievable, but also with manual
pouring high reproducibility is obtained. Because of the changing heat input caused
by a varying pouring rate or pouring temperature, the thickness of the remaining
solidified alloy on the channel also changes. Due to the isolation effect of a thicker or
thinner metal layer between the melt and the channel, the resulting temperature
remains approximately constant (Table 9.4).