Thixoforming : Semi-solid Metal Processing

Figure 9.33 So-called Constant Temperature Process shown schematically.
9.4.1
Cooling Channel Process
9.4 Rheoroutej343
The cooling channel process (Figure 9.34) was developed to deliver a fine and globular
microstructure for batch processing with an HPDC machine. The significant
constituent parts are the cooling channel and the ceramic mould. Accordant to this
configuration (Figure 9.34), the cooling channel process is separable into two process
steps with essential effects on the resulting microstructure of the precursor billets:
the pouring of the melt over the inclined steel channel and the decelerated cooling of
the liquid to the casting temperature.
The cooling channel process will be considered by means of the conventional
alloy A356 (AlSi7Mg0.3). A strontium-refined and titanium grain-refined alloy,
Anticorodal-70 dv from Rheinfelden, was used.
The process step pouring includes the melt flow of the liquid metal over the
inclined, 480 mm long and oil-tempered steel channel. The effect of this first process
step on the microstructure becomes apparent by pouring into the mould with and
without the channel. If the melt is poured directly into the ceramic mould with
otherwise the same process parameters, a dendritic microstructure will be obtained.
Using the channel, the microstructure forms globularly (Figure 9.35). In this
respect, the mechanisms concerning the channel have an essential effect on the
microstructure.
While flowing over the channel, the slightly superheated melt cools below the
liquidus temperature (T L ¼ 615 C), forming seed crystals, and in the ceramic mould.
As a result of the heat extraction, a thin, solidified metal layer remains on the channel.
Additional seed crystals are formed during first contact of the melt with the cold