Thixoforming : Semi-solid Metal Processing

34j 2 Metallurgical Aspects of SSM Processing
liquid phase) and a high slope of the liquidus line, ml. According to Easton and
StJohn [4], a high product ml(k 1) reflects on the one hand a high grain growth
restriction, which can be considered to be beneficial, but on the other a high
constitutional undercooling DTC. A high DTC, however, results in increased proneness
to dendritic solidification, which is a disadvantage for SSM processing.
2.3
Slurry Formation in the Rheo- and Thixo-routes
As already mentioned, two major categories of SSM slurry-making processes exist.
The first group comprises all processes in which slurries are produced directly from
the liquid phase. These routes are termed the rheo-route or rheocasting, or slurry-ondemand
(SoD) technology. The second category of SSM processing – the thixo-route
– deals with solid feedstock that was agitated during solidification and exhibits a
rosette-like structure. It is then reheated into the freezing range in order to form a
non-dendritic, globular structure of the primary phase.
From the microstructural point of view, there is a significant difference between
the thixo- and rheo-materials. Spheroidization of the thixo-material occurs during
reheating and final holding at the casting temperature. During this process, liquid
pockets are entrapped in the primary globules and cannot contribute as a lubricant
to the final forming step. In SoD processes, this entrapment never occurs during
solidification of the primary aluminium phase, and therefore the full amount of
liquid phase can contribute to the forming operation (Figure 2.2).
When talking about the globular primary phase , it implies that also a secondary
phase exists. For most light metal casting alloys this is in fact the case; the secondary
phase is eutectic, as for A356, A319 and AZ91. But complex steel grades also exhibit
pronounced eutectic solidification; see Figure 2.1, label e . During reheating, the
eutectic transforms to liquid and no anomalous remelting occurs. In contrast to this
Figure 2.2 Microstructure of the alloy A356; thixo-route (a)
and rheo-route (b). Entrapped liquid pockets are visible in the
thixo-sample.