Thixoforming : Semi-solid Metal Processing

342j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
bulged part of the blade. This location already stood out as a sustained hotspot in the
simulation and a simple local reduction of the blade strength could overcome this.
The metallographic examination shows only a small trend for segregation of liquid
material in the tip. The hardness increase at the blade tip is related to the increased
solidification rate. The steel X46Cr13 was also examined and achieved full filling only
at liquid states of more than 50% because of its mushy solidification morphology. The
process temperature is, at 1485 C, again 200 K higher than for X210 and led to much
more presolidification for the same tool temperature. The parts produced are only
about 95% filled and show strongly segregated areas consisting exclusively of forward
slipped liquid phase. For these low-carbon steels the self-heated ceramic tool could be
a working solution.
9.4
Rheoroute
To overcome the disadvantage of the expensive reheating step in thixocasting process
routes, the development of rheo-processes in recent years was aimed at producing
semi-solid slurries directly from the melt. In manufacturing rheo-feedstock material,
the dominant factors to achieve a fine and globular microstructure are the grain
density and the cooling rate. Slow cooling and a high grain density result in a globular
microstructure. This is caused by the longer time for diffusion of alloying elements in
the melt, whereas the risk of treeing as a consequence of constitutional supercooling
rises with an increase in cooling rate. In addition, a high grain density leads to a fine
and spheroidal microstructure, not least because of the smaller space between the
grains, whereby the formation of wide dendrite arms is limited. Another influence is
the increasing latent heat per unit time due to the larger solid–liquid interface, which
is caused by the higher grain density. Hence the supercooling decreases and the
formation of dendrites is inhibited.
Constant Temperature Process (CTP) The development of the so-called Halteverfahren
(Figure 9.33) to manufacture a globular microstructure directly from the melt
is based on technical expertise that were acquired during the first period of the
collaborative research centre SFB289. In this process, liquid aluminium (here A356) is
cooled to the semi-solid state under controlled conditions. For this purpose, the
slightly superheated melt is poured into a steel mould, located in an electric furnace to
keep the melt temperature slightly above the solidus temperature. Due to the slow
cooling, a globular microstructure will be obtained. At the required solid fraction of the
semi-solid slurry, the billet is pushed out of the mould using an ejector (Figure 9.33) to
assure continuous HPDC processing.
With respect to the process stability and the quality of the manufactured semi-solid
slurry, the results could not meet expectations (the reasons for this are described
later). In contrast, the developed cooling channel process provides much better semisolid
precursor material, hence the cooling channel process was mainly focused on in
the following.