Thixoforming : Semi-solid Metal Processing

9.4 Rheoroutej347
If rounding of the grains is noticeable, in contrast to reports [39, 40], an increase in
the grain density during channel contact is to be expected.
The remnant solidified metal from the cooling channel and the cooling slope was
investigated by microscopy. Images from the top and bottom of slope and channel
were compared. In contrast to the 45 -tilted cooling slope described in Ref. [41], no
globularization of the grains is noticeable during flow in the cooling channel process,
notwithstanding the double flow length. However, grain coarsening occurs. The
decisive factor for the spheroidization seems to be the inclination of the slope of 45
compared with only a 5 tilt of the channel. Hence it can be assumed that the main
effect on nucleation in the cooling channel process (only 5 inclination) is the cooling
of the melt below the liquidus temperature and not the shearing.
When melt flows into the mould, additional cooling of the melt takes place and
nucleation appears. To evaluate the effect of this nucleation on the microstructure,
samples were taken from precursor material, with and without using the cooling
channel, one cast into a ceramic mould and another cast into a steel sheet mould
(Figure 9.35).
Without the channel, on pouring directly into both moulds the results differ
distinctly. With the ceramic mould, a coarse and dendritic microstructure appears,
whereas the microstructure in the steel sheet mould is globular. From this it follows
that in the steel sheet mould an explicitly higher nucleation takes place than in the
ceramic mould. The reason for this is in all probability the higher heat conductivity of
the steel, which leads to stronger supercooling. In contrast, the heat conductivity of
the ceramic is so low that the surface area of the ceramic mould heats up rapidly
and nucleation is diminished. As a result of the small grain density, a dendritic
microstructure is formed.
On pouring over the channel, the grain density is additionally raised. In consequence,
the microstructures formed are fine and globular. Considering the average
globule density (always determined by image analyses from five samples), the use of
the channel changes the microstructures to higher values, from 65 mm 2 (without
channel, steel sheet mould) to 74/77 mm 2 (steel/ceramic) [note: the standard
deviation (SD) was always about 3 mm 2 ] with the channel. In this case, it seems
that it does not matter if it is cast into the ceramic mould or into the steel sheet mould.
This leads one to assume that nucleation only has an effect on the grain density to a
certain steady state, because the additional nucleation from the steel sheet mould
seems to have no effect on the microstructure. Additional nucleation will probably be
eliminated by the increased latent heat, at least with actual process and cooling
parameters. Another consequence is that an additional seed crystal enhancement due
to the shearing on the channel is of no use.
These and further experiments accomplished with the RCP (cf. Section 9.1.2) show
that a globular microstructure is also achievable without pouring over a channel.
Indeed the channel leads to a finer microstructure, but it does not differ concerning
the roundness of the globules. Hence a good workability is given.
A decisive role when using the steel sheet mould is played by the wall thickness of
the mould. In the Constant Temperature Process, a massive steel mould with about a
1 cm wall thickness was used. Thereby an amount of dendrites leads to a worsened