Thixoforming : Semi-solid Metal Processing

9.4.1.2 Parameters: Cooling to the Process Temperature
The second important process step of the cooling channel process is the decelerated
cooling to the process temperature. Regarding sufficiently slow cooling, the mould
material ceramic was chosen for homogeneous cooling of the precursor billets. The
mould was designed in such a way as to ensure easy handling and transfer to the shot
sleeve of the HPDC machine. To evaluate the influence of the mould temperature on
the resulting microstructure, the mould temperature was varied between 20 and
400 C. Heating to 400 C resulted in doubling of the cooling time from about 500 to
1000 s. Furthermore, the increased temperature unfortunately led to a coarser
microstructure. It was found that the mould should be cooled to below 80 C before
pouring to prevent disadvantageous grain coarsening.
In contrast to the decelerated cooling, the cycle time should be as short as possible
for useful process times. As a result, it has to be evaluated how fast cooling is possible
to achieve nevertheless a globular microstructure. Sufficiently spherical and fine
globules (diameters less than 100 mm) are necessary in order to fill homogeneously
thin die cavities during casting [42]. Otherwise, if dendrites are formed, segregation
in the wake of the sponge effect occurs, which results in inhomogeneous material
properties [43].
Concerning this issue, simulations using the software package MICRESS [44]
coupled with ThermoCalc were carried out [55]. To perform the simulations, some
starting parameters had to be determined: the real cooling rate, the seed density and
the alloy composition. Using the ceramic mould without additional cooling, the
solidification lasts 6.5–8 min, depending on the aimed for solid fraction. To determine
the parameters of the cooling channel process for the simulations, five
experiments were carried out. The casting temperature was always 630 C. From
the measured cooling curves, an average cooling rate of 0.07 K s 1 was determined,
and considering the latent heat a heat flux of 0.94 J s 1 was approximated with the
ThermoCalc software.
Due to the effect of alloying elements on the microstructure evolution, the exact
alloy composition was needed. Hence the precursor material was analysed at
60 positions, using a spectrometer (Table 9.6). All alloying elements with concentrations
less than 0.1 wt% were excluded from the simulations. The thermodynamic
database TTAL5 from ThermoTech was used. The diffusion coefficients for the solid
and liquid phases were taken from [45] (Table 9.7) and modelled by means of an
Arrhenius plot. Cross-diffusion effects were neglected during the simulations. The
initial temperature was chosen as 615 C (a cooling of 15 C during channel contact
was assumed, based on previous investigations) and the release of latent heat was
Table 9.6 Results of the spectroscopic analysis of the precursor material (wt%).
Si Fe Cu Mn Mg Ti Ag Sr V Ga
9.4 Rheoroutej349
Average 7.11 0.0855 0.0023 0.0017 0.340 0.1041 0.0013 0.0294 0.0119 0.0147
SD 0.28 0.0106 0.0004 0.0003 0.024 0.1177 0.0001 0.0020 0.0002 0.0007