Thixoforming : Semi-solid Metal Processing

Figure 9.36 Results of the 2D-MICRESS simulation.
9.4 Rheoroutej351
fine microstructure (F ¼ 1.21, D ¼ 97 mm) was obtained (Figure 9.35). With a
decrease in the grain density and an increase in the cooling rate, the microstructure
changes from globular to dendritic. Increasing the cooling rate with the grain density
from the experiments, the morphology changes to a more dendritic form, but the
postulated criteria were nevertheless maintained. Comparing Figure 9.37, in reality a
higher coagulation of the grains is expected and hence the castability diminishes. The
reason is that the simulation does not take fluid flow and thermal convection into
account, but coagulation is forced by grain impact. Apart from the observed
coagulation, minimization of the cooling time to less than 3 min seems to be possible.
The increase in the grain density has a particularly positive effect on the microstructure,
because very fine and spherical globules were formed. Moreover, they
remain relatively round at high heat fluxes (Q ¼ 10 J s 1 , F ¼ 1.25). At low heat fluxes
and high grain densities, coagulation appears; hence the real grain diameters and the