Thixoforming : Semi-solid Metal Processing

354j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.39 At different cooling rates: (a) microstructures;
(b) frequency distribution of the shape factor normalized on
100%. The distribution flattens to higher heat fluxes. The profiles
at 0.066 and 0.147 K s 1 are very similar to each other, in
contrast to the water-cooled sample (0.452 K s 1 ). This indicates
the probable castability: sample 1, very good; sample 2, good;
sample 3, bad.
graph. Comparing Figure 9.39, the water-cooled sample forms a dendritic microstructure
containing globules.still
The comparatively large change in the shape factor from 0.78 at a cooling rate of
0.147 K s 1 to 0.69 at 0.166 K s 1 leads to the supposition that cooling should not be
faster than 0.150 K s 1 . In that case, the process time will be reduced to about 180 s.
This matches the simulation results fairly well.
The difficulty in appraising an exact borderline is the two-dimensional view. The
more significant 3D structure is in contrast not observable. To verify the process
window found, forming experiments regarding segregation and mechanical properties
have to be performed. In addition, the effect of the cooling on the mechanical
properties, regarding the finer microstructure, should be demonstrated.