Thixoforming : Semi-solid Metal Processing

338j 9 Rheocasting of Aluminium Alloys and Thixocasting of Steels
Figure 9.28 (a) Tool insert of the moveable die-half: shown are
the knife cavity, the oxide wiper, the gating and the boreholes of the
ejectors. The knife blade is 200 mm long and 3 mm thick.
(b) Temperature distribution of the cast metal (X210CrW12;
T c ¼ 1290 C; v piston ¼ 0.3 m s 1 ) at 100% filled form. (c) X-ray
analysis confirms the good quality of the cast parts.
of the knife, laminar filling takes place with Vmetal 10 m s 1 . The metal speed
increases with reduction in the cross-section to the knife shaft. The filling of the shaft
area is favoured by the further diminution of the crosscut to the knife blade and the
ram pressure effect resulting from it. The 3 mm thick blade then becomes filled at
Vmetal 20 m s 1 durchstr€omt. The analysis of the temperature development of
the semi-solid slurry during the form filling shows the formation of a channel flow
in the area of the knife handle. The described ram pressure effect at the transition
from the shaft to the thin blade leads to a strong reduction in the average metal speed
in the knife handle, which leads to the formation of a solid shell in connection with
the high undercooling at the form surface. Through this, a reduction in the effective
flow crosscut to a middle channel arises.
The step-shot experiments carried out for the validation of the simulation correlate
with the numerical calculations. In the course of the experiments the piston speed
was increased for the optimization of the surface quality of the components at
vpiston ¼ 0.4 m s 1 . The geometry showed high precision and the contour acuity filled
completely and is almost free of burrs. The surface quality is influenced strongly by
ceramic mould release agents sprayed on before the shot (Centricoat).
A first check of the component quality was carried out with X-ray analysis. The
overview representation of the thixocast structure of the ingate area shows a
distinctive hem of increased liquid phase which correlates with the shell formation
observed in the results of simulation and the step-shot experiments and the channel
flow resulting from it.
There is occasional formation of micro porosity at the top of the knife blade due to
the high metal speed and lack of feeding. Furthermore, ceramic particles washed into