Thixoforming : Semi-solid Metal Processing

212j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.46 Simulation results in the container after 553 s:
(a) temperature, K, (b) solid fraction, (c) grain density, m 3 ,
(d) grain size, mm, (e) solute concentration, wt%, (f) liquid
velocity, m s 1 , after 93 s.
sudden rise at about 0.6 s as a result of the high supercooling that occurred when the
hot liquid metal reached CP1 and CP2. It fluctuates afterwards as a result of latent
heat release, flow drift and grain transport. The slight decrease in cooling rate at CP1
as per Figure 6.47a results in gradual reduction in grain production rate and n
(Figure 6.47b).
The values of T, f S and n were tracked in the container and the results (Figure 6.48)
manifested the role of the container in this process. The cooling curves in
Figure 6.48a show a relatively higher temperature at the container top compared
with the bottom and the difference diminishes as time proceeds. These results agree
with the experiments of B€unck [47]. The grain size (Figure 6.48b) exhibits a
continuous increase due to temperature losses and grain growth. The grain density
reaches a steady state value of 4.39 10 10 m 3 after complete filling of container at
about 5 s, where the cooling rate is slower and the expected undercooling is much
lower. If the grain density in the container is compared with the outgoing one from
the channel (Figure 6.47b), the former was found to have 1.0 10 10 m 3 more grains.
This means that further nucleation took place during container filling, where its
initial temperature was 60 C.