Thixoforming : Semi-solid Metal Processing

208j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.43 Simulated velocity profile in a cross-section of the vertical duct.
6.2.5
Summary
The one-phase computational model based on the Herschel–Bulkley approach takes
into account yield stress, thixotropy and shear-thinning behaviour, and can be used in
some cases to simulate the flow front of the die filling experiments, but the
parameters have to be adjusted for each set of process conditions. Since this model
makes the approximations that material is homogeneous, the results cannot always
be physically meaningful. The mathematical model based on the two-phase
approach, in contrast, takes into account all time-dependent phenomena occurring
during casting but, most importantly, it accounts for non-isothermal phase change in
addition to phase segregation, which is one of the important issues and often the
main reason for an unsuccessful casting.
6.3
Simulation of Cooling Channel
Simulation of feedstock formation for thixoforming has attracted little interest
compared with the simulation of thixotropic-behaviour, and thixoprocessing of
semi-solid metals [2]. The aim of this work was to study the effect of the different
process parameters on the quality of the slugs obtained from the cooling channel [37].
Grain density and size, solid fraction and macro-solute distributions were used as
quality measures.
Simulation was carried out with the CFD software FLUENT 6.3 combined with the
multiphase flow and solidification simulation code MeSES, which was first developed
at the Foundry Institute at Aachen University within the scope of SFB370. Application
of the MeSES code requires many thermophysical parameters which were
obtained from the literature, while others that were not available were determined
experimentally.