Thixoforming : Semi-solid Metal Processing

210j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Table 6.2 Results of spectrometric analysis for aluminium alloy A356.
determined. With the help of image analysis, the grain density was deduced at each
thermocouple. The accumulative density function was fitted to the n–DTmax plot and
correspondingly the Gaussian curve was generated. The characteristic nucleation
parameters obtained are summarized in Table 6.3.
The nucleation parameters obtained, together with the experimental boundary
conditions, were used to investigate the models provided within MeSES code
numerically. An axisymmetric grid cell was constructed and the Eulerian multiphase
approach implemented in FLUENT was applied. The simulation results
exhibited good qualitative and quantitative agreement with the experimental results.
6.3.3
Process Simulation
Element
Si Fe Cu Mn Mg Zn Ni Cr Ti Ag Sr V Ga
wt% 7.19 0.148 0.014 0.010 0.358 0.012 0.007 0.005 0.082 0.001 0.035 0.009 0.008
Based on the previous simulation work of Wang et al. [46], a new 2D grid (Figure 6.44)
was built with a finer mesh. Two solid zones (channel and container walls) and two
fluid zones (channel and container domains) were employed. The thermophysical
properties provided in Ref. [45] were utilized together with the boundary conditions
from the experiments.
The process variables during experiments and simulation were the inlet temperature,
Ti, inletvelocityvi, channel wall temperature Tw(ch) and container wall
temperature Tw(cont). Forinstance,ifvi ¼ 0.15 m s 1 , Ti ¼ 630 C, Tw(ch) ¼ 120 C
and Tw(cont) ¼ 60 C, the calculated pouring time to fill the container was found to be
4.3 s and, after that pouring time, pouring at the inlet is turned off. The rest of
the metal over the channel is allowed to drain off for an additional 0.4 s to complete
the container filling. After these two stages, simulation is continued in the
container only up to 550 s. Figure 6.45 shows the resulting liquid fraction after
a3.7sflow time.
The profiles of temperature, solid fraction, grain density, grain diameter and solute
concentration in the container after a solidification time of 553 s are illustrated in
Figure 6.46 . The results reveal the great macro-homogeneity of all quantities inside
Table 6.3 Characteristic grain nucleation parameters for A356 aluminium alloy.
Liquidus temperature ( C) DT N ( C) DT s ( C) n max (m 3 )
614.6 7.889 0.346 2.17 · 10 11