Árbók VFÍ/TFÍ - 01.06.2000, Blaðsíða 329

Ritrýndar vísindagreinar 327
tlirce material coefficients involved in the thermal analysis. They are thermal conductivity k,
density p and specific heat cp. For the stress analysis there are also another three parameters
necessary, Young's Modulus E, Poisson's ration v, and the linear coefficient of thermal
expansion, a.
For FeSi only properties necessary for thermal analysis are defined, because the stress in
the solidified FeSi is not of interest. FeSi property values are based on data from the research
lab at IA.
Values of boundary parameters are determined by values listed in [3] and by comparing
simulation temperature with mould temperature measured by Balzer [2]. Simulation mould
temperature is compared with temperature graphs printed in the reference.
Process Simulation
When using the 1D modcl all the process parameters are considcred but only one geometrical
parameter, namely zp mould bottom thickness. Thc thickness of thc IA mould bottom is 150
mm and each FeSi layer cast is considered to be 40 mm in thickness. A time step of At = 0.1
s is used.
The casting process (process 1) is simulated for one casting cycle and dcfíned by the
values shown in table 2.
l\ l2 l3 l4 l5 l6
80__________200___________200____________140___________60___________240__________________
Table 2: Sub-cycle duration values [sj (see table 1) for one layered casting, process 1.
This process represents a short casting cycle. The duration of the FeSi water cooling sub-
cycle is only t2 = 200 seconds or just over 3 minutes. Simulation results are shown in fig-
ures 4, 5 and 6.
Figure 4 shows the temperature during process 1 at the mould hot side (surface), mould
core (middle) and mould cold sidc (back). The broken vertical lines represent the start of
each sub-cycle, i.e. sub-cycle 1 starts at 0 seconds and sub-cycle 2 starts at the first broken
vertical line (80 seconds) etc.
As expected the surface is most affected by the casting process while the effect of the
process is not as vigorous in the mould core or at the back. The surface temperature
increases rapidly during casting (sub-cycle I) and continues to rise during water cooling
(sub-cycle 2) until all the FeSi melt has solidified. As a consequence, the surface tempera-
ture changes abruptly as depicted near the end of sub-cycle 2 and starts decreasing due to
the fact that thc heat transfer into the niould is less than the heat absorbcd by the core and
cold side. This demonstrates that energy equilibrium is bcing reached in the mould. This
trend continues during FeSi air cooling and mould hot sidc air cooling (sub-cycles 3 and 4
respectively) until water-based coating is applied to the mould (sub-cycle 5). Ininiediately,
the surface temperature drops violently and gradually the temperature increase at the back
side of thc mould comes to a halt.
After the water has evaporated (sub-cycle 6) the surface starts heating up while the back
side cools down due to the fact that the back sidc is now at a greater temperature. This trcnd
continues until the mould is virtually at a homogeneous temperature.