Thixoforming : Semi-solid Metal Processing

184j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.16 Equilibrium viscosity curves for different solid fractions. Material: X210CrW12.
become apparent. The agglomeration state is assumed to remain constant momentarily
if the shear rate is suddenly changed from a basis shear rate to a higher or lower
one. As a result, Newtonian flow behaviour is anticipated, but, as already observed for
tin–lead [12, 27] and aluminium alloys [13, 22]. So far, this phenomenon cannot be
explained. In the model approach (Equation 6.3) the flow exponent m describes the
isostructural behaviour when k remains constant.
Figure 6.16 shows the steady-state flow curves generated from Figure 6.14.
Figure 6.17 more clearly displays the flow curve for the liquid material ( fs ¼ 0%),
which shows as expected Newtonian properties with a viscosity of 50 mPa s. The
high fluctuations in the liquid measurements result from the bearing of the rod as the
momentum of the bearing is in the range of that from the liquid metal.
The shear rate jump experiments are used to calculate initial model parameters
from Equation 6.3, which in turn can be utilized for the more exact inverse parameter
determination described in Section 6.2.4.2. Figure 6.18 exemplarily shows a comparison
between experimental data and the modelling approach for a solid fraction of
33%. The model is able to predict the flow behaviour in a reasonable range. The main
disagreement is observed for the jump downwards immediately after material
preparation. This phenomenon can be discerned also for aluminium or tin–lead
alloys but cannot be explained at the moment.
Figure 6.17 Flow curve for liquid X210CrW12.