Thixoforming : Semi-solid Metal Processing

188j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
magnitude of the dynamic yield stress. This enables us to predict the isostructural yield
stress which appears at t ¼ 0. If the curves are extrapolated to zero, the isostructural
yield stress also tends to zero, independent of the solid fraction. As a consequence, no
stable particle network exists directly after stirring and therefore semi-solid alloys do
not exhibit an isostructural yield stress.
Oscillation Experiments Oscillation experiments are applied in suspension rheology
to gain information about structural changes over a wide range of shear rates. It is
known that these changes depend on particle size. In the case of a metallic
suspension, the occurrence of Ostwald ripening (see above) is used to adjust different
particle diameters at a constant solid fraction. The material preparation shown in
Figure 6.4 is stopped after a certain time and oscillation is conducted for about a
30 min. After this, shearing is continued until another oscillation period at a different
particle diameter took place. Oscillation was performed within the linear viscoelastic
regime with an angular velocity of 25 s 1 and an amplitude of 8 mrad for the
sinusoidal shear rate. Quenching experiments at the beginning and end of the
oscillation period showed that particle growth during this time is negligible. The loss
modulus G 00 and the storage modulus G 0 were monitored and the loss angle was
calculated from the phase shift between signal and stimulation.
The development of the storage modulus together with the loss angle is presented
in Figure 6.23. The particle diameter of the metallic suspension was adjusted up to
550 mm. The storage modulus increases with time during the oscillation period. The
loss modulus behaves in the same manner. At the beginning, the slope of the curve is
very high. This is an indicator of rapid structural changes of the material. With time
the slope decreases and approaches a steady-state value.
The loss angle is the ratio between storage and loss modulus. For pure viscous
materials it is 90 and for pure elastic materials it is 0 . Figure 6.24 shows that the
material properties change from a more viscous-like to a more elastic-like flow
Figure 6.23 Influence of particle diameter on the storage modulus
G 0 and the loss angle d. Material: Sn–15%Pb.