Thixoforming : Semi-solid Metal Processing

186j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.20 Comparison between the flow behaviour of steel alloy
X210CrW12 and aluminium alloy A356 for the same solid fraction.
Consequently, it is assumed that the solid particles in the steel alloy are smaller than
those in aluminium, which in the present case – according to quenching tests – are
around 260 mm. Due to device-related limits, quenching experiments for the steel
alloy to prove this assumption are not possible so far.
Yield Stress of Metallic Suspensions As mentioned above, the yield stress is a
property extensively discussed in the literature. Creep experiments using a tin–lead
alloy performed with the UDS200 Couette rheometer from Anton Paar Germany
allowed a closer look at cases in which yield stress can be expected. Due to the
presence of the additional bearing of the rod in the aluminium and steel rheometer,
these investigations with very small stresses and deformations could not be conducted
for the high-melting alloys so far. Anyhow, the results are expected to be
qualitatively the same as those obtained for tin–lead, because in principle the
rheological behaviour of metallic suspensions is independent of the alloy under
investigation.
After material preparation, the yield stress is usually measured using shear stress
ramps. During the ramp time, the thixotropic properties of metallic suspensions lead
to a change in microstructure. Hence the measured yield stress depends on the slope
of the ramp and also on the resting time immediately after material preparation, as
illustrated schematically in Figure 6.21a. The change of the internal structure can be
measured with oscillation experiments. It can be observed that the evolution of the
storage modulus (indicating the increase in internal strength) versus resting time is
similar to the slope of the yield stress measured with stress ramps (Figure 6.21b). The
build-up of this so-called dynamic yield stress is based on the formation of a particle
skeleton in the slurry. This hardening process approaches the static yield stress when
the skeleton forms a stable network. Further details on oscillation experiments are
given in the next section.