Thixoforming : Semi-solid Metal Processing

load on the temperature-sensitive measurement module is minimized. The whole
spindle of length 550 mm needs to be pivoted to prevent an eccentric or vibrating
rotation. Additionally, the rod is centred in the fixed cup as the tip of the rod
(Figure 6.6) rotates in a small depression in the bottom of the cup. The momentum of
the bearing is predetermined and subtracted from each measurement. Oxidation of
the sample surface is avoided by flushing the void above with preheated argon. The
measuring system was calibrated using silicon oil of known viscosity.
During the so-called material preparation (see also Section 6.1.1), the semi-solid
material in the rheometer was generated by first completely liquefying solid material
followed by cooling to the desired temperature in the mushy state at a fixed cooling
rate and a shear rate of 100 s 1 . After isothermal shearing for 1 h, the experiments
described in Section 6.1.5.1 were carried out.
6.1.5.2 Capillary Rheometer
In capillary rheometry, the fluid is extruded through a slot capillary of a circular or
rectangular cross-section at a constant flow rate. When the flow reaches steady-state
conditions, the pressure drop Dp along the capillary length L is measured experimentally.
Hence the true shear stress at the capillary wall Dw can be determined by the
following equation:
tw ¼ HDp
ð6:11Þ
2L
In terms of a slit capillary of rectangular cross-section with a flow rate Q ¼ uHW,
where u is the average velocity of the slurry, H is the height and W is the width of the
capillary, the apparent shear rate g_ ap can be determined by
g_ ap ¼ 6Q
WH2 The apparent viscosity hap of the slurry is defined as
h ap ¼ tw
_
g ap
6.1 Empirical Analysis of the Flow Behaviourj177
ð6:12Þ
ð6:13Þ
According to Equation 6.11, the viscosity is proportional to the measured pressure
drop Dp and inversely proportional to the steady-state flow rate Q. Capillary rheometry
is a single-point method, that is, it only provides one viscosity value and its
corresponding shear rate for each experiment. The results are extracted under highly
transient conditions since the measuring time for these experiments varies in the
range of seconds. Hence no steady-state values can be extracted, although the
information gathered from these experiments is probably close to isostructural data.
Compared with rheological studies which have been carried out using concentric
cylinder rheometers (see Section 6.1.4), the length of the experimental time in
capillary rheometers is orders of magnitudes smaller and the applied shear rates are
orders of magnitudes higher (Table 6.1). Furthermore, experiments in rotational
rheometers can be conducted under steady-state conditions and time-dependent
effects can be measured using discontinuous or linear variances of the shear rate [23].