Thixoforming : Semi-solid Metal Processing

Figure 6.8 Experimental results exhibit a quasi-stationary flow in
pressure–time curve although a downward drift is observable
for the first sensor located near the ingate. (mean velocity of the
slurry u ¼ 6ms 1 ).
6.1 Empirical Analysis of the Flow Behaviourj179
one pressure sensor and the effective plunger pressure, a quasi-stationary flow is
assumed, although a downward drift is observable in the pressure–time curve for the
first sensor located close to the ingate system (Figure 6.8).
6.1.5.4 Horizontal Capillary Rheometer
Since the operating temperature of the vertical capillary viscometer is limited to a
maximum temperature of 250 C, the experimental setup is not appropriate for
rheological measurements of semi-solid alloys with an excessive solidus–liquidus
interval. A horizontal capillary viscometer has been designed particularly for rheological
measurements of higher melting alloys such as aluminium and magnesium.
The application of electric heating cartridges ensures significantly higher operating
temperatures of up to 650 C while special attention is paid to the heat balance of the
tool assembly. The modular design of the viscometer in combination with the design,
position and power dimensioning of the electric heating system ensures a continuous
flow of the material under investigation through the capillary without solidification
occurring during the experiment, and also short setup times through a simple
assembly. Compared with the vertical viscometer tendencies of time-dependent
viscosity behaviour can be detected with the installation of several sensors over the
flow length (Figure 6.9).
Based on the design experience and the thermal requirements, especially the
design of the measurement system had to be changed. Quartz sensors, directly
mounted on the capillary surface, are in common use in pressure die casting. For
sensitivity reasons, these sensors are replaced with highly sensitive force sensors.
Because the operating temperature is limited to a maximum of 250 C, the sensors
are mounted in a water-cooled rack on top of the tool assembly. They are connected
to the capillary surface with pins of a specific diameter for load transmission
(Figure 6.10). In order to detect time dependency in the rheological behaviour, the
capillary is equipped with four pressure sensors in an equidistant arrangement over
the flow length. If time dependency occurs, the sensors will detect a non-linear