Thixoforming : Semi-solid Metal Processing

178j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
The determination of time dependency in capillary flows requires the use of several
pressure transducers along the capillary length.
Nevertheless, the experimental concept is advantageous as it is closely related to
the thixocasting process. If similar flow conditions occur in real semi-solid processing
due to comparable geometric boundary conditions, the results extracted from
these experiments are relevant in terms of viscosity, morphology of the flow front and
phase separation under laminar flow conditions [24].
Two different types of capillary viscometers have been designed in order to study
the rheological behaviour of both Sn–15%Pb alloy and aluminium alloys in the semisolid
state under processing conditions similar to those in real semi-solid metal
forming. In capillary rheometry, the partially liquefied alloy is extruded through a slot
capillary and the pressure difference and the corresponding flow rate are measured in
order to evaluate the viscosity and the shear rate. The capillary must be kept under
isothermal conditions to realize appropriate experimental conditions.
6.1.5.3 Vertical Capillary Rheometer
An experimental setup has been designed using a Buhler shot-controlled highpressure
die-casting machine (H-630 SC) with a vertical slot-capillary built into a
modular tool, Figure 6.7. Two piezoelectric pressure sensors have been installed
along the capillary length. These type f sensors and also an additional inductive flow
front sensor are in common use in high-pressure die-casting technology. Regarding
the sensitivity of the pressure sensor, the measured flow pressures vary in the region
of 1 10 5 Pa while the cavity pressure sensors applied exhibit a maximum of about
2 10 8 Pa with an error in linearity of 2%. The vertical capillary viscometer can be
heated with conventional heat transfer oil to a maximum temperature of 250 C,
which is necessary to ensure isothermal conditions for the experiments with the
Sn–15%Pb alloy. Analysing the signal of the two piezoelectric sensors or the signal of
Figure 6.7 Illustration of the experimental setup. The viscometer
is integrated in a modular high-pressure die-casting tool using
a vertical design of the slot capillary.