Newsletter - Aachener Verfahrenstechnik - RWTH Aachen University

parts etc. If you observe a cross section of a thixo
formed part (or the starting material used for
thixo casting process) through a microscope, the
structure of the material may look like the picture
below to the right. If you look at a material of the
same chemical composition, but for a traditional
pressure casting process it may look like the picture
to the left (Fig.11.
Fig.11: Samples of the aluminum alloy A356.
Left: quenched from liquid, middle:
sheared in the semi solid state for 5 minutes,
right: sheared in the semi solid
state for 3 hours.
The thixo forming material goes through a process
to obtain this desired structure, which results
in completely different flow properties and
hence product properties. As rheologists we are
investigating the rheological (flow) properties of
the material using a rheometer with an integrated
oven shown in Fig.12.
The material’s flow properties are changing due
to the Ostwald ripening effect. We are working
in the constitution of a model to describe this
effect in the thixo forming material. The rate of
the Ostwald ripening is influenced by temperature,
amount and duration of shear and other
chemical and physical parameters of the alloy.
We are investigating these influences: The rheological
properties are being measured, then the
samples are quenched (shock frozen) to solid
and investigated under a microscope. Through
the rheological and optical investigations, we are
coupling the parameter influence on the ripening
process as well as the influence of the ripening
process on the flow properties of the material.
The results will be used to verify the model as
the constitution of this proceeds.
Fig.12: Measurement set-up for rheological
preparation and measurements. Left:
Rheometer with integrated oven, middle:
rheometer cup and rotational body,
left: aluminum sample after measurement.
The AVT Newsletter Mechanical Process Engineering 14