Thixoforming : Semi-solid Metal Processing

196j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.34 Overview about the flow stress (steady values) and
liquid phase fraction depending on temperature with related
micrographs.
area emphasizes the significant dependence of the globular microstructural formation
(Figure 6.34).
6.2
Numerical Modelling of Flow Behaviour
6.2.1
Motivation
When filling the die cavities, the semi-solid alloy shows complex shear-thinning
thixotropic flow behaviour, influenced by the occurrence of yield stress and segregation
effects. Additionally, the flow is greatly affected by the change in the slurry
temperature causing changes in solid fraction distribution, and also solidification
effects in the contact of the material with the die wall (see Section 6.1.1). Since the
mechanical properties of the thixoformed components are strongly influenced by
the spatial distribution of the solid content, the mentioned phenomena directly
influence the quality of the final product, The result of the work of Noll et al. [31] is that
the degree of segregation is influenced by the geometry and the process conditions,
and therefore the die geometry and also the process conditions have to be adjusted in
such a way that segregation is minimized. So far, the development of new dies and
thixoforming processes has been done experimentally, by trial and error. This way of
designing new dies leads to high development costs, which set a demand for
improvement of the process by use of numerical modelling, in terms of parameter
determination and process optimization.