Thixoforming : Semi-solid Metal Processing

170j 6 Modelling the Flow Behaviour of Semi-solid Metal Alloys
Figure 6.1 Viscosity as a function of solid content and particle shape.
Figure 6.2 Shear-thinning flow behaviour of a suspension.
X210CrW12 complemented by investigations on the aluminium alloy A356. The lowmelting
tin–lead alloy is just used for supplementary analysis in terms of yield stress
of metallic suspensions and structural evolution during rest.
6.1.1
Structural Phenomena Influencing the Flow Behaviour
The complex flow behaviour of suspensions is directly related to their internal
structure. Figure 6.3 exemplarily shows for different states of the solid phase how
structural changes depend on shear rate and result in changes in viscosity. With
increasing shear rate, the particle size may change or particle agglomerates break up.
Emulsions (left field in the boxes in Figure 6.3) even allow for a change in particle
shape with increasing shear rate. Depending on the shear rate, the structural changes
are caused either by structure forces or by hydrodynamic/viscous forces.
In metallic suspensions, the same phenomena can be observed. By decreasing
shear rate agglomeration occurs because particles collide and bonds between them
are formed. The strength of the bonds increases with time. A further lowering of the
shear rate intensifies this process whereas an increase breaks the bonding and the
average agglomeration size diminishes. The inner network of a semi-solid alloy billet