Thixoforming : Semi-solid Metal Processing

decreasing process temperatures with increasing carbon concentration, high-carbon
steels are advantageous pertaining to the tool strain. In addition to the location of the
solidification interval, the size or rather the development of the solids fraction with
temperature is of importance. To allow easy adjustment of the liquid fraction, the
increase of the cooling curve should be as low as possible. A prerequisite for a
successful semi-solid forming is a particular microstructural formation in the semiliquid
state, so that the material is available as a solid–liquid suspension with a low
viscosity during the forming.
3.2.1.1 Rheological Behaviour of Thixotropic Metal Suspensions
In contrast to Newtonian fluids, for instance water, which shows an increase in viscosity
with increase in shear stress, metal alloys in the temperature interval between the
solidus and the liquidus temperature exhibit thixotropic flow behaviour, when solid
particles exist in a globular form within a liquid matrix and are able to move freely
against each other(Figure 3.1). Partial liquid metals in the unstressed state can be
treated similarly to solids, because of the developing coalescence of the solids particles.
In the primary examination concerning the behaviour of metallic suspensions, an
increase in shear stress and a decrease in cooling rate during the process lead to denser
and rounder primary particles. The mechanisms that occur in globulitic metal
suspensions are not entirely resolved, but according to the theory developed from
experiments agglomerations of particles leading to a higher viscosity accumulate in the
quiescent state due to interfacial tensions [2, 3]. The shape of the solid globules is a
crucial parameter for the description of the microstructure in the partial liquid state.
The morphology of the solid particles alters, for example, during stirring with a change
in the stir level (Figure 3.2). Whereas the material in the unstressed state is characterized
by cohesive grain clusters, the particles developed with increasing shearing rate,
increasing shearing time and decreasing cooling rate are no longer dendritic but
rosette-shaped and ultimately globular. Partial liquid bodies can be handled like solids
due to the developed solid network, provided that the shearing forces are too small to
Figure 3.1 Depiction of the different mechanisms to explain the
flowability of partial liquid metallic suspensions [5].
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