Thixoforming : Semi-solid Metal Processing

Due to the multitude of different experimental equipment and rheometer designs
for the determination of the rheological characteristics of partial liquid metallic
suspensions, these are not discussed here in detail and readers are rather referred to
the literature [4, 8–12]. A model with which the structural changes in the semi-liquid
state can be simulated is presented in [13].
3.2.1.2 The Sponge Effect
As already mentioned, in addition to the phase fraction, the morphology of the
phases is of crucial importance for the flow behaviour of partial liquid suspensions.
In this way, the particles are able to slide past each other or to move in addition to
that by means of translation and rotation movements. The microstructural development
during the reheating and the holding in the semi-liquid state are furthermore
characterized by an initial spheroidization and a subsequent diffusion controlled
microstructural coarsening. This can be described according to the Ostwald
ripening mechanism and the coalescence process [15]. Figure 3.3a shows an
increase in the carcass degree. The different material currents and diffusion
processes occurring during the coalition of two equally sized particles are illustrated
in Figure 3.3b.
Parallel to the spheroidization and the growth of the solid phase, the connectivity
increases, that is, the strength of the steric carcass increases, and this bestows solid
material character and a very high viscosity on the material. If this cannot be broken
up with a high enough shearing stress, no flowable suspension develops. A structural
viscous, thixotropic flow can only be observed if the operating shearing strengths
Figure 3.3 Growth of the carcass volume and the viscosity by
coagulation of the globulites during the isothermal halt or
temperature decreasing in the two-phase area (a) and diffusion
activity during coagulation (b) [16].
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