Thixoforming : Semi-solid Metal Processing

46j 3 Material Aspects of Steel Thixoforming
Figure 3.2 Structural changes during the solidification and shearing of metallic suspensions [14].
destroy them. The development of the network permits a relatively easy insertion of
partial liquid billet into the casting or forging tools. On applying a sufficient shear
stress, the solid framework breaks open and the previously formed agglomerations can
be dissolved. A fluid suspension of solid particles within a liquid matrix-phase
develops, resulting in a reduction in viscosity. The structure formation of partial liquid
casts under convection, or generally under external strain, is therefore the result of the
interplay of agglomeration and destruction of agglomerates through induced shear
forces. This interplay can be examined with different methods, where, depending on
the particular method, shearing rates of up to 1 s 1 with shear rate leaps of up to
1500 s 2 can be realized [6]. With low shearing rates, large agglomerates are generated,
which disintegrate with higher shearing rates and exist as isolated particles in the liquid
phase. The proportion of the intra-globular, immobile liquid phase rises with increasing
agglomerate size, so that in this case higher viscosities are measured due to the
lower liquid-phase fraction available for sliding.
As a result of the multitude of different rheological phenomena, it is usually not
reasonable to describe a material using only one measurement. Therefore, most
materials are characterized using viscosity curves, that is, viscosities subject to the
shearing incline, or using flow curves or the shearing incline subject to shear stress.
For metallic alloys, these curves have to be determined for each temperature in the
semi-solid state. Therefore, three approaches are in particular used for the analysis of
the rheological properties of partially solidified metal alloys [7]:
1. isothermal and stationary viscosity measurements for the identification of the flow
curve;
2. experimental study of the time-dependent behaviour under isothermal conditions
(ramp experiments to investigate hysteresis loops, leap experiments and measurements
of different initial viscosities); and
3. cooling or heating experiments to measure the viscosity under a constant cooling
or heating rate and shear rate.