Thixoforming : Semi-solid Metal Processing

48j 3 Material Aspects of Steel Thixoforming
exceed the cohesiveness and stiffness of the solid-phase carcass. If the externally
applied forces are too low, the body is only deformed elasto-plastically and no
thixotropy occurs. The frame firmness of the solid phase can be described by the
degree of carcass creation. If the shearing forces operating during the transformation
of a partial liquid material are not sufficient to destroy the steric cohesive carcass, a
mere plastic deformation and a compression of the solid phase carcass result. The
reheated billet will be plastically compressed and no thixotropy occurs, so that the
liquid phase, which is embedded in the spaces between the solid particles, is extruded
from the compacted solid phase. Because this is similar to the wringing out of a waterfilled
sponge, this effect is also referred to as the sponge effect . It can, for instance, be
quantified by quantitative measurements of the chemical composition within the
unit [15]. In re-extrusion experiments with the alloy AIMgSi1, a homogeneous
deformation without noteworthy phase separation occurred only with a liquid-phase
fraction between 40 and 50%. The segregation due to the sponge effect should
generally be avoided because of the resulting poor component-properties. In particular
cases it can be positively used as an alternative option for the production of
gradient components.
3.2.1.3 Temperature Interval Size and Temperature Sensitivity
For thixoformability of a metal, the possibility of the defined adjustment of the
solid–liquid fraction in the solidification-interval is a fundamental criterion. This is
independent of whether the partial liquid state of an alloy is reached by heating a solid
body as in conventional thixoforming or by directed cooling of a melt as in the rheoroutes.
Because pure metals and alloys of eutectic composition do not have a
solidification interval but rather a specific transformation temperature, these are
only restrictedly or not at all suitable for thixoforming. Due to the different requirements
of the thixo-procedures on the primary material, no explicit threshold for the
solid- or liquid-phase portion is given in the literature. Recent studies show that for
thixoforging solid-phase fractions of 60–80% and for thixocasting 30–50% are
used [4]. The determination of the optimal concentrations is subject of current
research. It cannot be finally indicated independent of the structure of the primary
metal and the resulting microstructural formation.
Apart from the plant-specific calibration of an exact and homogeneous temperature
distribution within the to-be-moulded billet, further factors such as the temperature
sensitivity of the solid-phase fraction S (Equation 3.1) and the temperature
sensitivity of the enthalpy of the solid–liquid system L (Equation 3.2) are relevant for
the adjustments of the designated phase fractions.
S ¼ df s =dT ð3:1Þ
L ¼ dH=dT ð3:2Þ
where fs ¼ solid-phase fraction, H ¼ enthalpy and T ¼ temperature.
Table 3.1 shows some values for technically interesting aluminium and magnesium
alloys and steels [17, 18]. It has to be considered here that the L value is strongly
dependent on the observed temperature interval. It is, for instance, 58.8 for the alloy