Thixoforming : Semi-solid Metal Processing

56j 3 Material Aspects of Steel Thixoforming
The advantage of the thermal procedures is the easy and cost-efficient specimen
preparation, but a systematic error has to be considered because of the use of peak
reference surface integration. In comparison with industrial application, the disadvantages
of thermal procedures concerning the direct transferability of the results
for semi-solid technologies (considerably quicker inductive heating) is the lower
feasible heating rate of DTA and DSC devices.
For the metallographic determination of the average phase contents by means of
quenching experiments, the random and uniform distribution of the liquid and solid
phases in the sample volume has to be ensured. Furthermore, the phase concentrations
should not change significantly during quenching. This is generally the case if
the liquid phase transforms in such a way that it can be easily distinguished from the
already present solid-phase components. If the alloys are quenched from temperatures
slightly above the eutectic temperature, the contour of the round globulites
and the two-phase eutectic can easily be distinguished. The characterization of the
microstructure is more difficult in cases in which the quenching temperature is
much higher than the eutectic temperature. In that case, secondary, solid particles are
formed, which can hardly be differentiated from the existing globulites, while the
remaining cast solidifies eutectically. Remarkably, research effort is focused on this
issue. In special cases, solid particles present in the partial liquid state can be
differentiated from secondary phase fractions by means of special etching techniques
due to their chemical composition. However, the boundary of the different solid
phases is often very blurred so that no precise characterization is feasible.
Grain Size and Grain Size Distribution in the Solid Phase The influence of the grain
size on the rheological properties of the partial liquid material is the object of current
research [4]. It should, on the one hand, be large enough to build a dimensionally rigid
solid-phase carcass. On the other, it should be small enough that also solid material can
flow into thin component areas. It is usually assumed that the smallest to-be-filled
component widths without causing noteworthy separations of solid- and liquid-phase
components should not be less than 20–30 times the grain radius [31]. For a laminar
mould filling without pore or entrapment formation, the globulites should preferably
be round and separated from each other, so that the formation of segregations due to
separations of the solid and liquid phases can be reduced to a minimum.
The grain size distribution by means of 2D analysis is difficult to define because of
the random view of the grain sections. To take this influence into consideration, a
Saltykov correction can be conducted. Furthermore, by means of extensive serial
sections, the three-dimensional solid-phase carcass can be reconstructed. An overview
of the hitherto conducted research with different materials can be found in [32].
A largely error-free analysis of the grain size distribution can, however, only be determined
by means of 3D examination methods. X-ray micro-tomography permits a
destruction-free determination of the structural parameters with dissolution of 2 mm
within 15 min [31–34].
Form Factor of the Solid Phase The globulite form factor, F, is an important parameter
for thixoforming because it strongly influences the flowability and the viscosity