Thixoforming : Semi-solid Metal Processing

It is important to note that only the interglobular liquid phase contributes to shear
thinning and therefore thixotropic behaviour. Intraglobular liquid phase that may
form during reheating of the thixo-feedstock does not contribute to the viscosity of
the slurry and therefore the numbers in Table 2.1 are applicable only for ideal
slurries.
While the usability of the criteria solid fraction and temperature sensitivity has
been confirmed for Al and Mg alloys, the situation with Fe-based alloys is more
complex. Here, due to technological difficulties (tooling problems and oxidation), the
absolute temperature level must also be taken into account. High-alloyed steels with a
complex melting behaviour mostly exhibit a wide solidification interval at relatively
low temperatures and are therefore less sensitive to temperature changes and easier
to thixo-form. Figure 2.1 illustrates the situation for different steel grades by means of
solidification curves (DTA measurements) [3]. It is worth noting that the complex
steels (X220CrVMo134, X210CrW12, HS6-5-3) also exhibit eutectic solidification.
The benefit of such phase transformation is addressed in Section 2.4.
As for the solid fraction adjustment, it is recommended also to take the absolute
temperature at f S ¼ 50% into account, which is T50 850 K for AlSi7Mg, T50 910 K
for AlSi1Mg, T50 1565 K for X210CrW12 and T50 1700 K for 100Cr6. As a rough
appraisal for the evaluation of the thixo-ability of an alloy, we propose a simple
multiplication of S and T50. For AlSi7Mg S T50 is 7.5, for X210CrW12 it is 7.0, for
100Cr6 it is 17.0 and for AlSi1Mg it is 24.6. According to this simple rule, AlSi7Mg
and X210CrW12 are of comparable capability, whereas the adjustment with 100Cr6
and AlSi1Mg is much more complicated.
As mentioned above, from the temperature sensitivity a wide freezing rage can be
considered to be beneficial. A wide freezing rage is achieved in systems with a low
element partitioning ratio, k (element content in solid phase/element content on
Figure 2.1 Solidification curves for various steel grades;
the complex steels exhibit eutectic solidification at relatively
low temperatures [4].
2.2 Temperature Sensitivity S and Solid–Liquid Fractionj33