Thixoforming : Semi-solid Metal Processing

158j 5 Thermochemical Simulation of Phase Formation
Figure 5.8 (a) Amount of liquid for X210CrW12
as a function of temperature. The solid line
shows the equilibrium fraction. The dotted line
shows a DICTRA simulation of the solidification.
The dashed line shows a DICTRA simulation of
the melting, directly heating after solidification
and cooling to 1100 C. The dash-dotted line
diffusion in the liquid phase. Complete melting will then occur above the liquidus
temperature.
5.2.8
Suitability for Semi-solid Processing
shows a DICTRA simulation of the melting after
complete homogenization at 1100 C. The
composition used for the simulation was
different from that in Table 5.1, so that the
equilibrium and DICTRA solidification curves are
slightly different from those in Figure 5.3. (b)
Detail of (a).
A major criterion to determine if a particular material is suitable for semi-solid
processing is usually the temperature sensitivity of the liquid fraction (Chapter 2) or,
as suggested by Balitchev et al. [1], the temperature interval in which a certain range of
liquid fraction can be achieved. Applying this to X210CrW12, the conclusion is that it
is well suited provided that the desired liquid fraction is above about 40%. If it is below
40% it is nearly impossible to control if temperature alone is used to control the
process (see Figures 5.3 and 5.8). The solidification behaviour of X210CrW12 is
actually similar to that of the aluminium alloy A356, except that the temperatures are
much higher. They both form a primary metallic phase and around 40% eutectic
structure. The eutectic structure is formed in a narrow temperature interval. A356 is
currently used extensively for semi-solid processing, suggesting that X210CrW12
is also fairly suitable. At the knee in the liquid fraction curve, the apparent heat
capacity changes drastically. This can (and is being) be used to control the process
(Chapter 10). Using a combination of temperature and heat flow control, it may even
be possible to reach reproducibly liquid fractions below 40%, thus contradicting the
standard suitability criterion.