Thixoforming : Semi-solid Metal Processing

40j 2 Metallurgical Aspects of SSM Processing
be helpful in reducing TFR. At first sight, it is the increased amount of eutectic that
helps in reducing the alloy s susceptibility to hot tearing.
In summary, from a simplified metallurgical view, choosing SSM material with a
significant amount of eutectic can reduce the proneness to segregation and hot
tearing. For this reason, Al and Mg wrought alloys are considered to be rather
unfavourable. Among the Fe-base alloys, use of the steel grades X210CrW12 and
HS6-5-2 can be recommended [12].
As for segregation, only the microscopic aspect was considered in this section.
Macroscopic segregation due to long-range decomposition of the liquid and solid
phases during the SSM forming process is mostly caused by inappropriate technological
conditions and is due to only to a minor extent solely to metallurgical
circumstances. In this respect, one important aspect is discussed in the next section.
2.5
Impact of Variations in Alloy Composition
Fluctuations in alloy composition affect all casting processes, from gravity casting to
high-pressure die casting. While this is a problem in fully liquid casting processes, it
is even more so in semi-solid casting processes. With process stability in mind, the
solidification range and in particular the DT 40–60 temperature interval and the
sensitivity to temperature fluctuations S in this temperature range have already
been described as important factors influencing the choice of alloys for SSM
production. For the alloys AlSi7Mg and AZ91 it was shown that DT 40–60 is 17 and
22 C, respectively, and that the sensitivity to temperature change, S ,atfL ¼ 0.5 is
0.83 and 0.87% K 1 . International alloy standards tolerate significant fluctuations in
the content of major and minor alloying elements. For the alloy A356, the tolerance
field for Si is 6.5–7.5% and for Mg 0.25–0.45%. In the magnesium alloy AZ91, the
aluminium and zinc contents can vary between 8.5 to 9.5% and 0.45 to 0.9%,
respectively. With the upper and lower limits of these alloy compositions, the
solidification curves shift for by degrees (Figure 2.10).
Figure 2.10 Calculated cooling curves for AlSi7Mg (a) and AZ91
(b) at the upper and lower limits of tolerated alloy composition,
indicating the shift of temperature at f L ¼ 0.5 [13].