Thixoforming : Semi-solid Metal Processing

For A356, there is a temperature shift of 7.5 C, and for AZ91 the shift is 12 C.
For the alloy AlSi7Mg, it must also be noted that a 50% solid fraction at the upper
compositional limit cannot be reached without any eutectic solidification in the slug.
If eutectic is present during the holding period of the slug, a very coarse eutectic
phase will be formed. This reduces the mechanical properties, especially elongation
values. However, in general, it can be stated that both alloys are well suited for semisolids.
Let us assume that SSM forming takes place at 570 C, aiming for a 50% solid
fraction in the alloy AZ91. If the alloy is at the upper limit of the tolerance field, the
actual solid fraction would only be around 44%, whereas if the alloy is at the lower
limit fS would be about 55%, and only a little influence on product quality can be
assumed.
For the steel X210CrW12, fluctuations of 2.0–2.3% C, 11–13% Cr and 0.6–0.8% W
are tolerated, resulting in a temperature shift of about 35 C. This corresponds to a
solid fraction variation from about 42% to 57%, which is also considered to be not
very harmful.
The situation is more complicated for the Al wrought alloy AA6082, with
tolerated fluctuations of 0.7–1.3% Si, 0.6–1.2% Mg and 0.1–0.45% Mn. Here the
temperature shift is approximately 11 C, but with the high S value of 0.027 K 1
this corresponds to an intolerable solid fraction variation of about 30%. Similar
problems are expected for the steel grade 100Cr6. The accepted chemical variations
are 0.97–1.1% C, 1.3–1.6% Cr, 0.15–0.3% Si and 0.25–0.45% Mn. This implies a
possible temperature shift of about 18 C or a variation in solid fraction of almost
20%.
2.6
Conclusion
2.6 Conclusionj41
The impressive bundle of advantages mentioned in the Introduction has been the
driving force for continued SSM research worldwide. In contrast to what was
expected several years ago, the acceptance of SSM technology in industry is still at a
relatively low level. The essential demand is to produce components at a constant
high-quality level. From a metallurgical point of view, only a few alloy systems fulfil
the requirements for high-quality SSM production. However, also in case of
optimal alloy selection, unintentional fluctuations in solid fraction due to
fluctuations in alloy composition may cause quality problems. For a given set of
formingparameters,analloywithacompositionatthelowerorupperendofits
standard may generate severe defects such as macro-segregation. This does not
mean that SSM processing at high or low solid fractions is generally bad, or that it
cannot yield high-quality components. If variations in solid fraction occur, the
forming parameters have to be adjusted properly. On the other hand, if the process
parameters are kept constant, only a limitation in compositional fluctuation
can help to reduce scrap production. In order to stabilize the SSM process, the
use of temperature control in combination with chemical monitoring is highly
recommended.