Thixoforming : Semi-solid Metal Processing

36j 2 Metallurgical Aspects of SSM Processing
2.4
Proneness to Segregation and Hot Tearing
A significant difference in microstructural development of partly eutectic alloys on
the one hand and single-phase alloys on the other is observed not only for slurry
formation but also for the solidification after the forming process. Figure 2.4
illustrates the freezing behaviour for various alloying systems, calculated for nonequilibrium
solidification using the Scheil model. Commercial forming processes do
not allow equilibrium solidification, which is why a eutectic is also often found in
single-phase alloys (see Figure 2.4: AA6082, AZ91 and 100Cr6).
The presence and morphology of the eutectic phases significantly influence the
propertiesofthecomponents.AcomparisonoftheAlalloyA356withtheMgalloyAZ91
illustrates this peculiarity. A356 is SSM processedjust above the eutectictemperature of
575 CwhereitfeaturesanadequateliquidfractionfLof51%.Duringcooling,thisliquid
solidifiesasaeutecticandthefinalmicrostructureconsistof49%a-Aland51%eutectic.
AZ91 has to be processed at a temperature of about 570 C(fL 50%) and nonequilibrium
solidification ends with alloying element enrichment in the remaining
Figure 2.4 Non-equilibrium (Scheil) solidification behaviour of
various alloying systems; partly eutectic alloy A356 and singlephase
alloys AA6082, AZ91 and 100Cr6.