Thixoforming : Semi-solid Metal Processing

Figure 2.5 IMP formation along the grain boundaries. Mg17Al12 in
AZ91 (a), Si and AlFeMnSi in AA6082 (b).
2.4 Proneness to Segregation and Hot Tearingj37
liquidandthustheformationofabout16%eutectic.However,itisnotthetotalamountof
eutectic that most controls the properties but the amount of the brittle phase, Si in A356
and Mg17Al12 in AZ91, and also its distribution and morphology. The Al–Si eutectic
contains11.3%SiandtheeutecticinAZalloyscontainsasmuchas66%ofthebrittleIMP
Mg17Al12. The globular shape of the primary Mg phase facilitates the formation of a
contiguous network of eutectic.Asa result, the microstructure is characterizedbyabout
10% of coarse and very brittle intermetallic Mg17Al12 concentrated along the grain
boundaries. Non-equilibrium segregation into the liquid phase occurs not only for the
main alloying elements but also for trace elements such as Fe. In AA6082, for example,
Fe enrichment in the liquid leads to the formation of eutectic Si and brittle Fe-rich
IMPs along the grain boundaries. Figure 2.5 illustrates the IMP morphology in
SSM-processed AZ91 and AA6082.
Such IMP formation due to non-equilibrium segregation significantly reduces the
properties of the SSM-processed alloys. A356 with 50% eutectic exhibits an elongation
to fracture of typical 12 5% in the T6 condition, whereas it is only about 7 3%
for AZ91 (F and T4; 15% eutectic) and 7 3% for AA6082 (T6; 2.5% eutectic) [6].
Element segregation is also an important issue in SSM processing of single-phase
steels such as 100Cr6. As already indicated in Figure 2.4 (Note: the appearance of a
eutectic phase should be treated with caution since the calculation was performed
without considering C to be a fast-diffusing element [7]), and clearly disclosed by
micrographs, X-ray maps and diffusion simulations in Figures 2.6–2.8, pronounced
segregation of the alloying elements (Cr, Mn and Si) and slight C enrichment towards
the intergranular regions occur during cooling from the semi-solid range. In these
regions, the Cr content reaches a value well above 3% and therefore the martensite
start temperature decreases to below room temperature [8, 9] . This generates the
formation of a unique microstructure: martensite with an intergranular network of
retained austenite (Figure 2.3b), accompanied by the sporadic formation of carbides
at the grain boundaries.