Fig. 1. This is the Al-Cu phase diagram.
Fig. 2. Detail of the Al-Cu phase diagram is shown.
feeding. Macrosegregation is most easily seen in copper-containing alloys. An example is shown in Fig. 4. This X-ray shows a section of a cylinder head cast in 319 alloy. Two risers are visible at the top. Both show shrinkage cavities. Underneath the risers are dark bands in the casting, where copper- rich liquid has been “sucked into” the casting. Risers are placed in the
mold to feed liquid to the solidifying metal, whose volume shrinks by 5-7% during solidification. When the risers are not sufficiently large, they will feed the cast- ing late in the solidification when the remaining liquid is enriched in copper (and other solute elements). Macrosegregation of
this sort also can develop in Al-Si casting alloys, but it is more difficult to see this in casting X-rays. (The density difference between aluminum and silicon is much smaller than between aluminum and copper.)
Calculating Solidification Paths Equations for microsegregation
during solidification can be used to calculate solidification paths for casting alloys. The easiest way to proceed is to assume the distribution coefficients
for the binary alloy systems can be used in ternary alloy systems. The starting point for our calculations is the aluminum rich corner of the Al-Si-Fe ternary. Eleven different intermetallic compounds have been identified in this system. Four of them occur in the aluminum- rich portion of the ternary presented in Fig. 5. They are: • FeAl3, which is found in the Al-Fe binary and in alloys low in silicon.
• α-FeSiAl, which has a com- position close to Fe2SiAl8.
• δ-FeSiAl, which has the composition FeSi2Al3. The compound which is of most concern here is the β phase. This is the intermetallic compound normally observed in commercial castings. The calculated solidifica-
• β-FeSiAl, which is usually represented by the composi- tion FeSiAl5.
Fig. 3. This diagram shows the distribution of copper in Al-4-5% Cu alloy. (Local solidification time is 750 seconds.)
36 | MODERN CASTING June 2014
tion paths for an AA309 alloy having 5% silicon and vari- ous iron contents have been calculated. (This alloy also has 1.2% copper and 0.5%
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