Ja. Taylor et al., The role of iron in the formation of porosity in Al-Si-Cu-Based casting alloys: Part II. A phase-diagram approach, MET MAT T A, 30(6), 1999, pp. 1651-1655
The mechanism by which iron causes casting defects in the AA309 (Al-5 pet S
i-1.2 pet Cu-0.5 pet Mg) may be related to the solidification sequence of t
he alloy. Superimposing calculated segregation lines on the liquidus projec
tion of the ternary Al-Si-Fe phase diagram suggests that porosity is minimi
zed at a critical iron content when solidification proceeds directly from t
he primary field to the ternary Al-Si-beta Al5FeSi eutectic point. Solidifi
cation via the binary Al-beta Al5FeSi eutectic is detrimental to casting in
tegrity. This hypothesis was tested by comparing the critical iron content
observed in the standard AA309 alloy to that of a high-silicon (10 pet Si)
variant of this alloy.