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I. INTRODUCTION

IRON is the most common and detrimental impurity present in aluminum casting alloys and has long been associated with an increase in casting porosity. After examining the effect of iron on casting porosity, Taylor et al.[1,2,3] found that porosity in Al-5Si-1Cu and Al-10Si-1Cu alloys starts to increase at intermediate iron contents. By using the same techniques, Otte[4] examined the effect of iron on porosity formation in an Al-9Si-3Cu alloy. He found that the total level of porosity increased slightly with iron content and that a large region of interconnected “sponge-like” porosity formed at high iron contents. Iwahori et al.[5] also studied the relationship between porosity and melt iron level. They suggested that feeding decreased dramatically at iron contents above a critical value. In addition to its negative effect on casting porosity, iron was also reported to reduce the castability, the mechanical properties (particularly ductility), the corrosion resistance, and the machinability of Al-Si casting alloys.[6,7] While the negative effects of iron are clear, the mechanism involved is not fully understood. It is generally believed to be associated with the formation of Fe-rich intermetallic phases during solidification. A number of Fe-rich intermetallic phases, including  (Al8Fe2Si or Al15(Fe,Mn)3Si2),
(Al5FeSi),  (Al8Mg3FeSi6), and  (Al4FeSi2), have been identified in Al-Si casting alloys.[6–10] Among them, the
phase was reported to be particularly deleterious to the castability and mechanical properties.[6,7,8] The platelet-like mor-

L. LU, Research Fellow, and A.K. DAHLE, Associate Professor, are with the CRC for Cast Metals Manufacturing (CAST), Division of Materials Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted December 7, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A

phology of this phase is expected to cause severe feeding difficulties during solidification, and eventually to increase the tendency to porosity formation. It has been suggested that the formation of porosity at high iron contents is due to the precipitation of coarse
platelets, which block the interdendritic flow channels.[1–5,11,12] Taylor et al.[1,2,3] further suggested that the formation of large
platelets at high iron contents facilitates the nucleation of eutectic Si, therefore leading to a rapid deterioration of the interdendritic permeability. The nucleation of eutectic Si on
platelets has been proposed in the literature.[12,13] However, no direct evidence has been presented yet. It has also been suggested that the
platelets are effective pore nucleation sites and therefore likely increase gas porosity in castings.[11] Since iron is inevitable and cannot be economically removed from the molten aluminum, strategies have to be developed to neutralize its negative effects. It is well known that addition of such elements as Mn, Cr, Be, and Ni can neutralize the embrittling effect of iron by modifying the platelet-l