The role of oxides in the formation of primary iron intermetallics in an Al-11.6Si-0.37Mg alloy
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UCTION
THE iron impurity that is commonly present in commercial aluminum casting alloys generally manifests itself by the formation of intermetallic phases. Two of the phases that are commonly encountered in commercial aluminum alloys and have been the subject of much research are the a-Fe phase, which usually has a blocky or Chinese-script morphology, and the b-Fe phase, which has a plate-like morphology. Both these phases exist with a range of compositions and crystal structures. The a-Fe phase is generally described as having a cubic or hexagonal crystal structure, and the possible compositions include Al8Fe2Si, Al8(Fe,Mn)2Si, Al15(Fe,Mn)3Si2, and Al19(Fe,Mn)5Si2.[1–5] The b-Fe phase is usually considered to be monoclinic Al5FeSi phase but has sometimes been described as tetragonal Al3(Fe,Mn)Si2.[1–5] The composition and type of ironrich intermetallics that form is highly dependent on the melt composition and cooling conditions. For instance, the presence of manganese in the melt and high cooling rates lead to the preferential formation of the a-Fe phase at the expense of the b-Fe phase.[6,7] The presence of these precipitates, particularly b-Fe, is generally considered to have a negative impact on the quality of castings due to their brittle nature and effect on porosity formation.[8–11] However, recent work by Cao and Campbell has identified a process whereby the formation of these intermetallic particles is harnessed to remove oxide films from the castings, therefore leading to improved quality of the molten aluminum alloys.[1,2] Oxide films have long been identified as cracks in the castings that lead to porosity problems and reduced ductility and fatigue properties of castings. Therefore, limiting the introduction of oxide films into the liquid aluminum has D.N. MILLER, Research Fellow, is with the School of Chemistry, University of St. Andrews, St. Andrews, Fife, KY16 9ST, U.K. Contact e-mail: [email protected] L. LU, Senior Research Scientist, is with CSIRO Minerals, P.O. Box 883, Kenmore, Queensland, 4069, Australia. A.K. DAHLE, Associate Professor, is with the Division of Materials Engineering, University of Queensland, Brisbane, Queensland, 4072, Australia. This article is based on a presentation made in the John Campbell Symposium on Shape Casting, held during the TMS Annual Meeting, February 13–17, 2005, in San Francisco, CA. METALLURGICAL AND MATERIALS TRANSACTIONS B
been a major research effort in the aluminum industry. Many techniques have therefore been developed. A new technique has recently been proposed by Cao and Campbell[1] to remove oxide films from the liquid aluminum by nucleation of Fe-rich intermetallics on the oxide films, causing them to sink under the weight of these Fe-rich intermetallics. According to Cao and Campbell,[1,2] the oxide film on the surface becomes folded and entrained within the melt during the pouring operation of liquid aluminum. These folded films in the liquid aluminum have an external surface that is wetted by the melt and an internal surface that is nonwetted. It was
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