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DOUBLE oxide films have become recognized as significant defects in Al and Mg alloys, leading not only to a reduction in mechanical properties but, perhaps just as detrimental, an increase in the scatter of properties.[1] The defects are caused during the casting process when liquid metal is transferred, e.g., from a melting or holding furnace to a crucible, or during mold filling. The defects occur when the oxidized liquid metal surface is folded over onto itself, trapping the local mold atmosphere, and is then carried into the bulk liquid metal.[2] If parts of the oxide film come into contact, it is thought that they should not bond, owing to their relatively low temperature in the liquid metal. Once the defects become submerged into the bulk liquid, they may be

WILLIAM DAVID GRIFFITHS, Senior Lecturer, is with the School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Contact e-mail: W.D.Griffi[email protected] AHMED ELSAYED, Research Associate, is with the Department of Mechanical Engineering, Alexandria University, Alexandria, Egypt, and also with the Department of Mechanical Engineering, Institute for Advanced Manufacturing and Engineering (AME), School of Engineering and Computing, Coventry University, Coventry CV6 5LZ, U.K. MAHMOUD AHMED EL-SAYED, Assistant Professor, is with the Department of Industrial and Management Engineering, Arab Academy for Science and Technology and Maritime Transport, Abu Qir, P.O. Box 1029, Alexandria 21599, Egypt. Manuscript submitted April 1, 2016. Article published online September 7, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B

carried into the mold cavity and trapped during solidification. Their variable effect on the mechanical properties depends upon various factors such as whether the section of the casting in which they reside is under load or not in service, the orientation of the defect to the applied load, and its morphology.[1–3] Double oxide film defects are seen as symmetrical defects on either side of a fracture surface in tensile test bars.[1] They are identified as oxides by EDX analysis during scanning electron microscopy. Different alloys are associated with different oxides on the liquid metal surface; for example, commercial-purity Al should form alumina (Al2O3), while a Mg content of between 0.02 and 1.5 wt pct has been suggested to form MgAl2O4 spinel. With a Mg content of more than 1.5 wt pct, magnesia, MgO, would be expected to form.[4] Once a double oxide film defect has become incorporated into the melt, it would be expected to contain an atmosphere that would be predominantly air (although other mold gases may also be present), which would be expected to react with the surrounding liquid metal. It was proposed by Nyahumwa et al.[5] that oxygen in the internal atmosphere would react first, followed by nitrogen. As the interior gases were consumed, the sides of the oxide film defect would come together, perhaps making the defect less detrimental. Studies by Griffiths et al.[6,7]