Modelling the temperature change of bifilm defects in Al cast alloys

Double oxide films (bifilms) are significant defects in the casting of light alloys, and have been shown to decrease tensile and fatigue properties, and also increase their scatter, making casting properties unreproducible and unreliable. Recent research has suggested that the nature of oxide film defects may change with time, as the air inside the bifilm would react with the surrounding melt leading to its consumption, which may enhance the mechanical properties of Al alloy castings. It was suggested that in a pure Al melt, oxygen within the bifilm atmosphere would be consumed first to form alumina, then nitrogen would react to from AlN. A CFD model of the heat distribution associated with the reactions between the interior atmosphere of a double oxide film defect and the surrounding liquid alloy suggested that highly localized increases in temperature, up to 5000, could occur, over a scale of a few hundred micrometers. Such localized increases in temperature might lead to change the nature of the bifilm causing it to be less harmful to the properties of Al cast alloys.