Abstract
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 to increase their scatter, making casting properties unreproducible and unreliable. A bifilm consists of doubled-over oxide films containing a gas-filled crevice and is formed due to surface turbulence of the liquid metal during handling and/or pouring. Previous studies has shown that the nature of oxide film defects may change with time, as the atmosphere inside the bifilm could be consumed by reaction with the surrounding melt, which may enhance the mechanical properties of Al alloy castings. As a proxy for a bifilm, an air bubble was trapped within an Al-7wt.%Si-0.3wt.%Mg (2L99) alloy melt, subjected to stirring. The effect of different parameters such as the holding time, stirring velocity and melt temperature on the change in gas composition of the bubble was investigated, using a design of experiments (DoE) approach. Also, the solid species inside the bubbles solidified in the melt were examined using SEM. The results suggested that both oxygen and nitrogen inside the bifilm would be consumed by reaction with the surrounding melt producing MgAl2O4 and AlN, respectively. Also, hydrogen was suggested to consistently diffuse into the defect. The reaction rates and the rate of H diffusion were shown to increase upon increasing the holding time and temperature, and stirring velocity. Such significant effect of the process parameters studied on the gaseous content of the bubble suggesting that a careful control of such parameters might lead to the deactivation of bifilm defects, or at least elimination of their deteriorous effect in light alloy castings.
Highlights
In response to the raising consumer demands for increased performance and fuel economy, the use of aluminium to replace heavier materials in the automotive and aerospace industries was a justified choice
Previous studies has shown that the nature of oxide film defects may change with time, as the atmosphere inside the bifilm could be consumed by reaction with the surrounding melt, which may enhance the mechanical properties of Al alloy castings
The reaction rates and the rate of H diffusion were shown to increase upon increasing the holding time and temperature, and stirring velocity. Such significant effect of the process parameters studied on the gaseous content of the bubble suggesting that a careful control of such parameters might lead to the deactivation of bifilm defects, or at least elimination of their deteriorous effect in light alloy castings
Summary
In response to the raising consumer demands for increased performance and fuel economy, the use of aluminium to replace heavier materials (steel or copper) in the automotive and aerospace industries was a justified choice. Many Al alloys have high strength to weight ratio [1,2,3] Such properties made it very suitable for casting and today, significant amounts of cast aluminium alloys are PLOS ONE | DOI:10.1371/journal.pone.0160633. The mechanical properties of Al castings were greatly affected by their inclusion contents, double-oxide film defects, or bifilms, which was first introduced by John Campbell since the early nineties [4]. These defects are created due to surface turbulence of the liquid Al, which is a common foundry practice during the metal pouring. Submerged oxide films were recognized to initiate hydrogen porosity [6,7], and to act as a nucleation site for Fe-rich intermetallics [8]
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