Double oxide film defects and mechanical properties in aluminium alloys

Double oxide films (bifilms) are significant defects in light alloy castings which were reported to have detrimental effects on the reliability of the castings. The research reported here was aimed at studying how these defects develop with time. The results suggested that both O and N inside the bifilm would be consumed by reaction with the surrounding melt, and that H might be diffused into the defect. Based on the estimated reactions rates the time required for the consumption of the atmosphere inside a bifilm entrained in pure Al, Al-7wt.%Si-0.3wt.%Mg and Al-5wt.%Mg alloy melts, was determined to be 538, 1509 and 345 seconds respectively. The results also suggested the occurrence of two competing mechanisms during holding of the castings in the liquid state before solidification. The first mechanism was related to the consumption of the bifilm atmosphere, which might reduce the size of bifilms and therefore increase the Weibull moduli the UTS and the % elongation. The other mechanism was the diffusion of H into the bifilms, which would be expected to increase their sizes and reduce the moduli. This research therefore could lead to the development of new techniques by which bifilms might be deactivation in light alloy castings.