The behaviour of double oxide film defects in liquid Al alloys under atmospheric and reduced pressures

The effect of holding an Al melt containing double oxide film defects, under 1000 and 80 mbar for extended periods of time up to 60 min, was investigated by using a Weibull statistical analysis. This was aimed at determining any resulting changes in the number and morphology of oxide film defects, and any changes in their effect on the mechanical properties of the solidified castings. The results suggested that the number of oxide film defects in the melt would gradually decrease due to their continuous migration towards the upper surface, and that holding the melt under a vacuum would increase this removal rate due to enhanced floatation. The reproducibility of the mechanical properties of the castings was observed to decrease during the first 10 min of holding under both normal and reduced atmospheric pressures, but increase thereafter. This was attributed to the simultaneous expansion of smaller defects by the diffusion of H into their atmosphere, coupled with floatation of larger defects out of the melt leading to a gradual decrease in the H content dissolved in the melt. (C) 2009 Elsevier B.V. All rights reserved.