A study of double oxide film defect behaviour in a quiescent aluminium melt

The change in the volume of an air bubble held within commercially pure liquid aluminium was recorded by real time x-ray radiography, to estimate the behaviour of double oxide film defects after entrainment in liquid aluminium. This showed that first oxygen and then nitrogen were consumed by the melt to produce Al ₂0 ₃ and AIN respectively. Varying the initial hydrogen contents of the melt also showed that hydrogen could diffuse into the atmosphere within a trapped bubble, and cause it to expand. A mathematical model was developed using the consumption rates obtained from this experiment that included the diffusion of hydrogen, which suggested that the atmosphere within a double oxide film defect should be consumed by the surrounding AI melt in less than two minutes. In addition, a double oxide film-free liquid aluminium melt was poured into ceramic test bar moulds which then held in the liquid state for 20 minutes. Study of the fracture surfaces suggested that the two non-wetting surfaces of a double oxide film defect might bond to each other, after the internal atmosphere of the film was consumed, and that this could occur in 20 minutes or less.