When the oxidised surface of a liquid metal is folded over onto itself and entrained double oxide film defects are formed, which form crevices or cracks in the solidified casting, of varying sizes and orientations. These defects not only reduce mechanical properties, but also increase the scatter of properties. This paper reports an analog experiment to study the behavior of the interior atmosphere of double oxide film defects in Al alloy melts of varying Mg content. Air bubbles were trapped in melts of liquid Al alloy which were then solidified after holding for varying periods of time. The composition of the bubbles was subsequently measured using mass spectroscopy. This showed the reaction of oxygen from the bubble atmosphere to form oxides, followed by the consumption of nitrogen to form AlN. Simultaneously, hydrogen from the melt diffused into the air bubble. The changes in composition were used to estimate the rate of change in composition of the interior atmosphere of a typical double oxide film defect of an estimated size. This suggested that double oxide film defects may quickly achieve an interior atmosphere that would consist of a mixture of mainly nitrogen and hydrogen, and that this atmosphere could exist for periods of time greater than the typical solidification times of light alloy castings. In other words, oxide film defects created during mould filling should persist into the solidified casting. In addition, SEM analysis of oxide film defects also suggested the presence of oxide whiskers, which seem to have formed during holding in the melt.

A new contactless ultrasonic sonotrode method was previously designed to provide cavitation conditions inside liquid metal. The oscillation of entrapped gas bubbles followed by their final collapse causes extreme pressure changes leading to de-agglomeration and the dispersion of oxide films. The forced wetting of particle surfaces and degassing are other mechanisms that are considered to be involved. Previous publications showed a significant decrease in grain size using this technique. In this paper, the authors extend this research to strength measurements and demonstrate an improvement in cast quality. Degassing effects are also interpreted to illustrate the main mechanisms involved in alloy strengthening. The mean values and Weibull analysis are presented where appropriate to complete the data. The test results on cast Al demonstrated a maximum of 48% grain refinement, a 28% increase in elongation compared to 16% for untreated material and up to 17% increase in ultimate tensile strength (UTS). Under conditions promoting degassing, the hydrogen content was reduced by 0.1 cm3/100 g.

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Inclusions have a detrimental effect upon casting properties but it is known empirically that a slow liquid metal flow has a beneficial effect by reducing the number of inclusions entering the casting. Positron Emission Particle Tracking (PEPT) is a method that can be used to track the path taken by radioactive particles, and can be used to follow the behaviour of inclusions as they make their way from a furnace, along the launder and into the casting. A new PEPT camera geometry was developed and used to track radioactively labelled γ-alumina particles, in the region of 600 μm size. The camera detectors were arranged radially around a launder, into which was poured 150 kg of liquid Al alloy, the radioactive particles being released at intervals during the pour. The positron camera was 0.7 m in length and 0.1 m2 in cross-sectional area, and this matched the dimensions of an industrial launder. A model of inclusion behaviour in a flowing liquid Al alloy was also developed, and the Positron Imaging system described was used to validate this model.

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