Measurements of oxide films in Al-(6–17) wt%Si foundry alloys using the Qualiflash filtration technique

Abstract

The Qualiflash technique was used to assess the melt cleanliness of Al-Si foundry alloys. This is a simple filtration technique where the melt cleanliness is gauged by the number of steps covered by the liquid metal that filters through into a step-like ingot mould placed underneath the filter. The present work was performed on A356 and A390 aluminum foundry alloys. The main objectives were to validate the reliability of the Qualiflash technique in measuring the oxide content, and to determine the role of grain-refining and Si-eutectic modification agents in relation to oxide formation. The results indicate that Qualiflash is a quick and simple technique to determine the concentration of aluminum oxides (films and massive particles), as well as other oxides, i.e. MgO, MgAl2O4, in terms of the number of ingot steps of filtered metal obtained. Grain-refining or modification agents themselves do not create oxides. However, the method of their introduction into the molten metal contributes to oxide formation. The number of ingot steps varies linearly with the alloy melt temperature. The slope is strongly related to the amount of oxide films that exist in the liquid metal prior to filtration. Such a relationship is expressed in terms of the Quality Temperature Index (QTI). The effect of oxide concentration on the QTI was investigated in detail for the two alloys studied. The results show that the relationship between the amount of oxides and the number of ingot steps is exponential, with a fitness coefficient of the order of 0.87.