Abstract

EL-SAYED, M.A.  and  GHAZY, M.. Entrained defects in light metal cast alloys. J. S. Afr. Inst. Min. Metall. [online]. 2017, vol.117, n.7, pp.656-662. ISSN 2411-9717.  http://dx.doi.org/10.17159/2411-9717/2017/v117n7a7.

The properties of light alloy castings are strongly affected by their inclusion content, particularly double oxide film defects (bifilms), which not only decrease the tensile and fatigue properties, but also increase their scatter. Recent research has suggested that oxide film defects may alter with time, as the air inside the bifilm would react with the surrounding melt. In this work, the effect of time on double oxide film defects is studied for different Al alloys. The results suggest that bifilm defects, once entrained, experience changes in their internal atmospheres which significantly affect their morphology and their influence on the alloy's mechanical properties. These changes involve the consumption of both oxygen and nitrogen inside the defect (with the former occurring first), which enhances the mechanical properties, but this is followed by hydrogen diffusion into the bifilms with a corresponding adverse effect on the properties.

Keywords : double oxide film defects; aluminium; casting; hydrogen.

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