Abstract
The nature of enhanced pore formation in Sr-modified A356 Al alloy melts has been investigated, leading to the observation of the precipitation of Sr-rich compounds on the wetted side of oxide films. Scanning electron microscopy of casting surfaces showed that the surface oxide film might be a good possible nucleation site of Sr-rich compounds. The observation of internal pores using secondary electron microscopy and X-ray microanalysis mapping showed precipitation of Sr-rich compounds on the wetted side of the oxide films in most pores. The possibility of Sr-rich intermetallics being nucleated on oxide films was assessed by determining their planar disregistry factor; good matching planes were found between Al2Si2Sr intermetallics and γ-Al2O3 and spinel oxide films. The crystallographic alignment revealed in scanning electron microscope (SEM) images also corroborates an epitaxial relationship. Mechanisms by which Sr in Al-Si alloys may contribute to the formation of porosity are discussed.
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