Influence of oxides on porosity formation in Sr-treated Al-Si casting alloys

The strength and quality of an Al-Si alloy casting are determined by its microstructure and the amount of porosity present in the casting. Modification is one of the processes used to improve the microstructural quality, where the addition of a modifying agent alters the shape of the eutectic Si from an acicular to a fibrous form that is extremely beneficial to the mechanical properties. Among various modifiers, strontium has been used extensively, as it is easier to handle and more resistant to fading. However, its addition is also associated with porosity formation in these alloys. The porosity formed in Sr-modified castings has been variously related to an increase in the hydrogen level of the melt, feedability problems in the mushy zone during solidification, and changes in the mode of eutectic nucleation—from near the -Al dendrites in the Sr-free alloy, to within the eutectic liquid itself in the Sr-containing alloy. The present study was carried out to determine the influence of oxides on the porosity characteristics observed in Al-Si alloys containing strontium. A series of experimental and industrial alloys viz., Al-7%Si, Al-12%Si, 319 and 356 were selected, to cover a variety of alloy freezing ranges. The techniques of thermal analysis, optical microscopy, and SEM/EDX and EPMA analyses were employed to obtain the results presented here. It is seen how the presence of oxides (Sr and Al) is responsible for the porosity formation observed in Al-Si alloys, and that the difference in porosity characteristics with the addition of Sr depends on the amount of Sr oxides present in the solidified structure. The presence of aluminum oxide films leads to the formation of large pores that are often linked together. Both aluminum and strontium oxides are favorable sites for the nucleation of other microconstituents.