Role of iron in relation to silicon modification in Sr-treated 319 and 356 alloys

Abstract

The low specific gravity, excellent castability and good corrosion resistance of aluminium—silicon casting alloys makes them important materials for use in automotive applications. The mechanical properties of these alloys are controlled by their chemical composition as well as the quality of their microstructure. To this end, ‘modifiers’ such as sodium, strontium, etc. are normally added to the alloys, whereby large improvements in properties (mainly ductility) are observed, as a result of the ‘modification’ or change in morphology of the Si phase of the Al-Si eutectic from its normally brittle, acicular form to a fibrous form, accompanied by a reduction in the Si particle size. Another important microstructural consideration is control of the iron content in order to minimise the formation of the brittle β-Al5FeSi phase which, owing to its plate-like nature, is also deleterious to the alloy properties. In view of the fact that Sr modification is normally carried out in Al-Si alloys, and the Fe levels are usually minimised, the present work was carried out determine to what extent the presence of Fe would affect the Si modification in such alloys. Alloys 319 and 356 were selected for study, where the Fe levels in the two alloys were varied from 0·2 to 0·8 and from 0·12 to 0·6 wt-%, respectively, to cover the minimum and maximum limits specified by industry. The effects of Fe and cooling rate (using directional and conventional solidification) on the Si particle characteristics in the unmodified and Sr-modified alloys were studied, using optical and scanning electron microscopy, and quantified using image analysis. Temperature-time parameters of reactions taking place during solidification were determined using thermal analysis. The role of Al2Si2Sr precipitation in the Sr-modified alloys is discussed, and a mechanism for the effect of Fe on Si modification in such alloys is also presented. IJCMR/442