Effect of cooling rate and Sr-modification on porosity and Fe-intermetallics formation in Al-6.5% Si-3.5% Cu-Fe alloys

Abstract

Together with porosity, the problems created by the formation of iron intermetallics in Al-Si alloys pose serious concerns in regard to their use in structural components. The presence of these intermetallics, particularly the brittle β-Al₅FeSi phase, tends to reduce the alloy properties and casting quality. In order to optimise the iron intermetallics in Al-6.5% Si-3.5% Cu-Fe alloys, the effects of cooling rate, Sr modification, other melt additions, and heat treatment were studied, with a view to determining the means of their formation and dissolution, in relation to their effect on porosity formation and alloy properties. The iron levels selected for study were 0.5%, 1.0% and 1.5%, covering the range of levels normally observed in foundry alloys. Castings were prepared using a cooling fin sand mould (DAS range: 25–85 μm) for microstructural analysis, and a Stahl permanent mould for tensile testing purposes. Samples of molten metal quenched in cold water provided much smaller DASs. Microstructural characterisation was carried out using optical microscopy and thermal, image and electron microprobe analyses. Tensile properties (T6 condition) were determined at room temperature and a strain rate of 4 × 10⁻⁴/s using an Instron Universal testing machine. The results show that at any Fe level, the average β-Al₅FeSi platelet length is strongly determined by the DAS. Strontium (250–350 ppm) is effective in reducing the β-platelet size through its fragmentation/dissolution effect on the latter. At higher Sr concentrations, both coarsening of the β-platelets (i.e., “overmodification”) and their decomposition (through Si rejection ahead of the platelets) occur. Addition of P increases β-phase formation (due to the presence of AIP particles that act as possible nucleation sites). Melt superheating (900 °C) thereafter reduces the effect. At Fe levels of 1% or more, α-Chinese script or sludge particles form only at slow cooling rates or with superheating. Tensile properties of alloys containing up to 1 % Fe are improved with Mn, Mn + Cr, Sr or Be additions. Best overall properties are achieved with Mn at Mn/Fe = 0.7. Also, Sr additions of 200–300 ppm are best, higher levels giving rise to increased porosity levels and, hence, lower properties. For a given Fe level, the optimum Sr concentration depends on the cooling rate and Si content of the alloy. Iron levels of 1.5% are not recommended for Al-6.5% Si-3.5% Cu (319.2) alloys as the properties cannot be improved by any of the above means.