Many commercial Al-Si foundry alloys contain silicon of near-eutectic composition for good castability. Cu, Mg and Fe are common alloying elements found in Al-Si alloys. The Al-Si eutectic phase represents the major part of the alloy’s volume and differences in eutectic nucleation and growth mechanisms significantly influence the formation of casting defects and their distribution.
The purpose of this research work is to investigate the influence of alloying elements (Cu, Mg, Ni and Fe) on eutectic nucleation, grain morphology and final microstructure. Thermal analysis, quenching and in-situ X-ray tomography experiments were performed with commercial purity Al-10Si alloy with increasing additions of Cu, Mg, Ni and Fe in unmodified and Sr-modified conditions.
In the context of the present work the ternary alloying elements when added to the Al-Si alloy in sufficient quantity, formed intermetallics by reacting with either Al or Si or both. It was identified that in cases where an intermetallic nucleated prior to the onset of the Al-Si eutectic reaction, the eutectic nucleation frequency was affected by changes to either the available nuclei population or the silicon concentration. The nucleation mechanism of these intermetallics was studied in order to understand their influence on Al-Si eutectic nucleation and growth.
In cases where the intermetallic nucleated after the Al-Si eutectic, segregation of the ternary solutes in front of the Al-Si eutectic interface changed the nucleation and macroscopic growth dynamics considerably. At low solute additions, the interface remained stable with slight destabilisation occurring in the final stages of eutectic solidification and near-spherical eutectic grains were observed. As the solute concentration increased destabilisation of the eutectic interface occurred earlier during solidification. As the solute concentration increased further the grain shape became more complex with the appearance of primary and secondary arms.
In addition, the present work identified that changes in nucleation and growth dynamics of Al-Si eutectic due to the presence of solute, influenced the eutectic silicon morphology considerably. In cases where the eutectic nucleation frequency was sufficiently reduced, a change in the eutectic morphology from flake-like to a mixture of flake-like and fibrous
morphologies was observed irrespective of the amount of Sr present in the alloy. When the eutectic nucleation frequency increased, a fibrous to flake-like transition within the eutectic grain was observed. A constitutional undercooling model was introduced in order to explain these changes in eutectic nucleation, interface stability and microstructure.