Influence of grain refinement on the columnar-to-equiaxed transition in binary Al alloys

In the solidification of metallic alloys, columnar or equiaxed growth may occur depending on, among others, the experimental conditions, the thermophysical properties of the alloy and the presence of grain refining particles. In transient processes, typical for many casting situations, a columnar-to-equiaxed transition (CET) is often observed. We have investigated the CET experimentally in AlSi7 alloys with and without inoculants within the framework of the ESA microgravity application promotion programme (MAP) columnar-to-equiaxed transition in solidification processing (CETSOL). The effect of different cooling rates on the CET was studied in addition to the effect of grain refinement in directional solidification. From measured temperature profiles in the samples during solidification and from the analysis of longitudinal and transversal sections of the samples, critical parameters at the CET were determined and compared to predictions from numerical models following Martorano et al. and from Hunt. It was found that the results are in good agreement with the former for non-refined AlSi7 alloys and that grain refinement shifts the critical parameters significantly, concerning critical gradient and nucleation undercooling.