Abstract
Abstract The presence of Fe aids in establishing the mechanical and physical properties of AlSi alloys and is also one of the main impurities leading to formation of β-Al5FeSi intermetallics. This study aims to understand the effect of fluid flow on the dendritic microstructure with intermetallics in Al-5/7/9 wt pct Si-0.2/0.5/1.0 wt pct Fe alloys that are directionally solidified under defined thermal and fluid flow conditions. We made extensive use of 3D X-ray tomography to obtain a better insight into the morphology and formation of the intermetallics. Three-dimensional (3-D) distribution of intermetallics presented here shows that the growth of large β-Al5FeSi due to forced flow occurs in the eutectic specimen center and together with an increase in the number density of β precipitates. The 3D reconstructions have verified the β shaped to be curved, bent with twining, branched, and to have imprints, holes, and propeller-shaped platelets. The 3D views showed that hole-shaped β arose from the lateral growth around α-Al dendrites. These views also confirmed the phenomenon of shortening of β as an effect of flow in the dendritic region, where β could be fragmented or completely remelted, and ultimately resulting in microstructures with shorter β-Al5FeSi and increases in number density. The analysis revealed an interaction between melt flow, 3D distribution, and the morphology of β-Al5FeSi. The growth of a large and complex group of β intermetallics can reduce the melt flow between dendrites and strengthen pore nucleation and eutectic colonies nucleation, leading to lower permeability of the mushy zone and increased porosity in the castings.
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