Thermosolutal convective effects on dendritic array spacings in downward transient directional solidification of Al-Si alloys

Solidification thermal variables, such as tip growth rate, tip cooling rate, local solidification time and dendrite arm spacings, have been measured in Al-Si hypoeutectic alloys directionally solidified under downward transient heat flow conditions. It is a well-known fact that the dendritic spacings can affect not only microsegregation profiles but also the formation of secondary phases within interdendritic regions, which influences mechanical properties of cast structures. A reduced number of studies have been carried out in order to analyze the effects of melt convection within the interdendritic region or to verify the influence of growth direction on dendritic arm spacings. In this work, an experimental approach is used to quantitatively determine the solidification thermal variables. The work also focuses on the dependence of dendrite arm spacings on these solidification thermal variables. The experimental data concerning the solidification of Al 5, 7 and 9 wt.% Si alloys are compared with the main predictive dendritic models from the literature. A comparison between upward and downward unsteady-state results for dendritic spacings has also been conducted.