The morphology and distribution of primary dendrites have been examined in Pb-2.2, 5.8 and 10.8 wt. pct. Sb alloy samples directionally solidified (DSed) in ampoules shaped like an hour-glass to examine the influence of cross-section change induced advective flow on the cellular/dendritic interface. This sample design increases the advective flow of the melt towards the array tips, as the liquid-solid interface enters the neck of the ampoule, and then decreases it as the interface exits the neck. The warm solute-rich melt flowing towards the growth front suppresses the extent of side-branching, decreases the primary dendrite spacing, and increases the primary dendrite trunk diameter as observed in the Pb-5.8 and 10.8 Sb alloys. The flow appears to suppress the formation of cells. A cellular interface growing in the Pb-2.2Sb alloy became planar as the solidification front entered the neck, becoming cellular again as it exited the neck.
Under a NASA (National Aeronautics and Space Agency)-ESA (European Space Agency) collaborative research project, MICAST (Microstructure formation in casting of technical alloys under a diffusive and magnetically controlled convection conditions), three Al-7wt% Si samples (MICAST-6, MICAST-7 and MICAST2-12) were directionally solidified at growth speeds varying from 10 to 50 µm s-1 aboard the International Space Station to determine the effect of mitigating convection on the primary dendrite array. The observed primary dendrite trunk diameters during steady-state growth of MICAST samples show a good agreement with predictions from a coarsening based model developed by the authors. The trunk diameters in the terrestrial-grown equivalent samples were larger than those predicted from the model. This suggest that thermosolutal convection increases the trunk diameter of primary dendrites, perhaps by increasing their tip radius due to compositional changes.
Formation of spurious grains during directional solidification (DS) of Al-7 wt.% Si and Al-19 wt.% Cu alloys through an abrupt increase in cross-sectional area has been examined by experiments and by numerical simulations. Stray grains were observed in the Al-19 wt.% Cu samples and almost none in the Al-7 wt.% Si. The locations of the stray grains correlate well where numerical solutions indicate the soluterich melt to be flowing up the thermal gradient faster than the isotherm velocity. It is proposed that the spurious grain formation occurred by fragmentation of slender tertiary dendrite arms was enhanced by thermosolutal convection. In Al-7 wt.% Si, the dendrite fragments sink in the surrounding melt and get trapped in the dendritic array growing around them, and therefore they do not grow further. In the Al-19 wt.% Cu alloy, on the other hand, the dendrite fragments float in the surrounding melt and some find conducive thermal conditions for further growth and become stray grains.
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