Macrosegregation in Al–7Si Alloy Caused by Abrupt Cross-section Change During Directional Solidification

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Journal of Crystal Growth


Hypoeutectic Al-7 wt .% Si alloys were directionally solidified vertically downward in cylindrical molds that incorporated an abrupt cross-section decrease (9.5 mm to 3.2 mm diameter) which, after 5 cm, reverted back to 9.5 mm diameter in a Bridgman furnace; two constant growth speeds and thermal gradients were investigated. Thermosolutal convection and cross-section-change-induced shrinkage flow effects on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation was seen, particularly in the larger cross-sections, before contraction and after expansion, this more evident at the lower growth speed. This alloy shows positive longitudinal macrosegregation near cross-section decrease followed by negative macrosegregation right after it; the extent of macrosegregation, however, decreases with increasing growth speed. Primary dendrite steepling intensified as solidification proceeded into the narrower section and negative longitudinal macrosegregation was seen on the re-entrant shelves at expansion. A two-dimensional model accounting for both shrinkage and thermo-solutal convection was used to simulate solidification and the resulting mushy-zone steepling and macrosegregation. The experimentally observed longitudinal and radial macrosegregation associated with the cross-section changes during directional solidification of an Al–7Si alloy is well captured by the numerical simulations.


This work was supported by NASA Grant NX10AV40G and NNX14AM18G. M. Lauer would like to acknowledge support from the Sandia National Laboratories Campus Executive Fellowship program.

Original Citation

Ghods M, Johnson L, Lauer M, Grugel RN, Tewari SN, Poirier DR. Macrosegregation in Al–7Si alloy caused by abrupt cross-section change during directional solidification. J Cryst Growth. 2016;449:134-147.