Macrosegregation in Al–7Si Alloy Caused by Abrupt Cross-section Change During Directional Solidification
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.
Ghods, M.; Johnson, L.; Lauer, M.; Grugel, R. N.; Tewari, Surendra N.; and Poirier, D. R., "Macrosegregation in Al–7Si Alloy Caused by Abrupt Cross-section Change During Directional Solidification" (2016). Chemical & Biomedical Engineering Faculty Publications. 127.
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.
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.