Macrosegregation in Al-7Si alloy caused by abrupt cross-section change during directional solidification

M. Ghods, L. Johnson, M. Lauer, R. N. Grugel, S. N. Tewari, David R Poirier

Research output: Contribution to journalArticle

9 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)134-147
Number of pages14
JournalJournal of Crystal Growth
Volume449
DOIs
StatePublished - Sep 1 2016

Keywords

  • A1. Computer simulation
  • A1. Cross-section change
  • A1. Directional solidification
  • A1. Fluid flows
  • A1. Segregation
  • B1. Aluminum alloys

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Materials Chemistry
  • Inorganic Chemistry

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