Spurious Grain Formation at Cross-Sectional Expansion During Directional Solidification: Influence of Thermosolutal Convection
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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 solute-rich 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.
Keywordsdirectional solidification modeling and simulation thermosolutal convection grains formation dendrite fragmentation
This work was supported by NASA Grants NX10AV40G and NNX14AM18G. The Al-19% Cu and Al-7% Si alloys for this study were kindly provided by Dr. Men G. Chu at ALCOA Technical Center.
- 23.M.A. Lauer, Modeling Macrosegregation in Directionally Solidified Aluminum Alloys, Ph.D. dissertation, The University of Arizona, Tucson, AZ, 2015Google Scholar