Spurious Grain Formation at Cross-Sectional Expansion During Directional Solidification: Influence of Thermosolutal Conv
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Spurious Grain Formation at Cross-Sectional Expansion During Directional Solidification: Influence of Thermosolutal Convection M. Ghods
, M. Lauer, S.R. Upadhyay, R.N. Grugel, S.N. Tewari, and D.R. Poirier
(Submitted December 3, 2017; in revised form February 16, 2018) 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. Keywords
directional solidification, modeling and simulation, thermosolutal convection, grains formation, dendrite fragmentation
1. Introduction Single-crystal superalloy gas turbine blades, having varying cross-sections along their length, are manufactured by directional solidification in investment casting ceramic shell molds. Alignment of [100]-oriented primary dendrites along the entire length of these blades is critical to maximize their life at elevated temperatures and high stresses. The presence of any misaligned (spurious) grains is highly detrimental and leads to an early failure. Spurious grain formation in directionally solidified blades is often associated with solidification through an area expansion (Ref 1). Ideally, when the mushy-zone growing in the smaller area portion of the mold enters the section increase, the secondary arms of the peripheral dendrites in the mushy-zone grow and spread sideways on the above platform, which then generate tertiary arms. Some of these tertiary arms ultimately become aligned primary dendrites that then fill the larger cross-sectional portion of the mold. Here the melt can locally undercool which can result in the nucleation of new stray grains. This mechanism has been successfully used to
M. Ghods, Chemical Engineering Program, Middle East Technical University - Northern Cyprus Campus, Gu¨zelyurt via Mersin 10, Turkey; M. Lauer, Department of Materials Science and Engineering, The University of Arizona, Tucson, AZ 85721, USA; and ME Elecmetal Inc., Duluth, MN 55808, USA; S. R. Upadhyay and S. N. Tewari, Chemical and Biomedical Engineering Department, Cleveland State University, Cleveland, OH 44114, USA; R. N. Grugel, NASA-Marshall Space Flight Center, Huntsville, AL 35811, USA; and D. R.
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