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Macrosegregation Due to Convection in Al-19Cu Alloy Directionally Solidified Through an Abrupt Expansion in Cross-Section: A Comparison with Al-7Si

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Abstract

Hypoeutectic Al-19 wt.% Cu alloys were directionally solidified at two different growth speeds in cylindrical molds that featured an abrupt increase in cross-section, from 3.2 to 9.5 mm in diameter. The effects of thermosolutal convection and shrinkage flow induced by the cross-section change on macrosegregation were investigated. Dendrite clustering and extensive radial macrosegregation were seen, particularly in the larger cross-section after expansion. Negative longitudinal macrosegregation right after the cross-section increase was observed; the extent of macrosegregation, however, decreases with increasing growth speed. Both thermal and flow effects due to cross-section change were seen to influence the radial macrosegregation immediately before, and after the expansion. Radial macrosegregation pattern was found to be changing as the mushy zone enters the larger cross-section region above the cross-section change where the solidification is in its unsteady state. The effect of the solutal expansion coefficient on macrosegregation was studied by comparing the degree of thermosolutal convection in Al-19 wt.% Cu with a previous study in which we investigated Al-7 wt.% Si. A two-dimensional model accounting for both shrinkage and thermosolutal convection was used to simulate the resulting steepling, as well as the axial and radial macrosegregation. The experimentally observed macrosegregation associated with the expansion during directional solidification is well predicted by the numerical simulations.

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Notes

  1. In this article, “wt.%” is dropped, so Al-4.5 wt.% Cu is simply designated as Al-4.5Cu and so on.

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Acknowledgments

This work was supported by NASA Grants NX10AV40G and NNX14AM18G. The Al-19% Cu and Al-7% Si alloys for this research were kindly provided by Dr. Men G. Chu at ALCOA Technical Center. M. Lauer would like to acknowledge support from the Sandia National Laboratories Campus Executive Fellowship program.

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Author notes

  1. M. Lauer

    Present address: ME Elecmetal Inc., Duluth, MN, 55808, USA

Authors and Affiliations

  1. Chemical and Biomedical Engineering Department, Cleveland State University, Cleveland, OH, 44114, USA

    M. Ghods & S. N. Tewari

  2. Department of Materials Science and Engineering, The University of Arizona, Tucson, AZ, 85721, USA

    M. Lauer & D. R. Poirier

  3. NASA-Marshall Space Flight Center, Huntsville, AL, 35812, USA

    R. N. Grugel

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  1. M. Ghods
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Correspondence to M. Ghods.

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Ghods, M., Lauer, M., Grugel, R.N. et al. Macrosegregation Due to Convection in Al-19Cu Alloy Directionally Solidified Through an Abrupt Expansion in Cross-Section: A Comparison with Al-7Si. J. of Materi Eng and Perform 26, 4876–4889 (2017). https://doi.org/10.1007/s11665-017-2925-y

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  • DOI: https://doi.org/10.1007/s11665-017-2925-y

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