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Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification

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Abstract

Unmodified, Na-modified, and Sr-modified castings of Al-7 pct Si and Al-12.5 pct Si alloys were cast in molds in which it was possible to create different cooling conditions. It is shown how solidification influences the distribution of porosity at the surface and the center of the castings as a function of modification and Si content in sand- and chill-cast samples. Eutectic modification, Si content, and cooling conditions have a great impact on the distribution of porosity. Unmodified and Na-modified castings are more easily fed with porosity tending to congregate near the centerline of the casting, while Sr-modified castings solidify in a mushy manner that creates a more homogeneous distribution of porosity in the casting. The amount of porosity was highest in the Sr-modified alloys, lower in the Na-modified alloys, and lowest in the unmodified alloys. The size of the porosity-free layer and the effectiveness of the feeders were greater in the castings made with the steel chills due to the increased thermal gradients and consequent increase in the directionality of solidification.

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Notes

  1. To avoid confusion, in this article, the term “grain” (grain size, grain shape/morphology, etc.) will be used exclusively to describe a discrete entity of primary α-Al phase, i.e., dendritic or rosette-like, equiaxed or columnar in form, or formed from a unique nucleation event. The term “cell” (cell size, cell shape/morphology, etc.) will be exclusively used to describe a discrete entity of Al-Si eutectic, i.e., the related Al and Si components, formed from a unique eutectic nucleation event.

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Acknowledgments

CAST Cooperative Research Centre (CAST CRC) is established under and is funded in part by the Australian Federal Government’s Cooperative Research Centers Scheme. Niels Skat Tiedje gratefully acknowledges the support of CAST CRC during his period of research work at the University of Queensland. The project was also partly funded by the Danish Foundry Organisation, Otto Mønsted’s Foundation, and Julie Damm’s Foundation.

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Authors and Affiliations

  1. Department of Mechanical Engineering, Technical University of Denmark (DTU), DK-2800, Kgs. Lyngby, Denmark

    Niels Skat Tiedje (Associate Professor)

  2. CAST Co-operative Research Center, School of Mechanical & Mining Engineering, The University of Queensland, Brisbane, 4072, Australia

    John A. Taylor (Associate Professor)

  3. Department of Materials Engineering, CAST Co-operative Research Centre, Monash University, Melbourne, 3800, Australia

    Mark A. Easton (Program Manager)

Authors
  1. Niels Skat Tiedje
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  2. John A. Taylor
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  3. Mark A. Easton
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Correspondence to Niels Skat Tiedje.

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Manuscript submitted October 7, 2011.

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Tiedje, N.S., Taylor, J.A. & Easton, M.A. Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification. Metall Mater Trans A 43, 4846–4858 (2012). https://doi.org/10.1007/s11661-012-1308-0

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