Feeding and Distribution of Porosity in Cast Al-Si Alloys as Function of Alloy Composition and Modification
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are commonly used for the manufacture of a wide range of commercial products. They are generally characterized by good castability; i.e., they flow well in the mold and reproduce the mold geometry and detail well with minimal casting defects. With the use of properly designed gating and feeding systems, it is relatively easy to make sound castings that have good mechanical properties.[1] As such, Al-Si alloys with Si content ranging from 5 to 13 pct have been popular for many decades. All of these alloys form an Al-Si eutectic at the end of solidification, the amount depending on the Si content. At 2 pct Si (lower than typical foundry levels), the alloy contains almost entirely a-Al phase, whilst at 12 to 13 pct Si, the alloys are almost completely Al-Si eutectic.[2] The amount, grain size, and morphology of the primary a-Al phase have been shown to be important to the castability of alloys.[1,3–5] In addition, the role of eutectic modification, including the influence on eutectic
NIELS SKAT TIEDJE, Associate Professor, is with the Department of Mechanical Engineering, Technical University of Denmark (DTU), DK-2800 Kgs. Lyngby, Denmark. Contact e-mail: nsti@mek. dtu.dk JOHN A. TAYLOR, Associate Professor, is with the CAST Co-operative Research Center, School of Mechanical & Mining Engineering, The University of Queensland, Brisbane 4072, Australia. MARK A. EASTON, Program Manager, is with the CAST Co-operative Research Centre, Department of Materials Engineering, Monash University, Melbourne 3800, Australia. Manuscript submitted October 7, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
cell* size and morphology, has also been shown to have *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 a-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.
a very significant effect on the presence of casting defects.[6–9] The Al-Si eutectic is relatively coarse with highly faceted Si particles in untreated (unmodified) alloys. Therefore, it is common in the industry to add chemical modifiers, typically Na or Sr, in small amounts to Al-Si alloys to refine the size and change the shape of the Si particles, which has been shown to improve the mechanical properties, especially the ductility of the alloys.[1,10] Modification increases ductility from approximately 1 pct to up to 10 pct in well-treated alloys cast under the best possible conditions.[1] Other elements such as Sb and Bi are also known to be modifiers; however, they are not readily available in forms that are useful to the industry.[10–12] Sodium is an excellent modifier that has been used for decades in the industry. Recent research has shown that it acts to reduce the number of nuclei a
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