Effects of Individual and Combined Additions of Pb, Bi, and Sn on the Microstructure and Mechanical Properties of Al-10.
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ALUMINUM alloys are widely used in the automotive industry nowadays, since recent trends incline toward achieving higher performance without increasing weight. Thus, automotive components are increasingly being made of aluminum alloys in order to reduce overall weight, while at the same time maintaining or improving mechanical properties. Apart from their excellent casting characteristics, as well as their wear and corrosion resistance, aluminum-silicon (Al-Si) casting alloys are used so extensively because they also impart a wide range of mechanical properties and high strength to weight ratio.[1–3] Near-eutectic Al-Si casting alloys have increasingly attracted attention because of their excellent castability and lower cost compared with A356 alloys.[3–5] The production of near-eutectic Al-Si alloys with improved quality (better structure and mechanical properties) involves the application of two major processes: (1) addition of alloying elements during the melting stage and melt treatment (grain refining and modification) or (2) heat treatment.[4,5] The addition of certain elements, such as calcium (Ca), antimony (Sb), sodium (Na), and strontium (Sr), to near-eutectic Al-Si alloys has the effect of altering or modifying the A.M.A. MOHAMED, Doctor, and F.H. SAMUEL and A.M. SAMUEL, Professors, are with De´partement des sciences applique´s, Universite´ du Que´bec a Chicoutimi, 555, boulevard de l’Universite´, Chicoutimi, PQ, Canada G7H 2B1. Contact e-mail: [email protected] H.W. DOTY, Doctor, is with GM Powertrain Group, Metal Casting Technology, Inc., Milford, NH 03055. Manuscript submitted May 2, 2008. Article published online November 12, 2008 240—VOLUME 40A, JANUARY 2009
morphology of eutectic silicon from an acicular platelike form to a fibrous form. This change in Si morphology enhances the mechanical properties of the alloy, in particular, its ductility.[6] In recent years, the use of Sr as a modifier, instead of Na and Sb, has become a widely accepted practice, although Sr has been reported to have an incubation-time problem.[5,7,8] However, it is worth noting that at the same time as eutectic silicon particles change from acicular to fibrous, the amount, morphology, and size of the dendritic a-Al phase are also affected. It was thought in error that it was enough to only modify the eutectic Si phase in near-eutectic Al-Si alloys, and not necessary to refine the dendritic a-Al phase; but in fact, at the same time that the morphology and size of the eutectic silicon phase is transformed by the modification treatment, considerable changes to the amount, shape, and size of the a-Al phase also occur.[9] Previous investigations suggest that the addition of Sr to hypoeutectic and near-eutectic Al-Si alloys may promote the columnar growth of the dendrites, which are present in the form of fine slender highly-branched columnar grains; addition of Sr may also result in a noticeable increase in the amount of the dendritic a-Al phase.[2,6] Thus, with regard to such alloys dendrite refinement is a necessary step. The master all
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