Influence of calcium on the microstructure and properties of an Al-7Si-0.3Mg- x Fe alloy
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-SI castings are widely used in engineering applications due to their excellent combination of properties such as good castability, low coefficient of thermal expansion, high strength-to-weight ratio, good corrosion resistance, and minimum energy requirement for recycling. Recently, sustainability and recycling of resources is becoming increasingly important with increasing public awareness of environmental issues, energy, and depleting natural resources. Recycling of aluminum brings potential energy savings of up to 95 pct and generates 99 pct less emission compared to primary aluminum production.[1] Therefore, secondary aluminum alloys are receiving widespread acceptance worldwide. Furthermore, increased use of recycled aluminum casting alloys requires strict microstructural and compositional monitoring to control any deleterious effects arising from impurity elements in the recycled metal.[2] Iron is the most common deleterious impurity in aluminum alloys in general, and in recycled Al-7Si-0.3Mg alloys in particular, since it forms different intermetallic compounds with Al, Si, and other elements present in the alloy.[3] Of the various intermetallic phases, the presence of b phase is considered to be particularly detrimental to the ductility and fracture toughness of the alloy. The platelet and needlelike morphologies of the b-Al5FeSi phase make it acts as a stress raiser in the microstructure, thereby reducing the mechanical properties of the alloy. Among the various techniques available to minimize the detrimental effect of iron, namely, rapid cooling, melt superheat treatment, addition of trace elements, and low-temperature working, the addition of trace elements is the most commonly used method.[4,5] Neutralization of the negative effects of iron by addition of Be, Mn, and Cr individually or in combination has been S.S. SREEJA KUMARI, Research Associate, R.M. PILLAI, Senior Deputy Director and Head MMD, and B.C. PAI, Senior Deputy Director and Head, RPBD, are with the Regional Research Laboratory (CSIR)Trivandrum, Materials and Minerals Division, Industrial Estate P.O., Pappanamcode, Kerala 695 019, India. Contact e-mail: rmpillai_rrl@yahoo. com. K. NOGITA, Senior Research Fellow, and A.K. DAHLE, Associate Professor, are with the Division of Materials Engineering, The University of Queensland, Brisbane Qld 4072, Australia. Manuscript submitted November 21, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
found to be the most effective means of improving the ductility and tensile strength of Al-7Si-0.3Mg alloys.[6] However, the addition of Mn results in other problems such as hot tearing and damage of melt quality and furnace linings due to the crystallization of a coarse Al15(FeMn)3Si2 compound, usually termed ‘‘sludge’’ or ‘‘fallout.’’ The Mn and Cr also do not reduce the negative effect of Fe on fracture toughness. Furthermore, Be is carcinogenic and companies are therefore reluctant to use even small amounts of Be.[7,8,9] On the other hand, there is considerable evidence that Ca modifies and refines the detri
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