Solidification and Microstructural Evolution of Hypereutectic Al-15Si-4Cu-Mg Alloys with High Magnesium Contents
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HYPEREUTECTIC Al-Si alloys are widely used in automotive and aerospace applications because of their low coefficient of thermal expansion, good wear resistance, and high hardness.[1–3] Alloys such as A390, containing 17 pct Si, 4.5 pct Cu, and 0.5 pct Mg, are widely used in the manufacture of pistons, cylinder blocks, and AC compressors[3] (all alloy compositions are given in weight percent (wt pct) unless otherwise stated). The presence of primary silicon in the matrix provides good wear resistance and high hardness. In this respect, hypereutectic Al-Si alloys have often been considered as in situ metal matrix composites (MMCs), with the primary Si acting as the reinforcement.[4–6] However, the size and the morphology of the Si phase can affect the mechanical properties of the alloy. The addition of a grain refiner and optimization of the cooling rate have been proposed to reduce the size of silicon particles and thus improve the mechanical properties of these alloys.[7] M. TEBIB, Ph.D. Candidate, A.M. SAMUEL, and X.-G. CHEN, Professors, are with the Department of Applied Science, University of Que´bec at Chicoutimi, 555, Boulevard de l’Universite´, Saguenay, QC G7H 2B1, Canada. Contact e-mail: [email protected] F. AJERSCH, Professor, is with the E´cole Polytechnique de Montre´al, Montreal, QC H3T 1J4, Canada. Manuscript submitted December 6, 2012. Article published online April 30, 2013 4282—VOLUME 44A, SEPTEMBER 2013
Ba¨ckerud et al.[8] studied the solidification of A390 alloy. The solidification reactions reported by them are listed in Table I. Solidification begins with the precipitation of primary silicon. The second phase precipitated is a small volume fraction of dendritic aluminum phase, followed by the eutectic reaction wherein Al, Si, and b-Al5FeSi phases are formed. Precipitations of Mg2Si and h-Al2Cu phases take place in later reactions, as shown in Table I. Hekmat-Ardakan et al.[9,10] and Mandal and Makhlouf[11] observed that Mg addition changed the morphology and size of the primary silicon and the eutectic phase in hypereutectic A390 alloy, and led to the formation of both eutectic and primary Mg2Si phase particles. Although the tensile strength of these alloys was superior to that of A390 alloy, they exhibited a lower hardness. Zeng et al.[12] examined the effects of both Si and Mg contents in Al-Si-Mg alloys, for Si and Mg levels ranging from 8 to 17 pct and from 2 to 4 pct, respectively. The Mg addition ranging from 2 to 4 pct changed the brittle b-Al5FeSi phase to the p-Al8Si6Mg3Fe phase, and improved the castability and mechanical properties at high Mg content. Lin et al.[13] investigated the effects of Si and Mg contents on the microstructures of hypereutectic Al-Si-Mg alloys. They reported that primary Mg2Si and primary Si are formed in the hypereutectic Al-Si alloy when the contents of Si and Mg are equal to, or more than, 19 and 4 pct, respectively. It was also found that increasing METALLURGICAL AND MATERIALS TRANSACTIONS A
the amounts of Si and Mg increases the size of primary Si and prim
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