Effect of Mg and Sr additions on the formation of intermetallics in Al-6 Wt pct Si-3.5 Wt pct Cu-(0.45) to (0.8) Wt pct
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I.
INTRODUCTION
THE fact that they possess the essential characteristics of good castability, corrosion resistance, and machinability makes Al-Si foundry alloys excellent candidates for automotive applications. The silicon content in these alloys varies from 5 to 20 pct, with the eutectic occurring around 12 pct Si. Prominent among such alloys is the 319 alloy, with a silicon content of 6.0 to 6.5 pct, copper (Cu) in the amount of 3.5 pct, and magnesium (Mg) often being added for strengthening purposes. The mechanical properties are controlled by the cast structure, i.e., by the microstructural constituents that precipitate upon solidification, depending upon the composition and solidification conditions of the alloy.[1] These include, in addition to the a-aluminum dendrites, the main Al-Si eutectic (where the eutectic Si particles are generally present as brittle, acicular platelets in the as-cast, nonmodified condition), Fe intermetallics (commonly, the aAl15(Fe,Mn)3Si2 and b-Al5FeSi phases), secondary eutectic phases such as Mg2Si and Al2Cu, and other complex intermetallics that result from the remaining liquid during the last stages of solidification.[2,3] To enhance the alloy properties, modification of the euF.H. SAMUEL, Professor, and A.M. SAMUEL, Research Associate, are with the Department of Applied Sciences, University of Quebec at Chicoutimi, Chicoutimi PQ, Canada G7H 2B1. P. OUELLET, formerly Graduate Student, Department of Applied Sciences, University of Quebec at Chicoutimi, is Engineer with Montupet Lte´e, Rivie`re-Beaudette, PQ, Canada J0P 1R0. H.W. DOTY, Materials Engineer, is with General Motors Corporation, GM Powertrain/MCT, Milford, NH 03055. Manuscript submitted April 28, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
tectic silicon is usually carried out using strontium (Sr), to convert the silicon morphology from an acicular to a fibrous form. Among the intermetallics appearing in 319-type alloys, the b-Al5FeSi platelet/needlelike iron intermetallic is considered the most critical, as it significantly reduces the alloy ductility and fracture toughness, and different measures are adopted to neutralize its harmful effect.[4] In comparison, the a-iron Al15(Fe,Mn)3Si2 phase, due to its compact morphology (Chinese script–like in appearance), is less harmful to the mechanical properties. However, a coarser variation of the a phase, termed sludge, can also appear, in the form of polyhedral or starlike shaped particles,[5] where their hard, brittle nature can cause machining problems and tool wear. The addition of Sr to the 1XXX and 6XXX series of wrought aluminum alloys has been reported to transform a major proportion of the b-Al5FeSi phase occurring in the ingots into the a-Al8Fe2Si scriptlike phase.[6,7] The Sr adsorbs to the latter and prevents the diffusion of silicon (which builds up at the Al matrix/a-AlFeSi interface) into the a-Al8Fe2Si phase, resulting in an absence of b-Al5FeSi needles in the microstructure.[7] The present authors have observed, however, that rather than being adsor
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