Influence of composition, Sr modification, and annealing treatment on the structure and properties of cast Al-4 pct Mg a
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DUE to their high strength and formability, the 5XXX series of aluminum-magnesium alloys is used extensively in can-end stock and automotive structural sheet applications. The iron (Fe) and silicon (Si) contents of the alloys determine the amount, size, and type of intermetallics that occur. These, in turn, influence the alloy performance. Parameters such as the alloy Fe/Si ratio, melt treatments such as modification and grain refinement, and the heat treatment applied to the alloy are various ways in which the alloy formability and mechanical properties can be controlled. As has been reported by Mondolfo,[1] under normal equilibrium conditions, aluminum alloys containing Mg and Si and low Fe contents should exhibit microstructures consisting of the ␣ -Al8Fe2Si phase. In direct-chilled castings, however, the iron has a tendency to precipitate in the  Al5FeSi plateletlike form, whose hard, brittle nature reduces F.H. SAMUEL, Professor, and A.M. SAMUEL, Research Professor, are with the Department of Applied Sciences, University of Quebec at Chicoutimi, Chicoutimi, PQ, Canada G7H 2B1. Contact e-mail: [email protected] H.W. DOTY, Project Manager, Materials Engineering, is with General Motors Powertrain, Metals Casting Technology, Inc., Milford, NH 03055. S. VALTIERRA, Research and Development Manager, is with Corporativo Nemak S.A. de C.V., Garza Garcia, N.L. 66221, Mexico. Manuscript submitted November 15, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the hot workability and extrudability of the alloy, and enhances pickup, leading to a poor surface finish. The alloy extrudability can be recovered by converting the  phase to the more compact ␣ phase during homogenization by the addition of small amounts of manganese.[2] Flood et al.[3] studied the effects of casting speed, grain refinement, and variations in Fe and Si content on the structure of can-end stock. At low Fe:Si ratios, the ␣ -Al8(Fe, Mn)2Si intermetallic is more prominent than Al3(Fe,Mn). During preheating and mechanical processing, these phases partly convert to Al6(Fe,Mn). The volume fraction of the Fe phase is proportional to the iron content of the alloy, whereas that of the Mg2Si phase is related to its Si content. The effect of grain refining additions on the type of intermetallics formed in dilute aluminum alloys has also been investigated by Meredith et al.[4] The authors have reported that the addition of grain refiner to Al-Fe alloys slightly alters the balance of the intermetallic phases. The dominant phase is always Al6Fe, but its morphology is changed significantly, from rodlike to curved and platelike with the addition of the grain refiner. Marshall et al.[5,6] examined the relationship between the microstructure and properties of AA5182 alloy sheet containing various amounts of Fe and Si. From a detailed quantitative metallographic analysis, they found the following: (1) at a low Fe:Si ratio, the main phase formed is Al6(Fe,Mn), VOLUME 34A, JANUARY 2003—115
with a small amount of Al15(Fe,Mn)3Si2 (in the annealed condition); (2) frag
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