Tensile Properties of Al-Cu 206 Cast Alloys with Various Iron Contents
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TRODUCTION
THERE has been increasing interest in heat-treatable Al-Cu 206 cast alloys for aerospace and automobile industries because of their high strength and good elevated temperature properties.[1] As is well known, however, high iron contents in these alloys can cause significant degradation in mechanical properties (particularly ductility). To this end, an extremely low upper limit for iron is usually required, e.g., 0.15 pct in 206.0 or 0.07 pct in A206.0[2] (all alloy compositions are in weight percent unless indicated otherwise). As a consequence, the extremely low iron requirements in Al-Cu cast alloys do not encourage the use of iron-rich recycled aluminum alloys and, thus, significantly increase the manufacturing costs. Finding a solution to increase the allowable iron content has been a great challenge for the design and development of Al-Cu 206 cast alloys. Because of its extremely low solubility in solid aluminum (0.005 pct at 723 K (450 C)[3]), iron often precipitates in the form of iron-rich intermetallic phases during solidification. The most reported iron-rich intermetallics, which may appear in the finally solidified 206 cast alloys, include plateletlike Al7Cu2Fe/Al7Cu2(FeMn) (b-Fe)[4–8] and Chinese script Al15(FeMn)3(SiCu)2 K. LIU, Student, and X.-G. CHEN, Professor, are with the Department of Applied Science, University of Quebec at Chicoutimi, Chicoutimi, QC G7H 2B1, Canada. Contact e-mail: [email protected] X. CAO, Adjunct Professor, is with the Department of Applied Science, University of Quebec at Chicoutimi, and also Research Officer, Structures, Materials and Manufacturing Laboratory, National Research Council Canada, Montreal, QC H3T 2B2, Canada. X. CAO—This manuscript is published with the permission of the National Research Council of Canada, by right of the Crown in Right of Canada. Manuscript submitted June 11, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
(a-Fe),[4–6,8,9] depending on the alloy composition, such as the contents of Fe, Mn, and Si. Recently, two other iron-rich intermetallics, i.e., plateletlike Al3Fe/Al3 (FeMn) and Chinese script AlmFe/Alm(FeMn), were also observed to precipitate in the 206 cast alloys at a high iron content of 0.5 pct.[8,10–12] These iron-rich intermetallic phases, particularly for those with platelet morphology, can appear to act as stress risers and crack initiators,[13–15] and thus have usually been thought to be harmful to the mechanical properties of cast Al alloys. In order to neutralize the detrimental effect of the platelet iron-rich intermetallics in Al-Cu cast alloys, Mn or Si is added to transfer the iron-rich intermetallics from platelet into Chinese script, which is thought to be less harmful to the mechanical properties.[15–19] Though the transition from platelet to Chinese script during solidification has been well realized from the point of microstructural evolution,[4–6,11] very limited investigation has been performed to quantify the effect of iron-rich intermetallics on the mechanical properties of the 206 cast alloys. Sigworth and Major
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