Effects of Iron-Rich Intermetallics on Tensile Deformation of Al-Cu 206 Cast Alloys
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INTRODUCTION
IN Al-Cu 206 family cast alloys, iron is one of the major impurities and thus is strictly limited to less than 0.15 pct, or even 0.07 pct in A206.0 cast alloy[1] (all alloy compositions are given in wt pct unless otherwise indicated). However, the extremely low iron levels will discourage the utilization of the iron-rich recycled aluminum alloys and thus significantly increase the manufacturing cost. Iron often precipitates in the form of iron-rich intermetallics during solidification due to its extremely low solubility in solid aluminum (0.005 pct at 723 K (450 C)[2]). The precipitated iron-rich intermetallics include platelet Al7Cu2(FeMn) (b-Fe)[3–5] and Al3(FeMn),[6,7] Chinese script Al15(FeMn)3(SiCu)2 (a-Fe),[3–5] Al6(FeMn)[7] and Alm(FeMn),[7,8] depending on the alloy composition and solidification conditions. As is well known, the iron content and resultant ironrich intermetallics have significant influences on the mechanical properties of the Al-Cu 206 cast alloys.[5,9–12] It was found that the tensile properties decrease when the iron content increases from 0.03 to 0.24 pct and the extent of the reduction increases with increasing iron content.[9] Tseng et al.[5,11,12] reported a linear decrease with increasing iron content ranging from 0.05 to 0.2 pct. In the study of Kamga et al.,[10] the effect of heat treatment and alloy compositions (Fe and Si) on the 206 alloys with 0.1 to 0.3 pct Fe was investigated. It was found that the tensile properties were highly influenced by the ratio of iron to silicon and the best tensile properties were obtained in alloys that had low iron contents and an iron/silicon ratio
of approximately 1 to obtain the dominant Chinese script iron-rich intermetallics. The studies mentioned above have usually been limited to iron contents less than 0.3 pct. However, the Al-Cu 206 cast alloys tend to contain increasing levels of iron due to the utilization of recycled aluminum. Consequently, the present authors have systematically investigated the effect of the iron-rich intermetallics on the tensile properties of the 206 family cast alloys at iron contents up to 0.5 pct.[13] It was confirmed that the tensile properties tend to decrease with increasing iron content at low levels of Mn and Si. However, it was demonstrated that the upper limit of the iron can be as high as 0.5 pct and the newly developed high-iron Al-Cu 206 cast alloys can still have a good combination of both tensile strength and ductility if their chemistries, casting processes, and microstructures are well controlled.[13] To date, limited information about the effects of iron and iron-rich intermetallics on the tensile fracture mechanisms has been disclosed in the open literature and thus it is essential to understand how the iron-rich intermetallics influence the tensile behavior to better develop and optimize cost-effective high-iron 206 cast alloys. To this end, the tensile fracture surfaces of the 206 cast alloys containing various iron contents and associated iron-rich intermetallics are systematically investigat
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