Effect of Melt Conditioning on Removal of Fe from Secondary Al-Si Alloys Containing Mg, Mn, and Cr
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TRODUCTION
THE use of secondary Al alloys is an important step towards circular economy. The production of secondary Al consumes only approx. 5 pct of the energy required to produce primary Al.[1] However, impurity elements enrich in secondary Al. Especially, the presence of Fe has detrimental effects on the casting process and on the mechanical properties of cast secondary Al-Si alloys. The presence of Fe can lead to the formation of large plate-shaped primary Fe-containing intermetallic b-phase particles. For detailed information on the phases, see Table I and what follows. These plateshaped b-phase particles hinder effective feeding and, thus, cause shrinkage porosity.[2–8] Furthermore, the edges of the plate-shaped b phase can act as stress concentration points and lead to a decrease in ductility.[9–12] Therefore, in order to deal with Fe impurity, a
lot of effort has been spent on optimizing the microstructure, e.g., by promoting the formation of the ac phase with less detrimental morphology by the application of high cooling rates and/or by the addition of further elements like Mn and Cr.[6,9–11,13–22] However, an increased overall intermetallic phase fraction due to Mn and Cr addition can overrule the beneficial effect of ac-phase formation instead of b-phase formation promoted by these metals.[9,12,17,23,24] Consequently, true removal of Fe from the secondary Al-Si alloys is required. For that purpose, attempts have been made to separate primary Fe-containing intermetallic particles from the Al melt,[25–31] e.g., by metal-melt filtration technique.[26] In addition to the obtained Fe-depletion, the refined Al alloy can subsequently be subjected to further microstructure optimization. The conditions for the formation of primary Fe-containing intermetallic particles, the so-called sludge, have been estimated based on the sludge factor SF[32–36] SF ¼ wFe þ 2wMn þ 3wCr ;
HANKA BECKER, ANGELA THUM, BENEDIKT DISTL, MARIO J. KRIEGEL, and ANDREAS LEINEWEBER are with the Institute of Materials Science, TU Bergakademie Freiberg, GustavZeuner-Straße 5, 09599 Freiberg, Germany. Contact e-mail: [email protected]. Manuscript submitted May 7, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
½1
where w are the mass percentages of the elements in the index. The idea behind the sludge factor is that the effect of the transition metals Fe, Mn, Cr on sludge formation may be estimated solely or at least predominantly on the basis of this weighted sum of the mass percentages of these metals. Thereby, the effect of Mn
Table I.
Summary of Primary Intermetallic Phases Forming in Hypoeutectic Al-Si Alloys with xM < 5 Atomic Percent and M = Fe, Mn, or Cr and Reported Solubilities
Phase
Space Group
Al-Fe-Si ah-Al7.1Fe2Si
P63/mmc
b-Al4.5FeSi
A2/a
h-Al13Fe4
C2/m
Lattice Parameters
References
a = 12.346 A˚ c = 26.210 A˚ a = 6.1676 A˚ b = 6.1661 A˚ c = 20.8093 A˚ b = 91.0 deg a = 15.492 A˚ b = 8.078 A˚ c = 12.471 A˚ b = 107.69 deg
67 68
Notes Narrow homogeneity range[67] Relative solubility of Mn: Fe0.97Mn0.
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