Solidification of Iron-Rich Intermetallic Phases in Al-4.5Cu-0.3Fe Cast Alloy
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TRODUCTION
IRON is always present in commercial aluminum alloys, which mainly originated from the (1) primary metal and master alloys; (2) iron tools and equipments used during melting, transferring and casting processes; and (3) recycled aluminum alloys.[1] On the one hand, iron is one of the major alloying elements in some aluminum alloy systems. For instance, iron is added to increase the strength at an elevated temperature in Al-Cu-Fe-Ni alloys, such as AA2219 and AA2618.[2,3] However, iron is thought to be one of the most harmful impurities in most of the commercial aluminum alloys. According to Lyakishev,[4] the solubility of iron is approximately 1.8 wt pct at 928 K (655 C) in liquid pure Al but only 0.0052 wt pct at 723 K (450 C) in solid pure Al. Therefore, almost all the iron will precipitate from liquid Al alloys in the form of ironrich intermetallics. To date, many researchers[5–10] have completed abundant work on iron-rich intermetallics in Al-Si cast alloys. Generally speaking, iron-rich intermetallics can be grouped into three kinds of morphologies: polyhedral or star-like, Chinese script, and platelet. Hwang et al.[11] reported that the polyhedral iron intermetallics usually form at high Mn contents. These polyhedral iron K.LIU, Ph.D. Candidate and X.-G. CHEN, Professor, are with the Department of Applied Science, University of Que´bec at Chicoutimi, Chicoutimi, Que´bec G7H 2B1, Canada. X. CAO, Adjunct Professor, is with the Department of Applied Science, University of Que´bec at Chicoutimi, and is also a Research Officer with the Aerospace Manufacturing Technology Center, Institute for Aerospace Research, National Research Council Canada, Montreal, Quebec H3T 2B2, Canada. Contact e-mail: [email protected] Manuscript submitted August 5, 2010. Article published online December 16, 2010 2004—VOLUME 42A, JULY 2011
intermetallic particles are also called a-Fe phase. Shabestari[12] found that this polyhedral a-Fe phase is detrimental to the mechanical properties because of the formation of ‘‘sludge’’ in the presence of excessive Mn and/or Cr. In addition, the a-Fe phase can also precipitate in Chinese script. Cameron et al.[13] reported that the Chinese script phase is Al8Fe2Si at low Mn contents and high cooling rates but Al15(FeMn)3Si2 phase at high Mn contents and low cooling rates. For platelet iron-rich intermetallic phase, it is thought to be particularly deleterious to the mechanical properties because it is brittle and can act as the stress riser. This platelet phase is usually identified to be Al5FeSi and can block the feeding when it is precipitated interdendritically, leading to the formation of shrinkage porosity.[14] In addition, p-Fe (Al8Mg3FeSi6) and d-Fe (Al4FeSi2) are also reported in some Al-Si casting alloys.[5] For the nucleation of these Fe-rich intermetallics, some different opinions exist: Khalifa et al.[10] reported that a-Al2O3, c-Al2O3, CaO, MgO, TiB2, TiC, Al4C3, and SiC are all the potential nucleation sites for ironcontaining intermetallics, depending on the composition
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