Formation Mechanism of Nanoscale Al 3 Fe Phase in Al-Fe Alloy During Semisolid Forming Process
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TRODUCTION
ALUMINUM alloys are widely used in automotive, aerospace, and transportation because of their low density, high specific strength, and excellent corrosion resistance.[1–4] However, iron in Al alloys always results in the formation of Fe-bearing phase, and the shape of Fe-bearing phase is generally needlelike. The needlelike Al-Fe phase easily causes stress concentration and dissevers Al matrix during plastic deformation.[5,6] Further, Fe is one of the most common elements in Al alloys, and the solid solubility of Fe in Al is relatively low.[7,8] In order to avoid the formation of Fe-bearing phase, the most ideal method is to reduce the content of Fe in Al melt to an extremely low level, or even to completely remove Fe from Al melt. However, the fact is that Al melt is often exposed to Fe container during the forming process, and the inclusion of Fe in Al is almost inevitable; besides, the Fe removal process is complex and costly. Also, it has been reported that the needlelike Fe-bearing phase forms in Al alloys even if the content of Fe is only 0.1
XIANG WANG and YU WANG are with the School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China. REN-GUO GUAN is with the School of Materials Science and Engineering, Northeastern University and also with the School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China. Contact email: [email protected] Manuscript submitted August 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
wt pct,[9] but the content of Fe in some recycled Al alloys can be as high as 2 wt pct. Therefore, Fe has become a key factor in restricting the performance of Al alloys and limiting their application. In recent years, the research topic has shifted from removing Fe to refining the Fe-bearing phase. It has been widely reported that Fe-bearing phase can be effectively refined and its morphology can be changed by some measures, and the negative effect of Fe-bearing phase can be partly avoided and Fe can even serve as a reinforcing particle.[9] The measures can be divided into two categories. One is refining the Fe-bearing phase during liquid forming and includes magnetic stirring,[10,11] rapid solidification,[12–14] element modification,[9] and so on. Ban et al. studied Al3Fe phase in hypereutectic Al-Fe alloy during solidification under the function of magnetic field and found that Al3Fe phase could be refined by AC magnetic field, while DC magnetic field had the opposite effect.[11] Li et al. found that primary Al3Fe phase was refined because the nuclei density of primary Al3Fe was increased by the magnetic field.[15] However, the magnetic field that can effectively refine Al3Fe phase has a high requirement of magnetic field strength and stability, so it is costly and not suitable for large-scale industrial application. Some studies showed that Al3Fe phase could be refined by rapid solidification because the solid solubility of Fe in Al matrix was expanded by increasing the cooling rate.[16] Nevertheless, the research of Zhang
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