The Effect of Holding Liquid Aluminum Alloys on Oxide Film Content
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ALUMINUM alloy castings are increasingly commonplace in applications such as the automotive and aerospace industries where their mechanical properties must be reliable and reproducible. A common casting defect with a random nature that significantly reduces the reliability of Al (and Mg) alloy castings is the double-oxide film defect, the formation of which was discussed by Campbell.[1] During the casting process, the surface of the liquid metal can fold over on itself, and the opposing surface oxide films come into contact with each other, trapping a volume of gas (presumably predominantly air). This forms a discontinuity or crack in the matrix of the solidified casting (Figure 1), reducing mechanical properties, but not in a consistent way. It has been proposed that, once the doubled-over oxide film defects become entrained in the melt, they could become convoluted because of turbulence in the bulk liquid as the liquid metal fills the mold. However, the oxide film defects could also be expanded again because of a variety of effects, such as the precipitation of dissolved hydrogen from the surrounding melt into the internal atmosphere of the defect and strains in the liquid metal because of solidification shrinkage.[1–4] RAMIN RAISZADEH, Assistant Professor, is with the Department of Metallurgy and Materials Science and Mineral Industries Research Center, Shahid Bahonar University of Kerman, Kerman 76169, Iran. Contact e-mail: [email protected] WILLIAM D. GRIFFITHS, Lecturer, is with the School of Metallurgy and Materials, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom. Manuscript submitted September 4, 2010. Article published online October 5, 2010. METALLURGICAL AND MATERIALS TRANSACTIONS B
To understand the behavior of entrained double oxide film defects Raiszadeh and Griffiths[5] held a known volume of air in a melt of commercially pure liquid aluminum, and they recorded the change in its volume with time using real-time X-ray radiography. These results showed that, first, the oxygen of the trapped air bubble reacted to form Al2O3, and then, second, the nitrogen reacted to form AlN. These reaction processes were continuous, with no incubation time required. A semiempirical mathematical model was derived using the oxygen and nitrogen reaction rates estimated in this experiment, and it was used to estimate the duration of the atmosphere within double-oxide film defects.[2] Only a general estimate of the size of the latter was possible, but this work suggested that the atmosphere of a typical double-oxide film defect in liquid commercial purity aluminum (with a low initial hydrogen content), might be consumed by reaction with the surrounding melt in about 7 to 77 seconds, depending on the assumptions made about volume and surface area of the defect. After consumption of the internal gases within an oxide film defect, only the residual argon from the original entrained air (about 1 vol pct) should remain. It has been suggested that once the interior gases
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