Effect of Ca on the Behavior of Double Oxide Film Defects in Commercially Pure Aluminum Melt
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TRODUCTION
FORMATION of a double oxide film defect in reactive liquid alloys such as Al or Mg was first described by Campbell.[1] During the casting process, every time the surface of the liquid metal folds upon itself, two surface oxide films come into contact with each other on their unwetted sides and a volume of gas (presumably predominantly air) is trapped between them. This defect (i.e., two oxide layers with the volume of air trapped between them) which leads to a crack in the solidified casting not only deteriorates the mechanical properties of the casting, but also could act as an initiation site for the formation of other defects such as hydrogen pores[2] and Fe-rich phases[3] before solidification. Raiszadeh and Griffiths[4] demonstrated that when a double oxide film defect moved in the liquid metal, cracks were formed on its oxide layers. First oxygen and then nitrogen of the trapped atmosphere of the defect came into contact and react with the surrounding Al melt through these cracks to produce Al2O3 and AlN, respectively. Using a semi-empirical mathematical model,[5] they suggested that this process took a few seconds to 2 minutes, depending on the size of the oxide defect and the severity of the turbulence in the melt.
FARZANEH KHALEGHIFAR, Postgraduate Student, RAMIN RAISZADEH, Associate Professor, and HAMID DOOSTMOHAMMADI, Assistant Professor, are with the Department of Metallurgy and Materials Science and Mineral Industries Research Centre, School of Engineering, Shahid Bahonar University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran. Contact e-mail: [email protected] Manuscript submitted July 31, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
It is suggested[6] that, if the two layers of an oxide film defect bond with each other, the most deleterious characteristic of the defect (i.e., its crack-like nature) is eliminated, and therefore, the mechanical properties of the produced casting should be dramatically enhanced. Aryafar et al.[7] showed that after O and N of the trapped atmosphere within a defect that was held in an A356 aluminum melt was consumed, the two oxide layers of the defect gradually bonded with each other during the transformation of Al2O3 to MgAl2O4 and then to MgO. The occurrence of such bonding (dubbed healing[1]) was later confirmed for other aluminum alloys such as commercially pure,[8] Al-Mg,[9,10] and Al-0.05 wt pct Sr.[11] Najafzadeh and Raiszadeh[8,9] concluded that, for the two layers of an oxide defect to bond with each other, two criteria must be met (1) complete consumption of O and N within the defect by the reaction with the surrounding melt, and (2) occurrence of a transformation involving the rearrangement of atoms at the internal surfaces of the oxide layers. They also observed that the two oxide layers of a double oxide film defect began to bond together when maintained in commercial purity Al melt for 5 hours. They attributed the bonding of the oxide layers to the transformation of c- to a-Al2O3. The extent of the bonding was gradually increased by holding time so
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