Oxide skin strength on molten aluminum
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I. INTRODUCTION
THE properties of the oxide skin formed on molten aluminum alloys are important in extractive processes and processes such as refining, remelting, and casting of molten aluminum where the metal is in contact with oxygen. Dross formation during aluminum handling is a major source for metal loss during production. For recycled aluminum, the losses can be up to 10 pct of the processed metal. The dross contains 60 to 90 pct aluminum.[1] Not only does a high oxide growth rate increase the aluminum loss, but also a high oxide strength increases the metal loss. The reason for this is that a strong oxide layer makes it more difficult to separate oxide and metal from the dross. Thus, a slow growth rate and (in this case) low oxide strength is preferable. When handling molten aluminum, entrainment of oxides from the surface will occur if precautions are not taken. In continuous air slip DC casting of aluminum, the oxide film is believed to influence the castability and surface structure of the extrusion billets. The principle of the air slip casting process is illustrated in Figure 1. In the DC casting process, the time when the molten metal is in contact with gas inside the mold is lower than 20 seconds. This is more than enough to create an oxide layer on the surface of the meniscus where oil and gas meet the liquid metal. Due to variations in the metal pressure, resulting from friction in the hot top, waves on the metal surface, etc., the meniscus will oscillate during casting and thus affect the surface and outer layer of the cast.[2] The strength of the oxide skin will then affect the dynamics and movements of the meniscus during casting. In die casting, surfaces are coated so the cast is easy to get out of the mold. One unfavorable implication of this is that surface tension of the liquid will create a force barrier against flowing into thin sections.[3] Thus, when thin and thick sections are cast from one single gate, the thinner section MARTIN SYVERTSEN, Research Scientist, is with SINTEF Materials and Chemistry, NO-7465 Trondheim, Norway. Contact e-mail: martin. [email protected] Manuscript submitted December 27, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
will not be filled as easily as the thicker ones. As explained later, it is believed that the effective surface tension is a combination of the interface tension between the molten aluminum and an oxide layer, the mechanical strength of the oxide layer, and the surface tension of the oxide layer. Based on this, a reduction of the oxide skin strength will increase the ability for metal to penetrate thinner sections. To measure the oxide skin strength, a method developed by Professor E. Fromm and his former student Dr. W. Kahl in the 1980s[4] is used. This method implies stretching the oxide skin (by rotation) and measuring the force (torque) when the skin breaks or cracks. II. THEORY A. Oxidation of Aluminum and Aluminum Alloys It has been shown earlier that the oxide film that forms first on a pure aluminum surface is )-Al2O3.[5] After some
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