Formation and Evaluation of Protective Layer Over Magnesium Melt Under SF 6 /Air Atmospheres
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INTRODUCTION
OXIDATION of liquid metals in various atmospheres causes many problems.[1] Molten magnesium and its alloys have a high affinity to oxygen as well as nitrogen in the atmosphere. The vapor pressure of the molten metal is high, and the oxide layer formed on the surface of the melt in air is not protective. It is attributed to the porous nature of the oxide layer and a small Pilling-Bedworth ratio,[2] which for magnesium oxide is 0.81 at 298 K (20 C). Thus, molten magnesium needs to be protected by other means. Melt loss during the oxidation of magnesium melt and contamination caused by the presence of the oxide particles and inclusions make the handling of melt difficult in industry. When liquid magnesium is exposed to ambient air, oxidation starts at a rapid rate. If it is not inhibited the melt will ignite and burn with an intense white flame. Therefore, to avoid the melt loss, contamination, and handling difficulties it is necessary to reduce the reactivity of the metal by some means such as adding alloying elements to the melt, covering the melt by certain salts, and handling the melt under an inert or a protective gas.[1,3,4] Since low viscosity of the melt is required to obtain accurate surface and shape of the parts, casting of magnesium melt generally takes place at temperatures
SAMAR EMAMI, formerly Graduate Student with the Department of Metallurgical Engineering, University of Utah, Salt Lake City, UT 84112, is now with United States Gypsum (USG), 700 North US Highway 45, Libertyville, IL 60048. HONG YONG SOHN, Professor, is with the Department of Metallurgical Engineering, University of Utah. Contact e-mail: [email protected] HANG GOO KIM, formerly Research Associate Professor with the Department of Metallurgical Engineering, University of Utah, is now Senior Vice President with POSCO M-Tech, Pohang, Korea. Manuscript submitted November 11, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
above 923 K (650 C). Depending on the casting process used, the temperature can go up to 1123 K (850 C). However, since an unprotected melt ignites immediately in air at these elevated temperatures, this becomes a safety hazard in the casting process and care must be taken in handling of the melt.[5] Recently, the use of inert or reactive gases has been seen in industrial practice, especially in the magnesium die-casting foundries. Among the first practical and commercially available gases used, SO2 showed an effective protection when 1 to 2 vol pct was added to the air. Due to the toxicity and corrosive nature of this gas, however, efforts were made to find a suitable replacement. Another example of a protective gas is argon. However, it does not form a protective layer on the melt surface and thus is not sufficiently effective due to the high vapor pressure of the magnesium at elevated temperatures. Since the early 1970s, utilization of cover gas mixtures entered the industry. SF6 and CO2 were known as successful solutions to the melt protection issue by forming a thin, coherent, and stable film on the melt surface.
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