Electrochemical Deoxidation of Titanium and Its Alloy Using Molten Magnesium Chloride
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INTRODUCTION
OXYGEN is a major impurity in titanium and its alloys,[1] and its direct removal from metallic titanium is difficult because oxygen has a strong affinity with titanium and readily dissolves into solid titanium at elevated temperatures.[2–4] Commercial titanium and its alloys contain oxygen at approximately 1000 mass ppm (0.1 mass pct) and are produced industrially using an oxygen-free reduction process known as the Kroll process.[5–7] The Kroll process consists of four major steps: carbo-chlorination to convert a TiO2 feed into TiCl4, magnesiothermic reduction of TiCl4 to produce a titanium sponge, melting of the titanium sponge with other feedstocks to produce ingots, and molten electrolysis of MgCl2 to regenerate magnesium and chlorine gas. The magnesiothermic reduction of TiCl4 has the essential advantage of producing high-purity titanium with low oxygen contents. However, the high-grade titanium sponge produced typically contains 300 to 1000 mass ppm oxygen, even though the reduction process is carried out in a gas-tight oxygen-free system. Ingots of titanium and its alloys are produced by melting feedstocks such as titanium sponge, additive metals, and titanium scraps in a vacuum arc furnace and/or an electron beam furnace. This melting process is usually repeated several times to ensure chemical and mechanical homogeneity. However, common melting techniques have no oxygen removal capability, and the YU-KI TANINOUCHI, Research Associate, and TORU H. OKABE, Professor, are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguroku, Tokyo 153-8505, Japan. Contact email: [email protected] YUKI HAMANAKA, formerly Graduate Student with the Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, is now Engineer with Mitsubishi Materials Corporation, 1511 Furumagi, Joso-shi, Ibaraki 3002795, Japan. Manuscript submitted February 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
oxygen level in titanium sometimes increases during the melting process. As a result, low oxygen-containing feed, such as a high-purity titanium sponge, must be used to produce high-grade titanium products with low oxygen contents. Metal scraps of titanium and its alloys are often contaminated by oxygen. Because the common melting techniques have no oxygen refining capability, the use of large amounts of scraps as ingot feedstock is hindered. Figure 1 shows the material flow of metallic titanium in terms of the oxygen level. Low-grade scraps unsuitable for remelting are reused as additives in the production of steel, aluminum, etc., or are discarded as waste.[8] Oxygen dissolution into titanium often occurs during metalworking of ingots to final products, especially when titanium products are exposed to oxygen or air at elevated temperatures. For example, hot rolling causes the formation of thin layers of not only oxides but also oxygen-enriched metallic phase (or alpha case) on the surfaces of titanium products.
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