Thermodynamic Analysis of Deoxidation of Titanium Through the Formation of Rare-Earth Oxyfluorides
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INTRODUCTION
OXYGEN dissolved in metallic Ti cannot be easily removed by the currently used vacuum melting techniques such as electron beam and vacuum arc melting. The oxygen content in Ti usually increases during the melting, casting, and machining processes. Ti scrap often contains oxygen impurities with a concentration of 2000 to 4000 mass ppm (0.2 to 0.4 mass pct), which is higher than that in virgin metals (300 to 2000 mass ppm) such as Ti sponges produced by the Kroll process. Because the production cost of metallic Ti is high, it is desirable to remelt scraps with virgin metals to manufacture primary ingots of Ti or its alloys. However, the use of scrap as a raw material increases the oxygen content in the produced Ti ingots. As of today, effective commercial techniques capable of directly deoxidizing metallic Ti have not been developed yet. Therefore, scraps with relatively high oxygen contents cannot be reused as the raw materials for the production of Ti ingots; they are either used as the additives in the production of steel or other metals (such as Al and Cu) or simply treated as non-recyclable waste.[1,2]
TORU H. OKABE, YU-KI TANINOUCHI, and CHENYI ZHENG are with the Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan. Contact e-mail: [email protected] Manuscript submitted May 3, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
The demand for Ti and its alloys has been increasing in various fields (especially in the aerospace industry). Thus, recycling Ti scraps for producing primary Ti ingots represents an important task that requires the development of effective deoxidation method. Metallic Ti has a strong binding affinity for oxygen.[3] Furthermore, oxygen is highly soluble in metallic Ti. At 1300 K (1027 C), the solubility limits of oxygen in aand b-Ti are approximately 14 and 1 mass pct, respectively,[4] which make the deoxidation of metallic Ti very difficult. In the past, various techniques for the direct removal of oxygen from Ti have been proposed and studied.[5–37] Using some of these approaches, the oxygen concentration in Ti can be theoretically reduced to 500 mass ppm or less. However, the efficiency of these techniques remains relatively low, and none of them is used as a large-scale industrial process. For example, slow oxygen diffusion in Ti often becomes a problem when deoxidizing a large specimen of bulk solid-state Ti; the aforementioned techniques are applicable only in processing swarf and powders. Furthermore, the existence of large industries that effectively consume Ti scraps (e.g., steel production uses scraps as additives) has suppressed interest in commercializing deoxidation techniques. Since rare-earth metals exhibit extremely strong binding affinities for oxygen, they can be potentially utilized as deoxidants for Ti. For example, under the Y/ Y2O3 equilibrium at approximately 1200 K (927 C), Ti metal containing around 100 mass ppm of oxygen was obtained.[38,39] Furthermore, a highly reducing atmosphere can be obtained und
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