Aluminum volatilization and inclusion removal in the electron beam cold hearth melting of Ti alloys
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I. INTRODUCTION
INCREASED demand for higher performance products requires greater control of alloy processing in order to yield (1) a reduction or control of residual elements such as oxygen and nitrogen, (2) a controled amount of defects (porosity, inclusions, etc.), and (3) an improved control of alloy composition with minimized macrosegregation in the solidified ingot. These goals can be reached by improving standard liquid metal treatment and remelting processes, but also by developing new processing techniques. For safety and performance reasons, the aerospace industry, one of the high-tech sectors, is aiming to take full advantage of this approach. In the case of the titanium alloys, that make up nearly 30 pct of the weight of aeroengines, the random occurrence of the hard-alpha defect has been one of the main materials problems in the manufacture of highquality titanium products. These defects having 100 to 1000 mm in particle size are composed of a titanium matrix, which is highly enriched in interstitial elements (mainly nitrogen) that locally stabilize the alpha phase. These significantly hard defects can initiate the rupture of rotating turbine components during fatigue cycles. With conventional remelting processes such as vacuum arc remelting it is extremely difficult to completely eliminate these inclusions, which remain solid due to their high melting point. Since the failure of components due to this type of defect has been identified as the cause of aircraft accidents,[1] the assurance of their total elimination remains a big challenge for aerospace metallurgists. The electron beam cold hearth melting (EBCHM) process became very successful in the past few years because J.P. BELLOT, Associate Professor, and D. ABLITZER, Professor, are with the Laboratoire de Science et Ge´nie des Mate´riaux Me´talliques (UMR 7584), Ecole des Mines, 54042 Nancy, Cedex, France. E. HESS, Engineer, formerly with the Ecole des Mines, is with IRSID Company, Maizieresles-Metz, France. Manuscript submitted October 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS B
of its promising abilities to eliminate all defects, as is claimed by its supporters. In the EBCHM technique, the purification and solidification stages are clearly separated, as shown schematically in Figure 1. A first electron gun melts the charge, which can have a variety of forms (ingot, sponge, or scrap). The molten metal flows into a shallow water-cooled crucible (the cold hearth), where one or more electron guns maintain the temperature of the liquid metal. At the other end of the hearth, the molten metal flows over the outlet lip into a water-cooled copper mold to form the secondary ingot. One of the major functions of the cold hearth furnace is to separate inclusions heavier or lighter than the liquid alloy, while at the same time increasing the residence time of lower density particles that have a high melting point in order to ensure their complete dissolution. All these operations are performed inside a high vacuum chamber in order to ensure proper o
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