Titanium Casting with Sand Foundry Techniques

Abstract

The ability to produce a east titanium configuration has a history of approximately 10 years. It is presently being done in the United States by six companies. Casting of titanium is no simple task when one considers the thousands of foundries in existence producing cast parts from other metals. The difficulties are that titanium must be handled more carefully because molten titanium is very reactive to liquids, gases, and solids. At present, consumable vacuum arc melting offers the only suitable commercial method of producing titanium castings. The rapid contamination of titanium at elevated temperature by oxygen and nitrogen and the severe damage to ductility by small percentages of such contaminants requires that melting of titanium be done in the absence of air. Because liquid titanium is an extremely effective solvent, problems of containing the molten metal within a crucible during the melting operation are encountered. A water-cooled copper crucible is used to extract heat so rapidly from the liquid metal that it solidifies before its solvent reaction can take effect. This provides a thin film of solid titanium between the molten titanium and the copper crucible. This is referred to as skull melting. The liquidus to solidus range of titanium is quite narrow and problems of fluidity exist. When enough molten titanium is accumulated in the crucible, the power is cut, the electrode is rapidly withdrawn, and the pot is tilted, all within a few seconds or the pool will solidify. A centrifuge is used within the furnace vacuum chamber during the pouring of the melt. The high g forces during the casting operation help overcome the lack of fluidity. The centrifuge action assists in filling mold cavities with metal densities vastly superior to those from conventional sand foundry static casting. Conventional materials such as sand, shell, or ceramic molds are unsatisfactory for titanium since titanium above or near its melting point will violently react with these materials. A water soluble, expendable graphite mold system is a relatively new development in titanium casting and configurations with close tolerances and excellent surface finish are obtainable. New information related to the production of titanium castings with the latest applied technology was presented.