Evaluation of the α-Case with TiO₂ in Mold for Titanium Investment Casting

Abstract

Molten titanium easily reacts with not only oxygen, carbon, and nitrogen but also the ceramic mold to form an α-case on the casting surface. However, the brittle α-case must be removed by chemical milling because it weakens the mechanical properties of the casting. The aim of this study is to evaluate and suppress the formation of the α-case by including TiO₂ in the mold. Titanium powder was added into the alumina mold in order to facilitate the formation of TiO₂ by the oxidation of titanium during mold firing. Another way to include TiO₂ in the mold was to simply add TiO₂ into the alumina mold. Optical microscopy and the micro-Vickers hardness test were used to characterize the α-case. The composition and morphology of the molds were confirmed using X-ray diffraction and field emission scanning electron microscopy. The mold with added titanium (Al₂O₃ + Ti) was effective in controlling the α-case. However, the interfacial reactions of the mold with the added TiO₂ (Al₂O₃ + TiO₂) exhibited distinct differences in α-case formation compared to the Ti-added mold (Al₂O₃ + Ti), while having almost identical compositions.