Variation in Alpha-Case Thickness of Ti-xAl Castings

Alpha-case formation, originating from interfacial reactions between molten titanium and oxide molds, remains a critical issue limiting the surface integrity and mechanical performance of titanium castings. In this study, the effect of aluminum content (0-52 at%) on alpha-case formation was systematically investigated using plasma arc melting and drop casting with alumina-based molds. The reaction kinetics between titanium melts and alumina molds were evaluated through cooling rate measurements and thermodynamic modeling. Microstructural and compositional analyses using optical microscopy, hardness testing, and electron probe microanalysis revealed that increasing aluminum content effectively suppressed alpha-case development. No distinct reaction layer was observed when the aluminum concentration exceeded 30 at%. The alpha-case consisted primarily of Ti₃Al, TiO₂, and Ti₅Si₃ phases, indicating that the molten titanium reacted with both alumina and silica constituents of the mold. Oxygen was identified as the dominant element controlling the reaction depth, consistent with its diffusion behavior across titanium phases. Calculated alpha-case thicknesses showed excellent agreement with experimental measurements, confirming that the reduction in alpha-case depth with increasing aluminum content results from decreased oxygen diffusivity, shorter reaction time, and lower interfacial temperature. These findings establish aluminum addition as a key strategy for minimizing interfacial reactions during titanium investment casting, thereby improving dimensional accuracy and surface quality in high-temperature components.