Abstract
This paper proposes a newly developed alpha-case controlled mold material for Ti castings. An Al2O3 mold containing alpha-case reaction compounds, titanium oxide (TiO2, Ti2O and Ti6O) and titanium silicide (Ti5Si3) was manufactured via a reaction between Al2O3 and Ti powder under different firing conditions in air and a vacuum. In comparison with the Al2O3 and Al2O3+Ti mold fired in a vacuum, the micro-Vickers hardness and nano indentation profiles of Al2O3+Ti indicated that the alpha-case thickness was significantly reduced from 350 μm to ∼45 μm. The alpha-case formation in the Al2O3+Ti mold was reduced due to the presence of TiO2, which formed the TiO intermediate phase that acted as a diffusion barrier. In addition, Ti5Si3 was effective in minimizing Si contamination at the casting surface due to the reaction between Ti and the colloidal SiO2 binder. Therefore, alpha-case reaction compounds, such as TiO2 and Ti5Si3 in Al2O3, can effectively reduce alpha-case formation at the casting surface.
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