Stability of yttria for titanium alloy precision casting mold

Abstract

Superior properties of yttrium oxide for mold material use in precision casting of Ti-6Al-4V alloy have been proved in experiments under simulating process conditions and by assuming a model for formation of a hardened layer in the surfaces of the castparts. The model consists of instantaneous dissociation of oxide in the mold and rapid absorption and diffusion of dissociated oxygen into Ti-6Al-4V alloy castparts during mold filling, solidification and cooling after solidification. The maximum hardness in the surface hardened layer correlates directly with the stability of oxide which depends on the activities of elements consisting of the oxide in the molten titanium alloy. The hardness profiles in the surface hardened layer are determined by diffusion of oxygen in the alloy during cooling and solidification of the castparts.