Modeling of Alpha-Case Formation and Its Effects on the Mechanical Properties of Titanium Alloy Castings

Titanium (Ti) castings generally contain a hard surface layer of alpha-case (α-case) that is enriched of oxygen. For dental Ti prostheses, the hard and brittle α-case can be a significant fraction of the thickness of the entire work piece and must be controlled properly to ensure minimum properties. The objective of this article is to present a theoretical framework for predicting α-case formation in dental Ti castings as a function of cooling rate and the influence of α-case on the tensile ductility and wear resistance. First, a diffusion analysis is presented to describe the oxygen profile and microhardness distribution within the α-case in Ti castings of CP-Ti and Ti-6Al-4V. The reduction in tensile ductility by the α-case is then predicted using an existing tensile ductility model. Furthermore, the improvement in wear resistance by the α-case is predicted using a recent wear model. These results are then used to assess the feasibility of controlling the α-case thickness to optimize both tensile ductility and wear resistance in Ti castings.