The influence of face coat material on reactivity and fluidity of the Ti6Al4V and TiAl alloys during investment casting

Abstract

Ti6Al4V alloy belongs to the most significant alloys within the conventional titanium alloys, namely for producing turbochargers impellers and human prostheses. TiAl alloys, because of its attractive properties, such as half density of any nickel-based alloys and excellent high temperature properties, exhibit excellent potential for aerospace turbines and turbocharger turbines application. Investment casting is a near net shape process with great interest for these kind of complex parts, but the processing of these alloys using this technique is still a challenge. In spite of these advantages, these alloys are highly reactive in their molten state, reacting with the ceramic shells used in investment casting, forming a hardened and brittle layer called alpha case on the cast alloy surface, rich in interstitial elements such as oxygen. It is commonly accepted that yttria-based face coats are the best solution for minimizing metal mold reaction, but this ceramic oxide is very expensive. So, the aim of this work is to test alternative materials to produce ceramic shells face coats. A test sample simulating both compressor wheels and turbines was developed and assembled in a wax tree for alpha case and fluidity evaluation. Reactivity studies were conducted based on microhardness measurements and microstructural analysis of γ-TiAl and Ti6Al4V standard test samples, casted in shells with different face coat materials: fused Y₂O₃, ZrSiO₄, Al₂O₃, yttria (6%) stabilized ZrO₂ and yttria stabilized ZrO₂ with 10% fine Y₂O₃ (3–7 µm). The results obtained showed that fused Y₂O₃ face coat eliminates the alpha case, although affecting the fluidity, and γ-TiAl castings have more misruns blades than Ti6Al4V castings.