Influence of Al 2 O 3 concentration in yttria based face coats for investment casting Ti–45Al–2Mn–2Nb–0·2TiB alloy

Abstract

An Al₂O₃–Y₂O₃ based face coat material was researched by adding Al₂O₃ into yttria in different concentrations to reduce the mould cost whilst retaining the face coat chemical inertness. The face coat compositions and sintering properties at different temperatures were investigated using X-ray diffraction, friability and dilatometers. Meanwhile, the chemical inertness of the face coats was identified using a sessile drop test method to interact with a Ti–45Al–2Mn–2Nb–0·2TiB alloy. The results show that at different firing temperatures, a series of phase transformations have taken place between Y₂O₃ and Al₂O₃, resulting in different face coat compositions and sintering properties. However, the chemical inertness of the face coat against the molten TiAl alloy was not influenced by the phase transformations, and it was dependent on the amount of Al₂O₃ added to the face coat. In this experiment, no interaction was observed between the Al₂O₃–Y₂O₃ face coat material and TiAl alloy when Al₂O₃ additions were lower than 33·33 mol.-; however, increasing addition amount of Al₂O₃ will increase the chance of face coat and metal interaction.