Abstract
Research was carried out to determine the interactions between the filler and stucco materials in CaZrO3 based facecoats during shell firing as well as between the facecoat and a TiAl alloy during the casting process. A ‘flash re-melting’ technique, which gives a similar heating profile to the actual investment casting process, was used to study the phase transformations in the shell moulds. The chemical inertness of the facecoat was then investigated using a sessile drop test using a Ti–45Al–2Nb–2Mn–0.2TiB alloy. In this study, the facecoat compositions and the interaction products between metal and shells were characterized using x-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A severe interaction was found between CaZrO3 filler and Al2O3 stucco, which rapidly damaged the shell surface. As well as oxygen, zirconium and silicon ions from the shell moulds were also observed to penetrate into the TiAl metal to form (Ti, Zr)5(Al, Si)3 phases in the metal/shell interfacial areas.
Highlights
Gamma titanium aluminides are a family of low density, high performance alloys with the potential to replace current Ni-base superalloys used in the production of aero-engine components
The moulds were made by first investing the wax pattern with the primary slurry which consisted of a colloidal ZrO2 binder (Remet, Ti-coat), CaZrO3 filler (ABSCO, -325mesh), liquid polymer (EVO STIK, PVA), wetting agent (Remet, Victawet 12) and anti-foam (Remet, Burst RSD-10)
A CaZrO3 based material was used as the facecoat material to investment cast a TiAl alloy
Summary
Gamma titanium aluminides are a family of low density, high performance alloys with the potential to replace current Ni-base superalloys used in the production of aero-engine components. Due to the high affinity of elements such as oxygen, nitrogen etc., titanium and its alloys can interact with mould materials during the investment casting process, resulting in an interaction hardened layer being generated at the metal surface [3,4]. This hardened layer contains a large amount of dissolved oxygen, and it is very brittle and susceptible to crack generation and propagation [5]. Yuan et al / Materials Chemistry and Physics 155 (2015) 205e210 alternative, cheaper facecoat materials based on nitride compounds such as AlN and BN have been studied [12,13] but the poor chemical inertness of these materials made them unsuitable for use in TiAl alloy casting
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