Abstract
Casting of titanium based alloys presents considerable problems including the extensive interactions that occur between the metal and refractory. An investigation was undertaken to develop a zirconia facecoat suitable to replace current zircon/silica facecoat used in investment casting titanium aluminide alloys. The stability of the zirconia slurry was assessed using pH, viscosity and plate weight, and the mould properties such as friability, strength and permeability were measured. The interaction between the zirconia face coat and a Ti–46Al–8Nb–1B alloy was studied by centrifugal investment casting at three mould pre-heating temperatures. Computer simulation of metal cooling profiles during casting was also carried out to assist the analysis. In this work, a stable zirconia slurry was developed, and the moulds produced using a zirconia facecoat have comparable mechanical properties and better permeability than those made of the traditional zircon/silica facecoat. The friability of the zirconia facecoat was much improved in comparison to that made of yttria face coat. During casting, metal non-fill defects were presented at a low preheat mould temperature of 500°C. The interaction products found between the metal and mould included a combined (Ti, Zr)5(Si, Al)3 and ZrAl2 phases, a re-precipitated ZrO2 phase, and a Al2O3 film at the interface. The interaction between mould and metal also caused a high hardness at the interface due to oxygen penetration, which varied with samples using different mould pre-heat temperatures. The suggestion has been made that the mould pre-heat temperature should be less than 1200°C.
Highlights
The first attempts to cast titanium alloys were made in 1950 [1] and these alloys are widely used in the aerospace, energy and chemical industries
In terms of green and fired strength, the properties of the zirconia shell system were all comparable to that of the standard zircon/silica facecoat shell used for casting steel
A significantly higher hardness value at the surface of the metal cast into the mould pre-heated at 1200 1C was likely to be related to the forming of the Al2O3 oxide layer, which increased from 1–2 μm thick at 500 1C and 1000 1C mould preheat temperatures to 15 mm thick at 1200 1C
Summary
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 [2]. Titanium aluminides are difficult to process mainly due to [3]: high melting temperature and poor fluidity, strong chemical activity of the molten metal, low density combined with high viscosity in the molten state. In order to further eliminate the interaction between the mould and metal, mould materials with high chemical inertness have been used as the face coat material during the investment casting process. An investigation was undertaken to develop a zirconia facecoat suitable to replace current zircon/silica facecoat in investment casting of titanium aluminide alloys. The interaction between the zirconia face coat and a Ti–46Al–8Nb–1B alloy was studied at a range of preheat temperatures between 500 1C and 1200 1C using full-scale casting.
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