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Yttria is the best face coat material in the shell mold for the investment casting of γ-TiAl alloys. However, yttria inclusions may occur in the cast products and there is a lack of knowledge about the influence of yttria inclusions on the mechanical properties of TiAl. In the present study, the effect of yttria inclusions on room temperature tensile properties of two investment cast TiAl alloys, Ti-45Al-2Mn-2Nb (at%) + 0.08 vol% TiB2 and Ti-46 Al-2 Nb-2Cr-0.15B (at%), were studied. The results show that tensile failure tends to initiate from yttria inclusions, resulting in reduced plastic strain and ultimate strength. Yttria inclusions differ in location (surface /internal), size, and morphology (agglomerate /whole piece), those with larger sizes or located at the sample surface being more harmful. The tensile properties of inclusion containing TiAl parts can be estimated using the size and location determined by X-ray inspection. The critical size at which inclusions become detrimental to the mechanical properties of TiAl was discussed.

Using X-ray diffraction, hardness evaluation and scanning election microscopy, the interaction between a Ti–46Al–8Nb–B alloy and an yttria facecoat was analysed after investment casting with different cast bar diameters. Results showed that the interaction between an yttria facecoat and alloy can cause the oxygen in the facecoat to diffuse into the alloy to form an interaction layer at the metal/shell interface. Aluminium was observed to evaporate at the alloy surface, and the remaining Al ions at the interaction layer interacted with oxygen to form Al2O3 whiskers. Meanwhile, some Nb rich precipitates were found at the metal/shell interface. By increasing the casting bar diameters, the interaction layer thicknesses were increased with the associated growth of Nb rich precipitates.

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 ZrAl 2 phases, a re-precipitated ZrO 2 phase, and a Al 2 O 3 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.

Investment casting is widely used to cast near-net shape components, reducing material waste and process cost. Due to the high reactivity of titanium and its alloys, in order to reduce the interaction between the mold and molten titanium, very costly materials are used in the mold face coat. In order to reduce the material cost while maintaining the chemical inertness of the face coat, ZrO2 was added into an yttria-based face coat in different concentrations in this study. The face coat properties of the different systems were analyzed using dilatometry and XRD. The chemical inertness of the different face coat systems were tested using a sessile drop test using a Ti-45Al-2Mn-2Nb-0.2TiB alloy, and the interactions between the face coat and the alloy were analyzed by the interfacial microstructures, contact angle, and hardness changes. The results showed that small amounts of ZrO2 can be added into yttria without changing the chemical inertness of the face coat. High amounts of ZrO2 in the face coat can interact with TiAl alloy to form different interaction products. Meanwhile, both Y2O3 and ZrO2 filler materials were observed to dissolve in molten TiAl during the sessile drop test.

In order to reduce the interaction between the Ti alloys and ceramic shell during the casting, materials with high thermal and chemical inertness were used in investment casting. An investigation was undertaken to analyze the influence of the change of binder systems on the slurries, facecoats and the thermo-chemical properties of the facecoat systems using an Y2O3–ZrO2 filler material. The results showed that, using alumina-sol as the binder in the slurry gave the longest life of around three days followed by that using the commercially available zirconia-sol at around 6 h, and the yttria sol based slurry giving a shortest life of around 1.5 h. Meanwhile using the alumina-sol can also enhance the facecoat sintering properties. There was no obvious evidence observed that the change of the binder system influenced the facecoat chemical inertness.

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 we...

Investment casting is an economical method to manufacture near net-shape metal components. Due to the very high thermal and chemical inertness, yttria has been widely used as the mould face coat material for the investment casting of titanium alloy for many years. An investigation was undertaken to study the oxidation behaviour of TiAl alloy during casting in a mould using pure yttria as the face coat. This research shows that the TiAl alloy was still oxidized in the mould during casting when using yttria as the face coat. During high temperature casting, the yttria in the face coat was dissolved by high temperature molten metal flow. The oxygen from the yttria face coat diffused into TiAl and interacts with TiAl to form different microstructure and phases (e.g. precipitates such as oxygen enriched Ti3Al and Al2O3 phases). Meanwhile, the dissolved yttrium was then re-precipitated at the metal interfacial area as yttria inclusions after metal cool down.

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B2O3, YF3, Al2O3, ZrO2, and TiO2, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B2O3 and YF3 could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO2, Al2O3 and ZrO2 appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La2O3 added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.

Yttria is widely used as a key material in the manufacture of the mould face coat for the investment casting of Ti or titanium aluminide alloys, as it has the required thermal and chemical inertness. Because of the high melting temperature of yttria, moulds manufactured using an yttria face coat need high sintering temperatures to achieve a good surface finish, which increases the production cost of the mould. In this research, small amounts of sintering aids, B2O3, YF3, Al2O3 and ZrO2, were added into a simple yttria face coat and sintered at temperatures from 1000 to 1400°C to investigate the influence on the sintering properties of yttria. These experiments found that the sintering additive B2O3 showed the best performance in enhancing yttria face coat sintering over the test temperatures. Sintering aids such as YF3 only slightly altered the sintering properties of yttria at temperatures around 1000–1200°C, and powders such as YAZ were seen to be more suitable for sintering at temperatures arou...

Yttria is an important primary coat material in investment shells for casting extremely reactive TiAl alloys and polymer is usually added to slurry to improve strength of the shell. In this investigation, systems that vary the polymer content through 0%, 6% and 30% were produced and the samples were sintered at 1000 °C, 1200 °C and 1400 °C. The results suggest that polymer content and sintering temperature appear to have little effect on the hardened alpha layer of the cast alloy, which is elevated by near-surface oxygen content. Silica from the backing coat is seen to travel through the primary coat and diffuse into the alloy regardless of the shell system. Firing temperatures above 1200 °C increase the shell strength undesirably but the friability of the primary coat decreases as firing temperature increases. Higher mould permeability was found in the mould containing higher polymer level and this would be advantageous for complete mould filling.

The investment casting process is an economic production method for engineering components in TiAl-based alloys and offers the benefits of a near net-shaped component with a good surface finish. An investigation was undertaken to develop three new face coat systems based on yttria, but with better sintering properties. These face coat systems were mainly based on an yttria-alumina-zirconia system (Y2O3-0.5 wt pct Al2O3-0.5 wt pct ZrO2), an yttria-fluoride system (Y2O3-0.15 wt pct YF3), and an yttria-boride system (Y2O3-0.15 wt pct B2O3). After sintering, the chemical inertness of the face coat was first tested and analyzed using a sessile drop test through the metal wetting behavioral change for each face coat surface. Then, the interactions between the shell and metal were studied by centrifugal investment casting TiAl bars. Although the sintering aids in yttria can decrease the chemical inertness of the face coat, the thickness of the interaction layer in the casting was less than 10 μm; therefore, these face coats still can be possible face coat materials for investment casting TiAl alloys.

The investment casting process offers great freedom of design with the economic advantage of near net shape production and has been widely used to process TiAl alloys. An investigation was undertaken to develop stable and cheap refractory slurry systems, suitable for use as a face coat in the investment casting of titanium aluminides. A TiO2–Al2O3 (TiAlO) based face coat slurry was investigated and compared with a traditional yttria based face coat slurry. After sintering, the chemical inertness of the manufactured shells in contact with molten TiAl alloys was tested by simulating the casting process using a flash re-melting test, in which sample shells were in contact with titanium aluminides at given temperatures for varying time durations and subsequently re-solidified. Compared with yttria, the TiAlO face coat had relatively lower chemical inertness against molten alloys, forming a thick hardened layer at the metal/shell interface with massive interaction products along grain boundaries. The reaction also changed the wetting behaviour of the TiAl on the TiAlO face coat with the contact angle decreasing as a function of interaction time.