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Content archived on 2024-06-10

Enhanced processing of orthorhombic titanium aluminide components


A particular orthorhombic titanium aluminide-based alloy was found to have a better balance of mechanical, physical and environmental properties than two other candidate alloys, and was selected for scaled-up processing studies The fundamental relationships between phase equilibria, microstructural development and mechanical properties have been established. Processing by three routes has been extensively investigated: conventional radial forging and longitudinal forging; plate forging and sheet rolling; "van Gogh skies" (VGS) extrusion. The material responds well to each processing technique and produces good results under a range of conditions.
The VGS microstructure can be obtained by extruding even at relatively low extrusion ratios. Forging after VGS-extrusion reduces strength and improves ductility. The effect is more significant at higher forging temperatures. An investigation of joining techniques indicated that diffusion bonding gave better results than a range of alternative welding techniques. Machining of the alloy is difficult. Cutting techniques (turning, milling and drilling) generally result in a very high rate of tool wear. Grinding appears to be a more economical method.
The oxidation resistance in terms of mass gain is good at temperatures that are higher than those at which current materials can be used, but sub-surface embrittlement and the associated notch-sensitivity are an obstacle to use at lower temperatures. An optimised processing route gives a good balance of properties between strength and ductility.
A final demonstrator component, a centrifugal impeller, has been produced by forging and machining. The machining was difficult but not impossible. It has been demonstrated that a turbocharger blade could be produced by VGS-extrusion, forging and machining.

In summary, the mechanical properties and the processability of the material are generally very good. It is other properties, particularly the oxidation resistance and notch embrittlement, which are obstacles to its use in service.
Objectives and content:

Future developments in gas turbine aero-engine technology, both in the civil and military sectors, are being driven by the requirement for higher thrust to weight ratios and improved fuel efficiency, while maintaining or improving the safety and reliability of the engine. These requirements can be met by decreasing the overall weight of the engine and by increasing the engine operating temperature. Similarly in the industrial power generation sector there is a need to replace current nickel-based turbochargers with alight weight material to improve engine response times and minimise harmful emissions. Therefore there is a strong industrial demand from both the aero-engine and industrial power generation sector for lightweight, fire resistant engine materials with a temperature capability of up to 700ÐC. The recent development of orthorhombic titanium aluminise alloys, based on the composition Ti2AINb, has the potential to meet these demands. Furthermore these alloys offer the potential for conventional processing combined with the temperature capability of y-TiAI based alloys. It is envisaged that orthorhombic alloys have the potential to be used to fabricate lightweight casings and compressor impeller structures in gas turbine aero-engines and turbochargers in industrial power units. However, to date very limited data is available amongst European manufacturers concerning the inter-relationship between thermo-mechanical processing, microstructural development and mechanical properties in these alloys; while the viability of producing large scale forged and sheet components still needs to be demonstrated.

Therefore it is planned to carry out a detailed program of research to define the processing microstructure-property relationships in orthorhombic alloys with a view to identifying an optimum processing route for the production of forged and sheet components. Initially the processing-microstructure-property relationships in three selected alloys will be evaluated under identical processing conditions. The alloy with the most promising balance of properties will subsequently be selected for scaling-up to an industrial scale ingot. Extensive forging and rolling trials will be carried out on this alloy, including the development of constitutive equations to model the materials behaviour, to optimise the thermo-mechanical processing conditions. This thermo-mechanical processing study will be followed by a detailed evaluation of the effects of heat treatment on the microstructure and mechanical properties of the alloy, and an optimised processing heat treatment route will be defined. Machining and joining studies will be performed to identify viable fabrication procedures. The viability of component production using the optimised processing-heat treatment routine, and the joining and machining exercises will be confirmed by the production and testing of a generic demonstration compressor impeller and the fabrication of a flanged casing typical of high temperature engine compBE97-4019

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