Concept and Objectives:
The aim of this research project on Casting of Large Ti structures (COLTS) is to build on recent work on casting of Ti-based alloys to further develop centrifugal and gravity casting, so that large components of Ti alloys can be manufactured cost-effectively.
This project is in response to call AAT.2010.4.1-8 which aims to enhance strategic international co-operation between China and Europe in the field of casting of large titanium aerostructure components. The background to the project is the very significant weight saving and thus to improved fuel economy and reduced emissions which can come about through the use of Ti alloy components in some airframes and spacecraft and in aeroengines.
The potentially most cost-efficient method of production of such components is casting, and the focus of this proposal is casting of Ti alloys using clean-melting technology, skull melting, which limits the superheat of the molten alloy to about 40°C above its melting point.
Because of this limited superheat, it is necessary to use centrifugal casting or very sophisticated gravity casting, so that mould-filling can be achieved.There is considerable experience in China in centrifugal casting and gravity casting of components up to 1.5m in length and larger casting tables up to 2 and 4m in length are now in development. This proposal is thus an excellent fit for an FP7 collaborative project with China.
Two demonstrator components have been identified which will be cast from Ti6Al4V. A comprehensive data-base of the mechanical properties specified by endusers for the cast components will be obtained and an important part of the work is to further improve the properties of welded Ti structures in order to allow large components to be built up via welding if that is necessary.
Underpinning all of the experimental work there will be a comprehensive process-modelling activity.
COLTS Results in brief:
Aircraft casting overcomes reactivity
Reducing production costs and enhancing sustainability and performance are key goals of the aerospace sector. Novel casting technology developed with EU support will enable improved manufacture of titanium (Ti)-based components to address all three.
Ti alloys are widely used in the aerospace industry given their excellent strength, low density, corrosion resistance and durability. Large Ti components are typically manufactured using thermochemical processing. Although it is expensive, it is currently preferable to more cost-effective casting due to superior quality and reliability of the product.
A Chinese and European consortium initiated the EU-funded ‘Casting of large Ti structures’ (COLTS) project to improve the quality of cast products and reduce the cost even further. Casting is a process whereby molten metal is poured into a hollow mould and the material is cooled into a solidified shape. The Chinese team brought extensive experience in casting and in electron beam welding of Ti alloys. This was complemented by European group expertise in computer modelling of casting and welding and in assessment of materials’ microstructures and properties. Selected demonstrators were a large cylinder, a cubic space frame, a doorframe and cross-connectors containing many thin-walled components.
Molten Ti alloys are highly reactive and must be melted under special conditions that limit their temperature. This in turn makes filling of mould cavities difficult, particularly for thin and/or large castings. Researchers applied metal-based coating to strong waxes developed by the consortium to make the final moulds for casting the parts with limited wax distortion. The coatings significantly inhibited reactivity with the molten alloy.
Welded and cast components showed promising ability to meet requirements of dimensions, surface finishes and mechanical properties. Subsequent modelling suggests that processes can be optimised to do so completely. Project components are now on display at the European Space Agency’s museum in the Netherlands, attesting to the significance of outcomes.
COLTS Ti casting technology will soon result in lighter, lower-cost and high-performance components for the aerospace sector with lower environmental impact all along the product life cycle. Results will provide a boost to the EU aerospace sector and have important applications to numerous other fields in which Ti is being used.