COLTS was a project initiated by a Chinese and European consortium which aimed to improve on centrifugal and gravity casting of large thin-walled components made of Ti alloy Ti6Al4V. Ti alloys are commonly used in the aerospace industry due to their excellent strength, corrosion resistance and durability, as well as their low density. Ti components are commonly manufactured by thermochemical processing, because this method offers superior properties of the final parts. Casting is potentially the most cost-effective way of manufacturing such components, but it comes with several setbacks. The COLTS project aimed to improve the quality of TI castings and develop a casting process which could be regularly used in the industry.
The project focused on a clean-melting method called skull melting. Ti alloys are extremely reactive, therefore they have to be melted in a water-cooled induction furnace to keep a skull of solid alloy between the molten alloy and the induction coil. This limits the superheat to about 40°C, which makes mould filling difficult, especially in thin sections or large castings. Because of this, it was necessary to develop a more refined casting technology, including development of strong waxes, improved yttria-based mould coatings, and appropriate mould design. The process design was supported by extensive use of computer modelling.
The demonstrator components selected by ESA and Airbus were a large cylinder, a cubic space frame, a doorframe and cross-connectors containing many thin-walled components. The components required by ESA were meant to replace Al components because of the higher modulus and lower coefficient of expansion of Ti. The components designed by Airbus were aimed at improving the fly-to-buy ratio by using castings rather than forgings.
The manufactured parts were close to the specifications set by both companies. Although further work is required on some aspects, for instance dimensional tolerances and yttria inclusions, subsequent modelling suggested that it would be possible to optimise the process to meet the requirements completely. Unfortunately this was found to be impossible within a three-year project. Examples of these components are exhibited in ESA’s museum in the Netherlands.