Jonathan Newell examines whether cold spray deposition technology is ready to extend the life of metal alloy aircraft components.
Many of the aircraft fleets of the world’s airlines are reaching middle-age and as austerity continues to bite, expectations for extended life cycles are being raised creating a difficult equation to be solved between continued operation of ageing equipment and the overarching need for reliability in safety critical parts, which are typically replaced by new components during major overhauls.
Metals vs composites
Composite technology that’s suitable for aerospace usage is still relatively modern and considerable advances have been made in the development of composite materials over the last few decades. However, although the latest airliners such as the Boeing 787 Dreamliner and the Airbus A350XWB can boast as much as 50% composite content in their structures, older aircraft have far less with those built in the 1980s having as little as 5%.
Despite the more extensive use of composites in new-builds, metal alloys are still of enormous importance and the aerospace industry makes extensive use of aluminium, magnesium and titanium alloy components, mainly for structural airframe parts and engine components. This continuing reliance on metal parts means that metal alloys are certain to remain a subject of critical importance in the aerospace industry for the foreseeable future.
As aerospace companies increasingly seek to update existing air fleets rather than purchase new stock, more reliable and economic life extension strategies are needed in order to safely continue operating ageing aircraft. It is much more environmentally friendly and less costly to repair rather than replace critical parts, the development of environmentally sound, inexpensive, reliable and safe repair technologies is a key part of this process.
Work is being done by a number of groups to research and develop life extension processes for the aerospace industry, including the EU funded CORSAIR consortium investigating the use of cold spray deposition on metallic parts.
Cold spray metal deposition
The technique of cold spray metal deposition is to fire powdered metal at high velocity through a nozzle at a metal substrate. The term “cold” is used as the process operates at a temperature significantly below the melting point of the powdered feedstock or the substrate.
When the powder hits the substrate, there is sufficient kinetic energy for it to form a metallurgical bond with the substrate material. Repairs can therefore be made to component parts without the need for cutting and joining and without disturbing the mechanical or metallurgical properties of the base material.
Despite the apparent simplicity of the concept, the use of the technology requires significant work in terms of verification and certification on specific alloys and component types in order for it to be approved as a repair technology for aerospace components.
So why not use thermal spraying or laser metal deposition techniques? The problem lies in the types of alloys used in the aerospace industry. Magnesium, Aluminium and Titanium alloys exhibit high levels of sensitivity to elevated temperatures, oxidation and the effects of heating and cooling the material quickly.
Examples of the limitations of thermal spray repairs on aircraft components include mechanical distortion, poor adhesion to the substrate, oxidation, porosity and residual stresses that could lead to subsequent failure in service. Cladding techniques such as laser deposition on the other hand tend to introduce thermally-induced changes in the substrate.
The CORSAIR project
CORSAIR (COld spray Radical Solutions for Aeronautic Improved Repairs) is a European consortium made up of 11 organisations stretching from Eastern Ukraine to the North of England. Comprising experts in aviation, materials science and cold spray deposition, the consortium includes Iberia Airlines, Airbus, three academic institutions and a number of industry suppliers.
Using funding allocated by the European Union Framework 7 programme (Project 605027), the consortium is examining every aspect of the cold spray process from powder feedstock through the deposition system and spraying parameters to post-processing in order to optimise its utilisation for the repair of aerospace components.
Starting in mid 2013, the 42-month project began with the selection of materials and components that would be subjected to investigations into the feasibility of repairs using the cold-spray deposition process. This took existing technology beyond the state of the art and required the development of new cold spray guns, nozzles and the ability to repair previously unreachable places such as the inner diameters of components.
The project’s second year focused mainly on the larger scale manufacturing of powder feedstock beyond the laboratory conditions used for the initial studies. Year two also saw the further development of equipment to enable it to be portable and therefore suitable for in-situ repairs.
All the CORSAIR project stakeholders are descending on the Greek island of Skiathos in mid-June to attend the 4th International Conference of Engineering Against Failure (ICEAF) at which it will discuss the second year milestones and their readiness to enter the final crucial 18 months of the project in which test repairs will be carried out on the chosen combinations of materials and components which will then be subject to a validation process. If this is successful, by late 2016, cold spray deposition technology will have been demonstrated to be viable for aircraft component repairs.
The portable cold spray system that was built during the last year will have been fully tested by that date and the consortium expects to announce its readiness to enter the final phase of the project.
The latest position on powder feedstock readiness was recently presented at the AeroMat 2015 event in Long Beach for the Aerospace Materials industry, where Heidi Lovelock of consortium member TWI delivered a presentation on the effect of powder feedstock properties on Ti-6Al-4V cold sprayed coating characteristics, which detailed work relating to the optimisation of coatings on Ti-6Al-4V substrates, by the careful control of Ti-6Al-4V powder properties and spraying parameters.
Impact on future of aviation
The successful adoption of cold spray deposition technology as a mainstream repair system for aerospace components could have a significant impact on the aviation industry during the next decade and beyond.
The main benefits of using such repair techniques are the extended life expectancy of component parts, the ability to repair rather than replace structurally significant parts and overall improvements in aircraft reliability and utilisation.
In addition, the cold spray technique goes some way towards reducing the usage of existing hazardous and environmentally unsafe repair processes such as hard anodising and chromate conversion. Such processes require expensive measures to protect the environment from harmful waste and to protect workers from exposure to hazardous chemicals.
The economic benefits for the airline industry have the potential to be enormous. According to the CORSAIR consortium, maintenance bills for larger carriers can amount to as much as a billion Euros annually and therefore any sustainable reduction in maintenance costs directly affects the bottom line for the highly competitive airline industry.
The use of cold spray deposition as a repair technology enables components to be repaired and continued to be used when they would otherwise have to be replaced. The repair can also take place in-situ reducing the overall time needed to carry out the repair.
A short horizon
With the CORSAIR project due to end in November 2016 and with progress so far being on target and having achieved expected and consistent results, it is likely that the airline industry won’t have to wait too long to be in a position to validate the technology and start achieving the benefits that it promises.
Further information can be found on the CORSAIR project website