Cold Spray of Al Alloys as Repair Technology in Aeronautics

In view of the high cost for manufacturing high performance aero engine components, the importance of maintenance and repair is crucial. In particular, the use of thermal techniques such as welding or thermal spray deposition to repair aluminium aircraft components could affect noticeably the substrate-coating interface as well as the residual stress state and microstructure of the whole zone subjected to the thermal input. In this field, the Cold Spray  technology is gaining more and more interest from the aeronautic industry for the deposition of thick coatings with a low temperature approach and the attention of European Union on this technological development is very high as confirmed by the 7^thFP CORSAIR project wherein this study is framed.

Currently, a wide range of Al alloy based coatings are deposited by high-pressure cold spray using a polymeric nozzle that limits the maximum operating gas temperature to 350°C. This study goes beyond this limit by employing a water cooled SiC nozzle that allows to preheat the process gas to  temperatures higher than 350°C and therefore to reduce significantly the critical velocity of sprayed materials. The deposition of A2024 and A357 coatings on similar base materials was studied as the base interest for aeronautic MRO processes of aircraft parts (A2024), and housings and gearboxes (A357), respectively. The evolution of morphology, microstructure and mechanical properties with the spray temperature were studied. Furthermore, the coating adhesion and residual stress profiles were determined for the coatings sprayed with the polymeric nozzle at 350°C and those sprayed with the SiC nozzle at higher temperatures. Finally, repair trials were performed on industrial prototypes following standard procedures and recommendations by the aerospace partnerns.