The prediction of interfacial crack growth of cold sprayed coatings is an important part of their lifetime assessment. Currently, there are no such models available for cold sprayed coatings and hence, coating delamination is described by cohesive law implemented in Finite Element (FE) software. The FE damage model requires, for both normal and tangential direction to the interface, two cohesive material parameters: strength and energy.
A new Cold Spray nozzle design approach has been developed, which takes into account the proper gas expansion through the divergent part of the nozzle, as well as the maximum acceleration achieved by the particles. Short length (53 mm) and low gas consumption nozzles were designed for specific aeronautical applications. In particular, short nozzles were designed for mobile unit applications, allowing longer spraying operation times and reduced operation costs. A new numerical tool was specially designed to rapidly iterate between 480 possible process combination parameters using a data source of more than 6,000 data entries. The tool also provides technological recommendations for the following nozzles: MOC24, ImpactTechNitro, and ImpactTechV7.
The CORSAIR New Industrial Portable Cold Spray Unit was successfully developed. The system comprises of a removable powder feeder and gas preheater. It requires an N2 gas supply, compressed air and electrical connectivity, and allows access up to 7m from the unit itself thus enabling the repair of areas with limited accessibility (in-situ applications). Repairs are being realized on various aircraft components provided by the industrial partners of the consortium and the results are being compared against traditional fixed Cold Spray.
Cold spray repairs on Al and Ti alloys were realized using process parameters beyond the state of the art. Throughout these investigations we were able to assess the optimal temperature and stand-off-distance required to get the maximum deposition efficiency, as well as improved mechanical properties and porosity, with respect to the alloys being studied. . The end-users have provided selected samples and have defined the surface preparation and deposition parameters to be used for the repairs. The first repairs on real parts provided by the industrial partners of the project have started by considering state of the art parameters and will continue by using the optimal parameters identified. Additional components are being used for the evaluation of the tooling and fixture capabilities, and the validation of the first deposition results. In the end, the results will be critically compared.
Cold Spray trials carried out on Al alloys using the CGT Kinetiks 4000 and Impact Innovations 5/11 systems, suggest that process conditions of 500°C and 60bar result in ultra-high density coatings (<0.4% porosity) for the tested powder feedstock. The metallographic analysis indicates a 50% reduction in coating porosity compared to previous best coatings deposited using the CGT Kinetiks 4000/47 system. A different Al alloy C355 powder feedstock in the -45+20µm size range and spherical particle morphology was manufactured and utilized for Cold Spray trials at TWI Ltd. The trials concentrated on maximum process parameters in order to obtain higher density Al alloy coatings with improved mechanical properties. Indeed, the metallographic examination of coatings using this powder, confirmed previous findings that increasing gas temperature and pressure, significantly reduces the coating porosity levels to approx. 2.5%.
Cold Spray trials were also carried out on Ti alloys, using the maximum available gas temperature and pressure of the CGT Kinetiks 4000/47 system. Under these conditions, the coating porosity is relatively high, while coating adhesion remains low. The deposition of coatings at higher process temperature and pressure is realized using the Impact Innovations 5/11 system. Under selected conditions, the coating porosity was <2.0% and the coating adhesion was >70MPa with minimal surface preparation. The cohesive strength of the coatings increased to 819 MPa following heat treatment.
The Cold Spray coating development activity and the extensive testing using selected powder feedstocks will help the decision making process towards achieving successful repair procedures of aircraft components. The evaluation of the repaired samples in terms of NDT, Structural and Microstructural examination, fatigue testing and surface protective treatments (anodizing) is expected in the next months.
The design oif the specimens for the fatigue tests have been reviewed since the previous onceones showed to be not perfectly suitable for achieving adequate and repeatable properties. Now the specimens have been machined and the psuhpush-pull fatigue tests are running.
The aforementioned results were presented at numerous dissemination forums and high-impact events, including the 4th International Conference of Engineering Against Failure, the 5th EASN Workshop on Aerostructures and the AERODAYS 2015 Conference.