Characterisation Of Cold Sprayed Ni Alloy 718 Coatings

Repairing of Ni-alloy components using cold spray is being increasingly considered as an option in the aerospace industry. To further the understanding of the microstructure of Ni-alloy coatings and the bonding mechanism, gas atomised alloy 625 and 718 were sprayed on carbon steel substrates to form 0.5 mm thick coatings and single particle impacts. Spray trials were performed with different process parameters to compare the splat and coating morphology/microstructure and to optimise the parameters. The un-sprayed powder, single particle impacts and coatings were characterised using SEM, EBSD, TEM and nano-scale XRF and XRD. Four-point bend tests were performed to test strength, ductility, cracking and de-bonding. Fine grains were observed in the substrate-particle interfaces caused by particle fragmentation, deformation and dynamic recrystallisation. Low angle grain boundaries and sub-grains form in the substrate due to strain induced by high energy impacts. The deposition efficiency, thickness, porosity, hardness and surface roughness of the coatings were measured and compared across all parameters. The porosity decreases notably and the hardness increases with the increase in gas temperature and pressure. The results indicate that temperature has a larger effect on the coating properties compared to the pressure and that deformation has an important role in bonding.