Fatigue Properties and Crack Behavior of Cold Spray Coatings

Abstract

Cold spray is a promising innovative technology based on the effect of kinetic energy transferred to fine particles impacting on the substrate. The energy is provided by high-pressure carrying gases (Air, He, N2) heated at temperatures well below the melting point of the sprayed materials. The severe plastic deformation due to the high impact velocity of cold-sprayed particles leads to very interesting microstructural features and surface modifications allowing to obtain promising properties of the substrates in terms of fatigue resistance and crack behavior. In order to acquire long-term safety and reliability of materials modified by cold spray, the fatigue properties of coated substrates must be clarified. The present work is aimed to show the effect of various cold-sprayed particles (Al alloys, Ni, and Ni-based alloys) on the microstructural behavior of the coatings, on the coated substrates fatigue properties, and on the crack behavior of pre-notched and coated substrates. A literature review regarding cold spray fatigue properties and crack behavior is presented. The effect of cold spray processing parameters on fatigue properties was analyzed for different materials such as AA7075 sprayed on AA7075 substrate, AA2024 sprayed on AA7075 substrate, and AA2024 sprayed on AZ91 magnesium alloy substrate. They were analyzed those conditions leading to an increase in fatigue life. In the work paper, the possibility of repairing cracks, in aluminum panels, through cold spray is demonstrated. 2099 aluminum alloys with a surface 30° V notch were filled with 2198 and 7075 aluminum alloy powders via cold spray. The crack behavior of V-notched panels subjected to bending loading was studied by FEM and mechanical experiments. The simulations and the mechanical results showed a good behavior in terms of K factor reduction and crack nucleation and growth behavior of the repaired panels. The study was finalized to predict the failure initiation locus in the case of repaired panels subjected to bending loading and deformation. The way stress concentration was quantified, residual stress field and failure are affected by the mechanical properties of the sprayed materials and by the geometrical and mechanical properties of the interface with particular effect of the adhesion strength and of the local residual stresses. The coating benefits are also demonstrated for pure Ni and IN625 particles sprayed on various substrates. The crack behavior is, in this case, shown for 30°, 60°, and 90° notches filled with cold-sprayed particles; the analyses are conducted through FEM and experiments.