Fatigue behavior of cold spray coatings: The effect of conventional and severe shot peening as pre-/post-treatment

Abstract

Abstract Cold spray coating is gaining increasing attention as a solid state additive technique for repairing structural components. The fatigue behavior of Al 6082 cold spray coating with the same material as substrate is studied. Furthermore, the possibility of improving the load bearing capacity of the cold spray coated specimens by shot peening process is explored. Conventional and severe shot peening were applied before and after cold spray depositions. The coated, coated-peened and peened-coated specimens were characterized by optical microscopy observation, surface roughness and X-ray diffraction residual stress measurements. The specimens were subjected to rotating bending fatigue tests in order to determine the fatigue strength. Unlike the fixed-cantilever, constant-deflection thin specimens, often used to measure fatigue strength of cold spray coatings, the present approach does not considerably reduce the applied stress at the interface compared to the nominal stress and thus represents a more critical and realistic response of the coatings to fatigue loading conditions. The fractured surfaces were examined by scanning electron microscopy. Cold spray coating increased the fatigue strength by 15% compared to the as-received specimens. Further improvement was achieved by the hybrid peening and coating treatment. It was found that shot peening is more efficient if it is performed prior to cold spray deposition. A 26% improvement in the fatigue strength was obtained by performing severe shot peening prior to cold spray deposition. This improvement was attributed to the integrity of coating/substrate interface without any signs of delamination, crack initiation from the surface instead of the interface, crack propagation mainly by transcrystalline mechanism and not through particle boundaries and finally high depth of compressive residual stresses.