Effect of Heat Treatment on Residual Stress of Cold Sprayed Nickel-Base Superalloys

Abstract

Residual stress can be developed in most thermally sprayed coatings due to the momentum of molten particles during impact; and heat transfer during solidification of the splats. Another reason for residual stress built-up in thermally sprayed coatings is due to splat curl-up during solidification and the differences in thermal expansion coefficients between the coating and the substrate. However; in the cold spraying process; it is believed that the main reason for residual stress formation is plastic deformation during impact and flattening of solid particles. Residual stresses can drastically influence coating quality and reduce its service time. In this study; residual stress is measured for two well-known nickel based super alloys (Inconel 625 and Inconel 718) deposited on 7074 aluminum alloy substrates by the cold spraying technique. Residual stress in Inconel 625 was found to be highly tensile on the surface and compressive on the subsurfaces. After heat treatment the residual stress was relieved and was compressive in nature. Whereas for Inconel 718; residual stress was compressive on the surface and tensile on the subsurfaces in the as-sprayed condition. After heat treatment; the residual stress was compressive with increased magnitude. The heat treatment at 800°C made the residual stress more compressive. The porosities of both Inconel 625 and Inconel 718 were reduced after heat treatment.