Optimization of Inconel 718 thick deposits by cold spray processing and annealing

Cold Spraying of high strength materials, i.e., Inconel 718 is still challenging due to the limited deformability of the material restricting the quality of deposits. Thus, process parameters must be tuned for reaching higher particle impact velocities and temperatures to allow for maximum amounts of well-bonded particle-substrate and particle-particle interfaces. In the present study, Inconel 718 powder was cold sprayed under varied process gas temperatures for a systematic study of the influence on the quality of thick deposits. In addition, one set of samples of each batch was exposed to post heat treatment procedures by hot isostatic pressing, thermal soft annealing, and aging for attaining hard bulk material properties. Deposits microstructure, porosity, electrical conductivity, hardness, and residual stress were analyzed in as-sprayed and as-heat treated conditions. Results are discussed in terms of the coating quality parameter, defined as the ratio between particle impact velocity and critical velocity. As-sprayed deposits exhibit microstructures with highly deformed particles and well bonded internal interfaces. X-ray diffraction reveals that powder and deposits present a gamma-solid-solution phase, allowing to assume conventional softening behavior for estimating critical conditions for bonding. Increasing the process gas temperature leads to lower coating porosity and higher electrical conductivity. Deposits showed similarly high microhardness and compressive residual stresses, both caused by work hardening during cold spraying. Subsequent heat treatments improved the quality of internal interfaces, mostly for deposits with high values of coating quality parameters. By distinguishing influences on several coating properties, these results contribute to gain basic knowledge for successful manufacturing of Inconel 718 thick deposits by cold spraying, particularly concerning needed coating quality parameters for adjusting desired properties.