Microstructural Characterization, Mechanical Properties and Thermal Stability of Friction Surfaced Inconel 718 Coatings

In the present work, coatings of Inconel 718 were deposited on a self-mating substrate by friction surfacing process. The bonding between the coating and the substrate was sound. The microstructure in the coating volume was analysed in detail through electron back-scattered diffraction. The coating microstructure contained fine grains (1-2 mu m) and majority (> 74 pct) of boundaries had high angle character. The fine grains were formed by discontinuous dynamic recrystallization process. Local occurrence of continuous dynamic recrystallization was also observed in the microstructure. The microstructure was heterogeneous across the coating width. On the retreating side, the grain size distribution was wide and the grains had higher retained deformation (strain) inside them. There was variation in the fraction of twin boundaries across the coating width. The difference in the microstructural features is attributed to the variation in strain rate and temperature across the coating width during friction surfacing. A temperature variation of 50 degrees C was recorded across the coating width through infrared camera. The coating hardness was significantly higher than that of the substrate. The coating microstructure was stable against the double ageing treatment, which was carried out to generate the strengthening precipitates, post friction surfacing. The coating hardness further improved on the double ageing treatment. Annealing at 800 degrees C for 1 hour did not change the grain size in the coating, however, the hardness dropped. This was possibly due to over-ageing/dissolution of the strengthening precipitates. A further decrease in hardness and grain growth was observed in the coating when annealed at 900 degrees C and 1000 degrees C for 1 hour.