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

Residual stress formation during cold spraying process may result in deteriorative effects on the performance of coating materials. The objective of this investigation is to characterize residual stress built-up in two well-known nickel-based superalloys (Inconel 625 and Inconel 718) deposited using cold spraying technique. To this end, the residual stress was precisely measured using x-ray diffraction method. Here, residual stress in the subsurface regions was only studied because the surface properties may alter during sample preparation. The average residual stress was slightly higher in Inconel 625 compared to the Inconel 718 sample. Heat treatment at 800 °C helped in the reduction of porosities which exerted tensile stress in subsurface regions of both coatings. Stresses with opposite signs could cancel each other and result in reduction of residual stress after heat treatment. However, the recovery of residual stress was higher for Inconel 718 coating. In the next step as-sprayed and heat-treated coating samples were subjected to microindentation test to measure their hardness and study the crack formation in the samples. The as-sprayed Inconel 625 exhibited higher hardness than Inconel 718, but the hardness of Inconel 625 decreased more drastically after heat treatment. While the cracks were formed on both as-sprayed samples around indents, no cracks were found in the heat-treated samples. The results from this study will contribute to better understanding the performance of cold spray deposited superalloys under service conditions and the effect of stress relaxation heat treatment on elimination of residual stress.