Aircraft engine components endure harsh operating conditions, resulting in notable damage, such as compressor blades erosion, reduction of trailing edge thickness in HP turbines due to erosion and oxidation etc. The frequent replacement of these damaged components can be expensive and as a result, the aerospace industry emphasizes on repair and restoration. A significant number of aero-engine components are manufactured using Inconel 718; however, repairing such superalloy materials presents significant challenges. Traditionally, various thermal spray processes have been widely utilized to repair worn-out parts. Nonetheless, these processes come with inherent limitations, primarily due to the involvement of high temperatures. This can lead to issues such as oxidation, grain growth, the formation of a heat-affected zone/brittle phases, and segregation during solidification. Recent advancements in repair technologies have indicated a promising solution to these issues through the use of "cold spray," a solid-state coating process.
The objective of this research is to effectively apply an IN718 coating onto an IN718 substrate using a high-pressure cold spray (PCS-100), aiming for a well-bonded and highly dense deposit with favorable mechanical properties. Particle velocity measurements conducted with Hi-Watch CS2 equipment revealed that an increase in pressure at a constant temperature resulted in higher particle velocities. A thick coatings (>4 mm) was successfully deposited and it was observed that deposition efficiency decreases in expense of built thickness. Additionally, the study explored the effects of gas pressure and traverse speed on the thickness, deposition efficiency, microstructure, adhesion strength, and mechanical properties of cold-sprayed coatings. The splat analysis indicated well-deformed particles with visible jetting, and an escalation in particle deformation was noted with an increase in pressure. The coating density exhibited improvement with an increase in pressure, which further enhanced to a porosity level of 0.5% after post-spray heat treatment (PSHT). The hardness of the as sprayed coatings increased from 397  28 HV to 436  25 HV with increase in gas pressure from 5 to 7 MPa. The hardness was further increased after double aging heat treatment due to γʹ and γʺ precipitation. During adhesion tests it was found that as-sprayed samples were not able to surpass glue strength (74.5 ± 2.5 MPa). However, PSHT played a crucial role in enhancing adhesion beyond glue strength, helping the coating to pass the repair criteria. The tensile test results of the as-sprayed coatings showed inconsistency due to premature failure caused by inadequate metallurgical bonding. Although there was a slight improvement in tensile strength with an increase in pressure, a notable and more substantial enhancement was observed after PSHT. Despite the improved strength via double aging, it failed to provide significant ductility. The combination of solution treatment along with the double aging treatment yielded strength (1325 ± 10 MPa) close to bulk materials but failed to achieve sufficient ductility that could match the bulk material. Thus, despite the improvement in the properties further optimization is suggested.
Fretting wear tests were conducted at RT and 500°C and the effect of PSHT on cold spray coating was studied. Tests were conducted on an apparatus (RTEC MFT5000) and the results were analyzed using profilometer and SEM imaging. Initial trials were done for 5 MPa as sprayed sample and it was found that wear regime changes from full stick to partial slip to gross slip regime as the stroke length is increased. The CoF showed higher value at RT compared to 500°C test due to lack of oxidation wear as present at high temperature which provides lubrication against wear via oxide debris. Further analysis has to be done for reaming samples to study the effect of PSHT and to conclude the wear mechanism.
In conclusion, the study demonstrated that cold spray can be a viable choice for the repair application but to achieve that the desired bulk like properties must be attained. It is suggested to opt options like hot isostatic pressing, induction heating etc. to achieve that and to make cold spray more feasible for the repair of aero engine components.