Assessment of the grindability of Inconel 718 under different coolant delivery techniques

Abstract

Nickel-based Inconel 718 stands out from other materials due to its peculiar combination of properties such as high mechanical strength, high hardness, high chemical inertia and low thermal expansion, as well as its ability to operate at high temperatures and in cryogenic conditions. These properties make this material suitable for aeronautic applications, especially for hot structural components, such as blades and discs of aircraft engines in which a high dimensional accuracy and low surface roughness are required. Such manufacturing requirements in general can be achieved through the grinding process. However, due to low thermal conductivity of both the Inconel 718 alloy and the conventional abrasive grinding wheels, most of heat generated during machining concentrates on the workpiece–grinding wheel interface. The usual practice for the removal of heat from grinding zone is by the application of high volume of coolants, but such technique can cause injury to the operator and harm to the environment, if special care with its use and disposal is not taken, which, in turn, increases the cost of the process. Furthermore, due to the poor machinability of Inconel 718, cutting conditions need to be different from those generally employed when grinding steels and cast irons, especially the depth of cut. Within this context, this study presents an approach for determining the grindability of Inconel 718 under different coolant delivery techniques (conventional and minimum quantity lubrication—MQL) and cutting conditions. Grinding tests were also carried out on a grey cast iron material for comparison purposes. Surface roughness, microhardness and surface texture of machined surfaces were the output parameters used to access the grindability of Inconel 718. Results showed that higher values of surface roughness were recorded after grinding the Inconel 718, compared to grey cast iron, thereby indicating the lower grindability of Inconel 718. No evidence of reduction in hardness due to thermal damage was observed after grinding both materials under the established cutting conditions.