The effect of microstructures and precipitates (γ', γ″, δ) on machinability of Inconel-718 nickel-based superalloy in turning process

Nickel-based superalloys have been extensively used in the aerospace and energy sector due to their excellent mechanical properties at high pressures and temperatures. The exotic mechanical properties of Inconel-718 majorly depend on its microstructure and precipitates, which can be controlled using a heat-treatment process. In this work, a wide range of microstructures with micro-hardness (210–418 HV) and average grain size (12–88 μm) was achieved by solution treatment and aging. Electron microscopy (EBSD, SEM, EDS and TEM), analysis revealed the precipitates and microstructures. A high-volume fraction of γ' and γ″ increases the strength while δ reduces the strength of the sample. The role of microstructure, precipitates and machining parameters (such as feed and depth of cut) on the machinability of Inconel-718 was investigated. There was no direct correlation observed between average grain size and machinability due to the presence of precipitates. The solution treatment of the as received sample enables dissolution of all the precipitates (γ', γ″, δ), leaving only the matrix (γ) phase, thus improving the machinability. The machined surface was characterized using Kernel average misorientation (KAM) and Image quality (IQ) map to quantify the machined affected zone (MAZ). Both the KAM and the IQ, parameters consistently reveal MAZ with large plastic deformation, with no grain fragmentation, micro cracks, and strain localizations in the solution treated samples which are in contrast with the aged samples.