ECCI and EBSD study of subsurface damages in high speed turning of inconel 718 under different tools and machining parameters

Inconel 718 is a Ni-based superalloy that can perform excellently at elevated temperatures. However, surface and subsurface damages in the form of microstructural and property changes and tensile residual stresses are common in a machined Inconel 718 component because of its poor machinability. Such damages have a significant influence on performance and the life time of the part. To characterise microstructural damages and understand how they are correlated to machining conditions are not only important for the evaluation of surface integrity but also for the optimization of machining operations to minimise effects from the machining process. This paper uses the ECCI (electron channelling contrast imaging) and EBSD (electron back scatter diffraction) methods to study the effect of cutting tools and cutting speeds on subsurface plastic deformation of machined Inconel 718. When turning at 200 m/min, a comparable level of plastic deformation was found under the surface machined with uncoated cubic boron nitride (CBN), titanium nitride coated CBN and whisker reinforced alumina (WRA). With an increase in cutting speed, the plastic deformation depth increased, and uncoated CBN tools showed superior performance in term of subsurface microstructure alterations compared to the other tool materials.