Mechanisms in the machinability improvement of Inconel 718 superalloy through ultra-high-speed grinding

Inconel 718 (IN718) superalloy is a typical difficult-to-machine material characterized by low thermal conductivity and severe work hardening. Ultra-high-speed machining (UHSM) exhibits characteristics of material embrittlement and the skin effect of machining damage, which may address the above dual machining issues. This paper investigates the speed effect on the formation of machined surface and, for the first time, achieves ultra-high-speed grinding of IN718 superalloy at a speed up to 240 m/s. The grinding forces and surface integrity across various speed ranges are examined in detail. Multiple techniques are employed to characterize and analyze the subsurface microstructure. The results demonstrate that brittle-mode removal of IN718 superalloy occurs at a grinding speed exceeding 190 m/s, effectively mitigating work hardening and heat generation resulting from intensified plastic deformation. Furthermore, the machining speed influences the formation mechanism of recrystallization layer, gradually transitioning from discontinuous dynamic recrystallization (dDRX) domination to continuous dynamic recrystallization (cDRX) domination with an increase in grinding speed. Meanwhile, multifold nano-twins with a wide range of 4–5 nm form within tens of nanometer grains under UHSM conditions, further altering the subsurface microstructure. These findings provide valuable scientific insights for enhancing the machinability of other difficult-to-machine materials.