Grain refinement mechanism in chip and the machined subsurface during high-speed machining of Inconel 718 alloy

Abstract

Microstructural defects on the machined surface of Inconel 718 alloy can be a significant source of fatigue cracks, potentially leading to failures of the structural components of aerospace engines. This work focused on the grain refinement mechanism induced by the dynamic recrystallization (DRX) behavior in both the chip and machined subsurface during high-speed machining (HSM) of Inconel 718 alloy. Multiscale metallurgical observations were conducted using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and precession electron diffraction (PED) to characterize the crystal structures, including the grain size, grain boundary distribution, dislocation density, and grain orientation distribution. Finite element (FE) simulations were performed to obtain the strain, strain rate, and cutting temperature to analyze the microstructure evolution mechanism under the effect of severe deformation conditions in HSM. The results indicate that grain refinement in the chip primarily occurs in the shear band and secondary deformation zone. The primary cause of the variation in the final grain size is attributed to the distinct thermomechanical loading conditions experienced by the material. The microstructure observed in the machined subsurface reveals a significant gradient distribution, characterized by the presence of an amorphous structure, nanocrystallites, refined grains, and elongated grains along the depth direction. The equiaxed ultrafine nanocrystals observed in the machined near-surface result from both continuous DRX (cDRX) and discontinuous DRX (dDRX) mechanisms. In the deeper zone, the presence of numerous subgrains and a high geometrically necessary dislocation (GND) density reaching up to 1016/m2 within the grains indicate that the refined grains in this zone result from the cDRX mechanism. The variation in DRX mechanisms observed in the machined subsurface are closely related to the gradient distribution of the thermomechanical load generated in HSM.