Phase transition and dynamic recrystallization mechanisms of white layer formation during turning superalloy Inconel 718

Abstract

White layer is commonly formed on the machined surface of nickel-based Inconel 718 superalloy, which probably induces the generation of cracks, leading to the fatigue life reduction of the workpieces. Nevertheless, the microstructure of the white layer is still not revealed clearly. The core issues, which this paper aims to reveal, are the transition of γ, γ′, γ" and δ phases and dynamic recrystallization mechanism within the white layer formed during turning Inconel 718. Therefore, the dry orthogonal cutting tests with worn tools were conducted to make the white layer first. After that, the morphology and microstructure of the white layer are studied with the aid of scanning electron microscope (SEM), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and electron backscatter diffraction (EBSD). The results indicate that the thickness of the white layer is uneven and varies within three microns. Meanwhile, the amount of γ′ phase decreases, and δ phase is crushed and refined within the white layer. The lattice mismatch between γ and γ" phases decreases and the white layer is more homogeneous than the original material. Furthermore, the grains were refined to submicron, and continuous dynamic recrystallization (CDRX) occurs in the white layer. The CDRX results in a recrystallized structure with significantly reduced defect density in the white layer formed during the turning of Inconel 718.