Formation mechanism of white layer in high-temperature alloy machining via PCBN tools under high-pressure cooling regimes

Abstract

This study deals with the white layer formation mechanism on GH4169 nickel-based superalloy machined surfaces during PCBN tool turning under high-pressure cooling (HPC). GH4169 is widely used in critical aerospace components for its superior high-temperature mechanical properties, corrosion resistance and microstructural stability, but machining-induced brittle white layer severely degrades component fatigue strength and service life. HPC has shown great potential to suppress white layer formation, yet its underlying mechanism and influencing rules remain unclear. To address this, we conducted 2D finite element cutting simulations and HPC turning experiments of GH4169. Simulations quantified the cutting thermo-mechanical loads via transient temperature, strain and strain rate fields of the machined surface. Experiments explored the effects of cutting speed and cooling pressure on surface work hardening, characterized the element distribution, phase composition and microstructure evolution of the white layer and matrix using EDS, XRD and EBSD, and finally quantified the influences of cutting parameters on white layer thickness. Results show that element diffusion, phase transformation and severe grain refinement occur in the white layer, with the coupling of phase transformation and grain refinement dominating its formation. Under HPC, white layer thickness increases with rising cutting speed and decreases with increasing cooling pressure.