Investigation on chip formation and surface morphology of forged CrMnFeCoNi high entropy alloy during orthogonal cutting

The CrMnFeCoNi high entropy alloy (HEA) is the candidate for advanced structural materials used in extreme service environments due to the excellent mechanical properties. Good machinability and excellent surface quality are of great significance for realizing the service performance of the material. Chip formation is closely related to machining efficiency, surface integrity and machining stability during cutting. Thus, this work focused on chip formation and surface morphology during high-speed machining of the forged CrMnFeCoNi HEA. The results showed that continuous chips were formed under the given cutting parameters, and the free surface of chips was sawtooth. The degree of serration was closely related to the processing parameters. The chip formation model was established. Under the double action of shear and extrusion, thermoplastic instability occurred along the shear plane, adiabatic shear zone was formed, and integral sliding occurred to form a serrated free surface. The chips were separated from the machined surface by plastic deformation and ductile fracture. A deformed layer with the thickness of about 20 μm was formed on the machined surface. The deformed layer was coordinated with deformation twins and dislocations. The outermost part of the surface was a nanocrystalline region with the thickness of about 2 μm, which played a role in fine crystal strengthening. The thickness of deformed layer and nanocrystalline region decreases with the increase of cutting speed, and increases with the increase of cutting thickness. This paper provides a reference for enriching the machining mechanism of high entropy alloy by deeply understanding the chip characteristics, forming and fracture mechanism and the microstructure of the processed surface.