Tool wear and chip formation during dry high speed turning of direct aged Inconel 718 aerospace superalloy using different ceramic tools

Direct aged Inconel 718 superalloy is used in manufacturing critical cross-sections of aero-engine components. It is a hard-to-machine material, especially in dry conditions. To perform successful machining operations for this alloy, cutting tools with high hot hardness and chemical stability are required. The present study investigates the tool wear and chip formation during dry finish turning of direct aged Inconel 718 superalloy (51-53 HRC) using different ceramic tools. Pure alumina with added ZrO2 and alumina matrix reinforced with silicon carbide whiskers tools were used at cutting speeds of 150 and 250 m/min. A scanning electron microscope and energy dispersing spectroscopy were utilized to study the tool wear mechanisms. Structural and phase transformations during cutting on the tool-chip interface at the higher cutting speed were analyzed with X-ray photoelectron spectroscopy. Chip-undersides and cross-sections were studied with scanning electron microscope to investigate the chip formation mechanism. Results reveal that pure alumina with added ZrO2 can be an adequate choice for machining direct aged Inconel 718 because of its higher abrasive wear resistance and the formation of a sapphire protective tribo-layer at the tool-chip interface under severe cutting conditions. Chipping and notching of the cutting edge was found to decrease with the rise of the cutting speed for this tool material. As confirmed by X-ray photoelectron spectroscopy, alumina reinforced with silicon carbide whiskers tool was found to have a lower performance due the chemical degradation of the whiskers, especially at the higher cutting speed (250 m/min).