High-Speed Dry Milling of IN718 with Advanced Ceramic Tools: A Machinability Study

Abstract

Inconel 718 (IN718) is a widely-used nickel-based superalloy in the aerospace industry, which is known for its superior mechanical properties and microstructural reliability at elevated temperatures. However, the poor machinability of this heat resistant alloy has always been a challenge for the manufacturing industry. The high shear strength, low thermal conductivity and intensive work hardening properties of IN718 induce high thermo-mechanical stresses on the tool’s cutting edge which leads to rapid tool wear even at extremely low cutting speeds. Hence, limited scientific data is available around high-speed machining of IN718, especially on more demanding applications such as interrupted cutting during milling. This research is focused on IN718 machinability studies at extreme conditions, using a dry high-speed face milling process with exotic tool materials like advanced SiAlON ceramics. Comprehensive material characterization techniques and machinability studies have been performed using tool wear studies, micro and nanomechanical indentations, optical 3D imaging, SEM/EDS studies and finite element modelling to identify the dominant tool wear mechanisms over a wide range of cutting parameters. Also, further investigations have been done on the mechanisms of chip formation, shear deformation at extreme conditions and tribological aspects of tool-chip-workpiece interactions such as the formation of built-up layers and tribofilms. Results suggest that there is a narrow operational window (around 1000 m/min cutting speed) in which high-speed machining of IN718 superalloy can be performed using a carefully selected ceramic tool material. Experimental results under this cutting speed show a drastic transformation in the machinability of inconel 718 when surpassing a high level of temperature and strain rate. Following further surface integrity studies on the machined workpiece, it can be concluded that such a process is suitable for rough or semi-finishing of IN718 parts at a four times higher material removal rate as compared with conventional practices currently used in industry.