Tool wear and chip morphology in high-speed milling of hardened Inconel 718 under dry and cryogenic CO2 conditions

This paper details an investigation into the performance of PVD tungsten carbide coated ball nose milling inserts when conducting high-speed cutting of Inconel 718 under eco-friendly machining methods of cryogenic carbon dioxide (CO2) and dry cutting conditions. The experiments were performed at varying cutting parameters of; cutting speed: 120-140 m/min, feed rate: 0.15-0.25 mm/tooth, and axial depth of cut: 0.3-0.7 mm. The radial depth of cut was kept constant at 0.4 mm. A new cryogenic CO2 cooling system was introduced for efficient and consistent cooling performance during cutting. The analysis includes the tool life, tool wear patterns and mechanisms as well as its relationship with the chips' morphology. The experimental results showed that cryogenic and dry cutting conditions reported approximately similar tool wear patterns. The tool wear started with smooth abrasion and chipping around the depth of cut line, which then progressed into flank wear and finally notching and flaking via mechanisms of abrasive and adhesive wears. However, severe BUE was repeatedly observed under dry cutting, which widened the flaking and accelerated the notching. Hence, cryogenic CO2 showed significant improvement towards increasing the tool life to a maximum of 70.8% relative to dry cutting. The consistent cooling effect by the cryogenic CO2 managed to efficiently reduce the cutting temperature at the cutting point to 80% compared to dry cutting, which is believed to be the main factor causing the aforementioned improvement. The strong influence of cutting conditions and tool wear patterns upon the chip morphology was also evident. Compared to cryogenic cutting, the shape and colour of the chips were found to be severe, distorted, and darker in dry cutting, which confirmed that it was thermally affected by the high cutting temperature.