Chip formation and wear mechanisms of SiAlON and whisker-reinforced ceramics when turning Inconel 718

Inconel 718 has found use in many demanding applications due to its high temperature fatigue strength, hardness and low thermal conductivity. These material properties present a challenge to productive and high quality (surface finish) machining and promote rapid tool wear. The work presented here describes the chip formation and wear mechanisms of silicon nitride (Si3N4) based ceramic SiAlON and silicon carbide whiskers reinforced alumina (WRA) (Al2O3 + SiCw) round (RNGN) inserts, when turning solution-annealed Inconel 718 (27<HRC<30) with a 10% concentration cutting fluid. Four sets of cutting trials were conducted at a cutting speed of 250 m/min and another four at 300 m/min. The results show that using SiAlON turning inserts at a cutting speed of 300 m/min delivered the best results in terms of tool flank wear, total tool life, and work-piece surface finish. The morphology of the Inconel 718 chip, at cutting speeds above 250 m/min, presents an intense shear band localisation in the primary shear zone of the chip/tool interface, which leads to chip segmentation.