Influence of milling direction in the machinability of Inconel 718 with submicron grain cemented carbide tools

The nickel-based alloys have a growing demand in many fields due to their outstanding properties at high temperatures. These properties lead to relatively low machinability, one of the main obstacles to its more extensive use. Improvements in cutting tool quality are one of the key points to overcome the challenges. The decrease to a submicron scale of the grains of cemented carbide tools is one of the alternatives to improve the machinability of the nickel-based superalloys. In this paper, two different grades of submicron grains of uncoated cemented carbide tools, TMG30 (10% Co, S30-40) and CTS18D (9% Co, S20-40), were evaluated in the end milling process of Inconel 718, through a 24 factorial design of experiments having as parameters the cutting speed, feed rate, machining direction (up and down milling), and tool grade. The tool life, machining power, and surface roughness were used as machinability evaluators. It was found that the machining direction, cutting speed, and feed rate had a significant influence on the machinability output variables, with the machining direction being the most significant one. The differences in the two tool grades were too small to be statistically significant. Simulations using the finite element method of the effective plastic strain, validated by the measurement of experimental machining power, showed that the up milling presented around 14% more plastic deformation than the down milling, which combined with the work-hardenability of the Inconel 718 explains the shorter tool life of this condition.