Experimental and computational contribution to chip geometry evaluation when micromilling Inconel 718

Abstract

The spheroidal chip, or specifically dendritic spherical chip, was recently reported during Inconel 718 micromilling. It is believed that this kind of chip is obtained when oxidation is induced due to the high specific energy in a small volume of chip. If a high rate oxidation is possible, the exothermic reaction will allow the chip to obtain enough heat and therefore, change its shape to a spherical geometry. It has been reported that Inconel 718 favor the occurrence of this chip once it has low thermal conductivity and high oxidation rates. However, it is a new kind of chip and still needs literature consolidation. In this sense, this work aims to contribute in the comprehension of the different chip geometries obtained in the micromilling process. To evaluate chip formation, micro slots were machined in Inconel 718 workpiece, with a 400 μm diameter WC micromill tool. The chips were collected and analyzed into the scanning electron microscope. Considering the small chip dimensions and the inability of accessing the cutting zone, computer simulation analyses were performed to complement the discussion. Three main types of chips could be identified: helicoidal, ribbon and spheroidal. Helicoidal chip is formed by the cutting properly occurring in the main cutting edge and the ribbon (or arc) chips are attributed to the minor cutting edge. Despite unexpected, both chip types are formed in the same amount which indicates that the cut in the minor cutting edge is higher than expected, leading to higher wear in this region of the micromill. This action could be caused by an insufficient end cutting edge angle in the micromill and could also be associated with a higher friction, and therefore generating the higher energy that contribute to the sphere's formation. Thus, the presence of spheroidal chip could indicate that the process needs improvement.