Influence of Microstructure on Chip Formation - Machining Aero Engine Alloys
Aero engine materials such as nickel-iron and titanium based alloys are known to be very difficult to machine. They are also known to produce shear-localized chips; a type of chip which is associated with both an increase of the tool wear and the vibration levels during machining. Robust machining operations are crucial for maintaining high productivity in the machining shop and variations in the actual work material, e.g. between batches or when the material supplier is changed, are known causes for unwanted disturbances. It goes without saying that a deeper understanding of how the work material microstructure interacts with the machining operation will lead to more robust processes.
The aim of this work was to increase the understanding of how the microstructure of work material influences shear-localized chip formation. This was studied through transverse and orthogonal turning in two alloys, the superalloy Alloy 718 and the titanium alloy Ti-6Al-4V, for both of which the microstructure was varied by means of heat treatment.
It was found that if the microstructures were finer than the uncut chip thickness, the machining behaviour was isotropic with a gradual increase of the shear-localization in chips when the cutting speed was increased in Alloy 718, and when feed rate was increased in Ti-6Al-4V. However, when the size of the microstructural constituents was increased through heat treatments and to be in the same order of magnitude as the chip thickness, anisotropic effects were found. The result was serrated chips at all cutting speeds in the case of Alloy 718, due to anisotropic deformation of grains at low speeds and with an increased influence of shear localization at higher speeds. The amount of shear localization was found to vary with the crystallographic orientation. In Ti-6Al-4V the orientation of the lamellar structure in the coarse Widmanstätten microstructure type affected the shear localization and this anisotropy could explain the presence of large lamellae in chips even at low feed rate.