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The contact stress and heating effect between the cutting tool and workpiece in metal machining is symmetrical. However, the symmetry may be destroyed by changes in the workpiece material mechanical properties, such as ductility. The goal of this study is to reveal the wear characteristics of the cutting tool in machining a ductile metal with the cutting speed at which the metal is embrittled by the high-strain-rate-embrittle effect (HSREE). Orthogonal high-speed turning experiments were carried out. Pure iron type DT8 was cut at different cutting speeds, ranging from 1000 m/min to 9000 m/min. The shape and morphology of the chips obtained in the experiment were observed and analyzed by optical microscope and scanning electron microscope (SEM). Tool wear characteristics at different cutting speeds were observed. It shows that the pure iron becomes completely brittle when the cutting speed is higher than 8000 m/min. On the rake face, the coating of the cutting tool bursts apart and peels off. A matrix crack originates in the cutting edge or rake face and extends to the flank face of the cutting tool. The effects of HSREE on the tool wear is discussed. The findings of this study are helpful for choosing a suitable tool for brittle cutting of the ductile metal pure iron with very high cutting speed and solving the problems in machining due to its high ductility and high stickiness. [ABSTRACT FROM AUTHOR]

This paper investigates the brittle removal mechanism of ductile materials in ultra-high speed machining (UHSM). Firstly, a predictive model of critical cutting speed for UHSM is proposed with the theory of stress wave propagation. The predicted critical cutting speed for ductile-to-brittle transition of ductile materials is then validated by machining experiments of 7050-T7451 aluminum alloy at the cutting speeds ranging from 50 m/min to 8000 m/min. The experimental results show that fragmented chips are produced above the critical cutting speed of 5000 m/min for 7050-T7451 aluminum alloy. The scanning electron microscopic (SEM) images of chips, chip roots and finished workpiece surfaces are observed and analyzed. Large amounts of brittle cracks and cleavage steps are observed on the fragmented chip surface obtained under the ultra-high cutting speed. Due to the remained brittle cracks, the finished surface quality obtained with UHSM is worse than that obtained with high speed machining. Secondly, the specific energy models for the chip formation are proposed and validated by experiments under ductile regime machining and brittle regime machining, respectively. The specific energies consumed for continuous and serrated chip formation mainly include plastic deformation energy located in the primary shear zone, the friction work between the tool–chip interface, and the chip kinetic energy. The plastic deformation energy accounts for the largest proportion among the total specific energy. Comparatively, the specific energy consumed during fragmented chip formation mainly includes the local kinetic energy of fragments and fracture surface energy. When the chip morphology evolves from serrated to fragmented one, the specific energy consumed reduces substantially, which demonstrates that the UHSM is beneficial for the energy saving. Lastly, taking both of the material removal efficiency and machined surface quality into consideration, the UHSM is recommended to be applied in rough machining or semi-finishing, while high speed machining is recommended to be applied in finishing process. This research firstly reveals the control mechanism for ductile-to-brittle transition of ductile materials under critical cutting speed (i.e. critical strain rate) considering solid mechanics and metal cutting principles as well as energy consumption simultaneously. This paper is enticing from both engineering and analytical perspectives aimed at revealing the mechanism of UHSM and instructing the optimization of machining parameters. [ABSTRACT FROM AUTHOR]