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Since Dr. Carl Salomon proposed the well-known hypothesis on cutting temperatures in 1931, the debate on the hypothesis has never stopped. It shows that evolutions of temperatures with the increase of cutting speed measured at different points are different. From the tool wear and surface integrity point of view, this paper focuses on the evolutions of chip temperature (Chip temp.), tool-chip contact temperature (T-C temp.), and tool-workpiece contact temperature (T-W temp.) with the increase of cutting speed from low (100 m/min) to very high (7000 m/min). First, the cutting heat generation and the temperature increment in machining are theoretically analyzed. Then the influences of chip shape and mechanical property variations with the increase of cutting speed on the temperatures are analyzed. Finally, the cutting temperatures are semiquantitatively derived from chip colors and other heat characteristics in the chip and cutting tool obtained in cutting experiments: evolutions of Chip temp., T-C temp., and T-W temp. with the increase of cutting speed are presented based on the experiment results. The influence of chip shape and temperature variation on tool wear are also discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Chips produced in cutting of pure iron turns into grains at cutting speed 8000 m/min. • Above 8000 m/min chips are formed by transgranular and intergranular fractures. • High ductile metals can be fabricated brittlely with a high deformation rate. Many materials have high-strain-rate-embrittlement effect (HSREE). This study tries to find weather the HSREE takes place in the high-strain rate deformation of a high ductile material pure iron DT8 to assess the possibility to cut the pure iron in its brittle state. Orthogonal high-speed cutting experiments were carried out with the highest cutting speed up to 9000 m/min. Chip formation by brittle fracture mechanism was captured when the cutting speed is higher than 8000 m/min. Brittle fracture characteristics, including transgranular and intergranular fracture morphologies in the chips at 8000 m/min show the pure iron becomes completely brittle at cutting speed of 8000 m/min. The result shows that it is possible to cut the high ductile material pure iron in its brittle state to avoid the problems incurred by its high ductility. [ABSTRACT FROM AUTHOR]

Saw tooth chip is a typical characteristic encountered in high-speed machining. It consists of nearly undeformed segments and highly sheared concentrated shear bands. However, no consensus on the formation of the concentrated shear band has been reached though several saw tooth chip formation models have been built. In this paper, the concentrated shear band formation procedure is analyzed based on a new proposed saw tooth chip formation model. Cutting experiments have been conducted to validate the proposed mode. It shows that the plastic side flow and elastic compression of the uncut chip are crucial for the concentrated shear band formation. The localized shear at the inner end (tool tip side) of the primary shear plane firstly takes place under the indentation of cutting tool. Then, the second localized shear is produced at the outer end (free surface side) of the primary shear plane due to stress concentration there, after which the whole concentrated shear band forms. The stress transition at the outer end of the primary shear plane makes the second localized shear easier evolve into cracks. Influences of material brittleness and cutting speed on initiation and propagation of the concentrated shear band are also analyzed. [ABSTRACT FROM AUTHOR]

Abstract: In this paper cutting experiments of AerMet100 (52 HRC) using nano TiAlN-coated carbide tools with cutting speed from 250m/min to 7000m/min are carried out. The cutting tool wear characteristics under ultra-high cutting speeds are examined and the corresponding mechanisms are analyzed. The focus is put on the tool wear characteristics and mechanisms at cutting speed above 500m/min. The influences of cutting temperature and workpiece dynamic mechanical properties on tool wear are discussed. The results show that the tool wear mechanism is highly influenced by cutting temperature and workpiece mechanical properties. At cutting speed of 500m/min the dominate wear mechanism is micro chipping and fracture due to the cutting force vibration and cold welding of contacting materials between cutting tool and workpiece. At higher cutting speed from 1000m/min to 7000m/min the predominant tool wear mechanisms are abrasion, adhesion, and fracture due oxidation, and thermal softening, and grooving by hard grains. The roughness of tool flank at this cutting stage decreases with the increase of cutting speed due to the high thermal softening. The increase of workpiece material brittleness with the increase of cutting speed lowers the elastic rebound of machined surface and reduces the tool wear rate. [Copyright &y& Elsevier]

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]