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Research Papers

Chip Morphology and Chip Formation Mechanisms During Machining of ECAE-Processed Titanium

[+] Author and Article Information
Brian Davis, David Dabrow, Andrew Miller, John K. Schueller

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611

Ryan Newell, Yongho Sohn

Department of Materials Science
and Engineering,
University of Central Florida,
Orlando, FL 32816

Guoxian Xiao

General Motors Global R&D,
Warren, MI 48090

Steven Y. Liang

George W. Woodruff School of
Mechanical Engineering,
Georgia Institute of Technology,
Atlanta, GA 30332

Karl T. Hartwig

Materials Science and Engineering Department,
Texas A&M University,
College Station, TX 77843-3003

Nancy J. Ruzycki

Department of Material Science and Engineering,
University of Florida,
Gainesville, FL 32611

Yong Huang

Department of Mechanical and
Aerospace Engineering,
University of Florida,
Gainesville, FL 32611;
Department of Material Science
and Engineering,
University of Florida,
Gainesville, FL 32611
e-mail: yongh@ufl.edu

1Corresponding author.

Manuscript received June 3, 2017; final manuscript received October 30, 2017; published online December 21, 2017. Assoc. Editor: Guillaume Fromentin.

J. Manuf. Sci. Eng 140(3), 031008 (Dec 21, 2017) (12 pages) Paper No: MANU-17-1355; doi: 10.1115/1.4038442 History: Received June 03, 2017; Revised October 30, 2017

Severe plastic deformation (SPD) processing such as equal channel angular extrusion (ECAE) has been pioneered to produce ultrafine grained (UFG) metals for improved mechanical and physical properties. However, understanding the machining of SPD-processed metals is still limited. This study aims to investigate the differences in chip morphology when machining ECAE-processed UFG and coarse-grained (CG) titanium (Ti) and understand the chip formation mechanism using metallographic analysis, digital imaging correlation (DIC), and nano-indentation. The chip morphology is classified as aperiodic saw-tooth, continuous, or periodic saw-tooth, and changes with the cutting speed. The chip formation mechanism of the ECAE-processed Ti transitions from cyclic shear localization within the low cutting speed regime (such as 0.1 m/s or higher) to uniform shear localization within the moderately high cutting speed regime (such as from 0.5 to 1.0 m/s) and to cyclic shear localization (1.0 m/s). The shear band spacing increases with the cutting speed and is always lower than that of the CG counterpart. If the shear strain rate distribution contains a shift in the chip flow direction, the chip morphology appears saw-tooth, and cyclic shear localization is the chip formation mechanism. If no such shift occurs, the chip formation is considered continuous, and uniform shear localization is the chip formation mechanism. Hardness measurements show that cyclic shear localization is the chip formation mechanism when localized hardness peaks occur, whereas uniform shear localization is operative when the hardness is relatively constant.

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Figures

Grahic Jump Location
Fig. 1

(a) Schematic representation of the experimental setup and (b) high-speed camera system implemented on the Whacheon lathe

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Fig. 2

Representative chip morphology variations of ECAE 1, ECAE 2, and CG grade 4 Ti chips as a function of cutting speed

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Fig. 3

Surface topographies of ECAE-processed Ti chips (perpendicular to the chip surface): (a) aperiodic saw-tooth chip formation (0.1 m/s), (b) continuous chip formation (0.5 m/s), and (c) periodic saw-tooth chip formation (10 m/s). For comparison, the corresponding surface topographies of CG Ti chips are also shown in (d)–(f).

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Fig. 4

Shear band spacing of ECAE-processed and CG Ti chips (± sigma error bars)

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Fig. 5

Segmentation ratio of ECAE-processed and CG Ti chips (± sigma error bars)

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Fig. 8

Free surfaces of ECAE-processed Ti chips: (a) schematic of a saw-tooth segment showing the viewing direction, (b) rippled texture at 0.1 m/s, (c) fine lamellae at 0.5 m/s, and (d) smooth surface/elongated dimples at 10.0 m/s. For comparison, the corresponding free surfaces of the CG Ti chips are also shown in (e)–(g).

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Fig. 6

Progression of a single saw-tooth segment of ECAE-processed Ti machined at a cutting speed of 0.1 m/s. The schematics illustrate the four deformation stages: (a) stage 1, (b) stage 2, (c) stage 3, and (d) stage 4.

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Fig. 7

Progression of continuous chip formation of ECAE-processed Ti machined at a cutting speed of 0.5 m/s

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Fig. 9

Shear strain rate (γ˙) progression for ECAE-processed Ti during (a) formation of a single saw-tooth segment (aperiodic saw-tooth chip), (b) formation of a single continuous chip, (c) formation of several aperiodic saw-tooth segments, and (d) formation of a single continuous chip over a longer period than in (b) (SB: shear band)

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Fig. 10

Nano-indentation measurements across the PSZ: (a) Schematic representation of the indents for (b) ECAE-processed Ti and (c) CG Ti

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