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Research Papers

Microstructural Characterization of Chip Segmentation Under Different Machining Environments in Orthogonal Machining of Ti6Al4V

[+] Author and Article Information
Shashikant Joshi, Asim Tewari

Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, India

Suhas S. Joshi

Department of Mechanical Engineering,
Indian Institute of Technology Bombay,
Powai, Mumbai 400 076, India
e-mail: ssjoshi@iitb.ac.in

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 8, 2014; final manuscript received October 9, 2014; published online November 7, 2014. Assoc. Editor: Toshio Nakamura.

J. Eng. Mater. Technol 137(1), 011005 (Jan 01, 2015) (16 pages) Paper No: MATS-14-1009; doi: 10.1115/1.4028841 History: Received January 08, 2014; Revised October 09, 2014; Online November 07, 2014

Segmented chips are known to form in machining of titanium alloys due to localization of heat in the shear zone, which is a function of machining environment. To investigate the correlation between machining environments and microstructural aspects of chip segmentation, orthogonal turning experiments were performed under three machining environments, viz., room, LN2, and 260 °C. Scanning electron and optical microscopy of chip roots show that the mechanism of chip segment formation changes from plastic strain and mode II fracture at room temperature, to predominant mode I fracture at LN2 and plastic strain leading to shear band formation at 260 °C. The chip segment pitch and shear plane length predicted using Deform™ matched well with the experimental values at room temperature. The microstructural analysis of chips show that higher shear localization occurs at room temperature than the other two temperatures. The depth of machining affected zone (MAZ) on work surfaces was lower at the two temperatures than that of at the room temperature at a higher cutting speed of 91.8 m/min.

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Figures

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Fig. 1

Experimental details: (a) plane strain condition in machining, (b) orthogonal turning, (c) quick withdrawal of tool, and (d) frozen chip root mounted on epoxy mould

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Fig. 2

Experimental set up for (a) LN2 cooling and (b) flame heating

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Fig. 3

Thermal images at a cutting speed of 91.8 m/min (a) at LN2 cooling, (b) at room temperature, and (c) at 260 °C

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Fig. 4

Chip microstructure showing mechanism of segment formation

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Fig. 5

SEM images of chips, segments and modes of fracture under the three machining environments at a cutting speed of 23.4 m/min. (a)(c) at room temperature, (d)(f) at LN2 temperature, and (g)(i) at 260 °C.

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Fig. 6

Chip and segments at three initial work piece temperature at cutting speed of 91.8 m/min. (a) and (b) at room temperature, (c) and (d) at low temperature and (e) and (f) at 260 °C.

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Fig. 7

Segment formation at (a) room temperature, (b) LN2 temperature, and (c) at 260 °C

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Fig. 9

Influence of machining environment on segment ratio

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Fig. 10

Influence of machining environment on segmentation frequency

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Fig. 11

Tool-work kinematic relation and contact condition used in Deform™ model

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Fig. 12

Predicted and experimental chip morphology at a cutting speed of 91.8 m/min at (a) and (b) room temperature, (c) and (d) LN2 temperature, and (e) and (f) 260 °C

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Fig. 13

Influence of cutting speed on (a) average shear plane length and (b) average segment pitch at room temperature

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Fig. 14

Comparison of (a) average shear plane length and (b) average segment pitch of experimental and predicted segment dimensions under the three machining environments at a cutting speed of 91.8 m/min

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Fig. 15

Optical images of chip roots and chips obtained under three machining environments at cutting speed of 23.4 m/min (a) and (b) at room temperature, (c) and (d) at LN2 temperature, and (e) and (f) at 260 °C

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Fig. 16

Optical images of chip roots and chips obtained under three machining environments at a cutting speed of 91.4 m/min, (a) and (b) at room temperature, (c) and (d) at LN2, and (e) and (f) at 260 °C

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Fig. 17

Parameters used to analyze MAZ

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Fig. 18

Grain deformation in the MAZ: (a) at room temperature, (b) at LN2 temperature, and (c) at 260 °C

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Fig. 19

Variation in the strain induced in the grain beneath the machined edge: (a) at 23.4 m/min and (b) at 91.8 m/min

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Fig. 20

Influence of machining environment on depth of MAZ

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Fig. 21

Variation in the residual stresses under three machining environment

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