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Research Papers

Viscous Shear Banding in Cutting of Metals

[+] Author and Article Information
Dinakar Sagapuram

Department of Industrial and
Systems Engineering,
Texas A&M University,
College Station, TX 77843
e-mail: dinakar@tamu.edu

Koushik Viswanathan

Department of Mechanical Engineering,
Indian Institute of Science,
Bangalore 560012, India
e-mail: koushik@iisc.ac.in

1Corresponding author.

Manuscript received June 7, 2018; final manuscript received July 10, 2018; published online July 31, 2018. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 140(11), 111004 (Jul 31, 2018) (7 pages) Paper No: MANU-18-1434; doi: 10.1115/1.4040875 History: Received June 07, 2018; Revised July 10, 2018

Shear banding is a type of plastic flow instability with often adverse implications for cutting and deformation processing of metals. Here, we study the mechanics of plastic flow evolution within single shear bands in Ti- and Ni-based alloy systems. The local shear band displacement profiles are quantitatively mapped at high resolution using a special micromarker technique. The results show that shear bands, once nucleated, evolve by a universal viscous sliding mechanism that is independent of microstructural details. The evolution of local deformation around the band is accurately captured by a momentum diffusion equation based on a Bingham-type flow rule. The predicted band viscosity is very small, compared to those of liquid metals. A plausible explanation for this small viscosity and fluid-like behavior at the band, based on phonon drag, is presented.

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Figures

Grahic Jump Location
Fig. 1

Schematic of chip formation in plane-strain cutting: (a) uniformly deformed continuous chip under steady plastic flow conditions, and (b) saw-tooth shaped shear banded chip as a result of periodic localization of the plastic flow

Grahic Jump Location
Fig. 2

SEM images of markers in the (a) undeformed state (workpiece) and (b) deformed state (shear banded chip). Sharp displacement of markers between two adjacent segments, due to localized (plastic) sliding at the shear band, is evident from(b).

Grahic Jump Location
Fig. 3

SEM image of an etched chip cross section illustrating the extensive plastic flow within shear band. Note highly sheared, fluid-like flow morphology at the band. No evidence was found for fracture at the band.

Grahic Jump Location
Fig. 4

Shear band displacement profiles in (b) Ti-6Al-4V and (c) Inconel 718 over a range of cutting velocities (V0). The schematic in (a) shows the reference axes used for the plots. U is the displacement parallel to the sliding interface (shear band) and y is perpendicular distance from the interface. VS is the net relative sliding velocity between the adjacent material blocks (chip segments).

Grahic Jump Location
Fig. 5

Comparison of experimental shear band displacement profiles with the viscous slider model predictions in (a) Ti-6Al-4V and (b) Inconel 718. The model-predicted displacement profile (Eq. (3)) is shown as a solid black line. The experimental displacement profiles are fitted to the model curve by appropriately scaling the x-axis. Displacement profiles at different V0 are seen to closely cluster around the model curve. The shear band viscosity (μ) values, obtained from the fits, are shown on the right as a function of V0.

Grahic Jump Location
Fig. 6

Shear band microstructure in (a) Ti-6Al-4V and (b) CP Ti (grade 2. The top row shows bright-field images of the highly refined grain structure with a typical grain size of sub-100 nm in Ti-6Al-4V and ∼ 200 nm in CP Ti. The corresponding diffraction patterns are given in the bottom row.

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