Unusual Applications of Machining: Controlled Nanostructuring of Materials and Surfaces

[+] Author and Article Information
C. Saldana, T. L. Brown, W. Moscoso, W. D. Compton

Center for Materials Processing and Tribology, School of Industrial Engineering, Purdue University, West Lafayette, IN 47907

S. Swaminathan1

Center for Materials Processing and Tribology, School of Industrial Engineering, Purdue University, West Lafayette, IN 47907

J. B. Mann2

Center for Materials Processing and Tribology, School of Industrial Engineering, Purdue University, West Lafayette, IN 47907

S. Chandrasekar3

Center for Materials Processing and Tribology, School of Industrial Engineering, Purdue University, West Lafayette, IN 47907chandy@ecn.purdue.edu

RFN—rake face normal, CFD—chip flow direction; a detailed description of the different planes can be found in Ref. 25.


Present address: GE Global Research, Bangalore, India.


Also at M4 Sciences, West Lafayette, IN 47906.


Corresponding author.

J. Manuf. Sci. Eng 132(3), 030908 (May 26, 2010) (12 pages) doi:10.1115/1.4001665 History: Received August 14, 2009; Revised April 17, 2010; Published May 26, 2010; Online May 26, 2010

A class of deformation processing applications based on the severe plastic deformation (SPD) inherent to chip formation in machining is described. The SPD can be controlled, in situ, to access a range of strains, strain rates, and temperatures. These parameters are tuned to engineer nanoscale microstructures (e.g., nanocrystalline, nanotwinned, and bimodal) by in situ control of the deformation rate. By constraining the chip formation, bulk forms (e.g., foil, sheet, and rod) with nanocrystalline and ultrafine grained microstructures are produced. Scaling down of the chip formation in the presence of a superimposed modulation enables production of nanostructured particulate with controlled particle shapes, including fiber, equiaxed, and platelet types. The SPD conditions also determine the deformation history of the machined surface, enabling microstructural engineering of surfaces. Application of the machining-based SPD to obtain deformation-microstructure maps is illustrated for a model material system—99.999% pure copper. Seemingly diverse, these unusual applications of machining are united by their common origins in the SPD phenomena prevailing in the deformation zone. Implications for large-scale manufacturing of nanostructured materials and optimization of SPD microstructures are briefly discussed.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

Plane-strain machining of OFHC copper showing (a) velocity and (b) strain rate fields obtained by PIV. α=+20 deg, t0=0.350 mm, tw=3 mm, and V0=5 mm/s. α, t0, tc, and V0 are defined in the left hand side of the figure. tw is the width of the chip measured into the page.

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Figure 2

Deformation conditions accessed in the SPD of pure copper. Contours of constant strain rate (dε¯/dt) are shown overlaid on the graph. The “coordinates” (a, b) of the points A, B, C, and D represent the strain rate, dε¯/dt, and the deformation zone temperature T, respectively.

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Figure 3

TEM images of microstructures in pure copper generated under different SPD conditions: (a) elongated grains and subgrains (γ∼5,α=+20 deg,V0=13 mm/s), (b) equiaxed grains (γ∼7,α=0 deg,V0=13 mm/s), (c) densely nanotwinned structure (γ∼1.5,α=+50 deg,V0=1.67 m/s), and (d) partially recrystallized microstructure (γ∼4, α=+20°, V0=2.5 m/s)

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Figure 4

OIM analysis, including IPF map, pole figure, and misorientation distribution, for copper subjected to shear strain of (a) 3, (b) 7, and (c) 11

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Figure 5

TEM image of the copper after cryo-SPD at γ∼2.5 and dε¯/dt∼103 s−1. Inset SAD pattern shows operating reflections in the lattice, indicative of twinning.

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Figure 6

Deformation-microstructure map for copper showing schematically, microstructures characteristic of the different SPD conditions. The dotted curves demarcate roughly, regions with different microstructures.

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Figure 7

Mechanics of LSEM: (a) schematic showing geometry of constrained chip formation. The constraint fixes the chip thickness tc a priori. (b) Variation in shear strain and normalized hydrostatic pressure (p/2k) in the deformation zone with chip thickness ratio (λ). k is the shear yield strength of the material.

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Figure 8

Bulk forms produced by LSEM: (a) Ta plate (λ=1.4, γ∼1), (b) foils of Al 6061-T6 (λ=1.6, γ∼2), CP-Ti (λ=1.6, γ∼2), OFHC Cu (λ=4.2, γ∼4), and Inconel 718 (λ=3.5, γ∼4) in that order from top to bottom, and (c) rod of OFHC Cu

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Figure 9

Microstructure of bulk alloys produced by LSEM for (a) Al 6061-T6 (γ∼3), (b) Inconel 718 (γ∼4), and (c) Fe (γ∼4)

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Figure 10

A pair of microscale gears created from nanostructured Inconel 718 foil using micro-EDM (Agie AC Vertex 2F). The foil was produced by LSEM with λ=3.5, γ∼4, and V0=0.5 m/s. The microstructure of the foil is shown in Fig. 9(36).

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Figure 11

Schematic of machining with modulation superimposed in the direction of undeformed chip thickness. The surfaces created in two successive machining passes of the tool are shown. In turning, each machining pass would be along the circumference of the workpiece with V0=πdfw, where d is the diameter of the workpiece and fw is its frequency of rotation. A particle forms whenever the tool disengages from the workpiece. The size and shape of each particle are determined by the intersection of two successive machining passes, as shown. The schematic on the right describes the chip/particle dimensions (tc, lc as shown, tw is the width of the chip into the page) in a conventional machining framework for ease of understanding.

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Figure 12

Scanning electron micrographs of Al 6061-T6 particulate produced by MAM: (a) equiaxed, (b) fiber, and (c) platelet shapes. Note uniformity in size and shape of the particles. The corresponding MAM conditions (d,fw,t0,fm) were as follows: (a) 6 mm, 3 rps, 0.150 mm, 0.050 mm/rev, 181.5 Hz; (b) 6 mm, 3 rps, 0.600 mm, 0.01875 mm/rev, 325.5 Hz; (c) 2 mm, 2 rps, 0.050 mm, 0.0025 mm/rev, 25 Hz.

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Figure 13

SEM pictures of Al6061-T6 fiber particles with aspect ratio ∼7. The smooth surface is that in contact with the rake face of the tool.

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Figure 14

TEM image showing sub-100 nm grains in the microstructure of an Al 6061-T6 fiber particle (aspect ratio ∼7). Inset is the SAD pattern, which shows evidence of preferred crystal orientations.

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Figure 15

Variation in strain with depth from the machined surface in copper and brass. The dotted curves are extrapolations used to estimate the strains at the surface. The strain values in the chip are marked as A (brass, +10 deg rake), B (copper, +20 deg rake), and C (brass, −30 deg rake).

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Figure 16

Variation in hardness with depth from the machined surface in OFHC copper for γ∼1 (rake angle=+50 deg), γ∼8 (rake angle=−10 deg), and γ∼4 (rake angle=+20 deg, cryo-SPD). Points A, B, and C on the vertical axes represent the corresponding average chip hardness values. Inset shows TEM pictures of microstructure on the machined surface (point B′) and of the chip (point B) created under a similar deformation condition (γ∼8). Both of these microstructures are seen to be very similar, highlighting the equivalence between chip and surface microstructures for the same process conditions.




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