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TECHNICAL PAPERS

# Study of the Size Effects and Friction Conditions in Microextrusion—Part II: Size Effect in Dynamic Friction for Brass-Steel Pairs

[+] Author and Article Information
Lapo F. Mori, Neil Krishnan, Jian Cao

Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208

Horacio D. Espinosa1

Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208espinosa@northwestern.edu

Fuji prescale medium pressure has a sensitivity range from $10MPato50MPa$.

MATLAB is a trademark of The MathWorks, Inc.

For the small contact area samples, the scan area was reduced in order to have the whole field of view covered by the ring trace.

The average roughness $Ra$ is the mean vertical height deviation of the asperities measured from the centerline of the surface between peaks and valleys (15); the root-mean-square value $Rq$ is defined as the square root of the deviations and represents the standard deviation of the asperity height distribution (14).

Without the assumption of equal variances.

In our experiment, because of the geometry of the samples, there was a difference in sliding velocity between the large contact area samples $(∼5m∕s)$ and the small contact area samples $(∼1m∕s)$.

It is reported the average value, the standard deviation of the mean and the percentage of the mean represented by the standard deviation of the mean: $μ±σ(σ%)$.

1

Corresponding author.

J. Manuf. Sci. Eng 129(4), 677-689 (Mar 20, 2007) (13 pages) doi:10.1115/1.2738131 History: Received August 09, 2006; Revised March 20, 2007

## Abstract

In this paper, the results of experiments conducted to investigate the friction coefficient existing at a brass-steel interface are presented. The research discussed here is the second of a two-part study on the size effects in friction conditions that exist during microextrusion. In the regime of dimensions of the order of a few hundred microns, these size effects tend to play a significant role in affecting the characteristics of microforming processes. Experimental results presented in the previous companion paper have already shown that the friction conditions obtained from comparisons of experimental results and numerical models show a size effect related to the overall dimensions of the extruded part, assuming material response is homogeneous. Another interesting observation was made when extrusion experiments were performed to produce submillimeter sized pins. It was noted that pins fabricated from large grain-size material $(211μm)$ showed a tendency to curve, whereas those fabricated from billets having a small grain size $(32μm)$, did not show this tendency. In order to further investigate these phenomena, it was necessary to segregate the individual influences of material response and interfacial behavior on the microextrusion process, and therefore, a series of frictional experiments was conducted using a stored-energy Kolsky bar. The advantage of the Kolsky bar method is that it provides a direct measurement of the existing interfacial conditions and does not depend on material deformation behavior like other methods to measure friction. The method also provides both static and dynamic coefficients of friction, and these values could prove relevant for microextrusion tests performed at high strain rates. Tests were conducted using brass samples of a small grain size $(32μm)$ and a large grain size $(211μm)$ at low contact pressure $(22MPa)$ and high contact pressure $(250MPa)$ to see whether there was any change in the friction conditions due to these parameters. Another parameter that was varied was the area of contact. Static and dynamic coefficients of friction are reported for all the cases. The main conclusion of these experiments was that the friction coefficient did not show any significant dependence on the material grain size, interface pressure, or area of contact.

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## Figures

Figure 1

Samples of pins extruded using the 0.76:0.57mm die and work pieces having a grain size of 32μm and 211μm (modified from (2))

Figure 2

Drawing of the CuZn30 brass disk

Figure 3

Drawing of the cup-shaped 1018 steel piece for (a) the case of large contact area and (b) the case of small contact area

Figure 4

AISI 1018 cold-drawn steel cup after lapping and polishing: (a) reports the large contact area cup (nominally 75.40mm2) and (b) the small contact area cup (nominally 13.35mm2)

Figure 5

CuZn30 brass disk after lapping and polishing: (a) reports the disk without indent, (b) the disk with the small area indent, (c) the disk with the small area indent after the test, (d) the disk with a large area indent, and (e) the disk with a large area indent after the test. The pictures taken after the test clearly show that the contact area (brighter ring) is inside the indent.

Figure 6

Typical pattern generated by pressure-sensitivite film placed between the ring and the disk of the friction pair; (a) represents a large contact area pair (nominally 75.40mm2) and (b) a small contact area pair (nominally 13.35mm2). Both images show a uniform shaded area, which corresponds to uniform pressure on the contact area.

Figure 7

Scheme of the stored-energy Kolsky bar used for the dynamic friction analysis

Figure 8

Typical signal from the Wheatstone bridges on the incident and on the transmission bar recorded by the oscilloscope. The incident, reflected, and transmitted waves are highlighted by arrows.

Figure 9

Indent heights hi caused by pretest compression at a pressure of 150MPa. The mapped surface for the different high-pressure tests is reported: (a) 32μm grain size, large contact area, hi≃10μm, (b) 211μm grain size, large contact area, hi≃20μm, and (c) 211μm grain size, small contact area, hi≃30μm.

Figure 10

Surface profiles of the brass sample and surface roughness values for the brass and steel samples before (a) and after (b) the friction test for the 32μm grain size brass at low pressure. In (c) the average values and the standard deviation of the mean of the surface roughness are reported for each case.

Figure 11

Graph of the friction coefficient μ as a function of time t for different axial pressures, brass grain sizes, and contact areas: (a) low-pressure (22MPa), 32μm grain-size, large area, (b) low-pressure (22MPa), 211μm grain-size, large area, (c) high-pressure (245MPa), 32μm grain-size, large area, (d) high-pressure (251MPa), 211μm grain-size, large area, and (e) high-pressure (291MPa), 211μm grain-size, small area

Figure 12

Box plots representing the effect of grain size on the friction coefficients: (a) static coefficient of friction and (b) dynamic coefficient of friction

Figure 13

Box plots representing the effect of contact pressure on the friction coefficients: (a) static coefficient of friction and (b) dynamic coefficient of friction

Figure 14

Indent heights hi caused by the friction test after pretest compression. The mapped surface for the different high-pressure tests is reported: (a) 32μm grain size, large contact area, hi≃30μm, (b) 211μm grain size, large contact area, hi≃50μm, and (c) 211μm grain size, small contact area, hi≃50μm.

Figure 15

Box plots representing the effect of contact area on the friction coefficients: (a) static coefficient of friction and (b) dynamic coefficient of friction

Figure 16

Surface profiles of the brass sample and surface roughness values for the brass and steel samples before (a) and after (b) the friction test for the 211μm grain size brass at low pressure. In (c), the average values and the standard deviation of the mean of the surface roughness are reported for each case.

Figure 17

Surface profiles of the brass sample and surface roughness values for the brass and steel samples before (a) and after (b) the friction test for the 32μm grain size brass at high pressure. In (c), the average values and the standard deviation of the mean of the surface roughness are reported for each case.

Figure 18

Surface profiles of the brass sample and surface roughness values for the brass and steel samples before (a) and after (b) the friction test for the 211μm grain size brass at high pressure. In (c), the average values and the standard deviation of the mean of the surface roughness are reported for each case.

Figure 19

Surface profiles of the brass sample and surface roughness values for the brass and steel samples before (a) and after (b) the friction test for the small contact area 211μm grain size brass at high pressure. In (c), the average values and the standard deviation of the mean of the surface roughness are reported for each case.

Figure 20

Surface profiles of the brass sample showing two distinctly different patterns at two locations after the friction test for the small contact area 211μm grain size brass at high pressure: (a) profile after the test at location 1 and (b) profile after the test at location 2

Figure 21

Surface profiles at eight different locations of the small contact area brass specimen after the high-pressure friction test: (a) shows the locations in which the surface is scanned and (b)–(i) represent an area of 430μm by 320μm. (b) Location 1, (c) Location 2, (d) Location 3, (e) Location 4, (f) Location 5, (g) Location 6, (h) Location 7, (i) Location 8.

Figure 22

Surface profiles of the small contact area brass specimen after the high-pressure friction test. The images show regions where material appears to have been gouged out leading to excessive surface damage.

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