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TECHNICAL PAPERS

# Study of the Size Effect on Friction Conditions in Microextrusion—Part I: Microextrusion Experiments and Analysis

[+] Author and Article Information
Neil Krishnan

Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60201

Jian Cao1

Department of Mechanical Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60201jcao@northwestern.edu

Kuniaki Dohda

Department of Engineering Physics, Electronics & Mechanics, Nagoya Institute of Technology, Gokisocho, Showa, Nagoya 466-8555, Japan

1

Corresponding author.

J. Manuf. Sci. Eng 129(4), 669-676 (Aug 11, 2006) (8 pages) doi:10.1115/1.2386207 History: Received December 05, 2005; Revised August 11, 2006

## Abstract

Microforming is a relatively new realm of manufacturing technology that addresses the issues involved in the fabrication of metallic microparts, i.e., metallic parts that have at least two characteristic dimensions in the sub-millimeter range. The recent trend towards miniaturization of products and technology has produced a strong demand for such metallic microparts with extremely small geometric features and high tolerances. Conventional forming technologies, such as extrusion, have encountered new challenges at the microscale due to the influence of “size effects” that tend to be predominant at this length scale. One of the factors that of interest is friction. The two companion papers investigate the frictional behavior and size effects observed during microextrusion in Part I and in a stored-energy Kolsky bar test in Part II. In this first paper, a novel experimental setup consisting of forming assembly and a loading stage has been developed to obtain the force-displacement response for the extrusion of pins made of brass ($Cu∕Zn$: $70∕30$). This experimental setup is used to extrude pins with a circular cross section that have a final extruded diameter ranging from $1.33mm$ down to $570μm$. The experimental results are then compared to finite-element simulations and analytical models to quantify the frictional behavior. It was found that the friction condition was nonuniform and showed a dependence on the dimensions (or size) of the micropin under the assumption of a homogeneous material deformation. Such assumption will be eliminated in Part II where the friction coefficient is more directly measured. Part I also investigates the validity of using high-strength/low-friction die coatings to improve the tribological characteristics observed in micro-extrusion. Three different extrusion dies coated with diamondlike carbon with silicon (DLC-Si), chromium nitride (CrN), and titanium nitride (TiN) were used in the microextrusion experiments. All the coatings worked satisfactorily in reducing the friction and, correspondingly, the extrusion force with the DLC-Si coating producing the best results.

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## Figures

Figure 1

Microextrusion setup: (a) segmented dies, (b) forming assembly, (c) loading stage, and (d) force-displacement response

Figure 2

Geometry of the extrusion die with the corresponding dimensions for the four sets of dies used in the experiments. “L” is the length of the bearing surface and is equal to half the inlet diameter of the die (1=0.5a).

Figure 3

Deformed mesh for FEM simulations of extrusion with different coefficients of friction

Figure 4

Extruded pins obtained from experiments using the four different dies: (a) 0.76:0.57mm die, (b) 1.50:1.00mm die, (c) 1.75:1.17mm die, and (d) 2.00:1.33mm die

Figure 5

(a) Force-displacement response for 2.00:1.33mm diameter extrusion. (b) Force-displacement response for 1.75:1.17mm diameter extrusion. (c) Force-displacement response for 1.50:1.00mm diameter extrusion. (d) Force-displacement response for 0.76:0.57mm diameter extrusion.

Figure 6

(a) Friction coefficients from comparison of experiments and FEM simulations for the 211μm grain size material. (b) Friction factors from comparison of experiments and analytical models for the 211μm grain size material.

Figure 7

Samples of pins extruded using the 0.76:0.57mm die and work pieces having grain sizes of 32 and 211μm

Figure 8

Optical micrographs of the dies: (a) uncoated, (b) CrN coating, (c) TiN coating, and (d) DLC-Si coating

Figure 9

Samples of pins produced using uncoated and coated dies: (a) uncoated, (b) CrN coating, (c) TiN coating, and (d) DLC-Si coating

Figure 10

(a) Force-displacement response for the uncoated die. (b) Force-displacement response for the die coated with CrN. (c) Force-displacement response for the die coated with TiN. (d) Force-displacement response for the die coated with DLC-Si.

Figure 11

Averaged force-displacement response for all dies

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