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

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 1.33 mm down to 570 m. 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.

Microforming Processes

“…It is revealed that the coating of DLC-plasma can significantly decrease the friction, while the coatings of TiN and CrN increase the friction. A similar study was done by Krishnan et al [78] using brass in micro forward extrusion process, but different results were obtained. Their results indicate that the coatings of DLC-Si, CrN and TiN can reduce the extrusion force.…”

“…4.37a. To overcome the difficulty in part ejection, separating the extrusion die into two halves was proposed by Krishnan et al [78] and the authors of this monograph. However, such design could lead to the formation of flash at the parting line, as shown in Fig.…”

“…Their experimental and simulation results suggest that there is an interactive effect among the interfacial friction, the plastic properties of micro-sized material, and grain and part sizes on the deformation load and the geometry of micro-formed part. Therefore, the conventional friction evaluation methods based on the material deformation load [78,[83][84][85][86] or the deformed part geometry [87,88] may not be very efficient in evaluation of the friction condition in microforming processes. …”

Impact of Surface Topography of Tools and Materials in Micro-Sheet Metal Forming

“…Especially if the non-uniform pressure distribution was Although these applied researches on the field of dry-microforming are well developed for the practical application, the basic research on the dry friction behaviour during microforming was quite few and it was only worked for bulk metal forming (Vollertsen et al, 2009). Krishnan et al investigated the scale effect on dry friction by scaled forward extrusion test of brass (Cu:Zn 70:30) with the scale range of 0.57-1.33mm in outer diameter of extruded pin (Mori et al, 2007;Krishnan et al, 2007). In addition, the other author conducted the scaled ring compression test of CuZn15 alloy with different grain size.…”

Development of in-situ observation system of dynamic contact interface between dies and materials during microforming operation

“…First application was reported by Yang et al, who applied to the micro-extrusion die using pulse plasma chemical vapor deposition (CVD) method [4]. Similarly, the friction properties of DLC coated microdies were followed by Krishnan et al, for micro-extrusion process [5] and by Fujimoto et al for microbending process [6], Aizawa et al for micro-bending with ironing [7], and Hu et al [8] and by Wang et al [9] for micro-deep drawing process.…”