Microforming: Study of Friction Conditions and the Impact of Low Friction/High-Strength Die Coatings on the Extrusion of Micropins

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 micro-scale due to the influence of ‘size effects’ that tend to be predominant at this length scale. One of the factors that shows a strong influence is friction. This paper focuses on the frictional behavior observed at various sample sizes during micro-extrusion. 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 microns. 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 non-uniform and showed a dependence on the dimensions (or size) of the micropin. The paper 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 diamond-like carbon with silicon (DLC-Si), chromium nitride (CrN) and titanium nitride (TiN) were used in the micro-extrusion experiments. All the coatings worked satisfactorily in reducing the friction and correspondingly, the extrusion force with the DLC-Si coating producing the best results.