Investigation on friction characteristics of micro double cup extrusion assisted by different ultrasonic vibration modes

Abstract

Ultrasonic-assisted plastic micro-forming is a research hotspot in metal-forming process. The friction characteristic is a key factor affecting the micro-forming properties of metal in ultrasonic-assisted micro-forming process. However, the existed researches were mostly focused on the friction characteristics of free surfaces, while few were studied on the friction characteristics between sample and mold cavity. In this paper, the micro double cup extrusion experiments of copper T2 were conducted to investigate the friction characteristics between sample and mold cavity with multiple ultrasonic vibration modes. Furthermore, the numerical model was developed to quantify the friction stress reduction caused by multiple ultrasonic vibration modes and estimate its contribution to decreasing the forming stress, which was usually considered mainly affected by acoustic softening, stress superposition, and friction reduction. The results show that the forming stress and the surface roughness of extruded samples are decreased sequentially with the multiple ultrasonic vibration modes of tool vibration (TV), workpiece vibration (WV), and compound vibration (CV). The cup height ratios of double cup extrusion are also increased sequentially with the ultrasonic vibration modes. But the increase of cup height ratio does not indicate the increase of friction coefficient. The friction stress reduction between sample and mold cavity is increased sequentially with TV, CV, and WV modes, and its contribution to decreasing the forming stress is 48%, 15%, and 49%, respectively.