The Investigations of Friction under Die Surface Vibration in Cold Forging Process

The objective of this thesis is to fundamentally study the influence of die surface vibration on friction under low frequency in metal forging processes. The research includes vibrating tool system design for metal forming, theoretical and experimental investigations, and finite element simulations on die surface vibration in forging process. After a general introduction to friction mechanisms and friction test techniques in metal forming, the application of ultrasonic vibration in metal forming, the influence of sliding velocity on friction is described. Some earlier investigations of the application of ultrasonic vibration on drawing, rolling and other metal forming process show that the load and friction coefficient would be decreased with the presence of ultrasonic vibration. Investigations on forging processes and under low frequency, especially the quantitative analysis of friction reduction are, however, quite limited. The thesis describes the two types of vibrating tool system for implementing the die surface vibration in the forging process. The tool developing can be divided into two stages: the first stage is to design and manufacture a prototype tool system for the essential investigation, and the second stage is to design and manufacture a more practical tool system which can be used to forging some industrial components with larger capacity. The high performance and power piezoelectric actuator stack as the vibration source will be used for designing the vibration system in order to reduce the size of the vibration system. The prototype experiments were conducted with cylinder upsetting and ring compression test. Further investigations focus on rectangular specimen compression test in order to investigate the metal flow behaviour for lower friction conditions while the specimen is undergoing vibration. In the experiments, die surface orientation, frequency and amplitude of vibration, vibrating wave form and the direction of vibration has been taken into account as the parameters which influence friction behaviour in forging process. The results reveal that friction could be reduced up to 50% with vibration being applied in forming process. Furthermore, by using finite element method, a series of the simulations of the cold forging process under die surface excitation have been implemented in order to further understand the influence of vibration on friction, especially the influence on metal flow, stress and strain distribution. The results from FEM simulation show that die surface vibration have significant influence on the metal flow, strain and stress distributions in a deformed work piece. Finally, a number of validation tests with or similar to industrial components are presented to verify the possibility of applying vibration to a forging process and effectiveness of vibration on metal forming.