Evaluation of interfacial friction characteristics in multi-mode ultrasonic vibration assisted micro-extrusion process

Ultrasonic vibration assisted micro-forming process has been widely used for its advantages of increasing forming performance, reducing forming stress and interface friction. Existed studies were concentrated on the effect of ultrasonic vibration on the tool-sample interface friction, but few studies concerned the mold cavity-sample interface friction. In this paper, the variation characteristics of the mold cavity-sample interface friction during ultrasonic-assisted micro-extrusion were studied. The multi-mode ultrasonic vibration assisted copper T2 micro-extrusion experiments were conducted, and a theoretical model was developed to predict the changing rule of the interface friction coefficient and quantify the ultrasonic friction reduction effect during the ultrasonic vibration assisted micro-extrusion forming process. The results show that the mold cavity-sample interface friction coefficient was decreased with the ultrasonic amplitude increasing, and decreased sequentially with tool ultrasonic vibration (TUV), mold ultrasonic vibration (MUV) and compound ultrasonic vibration (CUV). The extrusion stress of the theoretical model agrees well with that of the experiments and the maximum relative errors of TUV, MUV and CUV are 6.9 %, 7.6 % and 11.6 %, respectively. This research result will be beneficial for further investigation on the friction mechanism in the ultrasonic vibration assisted micro-forming processes.