Surface and volume effects in multimodal ultrasonic vibration assisted micro-extrusion forming: Experiments and modelling

Ultrasonic vibration (UV) has been widely used in microplastic forming with its advantage of improving the metal plasticity. However, the coupling mechanism of the surface and volume effects of UV is still unclear. In this study, the multimodal ultrasonic vibration (tool vibration (TV) and mold vibration (MV)) assisted micro-forward extrusion (MFE) and micro-backward extrusion (MBE) experiments of copper T2 were conducted. An analytical method for ultrasonic surface effect in multimodal UV assisted micro-extrusion forming process was proposed, and a decoupling analysis method of ultrasonic surface effects and volume effects was also presented. The results indicated that the extrusion stress reduction caused by surface effect and volume effect were increased with the increase of ultrasonic amplitude, and the extrusion stress reduction of MV was greater than that of TV. The surface quality of the sample was improved by UV, and the surface roughness with MV was lower than that with TV. In addition, the coupling mechanism of ultrasonic surface effect and volume effect was revealed, and a comprehensive analytical model was developed considering both ultrasonic surface effect and volume effect. The predicted results of the extrusion stress were well agreed with the experimental results, with a maximum error was only 3.2%. These results provide a theoretical basis for further studying the mechanism of UV assisted micro-forming.