Deformation Properties And Microstructure Evolution During Ultrasonic Vibration-assisted Microextrusion

Along with the widely use of micro-parts in various fields such as communication, electronics, aviation, precise instrumentation, and automobile. The demand of multi-functional integrated micro-parts is increasing. Net forming and near net forming which have advantageous of high production, high material utilization, and production with excellent mechanical properties, such as extrusion, are provide a good way to produce microparts abundantly. However, the friction force increased and the formability decreased in micro-forming, because of the size effects. Applying ultrasonic vibration in forming process may be a feasible approach for improving the micro-forming property of material, as the advantages found in ultrasonic assisted macro-forming.In this thesis, a series of ultrasonic assisted micro-extrusion experiments which with various process parameters of extrusion diameter D, grain size L and vibration amplitude A, were performed to study the effects of ultrasonic vibration on size effects. The mechanisms of ultrasonic assisted micro-extrusion and its size effects were studied. The effects on micro-forming property of material and its microstructure were studied respectively. The ratio of average extrusion height H to die extrusion diameter D was used as the micro-formability evaluation parameter.It is found that stress superposition and acoustic softening effect are the main reasons for the formability increasing and the decreasing of forming force in ultrasonic assisted micro-forming process. After comparing with the conventional micro-extrusion, it’s found that vibration contribute to improving the formability of material. The formability of pure copper increases and forming force decreases with the increasing amplitude. The friction condition is also improved by vibration. Besides, the vibration doesn’t influence the effects of extrusion diameter D, grain size Lon the change rule of formability of material. The relative grain size L/D(the ratio of grain size L to die extrusion diameter D) has a same effect on the ratio H/D, both in conventional micro-extrusion and ultrasonic assisted micro-extrusion. The effect curves can be divided into the same three parts: when L/D≤0.15, micro-formability decreases with increasing L/D for the increasing resistance of material flow and the worse harmony deformation between grains; When 0.15<L/D≤0.4, the average of H/D reach the lowest, and have a worse repeatability, because of the worst harmony deformation between grains and the randomness of orientation and distribution of grains. However when L/D>0.4, it becomes to single grain deformation gradually, the ratio H/D increases slowly. The experiment of XRD indicate that vibration contribute to grain rotating, a big amplitude has more good effects on the preferred of orientation of(1, 1,1) plane of pure copper.By analyzing the surface topography of micro-extrusion samples, a better surface is obtained by applying ultrasonic vibration, but the quality of surface doesn’t become better obviously with the increasing amplitude. And a phenomenon of end concave on the sample end is found in ultrasonic assisted micro-extrusion, which is on the contrary of the conventional micro-extrusion. In this thesis, we also found that from the metallographic, a wider shear band is found with big amplitude.In short, introducing the ultrasonic vibration into the micro-forming process to improve the formability, the precision and the quality is a new technology worth to explore.