Investigation On Titanium Plastic Flow Mechanism In Mesoscale Under Ultrasonic Vibration And Ultrasonic-Assisted Microneedle Array Microforming

With the increasing miniaturization of modern industrial products,microforming shows great potential for development in micro-parts manufacturing.However,since many size effect phenomena of material will occur under mesoscale,the miniaturization of size feature makes process design and die design more complex.To solve those problems in microfoming,this paper proposed the ultrasonic-assisted microfoming technology to improve the quality of microformed-parts.Applying ultrasonic vibration excitation in metal material plastic deformation can soften material and generate some other macroscale and microscale phenomena.However,about the effect of ultrasonic vibration on material plastic flow mechanism,most researches focused on the flow stress decrease caused by ultrasonic vibration,few researches have focused on the micro-structure evolution.The microscopic physical mechanism of ultrasonic vibration on material plastic deformation is still unclear.Firstly,a set of ultrasonic vibration test system equipment was designed and assembled,which its' resonant frequency is 20 KHz and the rated power is 1200 W.The effect of strain rate,grain size,specimen size and ultrasonic amplitude on mesoscale titanium microcylinder plastic deformation behavior was studied using the built test system,a material constitutive model of mesoscale titanium was established,which considers the effect of specimen size and ultrasonic amplitude.Secondly,topographies of deformed specimens were analyzed using SEM,and twinning distribution of deformed specimens were analyzed with EBSD,the non-uniform deformation behavior of mesoscale titanium and the mechanism of ultrasonic ”softening” material were revealed.Finally,the microneedle array extrusion forging die was designed,and the effect of ultrasonic vibration on the qualities of microformed-parts was simply studied.By analyzing the compression test results of titanium microcylinder,it can be seen that the strain rate and the grain size have little effect on plastic flow of titanium;the flow stress increases with the decrease of specimen size,and the flow stress decreases with the increase of ultrasonic vibration amplitude.From the topographies of deformed specimens,it shows that with the decrease of specimen size,the end surface of deformed specimens changes from a circular contour to an irregular shape.The side surface changes from a relatively smooth surface to uneven.This reveals that the extent of non-uniform deformation increased with the decrease of specimen size.By analyzing the twinning distribution of deformed specimens in different reduction with and without ultrasonic vibration excitation,it can be seen that with the increase of specimen reduction,twinning volume fraction percentage increases,which means material has to continuously produce twinning for deformation coordination when the deformation is small.In the condition of same reduction,the length of twinning boundary with ultrasonic is less than the that without ultrasonic excitation,which means when applying ultrasonic vibration excitation,the specimen needs less twinning to reach the same reduction.It shows that the ultrasonic excitation improves dislocation glide and”soften” material.Based on the above research,the titanium microneedle array ultrasonic-assisted extrusion forging process were carried out experimentally.The die with 9 ?0.1mm array holes were designed,and were coupled with the ultrasonic vibration test system designed in this paper.The effect of ultrasonic vibration on microneedle array forming quality was simply studied.From the topographies of microformed microneedle array,it can be seen that the roughness of the microneedle array was increased when applying ultrasonic vibration.