| With the rapid development of MEMS technology,the demand for micro parts is increasing,which makes micro and nano manufacturing a high-end technology that countries are competing to develop.Plastic microforming is a micro and nano manufacturing process with high processing efficiency and excellent product performance.The existence of size effect in the microforming process makes the formability of materials significantly reduced,and the dimensional accuracy of micro components is difficult to guarantee.Electromagnetic forming technology is a kind of single-sided die sheet forming technology that uses Lorentz force to directly form thin plates,which has the advantages of fast forming speed,good uniformity of thin plate deformation,high dimensional accuracy of formed parts,and has the potential ability to overcome the size effect in the microforming process.This thesis aims at studying the coupling mechanism of strain rate and size effect in the process of electromagnetic microforming;analyzing the influence of electromagnetic assisted forming on the forming properties of pure titanium sheets;clarifying the influence of microstructure evolution on the mechanical behavior of pure titanium sheets during electromagnetic microforming;exploreing new technology and method of electromagnetic-assisted micro-forming of pure titanium bipolar plate.The strain rate effect and size effects on the mechanical behavior of pure titanium sheets was firstly studied.It was found that with the increase of grain size or the decrease of sample thickness,the flow stress and work hardening rate of pure titanium sheet decreased,and with the increase of strain rate,the yield strength and work hardening rate of pure titanium sheet increased significantly.The increase of strain rate weakened the grain size effect,but strengthened the sample thickness effect.The strain localization behavior of pure titanium sheets under different tensile directions and different strain rates was analyzed by image digital correlation(DIC).It was found that a high strain rate tension in the RD direction can generate multiple high strain regions,thereby enhancing the uniform deformation behavior of pure titanium sheets.While tension along the TD direction,the strain rate has little effect on the strain localization behavior of the pure titanium sheet.Based on the Johnson-Cook model,a constitutive model of the mechanical behavior of pure titanium sheet coupled with strain rate and size effect is proposed.The influence of size effect on the twinning behavior of meso-scale pure titanium sheet during electromagnetic high-speed deformation was systematically studied,and the main mechanism of dislocation pilling-up on the size effect of twinning tendency was clarified.Based on this,the coupling mechanism of size effect and strain rate on mechanical behavior of pure titanium sheet is revealed.Further analysis was carried out on the size effect on the selection of twinning variants in pure titanium sheet,and it was found that the low Schmid twin variants gradually decreased with the increase of grain size,which clarified the strain coordination mechanism on the selection of twin variants.The forming limit of pure titanium sheet under quasi-static(QS),electromagnetic forming(EM)and pre-electromagnetic-quasi-static(EMPB-QS)hybrid forming methods is studied.Compared with quasi-static forming,electromagnetic-driving forming can significantly reduce the forming limit of uniaxial tension specimen,while hybrid forming can significantly improve the forming limit and elongation of uniaxial tension specimen.Microstructure studies show that the enhancement of twinning behavior during EMPB of uniaxial specimens contributes to the improvement of uniform deformation behavior of pure titanium sheets,thereby increasing the elongation and forming limit of pure titanium uniaxial tension specimen.However,in the EM forming process,the twinning behavior is prone to be early saturated,resulting in a low forming limit.The influences of size effects on the forming limit of the sheet were further analyzed,and it was found that with the increase of the grain size or the decrease of the thickness of the sample,the forming limit of the pure titanium sheet decreased.The ultimate aspect ratio and forming quality of pure titanium array microchannels under the QS forming,EM forming and EMPB-QS hybrid forming process were comparatively studied by the combination of finite element simulation and experiment.The optimum pre-discharge voltage for the hybrid forming process is determined to be10 k V when the hybrid forming process can increase the limit depth of the pure titanium array microchannel by more than 20%,and can significantly reduce the unloading rebound of the pure titanium array microchannel and improve its dimensional accuracy.In addition,the hybrid forming process can combine the advantages of the excellent flow channel depth uniformity of the QS forming process and the excellent thickness reduction uniformity of the EM forming process,and realize the high-aspect-ratio pure titanium array microchannel forming with high precision and high quality.In this study,the coupling effects of strain rate and size effects on the mechanical behavior of pure titanium sheet was clarified.The influence of size effects on the twin behavior of electromagnetic driven forming of pure titanium sheet is clarified.Based on this,the physical mechanism of size effect on mechanical behavior of pure titanium sheet is revealed;The influence of electromagnetic drive forming on the formability of pure titanium sheet is analyzed.A new method of electromagnetic preforming-molding hybrid forming process for pure titanium array microchannels was proposed,which significantly improved the ultimate aspect ratio and forming accuracy of pure titanium array microchannels.Thus this study provides a new method for the manufacture of threedimensional complex thin-walled structural parts with high aspect ratio and dimensional accuracy. |
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