Research On Micro-tension Experiment And Micro-bulging Finite Element Simulation Of Pure Copper Sheet At Cryogenic Temperature

Product miniaturization is a global trend in the use of products today,enabling product functions to be realized with micro-scale geometric shapes.Driven by the continued miniaturization of micro-electromechanical systems(MEMS)and microfluidic systems(MFS),the demand for micro-products has increased significantly.Due to the unique characteristics of the size effect at the microscale,when there are only a few grains in the thickness direction of the sheet,the strain coordination ability between the grains decreases,resulting in poor uniform plastic deformation ability of the material;due to the limited formability of the material itself,this This limits the forming range of microshaped components.At present,methods such as increasing the forming temperature and refining the microstructure of the material are used to improve the micro-forming ability of the material.However,these methods have their own limitations,and it is difficult to meet our comprehensive performance requirements for micro-formed parts.This topic focuses on studying the deformation mechanism and forming performance of pure copper under cryogenic temperature conditions,and has a guiding role in studying the forming ability and service ability of various metals and alloys in cryogenic temperature microforming processes.In the study of cryogenic temperature micro-tension experiment and micro-bulging simulation of pure copper sheets,firstly,a cryogenic temperature micro-tension experimental device and a cryogenic temperature micro-bulging experimental device were designed,and cryogenic temperature micro-tension pre-experiment was carried out.The cryogenic temperature micro-tension experimental device was modified to meet the experimental requirements.Then,0.2 mm thick pure copper was recrystallized annealing to obtain a tensile sample of a desired crystal grain size.Through the cryogenic temperature unidirectional micro-tension experiment,the influence of the grain size(d),the characteristic size(t/d)and the deformation temperature on the flow stress of the material was analyzed.and the room temperature(RT)and cryogenic temperature(CT)Work hardening behavior and fracture mechanism of the material were studied at the room temperature(RT)and cryogenic temperature(CT).It was found that due to the existence of size effect at mesoscale,when the thickness of the sample is fixed,the relationship between the flow stress of the material and the grain size deviates from Hall-Petch.relationship;when the temperature is reduced to cryogenic temperature,the recovery process of dislocations is suppressed,the dislocation density increases,the flow stress of the material increases,the flow stress of the sample with small grain size increases greatly,and the size effect weakens.The plasticity of samples with large grain size is significantly improved under cryogenic temperature conditions,but the plasticity of samples with small grain size is not obvious.Based on the crystal dislocation theory,Taylor relationship and surface layer theory,by considering the influence of the deformation temperature on the dislocation recovery process in the deformation process,the constitutive model of tensile flow stress of pure copper sheet at mesoscopic scale is established and verified.Based on the dislocation recovery constitutive model established in this paper,a micro-bulging simulation model of pure copper thin sheet.The influence of grain size(d),deformation temperature,punch diameter and grain size on the micro-bulging technology were analyzed.The effect of the ratio(D/d)on the maximum bulging depth,maximum bulging load,and the amount of thinning in each deformation zone of the micro-bulging process.The simulation results show that the cryogenic temperature conditions can enhance the maximum bulging depth and the maximum bulging load.And as the ratio of the diameter of the bulging punch to the grain size(D/d)decreases,the maximum bulging load curve of the 90.7?m sample has a tendency to show an inflection point,which is similar to the size effect of the tensile flow stress,called the size effect of the maximum bulging load.