Research On The Mechanism Of The Micro-forming And Filling Behavior Of Ultrafine-grained Pure Copper

Microforming technology,due to the high processing efficiency,low manufacturing cost and good performance and dimensional accuracy,has become one of the important research directions in the micromanufacturing field.However,the application of traditional metal materials with coarsed grain(CG)is limited because of the existence of the size effects which can reduce the dimensional accuracy and enhance the performance scatters of micro-parts.The ultrafine grained(UFG)materials with homogeneous sub-micron grains,may have provided new ideas and prospects for improve this situation during microforming.But up to now,there is still no further research on the application in ultrafine grained materials.Therefore,the study of plastic deformation behavior and size effects of UFG materials is of great theoretical significance and practical value.This thesis aims to research on the microforming mechanisms of UFG pure copper,and expand the application of UFG materials in microforming area.The UFG pure copper were fabricated by severe plastic deformation(SPD)technologies of the equal channel angular pressing(ECAP)processing and high pressure torsion(HPT)processing in this thesis.Pure copper was processed by ECAP processing through 0-12 passes using route Bc with a die channel angle of 110°.UFG pure copper was achieved with homogeneous microstructure,an average grain size of 0.41 ?m and microhardness values of 134.Pure copper is subjected to HPT processing at room temperature using quasi-constrained high-pressure torsion through 0 to 10 turns.UFG pure copper was achieved with homogeneous microstructure,an average grain size of 0.25 ?m and microhardness values of 140.Both of UFG pure copper materials has simple shear texture at a certain level.Based on the built in situ micro tension testing system for digital image correlation(DIC),the local strain of UFG anc CG pure copper was quantitatively multi-scale researched by combining the DIC technique with the ex situ EBSD technique,respectively.Based on optical microscope in situ micro tension testing,the DIC techniques observed the formation of narrow shear bands for UFG Cu at the early stage of tension,and local strain is uneven distributed within it and the values show bimodal distribution.However,the formation of wide shear bands for CG Cu and unevenly distributed local strain seems more dispersed within it and the values shows unimodal distribution.For the strain localization for the UFG Cu,based on SEM in situ micro tension testing,it is shown that the inhomogeneous behavior become much more severe.Moreover,by combining based on EBSD ex situ tension testing for the UFG Cu,the change features of microstructure is analyzed,and the strain localization induced by grain boundary movement results in the low elongation at room temperature for UFG Cu.The compression behaviors of micro samples with a diameter of 2 mm for UFG and CG Cu are studied.Compared with the typical hard working behavior of CG Cu,the strain softening phenomenon was found in UFG pure copper.It was analyzed that the microstructure evolution of UFG and CG Cu by comparing the grain morphology,grain boundary feature and the substructure of grain interior and so on.Based on grain rotation and grain boundary sliding,a model of softening mechanism at room temperature was established.At the same time,the UFG sample showed anisotropy behavior at a certain level,but the inhomogeneous deformation behavior was weak and the surface quality of the compressed sample improved with the UFG structure also well retained,which would build a foundation for the fabrication of micro-part with high performance.A series of studies on the fracture behavior of UFG and CG Cu conducted by using DIC technique and X-ray synchrotron radiation technology,respectively.Analyzed the shear fracture mode in UFG Cu and the ductile fracture mode in CG Cu,respectively.DIC technique is used for characterizing the relationship between the surface cracks evolution and the local shear strain,and it is found that the surface cracks is strongly affected by the shear stress.X-ray synchrotron radiation technology realize no damage and 3D visualization and quantization research in the samples with UFG structure.It is clear that the angles between the voids growth/linkage and the tensile axis is 55 o,and the fracture mechanism model induced by shear texture is established.At the same time,the X-ray synchrotron radiation technology prove that numbers of voids decrease with the grain size increase,and voids density maintain the exponential growth with the increasing strain in UFG pure copper.The mechanisms of size effects of metal materials in micro-forming are discussed.Based on this,the compression specimens with the diameter size of 0.5 mm-2 mm are fabricated by wire electrical discharge machining,and the scale effects caused by specimen size and grain size are studied.Scale effect is not obvious in the meso scale,which would provide a theoretical support for the application of UFG materials in micro forming.The deformation behavior in the microscale was explored,showing the “smaller is weaker” regime occurring with decreasing the ratio of specimen size to grain size(D/d),and pronounced weakening with decreasing D/d below ~ 17 for the average grain size ~ 0.28 ?m.Based on the surface layer theory,the relationship between the yield stress and D/d is built by considering the deformation mechanism of grain boundary sliding.Apply the UFG pure copper to fabricate the micro gear.Using micro array channel testing for evaluating the filling properties of UFG and CG Cu respectively,results show that both of them have a good filling effect,and embossing parts of UFG Cu needed bigger load at the same filling height,but its surface quality is better and the nanohardness is higher and more stable.Based on this,by combination with the filling behavior by finite element simulation,the effects of loads,grain sizes and load ways on the filling rules of micro gears are studied.The fabricated UFG micro gears with higher contour precision possess the higher average nanohardness ~ 1.67 GPa,and better wear resistance.The results validate of the potential application of UFG materials in miroforming area.