Research Of Size Effcet On Springback Behavior In Forming Of Micro/Meso Scale Sheet

In recent years,people's demand for miniaturized,integrated and intelligentized products has been more and more urgent.This trend accelerated the rapid development of precision machinery and electronics industries.Microproducts are extensively used in the automotive,telecommunications,micro-electromechanical systems(MEMS),micro systems technology(MST),bio-medical,aviation/aerospace industry etc,which leads to a stronger demand for different kinds of microparts.For example,metal sheet with micro structure features are widely used in different fields,such as PEMFC bipolar plates,micro reactor,micro radiator.etc.How to manufacture this kind of microparts stably and efficiently has become a major problem remaining to be solved.Microforming is a manufacturing approach using traditional plastic forming techniques to produce microparts with at least two dimensions under sub-millimeter level.It is widely used in the manufacture of metal microparts due to its low material cost,high productivity and strengthened mechanical properties of parts.However,because elastic deformation exists with plastic deformation in metal,springback will occur inevitably after unloading,which will lead to a low precision in forming accuracy.Springback is affected by many factors.Especially when the part is miniaturized to micro/meso scale,size effect will occur to make the problem different from macro scale.Thus techniques and theories in macro-scale forming cannot be applied to microforming directly,which leads to a severe dimension inaccuracy.This paper focused on the micobending of micro/meso scale copper sheet,studied the size effect on springback behavior through experiments and finite element analysis.The main work and conclusions are as bellow.1)Mechanical property experiments of copper sheetHeat treatment was conducted firstly to reveal the effect of temperature and holding time on grain size.Then three degrees of gain sizes were chosen for copper sheets with different thicknesses respectively,and tensile specimen was prepared.Through uniaxial tensile test,the flow stress curves were obtained for different copper sheets.The composite model was adapted to explain the flow stress results.FE model of uniaxial tensile test considering surface layer model was established.The simulation results validated the accuracy of the composite model.Young's modulus was obtained by nanoindentation test,and yield stress was calculated then.It showed that with the decrease of sheet thickness,yield stress firstly decreases and then increases.2)Microbending experiment of copper sheet and FE analysisA mould for microbending was designed,the experiment system was then established,and the effect of sheet thickness,grain size,forming angle,thickness/grain size ratio on springback angle was investigated.The result shows: springback increases with the decrease of copper thickness,and when the thickness is small,the trend of spingback decrease is stronger;springback angle decreases with the increase of grain size;springback angle increase with the increase of forming angle,and when the copper is thicker or the grain size is bigger,this kind of trend is weaker;when the thickness is the same,springback angle decreases with the increase of thickness/grain size ratio,but when the ratio remains similar,springback angle increases with the decrease of thickness,which means that thickness is the main factor that affects the springback angle.A finite element model considering the surface layer effect was established.And the effect of surface layer on springback was calculated.The springback angle calculated by the FE model that considered surface layer effect is more accurate.3)Investigation of micro groove forming of metal sheetIn order to improve the forming accuracy of PEMFC unipolar plate,a FE model was established by using the method in chapter 3.The effect of blank holder force,punch-nose radius,gap between punch and die,grain size on springback angle was investigated,respectively.And a respond surface was calculated to provide a reference for the design of PEMFC unipolar plate.