Study On Numerical Simulation Modeling Of Micro Thickness Sheet Metal Forming And Size Effect Of Bending Springback

With the rapid development of electronics industry and precision machinery, metal sheet and foil are widely used in electronics industry,MEMS,medical care and the field of new energy.Micro sheet metal forming technology becomes preferred in mass production of micro parts for its high productivity,low material loss, excellent mechanical properties and small errors.However, micro sheet metal forming has big difference with macro size metal forming in mechanical properties,shaping characteristic and other aspects due to existence of the size effect. The mature theory and technology of traditional macro-forming process can not be directly applied to the field of micro-forming process, which seriously restrains the development of micro forming manufacturing industry. Therefore, it is necessary to carry out the research on simulation modeling method of micro sheet metal forming and size effect of springback to promote the development of micro sheet metal forming technology.In this dissertation, T2copper sheet and foil were selected as the research object. The influence of material supply condition, heat treatment process,oxidation film and other factors on the mechanical properties and micro forming size effect was studied through the unidirectional micro-tensile test and material microstructure observation. Pure copper sheet and foil whose original state is rolling states, respectively,were conducting unidirectional micro tensile tests with the rolling direction of the flow line into0°,45°and90°sampling.Experimental results show that under the same circumstances of material thickness,with the materials sampling direction and rolling direction changing to0°,45°,90°,the overall flow stress and elongation were showing a downward trend, that is, for sheet metal,mechanical properties along the rolling direction was superior to perpendicular to the rolling direction,and with the reduction of material thickness and the number of crystal grains in the thickness direction,it shows the size effect just weakening as the material thickness thinning.And sample materials at same rolling direction also show yield ratio rises substantially with the thickness and the elongation decreases.Pure copper sheet and foil of different thickness of0.20mm,0.15mm,0.10mm,0.05mm and0.03mm were conducted annealing process under protective atmosphere,and annealing temperatures were400℃,500℃,600℃and700℃.By observing the microstructure of the material under different annealing temperature and conducting uniaxially stretching experiment,results show that the flow stress increases with the decreasing of grain size as the specimen thickness is constant,but the materials with different thickness exhibit two distinct size effect as the annealing temperature is identical.For the sheet of t>0.10mm,it shows "the smaller the weaker "phenomenon and for the foil of t<0.10mm it shows" the smaller the stronger".For the size effect of the smaller the stronger,some literature suggests the oxide film strengthening speculation,but no specific research reports.By analyzing the advantages and disadvantages of several copper surface Cu2O films prepared methods,this article proposed copper oxide films thicker samples were prepared using electro-deposition and thermal oxidation methods with different heating temperature,and the samples with thick oxide film and in the natural state were conducted uniaxial tensile comparative tests,and studying the impact of oxide film on the mechanical properties of materials.Experimental results show that oxide passivation film at the micro molding has a significant influence on the mechanical properties of the materials.By analyzing the inner mechanism of micro-sheet forming,the essential cause of size effect in material behavior of micro metal sheet forming was revealed by dislocation pile up theory, surface model theory and metal crystal plasticity theory.The different scale parameter and ratio parameter were introduced to elaborate the influence of the surface layer on the grain boundary,grain boundaries and the performance of the material from different angles, modify the Hall-Petch formula which is applicable to multi-crystal plastic deformation,and construct the grain boundary model considering the impact of the surface layer.The size effect of pure copper oxide surface passivation film on material properties was analyzed,and the scale parameters were introducing which express the influence of oxide film thickness on the material properties,and oxide film reinforced material constitutive model was established.According to the shape and orientation distribution of the grain,the board material analysis was divided into different regions and given the mechanical properties alone,regional models were established considering the surface layer and grain anisotropy.By processing the micro-tensile tests and experimental data a variety of material constitutive models established above were verified.According to the average grain size the dimensions and shape are randomly divided,and the different orientation of grains were given different elastic properties and plastic properties using the regional model established in this paper considering the surface layer and grain anisotropy.Regional finite element model was established considering the impact of surface layer and grain anisotropy.Bending process and unloading rebound process of T2pure copper micro components was simulated combining dynamic explicit with static implicit algorithm.And meso-bending springback rule and traditional modeling methods were analyzed and it was found that after two modeling methods simulate bending rebound,rebound angles were thinning with increasing thickness of the materials,but with mesoscopic modeling simulating micro-bending resilience of pure copper can more accurately exhibit the characteristics of micro molding,that is,with thickness reducing,residual stress’ reduces,springback angles increase.Different grain bits were combined in angle bend region,and bending springback was simulated for four different situations of the bending angle area,which are grain random distribution,[111] orientation,[110] orientation,[100] bits.And the rebound angles and forming force are calculated.The anisotropy of material properties caused by different grain orientation was studied.Material models considering the impact of the surface layer and the oxide film were built.Size effect of surface layer affecting material properties and oxide film affecting material properties were studied.Besides factors influencing springback just as material thickness,relative thickness,crystal orientation,size of die working part,bending angle etc were studied basing on the bending process simulation of pure copper micro-components and measures to reduce springback were discussing in this paper. According to the characteristics of the micro-bending process L-shaped mold of unilateral micro-bending experiments was designed and processed,and micro bending process of copper sheet and foil was investigated experimentally by changing the blank thickness,grain sizes and bending angle.Through experimental study,the influence of blank thickness,grain sizes,bending angle and other factors on micro bending springback was obtained,and the experimental data and simulation results and theoretical calculations were compared,which showed the obtained bending springback law and numerical simulation results were consistent,and verifying the correctness of the established theoretical models and simulation results.