| Metal sheet components with surface microstructures like microchannels,microriblets,microdimples,show notable advantages in energy saving,drag/friction reduction and energy efficiency enhancement.So,these components are widely used in microchannel reactors,microchannel heat exchanger,circraft's skin,sliding contact.However,the existing fabricating methods,such as micromilling,microEDM,laser ablation,have the limitations of high cost,low efficiency and are not suitable for mass production.Compared with the material-removed methods,the microplastic deformation based coining technique has the superiority of high productivity and material utilization,better mechanical properties of formed parts,etc.But,the problems of great load and heavy mould wear come with the the traditional coining process.In this thesis,the R2P(roll-to-plate)imprinting process for the fabrication of microstructure on the surface of metal sheet is studied by combining the traditional plate coining with the high-efficiency rolling.This technique could reduce the forming load and improve high productivity.Nevertheless,the plastic deformation behaviors of material are influenced by the grain size,orientation and die cavity dimension due to reduction of the surface feature sizes.The size effects occur which lead to the difficulty of material flowing and filling,and the larger scattering of microstructure dimensions.This kind of size effects cannot be difficultly taken into account in the traditional plastic deformation theory.Therefore,the conventional forming theory cannot be used to effectively guide the roller microimprinting process design,which brings about the lower quality of formed parts.Aimed at the above-mentioned problems,the R2 P imprinting process is taken as the research object and the deep investigations are implemented in material constitutive modeling,numerical modeling of process and experiments.First,the effects of grain size and orientation on the material properties are investigated.Moreover,the material constitutive model about size effects is established with the consideration of grain orientations.Second,the numerical model of R2 P microimprinting process is built.The flowing and deformation of material are analyzed in the forming of R2 P imprinting.The size effects and the influence of process parameters on the forming quality are studied.At last,the R2 P microimprinting process system is developed in the lab.The feasibility of this process is validated.Major research work of this thesis covers the following parts:1)Material constitutive modeling of size effects considering grain orientationsAimed at the effects of grain orientations on the material mechanical properties in microforming,the tensile testing of polycrystal pure copper with different grain sizes is carried out and the orientations before and after tension are tested by the EBSD device.The effects of grain size and orientation on the flowing stress are revealed.The parameter of grain orientation change is introduced and the change rules and distribution of three representative orientation grains in the deformation direction of polycrystal pure copper are described.Comprehensively considering the effects of grain size and orientation change,the material size effect constitutive model is established with considering the grain orientations.The accuracy of model is verified with the experiment and numerical simulation.2)Numerical modeling of R2 P micro/meso imprinting processConsidering the characteristics of grain microstructures and anisotropy,the method of Voronoi diagram is adopted.The mapping relation between the material grain geometry model and real microstructures is constructed.According to the characteristics of local plastic deformation in the R2 P imprinting,the material is divided into two layers in the direction of depth.The material is discretized into virtual grains in the local forming region of microfeatures.And then,the model of R2 P microimprinting process is created.Through simulations and analysis,the overall deformation and local microflowing rules of material are explored.The effects of grain size,orientation and thickness dimension on the forming height,microhardness and surface roughness are investigated with the different grain size materials.The accuracy of model is validated with the roller imprinting experiments of microgrooves.3)Process parameters analysis of R2 P micro/meso imprintingOn the basis of finite element model of R2 P microimprinting process,the forming of microgrooves in the two kinds of different loading paths,the direction perpendicular to and parallel to the grooves,is investigated combining with the experiments of R2 P microimprinting.The flowing and filling rules of material in the microgrooves of flat die are revealed in different loading paths.The influences of process conditions,parameters and die structures on the forming quality are investigated with the size effect and material.These would provide the references for forming process design of functional surface microstructures.4)Forming study of surface microstructures in firction reduction by R2 P imprinting processTaking the typical microdimple of surface friction reduction as an example in industrial fields,the experimental system of R2 P microimprinting process is developed.By the experimental system,the forming experiments of different size microdimples are conducted.Combined with the simulation model,the flowing and filling rules of material in the rolling and transverse direction are explored and the reasons of material pileup on the two sides of die cavity in the rolling direction are analyzed.The effects of process parameters on the geometry dimensions,surface roughness of workpiece,hardness profile and transfer ratio are investigated in detail.The forming experimental results validate that it is feasible for the more efficient manufacturing of surface microstructures using R2 P imprinting process. |
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