| In this thesis,in order to study micro-behavior of particles during compaction densification under external energy,the discrete element method(DEM)model has been firstly used to generate the data of initial disordered packing and various ordered packings of equal spheres,followed by the characterization of these packing structures.Then the DEM created data(mainly the coordinates of the particles and their coordination number)were imported into the finite element model(FEM)to realize their coupling.On this basis,the Multi-particle FEM(MPFEM)simulation was carried out to identify the densification of different initial packing structures compacted under external energy(in this work the single action die compaction process was applied).The packing models generated by DEM simulation include two-dimensional(2D)composite powder packing structures(i.e.SiC particulate reinforced Al powders)and three-dimensional(3D)mono-sized sphere packing structures.In 2D simulation,the effect of initial structures(disordered packing and various ordered packing structures)and the content of hard particles(the volume fraction of SiC:0%,20%,40%,60%)on the compaction densification have been studied.In 3D modeling,the initial packing structures of equal spheres include simple cubic(SC),body-centered cubic(BCC),face-centered cubic(FCC),hexagonal close packed(HCP)and random packing structures.In addition to the FEM modeling on the compaction process under various pressures,more emphasis in current work was focusing on the characterization of packing structure from macro-and microscopic point of view and on the analysis of densification mechanism during compaction.The effects of the initial packing structure and the pore shape therein on the parameters in Shima model have been studied.The results indicate that:1.In 2D modeling,the initial packing structure and the distribution of SiC particles can both affect the pressure-relative density relationship.The pores among particles are difficult to be filled if the framework was formed by SiC particles.2.During the forming of SiC/Al composite powders,the Mises stresses of SiC particles are large,however,the particle deformation is not obvious,only small elastic deformation occurs at contact area.In contrast,the Mises stress of Al particles are small,while the particle deformation is large.And finally,the shape of Al particles after compaction becomes concave.3.For 2D random packing structure,the more content of SiC,the larger the compacting pressure needed.When the relative packing density is low,the effects of SiC content on the compaction is not obvious;however,with a high relative packing density,the SiC content effects become significant.4.3D simulation results are in good agreement with the physical experiments.The face number for each particle in BCC initial packing changes from 8 to 12 and then to 14 during compaction.The shape of each particle in FCC initial packing becomes tetrakaidecahedron after compaction,while that in HCP becomes dodecahedron.During compaction,the unstable SC results in the non-uniform deformation to form irregular polyhedra.The compaction on the FCC initial packing can realize higher packing density than other initial structures at each fixed pressure.5.For 3D random initial packing,sliding occurs at the early stage of compaction;when particles are jammed,the contact between adjacent particles changes from point to plane,and with the further increase of the compaction pressure,the contact area extends.6.The parameters of 1/f and f' in Shima's model are affected by the initial packing structure and the internal pore shape.Hence,for the entire densification process a single function cannot adequately represent the variation of the yield function parameters in terms of relative density.7.The mechanism of individual particle deformation and the factors affecting the yield function parameters can be studied accurately by MPFEM,which are very difficult to be identified in both the physical experiments and other numerical simulations. |
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