Papers by Author: Hyoung Seop Kim

Abstract: A phase mixture model (PMM) was considered in which materials are treated as a mixture of grain interior phase, grain boundary phase and pores (if the material is porous) for the elasticity and plasticity of nanostructured materials (NSMs). In order to investigate the effects of grain size and porosity on the elastic modulus, a self-consistent method in conjunction with PMM was employed. The calculated results are compared with the experimental measurements in the literature. The elastic modulus of NSMs decreases with a decrease of the grain size and the decrement is relatively large at grain sizes below about 10 nm. The effect of porosity, however, is substantially greater than the grain size effect. For the plasticity of NSMs, grain size effects were introduced both via the dislocation glide mechanism and through the diffusion mechanisms providing mass transfer via grain boundaries. A good agreement between the calculated deformation behavior and experiment was found. The quality of the above predictions with regard to strength, strain hardening, strain sensitivity and ductility behavior testify the adequacy of the model. It is concluded that the model can be used as a convenient tool for simulating the deformation behavior of NSMs.

Abstract: Equal channel angular pressing (ECAP) is a convenient forming procedure among various severe plastic deformation processes. It is based on extruding material through specially designed entry and exit channel dies to produce an ultrafine grained microstructure. The properties of the materials obtained depend on the plastic deformation behaviour during ECAP, which is governed mainly by the die geometry, the material itself and the processing conditions. As the mechanical properties of the severely deformed material are directly related to the deformation history, understanding the phenomena associated with strain and strain rate development in the ECAP process is very important. In this study, the results of continuum modelling of ECAP are described in order to understand strain and strain developments. For this purpose, the results of modelling ECAP using the finite element method and analytical solution are presented for various geometric conditions. It was concluded that although deformation is nonuniform due to geometric effects, the strain and strain rate values obtained by the analytical solutions are not much different from the average results of the finite element method.

Abstract: Plastic deformation behavior during equal channel multi-angular pressing (ECMAP) was analyzed using the three dimensional finite volume method of the commercial code MSC.Superforge. In order to understand local and global deformation characteristics, effective strain and pressing load histories were investigated. The predicted plastic deformation behavior of the workpiece material during ECMAP of route A, route B and route C with a theoretical total strain of ~2.2 upon a single pass at three different friction factors (m=0, 0.1 and 0.2) was compared. The predicted strain results show different values in outside and similar values in central regions of the processed workpieces with different friction and forming routes. The pressing loads are higher under higher friction condition, showing almost no difference with three different pressing routes.

Abstract: Effect of equal-channel angular pressing (ECAP) on the corrosion and mechanical properties of Cu-35%Zn alloy were studied. Two types of feed direction were selected. One is parallel pass and the other is 180°degree rotated ECAP pass after each pass. Both ECAP passes made texture in each specimen in which shear band with 45 degree on transverse direction and twins exist. The specimen prepared by parallel ECAP pass has finer shear band. Relative amount of twins to shear band on the microstructure becomes decrease with number of ECAP pass. Microhardness increased from 75 Hv to 210 Hv by ECAP. The corrosion potential and rate of the ECAPed Cu-35%Zn alloys in aerated aqueous 1 M-H2SO4 solution were –92.3 mVSHE and 3.72x10-2 A/cm2 for route- A and –38.6 mVSHE and 5.08x10-2 A/cm2 for route-C, respectively. The corrosion potential and rate of depended on the feed direction and number of pass.

Abstract: A model describing the evolution of the misorientation angle between dislocation cells with plastic strain is proposed. The model is applied to the case of equal channel angular pressing (ECAP) of copper. In a basic version of the model, the evolution of the average misorientation angle is traced. A way of handling the evolution of the misorientation angle distribution function using a probabilistic description is also outlined.

Abstract: In this study, bottom-up type powder processing and top-down type SPD (severe plastic deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders with least grain growth, which was considered as a bottle neck of the bottom-up method using the conventional powder metallurgy of compaction and sintering. ECAP (Equal channel angular pressing), one of the most promising method in SPD, was used for the powder consolidation. In the ECAP process of not only solid but also powder metals, knowledge of the density as well as internal stress, strain and strain rate distribution is important for understanding the process. We investigated the consolidation, plastic deformation and microstructure evolution behavior of the metallic powders during ECAP using experimental and theoretical methods. Almost independent behavior of powder densification in the entry channel and shear deformation in the main deformation zone was found by the finite element method in conjunction with a pressure dependent material yield model. It was found that high mechanical strength could be achieved effectively as a result of the well bonded powder contact surface during ECAP process of gas atomized Al-Si powders. The SPD processing of powders is a viable method to achieve both fully density and nanostructured materials.

Abstract: Equal-channel angular (ECA) pressing is the main technique of the severe plastic deformation (SPD) method, applied for fabrication of bulk nanostructured metal materials. At the same time the practical realization of this technique is a rather challenging task. This is connected with the fact that the material during the ECA pressing is subjected to large strains under high imposed pressure at relatively low temperatures. Simulation with the help of the finite element method (FEM) or the variation-difference (VDM) method is widely applied to analyze the process of ECA pressing. A variety of as commercial as well as in-house developed programs are used by researches, when conducting this analysis. As a result the correlation between the modeling results, obtained at different laboratories as well as their adequacy, i.e. possibilities of their application for the analysis of the experimental data become topical issues. In order to find answers to the questions put by there has been performed computer simulation of 1st pass of ECA pressing by an example of pure copper at 4 different laboratories, engaged in SPD problems. Meanwhile, the investigators used different software packages, however, initial simulation conditions were set equal. This refers in particular to geometry sizes and the form of the die-set possessing square transverse section of the channels, as well as to the inner and outer curvature radii of the channels in the point where they intersect, and to the form and dimensions of the billet, strain rate, strain curve, isotropic model of the material. The modeling temperature was ambient. The die-set and the punch were assigned as absolutely solid non-deformable bodies. Taking into account the symmetry of the solving task, the modeling was conducted for a half of the billet, cut along the vertical plane, coming through its geometrical center. The friction coefficient was assigned equal to zero, in order to avoid influence of friction on the character of the material flow as well as not to complicate the problem at the given stage of comparison. Other modeling parameters were chosen by each researcher on his own, basing on his experience and conventional approaches to modeling. Comparison of the obtained modeling results was made by means of matching of the calculated values of the level of the accumulated strain along the bulk of the billet, pressing efforts, and the geometrical form of the billet after ECA pressing. Modeling results were compared with the results of the experimental researches.

Abstract: In this study, equal channel angular pressing was carried out on Cu-Fe-Cr composites at room temperature. The microstructure and hardness of Cu-Fe-Cr pressed using different ECAP routes were investigated. All Cu-Fe-Cr specimens showed ultrafine-grained microstructures with the shape and distribution of Fe-Cr phase dependent on the processing routes. As the number of pressing increased by route A, the initial denfrite of Fe-Cr phase were elongated along the shear direction and developed into filaments. On the other hand, as the number of pressing increased by route Bc, the initial dendrite became finer by fragmentation with no pronounced change of the shape. In route C, the shearing of the second phase in the first pass can be reversed by the shearing in the reverse direction in the second pass and the morphological change of Fe-cr particles is minimal. The hardness increased more rapidly in route Bc and route C than in route A. In ECAPed Cu-Fe-Cr, the spacing between Fe-Cr filaments did not decrease appreciably with strain unlike the cold-drawn Cu-Fe-Cr in which the spacing between Fe-Cr filaments decreases rapidly with strain. The higher strength in route C can be associated with the sub-divided microstructure resulting from the activation of various slip systems enhanced by the presence of larger strong particles. This result suggests that the microstructural development in Cu matrix is more important in strengthening than the morphological development of Fe-Cr phase in ECAPed Cu-Fe-Cr.