• Transactions of Materials Processing
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• v.5 no.1
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• pp.37-46
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• 1996

Superplastic forming/diffusion bonding (SPF/DB) processes were analyzed using a rigid visco-plastic finite element method. The optimum pressure-time relationship for a target strain rate and thickness distributions were predicted by two-node line elements based on the membrane approximation for plane strain. Material behavior during SPF/DB of the integral structures having complicated shapes was investigated. The tying condition is employed for the analysis of inter-sheet contact problems. A movement of rib structure is successfully predicted during the forming.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.06a
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• pp.100-123
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• 1995

The relationship between plastic deformation and crystal grain change in warm forging processes of SM100 carbon steel is studied. If the carbon steel is deformed in warm forging temperature (about recrystallization range), the crystal grain and cementite of the internal part are changed, so material properties are changed. Some experimental values, such as the elliptic degree of cementite, the grain size of cementitie and ferrite grain size, are investigated. When the plastic deformation proceeds, the elliptic degree of cementite becomes large, the grain size of cementite particle is small, and the size of ferrite grain appears fine by recrystallization. The elliptic degree of cementite has a considerable effect on formability. The distribution of effective strain in the forging is calculated by the rigid visco-plastic FEM analysis. The effective strain distribution obtained from the FEM simulation is compared with the experimental result. At effective strain 0.3 dynamic recovery and dynamic recrystallization begin, over 2.5 the organization of material has better quality that is suitable for the following cold forming.

• Transactions of Materials Processing
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• v.26 no.5
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• pp.286-292
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• 2017

Magnesium alloy shows strong anisotropy and asymmetric behavior in tension and compression curve, especially at room temperature. These characteristics limit the application of finite element method (FEM) which is based on conventional continuum mechanics. To accurately predict the material behavior of magnesium alloy at microstructural level, a methodology of fully coupled multiscale simulation is presented and a crystal plasticity model as a constitutive equation in the simulation of metal forming process is introduced in this study. The existing constitutive equation for rigid plastic FEM is modified to accommodate deviatoric stress component and its derivatives with respect to strain rate components. Viscoplastic self-consistent (VPSC) polycrystal model was selected as a constitutive model because it was regarded as the most robust model compared to Taylor model or Sachs model. Stiffness matrix and load vector were derived based on the new approach and implemented into $DEFORM^{TM}-3D$ via a user subroutine handling stiffness matrix at an elemental level. The application to extrusion and rolling process of pure magnesium is presented in this study to assess the validity of the proposed multiscale process.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.2
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• pp.461-472
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• 1996

The relationship between plastic deformation and crystal grain change in warm forging processes of SM10C carbon steel is studied. If the carbon steel is deformed at warm forging temperature(about recrystallization range), material properties are changed due to microstructural chanre of the crystal grain and cementite of the internal part. Some experimental values are investigated in terms of the elliptic degree of cementite, the grain size of cementite and ferrite grain size. When plastic deformation proceeds, the elliptic degree of cementite becomes larger and the grain size of cementite particle becomes small. In addition, the size of ferrite grain becomes fines by recrystallization. The elliptic degree of cementite has a considerable effect on formability. The distribution of effective strain in the forging was calculated by the rigid visco-plastic FEM analysis. The effective strain distribution obtained from the FEM simulation is compared with the experimental result, At the level of effective strain 0.3, dynamic recovery and dynamic recrystallization begin and at the level of over 2.5, the organization of material has better internal structure that is suitable for the following cold forming.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.6
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• pp.1411-1421
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• 1995

A rigid visco-plastic finite element method has been developed for modeling superplastic forming processes. The optimum pressure-time relationship for a target strain rate and thickness distributions was predicted using two-node line element based on membrane approximation for plane strain and axisymmetric condition. Analysis of superplastic forming was carried out using the developed program and the numerical results were compared to the values available in the literature for plane strain problems. For description of the contact between the dies and sheet, the direct projection method was applied to the complicated problem and the validity of the scheme was tested. Experiments for the various geometries such as hemisphere and cone were performed with the developed forming machine using the calculated optimum pressure-time curves. Comparison between analysis and experiments showed good agreement.

• Transactions of Materials Processing
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• v.11 no.5
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• pp.439-446
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• 2002

This study presents a new computational model to analyze the grain deformation in a polycrystalline aggregate in a discrete manner and based directly in the underlying physical micro-mechanisms. When scaling down a metal forming process, the dimensions of the workpiece decrease but the microstructure of the workpiece remains the similar. Since the dimensions of the workpiece are very small, the microstructure especially the grain size will play an important role in micro forming, which is called size effects. As a result, specific characteristics have to be considered for the numerical analysis. The grains and grain boundary elements are introduced to model individual grains and grain boundary facets, respectively, to consider the size effects in the micro forming. The constitutive description of the grain elements accounts for the rigid-plastic and the grain boundary elements for visco-elastic relationships. The capability of the proposed approach is demonstrated through application of grain element and grain boundary element in the micro forming.