Hemming is the last farming process in stamping and determines external quality of automotive outer panels. Few numerical approaches using 3-dimensional finite element model have been applied to a hemming process due to small element size which is needed to express the bending behavior of the sheet around small die comer and comparatively big model size of automotive opening parts, such as side door, back door and trunk lid etc In this study, part model assembling method is suggested and applied to the 3-dimensional finite element simulation of flanging and hemming process far an automotive front hood.

In order to make a doubly-curved sheet metal effectively, a sheet metal forming process has been developed by adopting the flexibility of the incremental forming process and the principle of bending deformation which causes slight deformation in thickness. The developed process is an unconstrained forming process with no holder. For this study, the experimental equipment is set up with the roll set which consists of two pairs of support rolls and one center roll. In the experiments using aluminum sheets, it is found that the curvature of the formed sheet metal is determined by controlling the distance between supporting rolls in pairs and the forming depth of the center roll and it also depends on the thickness of the sheet metal. In order to check the effect of process parameters on the generation of sheet metal curvatures in this process, the orthogonal array is adopted. From the experimental results, among the process parameters, the distance between supporting rolls in pairs along the direction of one principal radius of curvature as well as the forming depth and the thickness of the material is shown to influence the generation of curvature in the same direction significantly. That is, the other distance between supporting rolls in pairs which are not located in the same direction of one principal radius of curvature, does not have an significant effect on the generation of the curvature in that direction. It mainly affects the generation of curvature in its own direction with the forming depth and the thickness of the material.

Recently, instead of a matched die forming method requiring a high cost and long delivery term, a multi-point dieless forming method using a pair of matrix type punch array as flexible dies has been developed. Since the conventional multi-point dieless forming method has some disadvantages of difficulty in precise punch control and high-cost of equipment, a new concept of multi-point dieless forming method combined with an elasto-forming method has been suggested in this study. For optimal selection of elastomers, compression tests of rubbers, polyethylene and foams were carried out together with FEM analysis of the deformation behavior during sheet forming process using a rigid punch and elastomers. Compressive strain was concentrated on the upper central area of the elastomer under the punch, and the rubber exhibited higher concentration of the compressive strain than foams. Two-dimensional curved surface was formed successfully by the multi-point elasto-dieless forming method using an optimal combination of rubber and foam materials.

The large deformation at cryogenic temperature is expected as one of the effective methods to produce large bulk ultrafine grained materials. The effects of annealing temperature, 150∼$300^{\circ}C$, on microstructures and mechanical properties of the sheets received 85% reduction at cryogenic temperature were investigated, in comparison with those at room temperature. Annealing of 5083 Al alloy deformed 85%, at $200^{\circ}C$ for an hour,. resulted in the considerable increase of tensile elongation without the great loss of strength and the occurrence of equiaxed grains less than 300nm in diameter.

Three-dimensional rigid-plastic finite element analysis has been performed to optimize open die forging process to make round bar. In the round bar forging, it is difficult to optimize process parameters in the operational environments. Therefore in this study, finite element method is used to analyze the practice of open die forging, focusing on the effects of reduction, feeding pitch and rotation angle for optimal forging pass designs. The soundness of forging process has been estimated by the smoothness and roundness of the bar at various combination of feeding pitches and rotation angles. From the test result, process conditions to make round bar having precise dimensional accuracy have been proposed.

Finite element analysis of open die forging process to make rectangular billet has been performed in this study. Three dimensional rigid-plastic finite element method was used to analyze the effects of process variables, forging pass design and die configurations on the void closure phenomena to maximize the internal deformation for better structural homogeneity and center-line consolidation of the rectangular billet. The effect of anvil width ratio, anvil pitch, anvil shape and number of pass has been estimated by the degree of void closure ratio. Although it is difficult to optimize process parameters in the operational environments, favourable process conditions are suggested for better product quality.

Deformation surrounding the hole in the tube during the hydropiercing process has been investigated in this study. The tube is expanded and internally pressurized between upper and lower dies, and a piercing punch is driven forcefully through a cross passage in the die and through the wall of the tube. The pressurized fluid within the tube provides support to the wall of the tube during a piercing step to form a hole in the tube having less deformation surrounding the hole in the tube. The deformation area may be fully retracted to a substantially flat form or partially retracted to a countersunk form. In this study, a mathematical model that can predict deformation surrounding the hole has been proposed and experimentally verified by actual hydropiercing test.

In order to theoretically analyze the creep behavior of high Cr steel at $600^{\circ}C$, a unified elasto-viscoplastic constitutive model based on the consideration of dislocation density is proposed. A combination of a kinetic equation describing the mechanical response of a material at a given microstructure in terms of dislocation glide and evolution equations for internal variables characterizing the microstructure provides the constitutive equations of the model. Microstructural features of the material such as the grain size and spacing between second phase particles are directly implemented in the constitutive equations. The internal variables are associated with the total dislocation density in a simple model. The model has a modular structure and can be adjusted to describe a creep behavior using the material parameters obtained from uniaxial tensile tests.

Microforming can be a good application for bulk metallic glasses. It is important to simulate the deformation behaviour of the bulk metallic glasses in a supercooled liquid region for manufacturing micromachine parts. For these purposes, a correct constitutive model which can reproduce viscosity results is essential for good predicting capability. In this paper, we studied deformation behaviour of the bulk metallic glasses using the finite element method in conjunction with the fictive stress constitutive model which can describe non-Newtonian as well as Newtonian behaviour. A combination of kinetic equation which describes the mechanical response of the bulk metallic glasses at a given temperature and evolution equations fur internal variables provide the constitutive equation of the fictive stress model. The internal variables are associated with fictive stress and relation time. The model has a modular structure and can be adjusted to describe a particular type of microforming process. Implementation of the model into the MARC software has shown its versatility and good predictive capability.

The effect of grain refinement on the plastic deformation behaviour of nanocrystalline metallic materials is investigated. A phase mixture model in which a single phase material is considered as an effectively two-phase one is discussed. A distinctive feature of the model is that grain boundaries are treated as a separate phase deforming by a diffusion mechanism. For the grain interior phase two concurrent mechanisms are considered: dislocation glide and mass transfer by diffusion. The proposed constitutive model was implemented into a finite element code (DEFORM) using a semicoupled approach. The finite element method was applied to simulating room temperature tensile deformation of Cu down to the nanoscale grain size in order to investigate the pre- and post-necking behaviour.

A computer code was developed to simulate the filling stage of the injection/compression molding process by a finite element method. The constitutive equation used here was the compressible Leonov model. The PVT relationship was assumed to follow the Tait equation. The flow-induced birefringence was related to the calculated flow stresses through the linear stress-optical law. Simulations of a disk part under different process conditions including the variation of compression stroke and compression speed were carried out to understand their effects on birefringence variation. The simulated results were also compared with those by conventional injection molding.