• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2007.05a
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• pp.160-164
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• 2007

In the steel industry, there is a need to produce large forged parts for the automobile industries, the flight and shipping industries ad military industries. In the steel-industry application, a cogging technique for cast ingots is required, because the major parts are needed as one large body in order to obtain higher quality. Therefore, cogging process is the primary step in manufacturing of practically large open-die forging. In the cogging process, internal voids have to be eliminated as defects, The present work is concerned with the elimination of the internal voids in large ingots so as obtain sound products. In this study, hot compression tests were carried out to obtain the flow stress of cast microstructure at different temperature and strain rates. The FEM analysis are performed to investigate the overlap defect of cast ingots during cogging stage. The measure flow stress data were used to simulate the cogging process of cast ingot using the practical material properties. Also the analysis of void closure are performed by using the $DEFORM^{TM}$-3D. The calculated results of void closure behavior are compared with the measured results before and after cogging, which are scanned by the X-ray scanner. From this result, the criteria for deformation amounts effect on the void closure can be investigated by the comparison of practical experiment and numerical analysis.

• Transactions of Materials Processing
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• v.22 no.5
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• pp.269-274
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• 2013

Experiments and numerical simulations of the incremental upsetting test were carried out to investigate void closure behavior and mechanical characteristic of a 1.5wt%C ultra-high carbon steel. The experimental results showed that the voids become quickly smaller as the reduction ratio increases. The simulation results confirmed this behavior and indicated that the voids were completely closed at a reduction ratio of about 40~45% during incremental upsetting. After the completion of the incremental upsetting tests, the process of diffusion bonding was employed to heal the closed voids in the deformed specimens. To check the appropriate temperature for diffusion bonding, deformed specimens were kept at 800, 900, 1000 and $1100^{\circ}C$ for an hour. In order to investigate the effect of holding time for diffusion bonding at $1100^{\circ}C$, specimens were kept at 10, 20, 30, 40, 50 and 60minutes in the furnace. A distinction between closed and healed voids was clearly established using microstructural observations. In addition, subsequent tensile tests demonstrated that complete healing of a closed void was achieved for diffusion bonding temperatures in the range $900{\sim}1100^{\circ}C$ with a holding time larger than 1 hour.

• Transactions of Materials Processing
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• v.22 no.8
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• pp.463-469
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• 2013

In the previous work, a new forging process design, which included incremental upsetting, diffusion bonding and cogging, was suggested as a method to manufacture 1.9wt%C ultrahigh carbon workrolls. The previous study showed that incremental upsetting and diffusion bonding are effective in closing voids and healing of the closed void. In addition, compression tests of the 1.9wt%C ultrahigh carbon steel revealed that new microvoids form within the blocky cementite at temperatures of less than $900^{\circ}C$ and that local melting can occur at temperatures over $1120^{\circ}C$. Thus, the forging temperature should be controlled between 900 and $1120^{\circ}C$. Based on these results, incremental upsetting and diffusion bonding were used to check whether they are effective in closing and healing voids in a 1.9wt%C ultrahigh carbon steel. The incremental upsetting and diffusion bonding were performed using sub-sized specimens of 1.9wt%C ultrahigh carbon steel. The specimen was deformed only in the radial direction during the incremental upsetting until the reduction ratio reached about 45~50%. After deformation the specimens were kept at $1100^{\circ}C$ for the 1 hour in order to obtain a high bonding strength for the closed void. Finally, microstructural observations and tensile tests were conducted to investigate void closure behavior and bonding strength.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.197-198
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• 2008

The forging process produces complicated and designed components in a die at high productivity for mass production and minimizes the machining amount for favorable material utilization; the forging products used at highly stressed sections are well accepted at a wide range of industry such as automobile, aerospace, electric appliance and et cetera. Accordingly, recent R&D activities have been emphasized on improvement of forging die-life and near net shaping technology for cost effectiveness and better performance. Usually closing and consolidation of internal void defects in a ingot is a vital matter when utilized as large forged products. It is important to develop cogging process for improvement of internal soundness without a void defect and cost reduction by solid forging alone with limited press capacity. For experiments of cogging process, hydraulic press with a capacity of 800 ton was used together with a small manipulator which was made for rotation and overlapping of a billet. Size of a void was categorized into two types; ${\phi}$ 6.0 mm and ${\phi}$ 9.0 mm to investigate the change of closing and consolidation of void defects existed in the large ingot during the cogging process. In addition for forming experiment of piston grown air drop hammer with a capacity of 16 ton was used. The experiment with piston crown was carried out to show the formability and void closing status. In this paper systematic configuration for closing process of void defects were expressed based on this experiment results in the cogging process. Also forging defects through forming process for piston crown was improved using the experiment results and FE analysis. Consequently this paper deals with the effect of radial parameters in cogging process on a void closure far large forged products and formability of piston crown.

• Transactions of Materials Processing
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• v.21 no.5
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• pp.319-323
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• 2012

In order to predict the internal void closing behavior in open die forging process, multiple scale modeling has been developed and applied. The huge size difference between ingot and inner void makes it almost impossible to simultaneously model the actual loading conditions and the void shape. Multiple scale modeling is designed to integrate macro- and micro- models effectively and efficiently. The void closing behavior was simulated at 39 different locations in a large ingot during upsetting and cogging. The correlation between the closing behavior and variables such as effective plastic strain and maximum compressive strain was studied in order to find an efficient measure for predicting the soundness of the forging.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1996.10a
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• pp.89-94
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• 1996

The use of shield is increasing day by day, because it's method is advantageous tunneling method to soft and collapsible ground. In case of analyzing shield tunnel by FEM, short term behavior of ground by initial heaving and tail void closure and long terms of it because of consolidation by changes of pore pressures in clay must be considered. In this paper, the shield tunneling construction stages was analyzed from 2 dimensional elasto - viscoplastic finite element program used Mohr - Coulomb yield criterion but not considered the changes of pore pressures. The object of investigation was N - 2 Tunnel. Since the good results of analysis compared to the measured behavior of ground for heaving, tail void closure and liner installation, this results can be applied to design and construction of shield tunneling for the subways, sewage lines etc.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2013.05a
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• pp.1535-1536
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• 2013

• Tunnel and Underground Space
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• v.12 no.4
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• pp.304-311
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• 2002

This study reports the influence of vocid geometry on fracture closure and permeability from numerical experiments. As the aperture distributions of rock fractures are characterized by statistical methods, synthetic fractures have successfully been simulated in this way. Based on the generated fracture models, models for fracture closure and flow calculation have been developed. A fracture closure model has been developed by considering the asperity compression and half-space deformation, and flow calculations have been performed using a finite difference method adopting a local cubic law. The results of numerical experiments have shown that the increase in the aperture spatial correlation leads the fracture closure and the decrease in fracture permeability to increase. Also, it has been indicated that there is an implicit relation between fracture normal stiffness and permeability. The importance of this study is to enhance the understanding the hydro-mechanical behavior of fractured rock massed due to engineering projects.

• Transactions of Materials Processing
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• v.4 no.1
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• pp.48-58
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• 1995

In this study, a contact algorithm for the finite element analysis of free surface contact problem in materials forming is presented. The proposed contact algorithm consists of two parts. The first is the contact searching part, and the second, the constraint part. The contact searching algorithm does not require any a priori knowledge of the pairs of contact nodes or segments and the impenetrability constraint is satisfied using the penalty function scheme. void colsure in open-die forging was simulated to verify the accuracy and capability of the currently developed contact algorithm. The simulation results, obtained from ABAQUS simulation, were compared well to the experimental data available in the literature.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.81-86
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• 2001

Interfacial pressure jump terms based on the physics of phasic interface and bubble dynamics are introduced into the momentum equations of the two-fluid model for bubbly flow. The pressure discontinuity across the phasic interface due to the surface tension force is expressed as the function of fluid bulk moduli and bubble radius. The consequence is that we obtain from the system of equations the real eigenvalues representing the void-fraction propagation speed and the pressure wave speed in terms of the bubble diameter. Inversely, we obtain an analytic closure relation for the radius of bubbles in the bubbly flow by using the kinematic wave speed given empirically in the literature. It is remarkable to see that the present mechanistic model using this practical bubble radius can indeed represent both the mathematical well-posedness and the physical wave speeds in the bubbly flow.