• Transactions of Materials Processing
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• v.19 no.8
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• pp.480-485
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• 2010

In this work, the closing behavior of cylindrical-shaped voids was experimentally investigated according to various parameters such as reduction ratio in height, initial void size and billet rotation during hot open die forging process. The reduction ratio in height, number of path, and billet rotation were chosen as key process parameters which influence the void closing behavior including the change of void shape and size. On the other hand, values of die overlapping and die width ratio were set to be constant. Void closing behavior was estimated by microscopic observation. Based on the observations, it was confirmed that application of billet rotation is more efficient to eliminate the void with less reduction ratio in height. The experimental results obtained from this study could be helpful to establish the optimum path schedule of open die forging process.

• 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.

• Transactions of Materials Processing
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• v.20 no.5
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• pp.339-343
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• 2011

This paper presents methods for analyzing the extent of cylindrical-shaped void closure. In addition, a quantitative relationship between change in void fraction and height reduction ratio of a compressed specimen is proposed. The height reduction ratio, number of deformation steps and billet rotation were chosen as key process parameters influencing the void closing behavior, namely, the changes in void shape and size during hot open die forging of a large ingot. The extent of void closure was analyzed from microscopic observations and estimated from tensile test results. The tensile strengths of specimens with closed voids and those without were compared for various reduction ratios in height. The results confirmed that void closure occurs at reduction ratios greater than 30 %. The void closing behavior could be expressed as a hyperbolic tangent function of reduction ratio in height, number of paths, and billet rotation. The knowledge presented in this paper could be helpful for optimizing deformation paths in open die forging processes.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.114-117
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• 2009

In this work, closing behavior of the voids generated in a casting process was investigated for various parameters such as reductions in height void size and billet rotation during hot open die forging process. The reduction in height and path schedule including the number of paths and billet rotation were chosen as key process variables to express the change of geometrical void shape and void closing behavior. On the other hand, values of die overlapping and die width ratio were set to be constant. Extend of void closure was observed and evaluated using tensile test and microscope. Based on the experimental result, it is ensured that void closure do not occur at 15% and 30% reduction in height as well as one or two rotations of a billet. The useful datum obtained from this study could be utilized to establish an optimum path schedule in the open die forging process.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.406-409
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• 2005

In the forging operation of large ingot two break-down process are upsetting and cogging. The first purpose of upsetting is to ensure sufficient forging ratio for subsequent cogging operations and consolidate the voids along the centerline. The second purpose is related to improve the physical properties for a final product. Voids which are generated during the casting process can be one of the decisive defects of materials. So it is necessary to know the standard of Judgment for void-closure in upsetting operation. In practical conditions, FEM analysis(DEFORM 2D 8.1) was carried out to decide how much effective strain has influence on void-closure. It is finally suggested that the function consists of the effective strain of analysis data and the area rate of void.

• 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.