• Transactions of Materials Processing
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• v.16 no.2 s.92
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• pp.126-131
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• 2007

Voids in a large rotor are formed in solidification process of a cast ingot. The voids have to be eliminated from the rotor by a forming process, because they would became stress-intensity factors which suddenly fracture the rotor in the operation. Previous studies on void-elimination of a large rotor have mainly focused on finding the process variables affecting the void-closure. But the study on the amount of void closure in a large rotor has been very rare. This study was performed to obtain an equation which predicts the amount of void-closure in a forging process of a large rotor and to evaluate the availability of the void-closure equation through finite element analyses. Firstly, 2D FE analysis was carried out to find effects of time integral of hydrostatic stress and effective strain on void volume rate of a large rotor in the upsetting process for various diameters and shapes of void, and material temperature. From the 2D FE analysis, we found that effective strain was suitable for predicting the void-closure of a large rotor, because there was a constant relationship between void volume rate and effective strain. And a void-closure equation was proposed fur predicting void-closure of a large rotor in the upsetting process. Finally, ken the 3D FE analysis, the proposed void-closure equation was verified to be useful for upsetting and cogging processes.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.57-60
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• 2008

The studies for internal void closure have been conducted experimentally and numerically for open die forging. The FEM analysis is performed to investigate the deformation behavior of some internal voids in cast ingots during two upsetting stages. The calculated results of void closure behavior are compared with the measured results before and after upsetting. The shapes and sizes of each internal void 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 types of void. Closed voids could be compressed and eliminated after forging when the applied deformation amounts were larger than the critical effective strains. On the other hand, open voids could not be compressed and removed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.75-78
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• 2008

The predictive equation of void-closure was developed to evaluate void crush ratio with respect to the process variables in the cogging process of a large round bar. The comprehensive finite element analysis with the process variables such as reduction ratio and die width ratio was carried out. The predictive equation of void-closure for cogging process was established on the basis of the regression analysis with the extensive FE analysis results and verified by comparing the predicted results with FEA results with various forging passes.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.05a
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• pp.67-70
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• 2008

This paper deals with the effect of radial parameters in cogging process such as reduction in height (Rh) and rotational angle ($\theta$) of a billet on a void closure for large forged products. Usually closing and consolidation of internal void defects in a ingot is a vital matter when utilized as large forged products, using a press with limited capacity and the sizes of the ingots becoming larger. Consequently, 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; $\emptyset$ 6.0 mm and $\emptyset$ 9.0 mm to investigate the change of closing and consolidation of void defects existed in the large ingot during the cogging process. Also open void and closed void in the ingot were tackled to show the differentiation of closing process of internal voids with respect to void sizes. In this paper systematic configuration for closing process of void defects were expressed based on this experiment results in the cogging process.

• Transactions of Materials Processing
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• v.13 no.2
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• pp.148-153
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• 2004

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.

• 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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• 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.

• 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.

• Transactions of Materials Processing
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• v.5 no.1
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• pp.18-26
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• 1996

In order to investigate the effect of pre-cooling of ingot on void closure in hot plate forging the internal strain and stress distributions are examined quantitatively by using ABAQUS. Simula-tions are carried out on a large slab ingot having the same temperature and the temperature gradient induced by air-cooling. It is shown that pre-cooling produces little effect on the strain behavior but remarkable effect on the hydrostatic stress at the central zone of ingot. The main factors for crushing micro-voids are the effective strain and the time integral of hydrostatic stress in the region surrounding the voids. Based on regression analysis it was found that the distortion of void can be expressed as a polynomial function of the two factors.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.10
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• pp.1877-1889
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• 1992

Upsetting is performed in open-die press forging to deform metal in all directions in order to enhance soundness of a product and reduce directionality of properties caused by casting. It is necessary to ensure sufficient forging ratio for subsequent cogging operations and consolidate the void along the centerline. To obtain these benefits, the upper die shape (dome and dished shape) is considered as an upsetting parameter. Thermo-viscoplastic finite element analysis has been carried out so as to understand the influence of upper die shape on the effective strain, hydrostatic stress and temperature in the upset-forged ingots without internal defects. The analysis is focused on the investigation into internal void closure in ingots with pipe holes and circular voids. The computational results have shown that the volume fraction of the void is independent of the circular void size and the closure of internal voids is much more influenced by the effective strain than the hydrostatic stress around the void. It is finally suggested that the height reduction must be over 35% for consolidation of internal voids.