• Transactions of Materials Processing
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• v.13 no.8
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• pp.663-670
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• 2004

The bending phenomenon has been known to be occurred by the difference of velocity at the die exit. The difference of velocity at the die exit section can be obtained by the different velocity of billets through the multi-hole container. The difference of velocity at the die exit can be controlled by the two variables, the one of them is the different velocity of extrusion punch through the multi-hole container, the other is the difference of hole diameter of muliti-hole container. In this paper the difference of hole diameter is applied. So it can bend during extruding products because of the different amount of two billets when billets would be bonded in the porthole dies cavity. And the bending curvature can be controlled by the size of holes. The experiments with aluminum material for the curved tube product had been done for circular or rectangular curved tube section. The results of the experiments show that the curved tube product can be formed by the extru-bending process without the defects such as distortion of section and thickness change of wall of tube and folding and wrinkling. The curvature of product can be controlled by shape of cross section and the difference of billet diameters. And it is known that the bonding and extruding and bending process can be done simultaneously in the die cavity by the experiments that rectangular hollow curved tubes could be extruded by porthole dies with four different size billets made of aluminum material. And it shows that bending phenomenon can happen during extruding with for different billets from the analysis by DEFORM-3D.

• Journal of the Korean Society for Precision Engineering
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• v.11 no.4
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• pp.77-87
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• 1994

This paper deals with an automated forming sequence design system by which designers can determine desirable operation sequences even if they have little experience in the design of cold forging process. The forming sequence design in the cold forging is very important and requires many kinds of technical and empirical knowledge. They system isproposed, which generates forming sequence plans for the multistage cold forging of axisymmtrical solid products. Since the process of metal forming can be considered as a transformation of geometry, treatment of the geometry of the product is a key in planning process. To recognize the geometry of the product section, section entity representation and primitive geometries were used. Section entity representation can be used for the calculation of maximum diameter, maximum height, and volume. Forming sequence for the part can be determined by means of primitive geometries such as cylinder, cone, convex, and concave. By utilizing this geometrical characteristics (diameter, height, and radius), the product geometry is expressed by a list of the priitive geometries. Accordingly the forming sequence design is formulated as the search problem which starts with a billet geometry and finishes with a given product one. Using the developed system, the sequence drawing with all dimensions, which includes the proper sequence of operations for the part, is generated under the environment of AutoCAD. Based on the results of forming sequence, process variables(strain, punch pressure, die inner pressure, and forming load) are determined.

• Transactions of Materials Processing
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• v.30 no.3
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• pp.142-148
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• 2021

In hot forging analysis, the interfacial heat transfer coefficient (IHTC) is a very important factor defining the heat flow between the die and the material. In particular, in the hot forging analysis of aluminum 6xxx series alloy, which are used in automobile parts, differences in load and microstructure occur due to changes in surface temperature according to the IHTC. This IHTC is not a constant value but changes depends on pressure. This study derived the IHTC under low load using aluminum 6082 alloy. An experiment was performed by fabricating a compression die, and a heat transfer analysis was performed based on the experimental data. The heat transfer analysis used DEFORM-2D, a commercial finite element analysis program. To derive the IHTC, heat transfer analysis was performed for the IHTC in the range of 10 to 50 kW/m²℃ at intervals of 10kW/m²℃. The heat transfer analysis results according to the IHTC and the actual experimental values were compared to derive the IHTC of the aluminum 6082 alloy under low load.

• Transactions of Materials Processing
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• v.31 no.5
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• pp.253-260
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• 2022

In this study, the microstructural characteristics of a high-speed-extruded Mg-5Bi-3Al (BA53) alloy and its tensile, compressive, and high-cycle fatigue properties are investigated. The BA53 alloy is successfully extruded at a die-exit speed of 16.6 m/min without any hot cracking using a large-scale extruder for mass production. The homogenized BA53 billet has a large grain size of ~900 ㎛ and it contains fine and coarse Mg₃Bi₂ particles. The extruded BA53 alloy has a fully recrystallized microstructure with an average grain size of 33.8 ㎛ owing to the occurrence of complete dynamic recrystallization during high-speed extrusion. In addition, the extruded BA53 alloy contains numerous fine lath-type Mg₃Bi₂ particles, which are formed through static precipitation during air cooling after exiting the extrusion die. The extruded BA53 alloy has a high tensile yield strength of 175.1 MPa and ultimate tensile strength of 244.4 MPa, which are mainly attributed to the relative fine grain size and numerous fine particles. The compressive yield strength (93.4 MPa) of the extruded BA53 alloy is lower than its tensile yield strength, resulting in a tension-compression yield asymmetry of 0.53. High-cycle fatigue test results reveal that the extruded BA53 alloy has a fatigue strength of 110 MPa and fatigue cracks initiate at the surface of fatigue test specimens, indicating that the Mg₃Bi₂ particles do not act as fatigue crack initiation sites. Furthermore, the extruded BA53 alloy exhibits a higher fatigue ratio of 0.45 than other commercial extruded Mg-Al-Zn-based alloys.

• Journal of the Korean Society of Industry Convergence
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• v.26 no.1
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• pp.95-103
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• 2023

The domestic remanufacturing industry is concentrated in auto parts, so it is necessary to expand into various industries. In the domestic aluminum industry, the extrusion process accounts for more than 40% of the total, but the old and management of the extrusion equipment is not done properly. In particular, the extruder has a structure in which equipment is not replaced until major parts are damaged or worn, so there are problems such as lower process precision, productivity and production efficiency compared to new equipment, and high maintenance costs. In this study, the old extruder was remanufactured for major high-risk parts through Failure Mode and Effect Analysis(FMEA), and the process level and performance of the extruder were evaluated before and after remanufacturing. Compared to the existing extruder, the standard deviation of the remanufacture extruder was reduced by 93.5%, 57.9%, and 70.0%, respectively, in major process control items such as container temperature, billet temperature, and ram speed, keeping performance constant. In addition, it was possible to produce products with complex shapes that could not be produced before due to problems such as dimensional deviation within tolerances. In this study, remanufacturing guidelines were presented by analyzing the effect of failure modes of the old extruder, and the performance improvement of the extruder was confirmed.

• Analytical Science and Technology
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• v.24 no.5
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• pp.395-400
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• 2011

This study has been carried out in order to effectively manage three groups of unregulated hazardous organic substances (PCDD/DFs, PAHs, PCBs) in South Korea. The investigated substances have been analyzed according to the test methods for hazardous substances in specified wastes provided by the National Institute of Environmental Research, Korea. Total contents of the organic compounds have been determined for hazardous wastes from three major industrial categories (paint, iron removal, steelmaking), such as waste organic solvent, waste paint or dust. By investigating the waste samples for 7 PAHs using GC/MSD, Naphthalene has been detected (N.D~1631.33 mg/kg). The highest Naphthalene concentration, which exceeded the korean marine dumping waste standard, was found in waste organic solvents and waste paints. Although a content analysis of 7 PCB isomers has been conducted, none of these compounds was detected. The analysis of PCDD/DFs has revealed that all samples meet the criterion for low POP content defined in the technical guidelines developed under the Basel Convention. The PCDD/DFs content in dust samples deriving from 10 manufacturing processes (billet, bloom) was determined to be in the range of N.D~5.66 ng I-TEQ/g waste.

• Journal of the Korean Society for Nondestructive Testing
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• v.16 no.3
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• pp.162-173
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• 1996

Dry magnetic particle inspection(MPI) was performed to detect the surface defects of steel ingot cast billets. Magnetic properties of several materials were characterized by the measurement of the B-H hysteresis curve. The inspection results were evaluated in terms of the magnetizing current, temperature, and the amount of magnetic particles applied to billets. Magnetic flux leakage near the defect site of interest was measured and compared with the results of calculation by the finite element method in the case of direct magnetizing current. Direct and alternating magnetizing currents for materials were deduced by the comparison of the inspections. Results of the magnetic particle inspection by direct magnetizing current were compared with those of finite element method calculations, which were verified by measuring magnetic leakage flux above the surface and the surface defects of the material. For square rods, due to the geometrical effect, the magnetic flux density at the edges along the length of the rods was about 30% of that at the center of rod face for a sufficiently large direct magnetizing current, while it was about 70% for an alternating magnetizing current. Thus, an alternating magnetizing current generates rather uniform magnetic flux density over the rods, except for the region on the face across about 10 mm from the edge. The attraction of the magnetic particle by the magnetic leakage field was nearly independent of the surface temperature of the billets up to $150^{\circ}C$. However, the temperature should have been limited below $60^{\circ}C$ for an effective fixing of gathered magnetic particles to the billet surface using methylene chloride. We also found that the amount of applied magnetic particles tremendously affected the detection capability.

• Korean Journal of Materials Research
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• v.5 no.8
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• pp.953-959
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• 1995

It was investigated that reinforced species, billet condition and extrusion variation in Al 6061 composite material effected on extrusion process of particulate reinforced composite material. The strength of composite material with reinforcement species revealed SiC$\sub$w/> A1$_2$O$\sub$3f/ > A1$_2$O$\sub$3f/ > A1$_2$O$\sub$3f/ orderly. K$\sub$w/ increased as volute fraction increased in all composite material. The composite materials reinforced by A1$_2$ $O_3$required the larger pressure in hot extrusion process than those by SiC$\sub$p/ at all condition. Extrusion process tended to decrease as the semi-angle of extrusion dies increased because larger contact area caused larger shear friction. Extrusion temperature went up about 50$^{\circ}C$ in low elevated deformation temperature. In extrusion temperature above 500$^{\circ}C$, severe tearing occurred on extrusion surface. More reinforcement in volume fraction, more hot tearing.

• The Korean Journal of Financial Management
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• v.25 no.1
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• pp.51-84
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• 2008

We investigate the signaling effect of repurchase preceding new equity issue using Korean data. In a short time span, firms announce stock repurchases and equity offerings. The proximity of two events in Korean firms indicates that those are not independent of each other. In this paper, we test the signaling effect of repurchase on equity offerings on the two measures. One is announcement effect, which is measured as CAR(0, +2). The other is the effectiveness which is measured as CAR(0, +30) because the price movement during this window influences on the price of new issues. Previous studies that stock repurchase convey positive signal to equity offerings-Billet and Xue(2004) and Jung(2004)-construct sample without the limit of time interval between two events. This causes the unclear relation between those because of the long time interval. In this study we consider only samples of being within one year each other to reduce this problem and clarify the signal of repurchase on equity offerings. Korean firms are allowed to repurchase own shares with two different method. One is direct repurchase as same as open market repurchase. The other is stock stabilization fund and stock trust fund which trust company or bank buy and sell their shares on the behalf of firms. Generally, the striking different characteristic between direct repurchase and indirect repurchase is following. Direct repurchase is applied by more strict regulation than indirect repurchase. Therefore, the direct repurchase is more informative signal to the equity offering than the indirect repurchase. We construct two sample firms- firms with direct repurchase preceding-equity offerings and indirect repurchase-preceding equity offering, and one control firms-equity offerings only firms-to investigate the announcement effect and the effectiveness of repurchases. Our findings are as follows. Direct repurchase favorably affect the price of new issues favorably. CAR(0, +2) of firms with direct repurchase is not different from that of equity offerings only firms but CAR(0, +30) is higher than that of equity offerings only firms. For firms with indirect repurchase and equity offerings, Both the announcement effect and the effectiveness does not exist. Jung(2004) suggest the possibilities of how indirect stock repurchase can be regarded as one of unfair trading practices on based on the survey results that financial managers of some of KSE listed firms have been asked of their opinion on the likelihood of the stock repurchase being used in unfair trading. This is not objective empirical evidence but opinion of financial managers. To investigate whether firms announce false signal before equity offerings to boost the price of new issues, we calculate the long-run performance following equity offerings. If firms have announced repurchase to boost the price of new issues intentionally, they would undergo the severe underperformance. The empirical results do not show the severer underperformance of both sample firms than equity offerings only firms. The suggestion of false signaling of repurchase preceding equity offerings is not supported by our evidence.