• Transactions of Materials Processing
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• v.33 no.3
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• pp.161-168
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• 2024

Aluminum alloy sheets, compared to conventional steel sheets, face challenges in press forming due to their lower elongation. To enhance their formability, extensive research has focused on forming technologies at elevated temperatures, specifically warm forming at around 300℃ and hot forming at approximately 500℃. This study proposes that the formability of aluminum alloy sheets can be significantly enhanced using a multi-stage hot forming technique. The research also investigates whether the strength of the A6061 aluminum alloy, known for its precipitation hardening, can be maintained when formed below the precipitate solid solution temperature. In the experiments, the A6061-T6 sheet underwent heating and rapid cooling between 250 and 500℃. The mechanical properties were evaluated at each stage of the process. The findings revealed that when the initial heat treatment was below 350℃, the strength of the material remained unchanged. However, at temperatures above 400℃, there was a noticeable decrease in strength coupled with an increase in elongation. Conversely, when the secondary heat treatment was conducted at temperatures of 350℃ or lower, the strength remained comparable to that of the initial heat treated material. However, at higher temperatures, a reduction in strength and an increase in elongation were observed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2006.05a
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• pp.226-229
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• 2006

Process design has been performed for the warm hydroforming of light weight alloy tubes. For the heating of tubes, specially designed induction heating system has been adopted to ensure rapid heating of tubes. The induction heating system uses 30kHz frequency induction coil in order to concentrate the energy in the tube and prevent the energy loss. But the induced heat by the integrated heating system, consisting of induction coil, tube, pressure oil and dies, was normally not equally distributed over the length and circumference of the tube specimen, and consequent temperature distribution was non-uniform. So additional heating element has been inserted into the inside of the tube to maintain the forming temperature and reduce temperature drop due to heat loss to the molds. And for that heat loss, a heat insulation system has also been installed. The drop in flow stress at elevated temperatures results in lower internal pressure for hydroforming and lower clamping forces. The proposed warm hydroforming process has been successfully implemented when applying 6061 aluminum extruded tubes.

• The Bulletin of The Korean Astronomical Society
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• v.44 no.1
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• pp.34.1-34.1
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• 2019

To study the formation and evolution of stellar bars and gaseous nuclear rings in barred galaxies in realistic environments, we run fully self-consistent three-dimensional simulations of isolated disk galaxies. We consider two groups of models with cold or warm disks that differ in the radial velocity dispersion. We also vary the gas fraction of the disks. We found that a bar forms earlier and more strongly as the gas fraction increases in the cold disks, while the gas delays the bar formation in the warm disks. The bar formation enhances a central mass concentration which in turn weakens the bar strength temporarily, after which the bar regrows to become stronger in a model with a smaller gas fraction in both cold and warm disks. Although all bars rotate fast in the beginning, they rapidly turn to slow rotators. Gas infalling to the central region forms a dense star-forming nuclear ring. The ring size is very small when it first forms and grows over time. The ring star formation is episodic and bursty due to star formation feedback, and has a good correlation with the mass inflow rate to the ring. Some expanding shells produced by star formation feedback are sheared out in the bar regions and collide with dust lanes to appear as filamentary interbar spurs.

• Transactions of Materials Processing
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• v.14 no.7 s.79
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• pp.628-634
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• 2005

The drawability of AZ31B magnesium sheet is estimated at various temperatures (200, 250, 300, 350, $400^{\circ}C$), forming speeds (20, 50, 100mm/min), thicknesses (0.8, 1.4mm) and blank holding forces (2.0, 2.8, 3.4kN). The deep drawing process (DDP) of circular cup is used in forming experiments. The results of deep drawing experiments show that the drawability is well at the range from 250 to $300^{\circ}C$, 50mm/min forming speed and 2.0kN blank holding force. The 0.8mm magnesium sheets were deformed better than 1.4 mm. Blank holding force was controlled in order to improve drawability and prevent the change of cup thickness. When blank holding force was controlled, tearing and thickness change were decreased and limit drawing ratio was improved from 2.1 to 3.0.

• Transactions of Materials Processing
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• v.6 no.5
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• pp.446-455
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• 1997

A Combined extrusion operation consists of forward and backward extrusion forming and it is possible to make the process be simple by employing it. But the metal flow pattern induced by the operation is hard to analyze accurately because the flows are non-steady, which have at least two directions dependent upon each other. So engineers in the industrial factories had conducted the two extrusion operations separately. A new process was designed by the industrial expert for forming of an alu-minum preform using the combined extrusion operation. In this study, experiments and finite element analysis was carried out to determine the process parameters. Through the preliminary experiment, it was shown that warm forming condition was more desirable than cold or hot ones. And optimal shape of initial billet could be also determined. From the compatibility test, bonde-lube was chosen as the optimal lubricant and 20$0^{\circ}C$ as the material temperature by the inspection of micro-structure. The operation was simulated by the rigid-plastic finite element method to examine the metal flow. Disap-pearing of dead metal zone was observed as the punch fell down and desirable shape was obtained from the one operation. As a result of this study, 7 operations could be reduced and 225% of material saved.

• The Bulletin of The Korean Astronomical Society
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• v.43 no.1
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• pp.44.2-44.2
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• 2018

G33.92+0.11, classified as a core-halo UC HII region at a distance of 7.1 kpc, contains several sub-clumps (~20-200 solar masses) as identified by dust continuum emission. This source shows very complicated features associated with vigorous massive star-forming activities with a nearly face-on projection. The ambient gas is still accreting to the massive molecular clumps dynamically, while the whole cloud is under disruption by newly formed stars. Using the recent high resolution (< 0.2") ALMA observations, we investigate the detailed structure associated with the star-forming activities by comparing different chemical tracers. The sub-clumps having extremely complex morphologies still preserve cold dense gas together with the turbulent and dense warm gas resulted by newly formed stars and interaction with accreting gas. The accretion of the ambient gas may have occurred episodically to this source. Most recent star formation, which probably the third generation of star formation in this region, is taking place in the northern part (A5 clump). The relatively small mass (~ 1/3 of A1 or A2) and the lack of turbulent gas of this star-forming core may suggest that this core was formed already during the overall collapse of the whole cloud for the first star formation. We think that gravitational collapse of these sub-clumps appears as sequential star formation of this region. The later interaction with accreting gas may have not been a direct cause of the star formation activities of this source.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.407-410
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• 2005

The drawability of AZ31B magnesium sheet is estimated according to the variable temperatures (200, 250, 300, 350 and $400^{\circ}C$), forming speed (20, 50, 100 mm/min), thickness (0.8, 1.4 t), blank holding force (1.0, 1.4, 1.7kN). The deep drawing process (DDP) of circular cup is used in forming experiments. The results of deep drawing experiences show that the drawability is well at the range from 250 to $300^{\circ}C$, 50mm/min forming speed and 1.4kN blank holding force. The 0.8t magnesium sheets were deformed better than 1.4t. BHF was controlled in order to improve drawability and protect the change of cup thickness. When BHF was controlled, tearing and thickness change were decreased and LDR. was improved from 2.1 to 3.0.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2008.10a
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• pp.71-74
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• 2008

The clad sheet is the sheet metal that joined the one or more material with the different property by rolling process. In this study, it is investigated about the mechanical property or formability of SUS-Al-Mg clad sheet. The clad sheet was formed at elevated temperature because of their poor formability at room temperature. The tensile test was confirmed at various temperature and the reduction of strain rate above $250^{\circ}C$. LDR(Limited Drawing Ratio) was obtained through deep drawing test to confirm the formability of the clad sheet. The FE analysis is performed to compare prototype products.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.03a
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• pp.203-210
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• 1995

The rapid progress of automobile industry has led to a demand for sheet metal products and the new forming technology for complex product is required. In Deep Drawing process of steel sheet, especially stainless steel sheet at room temperture, some intermediate annealing need to be added to lessen strain hardening effect. The present study is concerned with the wram deep drawing of stainless steel sheet. In order to reduce the number of working process. the limit drawing ration is considered as main parameter. In this study, the effect of process variables such a sblank holder force, working temperature and lubricant on limit drawing ration is investigated . Experiments are carried out for the hemisherical and sqare shape at room and warm temperature respectively. The drawing loads and thickness deviation accoring to process variables measured . As the result of apploying those experimental data to the commerical butt product, the number of process can be reduced and good quality of products can be obtained.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.04b
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• pp.171-177
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• 1995

In this paper, the deformation method for inner and outer races of rollr-bearing bymeans of the warm precision forging is investigated. We adapted the process designsuch as following that, toincrease Die life, reduce heat transfer through conduction and the eccentricity of preform in warm forging of bearing gace, the bottom portion of billet is formed during upsetting process. Then it is backward extruded, and thus obtained ring preform is formed by combined extrusion. Also, we compared it with the froming method in China and Japan, and we have known it is more excellent method. Basides, this forming method is simulated by UBST which is based on the merits of UBET nd FEM. The results show that it is easy to know the exact location of neutral surface through the inspection of streamline during combined extrusion, and the velocity vector distribution along the surface of velocity discontinuity is investigationed. Also the effectiveness of this method is proved by te experiment using model material that is Plasticine.