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42 results on '"Yang, Dong‐Yeol"'

11. Simultaneous control of mechanical strength and hierarchical structure in freeze‐casted porous alumina by two‐step sintering.

Author

Jeon, Sang‐Chae, Woo, Jong‐Won, Kim, Sung‐Hyun, Ha, Jang‐Hoon, Yang, Dong‐Yeol, Moon, Kyoung‐Seok, Wu, Haoyue, Yang, Jae‐Hwan, and Kim, Dong‐Kyu

Subjects
SINTERING, FINITE element method, COMPRESSIVE strength, ACTIVATION energy
Abstract

To increase mechanical strength of porous ceramics, here, an effective two‐step sintering (TSS) technique capable of producing highly porous alumina with enhanced mechanical strength is suggested. Based on the sintering theories, here, a significantly lower activation energy for densification at low temperature region allowed a beneficial temperature range for the TSS to be deduced. With a specific TSS regime (T1 = 1550°C and T2 = 1400°C), significantly higher compressive strength levels (8.00–15.24 MPa) were measured with an apparent porosity of 56.49% compared to conventional sintering (1.03–1.86 MPa) with similar apparent porosity of 57.84%. Specifically, another TSS regime (T1 = 1550°C and T2 = 1380°C) left submicron‐sized open pores within the lamellar walls, providing a hierarchical porous structure with enhanced mechanical strength. An evaluation of the mechanical stability by a finite element analysis indicated outstanding compressive strength even with small pores in the lamella walls. [ABSTRACT FROM AUTHOR]

Published
2024
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16. Effects of expanded graphite content on the performance of MgO‐C refractories.

Author

Kim, Junseong, Jeong, Seunghwa, Lee, Minsuk, Kang, Dong Jae, Park, Hong‐woo, Lee, Hwa‐In, Yang, Dong‐Yeol, Kim, Eun Hee, Lee, Soonil, Yang, Seung‐Cheol, and Jeon, Sang‐Chae

Subjects
THERMAL shock, HEAT treatment, REFRACTORY materials, PHENOLIC resins, THERMAL expansion, GRAPHITE, POWDERS
Abstract

Expanded graphite was introduced into a MgO‐C refractory to suppress its thermal expansion and thus enhance thermal shock resistance. The refractory was prepared by mixing MgO powders (particle size = 75 μm and 1 mm), antioxidant and curing agents, flake and expanded graphite, and a novolak‐type phenolic resin at 50°C. This was followed by aging at 20°C for 24 h, compacting by uniaxial pressing, curing at 210°C for 5 h, and heat treatment at 1500°C. With an increase in expanded graphite content from 0 to 4 wt.%, the bulk density decreased, apparent density remained unchanged, and apparent porosity increased. The gaps created in the vicinity of MgO particles because of this increase in apparent porosity buffered the thermal expansion of MgO. This increased resistance to thermal shock up to 1500°C. However, the increase in expanded graphite content also had a detrimental effect reflected in the decrease in fracture strength and increase in the residual strain after repeated thermal shock. This contradiction indicates that composition optimization is important for the practical performance enhancement of MgO‐C bricks. The optimum content of the expanded graphite was determined as 2 wt.%. [ABSTRACT FROM AUTHOR]

Published
2023
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17. A Novel Route for Predicting the Cracking of Inoculant-added AA7075 Processed via Laser Powder Bed Fusion

Author

Choe, Jungho, primary, Kim, Kyung Tae, additional, Yu, Ji Hun, additional, Park, Jeong Min, additional, Yang, Dong Yeol, additional, Jung, Soo ho, additional, Jo, Seungki, additional, Joo, Hyomoon, additional, Kang, Mungu, additional, Ahn, Soung Yeoul, additional, Jeong, Sang Guk, additional, Kim, Eun Seong, additional, Lee, Haksung, additional, and Kim, Hyoung Seop, additional

Published
2022
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25. Densifying method in additive manufacturing process of H13 tool steel : laser re-melting

Author

Choe, Jungho, Jung, Im Doo, Yun, Jaecheol, Yang, Sangsun, Yu, Ji-Hun, Yang, Dong-Yeol, Kim, Yong-Jin, School of Mechanical and Aerospace Engineering, Proceedings of the 3rd International Conference on Progress in Additive Manufacturing (Pro-AM 2018), and Singapore Centre for 3D Printing

Subjects
Engineering::Mechanical engineering::Prototyping [DRNTU], Additive Manufacturing, Selective Laser Melting
Abstract

AISI H13 tool steel structures were fabricated by selective laser melting which is one of the metal additive manufacturing process. Scanning speed of laser was controlled in the ranging of 100-1000 mm/s to optimize the process parameter to fabricate the highly densified structure. Laser power, hatch space and layer thickness were fixed at 90 W, 80 μm and 25 μm. Under 200 mm/s, spherical shape pores were observed and irregular shape pores were observed in over 400 mm/s samples. Residual pores were not easily removed by changing laser scanning speed alone and laser re-melting technique was applied to solve the pore problem. Each layers were exposed twice to laser with various scanning speed before re-coating the powder for next layer. Spherical shape pores were observed from all of the specimen exposed under 200 mm/s irrespective of another exposed laser speed. While re-melting over scanning speed of 800 mm/s did not showed effect on the densification, the combination of first scanning speed (400 mm/s) and second scanning speed (600 mm/s) showed the significant reduction of pores. Published version

Published
2018

32. Nano-mechanical Behavior of H13 Tool Steel Fabricated by a Selective Laser Melting Method.

Author

Nguyen, Van Luong, Kim, Eun-ah, Yun, Jaecheol, Choe, Jungho, Yang, Dong-yeol, Lee, Hak-sung, Lee, Chang-woo, and Yu, Ji-Hun

Subjects
STEEL metallurgy, STEEL analysis, NANOINDENTATION tests, NANOFABRICATION, TENSILE strength
Abstract

Nano-mechanical properties of selective laser melted H13 steel at a scan laser speed of 100 mm/s were investigated using nanoindentation tests. The findings shed light on the interrelationship between the nanoindentation strain rate and hardness. It was found that the strain-rate sensitivity exponent (m = 0.022) of this material indicated that the nanoindentation hardness increased in a range of (8.41 to 9.18) GPa with an increase in the strain rate ranging from 0.002 to 0.1 s−1. [ABSTRACT FROM AUTHOR]

Published
2019
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42. A novel route for predicting the cracking of inoculant-added AA7075 processed via laser powder bed fusion

Author

Choe, Jungho, Kim, Kyung Tae, Yu, Ji Hun, Park, Jeong Min, Yang, Dong Yeol, Jung, Soo ho, Jo, Seungki, Joo, Hyomoon, Kang, Mungu, Ahn, Soung Yeoul, Jeong, Sang Guk, Kim, Eun Seong, Lee, Haksung, and Kim, Hyoung Seop

Abstract

Solidification cracks caused by columnar grains in precipitation-hardenable Al alloys such as Al–Zn–Cu–Mg limits the applicability of laser-based additive manufacturing. Recently, cracking has been effectively reduced by introducing equiaxed grains through inoculant addition to Al alloy powder. However, the mechanisms through which equiaxed grains can prevent cracking have not been explained from the viewpoint of both microstructure and process parameters. Thus, the control over the cracking behavior of Al alloys during the laser powder bed fusion (LPBF) process has remained limited because of a lack of theoretical understanding. In this study, a solidification cracking model was proposed using the parametric LPBF results of ZrH2particle-added AA7075, where equiaxed grains were formed as a function of energy density. In the proposed model, the cracks in the previously formed layer were healed by liquid backfilling during remelting under appropriate solidification conditions. The synergistic effects of both repeatable melting (layer-by-layer) and equiaxed grain formation on the cracking behavior were revealed. Consequently, the proposed model opens a novel route to cracking prevention in laser-based metal additive manufacturing processes.

Published
2023
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