• Proceedings of the KWS Conference
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• 2009.11a
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• pp.113-113
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• 2009

세계적으로 환경문제에 대한 관심이 커짐에 따라 연료 효율 향상과 매연을 줄이기 위해 차량 경량화 요구가 증가하는 동시에 안전 규제가 강화되고 있어 높은 강도를 가지는 소재의 적용이 증가하고 있다. 충돌 시 차량 구조를 유지시켜주는 범퍼나 B-필러와 같은 부품에 1500MPa급의 초고강도강이 적용되고 있으며 레이저 용접이 가지는 장점이 많이 알려지면서 그 사용이 증가하는 추세이다. 따라서 레이저 용접에 의한 초고강도강의 용접 특성에 대한 연구가 필요하다. 본 연구에서는 1500MPa급 초고강도강의 레이저 맞대기 용접을 실시하여 단면, 경도 측정, 인장시험을 통해 용접부 특성을 파악하였다. 파이버 레이저와 디스크 레이저를 사용하여 각각의 레이저빔 직경 변화에 따른 입열량 변화에 따라 용접성에 미치는 영향에 대해 알아보았다.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.64-64
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• 2009

The use of ultra high strength steels (UTSS) is a natural result with increasing the demands for the lightweight materials and developing an innovative steel technology. Recently it has been used a 1500MPa grade hot stamping steel as automobile bodies, reinforcement parts, and seat frame parts in the automotive industry. It is a quenchenable steel manufactured by hot stamping process. It is well known that UTSS welding has softening in the heat affected zone(HAZ). Because welding is a sort of process applying heat, it should change the heat treated features and degrade the strength. This study was performed to investigate the influence of the heat input on the softening of the HAZ in the GMAW process. Each experiment was compared with that in the conditions having a different current and voltage at a same heat input. In order to analysis characteristics of the HAZ, optical microscope was used to observe microstructure and vickers hardness tests were carried out across the welds. Applying low heat input means a fast cooling rate. It leads to high hardness in the HAZ. It is found that characteristics of the HAZ are determined by microstructure obtained by different cooling rate.

• Journal of the Korean Ceramic Society
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• v.51 no.1
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• pp.19-24
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• 2014

Three-layer porous alumina-mullite composites with a symmetric gradient porosity are prepared using a controlled freeze/gel-casting method. In this work, tertiary-butyl alcohol (TBA) and coal fly ash with an appropriate addition of $Al_2O_3$ were used as the freezing vehicle and the starting material, respectively. When sintered at $1300-1500^{\circ}C$, unidirectional macro-pore channels aligned regularly along the growth direction of solid TBA were developed. Simultaneously, the pore channels were surrounded by less porous structured walls. A high degree of solid loading resulted in low porosity and a small pore size, leading to higher compressive strength. The sintered porous layered composite exhibited improved compressive strength with a slight decrease in its porosity. After sintering at $1500^{\circ}C$, the layered composite consisting of outer layers with a 50 wt% solid loading showed the highest compressive strength ($90.8{\pm}3.7MPa$) with porosity of approximately 26.4%.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.38 no.12
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• pp.1373-1377
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• 2014

Hot-stamping is a method of obtaining ultrahigh-strength steel by simultaneously forming and cooling boron steel in a press die after it has been heated at $900^{\circ}C$ or above. After heat treatment, boron steel has a strength of 1500 MPa or more. This material ensures a high level of quality because it overcomes the spring-back phenomenon, which is a problem associated with high-strength steel materials, and the degree of dimensional precision is improved by 90 or more because of the good formability compared with existing types of steel. In this study, the welding characteristics were identified through the butt and lap welding of hot-stamped steel using a disk laser. Full penetration was obtained at a faster speed with butt welding compared to lap welding, and a white band was observed in every specimen.

• Laser Solutions
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• v.13 no.3
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• pp.19-26
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• 2010

In this study, fundamental experiment was conducted with various strength of UHSS (Ultra High Strength Steel) by $CO_2$ laser. And then, butt and lap joint laser welding with boron alloyed steel and Al-Si coated boron alloy steel have been done by changing laser beam feature, existence of gap and existence of coating layer to know welding characteristics of those materials. As a result, in case of fundamental experiment with various strength steel, hardening was found in the weld metal of all tested materials and softening was found at the heat affected zone of SGAFC 1180. In case of laser butt welding of UHSS, mechanical properties was improved by using small laser beam diameter and Al-Si coating layer caused fracture of weld metal. In case of laser lap welding of UHSS, Al-Si coating layer resulted in formation of intermetallic compound at the fusion boundary where fracture occurred. Al-Si coating layer caused lowering mechanical properties of weld metal.

• Journal of Electrical Engineering and Technology
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• v.6 no.4
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• pp.543-550
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• 2011

Conductive SiC-$ZrB_2$ composites were produced by subjecting a 40:60 (vol%) mixture of zirconium diboride (ZrB2) powder and ${\beta}$-silicon carbide (SiC) matrix to spark plasma sintering (SPS). Sintering was carried out for 5 min in an argon atmosphere at a uniaxial pressure and temperature of 50 MPa and $1500^{\circ}C$, respectively. The composite sintered at a heating speed of $25^{\circ}C$/min and an on/off pulse sequence of 12:2 was denoted as SZ12L. Composites SZ12H, SZ48H, and SZ10H were obtained by sintering at a heating speed of $100^{\circ}C$/min and at on/off pulse sequences of 12:2, 48:8, and 10:9, respectively. The physical, electrical, and mechanical properties of the SiC-$ZrB_2$ composites were examined and thermal image analysis of the composites was performed. The apparent porosities of SZ12L, SZ12H, SZ48H, and SZ10H were 13.35%, 0.60%, 12.28%, and 9.75%, respectively. At room temperature, SZ12L had the lowest flexural strength (286.90 MPa), whereas SZ12H had the highest flexural strength (1011.34 MPa). Between room temperature and $500^{\circ}C$, the SiC-$ZrB_2$ composites had a positive temperature coefficient of resistance (PTCR) and linear V-I characteristics. SZ12H had the lowest PTCR and highest electrical resistivity among all the composites. The optimum SPS conditions for the production of energy-friendly SiC-$ZrB_2$ composites are as follows: 1) an argon atmosphere, 2) a constant pressure of 50 MPa throughout the sintering process, 3) an on/off pulse sequence of 12:2 (pulse duration: 2.78 ms), and 4) a final sintering temperature of $1500^{\circ}C$ at a speed of $100^{\circ}C$/min and sintering for 5 min at $1500^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.33 no.10
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• pp.1177-1185
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• 1996

Y-TZP/Ce-TZP ceramics having relative sintered densities of>95% average grain sizes of 0.36$\mu\textrm{m}$ microhar-dness of 1150 kg/mm2 fracture strength of 390-830 MPa and toughness of 6.4-10.2 MPa$.$mm1/2 were prepared by conventional sintering of 3 mol% Y2O3-ZrO2 and 12 mol% CeO2-ZrO2 powders at 1400 and 1500$^{\circ}C$ The average grain sizes of Y-TZP/Ce-TZP ceramics were mainly governed by those of Ce-TZP. White increasing Ce-TZP content toughness increased while microhardness and fracture strength decreased. With comparing microhardness and toughness fracture strength was more sensitive on not only grain size but also other factors such as microstructural and compositional variations. The densification of Y-TZP/Ce-TZP cermaics was not greatly affected by composition and soaking time at temperature over 1400$^{\circ}C$ With increasing CE-TZP content the stability of t-ZrO2 decreased under thermal aging in air whereas increased in hydrothermal atmosphere and aqueous solution.

• Korean Journal of Materials Research
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• v.26 no.2
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• pp.61-66
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• 2016

In order to fabricate porous mullite ceramics with controlled pore structure and improved mechanical strength, a freeze casting route has been processed using camphene mixed with tertiary-butyl alcohol (TBA) and coal fly ash/alumina as the solvent and the ceramic material, respectively. After sintering, the solidification characteristics of camphene and TBA solvent were evident in the pore morphology, i.e., dendritic and straight pore channels formed along the solidification directions of camphene and TBA ice, respectively, after sublimation. Also, the presence of microcracks was observed in the bodies sintered at $1500^{\circ}C$, mainly due to the difference in solidification volume change between camphene and TBA. The compressive strength of the sintered bodies was found generally to be dependent, in an inverse manner, on the porosity, which was mainly determined by the processing conditions. After sintering at $1300{\sim}1500^{\circ}C$ with 30~50 wt% solid loading, the resulting mullite ceramics showed porosity and compressive strength values in ranges of 83.8~43.1% and 3.7~206.8 MPa, respectively.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.18 no.1
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• pp.210-215
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• 2017

This study examined the mechanical design and the structural strength analysis of the rake used in a rotary screener. For the case of the mid-water part, an additional load condition was given by calculating the underwater frictional force generated by the water. For the upper-water part, the analysis was done by dividing the 500kg additional load into the left-concentrated load, center-concentrated load, and right-concentrated load depending on the location of the attached narrow material, and it was determined that all 3 additional load conditions were free from plastic deformation and were secure. The additional load that can occur from the weight of the attached narrow material was divided into three, 1,000kg, 1,300kg, and 1,500kg, and then analyzed. The results of the analysis show that in the case of 1,000kg and 1,300kg, an equivalent strength of 143.6MPa and 186.6MPa occurred, respectively. These figures are lower than the yield strength of the STS304 used in the rotary screener. Therefore, plastic deformation does not occur and is considered to be secure. In the case of 1500kg, however, the equivalent strength that occurred was 215.41MPa, which is greater than the yield strength of STS304, and was judged to be unsafe.

• Computers and Concrete
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• v.7 no.1
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• pp.53-62
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• 2010

Dredged silt from reservoirs in southern Taiwan was sintered to make lightweight aggregates (LWA), which were then used to produce high-performance lightweight aggregate concrete (HPLWC). The HPLWC was manufactured using different amounts of mixing water (140, 150, and 160 $kg/m^3$) and LWA of different particle densities (700, 1100, and 1500 $kg/m^3$) at different W/b ratios (0.28, 0.32, and 0.4). Results show that the lightweight aggregates of dredged silt taken in southern Taiwan perform better than the general lightweight aggregates. In addition, the HPLWC possessed high workability with a slump of 230-270 mm, and a slump flow of 450-610 mm, high compressive strength of over 40 MPa after 28 days of curing, good strength efficiency of cement exceeding $0.1MPa/kg/m^3$, low thermal conductivity of 0.4-0.8 $kcal/mh^{\circ}C$, shrinkage of less than $4.8{\times}10^{-4}$, and high electrical resistivity of above 40 $k{\Omega}-cm$. The above findings prove that HPLWC made from dredged silt can help enhance durability of concrete and provide and an ecological alternative use of dredged silt.