• 한국재료학회지
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• 제17권1호
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• pp.11-17
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• 2007

Turbulent in-situ mixing process is a new material process technology to get dispersed phase in nanometer size by controlling reaction of liquid/solid, liquid/gas, flow ana solidification speed simultaneously. In this study, mixing which is the key technology to this synthesis method was studied by computational fluid dynamics. For the simulation of mixing of liquid metal, static mixers investigated. Two inlets for different liquid metal meet ana merge like 'Y' shape tube having various shapes and radios of curve. The performance of mixer was evaluated with quantitative analysis with coefficient of variance of mass fraction. Also, detailed plots of intersection were presented to understand effect of mixer shape on mixing. The simulations show that the Reynolds number (Re) is the important factor to mixing and dispersion of $TiB_2$ particles. Mixer was designed according to the simulation, and $Cu-TiB_2$ nano composites were evaluated. $TiB_2$ nano particles were uniformly dispersed when Re was 1000, and cluster formation and reduction in volume fraction of $TiB_2$ were found at higher Re.

• 한국주조공학회지
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• 제31권2호
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• pp.66-70
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• 2011

Hot dip aluminizing (HDA) is widely used in industry for improving corrosion resistance of material. The formation of intermetallic compound layers during the contact between dissimilar materials at high temperature is common phenomenon. Generally, intermetallic compound layers of $Fe_2Al_5$ and $FeAl_3$ are formed at the Al alloy and Fe substrate interface. In case of cast iron, high contact angle of graphite existed in the matrix inhibits the formation of intermetallic compound layer, which carry with it the disadvantage of a reduced reaction area and mechanical properties. In present work, the process for the removal of graphite existed on the surface of specimen has been investigated. And also HDA was proceeded at $800^{\circ}C$ for 3 minutes in aluminum alloy melt. The efficiency of graphite removal was increased with the reduction of particle size in sanding process. Graphite appears to be present both in the region of melting followed by re-solidification and in the intermetallic compound layer, which could be attributed to the fact that the surface of cast iron is melted down by the formation of low melting point phase with the diffusion of Al and Si to the cast iron. Intermetallic compound layer consisted of $Fe(Al,Si)_3$ and $Fe_2Al_5Si$, the layer formed at cast iron side contained lower amount of Si.

• 동력기계공학회지
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• 제20권6호
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• pp.40-45
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• 2016

The manufacturing process of large scaled Al 7050 alloy is difficult for the occurrence of solidification crack during casting. The aims of this study are the evaluations of microstructure and mechanical properties of extruded Al 7050 billet and ring forged one with large scale. Large scaled Al 7050 billet was casted by direct-chill casting process. The extruded and ring forged specimens were prepared from the casted ingot after residual stress relief and homogenization heat treatment, respectively. Microstructures, hardness and tensile test of the surface, middle and center part of each specimen were performed at room temperature. Sheared and elongated type grains were observed at the edge parts of surface and center area and its aspect ratios of grains were low and similar as 0.21 while that of middle area was closed to 0.92 value in ring forged Al 7050 alloy. The mechanical properties of extruded Al 7050 alloy were superior than those of ring forged one. The hardness values of surface and center part were slightly higher than that of middle part in ring forged Al 7050 alloy.

• 한국정밀공학회지
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• 제17권9호
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• pp.189-195
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• 2000

RP techniques have their unique characteristics according to the working principles : star-stepped surface of parts due to layer-by-layer stacking low build speed caused by line-by-line solidification to fish one layer and post processing to improve surface finish etc The objective of this study is to propose a new RP technique Variable Deposition Mnanufacturing (VDM) which can make up for the disadvantages of the existing RP techniques and to develop an apparatus to implement the technique. The proposed process can greatly reduce the build time and improve the surface finish of parts generated. Experiments are carried out to obtain the range of temperature of molten material to maintain its fluidity and to investigate the effect of gas cooling on the preservation of the slopes. Some simple shapes such as a line-shape an S-shape and a circle-shape are fabricated from Ethylene Vinyl Acetatecopolymer(EVA) In order to examine the applicability of VDM to more general shapes a tensile specimen and a yo-yo shape were manufactured by the proposed RP method using EVA material as a trial approach. The current basic study shows a high potential of practical use of the proposed VDM process to prototyping of a general three-dimensional shape.

• 한국재료학회지
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• 제27권9호
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• pp.495-500
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• 2017

Oxide layers were formed by an environmentally friendly plasma electrolytic oxidation (PEO) process on AZ91 Mg alloy. PEO treatment also resulted in strong adhesion between the oxide layer and the substrate. The influence of the KF electrolytic solution and the structure, composition, microstructure, and micro-hardness properties of the oxide layer were investigated. It was found that the addition of KF instead of KOH to the $Na_2SiO_3$ electrolytic solution increased the electrical conductivity. The oxide layers were mainly composed of MgO and $Mg_2SiO_4$ phases. The oxide layers exhibited solidification particles and pancake-shaped oxide melting. The pore size and surface roughness of the oxide layer decreased considerably with an increase in the concentration of KF, while densification of the oxide layers increased. It is shown that the addition of KF to the basis electrolyte resulted in fabricating of an oxide layer with higher surface hardness and smoother surface roughness on Mg alloys by the PEO process. The uniform thickness of the oxide layer formed on the Mg alloy substrates was largely determined by the electrolytic solution with KF, which suggests that the composition of the electrolytic solution is one of the key factors controlling the uniform thickness of the oxide layer.

• 한국세라믹학회지
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• 제55권5호
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• pp.446-451
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• 2018

In this study, different formulations of magnesium oxide and various modifiers (phosphoric acid, ferrous sulfate, pure acrylic emulsion, silicone acrylic emulsion, glass fiber, and polypropylene fiber) were used to prepare magnesium oxychloride cement composites. The compressive strength of the magnesium oxychloride cement was tested, and the softening coefficients of the composites after soaking in water were also calculated. The results showed that a magnesium oxychloride cement sample could not be coagulated when the MgO activity was 24.3%, but the coagulation effect of the magnesium oxide cement sample was excellent when the MgO activity was 69.5%. While pure acrylic emulsion, silicon-acrylic emulsion, and glass fiber showed insignificant modification effects on the magnesium oxychloride cement, ferrous sulfate heptahydrate, phosphoric acid, and polypropylene fiber could effectively improve its water resistance and compressive strength. When the phosphoric acid, ferrous sulfate heptahydrate, and polypropylene fiber contents were 0.47%, 0.73%, and 0.25%, respectively, the softening coefficient of a composite soaked in water reached 0.93 after 7 days, and the compressive strength reached 64.3 MPa.

• 한국분말재료학회지
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• 제15권2호
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• pp.95-100
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• 2008

The investigation is to modify the mechanical and chemical properties of Mg alloys using a combination of rapid solidification and surface treatment. As the first approach, $Mg_{95}Zn_{4.3}Y_{0.7}$ was gas atomized and pressure sintered by spark plasma sintering process (SPS), showing much finer microstructure and higher strength than the alloys as cast. Further modification was performed by treating the surface of PM Mg specimen using Plasma electrolytic oxidation (PEO) process. During the PEO processing, MgO layer was initiated to form on the surface of Mg powder compacts, and the thickness and the density of MgO layer were varied with the reaction time. The thickening rate became low with the reaction time due to the limited diffusion rate of Mg ions. The surface morphology, corrosion behavior and wear resistance were also discussed.

• 한국산학기술학회논문지
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• 제7권5호
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• pp.811-816
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• 2006

이방성 자기저항효과(anisotropic magnetoresistance : AMR)는 단층 자성박막으로 구성되므로 경제성 있게 박막화 시켜서 소형화가 가능하다. 기존의 급속응고법으로 생산된 리본형 MCo(M=Cu, Ag) 소자가 경제적으로 공업적 목적을 달성하였으나, 리본형 특유의 두께와 가공성이 부족하여 소형 소자에 함께 집적하기 곤란한 단점이 있었다. 새로운 박막형을 쓰면 추가 열처리 없이 기존의 리본형 소자와 비교하여 박막상태로 적절한 AMR 특성이 나오는지와 최적 AMR을 얻기 위한 Co의 조성을 아는 것이 중요하다. 열증착기를 써서 100nm의 $Cu_{1-x}Co_x$와 $Ag_{1-x}$ 박막을 Co의 조성을 $10{\sim}70wt%$로 달리하며 제작하여 이때의 자기적 특성을 확인하였다. CuCo는 40% Co에 0.5T에서 1.4%, AgCo는 30% Co에 2.6%의 MR비를 얻었고 이는 리본형 소자보다는 표면 산란 효과에 의해 MR비는 작지만 표면산화막 없이 직접 다른 소자공정과 함께 진행할 수 있는 장점이 있음을 확인하였다.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2000년도 추계학술대회논문집
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• pp.396-402
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• 2000

Porous structures of aluminum foam have been studied. The apparent foam shape, foam hight, density, pore size, shape, and their distributions in various section areas of the experimental samples have been investigated. The sample have been cast into metallic mold, using aluminum foam prepared from a precursor based on pure Al ingot mixed with various amount of 1-2wt% increasing viscosity and foam agent materials. The process provides for flexibility in design of foam structures via relatively easy control over the amount of hydrogen evolution and the drainage processes which occur during foam formation. This is facilitated by manupulating parameters such as the foaming agent, thermal histories during solidification and mix melt viscosities. The acoustical performance of the panel made with the foamed aluminum is considerably improved; its absorption coefficient shows NRC 0.6-0.8. It has been found that the Al foam is very preferable for the compactness of the thermal system.

• 마이크로전자및패키징학회지
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• 제26권2호
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• pp.45-49
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• 2019

높은 신뢰성의 n형 Bi-Te-Se계 열전소자 제조를 위한 열간압출 공정을 확립하였다. 용융-응고 공정을 이용하여 Bi-Te-Se 원료잉곳을 합성하였으며, 고에너지 볼밀을 이용하여 평균 ${\sim}30{\mu}m$ 크기의 분말로 분쇄하였다. 일축가압 공정으로 분말을 직경 20 mm의 디스크 형태로 성형한 후 압출용 몰드 설계-제작 및 열간압출 공정 온도와 압력을 제어하여 성형체로부터 00l 방향으로 결정 배향된 지름 1.8 mm의 원통형 고밀도 압출체를 제조하였다. 상온에서 최대 ${\sim}4.1mW/mK^2$의 높은 파워팩터를 나타냈으며, zone melting 공정으로 제조한 상용 열전소재와 비교하여 2배 이상 향상된 기계적 강도 (~50 MPa)를 구현하였다.