• 한국전기전자재료학회논문지
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• 제18권5호
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• pp.468-472
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• 2005

High-temperature Superconductor(HTS) tubes were fabricated in term of different processing variables such as preheating temperature, speed of mold rotation and cooling rate by centrigugal forming method. For powder melting by induction the optimum range of melting temperatures and preheating temperature were $1050{\circ}C{\sim}1100{\circ}C\;and\;550{\circ}C\;for\;30\; min$, respectively The mould roating speed was 1000 rpm. A tube was annealed at $840 {\circ}C$ for 72 hours in oxygen atmosphere. The plate-like grains were well developed along the loafing direction and typical grain size was about more than $40{\mu}$. It was found that Ic values increased with increasing the Preheating temperature and speed of mold rotation. While Ic decreased with increasing the cooling rate. The measured Ic in $50mm{\times}70mm{\times}2.5mm$ tube was about 896 Amp.

• 한국정밀공학회지
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• 제16권9호
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• pp.41-47
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• 1999

The scale of dendritic structure of a cast preform plays a key role in determining the mechanical properties of cast/forged products. In this study, casting experiments are carried out to reduce dendrite arm spacing (DAS) to smaller than 20 ${\mu}$m by increasing cooling rate of the mold and then to spheriodize dendritic structures by addition of alloying elements such as Zr and Ti-B. From the casting experiments, appropriate casting conditions for producing the cast preform of a motorcycle connecting rod are obtained. To obtain fine microstructures of the cast preform, mold temperature must set to be low whilst cooling rate being high. When cooling rate is 10 $^{\circ}C$/s, the size of DAS is 17.4 ${\mu}$m. And the degree of spheriodization of a grain in the cast preform is described by aspect ratio, which is defined as the ratio of major and minor radii of an elliptical grain. When 0.5% Zr and 0.24 % Ti+B are added to the molten aluminum alloy, the best aspect-ratio 0.75 is obtained. After forging the cast preform of a motorcycle connecting rod, the microstructure and mechanical properties of the cast preform are compared with those of the cast/forged product. Cast/forged products are superior in microstructure and in mechanical properties such as ultimate strength, elongation, and hardness.

• 한국주조공학회지
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• 제17권6호
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• pp.608-614
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• 1997

The effect of mold preheating temperature on the microstructure such as grain size, eutectic silicon morphology was investigated for the Al-7wt%Si-0.3wt%Mg alloy. Microstructural variations have been characterized as a function of Sr addition and cooling rate during solidification. Microstructures were correlated with cooling rate, local solidification time and eutectic nucleation temperature, etc. In this study, Sr addition caused increase of local solidification time, undercooling and reduction of eutectic plateau temperature. In logarithmic scale, local solidification time was in inverse proportion to cooling rate. Eutectic nucleation temperature was in inverse proportion to cooling rate of logarithmic scale. Increasing the cooling rate refined dendrite arm spacing and eutectic silicon. Dendrite arm spacing was logarithmically in inverse proportion to cooling rate. Without modifier addition, eutectic silicon was modified at cooling rate of $7^{\circ}C/s$ or higher.

• 한국재료학회지
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• 제17권10호
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• pp.560-566
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• 2007

AlN plates were fabricated by hot pressing at $1700-1900^{\circ}C$ using yttria and alumina (3 and $10\;{\mu}m$ particle size) powders as additives and characterized: density, thermal conductivity, transverse rupture strength, and grain size measurement by SEM and EDS. Density values of $3.31-3.34\;g/cm^3$ are largely attributed to hot pressing of powder mixtures in carbon mold under $N_2$ atmosphere which caused effective degree of oxygen removal from yttrium-aluminate phase expected to form at $1100^{\circ}C$. The grain size of hot pressed AlN was almost homogeneous, with size approximately from 3.2 to $4.0\;{\mu}m$ after hot pressing. $Al_2O_3$ powder of $3\;{\mu}m$ particle size resulted in better transverse rupture strength and finer grain size compared to $10\;{\mu}m$ $Al_2O_3$ powder. The thermal conductivity of AlN ranged between $83-92.7\;W/m{\cdot}K$ and decreased with $Al_2O_3$ addition. Fine grain size is preferred for better mechanical properties and thermal conductivity.

• 한국분말재료학회지
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• 제27권5호
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• pp.373-380
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• 2020

The purpose of this study is to investigate the densification behavior and the corresponding microstructural evolution of tantalum and tantalum-tungsten alloy powders for explosively formed liners. The inherent inhomogeneous microstructures of tantalum manufactured by an ingot metallurgy might degrade the capability of the warhead. Therefore, to overcome such drawbacks, powder metallurgy was incorporated into the near-net shape process in this study. Spark plasma-sintered tantalum and its alloys with finer particle sizes exhibited higher densities and lower grain sizes. However, they were contaminated from the graphite mold during sintering. Higher compaction pressures in die and isostatic compaction techniques also enhanced the sinterability of the tantalum powders; however, a full densification could not be achieved. On the other hand, the powders exhibited full densification after being subjected to hot isostatic pressing over two times. Consequently, it was found that the hot isostatic-pressed tantalum might exhibit a lower grain size and a higher density as compared to those obtained in previous studies.

• 한국재료학회지
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• 제31권12호
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• pp.672-676
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• 2021

In this paper, the effect of Ni (0, 0.5 and 1.0 wt%) additions on the microstructure, mechanical properties and electrical conductivity of cast and extruded Al-MM-Sb alloy is studied using field emission scanning electron microscopy, and a universal tensile testing machine. Molten aluminum alloy is maintained at 750 ℃ and then poured into a mold at 200 ℃. Aluminum alloys are hot-extruded into a rod that is 12 mm in diameter with a reduction ratio of 39:1 at 550 ℃. The addition of Ni results in the formation of Al₁₁RE₃, AlSb and Al3Ni intermetallic compounds; the area fraction of these intermetallic compounds increases with increasing Ni contents. As the amount of Ni increases, the average grain sizes of the extruded Al alloy decrease to 1359, 536, and 153 ㎛, and the high-angle grain boundary fractions increase to 8, 20, and 34 %. As the Ni content increases from 0 to 1.0 wt%, the electrical conductivity is not significantly different, with values from 57.4 to 57.1 % IACS.

• 한국주조공학회지
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• 제15권6호
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• pp.574-587
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• 1995

The $SiC_p-reinforced$ preforms fabricated by spray casting process were hot-extruded and subsequently T6-treated, and the morphology of the silicon phase and the grain size for these preforms and extruded samples were examined by Image Analyzer. Experimental observation revealed that with increase in volume percent of SiC particles, the grain size and silicon phase of the $Al-Si/SiC_p$ composites become finer, the shape of Si phase is changed from blocky to granular type, and aspect ratio of Si phase tend to become unity. Wear-tests with various sliding velocities, show that the wear resistance of spray cast specimen is increased remarkably compare to the permanent mold cast specimen at the sliding velocity range of $1.98{\sim}2.38m/sec$.. Microstructural observations for the worn surfaces of specimens revealed that wear resistance of Al-Si alloys at certain sliding velocities could be improved not only by the fine grain size of aluminum matrix but also the fine size and granular shape of silicon phases. The wear resistance of $SiC_p$ reinforced aluminum composites was found to be sensitive to the volume percentage of the reinforcing particles. The worn surfaces with various sliding velocities, show that change in wear mechanism seems to occur at the sliding velocity of near 2m/sec for all samples, and such a change in mechanism is delayed with increase in $SiC_p$ volume fraction.

• 한국주조공학회지
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• 제4권1호
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• pp.12-20
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• 1984

In order to investigate the effect of size of sand grains, bentonite content and moisture on mixing power, standard mixing power, permeability, green compressive strength and green mold hardness were measured with mixing time, and also coated layer of mixed sand with time was observed by optical microscope and scanning electron microscope. From this experiment, the results were summarized as follows. 1. Mixing power increased as size of sand grains decreased. 2. Mixing power increased gradually as bentonite content increased and in particular, increased rapidly in 7-10% bentonite. 3. Mixing power increased as moisture content decreased. 4. The mixing time required to get the optimum mixing power decreased as moisture content and grain size increased, but increased as bentonite content increased.

• 소성∙가공
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• 제27권6호
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• pp.379-385
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• 2018

This study investigated the effect of extrusion conditions on microstructures and mechanical properties of AM80 magnesium alloys. The billets of magnesium alloy used for hot extrusion were prepared by permanent mold casting method, and its extrusion was hot direct extrusion with different extrusion conditions. The results of microstructural analysis showed that the main phases in the as-casted alloys were ${\alpha}-Mg$, ${\beta}-Mg_{17}Al_{12}$, and lamella $Mg_{17}Al_{12}$. Hot extrusion results, The tensile strength of the most soundly manufactured extruded bars (extrusion temp: $350^{\circ}C$, extrusion ratio: 27:1, ram speed: 2mm/s) was approximately 327MPa at room temperature. The increase in the mechanical properties of hot-extruded alloys was as a result of grain refinement by dynamical recrystallization during hot extrusion.

• 한국주조공학회지
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• 제21권5호
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• pp.296-303
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• 2001

The effects of the revolution speed on the mechanical properties and microstructure of Cr-Mo steel during vertical centrifugal casting using investment mold were investigated. The casting yield, tensile strength, hardness and absorbed energy during impact test were increased with increasing revolution speed. The effect of mass variation caused by the change in the revolution speed on the grain size was the most significant.