티타늄의 소결특성에 미치는 산소함량의 영향

최종석,이동희,최국선,길대섭,서창열,김원백,하호,Choe, Jong-Seok,Lee, Dong-Hi,Choi, Good-Sun,Kil, Dae-Sup,Suh, Chang-Youl,Kim, Won-Baek,Ha, Ho 한국재료학회 2000 한국재료학회지 Vol.10 No.7

스폰지 티타늄으로부터 수산호-탈수소화법(HDH)법으로 제조된 부말에 고상탈산법(DOSS)을 적용시켜 만든 산소 농도 범위 1980~8450 ppm, 입경 $25\mu\textrm{m}$ 내외의 불규칙 티타늄 분말의 성형 및 소결성을 조사하였다. 250MPa의 가압력으로 냉간압축성형한 결과, 성형밀도는 69.0%~62.3% 범위 내에 있었고 산호함량 증가에 따라 직선 또는 완만하게 감소하였다. 이러한 경향은 티타늄 분말의 경도변화로 설명할 수 있었다. 최고 7%까지의 차이를 보였던 성형밀도에도 불구하고 $1100^{\circ}C$에서 2시간동안 소결한 결과, 산소함량에 무관하게 소결밀도는 90.5$\pm$0.5%를 보였으며, 결정립의 크기는$60\mu\textrm{m}$ 내외의 균일하였고, 가공크기 및 분포도 유사하였다. 소결체의 경도에 미치는 산소의 영향은 실험범위 내에서 VHN(sintered)=135.5+64.3$\times$(wt%$O_2$)의 실험식을 얻었다. 소결체의 파단면 관찰한 결과, 연성에서 취성파괴로의 천이는 소결체에의 산소함량이 2987~5582ppm 사이에서 일어나는 것으로 나타났다. The compacting and sintering behavior of titanium powders containing oxygen in the range of 1980~8450 ppm was examined. The powders were prepared by the hydride-dehydride (HDH) and by the deoxidation by solid state(DOSS) methods. Their compaction density ranged from 69.0% to 62.3% and decreased with the increase in the oxygen content. It was explained by the effect of oxygen on the hardness of powders. Unlike the compaction density, the oxygen content did not affect the apparent density greatly being 90.5$\pm$0.5% after sintering at $1100^{\circ}C$ for 2 hours. Their average grain size was $60\mu\textrm{m}$ and the size and distribution of pores were about the same for all cases. The hardness of sintered samples showed a linear increase with oxygen and could be expressed as VHN(sintered)= 135.5+64.3$\times$$(wt{\%}O_2)$ The exami-nation of fracture surface revealed that the ductile-brittle transition occurs at oxygen contents of 2987~5582 ppm.

금속간화합물 Ti-51at%Al 분말 내의 불순물 분석

최국선,이동희,Choi, Good-Sun,Lee, Dong-Hui 한국재료학회 1992 한국재료학회지 Vol.2 No.2

플라즈마 회전전극법으로 제조된 Ti-51at%Al 금속간화합물 분말에 함유된 불순물의 함량변화를 분석하였다. 산소와 탄소의 함량은 각각 600 및 200ppm 정도로 분말입도에 따라 변하였으며, 이는 주로 원재료에 포함된 불순물의 영향이었다. AES 분석에 의하면 분말표면 산화물은 주로 Ti 및 Al과 산소 및 탄소의 복합화합물로 구성되었으며, 그 두께는 약 100 및 $200{\mu}m$ 크기의 분말에 대하여 각각 7 및 3.4nm 정도로 측정되었다. 분말의 제조로 산소와 탄소의 함량이 오히려 감소되는것의 구체적인 기구는 아직 확인되지 않았으나 플라즈마 고열에 의한 정련현상에 기인하는 것으로 생각된다. Unusual surface impurity levels of PREPed Ti-51at%Al powders were analyzed using Auger spectroscopy and they were compared with these obtained from bulk starting electrode. Oxygen and carbon contents were varied very much with particle size. Powder surfaces were believed to be mainly covered by a complex compound containing Ti, Al, O, and C. The decrease in contents of oxygen and carbon of powders were attributed to the certain refining reaction of transfer type DC Ar plasma during the powder production.

전자빔 drip 용해횟수가 Mo 잉고트 특성에 미치는 영향

최국선,이강인,이동희 ( Good Sun Choi,Kang In Rhee,Dong Hi Lee ) 한국주조공학회 1995 한국주조공학회지 Vol.15 No.3

N/A Molybdenum ingot of 50mm in diameter were obtained from sintered Mo bars by EB drip melting technique. Macroscopic observation of EB remelted ingot indicates that coarse and columnar grains grow in the direction parallel to ingot pulling direction. This can be explained by slow solidification (3㎚/min), large temperature gradient and heat flow to this direction. The orientation of columnar structure was found to be <110>, <200> and <211> by the analysis of X-ray diffraction patterns. The contents of typical metallic impurities in Mo sintered bar are 1.2ppm Cr, 3ppm Fe, 44ppm Zr, 150ppm W. Most of metallic impurities were reduced below the order of ppm except zirconium and tungsten by the selective evaporation. In the removal of nonmetallic impurities, oxygen and carbon impurities were lowered from 120 to 6ppm and from 157 to 106ppm, respectively, after first melting. Although the purification effect was not significant with the number of remelting, Vickers hardness was reduced from 217 to 195 and 184 in sequence with increasing the number of remelting.

플라즈마 회전전극법에 의해 제조된 TiAl 분말의 응고특성

이동희,최국선 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.8

The impurity levels, shapes and solidification characteristics of PREPed Ti-51at%Al powders were investigated. Oxygen and carbon contents were varied very much with particle size. The appearances and microstructures of powders were found to be depended on cooling rate which is in turn particle size. The estimated cooling rate of powders ranging 200 to 50㎛ in size was 1.6×10⁴∼1.7×10^5 K/s. The coarse powders were solidified as primary α dendrite. From about 30% of ◎50㎛ in size martensite -like microstructure was found, which was believed to result from the solid state transformation of primary β phases during cooling.

플라즈마 회전전극법에 의한 Al-Cu 합금 분말의 제조

최국선,김진영,이동희 연세대학교 대학원 1991 延世論叢 Vol.27 No.1

The production and characterization of Al-Cu alloy powders by the plasma- rotating-electrode process(PREP) was carried out to determine the influence of operating conditions(rotating speed, and plasma current) and alloy compositions on such particle characteristics as particle size distributon, shape, microstructure, and chemical composition. Each experimental condition showed a particle size range of 75∼353μm with mean sizes of 140∼160μm. The mean particle size of the powders changed with alloy compositions at the same rotating speed and plasma current. Particle shapes were mostly of the ligament type. Microstructures consisted mainly of dendrites, but also lamellar cell in the vicinity of eutectic composition. Al -20wt.% Cu powders of the same size, from even a single atomizing run, showed a wide variety of microstructure due to different thermal and nucleation history. Chemical compositions of alloy powders varied with particle size as a result of the interaction between high temperature plasma gas and material constituents of alloys during powder formation.

플라즈마 회전전극법에 의한 STS 304 및 STS 316 스테인리스강 분말의 제조 및 분말특성 연구

이동희,최국선 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.12

The plasma rotating electrode process was applied to the production of type 304 and 316 stainless steel powders in order to determine the effect of operating conditions on the particle size and its distribution, and to analyze their microstructures, cooling rate and relative amount of ferrite phase for the as-prepared powders. For the operating conditions of plasma arc current : 140∼240 A and anode rotating speed : 17,500∼25,000rpm, stainless steel powders produced showed the typical bimodal size distribution ranging 50∼425㎛ and mean volume-surface diameter ranging 143∼238㎛. The appearances and microstructures of powders were found to be solidified with cellular or dendrite structures which have being depended on cooling rate(particte size). The estimated cooling rate of powders ranging 150 to 50㎛ in size was about 10³∼10^5K/s. The weight traction of ferrite phase in type 316 powders was found to be increased with decreasing particle size, due to the difference in the solidification behavior. For type 304 powders, however, the ferrite phase was not detected by X.R.D.

304L 오스테나이트 스테인레스강 분말의 소결성에 미치는 Mo의 첨가 영향

최국선,이동희,최왕규 연세대학교 산업기술연구소 1989 논문집 Vol.21 No.2

An investigation of Mo addition effect on the sinterability of 304L austenitic stainless steel powders was carried out by using the compacts of three different Mo contents(0.2 and 5 wt%) and using diffusion couple of same stainless steel and Mo powders. The compacts of pure stainless steel powder showed conventional sintering process. And also. we found the diffusion (alloying) layer, 60㎛, and porosity formation toward stainless steel compact in the vicinity of coupled zone by means of SEM and EDS analysis. The porosity of compacts to which Mo was added, was high compared with no Mo added compacts except early stage of sintering. But in compacts sintered at 1200℃ for 45 min, the abnormal increase of porosity could be understood by "bridging" effect of alloying layer formed during sintering process.

가압통전소결법에 의한 금속간화합물 Tial 분말의 소결성

김진영,이동희,최국선 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.7

The effects of current density and the mode of applied current(DC and/or AC) on the resistance/spark sintering of TiAl powders were investigated to develope a new consolidation technique for this intermetallic compound. PREPed r-TiAI powders ranging 180∼210㎛ in size were sintered for short time of period(∼2mins) applying two different pressure by step by step during the flow of high density current of DC and/or AC. It was found that the mixed mode of DC+AC was more effective for the densification than DC mode because the former mode was believed to allow TiAl powders inside graphite dies to reach higher temperature due to the sparking at the neck area and uniform current distribution in the powder compact by AC effects. Compacts of high relative density($gt;99%) and uniform lamellar structure were obtained at the condition of DC(540A/㎠)+AC(50 A/㎠ and 5 ㎑).

평면상 응고계면의 형태학적 안정성에 대한 비평형 열역학적 고찰

한정호,이동희,최국선 대한금속재료학회(대한금속학회) 1986 대한금속·재료학회지 Vol.24 No.1

This theoretical study was carried out on the determination of the morphological stability criterion of planar solid-liquid interface. The system of a directional solidification under the nonequilibrium steady-state was defined as a continuous open system and the basic assumptions for the perturbation on the planar interface were introduced in order to analyze the pattern formation. The pattern formation was also interpreted by applying the marginal stability hypothesis. The results obtained can be summarized as follows. 1. The morphological stability criterion was T_mΓω²+ G_L-mG_c $gt; 0 and this could be reduced to the CS criterion. 2. The critical wavelength of perturbation, λ_c, was calculated by applying the marginal stability. From this result, it could be predicted that the shapes of the interface were changed with the wave length of perturbation. 3. It was suggested that the cell size, ℓ, can be limited by the critical wavelength of perturbation.