• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.187-192
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• 2002

Applicability of laser micro welding process to the fabrication of medical devices was investigated. Austenitic stainless steel wire (SUS304) was spot melted and crosswise welded, which is one of the most possible welding process for the fabrication of medical devices, by using a Nd-YAG laser. Effects of welding parameters on the microstructure, tensile strength and corrosion resistance were discussed. In the spot melting, melted metal width decreased with decreasing the input energy and pulse duration. Controlling the laser wave to reduce laser noise which occurred in the early stage of laser irradiation made reasonable welding condition wider in the welding condition of small pulse duration such as 2ms. The microstructure of the melted metal was a cellular dendrite structure and the cell size of the weld metal was about 0.5~3.5 ${\mu}{\textrm}{m}$. Tensile strength increased with the decrease of the melted metal width and reached to a maximum about 660MPa, which is comparable with that for the tempered base metal. Even by immersion test at 318K for 3600ks in quasi biological environment (0.9% NaCl), microstructure of the melted metal and tensile strength hardly changed from those for as melted material. In the crosswise welding, joints morphologies were classified into 3 types by the melting state of lower wire. Fracture load increased with input energy and melted area of lower wire, and reached to a maximum about 80N. However, when input energy was further increased and lower wire was fully melted, fracture load decreased due to the burn out of weld metal.

• 한국재료학회지
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• 제12권2호
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• pp.140-145
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• 2002

This study was conducted to investigate the brazing characteristics between Zircaloy-4 nuclear fuel cladding tubes and bearing pads with filler metals of amorphous $Zr_{1-x}Be_x$(0.3$\leq$x$\leq$0.5) binary alloy, in which they were produced in the ribbon form by the melt-spinning metod. The crystallization behavior, stability, hardness and micro-structure of brazed zone were examined by X-ray diffraction, differential scanning calorimetry, micro-Vickers hardness test, optical microscopy, and transmission electron microscopy. $Zr_{1-x}Be_x$(0.3$\leq$x$\leq$0.4) amorphous alloys were crystallized to $\alpha$-Zr with increasing the temperature, and the rest were transformed to ZrBe$_2$at higher temperatures. On the other hand, $Zr_{1-x}Be_x$(0.4$\leq$x$\leq$0.5) amorphous alloys were crystallized to $\alpha$-Zr and ZrBe$_2$, simultaneously. The thickness of the layer brazed with amorphous alloy was increased with increasing the beryllium content due to the higher diffusion of Be. The morphology of brazed layer with PVD Be filler metal showed dendrite while that brazed with amorphous alloys appeared globular. Micro-Vickers hardness of brazed zone increased as the beryllium content of filler metal was decreased.

• 동력기계공학회지
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• 제14권2호
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• pp.71-77
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• 2010

This paper was studied on microstructure, mechanical properties and corrosion characteristics of 316L stainless steel pipe welds was fabricated by orbital welding process. S-Ar specimen was fabricated by using Ar purge gas and S-$N_2$ specimen was fabricated by using $N_2$ purge gas. Ferrite was not detected in weld metal of S-$N_2$ specimen but the order of 0.13 Ferrite number(FN) was detected in weld metal of S-Ar specimen. Oxygen and Nitrogen concentration of S-$N_2$ specimen was higher than S-Ar specimen on HAZ and inner bead. The welds microstructural characteristics of S-Ar and S-$N_2$ specimens are similar. The microvickers hardness values of S-Ar and S-$N_2$ specimens welds were similar and average values of each regions were in the range of 174~194. The microstructures of S-Ar and S-$N_2$ weld metal were full austenite by primary austenite solidification. The Solidification structures of S-Ar and S-$N_2$ weld metal were formed directional dendrite toward bead center. The potentiodynamic polarization curve of STS 316L pipe welds exhibited typical active, passive, transpassive behaviour. Corrosion current density$(I_{corr.})$ and corrosion rate values of S-Ar specimen in 0.1M HCl solution were $0.95{\mu}A/cm^2$ and $0.31{\mu}A$/year respectively. The values of S-$N_2$ specimen were $1.4{\mu}A/cm^2$ and $0.45{\mu}m$/year.

• 한국재료학회지
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• 제23권6호
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• pp.339-343
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• 2013

Porous Ti-systems with unidirectionally aligned channels were synthesized by freeze-drying and a heat treatment process. $TiH_2$ powder and camphene were used as the source materials of Ti and sublimable vehicles, respectively. Camphene slurries with $TiH_2$ content of 10 and 15 vol% were prepared by milling at $50^{\circ}C$ with a small amount of oligomeric polyester dispersant. Freezing of the slurry was done in a Teflon cylinder attached to a copper bottom plate cooled at $-25^{\circ}C$ while unidirectionally controlling the growth direction of the camphene. Pores were generated subsequently by sublimation of the camphene during drying in air for 48 h. The green body was heat-treated at $1100^{\circ}C$ for 1 h in a nitrogen and air atmosphere. XRD analysis revealed that the samples composed of TiN and $TiO_2$ phase were dependent on the heat-treatment atmosphere. The sintered samples showed large pores of about 120 mm which were aligned parallel to the camphene growth direction. The internal wall of the large pores had relatively small pores with a dendritic structure due to the growth of camphene dendrite depending on the degree of nucleation and powder rearrangement in the slurry. These results suggest that a porous body with an appropriate microstructure can be successfully fabricated by freeze-drying and a controlled sintering process of a camphene/$TiH_2$ slurry.

• 열처리공학회지
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• 제12권2호
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• pp.145-156
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• 1999

The distribution of the second phase, the change of transformation temperature and mechanical properties with thermomechanical treatment conditions were investigated by metallography, calorimetry, EDS, tensile test and fractography in a Cu-Al-Ni-Ti-Mn alloy. The cast structure revealed Ti-rich precipitates($X_L$ phase) between dendrite arms, which have been identified as $(Cu,Ni)_2TiAl$ intermetallic compounds. By homogenizing above $900^{\circ}C$, the $X_L$ phase was melted in the matrix, while the Xs phase was precipitated in matrix and the volume fraction of it was increased. When hot-rolled specimen was betatized below $750^{\circ}C$, recrystallization could not be observed. However, the specimen betatized above $800^{\circ}C$ was recrystallized and the grain size was about $50{\mu}m$, while Xs phase was precipitated in matrix. With raising betatizing temperature, $M_s$ and $A_s$ temperatures were fallen and transformation hysteresis became larger. The strain of the specimen betatized at $800^{\circ}C$ was 8.2% as maximum value. The maximum shape recovery rate could be obtained in the specimen betatized at $800^{\circ}C$ but it was decreased due to the presence of Xs phase with increasing betatizing temperature.

• 한국재료학회지
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• 제9권9호
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• pp.883-889
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• 1999

직경이 각각 4,6,8,12 및 15mm인 순Al 또는 Al-(1,2,3wt%) Cu합금 내부냉금이 95mm ø$\times$200mm H 크기의 흑연주형 중심부에 설치되었다. 이 내부냉금과 같은 조성의 용융금속이 $750^{\circ}C$의 주입온도에서 주형에 주입되었고, 냉각속도, 응고조직 및 성분편석이 분석되었다. 주괴직경/내부냉금직경의 비가 8까지 증가될수록 결정립도, 2차수지상간격 및 성분편석이 현저하게 감소됨이 확인되었다. 그러나 이 비가 8이상이 되면, 미용해 내부냉금이 발생되고, 내부냉금의 효과는 급속히 감소되었다. 순Al 및 Al-(1,2,3wt%)Cu 합금에서 최대 내부냉금 효과를 얻을 수 있는 적정 주괴직경/내부냉금직경의 비는 $750^{\circ}C$주입온도에서는 8이었다.

• 열처리공학회지
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• 제13권4호
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• pp.246-252
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• 2000

Stellite 12 alloy-powder was overlaid on 410 stainless steel valve seat using plasma transferred arc(PTA) process. Variation of characteristic of microstructure and hardness of deposit with current(90~150 A) and preheat temperature(R.T.~$400^{\circ}C$) was investigated. Important conclusion obtained are as follows; All welding conditions used produced a sound deposit layer with no defect in single pass welding. The maximum deposit had 4.0~4.8 mm in thickness and its bead width was increased with increase of current and preheat temperature. The deposit showed hypoeutectic microstruture, which was consisting of primary cobalt dendrite and networked $M_7C_3$ type eutectic carbides. The amount of eutectic carbides was decreased and its dendritic secondary arm spacing was increased with increase of current. Hardness of the deposit was decreased with increase of current. Preheat temperature up to $400^{\circ}C$, however, showed little influence on the hardness and microstructure. The hardness was also influenced by diluted Fe content near the interface in addition to microstructure and dendritic secondary arm spacing. Hot hardness at $500^{\circ}C$ showed higher than 300 HV.

• Korean Chemical Engineering Research
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• 제45권6호
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• pp.591-595
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• 2007

기존 마이크론 금속섬유 필터를 지지체로 그 표면 위에 나노입자 증착 후, 열처리를 하여 나노구조 기공층이 표면에 부착 형성된 멤브레인 필터를 제작하였다. 가지상 구조의 나노입자 응집체를 마이크론 금속섬유 필터상에 부착함으로써 기존 금속 멤브레인 필터에 비하여 여과성능이 향상된 나노구조 기공층 멤브레인 필터를 개발하였다. 증착한 나노구조 기공층을 지지체 필터 표면상에 부착시키기 위한 열처리 온도가 증가함에 따라 나노입자 응집체의 수축 현상으로 인하여 나노구조 기공층 멤브레인 필터의 차압은 감소하였지만, 여과효율의 감소는 미미하였다.

• 한국주조공학회지
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• 제40권4호
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• pp.118-127
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• 2020

Microstructural evolution during a heat treatment and high-temperature tensile properties have been investigated in conventionally cast CM247LC. In as-cast specimens, MC carbides with high amounts of Ta, Ti, Hf, and W were found to exist in the interdendritic regions, and γ' was observed in the form of cubes and octocubes prior to decomposition into cubes. In the heat-treated condition, some portion of eutectic γ-γ' remained, and uniform cubic γ' was observed in both interdendritic regions and dendrite core. Three types of carbides with different stoichiometries and compositions were found at the grain boundaries. MC carbides with high Hf contents were observed in the vicinity of eutectic γ-γ'. The highest tensile strength value was found at 750℃, whereas the greatest ductility appeared at 649℃. The effect of the temperature on the tensile properties was closely related to the dislocation structure. With increase in the test temperature, the density of dislocations inside γ' decreased, whereas that in the γ matrix increased. Stacking faults generated in γ' at 750℃ had a strengthening effect, whereas thermally activated dislocation motion at a high temperature was considered to have the opposite effect.

• 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.