• Journal of Welding and Joining
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• 제17권4호
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• pp.60-68
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• 1999

The previously developed two dimensional model was modified in order to predict more accurately the degree of microsegregation and eutectic fraction on in weld metal whose solidification rate is very fast. The model employed the same assumptions with previous model but considered of a tip undercooling. The previously predicted microsegregation and eutectic fraction has the discrepancies between simulated and examined results in the weld metal solidification. The experiments for the weld metal solidification of 2024 A1 and Fe-Ni alloy were carried out in order to examine the reasonability and feasibility of this modified model. The concentration profile of the solute and eutectic fraction predicted by the simulation agreed well with those found from experimental works. According to the results, it was believed that the dendrite tip undercooling considered in the modified model be reasonable for predicting the degree of microsegregation more accurately in weld metla solidification. In the GTA welds, degree of dendrite-tip undercooling increases with increasing solidification rage(welding speed). This serves to increase the concentration of dendrite core and thus result in reducing the degree of segregation. And solid state diffusion(back diffusion) during solidification is very low in the weld metal solidification so that little additional homogenization of solute occurs during solidification. With consideration of tip undercooling this modified model can predict exactly degree of microsegregation and eutectic fraction from slow solidification(casting) to fast solidification(welding).

• 한국자동차공학회논문집
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• 제7권8호
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• pp.143-149
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• 1999

In this study , the new method which is used by magneticfluid for undercooling ends and continuous ice making is introduced to ice making in the ice stroage system and its validity is studied experimentally . This study made sure how shape control of magniticfluid and instant explosure method can effect on the undercooling degree and it exchange by time change at the moments of undercooling ends and acquired the fundamental knowledge for control method about undercooling ends.

• 대한기계학회논문집B
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• 제22권5호
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• pp.698-707
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• 1998

A simplified model for predicting microsegregation during columnar-dendritic solidification of binary alloys is developed, in which back diffusion, dendrite arm coarsening and dendrite tip undercooling are simultaneously incorporated. The inclusion of tip undercooling is accomplished by modifying the initial conditions of the existing solute diffusion model, in such forms that tip undercooling depresses the beginning of solidification below the liquidus temperature, and that the secondary arm spacing evolves in accordance with the minimum undercooling theory. Sample calculations for the well-known benchmark system show that the present predictions not only consist with the extablished limiting cases, but also agree favorably with the available experimental data within a reasonable tolerance. In particular, a typical decreasing trend in the eutectic fraction at high cooling rates is successfully resolved. Comparison of the individual and combined effects of characteristic parameters in reference with the limiting cases reveals the interactions among parameters. Every parameter plays the role of reducing the eutectic fraction, and the degree of influence depends primarily on the cooling rate. Coarsening enhances the effect of tip undercooling, while suppressing that of back diffusion. A vigorous back diffusion seems to restrain the apperance of the undercooling effect. Overall, each contribution of the three parameters to microsegregation is estimated to be of the same order, which suffices to justify the present study.

• 대한기계학회논문집
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• 제18권9호
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• pp.2404-2412
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• 1994

A mathematical model has been developed for predicting kinematic, thermal, and solidification histories of atomized droplets during flight. Liquid droplet convective cooling, recalescence, equilibrium-state solidification, and solid-phase cooling were taken into account in the analysis of the solidification process. The spherical shell model was adopted where the heterogeneous nucleation is initiated from the whole surface of a droplet. The growth rate of the solid-liquid interface was determined from the theory of crystal growth kinetics with undercooling caused by the rapid solidification. The solid fraction after recalescence was obtained by using the integral method. The thermal responses of atomized droplets to gas velocity, particle size variation, and degree of undercooling were investigated through the parametric studies. It is possible to evaluate the solid fraction of the droplet according to flight distance and time in terms of a dimensionless parameter derived from the overall energy balance of the system. It is also found that the solid fraction at the end of recalescence is not dependent on the droplet size and nozzle exit velocity but on the degree of subcooling.

• 한국분말재료학회지
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• 제1권1호
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• pp.85-94
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• 1994

In order to predict droplet velocity and temperature profiles and fractional solidification with flight distance during spray forming, the Newtonian heat transfer formulation has been coupled with the classical heterogeneous nucleation and the specific solidification process. It has been demonstrated that the thermal profile of the droplet in flight is significantly affected by process parameters such as droplet size, initial gas velocity, undercooling. As the droplet size and/or the initial gas velocity increase, the onset and completion of solidification are shifted to greater flight distances and the solidification process also extends over a wider range of flight distances. The amounts of solid fractions formed during recoalescence, segregated solidification and eutectic solidification are insensitive to droplet size and initial gas velocity whereas those are strongly affected by the degree of undercooling. There are good linear relations between the undercooling and the corresponding solid fractions generated during recoalesced, segregated and eutectic stages.

• 한국주조공학회지
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• 제11권4호
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• pp.314-322
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• 1991

In this study, Na, Li, Sb, Sr were added as the modifiers to improve the mechanical proper ties of Al-Si alloys and their effects were compared one another Thermal analysis was carried out to measure the degree of undercooling, that is, the basic mechanism of modifying effect. The measurements of holding time and analyses of components of Al-Si alloys were carried out to compare the fading of the modifiers. Greater degree of undercooling was measured from the modified alloy than the normal alloys. In the modified alloy, the shape of Si was changed into a fine fiber phase or a lamellar phase from the coarse plate phase. There are not modifying effect of Sb below the $1.5^{\circ}C$ /sec of cooling rate. As the cooling rate increases, similar effects appears in the non-modified alloys like the modified alloys. Na and Li were faded in short time and were difficult to control on the melt, but Sb and Sr were scarcely faded for 6 hours of holding and were easy to control on the melt. Na and Sr are the good modifiers in terms of microstructure. Sb is the best modifier, but Na, Li are the inadequate modifiers in terms of fading.

• 암석학회지
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• 제6권3호
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• pp.185-209
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• 1997

청산지역의 화강암류는 기재적인 특징으로부터 백록 화강섬록암, 청산 반상 화강암 및 청산복운모 화강암으로 분류할 수 있다. 백록 화강섬록암의 각섬석은 외각섬석군에 속하고, 중심부의 Mg- 보통각섬석에서 주변부의 양기석질 보통각섬석으로 조성적 변화를 보인다. 청산 지역 화강암류의 흑운모는 금운모와 애나이트의 중간 조성을 나타낸다. 복운모 화강암에서 산출되는 백운모는 그 산출상태와 조성으로부터 마그마 정출의 1차적인 백운모로 판단된다. 주성분 산화물의 변화 경향에서 각각의 화강암류는 체계적인 변화를 나타낸다. 백록 화강섬록암은 청산 반상 화강암 및 복운모 화강암과는 주성분의 변화 경향에서 매우 뚜렷한 차이를 보인다. 반상 화강암과 복운모 화강암의 경우 일부 성분에서는 연속성이 관찰되지만, 몇몇 원소에서는 뚜렷한 차이를 보인다. 따라서 청산 지역의 세 화강암류는 각각 이질적 성인의 다른 조성의 마그마로부터 형성된 것으로 판단된다. 화강암류의 전암 화학조성으로부터 청산 지역 화강암류는 칼크-알칼리질임을 알 수 있다. 한편, 백록 화강섬록암과 청산 반상 화강암은 그 전암 조성이 I-타입과 메타알루미나질임에 비해, 청산 복운모 화강암은 I-/S-타입 양쪽과 과알루미나질을 나타낸다. 불투명광물의 함량과 전암 대자율값으로부터 청산지역 화강암륜느 모도 티탄철석계열에 속해 비교적 환원적인 환경에서 관입·고결되었음을 알 수 있다. 또한 세 화강암류는 활동성 대륙연변부에서 일어난 화성활동의 결과로 형성되었음이 추정된다. 청산 반상 화강암에 나타나는 알칼리장석의 거정은 그 결정의 크기, 형태, 배열 및 포유물의 분포 양상 등으로부터 마그마 기원임을 추정할 수 있다. 반상 화강암의 조직적 특징은 2단계 정출작용으로 설명된다. 즉, 알칼리장석 거정의 크기와 형태는 과냉각 정도가 적은 상태에서 결정의 느린 핵형성과 빠른 성장속도로 말미암아 형성된 반면, 석기는 과냉각 정도가 큰 상태에서 핵형성 밀도와 속도가 증가하여 형성된 것으로 판단된다.

• Nuclear Engineering and Technology
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• 제54권5호
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• pp.1851-1859
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• 2022

Experimental research on flow characteristics of open natural circulation system was performed, to figure out the mechanism of the open natural circulation behaviors. The influence factors, such as the heating power, the inlet subcooled and the level of cooling tank on the flow characteristics of the system were examined. It was shown that within the scope of the experimental conditions, there are five flow types: single-phase stable flow, flash and geyser coexisting unstable flow, flash stable flow, flash unstable flow, and flash and boiling coexisting unstable flow. The geyser flow in flash and geyser coexisting unstable flow is different from classic geysers flow. The flow oscillation period and amplitude of the former are more regular, is a newly discovered flow pattern. By drawing the flow instability boundary diagram and sorting out the flow types, it is found that the two-phase unstable flow is mainly characterized by boiling and flash, which determine the behavior of open natural circulation respectively or jointly. Moreover, compared with full liquid level system, non-full liquid level system is more prone to boiling phenomenon, and the range of heat flux density and undercooling degree corresponding to unstable flow is larger.

• 한국주조공학회지
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• 제30권5호
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• pp.161-166
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• 2010

In this examination, the effect of Sr addition and holding time on microstructure of Al-10.5wt%Si-2wt%Cu secondary die-casting alloy was investigated. Degree of undercooling was improved with increasing the Sr content in this alloy. Up to 0.02wt%Sr addition, acicular and lamellar eutectic structure was observed in the microstructure. Meanwhile, the eutectic Si was modified toward the fine fibrous form by increasing Sr content with more than 0.03wt% and holding time of the melt. The well- modified alloys showed decreased eutectic silicon size from 3.25 ${\mu}m$ to less than 0.8 ${\mu}m$. From these results, the optimal strontium content and holding time were identified on the Al-10.5wt%Si-2wt%Cu secondary die-casting alloy.