• 대한기계학회논문집
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• 제15권2호
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• pp.584-595
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• 1991

In the direct rolling processes for the mushy state alloy, a mushy state material which simultaneously contains liquid-solid phase is obtained from the exit port of stirring apparatus with a given solid fraction. This solid fraction is dependent on the temperature of within the solid-liquid range which shows to be controlled accurately by the experimental conditions for a given stirring apparatus. Rolling conditions for fabrication the fine surface strip were obtained from direct rolling experiment with mushy state alloys of Sn-75%Pb and aluminum alloy. Influence of solid fraction, rolling speed and initial roller gap on the state of strip surface and solidified structure was observed. We proposed theoretical model for prediction of rolling force, and we compared calculation result and experimental value measured with load cell.

• 대한기계학회논문집B
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• 제22권12호
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• pp.1657-1668
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• 1998

In the analysis of a mushy solidification system with natural convection using a fixed grid method, the enthalpy method has been used to account for the release of latent heat. The variable viscosity, Darcy source, and hybrid methods have been employed for the velocity suppression in a mushy region. The choice of the values of solid viscosity and permeability constant in conjunction with the Darcy source term plays an important role in forming the location and shape of the phase boundaries. In this work the effects of these major parameters related to steady-state behavior in the system of mushy solidification are investigated through a simple test problem. The effective specific heat based on the spatial gradients of the enthalpy and temperature is adopted for the treatment of the release of latent heat. The effects of the Prandtl and Rayleigh numbers on the shape of mushy region are examined using the hybrid method.

• Korean Chemical Engineering Research
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• 제52권2호
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• pp.199-204
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• 2014

2성분 응고계에서 다공성 mush 층에서의 조성 대류는 생성되는 제품의 질에 영향을 준다. 본 연구에서는 일정한 속도로 응고되는 mush 층을 고려하였다. 선형 안정성 이론을 사용하여 mush 층에 대한 교란방정식을 유도하였고, 기본상태 온도장과 mush 층에서 기공률의 분포를 수치해법으로 조사하였다. 과열량이 클 때 mush 층의 두께는 열경계층의 두께에 비해 상대적으로 작았다. 과열량이 감소함에 따라 mush 층의 두께를 기준으로 한 Rayleigh 수는 증가하였고, mush 층은 조성 대류에 대해 안정해졌다. mush 층의 윗면에 등온조건을 적용한 경우보다 온도 및 열속의 연속조건을 액체-mush 계면에 적용한 경우에 임계 Rayleigh 수가 더 작게 얻어졌다.

• 대한기계학회논문집B
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• 제24권9호
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• pp.1166-1174
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• 2000

This work presents a two-dimensional quasi-steady state model to study the fluid flow and heat transfer in high-power density welding process of thin AISI-304 stainless steel plates. The enthalpy method and the finite volume method were used for a numerical analysis of the mushy region phase change as well as the heat flow at the weld pool and the heat-affected zone. The results show that the mushy region distributed around the weld pool becomes wider downstream and the surface heat losses by convection and radiation can be significant factors in welding process especially when a welding speed is relatively low.

• 한국정밀공학회:학술대회논문집
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• 한국정밀공학회 1997년도 추계학술대회 논문집
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• pp.167-170
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• 1997

For the optimal net shape forging of semi-solid materials (SSM), it is important to predict the deformation for variation of strain rate. It should be necessruy to conduct a formation of stress-strain curve in semi-solid alloys for analysis of the thixoforming process. Particularly, important problem to application of computer aided engineering in SSM processing is to prevent a segregation of liquid component during compression process. The liquid segregation is studied as multistage change of the strain rate and test piece size to prevent the liquid segregation during the compression process. The compression test for semi-solid aluminium alloy with a controlled solid fraction is performed by dynamic material test system with a furnace. Moreover morphology of structure and fraction of pore are investigated through compression test.

• 열처리공학회지
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• 제25권3호
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• pp.126-133
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• 2012

Semi-solid forming of the high melting point alloys such as steel is a promising near-net shape forming process for decreasing manufacturing costs and increasing the quality of the final products. This paper presents the microstructure characteristics of SKH51 (high speed tool steel) during heating and holding in the mushy zone between $1233^{\circ}C$ and $1453^{\circ}C$, which has been measured by differential scanning calorimetry (DSC). The results of heating/holding experiments showed that the grain size and the liquid fraction increased gradually with temperature up to $1350^{\circ}C$. The drastic grain growth occurred at heating above $1380^{\circ}C$. The strain-induced melt-activated (SIMA) process has been applied to obtain globular grains in the billet materials. Working by mechanical upsetting and successive heating of SKH51 into the temperatures in the mushy zone resulted in globular grains due to recrystallization and partial melting.

• 소성∙가공
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• 제12권2호
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• pp.94-101
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• 2003

For the optimal net shape forging of S semi-solid materials (SSM), it is important to predict the deformation behavior and defects of materials. Among these defects, liquid segregation is detrimental to produce products with good mechanical properties. Moreover, to apply a numerical method to thixoforging, it is very important to prevent a liquid segregation during forming process. The liquid segregation phenomena in deformation behavior of semi-solid material with variation of test specimen size were studied. The SSM compression tests were performed by dynamic material test system with a furnace. Stress-strain curves and microstructures of SSM were investigated, and Porosities were analyzed to evaluate the effects of experiment parameters on liquid segregation.

• 열처리공학회지
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• 제27권3호
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• pp.127-132
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• 2014

The semi-solid metal forming process has been applied to realize a near-net shape fabrication of a high speed tool steel. A complicatedly shaped part out of SKH51 was successfully manufactured by introducing pertinent materials, tooling and processing conditions. A SKH51 billet with globular grains was heated at temperatures between 1300 and $1350^{\circ}C$ using high frequency induction heater to get semi-solid microstructure before high rate injection of mushy metal into a die cavity for the forming process. It was necessary to control the preheating of dies between 300 and $400^{\circ}C$ to maintain the homogeneous microstructure during the semi-solid metal forming process. Significant defects such as pores, high fraction of liquid fraction and segregation could be removed from the part by using air vents.

• 대한소아치과학회지
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• 제30권2호
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• pp.308-319
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• 2003

기능력은 뼈의 형성, 유지 및 개조 이외에도 치아이동, 교정치료, 기능성 악교정장치의 사용 등과 관련한 치주인대세포의 특성 변화 등에 중요한 영향을 미친다. 또한, 하악과두에서도 기능력의 변화에 따라서 다양한 특성의 변화가 나타난다. 본 연구에서는 ICR 생쥐를 8주 동안 soft-diet와 hard-diet의 식이 조절을 통해 기능력의 변화를 유도하여, 기능력의 변화와 관련한 특이 유전자를 검출하기 위하여 subtractive hybridization, northern 분석 및 mRNA in-situ hybidization 등의 실험을 시행하여 다음과 같은 결과를 얻었다. 1. Soft-diet군과 hard-diet군의 subtractive hybridization을 통하여 총 39개의 clone을 얻었고 이중에서 11개의 서로 다른 hard-diet군 특이 후보 유전자들을 분리하였다. 2. 11개의 후보 유전자들 중에서 homology 검색과 northern 분석을 통하여 기능력과 관련이 있을 것으로 생각되거나 hard-diet군에서 mRNA가 선택적으로 발현되는 FS-s2, FS-s5, FS-s18 및 Fs-s22 유전자를 선택하였다. 3. Soft-diet군과 hard-diet군의 형태학적 분석에서 soft-diet군은 hard-diet군에 비하여 골모세포의 활성과 골개조의 특성은 저하된 것으로 관찰되었으나, 하악과두의 연골성골화 과정은 오랜 시간동안 지속되는 것으로 나타났다. 4. FS-s2, FS-s5, FS-s18 및 Fs-s22 유전자의 cRNA 탐침자를 이용한 in-situ hybridization에서 4개의 유전자의 mRNA들은 hard-diet군의 세포들에서는 강하게 발현되었으나 soft-diet군의 세포들에서는 그 발현이 미약하거나 나타나지 않았다. 이상의 결과를 종합하여 FS-s2, FS-s5, FS-s18 및 Fs-s22 유전자들은 대부분이 기능이 거의 알려져 있지 않는 것들로 기능력의 변화 과정에서 중요한 역할을 할 것으로 생각되나 앞으로 보완 연구를 통하여 각 각의 유전자들의 보다 정확한 기능을 이해하는 것이 필요 할 것으로 사료된다.