가스분무한 Al-8wt.%Fe 합금분말의 급속응고과정에 대한 수치해석

김성균,최회진,나형용 ( Seong Gyoon Kim,Hoi Jin Choi,Hyung Yong Ra ) 한국주조공학회 1993 한국주조공학회지 Vol.13 No.5

N/A A numerical analysis on the microstructural evolutions of microcellular and cellular α -aluminum phase in the gas-atomized Al-8wt. pct droplets was represented. The 2-dimensional non-Newtonian heat transfer and the dendritic growth theory in the undercooled melt were combined under the assumptions of a point nucleation on droplet surface and the macroscopically smooth solid-liquid interface enveloping the cell tips. It reproduced the main characteristic features of the reported microstructures quite well. It predicted a considerable volume fraction of segregation-free region in a droplet smaller than l0㎛ if an initial undercooling larger than 100K is given. The volume fractions of the microcellular region(g_A) and the sum of the microcellular and cellular region(g_a) were predicted as functions of the heat transfer coefficient, h and initial undercooling, ΔT. It was shown that g_A and g_a, in the typical gas-atomization processes with h=0.1-1.0W/㎠K, are dominated by ΔT and h, respectively, but for h larger than 4.0W/㎠K, a fully microcellular structure can be obtained irrespective of the initial undercooling.

평면 고-액계면상에 존재하는 Perturbation의 morpohology 변화에 대한 이론적 고찰

나형용,김성균 대한금속재료학회(대한금속학회) 1984 대한금속·재료학회지 Vol.22 No.5

By using the quasi-steady state solution of the diffusion equation, we discussed the time evolution of a perturbation on the flat solid-liquid interface during solidification of dilute binary alloy or pure materials and the interaction between solute diffusion effect and interface curvature effect. As the results, we showed that the perturbation is propagated to the lateral directions with cell spacing, λ_s (=2π√(3D)_o) irrespective of its initial morphology and at this spacing, the interaction between diffusion and curvature effect reached equilibrium state. Also, appling these results to the secondary arm branching of dendrite, we showed that the branching mechanism can be expressed by the perturbation propagation process.

두 벽사이에서의 방향성응고시 고-액계면의 형태

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1994 대한금속·재료학회지 Vol.32 No.10

The steady state shapes of solid-liquid interface under various conditions, during directional solidification of pure materials between two straight walls, were calculated by numerical analysis. The transition from the interface shapes governed by the interface energy to that governed by the mobility can occur as the pulling rate changes. At a high pulling rate where the interface shapes are governed by the mobility anisotropy, there is a critical mobility anisotropy above which the faceted interface appears. At a high pulling velocity and with a high mobility anisotropy, the faceted interface shapes depend on the wetting angle, ψ between the interface and the wall only at the range of π/4$lt;ψ$lt;5π/4. At the other range of wetting angle the macroscopic interface shape is independent of the wetting angle. The faceted interface can be composed of the planes with high crystallographic indices if the mobility anisotropy, at the high pulling velocity, is slightly larger than the critical value or if the wetting angle, at the high pulling velocity and with the high mobility anisotropy, is at a range of π/4$lt;ψ$lt;π2.

열교환법에 의한 구리단결정 성장

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.5

The effects of cooling velocity, melt weight and purity of copper were reported on copper single-crystal growth by Heat Exchanger Method in this paper. Copper single-crystals could be grown at the cooling velocity lower than 1℃/min. Higher purity(99.995wt%) of copper single crystal less than 7㎝ height(radius 3.5㎝) was grown when the cooling velocity was 0.34℃/min. and the height less than 3.5㎝(radius 3.5㎝) was grown with the plane front of solid-liquid interface and of semi-ellipse, and the free surface of single crystal was solidified in the end. However in the lower purity(99.93wt%) of copper, it should be solidified with plane front at the initial stage, and cell, dendrite formed finally at the cooling velocity lower than 0.5℃/min. The significant segregation were caused when the solid-liquid interface was changed from plane front to cell structure.

급속응고에서의 밴드조직

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.2

The microstructural evolutions during the transition between the supersaturated solid solution and the solutal dendrite in rapid solidification were qualitatively examined using the model of banded structure formation proposed by Carrard et al., It was shown that the positive average temperature gradient at solid-liquid interface is a necessary condition for the banded structure formation, and the banded structure can not be formed during rapid undercooled solidification. Also, it was shown that the banded structure is hardly anticipated in melt-spun aluminum alloy and, on the other hand, it can be formed at the free surface side of the melt-spun Ag-Cu alloy ribbons. These predictions are in good agreement with the reported experimental results on banded structure formation.

열교환법에 의한 실리콘 단결정 성장과정의 수치해석

김성균,나형용,김동익,최회진 대한금속재료학회(대한금속학회) 1995 대한금속·재료학회지 Vol.33 No.11

Numerical simulations including the fluid flow, conduction and radiation heat transfer have been performed using the geometry of real furnace for the single-crystal growth by heat exchanger method. The finite difference method based on the control volume approach and SIMPLE algorithm were used to solve the momentum and energy equations. Almost all parts of the furnace including the heater, insulating materials and crucible were considered in the calculation domain and the latent heat was accounted by an iterative heat evolution method. Silicon with low thermal conductivity was selected as a model material in order to compare the results with the previous report on the copper single-crystal growth. The effects of cooling rate of the heater, crucible material, crucible shape and melt weight on single-crystal growth were investigated together with the role of natural convection in melt. The optimum process conditions such as the critical cooling rate and the critical ratio of the height to the radius of crystal for the silicon single-crystal growth by heat exchanger method were determined. According to the simulations, among many parameters crucible shape was a dominant processing parameter to control single crystal growth when a thermal conductivity of the specimen was low. The lower the thermal conductivity of base material of crucible was then that of the specimen, the less the region of poly crystal formed at the edge of the crystal was.

Fe40Ni40P14B6 비정질합금에서 결정의 핵생성기구 (2)

김성균,나형용 대한금속재료학회(대한금속학회) 1989 대한금속·재료학회지 Vol.27 No.12

The nucleation rate with temperature and time during crystallization of Fe_(40)Ni_(40) P_(14)B_6 metallic glass investigated in the preceding paper( I ) was discussed quantitatively using the classical homogeneous nucleation theory. The diffusion coefficient, glass-crystal interface energy(σ) and dσ/dT(T : temperature) obtained from the non-steady state homogeneous nucleation theory agree well with the reported values, which means that the nonsteady state homogeneous nucleation occurs during crystallization. We also showed that the glass-crystal interfacial energy in this alloy must decrease with temperature and discussed the exponent n of Johnson-Mehl-Avrami equation.

비정질 Fe40 Ni40 P13 C7 합금의 기계적 성질에 미치는 가공의 영향

김성균,나형용 대한금속재료학회(대한금속학회) 1985 대한금속·재료학회지 Vol.23 No.5

In this study, the influence of deformation on the mechanical properties of amorphous Fe_(40)Ni_(40)P_(13)C_7, ribbon made by single roll melt spinning was investigated. As-quenched ribbon was rolled about ∈_T (thickness reduction ratio)=1∼6% and continuous bending deformed about ∈_M(Max. plastic strain)=1.0∼2,5%. The fracture of rolled ribbon occurs along the plane of shear band formed by rolling. Tensile strength decreases with thickness reduction. These results are attributed to the work softening in the shear band by rolling. The fracture plane of continuous bending deformed ribbon is not a single plane, but double plane. (The shear band plane formed by bending and the flat plane orientated with 45˚∼50˚to the tensile axis). In this case, the shear band present obstacle to the propagation of the slip during tensile test and tensile strength increases.

가스분무시킨 Al-Fe 합금의 급속응고과정에 대한 이론적 해석

김성균,나형용,신성호 대한금속재료학회(대한금속학회) 1992 대한금속·재료학회지 Vol.30 No.3

A new model for the continuous structural evolution during rapid solidification which includes the interactions between the external cooling, release of latent heat, solute diffusion and the capillary effect of solid-liquid interface was suggested. This model where the heat transfer and the solute diffusion procedure were calculated by a numerical and an analytic method each other, was based on the facts that the boundary layer thickness of the thermal diffusion field is much larger than the cell or dendrite spacing, while that of the solute diffusion field is smaller. The microstructural evolutions in the gas -atomized Al-Fe(2∼8wt%) droplets were obtained by this model. The main results are as follows. The discontinuous microstructural change from 'Zone A' to 'Zone B' with increasing solid fraction was predicted and their microstructural scales were calculated quantitively. In 'Zone A', the interface velocity is governed mainly by the solutal diffusion and the non-steady state solidification occurs. In 'Zone B', the interface velocity is determined by the heat removal rate of external coolant rate irrespective of the solute concentration and initial undercooling, and the steady state solidification occurs. Irrespective of the initial undercooling, droplet size(>10㎛) and Fe Concentration(2∼8wt%), the 'Zone A' and 'Zone B' structures appear simultaneously during gas-atomization of AI-Fe alloys.

비정질합금의 결정화과정의 해석에 이용한 Johnson-Mehl-Avrami 식에 대한 고찰

김성균,나형용 대한금속재료학회(대한금속학회) 1987 대한금속·재료학회지 Vol.25 No.7

By modifing Johnson-Mehl-Avrami equation applicable to the isothermal phase transformation of space-random nucleation and constant growth rate, the time dependent nucleation rate could be expressed as a function of the transformation volume fraction. Using this modified J-M-A equation, we discussed the problems in evaluating the nucleation rate from In t vs ln (-ln (1 -x)) plot. Because a small chance in the shape of the In t vs ln (-ln (1-x)) plot makes extremely large variation in nucleation rate, J-M-A exponent n may lose it's well-known meaning. One can not infer the whole nucleation procedure during phase transformation by any method using only the J-M-A equation.