• Korean Journal of Materials Research
• /
• v.5 no.4
• /
• pp.405-411
• /
• 1995

급냉응고된 Al-3.51wt%Li-0.34wt%Zr 합금의 시효거동을 시차주사열량계(DSC)에 의한 열분석 방법으로 조사하였다. DSC에 의한 비열측정 결과 $\delta$’의 석출에 의한 발열반응과 $\delta$, $\beta$ 및 복합석출상의 석출에 의한 발열반응을 확인하였으며 $\delta$’ 및 $\delta$의 재고용에 의한 2개의 흡열반응을 확인하였다. 7$0^{\circ}C$ 저온시효시 $\delta$’의 석출에 의한 발열반응 이전에 흡열반응이 나타났으며 이것은 $\delta$’ 석출 이전에 $\delta$’ 전구생성물이 형성되었음을 의미한다. DSC 곡선상에 나타난 발열과 흡열곡선을 해석하여 얻은 $\delta$’상 석출과 재고용의 활성화에너지값은 각각 83KJ/mol과 98KJ/mol로서 Al-Li 2원계 및 Al-Li-Mg에 비해 높은 값을 나타내엇으며, 시효에 의한 강화가 일어나 DSC에 의한 비열변화 조사결과 나타난 $\delta$’상 석출 완료 시효조건점 (21$0^{\circ}C$, 1시간)에서 최고경도값(Hv 160)을 나타내었다.

• Journal of Korea Foundry Society
• /
• v.9 no.1
• /
• pp.67-72
• /
• 1989

Solidification analysis was conducted on large steel castings and ingots by a modified finite difference method. Auto-mesh generation system was developed for improving the application of the computer analysis system to casting disign. Combined use of the prediction parameters, solidification time and temperature gradient, and an auxiliary parameter, shrinkage potential, were used to predict the formation of shrinkage defects. Several examples on the prediction of shrinkage cavity by this method were campared with the experimental reslts. It was found that a quantitative design of large steel castings and ingots can be made by the computer aided analysis of solidification process.

• Journal of Korea Foundry Society
• /
• v.18 no.6
• /
• pp.578-585
• /
• 1998

A multi-purpose code MAGMA was employed for mold design and process control in producing Al wheel by lowpressure die casting. Three-dimensional solid modeling was followed by mesh generation of casting and molds(top, bottom and side). The simulation of stability of casting cycle time, mold filling simulation with pressure variation from P1 to P2, solidification simulation by solidification time and feeding criteria, and temperature distribution of molds during processes were studied in this research. The thermal stability of molds was attained after 5 cycles when molds were preheated at $400^{\circ}C$. The pressure increase from P1 to P2 for mold filling was evaluated as slightly higher, and 6 seconds were taken for the mold filling. The cycle time was believed to be designed properly judged from the solidification time of casting and open/close time of molds.

• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 1997.04a
• /
• pp.597-601
• /
• 1997

The technology of Semi-Solid Forging(SSF) has been actively developed to fabricate near-net shape products using light and hardly formable materials. Generally, the SSF process is composed of slug heating,forming,compression holding and ejecting step. After forming step in SSF, the slug is comperssed during a certain holding time in order to be completely filled in the die cavity and be accelerated in solidification rate. This paper presents the analysis of temperature,solid fraction and shrinkage at compression holding step for a cylindrical slug,then predicts the solidification time to obtain the final shaped part. Enthalpy-based finite element analysis is performed to solve the heat transfer problem considering phase change in solidification.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.1
• /
• pp.12-24
• /
• 1999

A computer program has been developed for analyzing the two-dimensional, unsteady conservation equations for transport phenomena in the molten region of twin-roll continuous casting in order to predict the turbulent velocity, temperature fields, and solidification process of the molten steel. The energy equation of the cooling roll is solved simultaneously with the conservation equations of molten steel in order to consider heat transfer through the cooling roll. The results show the velocity, temperature and solidification pattern in the molten region with roll temperature as a function of time. The results for velocity and temperature fields with solidification are compared with those without solidification, giving different thermofluid characteristics in the molten region. We also investigated the effects of revolutional speed of roll, superheat and nozzle geometry on the turbulent flow, temperature and solidification in the molten steel and temperature fields in the cooling roll.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.26 no.11
• /
• pp.1597-1604
• /
• 2002

In this study, numerical investigation has been performed on the impingement, spreading and solidification of a coating material droplet impacting onto a solid substrate in the thermal spray process. The numerical model is validated through the comparison of the present numerical result with experimental data fer the flat substrate without surface defects. An analysis of deposition formation on the non-polished substrate with surface defects is also performed. The parametric study is conducted with various surface defect sizes and shapes to examine the effect of surface defects on the impact and solidification of the liquid droplet on the substrate.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.18 no.4
• /
• pp.1009-1018
• /
• 1994

An attempt is made to develop a kind of hybrid numerical method for computations of the thermal stresses during a solidification process. In this algorithm, the phase-change heat transfer analysis is perrformed by a finite volume method(FVM) and the thermal stress analysis in a solidifying body by a finite element method(FEM). The temperatures at the grid points calculated in the heat transfer analysis are transferred to those of gauss points in elements by a bi-cubic surface patch technique for the thermal stress analysis. A hyperbolic-sine constitutive law is used to prescribe the inelastic strain rate of material. Results for the unidirectional solidification process of a pure aluminum are compared with those of others and shows good agreement.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.24 no.6
• /
• pp.792-801
• /
• 2000

The problem of phase change from liquid to solid in the inviscid plane-stagnation flow is theoretically investigated. The solution at the initial stage of freezing is obtained by expanding it in powers of time, and the final equilibrium state is determined from the steady-state governing equations. The transient solution is dependent on the three dimensionless parameters, but the equilibrium state is determined by one parameter of (temperature ratio/conductivity ratio). The effect of the fluid flow on the growth rate of the solid in the pure conduction problem can be clearly seen from the solution of the initial stage and the final equilibrium state. The characteristics of the transient heat transfer at the surface of the solid and the liquid side of the solid-liquid interface for all the dimensionless parameters are elucidated.

• Korean Journal of Materials Research
• /
• v.9 no.2
• /
• pp.139-143
• /
• 1999

Solidification simulation of gray cast iron sheave product was conducted with the consideration of graphite precipitation during solidification, and of casting processes such as post-inoculation method, treatment and rate, the result of which was aimed to be adopted in the field. In risering design(I), shrinkage cavities were predicted to occur in the part below the risers and center part of the product. While the former defect was considered to be due to the solidification behaviour of riser neck, the latter was due to the feeding channel. In design(II), the length of the riser neck was reduced and one top open riser was attached in the center of the product to prevent the formation of shrinkage cavities, whereby defect-free product was produced.

• Journal of Korea Foundry Society
• /
• v.13 no.5
• /
• pp.424-431
• /
• 1993

A Numerical technique has been developed for the coupled heat transfer and fluid flow calculation during the casting process. In this method the SMAC technique was coupled with the concept of Volume of Fluid(VOF) to calculate melt free surface and velocity profiles within the melt, and the Energy Marker method coupled with the finite difference method was proposed for the convective and conductive heat transfer analysis in the casting. When comparing numerical calculations with experimental observations, a close correlation was evident. It has been shown that this technique is useful for proper gating and casting design, especially for thin-walled castings.