• 연구논문집
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• s.26
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• pp.95-102
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• 1996

The carbide growth behavior of MAR-M247 LC alloy was investigated by directional solidification and quench method. The carbide volume fraction, trapping and growth behavior were correlated with the growth rate. It was found that the carbide volume fraction decreases at slower growth rate. This decreasing was caused by lower solid-liquid interface trapping ability at the slower growth rate.

• Korean Journal of Materials Research
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• v.17 no.12
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• pp.654-659
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• 2007

Austenite precipitation behavior was studied with solidification rates and alloying contents, N and Cr, in duplex stainless steels by directional solidification. Directional solidification experiments were carried out with solidification rates, $1{\sim}100mm/s$, and N and Cr contents, $0{\sim}0.27wt.%,\;25{\sim}28wt.%$ respectively, in a duplex stainless steel, CD4MCU. As the solidification rate increases, the dendrite spacing reduced and the austenite phase in the ferrite matrix became finer. The volume fraction of austenite phase increased and its shape went to be round with increasing nitrogen contents in duplex stainless alloys. The Cr alloying element, even though it is a ferrite former, showed to enhance the nitrogen solubility in the alloy and caused the austenite round and finer. Also, Cr was supposed to decrease the austenite volume fraction, but it increased the austenite slightly due to increasing nitrogen solubility during solidification.

• Transactions of the Korean Society of Mechanical Engineers
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• v.19 no.4
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• pp.1095-1101
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• 1995

Numerical analysis of vertical solidification process allowing solid-liquid density change is performed by a hybrid method between a winite volume method (FVM) and a finite element method (FEM). The investigation focuses on the influence of solid-liquid density change and cooling rates on the motion of solid-liquid interface, solidified mass fraction, temperatures and thermal stresses in the solid region. Due to the density change of pure aluminium, solid-liquid interface moves more slowly but the solidified mass fraction is larger. The cooling rate of the wall is shown to have a significant influence on the phase change heat transfer and thermal stresses, while the density change has a small influence on the motion of the interface, solidified mass fraction, temperature distributions and thermal stresses. As the cooling rate increases, the thermal stresses become higher at the early stage of a solidification process, but it has small influence on the final stresses as the steady state is reached.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.7
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• pp.1818-1832
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• 1994

Transport process during solidification of an AI-CU alloy in a vertical annular mold of which inner wall is cooled is numerically simulated. A model which can take account of local density dependence on the solute concentration is established and incorperated in the analysis. Results show that thermally and solutally induced convections are developed in sequence, so that there is little interaction between them. Thermal convection effectively removes the initial superheat from the melt and vanishes as solidification proceeds from the cooling wall. On the other hand, solutal convection which is developed later over the meshy and the pure liquid regions leads to large-scale redistribution of the consituents. The degree of the initial superheating hardly affects overall solidification behavior except the early stage of the process, when the cooling rate is kept constant. Macrosegregation is reduced remarkably with increasing cooling rate, because not only the liquidus interface advances so quickly that time available for the solute transport is not enough, but also the interdendritic flow is strongly damped by rapid crystal growth within the mushy region.

• Korean Journal of Materials Research
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• v.11 no.6
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• pp.502-507
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• 2001

The effect of solidification rate on the microstructure of directionally solidified IN792+ Hf superalloy has been studied. Solidification sequence and precipitation behavior of the alloy have been analysed by microstructural observation. The script carbide transformed to faceted carbide with decreasing solidification rates. The incorporation of ${\gamma}$ phase into the faceted carbide was due to dendritic growth of carbides. Some elongated carbide bars formed along the grain boundaries at a solidification rate of 0.5$\mu\textrm{m}$/s. Two zones, ${\gamma}$' forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic ${\gamma}$/${\gamma}$' nucleated in the f forming elements enriched zone. Formation of eutectic ${\gamma}$/${\gamma}$' increased the ratio of (Ti+Hf+Ta+W)/Al and induced η phase precipitation. The ratio of (Ti+Hf+Ta+W)/Al decreased at lower solidification rates due to sufficient back diffusion in the residual liquid area. Hence, the Precipitation of the η Phase efficiently suppressed at the lower solidification rate.

• Korean Journal of Materials Research
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• v.12 no.12
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• pp.897-903
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• 2002

Morphological evolution and growth mechanism at the solid/liquid interface during solidification were investigated in the Ni-base superalloy GTD111M by directional soldification and quenching(DSQ) technique. The experiments were conducted by changing solidification rate(V) and thermal gradient(G) which are major solidification process variables. High thermal gradient condition could be obtained by increasing the furnace temperature and closely attaching the heating and cooling zones in the Bridgeman type furnace. The dendritic/equiaxed transition was found in the G/V value lower than $0.05$\times$10{^3}^{\circ}C$s/$\textrm{mm}^2$, and the planar interface of the MC-${\gamma}$ eutectic was found under $17 $\times$ 10{^3}^{\circ}C$ s/$\textrm{mm}^2$. It was confirmed that the dendrite spacing depended on the cooling rate(GV), and the primary spacing was affected by the thermal gradient more than solidification rate. The dendrite lengths were decreased as increasing the thermal graditne, and the dendrite tip temperature was close to the liquidus temperature at $50 \mu\textrm{m}$/s.

• Journal of Korea Foundry Society
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• v.30 no.3
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• pp.100-110
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• 2010

The solidification sequence and formation of intermetallic phase of Fe-rich Al-Si-Cu alloy were investigated by using real-time imaging of synchrotron X-ray radiation. Effects of cooling rate during uni-directional solidification on the resultant solidification behavior was also studied in a specially constructed vacuum chamber in the SPring-8 facility. The series of radiographic images were complementarily analyzed with conventional analysis of OM and SEM/EDX for phase identification. Detailed solidification sequence and formation mechanisms of various phases were discussed based on real-time image analysis. The growth rates of $\alpha$-AlFeMnSi and ${\beta}-Al_5FeSi$ were measured in order to understand the growth behavior of each phase. It is suggested that real-time imaging technique can be a powerful tool for the precise understanding of solidification behavior of various industrial materials.

• Korean Journal of Metals and Materials
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• v.47 no.7
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• pp.440-446
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• 2009

Co base superalloys have been widely used for the parts of gas turbine due to their excellent strength, thermal fatigue, oxidation resistance and weldability at high temperature. In this study, directional solidifications were carried out at various solidification rates, including $0.5{\sim}300{\mu}m/s$ in the Co base superalloy FSX-414. The cellular interface were formed at a low solidification rate, $1{\mu}m/s$, and the dendritic interface was found at higher solidification rates, $5{\sim}300{\mu}m/s$. As the spacing of dendrite structure decreased, the size and spacing of eutectics decreased. Dendrite arm spacing decreased with increasing solidification rates and temperature gradient. It was interesting to find the $M_{23}C_{6}$ eutectic microstructure formed between $\gamma$ dendrites. Composition analysis showed that Cr and W were segregated severely between the dendrites, which resulted in the formation of Cr-rich $M_{23}C_{6}$ and W-rich MC carbides.

• Journal of Welding and Joining
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• v.23 no.2
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• pp.52-58
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• 2005

This study was carried out to investigate the effect of bonding temperature and heating rate on transient liquid phase diffusion bonding of Ni-base superalloy. The heating rate was varied by $0.1^{\circ}C$/sec, $1^{\circ}C$/sec, $10^{\circ}C$/sec to the bonding temperatures $1100^{\circ}C,\;1150^{\circ}C,\;1200^{\circ}C$ under vacuum. As bonding temperature increased, maximum dissolution width of base metal increased, but a dissolution finishing time decreased. The eutectic width of insert metal in the bonded interlayer decreased linearly in proportion to the square root of holding time during isothermal solidification stage. The bonding temperature was raised, isothermal solidification rate slightly increased. As the heating rate decreased and the bonding temperature increased, the completion time of dissolution after reaching bonding temperature decreased. When the heating rate was very slow, the solidification proceeded before reaching bonding temperature and the time required for the completion of isothermal solidification became reduced.

• Journal of Welding and Joining
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• v.17 no.4
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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).