• Journal of Korea Foundry Society
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• v.17 no.5
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• pp.494-501
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• 1997

In order to manufacture large rod preforms of 2014 Al alloy with a good mechanical property by spray forming method, it was spray-formed at a droplet temperature of $715^{\circ}C$, a droplet flight distance of 400mm, and a spraying angle of $35^{\circ}$. The rod preforms were extruded at $397^{\circ}C$ with the die temperature of $420^{\circ}C$ under the hot extrusion ratio 21:1 and T6 heat treatment was performed. The 2014 Al alloys cast by hot top process were also extruded and heat-treated at the same condition as a reference material. Microstructural observation and tensile test were carried out to investigate the effects of extrusion on microstructure and mechanical property of spray-formed Al alloy. Spray-formed Al alloys had many porosities due to inappropriate process conditions such as long droplet flight distance and low droplet temperature but have fine equiaxed grain. These porosities were reduced with decreasing in grain size by hot extrusion. Ultimate tensile strength and yield strength of spray formed-extruded 2014 Al alloy were inferior to those of the normal cast-extruded 2014 Al alloy, but elongations were superior. The control of porosity was important to get spray formed preform with a good mechanical property.

• Nuclear Engineering and Technology
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• v.52 no.3
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• pp.508-519
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• 2020

During transients or accidents, the reactor core is uncovered, and droplets entrained above the quench front collides with the uncovered fuel rod surface. Droplet impact cooling can reduce the peak cladding temperature. Besides zirconium-based cladding, versatile accidental tolerant fuel (ATF) claddings, including FeCrAl, have been proposed to increase the accident coping time. In order to investigate the effect of surface properties on droplet impact cooling of cladding surfaces, the droplet impact phenomena are photographed on the FeCrAl and zircaloy-4 (Zr-4) surfaces under different conditions. On the oxidized FeCrAl surface, the Leidenfrost phenomenon is not observed even when the surface temperature is as high as 550 ℃ with We > 30. Comparison of the impact behaviors observed on different materials shows that nucleate and transition boiling is more intensive on surfaces with larger thermal conductivity. The Leidenfrost point temperature (LPT) decreases with the solid thermal effusivity (${\sqrt{k{\rho}C_p}}$). However, the CHF temperature is relatively insensitive to the surface oxidation and Weber number. Droplet spreading diameter is analyzed quantitatively in the film boiling stage. Based on the energy balance a correlation is proposed for droplet maximum spreading factor. A mechanistic model is also developed for the LPT based on homogeneous nucleation theory.

• Journal of the Korean Society of Visualization
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• v.18 no.1
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• pp.18-25
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• 2020

The droplet impact behavior is dominated by some parameters such as surface temperature, We number, surface and fluid property. Especially, Leidenfrost effect which prevents the contact between surface and droplet is very powerful phenomenon for determining droplet impact behavior. Due to this effect, the impact regime is divided into contact boiling regime and film boiling regime whether the droplet contact with the surface. Many studies have found that surface micro-structures which processed by surface processing are effective to overcome the Leidenfrost effect. In this study, droplet impact behaviors were compared using ethanol both on flat and laser-ablated Al surface. On the flat surface, impact regime was mainly divided by surface temperature. And there is key dominant parameter for each regime. On the laser-ablated surface, we could see changed impact regime and different impact behavior such as jetting and ejection of tiny droplets despite of same impact conditions.

• Journal of the Korean Ceramic Society
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• v.24 no.6
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• pp.593-601
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• 1987

A thermodynamic model to predict the stability of the water-in-oil type emulsion and the size of the droplets in stable emulsions was developed. Using this model, the effects of various factors government the droplet size in the metal salt solution-kerosene-span 80 system for the preparation of Al2O3-ZrO2 composite powders were investigated. It was shown that the given emulsion systems were thermodynamically unstable in every case but could be kinetically meta stable. When radius ofthe droplet was below nm, the increase in entropy change due to the configurational contribution of small droplets dominated the total free energy change for emulsification. The optimum conditions under which smaller deoplet was obtained were proposed and the validity of the model was proved with diameters of the droplet and composite powders experimentally determined.

• Corrosion Science and Technology
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• v.17 no.3
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• pp.129-137
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• 2018

The resistance to corrosion of additive manufactured (3D printing) Ti-6Al-4V alloys was investigated using micro-electrochemical tests. In terms of corrosion resistance, the acicular martensitic ${\alpha}^{\prime}$ phase in such additive manufactured Ti-6Al-4V was the focus of attention, and its behavior was distinct from that of conventional subtractive manufactured Ti-6Al-4V. To order to identify ${\alpha}^{\prime}$ phase, XRD tests were performed and micro Vickers hardness was measured for different grains (bright and dark grains) in the additive manufactured Ti-6Al-4V alloy. Micro-electrochemical tests were performed to measure corrosion resistance of bright and dark grains in the additive manufactured Ti-6Al-4V alloy with specially designed electrochemical micro-droplet cell. Critical pitting temperature (CPT) measurement was performed to evaluate the resistance to pitting corrosion of additive manufactured Ti-6Al-4V alloys with different volumes of ${\alpha}^{\prime}$ phase and subtractive manufactured Ti-6Al-4V alloy. The dark grains of the laminated Ti-6Al-4V alloy distributed broader than the bright grains measured with low microhardness. The dark grains of the Ti-6Al-4V alloy, which was rich in martensite ${\alpha}^{\prime}$, had lower general corrosion and pitting resistance than bright grains. As the fraction of martensite ${\alpha}^{\prime}$ phase increased, the resistance to the pitting corrosion decreased.

• Corrosion Science and Technology
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• v.19 no.6
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• pp.310-317
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• 2020

This study investigated the damage to the specimen due to liquid droplet impingement erosion corrosion, which improved the corrosion resistance and durability via hard anodization of 5083-H321 aluminum alloy, which is widely used for small ships and marine structures. The experiment combined liquid droplet impingement erosion and electrochemical equipment with the flow rates in natural seawater solution. Subsequently, Tafel extrapolation of polarization curves was performed to evaluate damage due to the liquid droplet impingement erosion corrosion. The damaged surface was observed using a 3D microscope and a scanning electron microscope. The degree of pitting damage was measured using the Image J program, and the surface hardness was measured using the micro-Vickers hardness tester. The corrosion current density, area, depth, and ratio of the damaged areas increased with the increase in flow rate. The grain size of the damaged area at a flow rate of 20 m s-1 showed fewer and minor differences in height, and a smooth curved shape. The hardness of the damaged surface tended to decrease with increase in flow rate.

• Applied Microscopy
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• v.43 no.2
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• pp.98-101
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• 2013

In the present study, formation of yard and shell has been investigated in as-melt-spun $Gd_{30}Ti_{25}Al_{25}Co_{20}$ alloy using a variety of transmission electron microscopy techniques. The phase separation during cooling leads to the formation of the microstructure consisting of amorphous droplets with different size scales embedded in the amorphous matrix. Due to the interdiffusion at the interface after the first-step phase separation, ~50 nm-thick yard develops on the surface of the primary droplet particle. Due to the critical wetting phenomenon, ~5 nm thickness shell enveloping the droplet forms. The sell is enriched in Co and Ti, implying that the composition is close to that of the droplet.

• Journal of the Korean Society of Safety
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• v.18 no.3
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• pp.29-33
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• 2003

The objective of the present work is to examine evaporation cooling of droplet due to surface roughness on a heated surface. The surface temperatures varied from 80-$160^{\circ}C$ on aluminum alloy (AL 2024) md surface roughness was 0.l8$\mu\textrm{m}$ 1.36$\mu\textrm{m}$. The results are as follows; Regardless of surface roughness, as droplet diameter is bigger, the in-depth temperature of solid decreases and evaporation time increases. In the case of same initial temperature on the heated surface, as droplet diameter is smaller and small surface roughness is bigger, evaporation time decreases and time averaged heat flux increases.

• Journal of ILASS-Korea
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• v.17 no.1
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• pp.14-19
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• 2012

The present study conducts experimental investigation on spreading and deposition characteristics of a $4.3{\mu}l$ de-ionized (DI) water droplet impacting upon aluminum (Al 6061) flat and textured surfaces. The micro-textured surface consisted the micro-hole arrays (hole diameter: $125{\mu}m$, hole depth: $125{\mu}m$) fabricated by the conventional micro-computer numerical control (${\mu}$-CNC) milling machine process. We examined the surface effect of texture area fraction ${\varphi}_s$ ranging from 0 to 0.57 and impact velocity of droplet ranging from 0.40 m/s to 1.45 m/s on spreading and deposition characteristics from captured images. We used a high-speed camera to capture sequential images for investigate spreading characteristics and the image sensor to capture image of final equilibrium deposition droplet for analyze spreading diameter and contact angle. We found that the deposition droplet on textured surfaces have different wetting states. When the impact velocity is low, the non-wetting state partially exists, whereas over 0.64 m/s of impact velocity, totally wetting state is more prominent due to the increase kinetic energy of impinging droplet.

• Journal of the Korean Society of Safety
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• v.19 no.3 s.67
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• pp.14-19
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• 2004

The objective of the present work is to examine evaporation cooling of droplet due to surface roughness under radiative heat input condition. The surface temperatures varied from $80\~160^{\circ}C$ on aluminum alloy (AL 2024) and surface roughness was $0.18{\mu}m,\;1.36{\mu}m$. The results are as follows; Regardless of surface roughness under radiative heat input condition, as droplet diameter is larger, the in-depth temperature of solid decreases and evaporation time increases. In the case of $0.18{\mu}m\;and\;1.36{\mu}m$ of surface roughness, the larger the surface roughness is, the less the evaporation time is and the larger the temperature within the solid is. In the case of $Ra=0.18{\mu}m$ evaporation time and time averaged heat flux for radiative heat input case is shorter than for the conductive case.