• Transactions of Materials Processing
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• v.7 no.5
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• pp.450-458
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• 1998

The high temperature deformation behavior of Al 5083 alloy has been studied in the temperature range of 350 to 520 ${\circ}C$ and strain rate range of 0.2 to 3.0/sec by torsion test. The strain rate sensitivity(m) of the material was evaluated and used for estabilishing power dissipation maps following the dynamic material model. These maps show the variation of efficiency of power dissipation(${\eta}$=2m/(2m+1)) with temperature and strain rate. Hot restoration of dynamic recrystallization (DRX) was analyzed from the flow curve, deformed microstructure, and processing maps during hot deformation. Also, the effect of deformation strain on the efficiency of power dissipation of the alloy was analysed using the processing maps. Moreover relationship between the hot-ductility and efficiency of power dissipation of the alloy depending on thmperature and strain rate was studied using the Zener-Hollomon parameter(Z=${\varepsilon}$exp(Q/RT) It is found that the maximum efficiency of power dissipation for DRX in Al 5083 alloy is about 74.6 pct at the strain of 0.2. The strain rate and temperature at which the efficiency peak occurred in the DRX domain is found to be ∼0.1/sec and ∼450${\circ}C$ respectively.

• Transactions of Materials Processing
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• v.19 no.1
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• pp.25-31
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• 2010

To achieve desired microstructure and mechanical property of a manufacturing product, heat treatment process is applied as a secondary process after forging. Especially, quenching process is used for improving strength, hardness, and wear resistance since phase transformation occurs owing to rapid heat transfer from the surface of the specimen. In the present paper, a study on surface temperature measurement for water quenching of eutectoid steel was investigated. In order to determine the temperature history in experiments, three different measuring schemes were used by varying installation techniques of K-type thermocouples. Depending on the measured temperature distribution at the surface of the specimen, convective heat transfer coefficients were numerically determined as a function of temperature by the inverse finite element analysis considering the latent heat generation due to phase transformation. Based on the inversely determined convective heat transfer coefficient, temperature, phase, and hardness distributions in the specimen after water quenching were numerically predicted. By comparing the experimental and computational hardness distribution at three different locations in the specimen, the best temperature measuring scheme was determined. This work clearly demonstrates the effect of temperature measurement on the final mechanical property in terms of hardness distribution.

• Journal of Powder Materials
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• v.16 no.5
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• pp.342-350
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• 2009

Development of nanoparticulate materials technology is essential to processing of highly functional nanoparticulate materials and components with small and complex shape. In this paper, the effect of particle size on surface roughness and shrinkage of sintered Fe-8 wt%Ni nanopowder components fabricated by PIM were investigated. The Fe-8 wt%Ni nanopowder was prepared by hydrogen reduction of ball-milled Fe$_2$O$_3$-NiO powder. Feedstock of nanopowder prepared with the wet-milled powder was injection molded into double gear shaped part at 120$^{\circ}C$. After sintering, the sintered part showed near full densified microstructure having apparently no porosity (98%T.D.). Surface roughness of sintered bulk using nanopowder was less than 815 nm and it was about seven times lower than 7 $\mu$m that is typically obtainable from a sintered part produced from PIM.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.1
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• pp.35-40
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• 2007

The new PEI(poly(epoxy-imide))-nano Silica film has been synthesized via in situ CS sol process, and the chemical bonding and microstructure of nano silica dispersed in resin were examined by FT-IR, TAG and SEM. The dielectric properties of these hybrid films over a given temperature and frequency ranges have been studied in a point of view of stable chemical bonding of nano Silica filler. The results from IR spectra and SEM photograph indicated that PEI-Silica hybrid film prepared with nano CS sol process has been synthesized in uniform and chemical bonding. The decrease property of dielectric constant with CS content, tangent loss consistent of given frequency and temperature has been explained in terms of the chain movement of polymer through chemical bonging and size effect of nano silica. The new PEI-CS sol hybrid film with such stable chemical and dielectric properties was expected to be used as a high functional coating application in ET, IT and electric power products.

• Journal of Welding and Joining
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• v.11 no.4
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• pp.70-78
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• 1993

An attempt was made to investigate the effect of brazing time on microstructure, microhardness, and corrosion of Zircaloy -4as well as the beryllium diffusion into its sheet. The sheets were coated with beryllium and brazed at $1020^{\circ}C$ for 20-40 minutes in $2{\times}10^{-5}$ torr vacuum atmosphere. 1. Microstructurally the brazed zone was largely divided into three regions: a region of continuous or partially formed of eutectic liquid films along grain boundaries; a region of precipitation in both grains and grain boundaries; a region of elongated wide structure of .alpha.-laths, which was not affected by beryllium. 2. Due to the precipitates, the beryllium-migrated region was hardened and the width of the hardened region increased with increasing brazing time. 3. Beryllium brazed Zircaloy -4 sheets showed a higher corrosion rate than those of as-received and heat-treated at a brazing temperature. 4. Diffusion coefficient of beryllium into Zircaloy -4 at $1020^{\circ}C$ for 30 minutes was $7.67{\times}10^{-7}cm^2/sec.$ It seemed that Be penetrated Zircaloy -4 by forming eutectic liquid films along grain boundaries in the proximity of Be/Zr interface and it, thereafter, diffused into Zircaloy mainly by interstitial solid solution.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.22 no.3
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• pp.554-561
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• 1998

Vehicle body structures are formed by thousands of spot welds and fatigue failure of vehicle structures occur near the spot welds after driving a long way at a durability test road. It is necessary to know accurately the reason of the fatigue failure of the spot weld in the developing stage in order to reinforce it. Many investigations have been done regarding the strength of spot welded joints, contributing to understand its fatigue strength. In developing process, a fatigue failed spot welded area can be repaired by $CO_2$ welding or another method to continue the test. To know the effect of reinforcing these welds, several methods of welding were analyzed and compared to spot welding. With the results of this test, the appropriate repair method can be used instead of spot welding during the development of new car and best design guide can be given for the strength. In this study, fatigue and static tensile tests are made and microstructure is investigated for the purpose of estimating the strength of welded joints by using spot welded and $CO_2$ plug welded specimens. The tested specimens are of two types : Tensile-shear type(TS) and L-tension type(LT).

• International Journal of Concrete Structures and Materials
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• v.9 no.2
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• pp.207-217
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• 2015

The influence of powdered and colloidal nano-silica (NS) on the mechanical properties of cement mortar has been investigated. Powdered-NS (~40 nm) was synthesized by employing the sol-gel method and compared with commercially available colloidal NS (~20 nm). SEM and XRD studies revealed that the powdered-NS is non-agglomerated and amorphous, while colloidal-NS is agglomerated in nature. Further, these nanoparticles were incorporated into cement mortar for evaluating compressive strength, gel/space ratio, portlandite quantification, C-S-H quantification and chloride diffusion. Approximately, 27 and 37 % enhancement in compressive strength was observed using colloidal and powdered-NS, respectively, whereas the same was up to 19 % only when silica fume was used. Gel/space ratio was also determined on the basis of degree of hydration of cement mortar and it increases linearly with the compressive strength. Furthermore, DTG results revealed that lime consumption capacity of powdered-NS is significantly higher than colloidal-NS, which results in the formation of additional calcium-silicate-hydrate (C-S-H). Chloride penetration studies revealed that the powdered-NS significantly reduces the ingress of chloride ion as the microstructure is considerably improved by incorporating into cement mortar.

• Journal of the Korean Ceramic Society
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• v.38 no.8
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• pp.710-715
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• 2001

반응소결공정(RSP : Reaction Sintering Process)을 이용하여 YF댸₃와 Fe₂O₃의 성분에 소결첨가재 Bi₂O₃를 첨가하여 YIG를 합성하였다. Bi₂O₃첨가량과 소결온도에 따른 YIG 소결체의 미세구조 및 자기적 특성변화에 대해 주사전자현미경, X-선 회절분석기 및 시료 진동형 자력계를 이용하여 조사하였다. 소량의 소결첨가재 Bi₂O₃첨가시 YIG 소결체의 격자상수는 12.387에서 12.420 Å으로 증가하였다. 이는 상대적으로 이온반경이 큰 Bi 이온이 12면체 Y 이온 자리로 치환되었기 때문인 것으로 여겨진다. Bi₂O₃를 1.0 wt% 첨가하였을 때 비교적 균질한 미세구조를 보였으며, 1350℃에서 소결한 YIG의 밀도가 이론밀도의 98% 이상의 치밀화를 보였다. Bi₂O₃가 0.0 wt%에서 1.5 wt%로 첨가량이 증가함에 따라 상온에서의 포화자화값(M/sub s/)은 조금씩 증가하는 경향을 보였으나 큰 변화는 없었다. 반응소결공정을 이용 YIG 소결시 소결첨가제 Bi₂O₃가 1.0 wt%이고, 소결온도 1350℃에서 비교적 우수한 소결특성과 자기특성을 가지는 YIG 소결체를 얻을 수 있었다.

• Journal of the Korean Ceramic Society
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• v.25 no.4
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• pp.364-372
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• 1988

The mechanical properties and microstructure of ceramics of the system Al2O3-ZrO2-Y2O3 sintered at 1$650^{\circ}C$ for 2h after powder preparation by the precipitation method from Al2(SO4)3.18H2O, ZrOCl2.8H2O and YCl3.6H2O were investigated. The Al2O3-ZrO2-Y2O3 ceramics sintered at 1$650^{\circ}C$ for 2h after mixing alpha-Al2O3 and ZrO2-Y2O3 powders, both were separately precipitated and calcined, were found to have the relative density higher than 97.5% so that the strengthening and toughening mechanisms could be explained mainly as the stress-induced phase transformation. On the other hand, the sintered bodies prepared by co-precipitating the three starting materials were measured to have the relative density lower than 85% so that the degradation of strength were observed above 15 vol% ZrO2 contents due to the high porosity by which the effect of stress-induced phase transformation was assumed to be depressed.

• Proceedings of the KSME Conference
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• 2004.11a
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• pp.96-101
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• 2004

Conventionally, SMAW process was applied to join pipes of RCL, which caused lot of loss in time and cost due to excessive heat input and defects in joining section. Recently, narrow-gap welding(NGW) process was introduced to overcome the disadvantages of SMAW. However, the application of NGW to nuclear power plant is not yet common because safety of NGW process is not proven. In present paper, the welded coupons are made of carbon steel. They are manufactured under different processes; general welding(GW), post-weld heat treatment(PWHT) after GW, repair welding after GW and PWHT with repair welding after GW in carbon steel. Performed are various mechanical tests investigation of microstructure, hardness test, tensile test at room and high temperature, bending test, impact test and J-R test. It is verified that the mechanical properties of carbon steel are greatly changed after repair welding process due to applied heat flux, and that the effect of PWHT is beneficial.