• 한국정밀공학회지
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• 제29권6호
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• pp.632-639
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• 2012

A fully coupled thermo-mechanical model is adopted to study the temperature distribution and the material deformation in friction stir welding(FSW) process. Rotational speed is most important parameters in this research. Three dimension results under different process parameters were presented. Result indicate that the maximum temperature is lower than the melting point of the welding material. The higher temperature gradient occurs in the leading side of the workpiece. The maximum temperature can be increased with increasing the tool angular velocity, rpm in the current numerical modeling. In this research ABAQUS Ver.6.7 is to analyze a fully coupled thermo-mechanical model. ALE(Arbitrary Lagrangian-Eulerian) finite element formulation is used for the large deformation in FSW process and using the Mass scaling for the analysis time efficiency.

• Journal of Advanced Marine Engineering and Technology
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• 제11권4호
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• pp.50-58
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• 1987

At present it has been cleared that even carefully polished surfaces have irregularities on them which are large compared with molecular dimensions by the progress of a scanning electron microscope. When two solids are placed together, the real area of contact is very small, so that the local pressure is high and, in general, exceeds the yield pressure of the metal. Plastic flow of the solid occurs at the summits of the irregularities so that the real area of contact is proportional to the applied load. There is adhesion at local resions of contact and the friction is, in a large measure, the force required to shear them. On this view point, the friction experiment with different specimens which are carbon steel, copper and constantan was attempted to know the characteristics of temperature change in contact with different metals. Various experiments are summerized as follows; 1) With metals of high melting point, momentary 1000 .deg. C may last below $10^{-4}$ of a second. It is thought that above phenomena back up previous adhesion theory in wear. 2) As a general rule, surface temperature in contact with different metals becomes high when the load increases while it is observed that surface temperature decreases when the load increases with shapes of specimens.

• 한국전자통신학회논문지
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• 제15권2호
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• pp.313-318
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• 2020

300도 이상의 높은 용융점을 갖는 소위 엔지니어링 플라스틱은 기구적인 강성과 내화학성 및 마찰 및 마모성능이 우수하여 여러 산업에서 금속을 대체하는 소재로 각광받고 있다. 본 연구에서는 용융적층모델링 공법을 기반으로 하는 3D 프린터에서 높은 용융점을 갖는 엔지니어링 플라스틱을 조형할 수 있도록 방열특성이 우수한 3D 프린터 nozzle부의 구조를 설계하고 이를 해석적으로 검증하였다. 높은 온도로 가열되는 heat block과 필라멘트가 이송되는 nozzle상부 간의 단열 및 신속한 냉각을 위하여, 열전도계수가 낮은 열차단부(heat brake부)를 2중으로 구성하였고, 열차단부에 생성되는 열이 냉각핀을 통해 대기에 의해 냉각되는 구조를 적용하였다. 개선된 nozzle부 구조설계를 통해 종래 3D 프린터의 BCnozzle과 비교할 때, heat sink부에서의 온도를 50% 가량 낮출 수 있었으며, heat block에 직접적으로 연결된 heat brake부 최종단의 정상상태 온도를 14% 가량 낮출 수 있었다.

• 대한기계학회논문집B
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• 제24권9호
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• pp.1277-1284
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• 2000

To enhance heat transfer characteristics of water, micro-encapsulated octadecane of about $10{\mu}m$ diameter was added to water. Viscosity of the slurry was measured by using a capillary tube viscometer. The measured viscosity decreased as the temperature of the slurry increased, and it increased as the fraction of the capsules in the slurry increased. Thermal characteristics of the octadecane were studied by using a differential scanning calorimeter. The melting temperature and the melting energy of the octadecane were found to be $28.6^{\circ}$ and 34.4kcal/kg, respectively. The convective heat transfer characteristics of the slurry were investigated in a flow loop with a constant heat flux test section. Friction factor of the slurry flow was found to be similar to the expected curve by Petukhov. The Nusselt number of the slurry flow was highest when the octadecane melted. Effective thermal capacity of the 14.2% slurry was found to have 1.67 times of the thermal capacity of water.

• 대한기계학회논문집B
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• 제21권9호
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• pp.1165-1173
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• 1997

The steady state close-contact melting phenomenon occurring between a phase change material and an isothermally heated flat plate with relative motion is investigated analytically, in which the effects of vertical convection in the liquid film and solid-liquid density difference are incorporated simultaneously. Not only the scale analysis is conducted to estimate a priori qualitative dependence of system variables on characteristic parameters, but also an analytical solution to a set of simplified model equations is obtained to specify the effects under consideration. These two results are consistent with each other, in that the vertical convection affects both the solid descending velocity and the film thickness, and that the density difference alters only the solid descending velocity. While the effect of vertical convection can be characterized conveniently by a newly introduced temperature gradient factor which asymptotically approaches the unity/zero with decreasing/increasing the Stefan number, that of density difference is represented by the liquid-to-solid density ratio. It is shown that the solid descending velocity depends linearly on the density ratio, and that the ratios of solid descending velocity, film thickness and friction coefficient to the conduction solution are proportional to 3/4, 1/4 and -1/4 powers of the temperature gradient factor, respectively. Also, established is the fact that the effect of convection can be legitimately neglected in the analysis for the range of the Stefan number less than 0.1.

• 한국산학기술학회논문지
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• 제18권12호
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• pp.679-684
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• 2017

마찰 교반 용접(Friction Stir Welding)은 금속 소재 대상으로 용접 툴과 용접 재료의 마찰열을 이용하여 재료 융점 이하의 온도에서 접합하는 용접 기법이다. 이번 연구에서는 금속 접합시 쓰이는 마찰 교반 용접 기법을 활용하여 마그네슘 합금(AZ31)을 용접할 때, 용접시 발생하는 용접 대상인 마그네슘 합금의 온도 및 속도 변화에 대해 유동 해석 기법을 활용하여 분석하였다. 분석을 위해 유동 해석 툴인 플루언트를 활용하여 모델링 및 해석을 진행하였다. 먼저 용접 소재는 온도에 따라 변하는 고점도 뉴턴 유체로 가정하였으며, 나선형 홈이 있는 용접 툴의 회전에 의한 회전 유동 발생을 모사하기 위해 회전 영역과 정지 영역으로 구분하여 모사하였다. 용접 툴과 용접 재료 사이의 인터페이스는 마찰 및 미끄러짐 경계조건을 부여하여 용접 툴로의 열전달 효과를 고려하였다. 위의 유동 해석 모델링을 통한 과도 해석 결과로부터 시간의 변화에 따른 용접 소재의 속도와 온도 특성을 파악할 수 있었다.

• Steel and Composite Structures
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• 제28권6호
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• pp.759-766
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• 2018

Friction Stir Welding (FSW) is a solid-state process, where the objects are joined together without reaching their melting point. It has been shown that this method is a suitable way to join dissimilar aluminium alloys. The current article employed hole drilling technique to measure the residual stress distribution experimentally in different zones of dissimilar aluminium alloys AA6061-T6 and AA7075-T6 Butt welded using FSW. Results are compared with those of similar AA6061-T6 plates joined using a conventional fusion welding method called tungsten inert gas (TIG). Also, the evolution of the residual stresses in the thickness direction was investigated, and it was found that the maximum residual stresses are below the yield strength of the material in the shoulder region. It was also revealed that the longitudinal residual stresses in the joint were much larger than the transverse residual stresses. Meanwhile, Vickers micro hardness measurements were performed in the cross-section of the samples. The largest hardness values were observed in the stir zone (SZ) adjacent to the advancing side whereas low hardness values were measured at the HAZ of both alloys and the SZ adjacent to the retreating side.

• Tribology and Lubricants
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• 제34권6호
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• pp.270-278
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• 2018

STS 316L prepared by additive manufacturing (AM) exhibits deterioration of mechanical properties and wear resistance due to the presence of defects such as black-of-fusion defects, internal porosity, residual stress, and anisotropy. In addition, high surface roughness (integrity) of AM products remains an issue. This study aimed to apply ultrasonic nanocrystal surface modification (UNSM) technology to STS 316L prepared by AM to increase the surface hardness, to reduce the surface roughness, and to improve the friction and wear behavior to the level achieved by bulk material manufactured using traditional processes. Herein, the as-received and polished specimens were treated by UNSM technology and their resulting properties were compared and discussed. The results showed that UNSM technology increased the surface hardness and reduced the surface roughness of the as-received and polished specimens. These results can be attributed to grain size refinement and pore elimination from the surface. Moreover, the friction of the as-received and polished specimens after UNSM technology was lower compared to those of the as-received and polished specimens, but no significant differences in wear resistance were found.

• Journal of Advanced Marine Engineering and Technology
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• 제9권1호
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• pp.74-88
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• 1985

The wear becomes the most complicated cause of making various machineries short lives. In the studies by many scholors so far, the accurate and perfect methods to eliminate it completely are not found yet owing to many affected factors. On this view point of Depositional Wear, the wear test with seven different specimens from the part of frequent usage of which are carbon steel, nickel, copper, aluminum, zinc, lead and tin was attempted to make clear the relation between the wear of different metals and their melting point, crystal structure, sliding speed, load and lattice factors. The results of this study are summerized as follows; 1) Worn amount of each metal is maximum when each metal is same quality. 2) The easier becomes the the formation of solid solution in the course of friction proceeding, the more it results in worn amount of each metal. 3) The samer is each metal in crystal structure as well as lattice factor, the more it results in worn amount of each metal. These results might be of use in designing machineries, selecting materials and operating machineries.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 추계학술대회논문집B
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• pp.130-135
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• 2000

To enhance heat transfer characteristics of water, fine ice was added to it. The convective heat transfer characteristics of the ice slurry were investigated in a flow loop with a constant heat flux test section. The Nusselt number and Fanning friction coefficient of water flow were found to be similar to the expected curve by Petukhov. The Nusselt number of the ice sin flow was higher than the Nusselt number of water. Effective thermal capacity of the 10.84% ice slurry was found to have 2.39 times of the thermal capacity of water.