• Journal of Welding and Joining
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• v.27 no.6
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• pp.68-73
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• 2009

Recently, as titanium and titanium alloys are being increasingly used in wide areas, there are on-going researches to obtain high quality weld zone. In particular, growing interest is being drawn to laser welding, which involves low heat input and large aspect ratio in various welding processes and can facilitate shield in atmospheric condition compared with electron beam welding. The first report covered the analysis of embrittlement by the bead color of weld zone through quantitative analysis of oxygen and nitrogen and measurement of hardness as basic experiment to apply laser welding to titanium. Results indicated that the element that affect embrittlement the most was nitrogen, and as embrittlement and oxygenation go on, bead color changed to silver, gold, brown, blue and gray. This study performed butt welding of pure titanium and STS304 by using 1kW CW Nd:YAG laser, and to find out basic physical properties, evaluated welding performance by laser output, welding speed, root gap and misalignment etc, and examined mechanical properties through tensile stress and Erichsen test. The reason particles of pure titanium welded metal and HAZ are greater than STS304 is because they are pure metal and do not include many impure elements that work as nuclei in case of resolidification, thus becoming coarse columnar crystals eventually. In addition, the reason STS304 requires more energy during welding than pure titanium is because the particle size of base metal is smaller.

• Transactions of the Korean Society of Automotive Engineers
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• v.16 no.5
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• pp.8-14
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• 2008

For nondestructive inspection of electron beam (EB) welding part in automotive power transmission assembly, a pulse-echo ultrasonic testing apparatus in water immersion has been applied using the ultrasonic waves with a frequency of 10MHz. However various problems have appeared during the ultrasonic inspection, which led to some significant mistakes in automatic quality evaluation of the welding parts. Experimental study showed that the state of water couplant medium containing some amount of contaminants, rusts and anti-corrosion agents had considerable influences on the reduction of ultrasonic amplitudes during wave propagation. The amplitude reduction depending on the coupling medium state could bring about some mis-diagnoses for defects in the welding parts. The results proposed that for a reliable inspection of defects in welds the state of water medium should be kept in about 15 volume fractions (vol.%) of anti-corrosion agents and in minimized contaminants.

• Journal of Mechanical Science and Technology
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• v.16 no.12
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• pp.1627-1632
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• 2002

In this paper, a simplified ultrasonic CT system, which uses the information in three directions, that is, 90°, ＋45° and －45°about the inspection plane, is applied to the high strength steel, and the frequency analysis method for enhancing the C scan or CT image is developed. This frequency analysis method is based on the frequency response property of the material. By comparing the magnitudes in the frequency domain, the special frequency which shows a significant difference between the welded joint and base material was found and used to obtain a C scan or CT image. Experimental results for several kinds of specimens, having a welded joint by electron beam welding, a weld joint by arc welding, on a fatigue crack, showed that the obtained C scan or CT image has better resolution than the results of previous experiments using the maximum value of the received waveform.

• Journal of the Korean Society for Heat Treatment
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• v.6 no.4
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• pp.185-193
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• 1993

The effect and Condition of heat treatment on the tensile strength of welded joint was investigated in 300 grade 18% Ni-Co-Mo-Ti maraging steel sheets welded with electron beam. A good tensile strength of welded joint was obtained by following heat treatment cycle ; At $1100^{\circ}C$ the specimen was high temperature solution treated for 1 hour and then it was repeated solution treated at $900^{\circ}C$, $820^{\circ}C$ for 1 hour respectively to recrystallize the coarsened ${\gamma}$ grain. These heat treatment cycle was completed by an final aging heat treatment at $480^{\circ}C$ for 4 hour. Moreover, dissolution of dendrite, a significant decrease in seregation of Mo, Ti in weld metal were observed and also the coarsened ${\gamma}$ grain formed at $1100^{\circ}C$, $1200^{\circ}C$ changed to fine grain due to the effect of recrystallization.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.107-107
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• 2009

모듈 형 터빈 다이아프람은, 아우터 링(outer ring), 스팀 패스(steam path)와 이너 웹(inner web)의 원형 형상을 갖는 세 부분을 조립하여 원주 방향의 용접 조인트를 형성하는 기존의 다이아프람 형태가 아니라, 아우터 슈라우드(outer shroud), 베인(vane)과 이너 슈라우드(inner shroud)의 세 부분이 하나의 모듈을 이루고 이러한 모듈을 원주 방향으로 조립하여 방사 방향의 조인트를 형성한다. 전자빔 용접은 이와 같은 방사 방향의 조인트를 수직으로 가로지르는 용접 궤적을 따라 진행되며, 용접 패스에 따라 형성되는 용융 비드의 단면적만큼 인접하는 두 모듈을 접합시킨다. 이 경우 용융 비드의 단면적과 형상은 두 모듈의 결합 강도를 결정하는 중요한 요소가 되어, 제작 시 다이아프람의 크기와 두께에 따라 용입 깊이와 평균 단면 비드 폭을 규정하고 있다. 본 연구에서는 용입 깊이와 단면 비드 폭의 요구 조건을 만족하면서 결함이 없는 건전한 용접부를 얻을 수 있는 최적 용접 조건을 도출하는데 그 목적이 있다. 이를 위해 플레이트 시편과 모듈 시편을 사용한 기초 실험과 유사 시제품(semi-mockup) 실험을 실시하였다. 플레이트 기초 실험을 통해 전자빔 주요 변수인 빔 전류, 초점 위치, 용접 속도, 빔 진동 폭 변화에 따른 용융 비드 형상 변화를 관찰하였고, 빔 전류가 용입 깊이에 가장 큰 영향을 주는 인자임을 확인하여 요구 용입 깊이 별 적정 빔 전류 값을 설정하였다. 용접 속도는 생산성 측면에서 균열이 발생하지 않는 범위에서 가능하면 가장 큰 값을 사용하였고, 빔 진동 폭은 초점 위치와 함께 단면 비드 형상 결정에 많은 영향을 주는 인자로 확인되어 균열이 없는 가장 이상적인 단면 비드 형상인 완만한 쐐기 형태가 되도록 설정하였다. 이 후 실제 제품 폭과 용접 패스를 갖는 블록 모듈 실험을 통해 설정 용접 변수의 적용성과 균열 발생 여부를 확인하였고, 이 때 적용 제품 폭이 30 mm 이하이며 요구 용입 깊이가 50 mm 이상의 경우에서 비드 중앙부 균열이 발생함을 관찰하였다. 따라서 해당 영역의 제품에는 균열 저항성이 높도록 용접 속도와 빔 진동 폭을 줄여 최적 용접 변수를 새롭게 설정하였으며, 이를 유사 시제품 실험에 적용하여 최종적으로 용접 변수 안정성을 검증하였다. 이러한 실험을 통해 확인된 최적 용접 조건을 실 제품 제작에 적용하여 모듈 형 터빈 다이아프람 전자빔 용접 제작을 성공적으로 완료할 수 있었다.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.612-619
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• 2002

The dynamic behavior of Al-Mg alloys plasma was very unstable and this instability was closely related to the unstable motion of keyhole during laser irradiation. The keyhole fluctuated both in size and shape and its fluctuation period was about 440 ${\mu}{\textrm}{m}$. This instability has been estimated to be caused by the evaporation phenomena of metals with different boiling point and latent heats of vaporization. Therefore, the authors have conducted the spectroscopic diagnostics of plasma induced in the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. In the air environment, the identified spectra were atomic lines of Al, Mg, Cr, Mn, Cu, Fe and Zn, and singly ionized Mg line, as well as strong molecular spectrum of AlO, MgO and AIH. It was confirmed that the resonant lines of Al and Mg were strongly self-absorbed, in particular in the vicinity of pool surface. The self-absorption of atomic Mg line was more eminent in alloys containing higher Mg. These facts showed that the laser-induced plasma was relatively a low temperature and high density metallic vapor. The intensities of molecular spectra of AlO and MgO were different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectra were predominant in intensity, while the AlO spectra became much stronger in higher power density. In argon atmosphere the band spectra of MgO and AlO completely vanished, but AlH molecular spectra was detected clearly. The hydrogen source was presumably the hydrogen solved in the base Metal, absorbed water on the surface oxide layer or H$_2$ and $H_2O$ in the shielding gas. The temporal change in spectral line intensities was quite similar to the fluctuation of keyhole. The time average plasma temperature at 1 mm high above the surface of A5083 alloy was determined by the Boltzmann plot method of atomic Cr lines of different excitation energy. The obtained electron temperature was 3, 280$\pm$150 K which was about 500 K higher than the boiling point of pure aluminum. The electron number density was determined by measuring the relative intensities of the spectra1lines of atomic and singly ionized Magnesium, and the obtained value was 1.85 x 1019 1/㎥.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2011.11a
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• pp.515-519
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• 2011

Structural tests for the mixing head of a 75tonf class thrust chamber were performed to verify structural stability. The mixing head of a thrust chamber is loaded by high pressure with regeneratively cooled fuel and cryogenic liquid oxygen(LOx) as well as it transfers thrust load generated by liquid rocket engine. Therefore structural stability of mixing head is a very important factor to work without any plastic deformation or structural failure. In this study, two mixing heads were manufactured using different welding methods, Tungsten Inert Gas(TIG) welding and Electron Beam Welding(EBW) and evaluated a structural stability. The results of structural tests showed that the mixing head assembled by EBW can withstand the applied design load without any structural failures and be structurally more stable than that of TIG welding.

• Proceedings of the KWS Conference
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• 2006.10a
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• pp.161-163
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• 2006

포항가속기는 선형가속기 및 저장링을 통하여 2.5Gev의 고에너지 전자빔 생성과 저장, 유지 되고 있다. 저장링에서 안정적으로 방사광을 추출하여 각종 실험 및 연구를 위하여 이를 제공하고 있다. 이를 위해서 전자가 매우 안정적이고 높은 집적도를 가진 전자다발 형태로 유지되어야 한다. 전자다발의 집적도가 높을수록 전자간의 상호 간섭으로 보유에너지가 더 빠른 시간에 소진되어 전자빔의 수명이 짧아진다. 3차 하모닉 RF 공동기는 전자다발의 운동방향으로 길게 늘려서 전자밀도를 줄여 전자빔의 운전수명을 개선하기 위한 장치이다. 공동기의 제작은 소재준비, 성형, 가공, 용접, 연마, 열처리 등의 여러 공정을 거쳐 완료된다. 본 보고서에서는 공동기의 제작 공정 중 전자빔을 이용한 시험용접, 본용접, 용접부 검사 및 용접부의 연마와 결과에 대하여 기술하였다.

• Journal of Welding and Joining
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• v.33 no.3
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• pp.47-53
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• 2015

By the trends of electronic package to be more integrative, the fine Cu trace pitch of organic PCB is required to be a robust design. In this study, the short circuit failure mechanism of PCB with a Cl element under the Temperature humidity bias test ($85^{\circ}C$/85%RH/3.5V) was examined by micro-structural study. A focused ion beam (FIB) and an electron probe micro analysis (EPMA) were used to polish the cross sections to reveal details of the microstructure of the failure mode. It is found that $CuCl_x$ were formed and grown on Cu trace during the $170^{\circ}C$/3hrs and that $CuCl_x$ was decomposed into Cu dendrite and $Cl_2$ gas during the $85^{\circ}C$/85%RH/3.5V. It is suggested that Cu dendrites formed on Cu trace lead to a short circuit failure between a pair of Cu traces.

• Journal of Welding and Joining
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• v.34 no.1
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• pp.75-81
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• 2016

This study applied laser surface melting process using CW(Continuous wave) Yb:YAG laser and cold-work die steel SM45C and investigated microstructure and hardness. Laser beam speed, power and beam interval are fixed at 70 mm/sec, 2.8 kW and $800{\mu}m$ respectively. Depth of Hardening layer(Melting zone) was a minimum of 0.8 mm and a maximum of 1.0 mm that exceeds the limit of minimum depth 0.5 mm applying trimming die. In all weld zone, macrostructure was dendrite structure. At the dendrite boundary, Mn, Al, S and O was segregated and MnS and Al oxide existed. However, this inclusion didn't observe in the heat-affected zone (HAZ). As a result of interpreting phase transformation of binary diagram, MnS crystallizes from liquid. Also, it estimated that Al oxide forms by reacting with oxygen in the atmosphere. The hardness of the melting zone was from 650 Hv to 660 Hv regardless of the location that higher 60 Hv than the hardness of the HAZ that had maximum 600 Hv. In comparison with the size of microstructure using electron backscatter diffraction(EBSD), the size of microstructure in the melting zone was smaller than HAZ. Because it estimated that cooling rate of laser surface melting process is faster than water quenching.