Welding Filler Material Components for Reduction of Fumes and Hexavalent Chromium Generation from Shielded Metal Arc Welding and Flux Cored Arc Welding

정선교 서울대학교 대학원 2022 국내석사

Welding generates welding fumes and hexavalent chromium, which are classified as Group 1 carcinogens by the International Agency for Research on Cancer (IARC). In particular, due to the generation of high hexavalent chromium and fumes in shielded metal arc welding (SMAW) and flux-cored arc welding (FCAW), they impose a severe health risk upon exposure. Thus, this study aims to estimate the welding filler material components that can reduce the generation of fumes and hexavalent chromium in SMAW and FCAW. In the current study, nine welding rods for SMAW and eight flux-cored wires for FCAW were tested. Each type of welding was performed under uniform conditions in a fume-hood. Collected fume samples were analyzed by gravimetric analysis to calculate fume generation rate (FGR) and ion chromatography with the ultraviolet detection (IC-UV) for hexavalent chromium generation rate (HCGR). Welding filler materials were analyzed using wavelength dispersive X-ray fluorescence spectrometer (WDXRF). After performing statistical difference tests, a correlation analysis was conducted to estimate the statistical association between the generation rate and the content of filler component in the welding material in each type of welding. Based on the results of the correlation analysis, regression models were designed and then analyzed through multiple linear regression method. Finally, based on the results of correlation and multiple linear regression analyses, the component-combination formulas were designed and correlation analysis was conducted with fume generation rate and hexavalent chromium generation rate. For nine SMAW welding rods, FGR(per welding time) was in the range of 198.0–289.3 mg/min, and HCGR(per welding time) was in the range of 5.34–7.98 mg/min. By changing the welding filler material components under the same welding conditions, the generation rate was found to be reduced by approximately 26.7% (AVG = 20%) and 24.8% (AVG = 3.4%) compared to base FGR and HCGR, respectively. In the case of eight flux-cored wires, FGR was 590.4–821.1 and HCGR was 0.34–3.31 mg/min, which could be reduced by up to 23.5% (AVG = 10%) and 89.7% (AVG = 47.1%), respectively, by changing the welding material components under the same welding conditions. The results of correlation analysis of SMAW, with different elements as filler material, suggested a statistically significant correlation of fluorine (F), potassium (K), calcium (Ca), and sodium (Na) with FGR and chromium (Cr) and titanium (Ti) with HCGR. Whereas, in the case of FCAW, fluorine (F), potassium (K), and sodium (Na) with FGR and sodium (Na), potassium (K), silicon (Si), zirconium (Zr), and fluorine (F) with HCGR showed a statistically significant correlation. In most multiple linear regression models, the multicollinearity problem arises due to the interference among independent variables. That is, some specific elements did not strongly contribute to the change in the value of the dependent variable, and several elements made complex contributions in the fume and hexavalent chromium generation rate. So, this study proposed eleven component-combination formulas showing statistically significant correlation with dependent variables for SMAW and ten for FCAW. This study suggests that it is possible to reduce FGR and HCGR without affecting the performance of welding by using different components as welding materials. In order to reduce HCGR, it is recommended to reduce the FGR for SMAW and to reduce the content of hexavalent chromium in welding fumes for FCAW. Also, it is recommended to manufacture welding materials with components that can suppress oxidation of chromium and have higher electronegativity than metal chromium and chromium compounds. Thus, by considering the oxidation ability and electronegativity of the compound, HCGR can be reduced. If welding materials with low FGR and HCGR are manufactured and widely used in the field as per the suggested change in element content presented in this study, the problem of exposure to Group 1 carcinogens is expected to be fundamentally reduced. 용접 시에는 IARC 지정 Group 1 발암물질인 용접 흄과 6가 크롬이 발생하며, 선행연구에 따르면 전세계의 약 11,000만명의 근로자가 작업시간 동안 용접 흄에 노출된다 밝혀진 바 있다. 이러한 용접 흄과 6가 크롬은 용접 종류, 용접 조건, 환경영향, 용접 재료 등의 다양한 요인에 의해 발생 특성이 다르다. 많은 선행연구들을 통해서는 용접의 종류별 용접 흄 및 유해인자의 발생 특성에 대해 연구된 바 있고, 용접 전류, 전압 등의 용접 조건에 따라 용접 흄과 6가 크롬의 발생의 변화에 대해 기술한 바 있다. 하지만, 작업현장 내에서는 용접 시 용접의 성능이 가장 중요하며, 이러한 용접의 성능 유지를 위해 용접 조건을 변경하지 못하는 경우들도 더러 존재한다. 하지만, 이러한 용접 재료의 화학적 성분 함량에 따라 발암물질인 용접 흄과 6가 크롬의 발생이 어떻게 변하는 지 기술한 연구는 적다. 이에 따라, 본 연구에서는 시장 점유율이 높은 피복아크 용접과 플럭스 코어드 아크 용접을 대상으로 용접재료 성분을 달리하여 제조한 용접 재료들의 용접 시 흄 및 6가 크롬 발생량을 평가하여 통계적 추정을 통해 흄, 6가 크롬 저감에 영향을 끼치는 용가재 성분을 추정하고자 한다. 자체 성능 평가 기준을 만족하는 피복아크 용접봉 9 제품, 플럭스 코어드 와이어 8 제품에 대해 용접 종류별 통일된 용접 조건 하에서 용접을 실시 후, 흄을 포집했다. 포집된 흄은 중량 분석을 통해 흄 발생량을 산출하고, IC-UV/vis로 6가 크롬 함량을, 용접 재료에 대해 XRF로 성분을 분석하였다. 용접재료 내 각 화학적 성분들의 함량, 흄 발생량, 흄 중 6가 크롬 함량 및 6가 크롬 발생량을 산출한 후, 이들의 기존 제품 대비 값의 변화에 대한 통계적으로 유의한 차이가 있는지 검정하였다. 그 뒤, 흄 발생량, 흄 중 6가 크롬 함량 및 6가 크롬 발생량을 각각 종속변수로, 용접 재료 중 특정 성분 함량을 독립변수로 상관분석을 통해 흄, 6가 크롬 발생에 영향을 끼치는 용가재 성분을 통계적으로 추정하였다. 상관분석 결과를 바탕으로, 용가재에 일정 이상의 함량으로 포함된 성분에 대해 다중회귀분석을 위한 회귀모형을 설계해 분석하였다. 마지막으로, 통계적 차이를 보인 성분들 및 선행연구들을 통해 제시된 성분들, 개별원소간 상관분석 결과와 다중회귀분석 결과들을 바탕으로 용접재료 내 성분 배합 수식을 설계해 각 종속변수와 상관분석을 진행하였다. 9 종류의 피복아크 용접 제품의 용접시간당 흄 발생량은 198.0-289.3 mg/min으로 기존 대비 최대 26.7%, 평균 20%의 저감이 가능했다. 용접시간당 6가 크롬 발생량은 5.34~7.98 mg/min으로 기존 대비 최대 24.8%, 평균 3.4%의 저감이 가능했다. 8 종류의 플럭스 코어드 아크 용접 제품의 용접 시간당 흄 발생량은 590.4~821.1 mg/min으로 기존 대비 최대 23.5%, 평균 10.8% 저감이, 6가 크롬 발생량의 경우 0.34~3.31 mg/min으로 기존 대비 최대 89.7%, 평균 47.1% 발생량 저감이 가능했다. 또한, 피복아크 용접과 플럭스 코어드 아크 용접재료의 원소 함량별 흄 발생량과의 상관분석 결과, 피복아크 용접의 경우 F, K Ca, Na가 용접시간당 흄 발생량과, Cr, Ti가 용접시간당 6가 크롬 발생량과 통계적으로 유의한 수준으로 상관성을 보이는 것을 확인할 수 있었다. FCAW의 경우 F, K, Na 함량이 용접시간당 흄 발생량과, Na, K, Si, Zr, F가 용접시간당 6가 크롬 발생량과 통계적으로 유의한 상관성을 보이는 것을 확인할 수 있었다. 다중회귀분석 결과는 대부분의 회귀모형에서 독립변수 간의 간섭에 의한 다중공선성 문제가 도출되어 회귀모형의 적합도가 떨어지는 것으로 추정되었다. 이에 따라, 본 연구에서는 흄 발생량, 흄 중 6가 크롬 함량, 6가 크롬 발생량과 통계적으로 유의한 수준의 상관성을 보이는 성분 배합 수식을 피복아크 용접에 대해 11가지, 플럭스 코어드 아크 용접에 대해 10가지를 제시하였다. 종합적으로, 성분을 다르게 제조한 용접재료들에 대해 성능적 결함 없이 흄 발생량, 6가 크롬 발생량을 저감할 수 있다는 것을 증명하였다. 또한, 피복아크 용접의 경우 6가 크롬 발생량을 저감하기 위해서는 흄 발생량을 줄이는 것이, 플럭스코어드 아크 용접의 경우 6가 크롬 발생량을 줄이기 위해서는 흄 중 6가 크롬의 함량을 줄이는 것이 용이하다. 마지막으로, 용접 간 산화작용을 통해 발생되는 6가 크롬 발생을 저감하기 위해 용접 재료 제조 시 화합물들의 산화수, 전기음성도 등을 고려하여 금속 크롬 및 크롬 화합물보다 전기음성도가 높아 크롬의 산화를 억제하는 물질들로 용접재료를 제조하는 것을 권장하는 바이다. 본 연구에서 제시한 원소 함량변화에 따른 흄 발생량, 6가 크롬 발생량 식을 참고하여 흄 발생 및 6가 크롬 발생이 적은 용접 재료를 제조하여 현장에서 널리 사용된다면, 근로자의 1급 발암물질에 대한 노출 문제를 근본적으로 개선할 수 있을 것으로 기대되는 바이다.

Fatigue Design and Reliability Assessment on the Weld of Ni-base Alloy 617

박영수 성균관대학교 일반대학원 2015 국내박사

Alloy 617, Ni-base heat resistant and corrosion resistant material, was the target material for this study and has drawn much attention as a candidate material for A-USC thermal power generation whose creep strength was more than 220 MPa in 700℃ or higher to increase thermal power generation efficiency and reduce CO2 gas emission. The objective of this study was to obtain basic information with high reliability on the weld by developing a new welding method for Ni-based Alloy 617 materials and evaluating the welding strength and environmental strength and also based on the development, we intended to develop a new technology of fatigue design. This thesis focuses on acquiring basic information of welding technology development, weld-ability of material, mechanical properties of welding, corrosion fatigue characteristics and developing fatigue design technologies based on the above information with respect to Ni-based Alloy 617, which has been an interest as a candidate of the A-USC thermal power plant. And Acceleration life prediction and reliability evaluation based on neural network and acceleration life theories in addition to welding design development.

P92강과 alloy617 이종재료 용접부의 잔류응력해석 및 고온피로강도 평가

김태민 성균관대학교 일반대학원 2016 국내석사

우리나라는 전기 생산의 약 60% 이상을 화력발전에 의존하고 있고, 세계적으로도 전체 발전량의 대부분이 화력발전이기 때문에 화석연료를 주 원료로 하는 화력발전으로부터 발생하는 배기가스는 대기오염(air pollution)과 지구온난화의 주 원인으로 지목되어오고 있다. 이러한 문제를 개선하기 위한 가장 현실성 있는 방안은 화력 발전의 증기 온도를 현재보다 더욱 높여 발전 효율을 증대함으로써 배기가스 량을 줄이고 환경오염을 개선하는 것이다. 현재 선진국에서는 화력발전 증기터빈의 주 증기온도가 700℃이상에서 크리프강도(creep strength)가 220MPa 이상인 초초 임계급 화력 발전 기술 개발을 위해 다양한 연구가 진행 중에 있다. 주 증기 온도 700˚C이상인 초초 임계급 화력발전 기술 개발의 핵심은 온도 700℃이상에서 크리프강도가 220MPa 이상인 고온 내열, 내식 합금소재 개발과 응용 기술이다. 그러나 초초 임계급 차세대 화력발전의 발전 효율 증대를 위해 관심과 노력을 기울이고 있는 국내의 경우 현재로서는 소재 개발과 응용 기술 수준이 선진국에 비해 미흡한 실정이다. 고온 소재를 증기터빈(steam turbine) 등에 적용하기 위해서는 동종 및 이종 재료 용접기술(similar and dissimilar material welding technology)을 개발하고, 용접성(welding ability), 용접부(weld)의 기계적 성질(mechanical properties), 용접 야금학적 특성(metallurgical characteristics), 용접 잔류 응력(welding residual stress), 용접부 피로강도(welding fatigue strength) 등에 관한 기초정보가 확보되어야 한다. 따라서 본 연구에서는 차세대 초초 임계급 화력 발전용 고압단 후보 소재로 관심을 얻고 있는 P92강과 Ni기 Alloy 617을 대상으로 해서 이종 재료 용접기술(dissimilar material welding technology)을 개발하고, 용접부의 잔류 응력 해석 및 기계/금속학적 특성을 평가함으로서 이종 재료 용접부의 신뢰성에 관한 기초정보를 확보함으로써 신뢰성을 고찰하고자 하였다. Targeting Cr-based P92steel and Ni-based Alloy617 that are gaining attention as next-generation high pressure stage candidate materials for ultra super critical thermal power generation because their creep strength is over 220MPa at over 700℃, this research has been performed in the following ways for reducing CO₂emission and increasing thermal power generation efficiency. 1) Dissimilar material welding between P92steel and Alloy617 DCEN (Direct Current Electric Negative) TIG (Tungsten Inert Gas) welding technology was applied to dissimilar material welding between P92 steel and Alloy617. With monitoring system, the optimum welding condition of dissimilar material welding between P92steel and Alloy617 was deduced through various welding process applications and the welding groove shape was designed as U-groove. Especially, buttering welding technology was applied on this research for better dissimilar weldability. 2) Metallurgical Analysis on Weld Zone In dissimilar welding, different ingredients are diluted in a molten pool by convection. in this research, buttering welding was antecedent on P92. Because redistribution of ingredients and metallurgical micro-structure change were expected, ingredient analysis and metallurgical micro-structure observation were progressed. For metallurgical analysis on weld zone, micro-structure analysis was progressed by using SEM (Scanning Electron Microscope) and OM (Optical Microscope) and ingredient analysis was progressed by using EDS (Energy Dispersive x-ray Spectroscopy). 3) Welding Residual Stress Analysis of Dissimilar Material Welding Welding residual stress analysis of dissimilar material welding is very important for drawing design information of welding construction. In this research, heat transfer analysis and thermal stress analysis were connected by numerical analysis program, ABAQUS, and 3D model-based welding process was postulated to perform welding residual stress analysis. To improve reliability of the analysis value, the weld zone was etched to be modeled same as formed bead by multipass layer welding. 4) Mechanical Characteristic Evaluation of Weld Zone between P92steel and Alloy617 Mechanical characteristic of dissimilar material weld zone between P92steel and Alloy 617 at room temperature to high temperature has been comparative evaluated. By evaluating fatigue strength of the weld zone with purposing the 700℃ environment which is real A-USC running condition, especially, temperature dependent load and S-N Curve forward delivery related fatigue life was aimed to be drawn.

Development of CNN and Computer Vision Based Realtime Individual Welding Spark Detection Method to Avoid Fire Hazards in Construction Environments

진시 서울대학교 대학원 2023 국내박사

본 연구는 건설 현장에서의 용접 안전을 강화하기 위해 개별 용접 스파크를 자동으로 감지하는 방법을 소개합니다. 용접은 건설 현장에서 흔히 사용되는 공정이지만, 이로 인해 빛, 연기, 먼지, 열, 그리고 용융 금속 스파크가 발생하여 화재 재해의 위험이 있습니다. 이러한 문제를 해결하기 위해 기존의 건설 안전 계획에 추가적으로 행동 기반 안전 관리(BBS)를 도입합니다. BBS 는 작업자의 불안전한 행동과 관련된 건설 사고를 감소시키는데 기여합니다. 이 방법은 불안전한 행동을 식별하고, 일정 기간 동안 건설 현장에서 행동을 관찰한 후 원하는 행동에 대한 긍정적인 피드백과 불안전한 행동에 대한 부정적인 피드백을 제공하여 안전성을 향상시킵니다. 기존의 행동 기반 안전 방법은 수동 관찰에 의존하여 신뢰성과 정확성이 떨어지는 문제가 있었습니다. 제안하는 방법은 개별 용접 스파크를 정확하게 감지하고 위치와 관련 정보를 추출할 수 있도록 하였습니다. 제안하는 방법은 기존의 PPHT 기반 용접 화염 감지 방법과 CNN 및 CCTV 기반 실시간 건설 화재 감지(RCFD) 시스템과 비교하면 더 우수한 성능을 보입니다. 딥러닝 개체 감지와 유사한 출력도 갖지만 class confidence score 없어서 피쳐 엔지니어링 개체 검출과 딥러닝 개체 검출을 비교하는데 어려움이 있습니다. 이를 극복하기 위해 제안하는 API mfAP 를 도입하여 정확율 성능을 평가합니다. 실제 건설 현장에서는 제안하는 API 를 설치하여 용접 안전을 모니터링하고 용접 안전 뷰어를 보조할 수 있습니다. 또한 API 는 여러 용접 활동을 동시에 모니터링하여 효율성을 높일 수 있습니다. 결론적으로, 제안된 방법은 개별 용접 스파크를 정확하게 감지하고, 화재 재해를 방지하기 위한 중요한 지표인 용접 불꽃 비행 궤적 인디케이터와 스테이 브라이트 인디케이터를 제시합니다. 이러한 방법은 건설 현장의 용접 작업 안전성을 향상시키는데 큰 도움이 될 것으로 기대됩니다. Welding is a widespread and vital process at construction sites. Welding is a way to join two pieces of steel. The process will generate a large amount of light, smoke, dust, heat, and molten metal sparks. So the welding process can easily lead to fire disasters at construction sites. In recent years, South Korea and China have had big construction site fire disasters caused by welding. Welding can also hurt surrounding people, including welders and people who walk around. Welding-generated dust can cause lung damage. Welding intense light can damage eyes, and welding sparks can burn people's skin. Many researchers try to study these issues using behavior-based safety management and construction safety plans. Traditionally a construction safety plan is used to minimize the safety risks of welding flames. Generally, construction site safety plans are expected to form a safe climate. In China, Canada, and Australia, a traditional method of the fire safety plan is established as onsite fire management before the construction of buildings. A typical fire safety plan includes the roles and responsibilities of fire management, construction site layout, combustible material, emergency procedures, and evacuation plan (China State Construction Engineering. 2011; FWPA 2014; RMWB 2014). However, this kind of plan heavily relies on individual activities and engagement, which results in delays and less reliability. Behavior-based safety (BBS) management immediately contributes to the safety performance of construction welding. More than 80% of construction accidents are related to workers' unsafe behavior (H.W. Heinrich et al. 1980). According to Fam, et al. (2012), unsafe behavior is enacted when an employee does not respect safety rules, standards, procedures, instructions, and specified project criteria. BBS improves safety performance by 1) identifying unsafe behaviors 2) observing construction site behaviors over some time, and 3) providing positive feedback on desired behaviors and negative feedback on unsafe behaviors (Choudhry, R.M, 2014; Li, H., et al, 2015; Zhang, M., et al, 2013). However, conventional behavior-based safety methods rely on manual observation, which is time-consuming and labor-intensive. Seo, J. et. al. 2015 published that continuous monitoring of unsafe conditions and actions is essential to eliminate potential hazards promptly. The main limitation of current BBS methods is that they rely too much on manual observation, and hence many unreliable data records occur. To address such limitations, this dissertation proposed an automatic individual welding sparks detection method to study the activity of individual welding sparks. The proposed method is better than Chen, W. et al. 2020 proposed Progressive Probabilistic Hough Transform (PPHT) based welding flame detection method because it can detect welding sparks individually and know their position and other information. Also, the proposed method is better than Su, Y. et. al. 2021 published a CNN and CCTV-based real-time construction fire detection (RCFD) system. Because Su's system is designed for fire detection, including welding flame situations, the proposed method first uses an HSV color threshold to generate a color mask that only displays welding sparks. Then contour detection of welding sparks is processed. This method has specifically designed APIs to convert contour detection of individual welding sparks to the form of a bounding box (x, y, w, h). In this way, the proposed method has similar output as deep learning object detection but without class and confidence scores. So the proposed method can compare with a deep-learning object detector such as Yolo. However, the current mAP standard is hard to compare feature engineering object detection and deep learning object detection. So a helper API mfAP is proposed to make the comparison. Finally, two indicators are proposed using the individual welding sparks detection result. One is welding sparks flying trajectory indicator. Another one is welding spark hitting the ground and staying bright indicator. With these two indicators, we can monitor welding sparks' safety performance. To validate the proposed method, the author uses the same dataset to test individual welding sparks contour detection to train a Yolo customized object detector with sophisticated training parameters. Then compare the proposed individual welding sparks contour detection performance with Yolo's customize sparks object detector. As the proposed individual welding sparks contour detection can not use mAP to evaluate the performance, the proposed individual welding sparks contour detection uses the proposed mfAP to evaluate the performance and then compare it with the mAP result of the Yolo customized welding sparks object detector. After evaluating individual welding sparks detection, the author evaluates the performance of the welding sparks flying trajectory indicator. The way to evaluate the trajectory indicator is by comparing the gap between the prediction polynomial function and real welding sparks trajectory. The second indicator monitors whether welding sparks hit the ground and do not die for a while. Cause this phenomenon is related to construction fire disasters. The proposed API can monitor the average Y position of all individual welding sparks detection in a welding video. Then the API plot the average Y position of all welding sparks. We can see a mountain shape. So the API can monitor the hitting ground behavior by looking for the mountain shape pattern of the average Y position of all individual welding sparks detection. After the API reads the mountain pattern, the API counts the duration of the welding sparks' position that stays at the exact location for a while. In conclusion, the proposed method can do individual welding sparks detection precisely. Welding sparks flying trajectory indicators can correctly monitor contact with people or combustible materials at the construction site. The bright stay indicator can monitor the sparks hitting the ground and staying bright for a while. This is a critical issue for construction site fire disasters. Compared with Chen, W. et al 2020 proposed a Progressive Probabilistic Hough Transform(PPHT) based welding flame detection method, the proposed method can detect individual welding sparks and useful output information to other indicators development. Compared with Su, Y. et. al. 2021 published a CNN and CCTV-based real-time construction fire detection (RCFD) system, the proposed system studies explicitly the welding sparks that led to fire disasters. In real construction sites, the proposed APIs can be installed on the server of site CCTVs. So, the proposed system can monitor welding safety and assist a welding safety viewer. It also can increase efficiency as the system can monitor multiple welding activities simultaneously.

Fundamental Studies on Welding-induced Distortion in Thin Plates

한명수 The Ohio State University 2002 해외박사

The welding process, which is the most efficient and common joining technology in fabricating metallic structures, however, can cause various problems due to a nonlinear temperature distribution generated during the operation. Welding-induced distortion is one of the crucial detriments that go against fabrication accuracy. As higher strengthened materials and aggressive design optimization in recent years have promoted a wide employment of thinner plates in structures, welding of those thin plates causes extensive out-of-plane distortion such as angular and buckling distortion. Numerous efforts have been devoted to the investigation of welding distortion. Remarkable developments in computer technology in recent years have improved numerical prediction capability for welding distortion. Application to welding production is, however, prone to be unsatisfactory since the feasible three-dimensional analysis procedure for large spatial welded structures can not easily be found due to extensive computation time and limited resources available. Nevertheless, it is generally accepted that welding distortion is caused by inherent strains accumulated during welding, and it can be predicted if one can approximate the strains distributed in and around the weld region. In this dissertation, a fundamental study on the prediction and control of inherent strain fields was conducted through correlating inherent strains with welding process variables and the degree of constraints in order to develop comprehensive predictive methodologies for welding-induced angular and buckling distortion in thin plates. Experimental investigation was also performed to verify the numerical procedure for welding distortion analysis. Evolutional variation of longitudinal distortion was observed through numerically obtained transient stress and displacement diagrams. Thermal management techniques previously reported as effective control schemes for welding-induced buckling distortion were experimentally and numerically inspected to evaluate their applicability toward distortion mitigation planning for large welded structures. Significant findings observed through the study are summarized as follows: 1) Magnitudes of longitudinal inherent strains were uniquely determined by the peak temperatures and the nil-plasticity peak temperature as coupled parameters representing welding process variables and the degree of constraints. 2) The effective inherent strain responsible for angular distortion was produced under non-uniform temperature distribution through the thickness, and it was estimated by adding halves of longitudinal plastic strains to transverse inherent strains. 3) The shrinkage volume model considering effective inherent strains reasonably predicted magnitudes of angular distortion. 4) The onset of longitudinal distortion was strongly influenced by transient temperature fields. The more intensified thermal field was required for the onset in the smaller plate subjected to the smaller heat input. The growth of longitudinal distortion in small plates occurred for a limited cooling period. Longitudinal distortion in large plates continued to grow from the onset to the completion of the cooling cycle. 5) Eigenvalue analyses adopting the normalized inherent strain distribution as the perturbation load pattern resulted in the critical curve for the buckling instability. The buckling instability was manifested as either plate size or heat input increases. Buckling instability predicted by eigenvalue analyses was closely related to the longitudinal distortion continuing to grow until the completion of the cooling cycle. 6) Effectiveness of distortion mitigation techniques could be evaluated by their capability of controlling inherent strain fields. The accelerated cooling technique significantly reduced the inherent strain region, which in turn remarkably decreased the magnitude of compressive residual stresses directly influencing the buckling instability.

A methodology for predicting the phase fraction and microhardness of welded joints using heat transfer analysis and integrated modeling

송지효 서울대학교 대학원 2024 국내박사

Repeated heating and cooling during welding results in diverse microstructures and mechanical properties, which makes experimental analysis difficult. This study presents a methodology for predicting the phase transformation and fraction and microhardness at every location of welded joints. The methodology consists of the following steps: construction of an austenite decomposition database (AD data), heat transfer analysis, prediction of essential parameters, and prediction of the phase fraction. The AD data encompassed a comprehensive range of prior austenite grain size (PAGS) and t8/5 (cooling time between 800 and 500°C) values. The heat transfer model was developed to simulate the heat transfer during welding, obtain the thermal cycles, and calculate the critical parameters (austenite fraction, PAGS, and t8/5). Finally, the phase fraction was predicted by combining the AD data and parameters and the microhardness was calculated by considering microhardness value of each phase. This methodology, initially developed for single pass welding, has been successfully extended to multi pass welding. The developed methodology was refined to consider the effects of the PAGS and the phase fraction formed during the previous welding. This methodology was applied to single pass and multi pass welding. In both cases, the microhardness values were compared with the experimental results, and good agreement was observed, validating the precision of the calculated phase fraction and the reliability of the methodology. The results show that the methodology can provide valuable insights into phase transformation during welding. Furthermore, this study highlights the potential of this methodology to characterize and optimize the properties of multi pass welded joints under various welding conditions. 용접 시 반복적인 가열과 냉각은 용접부에 다양한 미세조직과 기계적 특성을 형성시키며, 이는 용접부의 특성을 실험적으로 분석하기 어렵게 한다. 본 연구에서는 용접부 모든 위치에서 상변화 거동과 상분율, 경도를 예측하는 방법론을 제시한다. 이 방법론는 오스테나이트 분해 데이터베이스(Austenite decomposition database, AD data)구축, 열 전달 해석, 상변화 변수 예측, 상분율 및 경도 예측 과정이 포함된다. 오스테나이트 분해 데이터베이스는 넓은 범위의 오스테나이트 입자 크기 (Prior austenite grain size, PAGS) 및 t8/5 (800도에서 500도까지의 냉각 속도) 값을 포함하므로 다양한 용접 조건에서 정확한 상 예측을 가능하게 한다. 열전달 모델을 활용해서는 용접 시 용접부에서의 열전달 현상을 해석해 온도이력을 얻는다. 이 온도 이력은 상변화 예측 모델에서 중요한 변수(오스테나이트 분율, PAGS 및 t8/5)를 계산하는데 활용된다. 최종적으로 오스테나이트 분해 데이터베이스에 상변화 변수를 적용해 상분율을 예측하며 예측된 상분율과 각 상의 경도 값을 결합하여 경도를 예측한다. 연구 초기 단일 패스 용접부 예측을 위해 설계된 방법론은 다층 패스 용접부에 활용될 수 있도록 발전되었다. 다층 패스 용접부 예측을 위해 발전된 방법론은 반복적인 용접 상변화 과정을 고려하였으며, 이전 용접 시 형성된 PAGS 및 상분율의 영향을 방법론에 적용하였다. 본 연구에서 제시한 방법론의 검증을 위해 단일 및 다층 패스 용접부에 초기 방법론 및 확장된 방법론을 각각 적용하였다. 단일 패스와 다층 패스 모두에서 예측한 경도값이 실험 결과와 일치하였으며, 이는 예측된 상분율 뿐만 아니라 초기 방법론 및 확장된 방법론의 신뢰성을 증명한다. 또한 이 방법론은 용접 시 상변화에 대한 중요한 이해를 제공하며, 다양한 용접 조건에서의 용접부 접합 특성을 효과적으로 분석하는데 활용될 수 있음을 보여준다.

LASER WELDING OF ZINC-COATED AND UNCOATED STEEL SHEETS AT ATMOSPHERIC AND SUBATMOSPHERIC PRESSURES

Jaehun Kim Graduate School of UNIST 2017 국내박사

In the automotive industry, zinc-coated steel is widely used because of its high corrosion resistance. Many automotive industry companies have tried to employ laser welding because of its many benefits, such as low heat input, high-intensity heat source, minimal distortion in heat affected zones, and high productivity. In lap joint laser welding of zinc-coated steel sheets, a proper gap needs to be maintained to avoid weld defects in weldment because the zinc vaporization temperature (1180 K) is lower than the steel melting temperature (1809 K). However, in this case, additional processes are required for application to actual industrial production lines, and it is difficult to precisely control the gap. Furthermore, although many researchers have investigated ways to mitigate the influence of high zinc vaporization pressure, it remains an issue because of erratic and unstable keyhole motion and melt pool behavior. Therefore, the purpose of this dissertation is to investigate the keyhole behavior and weldability of zero-gap laser welding of zinc-coated and uncoated steel sheets at atmospheric and subatmospheric pressures according to process parameters to develop the gap insensitive lap joint laser welding of zinc-coated steel. In this dissertation, firstly, a scaling law for predicting penetration depth was proposed, because the determination of penetration depth is the first consideration before the welding process. Moreover, then precisely observation method and analysis method were developed to observe clearly keyhole behavior, and effect of relative configuration of the laser beam and keyhole geometry on weldability for zero-gap lap laser welding of zinc-coated steel sheets. Also, the influence of ambient pressure on keyhole behavior and weldability were investigated to find solutions and possibilities for obtaining good welds for zero-gap lap laser welding of zinc-coated steel sheets by adjusting processing parameters (i.e. laser intensity and welding speed and ambient pressures).These studies can be summarized as follows. Firstly, a scaling law for predicting penetration depth was proposed that can be applied to both conduction mode and keyhole mode laser welding. The proposed scaling law was formulated based on a simple one-dimensional heat conduction model, and the effect of multiple reflections was accounted for. Because the scaling law was obtained from a laser heating problem, its physical meaning and why it needs to be formulated that way can be clearly explained. Experiments were conducted, and the obtained results were found to be in good agreement with the proposed scaling law. Secondly, in order to observe the keyhole behavior and reconstruct the keyhole geometry, a coaxial observation method was developed using a high-speed camera. A coaxial observation is a more useful and precise method to observe keyhole behavior than other lateral observation methods, and it was possible to study how the keyhole shape changes as the process parameters are varied. This chapter investigated the overall differences in the keyhole geometry between the zinc-coated and uncoated steels over a large process parameter space. Thirdly, using the obtained keyhole geometry data, the effect of keyhole geometry and dynamics on weldability was investigated by defining several key factors. It was found that the relative configuration of the keyhole and the laser beam is the most influential factor for obtaining good welds. For the zinc-coated steel, good welds were obtained at low welding speeds even zero-gap lap joint laser welding of zinc-coated steel sheets. Finally, based on the observation and analysis method from previous chapters, we investigated the laser welding of zinc-coated steel at subatmospheric pressures in order to compare between laser welding at atmospheric pressure and subatmospheric pressure. The purpose of this work is because the pressures inside the keyhole play a major role in weldability during zero-gap lap joint laser welding of the zinc-coated steel sheets. In this chapter, the main focus was to reconstruct time-averaged 3-D keyhole shapes and studying the influence of ambient pressures on keyhole behavior and weldability.

Gravitational effects on weld pool shape and microstructural evolution during gas tungsten arc and laser beam welding on 304 stainless steel, Ni, and Al-4 wt.% Cu alloy

강남현 Pennsylvania State University The Graduate School 2003 해외박사

본 연구는 Ni 200 합금 (99.5% Ni), 304 stainless steel, Al-4Cu 합금에서의 레이저 및 Gas Tungsten Arc Welding(GTAW)시 중력의 영향을 조사하였다. 중력벡터의 크기와 방향에 따른 용접부 형상, 미세조직 및 solute distribution의 거동을 연구하였다. 중력실험은 미국항공우주국(NASA) KC-135 비행기 실험을 통하여 무중력에서 1.8g까지의 중력크기 실험을 수행하였고, 중력방향에 따른 용접방향을 변화시키기 위한 ground 실험을 병행하면서 중력방향의 영향을 조사하였다. 중력벡터의 방위에 따른 실험은 용접 속도와 중력벡터의 방향을 체계적으로 변화시키면서 다음 3가지 형태의 용접 방식이 행하여졌다: welding upward in the direction opposing gravity (vertical-up 또는 ll-U weld), welding downward in the direction of gravity (vertical-down 또는 ll-D weld), and welding perpendicular to the direction of gravity (perpendicular 또는 ⊥ weld). 중력의 크기 및 방향에 따른 용접부 형상의 거동을 조사하여 “Stable" 용접부와 "Unstable" 용접부로 나눌 수 있었다. 일반적으로 높은 아크전류의 translational GTAW 공정에서 낮은 전류의 spot GTAW 공정에 비하여 중력방향에 따른 용접부 형상의 변화가 심하였다. 용접부 단면적은 용접부 형상의 안정성을 결정하는데 있어서 제1인자가 되지는 못했다. 304 STS GTAW의 "Stable" 용접부의 경우, ll-U 용접부는 용접풀 하단에서 less convexity 및 more free surface depression 현상을 보였다. 이로 인하여 ll-D 용접부에 비하여 10-20% 깊은 용입을 ll-U 용접부에서 볼 수 있었다. 용접풀 가운데에서의 용입 변화 이외에는 전체적인 용접부 형상에서 중력의 영향은 크지 않았다. 용접부의 크기가 커질수록 중력방향에 따른 용접부 형상의 차이가 많이 발생하였으며 이러한 상태를 "Unstable" 용접부로 규정하였다. 304 STS GTAW의 경우 ll-D 용접부를 ll-U 용접부와 비교하면 31% 용입 증가 및 28% 용융폭 감소를 보였다. 이러한 용접부 형상의 변화는 free surface deformation 및 convection flows 현상이 중력 방향에 따라 다르게 반응을 하였기 때문으로 생각한다. 중력방향에 따른 미세조직 거동을 조사하기 위하여 합금의 주요성분의 밀도차가 큰 Al-Cu 합금을 선택하여 GTAW를 수행하였다. 니켈 및 304 STS 경우와 같이 용접부 형상은 중력방향의 영향을 많이 받음을 알 수 있었다. 용접부 형상이 변한다는 것은 용접풀 내부의 convection flows의 변화를 의미하는 것으로 미세조직의 크기 및 성장방향을 결정하는 응고조직 앞에서의, 즉 solidification front에서의 응고속도(Vs) 및 열구배(GL)에 영향을 준 것으로 조사되었다. 응고방향, 미세조직 형상 및 primary dendrite arm spacing(λ1)은 ll-D 용접부의 경우 ll-U 용접부에 비하여 다른 현상을 보였다. ll-D 용접부 미세조직은 용접풀 표면에서 columnar 구조를 보였고, Vs의 굴골부를 용접풀의 가운데에서 발견하였다. 이는 용접풀 free surface의 변화에 따른 solidification front의 영향으로 설명이 가능하였다. 이러한 용접부 형상 및 미세조직에 대한 중력의 영향을 연구한 결과는 International Space Station(ISS)를 건설하거나 파이프 용접 등 거대한 구조물과 같은 열악한 용접 환경시 용접부의 크기와 질을 조절할 수 있는 중요한 데이터가 될 수 있을 것이다. 또한 수치해석(Numerical analysis)을 통한 용접부 해석은 실험자의 눈으로 확인하기 힘든 물리적/화학적 이론 및 현상을 검증할 수 있을 뿐 아니라 예측까지 가능하게 하는 중요한 역할이 기대된다. The objective of the present work was to investigate effects of gravitational (acceleration) level and orientation on Ni 200 alloy (99.5% Ni purity), 304 stainless steel, and Al-4 wt.% Cu alloy during gas tungsten arc welding (GTAW) and laser beam welding (LBW). Main characterization was focused on the weld pool shape, microstructure, and solute distribution as a function of gravitational level and orientation. This was accomplished in part through NASA's KC-135 aircraft experiment to produce a range of the gravitational level (microgravity to 1.8 g_(o)) and through a ground-based experiment. Welding for the ground-based experiment was conducted by varying the welding direction with respect to the gravity vector, i.e., welding upward opposing gravity (Ⅱ-U weld) on a vertical weld piece, welding downward with gravity (Ⅱ-D weld) on a vertical weld piece, and welding perpendicular to gravity (⊥ weld) on a horizontal weld piece. The welds were divided into two classes, i.e., 'stable' and 'unstable' welds, in view of the variation of weld pool shape as a function of gravitational level and orientation. In general, higher arc current and translational GTAW produced more significant effects of gravitational orientation on the weld pool shape than the case of lower arc current and spot welding. Cross-sectional area (CSA) was a secondary factor in determining the stability of weld pool shape. For the 'stable' weld of 304 stainless steel GTAW, the Ⅱ-U weld showed less convexity in the pool bottom and more depression of the free surface, therefore producing deeper penetration (10-20%) than the case of Ⅱ-D weld. Except for the penetration in the weld pool center, overall weld pool shape remained mostly constant with respect to gravity. As the weld pool size became larger, the weld pool shape was changed significantly as a function of gravitational orientation, i.e., the 'unstable' weld. The Ⅱ-D weld of 304 stainless steel showed 31% deeper penetration, 28% narrower width, and more hemispherical shape of the weld pool than the case of Ⅱ-U weld. Variations of the weld pool shape were due to the different degree of free surface deformation and convection flows with respect to the gravitational orientation. For GTAW on 304 stainless steel, gravitational level variation from low gravity (LG≈1.2 g_(o)) to high gravity (HG≈1.8 g_(o)) caused 10% increase in width and 10% decrease in depth while maintaining the overall weld pool volume. Furthermore, LBW on 304 stainless steels showed mostly constant shape of weld pool as a function of gravitational orientation. These results were because the weld pool size was not large enough to exhibit the gravity effects on the weld pool shape. For the LBW, there were other reasons of insignificant gravity effects; ⅰ) The keyhole formation dominated the weld pool shape; ⅱ) The laser beam diameter was not significantly changed as a function of the direction perpendicular to the weld piece, while the diameter of gas tungsten arc might be changed significantly due to its diverging phenomenon from the small electrode to the large weld piece. GTAW on Ni showed similar trends of weld pool shape compared with GTAW on 304 stainless steel, i.e., the weld pool became unstable by showing more penetration in the Ⅱ-D weld for slower arc translational velocity (V_(α)) and larger weld pool size. However, the Ni weld pool shape had greater stability of the weld pool shape with respect to the gravitational orientation than the case of 304 stainless steel, i.e., higher current boundary and no humping. These results were due to the combination of greater surface tension/thermal diffusivity and smaller viscosity for Ni compared with the properties of 304 stainless steel. Regardless of the gravitational level, the ferrite content and the distribution of the solutes (Cr and Ni) remained constant for GTAW on 304 stainless steel. However, for GTAW on Al-4 wt.% Cu alloys, the gravitational orientation changed the weld pool shape associated with convection flows. This variation on the convection flows influenced the shape of the trailing solid-liquid (s-1) interface. Correspondingly, the solidification morphology and primary dendrite spacing (λ_(1)) were modified because the solidification rate (V_(S)) and thermal gradient (G_(L)) were affected by the convection flow. For this reason, larger λ_(1) for the Ⅱ-U weld was observed near the solid-liquid boundary and surface than that of the perpendicular and Ⅱ-D weld. The Ⅱ-D weld exhibited different solidification morphology, e.g., more columnar structure near the weld pool surface and abnormal 'S' shape of the solidification rate (V_(S)) during its growth. In summary, gravity influenced the weld pool shape that was associated with convection flows and weld surface deformation for specific welding conditions. The variation of convection flows and weld pool shape played a role in modifying (V_(S)) and (G_(L)). Solidification orientation and morphology were affected because (V_(S)) and (G_(L)) were changed as a function of gravity. Studies of gravity on the welding process are expected to play a significant role in the space-station construction and circumferential pipe welding on the earth.

열가소성 수지 저항용접의 응력분포 및 강도평가에 관한 연구

김철인 전북대학교 대학원 2008 국내석사

This research is concerned with a study on application resistance welding in Polypropylene(PP), Polystyrene(PS), Polyetyrene(PE) thermoplastic resin. The resistance welding is just can apply to thermoplastic resin. The failure strength of resistance welded joint is changed by welding factor like as current(power level), welding time(total energy), pressure etc. and another heat element factor like as thickness, number of element line, orientation etc. Tensile-shear tests were carried out with the single-lap specimen using Polypropylene(PP), Polystyrene(PS), Polyetyrene(PE) thermoplastic resin. The failure mechanism was discussed in order to explain the tensile-shear strength evaluation on welding conditions at resistance welding. The results are summarized as follows: 1. When shear-tensile load is applied on the single-lap specimen used resistance welding, maximum load mainly distribute on the edge of lap joint. It is confirmed by the experiment and it is as documents. 2. The shear-tensile strength of resistance welded joint increases according to increase of the base material strength, and the shear-tensile strength of resistance welded joint is successful by increase of the weld line, but the increase is not linear. 3. The shear-tensile strength of resistance welded joint increases according to increase of heat element's thickness, because the enlarged welding line makes good welding strength. The shear-tensile strength of resistance welded joint of diameter 0.5mm is higher than the shear-tensile strength of resistance welded joint of diameter 0.2mm, because the stress intensity of diameter 0.5mm thickness specimen is small and welding line of diameter 0.5mm thickness specimen is big by the result of FEM. 4. Welding zone's tensile-shear strength is larged by voltage and electrified time increasing. But if the voltage and electrified time is very larged, the welding strength is small.

Shearography를 이용한 6061-T6 마찰교반용접부의 strain 측정에 관한 연구

김안드레이 전북대학교 일반대학원 2013 국내석사

ABSTRACT Study on the strain measurement of 6061-T6 Friction Stir Weld joints using the Shearography Kim Andrey Dept. of Mechanical Design The Graduate School Chonbuk National University Friction Stir Welding(FSW) is an relatively new solid-state joining process which was developed at The Welding Institute UK in 1991. This joint method uses a new principle of stirring materials and the integration of non-consumable rotating tools. Because of the many advantages of this new method, things such as cracks or pores do not occur during melting or solidification and low welding deformation. By focusing on transportation equipment since the middle of 1990s many industries such as Rail Rolling Stock, Shipbuilding, Automotive, and Aerospace have made practical advances in welding technology and many have been recognized as revolutionary. Traditionally the welding of aluminum alloys in the MIG(Metal Inert Gas) or TIG(Tungsten Inert Gas) have been used by inert gas but because of the deformation of welded joints and cracks in welding of aluminum alloys many issues have emerged. Various development processes, characteristics and organization of joints have been used to perform considerable research, but still much remains unanswered to grasp a full understanding of joint principles and conditions. As FSW does not require any filler material, the metallurgical problems associated with it can also be eliminated and good quality weld can be obtained. FSW involves complex material movement and plastic deformation. Welding parameters, tool geometry, and joint design exert significant effect on the material flow pattern and temperature distribution, there by influencing the microstructural evolution of material. These tool geometry is the most influential aspect of process development. The design of the shoulder and the pin is very important for the quality of the weld. Clearly, different materials and different thicknesses will require different pin profiles. The variations in tool design are infinite and combinations of shoulder diameter, shoulder profile, pin length, diameter and profile, are all important parameters in determining the speed of welding and the quality of the finished weld. In this paper two different(threaded triangle and quadrangular pin profiles) tools were developed with excellent welding performance compared to the conventional threaded circular one to improve the weldability of the friction stir welded joints of aluminum plate according to the welding conditions. The rotation speed and traverse speed conditions were changed in this study, the other welding conditions are constant. Next purpose in this study was to measure the local strain of the FSW joints using Shearography and compare it with the tension test strain results. In the case of Shearography method has advantages that the inspection can be perfomed at a real time measurement and is more insensitive to environmental noise. Also, it uses simple optical configuration compared to other optical interferometers. Shearography is a laser-based technique for full-field, non-contacting measurement of surface deformation(strain or displacement), hence, it is a partical tool that can be used in a factory environment. An aluminum plate with a thickness of 6.0 mm was used in this investigation. The Shearographic technique was utilized to test FSW joint samples under tension loading condition. Shearography method results show the great possibility of measuring the strain of FSW joints by comparing it with the tension test strain results, and determine the effect of weld zone. Keywords: 마찰교반용접(Friction Stir Welding; FSW), 용접 툴(welding tool), 전단간섭계(Shearography), 변형율(strain), 비파괴 검사(Nondestructive measurement)