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Microhardness profiling of Ti-6Al-4V components repaired through multiple laser additive re-melt technique
摘要： Outstanding properties inherent to Ti-6Al-4V have made it become an aerospace metal work horse and a metal giant in the titanium industry where it constitutes about 60% of titanium alloy usage. However, these properties can be influenced by high temperature processes like laser additive manufacturing (LAM). LAM is a solid free-form fabrication and a materials and process parameters dependent technique that generates solid components from computer aided design (CAD) files by using a laser beam to locally melt the powder and the substrate. In this work, LAM was used to repair narrow rectangular cracks in Ti-6Al-4V plates of 99.6 % purity by depositing similar grade Ti-6Al-4V powder and incorporating multiple laser re-melt treatments in between the deposited layers using argon as shielding gas at controlled deposition and re-melt power, gas flow rate, laser spot size, powder feed rate and scanning speed. Multiple laser re-melt treatments were made, each after every two (2) deposition tracks to collapse any irregularly deposited layers present within the groove, densely fusing the melted powder with the substrate and this was maintained until the multiple deposited welds filled up the cracks. The present work therefore investigates the effects of the multiple laser re-melt treatments on the microhardness of the laser additive repaired components. The obtained results show that laser re-melt treatments induced reheat effects onto the welds which generated a fine α+β martensitic structure within columnar prior beta grains and consequently enhanced the microhardness properties in both the heat affected zones and the fusion zones of the welds made.
关键词： microhardness,re-melt,Ti-6Al-4V,laser,Additive
更新于2025-09-12 10:27:22
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Origin of Bypass Diode Fault in c-Si Photovoltaic Modules: Leakage Current under High Surrounding Temperature
摘要： Bypass diodes have been widely utilized in crystalline silicon (c-Si) photovoltaic (PV) modules to maximize the output of a PV module array under partially shaded conditions. A Schottky diode is used as the bypass diode in c-Si PV modules due to its low operating voltage. In this work, we systematically investigated the origin of bypass diode faults in c-Si PV modules operated outdoors. The temperature of the inner junction box where the bypass diode is installed increases as the ambient temperature increases. Its temperature rises to over 70 ?C on sunny days in summer. As the temperature of the junction box increases from 25 to 70 ?C, the leakage current increases up to 35 times under a reverse voltage of 15 V. As a result of the high leakage current of the bypass diode at high temperature, melt down of the junction barrier between the metal and semiconductor has been observed in damaged diodes collected from abnormally functioning PV modules. Thus, it is believed that the constant leakage current applied to the junction caused the melting of the junction, thereby resulting in a failure of both the bypass diode and the c-Si PV module.
关键词： diode junction melt,bypass diode of PV module,leakage current,temperature inside the junction box
更新于2025-09-12 10:27:22
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Analytical prediction of laser mediated polymer melt and damage width
摘要： Far-field (remote) laser net-shape scanning has revolutionary potential across numerous applications which involve localized heating of materials. It offers a very high degree of manufacturing flexibility in concert with process repeatability, traceability and low cycle energy usage when compared to traditional tooling-based solutions if the material response can be accurately predicted. The functional mechanism of such processes is localized heating; in this work, an analytical model of the line width of phase change occurring between a 3mm thick virgin polypropylene, PP, sheet and a visually transparent 25μm thick PP film is presented. Validation of the model is provided empirically by the scanned application of a CO2 laser exhibiting a Gaussian beam profile onto reference materials at varying incident spot diameters, powers and traverse velocities. This work is of value for process parameter prediction, as this analytically based method is computationally light, enabling its real-time implementation in manufacturing environments.
关键词： Net-shape,Width,Prediction,Melt,Polymer
更新于2025-09-12 10:27:22
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Computational Investigation of Melt Pool Process Dynamics and Pore Formation in Laser Powder Bed Fusion
摘要： In the laser powder bed fusion additive manufacturing process, the presence of porosity may result in cracks and significantly affects the part performance. A comprehensive understanding of the melt pool process dynamics and porosity evolution can help to improve build quality. In this study, a novel multi-physics computational fluid dynamics (CFD) model has been applied to investigate the fluid dynamics in melt pools and resultant pore defects. To accurately capture the melting and solidification process, major process physics, such as the surface tension, evaporation as well as laser multi-reflection, have been considered in the model. A discrete element method is utilized to model the generation of powder spreading upon build plate by additional numerical simulations. Multiple single track experiments have been performed to obtain the melt pool shape and cross-sectional dimension information. The predicted melt pool dimensions were found to have a reasonable agreement with experimental measurements, e.g., the errors are in the range of 1.3 to 10.6% for melt pool width, while they are between 1.4 and 15.9% for melt depth. Pores are captured by both CFD simulation and x-ray computed tomography measurement for the case with a laser power of 350 W and laser speed of 100 mm/s. The formation of keyholes maybe related to the melt pool front wall angle, and it is found that the front wall angle increases with the increase in laser line energy density. In addition, a larger laser power or smaller scanning speed can help to generate keyhole-induced pores; they also contribute to produce larger sized pores.
关键词： additive manufacturing,melt pool,computational fluid dynamics (CFD),stainless steel,discrete element method (DEM),keyhole
更新于2025-09-12 10:27:22
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Melt Pool Size Control Through Multiple Closed-Loop Modalities in Laser-Wire Directed Energy Deposition of Ti-6Al-4V
摘要： Sensing and closed-loop control are critical attributes of a robust 3D printing process, such as Directed Energy Deposition (DED), in which it is necessary to manage geometry, material properties, and residual stress and distortion. The present research demonstrates multiple modes of closed-loop melt pool size control in laser-wire based DED, a form of large-scale metal additive manufacturing. First, real-time closed-loop melt pool size control through laser power modulation was demonstrated for intralayer control of bead geometry. Aspects such as controller tuning, response time, interaction with primary process variables, and disturbance rejection are presented. Next, an interlayer trend in laser power during the printing of layered components was documented, which inspired the development of novel modes of control. A controller that modulates print speed and deposition rate on a per-layer basis was developed and demonstrated, enabling the control of either average melt pool size alone or average laser power in coordination with real-time melt pool size control. This work demonstrates that accumulated heat in components under construction can be exploited to maintain process stability as print speed and deposition rate are automatically increased under closed-loop control. This has major implications for overall production efficiency. Control modes are characterized in terms of their effect on local bead geometry, global part geometry, and interlayer effect on energy density, among other factors.
关键词： Directed Energy Deposition,Monitoring,Metal,Closed-Loop,3D Printing,Ti-6Al-4V,Lasers,Control,Additive Manufacturing,Melt Pool
更新于2025-09-12 10:27:22
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Structural and luminescence properties of Dy3+-doped alkali fluoroborophosphate glasses for white LEDs applications
摘要： A new series of Dy3+-doped alkali fluoroborophosphate glasses were prepared by conventional melt quenching technique, and their structural and optical properties were investigated through XRD, FTIR, optical absorption, luminescence and decay measurements. The X-ray diffraction pattern reveals the amorphous nature of the prepared glasses. The fundamental stretching vibrations of various borate (BO3 and BO4) and phosphate (PO4) networks were identified through the FTIR analysis. The nature of the metal–ligand bonding and the electronic band structure has been investigated using the absorption spectra. The Judd–Ofelt (JO) intensity parameters (X2, X4 and X6) were evaluated, and the experimental oscillator strength values were also calculated. Using the JO intensity parameters, radiative properties like radiative transition probability (A), stimulated emission cross section (rE) and branching ratios (bR) for the emission transitions of the Dy3+ ions have been calculated. The luminescence spectra exhibit two visible bands of 4F9/2 → 6H15/2 (Blue) and 4F9/2 → 6H13/2 (Yellow). The decay time of the 4F9/2 level has been measured from the decay profiles and compared with the calculated lifetimes. The yellow-to-blue (Y/B) ratios and color coordinates have been calculated from the luminescence spectra and can be considered for white light emitting diodes applications.
关键词： Absorption,CIE diagram,Melt quenching technique,Rare-earth,Photoluminescence
更新于2025-09-11 14:15:04
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Laser-Induced Fluorescence Emission (L.I.F.E.) as Novel Non-Invasive Tool for In-Situ Measurements of Biomarkers in Cryospheric Habitats
摘要： Global warming affects microbial communities in a variety of ecosystems, especially cryospheric habitats. However, little is known about microbial-mediated carbon fluxes in extreme environments. Hence, the methodology of sample acquisition described in the very few studies available implies two major problems: A) high resolution data require a large number of samples, which is difficult to obtain in remote areas; B) unavoidable sample manipulation such as cutting, sawing, and melting of ice cores that leads to a misunderstanding of in situ conditions. In this study, a prototype device that requires neither sample preparation nor sample destruction is presented. The device can be used for in situ measurements with a high spectral and spatial resolution in terrestrial and ice ecosystems and is based on the Laser-Induced Fluorescence Emission (L.I.F.E.) technique. Photoautotrophic supraglacial communities can be identified by the detection of L.I.F.E. signatures in photopigments. The L.I.F.E. instrument calibration for the porphyrin derivates chlorophylla (chla) (405 nm laser excitation) and B-phycoerythrin (B-PE) (532 nm laser excitation) is demonstrated. For the validation of this methodology, L.I.F.E. data were ratified by a conventional method for chla quantification that involved pigment extraction and subsequent absorption spectroscopy. The prototype applicability in the field was proven in extreme polar environments. Further testing on terrestrial habitats took place during Mars analog simulations in the Moroccan dessert and on an Austrian rock glacier. The L.I.F.E. instrument enables high resolution scans of large areas with acceptable operation logistics and contributes to a better understanding of the ecological potential of supraglacial communities in the context of global change.
关键词： chlorophyll,glacial melt,laser-induced fluorescence emission (L.I.F.E.),ice,cryospheric habitats,phycoerythrin,non-invasive
更新于2025-09-11 14:15:04
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia, Spain (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Snow Cover Monitoring by Synergistic Use of Sentinel-3 Slstr and Sentinel-L Sar Data
摘要： The Sentinel satellite missions of the European Copernicus programme provide comprehensive data for long-term routine observations of the global environment. A key parameter for climate monitoring, hydrology and water management is the seasonal snow cover. We developed, implemented and tested a novel approach for monitoring snow extent and snowmelt area, exploiting the synergy of imaging radar of the Sentinel-1 mission and multispectral optical imagery of the SLSTR sensor on Sentinel-3. We describe the processing steps and algorithms for generating the synergistic snow cover product and show examples for snow cover maps over Europe based on single mission data as well as the final synergistic product.
关键词： Sentinel-1,snow melt area,Copernicus programme,Snow extent,Sentinel-3
更新于2025-09-10 09:29:36
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Structural stability and energy levels of carbon-related defects in amorphous SiO <sub/>2</sub> and its interface with SiC
摘要： We report the density-functional calculations that systematically clarify the stable forms of carbon-related defects and their energy levels in amorphous SiO2 using the melt-quench technique in molecular dynamics. Considering the position dependence of the O chemical potential near and far from the SiC/SiO2 interface, we determine the most abundant forms of carbon-related defects: Far from the interface, the CO2 or CO in the internal space in SiO2 is abundant and they are electronically inactive; near the interface, the carbon clustering is likely and a particular mono-carbon defect and a di-carbon defect induce energy levels near the SiC conduction-band bottom, thus being candidates for the carrier traps.
关键词： melt-quench technique,amorphous SiO2,carbon-related defects,density-functional calculations,SiC/SiO2 interface
更新于2025-09-10 09:29:36
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Reflectance variation in boreal landscape during the snow melting period using airborne imaging spectroscopy
摘要： We aim a better understanding of the effect of spring-time snow melt on the remotely sensed scene reflectance by using an extensive amount of optical spectral data obtained from an airborne hyperspectral campaign in Northern Finland. We investigate the behaviour of thin snow reflectance for different land cover types, such as open areas, boreal forests and treeless fells. Our results not only confirm the generally known fact that the reflectance of a melting thin snow layer is considerably lower than that of a thick snow layer, but we also present analyses of the reflectance variation over different land covers and in boreal forests as a function of canopy coverage. According to common knowledge, the highly variating reflectance spectra of partially transparent, most likely also contaminated thin snow pack weakens the performance of snow detection algorithms, in particular in the mapping of Fractional Snow Cover (FSC) during the end of the melting period. The obtained results directly support further development of the SCAmod algorithm for FSC retrieval, and can be likewise applied to develop other algorithms for optical satellite data (e.g. spectral unmixing methods), and to perform accuracy assessments for snow detection algorithms.
关键词： SCE,AISA,Scene reflectance,Boreal forest,Spectroscopy,FSC,Snow mapping,NDSI,MODIS,NDVI,Land cover classification,Snow melt,Fell,Reflectance
更新于2025-09-10 09:29:36