Temporal and Spatial Beam Shaping in LPBF for Fine and Porous Ti-Alloy Structures for Regenerative Fuel Cell Applications

Laser Powder Bed Fusion (LPBF) presents itself as a potential method to produce thin porous structures, which have numerous applications in the medical and energy industries, due to its in-process pore formation capabilities. Particularly, regenerative fuel cells, which are capable of both producing and storing energy through the use of hydrogen-based electrochemical fuel cell and electrolysers, respectively, can benefit from the LPBF-induced porosity for it porous layer components in the electrode. Numerous studies have reported that process parameters, such as laser power, scan speed and hatch spacing, are key factors affecting the formation of pores in LPBF material due to their control over the energy density and melt pool formation during the build. Contemporary fibre lasers offer novel temporal and spatial beam shaping capabilities. Temporal laser control means that the laser can use pulsed wave (PW) or single point exposure (SPE), and spatial beam shaping refers to variations in the intensity distribution of the laser, which can be modulated from Gaussian to ring shape via the use of multi-core fibers. These have seldom been studied in combination with LPBF. Therefore, the aim of this study was to utilise temporal and spatial beam shaping in LPBF to produce thin porous structures. To do this, PW and SPE laser temporal strategies were utilised and the duty cycle (which relates the on and off time of the laser) was varied between 50% and 100%. Beam shape indexes 0 (Gaussian), 3 and 6 (ring) were also investigated alongside more standard LPBF process parameters such as laser power and scan speed to manufacture thin porous walls, as well as fine struts. The thinnest wall obtained was 130 μm thick, while the smallest strut had a diameter of 168 μm. The duty cycle had a clear effect on the porosity of thin walls, where a duty cycle of 50% produced the highest number of porous walls and had the highest porosity due to its ability to control the intensity of the energy density during the LPBF process. The different beam shape indexes corresponded to different spatial distribution of the power density, and hence, modifying the temperature distribution in the meltpool during the laser material interaction. Beam shape index 6 (corresponding to a ring mode with lower peak irradiance) created more porous specimens and smaller meltpool sizes, with respect to its beam size. Overall, this study showed that temporal and spatial control of the beam (through duty cycle and beam shape index) are powerful tools which can control the distribution and intensity of the energy density during the LPBF process to produce thin porous structures for energy applications.</p

Coordination of spatial and temporal laser beam profile towards ultra-fine feature fabrication in laser powder bed fusion

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology that provides high shape and application flexibilities. Although dimensional flexibility is high in theory thanks to the non-contactless micro range processing tool (i.e., laser beam) and powder, the fabrication robustness of thin and ultra-thin features (dnominal?200 ?m) is still a challenge for the technology. In particular, geometrical fidelity and dimensional accuracy problems have been raising towards the ultra-thin fabrication segment. Although there were different studies that presented solutions for robust and sustainable fabrication strategy in ultra-thin segment in the literature, the vast majority of them focused on process itself directly, and the technological feasibility of the LPBF systems was not considered. However, without considering technological feasibility of the LPBF systems, the presented solutions in the literature are far from providing solid basis and they may mislead the users in the case of direct application. In this sense, this study mainly focused on the scanning capability of the systems for features under 200 µm dimensional range with temporal and spatial laser beam management in the case of conventional scanning strategy (contour and hatch). For this purpose, the fabrication process reduced to the two dimensions (scanning region) via the laser marking tests, and custom laser parameters, which are provided by the industrial grade open architecture LPBF system, has been used. Here, it has been reported that two different errors related to scanning performance and process parameters for continuous and pulsed wave laser emission modes, separation, and compensation of these two errors, and investigation methodology for technological feasibility of the LPBF machines. Moreover, in the pulsed wave laser emission mode, two linear parameters have been presented to optimize spatial energy distribution. Considering the results coming from the practical observations and measurements, it is possible to indicate that the technological feasibility of the utilized LPBF system should be key concern before laser or scan related parameters optimization. The results show that if correct scanning parameters have been selected in the technological feasibility window of the system, scan trajectories based on conventional hatching method can be carried out with sufficient geometrical accuracy

Comprehensive benchmarking of laser welding technologies including novel beam shapes and wavelengths for e-drive copper hairpins

Laser welding is the industrially accepted method for the joining of Cu hairpin windings in the production of electric drives. High brilliance laser beams are scanned over the bare ends of the Cu wires producing a rapid connection through deep penetration remote welding. Despite being an accepted manufacturing method, laser welding of Cu hairpins still requires detailed studies concerning manufacturing productivity and quality. As the availability of novel laser sources with higher power levels, new wavelengths, and beam shaping capabilities increase, the need for benchmarking studies emerges. In this work, six different laser welding systems were compared in terms of process productivity and quality during the welding of Cu hairpins used for automotive traction. The different solutions presented power levels from 3 to 6 kW, with wavelengths from near infrared (NIR) to visible, including in source dynamic beam shaping. The weld bead formation was observed through high-speed imaging. The welds were analyzed in terms of their geometry, internal defects, and most relevantly for their mechanical strength. The results showed advantages of each of the employed system while the laser systems providing the highest irradiance profile produced the fastest weld with more elevated mechanical strength independently from the wavelength

Defect-free Laser Powder Bed Fusion of Ti-48Al-2Cr-2Nb with a high temperature inductive preheating system

In the industrial panorama, Laser Powder Bed Fusion systems enable the near net shaping of metal powders into complex geometries with unique design features. This makes the technology appealing for many industrial applications, which require high performance materials combined with lightweight design, lattice structures and organic forms. However, many of the alloys that would be ideal for the realisation of these functional components are classified as difficult to weld due to their cracking sensitivity. γ-TiAl alloys are currently processed via Electron Beam Melting to produce components for energy generation applications. The Electron Beam Melting process provides crack-free processing thanks to the preheating stages between layers, but lacks geometrical precision. The use of laser powder bed fusion could provide the means for higher precision, and therefore an easier post-processing stage. However, industrial Laser Powder Bed Fusion systems employ resistive heating elements underneath the base plate which do not commonly reach the high temperatures required for the processing of γ-TiAl alloys. Thus, elevated temperature preheating of the build part and control over the cooling rate after the deposition process is concluded are amongst the features which require further investigations. In this work, the design and implementation of a novel inductive high temperature Laser Powder Bed Fusion system to process Ti-48Al-2Cr-2Nb is presented. Specimens were built with preheating at 800 °C and the cooling rate at the end of the build was controlled at 5 °C/min. Crack formation was suppressed and apparent density in excess of 99 % was achieved

18F-FDG PET and PET/CT in the localization and characterization of lesions in patients with ovarian cancer

Aim: The aim was to compare the imaging findings of 18F-fluorodeoxyglucose (18F-FDG) PET and integrated PET/CT in patients with primary, recurrent or metastatic ovarian cancer. Materials and methods. 21 women with ovarian cancer were evaluated. All patients had a integrated PET/CT scan. Localization, infiltration and uptake intensity of [18F]FDG were evaluated on PET and PET/CT. The certainty of localisation and characterisation was scored on a 3 point scale (L1 definite localisation; L2 probable localisation; L3 uncertain localisation; C1 benign; C2 equivocal; C3 malignant). Results. PET scored as L1 54 lesions (44%), as L2 51 (42%), and as L3 17 (14%). On the other hand, PET/CT scored as L1 120 lesions (98%), as L2 2 (2%), and none as L3. Thus PET/CT allowed a better localization in 54% of lesions. Moreover, PET scored as C1 25 lesions (20%), as C2 62 (51%), and as C3 35 (29%) . On the other hand, PET/CT scored as C1 57 lesions (47%), as C2 13 (11%), and as C3 52 (42%). Thus PET/CT allowed a sensible reduction in the number of equivocal lesions (40%). Even when patients were subgrouped on the basis of clinical stage of the disease, PET/CT was capable of better definition of the lesions either for localization and for characterization. Conclusions. In patients with ovarian cancer, PET/CT allows better anatomical localisation of pathologic uptake providing high accuracy for staging and restaging of ovarian cancer when compared with PET alone

CORE COLLECTION OF COMMON BEAN FORMED FROM TRADITIONAL BRAZILIAN GERMPLASM

O objetivo deste trabalho foi, através de estratégias de análise por modelos multivariado, selecionar acessos tradicionais, compor uma coleção nuclear de feijoeiro comum e avaliar sua representatividade em relação à coleção base de coletas hospedadas no BAG da Embrapa. Utilizando dados de caracterização de 2903 acessos de coletas representando todas as regiões geográficas do Brasil quanto a três descritores morfológicos (cor de semente, tipos de crescimento e tamanho de semente) e quatro descritores ecogeográficos (regiões geográficas, unidades federativas, altitudes e classes de solos), foram selecionados 400 acessos utilizando modelos multivariado aplicados aos dados transformados em valores multibinários. Os acessos amostrados apresentaram similaridade máxima (100,0%) com a coleção tradicional, diversidade fenotípica e heterogeneidade representativa em relação à coleção tradicional. Na coleção nuclear os acessos tradicionais representaram 9,5% dos acessos tradicional e equivalente a 3% dos acessos da coleção base. Com isso conclui-se que é possível formar uma coleção nuclear representativa da coleção base, no que diz respeito à diversidade genética e a conservação de alelos raros

Nonintrusive estimation of subsurface geometrical attributes of the melt pool through the sensing of surface oscillations in laser powder bed fusion

Molten pool geometry, whose surface parameters may be extrapolated through direct process observations, has been identified as a fundamental indicator of stability in laser powder bed fusion (LPBF). However, a parameter that cannot be directly measured on industrial systems by means of conventional sensing equipment is the molten pool depth. Indeed, methods based on x-ray imaging demonstrated in the literature have helped to better understand the process. However, retrofitting such solutions to industrial systems does not appear as a viable route currently. Within the present investigation, a nonintrusive sensing method for the indirect measurement of subsurface molten pool geometry based on the detection of surface oscillations is presented. The analysis of frames acquired using a high-speed camera and a secondary illumination light allows the identification of the crests of capillary waves through bright reflections on the surface of the molten pool. The characteristic oscillation frequency of the surface ripples may be correlated with the penetration depth or to other subsurface geometrical parameters. Proof of concept testing of the sensing principle was conducted on two different materials, namely, AISI316L and IN718, by means of single track LPBF depositions. Experiments were conducted at different levels of laser emission power to induce variations in molten pool characteristics. The process was observed by employing an off-axis illumination light and a high-speed camera, which allowed acquisitions with high spatial and temporal resolution. The acquired frames were postprocessed to extract the oscillation indicator, and analysis of the power spectral density of the signal allowed for the identification of the oscillation frequency. Results show that oscillation frequencies range from 3 to 5.5 kHz. Molten pool penetration depth and cross-sectional area could be correlated with the oscillation frequencies for the inline detection of these parameters during LPBF depositions. For both materials, higher oscillation frequencies corresponded to a shallower molten pool and a smaller mass of molten material. Moreover, different characteristic curves of oscillation frequency variations as a function of the melt pool cross-sectional area were determined for IN718 and AISI316L

Influence of pulsed and continuous wave emission on melting efficiency in selective laser melting

A wide proportion of the industrial Selective Laser Melting (SLM) systems are operated with high brilliance fiber laser sources. These sources are most commonly operated in continuous wave (CW). A smaller fraction of these systems employs pulsed wave (PW) emission by power modulation, resulting in pulses with μs level durations, kHz level repetition rates and comparable peak powers to CW emission. Clearly, the laser temporal emission mode can have an impact over temperature fields, which in return control the melt pool size, stability and densification behaviour. The aim of this paper is to investigate the effect of laser emission regime on the melting efficiency in SLM. In particular, an analytical model was developed to investigate the process efficiency in single track formation at fixed energy input when employing different emission modes. A single mode fiber laser installed on an in-house developed prototype powder bed fusion system was used as the experimental setup with AISI 316 L metallic powders. The effect of duty cycle was evaluated starting from CW (i.e. 100% duty) moving towards PW, at fixed energy density levels. Results show that at constant energetic input, CW increases process melting efficiency up to 3 times whilst the deposition stability is reduced with lower duty in PW regime. Although less efficient, at stable conditions the use of modulated emission produces narrower tracks providing higher process resolution