1,721,033 research outputs found
Tailoring the microstructure of Fe-2.9wt.%Si alloy in laser powder bed fusion using in-source beam shaping
Full text linkTuning the irradiance profile of the laser beam opens up new possibilities in terms of controlling the thermal field the material is subjected to during laser powder bed fusion (PBF-LB/M). This control can be exploited to manipulate the material's microstructure. In this work, a contemporary high power fibre laser with in-source beam shaping capability was used to test different irradiance profiles ranging from Gaussian to ring during the PBF-LB/M of Fe-2.9wt.%Si alloy. This soft magnetic alloy is used in electrical machines, and its magnetic properties are known to be correlated to the microstructure. Initially, the experimental work assessed the influence of peak and ring irradiance levels on densification and grain morphology. In a second experimental run, the influence of the beam profile on the conventionally employed energy density was verified. The results demonstrated that the use of combined Gaussian-ring profiles offers a method for manipulating grain size and shape from a columnar towards an equiaxed shape, without requiring a change in the scan strategy
The influence of novel beam shapes on melt pool shape and mechanical properties of LPBF produced Al-alloy
Full text linkNew generation of multi-core fiber laser sources provide in-source dynamic beam shaping. Such sources can switch between Gaussian and ring beams, providing new irradiance profiles. The new irradiance profiles add up to the process flexibility for controlling the temperature fields generated in the laser powder bed fusion (LPBF) process. On the other hand, they bring further complexity as new process parameters should be defined and their influence on the mechanical properties unveiled. Accordingly, this work studies the use of seven different beam shapes provided by a multi-core industrial fiber laser source with beam diameters varying between 47 μm to 144 μm during the LPBF of AlSi7Mg0.6 alloy. The beam spatial profiles were measured and descriptive irradiance parameters were defined. The Al-alloy constituted a challenging material type due to its high reflectivity and low melting point, hence, more prone to lack-of-fusion and keyhole defects. LPBF experiments were conducted at fixed energy density and peak irradiance levels investigating the influence of the beam shapes on the melt pool geometry in prismatic samples. The results showed that the peak irradiance and the ring intensity had a direct impact on the melt pool aspect ratio (AR) and melt pool depth to layer thickness ratio (h/z). With all the conditions providing adequately dense parts (>99.5 %), the mechanical properties were found to be correlated to the melt pool geometry. The results confirm that the beam shape can be tuned between the central peak and the ring to further manipulate the material properties in LPBF
Analytical modeling and characterization of ring beam profiles for high-power lasers used in industrial manufacturing
Full text linkActive fibre lasers are widely used in the industry for different manufacturing applications ranging from cutting, to welding and additive manufacturing. The recent introduction of the multiple-core fibre lasers allows these sources to flexibly change the Power Density Distribution (PDD) from conventional Gaussian profiles towards ring shapes. While the advantages of the novel beam shapes over the conventional ones are still being explored, the need for modeling tools to define the PDD shapes becomes more evident. This work studies the analytical modeling of Gaussian to ring profiles with the aim to move towards standardized parameters referable to the manufacturing processes. The proposed models combine Gaussian and annular components to define the novel beam shapes. Among the different models assessed, the Torus and Multi-Gaussian approaches exhibited the best fitting quality thus enabling the definition of descriptive metrics of the PDD. The modeling framework developed was validated on an industrial Laser Powder Bed Fusion (LPBF) system with a double-core light source. The beam shape variation along the propagation axis was assessed to analyze the effect of defocusing using the developed beam parameters. Eventually, the best performing model was furtherly validated with a bead on plate experiment to explain how the model coefficients can be jointly exploited to predict the material response using a Gaussian or a ring beam profile
Evaluation on the effectiveness of standards for preclinical mechanical characterization of spinal fixators
No full textIntroduction: Posterior spinal fixators are subjected to many load cycles after implantation due to walking, and failure events are continuously reported. To avoid this issue and evaluate preclinically the mechanical reliability of fixators a vertebrectomy (ASTM F1717 standard) and a physiological anterior support (ISO 12189) models are available. The aim of the study is to assess the international standards for the preclinical evaluation of posterior spinal fixators and to propose
improvements.
Materials and methods: Several anatomical/biomechanical parameters useful to describe the anatomy of the functional spine units were considered. Their value depending on the spinal level was obtained from literature or from direct measurements on biplanar stereoradiographies. Numerical models describing experimental setups were used to study the contribution of each parameter on the stress on the implant. The worst case condition was also determined.
Results: Vertebrectomy condition may guarantee high safety of the implant once implanted in an average patient from a physiological population. The worst-case combination of parameters demonstrates higher loads than those reached using the current standard (screw: +15 %; rod: +9 % at L1). The physiological condition may not be safe enough: despite the anterior support characteristics lay within the literature range of values, it may lead to a stress increase even beyond 350 %.
Discussion and conclusions: The study investigates the influence of biomechanical parameters on the stress on the fixator. Standards revision according to the anatomical worst-case condition (L1 level) would guarantee a higher safety for a greater range of patient population. Ongoing experimental testing partially corroborates numerical results
Low-temperature tensile properties of meta-stable β titanium Ti-5Al-5V-5Mo-3Cr alloy fabricated by pulsed laser powder bed fusion
Full text linkTi-5Al-5V-5Mo-3Cr (Ti-5553) alloy is increasingly used in structural airplane components, such as landing gear and fuselage, thanks to its high strength, good ductility, and low density. These components are consistently subjected to sub-zero temperatures during the flight. In this study, pulsed laser powder bed fusion (L-PBF) parameters were firstly optimized to fabricate highly dense (>99%) parts, and their effect on microstructure and microhardness was analyzed. Finally, tensile properties were evaluated under sub-zero temperature conditions and compared to the standard room ones to validate the feasibility of using Ti5553 under low-temperature environments
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
In-source dynamic beam shaping of Al alloys processed via LPBF: Effect of novel beam profiles on surface roughness and microstructure
Full text linkNext-generation fiber laser sources can dynamically combine Gaussian beams with ring distributions within the source. The irradiance manipulation may add flexibility to the Laser Powder Bed Fusion process, for instance to control microstructure or surface roughness. However, their use is limited by a lack of studies revealing how beam shaping can be exploited in a reproducible manner. Accordingly, this work shows the use of novel beam profiles provided by a multi-core fiber laser source in the LPBF of AlSi7Mg0.6. Irradiance profiles were characterized, then large experiments were conducted to investigate their effect on surface roughness and microstructure of produced parts
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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