1,720,959 research outputs found
Defining a new process window for LPBF of Ti-6Al-4V based on micro-warping phenomena
Full text linkDespite the many advantages that additive technologies have over subtractive ones, low porosity levels, good mechanical properties, and low residual stresses remain the most pressing issues that need further research. In particular, the latter can cause a mismatch between the desired geometry and the geometry that can be achieved.
In this work, a process window for the Laser Powder Bed Fusion (LPBF) process of Ti-6Al-4V alloys, has been identified. The micro-warping phenomenon, which causes the deformation of the printed part during the printing job and the failure of the process, was taken into account together with the parts’ strength, ductility, and porosity. The occurrence of micro-warping phenomena was assessed by the new Warping Alert (WA) parameter,
which depends on the parameters P and v. It was found that, before balling, micro-warping limits the process window in the laser power (P) – laser velocity (v) plane. However, optimal mechanical performances can be found in the proximity of the micro-warping zone, thus making it extremely important to determine the WA threshold value to the process design
Finite Element Method in L-PBF of Ti-6Al-4V: Influence of Laser Power and Scan Speed on Residual Stress and Part Distortion
Full text linkLaser powder bed fusion (L-PBF) is widely used in automotive, aerospace, and biomedical applications thanks to its ability to produce complex geometries. In spite of its advantages, parts produced with this technology can show distortion due to the residual stresses developed during the printing process. For this reason, numerical simulations can be used to predict thermal gradients and residual stresses that can result in part distortion. Thus, instead of performing experimental tests and using a trial and error approach, it is possible to use numerical simulation to save time and material. In this work, the effect of laser power and scan speed on residual stress and part distortion was analysed using a commercial finite element analysis (FEA) software DEFORM-3D™ with a layer-by-layer approach. Moreover, the accuracy of the numerical model with respect to process parameters and the utilised mesh was also studied. The results obtained from the numerical simulation were compared to the actual distortions to evaluate the accuracy of the FEM model. The predicted distortions using FEM analysis well fit the trend of the measured ones. The accuracy of the numerical model increases by considering a finer mesh
A new control parameter to predict micro‐warping‐induced job failure in LPBF of TI6AL4V titanium alloy
Full text linkLaser powder bed fusion (LPBF) includes a few printing techniques widely used, in recent years, concerning the additive
manufacturing of Ti6Al4V alloys. These produced parts, typically utilized in sectors such as aerospace and biomedical, are
characterized by very high added value. It is therefore fundamental to identify the influence of process parameters typical
of LPBF technology on the occurrence of warping leading to process failure. This study deals with the characterization of
single-track and “micro-scale” level warping phenomena which may lead to protrusion of material over the powder bed and
process failure before normal termination. This phenomenon was investigated as a function of process parameters, referring
also to the strength and ductility characteristics of the manufactured samples. With this purpose, several samples were
printed using variable process parameters both in terms of line energy density (LED) values and in terms of laser power and
speed combinations such as to guarantee constant LED values. For the samples that did not show significant micro-warping
phenomena, in addition to the transversal and longitudinal geometric characterization of the single track, tensile tests were
performed to determine both the resistance of the material and the ductility characteristics. The single tracks, for given process
parameters, were printed on a homogeneous material substrate. For every single track, a microstructural and morphological
transverse and longitudinal characterization has been carried out and the measured geometrical features were correlated to
the process parameters. The obtained results allowed the identification of a new threshold parameter, indicating the limit
operating conditions beyond which significant warping phenomena and process failure occur
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Ti-6Al-4V LASER POWDER BED FUSION PROCESS ENGINEERING
No full textIn recent years, additive manufacturing (AM) has taken increasing interest in many industrial sectors such as aerospace and biomedical thanks to its freedom of design. Thus allowing the production of complex and highly customized parts. Moreover, AM leads to a new way of thinking during the design phase, making possible new design strategies such as part consolidation and topology optimization to increase the functionality of the part. Among all the AM technologies, laser powder bed fusion (L-PBF) is one of the most suitable to produce metallic parts. In fact, L-PBF is widely adopted in industries for the fabrication of Ti-6Al-4V parts which usually find applications in the aerospace and biomedical fields. This is due to the high strength/weight ratio and corrosion resistance of the Ti-6Al-4V alloy. Despite all these advantages, parts produced with L-PBF can result in poor surface quality or mechanical properties. Thus, the Ti-6Al-4V L-PBF process engineering is of fundamental importance.In this dissertation, challenges for the fabrication of parts with the L-PBF process such as lack of fusion, gas pore, residual stress, and part distortion are presented. Following, the effect of process parameters on mechanical properties and part quality has been analyzed. Finally, prediction tools such as numerical simulations and machine learning are shown for the Ti-6Al-4V L-PBF process engineering. In order to investigate the influence of process parameters on L-PBF parts, mechanical tests were carried out together with microstructure analysis. Thus allowing a better understanding of the printing process. Moreover, reverse engineering techniques such as 3D acquisition systems were of fundamental importance to validate the results obtained from the numerical simulation. Overall, the results obtained from this thesis work suggest that: (i) build orientation is a key parameter when designing the L-PBF process due to its influence on thermal exchanges; (ii) line energy density alone is not enough to predict the success of the printing process, especially in the case of the warping phenomenon; (iii) machine learning can be an effective tool to predict the quality of the printed part when several process parameters have to be taken into account; (iv) numerical simulation can be used to study the development of residual stress during the L-PBF process and predict the part distortion
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