1,720,991 research outputs found
Probeless friction stir welding of dissimilar metal welds
Full text linkIn recent years, the number of lightweight parts made of high-strength aluminium alloys and advanced high strength steels (AHHS) has increased fivefold to further enhance engine efficiency and reduce CO2 emissions in automotive sector. Furthermore, lightweight assemblies increasingly utilize dissimilar structures combining both AHSS and high-strength aluminium alloys.
Widespread in automotive industry fusion method of resistance spot welding (RSW) triggers notable alterations in resolidified seam microstructure from its original state. Moreover, high thermal conductivity of aluminum alloys lead to rapid electrode wear, thereby enhance the formation of pores, voids, solidification and liquation cracking. Joining of dissimilar materials suffers even more challenges due to additional disparities in their electrical and thermal properties. At last, the formation of brittle intermetallic compounds (IMCs) detrimentally effects on the final mechanical properties. Temperature and holding time are two critical parameters that influence the formation of above-mentioned defects and formation of IMCs. Thus, reducing the amount of heat input and time of the welding process become an inevitable demand for both similar and dissimilar materials.
Being a solid-state welding method, Friction Stir Spot Welding (FSSW) involves joining between materials by plunging a non-consumable rotating tool involving a shoulder and a probe into the workpiece with further material heating and stirring. However, the crucial drawback of FSSW is the formation of a permanent central keyhole due to presence of probe, which significantly reduces the effective cross-sectional area.
Among all variations being invented to eliminate the keyhole, Probeless Friction Stir Spot Welding (P-FSSW) gains significant attention due to its process simplicity, high efficiently, and negligible tool wear. The weld formation is primarily influenced by the combined effects of vertical compression and horizontal material, which are exclusively driven by the shoulder. Thus, the contact between shoulder and workpiece surface plays a vital role in the heating and following material plasticization.
The featureless flat shoulder of P-FSSW has been primarily served as a reference point, while it demonstrated promising results in forming strong joints among different featured shoulders. Notably, it possesses minimal tool wear and contamination from plasticized material compared to other shoulder designs. Its main drawback is solely the requirement for a longer dwell time to attain the same level of strength as those achieved with featured shoulders.
The objective of this study is to investigate the feasibility of creating robust spot joints between dissimilar aluminum-aluminum and aluminum-steel materials using Probeless Friction Stir Spot Welding (P-FSSW) assisted by flat featureless shoulder. The study systematically evaluates heat generation, material flow, microstructural evolution and mechanical properties, aiming to optimize parameters and expand the scope of this technology.
Four sets of experiments on P-FSSW with different combination of dissimilar aluminum-aluminum and aluminum-steel material couples were performed at Hybrid Welding Laboratory at DMMM Polytechnic University of Bari. Optical microscopy, microhardness tests, tensile test, and scanning electron microscopy (SEM) equipped with the electron backscatter diffraction (EBSD) technique were employed to provide fundamental insight into the microstructural development. In regard to provide a better understanding of P-FSSW, the experimental findings were coupled with numerical results of thermal fields and material flow. Finite element model was developed in assist with Simufact Forming® 2021 software
Hybrid laser welding of stainless and high strength steels
Full text linkIn manufactory industry, welding is a joining technology with noticeable properties. Among fusion welding technologies, laser welding employs high power laser beam with high density. Laser beam welding debuted several decades later, after the invention of the laser in 1960, and has demonstrated to be especially useful in high-speed, automated welding. The laser beam produces a reduced melting zone, which has always been seen positive for the industrial products. Low thermal distortion, high speed and little heat input per unit of volume can be reached by fiber laser with some limitations like crack susceptibility, porosity, strict tolerances for edges preparation, and high reflectivity. Hybrid laser arc welding can overcome some of those limitations since its tendency to flexibility, low crack formation and porosity, high precision, narrow melting zones, high cooling and heating speeds, deep penetration. Mixing and diffusion phenomena can be controlled and that reduces the formation of intermetallic compounds in the case of dissimilar joints. In fact, dissimilar metal welding implicates joining two different metals or alloys by melting, which the addition of filler metal as an option. Among dissimilar metal weld, aluminum/steel (Al/Fe), advanced high strength steel (AHSSs) /austenitic stainless steel have increased the attention from aeronautical and automotive industry because of the properties such as reduced weight, good formability, high mechanical strength, and more. Dissimilar joining of hybrid Al-Fe structures is often challenging because of the inevitable formation of brittle intermetallic compounds (IMCs), which can compromise the mechanical properties of the weld due to their poor ductility. Therefore, it is necessary to limit their formation to obtain good quality welds. The major goal of this thesis is to analyze the microstructural and mechanical aspects of dissimilar joints used in the automotive industry, which were welded by different laser techniques. Particularly, great importance was given to the microstructure of the fuse (FZ) and heat affect zones (HAZ), being those zones mainly associated to the mechanical behavior of the weld. The mechanical and microstructural aspect of the joint were related with the process parameters to study the defects and to obtain the better mechanical properties. After the state-of-art on fusion welding processes, the research activity concerned the study of the fiber laser/TIG weldability of AISI 304 and AISI 410 stainless steel (SS), weldability of Twinning-induced plasticity steels (TWIP) and Dual Phase (DP) steel dissimilar joint by laser arc hybrid welding with austenitic filler and finally the assessment of weldability of TWIP and DP steels with AISI 316 stainless steel. Since the automotive industry has been reducing the vehicle weight and fuel consumption, advanced high strength steels and stainless steel can be help to this achievement. Research activity concerned also the study of the laser welding of the annealed alloy AA5754 with austenitic stainless steel AISI 316. Particularly, the laser off-set welding (LOW) was used to limit the formation of brittle intermetallic compounds during the welding process. The laser beam was directed onto the stainless steel side of the butt weld at a small distance from the edge of the aluminum sheet. The keyhole formed and the full penetration was reached in the SS side of the weld. Thin layer of intermetallic compounds of different stoichiometry was found at the interfaces between SS and Al fusion zone (FZ). Finite Element (FE) analysis was employed to evaluate both the temperature distribution, residual stress and distortions in the joints. Using Simufact commercial software, the numerical model of the joint was created and validated through the available experimental data. The target of this part of the research was to build a methodology to build a numerical model for laser and hybrid welding process, with a particular attention to the set-up of the heat source model
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
Singlet Oxygen as Key Reagent for Novel C-Nucleosides and Glycosides of Pharmacological Interest
No full textDye-Sensitized Photooxygenation of 2,5-Bis(glycosyl)furans
No full textThe dye-sensitized photooxygenation of 2,5-bis(glycosyl)furans followed by warming up to r.t. provides 1,1'- linked disaccharides separated by a functionalized spacer. Asymmetrical disubstituted glycosyl furans give the corresponding Bayer-Villiger type-rearranged products in a molar ratio depending on the sugars and the protecting groups. The migratory aptitudes have been rationalized by both theoretical calculations and experimental data. The 1,1'- disaccharides obtained are new sugar derivatives structurally related to some mimetics of Sialyl Lewis X
Dye-sensitized photooxygenation of sugar furans: novel bis-epoxide and spirocyclic C-nucleosides.
No full textDye-sensitized photooxygenation of 2-methyl 5-(2,3,5-tri-O-acetyl-b-D-ribofuranosyl)furoate leads to (1S,4R)-endo-peroxide, highlighting a high facial diastereoselectivity. This endo-peroxide rearranges into syn-(1R,2R:3S,4R)-diepoxide C-nucleoside, while by Et2S-reduction followed by NEt3 catalysis affords a spirocyclic C-nucleoside
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