1,720,966 research outputs found
Effect of Cu content on hot-crack resistance of Al-Cu-Mg alloys produced by laser powder bed fusion
No full textThe lack of high-strength Al alloys that can be processed by laser powder bed fusion (LPBF) without formation of cracks is considered as one of the main issues for a wider diffusion of this technology in many fields. In this study, the effect of Cu content on the hot-crack resistance of Al-Cu-Mg alloys produced by LPBF was investigated. Powder batches with different compositions were produced by mixing pre-alloyed gas-atomised 2024 alloy powder with increasing amounts (4, 6, 8, 10, and 12 wt.%) of Cu particles and then processed by LPBF. Thermodynamic simulation and microstructure investigations showed that the addition of Cu promotes the formation of abundant eutectic phase mixture, refines the grain structure and reduces the solidification temperature range, thus decreasing the solidification cracking susceptibility of the material. Samples produced with more than 10 wt.% of Cu featured relative density higher than 99.5% and no solidification cracks
Fluctuations of tracks and layers during aluminium laser powder-bed fusion
Full text linkLaser Powder-Bed Fusion (LPBF) is one method in Additive Manufacturing where layerwise complex structures can be built. However, although the LPBF machines produce promising parts, the efficiency and process speed are typically still low, which can make the process expensive and uneconomical. Recent research showed that volume elements in the parts can be melted several times, while only a small material volume is added, which indicates a high loss of energy. In order to understand the process better, in this work, theoretical modeling and smart powder-bed experiments were designed to explain the impact on the track dimensions based on the previously built tracks and layers. It was found that the powder availability varies for each track and has an alternating character within and between layers. The comparison of the simulation and experimental results indicates that the powder pick-up from neighboring powder volumes is the main reason for the variations of the powder availability
Development of Al-Cu-Mg and Al-Mg-Si-Zr Alloys with Improved L-PBF Processability
No full textMany Al alloys are susceptible to hot cracking when manufactured by Laser Powder Bed Fusion (L-PBF). In this study, small batches of Al powders were processed using a Reduced Build Volume device to target the optimal chemical composition of the alloy able to suppress hot cracks during solidification. Specifically, batches with increasing content of Cu and Zr were obtained through mechanical mixing of Al-4wt.%Cu-Mg and pure Cu and Al-Mg-Si and Al-Mg-Si-2wt.%Zr powders, respectively. The design strategy based on Cu relies on the segregation of an abundant Al-Al2Cu eutectic phase mixture during final stages of solidification, whereas the Zr addition promotes a fine equiaxed microstructure induced by heterogeneous nucleation of grains triggered by the precipitation of L1(2)-Al3Zr crystal nuclei in the liquid phase. The design of the new alloys was supported by thermodynamic simulations. The microstructures and phase transformations of the alloys were investigated through electron microscopy, X-ray diffraction, and differential scanning calorimetry
Design and Characterization of Al–Mg–Si–Zr Alloys with Improved Laser Powder Bed Fusion Processability
No full textAbstract: A key-factor for the industrial implementation of beam-based additive manufacturing technologies is the development of novel Al alloys characterized by enhanced hot-tearing resistance. Indeed, most of the standard Al alloys are susceptible to solidification cracking and can hardly be used to produce structural parts by laser-based additive manufacturing processes. In this study, we investigate the strategies to design high-strength Al alloys for Laser Powder Bed Fusion. The addition of Zr to the chemical composition of an Al–Mg–Si alloy (EN AW 6182) was carried out by following two different routes to promote the formation of equiaxed grains which are able to suppress hot cracking and enhance processability of the material. The first route is based on mechanical mixing of ZrH2 particles and gas-atomized Al alloy powder and on the in-situ reaction of the hydride to form Al3Zr nucleants. The second route relies on the use of pre-alloyed gas-atomized powders that feature Zr among the alloy elements. The specimens produced using pre-alloyed powder showed the best mechanical performance. After direct aging from the as-built condition, the alloy showed yield strength and ultimate tensile strength of 354 and 363 MPa, respectively, and elongation at fracture of 9.0 pct. The achieved properties are comparable to those of wrought 6182 alloy processed by conventional routes. Graphical Abstract: [Figure not available: see fulltext.]
Investigation and characterization of an Al-Mg-Zr-Sc alloy with reduced Sc content for laser powder bed fusion
Full text linkOnly few high strength Al alloys are processable by Laser Powder Bed Fusion due to the occurrence of hot cracks during solidification. In recent years, the addition of Zr and Sc in Al-Mg alloys revealed an effective solutions to suppress solidification cracking and improve strength. Nevertheless, since Sc is classified as a critical raw material by European Commission due to its high cost and supply risk, its content should be desiderably reduced. It is therefore necessary to focus on novel Al alloys featuring both enhanced processability and low amount of Sc. In this study, we investigated the microstructure and mechanical behavior of an Al-5.2Mg-0.8Zr-0.3Sc alloy, commercially available as m4pTM StrengthAl, produced by Laser Powder Bed Fusion. Simulations of equilibrium phase diagrams and Scheil solidification curves showed the precipitation of primary Al3Zr and Al3(Sc,Zr) in the liquid phase on cooling. These particles revealed able to act as nuclei for heterogeneous nucleation of grains, giving rise to a fine equiaxed structure which is able to suppress hot cracking and increase the processability of the Al-Mg-Zr-Sc alloy. Despite the reduced Sc content, the formation of secondary Al3(Sc,Zr) nano-phases during the annealing treatment led to a sharp increase of micro-hardness values, whereas a stress relief effect was monitored by residual stress measurements during aging. Both as-built and aged alloys show a bimodal grain size distribution and a similar crystallographic texture. Yield strength and ultimate tensile strength of 460 MPa and 485 MPa, respectively, were recorded in samples aged at 350 °C for 24 h and at 375 °C for 8 h
Investigation on two Ti–B-reinforced Al alloys for Laser Powder Bed Fusion
No full textOnly few medium: and high-strength aluminium alloys can be processed by Laser Powder Bed Fusion without forming solidification cracks. This constraint limits the diffusion of this technology in many industrial fields, including aerospace and motorsport sectors. In this study, a novel high-strength aluminium alloy for Laser Powder Bed Fusion was designed and its solidification behavior, microstructure and mechanical performance were investigated. The results were compared with those achieved by processing the high-strength A20X alloy processed with the same technology. The alloy was designed based on the chemical composition of the widely used 2618 Al alloy, a conventional high strength Al–Cu–Mg alloy for high temperature applications. The chemical composition of the 2618 alloy was modified by adding Ti and B, which form TiB2 particles that act as nuclei for the solidification of primary α-Al grains. The resulting microstructure made of equiaxed grains revealed resistant to hot cracking. A20X and 2618-modified alloys produced with optimized parameters featured relative densities higher than 99,7% and crack-free microstructures. The A20X-T6 showed yield strength and ultimate tensile strength of 428 MPa and 485 MPa, respectively, while the modified 2618-T6 revealed upper yield strength and ultimate tensile strength of 370 MPa and 468 MPa, respectively. The two alloys also showed a remarkably high strength at 150 °C and 250 °C, exceeding the typical strength values of the 2618 alloy produced by forging
Pseudoelasticity in FeMnNiAl shape memory alloy lattice structures produced by Laser Powder Bed Fusion
Full text linkThis work focusses on microstructure and mechanical properties of FeMnAlNi bulk and micro-lattice structures produced by Laser Powder Bed Fusion, an Additive Manufacturing technology. Microstructure investigation was conducted by means of Electron Back Scattered Electrons analysis, Optical Microscopy and X-Ray Diffraction. The mechanical behavior of the material was evaluated by compression tests and strains were measured by Digital Image Correlation. The material is characterized by coarse grains and by the presence of both austenitic (α-bcc) and martensitic (γ′-fcc) phases. The bulk material recovered an overall and local pseudoelastic strain of 1.3% and 2.9%, respectively. The lattice specimen exhibited ~2% strain recovery
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
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