Additive Manufacturing

Journal: Additive Manufacturing

Publisher

Elsevier

ISSN

2214-8604

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Publications 1 - 10 of 53

Analysis and compensation of shrinkage and distortion in wire-arc additive manufacturing of thin-walled curved hollow sections
Nguyen, Lam; Buhl, Johannes; Israr, Rameez; et al. (2021)
Additive Manufacturing
Curved hollow sections are used in numerous applications from lightweight space frames to pipings in process engineering. In conventional manufacturing, such structures are produced mainly by metal forming or casting. Despite ongoing efforts in rapid tooling technologies for casting and flexible forming processes, there is a demand for dieless manufacturing of curved hollow structures, as this would allow to produce parts with greatly reduced lead-times, e.g., as on-demand spare parts and on construction sites using mobile equipment. Wire-arc additive manufacturing (WAAM) processes with multi-axis deposition can be used in this case. This process typically draws upon the gas metal arc welding (GMAW) process with cold metal transfer (CMT) technology. Thin-walled metal parts produced by WAAM may show deviations to the target geometry due to material shrinkage and distortion, which may entail tedious trial-and-error compensation. This study analyses shrinkage and distortion in curved hollow sections with different cross sections and curvature radii both experimentally and with a thermo-mechanical finite element model. Based on these findings, a novel method for the compensation of shrinkage and distortion for thin-walled hollow parts is put forward and validated. The correction method draws upon the geometrical deviations that occur when the part is produced based on the CAD geometry, and computes a correction to the CAD geometry, which is used to define the welding path for a part with improved accuracy. The results show that the geometry of the manufactured part can be corrected to tolerances in the range of the waviness of the surface by applying the proposed correction.
Mechanical properties of 3D printed material with binder jet technology and potential applications of additive manufacturing in seismic testing of structures
Del Giudice, Lorenzo; Vassiliou, Michalis F. (2020)
Additive Manufacturing
Additive manufacturing can be used for the construction of small-scale specimens that are useful for the understanding of the seismic behavior of conventionally constructed masonry structures. In fact, it can provide useful information for the validation of the global level assumptions that numerical models of structures have to make, but are hard to validate as large-scale tests are very expensive. To this end, this paper suggests the use of a Binder Jet printer to manufacture small-scale masonry models. The first step for such a validation procedure is the determination of the mechanical properties of the bulk material printed with a Binder Jet printer. Compression and bending tests on a sand based printer that uses furan binder shows that the bulk material presents anisotropy in compression, but to a lesser degree than other powder based printers. In tension, the anisotropy is found to be statistically insignificant – in stark contrast with values reported in the literature for powder based printers. Aging is found to be crucial for the mechanical properties: They are found to reach a plateau after 15 days of curing time. No scale phenomena were observed for length scales between 50 and 100 mm.
Directed energy deposition of Inconel 718 powder, cold and hot wire using a six-beam direct diode laser set-up
Bambach, Markus; Sizova, Irina; Kies, Fabian; et al. (2021)
Additive Manufacturing
This study puts forward a new set-up for directed energy deposition (DED) with a six-beam direct diode laser head. The set-up implements three process variants, i.e. cold and hot wire as well as powder DED. The differences in the material behavior of INCONEL® alloy 718 (IN718) during additive manufacturing using the three processes is analyzed with respect to dilution, penetration into the base material, as well as solidification microstructure, second phase particles and texture. The results confirm that all three processes allow for defect-free DED of IN718, and that the hot wire process facilitates high-deposition rates with low dilution, whereas the cold wire and powder processes yield different dilution and penetration. The differences of the three process variants motivate to apply these processes interchangeably on the same part depending on the desired local microstructures, textures and thus, properties. The fundamental correlation between the processing conditions of the novel set-up used and the material behavior during DED is discussed.
Magnetically navigable 3D printed multifunctional microdevices for environmental applications
Bernasconi, Roberto; Carrara, Elena; Hoop, Marcus; et al. (2019)
Additive Manufacturing
Microrobotic prototypes for water cleaning are produced combining stereolithography 3D printing and wet metallization. Different metallic layers are deposited on 3D printed parts using both electroless and electrolytic deposition to impart required functionalities. In particular, by exploiting the flexibility and versatility of electrolytic codeposition, pollutants photodegradation and bacteria killing are for the first time combined on the same device by coating it with a composite nanocoating containing titania nanoparticles in a silver matrix. The microstructure of the microrobots thus obtained is fully characterized and they are successfully actuated by applying rotating magnetic fields. From the water cleaning point of view, devices show evident photocatalytic activity towards water pollutants and antimicrobial activity for gram negative bacteria.
Chemical recovery of spent copper powder in laser powder bed fusion
Speidel, Alistair; Gargalis, Leonidas; Ye, Jianchao; et al. (2022)
Additive Manufacturing
In laser powder bed fusion (LPBF), recovered unfused powder from the powder bed often degrades upon sequential processing through mechanisms like thermal oxidation and particle satelliting from ejected weld spatters and particle-laser interactions. Given the sensitivity of LPBF performance and build quality to powder properties, spent powder is generally discarded after a few build cycles, especially for materials that are sensitive towards surface oxidation. This increases feedstock material costs, as well as costs associated with machine downtime during powder replacement. Here, a new method to chemically reprocess spent LPBF metal powder is demonstrated under ambient conditions, using a heavily oxidised Cu powder feedstock recovered from prior LPBF processing as a model material. This is compared to an equivalent virgin Cu powder. The near-surface powder chemistry has been analysed, and it is shown that surface oxide layers present on spent Cu powder can be effectively reset after rapid reprocessing (from 5 to 20 min). Diffuse reflectance changes on etching, reducing for gas-atomised virgin Cu powder due to the formation of anisotropic etch facets, and increasing for heavily oxidised spent Cu as the highly absorptive oxide layers are removed. The mechanism of powder degradation for moisture sensitive materials like Cu has been correlated to the degradation of LPBF deposits, which manifests as widespread and extensive porosity. This extensive porosity is largely eliminated after reprocessing the spent Cu powder. Chemically etched spent powder is therefore demonstrated as a practical feedstock in LPBF in which track density produced is comparable to virgin powder.
The effect of multi-beam strategies on selective laser melting of stainless steel 316L
Heeling, Thorsten; Wegener, Konrad (2018)
Additive Manufacturing
In-situ controller autotuning by Bayesian optimization for closed-loop feedback control of laser powder bed fusion process
Kavas, Barış; Balta, Efe C.; Tucker, Michael Robert; et al. (2025)
Additive Manufacturing
Laser powder bed fusion (LPBF) additive manufacturing (AM) traditionally relies on static parameter assignment in an open loop, which can lead to defects when faced with complex thermal histories and process variability. Closed-loop control offers a promising alternative that can enhance stability and mitigate defects. However, controller performance relies heavily on precise parameter tuning, a process that is typically manual and system-specific. This study employs Bayesian Optimization (BO) as an automated, sample-efficient method for tuning in-layer controllers in LPBF, leveraging the repetitive nature of the process for either online (in-process) or offline (pre-process) tuning. We experimentally apply BO to tune an in-layer PI controller to modulate laser power, assessing its performance on wedge geometries prone to overheating. The results show that BO significantly reduces overheating, outperforming uncontrolled settings. Notably, this study presents the first microstructural analysis of parts produced with in-layer controlled tuning, identifying lack-of-fusion porosities caused by the controller’s corrective adjustments. In summary, BO demonstrates strong potential for automated controller tuning in LPBF, with implications for broader applications in AM, suggesting a path towards more adaptive and robust control across diverse machines and materials.
Selected design rules for material extrusion-based additive manufacturing of alumina based nozzles and heat exchangers considering limitations in printing, debinding, and sintering
Hadian, Amir; Morath, Benjamin; Biedermann, Manuel; et al. (2023)
Additive Manufacturing
Recent developments in the area of material extrusion-based additive manufacturing (MEX-AM) make it possible to produce complex-shaped ceramic parts using a layerwise printing process. To achieve a dense ceramic part, the green printed body is post-processed in subsequent debinding and sintering steps. Only a very limited amount of works exist that study the design limits of 3D printed structures by the MEX-AM process, especially considering the effects of the subsequent post-processing steps. The knowledge of such restrictions or design limits is key to fabricating complex parts without process failures or part defects. In this study, a set of geometric test specimens to determine the minimum build angle, printability of arches, minimum wall thickness, and shrinkage after debinding and sintering were printed with a filament-based extrusion head. Based on the test specimens, the work presents a first set of design rules to consider the process restrictions due to MEX-AM and post-processing for alumina parts. The work shows how simple printing structures can be used to derive design rules to guide the design of a complex-shaped multi-flow nozzle and a heat exchanger based on alumina without additional support structures. Finally, the work discusses potential improvements for future studies.
Electrochemical 3D printing of silver and nickel microstructures with FluidFM
van Nisselroy, Cathelijn; Shen, Chunjian; Zambelli, Tomaso; et al. (2022)
Additive Manufacturing
Electrochemical 3D printing of conductors with microscale resolution holds a great promise for a wide range of applications, but the choice of suitable metals for these technologies remains limited. Most efforts so far have been focused on deposition of copper, however, other metals are also of interest, especially when tuning of mechanical, electrical, or optical properties is required for a given application. Here we address the issue of a limited materials choice in electrochemical 3D printing by extending the materials library to silver and nickel. Free-standing microscale structures are fabricated in a single step via locally confined electrochemical 3D printing using FluidFM – a microchanneled cantilever nanopipette capable to deliver electrolyte through sub-microscale opening. The 3D printed structures are constructed in a layer-by-layer fashion, which allows complex geometrical shapes such as double rings, helices and tripods. We report the process performance in terms of printing speed (in the range 7–40 nm s−1 for silver and 27–42 nm s−1 for nickel) and reveal dense inner structure and chemical purity of the printed features
Stress-strain response of polymers made through two-photon lithography: Micro-scale experiments and neural network modeling
Diamantopoulou, Marianna; Karathanasopoulos, Nikolaos; Mohr, Dirk (2021)
Additive Manufacturing
Photopolymerization is the governing chemical mechanism in two-photon lithography, a multi-step additive manufacturing process. Negative-tone photoresist materials are widely used in this process, enabling the fabrication of structures with nano- and micro-sized features. The present work establishes the relationship among the process parameters, the degree of polymerization, and the nonlinear stress-strain response of polymer structures obtained through two-photon polymerization. Honeycomb structures are fabricated on a direct laser writing system (Nanoscribe) making use of different laser powers for two widely applicable, commercially available resins (IP-S and IP-Dip). The structures are then tested under uniaxial compression to obtain the corresponding stress-strain curves up to 30% strain. Raman spectroscopy is used to correlate the degree of conversion achieved upon different laser exposures of the base photoresist material with the selected mechanical properties (Young's modulus, tangent modulus, deformation resistance) after polymerization. Significant differences are recorded in the observed constitutive responses. Higher degrees of conversion result in higher elastic moduli and strength at large strains. Moreover, it is found that the IP-Dip resin yields higher degrees of conversion for the same laser power compared to the IP-S resin. A neural network model is developed for each resin that predicts the stress-strain response as a function of the degree of conversion. For each material, an analytical form of the identified constitutive response is provided, furnishing basic formulas for engineering practice.

Publications 1 - 10 of 53

Journal: Additive Manufacturing

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