Selective laser melting (SLM) manufactures components through the overlapping of multi-track and multi-layer molten pools of metal powders, resulting in two types of molten pool boundaries (MPBs), ...“layer–layer” and “track–track” MPBs, remaining in SLM parts. The microstructure of MPBs exhibits a complex and regular spatial topological structure. There is a coarse grain zone below the MPBs and nonmetallic elements (C, O, Si) near the MPBs are in an unstable state. Long and thin columnar grains with the same orientations distribute on two sides of the “layer–layer” MPBs, whereas the columnar grains on both sides of “track–track” MPBs have different orientations. The “track–track” MPBs are short and intersect with “layer–layer” MPBs at some points and form acute angles, where cracks are initiated when applied with external loads. The effect of the MPBs on microscopic slipping, macroscopic ductility and fracture mechanism of the SLM parts made along different directions, which were exerted a tensile loading in the as-built condition without heat treatment, was analyzed and evaluated using slip theory and experiments. The results reveal that the MPBs have a significant impact on the microscopic slipping at the loading, macroscopic plastic behavior and fracture mode, and are one of the main reasons for the obvious anisotropy and low ductility of SLM parts.
Powder-layering is an essential process of selective laser melting (SLM), but the underlying mechanisms of powder movement and packing at particle scale is unclear. Based on discrete element method ...(DEM), this study proposed a numerical model to investigate the flowing behavior of powder layered by a blade, where the contact force and cohesion force between individual particles were considered. DEM simulations gave visual morphologies of the flow profiles and velocity fields for powder-layering at particle scale, as well as the relationships between the quality of powder bed and the layering parameters. The model was validated by experiment results in terms of the macroscopic profiles of powder during layering, showing good prediction accuracy. Then, dynamic repose angle (DRA) and mass flow rate (MFR) were defined to make quantitative evaluation on the powder flow. Preliminary research shows that, the powder fluidity increases with the decreasing of particle friction coefficients, resulting in a denser and more uniform powder bed. The decreasing of particle radius R over the range of R > 21.8 μm can benefit the powder fluidity. However, when the particle radius decreases in the range of R < 21.8 μm, the weight of cohesion force rises and thus makes the powder fluidity worse. The increase of layering speed enhances the dilation of moving particles, and the decrease of layering height intensifies the local force-arches in particles. These will reduce the continuity and stability of the powder flow and is unfavorable for improving the density or uniformity of the layered powder bed.
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•Powder-layering in selective laser melting is modeled by discrete element method.•Powder flow related to the quality of powder bed is investigated at particle scale.•Effects of particle properties and layering conditions on powder flow are studied.•The model provides a powerful tool for powder-layering design and optimization.
Selective laser melting (SLM), as one of the additive manufacturing technologies, is widely investigated to fabricate metal parts. In SLM, parts are manufactured directly from powders in a ...layer-by-layer fashion; SLM also provides several advantages, such as production of complex parts with high three-dimensional accuracy, compared with other additive manufacturing technologies. Therefore, SLM can be applied in aeronautics, astronautics, medicine, and die and mould industry. However, this technique differs from traditional methods, such as casting and forging; for instance, the former greatly differs in terms of microstructure and properties of products. This paper summarizes relevant studies on metal material fabrication through SLM. Based on a work completed in Huazhong Univ. Sci Tech., Rapid Manuf. Center (HUST-RMC) and compared with characteristics described in other reported studies, microstructure, properties, dimensional accuracy, and application of SLM are presented.
Stable interfaces between immiscible solvents are crucial for chemical synthesis and assembly, but interfaces between miscible solvents have been less explored. Here the authors report the ...spontaneous water-on-water spreading and self-assembly of polyelectrolyte membranes. An aqueous mixture solution containing poly(ethyleneimine) and poly(sodium 4-styrenesulfonate) spreads efficiently on acidic water, leading to the formation of hierarchically porous membranes. The reduced surface tension of the polyelectrolyte mixture solution drives the surface spreading, while the interfacial polyelectrolytes complexation triggered by the low pH of water mitigates water-in-water mixing. The synergy of surface tension and pH-dependent complexation represents a generic mechanism governing interfaces between miscible solvents for materials engineering, without the need for surfactants or sophisticated equipment. As a proof-of-concept, porous polyelectrolyte hybrid membranes are prepared by surface spreading, exhibiting exceptional solar thermal evaporation performance (2.8 kg/m
h) under 1-sun irradiation.
It is believed that eye movements in free-viewing of natural scenes are directed by both bottom-up visual saliency and top-down visual factors. In this paper, we propose a novel computational ...framework to simultaneously learn these two types of visual features from raw image data using a multiresolution convolutional neural network (Mr-CNN) for predicting eye fixations. The Mr-CNN is directly trained from image regions centered on fixation and non-fixation locations over multiple resolutions, using raw image pixels as inputs and eye fixation attributes as labels. Diverse top-down visual features can be learned in higher layers. Meanwhile bottom-up visual saliency can also be inferred via combining information over multiple resolutions. Finally, optimal integration of bottom-up and top-down cues can be learned in the last logistic regression layer to predict eye fixations. The proposed approach achieves state-of-the-art results over four publically available benchmark datasets, demonstrating the superiority of our work.
Lack of monitoring of the in situ process signatures is one of the challenges that has been restricting the improvement of Powder-Bed-Fusion Additive Manufacturing (PBF AM). Among various process ...signatures.
In this work, a novel approach combining liquid deposition with selective laser melting (SLM) is used for fabricating reduced graphene oxide (RGO)/S136 metal matrix composites (MMCs). The grain ...sizes, crystallographic textures, phase compositions and mechanical properties can be tailored by controlling the RGO content in the RGO/S136 MMCs. The results show that the average grain size reaches its smallest size of 0.75 μm when 0.1 wt% RGO was added to the RGO/S136 MMCs. As the RGO content is increased from 0 wt% to 0.5 wt%, a continuous transition of the grains from the (001) orientation to the (101) and (111) orientations is observed. In addition, the cellular dendritic grains transform into equiaxed fine grains with increasing RGO content. The SLM-prepared RGO/S136 MMCs are dominated by high-angle grain boundaries (˃15°) and the martensite (bcc) phase. The hardness, ultimate tensile strength and yield strength of the SLM RGO/S136 MMCs exhibit trends that initially increase and then decrease, with maximum values of 580.6 HV, 535.3 MPa and 515.8 MPa, respectively. This paper highlights the possibility of controlling the RGO content to achieve the desired microstructural characteristics and mechanical properties of RGO/S136 MMCs fabricated by the SLM process.
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•The fully dense RGO/S136 composites via molecular-level mixing were firstly fabricated by selective laser melting.•EBSD patterns revealed the refined grain and tailored microstructure for SLM RGO/S136 composites.•SLM RGO/S136 composites with 0.1 wt% RGO exhibited maximum tensile properties and hardness.
Nickel-titanium alloys have been widely used in biomedical, aerospace and other fields due to their shape memory effect, superelastic effect, as well as biocompatible and elasto-thermal properties. ...Additive manufacturing (AM) technology can form complex and fine structures, which greatly expands the application range of Ni-Ti alloy. In this study, the development trend of additive manufactured Ni-Ti alloy was analyzed. Subsequently, the most widely used selective laser melting (SLM) process for forming Ni-Ti alloy was summarized. Especially, the relationship between Ni-Ti alloy materials, SLM processing parameters, microstructure and properties of Ni-Ti alloy formed by SLM was revealed. The research status of Ni-Ti alloy formed by wire arc additive manufacturing (WAAM), electron beam melting (EBM), directional energy dedication (DED), selective laser sintering (SLS) and other AM processes was briefly described, and its mechanical properties were emphatically expounded. Finally, several suggestions concerning Ni-Ti alloy material preparation, structure design, forming technology and forming equipment in the future were put forward in order to accelerate the engineering application process of additive manufactured Ni-Ti alloy. This study provides a useful reference for scientific research and engineering application of additive manufactured Ni-Ti alloys.
A TiAl-based alloy, Ti–45Al–2Cr–5Nb (at.%), has been processed by selective laser melting (SLM) using different energy density inputs. The experimental results show that when the energy density input ...increased from 250J/mm3 to 350J/mm3, the crystallographic texture varied from a strong (0001) orientation to a combination of (0001), 101̅1 and 112̅1 orientations. The SLM-processed TiAl alloy are dominated by high-angle (>15°) grain boundaries (HAGBs) and α2 (Ti3Al) phase. The contents of HAGBs and α2 are 92.8% and 90% respectively at the maximum density input of 350J/mm3. Moreover, a small amount of γ (TiAl) and B2 phases in a range of several hundred nanometers are uniformly distributed within the α2 matrix. The phase evolution mechanism in the SLM-processed TiAl alloy can be as follows: (210)β transformed to 202̅0α2 and (110)γ, and then the residual B2 and the incompletely transformed γ phase homogeneously distributed in the α2 phase matrix. The orientation relationship between B2, α2 and γ phases observed via HRTEM can be expressed as: 111B2//11̅0γ//112̅0α2. Those observations and discussions provide a deep insight into the microstructure characteristics and phase evolution in the SLM-processed TiAl alloy, and the findings would be a valuable reference for optimizing the energy density input in SLM to fabricate TiAl components with acceptable grain structure and phase compositions.
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•The crystal orientation and texture varied with the increased energy density input.•The contents of HAGBs and α2, γ, B2 phases were determined by EBSD.•An evolution mechanism of the α2, γ, B2 phases in SLM was discussed.•The orientation relationship between α2, γ, B2 phases were determined by HRTEM.
The microstructural stabilities, softening resistance, and high-temperature tensile properties of the H13 hot-work tool steel by selective laser melting (SLM) were systematically studied. A series of ...tempering procedures were performed on the as-SLMed specimens. Afterwards, the mechanism of softening resistance behavior was discussed based on the XRD, SEM, EBSD observations, hardness measurements, and high-temperature tensile tests. It was found that the as-SLMed H13 consisted of
α
-iron and
γ
-iron. The carbide-stabilizing elements aggregated as the cell-like substructures for the rapid solidification of the SLM process. After the softening resistance treatment, the retained austenite transformed to ferrite and carbide mixtures. The cell-like substructures dissolved slowly into the matrix when the temperature was below 550 °C. These factors increased the hardness and retarded the softening of the material. When the temperature was 600 °C, the microstructural constituents transformed to soft ferrite and globular carbides, which lead to a considerable decrease of the hardness. Due to the grain refinement, solid solution strengthening, and residual stress, the as-SLMed H13 exhibited better mechanical properties than that of the wrought counterparts.
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