Today, membrane technologies play a big role in chemical industry, especially in separation engineering. Tannic acid, one of the most famous polyphenols, has attracted widespread interest in membrane ...society. In the past several years, researches on the applications of tannic acid in membrane technologies have grown rapidly. However, there has been lack of a comprehensive review for now. Here, we summarize the recent developments in this field for the first time. We comb the history of tannic acid and introduce the properties of tannic acid firstly, and then we turn our focus onto the applications of membrane surface modification, interlayers and selective layers construction and mixed matrix membrane development. In those previous works, tannic acid has been demonstrated to be capable of making a great contribution to the membrane science and technology. Especially in membrane surface/interface engineering (such as the construction of superhydrophilic and antifouling surfaces and polymer/nanoparticle interfaces with high compatibility) and development of thin film composite membranes with high permselectivity (such as developing thin film composite membranes with ultrahigh flux and high rejection), tannic acid can play a positive and great role. Despite this, there are still many critical challenges lying ahead. We believe that more exciting progress will be made in addressing these challenges in the future.
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•Applications of tannic acid in membrane technologies are reviewed firstly.•The history and properties of tannic acid are introduced.•Tannic acid can help membrane technologies move forward.•Critical unsolved challenges are discussed and more exciting progress will be made in the future.
The prediction of thermal stress and distortion is a prerequisite for high-quality additive manufacturing (AM). The widely applied thermo-mechanical model using the finite element method (FEM) leaves ...much to be improved due to their oversimplifications on material deposition, molten pool flow, etc. In this study, a high-fidelity modelling approach by linking the thermal-fluid (computational fluid dynamics, CFD) and mechanical models (named as CFD-FEM model) is developed to predict the thermal stress for AM taking into account the influences of thermal-fluid flow. Profiting from the precise temperature profiles and melt track geometry extracted from the thermal-fluid model as well as the remarkable flexibility of the quiet element method of FEM, this work aims at simulating the thermal stress distribution by involving physical changes in the AM process, e.g., melting and solidification of powder particles, molten pool evolution and inter-track inter-layer re-melting. Unlike the conventional thermo-mechanical analysis, in this approach, thermal stress calculation is purely based on a mechanical model where the thermal loads are applied by using a linear interpolation function to spatially and temporally map the temperature values from the thermal-fluid model's cell centres into the FEM element nodes. With the proposed approach, the thermal stress evolution in the AM process of single track, multiple tracks and multiple layers are simulated, where the rough surfaces and internal voids can be well incorporated. Moreover, a conventional thermo-mechanical simulation of two tracks with predefined track geometry is conducted for cross comparison. Finally, the simulated thermal stress distribution can rationally explain the crack distribution observed in the experiments.
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•Modelling thermal stress in AM using temperature profile from thermal-fluid model.•Rough surfaces and internal voids are incorporated.•Thermal stress concentrations due to voids and rough surfaces are revealed.•Simulation results of thermal stress can rationally explain the cracks in experiments.
Powder spreading is an essential procedure in powder-bed-based additive manufacturing, and the resultant packing quality of the powder layer has important effects on the quality of the final ...products. In this work, the counter-rolling-type powder spreading is investigated by experiments and numerical simulations. Non-invasive in-situ measurements are performed to evaluate the packing qualities of the powder layer such as surface roughness and packing density, where the effect of the spreading speed is studied. It is found that both the surface quality and packing density of the powder layer decrease with the increase of spreading speed. Besides, the sensitivity of the surface roughness of the powder layer increases with the spreading speed, i.e., the higher the spreading speed is, the more remarkably the surface quality decreases. Numerical simulations using the discrete element method are performed to investigate the dynamics of the powder spreading in terms of the velocity, contact force and coordination number of powder particles, providing new insight to the physical mechanisms underlying the counter-rolling-type powder spreading at particulate scale.
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•Counter-rolling-type powder spreading process in additive manufacture is studied.•Surface roughness and packing density of thin powder layer are tested in-situ.•Packing qualities of powder layer decrease with increase of spreading speed.•The higher the spreading speed, the faster the decrease rate of surface quality.•Increase of pressure in powder pile is not beneficial for the packing qualities.
Metallic powder bed-based additive manufacturing technologies have many promising attributes. The single track acts as one fundamental building unit, which largely influences the final product ...quality such as the surface roughness and dimensional accuracy. A high-fidelity powder-scale model is developed to predict the detailed formation processes of single/multiple-track defects, including the balling effect, single track nonuniformity and inter-track voids. These processes are difficult to observe in experiments; previous studies have proposed different or even conflicting explanations. Our study clarifies the underlying formation mechanisms, reveals the influence of key factors, and guides the improvement of fabrication quality of single tracks. Additionally, the manufacturing processes of multiple tracks along S/Z-shaped scan paths with various hatching distance are simulated to further understand the defects in complex structures. The simulations demonstrate that the hatching distance should be no larger than the width of the remelted region within the substrate rather than the width of the melted region within the powder layer. Thus, single track simulations can provide valuable insight for complex structures.
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Auxetic composites, a kind of rationally artificial materials, possess superior multifunctional properties due to a mixture of materials. In this paper, an Isogeometric Topology Optimization (ITO) ...method is proposed for computational design of both the re-entrant and chiral auxetic composites in both 2D and 3D. The homogenization is numerically implemented using isogeometric analysis (IGA) to predict macroscopic effective properties of microstructures, where the periodic boundary formulation is imposed. An effective Non-Uniform Rational B-splines (NURBS)-based Multi-Material Interpolation (N-MMI) model is applied to compute material properties of all points in composite microstructures, mainly including the Fields of Design Variables (DVFs), Fields of Topology Variables (TVFs), and multi-material interpolation. A unified ITO formulation is developed for 2D and 3D auxetic composites, where an appropriate objective function with a weight parameter is defined to control the generation of different deformation mechanisms. Finally, several numerical examples are performed to demonstrate the effectiveness of the proposed ITO method, and a series of 2D and 3D auxetic composites with the re-entrant and chiral deformation mechanisms are found. The optimized composite structures are simulated using ANSYS to show the auxetic behavior.
•An ITO method is proposed for computational design of auxetic composites.•A unified ITO formulation for the re-entrant and chiral auxetic composites is developed.•A series of novel and interesting auxetic composites in 2D and 3D can be found.•The qualitative and quantitative analyses for 2D and 3D auxetic composites are addressed.
Direct energy deposition (DED) is a promising additive manufacturing technology for large-scale fabrications of high-value components. Grain structure control is challenging but meaningful for ...achieving desirable mechanical properties. A multi-scale three-dimensional (3D) Finite Volume Method-Cellular Automaton (CA-FVM) model is developed. The grain structure evolution in the transition-mode melting is investigated, and the simulated grain structures show fairly good qualitative and quantitative agreement with the experimental results. The influences of laser power and scanning speed on the formed grain structure are examined. A progressive columnar-to-equiaxed transition (CET) is found. The elongated grain is the primary grain morphology, even with the CET. The effects of high temperature gradient on the development of columnar structure are difficult to overcome. Moreover, nanoparticle reinforcement is numerically investigated as a promising technique to realize the site-specific grain structure control by interfering with the columnar growth. We expect this study to provide a deeper understanding of the DED-produced grain structure and improve confidence in the site-specific structure control.
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•A multi-scale three-dimensional Finite Volume Method-Cellular Automaton model is developed.•Columnar-to-equiaxed transition in transition-mode melting is simulated.•Elongated grain is primary grain morphology even with equiaxed nucleation.•Nanoparticle refinement is an efficient way to promote massive nucleation of equiaxed grains.
Abstract
A three-dimensional phase-field model is developed to simulate grain evolutions during powder-bed-fusion (PBF) additive manufacturing, while the physically-informed temperature profile is ...implemented from a thermal-fluid flow model. The phase-field model incorporates a nucleation model based on classical nucleation theory, as well as the initial grain structures of powder particles and substrate. The grain evolutions during the three-layer three-track PBF process are comprehensively reproduced, including grain nucleation and growth in molten pools, epitaxial growth from powder particles, substrate and previous tracks, grain re-melting and re-growth in overlapping zones, and grain coarsening in heat-affected zones. A validation experiment has been carried out, showing that the simulation results are consistent with the experimental results in the molten pool and grain morphologies. Furthermore, the grain refinement by adding nanoparticles is preliminarily reproduced and compared against the experimental result in literature.
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Metal 3D printing (3DP), a state-of-the-art manufacturing technology that brings the potential to fabricate complex structures at low cost and reduced energy consumption, has been ...extensively adopted in various industries. However, the porosity defects inherited from the printing process can significantly impede the mechanical properties and weaken the performance of as-printed components, potentially challenging this approach's reliability and reproducibility. The advancement of detection techniques currently opens up a more intuitive and deeper study of porosity defects. Given that, this review systematically states the 'restriction role' of porosity defects in metal 3DP by generalizing the detailed information on porosity defects, including their characterizations, formation and migration mechanisms, and their impacts on the performance of printed parts. Furthermore, feasible porosity mitigation measures are discussed to inspire more advanced methodologies for the next generation of metal 3DP.
Laser powder bed fusion (LPBF) is a 3D printing technology that can print metal parts with complex geometries without the design constraints of traditional manufacturing routes. However, the parts ...printed by LPBF normally contain many more pores than those made by conventional methods, which severely deteriorates their properties. Here, by combining in-situ high-speed high-resolution synchrotron x-ray imaging experiments and multi-physics modeling, we unveil the dynamics and mechanisms of pore motion and elimination in the LPBF process. We find that the high thermocapillary force, induced by the high temperature gradient in the laser interaction region, can rapidly eliminate pores from the melt pool during the LPBF process. The thermocapillary force driven pore elimination mechanism revealed here may guide the development of 3D printing approaches to achieve pore-free 3D printing of metals.
Selective Electron Beam Melting (SEBM) is a promising powder-based metallic Additive Manufacturing (AM) technology. However, most powder-scale modeling efforts are limited to single track process, ...while it is also difficult to experimentally observe the interaction between tracks and layers. In this study, we develop an integrated modeling framework to investigate the SEBM process of multiple tracks and multiple layers. This approach consists of a Discrete Element model of powder spreading and a Computational Fluid Dynamics (CFD) model of powder melting. These two models exchange 3D geometrical data as a cycle to reproduce the manufacturing process of multiple tracks along various scan paths in multiple powder layers. This integrated modeling approach enables further understanding of how current tracks and layers interact with previous ones leading to inter-track/layer voids. It also incorporates more influential factors, particularly the layer-wise scan strategy. The inter-layer/track voids due to the lack of fusion are systematically discussed in light of our simulation results which qualitatively agree with experimental observations in literature.
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•An integrated DEM-CFD modeling framework to simulate the powder being spread and then melted.•Meso-scale simulations of SEBM processes of multiple-layer multiple-track along various scan paths.•The formation of inter-layer/track voids due to lack of fusion and the influence of scan path.
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