Powder spreading process is to use a spreader such as blade or roller to spread powder layers for subsequent fusion in powder bed fusion additive manufacturing. In this work, the effects of various ...spreader geometries on powder spreading are examined by discrete element method (DEM). The results show that a compact region in the powder pile exists. Round and inclined surfaces of blade spreaders allow more particles in the compact region to be deposited compared with vertical blades, thus the powder layer formed is denser. However, they exert larger forces on the underlying part. Inhomogeneity of powder layers is caused by particle burst phenomenon, which is due to particle motion conflict in the compact region rather than large forces. Roller system has largest particle motion conflict thus powder layers formed are sparse and inhomogeneous with small layer gaps. Size segregation in blade systems is not as severe as roller systems.
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•Powder spreading process with different spreaders is simulated by DEM.•Round and inclined blades deposit more powders than vertical blades.•Round and inclined blades exert larger forces on the underlying part.•Large layer gap has better homogeneity and smaller force fluctuation.•Round blade system has the least size segregation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Powder bed fusion additive manufacturing has been applied to the fabrication of functionally graded materials. A new design that allows the material composition to change along the direction ...perpendicular to the powder spreading has been reported in the literature. Based on this design, this work examines the quality of the graded spread powder layer with two powders, which have a large difference of density. The results reveal that during the spreading of graded powders, the volume of particles on the heavy powder side is deposited less than that on the light powder side, indicating that heavy particles diffuse to the light powder side. This diffusion is affected by the spreading speed, but not much by the layer gap. Large spreading speed causes more significant deviation. The results also show that particle size affects diffusion, indicating that decreasing the particle size of the heavy powder may be a solution to reduce diffusion.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Available online Integrating transition metal centered MOFs with conductive materials is a feasible route to enhance electron transfer efficiency of materials. Herein, a composite porous structure ...CQDs10@NiFe-MOF-A was fabricated via introducing carbon quantum dots (CQDs) into porous NiFe-MOF. The CQDs would make partial loss of lattice in MOF during its growth, leading to the composite building block with the coexistance of crystalline region and amorphous region. The calcining treatment would produce an ultrathin protective layer as well as some lattice collapse. The synergy effect between NiFe ions effectively regulated electronic structure of metal active sites, and successful grafting of CQDs to NiFe-MOF significantly improved electrical conductivity. As expected, the catalyst exhibited outstanding OER performances with high mass activity of 91.6 A/g at overpotential of 300 mV and robust durability of 10,000 cycles in 1 mol/L KOH, which outperformed that of noble catalyst IrO2 of 25.2 A/g. The strategy paves a feasible and effective avenue for the non-noble metal catalysts.
A strategy of integrating porous NiFe-MOF with conductive carbon quantum dots (CQDs) was employed to enhance conductivity of the catalysts. The as-prepared CQDs10@NiFe-MOF-A exhibited outstanding OER performances with high mass activity of 91.6 A/g and robust durability of 10000 cycles in 1 mol/L KOH, which outperformed that of noble catalyst IrO2 of 25.2 A/g Display omitted
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
•Laser shape variation on melting and solidification is simulated.•The power density decreases with the incline of collimated laser rays.•The vortex within the melt pool changes due to the different ...laser radiation.•Thermal gradient and solidification rate are varied with laser incidence angles.
The shape variation of the laser beam is evidently observed in the laser powder bed fusion (LPBF) process because of changes in laser incidence angle and misalignment between the build plate and the laser focus plane. This issue is particularly relevant in large-scale LPBF systems where the laser beam needs to scan a large build area. However, most LPBF modeling studies assume vertical laser radiation. The heat transfer, melt pool, and solidification evolution due to the laser shape variation have not been well addressed and quantified. In the present study, the temperature distribution, melt pool geometry and flow dynamics are captured via numerical modelling, and the grain morphology is characterized under various laser incidence angles. The results show that the melt pool depth becomes shallower, and the width is near the beam size as the laser beam becomes more elongated. The beam shape variation can affect the liquid flow pattern with increasing incidence angle, resulting in a larger vortex at the front of the melt pool and a smaller vortex at the rear of the melt pool. The thermal gradient increases and the solidification rate decreases as the laser incident angle becomes larger. The present study enhances the understanding of multi-physics in the LPBF process.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Laser powder bed fusion (LPBF) is one of the most promising additive manufacturing (AM) technologies to fabricate metal components using laser beams. To understand the underlying thermal and physical ...phenomena in LPBF process, discrete element method (DEM) is applied to generate the randomly packed powder, then computational fluid dynamics (CFD) coupled with volume of fluid (VOF) is adopted to simulate the laser-powder interaction. The penetration and multiple reflection of laser rays is traced. The physics of melting and solidification is captured. The temperature profile indicates the laser travel path and the adsorption and transmission of laser rays with the powder. The wetting behaviour of the melt pool driven by the capillary forces leads to the formation of pores at the connection zone. It has been demonstrated that the developed model can capture the laser-powder interaction for further understanding of LPBF process.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The keyhole dynamics in the laser powder bed fusion (LPBF) process and its relationship with driving forces of surface tension, Marangoni force and recoil pressure have not been well addressed and ...quantified. In this work, through the modelling of melt pool dynamics, the keyhole life cycle including formation and drilling, fluctuation, and disappearance is captured. The results show that pores are sourced from the powder bed voids, the ejected keyhole protrusion, and the liquid eroded gas bubbles at the end of the track. An anticlockwise vortex is generated at the rear of melt pool, and the molten liquid travels in a clockwise path ahead of the melt pool. The variation of the powder layer thickness breaks the force equilibrium on the keyhole rim and leads to the depth fluctuation. The compressed rear keyhole is dominated by the surface tension, and the expanded front rim is controlled by the recoil pressure.
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•The keyhole lifetime includes drilling, fluctuation, and disappearance.•The variation of the powder layer thickness leads to the keyhole fluctuation.•The flow presents an anticlockwise vortex at the tail, and a clockwise path ahead.•Recoil pressure and surface tension dominate the keyhole dynamics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Particle scale modelling of the process physics involved in laser powder bed fusion (LPBF) is a recent research hotspot, and many efforts have been made in the literature. However, a comprehensive ...review of the physics in LPBF and the effects of key variables such as powder- and operation-related parameters, and the mechanisms of defects formation is still lacking. This paper aims to offer a state-of-the-art review on multi-physics related to metal powder recoating and further melting and solidification process. The studies on powder bed recoating explored by discrete element method are presented first, including model theory and validation, effects of process parameters, and physics of particle flow and size segregation. Then powder melting approach based on computational fluid dynamics and the involved phenomena such as melt pool dynamics, keyhole dynamics, defects formation mechanisms of pores, and balling and spattering are described. The needs for future research are also discussed.
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•Powder bed recoating explored by discrete element method (DEM) is reviewed.•Physics of powder flow and size segregation in powder spreading is presented.•Powder melting based on computational fluid dynamics (CFD) is summarized.•Melt pool flow dynamics and defects formation are discussed.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Discrete element method is used in this work to examine the mechanisms determining powder deposition efficiency during powder spreading in powder bed fusion additive manufacturing. The results reveal ...that powder flow in the powder pile is critical for the formation and break of transient jamming. The forces on the underlying part increase first with spreading speed then decrease with a large fluctuation. For varied spreader shapes, a small inclined angle of the spreader surface makes the force barrier farther from the discharging gap, creating a larger region which ensure enough powder supply to the gap. Furthermore, a small inclined angle of the spreader surface close to the gap results in less particle motion conflicts at the gap and ensures larger discharging rate through the gap. This mechanism explains why spreaders with inclined or round surfaces help increase powder deposition efficiency.
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•Powder deposition mechanisms in powder spreading are investigated by DEM.•Load on part increases first with spreading speed and then decreases.•Advantages of inclined and round spreaders are explained.•Small inclined angles create larger regions for storing and supplying powders.•Small inclined angles have less particle motion conflicts and larger discharging rate.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
A U-type reduction chamber (URC) is developed as the key component of the novel iron ore suspension roaster. It consists of a fluidization chamber (FC) and a supply chamber (SC), and functions dually ...as sealing valve and reduction chamber. This roaster has been applied successfully to iron ore reduction roasting, while only limited information is available on its working mechanism. The fluid dynamic behavior of solid particles within URC and effects of operating parameters, i.e., the fluidizing air velocity, aeration air velocity and delivery rate, on its performance were investigated in this study. Simulation experiments on the fluid behavior of feed ores were conducted in a cold operating URC apparatus using alumina particles with an average size of 147 μm. It was found that the feeding particles descend in moving-bed mode in the supply chamber, while the particles move upwards in fluidization mode in the FC under suitable operating conditions. The differential pressure fluctuation reaches the maximum at the middle region of the FC, indicating vigorous gas-solid interactions at this area. The solids holdup presents an “S” shape distribution along the height of the FC. The average solids holdup and axial nonuniformity index range from 0.35 to 0.50 and from 0.10 to 0.40, respectively, which are relatively higher than those found in conventional fluidized bed reactors, ensuring better gas-solids contacting and mixing and promotion of reduction performance. In addition, the solids height in the SC could self-adjust to maintain pressure balance in the URC system under various operating conditions, and the pressure difference produced by solids at a certain height in the SC is the main driving force to keep normal flow of solid particles. The information obtained from this study could be helpful, especially for the regulation and control of industrial iron ore suspension roasting.
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•A novel U-type reduction chamber (URC) has been presented and studied.•Impacts of operation parameters on fluid dynamic performance of URC was conducted.•Comprehensive analysis of the transient differential pressure signals.•Detailed description of axial solid holdup distribution profiles in the URC.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
Abstract
The use of Video Switching Matrix can greatly improve the utilization rate of internal space such as ships and submarines. This paper designs a multifunctional video switching matrix, which ...can realize 16-channel HDMI and VGA video signal input and 8-channel HDMI and VGA video signal output function. It can meet the complex scenes of HDMI and VGA integrated video input and output. This design realizes the conversion of input and output video formats through the front-end HDMI to VGA unit and the back-end VGA to HDMI unit. The middle RGB switching matrix is used to realize 16 input and 8 output selection of VGA video signal. It realizes the fusion of multi-channel dual-standard video switching. The module has the prospect of being applied to high reliability requirements in aerospace, navigation, and military industries.
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