Packed Powder Bed Research Articles

Experimental approach for development of a powder spreading metric in additive manufacturing

The powder spreading is a vital step of powder-based additive manufacturing (AM) processes. The quality of spread powder can considerably influence the properties of fabricated parts. Poorly packed powder beds with high surface roughness result in printed part layers with large porosity and low dimensional accuracy, leading to poor mechanical properties. Therefore, the powder spreadability and its dependence on process parameters and powder characteristics should be quantified to improve the efficiency of powder-based AM methods. This study proposes a novel dimensionless powder spreadability metric that can be commonly used in different powder-based AM processes. The quality of spread powder in terms of powder bed density and surface roughness was evaluated by adjusting the process parameters including recoating velocity and layer thickness, and powder characteristics including particle size distribution. In addition, the dynamic repose angle was proposed and examined as another powder spreadability metric. The results showed that these two proposed metrics were strongly correlated and lower recoating velocity and larger layer thickness led to higher spreadability and lower dynamic repose angle.

A molecular dynamics study of laser melting of densely packed stainless steel powders

Understanding the microstructure evolution mechanism of certain alloys during the laser based directed energy deposition (L-DED) process is key to reaching the goal of adjusting the proceeding parameters and controlling the properties of deposited structures. However, it is significantly difficult to capture the corresponding microstructure changes explicitly. In this study, a large-scale molecular dynamics (MD) simulation model, consisting of a densely packed powder bed and a cracked substrate, is built to show the grain growth and crack repairing process of the stainless steel by L-DED. A continuous laser track is employed, while the localized heating and solidification of meltpools are emulated directly by controlling the temperature distribution inside the meltpool areas temporally. By investigating the forming mechanism of the deposited layer, the simulation results show that the epitaxial growth of the grain stems from the substrate region. The defects, such as vacancies, stacking faults, and twin boundaries are usually formed near the cracked sites due to the partial dislocation and the glide of the atom. It is also found that the number of equiaxed grains is significantly larger than the number of columnar grains in the deposited layer, which results in the fact that the average grain size by the L-DED deposition is closely associated with the equiaxed grain size. Furthermore, the MD model (nanoindentation tests, uniaxial tensile tests) also demonstrates qualitative consistency with the experiments. It is found that the enhanced hardness and ultimate strength surface are attributed to the grain refinement on the energy-deposited layer.

Frugal Imaging Technique of Capillary Flow through Three-Dimensional Polymeric Printing Powders.

The development of novel imaging techniques of molecular and colloidal transport, including nanoparticles, is an area of active investigation in microfluidic and millifluidic studies. With the advent of three-dimensional (3D) printing, a new domain of materials has emerged, thereby increasing the demand for novel polymers. Specifically, polymeric powders, with average particle sizes on the order of a micron, are experiencing a growing interest from academic and industrial communities. Controlling material tunability at the mesoscopic to microscopic length scales creates opportunities to develop innovative materials, such as gradient materials. Recently, a need for micron-sized polymeric powders has been growing, as clear applications for the material are developing. Three-dimensional printing provides a high-throughput process with a direct link to new applications, driving investigations into the physio-chemical and transport interactions on a mesoscale. The protocol that is discussed in this article provides a non-invasive technique to image fluid flow in packed powder beds, providing high temporal and spatial resolution while leveraging mobile technology that is readily available from mobile devices, such as smartphones. By utilizing a common mobile device, the imaging costs that would normally be associated with an optical microscope are eliminated, resulting in a frugal-science approach. The proposed protocol has successfully characterized a variety of combinations of fluids and powders, creating a diagnostic platform for quickly imaging and identifying an optimal combination of fluid and powder.

Numerical simulation on evolution process of molten pool and solidification characteristics of melt track in selective laser melting of ceramic powder

A three-dimensional high-fidelity physical model for selective laser melting (SLM) of ceramic powder was created based on computational fluid dynamics (CFD) to examine the physical mechanism of molten pool and solidified tracks at mesoscopic scale. The discrete element method (DEM) was used to generate a randomly packed powder bed, and the volume-of-fluid method (VOF) was applied to dynamically monitor the free surface of the molten pool. The formation mechanism and evolution characteristics of the molten pool were found and analyzed, and the effects of laser power on the typical characteristics of solidified ceramic tracks of SLM were investigated. The molten pool was eventually solidified into a concave geometric shape track by surface tension. The laser power played a significant impact on the shaping quality of solidified ceramic track. When the laser power was too low, the melt track suffers from severe porosity and distortion defects, which can be effectively solved with increasing laser power. The simulation results were validated via single track selective laser melting of TiC ceramic powder.

Simulation of Powder Packing and Thermo-Fluid Dynamic of 316L Stainless Steel by Selective Laser Melting

A mesoscopic model was established to investigate thermo-fluid dynamic behavior in selective laser melting of 316L stainless steel. A powder packing model, including particle size distribution based on the discrete element method, was developed to describe the relative density variation of the powder bed with different layer thicknesses. A finite element method model with Gaussian laser beam was established to predict the dynamic thermal behavior and flow mechanism of the particles for a single-line scanning. The level-set method was used to trace the free surface of the molten metal with temperature-dependent surface tension. A function to describe the relative density of the powder bed with its thickness was obtained. An evaporation model considering the influence of mass, energy loss, and evaporation on the surface morphology of the molten pool was established. According to the temperature and velocity field in the molten pool, a vortex formed by the Marangoni effect might decrease the depth and increase the width of the molten pool. The molten pool was not disconnected by Plateau–Rayleigh instability due to the small length/diameter ratio at low scanning speeds. The model assuming regularly arranged powder bed underestimates the maximum temperature as compared to the model considering a randomly packed powder bed because a higher relative density of the former facilitates heat conduction. The simulated results are consistent with experimental results, including porosity, material loss, and surface defects.

Mesoscopic study of thermal behavior, fluid dynamics and surface morphology during selective laser melting of Ti-based composites

A mesoscopic simulation based on the randomly packed powder bed model was developed to study the thermal behaviors during selective laser melting (SLM) of Ti-based composites. Effects of processing parameters on the thermal behavior, fluid dynamics and surface morphology evolution within the molten pool were investigated. The obtained results revealed that the operating temperature, cooling rate and melt lifetime were highly enhanced as the laser power was increased. Meanwhile, the increased molten pool dimensions, the turbulent fluid flows, the improved escaping rate of the entrapped gas and the efficient rearrangement of reinforcing particles within the molten pool appeared at the application of the high laser power. At the optimized processing parameters, the peak of the operating temperature profile located in the laser and powder interaction area was apparently disappeared with the formation of the maximum temperature of 3300 K and, the mean operating temperature of the platform caused by the heat accumulation was as high as 1300 K. Moreover, the surface morphology of the molten pool predicted by the simulation showed a variation from continuous pores to fragments, then to the typical and regular liquid front, and finally to the turbulent liquid front and spatter and balling phenomenon as the laser power increased. At the laser power of 200 W and laser energy density of 140 J/m, the maximum velocity was located in the front and rear region and, the velocity vector located in the melt advanced front pointed to the rear region of the molten pool, indicating that the melt from the irradiation region would complete the efficient melt supplement and avoid the formation of residual pores and therefore, a good and flat surface with few spatters was obtained with the clear liquid front. The simulated surface morphology was found to be consistent with the experimental measurements.

Effect of particle size distribution on the packing of powder beds: A critical discussion relevant to additive manufacturing

Several additive manufacturing (AM) methods use powder feed materials. Selective laser sintering is an example of a versatile AM method, using feed material in powder form, capable of producing polymer and metallic parts. In the variations of this technique, a laser spot or an electron beam is used to locally sinter or melt a packed powder bed. After the completion of sintering on each layer, further powder is added on top of the existing bed so that the next layer may be joined. A major challenge in this method is controlling the porosity of the powder bed so that the final part has uniform and maximum density. Uniformity in the packing of bed from one layer to the other is important for optimizing the processing parameters. This review is focused on considering the packing characteristics of polydisperse hard particle beds and the determination of the expected density achievable for a given particle size and shape distribution. Models are presented for discrete mixtures as well as continuous distributions. The effect of the initial configuration of a particle bed on its ability to form a highly dense packing is also discussed. Blending of different particle sizes and shapes can be used to substantially increase the packing density, but can also lead to separation or segregation of the bed. Through appropriate control of the particle shape and use of wide distributions, packing densities close to 100 % can theoretically be achieved, but practicality and various effects that appear at small size scales prevent from achieving such high packing densities. Recent advancements have reduced the dependence of AM part quality on the density of the packed particle bed but the packing is still important for considerations such as thermal conductivity of the bed and absorption of laser power in the bed. Improved knowledge of packed bed characteristics can be helpful in developing AM methods for novel material systems.

Analytical modeling of part porosity in metal additive manufacturing

Porosity is a frequently observed defect in metal additive manufacturing due to the large thermal gradients caused by the repeated rapid melting and solidification. This work presents a physics-based model to predict the part porosity in powder bed metal additive manufacturing (PBMAM) per given process parameters, materials properties, powder size distribution. The molten pool dimensions were first calculated by a closed-form temperature solution considering the laser heat input and part boundary heat loss. The porosity evolution was calculated by subtracting the volume fraction of the preprocessed powder bed void from the postprocessed porosity. The volume fractions of powder and void in packed powder bed were calculated by advancing front approach (AFA) and image analysis considering powder size variation and statistical distribution. The porosity model was developed based on the correlation between molten pool dimensions and porosity evolution using regression analysis. Experimental validation of porosity values under different process conditions was included in PBMAM of Ti6Al4V. A close agreement was observed upon validation. Different from the previous statistical model, the presented model was developed based on the physical relationship between thermal behaviors porosity formation, which leads to an improved prediction accuracy with a small number of calibration data. Moreover, the short computational time of the closed-form temperature solution allows a fast porosity prediction for various processing window.

Fundamentals of radiation heat transfer in AlSi10Mg powder bed during selective laser melting

Purpose This paper aims to study numerically the influence of the applied laser energy density and the porosity of the powder bed on the thermal behavior of the melt and the resultant instability of the liquid track. Design/methodology/approach A three-dimensional model was proposed to predict local powder melting process. The model accounts for heat transfer, melting, solidification and evaporation in granular system at particle scale. The proposed model has been proved to be a good approach for the simulation of the laser melting process. Findings The results shows that the applied laser energy density has a significantly influence on the shape of the molten pool and the local thermal properties. The relative low or high input laser energy density has the main negative impact on the stability of the scan track. Decreasing the porosity of the powder bed lowers the heat dissipation in the downward direction, resulting in a shallower melt pool, whereas pushing results in improvement in liquid track quality. Originality/value The randomly packed powder bed is calculated using discrete element method. The powder particle information including particle size distribution and packing density is taken into account in placement of individual particles. The effect of volumetric shrinkage and evaporation is considered in numerical model.

Three routes to superinsulating silica aerogel powder

Silica aerogel is the archetypal thermal superinsulator and commonly applied to improve the thermal performance of on- and off-shore industrial infrastructure and buildings. Hitherto, the main products on the market are silica aerogel based blankets fabricated by casting a silica sol into a porous or fiber matrix, followed by gelation, hydrophobization, and (supercritical) drying. Considering the diffusion efficiency of the reagents and solvents used in the preparation, a reduced size of the gel bodies may accelerate and simplify the sol-gel, hydrophobization, and drying processes. Thus, particle based products, derived from silica aerogel granulate and powder additives and semi-finished products, are an attractive solution towards inexpensive aerogel applications. Here, we optimized the process parameters for silica aerogel powder production from three common silica precursors: waterglass (WG), ion-exchanged waterglass, and tetraethoxysilane (TEOS), including gelation pH, hydrophobization procedure, solvent system, processing temperatures, and ambient pressure drying protocol. Successful hydrophobization is confirmed by elemental analysis, FTIR and quantitative solid-state NMR spectroscopy. All three routes lead to silica aerogel powders with similar type IV isotherms, and BET surface areas above 700 m2/g. Importantly, the thermal conductivity of the packed powder beds does not exceed 20 mW/(m⋅K) for all three routes, implying even lower thermal conductivities of the aerogel phase itself. Total processing time, including gelation, aging, surface modification, and ambient pressure drying is between 2 and 4 h, depending on the selected route. Given that high quality silica aerogel powders can be produced from all investigated silica precursors and hydrophobization agents, the process selection for industrial upscaling can be based entirely on engineering and economic considerations.

Laser sintering of cold-pressed Cu powder without binder use

In this work, laser sintering (LS) of cold-pressed copper (Cu) powder without using a binder was demonstrated. To promote the densification of the final part, the Cu powder was cold pressed beforehand, thereby forming a densely packed powder bed for the LS. The densification and microstructural evolution study show that the cold-pressed powder led to an increase of up to 11% in the relative density and a decrease of 10 times in the surface roughness. The influence of the scan speed on the densification and sintering quality was studied. A relatively slow scan speed, 25 µm/s, allowed sufficient atomic diffusion during the LS process and ultimately produced dense Cu parts without cracks and open pores. The introduction of the cold-pressing step into the LS process is critical for achieving high-density powder metallurgy parts without using binders.

Thermal dynamic behavior during selective laser melting of K418 superalloy: numerical simulation and experimental verification

During selective laser melting (SLM) of K418 powder, the influence of the process parameters, such as laser power P and scanning speed v, on the dynamic thermal behavior and morphology of the melted tracks was investigated numerically. A 3D finite difference method was established to predict the dynamic thermal behavior and flow mechanism of K418 powder irradiated by a Gaussian laser beam. A three-dimensional randomly packed powder bed composed of spherical particles was established by discrete element method. The powder particle information including particle size distribution and packing density were taken into account. The volume shrinkage and temperature-dependent thermophysical parameters such as thermal conductivity, specific heat, and other physical properties were also considered. The volume of fluid method was applied to reconstruct the free surface of the molten pool during SLM. The geometrical features, continuity boundaries, and irregularities of the molten pool were proved to be largely determined by the laser energy density. The numerical results are in good agreement with the experiments, which prove to be reasonable and effective. The results provide us some in-depth insight into the complex physical behavior during SLM and guide the optimization of process parameters.

Quantifying and Understanding Mass Transfer in Redox Flow Batteries

Redox flow batteries (RFBs) are an attractive technology for grid-scale energy storage due to independent scaling of battery power and energy ratings. These devices utilize a liquid-phase electrolyte, with dissolved active species, which is pumped through a flow-through porous electrode, where the desired electrochemical reactions take place. Despite the critical role of mass transfer on flow cell performance, mass transfer rates within the porous electrodes of RFBs are rarely quantified. This modeling and experimental study quantifies and explains mass transfer rates in RFBs as a function of active species concentration, flow rate, and flow field design in a systematic fashion. Based on porous electrode theory,1we develop a steady-state, one-dimensional model to describe electrode polarization, considering losses due to the electrolyte resistivity, charge transfer, and convective mass transfer. Combining the Butler-Volmer kinetic equation, linear mass transfer relationships, Ohm’s Law and several assumptions for a model redox cell, a second-order, dimensionless, and ordinary differential equation emerges for the potential as a function of position and two dimensionless parameters. Solved numerically, the model produces a series of dimensionless plots that illustrate how the RFB electrode behaves across a range of exchange and limiting current values. For each steady-state cell polarization (I-V characteristic), the model reveals an explanatory spatial variation in overpotential and current distribution across the electrode. The dimensionless nature and reduction to two parameters enable facile curve fitting of the model to experimental polarization curves, such as shown in Figure 1. In conjunction with the model, we implement a single electrolyte diagnostic flow cell technique,2 with an iron chloride electrolyte, to probe the polarization performance as a function of the aforementioned experimental parameters. Quantitative mass transfer coefficients (km ) are extracted by fitting the two model parameters to experimental polarization curves, and the only additional experimental data required is the electrolyte conductivity. In this work, power-law proportionalities between mass transfer coefficient and electrolyte velocity are revealed for 4 flow field types: flow through (FTFF), interdigitated (IDFF), parallel (PFF), and serpentine (SFF). Quantifying mass transfer rates for 4 common RFB flow fields offers mechanistic insight into transport phenomena and provides tangible parameters for future engineering optimization. In terms of mechanistic understanding, the FTFF measurements indicate that traditional mass transfer coefficient correlations for packed powder beds are shifted relative to porous carbon paper electrodes used here. The small km values associated with the PFF and weak flow rate dependence confirms the findings of prior studies that the PFF does not promote forced convection in the porous electrode and is thus unsuitable for implementation in RFBs.2Additionally, the surprisingly high mass transfer rates associated with the SFF and the intermediate velocity dependence of the IDFF raise interesting questions as to the role of mixed transport in flow field designs. The mass transfer coefficient data and correlations in this work can serve as a basis for more advanced computational studies, for optimizing electrolyte flow rate to balance electrochemical performance and pump work, and for more detailed system-level descriptions of technical performance and cost. Moreover, this combined modeling and experimental approach offers transparency and applicability to a range of porous electrode materials, electrolyte compositions, and flow field geometries for other flow batteries, or other flowable electrochemical systems, with porous electrodes. Acknowledgements J. D. M. and K. M. T. contributed equally to this work. This project was supported by the Joint Center for Energy Storage Research (JCESR), an Energy and Innovation Hub funded by the United States Department of Energy. J. D. M. acknowledges additional funding from the National Science Foundation Graduate Research Fellowship, and K. M. T. recognizes support from the MIT summer research program.

Thermal behavior in single track during selective laser melting of AlSi10Mg powder

A three-dimensional model was developed to simulate the radiation heat transfer in the AlSi10Mg packed bed. The volume of fluid method (VOF) was used to capture the free surface during selective laser melting (SLM). A randomly packed powder bed was obtained using discrete element method (DEM) in Particle Flow Code (PFC). The proposed model has demonstrated a high potential to simulate the selective laser melting process (SLM) with high accuracy. In this paper, the effect of the laser scanning speed and laser power on the thermodynamic behavior of the molten pool was investigated numerically. The results show that the temperature gradient and the resultant surface tension gradient between the center and the edge of the molten pool increase with decreasing the scanning speed or increasing the laser power, thereby intensifying the Marangoni flow and attendant turbulence within the molten pool. However, at a relatively high scanning speed, a significant instability may be generated in the molten pool. The perturbation and instability in the molten pool during SLM may result in an irregular shaped track.

Numerical simulation and parametric analysis of selective laser melting process of AlSi10Mg powder

A three-dimensional numerical model was developed to investigate effects of laser scanning speed, laser power, and hatch spacing on the thermodynamic behaviors of the molten pool during selective laser melting of AlSi10Mg powder. A randomly distributed packed powder bed was achieved using discrete element method (DEM). The powder bed can be treated as a porous media with interconnected voids in the simulation. A good agreement between numerical results and experimental results establish the validity of adopted method. The numerical results show that the Marangoni flow within the molten pool was significantly affected by the processing parameters. An intense Marangoni flow leads to a perturbation within the molten pool. In addition, a relatively high scanning speed tends to cause melt instability. The perturbation or the instability within the molten pool results in the formation of pores during SLM, which have a direct influence on the densification level.

Effects of Hot Isostatic Pressing on Copper Parts Fabricated via Binder Jetting

Binder Jetting, an Additive Manufacturing process that fabricates parts via selective inkjet deposition of binder into a powder bed, is capable of cost-effectively producing complex metal and ceramic components without the need for support structures or anchors. Printed green parts are then sintered for added densification and strength. However, printed parts typically contain porosity due to the use of coarse powders and a loosely packed powder bed. While researchers have investigated many techniques (e.g., process parameter and powder morphology optimization) for achieving full theoretical density in binder jetted parts, 100% density is difficult to achieve without infiltration of a secondary lower melting point material. In this work, the authors investigate the use of Hot Isostatic Pressing (HIP) as a post-process heat treatment of sintered parts to evaluate its effect on density, porosity, and shrinkage of parts printed in Binder Jetting. The authors conduct this investigation in the context of copper. It is demonstrated that the use of HIP can improve the final part density from 92% (following sintering) to 99.7% of theoretical density.

Finite element simulations of temperature distribution and of densification of a titanium powder during metal laser sintering

Metal Laser Sintering (LS) is a powder bed fusion process that can be used to produce manufactured parts of complex shapes directly from metallic powders. One of the major problems of such powder bed fusion processes is that during the continuous movement of the laser beam, temperature distribution becomes inhomogeneous and instable in the powder. It leads to greater residual stresses in the solidified layer. Thus, temperature analyses must be performed to better understand the heating-cooling process of the powder bed as well as the interactions of different laser scanning paths within a sintering pattern. A transient 3D Finite Element (FE) model of the LS process has been developed with the commercial FE code ABAQUS. The model takes into account the different physical phenomena involved in this powder bed fusion technology (including thermal conduction, radiation and convection). A moving thermal source, modeling the laser scan, is implemented with the user scripting subroutine DFLUX in this FE code. The material’s thermal behavior is also defined via the subroutine UMATHT. As the material properties change due to the powder bed fusion process, the model takes it into account. In this way, the calculation of a temperature-dependent behavior is undertaken for the packed powder bed, within its effective thermal conductivity and specific heat. Furthermore, the model accounts for the latent heat due to phase change of the metal powder. Finally, a time- and temperature-dependent formulation for the material’s density is also computed, which is then integrated along with the other thermal properties in the heat equation. FE simulations have been applied to the case of titanium powder and show predictions in good agreement with experimental results. The effects of process parameters on the temperature and on the density distribution are also presented.

Discrete element simulation and experimental study of powder spreading process in additive manufacturing

Powders used in additive manufacturing (AM) are spread onto a compact layer of particles for sintering and this process is repeated layer by layer to form the final products. Spreading of rod-shaped particles in realistic AM settings is simulated using the discrete element method (DEM) to investigate the effects of particle shape and operating conditions on the bed quality, characterised by its surface roughness and solid volume fraction. It is discovered that larger particle aspect ratios, Ar, or higher spreader translational velocities result in a lower bed quality, i.e. a larger surface roughness and a smaller volume fraction. The surface roughness increases monotonically with Ar. However, the volume fraction exhibits a maximum at Ar=1.5 for randomly packed powder beds that are formed by the roller type spreaders moving at low translational velocities. It is also found that a roller outperforms a blade spreader in terms of the quality of the prepared bed at the same operating conditions. The micro-structural analysis of the beds also shows particle alignment in response to the induced flow, which is qualitatively confirmed by a set of purposely-designed experiments. In addition, a shape segregation is documented for powders with mixed aspect ratios (Ar) such that particles with larger Ar tend to accumulate on the upper layers of the bed.

Quantification of powder wetting by drop penetration time

We quantify the wetting step of rehydration of a food powder by studying drop penetration times (DPTs) in powder beds. A single droplet is placed on powder beds with various porosities and DPT is recorded by a digital microscope camera. DPTs for corn starch, potato starch and wheat starch are compared to the model proposed by Hapgood et al. (2002). Characterization of the internal structure of miniature powder beds is performed using a Phoenix microCT.Loosely packed powder beds show expected macrovoids and recorded DPTs are within an order of magnitude of the theoretical predictions. However, DPTs for starches deviate from predicted values. We study the effect of moisture content and internal porosity and suggest two model adaptions.Visual observations indicate that the drop penetration area within the bed exceeds the drawing area of the droplet itself. Adaptation of the model for the drop penetration area by constant factor increases DPTs, leading to better correspondence between experimental and theoretical DPTs. Most promising results are obtained by replacement of the effective pore radius predicted by the Carman–Kozeny equation with the experimentally observed pore radius seen in microCT imaging. Theoretical DPTs predicted by this semi-empirical model are in good agreement with experimental results. This is promising for the application of the current method to various food powders, to control quality, optimize properties or tackle problems during production.

The Kinetics of Isothermal Hydrogen Reduction of Nanocrystalline Fe<sub>2</sub>O<sub>3</sub> Powder

The kinetics of reduction of nanocrystalline Fe2O3 powder with hydrogen was investigated in the temperature range from 573K to 813K. A comparison between the reduction rates of the nanocrystalline powder (20­50nm) and those of the conventional powder (200­400nm) showed no discernible difference despite a substantially larger specific surface area of the former. Since observations under the scanning electron microscope revealed that the nanocrystalline powder forms large agglomerates, the kinetic results were theoretically tested for the possibility that the reduction of the powder was inhibited by intra-agglomerate pore diffusion. Alternatively, a potential influence of water vapor remaining in the packed powder bed is discussed. Activation energies of 47.2 and 51.5kJ/mol were obtained for the reduction of the nanocrystalline and the as-received powder, respectively. These values show fair agreement with those obtained by previous studies. [doi:10.2320/matertrans.M2014182]