Powder Flow Research Articles

Flowability of polymer powders at elevated temperatures for additive manufacturing

The flowability and spreadability of three different polymer powders for selective laser sintering were investigated by analysing the powder at different temperatures. These measurements were performed using both the Revolution Powder Analyzer and the Schulze Ring Shear Tester. The latter device was modified with a self-developed heating system to allow testing at temperatures up to 170 °C. Overall, the results of both tests showed that the powders' flowability decreases as the temperature increases, with the largest decrease observed near the materials' melting point. Afterwards, recoating tests were conducted on an EOS P395 machine, which utilized a self-built AI monitoring system to detect and evaluate the severity of defects in the powder bed. The tests revealed that the number and severity of defects also increases with temperature. When approaching the sintering window, the number and severity of defects greatly decreases, and the powder bed becomes stronger and more resistant to defects.

Investigation of the morphologies of chelate flame-sprayed metal oxide splats

This paper reports an investigation of Er2O3 splats formed by metal–ethylenediaminetetraacetic acid (EDTA) complex particles that were applied onto aluminum alloy substrates by flame spraying. The splat morphologies and coating microstructures were analyzed under different conditions. The effects of the in-flight particle temperature of the carrier gases and the impact particle deposition temperature on solidification of the molten droplets were evaluated with a rotating dodecahedron stage operated at velocities of 30, 75, and 90 rpm and with different temperatures and angles of incidence. Various techniques were used to analyze the surfaces and cross sections of the splats and coatings formed under the different conditions. Most of the splats underwent transitions from disk shapes (ratio > 85 %) to irregular shapes (circularity of 0.61–0.80) with increasing rotational velocity. The spray process provided a coating with a porosity of 23.3 % at a powder flow rate of 20 g/min. The temperature reached during the chelate flame spraying (CFS) process and the condensing velocity of the deposited splat, rather than the in-flight particle temperature, were found to control the splat morphology. This synergistic condensation effect paves the way to successful deposition of high-temperature structural ceramic coatings such as Er2O3 with the CFS method.

Ball Milling Treatment of Nb and B Nanoparticles Modified TiAl4822 Composite Powder and Its Effect on Powder Bed Quality and Powder Spreadability in Additive Manufacturing

Ball milling treatment is a low‐cost and achievable large‐scale production method for preparing the composite powder used for the laser powder bed fusion (LPBF) process. For the ball‐milled composite powder, good powder spreadability and composition homogeneity are essential for obtaining the LPBF‐fabricated part with good forming quality. In this article, the Nb and B nanoparticles modified TiAl4822 composite powder with a nominal composition of Ti‐47.53Al‐1.98Cr‐2.81Nb‐0.15B are prepared via ball milling. It is found that the physical properties of the composite powder are more sensitive to the milling time than the milling speed and ball‐to‐powder ratio. Long‐time milling is inclined to cause the formation of broken particles, poor powder flowability, and high oxygen contamination. Besides, the process maps between the ball milling energy E t and particle size distribution width, the static angle of repose, composition inhomogeneity, and oxygen content are established, respectively. Results show that when the applied E t is insufficient or excessive, the degree of sphericity or composition homogeneity always worsens. In these cases, deposition defects such as line or area defects are more prone to occur, due to the enhanced particle rearrangement resistance induced by the addition of nanoparticles.

Multiple-Instance Regression for Metal Powder Hall Flow Rate Prediction Using Augmented Particle Size and Shape Data

This study investigates the relationship between metallic powders and their flowability behavior (captured in terms of Hall flow rates using Hall flowmeters). Due to the many trait dependencies of powder flowability, which have made the formulation of a physical and mechanistic generalizable model difficult to resolve, this study seeks to develop an alternative data-driven framework based on powder size and shape characteristics for Hall-flow-rate predictions. A multiple-instance regression framework was both developed for processing multiple-instance powder data and compared with standard machine learning models. Data augmentation was found to improve the overall performance of the framework, although the limited dataset was a constraint. Still, the study contributes to ongoing efforts to identify traditional, associative, and generalizable patterns between powder properties and resultant flowability behaviors. The findings show promise for real-world applications with a larger dataset, such that this initial application of multiple instance regression frameworks for metal powder Hall-flow-rate predictions as a function of powder particle size and shape data can be scrutinized in full.

Influence of flow agent in laser sintering powders on powder behavior and critical process steps

Good powder flowability is a key prerequisite for laser sintering powders. In this paper, the influence of rheological properties on powder behavior during dosing and coating steps was investigated and related to the build behavior and part properties. Five mixtures of Polyamide 11 powder with 0–0.068 wt% flow agent were tested. An insufficient amount of flow agent resulted in less powder being dosed, which further decreased with subsequent dosing steps. Flow behavior of the powder in the recoater and the amount of powder deposited was evaluated both at room temperature and at 181 °C. At operating temperature, the filling level and amount of flow agent significantly influenced powder flow in the recoater and amount of powder deposited. Test jobs indicated that good flowability impacts the stability of the building process rather than part properties. In addition, the importance of temperature dependent powder flow measurements for powder development could be shown.

Comparison of Pin Mill and Hammer Mill in the Fine Grinding of Sphagnum Moss

Dried sphagnum moss was ground using a pin mill and a hammer mill under various operating conditions, i.e., changes in the rotor frequency and feed rate. The specific energy consumption of the size reduction was recorded. The ground powder was characterized by median particle size, width of size distribution (span), loose and tapped bulk densities, and the Hausner ratio. Pin milling used less energy for size reduction than hammer milling, especially when the target size was below 100 μm. In both milling methods, the specific energy consumption was mainly caused by the rotor frequency used. However, in pin milling, the specific energy consumption was also dependent on the production rate: the higher the rate, the higher the energy consumption. No such dependence was observed with the hammer mill. The span was wider in pin milling than hammer milling in the intermediate product size range although the difference decreased at the fine and coarse ends. A similar pattern was found for bulk densities. However, the flowability of powder, as characterized by the Hausner ratio, was comparable between the grinding methods.

Numerical modelling of SS316L powder flowability for laser powder-bed fusion

This work aims to improve the powder-bed spreading process for laser powder bed fusion additive manufacturing by gaining a greater understanding of metal powder flowability through numerical modelling and in-situ experimentation.Using the Discrete Element Method (DEM) to study the flowability of the powder and its intrinsic properties. A high-fidelity particle-scale model was developed to capture the dynamics of metal particle interactions in a virtual Hall flow meter based on a modified Beverloo law. The results are validated experimentally using the Hall flow static powder characterisation technique.For SS316L powder alloy with the hall-value of 29s/50g and with an angle of repose (AOR) of 32, the modelled powder that exhibited the same flow behaviour was found using 0.5 for both rolling and sliding coefficients resulting in simulated Hall value of 28.55s/50g with a simulated flow rate of 0.571 g/s, which is validated by AOR of the simulated powder [31.2-32.6]. However, rolling friction had minimal effect on the mass flow rate but increased the angle of repose. Sliding friction significantly decreased the mass flow rate and increased AOR.DEM is an ideal method to study flowability. However, there are certain constraints imposed on the computational power by a number of simulated particles and simulation time-step. Future research may involve investigating other dynamic flowability characterisation techniques.Enabling a better understanding of powder particle flow at a micro-scale by modelling powder flowability. This leads to simulating a more realistic powder bed and improving the powder spreading process, leading to better AM parts quality.This paper provides a unique approach for modelling the flowability of SS316L powder using a Beverloo law-based design of the Hall flow meter. This will improve the modelling of the spreading process needed for metal 3D printing.

Investigation on Microstructure and Flowability of Gas Atomized Heat-resistant KHR45A Alloy Powders for Additive Manufacturing

Investigation on Microstructure and Flowability of Gas Atomized Heat-resistant KHR45A Alloy Powders for Additive Manufacturing

Mechanical properties of laser powder directed energy deposited NASA HR-1 superalloy: Effects of powder reuse and part orientation

The feasibility of NASA HR-1 powder reuse for laser powder directed energy deposition (LP-DED) of thin-walled structures in the as-built surface condition was investigated in this study. Although continuously reusing the powder five times slightly reduced the number of fine particles in the powder and increased the mean particle size, changes in powder rheological properties were negligible. The similarities in powder flow between all reused conditions resulted in no variation in surface roughness, defects’ size and spatial distributions, and microstructure of the as-deposited parts and, thus no differences on tensile and fatigue properties. The effects of different specimen orientations, loading direction being perpendicular or parallel to the building direction, on mechanical properties were also investigated. While no change in tensile strength was noted, tensile ductility was lower for the parallel specimens because of surface micro-notches being perpendicular to the loading direction, which led to an early onset of fracture. Due to accelerated crack initiation from these micro-notches, parallel specimens were also less resistant to fatigue failures. The fatigue failures always initiated from the specimen surfaces, and all specimens experienced a ductile fatigue crack growth mechanism characterized by clear striations on the fracture surface.

Enhanced flowability and film properties of ultrafine powder coatings modulated by modified flow additive

Ultrafine powder coatings have superior film performance but poor flowability due to the high interaction between particles, which can be improved by the addition of inorganic flow additives. However, the improvement is limited and more importantly, the film performance deteriorates because of their incompatibility with the organic coating resin. In this study, a resin encapsulation approach was proposed to modify the commercial inorganic flow additives for the improvement of powder flowability and maintaining film quality. The effects of resin content and additive dosage on the flowability characterized by angle of repose (AOR) and avalanche angle (AVA), and film performance characterized by gloss, seed density and surface roughness were systematically investigated in two powder coating systems. It was found that the encapsulation of additives with the same resin of the host powder coating can enhance both flowability and film properties, compared to commercial nano-silica. With the increase of resin content, both the AOR and AVA first decrease to a minimal value at resin content of 5 % to 10 % and then increase when applying excessive resin encapsulation. Similar to the effect of resin content, there is also an optimal additive dosage at 0.3 to 0.5 wt% for the best flowability, while excessive additives lead to “bridge effect”. The negative effect raised by the additives on film quality can be mitigated by resin encapsulation. Higher gloss and lower roughness are obtained at larger resin content, while the smallest seed density is at resin content of 10 %. The study provides an effective and facile way to ameliorate the function of the flow additive and enhance the properties of powder coating films.

Bench to batch: Linking pharmaceutical powder flow characterisation, intermediate bulk container discharge and video observations

Five well known excipients and a model drug substance with varied particle properties and bulk behaviour were chosen for the study. Based on the results APAP, NaCMC-XL, mannitol and DCPA were selected for a design to understand the impact of different blends. Two pilot scale unvented IBCs were used in the study. The IBC discharge rates were measured using a catch balance and the mode of flow and powder behaviour inside the IBC was recorded using a camera. The videos inside the IBC showed that regardless of flow mode, for powder to flow from the IBC an air burst was necessary. This was similar to observations when emptying water from a bottle. The extent of the air flow inside the IBC was strong and could possibly result in fluidisation segregation. The discharge curves of 15° and 30° hopper half angles were very similar, which was explained by the vertical air movement in the steeper hopper, which reduces the particle acceleration. Several good indicators of flow/no flow in the IBCs were found. However, for predicting the discharge rate there was a linear correlation between flow through an orifice and IBC discharge rate.

Thermoplastic coating on fiber reinforced polymer composites by cold spray additive manufacturing

This work investigates the cold spray coating of nylon 6 on fiber-reinforced composite substrates. Deposition conditions are investigated by systematically varying the process parameters (impact velocity, temperature, carrier gas pressure, and flow rate) for two woven fiber-reinforced epoxy substrates with glass and carbon fiber reinforcements, resulting in deposition efficiencies exceeding 30%. Parametric studies are performed to identify the optimal system pressure and powder flow rates that lead to the successful deposition of the powder particles on composite substrates. It is observed that, upon impact, the particles undergo extensive plastic deformation without damaging the reinforcing fibers in the substrate. Microstructural observations reveal that the first (seeding) layer of nylon particles experiences a higher degree of plastic deformation due to the impact on a stiff substrate. The large plastic deformation in the impacting particles plays a key role in enabling adhesion to the substrate. The successive layers of deposited particles undergo less plastic deformation as they impact a comparatively softer pre-deposited nylon substrate. The underlying deposition mechanisms are further explored using multiscale finite element analyses. Results obtained in this work demonstrate the feasibility of cold spray deposition of thermoplastic particles on fiber-reinforced composites without imposing significant damage to the substrate.

Development and implementation of a pneumatic micro-feeder for poorly-flowing solid pharmaceutical materials

Consistent powder micro-feeding (<100 g/h) is a significant challenge in manufacturing solid oral dosage forms. The low dose feeding can well control the content consistency of the dosage forms, which improves drug efficiency and reduces manufacturing waste. Current commercial micro-feeders are limited in their ability to feed < 20 g/h of cohesive (i.e. powders of poor flowability) active pharmaceutical ingredients (API) and excipients (e.g. lubricants) with low fluctuation. To breach this gap, this study presents an advanced micro-feeder design capable of feeding a range of pharmaceutical-grade powders consistently at flow rates as low as 0.7 g/h with <20 % flow rate variation. This was possible due to a novel powder conveying concept utilising particle re-entrainment to minimise flow rate variations. This work details the design of this pneumatic micro-feeder and its excellent micro-feeding performance even for cohesive powders. The experimental studies investigated the influence of the process parameters (air pressure and air flow rate) and equipment configurations (insert size and plug position) on the feeding performance of different pharmaceutical-relevant powders, i.e., microcrystalline cellulose (MCC), croscarmellose sodium (CCS), crospovidone (XPVP) and paracetamol (APAP). It was shown that the system is capable of delivering consistent powder flow rates with good repeatability and stability.

Embedding ceramic components in metal structures with hybrid directed energy deposition

The combined benefit of both additive and subtractive manufacturing within the same gantry system enables hybrid directed energy deposition to create complex geometries with smooth surface finish and superior dimensional accuracy. Moreover, with layer-by-layer access to the structure during both the addition and subtraction of material, the insertion of components is now possible, assuming the components can survive the high temperatures associated with the subsequent metal deposition. Ceramic inserts are of interest for a variety of reasons including (1) to create complex interwoven ductile/brittle composites for ballistics or high-temperature applications or (2) to integrate high-temperature strain or temperature sensors protected within ceramic substrate subsumed into a larger metal structure. In this work, stainless steel substrates were machined to create an internal cavity for the insertion of a ceramic component. During the investigation of several different over-the-ceramic deposition strategies, components were inserted, and different process sequences were allowed to continue to envelop the inserted ceramic with varying success. Unmelted powder was used to serve both as a thermal buffer and to provide a flush surface upon which the laser cladding could continue. Subsequent depositions were attempted with both dry and wet powder (addition of machining coolant to wet). The wet powder has previously been demonstrated to not significantly impact the mechanical properties of a final structure and provided a thermal barrier to protect the ceramic piece from the extreme temperatures of the final metal deposition. The wetting of the powder provided stability and minimized displacement caused by the powder flow from the laser cladding head. Finally, the use of an oblique angle for laser cladding allowed for the redirection of some fraction of the introduced thermal energy away from the ceramic component and, consequently, improved the survival of the ceramic inserts. With this combination of techniques, ceramic inserts survived full embedding within a 3D-printed stainless steel structure.

Effect of HVOF processing parameters on [formula omitted] hard coatings deposited on AISI 4140 steel

High Velocity Oxygen Fuel (HVOF) thermal spray coating is a deposition process that is widely employed at present to improve surfaces and the remanufacturing of components subject to severe wear, offering an alternative to reduce the cost of spare parts and used as a substitute for electro-chromium, also generating greater thickness than chemical vapor deposition and physical vapor deposition. This study evaluates the influence of the projected application parameters for Cr3C2-20(Ni20Cr) coatings on AISI 4140 steel using the HVOF technique. To do so, a 24 factorial experimental design was used to evaluate the effect on the thickness, porosity, surface hardness, and microhardness in the cross-section, produced by four of the main operational factors in the HVOF process: the fuel/oxygen (F/O) ratio entering the combustion chamber, powder flow, the roughness of the substrate before deposition, and spray gun speed. The morphology of the Cr3C2-20(Ni20Cr) powders and the microstructure of the coatings were studied through a scanning electron microscope and energy dispersive spectroscopy, while the composition were analyzed by X-ray diffraction. It was determined that: (i) the F/O ratio is the operation parameter with the most significant influence on the properties studied; (ii) the gun speed has a significant effect on the thickness generated and porosity; and (iii) the powder flow and roughness of the substrate have a greater effect on the surface hardness and microhardness. Finally, it was determined that coatings with great thickness, low porosity, and great hardness, ideal for the recovery of parts, can be achieved from a F/O ratio of 0.45 and a powder flow with the system feeder rotating at 12 rpm, applied to a substrate with a roughness of Ra=18μm, combined with a spray gun speed of 5 mm/s.

Solid-state characterization of ibuprofen-isonicotinamide cocrystals prepared by electrospraying and solvent evaporation.

Aim: Electrospraying (ELS) was used to prepare micronized ibuprofen-isonicotinamide cocrystal (IBU-INA-ELS) and its properties were compared with the solvent evaporated cocrystal (IBU-INA-SE). Methods: Solid-state characterization of crystalline phase, production yield, particle size, powder flow, wettability, solution mediated phase transformation (SMPT), and dissolution rate were measured. Results: The ELS produced phase pure particles of IBU-INA with a size of 1.46μm and yield of 72.3%. This cocrystal improved the intrinsic dissolution rate and powder dissolution rate of IBU by 3.6- and 1.7-fold, respectively. Our experiments showed that the dissolution of IBU-INA was affected by particle size, solubility, SMPT and wettability. Conclusion: ELS produced micronized cocrystals for improving dissolution of ibuprofen with a high yield in a single step and mild conditions.

The preparation, characterization, and application of porous core–shell composite particles produced with laboratory-scale spray dryer

Objective To prepare porous core–shell composite particles (PCPs) in order to improve the flowability and compactibility of powder materials for direct compaction (DC), as well as the dissolution of tablets. Significance The results obtained are meaningful to boosting the development and further research of PCPs on DC. Methods: In this study, hydroxypropyl methylcellulose (HPMC E3) and polyvinylpyrrolidone (PVP K30) were selected as shell materials, the Xiao Er Xi Shi formulation powder (XEXS) was used as the core materials, ammonium bicarbonate (NH4HCO3), and sodium bicarbonate (NaHCO3) were employed as pore-forming agent. Using co-spray drying method to prepare composite particles (CPs). Then, the physical properties and comparison between different CPs were characterized comprehensively. Finally, the different CPs were directly compacted as tablets to explore the effect on the dissolution behavior of DC tablets, respectively. Results (i) The XEXS PCPs were prepared successfully by co-spray drying, and the yield of PCPs is almost 80%; (ii) The TS values of PCP-X-P-Na, PCP-X-P-NH4, PCP-X-H-Na and PCP-X-P-Na were 5.70, 7.56, 3.98, and 6.88 times higher than that of raw material (X); (iii) The disintegration time of PCPs tablets decreased 10–25% when compared with CPs tablets; (iv) The values of Carr’s index (CI), Hausner ratio (HR), Caking strength (CS), and Cohesion index (CoI) of PCP-X-H-NH4 were 19.16%, 19.29%, 40.14%, and 6.39% lower than that of X, respectively. Conclusions The PCPs prepared by co-spray drying did improve the flowability and compactibility of powder, as well as the dissolution of tablets.

Ongoing Challenges of Laser-Based Powder Bed Fusion Processing of Al Alloys and Potential Solutions from the Literature-A Review.

Their high strength-to-weight ratio, good corrosion resistance and excellent thermal and electrical conductivity have exponentially increased the interest in aluminium alloys in the context of laser-based powder bed fusion (PBF-LB/M) production. Although Al-based alloys are the third most investigated category of alloys in the literature and the second most used in industry, their processing by PBF-LB/M is often hampered by their considerable solidification shrinkage, tendency to oxidation, high laser reflectivity and poor powder flowability. For these reasons, high-strength Al-based alloys traditionally processed by conventional procedures have often proved to be unprintable with additive technology, so the design and development of new tailored Al-based alloys for PBF-LB/M production is necessary. The aim of the present work is to explore all the challenges encountered before, during and after the PBF-LB/M processing of Al-based alloys, in order to critically analyse the solutions proposed in the literature and suggest new approaches for addressing unsolved problems. The analysis covers the critical aspects in the literature as well as industrial needs, industrial patents published to date and possible future developments in the additive market.

Influence of weak electrostatic charges and secondary flows on pneumatic powder transport

AbstractThe dynamics of the transported powder determines the functionality and safety of pneumatic conveying systems. The relation between the carrier gas flow, induced by the flown‐through geometry, and the powder flow pattern is not clear yet for electrostatically charged particles. This paper highlights the influence of relatively minor cross‐sectional secondary flows and electrostatic forces on the concentration and dynamics of the particles. To this end, direct numerical simulations (DNS) capture the interaction of the continuous and dispersed phases using a four‐way coupled Eulerian–Lagrangian strategy. The transport of weakly charged particles in channel flows, where turbopheresis defines the particle concentration, is compared to duct flows, where additional cross‐sectional vortices form. For both geometries, the Stokes number () and the electrical Stokes number () are varied, and the turbulent carrier flow was fixed to . The presented simulations demonstrate that secondary flows, for the same , , and , dampen the effect of particle charge. In a duct flow, vortical secondary flows enhance the cross‐sectional particle mobility against the direction of electrostatic forces. Compared to a duct flow, in a channel, the wall‐normal aerodynamic forces are weaker. Thus, electrical forces dominate their transport; the local particle concentration at the walls increases. Further, electrostatic charges cause a stronger correlation between the gas and particle velocities. In conclusion, despite being weak compared to the primary flow forces, secondary flow and electrostatic forces drive particle dynamics during pneumatic transport.

Dry granulation of vitamin D3 and iron in corn starch matrix: Powder flow and structural properties

In this work, a twin-screw dry granulation (TSDG) was adopted to produce vitamin D3 (VD3) and iron blended dry granules using corn starch as an excipient. Response surface methodology was applied to determine the effect of the formulation compositions (VD3 and iron) on granule properties [tapped bulk density, oil holding capacity, and volumetric mean particle size (Dv50)]. Results indicated that the model fitted well, and responses, in particular flow properties, were significantly affected by the composition. The Dv50 was only influenced by the addition of VD3. The flow properties were characterized by the Carr index and Hausner ratio, which indicated very poor flow of the granules. Scanning electron microscopy with energy dispersive spectroscopy confirm the presence and distribution of Fe++ and VD3 in the granules. Overall, TSDG was proven to be a simple alternative method for the preparation of dry granules of VD3 and iron in a blend.