Gas Atomization Process Research Articles

Swirling supersonic gas flow in an EIGA atomizer for metal powder production: Numerical investigation and experimental validation

This paper is concerned with the modeling of the Electrode Induction Melting Gas Atomization (EIGA) process. This process employs a swirling supersonic inert gas jet to atomize a free-fall molten metal stream, with the aim to produce powders of highly reactive metals. As a first step towards the development of a complete model, this study aims to describe the behavior of the swirling gas flow through and downstream the EIGA gas nozzle and to highlight the influence of the main operating parameters on the jet characteristics. The developed model is based on the Reynolds-Averaged Navier-Stokes (RANS) approach using the Favre decomposition for compressible flow together with a k-ω SST turbulence model. The numerical results are partially validated against images of the gas flow patterns obtained using the Schlieren imaging technique. The simulation results show that the gas flow transits to a supersonic regime immediately before exiting the nozzle. In the atomization tower, it forms a swirling supersonic jet with an asymmetric geometry composed of several Mach diamonds. The inlet gas pressure has a significant effect on the jet structure and Mach number distribution. However, within the investigated range of pressure values (30–45 bar), the swirling motion of the jet, hence the jet geometry, remain little affected. In contrast, a small increase of the nozzle exit slit size significantly increases the swirling motion, which in turn strengthens the jet asymmetry. A reduction of the overpressure in the melting chamber has a similar effect, but to a lesser extent.

Size-dependent microstructure evolution and formation mechanisms during the gas atomization process of FGH4113A Ni-based superalloy powder

A novel microstructure evolution mechanism is proposed to optimize the preparation process of superalloy powders. This study focuses on the solidification segregation, as well as the correspondence between the powder sizes and the microstructure evolution mechanism of Ni-based superalloy fine powders. The quantitative analysis was systematically examined using thermodynamic analysis. The results indicated that dendrites and cellular microstructures formed on the surface and interior of the powders, but their key distribution characteristics were highly related to both the powder size and ultra-fine satellite particles that adhered to its surface. The number of ultra-fine satellite particles, outgrowth index, spacing between the secondary dendrite arms increased with the increasing powder size, exhibiting an ideal linear relationship. In particular, when the powder size was small, the length of the internal primary dendrite arms increased as the powder particle size increased, but interestingly, remained at approximately 30 μm when the particle size exceeded 45 μm. Finally, a model for the microstructural evolution mechanism of the surface and interior of alloy powders ≤100 μm was developed, which lays the theoretical foundation for the preparation of superalloy powders.

On the first hydrogenation kinetics and mechanisms of a TiFe0.85Cr0.15 alloy produced by gas atomization

This article reports the hydrogenation properties of a TiFe0.85Cr0.15 alloy prepared by gas atomization on one hand, and vacuum arc-remelting on the other hand for demonstration and reference, respectively. To evidence the excellent potential of ternary alloying combined with gas atomization process for improving the first hydrogen absorption kinetics, both the atomized powders and the reference crushed ingots were exposed to air from 1 h to as long as 10 days. In spite of the resulting surface oxide layer formation, all specimens still absorbed hydrogen partly thanks to the presence of the C14 Laves phase. As expected, the first hydrogenation kinetics decreased with increasing air-exposure time, but atomized powders not only outperformed the reference counterparts but also remained quite competitive despite surface oxidation. The remarkable kinetic enhancement observed was attributed to the formation of an additional Ti2Fe phase, to an increased amount of C14 Laves phase, as well as to a fine microstructure resulting from the rapid cooling rate involved in gas atomization. Even though different absorption behaviors were observed for all samples, the reaction mechanisms derived from solid-gas reaction models were ascribed to interface- and diffusion-controlled 3D processes in the early and subsequent stage of the first hydrogenation, respectively.

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Diameter of Free-Fall Gas Atomizer

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Diameter of Free-Fall Gas Atomizer

Fabrication and Soft Magnetic Properties of Fe81.3Si4B10P4Cu0.7 Amorphous Powders by Using the Spinning-water Atomization Process

Soft magnetic Fe81.3Si4B10P4Cu0.7 powders have been fabricated by using spinning-water atomization process (SWAP) under the water pressure of 17.5 MPa and gas pressure of 2 MPa. To clarify the amorphous forming ability, thermal stability, and the corresponding soft magnetism, the as-SWAPed powders have been sieved into 6 groups with different powder sizes from 0 – 150 μm. After the analysis of the amorphous and crystalline characteristics, the morphology, and soft magnetic properties of these 6 groups of as-SWAPed powders, it is concluded that the SWAPs with a high cooling rate of 105 K/s can improve the amorphous forming abilities of Fe81.3Si4B10P4Cu0.7 powders up to 53 μm, the saturated magnetic flux density as high as 170 – 173 emu/g and the thermal stabilities higher than 112.8 K. The characteristic parameters of as-SWAPed powders above mentioned are close to those of the counterpart rapid solidified ribbons. The surface oxide layers on as-SWAPed powders mainly consist of Fe2O3, and are 10 nm thick, much thicker than these counterpart ribbons, which might help to weaken the eddy effects accompanying with the slight decrease of the saturated magnetic flux density. Due to the higher cooling rates of SWAPs than gas atomization processes and the better spheroidization of powders for SWAPs than water atomization processes, it is key for NANOMET® family alloys to increase their amorphous forming abilities and better the soft magnetic performances.

Effect of powder size and geometry on consolidation of Mo30%W alloy by spark plasma sintering

Molybdenum alloys are important structural components for a variety of high temperature applications in the aerospace and nuclear industries. Spark plasma sintering has been successful in producing highly dense refractory metals with desirable microstructural characteristics, where the final microstructure and sintering mechanisms depend heavily on the starting powder size and morphology. To understand how powder size and morphology affect consolidation of a molybdenum 30 wt% tungsten alloy (Mo30W), three different powder feedstocks were sintered and the solid compacts characterized. These powders were prepared utilizing chemical reduction, ball-milling, and gas atomization processes. Additionally, to determine the effect of particle size on sintering, two different size distributions of the gas atomized material were also tested. In-situ sintering dilatometry data was analyzed to determine the apparent activation energy for the initial stage sintering of each powder. Powder geometry was found to have the greatest impact on the sintering of Mo30W powders, with the rougher, more angular powders densifying at lower temperatures compared with the smoother spherical powders densifying at higher temperatures. Powder size contributed much less to the densification of the powders using SPS. From these experiments, optimum sintering temperatures can be determined, with 1600 °C being sufficiently high to fully densify the chemical reduction and ball-milled materials and a maximum hold temperature of >1800 °C being required for the spherical atomized powders.

Simulation and validation of the gas flow in close-coupled gas atomisation process: Influence of the inlet gas pressure and the throat width of the supersonic gas nozzle

The effectiveness of a close-coupled gas atomisation process largely depends on the operational and the geometric variables. In this study, Computational Fluid Dynamics (CFD) techniques are used to model and simulate the gas flow in the melt nozzle area for a convergent-divergent, close-coupled gas atomiser in the absence of the melt stream. Firstly, a reference case, in which the atomisation gas is nitrogen at 50 bar and a supersonic gas nozzle with a throat width of L0 has been modelled, is presented. Then, the influence of both the inlet gas pressure and this design parameter are investigated, comparing the numerical results provided by simulations varying the inlet pressure from 5 to 80 bar and modelling different convergent-divergent gas nozzles with throat widths of 0.29∙Lo, 0.5∙Lo, 0.77∙Lo and 2∙Lo respectively. The simulation results show how similarly these two parameters modify gas mass flow rates, gas velocity fields, aspiration pressures in the melt delivery tube or the size of the recirculation zones below the melt nozzle. Therefore, it can be stated that this geometric variable of the gas nozzle may be as relevant as the inlet pressure in the atomisation process. The most important novelty of this study is related to experimental validation of the numerical results using the Particle Image Velocimetry (PIV) technique and through direct measurements of gas mass flow rates, with a clear correlation between simulated and measured data. Moreover, some results obtained with experimental atomisations using copper and nitrogen are also presented. The experimental results show that finer powders are produced by increasing the atomising pressure or the throat width of the supersonic gas nozzle, which can be directly related to the gas flow dynamics calculated numerically.

Synthesis of Refractory High-Entropy Alloy WTaMoNbV by Powder Bed Fusion Process Using Mixed Elemental Alloying Powder.

The growing interest in refractory high-entropy alloys (HEAs) in the last decade is mainly due to their thermal stability, outstanding mechanical properties, and excellent corrosion resistance. However, currently HEAs are still not considered for use as common structural materials due to their inherent drawbacks in terms of processing and machining operations. The recent progress witnessed in additive manufacturing (AM) technologies has raised the option of producing complex components made of HEAs with minimal machining processes. So far, this could be achieved by using pre-alloyed powders of HEAs that were mainly produced by a conventional arc melting furnace (AMF) in the form of small compounds that were transformed into powder via a gas atomization process. To significantly reduce the production cost, the present study aims to analyze the ability to synthesize HEA WTaMoNbV via a laser powder bed fusion (LPBF) process using mixed elemental alloying powder as the raw material. For comparison, a counterpart alloy with the same chemical composition was analyzed and produced by an AMF process. The microstructures of the tested alloys were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The physical properties were evaluated in terms of density and mechanical strength, while the electrochemical behavior was assessed by potentiodynamic polarization analysis. The results disclosed similarities in microstructure, physical properties and electrochemical behavior between HEA WTaMoNbV manufactured by the proposed LPBF process and its counterpart alloy produced by an AMF process.

Recycling of Ti6Al4V machining swarf into additive manufacturing feedstock powder to realise sustainable recycling goals

This paper addresses the imperative need to develop sustainable recycle technologies for high value machining swarf generated during the processing of Ti6Al4V alloy. A novel recycling process based on multi-stage ball milling is proposed. The process converts Ti6Al4V swarf into a powder feedstock suitable for additive manufacturing (AM). The powders produced from the cleaned swarf using an in-house designed and fabricated tumbler ball mill were characterised in terms of their morphology, particle size, flowability and spreadability. It was found that the dominant effect of milling with Ø 25 mm balls was particle size reduction (up to ∼ 40%) and the primary effect with smaller balls of Ø 6.25 mm was modification of particle morphology from irregular to rounded shape; thus, necessitating adoption of a multi-stage milling approach to achieve required size and morphology. Ti6Al4V powder having particle size in the range of 40–200 μm and near-spherical morphology was obtained after multi-stage ball milling up to 18 h. The powder characteristics were comparable or superior to the powder produced by generally used gas atomization (GA) process. The suitability of the powders for AM was established through direct metal laser sintering (DMLS). The proper melting of the optimally prepared powder occurs at 1000 mm/s scanning speed and 310 W of laser power. The developed multi-stage ball milling process was assessed vis-à-vis gas atomization using life cycle assessment (LCA). LCA revealed that the proposed ball milling method consumed lower energy (∼59%), had lower eco-cost (∼82%), and lesser global warming potential (GWP) (∼68%).

Strengthening the mechanical properties and wear resistance of CoCrFeMnNi high entropy alloy fabricated by powder metallurgy

In this study, an equiatomic CoCrFeMnNi high entropy alloy (HEA) was fabricated by a rapid solidified gas atomization process. Subsequently, the high-energy mechanical milling was carried out to further refine the microstructure of pre-alloyed powder to improve the sintering ability and strengthening of HEAs. The microscopic results show that the powder morphology significantly changed from spherical to flatten, flake, irregular, and partially spherical shape with increasing milling time. The XRD results exhibited HEA bulks consisting of major FCC and minor Cr7C3 phases. The hardness of HEA bulks increased from 270±10 Hv to 450±10 Hv with increasing milling time, while the compressive yield strength increased from 370 MPa to 1050 MPa due to grain boundary strengthening and dislocation strengthening. Meanwhile, the lowest coefficient of friction ∼0.283 and specific wear rate ∼1.03×10-5 mm3/Nm were obtained for the 60 min milled HEA due to increased surface hardness and oxidation behavior. The developed powder metallurgy approach could be considered as a promising way to improve the strength and wear resistance when compared to the conventional processed CoCrFeMnNi HEAs.

Study of the stabilization process of gas atomized Al0.5CoCrFeNi2Ti0.5 high-entropy alloy in phase transformation

The phase and microstructure evolution of a precipitation-hardening high entropy alloy (HEA), Al0.5CoCrFeNi2Ti0.5, were studied. In order to promote the applications in surface modification, the proposed HEA was made into powders by using a gas-atomization process. The as-obtained powders presented a spherical shape with uniform element distribution; moreover, their phase constitution transferred from BCC to FCC-dominated structure while particle size varied from several to 120 µm. Further annealing treatments were conducted with the metastable fine powders to investigate the size effect in the microstructure. Below 500 °C, the structure kept a major BCC phase, but the size of grains arranged along (110) plane were getting smaller with temperature. FCC crystal became the dominant structure while the temperature was higher than 550 °C, and the transitional sigma phase existed at 700–800 °C. After annealed at 900 °C, the metastable fine Al0.5CoCrFeNi2Ti0.5 powders fully transferred to a stable structure composed of a dominant FCC plus minor BCC phases. Elemental mappings showed that the Al0.5CoCrFeNi2Ti0.5 HEA transformed from a microstructure with uniform element distribution to a coarsened system comprised of (Fe, Cr)-rich FCC matrix and (Al, Ti)-rich order BCC precipitate. Furthermore, the hardness of the Al0.5CoCrFeNi2Ti0.5 HEA increased by more than 20% on annealing treatment, from 5.50 ± 0.75–6.91 ± 1.49 GPa due to the solid solution and the precipitation strengthening effect brought about by Ti addition.

Microstructure Evolution of FeNiCoCrAl1.3Mo0.5 High Entropy Alloy during Powder Preparation, Laser Powder Bed Fusion, and Microplasma Spraying.

In the present study, powder of FeCoCrNiMo0.5Al1.3 HEA was manufactured by gas atomization process, and then used for laser powder bed fusion (L-PBF) and microplasma spraying (MPS) technologies. The processes of phase composition and microstructure transformation during above mentioned processes and subsequent heat treatment were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and differential thermal analysis (DTA) methods. It was found that gas atomization leads to a formation of dendrites of body centered cubic (BCC) supersaturated solid solution with insignificant Mo-rich segregations on the peripheries of the dendrites. Annealing leads to an increase of element segregations till to decomposition of the BCC solid solution and formation of σ-phase and B2 phase. Microstructure and phase composition of L-PBF sample are very similar to those of the powder. The MPS coating has a little fraction of face centered cubic (FCC) phase because of Al oxidation during spraying and formation of regions depleted in Al, in which FCC structure becomes more stable. Maximum hardness (950 HV) is achieved in the powder and L-PBF samples after annealing at 600 °C. Elastic modulus of the L-PBF sample, determined by nanoindentation, is 165 GPa, that is 12% lower than that of the cast alloy (186 GPa).

Magnetocaloric particles of the Laves phase compound HoAl2 prepared by electrode induction melting gas atomization

Processing magnetocaloric materials into magnetic refrigerant particles is an essential issue in developing high-performance magnetic refrigerators. Here, we succeed in stably producing magnetocaloric particles of the promising material HoAl2 by a newly devised method based on electrode induction melting gas atomization process. The particle size range is on the order of submillimeters, which is suitable for practical refrigeration systems. The resulting particles with less contamination have good morphological, magnetic, and magnetocaloric properties: (i) almost spherical shapes with few internal pores, (ii) a sharp ferromagnetic transition around 30 K, and (iii) a large magnetocaloric effect comparable to the bulk counterpart. These features suggest the HoAl2 gas-atomized particles have the potential of use as a magnetic refrigerant. The presented method can be applied not only to HoAl2 but also to other brittle magnetocaloric materials with high melting points, facilitating the production of various magnetic refrigerants needed to develop magnetic refrigerators for hydrogen liquefaction.

Investigation of Mechanical Properties of Parts Fabricated with Gas- and Water-Atomized 304L Stainless Steel Powder in the Laser Powder Bed Fusion Process

The use of gas-atomized powder as the feedstock material for the laser powder bed fusion (LPBF) process is common in the additive manufacturing (AM) community. Although gas-atomization produces powder with high sphericity, its relatively expensive production cost is a downside for application in AM processes. Water atomization of powder may overcome this limitation due to its low cost relative to the gas-atomization process. In this work, gas- and water-atomized 304L stainless steel powders were morphologically characterized through scanning electron microscopy (SEM). The water-atomized powder had a wider particle size distribution and exhibited less sphericity. Measuring powder flowability using the Revolution Powder Analyzer (RPA) indicated that the water-atomized powder had less flowability than the gas-atomized powder. Through examining the mechanical properties of LPBF fabricated parts using tensile tests, the gas-atomized powder had significantly higher yield tensile strength and elongation than the water-atomized powder; however, their ultimate tensile strengths were not significantly different.

The influence of particle size and heat treatments on the transformation energies of a gas atomized Cu-Al-Ni-Mn shape memory alloy

Forward and reverse transformation temperatures of shape memory alloys are significantly affected by their thermal and processing history. Controlling these temperatures is of great importance when designing materials displaying shape memory effect. In this direction, the present work shows the influence of particle size and heat treatments on thermodynamic properties of a gas-atomized Cu-Al-Ni-Mn shape memory alloy. Commercially pure elements were used in the gas atomization process and differential scanning calorimetry was used to measure As, Af (austenitic start and final transformation temperatures), Ms, Mf (martensitic start and final transformation temperatures), transformation temperature hysteresis and chemical/non-chemical energies involved in the martensitic-austenitic transformation. The influence of quenched-in defects and annealing was also studied. Results show a correlation between particle size and transformation energies, which can be attributed to atomic ordering in the austenitic phase upon cooling, after the solidification process. This is further confirmed by the increase in the reverse and forward transformation temperatures after thermal cycling and annealing.

Flexible Powder Production for Additive Manufacturing of Refractory Metal-Based Alloys

The quality and properties of metal powders are essential for powder metallurgical (PM) processes in general and for additive manufacturing (AM) processing routes in particular. Thus, a variety of atomization technologies were established meeting the multiple needs of the different processing technologies. However, the production of refractory metal alloy powder remains challenging due to their high liquidus temperatures (>2000 °C), the formation of brittle intermetallic phases, as well as the reactivity with and sensitivity to interstitials of the constituting elements. In this contribution, powders made of Mo-20Si-52.8-Ti (at.%) were produced by a novel ultrasonic atomization (UA) process at laboratory-scale using an industrial electrode induction gas atomization (EIGA) process with a modified electrode concept for the first time. UA allows flexibility in alloy composition due to the arc melting-based principle, while the EIGA electrode is PM manufactured from elemental powders to provide similar flexibility on a larger scale. The powders resulting from these two processes were compared with respect to size distribution, sphericity, microstructure and phase constitution, chemical composition, and interstitial impurity content. In addition, several powder batches were produced with the UA process in order to assess the process reliability and stability. The properties, quality, and quantities of UA powders perfectly meet the requests for alloy development for powder bed fusion AM, while the modified EIGA process allows the upscaling of the alloy powder quantities.

Numerical investigation on flow process of liquid metals in melt delivery nozzle during gas atomization process for fine metal powder production

Based on volume of fluid (VoF) interface capturing method and shear-stress transport (SST) k-ω turbulence model, numerical simulation was performed to reveal the flow mechanism of metal melts in melt delivery nozzle (MDN) during gas atomization (GA) process. The experimental validation indicated that the numerical models could give a reasonable prediction on the melt flow process in the MDN. With the decrease of the MDN inner-diameter, the melt flow resistance increased for both molten aluminum and iron, especially achieving an order of 102 kPa in the case of the MDN inner-diameter ≤1 mm. Based on the conventional GA process, the positive pressure was imposed on the viscous aluminum alloy melt to overcome its flow resistance in the MDN, thus producing powders under different MDN inner-diameters. When the MDN inner-diameter was reduced from 4 to 2 mm, the yield of fine powder (<150 μm) soared from 54.7% to 94.2%. The surface quality of powders has also been improved when using a smaller inner-diameter MDN.

Effect of Ti and C alloyants on the microstructure of laser cladded cobalt-chromium coatings

The article presents the results of Ti and C additives on the microstructure of the Metcoclad 6 coating. Effect of chemical composition with optimal laser cladding parameters has been studied in the terms of modification of the carbide morphology. The experiments were carried out on TruDisk 3302 Yb:YAG disk, equipped with a disc powder feeder. As a deposited material Metcoclad 6 powder produced in inert gas-atomization process was used. To assess the impact of alloying elements on microstructure, SEM observations and EDS analysis were carried out. Vickers microhardness tests were provided on the cross-section of coatings on Wilson Wolpert 401 MVD device. It has been stated that addition of titanium changes the morphology of carbides from lamellar eutectic carbides to isolated primary TiC while addition of Ti and graphite allowed to maintain the structure typical for pure Stellite 6 alloy, but additional formation of primary carbides were observed.

Investigation of phase constitution and stability of gas-atomized Al0.5CoCrFeNi2 high-entropy alloy powders

In this study, Al0.5CoCrFeNi2 high-entropy alloy powders with uniform elements distribution were prepared by gas atomization process. The effects of heat treatment on the microstructure, phase transformation and mechanical properties were discussed. The FCC phase was observed in the as-atomized powder. The In-situ XRD results indicated that phase transform occurred at around 900 °C. Therefore, the annealing treatment was conducted at 1000 °C to observe the phase transformation. From the XRD of the different annealing times, additional diffraction peaks began to generate after 24 h. With SEM and EDS analyses, the results indicated the Cr element precipitated along the grain boundaries. The longer the annealing time, the more the precipitates. Afterward, EBSD was carried out to identify the crystal orientation of the phase. It was found that the FCC phase was aligned with the Ni3Al crystal structure. On the other hand, the Cr-rich phase formed after the annealing process tended to form the BCC crystal structure of NiAl and Fe, observed from the results of EDS and EBSD. Furthermore, the average hardness of as-atomized powder was 4.46 GPa. After the annealing process, the lattice distortion was eliminated. As a result, the hardness of FCC matrix was significantly decreased to 2.6 GPa. Whereas the hardness of segregated Cr-rich phase was 12.74 GPa. Thus, the average hardness of annealed powder was increased to 6.93 GPa due to the precipitation strengthening resulting from Cr-rich region.

The influence of gas pressure and temperature on the characteristics of aluminium powder produced by a novel low-pressure hot gas atomisation process

ABSTRACT A new low-pressure hot gas atomisation method exploiting the Venturi phenomenon is presented. The working gas is accelerated into a Venturi nozzle and is mixed with the molten metal that is atomised to transform to fine particles. The current work explores the case of pure aluminium that is atomised by carbon dioxide flowing through the Venturi nozzle, while the effect of operating gas temperature and pressure on the particle diameter distribution is investigated. At low temperatures, there is an almost linear relationship between the particle size and the gas pressure with a trend to produce smaller particles at higher gas pressures. This trend cannot be generalised at higher temperatures. The finer powder with average diameter d 50 of 129 µm and diametric range ΔH of 362 µm was produced at an operating gas pressure of 3 × 105 Pa (3 bar) and temperature of 903.15 K (630°C).