Electrode Induction Melting Gas Atomization Research Articles

Influence of evolution in reinforcement scale characteristic parameters on the microstructure and properties of Ti6Al4V-TiBw composites fabricated by electron beam powder bed fusion

The size, aspect ratio, and distribution of reinforcement, referred to as the scale characteristic parameters (SCP), are critical factors that influence the mechanical properties of discontinuous reinforced titanium matrix composites (DRTMCs). To investigate the impact of SCP evolution on the microstructure and mechanical properties of TiBw, this study employed spherical Ti6Al4V-TiBw composite powder prepared by electrode induction melting gas atomization (EIGA) as feedstock for fabricating Ti6Al4V-TiBw composites through electron beam powder bed fusion (EB-PBF) process. By implementing a two-step rapid cooling approach in EIGA and EB-PBF processes to “freeze” the TiBw at nanoscale within Ti6Al4V-TiBw composites, a significantly wider regulation window for microstructure and mechanical properties was achieved. Subsequently, heat treatment was conducted at temperatures ranging from 850 to 1200 °C to systematically elucidate the mutual influence regulation among SCPs, microstructure, and mechanical properties of TiBw. Based on our findings, it can be concluded that when subjected to heat treatment temperatures higher than 950 °C, an orientation relationship between TiBw and α-Ti is observed: {0001} α-Ti//{001} TiBw, {11 2‾ 0} α-Ti//{010} TiBw, {10 1‾ 0} α-Ti//{100} TiBw. Additionally, the cross-section of columnar-shaped TiBw exhibits semi-coherent interfaces with coherent interfaces bonded to Ti along its (100) crystal plane while displaying incoherent interfaces with Ti matrix along its (101) and (10 1‾) crystal planes. The strength of Ti6Al4V-TiBw composites exhibited a trend of initial increase followed by decrease with the evolution of TiBw scale characteristic parameters, while the elongation demonstrated an overall decreasing pattern. This research aims to establish a foundation for microstructure and properties control of DRTMCs and provide experimental references and theoretical basis for high-performance DRTMCs research.

Primary and secondary breakup of molten Ti64 in an EIGA atomizer for metal powder production

Electrode Induction melting Gas Atomization (EIGA) is a free-fall gas atomization process used to produce metal powders, where a swirling supersonic gas jet hits a molten metal stream atomizing it into droplets through various fragmentation mechanisms. Fragmentation mechanisms of molten Ti64 are investigated by high-speed video visualization. The role of gas pressure and melting chamber overpressure on the fragmentation mechanisms and on the particle size distribution is determined. The mechanisms observed are fiber breakup, bag breakup and Rayleigh breakup for primary fragmentation and bag breakup and shear breakup for secondary fragmentation. Increasing the gas pressure promotes shear breakup and creates finer droplets. A model based on log-stable law is proposed to fit the mass distribution according to the dominant fragmentation regime. A good agreement is observed between the different analysis (dominant atomization regime, mass cumulative distribution evolution, parameters of log-stable law). A criterion for prediction of nozzle clogging is devised.

Effects of gas pressure and catheter length on the breakup of discontinuous NiTi droplets in electrode induction melting gas atomization

NiTi powders used for selective laser melting have here been fabricated by the breakup of discontinuous droplets in electrode induction melting gas atomization (EIGA). The morphology, particle size distribution, and hollow ratio of the powder were characterized by scanning electron microscopy (SEM), laser particle size analyzer, and computed tomography (CT), respectively. The effects of gas pressure and catheter length on the particle size distribution and powder morphology were then studied. Furthermore, the effects of the classifier wheel speed on the particle size distribution and yield of the 15–53 μm powder in the classification process were also analyzed. The results showed that the average particle size (D50) of the NiTi powder first decreased and, thereafter, increased as the atomization gas pressure increased. This was also the situation with catheter length. Also, the yield of the 15–53 μm powder increased with an increase in the classifier wheel speed. The optimum parameters were a gas atomization pressure of 5 MPa, a tension length of 28 mm, and a classifier wheel speed of 660 r min−1. For this optimized condition, the D50 value and the yield of the NiTi powder were 57.54 μm and 46.4%. In addition, the flowability, hollow ratio, and oxygen content were 15.8 s/50 g, 0.31%, and 450 ppm, respectively.

Microstructure evolution of Ti64-TiBw composites via electron beam powder bed fusion using as-prepared composite powder as feedstock

A self-designed spherical Ti64 powder containing nanosized TiB whisker(TiBw) was developed by electrode induction melting gas atomization (EIGA), which was used as feedstock to prepare Ti64-TiBw composites by electron beam melting (EBM). Nano-scaled TiBw distributed along the grain boundaries, forming a discontinuous network structure in EBM-Ti64-TiBw composites. In order to elucidate the effect of nano-sized TiBw on microstructure and properties of EBM-Ti64-TiBw composites, heat treatment of as-fabricated EBM-Ti64-TiBw is performed from 800 to 1200 °C with interval of 100 °C, to regulate the size characteristics (diameter, aspect ratio) of TiBw, the revolution of microstructure and interface were studied in detail. The results indicate that Ti64/TiBw interface structure transforms from a semi-coherent interface to a coherent interface with increasing heat treatment temperature (HTT). Furthermore, TiBw begin to coarsen after the heat treatment above the β-transus temperature, leading to a weakening of its pinning effect on Ti64 matrix, and the discontinuous network structure disappears at 1000 °C. The growth and coarsening of TiBw are closely related to the diffusion rate of B atom in α-Ti and β-Ti phases. This work provides a theoretical foundation for size control of TiBw reinforcement and the elucidation of its evolution effect on microstructure in EBM-Ti64-TiBw composites.

CFD Modeling of Primary Breakup in an EIGA Atomizer for Titanium Alloy Powder Production.

Electrode induction melting gas atomization (EIGA) technology is a commonly used and effective method for producing spherical metal powders in additive manufacturing. In this paper, we aim to describe the atomization and fragmentation of liquid sheets from a typical swirl nozzle and highlight the primary breakup of titanium alloy powder production. We developed a computational fluid dynamics (CFD) approach to simulate the primary disintegration process of the molten metal using the volume of fluid (VOF) method coupled with the large eddy simulation turbulence model (LES). Our numerical results show that high-speed spraying creates supersonic airflow in the atomization chamber. Recirculation is the main area where primary atomization occurs. The formation of the recirculation zone is the direct driving force that allows atomization to proceed, which will increase turbulence intensity and achieve higher atomization efficiency. VOF-LES simulation can capture some qualitative results such as conical melt-sheet shape, wave formation, ligament formation, and perforation formation. The primary droplet size mainly ranges between 200 and 800 μm. Finally, with increasing gas pressure, the particle size of the atomized powder gradually decreases, and the particle size distribution becomes narrower.

Laser beam powder bed fusion of novel biomedical titanium/niobium/tantalum alloys: Powder synthesis, microstructure evolution and mechanical properties

The synthesis of spherical titanium/niobium/tantalum (TNT) alloy powders, namely Ti-20Nb-6Ta, Ti-27Nb-6Ta, Ti-35Nb-6Ta, and Ti-22Nb-19Ta (in wt-%) by electrode induction melting gas atomization is reported. The powder materials are characterized in detail using X-ray diffraction and scanning electron microscopy. Their processability via laser beam powder bed fusion (PBF-LB/M) is proven, and microstructure as well as mechanical properties of the additively manufactured specimens are assessed. All powders feature a dendrite-type microstructure with Nb/Ta-rich dendritic and Ti-rich inter-dendritic phases. Crystal structures of the powders are strongly composition-dependent. Nb- and Ta-rich Ti-35Nb-6Ta and Ti-22Nb-19Ta feature a body-centered cubic lattice, whereas Ti-rich Ti-20Nb-6Ta and Ti-27Nb-6Ta powders are characterized by multi-phase microstructures, consisting of non-equilibrium martensitic phases. Processing by PBF-LB/M causes significant changes in their microstructures: the dendrite-type morphologies vanish, and the formation of microstructures with a homogeneous element distribution can be observed in all additively manufactured parts. Ultimate tensile strength (UTS) as well as elongation at fracture are assessed by tensile testing. UTS values are found to be in a range from 651 MPa (Ti-35Nb-6Ta) to 802 MPa (Ti-20Nb-6Ta); strain-to-failure is between 21.3 % (Ti-35Nb-6Ta) and 31.7 % (Ti-22Nb-19Ta). Ductile fracture behavior is seen for all TNT alloys investigated.

In situ fabrication of spherical oxide dispersion strengthened Ti powder through gas atomization

Oxide dispersion strengthened (ODS) alloys exhibit excellent mechanical properties due to fine oxide dispersion, but they have great limitations due to utilizing powder fabrication via mechanical alloying. Therefore, this study examined the alloy composition and process that can fabricate ODS titanium (Ti) powder in situ through a gas atomization method. The composition and content of the oxide, which can dissolve in molten Ti during melting and precipitate inside powders during cooling in the gas atomization, were derived through thermodynamic calculations. The ODS Ti alloy composition of Ti + 1 wt% yttria (Y2O3) was derived, and the alloy was prepared as a powder through an electrode induction melting gas atomization (EIGA) method. First, rod-shaped ingots were prepared through vacuum arc remelting (VAR), and Y2O3 was coarsely precipitated along the grain boundaries due to the slow cooling rate. Then, the ODS Ti powder was fabricated by EIGA using the rod ingot and spherical powders could be continuously fabricated. The cross-section microstructure of the powder was observed, and the Y2O3 particles with several tens of nm were uniformly distributed inside the powder. These thermodynamic calculations and experiments confirmed that ODS Ti powder could be fabricated in situ using the gas atomization method.

Phase formation behavior and hydrogen sorption characteristics of TiFe0.8Mn0.2 powders prepared by gas atomization

In this article we have investigated the phase formation behavior and hydrogen storage characteristics of TiFe0.8Mn0.2 samples produced by electrode induction melting gas atomization (EIGA) as well as conventional vacuum arc-melting (VAM). TiFe (46Ti–46Fe–8Mn (wt%) with cubic, CsCl-type structure) and C14 Laves (37Ti–51Fe–12Mn (wt%) with hexagonal, MgZn2-type structure) phases were detected in both atomized powders and crushed ingots, the latter possessing a lower fraction of Laves phase. Microstructural characterization included electron probe micro-analysis (EPMA) and transmission electron microscopy (TEM), which evidenced the existence of a non-equilibrium Ti2Fe phase in the atomized powders. The formation of this additional secondary phase was attributed to the rapid cooling rate during powder manufacturing, based on earlier findings and phase formation thermodynamic calculations combined with Scheil solidification simulations. In addition, kinetics and pressure-composition isotherms (PCIs) were acquired to determine both absorption and desorption enthalpy/entropy, as well as to study the influence of the synthesis route (and hence the resulting microstructure) on kinetics, thermodynamics and reaction mechanisms of the TiFe0.8Mn0.2 alloy.

Effect of Electrode Induction Melting Gas Atomization on Powder Quality: Satellite Formation Mechanism and Pressure

Electrode induction melting gas atomization (EIGA) is a wildly applied method for preparing ultra-clean and spherical metal powders, which is a completely crucible-free melting and atomization process. Based on several experiments, we found that although the sphericity of metal powders prepared by EIGA was higher than that of other atomization methods, there were still some satellite powders. To understand the formation mechanism of the satellite, a computational fluid dynamics (CFD) approach FLUENT and a discrete particle model (DPM) were developed to simulate the gas atomization process, and several EIGA experiments with different argon pressures (2.5-4.0 MPa) were designed. A numerical simulation of the gas-flow field verified the formation trajectory of satellites, and the Hall flow rate of the powder produced under different pressures was 13.3, 13.8, 15.6, and 16.8, which were consistent with the prediction of the numerical simulation. This study provides theoretical support for understanding the satellite formation mechanism and improving powder sphericity in the EIGA process.

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Continuous Metal Melt Flow

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Continuous Metal Melt Flow

Preparation and Microstructure of High-Activity Spherical TaNbTiZr Refractory High-Entropy Alloy Powders.

High-activity spherical TaNbTiZr refractory high-entropy alloy (REHA) powders were successfully prepared by electrode induction melting gas atomization (EIGA) and plasma rotating electrode process (PREP) methods. Both the EIGAed and PREPed TaNbTiZr RHEA powders have a single-phase body-centered cubic (BCC) structure and low oxygen content. Compared with the EIGAed powders, the PREPed powders exhibit higher sphericity and smoother surface, but larger particle size. The average particle sizes of the EIGAed and PREPed powders are 51.8 and 65.9 μm, respectively. In addition, both the coarse EIGAed and PREPed powders have dendritic structure, and the dendrite size of the EIGAed powders is larger than that of the PREPed powders. Theoretical calculation indicates that the cooling rate of the PREPed powders is one order of magnitude higher than that of the EIGAed powders during the solidification process, and the dendritic structure has more time to grow during EIGA, which is the main reason for the coarser dendrite size of the EIGAed powders.

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.

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

Effect of powder size segregation on the mechanical properties of hot isostatic pressing Inconel 718 alloys

Pre-alloyed powder of Inconel 718 alloy was prepared by electrode induction melting gas atomization (EIGA) and powder metallurgy (PM) Inconel 718 alloys were made through hot isostatic pressing (HIPing) route. The change in volume of large size parts (special-shaped cylindrical capsule) has a much lower effect on the alloy than the powder size. The vibration of the powder filling process over a long time causes the particle size segregation phenomenon that the proportion of coarse particle powders in top part increases, and the proportion of finer particle powders in middle and bottom part increases. This phenomenon has little effect on tensile strength of PM Inconel 718 alloys at room temperature and 650 °C, but has a significant effect on elongation, room temperature impact properties, and stress rupture life at 650 °C/725 MPa. Compared to finer particle powders, the plastic deformation of coarse particle powders is small and a large number of under-deformed powder particles are retained, forming prior particle boundaries (PPBs) and reducing the bonding strength between powder particles. In order to avoid particle size segregation, optimized process parameters and reasonable particle size should be used.

Enhanced strength and ductility of nano-TiBw-reinforced titanium matrix composites fabricated by electron beam powder bed fusion using Ti6Al4V–TiBw composite powder

TiB whisker (TiBw)-reinforced Ti6Al4V (Ti64) based composite powder (as an alternative to premixed Ti64 +TiB 2 powders) was designed and produced via electrode induction melting gas atomization as the feedstock for electron beam powder bed fusion (EB-PBF), to synthesize nanosized TiBw reinforced Ti64 based composites with a quasi-continuous network microstructure and well-matched strength and ductility. To understand the interactions among the TiBw morphology, microstructures and mechanical properties, the phase composition, microstructural orientation, interface relationships, tensile properties, and fracture mechanism were investigated systematically. The formation of a quasi-continuous network in the microstructure, with nanosized TiBw distributed along the grain boundary, is primarily attributed to the high cooling rates and low solid solubility of boron in the Ti matrix. In addition, the (102) plane of α-Ti and the cylindrical plane of the nanosized TiBw had a semi-coherent interface, whereas the transverse face of the TiBw had an incoherent interface with the Ti matrix in the EB-PBF-fabricated Ti64–TiBw composites. The ultimate tensile strength of EB-PBF-fabricated Ti64–TiBw composites reached 1121 MPa, which is 27% higher than that of the EB-PBF-fabricated Ti64 alloy, and the elongation after fracture remains at 8.9%, which is 82% remarkable higher than that of forged composites with the same TiBw volume content. This study provides a novel strategy for producing Ti matrix composites with matched high strength–ductility by additive manufacturing employing Ti-based composite powder as the feedstock.

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.

Multi-physics coupling simulation of electrode induction melting gas atomization for advanced titanium alloys powder preparation

A numerical modeling method is proposed for the melting process of Titanium metals of Titanium alloys powder preparation used for 3D printing. The melting process simulation, which involves the tight coupling between electromagnetic field, thermal field and fluid flow as well as deformation associated during the melting process, is conducted by adopting the finite element method. A two-way coupling strategy is used to include the interactions between these fields by incorporating the material properties dependent on temperature and the coupling terms. In addition, heat radiation and phase change are also considered in this paper. The arbitrary Lagrangian–Eulerian formulation is exploited to model the deformation of Titanium metal during the melting process. The distribution of electromagnetic flux density, eddy current density, temperature, and fluid flow velocity at different time can be determined by utilizing this numerical method. In a word, the method proposed in this paper provides a general way to predict the melting process of electrode induction melting gas atomization (EIGA) dynamically, and it also could be used as a reference for the design and optimization of EIGA.

Pre-breakup mechanism of free-fall nozzle in electrode induction melting gas atomization

Based on the preparation of TC4 powder under electrode induction melting gas atomization in this study, the concept of pre-breakup in the free-fall nozzle channel is innovatively proposed. The computational fluid dynamics method was used to simulate the pre-breakup process, and the effect of initial droplet size on the pre-breakup process was investigated. The pre-breakup process of TC4 discontinuous droplets was simulated using a Eulerian-Eulerian volume of fluid multiphase model and the Reynolds stress model. In addition, the Eulerian-Lagrangian discrete phase model and the unstable breakup model were employed to simulate the entire atomization process of the discontinuous droplets and predict the size of the powder particles. Some simulation results were verified based on experimental measurements. The results exhibit that the pre-breakup process of discontinuous droplets can be mainly divided into the formation of ellipsoidal droplets, the formation of annular disk-shaped stacked droplets, the formation of disk-shaped liquid films, and the breakage of the disk-shaped liquid film. In addition, the pre-breakup of small droplets or droplets with appropriately increased size can effectively prevent the broken droplets from hitting and adhering to the inner wall surface of the nozzle channel. Finally, with the increase of initial droplet size, the atomized powder particle size becomes gradually coarse, and the powder particle size distribution becomes gradually concentrated.

Characterization of Ti6Al4V powders produced by different methods for selective laser melting

A comparative investigation was made into Ti6Al4V powders produced by electrode induction melting gas atomization (EIGA), plasma spheroidization (PS), and plasma atomization (PA) in terms of particle size distribution, shape, element distribution, microstructure, flowability and forming properties. The powders were characterized by a laser particle size and shape analyzer, a scanning electron microscopy (SEM), an energy dispersive spectrometer (EDS), an x-ray diffraction (XRD), a Hall-flow meter and SLM machine, respectively. Image analysis was used to quantitatively analyze the elongation and roundness of these Ti6Al4V powders. The results indicate that PA produces the smoothest powder with the lowest satellite sphere. The microstructure of the powders is composed of HCP-α‘ phase. The average elongation and roundness of EIGA Ti6Al4V powder is similar to those of PA Ti6Al4V powder. The flowability of the PA powder (26.23 s/50 g) is better than that of EIGA powder (32.16 s/50 g) and PS powder (35.30 s/50 g). The SLM Ti6Al4V samples produced by EIGA powder exhibit a well-balanced combination of strength (1047 MPa) and ductility (16.2%). In regard to the process of SLM, the PA and EIGA Ti6Al4V powders are more suitable than PS Ti6Al4V powder. The EIGA method is found to be the best choice among these three methods on account of cost and performance.

Direct Energy Deposition of Mo Powder Prepared by Electrode Induction Melting Gas Atomization

Direct Energy Deposition of Mo Powder Prepared by Electrode Induction Melting Gas Atomization