Plasma Rotating Electrode Process Powder Research Articles

Effect of Powder Characteristics on Relative Density and Porosity Formation During Electron Beam Selective Melting of Al2024 Aluminum Alloy

Abstract Defects, such as pores and cracks, can be found in parts fabricated by powder-bed additive manufacturing techniques. The origin of certain defects, such as some voids, can be linked to initial powder quality, which makes it an important factor in the process. Powders used in additive manufacturing processes are produced by different methods such as gas atomization (GA), plasma atomization (PA), and plasma rotating electrode process (PREP); each gives different powder quality. In this study, two different Al2024 powders, produced by electrode induction GA and PREP techniques, were used to investigate the effect of powder characteristics on defect formation during electron beam melting process (EBM). Powders were first characterized by using Hall flowmeter funnel and scanning electron microscope (SEM); then, the EBM process was carried out, and finally, samples were examined by density measurement using Archimedes method, SEM analysis, and tensile test. PREP powder showed higher levels of sphericity and surface smoothness without attached satellites. Consequently, a higher apparent density and decreased flowing time were achieved in PREP powder. Moreover, gas-induced internal pores were observed in GA particles. The results also revealed the average relative density of 96.7% and 99.4% for the parts built by GA and PREP powders, respectively. SEM micrographs confirmed the results of density measurement of the fabricated parts and showed higher degrees of both spherical and irregular-shaped pores in samples built by GA powder. Additionally, they showed deprived mechanical properties due to the higher porosity contents which can form stress-concentrated areas.

A comparative study on laser powder bed fusion of IN718 powders produced by gas atomization and plasma rotating electrode process

The effects of powder feedstocks prepared by different methods on the processibility of the powders and the performance of final components have largely been unexplored in metal additive manufacturing (MAM). In this paper, we conducted a comparative study on the laser powder bed fusion (LPBF) process of Inconel 718 (IN718) powders produced by gas atomization (GA) and plasma rotating electrode process (PREP), respectively. For the GA and PREP powders, the powder characteristics, such as chemical composition, particle size distribution (PSD), powder morphology and powder defects are initially characterized and compared. Parallel single-track experiments and cuboid components printing have also been designed and carried out in the LPBF process by using these two types of powders. Forming quality and mechanical properties of different components are systematically evaluated. The results show that the powder feedstocks do not significantly affect the mechanical properties under the optimized LPBF process parameters. However, the PREP powder, with smooth surface, higher sphericity, higher apparent density and better flowability, can obtain a larger processing window and more stable mechanical properties of the as-built parts than the GA powder.

Non-equilibrium solidification behavior associated with powder characteristics during electron beam additive manufacturing

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification, microsegregation and the formation of interdendritic phases significantly affects the material properties. Notably, the powder characteristics influence the heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of the powder feedstock on solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was fabricated using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, samples built using the two powders were characterized and compared. By performing multi-scale numerical simulations, melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that after appropriate size control, compared with the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate was facilitated, thereby suppressing microsegregation. These findings contribute to PBF knowledge related to feedstock, proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

Spreading behavior of Ti 48Al 2Cr 2Nb powders in powder bed fusion additive manufacturing process: Experimental and discrete element method study

The quality of powder beds is a key factor governing the density of bulk parts produced by the powder bed fusion–additive manufacturing (PBF-AM) process. Conventional assessments of powder flowability cannot be used to evaluate spreadability of the spreading process owing to different dynamic conditions used. Therefore, factors that have dominant effects on the powder bed properties must be investigated. In this study, two types of Ti 48Al 2Cr 2Nb powders, produced by the plasma rotating electrode process (PREP) and gas atomization (GA), were used to compare spreading behaviors and powder bed properties. The static and dynamic flowabilities of the PREP powder were better than those of the GA powder, while the packing density and surface roughness of the powder bed were worse. During the spreading process, the recorded recoating angle gradually increased for the GA powder, while it remained constant for the PREP powder. The increase in the recoating angle for the GA powder was due to the preferential segregation of fine particles during the spreading process. The discrete element method (DEM) model was calibrated by the fitting static and dynamic angle of repose. Based on the DEM model, three particle-flow regimes were identified: the vortex region, free flowing region, and shear stress region. In the shear stress region, the contact force between the GA powder particles was lower than that of the PREP powder, resulting in high packing density. Artificial manipulation of the particle size and shape in the DEM simulation revealed that the predominant factor governing the powder bed properties between the GA and PREP powders was the particle size distribution. We believe that our study provides an improved understanding of the powder-spreading mechanism and optimization of the powder bed properties. • Spreading behaviors and powder bed properties were investigated using experimental and discrete element method. • The static and dynamic flowabilities of the PREP powder were better than those of the GA powder, while the packing density and surface roughness of the powder bed were worse. • The predominant factor governing the powder bed properties was the particle size distribution rather than the particle sphericity distribution.

Controlling factors determining flowability of powders for additive manufacturing: A combined experimental and simulation study

Powder flowability affects dynamic powder behavior during additive manufacturing and varies depending on particle size distribution, particle morphology, and surface features. In this study, Inconel 718 alloy powders were prepared by gas atomization (GA), plasma atomization (PA), and plasma rotating electrode process (PREP). The flowability was experimentally evaluated by the avalanche angle. With the help of numerical simulations, the individual effects of particle size distribution, particle morphology, and particle surface features on flowability were analyzed. The results showed that the PREP powder possessed the best flowability among the three powders. The PSDs were slightly different, but these differences had almost no effect on their flowability. Nevertheless, the excellent flowability of the PREP powder was due to the high particle sphericity and the thin surface oxide film. If the environmental factors are well controlled during the handling, PREP has a natural advantage over GA and PA in terms of flowability.

Effects of process parameters and cooling gas on powder formation during the plasma rotating electrode process

The plasma rotating electrode process (PREP) is rapidly becoming an important powder fabrication method in additive manufacturing. However, the low production rate of fine PREP powder limits the development of PREP. Herein, we investigated different factors affecting powder formation during PREP by combining experimental methods and numerical simulations. The limitation of increasing the rotation electrode speed in decreasing powder size is attributed to the increased probability of adjacent droplets recombining and the decreased tendency of granulation. The effects of additional Ar/He gas flowing on the rotational electrode on powder formation is determined through the cooling effect, the disturbance effect, and the inclined effect of the residual electrode end face simultaneously. A smaller-sized powder was obtained in the He atmosphere owing to the larger inclined angle of the residual electrode end face compared to the Ar atmosphere. Our research highlights the route for the fabrication of smaller-sized powders using PREP.

Non-Equilibrium Solidification Behavior With Solute Trapping Associated With Powder Characteristics During Electron Beam Additive Manufacturing

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification–microsegregation and the formation of interdendritic phases significantly affects the mechanical properties and hot cracking resistance. Notably, the powder characteristics influence heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of powder feedstock on non-equilibrium solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was built using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, the samples built with the two powders were experimentally characterized and their compression properties were compared. By performing multi-scale numerical simulations, powder melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that upon appropriated size control, compared to the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate is facilitated, thereby suppressing microsegregation. The findings contribute to PBF knowledge related to feedstock, thus proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

The Nature of Anomalous Particles (Granules) in Rapidly Quenched PREP Powders—II. A Multiscale Study of PREP Powders and PM HIP Compacts of Ni-Based Superalloys and Stainless Steels

The formation of anomalous particles (granules) with a significantly different content of microalloying interstitial elements of carbon and boron characterizes the heterogeneity of the composition of rapidly quenched particles of high temperature Ni-based superalloys and stainless steels powders manufactured by the plasma rotating electrode process (PREP) technique. A detailed multiscale experimental study of the microstructure of PREP powders and PM HIP compacts of Ni-based superalloys and stainless steels is carried out in order to reveal the features of the microstructure of anomalous granules in the as-received state and consolidated state. Direct nuclear methods of activation autoradiography on carbon, track autoradiography on boron, metallography, SEM, EDX, and OIM are used. A significant effect of carbon and boron on the dendritic segregation of carbide-forming and boride-forming alloying elements in anomalous granules is revealed. The features of the behavior of carbon and boron, which determine the formation of the microstructure of anomalous granules and their retention in PM HIP compacts, are established.

Centrifugal granulation behavior in metallic powder fabrication by plasma rotating electrode process

In recent years, spherical powders with no or minimal internal pores fabricated by the plasma rotating electrode process (PREP) have been highly recommended for powder-type additive manufacturing. Most research on PREP is aimed at establishing relationship between PREP parameters and powder size. However, almost no dedicated research on granulation behavior has been conducted so far. In the present study, PREP experiments of Ti64 and SUS316 alloys were carried out. Numerical modeling based on computational thermo-fluid dynamics was developed to analyze the granulation behavior. In particular, the roles of the additionally introduced gas blast and the morphology of the electrode end surface in fluid granulation were preliminarily investigated. The study showed that in addition to the electrode's rotating speed and diameter, manipulating the plasma arc current (i.e., the melting rate) could also be an effective way to control the PREP-powder size. According to the simulation, there were competing actions of the gas blast affecting the powder size. The gas blast created disturbance on the fluid and deepened the depression of the electrode end surface, which facilitated powder refinement. However, the cooling effect enhanced the fluid stability and hindered fluid granulation. The conclusions indicated the possibility of using various methods to manipulate PREP-powder size.

Surface characterization of plasma rotating electrode atomized 30CrMnSiNi2A steel powder

Characterization of surface oxides on atomized steel powder is essential in order to determine the sintering conditions for providing strong metal bonding between the powder particles during consolidation. In the present study, it was the first time to systematically investigate the surface characterization of plasma rotating electrode atomized steel powder. China ultra-high strength low alloy steel 30CrMnSiNi2A powder produced by plasma rotating electrode process (PREP) was chosen as the model material. X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy and energy dispersive X-ray spectroscopy were performed to methodically investigate the morphology, size, composition and distribution of surface oxides of the PREP powder. The results indicated that the PREP powder surface was covered by dispersed oxide particulates and homogeneous Fe-oxide layer (about 4.6 nm) between the oxide particulates. MnO and FeO were the main forms of the oxide particulates and the Fe-oxide layer, respectively, and a little Fe2O3 existed on the outermost of the Fe-oxide layer. Besides, a small quantity of Cr2O3 and SiO2 existed in local areas of the oxide particulates and the Fe-oxide layer. At last the model of surface oxides distribution of the PREP steel powder was determined.

Characteristics and atomization behavior of Ti-6Al-4V powder produced by plasma rotating electrode process

In the present research, the characteristics and atomization behavior of Ti-6Al-4V powders produced by plasma rotating electrode process (PREP) with different rotation speeds were investigated. Three kinds of particles in the as-PREPed powders are observed: spherical particles, satellite particles and irregular particles. The mean particle size of the PREP powder decreases and its distribution becomes narrower gradually with increasing rotation speed. PREP powder at higher rotation speed demonstrates lower fractions of both satellite particles and irregular particles. By observing the residual electrode tip, it is considered that the irregular particles with corner or flat shape are possibly caused by the tearing of liquid film under the action of centrifugal force and shear force during the atomization process.

Nitinol powders generate from Plasma Rotation Electrode Process provide clean powder for biomedical devices used with suitable size, spheroid surface and pure composition

The nickel-titanium alloy (57Ni-43Ti in wt%) was atomized by the plasma rotating electrode process (PREP). The PREP parameters such as plasma arc current, rotating electrode speed with corresponding PREP powder size range in weight percentage analysis, powder morphology and biocapability of cells were studied by scanning electron microscopies, Inductively Coupled Plasma and X-ray diffraction techniques. From the electrode of the produced powders, the composition has no obviously changes. Weight percentage up to 31.8% of the range under 300 μm while the rotation electrode speed increase to 12k rpm. Spherical and flat with smooth surface were observed in different size range. Brittle phase was not observed of XRD data. The nitinol powder has high biocapability with cells showed no cytotoxicity and well cell adhesion in the in vivo assay.

Microstructures, tensile properties, and fracture mechanisms of Inconel 718 formed by HDR-LMD with PREP and GA powders

For the purpose of improving the productivity of laser metal deposition (LMD), the focus of current research is set on increasing the deposition rate for it, in order to develop high-deposition-rate LMD (HDR-LMD). Plasma rotating electrode process (PREP) and gas atomization (GA) are the most commonly used processes for producing LMD powders. In the presented work, the microstructures, the tensile properties, and the fracture mechanisms of Inconel 718 formed by HDR-LMD with PREP and GA powders have been examined. Initially, blocks were built up with different powders under identical experimental conditions. Afterwards, the samples built were heat treated, and then manufactured to tensile test specimens, after which they were tested in order to determine their tensile properties. Finally, microstructure and fracture analysis has been conducted. According to the results of the optical microscope observations for grain morphology, the EBSD analysis for grain orientations, and the analysis of twin boundary and grain boundary orientation, the GA sample has a higher recrystallization degree than the PREP sample. In addition, the intergranular δ phases are needle-like in the PREP sample, and some spherical δ phases have been observed in the GA sample. The tensile test shows that the quasistatic properties of the GA sample are slightly higher than those of the PREP sample. Based on the fracture analysis, different fracture mechanisms have been observed: in PREP sample, intergranular fracture has been identified; in GA sample, mixed fracture - both intergranular fracture and trans-granular fracture - has been observed. Furthermore, the pores found in the GA sample may be not definitely detrimental for the tensile properties. This hypothesis is supported by the following facts: the GA samples have significantly higher porosity, but meanwhile higher tensile properties; the dimples on the fracture surface that are around the pores develop well and fine; cracks formed in the pores did not extend to the outside of the pores.

Characterization of Prealloyed Ti–6Al–4V Powders from EIGA and PREP Process and Mechanical Properties of HIPed Powder Compacts

Prealloyed Ti–6Al–4V powders were prepared by electrode induction melting gas atomization (EIGA) and plasma rotating electrode process (PREP) in this work. A comparative study of EIGA and PREP powders for hot isostatic pressing (HIPing) compaction was conducted. Characterization of important technological parameters such as particle size distribution, powder surface morphology and flowability was carried out. Microstructure and mechanical properties of Ti–6Al–4V powder compacts HIPed from EIGA and PREP powders were also investigated. The results showed that the EIGA powder has a finer average particle size and higher tap density, while the PREP powder has better flowability and less pores. Micropores can be observed in heat-treated EIGA powder compacts by X-ray tomography and the porosity was found to be about 0.02%. There are no micropores (≥4 μm) to be detected in heat-treated PREP powder compacts. Transgranular fracture mode as well as micropores contributes to the scatter in fatigue property of heat-treated PREP powder compacts. The respective advantages and disadvantages of both EIGA and PREP powders for producing Ti-based complex parts through HIPing were also discussed.

A comparative study of Inconel 718 formed by High Deposition Rate Laser Metal Deposition with GA powder and PREP powder

A comparative study on the metallurgical properties and material microstructures of Inconel 718 (IN718) deposited by High Deposition Rate Laser Metal Deposition (HDR-LMD) using Gas Atomization (GA) and Plasma-rotating Electrode Process (PREP) powders has been carried out. Initially, powders of same nominal particle size produced by different methods were selected and the chemical composition, porosity and morphology of which have been characterized. After that, parallel experiments have been designed and performed, and the metallurgical properties and material microstructures have been analyzed. It is found: compared to IN718 deposited with PREP powder, material formed by GA powder has higher porosity and higher dilution zone whereas finer dendrite structure, lower Nb element segregation and lower volume fraction of Laves phase. In order to figure out the reasons, the mechanisms of pores formation, the laser energy allocation, heat dissipation and solidification of the processes using different powders have been qualitative and quantitative analyzed. It concludes: the high porosity of GA IN718 is due to that more gas has been drawn into the process because of the characteristics of GA powder; the microstructures of GA IN718 is superior to that of PREP powder, showing finer dendrite structure, lower Nb segregation and lower Laves phase fraction, which is due to its higher cooling rate.

プラズマ回転電極法を用いて作製した粉末焼結ニッケル超合金の組織と強度

As the material of turbine discs for the civil airplane, PM (powder metallurgy) superalloys have been in practical use. This is because the advantages of PM superalloys are more pronounced in homogeneous structure, mechanical properties and yield rate than the disadvantage in the cost of the processing cost such as HIP (Hot Isostatic Pressing) by the advance in technology to produce alloy powders. However, PPB (Prior Particle Boundary) is known to decrease the toughness and ductility of PM alloys. For these critical applications, the consolidated alloy powder particles are smashed up to break down carbide and oxide networks, known as prior particle boundaries (PPBs), and to refine the structure into a fine grain size by isothermal forging. High quality powder can be produced by Plasma Rotating Electrode Process (PREP). In this study, for getting the fine grain and high strength and elongation, the PREP powder was HIPed at a δ-subsolvus temperature because δ precipitate prevents grain growth. The relationship between the microstructure and mechanical properties was investigated using PREP powder and lower HIP temperature.

Microstructural evolution and mechanical properties of a β-solidifying γ-TiAl alloy densified by spark plasma sintering

This study deals with the densification of a pre-alloyed Ti–44Al–6Nb–1Mo–0.2Y–0.1B (at.%) powder by spark plasma sintering (SPS). The powder was produced by a plasma rotating electrode process (PREP), and then SPS densified at temperatures between 1200 and 1320 °C. At SPS temperatures below 1240 °C, the α2-dominated dendritic structure in the PREP powder particles disappeared and the fully dense microstructure mainly consisting of γ and B2 grains formed during SPS, but several original powder particle boundaries (OPBs) still remained. While sintered above 1240 °C, OPBs vanished entirely and an uniform duplex microstructure emerged. Furthermore, fully-lamellar (FL) microstructure with mean colony size smaller than 20 μm was produced via β-homogenization annealing. This FL microstructure renders a good tensile elongation of 1.25% and yield strength of 665 MPa at room temperature. However, instability of α2/γ lamellar structures was induced by final stabilization annealing, resulting in sharp reduction of both room-temperature ductility and high-temperature strength.

Effects of Initial Powder Compact Thickness, Lubrication, and Particle Morphology on the Cold Compaction Behavior of Ti Powder

This work investigates the compaction behavior of hydride–dehydride CP-Ti powder from green density/compaction pressure curves. These were obtained through a modification of selected processing conditions, such as variation in compact thickness, the use of internal lubrication, and additions of plasma rotating electrode process powder. A modified Cooper–Eaton equation, which treats the compaction process to be a combination of particle rearrangement (PR) and plastic deformation (PD) mechanisms, was used to simulate the curves. A comparison with aluminum and iron compaction is also carried out in this study. The research indicated that the cold compaction of titanium powder can be separated into two stages: a PR stage (stage I), which occurs at a compacting pressure in the range of 0 to 200 MPa, followed by a further PR stage initiated by PD, when the compaction pressure is in the range of 200 to 1000 MPa. The existence of stage II is due to the low plastic deformability of titanium and low density achieved at the end of stage I.

Microstructure and properties of hot isostatically pressed powder and extruded Ti25V15Cr2Al0·2C

The influence of process route on the microstructure and tensile behaviour of specimens prepared from hot isostatically pressed powders and extruded ingot of the burn resistant alloy, Ti–25V–15Cr–2Al–0·2C (wt-%), has been investigated. Samples based on gas atomised (GA) and plasma rotating electrode process (PREP) powders have been studied. Microstructural examination shows that many PREP powder particles are single crystals, whereas GA particles are polycrystalline. The mechanical properties of hot isostatically pressed specimens have been assessed using tensile testing monitored by acoustic emission, while microstructures have been characterised by synchrotron X-ray microtomography and optical and analytical scanning electron microscopy. Tomographic examination revealed a small fraction (<0·002 vol.-%) of pores in samples made from hot isostatically pressed GA powders, but no porosity was detected in samples made from hot isostatically pressed PREP powder. In view of their similar tensile behaviour, it is concluded therefore that the porosity does not contribute to the scatter and poor ductility in these hot isostatically pressed samples. These pores increased in size and volume fraction after heat treatment above the hot isostatic press temperature. The large scatter in tensile properties of both hot isostatically pressed GA and PREP samples was correlated with the presence of large (100–400 μm) circular crack initiation sites on the fracture surfaces, but the origin of these initiation sites has not been identified.

Production of Porous Sintered Co–Cr–Mo Alloys for Possible Surgical Implant Applications: Part 1: Compaction, Sintering Behaviour, and Properties

AbstractThe compaction and sintering behaviour of a coarse water atomised cobalt based alloy and a bimodal mixture of ball milled ASTM F75 plasma rotating electrode process powder with the cobaIt alloy powder is reported. The techniques employed were intended to generate a porous substrate, using cold compaction techniques, on which a polymeric surface layer could be mechanically bonded. Suitable combinations of compaction pressure and sintering temperature were shown to generate porosities in the bimodal mixture equivalent to those obtained with coarse water atomised powder, producing considerable improvements in green strength over the latter. Sintering in different atmospheres caused variations in surface chromium, nitrogen, and carbon content. Atmospheres that prevented decarburisation, e.g. argon, encouraged liquid phase sintering to occur at high temperatures (> 1340°C) via a carbide eutectic reaction; while nitriding and decarburisation effects also caused differences in hardness. The measured effe...