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Due to the increasing demand for high-precision products in metal additive manufacturing, such as powder bed fusion, there is growing expectation for high-quality fine powders produced by the Plasma Rotating Electrode Process (PREP) method. However, the PREP method currently faces a bottleneck due to its low acquisition rate of fine powders below 50 μm. In this study, we analyzed the generation and control of molten layers in PREP and their relationship with centrifugal force, aiming for size reduction. Through experiments using SUS316L and Ti6Al4V alloys, we demonstrated that process parameters such as material diameter, melting current, and cooling gas could control powder size. A significant increase in the acquisition rate of fine powders with an average particle size below 40 μm was observed using a production-scale PREP apparatus with specific parameters. Furthermore, we found that the gas cooling mechanism plays a crucial role in controlling parameters like the melting temperature of the molten layer. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of feedstock powders critically determine the quality of additive manufacturing (AM). The present study employs a plasma rotating electrode process (PREP) to produce non-equiatomic Co31.5Cr7Fe30Ni31.5 high-entropy alloy (HEA) powders and examines variations of their microstructure and mechanical properties with powder particle size in the range of 20–106 μm. It finds that the powders possess smooth surfaces, good sphericity, and equiaxed polycrystal grains under PREP electrode rod arc speed of 32000 r/min and main arc current of 760 A. A stable FCC structure is maintained regardless of the powder particle size. Moreover, the nano-hardness of the powders generally decreases with the increase of powder particle size except for that of 50–75 μm, which is positively correlated with the change of the grain size. The finest grain size is present for powders with particle sizes of 50–75 μm, which possess the highest average nano-hardness and reduced elastic modulus. Bulk alloys fabricated by cold spray (CS) and laser cladding (LC) AM maintain the FCC structure. The CSAM bulk alloy shows early brittle fracture due to the presence of pores and prior particle boundaries resulting from insufficient plastic deformation of the powder particles. The LCAM bulk alloy presents good tensile properties with the ultimate tensile strength (UTS) and elongation to failure being 481.2 MPa and 35.9%, respectively, which can be attributed to the good cohesion and grain characteristics. The present study shall provide the field of AM with useful information in the applications of non-equiatomic PREP HEA powders.

TiAl pre-alloyed powder is the foundation for additive manufacturing of TiAl alloys. In this work, TiAl pre-alloyed powder was prepared using a plasma rotating electrode process (PREP). The effects of electrode rotating speeds and current intensity on the microstructure and characteristics of TiAl pre-alloyed powder have been investigated in detail. The results show that the electrode rotating speeds mainly affected the average particle size of the powder (D50). As the electrode rotating speed increased, the D50 of the powder decreased. The current intensity mainly affected the particle size distribution of the powder. As the current intensity increased, the particle size distribution of the powder became narrower, which was concentrated at 45~105 μm. In addition, the current intensity had a significant effect on the sphericity degree of the powder with the particle size > 105 μm, but it had little effect on that <105 μm powder. TiAl pre-alloyed powder with a particle size > 45 μm demonstrated a dendritic + cellular structure, and the <45 μm powder had a microcrystalline structure. The powder was mainly composed of the α2 phase and γ phase. There were two kinds of phase structure inside the powder, namely the α2 + γ lamellar microstructure (particle size < 45 µm) and the α2 + γ network microstructure (particle size > 45 µm). The phase structure of the powder was related to the solidification path and cooling rate of molten droplets in the PREP. The average thickness of the α2 + γ lamellar was about 200 nm, in which the lamellar γ phases were arranged in an orderly manner in the α2 phase matrix with a thickness of about 20 nm. The network phase structure was corrugated, and the morphology of the γ phase was not obvious. [ABSTRACT FROM AUTHOR]

The plasma rotating electrode process (PREP) is an ideal method for the preparation of metal powders such as nickel-based, titanium-based, and iron-based alloys due to its low material loss and good degree of sphericity. However, the preparation of silver alloy powder by PREP remains challenging. The low hardness of the mould casting silver alloy leads to the bending of the electrode rod when subjected to high-speed rotation during PREP. The mould casting silver electrode rod can only be used in low-speed rotation, which has a negative effect on particle refinement. This study employed continuous casting (CC) to improve the surface hardness of S800 Ag (30.30% higher than mould casting), which enables a high rotation speed of up to 37,000 revolutions per minute, and silver alloy powder with an average sphericity of 0.98 (5.56% higher than gas atomisation) and a sphericity ratio of 97.67% (36.28% higher than gas atomisation) has been successfully prepared. The dense S800 Ag was successfully fabricated by laser powder bed fusion (LPBF), which proved the feasibility of preparing high-quality powder by the "CC + PREP" method. The samples fabricated by LPBF have a Vickers hardness of up to 271.20 HV (3.66 times that of mould casting), leading to a notable enhancement in the strength of S800 Ag. In comparison to GA, the S800 Ag powder prepared by "CC + PREP" exhibits greater sphericity, a higher sphericity ratio and less satellite powder, which lays the foundation for dense LPBF S800 Ag fabrication. [ABSTRACT FROM AUTHOR]

TiAl pre-alloyed powder is the foundation for additive manufacturing of TiAl alloys. In this work, TiAl pre-alloyed powder was prepared using a plasma rotating electrode process (PREP). The effects of electrode rotating speeds and current intensity on the microstructure and characteristics of TiAl pre-alloyed powder have been investigated in detail. The results show that the electrode rotating speeds mainly affected the average particle size of the powder (D50). As the electrode rotating speed increased, the D50 of the powder decreased. The current intensity mainly affected the particle size distribution of the powder. As the current intensity increased, the particle size distribution of the powder became narrower, which was concentrated at 45~105 μm. In addition, the current intensity had a significant effect on the sphericity degree of the powder with the particle size > 105 μm, but it had little effect on that 45 μm demonstrated a dendritic + cellular structure, and the 2 phase and γ phase. There were two kinds of phase structure inside the powder, namely the α2 + γ lamellar microstructure (particle size < 45 µm) and the α2 + γ network microstructure (particle size > 45 µm). The phase structure of the powder was related to the solidification path and cooling rate of molten droplets in the PREP. The average thickness of the α2 + γ lamellar was about 200 nm, in which the lamellar γ phases were arranged in an orderly manner in the α2 phase matrix with a thickness of about 20 nm. The network phase structure was corrugated, and the morphology of the γ phase was not obvious.

The plasma rotating electrode process (PREP) is an ideal method for the preparation of metal powders such as nickel-based, titanium-based, and iron-based alloys due to its low material loss and good degree of sphericity. However, the preparation of silver alloy powder by PREP remains challenging. The low hardness of the mould casting silver alloy leads to the bending of the electrode rod when subjected to high-speed rotation during PREP. The mould casting silver electrode rod can only be used in low-speed rotation, which has a negative effect on particle refinement. This study employed continuous casting (CC) to improve the surface hardness of S800 Ag (30.30% higher than mould casting), which enables a high rotation speed of up to 37,000 revolutions per minute, and silver alloy powder with an average sphericity of 0.98 (5.56% higher than gas atomisation) and a sphericity ratio of 97.67% (36.28% higher than gas atomisation) has been successfully prepared. The dense S800 Ag was successfully fabricated by laser powder bed fusion (LPBF), which proved the feasibility of preparing high-quality powder by the “CC + PREP” method. The samples fabricated by LPBF have a Vickers hardness of up to 271.20 HV (3.66 times that of mould casting), leading to a notable enhancement in the strength of S800 Ag. In comparison to GA, the S800 Ag powder prepared by “CC + PREP” exhibits greater sphericity, a higher sphericity ratio and less satellite powder, which lays the foundation for dense LPBF S800 Ag fabrication.

The influence of powder particle size on the microstructure of the powder metallurgy superalloy FGH97 was investigated. The powder atomized by the plasma rotating electrode process was sieved to three types: fine powder (0–53 μm), medium-sized powder (53–150 μm), and coarse powder (150–212 μm), which were all characterized by their nearly perfect spherical shape and low oxygen content. The powder was filled, degassed, sealed, and then densified by hot isostatic pressing. Then the microstructures of the fully dense compacts were characterized. The results show that fine powder led to limited plastic deformation of particles during densification, resulting in a minimum of Σ3 boundaries and modest PPB (Previous Particle Boundary) decoration. The fine powder compact showed that low impact toughness was caused by fracture along the PPB. However, the addition of Nb and Hf promoted the formation of the dispersed stable MC particles inside the grain, thus preventing the diffusion of C to the surface of the powder particles to form M6C carbides, and resulting in little tendency to present PPB for this alloy. Therefore, the medium-sized powder and coarse powder compact did not present PPB. Thus, the medium-sized powder is optimal for HIP compact preparation, given that fine powder produced PPB while the coarse powder increased the grain size.

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. [Display omitted] • Flowability of powders for AM was evaluated by experiment and simulation. • Effects of PSD, particle morphology and surface feature on flowability were evaluated. • Surface roughness that correlated to the oxidation altered powder flowability. • High sphericity and thin oxide film contributed to the superb flowability of PREP powder. [ABSTRACT FROM AUTHOR]

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. [Display omitted] • The limitation of increasing the rotational speed in decreasing powder size was clarified. • Cooling and disturbance effects varied with the gas flowing rate. • Inclined angle of the residual electrode end face affected powder formation. • Additional cooling gas flowing could be applied to control powder size. [ABSTRACT FROM AUTHOR]

In the present study, Ti6Al4V spherical powders were prepared by supreme-speed plasma rotating electrode process and the particle size fit log-normal distribution. The average diameter of the powders was successfully determined by a model developed in this work, suggesting that the particle size distribution is mainly affected by the rotating speed. The log-normal distribution factor of the particle size distribution maintains stable as the rotating speed ω varies. The particle size distribution indicates that the main atomization mode of Ti6Al4V under supreme-speed plasma rotating electrode process is of the characteristics of direct drop formation. The mechanical properties of the samples prepared by selective laser melting of Ti6Al4V powders were characterized, indicating that such Ti6Al4V samples with isotropy structure exhibit high yield strength and good ductility. Image 1 • A new simulation of random particle size ranges of PREP Ti6Al4V is proposed. • PREP can also provide applicable <45 μm titanium powder for SLM is proved. • The SLM properties of <45 μm PREP Ti6Al4V powders are reported. • The performance of <45 μm PREP Ti6Al4V in SLM matches with the current use. [ABSTRACT FROM AUTHOR]

This research aims to prepare high quality powder of Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn,which lays foundation for the following preparation of powder high temperature titanium alloy component.Ti-6.5Al-1.4Si-2Zr-0.5Mo-2Sn alloy ingots were prepared by vacuum arc remelting (VAR). The chemical compositions of ingots were examined. The loss of alloy elements, uniformity of composition, phase composition and microstructure of titanium alloy ingots were analyzed. Then powders were prepared by plasma rotating electrode process (PREP) with different rotating speeds. Alloy powders were sieved into different particle sizes. The relationship between rotating speed and physical and chemical properties of powder was studied. X-ray diffraction(XRD), scanning electron microscope(SEM) and metallurgical microscope(OM) were used to investigate the phase composition, morphology and microstructure of the powder. The results show that the composition distribution of titanium alloy ingot prepared by VAR is uniform and the content of alloy elements meets the requirements of national standard, through a unique pressing electrode. Microstructure of the ingot is lamellar structure with small amounts of silicide particles disperse in the matrix. The size distribution of the powders made by PREP method exhibits the normal distribution. The sphericity of the powder is excellent,and no hollow ball and satellite ball exists. With the increase of rotating speed, the proportion of small particles increases dramatically,the proportion of large particles decreases greatly. The phase composition of powder is mainly α' phase,and it is mainly composed of peritectic and small amount of dendrite structures. Compared with the alloy ingot, the elements of the powders have a slight loss, the content of O is less than 0.1%,which is beneficial to obtain high-performance powder titanium alloy.

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Abstract Two type products of IN600 alloy powders fabricated by Argon Atomizing (AA) and Plasma Rotating Electrode Process (PREP) were compared, in terms of microstructure, sphericity of powder, defects and mechanical properties. The interior morphology of AAed IN600 powder was mainly composed of cellular dendrites, and the dendrites were discontinuous and disordered. While, the interior morphology of PREPed IN600 powder was mainly composed of dendritic dendrites. The granularity of AAed IN600 powder ranged from 40 to 150 μm, in comparison that of PREPed IN600 powder ranged from 70 to 130 μm. The sphericity of PREPed powder particles were better than that of AAed powder particles. Large hollow powders existed in AAed IN600 alloy powder, while PREPed IN600 alloy powder did not exist. The AAed and PREPed IN600 alloy both exhibit equivalent tensile strength and yield strength after hot isostatic pressing and same heat treatment condition, but the section shrinkage rate of PREPed alloy (15.5%) was higher than that of AAed alloy (14.6%), indicating that the plasticity of PREPed IN600 alloy was better than that of AAed IN600 alloy. Highlights • Large hollow powders existed in AAed IN600 alloy powder. • The sphericity of PREPed powder particles were better than that of AAed powder particles. • The granularity of PREPed IN600 powder ranged from 70 to 130 μm. • The section shrinkage rate of PREPed alloy was higher than that of AAed alloy. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) becomes a potential way to prepare molds, since it provides an effective option to produce parts with complex geometries in fewer steps. Among the mold steel materials, DC53 is considered as a superior steel grade by die and mold making industries due to its high hardness and wear resistance. However, the fabrication of spherical DC53 powder and its AM processing feasibility has not yet been reported. In the present work, spherical DC53 powder and its AMed parts were prepared by plasma rotating electrode process (PREP) and laser-directed energy deposition (LDED), respectively. The feasibility of DC53 powder for LDED preparation and the mechanical properties of laser directed energy deposited (LDEDed) parts were investigated. It is found that the spherical DC53 powder prepared by PREP is well suited for LDED. The kinetic effects during PREP change the phase composition of DC53 powder (transformation from pearlite to austenite). Compared to commercially available DC53 (cast DC53 after heat treatment), the LDEDed part exhibits comparable relative density. Normally, the main phase in commercially available DC53 is pearlite. Since the LDED process is still a rapid prototyping process, the main phase in LDED parts is the same as that in the powder, which is γ-Fe. Interestingly, the phase transformation induced by kinetic effects could increase the hardness and wear resistance of DC53 effectively (the hardness has increased by 28.2 % and the wear rate has reduced by 47 %.), which is of great significance for the fast and efficient preparation of high performance molds. • This work provides a new method for the rapid fabrication of defect-free DC53 sample. • All phases in DC53 powder were changed to γ-Fe due to the rapid cooling during PREP. • Due to the relatively low cooling rate of LMDed, α-Fe phase was formed in LMDed DC53. • LMDed DC53 exhibits superior wear resistance to the traditional DC53. [ABSTRACT FROM AUTHOR]

• Composition and structure of surface oxides are analyzed by XPS combined with TEM. • A surface oxides distribution model of PREP steel powder is proposed. • Surface oxides mainly exist as oxide layer and oxide particulates. • FeO and MnO are the main forms of oxide layer and oxide particulates, separately. 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 Fe 2 O 3 existed on the outermost of the Fe-oxide layer. Besides, a small quantity of Cr 2 O 3 and SiO 2 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. [ABSTRACT FROM AUTHOR]