Plasma Rotating Electrode Process Research Articles

Comparative study of IN600 superalloy produced by two powder metallurgy technologies: Argon Atomizing and Plasma Rotating Electrode Process

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.

A novel model of calculating particle sizes in plasma rotating electrode process for superalloys

A novel PSD (particle size distribution) model to calculate the powder yields of plasma rotating electrode process (PREP) of nickel base superalloys powders was studied. The results showed that the meshing data of the nickel base superalloys powders like EP741NP and Inconel718 produced by PREP fitted the calculation of the novel PSD model well. The results also showed that not only rotating speed but also melting rate had significant influence on the fine powder yield in the centrifugal atomization system. The general PREP nickel base alloy powders showed characteristics of a bimodal Rosin-Rammler distribution mixed by the ligament disintegrating and direct drop formation. The fractions of the both mechanisms and the process factors of spraying were discussed. This fitting can help the engineers make the target powder yield increase by adjusting the process parameters.

Titanium-Tantalum Alloy Powder Produced by the Plasma Rotating Electrode Process (PREP)

Microstructure of Ti-28Ta powders produced by plasma rotating electrode process (PREP) was investigated by using scanning electron microscopy (SEM), optical microscopy (OM), and transmission electronic microscopy (TEM). Phase constituent of the PREP Ti-28Ta powders was analyzed by X-ray diffraction (XRD). It was found that microstructure of the PREP Ti-28Ta powders was dependent on the powder particle sizes. Predominant martensitic α”, some metastable β and trace athermal ω phases were observed in the powders with the small diameter. While, phase constituent of the PREP Ti-28Ta powders with the large particle size was predominant metastable β, some martensitic α” and trace athermal ω. With the reduction of the powder particle size, the amount of martensitic α” increased and the metastable β decreased. The martensitic α” was formed preferentially on the metastable β grain boundaries of the PREP Ti-28Ta powders. The increase of α” phase and decrease of β with reduction of the powder particle size is attributed to the increase of the volume of the grain boundaries due to the grain refinement.

A comparative study of Ti-6Al-4V powders for additive manufacturing by gas atomization, plasma rotating electrode process and plasma atomization

In this study, it is the first time to overall compare three types of spherical Ti-6Al-4V powders by gas atomization, plasma rotating electrode process and plasma atomization in terms of microstructure, porosity, argon gas content and pore spatial structure using scanning electron microscopy, mass spectrometer gas analyzer and synchrotron X-ray computed tomography (CT). It is found that the particle size plays a crucial role in the micro-morphology, porosity and argon gas content of the atomized powders. The experimental results show that the argon content of the gas atomized (GAed), plasma rotating electrode processed (PREPed) and plasma atomized (PAed) powders below 150 μm is 0.77 ± 0.06, 0.16 ± 0.06 and 0.70 ± 0.06 μg/g, while its porosity is 0.20 ± 0.01%, 0.08 ± 0.01% and 0.12 ± 0.01%, respectively. Both argon content and porosity within powders increase with increasing particle size for each type of powders. The results of three-dimensional reconstructed images from CT scanning present that the pore population, size and porosity within powders gradually increase with the increase of particle size. The pore sphericity of the GAed and PAed powders exhibits relatively higher than that of the PREPed powders due to different gas pressures inside of powders.

The structure and properties of pre-alloyed NiAl-Cr(Co,Hf) spherical powders produced by plasma rotating electrode processing for additive manufacturing

The structure and properties of pre-alloyed Ni41Al41Cr12Co6 spherical powders (the 40–160μm fraction) produced using the plasma rotating electrode processing technology were studied. The evolution of the fine structure of pre-alloyed Ni41Al41Cr12Co6 spherical powders under heating at 20, 350, …, 850°C was investigated. Disk-shaped chromium-rich pre-precipitation zones 5–30nm in size were detected in the temperature range of 450, …, 650°C; they precede the formation of the dispersed chromium precipitate. Prism-shaped chromium nanocrystals with the characteristic longitudinal dimension ranging from 15 to 100nm, whose nucleation process starts at prismatic dislocation loops, were found to be formed at 750, …, 850°C. The Young's modulus, hardness, and elastic recovery of powder material were determined by nanoindentation measurements. The resulting powders are characterized by bulk density of 3.8g/cm3, excellent flow rate (3.5s), and the angle of repose of 19.6°.

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.

Computational fluid dynamic investigation of the primary and secondary atomization of the free-fall atomizer in electrode induction melting gas atomization process

Nickel-based superalloy is mainly used for fabricating the important high temperature parts including the turbine disk, turbine baffle, compressor disk, and other critical components. Ceramic inclusions in powder metallurgy (PM) superalloy could promote fatigue crack initiation, and thus accelerating the crack propagation under certain conditions. In this case, the ultra-clean nickel-based superalloy powder is critical for PM superalloy components. Generally, there are two well-known methods of fabricating superalloy powders, i.e., argon gas atomization (AA) and plasma rotating electrode process (PREP). Electrode induction melting gas atomization (EIGA) process is a newly developed method of preparing ultra-clean metal powders. The EIGA process is a completely crucible-free melting and atomization process developed by ALD vacuum technologies. In this process, a slowly rotating prealloyed bar is fed into a conical induction coil. The end of the bar is inductively heated and molten alloys falls into an atomizer where the liquid alloy is atomized with a high-pressure inert gas. The EIGA prepared powders possess the advantages of AA (more fine powders) and PREP (ultra-clean powders) processes. Generally, there are two key issues in EIGA process, and the free-fall gas atomizer design is one of the critical issues for the powder yield and quality. Free-fall gas atomizers are some of the first two fluid atomizer designs to be used for molten metal atomization. In a simple open (unconfined stream) design a melt stream falls from a tundish exit via gravity into the convergence of focused atomization gas jets where it is disintegrated. The gas-melt interaction is complex, and it is difficult to characterize the interaction process directly. To have a good understanding of the atomisation technology, the physical break-up process instead of correlating the gas dynamics with droplet fragmentation indirectly must be able to be examined. And it will be desirable, if we input the atomization parameters, we can obtain the particles' distributions directly. In this work, a computational fluid dynamic approach to simulating the primary and secondary atomization processes is developed by using the volume of fluid method and discrete phase model. By integrating the metal stream break-up (in primary atomization) with the flow field and particles distribution simulation (in second atomization), this numerical simulation method is able to provide the direct assessment for the atomisation process. To verify the method performance, the melt stream is initialized into a 4 mm-diameter stream, which is then injected into the gas flow field for further fragmentation. The experimental results show that the simulated particles' diameter distribution is consistent with the experimental results in the same conditions.

Peculiar Features of Thermal Aging and Degradation of Rapidly Quenched Stainless Steels under High-Temperature Exposures

This article presents the results of comparative studies of mechanical properties and microstructure of nuclear fuel tubes and semifinished stainless steel items fabricated by consolidation of rapidly quenched powders and by conventional technology after high-temperature exposures at 600 and 700°C. Tensile tests of nuclear fuel tube ring specimens of stainless austenitic steel of grade AISI 316 and ferritic–martensitic steel are performed at room temperature. The microstructure and distribution of carbon and boron are analyzed by metallography and autoradiography in nuclear fuel tubes and semifinished items. Rapidly quenched powders of the considered steels are obtained by the plasma rotating electrode process. Positive influence of consolidation of rapidly quenched powders on mechanical properties after high-temperature aging is confirmed. The correlation between homogeneous distribution of carbon and boron and mechanical properties of the considered steel is determined. The effects of thermal aging and degradation of the considered steels are determined at 600°C and 700°C, respectively.

Shape memory TiNi powders produced by plasma rotating electrode process for additive manufacturing

Abstract This study aimed to produce spherical TiNi powders suitable for additive manufacturing by plasma rotating electrode process (PREP). Scanning electron microscopy, X-ray diffractometry and differential scanning calorimetry were used to investigate the surface and inner micro-morphology, phase constituent and martensitic transformation temperature of the surface and inner of the atomized TiNi powders with different particle sizes. The results show that the powder surface becomes smoother and the grain becomes finer gradually with decreasing particle size. All the powders exhibit a main B2-TiNi phase, while large powders with the particle size ≥178 μm contain additional minor Ti2Ni and Ni3Ti secondary phases. These secondary phases are a result of the eutectoid decomposition during cooling. Particles with different particle sizes have experienced different cooling rates during atomization. Various cooling rates cause different martensitic transformation temperatures and routes of the TiNi powders; in particular, the transformation temperature decreases with decreasing particle size.

Microstructures and Mechanical Properties of Hot Isostatic Pressed Parts of Ti-6Al-4VELI Alloy Spherical Powders with Three Different Diameter Distributions

We report the microstructure and mechanical properties of Ti-6Al-4VELI (extra low interstitial) alloy parts fabricated with three diameter distributions of Ti-6Al-4VELI spherical powders by hot isostatic pressing (HIP), which were manufactured via plasma rotating electrode processing (PREP). The powders were canned and pressed at temperatures 940 °C and pressure 120 MPa. α+β mixed phases in the specimens were observed through Optical Microscope (OM), Scanning Electron Microscopy (SEM) and energy dispersive spectroscopy (EDS) methods. The results show the microstructures of specimens are found to be strongly dependent on the different distributions of powders. The samples formed from fine powders (45∼100 μm) shows better microstructure properties than those formed from medium powders (100∼150 μm) and coarse powders (150∼250 μm). In addition, the billets formed from fine powders exhibit higher tensile strength and ductility than those formed via medium powders and coarse powders. These results are attributed to the initial microstructures of HIP samples, as confirmed by evaluating their fracture features.

Single track and single layer formation in selective laser melting of niobium solid solution alloy

Selective laser melting (SLM) was employed to fabricate Nb-37Ti-13Cr-2Al-1Si (at%) alloy, using pre-alloyed powders prepared by plasma rotating electrode processing (PREP). A series of single tracks and single layers under different processing parameters was manufactured to evaluate the processing feasibility by SLM, including laser power, scanning speed, and hatch distance. Results showed that continuous single tracks could be fabricated using proper laser powers and scanning velocities. Both the width of a single track and its penetration depth into a substrate increased with an increase of the linear laser beam energy density (LED), i.e., an increase of the laser power and a decrease of the scanning speed. Nb, Ti, Si, Cr, and Al elements distributed heterogeneously over the melt pool in the form of swirl-like patterns. An excess of the hatch distance was not able to interconnect neighboring tracks. Under improper processing parameters, a balling phenomenon occurred, but could be eliminated with an increased LED. This work testified the SLM-processing feasibility of Nb-based alloy and promoted the application of SLM to the manufacture of niobium-based alloys.

Microstructural characteristics and densification behavior of high-Nb TiAl powder produced by plasma rotating electrode process

In the present research, the microstructure and phase composition of high-Nb TiAl powder, produced by plasma rotating electrode process (PREP) have been investigated in details. PREPed powder with a mean particle size larger than 150μm demonstrates a dendrite structure, and that smaller than 75μm indicates a martensitic structure. PREPed powder is mainly composed of a hexagonal a2 phase, while in fine powder and coarse powders minor β phase and γ phase are detected, respectively, which is related to solidification path and cooling rate of molten droplets in PREP process. Phase composition and microstructure of the high-Nb TiAl powder have intrinsic effects on the resultant microstructure of hot isostatic pressed (HIPed) billets. After HIP processing, most martensitic and dendritic particles transformed into near γ structure, while few coarse dendritic powder was prone to introduce defects, such as residual primary particle boundaries (PPBs) or coarse lamellae structure, which were harmful to the mechanical properties of HIPed billets. The residual primary particle boundaries (PPBs) or coarse lamellae structure obviously decreased the ultimate tensile strength at room and elevated temperature.

Microstructural characterization and properties of Ti-28Ta at.% powders produced by plasma rotating electrode process

Ti-28 at.%Ta powders were produced by rapid solidification via plasma rotating electrode process (PREP). It is found that microstructure, morphology and phase constituents of Ti-28Ta powders are particle size dependent. Small powder particle exhibit the formation of predominant martensitic α” and minor metastable β, whilst large particles consist of primary metastable β with minor martensitic α” phase. The amount of martensitic α” increases while the amount of metastable β phase decreases with decreasing the particle size. The martensitic α” lath with the size of about 50 nm wide and 1 μm long are formed preferentially at the β grain boundaries. The martensitic transformation start temperature (Ms) and peak temperature (Mp) are ∼390 °C and ∼340 °C respectively for the Ti-28Ta powders with the particle size between 40 and 65 μm. An increased Vickers micro-hardness is obtained and is associated with decreasing particle size.

Microstructure of rapidly solidified Nb-based pre-alloyed powders for additive manufacturing

For powder-based additive manufacturing, sphere-shaped Nb-37Ti-13Cr-2Al-1Si pre-alloyed powders were prepared by plasma rotating electrode processing (PREP). The microstructure, surface oxidation and microhardness of the pre-alloyed powders were systematically investigated. Results showed that the main phases were Nb solid solution (Nbss) and Cr2Nb. The Cr2Nb phases were further determined using transmission electron microscopy (TEM). Fine dendrite structures were observed in the as-fabricated pre-alloyed powders, which transformed to large grains after heat treatment (HT) at 1450°C for 3h. With the increase of powder size, the secondary dendrite arm spacing (SDAS) increased and the microhardness (HV) decreased. A clean powder surface free of oxide particles was obtained by PREP and an oxide layer with 9.39nm in thickness was generated on the powder surface. Compared with Cr- and Nb-oxides, more Ti-oxides were formed on outmost powder surface with a higher content of Ti (up to 47.86 at.%). The differences upon the microstructure and microhardness of the pre-alloyed powders with different sizes were discussed.

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.

Microstructure and surface oxides of rapidly solidified Nb-Si based alloy powders

Spherical pre-alloyed Nb-20Si-24Ti-2Cr-2Al powders were prepared by plasma rotating electrode processing (PREP). The microstructure and surface oxides of Nb-Si based alloy powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and Auger electron spectroscopy (AES). Results showed that the main phases of Nb-Si based alloy powders were Nbss, βNb5Si3 and Nb3Si. Fine lamellar eutectic structure was formed during solidification, taking the central parts of eutectic branches. Anomalous eutectics were distributed at the edges of eutectic branches. The lamellar spacing (λ) ranged from 0.12μm to 0.38μm, increasing with the increase of powder diameter. An oxygen-enriched layer with 5.14nm in thickness, consisting of Nb2O5, SiO2, TiO2, Cr2O3 and Al2O3, was formed on the surface of a powder with 120μm in diameter. The amount of oxygen in the oxygen-enriched layer accounted for 49.66% of that in the whole single powder. Formation mechanism of eutectic structures and the surface oxidation mechanism were discussed.

Microstructure transition from lamellar eutectic to anomalous eutectic of Nb-Si based alloy powders by heat treatment and spark plasma sintering

With a nominal composition of Nb-18Si-24Ti-2Cr-2Al (at%), pre-alloyed Nb-Si based alloy powders were fabricated by plasma rotating electrode processing (PREP). The phases and microstructures of the pre-alloyed powders subjected to spark plasma sintering (SPS) and heat treatments (HT) at 900 °C, 1000 °C, 1200 °C and 1400 °C for 1 h were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that main phases of the pre-alloyed powders were Nbss, βNb5Si3 and Nb3Si in the as-received state, but evolved into Nbss and αNb5Si3 after either SPS or HT at >1000 °C for 1 h. The as-received powders processed by PREP were distinguished by novel refined lamellar eutectic structures. The microstructure transition from lamellar eutectic to anomalous eutectic was triggered by either SPS or HT at > 1000 °C for 1 h, accompanied by a significant drop in hardness. The silicides in the anomalous eutectics were coarsened and smoothened after HT at a higher temperature. The corresponding microstructure transition mechanism was discussed.

NiAl-based electrodes by combined use of centrifugal SHS and induction remelting

NiAl-based electrodes of a required size were fabricated by combined use of centrifugal SHS casting and induction remelting in an inert atmosphere and characterized by modern analytical methods. Thus produced electrode materials exhibiting high chemical purity and low content of impurity gases (0.005 wt % O, 0.0001 wt % N) can be recommended for use in centrifugal plasma sputtering of micro granules via the plasma rotating electrode process (PREP).

Influence of powder size on the crystallization behavior during laser solid forming Zr55Cu30Al10Ni5 bulk amorphous alloy

The Zr55Cu30Al10Ni5 bulk metallic glasses (BMGs) were prepared using laser solid forming (LSF) process from the plasma rotating electrode process (PREP) powder. The effect of the powder size on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) was investigated. It was found that the as-prepared powders were composed of the amorphous phase and Al5Ni3Zr2-type phase. The RZ mainly kept the amorphous state after LSF. The residual Al5Ni3Zr2-type phase could be observed in RZ only if the powder size was larger than 106 μm. Meanwhile, the NiZr2-type nanocrystals at the boundary of RZ primarily formed from the solidification of remelted liquid. With the increase of the powder size, the lower overheating temperature and shorter existing time of the molten pool enhanced the heredity of Al5Ni3Zr2 clusters and other intermetallic clusters in remelted alloy melt, which decreased the thermal stability of the already-deposited layer. The volume fraction of crystallization in the deposit increased with the increase in powder size. There was no crystallization occurred in the HAZ between the adjacent tracks and layers for the deposit prepared by the powder with the size range of 53–75 μm. However, the wide crystalline band with Al5Ni3Zr2-type faceted phase, CuZr-type dendrite, CuZr2-type spherulite and NiZr2-type nanocrystal were observed in the entire HAZ for the deposit prepared by the powder with the size range of 106–150 μm. The finer powder was benefit to prepare the BMGs by LSF.

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.