Plasma Rotating Electrode Process Research Articles

Fabrication of Cast Electrodes from Nanomodified Nickel Aluminide-Based High-Boron Alloy to Fabricate Spherical Powders Using the Plasma Rotating Electrode Process

A complex manufacturing method of the billets from the material based on high-boron nickel aluminide is proposed. The method includes manufacturing the semifinished alloy products using a combined method of self-propagating high-temperature synthesis and centrifugal casting from oxide feedstock and subsequent vacuum induction remelting with introducing Al-based foundry alloys containing nanosized ZrO2 and modifying the structure. The evolution of the microstructure and phase composition is investigated at all production stages. A cast ZrO2-modified cylindrical billet, which possesses high purity in regards to gas impurities (O – 0,005 wt.% and N – 0,0001 wt.%) and is suitable for the further production of powders by the plasma rotating electrode process, is fabricated according to the proposed technology.

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.

Influence of powdered state on crystallization during laser solid forming Zr55Cu30Al10Ni5 bulk metallic glasses

Laser solid forming (LSF) is a viable and promising manufacturing technique for preparing bulk metallic glasses (BMGs) without size limitation. Owing to the structural heredity of alloy melts, the crystallization characteristic of the powder has an important influence on that of the deposit during LSF process. In this work, the as-prepared Zr55Cu30Al10Ni5 (Zr55) alloy powder and the Zr55 alloy powder annealed at 1000 K are used for LSF of Zr55 BMGs. The influence of the crystallization characteristic of Zr55 alloy powder on the crystallization behavior of the remelted zone (RZ) and heat affected zone (HAZ) in the deposit are investigated. It is found that the as-prepared Zr55 powder prepared by plasma rotating electrode process (PREP) is composed of the amorphous phase and Al5Ni3Zr2 phase. When the heat input of laser is low, there exist some Al5Ni3Zr2 residual phases in the amorphous matrix in the RZ, and there appear some Cu10Zr7, CuZr2 and NiZr2 phases besides the Al5Ni3Zr2 phase in the HAZ for the deposit fabricated by as-prepared Zr55 powders. With the increase of the heat input of laser, the RZ remains the amorphous state since the Al5Ni3Zr2 phase is completely remelted, while there are a large quantity of Al5Ni3Zr2 phases and some other crystallization phases precipitated in the HAZ because the heating and cooling rate decrease in the HAZ during LSF. Fabricated by the fully crystallized annealed powder, the deposit is mainly of the amorphous phase, and almost no Al5Ni3Zr2 phase is found even if the incident laser power is low. It is shown that the crystallization of the deposit fabricated by the annealed powder at the low heat input does not change remarkably with the increase of the deposited layers. The Zr55 deposit with five deposited layers could still keep large volume fraction of amorphous phase. This is mainly because the powder experiences the structure relaxation entirely during the annealing treatment, and the volume fraction of the short/medium-range ordered structure associated with the Al5Ni3Zr2 phase in the powder is reduced. Therefore, the volume fraction of the Al5Ni3Zr2 clusters in re-solidified amorphous RZ in the deposited layer decreases during LSF, which is conducible to the increase of the thermal stability of the already-deposited layer. In result, the area of the HAZ in the subsequent deposition decreases and the precipitation of Al5Ni3Zr2 phase is suppressed. In conclusion, increasing the heat input of laser aggravates the crystallization of the deposited layers, and the Al5Ni3Zr2 cluster in the powder has an important influence on the crystallization behavior of the Zr55 deposited layers.

Investigations on processing powder metallurgical high-Nb TiAl alloy sheets

In this work, high Nb-containing TiAl alloy sheets were hot rolled starting from powder metallurgical billets. The prealloyed powder prepared by the plasma rotation electrode process (PREP) was characterized. Results show that there are two typical morphologies of powders: martensitic (M) and dendritic (D) powders, which are formed due to different local cooling conditions. The two PREPed powders lead to the formation of different microstructures in HIPing. The coarse D powder will introduce defects, such as residual primary particle boundaries and interfacial porosity in the as-HIPed microstructures. The deformation behavior of PM high-Nb TiAl alloys largely depends on γ phase. Besides the dynamic recrystallization and the superplastic deformation, due to the large amount of the γ phase in PM high-Nb TiAl alloy and the high rolling rate, the mechanical twinning in γ phase plays a critical role in the hot rolling process. The mechanical properties of high-Nb TiAl alloys can be significantly improved by the hot rolling, through the refinement of microstructures and the elimination of primary particle boundaries. However, a large deformation may introduce defects and coarsened microstructures, and thus 60% deformation may be suitable for the purpose of improving mechanical properties.

Surface fluorination effects on TiAl particle oxidation resistance

TiAl alloy particles (300μm) prepared using plasma rotating electrode processing (PREP) were treated by surface fluorination using fluorine gas (F2) at 25–200°C in pressure of 101kPa for 3h. Depending on the reaction temperature, the TiAl alloy surface components and structure differed markedly. Below 125°C, the oxides (TiO2 and Al2O3) on TiAl particles were gradually changed into oxyfluorides (TiOF2, AlOF, etc.). At temperatures higher than 150°C, the oxyfluorides were converted into fluorides (TiF4 and AlF3), which were hydrolyzed easily with moisture in air. Regarding the oxidation resistance, the oxyfluorides on the surface of TiAl were able to restrain less than 0.8% of the weight increase of TiAl particles, even at 1000°C. However, the fluorides created at temperatures higher than 150°C were affected negatively by the oxidation resistance. To optimize the beneficial effects of surface fluorination on the oxidation resistance of TiAl alloy, it is necessary to control the surface F/O contents.

Effect of Hf and Zr Contents on Stress-rupture and Fatigue Crack Growth Rate Performances in FGH96 PM Superalloy

Four experimental FGH96 alloys with various contents of Hf and Zr (0 and 0.04%, 0.3% and 0.04%, 0.6% and 0.04%, 0.3% and 0.06%, respectively) were produced using PREP (plasma rotating electrode process) + HIP (hot isostatic pressing) route. The unnotched and notched stress-rupture properties and fatigue crack growth rate (FCGR) of all the experimental alloys were investigated to study the effect of Hf and Zr. Relevant fracture morphology and microstructure were observed by scanning electron microscopy and transmission electron microscopy. The results revealed that appropriate content of Hf could lengthen stress-rupture life, eliminate notch sensitivity and slower FCGR. Microstructure analysis showed that the amount of γ′ phase should be increased or decreased by adjusting Hf and Zr contents, and MC carbide and oxide coupled growth should be increased by adding Hf content, which caused oxycarbide to precipitate along grain boundary and strengthen the alloy. It was found that excessive Zr in Hf-containing FGH96 alloy had certain deleterious effects on stress-rupture property because there was strong Zr segregation at prior particle boundary, leaving a long chain of large-size oxides along the boundary. The optimal content of Hf and Zr in FGH96 alloy was 0.6% and 0.04%, respectively.

Sintering and microstructure characteristics of 42CrMo4 steel processed by spark plasma sintering

Rapidly solidified micron sized 42CrMo4 steel powder with a size range of 150–250 μm produced by plasma rotating electrode process was consolidated using a recently developed spark plasma sintering (SPS) process. The relationship between sintering parameters (temperature and soaking time) and microstructural properties was investigated. The effect of slow and high heating regimes on the consolidation of sintered compacts has been also investigated. Maximum relative density (99.2%) was obtained at 1000 °C, under 50 MPa pressure, with 8 min holding time and 200 °C/min heating rate. The microstructure of sintered steel was influenced by carburization occuring inside the graphite SPS process die. The diffusion of carbon produced martensite structure near the surface region and hardened the surface. This effect was investigated in detail using optical microscopy and hardness measurement. Scanning electrone microscopy was also used to examine the fracture surface of sintered compacts. High heating rate promoted the relative density at low temperature compared to high temperature.

Solidification characterization of a new rapidly solidified Ni–Cr–Co based superalloy

Abstract The solidification characterization of a new rapidly solidified Ni–Cr–Co based superalloy prepared by plasma rotating electrode process was investigated by means of optical microscope, scanning electron microscope, and transmission electron microscope. The results show that the solidification microstructure changes from dendrites to cellular and microcrystal structures with decreasing powder size. The elements of Co, Cr, W and Ni are enriched in the dendrites, while Mo, Nb and Ti are higher in the interdendritic regions. The relationships between powder size with the average solid–liquid interface moving rate, the average interface temperature gradient and the average cooling rate are established. Microsegregation is increased with larger powder size. The geometric integrity of MC′ type carbides in the powders changes from regular to diverse with decreasing powder size. The morphology and quantity of carbides depend on the thermal parameters and non-equilibrium solute partition coefficients during rapid solidification.

Microstructure and Properties of Ti6Al4V Alloy Prepared by Hot Isostatic Pressing

Pre-alloyed Ti6Al4V powder produced by plasma rotating electrode process (PREP) was consolidated by hot isostatic pressing (HIP) under the parameters of 930°C/100MPa/3h. The microstructure is analyzed using scanning electron microscopy (SEM). The tensile property is measured at room temperature, the fracture surface morphology of the tensile specimens was investigated using SEM as well. The results show that the dense body has homogeneous microstructure and fine grain size. The average values of ultimate tensile strength (UTS), yield strength (YS) and elongation (EL) are 1119 MPa, 1043 MPa and 18% respectively. The fracture of specimen presents ductile fracture.

Solidification characteristics of atomized AlCu4Mg1-SiC composite powders

In this study, rapidly solidified metal matrix composite powders have been produced by PREP (Plasma rotating electrode process) atomization. AlCu4Mg1 alloy is used as the matrix material while SiC particles, with about 650 nm average particle size, are used as the reinforcement phase. The microstructural and solidification characteristics of composite particles are studied using optical and scanning electron microscope (SEM). The relationship between secondary dendrite arm spacing (SDAS) and particle diameter was examined, and these composite powders were found to have dendritic and equiaxed solidification with a fine eutectic phase. SDAS measurements using various sized particles show that secondary dendrite arm spacing slightly decreases with the decrease in particle size.

Microstructural investigation of as cast and PREP atomised Ti–6Al–4V alloy

A microstructure characterisation of Ti–6Al–4V is conducted for cast, extruded and micrometre sized particles. The plasma rotating electrode process is used to produce spherical Ti–6Al–4V powders from an alloy electrode. The process parameters and their impact on the material properties are described. The effects of electrode rotation speed on the particle size distribution, particle shape and crystal structure are investigated in detail. Optical microscopy and scanning electron microscopy are used for microstructural characterisation. The analysis shows that cast and extruded Ti–6Al–4V alloys have equiaxial α and α+β phase structures, while plasma rotating electrode processed powder from the same alloy compositions has an acicular or martensitic (α) structure. The microstructure scale depends on the particle size. Microhardness measurements are used to assess mechanical property dependence on the microstructure of this alloy. The rapidly cooled alloy particles have much higher hardness than cast or extruded bulk alloy.

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.

Evaluation of Mechanical Properties of a Superalloy Disk with a Dual Microstructure

The main purpose of this paper is to evaluate the mechanical properties of a FGH96 alloy disk with a dual microstructure. FGH96 is a powder metallurgy (P/M) processed disk alloy, which was developed in the 1990s in China. The manufacturing processes used to produce the FGH96 disk with a dual grain structure consisted of atomization by plasma rotating electrode process (PREP), hot isostatic pressing (HIP), isothermal forge, special heat treatment for obtaining dual grain struc-ture and final heat treatment. The disk was cut up and completely evaluated. Mechanical properties, including tensile, stress rupture, plastic creep, low cycle fatigue, fatigue crack growth rate, fracture toughness, impact and hardness, were tested at room and higher temperatures. In addition, a detailed grain characterization of the disk, from rim to bore, was also presented.

Comparative assessment of microstructure and compressive behaviours of PM TiAl alloy prepared by HIP and pseudo‐HIP technology

The present work discusses conditions of comparable microstructure and compressive behaviours of powder metallurgy (PM) Ti–46Al–(Cr,Nb,W,B) (at.‐%) alloy prepared by hot isostatic pressing (HIP) and pseudo‐HIP technology respectively. A PM route based on pseudo‐HIP of alloy powder prepared by plasma rotating electrode process was used for the rapid fabrication of high quality TiAl alloys. The pseudo‐hipped alloy shows a fine grained microstructure which is significantly finer than that of the hipped alloy consolidated under similar conditions. The pseudo‐hipped alloy exhibits more excellent compression properties than that of the hipped alloy at elevated temperatures. By using the kinetic rate equation, the stress exponent n and the apparent activation energy Q are determined and discussed respectively. The influence of microstructure on the flow behaviour during the total hot compressive deformation was studied. The role of submicrocrystalline forming and dynamic recrystallisation due to mechanical twinning and grain matrix deformation will be discussed.

Microstructure Characterization and Mechanical Properties of FGH 95 Turbine Blade Retainers

FGH 95 is a powder metallurgy (P/M) processed superalloy, which was developed in the 1980s in China. One of the applications of FGH 95 was high pressure turbine blade retainers. The manufacturing processes used to produce FGH 95 blade retainers consisted of atomization by plasma rotating electrode process (PREP), hot isostatic pressing (HIP) at super-solvus temperature and a sub-solvus solution heat treatment. The material had an equiaxed grain structure (ASTM 6, 5–7, 5). The γ' precipitates in as-HIP FGH 95 showed a tri-model distribution, Carbides in the alloy were MC type and precipitated at grain boundaries. The prior particle boundaries (PPB) in the material originated mainly from γ'z phase. Statistics of the mechanical properties data from batch production of the FGH 95 blade retainers were investigated. The as-HIP FGH 95 blade retainers showed high strength at room temperature and 650 °C, excellent creep resistance and outstanding stress rupture strength at 650 °C.

Effect of Nitrogen on Microstructure of Porous CoCrMo Alloy Compacts for Biomedical Applications

Porous Co-29Cr-6Mo alloy compacts were fabricated and their microstructures were examined. The Co-29Cr-6Mo alloy powders were atomized by using Plasma Rotating Electrode Process (PREP) in an Ar atmosphere. These powders were hot-pressed at 1223 K in Ar or nitrogen and annealed at 1473 K in vacuum or nitrogen. X-ray analysis revealed that the γ phase (f.c.c.) mostly exists in the compact hot-pressed in nitrogen although the ε phase (h.c.p.) is dominant in the compact hot-pressed in Ar. In addition, precipitates are observed at the interdendritic region in the compact hot-pressed in nitrogen. These results suggest that the Co-29Cr-6Mo alloy powders react with nitrogen to stabilize the γ phase. Accordingly, the constituent phase in the Co-29Cr-6Mo alloy is strongly affected by the atmosphere during hot-pressing.

Mechanical Properties and Biocompatibility of Porous Titanium Prepared by Powder Sintering

Porous Ti compacts were fabricated by spark plasma sintering (SPS) method and their Young's modulus and biocompatibility were investigated in this study. Ti powders were made from commercially available pure Ti (grade 2) using the plasma rotating electrode process (PREP) in an Ar atmosphere. Porous Ti compacts for biomedical applications were successfully fabricated in the porosity range from 5.1 to 39.2 vol%. Young's modulus of porous Ti compacts having porosity from 31 to 35 vol % is almost the same as that of human cortical bone. To investigate the biocompatibility of pure Ti, fibroblast-like L-929 and osteoblast-like MG-63 cells were cultured with the presence of the extract solution from pure Ti for 72 hours. The viabilities of cells cultured with the diluted extract solutions were almost the same compared to that of the control. Also, porous Ti compacts did not induce any morphologic damage of cells.

Mechanical Properties of Porous Titanium Compacts Reinforced by UHMWPE

Porous Ti compacts reinforced by ultra-high molecular weight polyethylene (UHMWPE) were fabricated and their mechanical properties were evaluated. Ti powder atomized by plasma rotating electrode process (PREP) was sintered at temperatures ranging from 1473 K to 1673 K for 7.2 ks in a vacuum. The porous Ti compacts contain the porosity of about 40%, irrespective of the sintering temperature. Porous Ti/UHMWPE composites were successfully fabricated by compressing UHMWPE powder into the porous Ti compacts. The compacts exhibit open pore structure and enables the penetration of UHMWPE into pores in the compacts. Young’s modulus of the composites is higher than that of the porous Ti compacts. The increment in Young’s modulus is not simply explained by the rule of mixture because Young’s modulus of the UHMWPE is approximately 1.3 GPa. Three-point bending strength of the composites is improved, presumably due to the local stress relief by UHMWPE in the vicinity of neck in the composites.

Osteoconductivity of Porous Titanium Having Young’s Modulus Similar to Bone and Surface Modification by OCP

The present study was designed to investigate whether porous titanium (Ti) having Young’s modulus similar to bone has osteoconductive characteristics in rat critical-sized calvarial bone defect. The effect of coating by octacalcium phosphate (OCP) was also examined. OCP is known as a precursor of initial mineral crystals of biological apatite in bones and teeth. Ti powder was prepared by plasma rotating electrode process in an Ar atmosphere. Then, porous Ti disks, 8 mm in diameter with 1 mm thick, were obtained using the particles ranging from 300 to 500 +m, by sintering at 1573 K without applied pressure. The disks had about 35 vol% in porosity and about 10 GPa in Young’s modulus which corresponds to that of human cortical bone. Newly formed bone was observed so as to fill the pore up at 12 weeks, confirming the ability to conduct the ingrowths of the bone tissue. Although in vitro study showed that proliferation of mouse bone marrow stromal ST-2 cells was inhibited on the dishes coated by OCP rather than the control dish, OCP coating on porous Ti seemed to stimulate the bone formation in vivo. Taken together, it seems likely that porous Ti having Young’s modulus similar to bone shows osteoconductive characteristics to conduct bone ingrowths. OCP could be a potential coating agent to assist bone regeneration on porous Ti.