Gas Atomization Process Research Articles

High-speed cinematography of gas-metal atomization

A high-speed cinematographic footage of a 304L stainless steel gas atomization, recorded at the National Institute of Standard and Technology (NIST), was analyzed using a discrete Fourier transform (DFT) algorithm. The analysis showed the gas atomization process possesses two prominent frequency ranges of melt oscillation (pulsation). A low-frequency oscillation in the melt flow occurring between 5.41 and 123 Hz, with a dominant frequency at 9.93 Hz, was seen in the recirculation zone adjacent to the melt orifice. A high-frequency melt oscillation range was observed above 123 Hz, and was more prominent one melt-tip-diameter downstream in the melt atomization image than upstream near the melt tip. This high-frequency range may reflect the melt atomization frequency used to produce finely atomized powder. This range also included a prominent high frequency at 1273 Hz, which dominated in the image further away downstream from the melt tip. This discrete high-frequency oscillation is most probably caused by the aeroacoustic “screech” phenomenon, intrasound (<20 kHz), a result of the atomizing gas jets undergoing flow resonance. It is hypothesized that this discrete intrinsic aeroacoustic tone may enhance melt breakup in the atomization process with evidence of this fact in the melt images.

Room-temperature tensile and high-cycle-fatigue strength of fine TiB particulate-reinforced Ti-22Al-27Nb composites

To further improve the mechanical properties of a Ti-22Al-27Nb (mol pct) alloy, based on the ordered orthorhombic Ti2AlNb (O phase), a TiB particulate-reinforced Ti-22Al-27Nb matrix composite was prepared using the gas-atomized powder metallurgy method. Because of the rapid solidification during the gas atomization process, the TiB particulates dispersed in the composite were extremely fine, with an average diameter of less than 1 µm and lengths ranging up to 5 µm. This composite (PM composite) showed higher tensile and high-cycle-fatigue properties at room temperature than both an unreinforced Ti-22Al-27Nb matrix alloy and a Ti-22Al-27Nb/TiB composite produced using a conventional ingot metallurgy method (IM composite) with relatively coarse (average diameter 5 µm and average length 40 µm) TiB particulates. These coarse TiB particulates in the IM composite were thought to provide only classical composite strengthening effects. On the other hand, the fine TiB particulates in the PM composite showed additional effects, such as blocking the movement of dislocations.

Thermoelectric properties of newly fabricated n-type 95%Bi 2Te 2–5%Bi 2Se 3 alloys by gas atomizing and extrusion process

n-Type 95%Bi 2Te 3+5% Bi 2Se 3 compounds were newly fabricated by gas atomization process and extruded under different extrusion ratios (16:1 and 25:1) at 450 °C. The effect of extrusion ratio on the microstructure and thermoelectric properties was investigated by a combination of microscopy, X-RD and thermoelectric property testing. Grains of extruded bars are refined due to the dynamic recrystallization during extrusion and also slightly orientated along the extrusion direction. The grain size of the extruded bar at an extrusion ratio 16:1 was measured to be ≈2–5 μm, while it is ≈5–10 μm for the extrusion ratio of 28:1. The compressive strength of hot extruded bar under 25:1 is 160 MPa and with decreasing the extrusion ratio to 16:1, the value is 250 MPa. Extruded bar under ratio of 16:1 shows the higher value of figure of merit ( Z=2.50×10 −3 K −1) than that of 25:1 ( Z=2.07×10 −3 K −1).

Microstructures and Mechanical Properties of Porosity-Graded Pure Titanium Compacts

Microstructures and mechanical properties ofporosity-graded Ti compacts were investigated in this study. To f the porositygraded compacts, Ti powders with three different particle sizes, 65, 189 and 374 mm were prepared by the plasma rotating electrode process (PREP) and the gas atomization process, and these powders were sintered with and without applied stress. Porosity and pore size ofporositygraded Ti compacts decrease significantly with decreasing initial powder size and by applying stress. The most porous layer (374 mm) is severely deformed in compression tests compared to other layers. The compressive strength of porosity graded-compacts is found to be in fairly good agreement with the strength ofmost porous layer. Bend strength ofcompact sintered at 1223 K and 1 MPa (223.3 MPa) shows a higher value than that ofhuman bone (156.9 MPa).

Advantages of innovative large-scale metal powder production

Powder metallurgy (PM) refers to a range of engineering techniques whereby net shape or near-net shape bodies are produced through the aggregation of a powder substrate. Specifically, the emergence of Additive Layer Manufacturing (ALM) is an exciting development in this field. However, the quality of any product produced by ALM is highly dependent upon the quality of the powder used. Gas atomisation is a specialised processing route in the PM field for the production of fine, spherical powders directly from a molten metal melt. The close-coupled gas atomisation process involves a melt stream being impacted by high velocity, under-expanded gas jets which initiate its break-up in two distinct phases; the second critical in determining the final particle size distribution (PSD) of the powder produced. However, fully understanding the mechanisms at work and exerting a high level of control over any produced powder is a challenge faced by the powder manufacturing industry; highlighted by the stringent requirements of the new ALM manufacturing technique.Utilising Ansys Fluent v14.5 computational fluid dynamics (CFD) software, a 3D axi-symmetric simulation has been developed of a close-coupled atomising gas jet nozzle configuration utilised by a powder manufacturer. Through the two-way coupling of the CFD with a Discrete Particle Model (DPM) using the Euler–Lagrange approach, an assessment has been made of the effect of process parameters on the final PSD of the metal powders produced. The most appropriate model for the simulation of the secondary break-up phase has been identified from the Kelvin–Helmholtz (KH), Kelvin–Helmholtz Rayleigh-Transport (KHRT) and Taylor Analogy Break-up (TAB) models. Subsequently, the validated simulation approach has been used for the qualitative assessment of the effect of altering the atomising nozzle geometry and process conditions on the average size of powders produced i.e. finer or coarser. It was also established that there is potential for the model to be used as a quantitative predictive tool of powder size; useful for quality control especially when manufacturing powder for use in ALM. However, further development and validation is required for confirmation of this function.

Gas atomization processing of designed dual-phase intermetallic hydrogen storage alloys

Two dual-phase nickel–metal hydride (Ni–MH) battery powders for battery applications were produced by gas atomization. They are characterized by a fine and uniform dispersion of rare-earth (RE)-containing second phase in the Zr-containing base alloy, achieved from gas atomizing molten mixtures of AB 2 and AB 5 alloys. The dual-phase alloy formulations were (AB 2) 100− x (AB 5) x , where x is 1 and 5. The new dual-phase alloys were found to have high C-rate discharge capacities. Crushing and annealing the dual-phase powders improved the discharge capacity of as-gas atomized powder. C- and C/8-rate discharge capacities of 383 and 425 mAh g −1 (or power density of 134 and 148 Wh kg −1), respectively, were measured for the crushed and annealed dual-phase MmZr05 alloy powder. The possible commercial powder production of the Ni–MH battery powder using a gas atomization process was demonstrated. Four 500 lbs lots of a Zr-based AB 2 alloy were successfully gas atomized in a pilot gas atomizer unit. The discharge capacities of this powder after processing exhibited 349 and 414 mAh g −1 at C- and C/8-rate discharges, respectively.

Progress toward gas atomization processing with increased uniformity and control

Many advanced technologies based on particulate materials demand the availability of fine spherical powders or spherical powders of a narrow particle size class. Generally, high-pressure gas atomization (HPGA) is a close-coupled discrete jet atomization method and is one of the most effective methods of producing such powders. Development of HPGA nozzles with discrete jets resembling convergent–divergent (C–D) rocket nozzle designs, instead of the previous cylindrical jets, was conducted to increase atomization efficiency and uniformity and to reduce the required gas supply pressures. Results of compressible gas flow measurements on both types of HPGA nozzles revealed a steadily increasing trend of gas mass flow with gas supply pressure and a positive deviation from isentropic behavior that increases for increasing supply pressure. This has been attributed to an insufficient volume in the atomization nozzle gas manifold that experiences enhanced expansion cooling at increasing pressures. In experiments on 316L stainless steel, the atomization efficiency of the HPGA nozzle with C–D jets was higher than that of the HPGA nozzle with cylindrical jets, reflecting a lower gas/metal mass flow ratio. In other words, while the powder size distributions were nearly the same for all of the HPGA experiments, the HPGA nozzle with C–D jets utilized atomization gas with a significantly reduced operating pressure and mass flow rate.

Effects of particle size and heat treatment on the electrode performance of a low-cobalt atomized AB 5-type hydrogen storage alloy

A low-cobalt hydrogen storage alloy with a composition of MLNi 3.8Co 0.3Fe 0.2Mn 0.4Al 0.3 was prepared by a vacuum induction casting method followed by gas atomization processing. The results showed that the atomized powders had lower discharge capacity and poorer activation kinetics but better cycling stability than the as-cast sample. The high amount of oxygen and nitrogen present in the atomized powder and the presence of amorphous phase were the main reasons for the observed low discharge capacity and poor activation performance. The atomized powder with a large particle size showed a lower oxygen content and therefore higher discharge capacity than that with a small particle size. However the favorable effect of the atomization processing on the durability of the hydrogen storage alloy was eliminated after heat treatment at 850°C for 6 h.

Microstructural features associated with spray atomization and deposition of Al-Mn-Cr-Si alloy

An inert gas atomization process was employed in production of rapidly solidified powders as well as disc-shape preform by spray deposition of an Al75Mn10Cr5Si10 alloy. Microstructural features of atomized powders and spray deposited preforms were evaluated by scanning and transmission electron microscopy and X-ray diffractometry techniques. Solidification structure of powders revealed cellular and dendritic morphology, depending on their size. The interdendritic regions consisted of second phase particles. In contrast the spray formed alloy exhibited microstructural homogeneity with distribution of ultra-fine second phase particles of intermetallic compound. The structure of second phase intermetallics was identified as a cubic α-Al(Mn,Cr)Si, in both the atomized powders and the spray-deposits. The formation of cubic phase is discussed as rational approximant structure of an icosahedral quasicrystal.

Characteristics of a low-cobalt AB 5-type hydrogen storage alloy obtained by a gas-atomization processing

A low-cobalt ML(NiCoMnAlSnSi) 5 alloy was prepared by a gas atomization processing. The discharge capacity of the atomized powder was around 150 mAh/g which could be increased to about 200 mAh/g after heat treated at 850°C for 6 h. A good activation performance was observed for the atomized powder particularly when the particle size was small. The small particle size and the presence of minor phases were responsible for the quick activation. The rate capability of the atomized powder was also improved by heat treatment.

Utilization of gas-atomized titanium and titanium-aluminide powder

A gas-atomization process has been developed producing clean, high-quality, prealloyed spherical titanium and titanium-aluminide powder. The powder is being used to manufacture hot-isostatically pressed consolidated shapes for aerospace and nonaerospace allocations. These include gamma titanium-aluminide sheet and orthorhombic titanium-aluminide wire as well as niche markets, such as x-ray drift standards and sputtering targets. The powder is also being used in specialized processes, including metal-matrix composites, laser forming, and metal-injection molding.

Ｍｎ‐Ａｌ‐Ｃ合金の磁気特性におよぼす急冷凝固速度の影響

Magnetic properties of Mn-AI-C alloy magnets produced from gas-atomized alloy powders were improved by using the finer particles because the cell size and the segregation of finer particles were very small. So, by using more rapidly solidified powders, the excellent characteristics of Mn-Al-C magnet was expected. In this work, the effect of the rapidly quenched ribbon on the characteristics of Mn-Al-C alloy was investigated, and discussed from the point of rapid solidification. The specimen were prepared by the rapid solidification apparatus with the single roll in the Ar gas atmosphere. Phase analysis of as-quenched and after heat treatment was done by X-ray diffraction analysis. Magnetic properties were estimated on the magnet made from the powder-extrusion process. Tile magnetic properties of Mn-Al-C alloy were improved by using of more rapidly solidified ribbons. Br, iHc, (BH)max and anisotropy of ferromagnetic T-phase were improved to higher level by using thiner thickness ribbons. At extrusion ratio 4, (BH)max value of 57.lkJ/m3 was obtained by the magnet prepared from rapidly solidified ribbons of 12gm average thickness, which was much higher than that by cast process or by gas atomization process. The improvement of magnetic properties of the extruded magnet from rapidly solidified ribbons was found to be caused by the finer microstructure and very small amount of non-magnetic phases such as γ2-phase and β-phase.

Characterization of Thermal Spray Coatings

The ability to characterize fully the microstructure of a coating is paramount for understanding the in-service properties and eventual optimization of the coating. This article discusses sample preparation and subsequent analytical techniques (LOM, SEM, XRD, WDS, and QIA) for several cermet thermal spray coatings and provides a detailed analysis of as-sprayed microstructures in addition to processing trends for several FeCrAIY-carbide coatings. It was found that the splats produced in these high velocity oxy-fuel (HVOF) coatings tended to exhibit a predominantly dendritic structure most likely retained from the gas atomization process that produced the original powder. Chemical analysis showed that the carbides tend to breakdown during spraying producing a complex mixture of oxides and various carbides. Finally, image analysis revealed that as the carbides in the pre-sprayed powder were increased, more carbides and oxides with less FeCrAIY and thinner coatings were found. These techniques allow the thorough characterization of thermal spray cermet coatings, which in turn should further the understanding of the thermal spray processes and help provide superior coatings in the future.

Microstructural features and heat flow analysis of atomized and spray-formed Al-Fe-V-Si alloy

Microstructural features of rapidly solidified powders and preforms of Al80Fe10V4Si6 alloy produced by spray forming process have been studied. The atomization and spray deposition were carried out using a confined gas atomization process and the microstructural features were characterized using scanning electron microscopy and transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques. The microstructure of a wide size range of atomized powders invariably revealed cellular and dendritic morphology. The extent of dendritic region and the dendritic arm spacing were observed to increase with powder particle size. The TEM investigations indicated the presence of ultrafine second-phase particles in the intercellular or interdendritic regions. In contrast, the spray deposits of the alloy showed considerable variation in microstructure and size and dispersion of the second-phase particles at specific distances from the deposit-substrate interface and the exterior regions of the deposit. Nevertheless, considerable homogeneity was observed in the microstructure toward the center of the spray deposit. The formation and distribution of a cubic phase α-Al(Fe,V)Si has been characterized in both atomized powders and spray deposits. A one-dimensional heat flow model has been used to analyze the evolution of microstructure during atomization and also during spray deposition processing of this alloy. The results indicate that thermal history of droplets in the spray on deposition surface and their solidification behavior considerably influence the micro-structural features of the spray deposits.

Gas atomization of polymers. I. Feasibility studies and process development

A new gas atomization process (GAP) was explored for mass producing high-quality spherical powders and very high aspect ratio microfibers from molten polymers. The process involves the use of high-pressure (7.6 MPa or 1050 psi) nitrogen gas and a specifically designed nozzle to atomize a molten stream of polymer into fine droplets that cool to form spherical powders. Polyethylene-based powders, ranging in size from 0 to 200 μm, were efficiently produced in short cycle times by changing a few process control variables such as atomization temperature and polymer melt stream size in a contamination-free environment. The crystallinity of the polymer favored formation of spherical powders. Analysis of the experimental data indicated that the maximum weight fraction of the powders at 0–53 μm can be produced by atomizing the more crystalline polymer, using a 3.175-mm melt stream size at 205°C. Using the GAP in an alternative route to mass producing powders from low-molecular-weight, polyethylene-based waxes that cannot be ground eliminates most of the problems of conventional grinding processes. These benefits of the process together with its flexibility, high throughput, and facile nature can be expected to make it worth considering for industrial processes that must be safe, be capable of mass production, and operate in an environmentally benign fashion. © 1998 John Wiley & Sons, Inc. Adv Polym Techn 17: 145–160, 1998

Gas Atomization Processing of LaNi5-x.Mm, Modified with Silicon and Tin

ABSTRACTA high pressure gas atomization (HPGA) process has been employed to study microsegregation in LaNi4.75 Sn0.25 and LaNi4.6Si0.4 alloy powders to serve as a basis for further investigations of low cost production of multicomponent alloys in combination with mishmetal (Mm). This investigation is an attempt to produce high quality powders for battery cathode fabrication that can be used in an as-atomized condition without prolong annealing, hydridingdehydriding, and grinding. Argon atomizing gas was used in the HPGA process of the LaNi4.75Sn0.25 and LaNi4.6Si0.4 alloys to investigate rapid solidification effects on microsegregation. Short annealing treatments of 5 minutes at 900°C were able to homogenize both alloy compositions, eliminating solidification micro-segregation along the grain boundaries phases. The rapid homogenization can be attributed primarily to the refined cell structures in gas-atomized particles that provide short diffusion pathways for the dissolving elements.

Introducing Metastability in Al-Cu-Zr Alloys by Non-Equilibrium Processes

Al-5Cu-5Zr alloys prepared by three different non-equilibrium processes, such melt-spinning, inert gas atomization and mechanical milling have been mechanically and microstructurally characterized. Melt-spinning and ball milling processes introduced a high degree of metastability in the material, which presented a high hardness even after thermal treatment at high temperature. On the other hand, the inert gas atomization process failed to introduce metastability in the material.

Transmission electron microscopy of as-quenched inert gas atomized particles: Effect of alloying additions in Nd2Fe14B

It is known that the addition of alloying elements to the Nd2Fe14B-based alloys significantly inhibits the onset of the crystallization in particles produced by the inert gas atomization (IGA) process. Transmission electron microscopy (TEM) was performed on a selection of as-atomized particles synthesized by IGA methods with the addition of 3 wt % TiC. TEM samples were successfully prepared using the wedge technique. Large particles (&amp;gt;10 μm) tend to consist of well-defined grains of Nd2Fe14B while the submicron size particles are generally amorphous and particles with diameter d in the size range, 1 μm&amp;lt;d&amp;lt;10 μm, contain nanocrystallites dispersed evenly throughout the amorphous matrix. The selected area electron diffraction pattern of the nanocrystallites in powder particles with diameters below 10 μm generally show three or more diffused diffraction rings that are indicative of α-Fe, with a particulate size of several tens of angstroms. These observations demonstrate that while the alloying addition of Ti and C in the IGA process significantly suppresses the formation of properitectic α-Fe in the Nd2Fe14B alloy, they do not completely prevent it.

Magnetic and mechanical properties of consolidated gas atomized and hydrogenation, disproportionation, desorption, and recombination Nd2Fe14B powders (abstract)

Magnetic and mechanical properties of consolidated (hot pressed) gas atomized powders of Nd2Fe14B with TiC additions will be reported and the results correlated with microstructure. The fracture toughnesses obtained will be compared with similar tests on hot pressed hydrogenation, disproportionation, desorption, and recombination (HDDR) material, made by the HDDR process and with previously reported results on a variety of commercially available materials. TiC additions, by controlling grain growth, aid the gas atomization process and should result in better processability of consolidated magnets. Improvements in the fracture toughness and processability may lead to the use of the Nd2Fe14B magnets in special situations where it is necessary for the magnet to be highly stressed. For example, in electric vehicle drive motors, high rpm and small clearances preclude the use of supporting elements and result in high tensile forces on the magnet.

High purity aluminium powder processed by high pressure gas atomization : J.F. Flumerfelt, I.E. Anderson. (Ames Laboratory, USA.)

Very fine (d < 10 μm) metal and alloy powders can be produced efficiently by a high pressure gas atomization (HPGA) technique. Rapid solidification processing, metal injection molding, thermal spraying and other emerging powder-processing approaches benefit from the use fine powders. This papers reports on the results of an investigation of gas atomization process dynamics with emphasis on the high pressure operating regime. The direct benefit of a better understanding of the process dynamics will be to permit a more informed selection of atomizer design and operating parameters as well as to further improve HPGA performance.