Gas Atomization Research Articles

Evaluation of a Laboratory-Scale Gas-Atomized AlSi10Mg Powder and a Commercial-Grade Counterpart for Laser Powder Bed Fusion Processing.

Laser powder bed fusion (LPBF) is an additive manufacturing technology that implies using metal powder as a raw material. The powders suitable for this kind of technology must respect some specific characteristics. Controlled gas atomization and post-processing operations can strongly affect the final properties of the powders, and, as a consequence, the characteristics of the bulk components. In fact, a complete characterization of the powders is mandatory to fully determine their properties. Beyond the most used tests, such as the volume particle size distribution (PSD) and flowability, the PSD number, the Hausner ratio and the oxidation level can give additional information otherwise not detectable. The present work concerns the complete characterization of two AlSi10Mg powders: a commercial-grade gas atomized powder and a laboratory-scale gas atomized counterpart. The laboratory-scale gas atomization allows to better manage the amount of the fine particles and the oxidation level. As a consequence, a higher particle packing can be reached with an increase in the final density and tensile strength of the LPBF bulk samples.

Removal of Pb(II) from Water by FeSiB Amorphous Materials

Amorphous materials have shown great potential in removing azo dyes in wastewaters. In this study, the performance of FeSiB amorphous materials, including FeSiB amorphous ribbons (FeSiBAR), and FeSiB amorphous powders prepared by argon gas atomization (FeSiBAP) and ball-milling (FeSiBBP), in removing toxic Pb(II) from aqueous solution was compared with the widely used zero valent iron (ZVI) powders (FeCP). The results showed that the removal efficiency of all the amorphous materials in removing Pb(II) from aqueous solution are much better than FeCP. Pb(II) was removed from aqueous solution by amorphous materials through the combined effect of absorption, (co)precipitation and reduction. Furthermore, FeSiBAP and FeSiBBP have relatively higher removal efficiencies than FeSiBAR due to a high specific surface area. Although the FeSiBBP has the highest removal efficiency up to the first 20 min, the removal process then nearly stopped due to aggregation.

Production of gas-atomized powders of [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 glass-forming alloy and their consolidation by hot extrusion

The present work is focused on the production of gas-atomized powders of the [(Fe0.6Co0.4)0.75B0.2Si0.05]96Nb4 (at%) glass-forming alloy and their consolidation within the supercooled liquid interval by hot extrusion. The thermal history of the powders during gas atomization was modeled using momentum and energy conservation equations. While these calculations predicted that the atomized droplets were subjected to solidification rates over 103 K/s, some crystalline phases were still experimentally observed in the atomized powders. The crystalline structure of these phases was determined using synchrotron x-ray diffraction experiments. The atomized powders with diameters between 53 and 75 µm were encapsulated in copper cans and hot extruded at 565 °C using an extrusion ratio of 3:1. The extrusion produced deformed powders, suggesting that the process was successfully conducted between the glass transition and crystallization temperatures. Minimal crystallization occurred during consolidation, as quantified by optical microscopy and differential scanning calorimetry techniques. Nanocrystals of about 10 nm, dispersed in a glassy matrix, were observed in the extruded powders. Furthermore, a discontinuous layer of α-(Fe, Co) phase was present between the extruded particles. The presence of hard micro-size crystalline particles among the powders led to a highly heterogeneous flow during extrusion. Due to stress concentration at the interface between glassy and crystalline particles, cracks sometimes nucleated and grew along the boundaries between the extruded particles. The results shown in this work give an important contribution to the understanding of the effect of crystalline particles on the processing of metallic glass matrix composites within the supercooled liquid interval.

Withdrawn: Research progresses in flow field of close-coupled atomizer and atomization mechanism

With the development of part manufacturing technology, many fields, such as medical treatment, military industry, machine and three-dimensional (3D) printing, propose increasingly higher requirements on performances of metal/alloy powder. Low oxygen content, accurate alloy composition, small particle size and high particle sphericity become characteristics of high-quality powder. With high atomization efficiency, low oxygen content in powder and high cooling rate, the close-coupled gas atomized powder preparation technology is an ideal choice to prepare high-quality powder. Nevertheless, powder preparation technology based on close-coupled gas atomization is a complicated process with mutual coupling effects of multiple-phases flows. However, the atomization mechanism still is not revealed fully yet. Close-coupled gas atomized powder preparation technology is facing with great challenges in mass preparation of high-quality powders at a low cost. It is expected to improve close-coupled gas atomized powder preparation technology and even achieve breakthroughs in atomizing principle. For this reason, this study summarized atomizer structure, gas atomization flow field test technology and gas flow field numerical simulation thoroughly according to existing associate studies. Moreover, the gas atomization mechanism of close-coupled atomizer was analyzed. Finally, further deep studies on atomizing characteristics and atomization mechanism of close-coupled vortical loop slit atomizer were prospected and several research directions were proposed. This paper summarized atomizer structure, gas atomization flow field test technology and gas flow field numerical simulation. Moreover, the gas atomization mechanism of close-coupled atomizer was analyzed. Further deep studies on atomizing characteristics and atomization mechanism of close-coupled vortical loop slit atomizer were prospected and several research directions were proposed.

On the first hydrogenation kinetics and mechanisms of a TiFe0.85Cr0.15 alloy produced by gas atomization

This article reports the hydrogenation properties of a TiFe0.85Cr0.15 alloy prepared by gas atomization on one hand, and vacuum arc-remelting on the other hand for demonstration and reference, respectively. To evidence the excellent potential of ternary alloying combined with gas atomization process for improving the first hydrogen absorption kinetics, both the atomized powders and the reference crushed ingots were exposed to air from 1 h to as long as 10 days. In spite of the resulting surface oxide layer formation, all specimens still absorbed hydrogen partly thanks to the presence of the C14 Laves phase. As expected, the first hydrogenation kinetics decreased with increasing air-exposure time, but atomized powders not only outperformed the reference counterparts but also remained quite competitive despite surface oxidation. The remarkable kinetic enhancement observed was attributed to the formation of an additional Ti2Fe phase, to an increased amount of C14 Laves phase, as well as to a fine microstructure resulting from the rapid cooling rate involved in gas atomization. Even though different absorption behaviors were observed for all samples, the reaction mechanisms derived from solid-gas reaction models were ascribed to interface- and diffusion-controlled 3D processes in the early and subsequent stage of the first hydrogenation, respectively.

Internal friction associated with ε martensite in shape memory steels produced by casting route and through additive manufacturing: Influence of thermal cycling on the martensitic transformation

Among the different families of shape memory alloys (SMA), the Fe-Mn-Si-Cr-Ni alloys have attracted a renewed interest because of its low cost, high corrosion resistance and high recovery strength during the shape memory effect, and the new technologies of additive manufacturing offer unforeseen possibilities for this family of SMA. In the present work, the reversible γ − ε martensitic transformation (MT), responsible for the shape memory effect, is studied in two Fe-Mn-Si-Cr-Ni alloys with high (20.2 wt%) and low (15.8 wt%) Mn content, produced by the conventional route of casting and rolling, in comparison with the MT in another similar alloy, with intermediate Mn content (19.4 wt%), which was produced by gas atomization and additive manufacturing through laser metal deposition. The forward and reverse γ − ε MT is studied by mechanical spectroscopy through the internal friction spectra and the dynamic modulus variation, together with a parallel microstructural characterization including in-situ observation of the γ − ε MT during cooling and heating at the scanning electron microscope. The evolution of the transformed fraction of ε martensite, evaluated through the integral area of the internal friction peak, was followed along thermal cycling in all three alloys. Both, the internal friction and the electron microscopy studies show that the ε martensite amount increases very fast during the first few cycles, and then decreases with a tendency towards its stabilization for many tens of cycles. The results show that the γ − ε MT is more stable on cycling in the additive manufactured sample than in the conventionally processed samples, opening new avenues for designing shape memory steels to be specifically processed through additive manufacturing. • Reversible martensitic transformation in shape memory steels. • Shape memory steels produced by additive manufacturing. • Internal friction reveals good shape memory in additive manufactured steels. • Shape memory stainless steel produced by laser metal deposition. • Shape memory steels characterized by mechanical spectroscopy.

Investigation of Effect of Various Hot Gas Atomisation and Melting Pot Temperatures on Tin Alloy Powder Product

This research investigates the effect of different types of hot gas atomisation (argon, nitrogen and oxygen) and melting pot temperatures on the particle size distribution, microstructure, density and phase of tin alloy (Sn-Cu-Ni-Ge) powder products. The tin alloy powder produced by hot argon gas atomisation had the greatest density (7.84 g/cm3) and the most spherical shape. While the tin alloy powder generated by hot oxygen gas atomisation had the lowest density (6.83 g/cm3), the highest endothermic area (60.41695 area unit) and the most elongated, irregular shape. Hot argon and nitrogen gas atomisation at a melting pot temperature of 800 °C produced a higher yield of 0-25 µm powder than at 700 °C. By contrast, hot oxygen atomisation produced the opposite result. However, all the powder products prepared at 800 °C had a higher spherical shape ratio in the range of 0-25 µm. Tin alloy powder produced by hot oxygen gas atomisation comprised only the elements of Sn and Cu, while the powder generated by hot argon and nitrogen gas atomisation consisted of elements such as the ingot of this powder.

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Diameter of Free-Fall Gas Atomizer

Effect of Electrode Induction Melting Gas Atomization Process on Fine Powder Yields: Diameter of Free-Fall Gas Atomizer

Experimental Investigation on the Spray Behaviour of Bluff Body Air-Assisted Atomizer Designs

This study investigates the gas dynamic effects and atomization behavior of a novel sonic bluff body-assisted two-fluid atomizer with three different geometric configurations based on airflow orifice diameters (d) of 2.0 mm, 3.0 mm, and 4.0 mm. Along with a 280 µm annular liquid sheet, atomizers that employed a central bluff body (cone) with 6.0 mm cone distance (Lc) are compared based on the range of different air and liquid (water) flow rates. The spray-bluff body-impacted secondary atomization was characterized through volume-normalized droplet size distribution (DSD) and cumulative droplet distribution, excentricity plots, Sauter mean diameter (SMD), and relative span factor (Δ). Droplet number density decreases with the increase in radial location, with lesser droplet density for the 3.0 mm atomizer. DSD and cumulative droplet distribution become less uniform with the increase in the radial locations with wider distribution for larger diameter atomizers (4.0 mm). Droplet excentricity follows an inverse relationship with the droplet diameter such that high diameter droplets have low excentricity (%) and vice versa. SMD and relative span factor (RSF) showed opposite trends when plotted (line plots) against the air-to-liquid ratio (ALR) with larger fluctuation in the SMD than the RSF (Δ) value. The spray pattern spread increases gradually with increasing liquid loading and with decreases in the ALR value for all atomizers.

Effect of powder size segregation on the mechanical properties of hot isostatic pressing Inconel 718 alloys

Pre-alloyed powder of Inconel 718 alloy was prepared by electrode induction melting gas atomization (EIGA) and powder metallurgy (PM) Inconel 718 alloys were made through hot isostatic pressing (HIPing) route. The change in volume of large size parts (special-shaped cylindrical capsule) has a much lower effect on the alloy than the powder size. The vibration of the powder filling process over a long time causes the particle size segregation phenomenon that the proportion of coarse particle powders in top part increases, and the proportion of finer particle powders in middle and bottom part increases. This phenomenon has little effect on tensile strength of PM Inconel 718 alloys at room temperature and 650 °C, but has a significant effect on elongation, room temperature impact properties, and stress rupture life at 650 °C/725 MPa. Compared to finer particle powders, the plastic deformation of coarse particle powders is small and a large number of under-deformed powder particles are retained, forming prior particle boundaries (PPBs) and reducing the bonding strength between powder particles. In order to avoid particle size segregation, optimized process parameters and reasonable particle size should be used.

In-situ spherical TiB2/Cu composite powder: A new method of liquid phase reaction coupled with gas atomization

The spherical TiB2/Cu composite powder was fabricated by a new method of liquid-phase in-situ reaction coupled with inert gas atomization. The powder morphology, characteristic indexes, microstructure and nanoindentation hardness were investigated. The effects of spheroidization time and solidification rate of atomized molten droplets on powder sphericity and the relationship between in-situ reaction control and TiB2 distribution were discussed. Results indicate that the powder has a good sphericity and d50 is 32.86 μm, in addition to 75% fine powder yield (<45 μm). The in-situ formed TiB2 particles (TiB2p) were uniformly distributed in grain boundaries and intragrain. The rapid solidification of molten droplets and the inhibition of copper grain growth by TiB2p refined the Cu matrix grain size (4.34 μm). The nanoindentation hardness of composite powder is 3.17 GPa, which is 1.9 times higher than that of spherical Cu powder, and the reasons for the increased hardness of powder are also discussed.

Suitability of Eroded Particles from Die-Sink Electro Discharge Machining for Additive Manufacturing—Review, Characterization and Processing

In this bipartite study, waste products of die-sink electro discharge machining (die-sink EDM) are investigated. EDM is based on an erosive character of discharges leading to material removal and molten material congeals in the dielectric. The aim is to show a theoretical suitability of these particles for a further usage as a secondary, recycled material in additive manufacturing (AM). Due to the energy- and cost-intensive process of gas atomization for AM powders, there is a need for alternative concepts for particle generation. The first part deals with an intensive review of references from the literature regarding particle size and circularity using image analysis. Secondly, real waste streams were investigated after washing and cleaning processes for oil removal via laser diffraction, dynamic image analysis, SEM with energy dispersive X-ray spectroscopy (EDX) as well as optical emission spectroscopy (ICP OES), categorized within the literature and compared to commercial AM powders. In general, it could be shown that, in principle, recycled particles fulfill main requirements for an AM usage regarding size and shape. Reference powders show median particle sizes of 30 µm to 34 µm and circularities of 0.90 to 0.93, whereas eroded particles exhibit an x50 value of 27 µm and circularity of 0.90, too. However, chemical purity, mainly caused by carbon contamination (5.4 wt% in eroded powder compared to 0.4 wt% in reference powder), must be improved before printing via AM machines. Additionally, several separation techniques have to be applied to remove undesired elements (alumina).

Fabrication and Soft Magnetic Properties of Fe81.3Si4B10P4Cu0.7 Amorphous Powders by Using the Spinning-water Atomization Process

Soft magnetic Fe81.3Si4B10P4Cu0.7 powders have been fabricated by using spinning-water atomization process (SWAP) under the water pressure of 17.5 MPa and gas pressure of 2 MPa. To clarify the amorphous forming ability, thermal stability, and the corresponding soft magnetism, the as-SWAPed powders have been sieved into 6 groups with different powder sizes from 0 – 150 μm. After the analysis of the amorphous and crystalline characteristics, the morphology, and soft magnetic properties of these 6 groups of as-SWAPed powders, it is concluded that the SWAPs with a high cooling rate of 105 K/s can improve the amorphous forming abilities of Fe81.3Si4B10P4Cu0.7 powders up to 53 μm, the saturated magnetic flux density as high as 170 – 173 emu/g and the thermal stabilities higher than 112.8 K. The characteristic parameters of as-SWAPed powders above mentioned are close to those of the counterpart rapid solidified ribbons. The surface oxide layers on as-SWAPed powders mainly consist of Fe2O3, and are 10 nm thick, much thicker than these counterpart ribbons, which might help to weaken the eddy effects accompanying with the slight decrease of the saturated magnetic flux density. Due to the higher cooling rates of SWAPs than gas atomization processes and the better spheroidization of powders for SWAPs than water atomization processes, it is key for NANOMET® family alloys to increase their amorphous forming abilities and better the soft magnetic performances.

Influence of Powder Plasticity on Bonding Strength of Cold-Sprayed Copper Coating

When cold spraying is performed at a velocity equivalent to or greater than a specific material-dependent critical velocity, powders suffer intensive plastic deformation and localized heating of interacting surfaces. The thermomechanical reaction of the sprayed powder upon impacting the substrate material triggers thermally dependent metallurgical bonding and/or mechanical interlocking mechanisms. In this study, three Cu feedstocks, fabricated through electrolysis (EP), gas-assisted water atomization (WA), and inert gas atomization (GA), were characterized and annealed before cold spraying. The electron back-scattered diffraction technique was used to analyze the grain structure and plastic microstrain within the powders and coatings. The plastic microstrains that originally existed in the Cu powders were released after 30 min of annealing at 500 °C. The influence of plastic deformation behavior (associated with the grain structure and plastic microstrain of powder feedstocks) on the bonding strength of the cold-sprayed Cu coatings on AA6061 aluminum alloy substrates was examined. The results indicate that EP powder with an asymmetric dendrite morphology was not conducive to the intensive plastic deformation that may cause recrystallized twin grains to form after cold spraying. Furthermore, the homogeneous microstructure of the spherical Cu feedstocks, which may be induced by strain release as recrystallized twin grains and low-angle boundary grain growth through annealing, caused the cold-sprayed Cu coating to have high ductility and low hardness. The findings reveal the low strain hardening and residual stress in the cold-sprayed coating—characteristics regarded as providing key advantages for the bonding strength of the coating.

Influence of Powder Characteristics on the Microstructure and Mechanical Behaviour of GH4099 Superalloy Fabricated by Electron Beam Melting

A Chinese superalloy, GH4099 (~20 vol.% γ′ phase), which can operate for long periods of time at temperatures of 1173–1273 K, was fabricated by electron beam melting (EBM). Argon gas atomized (GA) and plasma rotation electrode process (PREP) powders with similar composition and size distribution were used as raw materials for comparison. The microstructure and mechanical properties of both the as-EBMed and post-treated alloy samples were investigated. The results show that the different powder characteristics result in different build temperatures for GA and PREP samples, which are 1253 K and 1373 K, respectively. By increasing the building temperature, the EBM processing window shifts towards a higher scanning speed direction. Microstructure analysis reveals that both as-EBM samples show a similar grain width (measured to be ~200 μm), while the size of γ′ precipitated in the PREP sample (~90 nm) is larger than that of the GA sample (~130 nm) due to the higher build temperature. Fine spherical γ′ phase precipitates uniformly after heat treatment (HT). Furthermore, intergranular cracking was observed for the as-fabricated PREP sample as a result of local enrichment of Si at grain boundaries. The cracks were completely eliminated by hot isostatic pressing (HIP) and did not re-open during subsequent heat treatment (HT) of solution treatment and aging. The tensile strength of the PREP sample after HIP and HT is ~920 MPa in the building direction and ~850 MPa in the horizontal direction, comparable with that of the wrought alloy.

Laser Powder Bed Fusion of ODS 14YWT from Gas Atomization Reaction Synthesis Precursor Powders

Laser powder bed fusion (LPBF) additive manufacturing (AM) is a promising route for the fabrication of oxide dispersion strengthened (ODS) steels. In this study, 14YWT ferritic steel powders were produced by gas atomization reaction synthesis (GARS). The rapid solidification resulted in the formation of stable, Y-containing intermetallic Y2Fe17 on the interior of the powder and a stable Cr-rich oxide surface. The GARS powders were consolidated with LPBF. Process parameter maps identified a stable process window resulting in a relative density of 99.8%. Transmission electron microscopy and high-energy x-ray diffraction demonstrated that during LPBF, the stable phases in the powder dissociated in the liquid melt pool and reacted to form a high density (1.7 × 1020/m3) of homogeneously distributed Ti2Y2O7 pyrochlore dispersoids ranging from 17 to 57 nm. The use of GARS powder bypasses the mechanical alloying step typically required to produce ODS feedstock. Preliminary mechanical tests demonstrated an ultimate tensile and yield strength of 474 MPa and 312 MPa, respectively.

Two-fluid coaxial atomization in a high-pressure environment

We study the dynamics of atomization of a liquid column by a coaxial gas flow with varying gas pressures. Specifically, we analyse how the gas density increase associated with elevated gas pressures in the ambient and co-flowing gas jet influences the liquid destabilization and breakup process, as well as the resulting droplet formation and dispersion. We present new experimental results for a coaxial liquid–gas atomizer operating in a high-pressure environment, with gas–liquid momentum ratio in the range $M = 5\unicode{x2013}56$ and pressurized gas densities $\rho _g/\rho _0 = 1\unicode{x2013}5$ , where $\rho _0$ is the ambient gas density at standard conditions. High-speed shadowgraphy images are used to quantify the spatially and temporally varying liquid–gas interface in the spray near-field. Liquid core lengths, spreading angles and other spray metrics are presented, and the influence of gas density is identified from the comparison with atomization at atmospheric conditions. In the spray mid-field, phase Doppler interferometry is used in conjunction with laser Doppler velocimetry to quantify the droplet size and velocities, as well as their radial variations across the spray. Results show an increase in droplet size at elevated ambient pressures, when keeping the gas–liquid momentum ratio constant. Finally, we show that these observations are in line with predictions from the Kelvin–Helmholtz and Rayleigh–Taylor instabilities, both of which are relevant to the gas–liquid atomization process.

Formation mechanism of ceramic/metal composite spherical magnetic abrasive prepared via gas-solid atomization

In order to prepare good magnetic abrasive particles (MAPs) and make it play a better role in magnetic abrasive finishing (MAF), an experimental investigation on the formation mechanism of Al2O3/Fe-based composite spherical MAPs prepared via gas-solid atomization is presented. The morphology of MAPs and the distribution of Al2O3 abrasives inside and outside the iron matrix were characterized by scanning electron microscopy and energy-dispersive spectroscopy. The breaking mechanism of gas-solid two-phase flow to molten metal was analyzed by comparing with the metal powder prepared via gas atomization. The formation mechanism of MAPs was studied by analyzing and calculating the mutual movement and cooling process between Al2O3 abrasive and metal droplet. The results show that the breaking mechanism of gas-solid two-phase flow to molten metal is quite different from that of pure gas. Under the same experimental conditions, the average particle size of MAPs prepared by gas-solid atomization is smaller than that of metal powders prepared by gas atomization. After observing the surface and cross-section of MAP, it is found that Al2O3 abrasives are only evenly and densely distributed on the surface of iron matrix, and there is almost no Al2O3 abrasives inside the iron matrix. The calculation shows that in the process of MAPs’ formation, the time of Al2O3 abrasive moving from the inside to the surface of the droplet is less than that of droplet cooling.

A Comparative Study on Laser Powder Bed Fusion of Differently Atomized 316L Stainless Steel.

The significant growth of Additive Manufacturing (AM), visible over the last ten years, has driven an increase in demand for small gradation metallic powders of a size lower than 100 µm. Until now, most affordable powders for AM have been produced using gas atomization. Recently, a new, alternative method of powder production based on ultrasonic atomization with melting by electric arc has appeared. This paper summarizes the preliminary research results of AM samples made of two AISI 316L steel powder batches, one of which was obtained during Ultrasonic Atomization (UA) and the other during Plasma Arc Gas Atomization (PAGA). The comparison starts from powder particle statistical distribution, chemical composition analysis, density, and flowability measurements. After powder analysis, test samples were produced using AM to observe the differences in microstructure, porosity, and hardness. Finally, the test campaign covered an analysis of mechanical properties, including tensile testing with Digital Image Correlation (DIC) and Charpy’s impact tests. A comparative study of parts made of ultrasonic and gas atomization powders confirms the likelihood that both methods can deliver material of similar properties.

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

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