Gas Atomization Research Articles

Clogging of the delivery tube during preparation of copper powder by vacuum atomisation: coupling analysis of flow resistance and heat transfer

ABSTRACT During the production of micro-fine powders using gas atomization technology, the problem of clogging of the delivery tube is often encountered. Flow resistance inside the delivery tube and heat transfer of the delivery tube is one of the critical factors causing the clogging of the delivery tube. In this paper, the volume of the fluid model (VOF) coupled with the discrete phase model (DPM) were used to simulate the delivery tube-clogging process of the delivery tube involved in atomizing copper powder. The results indicate that, with the increase in the length of the delivery tube, the flow resistance increased; Shortening the length of the tube reduces the flow resistance and increases the superheat of the melt, the atomization process is uninterrupted. The atomized powder was qualitatively determined by standard deviation to determine the number of satellite balls. This research has guiding value in understanding the continuity of the atomization process.

CFD modelling of Close-Coupled Gas Atomisation (CCGA) process by employing the Euler-Lagrange approach to understand melt flow instabilities

This paper presents a CFD analysis of the melt and flow-field instabilities in a close-coupled gas atomisation process (CCGA) caused by the gas–melt interactions. The melt mass flow is coupled to the atomiser internal pressure, which varies over time due to the unsteady flow field. A two-phase flow of Argon gas and melt particles is modelled using a coupled Euler-Lagrange framework. Three different initial gas-to-melt ratios (GMRs) of 5.5, 2.6 and 1.32 are considered to study the gas–melt interaction. The results show that in the case GMR = 1.32, sustained instability both in the melt and in the flow-field are distinctly observed, resembling physical atomisation process. This melt fluctuation corresponded to an alternating flow-field fluctuation; alternating between open- to closed-wake condition, where the local maxima of the melt corresponded to an open-wake condition, and the local minima of the melt corresponded to a closed-wake condition.

High activity retention Al–Bi–Zn-base composite powder with mild hydrogen generation

Al–10Bi–7Zn and Al–10Bi–7Zn–1.5X (X: Cu, Fe, Ni, in wt.%) composite powders were designed and prepared through phase diagram calculation and gas atomization method. The effects of Cu, Fe, and Ni on the hydrolysis of Al–Bi–Zn-base composite powders for hydrogen production were investigated respectively. The composition and morphology of powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy disperse spectroscopy (EDS) analyses. Hydrolysis performance test results indicate that the additions of Cu, Fe, or Ni can modify the hydrogen production rate and enhance oxidation resistance in Al–10Bi–7Zn ternary alloy composite powder. In the quaternary composite powder system, Al–10Bi–7Zn–1.5Ni (wt.%) exhibits the best performance with the hydrogen generation yield of 75.3% (954.1 mL/g) within 500 min when reacting with distilled water at 60 °C and maintains a hydrogen yield of 57.9% (733.7 mL/g) within 1500 min after being stored (30 °C, relative humidity of 60%) for 7 d. In addition, the mechanism investigation suggested that the additions of Cu, Fe, or Ni in Al–Bi–Zn-base composite powders can stabilize the Al matrix to retain the high activity of powders resulting from inhibiting the cracking of composite powders in the air.

PRODUCTION OF Ni-HARD ALLOY POWDERS BY GAS ATOMIZATION

Ni-Hard cast iron materials are frequently used in equipment where high wear resistance is required in industrial applications. Generally, Ni-Hard is produced by conventional casting techniques. In this study, it was aimed to produce Ni-Hard powders to produce Ni-Hard cast iron materials with additive manufacturing techniques. To produce spherical and fine raw materials for additive manufacturing techniques, the gas atomization technique with a close-coupled nozzle system was preferred for the production of Ni-Hard powders. Ni-Hard alloy was melted in a high-frequency 10kW induction furnace under a protective atmosphere integrated into the gas atomization system. During the atomization process, 150 °C superheating was applied to prevent the liquid metal from freezing and clogging the melt delivery tube. In terms of the continuity of the atomization process, the negative pressure (aspiration pressure) values formed at the end of the melt delivery tube at different gas pressures were measured. An aspiration pressure of -50 mbar was obtained under an atomization pressure of 35 bar. Particle size distributions, hall flow behaviour and angle of repose properties of the produced powders were determined. Finally, the characterization of the powders was carried out by scanning electron microscopy. It was determined that the powders obtained as a result of atomization exhibited a spherical morphology and a narrow size range. The Hall flow rate test result of Ni-Hard powders was measured as 22 seconds for 50 g.

Microstructure and mechanical properties of Al0.5CoCrFeNi HEA prepared via gas atomization and followed by hot-pressing sintering

Pre-alloyed Al0.5CoCrFeNi high-entropy alloy (HEA) powders were prepared via gas atomization and followed by hot-pressing sintering (HPS) at 1200 °C for 1 h to fabricate bulk materials. The phase stability, microstructure evolution and mechanical properties of Al0.5CoCrFeNi HEA were mainly investigated. Results indicated that the atomized powders exhibited a fine-grained FCC + dendritic-like ordered BCC (B2) dual-phase structure. Rapid solidification was favorable to the metastable B2 phase formation during gas-atomization. Partial of the dendritic-like B2 phase dissolved after annealing (>600 °C), resulted in a homogeneous dual-phase structure with irregular B2 phase dispersed in the FCC matrix. The homogeneous structure was “inherited” in the sintered bulks and coarsened after HPS. Tensile test results suggested that the sintered bulks possessed excellent comprehensive mechanical properties with yield strength, ultimate tensile strength and elongation of 587.3 ± 18.7 MPa, 960.2 ± 23.6 MPa and 32.6 ± 5.1%, respectively. The excellent mechanical response during deformation was mainly attributed to the synergistic deformation of the FCC/B2 phase and considerable back stress hardening mechanism.

Computational insights into gas atomization of FeCoNiCrMoBSi high-entropy alloy: From droplet formation to rapid solidification

The intricate processes governing the gas atomization of molten metal droplets and their subsequent solidification hold paramount significance in materials science. Computational fluid dynamics simulations have emerged as indispensable tools for unraveling the complexities inherent in metal melt atomization. This study employed the volume of fluid method and the shear-stress transport k-ω turbulence model to simulate the primary atomization process of a FeCoNiCrMoBSi high-entropy alloy. Subsequently, the discrete phase model and Taylor analogy breakup model, based on the gas Weber number and incorporating primary atomization outcomes as initial conditions, were utilized to simulate droplet formation during the secondary atomization process. Analysis of the particle size distribution at the outlet of the geometric model demonstrated excellent agreement with experimental data for gas-atomized FeCoNiCrMoBSi high-entropy alloy powder. Furthermore, comprehensive characterization analyses were conducted to probe the phase constitution and structure of FeCoNiCrMoBSi powder produced by gas atomization. The results revealed that, irrespective of particle size, the powder exhibited a face-centered cubic structure and distinctive eutectic microstructural characteristics. The numerical simulation provided insights into the solidification process of secondary droplets by integrating inverse pole figure data for gas-atomized powder across varying particle sizes. The estimated cooling rate of secondary droplets ranged from 1.22 × 104 K/s to 2.64 × 105 K/s. These findings significantly advance the understanding of the gas atomization mechanism of the FeCoNiCrMoBSi high-entropy alloy, paving the way for the development of more efficient manufacturing processes for advanced materials.

Nitrogen alloyed austenitic Ni-free stainless steel for additive manufacturing

Nitrogen alloyed austenitic nickel-free stainless steel (ANFSS) is one of the most promising group of materials for consumer and healthcare products. They can be used to substitute not only conventional AISI 316L, but also more expensive titanium and Co-Cr alloys. Previously the utilization of nitrogen alloyed materials has been limited due to poor machinability resulting from high strength and ductility, and difficulties in manufacturing the nitrogen alloyed raw materials. Recent developments in powder metallurgy, for example, additive manufacturing (AM), offer feasible net shape manufacturing routes to go around machining and material related challenges. The present study investigates and introduces a viable processing route for an ANFSS by gas atomization, direct energy deposition (DED), and sintering. Special attention is paid to control the nitrogen content in different processing steps by investigating a Fe-16Mn-14Cr-0.27C-0.35N alloy with experimental and computational methods. The results show that the nitrogen content can be computationally estimated and maintained at desired level by proper selection of processing parameters, and thereby controlling the final nitrogen content of the ANFSS alloy.

Characteristic analysis of high-speed steel powder prepared by gas atomisation

The performance of high-speed steel (HSS) powder is a prerequisite for the preparation of high-performance powder metallurgy HSS. The physical properties, microstructure, and solidification characteristics of HSS powder prepared by nitrogen gas atomisation were systematically studied and analysed in detail. For the atomised TPM558 HSS powder, the morphology is spherical and the particle size distribution is wide. The smooth small particles of satellite powder attach to the surface of large particles. Shrinkage pits caused by solidification shrinkage exist on the surface of large particles. The average cooling rate is between 104 and 106 K·s−1, with the increase of cooling rate, the powder surface tends to be smooth, and the grain structure is transformed in the order of cellular to dendritic to radial. The single powder particle surface shows fine-equiaxed grains, while the internal presents columnar grains. The crystalline phases of the powder are austenite, ferrite, martensite, and carbide, and the carbide includes MC carbide with face-centred structure and M2C carbide with hexagonal structure. The average nano-hardness of the powder is 9.93 GPa, resulting in poor formability, and it can be pressed and formed only after adding binder.

Ultra strong FCC structured Ni8Cr4Co4Fe6W2 high entropy alloys with high strength and ductility by laser powder bed fusion

The high entropy alloys (HEAs) are widely used in high temperature service environment (such as aviation engine blades) with bright application potential in additive manufacturing. Laser powder bed fusion (LPBF) is technology currently offers the best reproducibility and dimensional accuracy for part production. The unique atomic lattice structure of HEA makes its plastic deformation mechanism different from that of traditional dilution alloy, resulting in the strengthening effect of face-centered cubic high-entropy alloy different from that of traditional metal alloy materials. In this paper, Ni8Cr4Co4Fe6W2 was designed by means of composition control. Its spherical powders with uniform composition were successfully prepared by high temperature remelting spheroidization (PHTR) method, compared with gas atomization (GA) method. The strain rate sensitivity m values of GA-Selective Laser Melting and PHTR-Selective Laser Melting samples were 0.030 and 0.027, respectively. The hardness of Ni8Cr4Co4Fe6W2 high-entropy alloy showed a trend of continuous increase with the decrease of grain size, while the elastic modulus didn’t change much (PHTR-SLM is 193 Gpa and GA-SLM is 189 Gpa). The simulated value of the contact stress decreased by 23 % compared with the theoretical value. The contact radius increased with the increase of plastic deformation in addition to the reduction of contact pressure. The printing sample value of the elongation, yield strength (YS) and Ultimate Tensile Strength (UTS) on PHTR-SLM has increased by 39.6 %, 33.7 % and 25.9 %, respectively in additive manufacturing, compared to GA-SLM. The ability to produce refractory HEAs through PHTR method will make it possible to manufacture complex geometry shapes at a reasonable cost.

Gas atomization of AA2017 aluminum alloy: Effect of process parameters in the physical properties of powders for additive manufacturing

Close-coupled gas atomization (CCGA) uses pressurized gas jets to atomize molten metals, producing powders suitable for additive manufacturing (AM). Optimization of processing parameters is crucial to achieve powders with good fluidity and high apparent density for laser powder bed fusion (L-PBF) and increase production yield. This study evaluated the influence of melt feed nozzle diameter, superheating temperature, and atomization pressure on AA2017 aluminum alloy powders' physical properties. Parameters variation poorly affected the median particle diameter (d50), while significantly altering the particle size distribution curves width (IDR = d90-d10). Suitable powders for AM/L-PBF were produced using a specific set of parameters (d0= 1.5 mm; ΔT= 150 °C; PG= 20 Bar), presenting appropriate fluidity and apparent density due to an optimal granulometric distribution and morphological characteristics. Mathematical analysis showed a good correlation between experimental and calculated mean particle size, suggesting an equation to predict percentile d90 based on previous correlations.

Manufacturing Oxide Dispersion Strengthened (ODS) steel plate via cold spray and friction stir processing

Oxide dispersion strengthened (ODS) steels, traditionally fabricated by ball milling and conventional powder metallurgy techniques to achieve bulk form, followed by intricate rolling and thermal treatment steps to achieve plate or sheet form. Here, we present a novel processing route that combines cold spray (CS) with friction stir processing (FSP) to manufacture ODS steel plate directly from gas atomization reaction synthesis (GARS)-prepared powder, thus no rolling steps involved. Microstructural and mechanical characterizations were performed to assess the quality and properties of the resulting ODS steel plate. Our findings demonstrate that the slightly porous CS deposited layer was fully consolidated after FSP, yielding a fully dense ODS steel plate that exhibited a favorable tradeoff between strength and ductility upon extraction from the substrate. Furthermore, through microstructural analysis, we revealed the presence of an appreciable density (∼1022/m3) of nano-sized oxide particles, with the majority being smaller than 5 nm via the combined CS + FSP fabrication route. This work serves as a first proof-of-concept demonstration of the manufacturing approach described herein, offering a possible alternative route for producing ODS steel plates.

Extremely low coercivity of powder Co-rich amorphous alloy obtained by gas atomisation

A novel Co-base soft magnetic powder with ultra-low coercivity was produced by inert gas atomisation. The as-atomised powder is fully amorphous owing to its high glass forming ability and the proper selection of atomisation conditions; it exhibits a very low coercivity (0.130 Oe) mainly due to the low magnetostriction coefficient of Co-based alloys. After production, the powder was annealed at different temperatures between 300 and 600 ºC for 30 minutes. Crystallisation started at around 575 ºC, which is the temperature of the first exothermic peak detected by differential scanning calorimetry. Annealing the powder at temperatures below 550 ºC develops only structural relaxation of the amorphous structure. The smallest coercivity (0.056 Oe) was found in the sample annealed at 400 ºC. On the other hand, an increment of the coercivity of four orders of magnitude occurred after full crystallisation at 600 ºC (394.320 Oe). From the analysis of the curves of anisotropy field distribution, it can be concluded that this Co-base alloy shows lower average anisotropy field, a more gaussian shape and a wider distribution than Fe-base alloys. This is explained by a lower magnetoelastic anisotropy and a more homogeneous distribution of the magnetic anisotropy. The powder exhibits a spherical shape that makes it very suitable as ferromagnetic phase to manufacture powder cores with extremely low hysteresis loss for medium-high frequency applications.

Phase transformation and strengthening of the gas-atomized FeCoCrNiMo0.5Al1.3 high-entropy alloy powder during annealing

Phase evolution and strengthening of the FeNiCoCrMo0.5Al1.3 powder alloy produced via inert gas atomization and annealed in the temperature interval of 300–800 °C have been studied by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, and microhardness testing. It was found that annealing at 300–600 °C leads to an increase of the element segregations between the several solid solutions with a rise of the lattice misfit (ε) to 1.5 % and microhardness growth to 1070 HV. It was assumed that elastic stress caused by the element partitioning is the main strengthening mechanism: microhardness rises linearly with misfit rise with dHV/dε = 43400 MPa. Sigma arises after the maximum elastic deformation (in 1.5 %) was reached. Formation of the dispersed coherent sigma phase in the annealing interval 600–800 °C results in the microhardness rise. Oxidation that began at 800 °C in 27 h is accompanied with FCC formation due to a depletion of the B2 in Al caused by Al2O3 formation. Estimation of the activation energy of the initial stage of the solid solution decomposition gives a very low value in 0.65eV, apparently caused by the high concentration of quenched vacancies. The activation energy of sigma formation approximately coincides with the activation energy of self-diffusion in BCC metals (about 2.60 eV).

Study on a novel Al-B-Li alloy powder with high enthalpy of combustion

ABSTRACT To develop aluminum-based alloy fuels with high energy release levels and good storage stability, the series of Al-B-Li alloy powders were prepared using high-temperature gas atomization. Al-4B-5Li meets the batch preparation requirements, with the mass combustion enthalpy of 31,101 J/g, exceeding the theoretical value of pure Al powder. XRD, SEM/EDS, XPS, TG-DTA, and other methods were used to characterize the alloy powder. The results showed that at 680℃~800°C, “channels” favorable for oxygen diffusion are formed inside the particles, which promotes the complete oxidation of Al-4B-5Li at 1300°C. Al-4B-5Li exhibits a total heat release of 6317 mJ/mg and an oxidation weight gain ratio of up to 95.0%, compared to just 8.39% for pure Al powder. In addition, Al-4B-5Li has good storage stability and lower energy attenuation compared to Al-5Li. After being placed in air for two weeks, the mass combustion enthalpy still exceeds 29,500 J/g, with a decay rate of only 4.52%. And the attenuation rate of Al-5Li is 10.17%. Al-4B-5Li has excellent combustion heat, thermal oxidation performance, and storage stability, and has good application prospects in the field of energetic materials.

Precise control of atomization initial stage to address nozzle clogging issue in the vacuum induction-melting gas atomization process

Vacuum induction-melting gas atomization (VIGA) is considered a widely applied method to prepare metal powders. However, the nozzle clogging problem in the VIGA process seriously affects the continuity of production and greatly increases the economic and time costs. In the atomization initial stage, the existence of unsteady state atomization mainly results in the nozzle clogging challenge. To understand the formation mechanism of nozzle clogging, a computational fluid dynamics (CFD) model was established to simulate the influence of unsteady state atomization sequence on the primary atomization of alloy melt. Typically, there are two kinds of sequence including starting the gas before pooling the alloy (namely, GA) and pouring the alloy before opening the gas (AG). The movement trajectory of the alloy melt droplet at the end of the delivery-tube was clarified. Based on CFD simulation, the atomization sequence of AG and GA was precisely controlled in the atomization pressure range of 2.5 MPa, 3.5 MPa, and 4.5 MPa to address the nozzle clogging issue. In the GA process, relatively finer powder was obtained at 3.5 MPa without nozzle clogging occurring. In contrast, the AG process greatly reduces the probability of nozzle clogging at 2.5–4.5 MPa, the final prepared powder is coarser. The predicted results of industrial experiments and numerical simulations are consistent. This study provides theoretical support for understanding the formation mechanism of nozzle clogging during the VIGA process.

Microstructural evolution and mechanical properties of the rapidly-solidified and hot-extruded 2196 Al-Li alloy

The microstructural evolution and mechanical properties of the 2196 Al-Li alloy prepared by the vacuum gas atomization, hot-isostatic-pressing (HIP) and hot-extrusion were investigated. The results show that prior particle boundaries (PPBs) exist in the microstructure of the HIPed alloy, and hot-extrusion effectively breaks PPBs to restrict the element micro-segregations. The as-extruded alloy exhibits fine grains, and bears higher solution treatment temperature due to the absence of low-melting-point micro-segregations. In the ageing process, the alloy hardness continues to rise, and reaches the peak value at 48 h. The precipitation sequence of the alloy is determined to be: supersaturated solid solution (SSS) → GP zone + fine T1 + δ′ → T1 + θ′ + δ′, where T1 phase dominates the whole ageing process. The diameter of the T1 platelets increases gradually with the increase of ageing time, meanwhile the T1 number density slightly drops in the over-aged alloy. The 3% pre-stretching promotes the nucleation of T1 phase, and the peak-ageing time is shortened to 28 h. Besides, no over-ageing phenomenon is observed in the pre-stretched samples. The Mg-Cu and non-Cu clusters formed on the dislocations accelerate the precipitation of T1 phase and prevent the nucleation of θ′ phase. The yield strength (YS) and ultimate tensile strength (UTS) of the T8 sample are 490 MPa and 515 MPa, respectively, which is notably higher than the T6 sample. The tensile strength of the present alloy is relatively higher than that of the Al-Li alloys with a comparable Li content.

Primary and secondary breakup of molten Ti64 in an EIGA atomizer for metal powder production

Electrode Induction melting Gas Atomization (EIGA) is a free-fall gas atomization process used to produce metal powders, where a swirling supersonic gas jet hits a molten metal stream atomizing it into droplets through various fragmentation mechanisms. Fragmentation mechanisms of molten Ti64 are investigated by high-speed video visualization. The role of gas pressure and melting chamber overpressure on the fragmentation mechanisms and on the particle size distribution is determined. The mechanisms observed are fiber breakup, bag breakup and Rayleigh breakup for primary fragmentation and bag breakup and shear breakup for secondary fragmentation. Increasing the gas pressure promotes shear breakup and creates finer droplets. A model based on log-stable law is proposed to fit the mass distribution according to the dominant fragmentation regime. A good agreement is observed between the different analysis (dominant atomization regime, mass cumulative distribution evolution, parameters of log-stable law). A criterion for prediction of nozzle clogging is devised.

High-Quality Spherical Silver Alloy Powder for Laser Powder Bed Fusion Using Plasma Rotating Electrode Process.

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

High strength and ductility on Ni6W2Cr4Fe8Ti1 high entropy alloys by selective laser melting

High entropy alloys (HEAs) have a good application potential in the preparation of high temperature service parts with complex shapes in additive manufacturing. Powder bed fusion (PBF) is a prominent type of additive manufacturing process that utilizes the thermal energy to bind powder materials together to build up plastic or metallic parts. However, the traditional gas atomization method (GA) often produce alloy powder (containing refractory element) with uneven composition, affecting the quality and performance of specimens. That is why innovative or opposite classical production powder methods are in demand for the PBF. It is worth noting that mechanical alloying technology overcomes the limitation of high temperature alloying of HEA containing refractory elements and realizes solid alloying of insoluble elements. In this paper, high entropy alloy Ni6W2Cr4Fe8Ti1 containing refractory elements is designed by means of composition control. Its spherical powders with uniform composition were successfully prepared by the new mechanical alloying - high temperature remelting spheroidization dual process (DP) method. The treated powder composition is evenly distributed with particle sizes ranging from 15 to 28 μm. And the spheroidization rate has exceeded 98.23% at 1943 K. The sample value of YS and UTS on Ni6W2Cr4Fe8Ti1 (DP-SLM) has increased by 35.7%, 45.5% and 27.3%, respectively in additive manufacturing, compared to GA-SLM. The ability to produce refractory HEA through DP method will make it possible to manufacture complex geometry shapes at a reasonable cost. It offers theoretical guidance to manufacture fine grain materials with high strength and ductility of new HEA components.

Study on Flowability Regulation of Vacuum Gas-Atomized Fe-Cr-Ni-W-B Spherical Powder.

High-quality Fe-Cr-Ni-W-B spherical powder is crucial for the powder metallurgy preparation of high-strength and tough Fe-Cr-Ni-W-B alloys. In this study, the controlled preparation of high-quality Fe-Cr-Ni-W-B spherical powder was achieved using the vacuum gas atomization method. The effects and mechanisms of atomization gas pressure, the melt nozzle inner diameter, and heat treatment temperature on the microstructure and flowability of Fe-Cr-Ni-W-B spherical powder were systematically investigated. By optimizing process parameters, spherical Fe-Cr-Ni-W-B powder with a sphericity of 95.1% and a flowability of 15.88 s/50 g was obtained, laying the foundation for the powder metallurgy preparation of high-strength and tough Fe-Cr-Ni-W-B alloys.