Revistas
Revista:
MATERIALS CHARACTERIZATION
ISSN:
1044-5803
Año:
2022
Vol.:
187
Págs.:
111824
Gas atomized Nd-Fe-B powders of several compositions were separated in different size fractions by sieving. These fractions were annealed between 1100 degrees C and 1150 degrees C for 24 and 96 h. The oxygen content of the powders was measured before and after annealing for the different size fractions. The oxygen concentration of the powders depends strongly on the particle size and increases significantly during annealing, particularly in the case of small particle sizes. The effect of particle size on the microstructural changes was analyzed in detail, particularly on grain growth, using high resolution scanning electron microscopy and transmission electron microscopy. Electron back scattering diffraction was used to measure grain size. When the particle size rises, the degree of sintering decreases and the higher solid/vapor surface area reduces the mobility of grain boundaries. Oxidation also reduces grain growth rate and its effect is more evident for particles sizes below 45-63 mu m and high Nd concentrations. Nb addition leads to the formation of intra- and intergranular precipitates. The size of these Nb-Fe-containing precipitates increases with the particle size for equivalent annealing conditions. At 1150 degrees C, Nb loses its effect as an inhibitor of grain growth in the particle size fractions larger than 45-63 mu m.
Revista:
POWDER TECHNOLOGY
ISSN:
0032-5910
Año:
2022
Vol.:
407
Págs.:
117688
The effectiveness of a close-coupled gas atomisation process largely depends on the operational and the geometric variables. In this study, Computational Fluid Dynamics (CFD) techniques are used to model and simulate the gas flow in the melt nozzle area for a convergent-divergent, close-coupled gas atomiser in the absence of the melt stream. Firstly, a reference case, in which the atomisation gas is nitrogen at 50 bar and a supersonic gas nozzle with a throat width of L0 has been modelled, is presented. Then, the influence of both the inlet gas pressure and this design parameter are investigated, comparing the numerical results provided by simulations varying the inlet pressure from 5 to 80 bar and modelling different convergent-divergent gas nozzles with throat widths of 0.29¿Lo, 0.5¿Lo, 0.77¿Lo and 2¿Lo respectively. The simulation results show how similarly these two parameters modify gas mass flow rates, gas velocity fields, aspiration pressures in the melt delivery tube or the size of the recirculation zones below the melt nozzle. Therefore, it can be stated that this geometric variable of the gas nozzle may be as relevant as the inlet pressure in the atomisation process. The most important novelty of this study is related to experimental validation of the numerical results using the Particle Image Velocimetry (PIV) technique and through direct measurements of gas mass flow rates, with a clear correlation between simulated and measured data. Moreover, some results obtained with experimental atomisations using copper and nitrogen are also presented. The experimental results show that finer powders are produced by increasing the atomising pressure or the throat width of the supersonic gas nozzle, which can be directly related to the gas flow dynamics calculated numerically.
Revista:
MATERIALS CHARACTERIZATION
ISSN:
1044-5803
Año:
2021
Vol.:
172
Págs.:
110844
Nd-Fe-B powders of different compositions were produced by gas atomization. These powders were annealed between 1000 and 1150 degrees C for several times to study the microstructural evolution. Differential scanning calorimetry was used to determine the thermal transitions on the as-atomized powders and after slow solidification. The microstructure was studied by high resolution scanning electron microscopy at each stage. Electron back scattering diffraction was used to measure grain size and confirm that gas atomized powders are isotropic. It was observed the formation of necks between the particles, densification, and grain growth due to liquid phase sintering. Grain growth and densification occur in parallel by a dissolution-reprecipitation mechanism. The effect of Nd content and Nb addition on the microstructural changes was analyzed in detail, particularly on grain growth. The degree of sintering increases with Nd content, as this element enhances the formation of the liquid phase. Nb addition leads to the formation of precipitates that delay densification and grain growth at 1000 degrees C, but promote abnormal grain growth at 1100 degrees C.
Revista:
MATERIALS & DESIGN
ISSN:
0264-1275
Año:
2021
Vol.:
199
Págs.:
109441
The effect of several operational and geometric variables on the particle size distribution of powders produced by close-coupled gas atomisation is analysed from a total of 66 experiments. Powders of three pure metals (copper, tin and iron) and two alloys (bronze Cu-15 wt% Sn and stainless steel SS 316 L) have been produced. Nitrogen, argon and helium were used as atomising gases. It is shown that the gas-to-metal ratio of volume flow rates (GMRV) is more relevant than the ratio of mass flow rates (GMR) in order to analyse the effect of atomisation variables on the particle size. Kishidaka's equation, originally proposed for water atomisation, is modified to predict the median particle size in gas atomisation. The accuracy of the new equation is compared with that of Lubanska, and Rao and Mehrotra. Kishidaka's modified empirical correlation is the most accurate in predicting the median particle size of the powders produced in this work. The morphology of the produced powders is studied by scanning electron microscopy (SEM) and it is observed that the melt superheat can play an important role in the aggregation of fine particles (< 10 mu m), which increases the fraction of large particles (> 100 mu m). (C) 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Revista:
NANOMATERIALS
ISSN:
2079-4991
We present the evolution of magnetic anisotropy obtained from the magnetization curve of (Fe0.76Si0.09B0.10P0.05)(97.5)Nb2.0Cu0.5 amorphous and nanocrystalline alloy produced by a gas atomization process. The material obtained by this process is a powder exhibiting amorphous character in the as-atomized state. Heat treatment at 480 degrees C provokes structural relaxation, while annealing the powder at 530 degrees C for 30 and 60 min develops a fine nanocrystalline structure. Magnetic anisotropy distribution is explained by considering dipolar effects and the modified random anisotropy model.
Autores:
Alvarez, K. L. (Autor de correspondencia); Baghbaderani, H. A.; Martín, José Manuel; et al.
Revista:
JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS
ISSN:
0304-8853
The present work demonstrates the high-frequency core loss performance of Fe-based amorphous and nanocrystalline powder cores, initially produced by gas atomised powder, consolidated using sieved particles <= 20 mu m, and isolated by a precise insulating layer of polymer to limit the inter- and intra-particle eddy currents to attain enhanced performance. The large glass forming ability (GFA) of the gas atomised powder, reflected by different glass forming instruments, such as the supercooled region (Delta T-x = 54 degrees C) and the reduced glass transition temperature (T-rg = 0.56), is consistent with the substantial amorphisation capability of the alloy. To the best of our knowledge, this is the first-ever report to reveal a large Delta T-x in the Finemet-type alloy powders, an essential parameter to gas-atomise the amorphous powders with significantly lower cooling rates compared to the melt-spun ribbons. Further, subsequent annealing of the amorphous powders, between the exothermic events guided by differential scanning calorimetry (DSC), lead to the growth of a fine nanocrystalline structure of grains <= 15 nm, thanks to the positive enthalpy of mixing of Cu with the constituents to act as a nucleation agent, to retain the excellent soft magnetic properties. The DC soft magnetic properties of the powders were significantly improved on thermal annealing, confirmed by hysteretic loops, quantified by reduced coercivity H-c < 1 Oe of annealed powders at < 575 degrees C, and attributed to the reduced magnetoelastic contribution due to zero/near-zero magnetostriction anisotropy, attained due to the homogenous nanocrystalline structure. The amorphous and nanocrystalline powder cores, consolidated by compression moulding, show ultra-high loss performance, due to the ultra-low coercivity attained on nanocrystallisation, and negligible eddy currents loss, owning to efficient insulation of small particles, for high-frequency power conversion applications, such as voltage regulator (VR), and resonant converters, in automotive industry and data storage centres.
Revista:
JOURNAL OF ALLOYS AND COMPOUNDS
ISSN:
0925-8388
Fe-Si-B-Nb-Cu alloy powders, with and without P additions, were produced by gas atomization. The particles smaller than 20 mm are fully amorphous, exhibiting good soft magnetic properties. The crystallization process was studied by differential scanning calorimetry, demonstrating that its kinetics changes dramatically with small variations in the composition. The (Fe0.76Si0.09B0.10P0.05)(97.5)Nb2.0Cu0.5 (at. %) alloy was annealed in the supercooled liquid region (480 degrees C) and at the first crystallization peak (530 degrees C). The structural characterization by means of differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy provided information that explained the excellent soft magnetic properties. Annealing at 480 degrees C produced an amorphous relaxed structure with improved soft magnetic properties. At 530 degrees C, a two-phase material formed by nanocrystals with an average grain size of 16-17 nm embedded in an amorphous matrix was developed. Partial nanocrystallization increased the saturation magnetization from 139 to 144 emu/g and reduced the coercivity from 2.24 to 0.69 Oe. These results can be understood in terms of the algebraic contribution of both phases to the magnetization and the application of the random anisotropy model to nanocrystalline soft magnetic materials. (C) 2019 Elsevier B.V. All rights reserved.
Revista:
IEEE TRANSACTIONS ON MAGNETICS
ISSN:
0018-9464
Año:
2018
Vol.:
54
N°:
11
Págs.:
1 - 5
We report on the magnetic properties of NdFeB powders produced by gas atomization, which is a powder manufacturing technology scarcely used in the past to produce such alloys. Using this technique, we have produced several ternary NdFeB alloys with Nd contents between 26.9 wt.% and 28.5 wt.%. The as-atomized powders were split into different size fractions by sieving. Subsequently, we measured the magnetic properties as a function of temperature, between 10 and 400 K, and particle size. The magnetic behavior depends strongly on the microstructure of the material, which in turn is determined by the particle size. It is reported a slope anomaly in the curve of magnetization as a function of temperature at around 150 K due to a spin-reorientation transition. Since gas-atomized powders are isotropic, this magnetic transition produces an increment of the magnetization below this temperature.
Revista:
IEEE TRANSACTIONS ON MAGNETICS
ISSN:
0018-9464
Año:
2018
Vol.:
54
N°:
11
In this paper, we report the effect of thermal treatment at different temperatures (523, 623, and 723 K) on the magnetic properties of an amorphous powder with composition Fe72.5Si12.5B15 obtained by gas atomization. Differential scanning calorimetry curves indicate that a significant structural rearrangement takes place during annealing before crystallization. As expected, the intrinsic coercivity decreases with the annealing temperature, being the most significant drop from 3.26 to 0.44 Oe for the powder with a particle size < 20 mu m, non-annealed, and annealed at 723 K, respectively. An analysis of the effect of annealing on the anisotropy field distribution obtained directly from the magnetization curve is presented for both the non-annealed and the annealed sample with the lowest value of coercive field. The probability density function of the anisotropy field exhibits a lower applied field value at the maximum and is better fit by a Gaussian distribution after the heat treatment, which is a sign of better amorphicity after structural relaxation.
Revista:
JOURNAL OF ALLOYS AND COMPOUNDS
ISSN:
0925-8388
Año:
2018
Vol.:
735
Págs.:
2646 - 2652
We report a research describing a new route for the production of Fe-Si-B powders, which is the so-called gas atomisation technology, being one of the challenges to study the conditions under which the glass structure is achieved. There have been produced powders of five different compositions in the Fe-Si-B ternary system. The particles with a diameter below 10 mm of the alloy Fe70Si18B12 (at%) were completely amorphous, exhibiting an excellent soft magnetic behaviour (coercivity of around 7 Oe), which is, without any doubt, of significant scientific merit and supports the use of this new route to produce soft amorphous magnetic materials. (C) 2017 Elsevier B.V. All rights reserved.
Revista:
POWDER METALLURGY
ISSN:
0032-5899
Año:
2016
Vol.:
59
N°:
2
Págs.:
128 - 141
Present empirical correlations to predict the median particle size of water atomised powders have a validity restricted to a particular atomiser and alloy family. This work proposes a mathematical function that takes into account the influence of the heat transfer coefficient and, therefore, of the solidification time on the median particle size. This equation is applied in combination with previously proposed empirical correlations to extend their validity to a broader range of alloys. Experiments were conducted with alloys of different melting point (Fe base, Cu base and Sn). Quantitative measurements of the median particle size, tap density and several shape factors, and qualitative observations of the particle shape confirmed the importance of the heat transfer rate. It is shown that the inclusion of the solidification time effect results in a better agreement between calculated and experimental data when both low and high melting temperature alloys are taken together.
Revista:
MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING
ISSN:
0921-5093
Año:
2010
Vol.:
527
N°:
16-17
Págs.:
3949 - 3966
The present paper focuses on the initial stage of the liquid phase sintering (LPS) of a commercially available P/M Al-Si alloy, with nominal composition A1-14Si-2.5Cu-0.5Mg (in wt%). The microstructural examination of the as-received powder showed that it is constituted by pure Al particles and master alloy particles with composition Al-28Si-5Cu-1 Mg (in wt%). Its compressibility is lower than for the conventional elemental P/M aluminium premixes, but it is still better than for prealloyed Al powders and other P/M powders. Thermogravimetry experiments showed that the elimination of the organic lubricant takes place between 275 and 490 degrees C during heating to the sintering temperature. The phase transformations leading to the formation of the liquid phase were studied by differential scanning calorimetry (DSC). The dimensional changes associated with the generation of the liquid were measured by dilatometry. Samples quenched into water from different temperatures (between 450 and 575 degrees C) and times (between 0 and 30 min) were studied to reveal the microstructural evolution of the alloy. The first liquid is formed inside the master alloy particles at around 505 degrees C. This liquid spreads across the compact, enhancing the chemical homogenization of the material. The alloying elements diffuse from the liquid inside the originally pure Al particles, reducing their melting temperature. This alloying process is almost concluded at around 535 degrees C. When the temperature is increased the liquid starts to be formed also in the originally pure Al particles. The melting of the FCC Al-rich phase finishes around 575-590 degrees C. The full melting of the alloy occurs at about 635-645 degrees C. After analyzing the different possible causes, it is concluded that the main swelling mechanism is the volume change associated with the melting of a fraction of the material when the temperature is increased. The phases detected by X-ray diffraction (XRD) in the as-received powder and in the sintered compact are FCC Al-rich solid solution, Si, theta-phase (CuAl(2)), and Q-phase (Cu(2)Mg(8)Si(6)Al(5)). (C) 2010 Elsevier B.V. All rights reserved.
