Fe-based Amorphous Alloy Powder Research Articles

Impact of gas pressure on particle feature in Fe-based amorphous alloy powders via gas atomization: Simulation and experiment

• The flow dynamics of the gas phase and gas-melt two-phase fields, and the break-up process were illustrated. • The particle size distributions of simulated results were consistent with the powder experimental tests. • The Fe-based alloy powders with diameters in the range of 25–45 μm were found to be a fully amorphous phase. Gas atomization is now an important production technique for Fe-based amorphous alloy powders used in additive manufacturing, particularly selective laser melting, fabricating large-sized Fe-based bulk metallic glasses. Using the realizable k-ε model and discrete phase model theory, the flow dynamics of the gas phase and gas-melt two-phase flow fields in the close-wake condition were investigated to establish the correlation between high gas pressure and powder particle characteristics. The locations of the recirculation zones and the shapes of Mach disks were analyzed in detail for the type of discrete-jet closed-coupled gas atomization nozzle. In the gas-phase flow field, the vortexes, closed to the Mach disk, are found to be a new deceleration method. In the two-phase flow field, the shape of Mach disk changes from “ S ”-shape to “ Z ”-shape under the impact of the droplet flow. As predicted by the wave model, with the elevation of gas pressure, the size of the particle is found to gradually decrease and its distribution becomes more concentrated. Simulation results were compliant with the Fe-based amorphous alloy powder preparation tests. This study deepens the understanding of the gas pressure impacting particle features via gas atomization, and contributes to technological applications.

Study on the Soft Magnetic Properties of FeSiB/EP Composites by Direct Ink Writing

Fe-based amorphous alloy has excellent soft magnetic properties; traditionally, Fe-based amorphous alloy such as soft magnetic devices was fabricated by insulation enveloping and suppression molding methods. In this process, the aging of organic envelope materials and the crystallization of Fe-based amorphous alloy were usually occurring, accompanying with low magnetic induction and poor mechanical properties. The direct ink writing (DIW) technique can make complex-shaped parts and needs no heating treatment after forming, which can avoid the effect of traditional molding process. In the present study, varying mass fraction FeSiB/EP composite parts were prepared by the DIW technique with the Fe-based amorphous alloy powder and epoxy resin, in which microscopic morphology, magnetic properties, and mechanical properties of FeSiB/EP soft magnetic composites were studied. The results indicate that the slurry with iron powder mass fraction of 92.3, 92.6, and 92.8 wt% has good printing performance and self-support ability, which is suitable for DIW. The density of the printed parts is about 4.317, 4.449, and 4.537 g/cm3, which is almost similar with the iron powder. The tensile strength and elongation of printing parts are significantly improved compared with the pure epoxy resin. From the photos of microscopic morphology of printing parts, it can be seen that FeSiB powders are evenly dispersed in EP, no pores, and defects, with the proportion increasing of powders; the insulation coating thickness decreases; and the magnetic performance improves. The optimal sample is 92.8 wt% FeSiB/EP, in which saturation magnetic induction strength is 137.9759 emu/g and coercivity is 4.6523 A/m.

The synergy of chromium and molybdenum on the stable catalysis of Fe-based amorphous alloy powder

The Fe41Co7Cr15Mo14C15B6Y2 amorphous alloy powder was prepared by gas atomization. It was interesting that rhodamine B can be completely degraded within only 6 min by 1 g/L Fe41Co7Cr15Mo14C15B6Y2 and 3 mM H2O2 at pH 5.0 at 313 K. This result can be repeated at least 30 times. It is worth mentioning that the catalyst surface spontaneously formed a double passivation film during the degradation process. The passivation film composed of Cr2O3 and Cr(OH)3 is located in the outer layer, while the passivation film composed of MoO2 and MoO3 is located in the inner layer. This double passivation film regulates the contact of ions on the catalyst surface with the solution, which makes the catalyst have a stable catalytic efficiency in the reaction. The findings have important implications in developing Fe-based amorphous alloys for functional application materials in the field of wastewater treatment.

Numerical Investigation of In-Flight Behavior of Fe-Based Amorphous Alloy Particles in AC-HVAF Thermal Spray Process

Computational fluid dynamics is used to investigate the in-flight behavior of particles of Fe-based amorphous alloy powder in an activated combustion high-velocity air fuel spray process. The continuity, momentum, energy, and species equations are solved with a renormalization group k–e turbulence model to predict the flow fields. A one-step chemistry model and eddy-dissipation model are used to simulate the combustion reaction. The processing of the Fe-based amorphous alloy particles in the gas flow is modeled based on the Lagrangian approach. The predictions show that the static pressure and flame temperature of the combustion products in the combustion chamber can reach up to 515,000 Pa and 1600 K, respectively, under the conditions considered in this study. Both the temperature and velocity of the alloy particles are strongly affected by the powder particle size. The particle injection position also has a great influence on the particle temperature. The greater the deviation of the injection point from the centerline, the higher the particle temperatures. The nitrogen flow rate and particle size were projected as two important parameters to avoid nozzle clogging, revealing that high gas flow rates and large particles favor expansion of the particle stream in the radial section during the spray process.

Novel Method for Making Surface Insulation Layer on Fe-Based Amorphous Alloy Powder by Surface-Modification Using Two-Step Acid Solution Processing

By introducing the SiC/GaN power devices, the high-frequency switching dc–dc converters are expected to have higher efficiency and lightweight. This paper focuses on the Fe-based amorphous alloy powder (Fe–Si–B–Cr–C based) used in the metal composite bulk magnetic core for high-frequency power inductor and transformer. In terms of eddy-current loss at MHz band or beyond, iron-based metal composites are required to have fine magnetic particles with surface insulation layer. In this paper, the finest amorphous powder with a median diameter of 2.57 $\mu \text{m}$ made by water atomization process was used. In order to fabricate the amorphous powder with surface insulation layer, a surface-modification scheme through the two-step acid solution processing using phosphoric acid and hydrochloric acid was introduced. As a result, the surface-modification scheme enabled to make a single-phase silica thin layer on the surface of the amorphous powder, and the silica thin layer was considered to be derivable from the silicon element in the amorphous (Fe–Si–B–Cr–C based) powder.

Pressure controlled micro-viscous deformation assisted spark plasma sintering of Fe-based bulk amorphous alloy

In this paper, the theoretical framework of viscous flow deformation is presented as a model to investigate the inherent role of applied pressure in the densification of Fe-based amorphous alloy powder during spark plasma sintering. The proposed model revealed that the evolution of the structural geometry of the powder compact resulted in an amplification of the applied pressure to a larger contact pressure. The resulting pressure controlled compressive viscous flow deformation of the particles exhibited the contribution of increased applied pressure towards enhancement of densification and thus confirmed the validity of the present model for analyzing pressure-assisted sintering of amorphous alloy powder. The application of this theoretical model correctly predicted the trend of increasing final density of the compacts with pressure while further improvement on its accuracy can be attained upon establishment of the relative contribution of mass flow and deformation of powder particles towards their consolidation.

Prediction of heating rate controlled viscous flow activation energy during spark plasma sintering of amorphous alloy powders

The viscous flow behavior of Fe-based amorphous alloy powder during isochronal spark plasma sintering was analyzed under the integrated theoretical background of the Arrhenius and directional structural relaxation models. A relationship between viscous flow activation energy and heating rate was derived. An extension of the pertinent analysis to Ti-based amorphous alloys confirmed the broad applicability of such a relationship for predicting the activation energy for sintering below the glass transition temperature (Tg) of the amorphous alloy powders.

Initial stage densification during spark plasma sintering of Fe-based amorphous alloy powder: Analysis of viscous flow

Understanding the mechanism of densification of amorphous alloy powders is important for determining the parameters during sintering of these materials. In this paper, we report on the analysis of densification of Fe48Cr15Mo14Y2C15B6 amorphous alloy powder during spark plasma sintering. Sintering up to 1000 °C resulted in two clearly distinguishable stages of densification: stage I from 320 °C to 740 °C and stage II from 830 °C to 1000 °C. The maximum densification rate during stage I was attained at 585 °C up to which the sample remained fully amorphous. During the entire stage II, the sample was fully crystalline, and the maximum densification rate attained at 935 °C was much lower than that observed during stage I. Viscous flow during stage I was analyzed, yielding an estimate of the activation energy to be 94.0 ± 0.2 kJ mol−1 in the temperature range from 525 °C to 580 °C.

Microstructure evolution and thermal stability of an Fe-based amorphous alloy powder and thermally sprayed coatings

High velocity oxy-fuel (HVOF) thermal spraying has been used to produce coatings of an Fe–18.9%Cr–16.1%B–4.0%C–2.8%Si–2.4%Mo–1.9%Mn–1.7%W (in at.%) alloy from a commercially available powder (Nanosteel SHS7170). X-ray diffraction (XRD), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM) were employed to investigate the powder, as-sprayed coatings and annealed coatings which had been heated to temperatures in the range of 550–925 °C for times ranging from 60 to 3900 min. Microhardness changes of the coatings were also measured as a function of annealing time and temperature. The powder was found to comprise amorphous and crystalline particles; the former had a maximum diameter of around 22 μm. The coating was composed of splat like regions, arising from rapid solidification of fully molten powder, and near-spherical regions from partially melted powder which had a largely retained its microstructure. The amorphous fraction of the coating was around 50% compared with 18% for the powder. The enthalpies and activation energies for crystallization of the amorphous phase were determined. Crystallization occurred in a two stage process leading to the formation of α-Fe (bcc), Fe 1.1Cr 0.9B 0.9 and M 23C 6 phases. DSC measurements showed that the first stage occurred at 650 °C. Annealing the coating gave a hardening response which depended on temperature and time. The as-sprayed coating had a hardness of 9.2 GPa and peak hardnesses of 12.5 and 11.8 GPa were obtained at 650 and 750 °C, respectively. With longer annealing times hardness decreased rapidly from the peak.

Research on the surface oxidation procedure of Fe-base metallic glass during wet oxidation treatment

Internal structure of magnetic core for high-frequency use could be divided into small insulating units to minimize eddy current loss. Accordingly, the powder size of Fe bal.Si 9.6B 14.4Nb 4 (numbers indicate at.%) amorphous alloy should be small and covered with some degree of insulating layer for producing high-frequency magnetic core. In this investigation, formation of the insulating layer on the surface of Fe-based amorphous alloy powder was studied using wet oxidation method. The size of Fe–Si–B–Nb amorphous alloy powder used in this research was less than 75 μm in diameter. Forming mechanism of surface cracks and consecutive oxide layers was explained and the thickness of insulating layer could be controlled by changing treatment time and concentration of HNO 3 in the oxidizing solution. Even though the oxide layer on the powder surface was partially flaked off during hot-compacting process, sufficient electrical resistance was maintained to lessen eddy current loss effectively.

철계 비정질 합금 분말코아의 자기적 특성에 미치는 분말 제조방식의 영향

철계 비정질 합금 리본을 분쇄한 뒤 상온 압축성형을 통하여 분말코아를 제조할 때 로터밀과 볼밀의 두 가지 분말제조 공정을 사용하여 만든 분말의 형상을 관찰하고, 또 이들을 이용한 분말코아의 자기적 및 전기적 특성과 미세구조를 조사하였다. 로터밀을 사용한 경우 실효투자율의 크기는 볼 밀링한 경우보다 낮았으나 이의 주파수 의존성 및 품질계수는 볼밀의 경우보다 우수하였으며 자심손실 또한 더 작았다. 한편 로터밀을 사용하여 만든 분말은 크기가 균일하고 가장자리는 강한 전단가공을 받아 완만한 곡선을 지닌 등축형을 이루고 있는 반면 볼밀을 사용한 경우는 크기가 상대적으로 불규칙하면서 작은 분말이 많고 가장자리는 날카로운 형상을 하고 있었다. 로터밀로 만든 분말코아의 낮은 투자율은 밀링시 심한 전단에 의해 발생하는 큰 내부응력의 축적에 기인하는 것으로 보여진다. 한편 로터 밀링을 통해 만든 분말코아의 경우 투자율은 상대적으로 낮지만 전기저항의 측정 및 미세구조의 관찰로부터 보다 양호한 절연상태가 확보되어 높은 주파수에 이르기까지 투자율이 유지되면서 적은 자심손실이 얻어진 것으로 판단되었다. 분말코아의 자기적 안정성을 나타내는 직류중첩특성 또한 로터밀의 경우가 볼 밀링한 경우보다 우수하였다. In the fabrication process of Fe-based amorphous alloy powder cores by pulverization of the melt-spun ribbons and cold compaction, the effects of powder preparation method on the magnetic & electric properties, powder shapes and microstructure of cores have been investigated. The powder cores made by using rotor mill showed low effective permeability as compared to the cores prepared by ball milling. However the frequency dependence and quality factor properties were superior in the case of rotor-milling. Further the powders prepared by rotor mill had homogeneous and round shapes through strong shearing in the sieve ring, while the ball milled powders were inhomogeneous and relatively small. The lower permeability of the powder cores fabricated with rotor mill was considered to be due to the high internal stress occurred by very intensive shearing. Moreover the powder cores produced by rotor-milling showed lower core loss and good frequency dependence of effective permeability possibly due to the higher electrical insulation between magnetic particles. The dc bias property of the powder cores made by rotor-milling was better than the one by ball-milling.

Plasma rotating electrode atomization of Cr base alloys: L.N.Belyanchikov, V.A.Grigoryan. (Moscow State Inst. of Steel and Alloys, Moscow, Russia.)

Gas atomization is now an important production technique for Fe-based amorphous alloy powders used in additive manufacturing, particularly selective laser melting, fabricating large-sized Fe-based bulk metallic glasses. Using the realizable k-ε model and discrete phase model theory, the flow dynamics of the gas phase and gas-melt two-phase flow fields in the close-wake condition were investigated to establish the correlation between high gas pressure and powder particle characteristics. The locations of the recirculation zones and the shapes of Mach disks were analyzed in detail for the type of discrete-jet closed-coupled gas atomization nozzle. In the gas-phase flow field, the vortexes, closed to the Mach disk, are found to be a new deceleration method. In the two-phase flow field, the shape of Mach disk changes from “S”-shape to “Z”-shape under the impact of the droplet flow. As predicted by the wave model, with the elevation of gas pressure, the size of the particle is found to gradually decrease and its distribution becomes more concentrated. Simulation results were compliant with the Fe-based amorphous alloy powder preparation tests. This study deepens the understanding of the gas pressure impacting particle features via gas atomization, and contributes to technological applications.

鉄系非晶質合金薄片状粉末の衝撃焼結

Dynamic compaction of Fe-based amorphous alloy powder was performed by using a gun-type momentum trap method. Residual shock effects on the amorphous alloy samples was characterized by microstructural analysis (i.e. optical, SEM, and TEM observations), X-ray diffraction analysis, tensile strength test, and measurement of it's soft magnetic properties. Two attempts have been conducted to control the shock duration time and compression rate in the shock compaction process. It is found that magnetic properties of shock consolidated Fe-based amorphous alloy powder are strongly affected by impactor thickness which influences on shock duration time. The difference of the magnetic properties depending on the shock duration time is, however, reduced by appropriate heat treatments at the post shock stage. On the other hand, it is also found that a stepwise shock wave front structure, which can be produced by using the buffer material, is preferred to produce the consolidated amorphous alloy powder having superior soft magnetic properties.The problems to produce the superior shock consolidated product of amorphous alloy were discussed, based on the results obtained in the present study.