Decreasing Powder Size Research Articles

Sunflower-like eutectic solidification in gas atomized Al0.3CrFeNiTi0.3 medium-entropy alloy powders: A comprehensive microstructural study

The addition of larger atomic size elements, such as Al and Ti, to CrFeNi medium-entropy alloys (MEAs) offers a strategic approach to overcome the strength-ductility trade-off. This study focuses on examining the morphological, solidification, phase, and elemental characteristics of Al0.3CrFeNiTi0.3 feedstock powders produced through gas atomization (GA). The rapid cooling rate inherent in GA results in sunflower-like eutectic solidification. This structure exhibits firstly solidified Cr-rich BCC phases filled with cuboidal-shaped L21-type precipitates, each measuring up to ∼20 nm. Their peripheral regions display outward and radial growth of L21 and Fe-rich BCC eutectic phases, while FCC phase develops at the remaining sites. Intermediate B2 layers between BCC and L21 phases are considered as kinetic buffers, which enhance the interfacial stability. The size and shapes of powders determine their cooling rates, leading to a transition from comparably large sunflower-like eutectic structures to finer equiaxed grains alongside reduced elemental segregation with decreasing powder size. Nano-hardness measurements demonstrate superior mechanical strength in the peripheral regions due to the higher volume fraction of the L21 phase and its optimal interfacial coherency with the Fe-rich BCC phase. Furthermore, the slightly higher concentrations of Al and Ti in Fe-rich BCC phase (compared to the Cr-rich BCC phase) induce a better lattice distortion effect.

Innovative Method for the Mass Preparation of α″-Fe16N2 Powders via Gas Atomization

The iron nitride materials, especially α″-Fe16N2, are considered one of the most promising candidates for future rare-earth-free magnets. However, the mass production of α″-Fe16N2 powders as a raw material for permanent magnets is still challenging. In this work, starting from iron lumps as a raw material, we have managed to prepare the α″-Fe16N2 powders via the gas atomization method, followed by subsequent nitriding in an ammonia–hydrogen gas mixture stream. The particle size was controlled by changing the gas atomization preparation conditions. X-ray diffractograms (XRD) analyses show that the prepared powders are composed of α″-Fe16N2 and α-Fe phases. The α″-Fe16N2 volume ratio increases with decreasing powder size and increasing nitriding time, reaching a maximum of 57% α″-Fe16N2 phase in powders with size below 32 ± 3 μm after 96 h nitridation. The saturation magnetization reaches the value of 237 emu/g and a reasonable coercivity value of 884 Oe. Compared to the saturation magnetization values of α-Fe powders, the α″-Fe16N2 powders prepared through our proposed approach show an increase of up to 10% in saturation and demonstrate the possibility of mass production of α″-Fe16N2 powders as precursors of permanent magnets without rare earths.

Preparation and Microstructure of Multi-Component High Entropy Alloy Powders Fabricated by Gas Atomization Method

As an attractive high-entropy alloy, AlCrCoNiCu high-entropy alloy has excellent corrosion resistance, wear resistance, and anti-bacterial capabilities, and is considered to be a potential substitute material for marine and nuclear industry materials with great potential. One key to further optimizing the performance of high entropy alloy was to prepare high entropy alloy powder materials with uniform composition, good flow-ability, and stable performance. In this work, the AlCrCoNiCu high entropy alloy powder was prepared by the gas atomization method. The results indicated that the powder was spherical in shape, homogeneous in composition, and composed of a face-center cubic (FCC) phase. After adding Fe and Mn elements, FCC and body-center cubic (BCC) phases appeared and the particle size of the powder was mainly located at 10–50 μm. Furthermore, the larger the particle size was, the more obvious the surface roughness was. With the decreasing powder size, its shape became relatively regular, and the surface roughness decreased. This work provided an experimental and theoretical reference for preparing high-performance single-phase and multi-phase high entropy alloy spherical powders.

Processing and Properties of Tungsten-Steel Composites and FGMs Prepared by Spark Plasma Sintering

Tungsten is the prime candidate material for the plasma-facing components of fusion reactors. For the joining of tungsten armor to the cooling system or support structure, composites or graded interlayers can be used to reduce the stress concentration at the interface. These interlayers can be produced by several technologies. Among these, spark plasma sintering appears advantageous because of its ability to fabricate fully dense parts at lower temperatures and in a shorter time than traditional powder metallurgy techniques, thanks to the concurrent application of temperature, pressure, and electrical current. In this work, spark plasma sintering of tungsten-steel composites and functionally graded layers (FGMs) was investigated. As a first step, pure tungsten and steel powders of different sizes were sintered at a range of temperatures to find a suitable temperature window for fully dense compacts. Characterization of the sintered compacts included structure (by SEM); porosity (by the Archimedean method and image analysis); thermal diffusivity (by the flash method) and mechanical properties (microhardness and flexural strength). Compacts with practically full density and fine grains were obtained; while the temperature needed to achieve full sintering decreased with decreasing powder size (down to about 1500 °C for the 0.4 μm powder). For fully sintered compacts, the hardness and thermal diffusivity increased with decreasing powder size. Composites with selected tungsten/steel ratios were produced at several conditions and characterized. At temperatures of 1100 °C or above, intermetallic formation was observed in the composites; nevertheless, without a detrimental effect on the mechanical strength. Finally, the formation of graded layers and tungsten-steel joints in various configurations was demonstrated.

Effects of process parameters and cooling gas on powder formation during the plasma rotating electrode process

The plasma rotating electrode process (PREP) is rapidly becoming an important powder fabrication method in additive manufacturing. However, the low production rate of fine PREP powder limits the development of PREP. Herein, we investigated different factors affecting powder formation during PREP by combining experimental methods and numerical simulations. The limitation of increasing the rotation electrode speed in decreasing powder size is attributed to the increased probability of adjacent droplets recombining and the decreased tendency of granulation. The effects of additional Ar/He gas flowing on the rotational electrode on powder formation is determined through the cooling effect, the disturbance effect, and the inclined effect of the residual electrode end face simultaneously. A smaller-sized powder was obtained in the He atmosphere owing to the larger inclined angle of the residual electrode end face compared to the Ar atmosphere. Our research highlights the route for the fabrication of smaller-sized powders using PREP.

Effects of powder material and process parameters on the roll compaction, sintering and cold rolling of titanium sponge

ABSTRACT Roll compaction is a powder metallurgy process that has the potential for cost reduction in the production of titanium parts. In this study, experiments were carried out to investigate the effects of powder type and size on the roll compaction and subsequent sintering and cold rolling behaviour of titanium powder. Powders used ranged in upper sieve sizes from 0.15 to 3 mm. Strip density was found to increase with decreasing powder size while thickness decreased. Following sintering in the range of 800–1200°C, cold rolling was performed with various levels of thickness reduction. Average (across the width) densities of up to 88% were achieved, with higher density regions in the middle of the strips reaching up to 99%. Roll force was measured during cold rolling and found to increase with increasing final (post-cold rolling) density and also with increasing roll gap reduction amounts.

An investigation of the effect of high-energy milling time of Ti6Al4V biomaterial on the wear performance in the simulated body fluid environment

ABSTRACTIn this study, the effect of milling time on wear behaviour of the Ti6Al4V alloy produced with the high-energy milling method was investigated. The Ti6Al4V alloy was milled at five different milling times in a mechanical alloying device. The milled powders were cold-pressed under 620 MPa pressure, sintered at 1300°C for 2 h and cooled to room temperature in the furnace. The sintered alloys were characterised with SEM, XRD and hardness and density measurements. Wear tests were performed using a pin-on-disc type wear testing device, under three different loads, at four different sliding distances in simulated body fluid environment. Results showed a decreasing powder size with increasing milling time. The highest decline in size occurred for the powders milled for 120 min. The result of hardness measurements and wear tests showed that samples milled for 120 min had both the highest hardness value and the lowest weight loss.

Effect of oil palm shell powder on the mechanical performance and thermal stability of polyester composites

This paper presents an experimental study on the development of polymer bio-composites. The composites were fabricated from unsaturated isophethalic polyester resin containing powdered oil palm shell (OPS) as a function of powder particle size. The influence of washing OPS powder in methanol to remove surface impurities was also investigated with the tensile and flexural strengths and moduli improving significantly (between 22.9% and 61.4%) for the composites containing washed OPS powder compared to the unwashed case. It was observed that the composite tensile and flexural strength generally increased with decreasing powder size with the strength of the composite containing 75–150μm OPS powder being similar to that of the pure matrix. However, the tensile and flexural moduli of the composites were found to be essentially independent of powder size. Thermogravimetric analysis (TGA) in flowing oxygen indicated that the addition of OPS powder shifted the thermal degradation peak of the bio-composite from 370°C to 418°C.

An alternative magnesium-based root canal disinfectant: Preliminary study of its efficacy against Enterococcus faecalis and Candida albicans in vitro

Organisms invading root canal systems result in serious pulpal and periapical disease. To eliminate microorganisms and restrain secondary infections, dental materials with antibacterial properties are urgently needed in endodontics. Magnesium is considered as a promising biodegradable and biocompatible implant material. However, there are barely researches about its application in endodontic therapy. This work investigated the in vitro efficacy of magnesium powder against Enterococcus faecalis and Candida albicans compared with a common disinfectant, calcium hydroxide. With Calcium hydroxide served as a comparison it demonstrated the qualified antibacterial and anti-fungus properties of Mg as root canal disinfectant due to its high alkalinity of degradation, and the antimicrobial efficacy enhanced with the decreasing powder size.

Solidification characterization of a new rapidly solidified Ni–Cr–Co based superalloy

Abstract The solidification characterization of a new rapidly solidified Ni–Cr–Co based superalloy prepared by plasma rotating electrode process was investigated by means of optical microscope, scanning electron microscope, and transmission electron microscope. The results show that the solidification microstructure changes from dendrites to cellular and microcrystal structures with decreasing powder size. The elements of Co, Cr, W and Ni are enriched in the dendrites, while Mo, Nb and Ti are higher in the interdendritic regions. The relationships between powder size with the average solid–liquid interface moving rate, the average interface temperature gradient and the average cooling rate are established. Microsegregation is increased with larger powder size. The geometric integrity of MC′ type carbides in the powders changes from regular to diverse with decreasing powder size. The morphology and quantity of carbides depend on the thermal parameters and non-equilibrium solute partition coefficients during rapid solidification.

Mechanical properties of directionally freeze-cast titanium foams

Titanium foams with aligned, elongated pores were created by directional freeze-casting of aqueous slurries of titanium powders, followed by ice sublimation and powder sintering. Increasing sintering times from 8 to 24 h and decreasing powder size from 20 to 10 μm resulted in improved densification within cell walls and decreased overall foam porosity, with a concomitant increase in compressive stiffness, yield strength and energy absorption. A simple model for foam stiffness and strength is in general agreement with experimental measurements of strength but overpredicts stiffness, probably because it does not take into account micro-plasticity occurring during measurements.

Gas Atomization of Amorphous Aluminum: Part I. Thermal Behavior Calculations

In this article, the thermal history and cooling rate experienced by gas-atomized Al-based amorphous powders were studied via numerical simulations. Modeling simulations were based on the assumption of Newtonian cooling with forced convection, as well as an energy balance, which involves gas dynamics, droplet dynamics, and heat transfer between gas and droplet. To render the problem tractable, phase transformations, crystal nucleation, and growth were not taken into account in the analysis of the solidification of Al droplets; instead, an energy balance approach was formulated and used. The numerical results and associated analysis were used to optimize processing parameters during gas atomization of Al-based amorphous powder. The results showed that the cooling rate of droplets increases with decreasing powder size and can reach in excess of 105 K/s for powder <20 μm in diameter. Gas composition has a more significant influence on cooling rate than gas pressure, and 100 pct He has the highest cooling effect. The results also showed that the cooling rate increases with increasing melt superheat temperature.

Analysis of microstructure formation in gas-atomised Al–12 wt.% Sn–1 wt.% Cu alloy powder

The microstructure of gas-atomised Al–12 wt.% Sn–1 wt.% Cu alloy powder has been investigated using scanning and transmission electron microscopy. Powder particles above approximately 8 μm in diameter exhibited a cellular–dendritic solidification morphology comprising α-Al dendrites and interdendritic Sn. In smaller powder particles, with diameters less than approximately 8 μm, the microstructure comprised an α-Al matrix with a dispersion of sub-micron Sn particles. The size of the dispersed Sn phase decreased with decreasing powder size (i.e. increased cooling rate). The formation of this dispersed phase microstructure is explained by the existence of a metastable liquid-phase miscibility gap in the system such that the reaction L → L 1 + L 2 occurred prior to the onset of solidification in particles below 8 μm in size. A heat transfer analysis was used to estimate the undercooling for the nucleation of α-Al in droplets cooled at different rates. The nucleation undercooling predicted for an 8 μm diameter droplet was 260 K and this represents a critical value which must be exceeded to enter the immiscibility region. The above value is in reasonable agreement with the temperature of the metastable immiscibility boundary, calculated from thermodynamic data, which occurs at an undercooling of approximately 280 K for Al–12 wt.% Sn.

Rapidly Solidified Microstructure and Failure Analysis of Powder Metallurgy Al-Si Alloys

Abstract The microstructure of the extruded bars showed a homogeneous distribution of eutectic Si and primary Si particles embedded in the Al matrix. The grain size of α-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si in the extruded bars was about 100–200 nm. The room temperature tensile strength of the alloy with a powder size &lt; 26 μm was 322 MPa, while for the coarser powder (45–106 μm) it was 230 MPa. With decreasing powder size from 45–106 μm to &lt; 26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultra-fine grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s) due to the lower strength of the bars with coarse powders. The fracture mechanism of failure in tension testing and wear testing was also studied.

Mechanical properties and fracture behavior of an ultrafine-grained Al-20 wt pct Si alloy

The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder size <26 µm was 322 MPa, while for the coarser powder (45 to 106 µm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 µm) was also higher than that of the Al-20 wt pct Si-3 wt pct Fe (powder size: 60 to 120 µm) alloys. With decreasing powder size from 45 to 106 µm to <26 µm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 µm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 µm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders.

Tensile Properties of HIP/DB'ed Ni-Base Superalloys

Hot isostatic pressing(HIP)/diffusion bonding(DB) was employed for cast superalloy MM247 and P/M superalloy Udimet720. The effects of P/M Udimet720 particle size, holding time for HIP/DB and post heat-treatment on the microstructure and tensile properties were investigated. Tensile properties of HIP/DB’ed MM247/Udimet720 were improved at room temperature and 650°C with decreasing powder size from –140mesh to –270mesh, and with increasing HIP/DB time from 2h to 3h. Post-HIP heat-treatments, which strengthen the soft MM247 with cuboidal ’ precipitation, are proposed to enhance the mechanical properties of the HIP/DB’ed MM247/Udimet720.

Influence of Voids on the Accuracy of Measurement of Vacancy Concentration in NiAl and CoAl Intermetallic Compounds

The vacancy concentrations cv in the B2-type alloys of Ni1-xAlx (0.45≤x≤0.54) and Co1-xAlx (0.46≤x≤0.52) were determined by the measurements of lattice constant with X-ray diffraction and bulk density ρA using the Archimedes method. The measurements were performed at room temperature for various sizes of powder samples. The obtained density of ρA increased with decreasing powder size, and the resultant vacancy concentration decreased. The size dependence of ρA and a scanning electron microscopic observation indicated that the increase in ρA was due to the presence of voids in the powder samples. The reliable values of cv were estimated using a model that reduced the influence of voids. The effective formation energies of vacancy and antistructure atom were also estimated from the composition dependence of cv. The results of the estimation showed that the Al antistructure is the ground state of the defect structure at 0 K.

Extrusion behavior of gas atomized nanostructured Al 88.7Ni 7.9Mm 3.4 alloy powders

The effects of powder size on the extrusion behavior of Al 88.7Ni 7.9Mm 3.4 alloy, prepared by gas atomization followed by degassing and hot extrusion, were studied by a combination of microscopy (optical, scanning and transmission electron) and hardness testing. The alloy powders exhibited different microstructures depending on the powder size. The powders −26 μm in diameter consisted of nanocrystalline particles of embedded in an amorphous matrix. The aspect ratio of the elongated band was in the range of 5–10 for the −26 μm powder extrudate, and 10–30 for the 45–53 μm powder extrudate, which is much smaller than the theoretically expected value of 125 at the extrusion ratio of 25:1. The lower aspect ratio of the 5–30 at smaller particles size indicates that the resistance to plastic flow increases with decreasing powder size. The elongated bands in the extrudate microstructure were found to be harder than the surrounding areas.

Effect of compaction pressure and powder grade on microstructure and hardness of steam oxidised sintered iron

Steam oxidation has proven to be an effective process to improve the properties of sintered iron components. The oxide formed on the surface and in the interconnected porosity strongly influences both the tribological and mechanical properties of these materials, for example through the extent of pore closure and the nature and morphology of the oxide produced. In this paper, the influences of compaction pressure and powder size on the microstructure, oxide content, hardness, and surface topography of steam treated sintered iron are analysed. Specimens prepared from atomised iron powders of different sizes (<65, 65–90, 90–125, and >125 µm) were compacted at four different pressures (300, 400, 500, and 600 MPa), sintered for 30 min at 1120°C and then subjected to a continuous steam treatment at 540°C for 2 h. A clear influence of the processing parameters on porosity was highlighted. Low porosity was always associated with high compaction pressure and greater powder size. Pore size was affected in the same way by compaction pressure, even though the effect of powder size acted in the opposite sense. Changes in compaction pressure and powder size had no significant effect on pore shape. Decreasing powder size always led to high hardness. The effect of compaction pressure on hardness is clear evidence of a compromise between porosity and blockage of the pore network by oxide. Samples produced with smaller powder sizes showed a continuous decrease in hardness as the compaction pressure increased, although for the large powder size there was a slight increase to a constant value of ultimate hardness. For the intermediate powder size a maximum hardness was obtained as the compaction pressure increased. X-ray diffraction showed that the oxide layer is composed of magnetite and haematite.

Microstructural behavior of rapidly solidified and extruded Al-14wt%Ni-14wt%Mm (Mm, misch metal) alloy powders

Abstract The effect of powder size on the microstructure and mechanical properties was studied in gas-atomized Al-14wt%Ni-14wt%Mm (Mm: misch metal) alloy powder and their extrudates. The alloy powders showed different microstructures depending on the powder size. With decreasing powder size, formation of intermetallic compounds such as Al 3 Ni, Al 11 Ce 3 and Al 11 La 3 was suppressed. The powders less than 26 μm in diameter consisted of nanocrystalline fcc-Al particles embedded in an amorphous matrix. Microstructural scale of the extruded bar decreased as the initial powder size was decreased. The ultimate tensile strength of the alloy bar has increased from about 512 to 728 MPa with decreasing initial powder size from 75 to 90 μm to −26 μm. The strength increase mainly results from structural refinement.