Μm Powder Research Articles

Effects of LDPE and PBAT plastics on soil organic carbon and carbon-enzymes: A mesocosm experiment under field conditions

Although the effects of plastic residues on soil organic carbon (SOC) have been studied, variations in SOC and soil carbon-enzyme activities at different plant growth stages have been largely overlooked. There remains a knowledge gap on how various varieties of plastics affect SOC and carbon-enzyme activity dynamics during the different growing stages of plants. In this study, we conducted a mesocosm experiment under field conditions using low-density polyethylene and poly (butylene adipate-co-terephthalate) debris (LDPE-D and PBAT-D, 500–2000 μm (pieces), 0%, 0.05%, 0.1%, 0.2%, 0.5%, 1%, 2%), and low-density polyethylene microplastics (LDPE-M, 500–1000 μm (powder), 0%, 0.05%, 0.1%, 0.5%) to investigate SOC and C-enzyme activities (β-xylosidase, cellobiohydrolase, β-glucosidase) at the sowing, seedling, flowering and harvesting stages of soybean (Glycine Max). The results showed that SOC in the LDPE-D treatments significantly increased from the flowering to harvesting stage, by 12.69%–13.26% (p < 0.05), but significantly decreased in the 0.05% and 0.1% LDPE-M treatments from the sowing to seedling stage (p < 0.05). However, PBAT-D had no significant effect on SOC during the whole growing period. For C-enzyme activities, only LDPE-D treatments inhibited GH (17.22–38.56%), BG (46.7–66.53%) and CBH (13.19–23.16%), compared to treatment without plastic addition, from the flowering stage to harvesting stage. Meanwhile, C-enzyme activities and SOC responded nonmonotonically to plastic abundance and the impacts significantly varied among the growing stages, especially in treatments with PBAT-D (p < 0.05). These risks to soil organic carbon cycling are likely mediated by the effects of plastic contamination and degradation soil microbe. These effects are sensitive to plastic characteristics such as type, size, and shape, which, in turn, affect the biogeochemical and mechanical interactions involving plastic particles. Therefore, further research on the interactions between plastic degradation processes and the soil microbial community may provide better mechanistic understanding the effect of plastic contamination on soil organic carbon cycling.

Microstructural evolution of Fe-25 W powder beds during CO2-H2 redox cycling at 800 °C

Fixed powder beds of Fe-25 W at% powders show excellent resistance to degradation during CO2-H2 redox cycling at 800 °C, releasing CO and H2O, respectively, with 100 % metal utilization. During cycling up to 165 full redox cycles (∼500 h), the reaction kinetics improve with cycle number and a stable, hierarchically-porous microstructure forms that shows neither sintering nor densification. The sintering inhibition provided by W addition to Fe stems from the formation of Fe-W mixed phases: as an intermetallic compound (Fe2W) in the reduced state and as a ternary oxide (FeWO4) in the oxidized state. In addition to the large channels running between coarse (100 µm) powder agglomerates, each agglomerate in the powder bed shows both microporosity due to sintering inhibition, and submicron porosity due to a chemical vapor transport reduction mechanism that is active when FeWO4 is reduced by H2 at high temperature. This hierarchical porosity enables fast reactions due to the short diffusion distance to free surfaces, and non-tortuous gas flow through the powder bed. As a result, under high CO2 flow, the CO2:CO ratio is sufficiently high to oxidize Fe3O4 to Fe2O3, improving the CO conversion capability on a per-mole basis.

Optimization of measured mechanical characteristics of selective microwave hybrid heating processed Inconel 625/ SS 304 weldments using multi-objective JAYA algorithm coupled with multi-attributes decision making R-method

This work focuses on the joining of Inconel-625/SS-304 using selective microwave hybrid heating (SMHH) technique. Input power, filler powder particle size, separator, and susceptor size are considered for experimentation according to the Definitive Screening Design. The multi-objectives measured are UTS, FS, and microhardness. XRD results show the intermetallic/secondary phases, and FESEM micrographs show the metallurgical bonding occurs between base metal and filler. The joint and interface region had an average microhardness of 204 ± 10 HV and 342 ± 18 HV, respectively. The UTS and FS of the weldments measured to 550 MPa and 805 MPa. MOJAYA technique is utilized for multi-objective optimization, and R-method determined the optimal process parameters. The optimal process parameters found to 2.2 kW, 25 μm powder, 120 grit and 0.804 mm separator. The confirmation test reveals UTS − 566 MPa, FS − 903 MPa, and microhardness − 365 HV, which closely matched with predicted observations.

Optimising corn (Zea mays) cob powder as an effective sorbent for diverse gel matrices: exploring particle size and powder concentration effects

SummaryThis study aims to valorise a plant waste, corn cobs (Zea mays) enriched with fibres (cellulose of 19.09–19.18% and hemicellulose of 34.04–60.13%), to create gel‐like sorbents. Corn cobs (CB) were dried, grounded and sieved to obtain 500, 250 and 125 μm powder size fractions. Various CB powder concentrations (10–40% w/w) were mixed with distilled water and rice bran oil for 2 min at ambient temperature to form hydrogel‐like and oleogel‐like sorbents, respectively. Visual appearance indicated that selected gels formed by 250 and 125 μm CB powders were self‐sustained right after mixing, while the largest particles (500 μm) could not fabricate gels at the CB concentrations studied. FTIR results suggested that the gelation process was primarily attributed to physical absorption rather than chemical binding. Comprehensive analyses of microstructure, physicochemical properties and rheological behaviour indicated that gelation was due to fibre–fibre interaction (125 μm) and oil/water–fibre interaction (250 μm). An increase in CB powder concentration enhanced the microstructural density and hardness of gels. Thus, 250 μm particles and higher absorbent concentrations resulted in brittle sorbents characterised by high hardness but low cohesiveness. The 250 μm particles also exhibit a superior antioxidant profile and lower oil/water loss than the 125 μm CB particles due to effective intra‐particle trapping mechanisms.

Mechanical and Structural Characterization of Resole Resin Composites Reinforced with Sabai Grass-Derived Cellulose Fibers

Cellulose fibers were extracted from Sabai grass (Eulaliopsis binata) using mechanical and chemical treatments. The grass was ground to ~250 µm powder and bleached with hydrogen peroxide and acetic acid. The bleached fibers were treated with 6% sodium hydroxide solution and incorporated into resole resin at various weight percentages to prepare the composites. The prepared samples were characterized by X-ray Diffraction (XRD), Fourier Transform Infra-Red (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), and compressive strength tests. The XRD and FTIR showed a nanometric crystallite size of 3.84 nm and effective removal of non-cellulosic compounds, respectively, leading to a compressive strength increase from 15.60 MPa to 25.26 MPa at 3 wt.-% cellulose fiber. The compressive strength improved up to 3 wt.-% cellulose fiber, which however declined upon adding more filler. SEM revealed modified surface morphology in the composites with cellulose fibers.

A novel coaxial heat pipe with an inner vapor tube for cooling high power electronic devices

Improving heat transfer limit of heat pipe is important for their application in high-power equipment system. Shear stress at the liquid–vapor interface affects the heat transfer capacity of heat pipe. In this study, a novel coaxial heat pipe is designed to eliminate vapor entrainment from interfacial shear stress for the first time. An inner thin-walled tube is sintered with the powder wick in the adiabatic section by one-step sintering process. The effects of filling ratio and powder size of sintered wick on the thermal performance of coaxial heat pipe are investigated experimentally. The results indicate that the optimal powder size range for the sintered wick of coaxial heat pipe is 106–125 μm, while the optimal filling ratio is about 125 %. It means that the sintered wick with 106–125 μm powder can achieve better trade-off between capillary performance and permeability. The coaxial heat pipe can operate at heat loads of 70 ∼ 140 W with the total thermal resistance less than 0.06 °C /W. Moreover, compared with the normal heat pipe, the heat transfer capacity of coaxial heat pipe is increased by 57 %, while the evaporation temperature is decreased by 9.6 ∼ 19.1 °C and the total thermal resistance is decreased by 41 %∼320 %. The inner tube can improve the replenishment efficiency of working liquid by eliminating interfacial shear stress, resulting in the significant heat transfer improvement. The design of coaxial heat pipe is an effective and low-cost solution to improve the thermal performance of heat pipe.

Effect of Preparation Process on the Microstructure and Characteristics of TiAl Pre-Alloyed Powder Fabricated by Plasma Rotating Electrode Process

TiAl pre-alloyed powder is the foundation for additive manufacturing of TiAl alloys. In this work, TiAl pre-alloyed powder was prepared using a plasma rotating electrode process (PREP). The effects of electrode rotating speeds and current intensity on the microstructure and characteristics of TiAl pre-alloyed powder have been investigated in detail. The results show that the electrode rotating speeds mainly affected the average particle size of the powder (D50). As the electrode rotating speed increased, the D50 of the powder decreased. The current intensity mainly affected the particle size distribution of the powder. As the current intensity increased, the particle size distribution of the powder became narrower, which was concentrated at 45~105 μm. In addition, the current intensity had a significant effect on the sphericity degree of the powder with the particle size &gt; 105 μm, but it had little effect on that &lt;105 μm powder. TiAl pre-alloyed powder with a particle size &gt; 45 μm demonstrated a dendritic + cellular structure, and the &lt;45 μm powder had a microcrystalline structure. The powder was mainly composed of the α2 phase and γ phase. There were two kinds of phase structure inside the powder, namely the α2 + γ lamellar microstructure (particle size &lt; 45 µm) and the α2 + γ network microstructure (particle size &gt; 45 µm). The phase structure of the powder was related to the solidification path and cooling rate of molten droplets in the PREP. The average thickness of the α2 + γ lamellar was about 200 nm, in which the lamellar γ phases were arranged in an orderly manner in the α2 phase matrix with a thickness of about 20 nm. The network phase structure was corrugated, and the morphology of the γ phase was not obvious.

Investigation of the Influence of Powder Fraction on Tribological and Corrosion Characteristics of 86WC-10Co-4Cr Coating Obtained by HVOF Method

Samples using powders of four different fractions, 15–20 μm, 20–30 μm, 30–40 μm and 40–45 μm, were fabricated to investigate the wear resistance, corrosion resistance and tribological properties of the 86WC-10Co-4Cr coating obtained using the HVOF method. The phase composition, microstructure and elemental distribution were analyzed using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. The hardness was measured on a Vickers microhardness tester, the friction coefficient and wear rate were investigated using a tribometer, and the corrosion resistance was evaluated on an electrochemical corrosion station. The results showed that the cross-sectional microstructure of the coating is mainly represented by multifaceted WC crystals embedded in the Co-Cr matrix and the presence of lower tungsten carbides, particularly W2C. The 15–20 μm fraction particles were subjected to superheating, contributing to the decarburization process. The 20–30 µm and 30–40 µm sized particles prevented overheating and had a more homogeneous structure. The 40–45 µm powder fractions did not reach sufficient temperature for complete melting, resulting in the formation of pores in the coating layers. The phase composition of the coatings included WC, W2C and CoO phases. According to the results of the study, it was found that the optimal powder fraction for coating the 86WC-10Co-4Cr composition with improved characteristics is the fraction of the 20–30 µm sized particles.

A Raman Spectroscopic Study of Lightning‐Induced Glass Produced From Five Mineral Phases

AbstractLightning‐induced volcanic spherules (LIVS) are glasses produced by the rapid melting and solidification of molten volcanic ash grains. High temperatures generated by lightning will alter the physical and chemical properties of minerals exposed to the discharge. Laboratory experiments reveal that LIVS glass composition varies depending on the starting material, exhibiting heterogeneous compositional features common in other glasses created by cloud‐to‐ground lightning, nuclear explosions, and high velocity impact events. This study uses scanning electron microscopy, energy dispersive spectroscopy, and Raman spectroscopy to investigate the structure and Raman signatures of lightning‐induced glass spherules manufactured from five igneous minerals (&lt;32 μm powders of albite, labradorite, augite, hornblende, and magnetite). LIVS were created through high‐current impulse experiments using peak currents of 25 and 40 kA. Analysis of the post‐experimental albite, labradorite, augite, and hornblende LIVS reveal primarily homogeneous silicate or aluminosilicate glasses with limited heterogeneity. Their amorphous Raman spectra are comparable to rhyolitic and mafic natural glasses along with Na2O‐K2O‐Al2O3‐SiO2, CaCO3‐Al2O3‐SiO2, and CaO‐MgO‐SiO2 synthetic glass networks. A few of the augite and hornblende LIVS spectra exhibit premelting effects, which occur below the melting point and represent the onset of cation disordering in phases that remain crystalline. Magnetite samples produced crystal‐rich, glass‐poor LIVS characterized by the growth of dendritic microcrystals and crystalline spectra that also contain a few bands alluding to the composition of their silicate–phosphate glass matrix. By understanding these chemical changes induced by lightning, we can extract information from other types of glasses produced during high temperature, short duration events.

A semantic segmentation algorithm for automated rapid melt pool identification from cross-sectional micrographs

Melt pool contours are an important empirical process measurement for metallic additive manufacturing. Melt pool boundaries indicate the maximal extent of the melting temperature contour. Melt pool boundaries characterize the effects of process parameters on melt pool size and variability and provide calibration and validation data for process simulations predicting thermal fields and microstructure. Serial cross sectioning of samples can produce numerous micrographs valuable for building robust melt pool contour datasets. However, the acquisition of contours from micrographs by hand is error prone and prohibitively time consuming for datasets large enough to capture variation within individual scan tracks and across a range of process conditions. In this work, we propose a semantic segmentation algorithm for automating melt pool contour acquisition from micrographs. The proposed algorithm is demonstrated on datasets from two different single scan track experiments conducted on a Powder Bed Fusion-Laser Beam platform using a bare Ti-6Al-4 V plate and an Inconel 718 plate with a 40 μm powder layer at different preheat temperatures. The algorithm was successfully trained and predicted the melt pool boundaries in the 300 image Ti-6Al-4 V dataset with 95% accuracy and an estimated reduction of 91% in processing time compared to data acquisition by hand. For the 1201 image Inconel 718 dataset, the algorithm achieved 89% accuracy and an estimated 99% time savings compared to hand measurements. The results for both datasets are demonstrated with a discussion about observed variability in the melt pool characteristics.

Effects of gas pressure and catheter length on the breakup of discontinuous NiTi droplets in electrode induction melting gas atomization

NiTi powders used for selective laser melting have here been fabricated by the breakup of discontinuous droplets in electrode induction melting gas atomization (EIGA). The morphology, particle size distribution, and hollow ratio of the powder were characterized by scanning electron microscopy (SEM), laser particle size analyzer, and computed tomography (CT), respectively. The effects of gas pressure and catheter length on the particle size distribution and powder morphology were then studied. Furthermore, the effects of the classifier wheel speed on the particle size distribution and yield of the 15–53 μm powder in the classification process were also analyzed. The results showed that the average particle size (D50) of the NiTi powder first decreased and, thereafter, increased as the atomization gas pressure increased. This was also the situation with catheter length. Also, the yield of the 15–53 μm powder increased with an increase in the classifier wheel speed. The optimum parameters were a gas atomization pressure of 5 MPa, a tension length of 28 mm, and a classifier wheel speed of 660 r min−1. For this optimized condition, the D50 value and the yield of the NiTi powder were 57.54 μm and 46.4%. In addition, the flowability, hollow ratio, and oxygen content were 15.8 s/50 g, 0.31%, and 450 ppm, respectively.

Effect of Milling Time and Reinforcement Volume Fraction on Microstructure and Mechanical Properties of SiCp-Reinforced AA2017 Composite Powder Produced by High-Energy Ball Milling.

The influence of milling time and volume fraction of reinforcement on the morphology, microstructure, and mechanical behaviors of SiCp-reinforced AA2017 composite powder produced by high-energy ball milling (HEBM) was investigated. AA2017 + SiCp composite powder with different amounts of SiC particles (5, 10, and 15 vol%) was successfully prepared from gas-atomized AA2017 aluminum alloy powder with a particle size of <100 μm and silicon carbide (SiC) powder particles with an average particle size of <1 μm. An optical microscope (OM), X-ray diffraction (XRD), and scanning electron microscope (SEM) were utilized to characterize the microstructure of the milled composite powder at different milling periods. The results indicated that the SiC particles were homogeneously distributed in the AA2017 matrix after 5 h of HEBM time. The morphology of the particles transformed from a laminar to a nearly spherical shape, and the size of the milled powder particles reduced with increasing the content of SiC particles. The XRD analysis was carried out to characterize the phase constituents, crystallite size, and lattice strain of the composite powders at different milling periods. It was found that with increasing milling time and SiC volume fraction, the crystallite size of the aluminum alloy matrix decreased while the lattice strain increased. The average crystallite sizes were reduced from >300 nm to 68 nm, 64 nm, and 64 nm after 5 h of milling, corresponding to SiC contents of 5, 10, and 15 vol%, respectively. As a result, the lattice strain increased from 0.15% to 0.5%, which is due to significant plastic deformation during the ball milling process. XRD results showed a rapid decrease in crystallite size during the early milling phase, and the minimum grain size was achieved at a higher volume fraction of SiC particles. Microhardness tests revealed that the milling time has a greater influence on the hardness than the amount of SiC reinforcements. Therefore, the composite powder milled for 5 h showed an average microhardness three times higher than that of the unmilled powder particles.

Effect of Starting Powder Particle Size on the Thermoelectric Properties of Hot-Pressed Bi0.3Sb1.7Te3 Alloys.

P-type Bi0.3Sb1.7Te3 polycrystalline pellets were fabricated using different methods: melting and mechanical alloying, followed by hot-press sintering. The effect of starting powder particle size on the thermoelectric properties was investigated in samples prepared using powders of different particle sizes (with micro- and/or nano-scale dimensions). A peak ZT (350 K) of ~1.13 was recorded for hot-pressed samples prepared from mechanical alloyed powder. Moreover, hot-pressed samples prepared from ≤45 μm powder exhibited similar ZT (~1.1). These high ZT values are attributed both to the presence of high-density grain boundaries, which reduced the lattice thermal conductivity, as well as the formation of antisite defects during milling and grinding, which resulted in lower carrier concentrations and higher Seebeck coefficient values. In addition, Bi0.3Sb1.7Te3 bulk nanocomposites were fabricated in an attempt to further reduce the lattice thermal conductivity. Surprisingly, however, the lattice thermal conductivity showed an unexpected increasing trend in nanocomposite samples. This surprising observation can be attributed to a possible overestimation of the lattice thermal conductivity component by using the conventional Wiedemann-Franz law to estimate the electronic thermal conductivity component, which is known to occur in nanocomposite materials with significant grain boundary electrical resistance.

How the Presence of Crystalline Phase Affects Structural Relaxation in Molecular Liquids: The Case of Amorphous Indomethacin.

The influence of partial crystallinity on the structural relaxation behavior of low-molecular organic glasses is, contrary to, e.g., polymeric materials, a largely unexplored territory. In the present study, differential scanning calorimetry was used to prepare a series of amorphous indomethacin powders crystallized to various extents. The preparations stemmed from the two distinct particle size fractions: 50-125 µm and 300-500 µm. The structural relaxation data from the cyclic calorimetric measurements were described in terms of the phenomenological Tool-Narayanaswamy-Moynihan model. For the 300-500 µm powder, the crystalline phase forming dominantly on the surface led to a monotonous decrease in the glass transition by ~6 °C in the 0-70% crystallinity range. The activation energy of the relaxation motions and the degree of heterogeneity within the relaxing matrix were not influenced by the increasing crystallinity, while the interconnectivity slightly increased. This behavior was attributed to the release of the quenched-in stresses and to the consequent slight increase in the structural interconnectivity. For the 50-125 µm powder, distinctly different relaxation dynamics were observed. This leads to a conclusion that the crystalline phase grows throughout the bulk glassy matrix along the internal micro-cracks. At higher crystallinity, a sharp increase in Tg, an increase in interconnectivity, and an increase in the variability of structural units engaged in the relaxation motions were observed.

Chemistry and microstructure of duplex stainless steel powders from recycled Z100 mixed with 316L steels

Recovered metallic waste can be used in additive manufacturing as a feedstock if the subsequent steps of the waste-to-product process are sufficiently mastered. In this study, impact of recycling of Z100 duplex steel mixed with 316L steel on the resulting powders microstructure and chemical composition was investigated. The utility of the original method of recycling stainless steels into a high-grade powder suitable for additive techniques has been demonstrated. By examining three gradations of powders, namely 20–50 μm, 50–100 μm and 125–250 μm, differences in selected properties in relation to the average particle size are shown. The results suggest that with increasing the particle diameter, fine-crystalline γ-austenite is favoured to precipitate at the boundaries and within the volume of the originally formed large δ-ferrite grains. It is reflected by a decrease of δ/γ fraction ratio from 0.64 in the 20–50 μm powders to 0.20 in the 125–250 μm, respectively. Obtained results indicate non-diffusional, shear or semi-shear character of δ → γ + δ phase transformation. The resulting fine-crystalline austenite is characterised by a significant dislocation density, which induces dislocation strengthening effect, responsible for an increase in Vickers hardness from 145 HV and Young's modulus from 29 GPa in the 20–50 μm group to 310 HV and 146 GPa in the 125–250 μm fraction, respectively.

Grain Size Effects on UV–MIR (0.2–14 μm) Spectra of Carbonaceous Chondrite Groups

Carbonaceous chondrites are among the most important meteorite types and have played a vital role in deciphering the origin and evolution of our solar system. They have been linked to low-albedo C-type asteroids, but due to subdued absorption bands, definitive asteroid–meteorite linkages remain elusive. A majority of these existing linkages rely on fine-grained (typically < 45 μm) powders across a limited wavelength range in the visible to near-infrared (0.35–2.5 μm). While this is useful in interpreting the fine-grained regolith of larger main-belt objects like Ceres, recent spacecraft missions to smaller near-Earth asteroids (NEAs), such as Bennu and Ryugu, have shown that their surfaces are dominated by larger grain size material. To better interpret the surfaces of these smaller, carbonaceous NEAs, we obtained laboratory reflectance spectra of seven carbonaceous chondrite meteorite groups (CI, CM, CO, CV, CR, CK, C2-ungrouped) over the ultraviolet to mid-infrared range (0.2–14 μm). Each meteorite contained five grain size bins (45–1000 μm) to help constrain spectral grain size effects. We find a correlation between grain size and absolute reflectance, spectral slope, band depth, and the Christiansen feature band center. Principal component analysis of grain size variation illustrates a similar trend to lunar-style space weathering. We also show that the Bus–DeMeo asteroid taxonomic classification of our samples is affected by grain size, specifically shifting CM2 Aguas Zarcas from a Ch-type to B-type with increasing grain size. This has implications for the parent body of the OSIRIS-REx target, Bennu. With Aguas Zarcas, we present results from Hapke modeling.

Magnetic Properties of Lightning‐Induced Glass Produced From Five Mineral Phases

AbstractThe physical properties of minerals are modified by the high temperatures of volcanic lightning during explosive eruptions. Alteration involves rapid heating and volatilization, melting, and fusion of ash grains within the discharge channel, followed by rapid quenching into new glassy textures. High current impulse experiments reveal that lightning alters not only the morphology and mineralogy of volcanic ash but also its magnetic properties. We investigate lightning‐induced magnetic changes in five igneous minerals (&lt;32 μm powders of albite, labradorite, augite, hornblende, and magnetite) by comparing hysteresis parameters before and after impulse experiments conducted at peak currents of 25 and 40 kA. Both the paramagnetic and ferrimagnetic behaviors of the samples were altered, which we interpret as a superposition of two processes. (a) Rapid melting allows iron contained within inclusions of Fe‐oxides and Fe‐bearing silicates to diffuse into the newly formed melt, thereby increasing the paramagnetism of the resulting glass. (b) Nucleation and growth of magnetite from the newly formed melt increases the ferrimagnetic behavior of the post‐experimental samples. Nominally non‐Fe‐bearing silicates like albite and labradorite have significantly increased paramagnetism and ferrimagnetism. Fe‐bearing silicates like augite and hornblende contain higher concentrations of ferrimagnetic inclusions, from which Fe diffuses into the newly formed melt, thereby increasing paramagnetism while decreasing ferrimagnetism. Experiments conducted on magnetite produced new magnetite crystals with dendritic habits. Although specific to volcanic ash, these results provide important insights into the magnetism of other materials affected by lightning on Earth, the Moon, and throughout the solar system.

Increased exothermic reactivity of polytetrafluoroethylene-coated aluminum powders: Impact of powder size reduction

Polytetrafluoroethylene (PTFE)-coated aluminum (PTFE/Al) powder was synthesized to evaluate its exothermic reactivity with the reduction in Al powder size. The exothermic enthalpy energies of uncoated Al powders clearly increase as the particle size decreases from 30 to 5 μm. Thus, the PTFE/Al (5 μm) powder exhibited the highest exothermic enthalpy energy of 4300 J/g, which is approximately two times higher than that of uncoated Al (5 μm) and PTFE/Al (30 μm) powders. The findings clarify that the combination of powder size reduction and PTFE coating results in both a considerable enhancement of exothermic reactivity and tailorable enthalpy energy values.

Cooling Rate Modeling and Evaluation during Centrifugal Atomization Process

Centrifugal atomization is a rapid solidification technique involving fast cooling rates to produce high-quality powders. The final microstructure of the atomized particles is closely linked with the thermal history and cooling rates experienced during the atomization process. However, there is insufficient research on the temperature evolution of metal particles produced by this technique, and most works evaluate the thermal history of the droplet through semi-empirical correlations that lie outside the conditions where they were derived. In this work, the cooling rate of centrifugally atomized Al-4%Cu was studied via mathematical modelling and experimental validation. A heat transfer model was implemented, and the value of the convective heat transfer coefficient was obtained from the Whitaker semi-empirical correlation considering three cases of study for the thermophysical properties of the gas. The validity of the Whitaker correlation was experimentally evaluated by means of cooling rates based on the Secondary Dendrite Arm Spacing (SDAS) technique. The Whitaker correlation with the Reynolds and Prandtl numbers evaluated at the ambient temperature and the gas conductivity evaluated at the film temperature gave the best agreement with the experimental results, with cooling rates in the order of 105 Ks−1 for &lt;32.5 µm powders atomized in He atmosphere.

Combinatorial Effects of the Recipes of the Initial Gas-atomized Powder Sizes on Microstructure and Passivation Characteristics of the SLM-ed Ti-6Al-4V Bulk Alloys

Bulk Ti-6Al-4V alloys have been fabricated by selective laser melting (SLM) in three recipes with different combinations of powder sizes. Combinatorial effects of initial gas-atomized powder sizes on microstructure and corrosion properties of SLM-ed Ti-6Al-4V alloys have been investigated by optical microscopy, X-ray diffraction, electrochemical measurements and surface free energy. The SLM-ed Ti-6Al-4V alloys are composed of acicular α' martensite and α phases. Many pinhole defects and twin boundaries exist in the SLM-ed Ti-6Al-4V alloys. According to electrochemical tests and surface free energy calculation, the SLM-ed Ti-6Al-4V alloys with 0 – 53 μm powders have the most positive corrosion potential, the lowest current density and the smallest surface free energy of 20.89 mJ m2. The passive film of SLM-ed Ti-6Al-4V alloys with 0 – 53 μm powders has superior protection ability due to their large thickness. The carrier concentration of SLM-ed Ti-6Al-4V alloys with 0 – 53 μm powders is the lowest by the Mott-Schottky curves. The SLM-ed samples with contact angles higher than 90º are hydrophobic, but the small contact angle of 46.98º reflects the hydrophilic features of as-rolled Ti-6Al-4V alloys. The combinatorial recipe of SLM-ed Ti-6Al-4V alloys with 0 – 53 μm powders is optimal for improvements on the corrosion resistance.