Preparation and oxidation of aluminum powders with surface alumina replaced by iron coating (original) (raw)
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Aluminum–Alumina Composites: Part Ⅰ: Obtaining and Characterization of Powders
Materials, 2019
The process of advanced aluminum-alumina powders production for selective laser melting was studied. The economically effective method of obtaining aluminum–alumina powdery composites for further selective laser melting was comprehensively studied. The aluminum powders with 10–20 wt. % alumina content were obtained by oxidation of aluminum in water. Aluminum oxidation was carried out at ≤200 °C. The oxidized powders were further dried at 120 °C and calcined at 600 °C. Four oxidation modes with different process temperatures (120–200 °C) and pressures (0.15–1.80 MPa) were investigated. Parameters of aluminum powders oxidation to obtain composites with 10.0, 14.5, 17.4, and 20.0 wt. % alumina have been determined. The alumina content, particle morphology, and particle size distribution for the obtained aluminum–alumina powdery composites were studied by XRD, SEM, laser diffraction, and volumetric methods. According to the obtained characteristics of aluminum–alumina powdery composites...
Oxidation of nano-sized aluminum powders
Thermochimica Acta, 2016
Oxidation of aluminum nanopowders obtained by electro-exploded wires is studied. Particle size distributions are obtained from transmission electron microscopy (TEM) images. Thermo-gravimetric (TG) experiments are complemented by TEM and XRD studies of partially oxidized particles. Qualitatively, oxidation follows the mechanism developed for coarser aluminum powder and resulting in formation of hollow oxide shells. Sintering of particles is also observed. The TG results are processed to account explicitly for the particle size distribution and spherical shapes, so that oxidation of particles of different sizes is characterized. The apparent activation energy is obtained as a function of the reaction progress using model-free isoconversion processing of experimental data. A complete phenomenological oxidation model is then proposed assuming a spherically symmetric geometry. The oxidation kinetics of aluminum powder is shown to be unaffected by particle sizes reduced down to tens of nm. The apparent activation energy describing growth of amorphous alumina is increasing at the very early stages of oxidation. The higher activation energy is likely associated with an increasing homogeneity in the growing amorphous oxide layer, initially containing multiple defects and imperfections. The trends describing changes in both activation energy and pre-exponent of the growing amorphous oxide are useful for predicting ignition delays of aluminum particles. The kinetic trends describing activation energies and pre-exponents in a broader range of the oxide thicknesses are useful for prediction of aging behavior of aluminum powders.
Intermetallics, 2020
In the present work, a novel technique has been introduced to obtain an aluminide coating by casting process and subsequent heat treatment. To do so, the aluminum sheet was placed at the bottom of a copper mold, then HH309 SS melt was poured into the mold. This technique was named Cast-Aluminizing (CA). The CA samples were heat-treated at the temperature range of 900-1050 � C for 0.5-5 h. The FE-SEM, XRD, and EDS were utilized to characterize the microstructure, phase analysis and chemical composition of cast-aluminized samples, respectively. Results showed that (Fe,Cr,Ni)Al 3 and (Fe,Cr,Ni) 2 Al 5 layers were formed at the Al/HH309 interface. FE-SEM analysis demonstrated a multi-layer aluminide coating on the heat-treated specimens. This coating consisted of (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) sub-layers. The growth kinetics investigation showed that the thickness of layers increased with the increase of the annealing temperature and time. The growth rate of layers obeyed a parabolic law. The activation energies for the growth of (Fe,Cr, Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers were about 203, 250 and 247 kJ/mol, respectively. Microhardness measurements revealed that (Fe,Cr,Ni) 2 Al 5 þ(Fe,Cr,Ni)Al 2 , (Fe,Cr,Ni)Al and α-Fe,Cr,Ni(Al) layers had a hardness of about 820-1040, 580-710 and 380-470 HV, respectively. The resistance to oxidation of castaluminized and heat-treated (CA þ HT) samples in the air at 1000 � C was studied. The CA þ HT samples exhibited higher oxidation resistance than uncoated samples due to the formation of a protective Al 2 O 3 layer on the surface.
A study on the formation of iron aluminide (FeAl) from elemental powders
Journal of Alloys and Compounds, 2015
The formation of iron aluminide (FeAl) during the heating of Fe-40 at.% Al powder mixture has been studied using a differential scanning calorimeter. The effect of particle size of the reactants, compaction of the powder mixtures as well as the heating rate on combustion behavior has been investigated. On heating compacted discs containing relatively coarser iron powder, DSC data show two consecutive exothermic peaks corresponding to precombustion and combustion reactions. The product formed during both these reactions is Fe 2 Al 5 and there is a volume expansion in the sample. The precombustion reaction could be improved by a slower heating rate as well as a better surface coverage of iron particles using relatively finer aluminum powder. The combustion reaction was observed to be weaker after a strong precombustion stage. Heating the samples to 1000°C resulted in the formation of a single and stable FeAl phase through the diffusional reaction between Fe 2 Al 5 and residual iron. DSC results for compacted discs containing relatively finer iron powder and for the non-compacted samples showed a single combustion exotherm during heating, with Fe 2 Al 5 as the product and traces of FeAl. X-ray diffraction and EDS data confirmed the formation of FeAl as the final product after heating these samples to 1000°C.
Aluminum particle reactivity as a function of alumina shell structure: Amorphous versus crystalline
Powder Technology, 2020
The objective of this study was to investigate Al particle reactivity as a function of the Al2O3 shell phase. Aluminum particles were thermally treated to transition the shell from amorphous to crystalline and each powder was combined with polytetrafluoroethylene (PTFE). Flame speeds were measured for Al + PTFE powder mixtures for two Al particle sizes that differ from micrometer (μAl) to nanometer (nAl) diameter and for both crystalline and amorphous Al2O3 shells encapsulating Al core particles. Results showed that μAl particles were more sensitive to shell phase than nAl particles. Reactions were modeled according to the melt dispersion mechanism (MDM), and altering the shell phase reduced the thickness, damaged the shell structure, impeded melt dispersion, and reduced flame speed for μAl particles by 45% and nAl particles by 12%.
Effect of the Alumina Shell on the Melting Temperature Depression for Aluminum Nanoparticles
Journal of Physical Chemistry C, 2009
The dependence of aluminum (Al) melting temperature on particle size was studied using a differential scanning calorimeter and thermogravitmetric analyzer for particles encapsulated in an oxide shell. Pressure generation within the Al core leads to an increase in melting temperature in comparison with traditional melting temperature depression calculated using the Gibbs-Thomson equation. On the basis of elasticity theory, the pressure in the Al core at the onset of melting is caused mainly by surface tension at the alumina-air and Al-alumina interfaces. This implies that pressure due to the difference in thermal expansion of aluminum and alumina relaxes. A possible relaxation mechanism is discussed. The static strength of the alumina shell and the maximum static generated pressure in aluminum were evaluated. Mechanically damaging the oxide shell was shown to reduce the melting temperature due to a decrease in generated pressure within the Al core. Thus, reduction in melting temperature can be used as a quantitative measure of damage to the oxide shell. Results from X-ray diffraction studies show that 17-nm diameter Al particles had a 2-nm thick alumina shell in the γ-phase, while for a flat surface Al had an amorphous alumina shell stable to a thickness of 4 nm. Thus, pressure due to surface tension promotes denser γ-phases. Since particles with shells initially in the amorphous or γ-phase show the same flame speed and ignition delay time, fast oxidation observed under high heating rates cannot be explained by a phase transformation in the alumina shell. These findings have important implications for the melt-dispersion mechanism for fast Al oxidation.
Melting and Oxidation of Nanometer Size Aluminum Powders
MRS Proceedings, 2005
Recently, nanometer-sized aluminum powders became available commercially and their use as potential additives to propellants, explosives, and pyrotechnics has attracted significant interest. It has been suggested that very low melting temperatures are expected for nano-sized aluminum powders and that such low melting temperatures could accelerate oxidation and trigger ignition much earlier than for regular, micron-sized aluminum powders. The objective of this work was to investigate experimentally the melting and oxidation behavior of nano-sized aluminum powders. Powder samples with three different nominal sizes of 44, 80, and 121 nm were provided by Nanotechnologies Inc. The particle size distributions were measured using small angle x-ray scattering. Melting was studied by differential scanning calorimetry where the powders were heated from room temperature to 750 °C in argon environment. Thermogravimetric analysis was used to measure the mass increase indicative of oxidation whil...
Development of Al-Fe 3 Al Composites by Powder Metallurgy Route
2016
Aluminium based MMCs are one of the most prominent materials due to their low density, high specific strength and stiffness and increased fatigue resistance which make them suitable for various wear and structural applications in various aerospace and automotive industries. Aluminium (Al) is widely used due to its excellent properties such as low density high thermal and electrical conductivity. However, Al has poor wear resistance behaviour, low hardness and poor fatigue properties. This is why Al is very often reinforced with hard materials like carbides, borides, nitrides, oxides and intermetallics. Rising interests in intermetallic compounds is connected with their high strength, corrosion resistance and wear resistance. Among the several intermetallic compounds available iron aluminide (Fe 3 Al) has been frequently considered for hightemperature structural applications because of their unique physical and mechanical properties. Fe 3 Al intermetallic compound has a high melting point, high hardness, low density and good oxidation and corrosion resistance. In the present work, an attempt has been made to study the effect of addition of Fe 3 Al as reinforcement in Al metal matrix composites. Here, in the present research work Al-10, 20, 30 vol. % Fe 3 Al composites have been developed by powder metallurgy route and their microstructure, hardness and wear properties have been investigated. In the present research work both as-received Fe 3 Al and Fe 3 Al developed by 40 h of mechanical alloying (MA) of Fe 75 Al 25 powder followed by heat treatment at 1100 o C for a period of 2 h in Ar atmosphere has been used as reinforcement. Nanocrystalline Al developed by milling Al powder for a period of 20 h has been used as the matrix for all the Al-Fe 3 Al composites developed in this study. The milled powders were analyzed using x-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive x-ray spectroscopy (EDX), high resolution transmission electron microscope (HRTEM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The 20 h milled nanocrystalline Al was mixed with both the as-received Fe 3 Al powder and the Fe 3 Al powder synthesized by MA in different vol. % and compacted under a uniaxial load of 222 MPa and sintered at 500 o C for a period of 2 h in Ar atmosphere. The microstructure of the various Al-Fe 3 Al sintered composites was analyzed using optical microscope, SEM and EDX. The relative density of the various sintered composites was determined by the Archimedes' principle. Dry sliding wear test of the various sintered composites was done on a ball-on plate tribometer to determine the wear behaviour of the composites. The hardness of the composites was determined using a Vickers microhardness tester. It was found that both the hardness and the wear resisiatnce of the various Al-Fe 3 Al sintered composites increased with the increase in Fe 3 Al content.