A numerical study of oxidation behavior during reactive atomization and deposition

et al. 2009

Metall Mater Trans B

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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 10 5 K/s for powder <20 lm 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.

Section: Modeling Results and Analysismentioning

confidence: 99%

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

Zheng

et al. 2009

Metall Mater Trans B

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“…Furthermore, the surfaces of oxidized droplets are always irregular. [26,27] These two factors cause a high frictional resistance between an incoming droplet and the deposited material's surface during spreading and flattening of the droplet. Second, the surrounding droplets exert a resistance to spreading and flattening of the droplet.…”

Section: A Impingement and Flattening Of Dropletsmentioning

confidence: 99%

An analytical model for the oxide size in Al alloys synthesized by reactive atomization and deposition

Lavernia

2004

Metall Mater Trans A

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This article presents the formulation of an analytical model to predict the size scale of oxide dispersoids in as-deposited Al alloys synthesized by reactive atomization and deposition (RAD). The proposed model formulation is primarily based on the assumption that all of the strain energy in the oxides is used to create interfaces between the oxide dispersoids and the matrix. It is also assumed that oxides are fragmented into plate-shaped dispersoids. Assuming three types of cross-sectional geometries for the oxide plates, i.e., circular (corresponding to disc-shaped oxide dispersoids), rectangular, and equilateral polygon, the following predictions are made on the basis of the analytical model. First, the curves for calculated effective cross-sectional diameters of plate-shaped oxide dispersoids vs droplet size and calculated effective volumetric diameter of plate-shaped oxide dispersoids vs droplet size can be divided into three distinct regions. These three regions are identified on the basis of two characteristic droplet sizes corresponding to the solid fraction equal to that on the deposited material's surface and to solid fraction of 0.6 (D 0.6 ), respectively, at impact. Second, the velocity of individual droplets at impact has a limited effect on the calculated effective cross-sectional diameter and effective volumetric diameter. With an increase in solid fraction on the deposited material's surface, the calculated effective cross-sectional diameter and effective volumetric diameter decrease significantly when droplet sizes are smaller than , whereas they remain almost unchanged when droplet sizes are higher than D 0.6 . Third, when plate-shaped oxide dispersoids with rectangular or equilateral polygon cross-sectional geometries are chosen, the calculated effective cross-sectional diameter and effective volumetric diameter are larger than the corresponding calculated diameter and effective volumetric diameter of disc-shaped oxide dispersoids. The calculated effective cross-sectional diameters, based on different cross-sectional geometries (e.g., circular, rectangular with up to 10 of the length/width ratio, and equilateral polygon), are all in reasonable agreement with the experimentally observed effective cross-sectional diameter of oxides. D f ss (D f ss )

“…During flight, the temperature and the solid fraction of a droplet can be calculated using the equation of energy conservation containing the contribution of oxidation reactions. [30] The oxidation behavior of the droplet is analyzed in detail in Reference 30. In the flight stage, thermal energy of a droplet is extracted by convection and radiation heat transfer between the droplet and the atomization gas.…”

Section: Numerical Modelingmentioning

confidence: 99%

“…[16,21,[35][36][37][38] Droplet size is represented using the diameter for a spherical SDN droplet and using the effective diameter on the basis of volume for a nonspherical RAD droplet (defined as the diameter of a spherical droplet with a volume equivalent to that of the nonspherical droplet). [30] There are two characteristic values in a lognormal distribution of droplet size: the mass median diameter (d 50 ) and the geometric standard deviation ( g ). The d 50 (d i defined as a droplet size below which the cumulative mass or volume percent is i) can be predicted by using a correlation originally developed by Lubanska, [35] which is closely related to the atomization gas velocity during interaction between gas jets and the melt stream and the ratio of the gas flow rate to the metal flow rate.…”

Section: Numerical Modelingmentioning

confidence: 99%

“…[30,52] Figures 5(a) through (c) show the calculated dynamics of typical small (30 m), medium (100 m), and large (300 m) droplets atomized by N 2 (i.e., SDN droplets) and 10 vol pct O 2 -N 2 (i.e., RAD droplets), together with the dynamics of N 2 and 10 vol pct O 2 -N 2 . It is indicated that N 2 and 10 vol pct O 2 -N 2 have almost the same curves of velocity vs axial flight distance.…”

Section: Calculated Dynamics Of the Atomization Gas And Individualmentioning

confidence: 99%

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On the influence of in-situ reactions on grain size during reactive atomization and deposition

Lavernia

2004

Metall Mater Trans A

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A fundamental study of the factors that govern grain size of 5083 Al processed via reactive atomization and deposition (RAD) is reported. Microstructural observation shows that the average grain size in RAD 5083 Al is slightly smaller than that in the material processed via N 2 spray deposition (SDN). A numerical approach, together with measurements of the temperature histories inside the deposited materials, is implemented to analyze the influence of in-situ reactions during RAD process on the evolution of grain size. The numerical results show that RAD 5083 Al possesses a slightly higher density of nuclei relative to that present in SDN 5083 Al on a per unit volume of deposited material basis at the beginning of the slow solidification of remaining liquid phase. Furthermore, the RAD material exhibits a slightly lower coarsening extent during the slow solidification. Grain growth is negligible during the solid-phase cooling. Accordingly, the calculated grain size in RAD 5083 Al is slightly smaller than that in SDN 5083 Al, consistent with the observed results.