Fig 19 Mean oxide thickness for aluminum powders

Spherical powders, atomized in an inert gas, contain substantially less oxygen compared to air-atomized powders. This is in part due to the smaller specific surface area of these powders. However, when compared to the mean oxide thickness, the oxide is actually between 2 and 5 nm thinner with relatively little dependence on the size of the powder.

While the mean oxide thickness is useful for comparison between different powders, it does not provide any information on the nature of the oxide skin. Indeed, recent observations have indicated that the oxide on aluminum powders is not a uniform thickness (Fig. 20). Most of the surface is covered with a thin oxide film with interspersed high hills up to 20 nm thickness (Fig. 21). The high hills represent the oxide that forms at discrete nucleation sites while the droplet is still in the molten state. As noted above, the distortion of the droplets by the nuclei is considered to be the cause of the irregular morphology of the air-atomized powders. The space between the mounts is covered by a thin skin that mostly forms in the solid state. A similar description has been offered for alloyed powders. The greater mean thickness observed in coarser air-atomized powders is consistent with this mechanism because they will have spent a longer period in the molten state exposed to oxygen.

Fig. 20 Simplified model of nonuniform oxide layer consisting of thick islands on a thin skin

Fig. 21 Model of oxide formed on gas-atomized Al5Mn6Cr powder particle. (a) In the as-atomized state. (b) After exposure to a humid atmosphere. Source: Ref 24

Temperature of the metal would obviously have a bearing on the amount of oxide formed in powders. The results relate to material obtained in regular atomizing conditions with a superheat of 150 to 200 °C. If higher temperatures were to be employed, higher oxide levels would be expected.

The surface of the aluminum powder is hygroscopic and will react with moisture to form Al(OH)3 by hydration of the oxide and corrosion of the metallic aluminum especially in fine powders. A minimum relative humidity of 60% is needed for hydration promoted by prolonged exposure to moisture and temperature. The presence of manganese and chromium has been found to sharply decrease the formation of Al(OH)3 in alloyed powders.

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