Aluminum powder metallurgy processing

Abstract

In recent years, the aluminum powder industry has expanded into non-aerospace applications. However, the alumina and aluminum hydroxide in the surface oxide film on aluminum powder require high cost powder processing routes. A driving force for this resezirch is to broaden the knowledge base about aluminum powder metallurgy to provide ideas for fabricating low cost aluminum powder components. The objective of this dissertation is to explore the hypothesis that there is a strong linkage between gas atomization processing conditions, as-atomized aluminum powder characteristics, and the consolidation methodology required to make components from aluminum powder. The hypothesis was tested with pure aluminum powders produced by commercial air atomization, commercial inert gas atomization, and gas atomization reaction synthesis (GARS). The conmiercial atomization methods are bench marks of current aluminum powder technology. The GARS process is a laboratory scale inert gas atomization facility. A benefit of using pure aluminum powders is an unambiguous interpretation of the results without considering the effects of alloy elements. A comparison of the GARS aluminum powders with the commercial aluminum powders showed the former to exhibit superior powder characteristics. The powders were compared in terms of size and shape, bulk chemistry, surface oxide chemistry and structure, and oxide film thickness. Minimum explosive concentration measurements assessed the dependence of explosibility hazard on surface area, oxide film thickness, and gas atomization processing conditions. The GARS aluminum powders were exposed to different relative humidity levels, demonstrating the effect of atmospheric conditions on post-atomization oxidation of aluminum powder. An Al-Ti-Y GARS alloy exposed in ambient air at different temperatures revealed the effect of reactive alloy elements on post-atomization powder oxidation. 6 Melt Chamber He + O2 Gas ^ Exhaust and Recirculation Spray Chamber Powder Collection (a) Melt Chamber o UHPN, Gas Q Spray Chamber Powder Collection (b) Vacuum Pump Exhaust Figure 2.2. a) Schematic of vertically downward atomization system used by Valimet for CIGA-VAL aluminum powders [9]. b) Schematic of vertically downward atomization system used for GARS aluminum powder processing. hydration of the oxide film [4]. For a point of reference, Bohlen et al. [5] state in their paper that the air atomized process used by Alcoa to produce a 7091 aluminum alloy powder for their research had a dew point of 4°C. After atomization of the melt, the droplets solidify into powder particles and are carried up and out of the spray chamber by the flowing gas. The powders are transferred to cyclones for particle size classification, followed by powder packaging and shipping to the P/M part producers. A similar atomization system shown is used by Alcan-Toyo America (ATA) in Napierville, Illinois, for CIGA of aluminum powders (Figure 2.1b) but the spray chamber is isolated from the ambient air. Before atomizing, the spray chamber of a CI...

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