Surface Reactions During Atomization, Handling and Consolidation of Al-powders
Surface reactions during atomization and consolidation of pure Al and the alloys Al2Mg, Al5Mn2.5Cr, Al5Mn6Cr and Al8Fe2Mo were studied by using ESCA, SAM and SEM. The powders were analysed in the as-atomized condition and after exposure to humid atmosphere. The powders were atomized by using the USGA (Ultra Sonic Gas Atomization)-technique. The cooling rate of the powder was in the order of 105-106 K/s. The rapid cooling increases the solubility of the alloying elements far beyond the isothermal solubility limits of the alloy system. Though the atomization is performed in a high purity He gas atmosphere, the oxygen potential is high enough to cause oxidation of the surface of the metal powder particles. At high temperatures, discrete Al2O3-particles are formed. During the subsequent cooling and storage of the powder a thin oxide layer develops between the oxide islands on the surface. The average oxide thickness is about 50 Å. The thickness of the oxide was determined by taking the effect of the spherical shape of the powder into consideration. The oxide thickness is found to be independent of the particle size, at least in the range 5 µm to 50 µm. Beside Al magnesium is the only element that oxidizes during atomization. It is enriched in the outer surface layers and distributed in a very irregular way over the surface.
Exposure of the powder to humid atmosphere causes hydration of Al2O3 and formation of Al(OH)3 on the surface. The amount of hydroxide increases with exposure time and relative humidity. The outer layers of Al(OH)3 are decomposed in ultra high vacuum to Al2O3 and H2O. The thickness of the decomposed layer of Al(OH)3 was found to be 17 Å, corresponding to 7 Å of Al2O3. The alloying elements Mn and Cr dissolved in the matrix of the powder tend to stabilize the surface oxide and thereby decrease the hydroxide formation.
Gases adsorbed on the powder are released during degassing and heating before extrusion. The gas reactions were analysed using mass spectroscopy. It was possible to detect desorption of H2O from the surface even after a minimum exposure to air. Alhydroxide is decomposed to Al2O3 and H2O during the heating process; longer exposure time to humid atmosphere gives a larger gas release from the powder surfaces. The results show the necessity of a careful degassing before consolidation of the powder.
Consolidation of the powder into compact products was done by hot extrusion. At a high reduction ratio the material formed becomes ductile. It is concluded that the surface oxide breaks up due to surface enlargement during extrusion, thereby achieving metallic bonding between the particles. Oxide products present on the boundaries at a lower area reduction ratio results in PPB (prior particle boundary) failure.