Abstract
During the rapidly solidified powder metallurgy processing of aluminum alloys, the vacuum degassing process should be carried out before powder consolidation in order to remove the adsorbed moisture and gaseous species that are harmful to consolidation from the powder-oxide skin. Therefore it is important to clarify the effect of vacuum degassing on surface characteristics of the gas-atomized aluminum alloy powders. Temperature-programmed desorption measurement on the Al-Ti-Fe-Cr and Al-Zn-Mg-Cu-Ag powders exposed to humid air, dry air and high purity argon gas was performed to determine the gas species evolved during degassing. It was found that the surface composition and processing atmosphere had a strong influence on the gas desorption behavior and on the surface characteristics of the powders. Adsorbed H2O on the surface of the powders brought about the formation of fresh Al2O3 layer by the reaction, 2Al + 3H2O = Al2O3 + 3H2, although adsorbed oxygen caused no formation of fresh Al2O3 during degassing. The surface oxide of the Al-Zn-Mg-Cu-Ag powder was more prone to hydration than that of Al-Ti-Fe-Cr powder. Accordingly, the formation of fresh Al2O3 on the Al-Zn-Mg-Cu-Ag powder at low temperature of 390 K occurred during vacuum degassing with much H2O adsorbed. On the other hand, Al-Ti-Fe-Cr alloy powders had less adsorbed H2O, resulting in the formation of fresh Al2O3 above 450 K during vacuum degassing.
