GAS ENTRAPMENT AND EVOLUTION IN PREALLOYED ALUMINUM POWDERS

Part of a comprehensive research programme involving different aspects of degassing of powder metallurgy (P/M) aluminium alloys carried out in the P/M Group of the Delft University of Technology, is reported. The fundamental aspects of moisture and gas evolution during degassing of a porous billet are described in a semi-quantitative manner using a kinetic approach. During degassing of Al-20Si-X P/M alloys, at temperatures up to 550-degrees-C, the partial pressures of moisture and hydrogen were within the range 10(-4) to 10(-7) mbar. The thermodynamics of gas desorption is mainly influenced by temperature which is the critical degassing parameter. It appears that the diffusion of aluminium through the oxide layer can explain, to a large extent, the kinetics of degassing of aluminium powders. A shift in the release of moisture and hydrogen towards higher temperatures is due to the presence of MgO in the surface layer, compared to the situation when only Al2O3 builds the oxide film. Thermodynamical data indicate that the reaction of magnesium with water vapour proceeds more intensely than that between aluminium and water vapour.