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The results reported in this paper showing the effect of different degassing modes on the Al-20Si-3Cu-1 Mg powder are a complement of our previous papers concerning the continuous and non-continuous degassing sequences of the same powder. This research was mainly directed at an improvement in the technique to remove volatile and gaseous contaminants from the porous compact made from the Al-20Si-3Cu-1 Mg powder. This improvement has been possible by modifying the conventional degassing process of the powder as reported previously, namely degassing of the powder assisted by flushing with depurative gases such as argon or nitrogen. It is apparent that flushing with argon or nitrogen increases the efficiency of moisture and hydrogen evolution in comparison with the conventional degassing mode.

The results reported here, showing the effect of a non-continuous degassing sequence on the Al-20Si-3Cu-1 Mg powder, are a complement of previous work concerning the continuous degassing of the same powder. The degassing experiments were carried out, under high vacuum, in the temperature range 20 to 550 °C in a horizontal furnace heated at a uniform heating rate of 2.5 °C min−1. The partial pressures of the released gases were monitored and analysed during the heating phase by a computerized Edwards EQ80F residual gas analyser (RGA). RGA measurements indicate that water and hydrogen are the main degassing products. A complete degassing can only be achieved if the sample is heated up to a temperature where the chemical reactions are finished in the applied time. Thermodynamical equations alone are not enough to explain the kinetics of degassing of aluminium powders. The diffusion of aluminium through its surface oxide layer (Al2O3), described by the self-diffusion of aluminium, can explain to a large extent the kinetics of degassing aluminium 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 °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.

Among the variety of new aluminium alloys, the Al-Si-X P/M system appears to be the most suitable for high-performance applications in the automobile industry. Our work concerns the research on the possible application of this system for products with enhanced wear and high-temperature resistance. This paper presents the characteristics of the air-atomized J1 (Al-20Si-3Cu-1Mg), J2 (Al-20Si-3Cu-1Mg-5Fe), J3 (Al-20Si-3Cu-1Mg-7.5Ni), K1 (Al-20Si-5Fe-2Ni), and the argon-atomized K2 (Al-20Si-5Fe-2Ni) powders, aimed at optimizing the processing conditions of the final products, in terms of production techniques, powder morphologies, powder sizes and size distributions, cooling rates, specific areas, surface oxide thicknesses and oxygen contents. Atomization in air (J1, J2, J3, K1) and atomization in argon (K2) resulted in morphologically different powders. Particle-size distributions were similar, indicating cooling rates of ∼104 to 106 K sec−1. This cooling range proves that the theoretical estimate presented in this work is sufficiently accurate. Al-Si-X P/M alloys consisted of primary and eutectic silicon crystals in an aluminium matrix (J1) plus intermetallic compounds (J2, J3, K1, K2). Air-atomized powders with different chemical composition showed an average oxide thickness of ∼30 to 40 nm. In powders with equal chemical composition, an inert atomization atmosphere produced powders with smaller surface area, lower amount of oxygen, and thinner total oxide thickness. The composition of surface oxides was strongly influenced by the chemical composition but the thickness was mainly influenced by the atomization atmosphere.

This paper describes some characteristics of an Al-20Si-X aluminium alloy, processed by the Osprey route, in terms of total oxygen content, porosity distribution and microstructure. A theoretical analysis of the solidification of the material, after a semi-liquid/semi-solid spray of atomized droplets-particles impacts the deposit, is presented. A heat flow calculation was conducted applying the forced convection method at quasi-steady conditions. Based on the calculation of the heat transfer coefficient the cooling rate was estimated within the range ∼102 to 104 K sec−1. Stroehlein OSA-MAT measurements showed that the total content of oxygen of the Osprey preform was 3.5 and 7 times lower than the corresponding values for argon (nitrogen) and air atomized Al-20Si-X powders, respectively. Light microscopic examination of the deposited material revealed a homogeneous microstructure with a porosity level as low as 1.3%. Microstructural features indicated that the Osprey process provided rapidly solidified material with an average cooling rate of 103 to 104 Ksec−1. This cooling range proves that the theoretical estimation presented in this work is sufficiently accurate.