Surface oxide and the role of magnesium during the sintering of aluminum
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I.
INTRODUCTION
MOST metal powders exposed to oxygen are covered by a surface oxide layer. This oxide prevents solid-state sintering in low-melting-point metals,[1] including aluminum,[2] but not in all metals.[3,4,5] This has been explained in terms of the relative diffusion rates through the oxide and the metal, for metals with stable oxides.[6,7] The oxide on aluminum, therefore, needs to be reduced or otherwise removed to enable effective sintering. The oxidation of a metal (M) may be represented by M 1 O2 ↔ MO2
[1]
The free energy of formation (DG) of the oxide is given by DG 5 2RT ln K1
[2]
where R is the gas constant, T is the temperature in Kelvin, and K1 is the equilibrium constant given by K1 5 (P O2)21
[3]
where P O2 is the partial pressure of oxygen when reaction [1] is at equilibrium. For aluminum at 600 7C, a P O2 , 10250 atm is required to reduce the oxide.[8] Atmospheres containing hydrogen are often used in powder metallurgy. Hydrogen can reduce a metal oxide by the reaction MO 1 H2 ↔ M 1 H2O
[4]
The equilibrium constant for this reaction (K4) is given by K4 5 P H2O/P H2
[5]
where P H2 and P H2O are the partial pressure of hydrogen and water vapor, respectively. The ratio of partial pressures can be converted to the dew point, which is effectively the water vapor content. A dew point of ≤2140 7C at 600 7C is required to reduce Al2O3.[9] Neither a dew point of 2140 7C nor a P O2 of 10250 atm is physically attainable, and, R.N. LUMLEY and T.B. SERCOMBE, Postgraduate Students, and G.M. SCHAFFER, Senior Lecturer, are with the Department of Mining, Minerals and Materials Engineering, The University of Queensland, QLD 4072, Australia. Manuscript submitted December 5, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
therefore, aluminum cannot be sintered in conventional atmospheres. The use of liquid phases is an alternative to solid-state sintering. An essential requirement for effective liquidphase sintering is a wetting liquid.[10] The wettability of a solid by a liquid is determined by the work of adhesion (Wa):[11,12] Wa 5 glv (1 1 cos u ) 5 gsv 1 glv 2 gsl
[6]
where glv is the surface tension of the liquid, gsv is the surface tension of the solid-vapor interface, gsl is the solidliquid interfacial tension, and u is the contact angle. A liquid is said to wet a solid when cos u . 0. High-meltingpoint metal oxides are generally poorly wetted by liquid metals, except above the wetting threshold, a temperature beyond which Wa increases sharply.[11] Liquid aluminum is not, therefore, expected to wet alumina near the melting point of the metal. Indeed, the contact angle is variously given as ;103 deg at 900 7C,[13] ;160 deg at 800 7C,[14] or ;162 deg at 950 7C,[15] although this is dependent on the partial pressure of oxygen and the presence of an oxide film on the molten metal.[16] It has been suggested that the AlCuAl2 eutectic can wet Al2O3 at 600 7C.[17] However, neither Mg, Ce, nor Ca additions to molten Al reduce the contact angle sufficiently to produce wetting.[13,14] Since the work
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