The oxidation of molten aluminum

Bergsmark, E.; Simensen, Christian J.; Kofstad, Per

doi:10.1016/0921-5093(89)90723-5

Cited by 49 publications

(29 citation statements)

References 2 publications

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“…The segregation of magnesium to the surface of liquid aluminium has been detected in a previous study on the oxidation of molten aluminium [36]. As an effective surfactant, Mg was found to reduce significantly the surface tension of liquid Al [39][40][41].…”

Section: Formation Of Oxide In Al-mg Alloysmentioning

confidence: 99%

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al–Mg alloys

Li¹,

Wang²,

Fan³

2012

Acta Materialia

177

120

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This paper investigates the mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl 2 O 4 , which displays an equiaxed and faceted morphology with {111} planes exposed as its natural surfaces. Depending on the Mg content, MgAl 2 O 4 particles exist either as oxide films in dilute Al-Mg alloys (Mg<1wt.%) or naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg>1wt.%). Such MgAl 2 O 4 particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl 2 O 4 and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl 2 O 4 particles with a lattice misfit of 1.4% and the increased number density of MgAl 2 O 4 particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl 2 O 4 particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl 2 O 4 particles have been naturally dispersed into individual particles by the increased Mg content.

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Section: Formation Of Oxide In Al-mg Alloysmentioning

confidence: 99%

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al–Mg alloys

Li¹,

Wang²,

Fan³

2012

Acta Materialia

177

120

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“…The self-diffusion coefficient of oxygen and Al in ␣-alumina at 800− 950°C is at most D =10 −19 and 10 −18 cm 2 /s, 8 the diffusion length l d =2 ͱ Dt f = ͑2−6͒ ϫ 10 −5 nm, which is 10 5 times smaller than the oxide shell thickness. Thus, the diffusion time t f would have to be increased by a factor of 10 10 to obtain a reasonable diffusion length l d =2−6 nm.…”

Section: Introductionmentioning

confidence: 99%

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

Levitas

Asay

Son

et al. 2007

Journal of Applied Physics

141

223

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An unexpected mechanism for fast reaction of Al nanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitation pressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.

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“…Several workers have explicitly mentioned this (see, for example [70,84,88,95]), with Field et al [88] stating that variations in specimen surface preparation can affect overall weight gains by an order of magnitude. Lawless [96] made the observation that despite it being widely understood that carefully prepared specimen surfaces were essential for studying oxidation, few studies have actually attempted to take the issue into account.…”

Section: The Effect Of Surface Treatment On Kineticsmentioning

confidence: 99%

“…Thiele [73], also found that water vapour suppressed oxidation, but did so by expediting the γ- Bergsmark et al [70] investigated the oxidation of molten commercial purity aluminium in pure oxygen at 800-950°C by a thermogravimetric method and characterised the resulting oxide. The oxidation kinetics could not be described by a single rate equation, but general trends were identified.…”

Section: Experimental Observations Of Oxidation Kineticsmentioning

confidence: 99%

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A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

Bonner¹

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Oxidation and dross formation on molten aluminium is estimated to directly cause losses of tens of millions of dollars per annum to the Australian smelting industry. Although there has been significant study of molten aluminium oxidation over the past few decades, the formation mechanisms are still not fully understood. There is also lack of reliable and useful kinetic data that can be applied to computational simulations of industrial melt handling processes involving the oxidation of either quiescent pools or turbulent transfers of molten aluminium. There is additional uncertainty in the literature relating to the physical processes underlying molten aluminium oxidation, and the microstructural evolution of the growing oxide film. Past oxidation studies have been confounded by the detrimental effect of the pre-existing oxide ubiquitously present on aluminium specimens. Recent studies conducted at The University of Queensland have indicated a way forward in this respect, with the development of an experimental technique that enables the melt surface to be skimmed free of pre-existing oxide prior to each oxidation experiment. This project is based on previous outcomes and expands on them to create new insights into the oxidative behaviour of molten aluminium in response to various key parameters, such as melt temperature and isothermal holding time. It also aims to resolve some ambiguities in the literature.Melts of commercial purity aluminium were isothermally held at various temperatures ranging from 750°C to 900°C, and exposed to ambient air for times ranging up to ~20 h. The oxide grown on each melt was removed and treated to separate the oxide itself from the underlying aluminium metal, using a molten salt flux. The mass of oxide was then determined as a function of melt temperature and exposure time. These gravimetric experiments were complemented by morphological and microstructural characterization of grown oxide films, using scanning electron microscopy, transmission electron microscopy, and electron diffraction. These two approaches were combined to develop a simple kinetic model based on the parabolic rate law.The amount of oxide formed was found to be significantly lower than what has previously been reported, and was attributed to the removal of the pre-existing oxide. The amount of oxide that formed increased with increasing temperature, and the oxidation rate continuously decreased with increasing exposure time. These observations are consistent with diffusion-limited growth.Microstructural characterisation showed two stages of oxidation. In the early stage, nano-sized, planar, polyhedral crystallites of γ-alumina nucleate at the metal/oxide interface. In the later stage, flake-like γ-alumina crystallites were observed to grow outwards from the oxide/air interface. The time at which the second stage of growth became active decreased with increasing temperature, and the flake-like crystallites continued to increase in size with increasing exposure time.iii Crystallographic texture was obser...

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The oxidation of molten aluminum

Cited by 49 publications

References 2 publications

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al–Mg alloys

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al–Mg alloys

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

A microstructural and kinetic study of molten aluminium oxidation in relation to dross formation

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