Cellular solids are a class of highly porous materials that covers a wide range of structures, such as foamed materials, replicated porous or additive manufactured structures, honeycombs, lattice structures, interconnected fibres or hollow spheres and syntactic foams. The pores can be more or less closed or completely open. Depending on their morphology and composition, these materials possess unique properties and are used as components in special and advanced engineering applications. These include filtering liquids and particles in gas streams, porous burners, biomedical devices, lightweight stiff structures, impact absorbers, sound damping modules, thermal management components, etc. The present paper gives an overview on the main fabrication methods and the resulting architectures, in relation to applications of metals and ceramics with >50 vol.-% porosity.

Highly porous metals and ceramics

Aluminium alloy castings are being used increasingly in safety-critical applications in the automotive and aerospace industries. To produce castings of sufficient quality, it is, therefore, important to understand the mechanisms of the formation of defects in aluminium melts, and important to have a reliable and simple means of detection. During the production of aluminium ingots and castings, the surface oxide on the liquid is folded in to produce crack-like defects (bifilms) that are extremely thin, but can be extremely extensive, and so constitute seriously detrimental defects. However, the presence of bifilms has not been widely accepted, because there has been no single metal quality test that has been able to resolve features that are only nanometres, or sometimes micrometres, in thickness. In the past, porosity has usually been held solely responsible for most failures in aluminium alloys, and hydrogen has been blamed as the actual cause. In this work it has been found that bifilms are the initiator and hydrogen is only a contributor in the porosity formation process. For the first time, evidence is presented for the contribution of air (or perhaps more strictly, residual nitrogen from air) as an additional gas, adding to hydrogen in pores in cast Al alloys. The Reduced Pressure Test (RPT) is used which is a simple and widely known test, is cost effective and involves no complicated equipment or consumables, thus recommending it for implementation on the foundry floor. On this basis, several Al-Si based alloys were studied. A quality index -Bifilm Index- is introduced to quantify the results of the reduced pressure test. In addition, mechanical tests were carried out to correlate bifilm index with mechanical properties.

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Determination of metal quality of aluminium and its alloys

https://rke.abertay.ac.uk/ws/files/8517709/WightForScIntAcceptedManus2010.pdf

The thermal visualisation of latent fingermarks on metallic surfaces.

High-temperature oxidation of molten aluminum was investigated by high-resolution electron microscopes in order to determine the possibility of heterogeneous nucleation of aluminum grains on oxide for the grain refinement and structural uniformity of intensively melt-sheared aluminum alloys. High-resolution observations detect initial amorphous phase and gamma-alumina phase and show fine clusters with size of about 150 to 200 nm composed of extremely fine aluminum grains and gamma-alumina or amorphous aluminum oxide. Furthermore, high-resolution lattice images and diffraction patterns show no orientation relationship, although there is a specific orientation between gamma-alumina and aluminum along (111)[110] with high potency of heterogeneous nucleation. The volumetric shrinkage by the transformation of gamma- into alpha-alumina causes the surface oxide films to repeatedly rupture and leads to the creation of channels to the base melt surface for further oxidation of fresh metal. Based on the observations, the mechanism of high-temperature oxidation of molten aluminum and formation of the fine clusters as well as the possibility of the heterogeneous nucleation of aluminum grains are discussed.

Formation of Fine Clusters in High-Temperature Oxidation of Molten Aluminum

The characteristics of oxide films formed within a short time of pouring were investigated using samples prepared via the oxide–metal–oxide sandwich (OMO) technique. Surfaces of the OMO sandwiches were studied using scanning electron microscopy. The thickness of the folds of oxide films was examined and compared with previously obtained values for Al–7Si–0··4Mg and Al–5Mg alloys. Results showed that the thicknesses of oxide films formed in pure aluminium and Al-7Si- 0.4Mg alloy are very close and in the range 100-500 nm.

Characteristics of dynamically formed oxide films on molten aluminium

In order to investigate the rate of oxidation of liquid aluminum and its alloys under conditions similar to those found industrially, carbon dioxide was bubbled through the melt and the change in gas composition monitored using a mass spectrometer. It was found that the dynamic oxidation of aluminum was a function of the carbon dioxide partial pressure and was considerably faster than oxidation under quiescent conditions. For alloys containing an appreciable amount of magnesium, the magnesium oxidized in two stages. The carbon dioxide was first reduced to carbon monoxide and then to carbon. At lower magnesium levels, aluminum is preferentially oxidized, but the reduction of the carbon dioxide proceeds only as far as carbon monoxide.

G. Wightman

Papers

The dynamic oxidation of aluminum and its alloys