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Stabilisation mechanisms in metal foams

Figure 1 Left: Photography of a pulled Film (AlSi9Mg0.6 + 3 vol.% TiB2). Middle: Particle trajectories of SiC particles and their movement from the centre of the film (B-D) into the Plateau border (A). The colour scale represents the time-wise evolution.
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Figure 2 Radioscopies of an AlSi9+ 6 vol.% TiB2 film at 21 % O2 and the prevention of an expanding rupture due to fixed particleclusters (black circles). The red circle indicates a free particlecluster next to the beginning rupture (black line).
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Figure 3 MgAl2O4 particles (spinel) up to 1 µm, down to 50 nm and their characteristic octahedral structure could be detected at the solid-gas interface (cell wall) of the foam via a) FIB tomography and b) TEM bright field microscopy.
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Manufacturing aluminium based metal foams through liquid metal route is simple and inexpensive due to few processing steps in comparison to other routes. However, stability of such foams is transient in nature due to liquid drainage (cell wall thinning) and corresponding coalescence till solidification. At present researchers are trying to answer the question, what stabilise a foam through particles in the molten Al, their attachment to the gas-solid interface and increase of bulk viscosity. Similarly the role of oxidation during foaming and its effect on foam stability is recently studied. In this regard an increasing amount of oxygen of the injection gas is advantageous, but its underlying stabilisation mechanism is still unrevealed. Thus a clear scientific understanding of foam stabilisation in the liquid state and during solidification is essential in order to achieve better foaming technology and production consistency.

1. Role of oxygen and solid particles on the stability of aluminium foams studied by single films under controlled atmosphere.

Keeping the aim of understanding foam stabilisation mechanisms as a motivation, the present work adopts a new course for metallic foams. Until now Al based foams have been studied both in the liquid (e.g. X-ray) and the solid state (e.g. metallography) by means of a large variety of methods which are mainly integral analyses of the entire foam. Thus we draw inspiration from the strategy of aqueous foam studies, whereby fundamental stabilisation theories were established through model systems which simplify the complexity of foam structures to a single element (like cell wall, Plateau borders and nodes). One such model is a free standing film, representing a single cell wall, vertically pulled from the liquid and subjected to light microscopic investigation. Transferring this idea to the field of metal foams is new, but might be the missing tool to understand various mechanisms related to foam stability.

Our first experiments of using a single circular wire frame have already shown that films of the highest achievable particle concentration (20 vol.% SiC) could not be stabilised without any oxygen. However if just a small amount of oxygen (1500 ppm O2) in the atmosphere is present and the alloy contains a little Mg (0.6 wt.%), a film could be stabilised even without any particles, compare figure 3 left. The necessity of oxygen and the stability only by its attendance was astonishing and hitherto new for metallic foams. By using more complex models to imitate artificial Plateau borders and keep them in the liquid state as well, particles were necessary though. The obvious influence of particles and oxygen in regard of stabilisation is the basis of our present study to uncover their fundamental mechanisms by various analysis methods.

The embedding of particles and their arrangement at the surface of solidified films is analysed using SEM/FIB or metallography. The 3D distribution of such particles in between the film is studied by tomography, based on the high brilliance and coherence of synchrotrons and its acquired radioscopies, see figure 1 right. By using Energy-Filtered-Transmission-Electron-Microscopy (EFTEM) we can have a closer look at the solid-gas interface and analyse the required thin oxygen layer (5-50 nm). Due to the achievement of keeping films liquid we are even able to study particles in the liquid, how they behave and stabilise the liquid film in-situ by synchrotron phase contrast. Hereof we have obtained the very first studies of tracking particle trajectories, analysing their velocities in between a liquid metal and the attachment behaviour at the solid-gas interface by followed image processing, see figure 1 middle. Furthermore even the moment of a bursting film can be studied by fast synchrotron radioscopies (up to 1000 fps), see figure 2. Due to this plenty of film pulling methods and analysing techniques we might pave the way to answer the question of the basic stabilisation mechanisms of metallic foams in its simplest way.

2. Application of grain refiners as submicron particles for stabilisation.

From the current knowledge it is known that pure aluminium melts cannot be blown to stable foams. Solid particles are essential for stable foams. But the way how these particles stabilise melt foams is still under dispute. Recent studies came forward with some particles stabilisation mechanisms which are listed below.

  1. Particle attachment to the gas solid interface which is important for stabilisation
  2. Presence of particles helps in drainage retardation
  3. Particles confined at the cell wall generate a mechanical barrier – a repulsive force called disjoining pressure against cell wall thinning.
  4. Self layering phenomenon of particles in the film which provides a structural barrier again bubble coalescence
  5. Partially wetted particles accumulate at bubble surface where they act as a mechanical barrier again coalescence.
  6. Stabilisation is due to particles interaction attached to one side of a cell wall. 

These mechanisms insist that the particle-particle interaction in the liquid film plays a major role in stabilisation. For producing long time stable metal foams the interaction between the particles in the liquid film should be improved. This allow us to decrease the particle size and hence the particle volume, which could be a cost effective solution. Our present study is aiming to use ultrafine particles in liquid Al at lesser volume to produce ultrastable foam. However introducing ultrafine particles in molten Al are bit difficult. Therefore in-situ formed ultrafine (ranges from 50 nm to 1 µm size) ceramic particles like TiB2, TiC or MgAl2O4 in Al is used as a foamable precursor, see figure 3. High quality foam with homogenous bubble distribution, smooth cell walls can be produced using this ultrafine particles reinforced Al foam precursors.

3. Gas Injection in a controlled atmosphere and without solid particles

Experiments of gas injected foams pursued the results of single films, whereby stable foams could only be stabilised by means of an oxygen-rich injection gas at ambient atmosphere. Was oxygen prevented by an argon-hemisphere, again no stable foam structure was possible, even by the highest achievable amount of particles.

Based up on the knowledge, that single films are stable without any kinds of particles and only by the presence of oxygen, this promising circumstance was adopted to be transferred to metallic foams. For this reasons different alloys, gases and injection methods in a controlled atmosphere are performed to improve the bubble size distribution and to develop foaming without any particles. By accomplishing this goal not only the negative recyclability of metal-ceramic composites can be avoided. It would also lead to an improvement of machinability and cost efficiency, due to a prevented production stage, which might be the missing step for a great industrial breakthrough to use metallic foams and their extraordinary mechanical properties in a wide range.

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