Gas Injection of liquid aluminium alloys

Metallic foams can be produced directly by gas injection or indirectly by foaming using a blowing agent and are mainly based on aluminium and its alloys. Both ways have their advantages depending on the desirable pore size and distribution, density and mechanical properties. One other factor is the price, whereby foams made by gas injection are way cheaper as expensive blowing agents are not being mandatory. Thus this research topic is following the liquid metal route to explore the economic potential producing non-expensive stable foams.
Our main study is concentrated on various Al-alloys containing different types and concentrations of particles. As a basic and reference the known well foamable composite F3S20S produced by Alcan, Montreal, Canada is used. This material consists of 20 vol.% SiC particles, mean particle diameter 10 µm, embedded in an AlSi9Mg0.6 alloy.

Gas injection furnace with movable injection cannula

The gas injection furnace, which we used, was constructed in-house and insist of two vertical heating plates (two heating coils per heating plate, power of each coil = 600 W) to heat up the melt to 680 °C, see the figure on the left. The heating plates are further embedded into insulating plates, to keep the heat loss at its minimum. The heating is measured and documented by a thermo couple, which is controlled via a self-developed program using the software "Labview". Perpendicular to the heating system, two X-ray transparent boron nitride plates are disposed to close the furnace and to study experiments in-situ via a microfocus X-ray source and a panel detector. The spot size is usually set to 5 µm by applying 100 kV leading to 100 µA. For injecting the gas and foaming the precursor, so far used static cannulas had several issues, like the required overpressure to break the oxide skin at the beginning of injection and the consequent possible contamination of the injection gas. Therefore a movable cannula system, powered by a stepper motor and controlled via the software "CME2" was developed to insert the cannula into the melt. The applied gas injection pressure is adjusted by a needle valve, measured by a sensor (company: MKS) and readout by "Capacitron DM 21". To investigate the influence of the oxygen level of the surrounding atmosphere, the chamber is further recessed into a gas tight steel chamber. Both oxygen concentrations of the gases (injection gas and atmosphere) can thus be controlled via an "Oxygen Service Controller II" of Prozess-Informatik GmbH, Germany. Before the foaming experiments could begin, the chamber is evacuated and purged with Argon (100 l/h) twice. Thereby moisture was removed and the same minimal oxygen content to the model system (10-8 ppm O2) can be achieved. The foam expansion process as well as the coalescence of bubbles can be recorded by the software "HiPic 7.1" and analysed via the software "ImageJ". The also the image and animation at the end of the page.

Alternative particles to stabilise foams created by gas injection

Although gas injected foams exhibit a great costeffective potential, they cannot be stabilised without the addition of stabilising particles (e.g. SiC or Al2O3), which again affect the price of the final product tremendously. Created bubbles in pure melts just rise to the surface and collapse, similar as it is known for a glass of champagne. Hitherto a minimum concentration of around 10 Vol.% SiC is required to allow foam expansion using particles of 5-20 µm in diameter. The smaller such particles are and the higher the oxygen concentration of the injected gas is the lower the required volume concentration is. But the background and understanding how such particles behave and what eventually stabilise foams is still under dispute. Ip et al. and Kumagai et al. made use of the surface tension acting as stabilisation factor of particle-reinforced foams. Former suggested that wetted particles are more useful to prevent cell wall thinning till extinction as the surface tension counteracts nearing film surfaces. A similar behaviour was also found for foams made using blowing agents. Körner et al. found that particles or oxide networks can establish a disjoining pressure inside films preventing film surfaces to merge, similar to known aqueous foam theories. The group of Kumagai followed a more theoretical approach whereby capillary suction into Plateau borders gets reduced by modifying the radius of the film by partial wetted particles. A total different starting point was published by Wübben and Banhart concerning a barrier mechanism. They showed that particles, which block cell wall channels or are captured by nearing film surfaces, establish a barrier and hinder the melt leaving the film. If such particles align ordered was theoretically proposed by Kaptay, but never approved by experimental results. All these approaches seem conceivable, but to stabilise a film effectively, and not only retards coarsening, particles have to be fixed at a certain position in the liquid state. Otherwise, viscosity is only enhanced by added particles, which decelerates drainage. However, this is sufficient to stabilise foams, made of high viscous melts by foaming using blowing agents, till solidification within only >100 s. Due to this short foaming time coalescence and coarsening are retarded and thus avoided, but long time stability cannot be given. In contrast, foams produced by gas injection using an oxidising medium (e.g. air) can be stabilised for 100 min almost without any coarsening. Why such foams are so stable was also attempted to be answered by a simpler model system, called single films. Thereby authors confirmed the importance of a present oxide layer, as it was already discussed concerning foams. Furthermore, they found that this oxide layer not only retards the retraction of a rupturing film, it even fixes particles at a certain position and hinders the rupture to expand any further. Therefore we developed different methods and composites to extend the knowledge about the role of particles, why they are required, how they act and if they have to be present anyway. Thus the present study can be seen as a first step towards particle-free foams, which might lead to an industrial breakthrough.

Audioslides of my paper: Stabilisation of aluminium foams and films by the joint action of dispersed particles and oxide films

K. Heim et al., Acta Materialia, 99:313-324, 2015