N. Babcsán

Fast processes in liquid metal foams investigated by high-speed synchrotron x-ray microradioscopy

Rupture of an individual film in an evolving liquid metal foam is investigated by means of high-speed x-ray radioscopy using white synchrotron radiation. At a frame rate of 5000frames∕s, the rupture event is spread over three to four images. The images show that the remnants of the rupturing film are pulled into the surrounding plateau borders in 600±100μs which conforms well with a liquid movement governed by inertia and not by viscosity. Within one order of magnitude, the viscosity of the liquid involved must be similar to the viscosity of pure liquid aluminium.

Study on aluminium-based single films

In the present paper the authors studied isolated metallic films made from the same material used for making metallic foams, and then characterised their properties. Metal films were made from a liquid aluminium alloy reinforced with ceramic particles of known concentration. Melts without such particles were also investigated. It is shown that stable films could not be made from Al-Si alloy having no particles, and just extremely thin and fragile films could be made from commercially-pure Al. In contrast, aluminium alloys containing particles such as SiC and TiB(2) allowed pulling thin, stable films, which did not rupture. Significant thinning of films was observed when the particle concentration in the melt decreased. By in situ X-ray monitoring of liquid films during pulling, film thickness and drainage effects within the liquid film could be studied. The morphology and microstructure of films was characterised after solidification. Our work shows that the question of how foams are stabilised can be studied using a simplified system such as a film, instead of having to deal with the multitude of different structural elements present in a foam.

Role of Oxide Particles in Aluminum Melt toward Aluminum Foam Fabrication by the Melt Route

The aim of this work is to elucidate the role and contribution of oxide particles to aluminum foam fabrication. The melts were internally oxidized by a thickening process in which pure aluminum melt was stirred with or without the addition of 1.5 wt.% calcium for maximum 25 min. After this, each thickened samples were melted again and mixed for 100 s by introducing 1.5 wt.% TiH2 as a blowing agent. In order to investigate the foam evolution, the foam samples were hold in the furnace for 50 to 500 s. The stirring torque (viscosity) of the calcium containing melt increases with thickening time and achieves the stationary value after 17 min. However, the torque of pure aluminum melt does not change during stirring. Oxides have been found on the microstructures of both stirred samples, although the content of oxides of calcium added sample is significantly more than that of pure aluminum. SEM observation results of samples thickened by calcium addition show that the melt contains calcium oxide and Al4Ca in addition to equiaxed aluminum, and the morphology of formed oxide is not granulous but wrinkled bifilm containing calcium and aluminum oxides. The oxides formed in the pure Al melt has less effect on the viscosity thus the foamability of the aluminum melt. It is found that the calcium oxides formed by stirring are responsible for the effective increase of melt viscosity. The foams using oxidized pure Al melt have dense layer at the bottom caused by drainage and coarse foam structure due to strong coalescence. In case of the Al-Ca alloy, uniform pore distribution, lack of the dense layer and homogeneous time dependent increase of the cell size were observed. Besides, the sample held for longer time has thicker cell wall at the bottom compared with that at the top. We confirmed that the oxide bifilms of Al and Ca contributes to decreased drainage rate and coalescence, namely stabilization. The insufficient amount of oxide particles in pure aluminum is the reason for the lack of stabile foam (significant drainage) in that case.

Liquid-Metal Foams – Feasible In Situ Experiments under Low Gravity

Metal foams are quite a challenge to materials scientists due to their difficult manufacturing. In all processes the foam develops in the liquid or semiliquid state. Liquid-metal foams are complex fluids which contain liquid metals, solid particles and gas bubbles at the same time. An X-ray transparent furnace was developed to monitor liquid metal foam evolution. Aluminium foams - similar to the commercial Metcomb foams - were produced by feeding argon or air gas bubbles into an aluminium composite melt. The foam evolution was observed in-situ by X-ray radioscopy under normal gravity. Drainage and rupture were evaluated during the 5 min foam decay and 2 min solidification. Argon blown foams showed significant drainage and cell wall rupture during the first 20 s of foam decay. Air blown foams were stable and neither drainage nor rupture occurred. We demonstrated the feasibility of experiments during parabolic flight or drop tower campaigns. However, the development of a foam generator for low gravity is needed.