Francisco Garcia-Moreno (Helmholtz-Zentrum Berlin / TU Berlin)
The structure of foams: An experimental tomographic study
Large pores in small foam components lead to pronounced statistical scatter of properties, while isolated centimetre-long cavities may even be a source of structure/mechanical failure. The frequent occurrence of such pores is a drawback for commercial applications. Therefore, it is desirable to have a more uniform cell size distribution and smaller cell sizes, preferably in the sub-millimetre range.
In this presentation we concentrate on the foaming processes of aqueous liquids and Al alloy melts following the gas injection route, and the foam structures that can be obtained. Several strategies of reducing the bubble size by gas injection techniques will be presented. Metal foams containing small equally sized bubbles are expected to be more stable in the liquid, with high liquid fractions. Alignment or even ordering of pores could be possible as is the case in aqueous foams, leading to new properties of the solid metal foam components. We discuss how the geometry of the gas injector, the gas pressure control and the vibration of the injector influence bubble size.
Time-resolved X-ray tomographic observation of the dynamic process, called tomoscopy, allows us to evaluate in-situ the foam formation and changes in bubble size during foam evolution. In the case of Al-based foams, once cooled and solidified, tomography allows the bubble size distribution to be measured and their structure characterised.
Finally, we compare the structure of metal with aqueous foams, emphasising similarities and differences, and discuss possible applications of micro cellular metal foams.
(Joint work with Paul Hans Kamm (Helmholtz-Zentrum Berlin) and Tillmann Robert Neu (TU Berlin).)