Visualisation of metal foaming process by synchrotron X-ray radioscopy

H. Stanzick, J. Banhart, L. Helfen, T. Baumbach
Fraunhofer Institutes in Bremen and Dresden, ESRF in Grenoble
see Appl. Phys. Lett. 78, 1152 (2001)

The foaming process of metals such as aluminium or zinc seems to be similar to the foaming of other more common systems such as water. However, as the internal structure of evolving metal foams has never been observed experimentally, the understanding of this process is still incomplete and rather speculative. An improvement of this situation could be achieded recently by looking at the foam formation process of various metals and also of sandwich composites with high-resolution synchrotron radioscopy in real time. The internal structure of such foams could be monitored with a spatial resolution of 40 microns and a time resolution of 333 ms. This way expanding pores, rupturing cell walls, melting face sheets and drainage effects could be investigated.

The following picture shows a AlSi7 foam which is fully expanded and already shows some signs of collapse. To visualise the differences in contrast (and also to create a nice artistic impression) the picture has been colour-coded. Blue, e.g., shows regions of high metal density.
Click to open a window and see a larger image.

The following series of images of an expanding Al/AlSi7/Al sandwich gives a first impression of the kinetics of metal foaming, although the data has been greatly reduced in spatial and time resolution showing only 11 of the 800 original radiographs:

More details can be seen in the x-ray movie of a foaming AlSi7 alloy (size=550 kB, you need an MPG player for this).
Click on to start the foaming process of the piece of foamable aluminium you see.
The movie has a higher (yet still reduced) spatial and time resolution. The original movie contains 25 times more pictures and has the full resolution of 40 microns. Moreover, the original foaming takes much longer: the movie is in fast motion running about 20 times faster.