Process control in aluminum foam production using real-time x-ray radioscopy
Aluminum alloy foams were created by expanding foamable precursors containing a gas-releasing blowing agent in a dense metallic matrix. The precursors were prepared in two different ways: either by hot-compaction of powder mixtures or by thixocasting of billets obtained by cold compaction of powder blends. Foam evolution was visualized by means of real-time X-ray radioscopy with image frequencies ranging up to 18 Hz and spatial resolutions down to 10 µm. The difference in pore formation between the two processing routes could be studied. Rupture of cell walls during foam expansion could be visualized, critical rupture thickness measured, and the time-scale of the rupture process estimated. By manufacturing foam precursors in which defects were incorporated deliberately, the question of the origin of very large pores in solid metal foams could be examined. By forced cooling of liquid metal foams while recording their structure, the importance of solidification-induced changes of foam morphology was illustrated.