Sub-mm sized bubbles injected into metallic melts

Liquid metal foams containing small equally sized bubbles are expected to be stable, with high liquid fractions. We concentrate on foaming processes of Al-alloy melts following the gas injection route and review briefly the theory about bubble formation in a liquid by gas injection. Several strategies of reducing the bubble size are presented. We discuss how the size and geometry of the gas injector, the gas pressure control and the oscillation of the injector influence bubble size. We also combine some of the strategies and demonstrate that it is possible to produce sub-millimetre bubbles in liquid Al-alloy by gas injection. X-ray observation of the process allows us to evaluate in situ the changes in bubble size during foam formation in the molten state. Through metallographic cross-sections we can corroborate the bubble size distribution once cooled down and solidified. Alignment of bubbles could be observed and, therefore, even ordering could be possible in future as it is the case in aqueous foams. Finally, we compare metal foam with aqueous foams, emphasising similarities and differences, and discuss possible applications of micro cellular metal foams. (C) 2014 Elsevier B.V. All rights reserved.