Abstract:Abstract:Coalescence is the rupture of a film between two adjacent bubbles in any type of liquid foam and has pronounced influence on the development of its macrostructure after solidification, mostly leading to larger pores and a wider size distribution. Foamable AlSi6Cu4 + 0.5 wt % TiH 2 precursors made following the powder metallurgical route were foamed in a gas-tight X-ray transparent furnace under 500 kPa (5 bar) pressure with subsequent pressure release. Coalescence was investigated by fast X-ray radios… Show more
“…The rearrangement of liquid caused by coalescence disturbs the local system and forces must be acting on neighbouring bubbles that lead to a cascade of such events similar to what is known for aqueous foams 28–30 . Radioscopy on metal foams has also shown such correlated events but owing to the superposition of features along the viewing direction it is not always clear whether temporally correlated ruptures belong to the same cascade 31 .Fig. 4Evolution of a group of five bubbles in a liquid metal foam.…”
The complex flow of liquid metal in evolving metallic foams is still poorly understood due to difficulties in studying hot and opaque systems. We apply X-ray tomoscopy –the continuous acquisition of tomographic (3D) images– to clarify key dynamic phenomena in liquid aluminium foam such as nucleation and growth, bubble rearrangements, liquid retraction, coalescence and the rupture of films. Each phenomenon takes place on a typical timescale which we cover by obtaining 208 full tomograms per second over a period of up to one minute. An additional data processing algorithm provides information on the 1 ms scale. Here we show that bubble coalescence is not only caused by gravity-induced drainage, as experiments under weightlessness show, and by stresses caused by foam growth, but also by local pressure peaks caused by the blowing agent. Moreover, details of foam expansion and phenomena such as rupture cascades and film thinning before rupture are quantified. These findings allow us to propose a way to obtain foams with smaller and more equally sized bubbles.
“…The rearrangement of liquid caused by coalescence disturbs the local system and forces must be acting on neighbouring bubbles that lead to a cascade of such events similar to what is known for aqueous foams 28–30 . Radioscopy on metal foams has also shown such correlated events but owing to the superposition of features along the viewing direction it is not always clear whether temporally correlated ruptures belong to the same cascade 31 .Fig. 4Evolution of a group of five bubbles in a liquid metal foam.…”
The complex flow of liquid metal in evolving metallic foams is still poorly understood due to difficulties in studying hot and opaque systems. We apply X-ray tomoscopy –the continuous acquisition of tomographic (3D) images– to clarify key dynamic phenomena in liquid aluminium foam such as nucleation and growth, bubble rearrangements, liquid retraction, coalescence and the rupture of films. Each phenomenon takes place on a typical timescale which we cover by obtaining 208 full tomograms per second over a period of up to one minute. An additional data processing algorithm provides information on the 1 ms scale. Here we show that bubble coalescence is not only caused by gravity-induced drainage, as experiments under weightlessness show, and by stresses caused by foam growth, but also by local pressure peaks caused by the blowing agent. Moreover, details of foam expansion and phenomena such as rupture cascades and film thinning before rupture are quantified. These findings allow us to propose a way to obtain foams with smaller and more equally sized bubbles.
“…7 and considering that all the roughly 3000 events depicted in each panel were created in 600 time steps in 5 samples, one can conclude that on average only one rupture is detected in one single foam in each time step and the chance that a second one is right in the line of sight of the first is negligible. Scenario iii) is well known for aqueous foams [53,54] and rupture avalanches were recently found in liquid metal foams too [55], but they are quite rare and require a big triggering rupture event to be followed by a number of smaller ruptures mostly involving bubbles smaller than 250 µm not counted here. Therefore above basic assumption can be expected to be valid.…”
“…When the first melt leaves the mold, the worker must quickly yet carefully remove the mold containing the formed liquid metallic foam. In addition to this, the formed liquid foam must be solidified immediately to prevent the collapse mechanisms (drainage and coalescence) and must be done in a very steady manner to avoid any turbulent movements that could damage the foam [24]. This is another dangerous task in which the worker is, once again, exposed to extremely high temperatures.…”
The paper presents an automated continuous production line (7 m × 1.5 m × 1 m) of high-quality metallic foams using a powder metallurgical method. This continuous production line was used to obtain metal foam parts and/or components by heating the foamable precursor material at melting temperatures close to the temperature of the metallic matrix and cooling the formed liquid metallic foam (in liquid state), which then results in a solid closed-cell metallic foam. This automated continuous production line is composed of a continuous foaming furnace, a cooling sector and a robotic system. This installation has enabled a technological breakthrough with many improvements solving some technical problems and eliminating the risks and dangers related to the safety of workers due to the high temperatures involved in this process. The whole process becomes automatic without any need for human intervention.
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