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Experiments on metallic foams under gravity and microgravity
Colloids and Surfaces A: Physicochem. Eng. Aspects 344 (2009) 101–106
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Experiments on metallic foams under gravity and microgravity Francisco García-Moreno a,b,∗ , Catalina Jiménez a,b , Manas Mukherjee a,b , Per Holm c , Jörg Weise d , John Banhart a,b a
Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany Helmholtz-Zentrum Berlin, Glienicker Strasse 100, 14109 Berlin, Germany c Swedish Space Corporation, Solna, Strandväg 86, 17104 Solna, Sweden d Fraunhofer Institut für Fertigungstechnik und Materialforschung, 28359 Bremen, Germany b
a r t i c l e
i n f o
Article history: Received 29 October 2008 Received in revised form 2 March 2009 Accepted 3 March 2009 Available online 17 March 2009 Keywords: Metal foam X-ray radioscopy Microgravity Drainage Parabolic flight
a b s t r a c t Aluminium foams were created on ground and under microgravity conditions and were allowed to evolve. The entire foaming process was monitored by in situ X-ray radioscopy, allowing for flow and drainage of the liquid of the foam to be monitored and quantified. A foam with a known low stability level was used to enhance the effects of microgravity. The flow of metal out of the foam under gravity during the 1.8 g phase was found to be fast, indicating a mobile liquid. The density profile evolved in a similar way as in aqueous foams. Reentry of the liquid melt into the foam was also observed. Under microgravity the foam re-established a uniform density and pore size distribution after nearly complete imbibition of the liquid. This also shows that the liquid is mobile. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal foams are presently materials that are produced industrially for a small market. In spite of the beginning commercialisation there are still many fundamental questions that have to be clarified to understand the phenomena governing growth and stabilisation of such foams. Gravity-induced drainage is a general problem in any type of foam and especially in metal foams due to both their higher density and liquid fraction as compared to aqueous foams. Drainage can induce undesirable density variations in the foam and also trigger film rupture. In particular for the study of phenomena such as coalescence rate, capillarity force, etc., where the foams have to be held in the liquid state for extended times, drainage is disturbing. The use of microgravity is the key to investigate less disturbed foams. Studies under microgravity were already performed on aqueous [1–5] and lead foams [6]. Gravity variations between 0 g and 1.8 g provided by parabolic flight campaigns (PFCs) allow us to analyse the response of liquid metal foams during gravitational changes, giving indications about the real viscosity of the system and the stability of films. X-ray radioscopy is a useful diagnostic tool for monitoring liquid metal foams in situ and it was a technological challenge to adapt
∗ Corresponding author at: Helmholtz-Zentrum Berlin, Materials, Glienicker Strasse 100, 14109 Berlin, Germany. Tel.: +49 30 80622761; fax: +49 30 80623059. E-mail address: [email protected] (F. García-Moreno). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.03.010
such a system to microgravity conditions and to study Al-based metallic foams for the first time. Previous work concentrated on lead and did not include in situ diagnostics [6]. The powder metallurgical (PM) foaming route – having been a successful production method for metallic foams for a long time [7] – is well suited for microgravity experiments due to the simplicity of the foaming step that requires only the heating of a solid precursor, compared to other metal foaming methods such as gas injection into a melt that involve the handling of liquid metals. Our first objective was to test the foaming system on ground to see the influence of gravitational conditions on foam expansion. In a second step, we aimed at preparing the system for a PFC and studying foam expansion and evolution under microgravity conditions, thus observing the effects of gravitational transition on foam structure and liquid fraction. Our third objective was to test the hardware under microgravity conditions for the foreseen sounding rocket campaign Maser 11. 2. Experimental 2.1. Sample preparation Foamable precursors having the nominal alloy composition AlSi6Cu4 (in wt.%) were prepared by blending elemental metallic powders with 0.6 wt.% of TiH2 acting as a blowing agent and subsequently compacting the powder mixture by cold isostatic pressing. The billets obtained were heated up to the semi-solid state, pressed
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Fig. 1. Metal foam setup for microgravity experiments, comprising a microfocus X-ray source, a furnace and a detector. Left: CAD drawing, right: photograph of the furnace.
into a die and allowed to solidify there. This method for precursor preparation by semi-solid (thixo)casting is explained in more detail in the literature [8,9]. Samples of 19.8 mm × 9.8 mm × 5 mm size were machined from the precursors to fit into the crucible. Two grooves on each precursor side were rasped in to hold them at the corresponding tongue in the crucible. 2.2. Foaming setup The microgravity foaming setup comprised a 80 kV microfocus source with 5 m spot size (Hamamatsu, Japan), a foaming furnace (SSC, Sweden) and a flat panel detector (120 mm × 120 mm, 12 bit dynamic range, 1 Hz image acquisition rate, exposure time set to 700 ms, 50 m pixel size), also from Hamamatsu, is shown in Fig. 1, left. All recorded images therefore represent a time interval of 700 ms. A photograph of the opened foaming furnace is shown in Fig. 1, right. The furnace is nearly X-ray transparent, with Al foil (2 × 0.3 mm), Ti foil (2 × 25 m) and again Al foil (2 × 17 m) serving as windows and IR radiation shielding in the direction of the X-rays. Inside the heating zone the crucible with inner dimensions of 20 mm × 20 mm × 10 mm is situated. It was machined from one piece of boron nitride (BN) together with a closing lid (2 walls, each 2 mm thick). Inside the crucible a piece of the foamed sample can be seen. The crucible has a tongue for holding the precursor. A Pt wire is wound around the BN crucible and allows us to heat up the sample up to 700 ◦ C. Four thermocouples are placed on each of the four sides of the crucible lateral to the X-ray direction to record the temperature profile in the sample. Cooling of the crucible during solidification is performed by blowing pressurised gas from an argon gas bottle onto the crucible through metallic tubes. Forced gas cooling is needed due to the short time of microgravity available. The system has low thermal mass and good thermal insulation. A gas outlet ensures constant gas pressure inside the crucible and serves as overflow for liquid metal in case of emergency. The gravity level in all three axes is recorded close to the furnace by three acceleration sensors. The entire system is shielded by a lead casing and mounted in a metallic frame damped against vibrations. The system was also used for reference experiments on the ground. Quantitative density analysis of X-ray images uses Beer–Lambert’s law of attenuation and neglects X-ray absorption of the gas contained in the bubbles [10]. The terms ‘relative density’ and ‘liquid fraction’ reflect the integral X-ray absorption by the metal in the solid and liquid states, respectively. The density of the solid precursor is set to 100%. Due to the symmetry given by the foaming direction the absorption values are integrated over a large number of bubbles in y-direction (perpendicular to gravity) and plotted as a function of foam height. For the PFC the setup was fixed to a metallic frame covered with soft protection (Fig. 2). All other control elements were placed on a separated frame to facilitate operation.
2.3. Foaming procedure Microgravity experiments were performed during the 46th ESA PFC in the specially conditioned ‘zero-g’ Airbus A300 operated by Novespace, France. As during the campaign the exchange of samples was not allowed due to safety regulations, a set of six exchangeable identical furnaces was used into which the samples had been loaded before the flight. The furnace exchange could be performed during flight in less than 1 min. The sample foaming procedure and the heating profile from the starting point of heating until the switching off of the heater was completely automatic and followed the profile pre-set before each experiment. Forced gas cooling was activated optionally. In this way, experimental reproducibility was ensured and the conditions resembling a sounding rocket experiment could be simulated. Reference experiments were carried out on ground with the identical setup. In addition, the furnace was tested and used for ground-based experiments using the X-ray lab-equipment described in the literature [11]. During each parabola around 20–22 s of microgravity were achieved, preceded and followed by periods during which around 1.8 g acceleration occurred. Fig. 3 gives an overview of the gravity level sequence and the foaming stages during a single parabola. The begin of sample heating was synchronised with the initiation of the previous parabola. The time between two parabolae was 3 min. During a series of five parabolae one single foaming experiment was performed. A constant heating rate of 3.66 K/s up to 350 ◦ C and of 2.77 K/s from 350 ◦ C to 650 ◦ C was employed. Foam expands during the first 1.8 g phase of each parabola with a velocity of ∼1–1.5 mm/s, that means it takes around 10–15 s to fully expand and fill the crucible. To adjust the foaming conditions to the desired gravity level,
Fig. 2. Experiment in the aircraft ready for the parabolic flight. Left: X-ray imaging facility, right: control unit.
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Fig. 3. Evolution of liquid metal foam during a parabola visualised by various X-ray radiographs. The temperature course and the gravity levels are also given.
adjustments in form of time offsets of ±5 s or ±10 s could be chosen during flight that shifted the onset of foaming to an earlier or a later time.
sample foamed under ‘+1 g’ collapsed more than the one foamed under ‘−1 g’.
3.2. Microgravity experiments 3. Results 3.1. Ground-based experiments Different ways of fixing the sample to the crucible were tested in order to provide good and reproducible thermal contact during microgravity experiments. Examples include a metallic spring, mechanical fixing with a tongue in the crucible or gluing the samples to the crucible with boron nitride (BN) paste. Foaming with the precursor fixed to the bottom or to the top of the crucible was compared under similar conditions (Fig. 4). Fixing the sample with two lateral slits and gluing it to the crucible with BN paste were found to be appropriate. In this way, the samples held at the top of the crucible started foaming at the same time as the ones fixed to the bottom. In contrast, a tantalum wire used as spring was found to disturb the X-ray images, to be unreliable and to induce temperature gradients in the sample. Therefore, only mechanical fixing and gluing were used. The heating profile for the parabolic fight was optimised on ground to be fast and reproducible (start of foaming within ±1 s of the target time) as already described in Section 2.3. The temperature within the expanded foam in the liquid state was measured to be stable by ±1 K. Foaming from top to bottom of the crucible is referred to as foaming under ‘−1 g’, meaning that gravity and foam expansion are in the same direction, which is different from the usual condition, called ‘+1 g’ in the following. Foam expansion experiments with thixocast AlSi6Cu4 samples fixed at the top (‘−1 g’) and at the bottom (‘+1 g’) were conducted under identical conditions. We found that while the starting point of foaming was the same, the sample under ‘−1 g’ filled the crucible faster, with an expansion velocity which was about 25% higher, as it is discernable in Fig. 4. For t = 180 s, the foam expanded under ‘−1 g’ almost filled the crucible, whereas the ‘+1 g’ it filled only ∼75% of it. From the X-ray images we can also observe that the density and pore size distribution of the solidified foam samples (at t = 400 s) is not same for the two conditions. The ‘−1 g’ sample has two denser regions, at the top and at the bottom however the ‘+1 g’ sample has only one at the bottom. Furthermore, pore sizes in the ‘−1 g’ sample are larger than in the ‘+1 g’ one and the
The automatic foaming procedure was successfully used to produce foam during the parabolae without the requirement of much operator action. The start of microgravity could be reached with fully expanded liquid metal foams. Foaming started 5 s earlier compared to the reference experiments on ground. Accordingly, the heating ramp was adjusted from 2.77 K/s to 2.63 K/s to reach the desired fully expanded condition during microgravity. During the 1.8 g period, foaming started homogeneously as on ground, but the strong gravity induced drainage more rapidly as shown in Fig. 3 (second image from left). Note that the term ‘drainage’ used in this work is referred to gravity-induced effects and does not consider other drainage mechanisms, e.g. by capillary forces. In Fig. 5a, a quantitative analysis of foam density evolution with time is plotted. Time starts at 550 ◦ C, around 20 s before the first 1.8 g phase. The mean liquid fraction of the uniform foam was around 40%. Almost 100% liquid fraction were found in the drained bottom region. During and after the transition from 1.8 g to 0 g, the liquid metal flowed back into the cellular structure towards the upper part of the crucible. This imbibition induced a homogeneous liquid fraction distribution in the foam during the microgravity period (Figs. 3 and 5). The foam volume continued expanding during imbibition beyond the volume increase one would expect from the inflow of fluid as the blowing agent was still active. At the end of the microgravity phase, an even stronger drainage appeared on the change to 1.8 g, leading to an increase from 40% to almost 100% liquid fraction on the bottom of the sample within a few seconds. This can be observed even more clearly in Fig. 6, where the liquid fraction at the bottom of a AlSi6Cu4 foam and the corresponding gravity level are plotted as a function of time. In this figure, the starting point of the 1.8–0 g transition is set to t = 0 s. The temperature (T = 650 ◦ C) was kept constant throughout this period. During the following period – lasting about 20 s – the liquid fraction increased in the bottom 5 mm of the sample until the foam solidified. It could be observed that especially the top part of the foam that still did not contact the crucible dried quickly, changing the appearance of the foam from uniformly convex to a structure exhibiting many protruding individual bubbles.
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Fig. 4. Foaming and drainage evolution of thixocast AlSi6Cu4 precursors foamed opposed to gravity ‘+1 g’ (left) and parallel to gravity direction ‘−1 g’ (right). Images for t = 0, 180, 250 and 400 s after initiation of heating are given. The grey arrows indicate foaming direction, the black arrows gravity direction.
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a precursor volume of Vp = 2 cm × 1 cm × 0.5 cm = 1 cm3 and a density = 2.85 g/cm3 for AlSi6Cu4 at room temperature, the resulting force is F ≈ 28 mN. The force difference between foaming under ‘−1 g’ and ‘+1 g’ conditions is then 2F ≈ 56 mN, high enough to lead to the difference. Still, the liquid metal does not fall out of the foam as it is held in it by the stabilising outer oxide layer and the inner oxides [14,15]. The very good temperature stability achieved (±1 K) after foam expansion and reproducible temperature profiles validated the hardware for microgravity experiments. 4.2. Microgravity experiments
Fig. 5. Density profile of a AlSi6Cu4 + 0.6 wt.% TiH2 foam during a parabola as a function of time. Gravity-induced drainage and melt imbibition into the foam are visible.
During the parabolic flights, foaming started 5 s earlier than on ground. A possible explanation is that the foaming temperature is reached earlier by the precursor due to an improved thermal contact between precursor and crucible during the early foaming stages taking place at 1.8 g. The strong gravity-induced drainage observed is probably a consequence of a reduced amount of stabilising oxides present in the precursor, caused by the specific manufacturing process in the semi-solid state. It is known that foam made from precursor manufactured by this production route exhibits drainage [8,9]. The fact that the drainage effect is very pronounced here was one reason for selecting this material instead of uniaxially compressed precursors that are much more stable due to the enclosed oxide films, as it promises to reveal more pronounced gravity phenomena in the limited microgravity time available. The observed fast liquid flow shows that the melt within the metallic films and Plateau borders is mobile and not a viscous gel as has been sometimes assumed. This mobile behaviour has recently been investigated in uniaxially pressed powders [16]. During the early foaming stage the increased drainage found at the bottom of the foam compared to normal conditions is explained by the first 1.8 g phase of a parabola. Imbibition of the liquid metal initially drained out of the foam back into the films and Plateau borders occurred during the transition from 1.8 g to microgravity, showing how capillary forces in the films and Plateau borders dominate at this stage. As a consequence, a liquid fraction of around 40% well distributed over the entire foam is observed in the X-ray images as well as a more uniform bubble size distribution. Dur-
Fig. 6. Liquid fraction at the bottom of a AlSi6Cu4 foam held at T = 650 ◦ C and gravity level during a parabola.
4. Discussion 4.1. Ground-based experiments Metal samples foamed under gravity have a reasonably good thermal contact to the crucible due to their own weight, so that heat flow into the sample is reproducible. Under microgravity, samples that are not clamped would levitate inside the crucible, leading to poor and irreproducible heating. The temperature profile and distribution in the precursor is very important for metal foam production due to the pronounced temperature sensitivity of the TiH2 decomposition [12,13]. Therefore, the two methods proposed here, namely holding with a groove and tongue on the side and gluing with X-ray transparent BN paste were both used to ensure uniform foaming while not interfering with X-ray imaging. The increase in foam expansion by ∼25% until filling the crucible under ‘−1 g’ conditions – visible in Fig. 4 – can be explained by the additional gravity force acting on the melt in parallel to foaming. For
Fig. 7. Liquid fraction as a function of foam height for a free drainage experiment under varying gravity level. Curves correspond to different times after transition from 0 g to 1.8 g. The horizontal broken line shows the average liquid fraction (∼40%) of the foam under microgravity.
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ing and after the subsequent transition from microgravity to 1.8 g, a high flow of liquid to the bottom of the sample can be observed, leading to high drainage. A detailed view of the liquid fraction distribution for different times is given in Fig. 7. This liquid fraction curve corresponds to the predicted and measured shape for liquid aqueous foams [17], although here we have changing gravity conditions. This demonstrates that there are a large number of similarities concerning drainage in both aqueous and metal foams. This gives us the chance to perform quantitative analyses and predictions using the existing theories. 5. Conclusions • The first successful microgravity experiments on liquid Al foam with in situ X-ray diagnostics were carried out on parabolic flights. • In ground-based foaming experiments with either parallel and anti-parallel orientation of gravity and foaming direction, a 25% increase of the foaming velocity for parallel orientation (‘−1 g’) was observed. • Strong drainage at 1.8 g was found, followed by complete imbibition of drained liquid metal into the foam after the onset of microgravity, the rate of both points at a highly mobile liquid. • Imbibition leads to more uniform density, pore size and shape distributions and to a average liquid fractions around 40%. • Strong and fast liquid flow during transition from 0 g to 1.8 was observed. • Vertical liquid fraction profiles for free drainage under varying gravity conditions resemble those of aqueous foams qualitatively. 6. Outlook No difference in the rate of film rupture in liquid metal foams was found comparing microgravity and normal gravity conditions. However, this was under conditions of insufficient time for reliable statistics. A longer experiment such as on a sounding rocket flight will be needed for that purpose. The actual parabolic flight campaign was also a successful verification of the hardware for the Maser 11 sounding rocket campaign scheduled for May 2008, where the bubble coalescence rate and corresponding pore size distribution under microgravity is the prime purpose of study. Acknowledgements Funding by ESA (Projects AO-99-075 and AO-2004-46) is gratefully acknowledged. We also thank Novespace and the Swedish
Space Corporation for the support during the 46th ESA Parabolic Flight Campaign. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2009.03.010. References [1] A. Saint-Jalmes, S. Marze, M. Safouane, D. Langevin, Foam experiments in parabolic flights: development of an ISS facility and capillary drainage experiments, Microgravity Science and Technology XVIII-1 (2006) 22–30. [2] J. Banhart, F. García-Moreno, S. Hutzler, D. Langevin, L. Liggieri, R. Miller, A. Saint Jalmes, D. Weaire, Foams and emulsions in space, Europhysics News 39 (2008) 26–28. [3] C. Monnereau, M. Vignes-Adler, B. Kronberg, Influence of gravity on foams, Journal of Chemical Physics 96 (1999) 958–967. [4] H. Caps, H. Decauwer, M.L. Chevalier, G. Soyez, M. Ausloos, N. Vandewalle, Foam imbibition in microgravity—an experimental study, European Physical Journal B 33 (2003) 115–119. [5] D.A. Noever, R.J. Cronise, Weightless eightless bubble lattices—a case of froth wicking, Physics of Fluids 6 (1994) 2493–2500. [6] T. Wübben, H. Stanzick, J. Banhart, S. Odenbach, Stability of metallic foams studied under microgravity, Journal of Physics: Condensed Matter 15 (2003) 427–433. [7] J. Banhart, Manufacturing routes for metallic foams, Journal of Metals 52 (2000) 22–27. [8] M. Haesche, J. Weise, F. Garcia-Moreno, J. Banhart, Influence of particle additions on the foaming behaviour of AlSi11/TiH2 composites made by semi-solid processing, Materials Science and Engineering A 480 (2008) 283–288. [9] J. Weise, O. Marchetto, M. Haesche, F. Garcia-Moreno, J. Banhart, Influence of alloying additions on foaming behaviour of thixocast AlSi11 precursor, in: H. Nakajima, N. Kanetake (Eds.), Porous Metals and Metal Foaming Technology, Japan Institute of Metals, 2006, pp. 123–128. [10] G.S. Vinod Kumar, F. García-Moreno, N. Babcsán, A.H. Brothers, B.S. Murty, J. Banhart, Study on aluminium-based single films, Physical Chemistry and Chemical Physics 9 (2007) 6415–6425. [11] F. García Moreno, M. Fromme, J. Banhart, Real-time X-ray radioscopy on metallic foams using a compact micro-focus source, Advanced Engineering Materials 6 (2004) 416–420. [12] F. von Zeppelin, M. Hirscher, H. Stanzick, J. Banhart, Desorption of hydrogen from blowing agents used for foaming metals, Composite Science and Technology 63 (2003) 2293–2300. [13] B. Matijasevic-Lux, J. Banhart, S. Fiechter, O. Görke, N. Wanderka, Modification of titanium hydride for improved aluminium foam manufacture, Acta Materialia 54 (2006) 1887–1900. [14] C. Körner, M. Arnold, R. Singer, Metal foam stabilization by oxide network particles, Materials Science and Engineering A 396 (2005) 28–40. [15] A. Dudka, F. García-Moreno, N. Wanderka, J. Banhart, Structure and distribution of oxides in aluminium foam, Acta Materialia 56 (2008) 3990–4001. [16] F. García-Moreno, A. Rack., L. Helfen, T. Baumbach, S. Zabler, N. Babcsán, J. Banhart, T. Martin, C. Ponchut, M. Di Michiel, Fast processes in liquid metal foams investigated by high-speed synchrotron X-ray micro-radioscopy, Applied Physics Letters 92 (2008) 134104. [17] D. Weaire, S. Hutzler, The Physics of Foams, Oxford University Press, Oxford, 1999.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Experiments on metallic foams under gravity and microgravity Francisco García-Moreno a,b,∗ , Catalina Jiménez a,b , Manas Mukherjee a,b , Per Holm c , Jörg Weise d , John Banhart a,b a
Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany Helmholtz-Zentrum Berlin, Glienicker Strasse 100, 14109 Berlin, Germany c Swedish Space Corporation, Solna, Strandväg 86, 17104 Solna, Sweden d Fraunhofer Institut für Fertigungstechnik und Materialforschung, 28359 Bremen, Germany b
a r t i c l e
i n f o
Article history: Received 29 October 2008 Received in revised form 2 March 2009 Accepted 3 March 2009 Available online 17 March 2009 Keywords: Metal foam X-ray radioscopy Microgravity Drainage Parabolic flight
a b s t r a c t Aluminium foams were created on ground and under microgravity conditions and were allowed to evolve. The entire foaming process was monitored by in situ X-ray radioscopy, allowing for flow and drainage of the liquid of the foam to be monitored and quantified. A foam with a known low stability level was used to enhance the effects of microgravity. The flow of metal out of the foam under gravity during the 1.8 g phase was found to be fast, indicating a mobile liquid. The density profile evolved in a similar way as in aqueous foams. Reentry of the liquid melt into the foam was also observed. Under microgravity the foam re-established a uniform density and pore size distribution after nearly complete imbibition of the liquid. This also shows that the liquid is mobile. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Metal foams are presently materials that are produced industrially for a small market. In spite of the beginning commercialisation there are still many fundamental questions that have to be clarified to understand the phenomena governing growth and stabilisation of such foams. Gravity-induced drainage is a general problem in any type of foam and especially in metal foams due to both their higher density and liquid fraction as compared to aqueous foams. Drainage can induce undesirable density variations in the foam and also trigger film rupture. In particular for the study of phenomena such as coalescence rate, capillarity force, etc., where the foams have to be held in the liquid state for extended times, drainage is disturbing. The use of microgravity is the key to investigate less disturbed foams. Studies under microgravity were already performed on aqueous [1–5] and lead foams [6]. Gravity variations between 0 g and 1.8 g provided by parabolic flight campaigns (PFCs) allow us to analyse the response of liquid metal foams during gravitational changes, giving indications about the real viscosity of the system and the stability of films. X-ray radioscopy is a useful diagnostic tool for monitoring liquid metal foams in situ and it was a technological challenge to adapt
∗ Corresponding author at: Helmholtz-Zentrum Berlin, Materials, Glienicker Strasse 100, 14109 Berlin, Germany. Tel.: +49 30 80622761; fax: +49 30 80623059. E-mail address: [email protected] (F. García-Moreno). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.03.010
such a system to microgravity conditions and to study Al-based metallic foams for the first time. Previous work concentrated on lead and did not include in situ diagnostics [6]. The powder metallurgical (PM) foaming route – having been a successful production method for metallic foams for a long time [7] – is well suited for microgravity experiments due to the simplicity of the foaming step that requires only the heating of a solid precursor, compared to other metal foaming methods such as gas injection into a melt that involve the handling of liquid metals. Our first objective was to test the foaming system on ground to see the influence of gravitational conditions on foam expansion. In a second step, we aimed at preparing the system for a PFC and studying foam expansion and evolution under microgravity conditions, thus observing the effects of gravitational transition on foam structure and liquid fraction. Our third objective was to test the hardware under microgravity conditions for the foreseen sounding rocket campaign Maser 11. 2. Experimental 2.1. Sample preparation Foamable precursors having the nominal alloy composition AlSi6Cu4 (in wt.%) were prepared by blending elemental metallic powders with 0.6 wt.% of TiH2 acting as a blowing agent and subsequently compacting the powder mixture by cold isostatic pressing. The billets obtained were heated up to the semi-solid state, pressed
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Fig. 1. Metal foam setup for microgravity experiments, comprising a microfocus X-ray source, a furnace and a detector. Left: CAD drawing, right: photograph of the furnace.
into a die and allowed to solidify there. This method for precursor preparation by semi-solid (thixo)casting is explained in more detail in the literature [8,9]. Samples of 19.8 mm × 9.8 mm × 5 mm size were machined from the precursors to fit into the crucible. Two grooves on each precursor side were rasped in to hold them at the corresponding tongue in the crucible. 2.2. Foaming setup The microgravity foaming setup comprised a 80 kV microfocus source with 5 m spot size (Hamamatsu, Japan), a foaming furnace (SSC, Sweden) and a flat panel detector (120 mm × 120 mm, 12 bit dynamic range, 1 Hz image acquisition rate, exposure time set to 700 ms, 50 m pixel size), also from Hamamatsu, is shown in Fig. 1, left. All recorded images therefore represent a time interval of 700 ms. A photograph of the opened foaming furnace is shown in Fig. 1, right. The furnace is nearly X-ray transparent, with Al foil (2 × 0.3 mm), Ti foil (2 × 25 m) and again Al foil (2 × 17 m) serving as windows and IR radiation shielding in the direction of the X-rays. Inside the heating zone the crucible with inner dimensions of 20 mm × 20 mm × 10 mm is situated. It was machined from one piece of boron nitride (BN) together with a closing lid (2 walls, each 2 mm thick). Inside the crucible a piece of the foamed sample can be seen. The crucible has a tongue for holding the precursor. A Pt wire is wound around the BN crucible and allows us to heat up the sample up to 700 ◦ C. Four thermocouples are placed on each of the four sides of the crucible lateral to the X-ray direction to record the temperature profile in the sample. Cooling of the crucible during solidification is performed by blowing pressurised gas from an argon gas bottle onto the crucible through metallic tubes. Forced gas cooling is needed due to the short time of microgravity available. The system has low thermal mass and good thermal insulation. A gas outlet ensures constant gas pressure inside the crucible and serves as overflow for liquid metal in case of emergency. The gravity level in all three axes is recorded close to the furnace by three acceleration sensors. The entire system is shielded by a lead casing and mounted in a metallic frame damped against vibrations. The system was also used for reference experiments on the ground. Quantitative density analysis of X-ray images uses Beer–Lambert’s law of attenuation and neglects X-ray absorption of the gas contained in the bubbles [10]. The terms ‘relative density’ and ‘liquid fraction’ reflect the integral X-ray absorption by the metal in the solid and liquid states, respectively. The density of the solid precursor is set to 100%. Due to the symmetry given by the foaming direction the absorption values are integrated over a large number of bubbles in y-direction (perpendicular to gravity) and plotted as a function of foam height. For the PFC the setup was fixed to a metallic frame covered with soft protection (Fig. 2). All other control elements were placed on a separated frame to facilitate operation.
2.3. Foaming procedure Microgravity experiments were performed during the 46th ESA PFC in the specially conditioned ‘zero-g’ Airbus A300 operated by Novespace, France. As during the campaign the exchange of samples was not allowed due to safety regulations, a set of six exchangeable identical furnaces was used into which the samples had been loaded before the flight. The furnace exchange could be performed during flight in less than 1 min. The sample foaming procedure and the heating profile from the starting point of heating until the switching off of the heater was completely automatic and followed the profile pre-set before each experiment. Forced gas cooling was activated optionally. In this way, experimental reproducibility was ensured and the conditions resembling a sounding rocket experiment could be simulated. Reference experiments were carried out on ground with the identical setup. In addition, the furnace was tested and used for ground-based experiments using the X-ray lab-equipment described in the literature [11]. During each parabola around 20–22 s of microgravity were achieved, preceded and followed by periods during which around 1.8 g acceleration occurred. Fig. 3 gives an overview of the gravity level sequence and the foaming stages during a single parabola. The begin of sample heating was synchronised with the initiation of the previous parabola. The time between two parabolae was 3 min. During a series of five parabolae one single foaming experiment was performed. A constant heating rate of 3.66 K/s up to 350 ◦ C and of 2.77 K/s from 350 ◦ C to 650 ◦ C was employed. Foam expands during the first 1.8 g phase of each parabola with a velocity of ∼1–1.5 mm/s, that means it takes around 10–15 s to fully expand and fill the crucible. To adjust the foaming conditions to the desired gravity level,
Fig. 2. Experiment in the aircraft ready for the parabolic flight. Left: X-ray imaging facility, right: control unit.
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Fig. 3. Evolution of liquid metal foam during a parabola visualised by various X-ray radiographs. The temperature course and the gravity levels are also given.
adjustments in form of time offsets of ±5 s or ±10 s could be chosen during flight that shifted the onset of foaming to an earlier or a later time.
sample foamed under ‘+1 g’ collapsed more than the one foamed under ‘−1 g’.
3.2. Microgravity experiments 3. Results 3.1. Ground-based experiments Different ways of fixing the sample to the crucible were tested in order to provide good and reproducible thermal contact during microgravity experiments. Examples include a metallic spring, mechanical fixing with a tongue in the crucible or gluing the samples to the crucible with boron nitride (BN) paste. Foaming with the precursor fixed to the bottom or to the top of the crucible was compared under similar conditions (Fig. 4). Fixing the sample with two lateral slits and gluing it to the crucible with BN paste were found to be appropriate. In this way, the samples held at the top of the crucible started foaming at the same time as the ones fixed to the bottom. In contrast, a tantalum wire used as spring was found to disturb the X-ray images, to be unreliable and to induce temperature gradients in the sample. Therefore, only mechanical fixing and gluing were used. The heating profile for the parabolic fight was optimised on ground to be fast and reproducible (start of foaming within ±1 s of the target time) as already described in Section 2.3. The temperature within the expanded foam in the liquid state was measured to be stable by ±1 K. Foaming from top to bottom of the crucible is referred to as foaming under ‘−1 g’, meaning that gravity and foam expansion are in the same direction, which is different from the usual condition, called ‘+1 g’ in the following. Foam expansion experiments with thixocast AlSi6Cu4 samples fixed at the top (‘−1 g’) and at the bottom (‘+1 g’) were conducted under identical conditions. We found that while the starting point of foaming was the same, the sample under ‘−1 g’ filled the crucible faster, with an expansion velocity which was about 25% higher, as it is discernable in Fig. 4. For t = 180 s, the foam expanded under ‘−1 g’ almost filled the crucible, whereas the ‘+1 g’ it filled only ∼75% of it. From the X-ray images we can also observe that the density and pore size distribution of the solidified foam samples (at t = 400 s) is not same for the two conditions. The ‘−1 g’ sample has two denser regions, at the top and at the bottom however the ‘+1 g’ sample has only one at the bottom. Furthermore, pore sizes in the ‘−1 g’ sample are larger than in the ‘+1 g’ one and the
The automatic foaming procedure was successfully used to produce foam during the parabolae without the requirement of much operator action. The start of microgravity could be reached with fully expanded liquid metal foams. Foaming started 5 s earlier compared to the reference experiments on ground. Accordingly, the heating ramp was adjusted from 2.77 K/s to 2.63 K/s to reach the desired fully expanded condition during microgravity. During the 1.8 g period, foaming started homogeneously as on ground, but the strong gravity induced drainage more rapidly as shown in Fig. 3 (second image from left). Note that the term ‘drainage’ used in this work is referred to gravity-induced effects and does not consider other drainage mechanisms, e.g. by capillary forces. In Fig. 5a, a quantitative analysis of foam density evolution with time is plotted. Time starts at 550 ◦ C, around 20 s before the first 1.8 g phase. The mean liquid fraction of the uniform foam was around 40%. Almost 100% liquid fraction were found in the drained bottom region. During and after the transition from 1.8 g to 0 g, the liquid metal flowed back into the cellular structure towards the upper part of the crucible. This imbibition induced a homogeneous liquid fraction distribution in the foam during the microgravity period (Figs. 3 and 5). The foam volume continued expanding during imbibition beyond the volume increase one would expect from the inflow of fluid as the blowing agent was still active. At the end of the microgravity phase, an even stronger drainage appeared on the change to 1.8 g, leading to an increase from 40% to almost 100% liquid fraction on the bottom of the sample within a few seconds. This can be observed even more clearly in Fig. 6, where the liquid fraction at the bottom of a AlSi6Cu4 foam and the corresponding gravity level are plotted as a function of time. In this figure, the starting point of the 1.8–0 g transition is set to t = 0 s. The temperature (T = 650 ◦ C) was kept constant throughout this period. During the following period – lasting about 20 s – the liquid fraction increased in the bottom 5 mm of the sample until the foam solidified. It could be observed that especially the top part of the foam that still did not contact the crucible dried quickly, changing the appearance of the foam from uniformly convex to a structure exhibiting many protruding individual bubbles.
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Fig. 4. Foaming and drainage evolution of thixocast AlSi6Cu4 precursors foamed opposed to gravity ‘+1 g’ (left) and parallel to gravity direction ‘−1 g’ (right). Images for t = 0, 180, 250 and 400 s after initiation of heating are given. The grey arrows indicate foaming direction, the black arrows gravity direction.
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a precursor volume of Vp = 2 cm × 1 cm × 0.5 cm = 1 cm3 and a density = 2.85 g/cm3 for AlSi6Cu4 at room temperature, the resulting force is F ≈ 28 mN. The force difference between foaming under ‘−1 g’ and ‘+1 g’ conditions is then 2F ≈ 56 mN, high enough to lead to the difference. Still, the liquid metal does not fall out of the foam as it is held in it by the stabilising outer oxide layer and the inner oxides [14,15]. The very good temperature stability achieved (±1 K) after foam expansion and reproducible temperature profiles validated the hardware for microgravity experiments. 4.2. Microgravity experiments
Fig. 5. Density profile of a AlSi6Cu4 + 0.6 wt.% TiH2 foam during a parabola as a function of time. Gravity-induced drainage and melt imbibition into the foam are visible.
During the parabolic flights, foaming started 5 s earlier than on ground. A possible explanation is that the foaming temperature is reached earlier by the precursor due to an improved thermal contact between precursor and crucible during the early foaming stages taking place at 1.8 g. The strong gravity-induced drainage observed is probably a consequence of a reduced amount of stabilising oxides present in the precursor, caused by the specific manufacturing process in the semi-solid state. It is known that foam made from precursor manufactured by this production route exhibits drainage [8,9]. The fact that the drainage effect is very pronounced here was one reason for selecting this material instead of uniaxially compressed precursors that are much more stable due to the enclosed oxide films, as it promises to reveal more pronounced gravity phenomena in the limited microgravity time available. The observed fast liquid flow shows that the melt within the metallic films and Plateau borders is mobile and not a viscous gel as has been sometimes assumed. This mobile behaviour has recently been investigated in uniaxially pressed powders [16]. During the early foaming stage the increased drainage found at the bottom of the foam compared to normal conditions is explained by the first 1.8 g phase of a parabola. Imbibition of the liquid metal initially drained out of the foam back into the films and Plateau borders occurred during the transition from 1.8 g to microgravity, showing how capillary forces in the films and Plateau borders dominate at this stage. As a consequence, a liquid fraction of around 40% well distributed over the entire foam is observed in the X-ray images as well as a more uniform bubble size distribution. Dur-
Fig. 6. Liquid fraction at the bottom of a AlSi6Cu4 foam held at T = 650 ◦ C and gravity level during a parabola.
4. Discussion 4.1. Ground-based experiments Metal samples foamed under gravity have a reasonably good thermal contact to the crucible due to their own weight, so that heat flow into the sample is reproducible. Under microgravity, samples that are not clamped would levitate inside the crucible, leading to poor and irreproducible heating. The temperature profile and distribution in the precursor is very important for metal foam production due to the pronounced temperature sensitivity of the TiH2 decomposition [12,13]. Therefore, the two methods proposed here, namely holding with a groove and tongue on the side and gluing with X-ray transparent BN paste were both used to ensure uniform foaming while not interfering with X-ray imaging. The increase in foam expansion by ∼25% until filling the crucible under ‘−1 g’ conditions – visible in Fig. 4 – can be explained by the additional gravity force acting on the melt in parallel to foaming. For
Fig. 7. Liquid fraction as a function of foam height for a free drainage experiment under varying gravity level. Curves correspond to different times after transition from 0 g to 1.8 g. The horizontal broken line shows the average liquid fraction (∼40%) of the foam under microgravity.
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ing and after the subsequent transition from microgravity to 1.8 g, a high flow of liquid to the bottom of the sample can be observed, leading to high drainage. A detailed view of the liquid fraction distribution for different times is given in Fig. 7. This liquid fraction curve corresponds to the predicted and measured shape for liquid aqueous foams [17], although here we have changing gravity conditions. This demonstrates that there are a large number of similarities concerning drainage in both aqueous and metal foams. This gives us the chance to perform quantitative analyses and predictions using the existing theories. 5. Conclusions • The first successful microgravity experiments on liquid Al foam with in situ X-ray diagnostics were carried out on parabolic flights. • In ground-based foaming experiments with either parallel and anti-parallel orientation of gravity and foaming direction, a 25% increase of the foaming velocity for parallel orientation (‘−1 g’) was observed. • Strong drainage at 1.8 g was found, followed by complete imbibition of drained liquid metal into the foam after the onset of microgravity, the rate of both points at a highly mobile liquid. • Imbibition leads to more uniform density, pore size and shape distributions and to a average liquid fractions around 40%. • Strong and fast liquid flow during transition from 0 g to 1.8 was observed. • Vertical liquid fraction profiles for free drainage under varying gravity conditions resemble those of aqueous foams qualitatively. 6. Outlook No difference in the rate of film rupture in liquid metal foams was found comparing microgravity and normal gravity conditions. However, this was under conditions of insufficient time for reliable statistics. A longer experiment such as on a sounding rocket flight will be needed for that purpose. The actual parabolic flight campaign was also a successful verification of the hardware for the Maser 11 sounding rocket campaign scheduled for May 2008, where the bubble coalescence rate and corresponding pore size distribution under microgravity is the prime purpose of study. Acknowledgements Funding by ESA (Projects AO-99-075 and AO-2004-46) is gratefully acknowledged. We also thank Novespace and the Swedish
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