Aluminum integral foams with tailored density profile by adapted blowing agents
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- Hartmann, J., Fiegl, T. & Körner, C. Appl. Phys. A (2014) 115: 651. doi:10.1007/s00339-014-8377-4
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The goal of the present work is the variation of the structure of aluminum integral foams regarding the thickness of the integral solid skin as well as the density profile. A modified die casting process, namely integral foam molding, is used in which an aluminum melt and blowing agent particles (magnesium hydride MgH2) are injected in a permanent steel mold. The high solidification rates at the cooled walls of the mold lead to the formation of a solid skin. In the inner region, hydrogen is released by thermal decomposition of MgH2 particles. Thus, the pore formation takes place parallel to the continuing solidification of the melt. The thickness of the solid skin and the density profile of the core strongly depend on the interplay between solidification velocity and kinetics of hydrogen release. By varying the melt and blowing agent properties, the structure of integral foams can be systematically changed to meet the requirements of the desired field of application of the produced component.
Aluminum integral foams are monolithic structures possessing a foamed cellular core, a compact, dense skin and a smooth transition region in between. At a constant relative density level, material distribution over the cross section of the produced component can differ. This can be due to a variation of the skin thickness, to a differing transition region or to a combination of both. This variation of the integral foam structure represents a decisive factor for the adaption of such components to the desired application such as flexural load, crash absorption or damping .
Thus, in order to improve the mechanical properties for employment as load bearing structures, an increase in skin thickness is favorable [2, 3]. For crash absorption, the compressive strength is an important factor which is at least partly dependent on the distribution of porosity within the foam. In this case, the density distribution should show a pronounced plateau. To benefit from the excellent damping properties of aluminum foams, the compact layer has to be as thin as possible since only porous material regions contribute strongly to damping .
There are two different existing process modifications in integral foam molding: the high pressure (HP-IFM) and the low pressure (LP-IFM) variant . The process steps are similar to those of the foam injection molding of polymers on the basis of which the steps are developed. A detailed description of the HP-IFM can be found in , the LP-IFM is discussed in detail in  (see also Fig. 3). In both cases, a certain amount of blowing agent, typically magnesium hydride (MgH2), is entrained into the cavity by the aluminum melt during mold filling at high velocity by a moving piston, where it decomposes thermally activated into magnesium (Mg) and hydrogen (H2). The released gas leads to the pore formation. In case of the low pressure process, mold filling and pore formation take place simultaneously as the cavity is underfilled with melt. At the cold mold wall, the melt solidifies to a compact skin due to a very high solidification rate, whereas the inner core region is foamed. In the case of HP-IFM, mold filling and foam formation are separate process steps. As the mold filling is completed, the volume of the cavity is increased by a core puller system after a certain delay time which initiates pore formation in regions where the alloy has not yet solidified. In both cases, there are two competing physical processes which by interaction define the integral foam structure: the decomposition of the blowing agent which is very sensitive to the local melt temperature [5, 8–11] and the velocity of the moving solidification front. In case of parts produced by HP-IFM, this interplay results in U-shaped density profiles whereas parts produced by LP-IFM show a more V-shaped profile.
Because of these disadvantages as well as the fact that the thickness is only variable within narrow limits as already shown by Wiehler , other approaches are necessary. That is why the goal of the current paper is to present on the one hand a way to increase skin thickness without any negative influence on surface quality and pore structure. And on the other hand, we also show the way how the skin thickness can be reduced. This as well as a complete modification of the porosity profiles of the foams is implemented by manipulation of the decomposition kinetics of the MgH2 powder. Furthermore, two different aluminum alloys are used, showing a strong difference in the solidification temperature influencing the decomposition kinetics of the entrained blowing agent .
2.1 Blowing agent modification: milling and oxidizing
The commercial blowing agent powder MgH2 (Tego Magnan, Evonik Goldschmidt) is treated in different ways. A powder fraction with a d50-value of 55 μm (d10 = 30 μm, d90 = 94 μm) is milled in batches of 40 g in a centrifugal ball mill (S1, Retsch; grinding balls to powder weight ratio of 10:1) for 1, 2, 3, 5 and 10 min at 370 rpm.
Powder as supplied (d10 = 25 μm, d50 = 50 μm, d90 = 92 μm) as well as milled powder (3 min at 320 rpm; d10 = 2 μm, d50 = 19 μm, d90 = 47 μm) is oxidized in a furnace at 270 °C and at 40 °C in a drying oven in a humid atmosphere (so-called “aging” ) for up to 23 h. A similar powder modification was already performed by Matijasevic-Lux et al. [13, 14] for titanium hydride (TiH2) to retard hydrogen release during the heating-up process and to improve the foaming properties of powder metallurgical aluminum foams. In our case, 270 °C is chosen to accelerate the oxidation process without reaching the theoretical decomposition temperature of MgH2 of about 280 °C. In addition, for the aging process under humidity, a much lower temperature is applied, but still slightly above room temperature to ensure a certain evaporation.
2.2 Analysis of particle size and powder reactivity
To quantify the properties of the so fabricated powders, particle size and decomposition temperature (θdec) are measured. The particle size distributions of the different powders are measured by laser diffractometry (Mastersizer 2000, Malvern Instruments), and pre-dispersed by a sonotrode in isopropanol. The resulting particle sizes (Fraunhofer model) are volume weighted and correspond to the diameter of spheres with equivalent cross-sectional areas as the real non-spherical particles. The mean particle size d50 is used as a representative value for the respective powder.
2.3 Characterization of the alloys by solidification simulation
The two different aluminum die casting alloys Al9Si3Cu(Fe) (226D) and Al5Mg2SiMn (Magsimal-59, Rheinfelden) are used to analyze the influence of the melting range on the integral foam structure of LP-IFM parts. Solidification simulations (Thermo-Calc, Access) according to Scheil–Gulliver are performed to gain information about the temperature as a function of solid phase fraction during cooling. Especially the temperature range around the liquidus temperature (θliq) is of interest as it represents the decisive value for the temperature of the melt surrounding the blowing agent particles during entrainment.
In addition, computational fluid dynamics simulation (CFD; Flow-3D, Flow Science) of particle entrainment and cavity filling is carried out to obtain further information on the temperature fields the blowing agent particles are exposed to. Altogether, this allows an estimation of the decomposition kinetics which helps to explain integral foam structures.
2.4 Foam fabrication by LP-IFM
Overview of the powders (particle size, θdec and oxidation treatment) and alloys used for the fabrication of integral foam parts
dry, 270 °C
dry, 270 °C
wet, 40 °C
dry, 270 °C
As the piston advances, the melt entrains the powder in a turbulent way into the mold where solidification and gas release take place simultaneously (Fig. 3, t2–t5). The melt wets the surface where it solidifies instantaneously whereby the resulting core density profile depends on the heat flow and decomposition kinetics in the inner region.
2.5 Foam characterization
The characterization of the produced parts comprises the preparation of cross sections from the 12 mm height sector to get an impression of the foam structure quality as well as of the skin thickness.
In addition, cylindrical samples with a diameter of 13 mm are prepared from the same sector (see Fig. 3, bottom right) for micro-computed tomography (μCT 40, Scanco Medical). With an X-ray acceleration voltage set to 60 kV, an initial current of 133 μA and an integration time of 300 ms, the samples are analyzed three dimensionally with a voxel resolution of 15 μm3. In that way, the relative density is recorded as a function of the distance to the surface of the samples which allows gaining information about the porosity distribution within the sample volume.
3 Results and discussion
3.1 Modification of the blowing agent reactivity
Hydrogen release takes place either by diffusion through the oxide layer and recombination on the surface [11, 15–18] or by breaking up the layer due to the formation of inner hydrogen gas pressure within the particle [19, 20] which occurs especially at very high heating rates . Thus, cracking the oxide layer accelerates desorption . In addition, the reduction of particle size increases the total particle surface and accelerates the hydrogen release by shorter diffusion paths [21, 22]. Not only the particle size changes during milling but also the grain size is reduced which also correlates with faster desorption . Finally, also incorporated impurities by abrasion processes during milling like iron (Fe) or iron oxide (Fe3O4) can act as catalysts for hydrogen desorption [23, 24]. On the whole, the combination of microstructure change, catalyst incorporation , particle size reduction  and breaking up of the hindering oxide layer  during milling leads to faster decomposition kinetics. There is a linear relation between d50 and θdec (Fig. 4, right). Thus, not only the desired particle size can be adjusted by milling but also the reactivity of the powder changes in a defined way.
Magnesium hydroxide [Mg(OH)2] forms on the surface  and the hydrogen content decreases. This Mg(OH)2 layer starts to decompose to MgO and H2O above 350 °C [28, 29]. Thus, it probably acts in the same way as the oxide layer but grows faster explaining the accelerated aging compared to dry oxidation.
These aging curves are used as master curves which help to exactly time the duration of oxidation of blowing agents to obtain powders with a defined decomposition temperature.
3.2 Solidification properties of the alloys Al9Si3Cu(Fe) and Al5Mg2SiMn
3.3 Modification of compact skin thickness and relative density profiles
Furthermore, due to the fact that strong hydrogen release already occurs before piston stops, the porosity level and pore size in the whole core region are homogeneous. The pores develop within the first 100 ms when the melt temperature over the cross section is still nearly constant. In contrast, in case of using the Al9Si3Cu(Fe)-alloy, the local pore size strongly depends on the distance to the mold wall as pores start to expand later when mold filling is already finished and a temperature gradient along the cross section is established (Fig. 10, dash-dotted line). As the MgH2 is very sensitive to the local surrounding temperature, the velocity of hydrogen release strongly depends on the distance from the center resulting in a graded pore size distribution (Fig. 8, middle left). A retarded decomposition by powder aging has a similar effect on the density profile of Al5Mg2SiMn foams as shown in Fig. 8, bottom right.
The structural design of aluminum integral foams is successfully implemented by alloy and blowing agent modification. The thickness of the solid skin—decisive for the respective field of application—can be varied without any intervention regarding the heat content of the melt. Instead of reducing the melt temperature or accelerating the solidification rate, which has negative effects on the surface and pore quality of the produced parts, the skin thickness is increased by slowing down the decomposition kinetics of the powder. It is possible to adjust the desired reactivity of MgH2 by either milling (higher decomposition rates) or oxidizing milled or non-milled powder (lower decomposition rates). Another parameter represents the alloy’s liquidus temperature which has direct influence on the hydrogen release velocity with the powder being very sensitive to the ambient conditions [5, 10]. By timing the decomposition and, therefore, the pore evolution within the process window, the density profiles over the component’s cross section can be varied according to the desired specifications. By combining the temperature fields from CFD with the decomposition kinetics of the powder the differences in the density profiles can be explained. This opens up new possibilities in the design of integral foam structures tailored to the needs for new components.
The authors gratefully acknowledge the financial support of the Deutsche Forschungsgemeinschaft (DFG), grant no. KO 1984/5-2. The authors also thank Dr. Andreas Borgschulte, EMPA, for his help and expertise concerning the decomposition behavior of magnesium hydride and Dr. Ralf Rettig for providing the Thermo-Calc calculations.