Fabrication of Porous Aluminum with Controllable Open-Pore Fraction
Aluminum with an open-pore structure was fabricated through nitridation of an AA6061-2 pct Mg-1 pct Sn powder mixture, where interconnected permeable AlN shells developed on each AA6061 particle and imparted strength to the assembly. The resulting intershell spaces form an open-pore structure. When such an open-pore structure is heated above the liquidus of the core, an open-closed pore transformation occurs, where the molten core in each shell spontaneously migrates to fill the open pores outside, leaving a closed pore inside each shell. Based on this finding, porous AA6061 with different open-pore fractions was fabricated by heating open-pore structures of AA6061 into the semisolid region, where the liquid fraction changes with temperature. The mechanism for the open-closed pore transformation is identified through detailed microstructural and thermodynamic analyses. Criteria for the open-closed pore transformation are specified. Additionally, net shape fabrication of porous aluminum with controlled pore features is realized using the novel concept.
Highly porous materials have found wide application in various industries[1, 2, 3] due to their interesting combinations of physical and mechanical properties,[4,5] such as high stiffness in conjunction with very low specific weight or high gas permeability combined with high thermal conductivity. Based on the connectivity of pores, porous metals are typically divided into two broad categories: (1) open-pore metals, known as metal sponges, in which pores are interconnected to form a percolating structure permitting the passage of gases and liquids; and (2) close-pore metals, known as metal foams, in which pores are isolated from each other by cell walls.
Open-pore metals are conventionally fabricated through two processing routes. One is through a replication process, which begins by creating a porous preform whose open pores are filled with the foam material. Then the preform is removed by leaching or shaking, leaving an interconnected porous network within the foam material. The other is through sintering of loose metal powder or fibers. The close-pore metals are overwhelmingly fabricated by foaming of liquid metal. First, a molten metal is oxidized or mixed with ceramics to increase its viscosity. Second, gases are introduced through blowing agents or external gas sources to form liquid metal foams. The subsequent cooling produces a close-pore metal through solidification of the liquid. While close-pore metals have found application as load bearing, structural components, and energy absorbers,[11,12] functions such as filtration, separation, catalyst support, bone replacement, and heat/mass exchange require desired open-pore structures.[13,14]
Encouraging progress has been made in the control of the size, shape, and uniformity of the pores in porous materials over the years.[15, 16, 17] However, it remains challenging to control the open-pore fraction in cellular metals. Furthermore, once a metal foam is produced, it is no longer possible to alter the pore morphology and the relative proportion of the open or closed pores. This work presents a novel method, which allows for an open-closed pore transformation in cellular aluminum, facilitating the control of pore structure in cellular aluminum materials.
The feed stock was prepared by mixing prealloyed AA6061 powder with 2 pct Mg and 1 pct Sn (all in wt pct). The aluminum powder was argon atomized, with a particle size range 15 to 75 μm. Both the Mg powder and the Sn powder are <45 μm. A mix of 400 g was poured into a steel crucible (dimensions: 200 mm × 200 mm × 60 mm) and covered with a loose fitting lid. The crucible was placed in an electrical resistance furnace, which was evacuated with a rotary pump to 10 Pa before being filled with flowing nitrogen (5 L/min). The furnace was subsequently heated to 833 K (560 °C) and held at temperature for 12 hours during which period nitridation occurred, leading to the formation of an AlN skeleton.[18, 19, 20, 21] The mix was thus transformed into an open-pore aluminum. After cooling to room temperature in the furnace, the sample was sectioned into equal parts. They were then treated at 893 K, 913 K, 923 K, and 973 K (620 °C, 640 °C, 650 °C, and 700 °C) for 2 hours in a vacuum of 10 Pa to transform the pore structure.
A demonstration part was fabricated to show the potential of the process for the fabrication of net shapes. AA6061-2 pct Mg-1 pct Sn-3 pct Nylon powder was used as a feed stock. A green part was built using a 3D System SinterStation Plus selective laser sintering machine (3D Systems Corporation, Rock Hill, SC). It was then subjected to 12 hours nitridation at 833 K (560 °C) to thermally remove the nylon and form an AlN skeleton. Finally, the part was treated at 973 K (700 °C) under vacuum for 2 hours to close the pores.
3 Results and Discussion
3.1 Open-Closed Pore Transformation
Figure 1(c) shows the attendant microstructure when the sample was further heated to 973 K (700 °C) (above the liquidus of AA6061) and held at temperature for 2 hours, followed by furnace cooling. A distinctly different microstructure is observed, with aluminum filling the interconnected channels outside the shells. As a result, the pores were transformed from largely open and interconnected to closed and isolated. Figure 1(d) shows the closed pores on the fracture surface of such a sample.
Densities and Porosities of As-Nitrided Samples and Such Samples Further Treated at Different Temperatures
Temperature [K (°C)]
Liquid Fraction (Wt Pct)
Bulk Density (g cm−3)
Total Porosity (Vol Pct)
Open Porosity (Vol Pct)
Open-Pore Fraction (Pct)
1.42 ± 0.01
49.2 ± 0.5
37.7 ± 0.4
76.6 ± 1.5
1.42 ± 0.01
50.9 ± 0.3
35.8 ± 0.9
70.4 ± 2.2
1.56 ± 0.00
44.2 ± 0.0
28.1 ± 0.6
63.5 ± 1.5
1.53 ± 0.04
45.4 ± 1.5
13.6 ± 3.0
30.0 ± 7.6
1.52 ± 0.07
45.7 ± 2.7
3.6 ± 1.6
7.8 ± 4.0
Mechanical Properties of Untreated Samples and Those Treated at 973 K (700 °C)
Yield Strength (MPa)
Compressive Strength (MPa)
Young’s Modulus (GPa)
60 ± 3
80 ± 6
4.3 ± 0.7
973 K (700 °C) treated
106 ± 4
147 ± 6
8.3 ± 0.6
3.2 Mechanism of Open-Closed Pore Transformation
Figure 5(b) schematically illustrates the microstructure of a porous aluminum structure prior to the open-closed pore transformation, with Al inside each AlN shell. There are concave regions outside the AlN shells where the radii of curvatures are infinitely small (labeled with open arrows in Figure 5(b)). This makes the system thermodynamically unstable. Upon melting, AA6061 will spontaneously migrate through the permeable AlN shells to fill these regions to minimize the overall Gibbs free energy of the system. In places where the AA6061 particles are closely packed so that the space outside the AlN shells is smaller than that inside, the infiltrant can completely fill the outside space. As a result, the open pores outside the AlN shells will be eliminated while closed pores will be created inside the AlN shells. In places where the AA6061 particles are loosely packed such that the space outside the AlN shells is greater than the inside (as seen in Figure 5(c)), only the concave regions (labeled with broken circles in Figure 5(c)) will be filled, leaving the rest of the outside space unfilled. Nevertheless, most open pores can be transformed into closed pores at an appropriate temperature.
The open-closed pore transformation occurs spontaneously, driven by the decrease in the overall interfacial energy of the system (Figure 5(a)). The phenomenon should not be limited to the Al-AlN system. Rather, we expect that similar open-closed cell transformations may be realized in other systems subject to the following conditions. First, the system has a core-shell structure where the shell has a higher melting point than the core and is well wetted by the molten core material. Second, the shell is permeable and permits the passage of the molten core material. Last, the space outside the shells has substructures whose radii of curvatures are smaller than that of the pores inside the shells, ensuring spontaneous redistribution of the molten core material from inside to outside the shell.
3.3 Porous Al with Controllable Open-Pore Fraction
3.4 Application of Open-Closed Pore Transformation to Net Shape Fabrication
A novel open-closed pore transformation was identified and realized for the fabrication of porous aluminum alloys with controllable pore structure. A powder mixture of AA6061-2 pct Mg-1 pct Sn is first nitrided in high-purity nitrogen at 833 K (560 °C) for 12 hours. This creates a permeable AlN shell on each AA6061 particle. As a result, the spaces outside the AlN shells form a percolating structure transforming the mix into an open-pore material. The AlN shells are interconnected, imparting strength to the open-pore material. When the porous structure is heated above the liquidus of AA6061 under vacuum, the molten core (AA6061) in each AlN shell flows through the permeable shell and fills the open pores outside, leaving a closed pore inside. The open-closed pore transformation is driven by a reduction in the interfacial energy of the system. Based on this finding, porous AA6061 with different open-pore fractions was fabricated by heating open-pore AA6061 in the semisolid region, where the liquid fraction depends on temperature. Consequently, the solid remains inside the AlN shell, while the liquid migrates to fill the open pores.
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The work was funded by the Australian Research Council.