Nanocrystalline Aluminum Truss Cores for Lightweight Sandwich Structures
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Substitution of conventional honeycomb composite sandwich structures with lighter alternatives has the potential to reduce the mass of future vehicles. Here we demonstrate nanocrystalline aluminum-manganese truss cores that achieve 2–4 times higher strength than aluminum alloy 5056 honeycombs of the same density. The scalable fabrication approach starts with additive manufacturing of polymer templates, followed by electrodeposition of nanocrystalline Al-Mn alloy, removal of the polymer, and facesheet integration. This facilitates curved and net-shaped sandwich structures, as well as co-curing of the facesheets, which eliminates the need for extra adhesive. The nanocrystalline Al-Mn alloy thin-film material exhibits high strength and ductility and can be converted into a three-dimensional hollow truss structure with this approach. Ultra-lightweight sandwich structures are of interest for a range of applications in aerospace, such as fairings, wings, and flaps, as well as for the automotive and sports industries.
Sandwich structures are unique enablers of lightweight design, as they offer an exceptional combination of low density and high bending rigidity. Sandwich panels are created by attaching thin, stiff facesheets to the top and bottom surfaces of a relatively thick, lightweight core. The thickness of the core serves to separate the facesheets, thus providing the sandwich construction with high area moment of inertia, while the low density of the core allows this increased area moment of inertia to be realized for minimal mass increase. Lightweight sandwich structures are widespread in aerospace applications (e.g., winglets, flaps, rudders, and rotor blades) but are also used in many other industries. State-of-the-art sandwich panels use carbon-fiber-reinforced polymer (CFRP) composite or aluminum alloy facesheets with honeycomb or foam cores. Typical honeycomb cores are made from aluminum alloys, meta-aramid fiber paper (Nomex®), or glass-fiber-reinforced polymer. Closed-cell polymer foams are alternatives to honeycomb, albeit with generally lower structural performance due to their stochastic cellular architecture.1
More recently, advanced core materials have been studied, among which the most compelling are tetrahedral and pyramidal truss structures.2,3 Hollow truss structures, when designed to suppress buckling, offer an opportunity for improved compressive and shear strengths versus honeycombs.4, 5, 6 Hollow truss structures are preferably fabricated by coating a polymer template of the truss structure, which is subsequently removed.7 This approach converts a two-dimensional (2D) thin film or coating into a three-dimensional (3D) cellular material, thereby redefining the applications of a range of thin film/coating materials.8 Thin film/coating materials with high strength and low density are sought to optimize the mechanical properties of the truss structure. Recently, an electrodepositable nanocrystalline aluminum-manganese alloy that exhibits unprecedented specific strength has been developed.9 In this study, we combine this high-strength nanocrystalline aluminum alloy with optimized truss architectures, resulting in sandwich cores with exceptional performance.
This study was conducted as part of the program on “Ultra-lightweight Core Materials for Efficient Load-Bearing Composite Sandwich Structures” funded by the National Aeronautics and Space Administration (NASA)’s Game Changing Development Program within the Space Technology Mission Directorate. The target application of this program was a fairing of the upper stage of NASA’s Space Launch System, which dictated a core thickness of 25.4 mm (1 in.) and a core density of 48 kg/m3. The state of the art for such fairings are lightweight sandwich structures of carbon-fiber-reinforced polymer (CFRP) facesheets bonded to a honeycomb core made of high-strength aluminum alloy 5056. Aluminum alloys offer excellent specific strength due to their low density. Xtalic Corporation has developed a nanocrystalline aluminum-manganese alloy that achieves yield strength over 1300 MPa,9 substantially higher than conventional wrought aluminum alloys; For example, Al 5056-H38 has yield strength of 345 MPa. While alloys with higher specific strength exist, they are typically difficult to roll into thin sheets and process into honeycombs due to their low ductility. Xtalic’s nanocrystalline aluminum-manganese alloy can be electrodeposited, which circumvents issues with formability and bodes well for fabrication of ultralight hollow truss cores. Grain sizes in the range of 10–100 nm are thermodynamically stabilized with addition of Mn, which preferentially segregates at grain boundaries and inhibits grain growth.10 More technical background on the nano Al-Mn material and processing method is provided in Ref. 11.
Polymer Template Fabrication
Polymer truss structures were used as low-cost templates to define the final architecture of the ultra-lightweight hollow metallic truss cores. Polymer templates for the core can be fabricated using various methods. We used the self-propagating photopolymer waveguide (SPPW) process for rapid, scalable, low-cost manufacturing of polymer truss structures.15 This process is fundamentally different from other additive manufacturing approaches in that it is not a layer-by-layer process; rather, the entire three-dimensional lattice architecture is formed in a single rapid exposure (30–60 s) with component area dictated solely by the size of the ultraviolet (UV) exposure source. In our SPPW process, a suitable liquid photomonomer is exposed to collimated UV light through a patterned mask, forming an interconnected three-dimensional network of self-propagating photopolymer waveguides. A wide array of different topologies with features on the desired length scale can be manufactured by altering the incident UV exposure angle and mask pattern. Significant benefits over other additive manufacturing techniques, such as stereolithography, include improved cost, time, scalability, and surface finish.
One of the unique attributes of a lattice design is its open cellular architecture, which allows access to the entire internal volume of the sandwich. In contrast to state-of-the-art honeycomb cores, neighboring lattice unit cells are connected in-plane via these open pores. This connected porosity was leveraged to integrate a second, sacrificial molding material into the open volume to support the pressure applied during composite facesheet consolidation. This compaction pressure is necessary to produce facesheets with high fiber volume fraction during co-curing and is typically higher than the in-service compressive loading on an ultralight core (<2 MPa). Thus, either parasitic mass must be added to the core or the facesheets must be consolidated separately (Fig. 9), which would increase adhesive mass and manufacturing costs.
In contrast, the novel approach utilized a sacrificial mold for in-series load bearing during consolidation (Fig. 10). An initially liquid mold material was injected into the open volume of the core, then allowed to solidify in net shape around the lattice. The interface between the facesheets and the core was controlled by means of interposer layers, which allowed for projection of the lattice nodes above the surface of the mold. For this study, a commercially available washout ceramic mandrel material (Aquapour, ACM) was chosen for the sacrificial mold.17 An additional 10% by weight of water-soluble polymer binder was added to the Aquapour material to ensure consistent filling around lattice members of different geometries.
To test the performance of truss core sandwich panels with CFRP facesheets, four-point bending and edgewise compression tests (ASTM C393 and C364) were performed in direct comparison with honeycomb core panels. Four-point bending testing showed failure of the core for both truss and honeycomb as expected. Edgewise compression testing showed adhesive failure in the truss core panel once the facesheets buckled. The honeycomb panel failed in the core by tension. The core–facesheet interface limited the strength of the truss core sandwich panel in edgewise compression, which could be overcome by choosing a stronger adhesive or increasing the interface area. A sandwich core with lower thickness H would allow for more nodes in contact with the facesheets for given area.
While nanocrystalline aluminum-manganese truss cores surpass the strongest commercially available aluminum honeycombs, simulations predict that even higher strength could be achieved. The discrepancy between the simulated and measured strength is due to porosity, surface roughness, and nonuniform thickness distribution. Additional process development is required to mitigate these issues, especially in such thick (200–300 µm) electrodeposited material. Cores with lower density and thickness H would require thinner aluminum deposits, facilitating reductions in porosity, surface roughness, and nonuniformity.
Galvanic corrosion has been an issue for sandwich panels with aluminum cores and CFRP facesheets due to their different electrochemical potential, especially in applications where moisture is present and long life is required. A thin layer of insulating glass scrim can be inserted between the core and the facesheet to block the electrochemical reaction and mitigate galvanic corrosion. Alternatively, a thin coating of a metal with higher potential such as copper or nickel can be electrodeposited on the surface of the aluminum truss to protect it from galvanic corrosion.
Over recent years, several new microarchitected cellular materials have been developed. While nanolattices21 and nano-honeycombs22 can take advantage of strengthening effects from nanoscale phenomena, it is not envisaged that fabrication of these materials at scales above 1 mm3 will be possible in the near future, and they are not therefore industrially relevant. Microlattices with multiple layers of unit cells cannot achieve the compressive strength of conventional aluminum honeycomb7,23 because of premature failure at the nodes. This is due to nodal rotation and stress concentration. The only other core materials that could surpass aluminum honeycombs are CFRP-based honeycombs or truss cores currently under development.24 However, composite processing adds a different set of challenges.
An approach to manufacture lightweight sandwich structures with high-strength, nanocrystalline aluminum truss cores has been demonstrated. By electrodepositing nanocrystalline aluminum-manganese alloy onto polymer templates, this extraordinary thin-film material can be converted into a three-dimensional hollow truss structure. Film thicknesses of 300 µm and more were achieved, and the scalability of the process was demonstrated by fabricating 30 cm × 30 cm × 3 cm sandwich panels. Mechanical testing was conducted in compression and shear, and improvements of 2–4 times over state-of-the-art aluminum honeycombs was realized. Simulations predict that further improvements are possible if the plating uniformity can be increased and the porosity and surface roughness decreased. While this study focused on large, flat sandwich panels with 3 cm thickness, this technology appears more advantageous for curved, net-shaped structures with thinner cores. The open-celled nature of the truss cores enables novel strategies for facesheet integration, such as co-curing, which eliminates the need for extra adhesive.
This work was supported by NASA’s Space Technology Mission Directorate under the Game Changing Development Program through Contract No. NNC15CA16C. The authors thank Shiyun Ruan (Xtalic) for useful discussions.
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