Experimental Investigation of Mechanical Properties of Metallic Hollow Sphere Structures
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IN recent years, metallic foams have become more attractive for load-bearing applications, and therefore, several manufacturing processes have been developed to produce high-quality closed-cell foams. However, imperfections in the foam structure, including cell wall curvatures, missing cell walls, cracks, and density inhomogeneities, cause a large drop of the mechanical properties.[1] Because of these imperfections, high-quality closed-cell foams are still far away from the theoretical limit in their mechanical properties and are situated in the range of comparable open-cell foams.[2] Hence, several efforts were undertaken in the last years to improve the foam quality or to produce regular structures (e.g., hollow sphere structures, miniaturized frameworks, etc.). O. FRIEDL, Materials Science Engineer, and R. PIPPAN, Professor, are with the Erich Schmid Institute of Material Science, Austrian Academy of Sciences, A-8700 Leoben, Austria, and the Christian Doppler Laboratory for Local Analysis of Deformation and Fracture, A-8700 Leoben, Austria. Contact e-mail: pippan@ unileoben.ac.at C. MOTZ, Senior Scientist, is with the Erich Schmid Institute of Material Science, Austrian Academy of Sciences. H. PETERLIK, Professor, and S. PUCHEGGER are with the Institute of Materials Physics of the University of Vienna, A-1090 Vienna, Austria. N. REGER is with Plansee SE, A-6600 Reutte, Austria. Manuscript submitted February 16, 2007. Article published online December 12, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS B
Metallic hollow sphere structures (HSSs) represent a ‘‘new kind’’ of metallic foams consisting of hollow spheres with almost constant sphere diameter and shell thickness made of various metals (e.g., mild steel, stainless steel, refractory metals). The single hollow spheres can be bonded together by, e.g., gluing, brazing, or sintering. Furthermore, it is possible to adjust the mechanical behavior of this material by varying the structural parameters for a certain application in a very defined way. Figure 1(a) depicts the structural parameters of the HSS, which are used in this article: the hollow sphere diameter D, the diameter of the sinter neck d, and the cell wall thickness t. In this kind of cellular material, three types of porosities can be found on different scales (Figure 1(b)). The macroporosity is defined as the volume inside the hollow spheres, whereas the mesoporosity displays the cavity between the single spheres. The porosity of the cell wall itself is called microporosity. Depending on the stacking of the hollow spheres, one can differ between regular (e.g., a facecentered-cubic (fcc) packing) and irregular (random packing) structures, which influences the mesoporosity. The bonding between the hollow spheres can be achieved by different mechanisms: due to adhesives, sintering of the hollow spheres or soldering of coated spheres. Depending on the size of the contact area of the sphere to sphere connection, the sphere structures can be ‘‘open,’’ i.e., there are free paths through the structure, or ‘‘
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