Technology Advances
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TECHNOLOGY ADVANCES Hollow Metal Spheres Created with Polymer Core Results in Strong, Porous, Lightweight Structures The Fraunhofer Institute for Manufacturing and Advanced Materials in Dresden, Germany, has introduced a powder metallurgy (P/M) approach for the production of lightweight, hollow metal spheres. In this process, a polymer core is coated with a powdered metal slurry to create the desired wall thickness, then conventional P/M binderremoval and sintering operations are used to produce hollow spheres with fully dense shells. Individual green (unfired) or sintered spheres can be sinterbonded, brazed, or sintered and then adhesively bonded together to fabricate porous structures with low bulk densities and very high specific strengths. A schematic representation of the production process for sintered components is shown in Figure 1. Sphere outside diameters from 0.5 mm to 10 mm with wall thicknesses of 20–1000 µm have been produced in a wide range of compositions including Fe, stainless steels, Ti alloys, Ni, and Ni-based superalloys. Since the process represents only a minor variation in traditional powdered metal and ceramic processing, the list of possible compositions is almost limitless. Compared to commercially available porous composite and polymer structures, the use of hollow spheres addresses applications where elevated operating temperatures, corrosive environments, or other factors must be considered. The fabrication of lightweight components using hollow spheres also provides more predicable macroscopic properties than do other cellular materials, due to the high degree of uniformity achievable in size, wall thickness, and porosity level. Furthermore, by combining multiple sphere sizes, structures with very specific mechanical, thermal, and microstructural Technology Advances provides upto-date reports of materials developments that show potential to bridge the gap between research innovation and application of advanced materials technologies. If you encounter or are involved with materials research that shows potential for commercialization and would like to present these developments, contact Renée G. Ford, Renford Communications, Ltd., P.O. Box 72, Harrison, NY 10528-0072; tel. 914-967-0955; fax 914-967-7927; or e-mail [email protected].
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properties can be developed. The mechanical properties of hollow sphere structures have been evaluated using cylindrical samples fabricated from different sphere compositions. The
results in Figure 2 show a compressive stress-strain curve and associated deformation behavior during compaction of 316L stainless-steel spheres. Compaction begins at about 60–70% strain with a rela-
Fluidized bed coating
Pre-expansion Polystyrene granules
Metal powder suspension
Pre-expansion styrofoam
Continuous hot-air counterflow Forming
Coated styrofoam spheres
• Heat treatment • Pyrolysis of styrofoam and binder • Sintering of metal powder
Hollow sphere body
Single metal hollow spheres
Figure 1. Process schematic for the production of hollow metal sphere
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