Strong cellular lattices with nitro-carburized stainless steel hollow trusses

Collinear lattice structures have been fabricated from 304 stainless steel hollow tubes via an alternating collinear lay-up process followed by bonding using a vacuum brazing method. By varying the tube wall thickness from 50 to 203 lm, square and diamond lattice topologies with relative densities between 0.03 and 0.11 were manufactured in this way and their compressive mechanical responses were characterized. A low temperature nitro-carburization treatment was then performed on a second set of the collinear lattice cores at a temperature of 440 8C for 20 h. The treatment created a thin (10 to 20 lm thick) extremely hard (∗12 GPa) surface layer on the interior and external surfaces of the hollow trusses. This significantly increased the compressive buckling resistance of individual trusses. Compressive strength enhancements (compared with untreated counterparts in the annealed (as brazed) condition) varied from 1.2 for thick walled tubes to 3.8 when the wall thickness was decreased to about twice the hardened layer depth. The moduli and strengths of all the lattices were found to increase with lattice relative density, and were well predicted by micromechanical models. The lowest relative density (thinnest wall) nitro-carburized hollow truss collinear lattice structures exhibited a specific compressive strength significantly higher than that of any other cellular metal reported to date. Nitro-carburized stainless steel collinear lattices therefore appear to be promising candidates for the cores of lightweight sandwich panels intended for elevated temperature and/or multifunctional applications