Multi-Material Additive Manufacturing of Metallic Auxetic Structures for High Energy Absorption.

This study investigates extrusion-based additive manufacturing of mono- and multi-material metallic auxetic lattice structures for energy absorption applications. Auxetic structures, characterised by a negative Poisson's ratio (NPR), exhibit superior energy absorption compared to conventional cellular materials. 2D re-entrant auxetic samples were fabricated from 316L and 17-4PH stainless steels, including a multi-material combination, using pam o2 HT pellet-based 3D printer followed by de-binding and sintering in an argon atmosphere. Quasi-static compressive testing, supported by full-field Digital Image Correlation (DIC), was used to characterize the mechanical performance of each system, with the aim of assessing how the combination of different materials within a single lattice affects mechanical performance. The 17-4PH lattice achieved the highest Energy Absorption (EA ≈ 21.1 J) and Specific Energy Absorption (SEA ≈ 2.48 J/g), but its stress-strain curve showed unstable oscillations, attributed to the material's stiffness and limited ductility, which cause struts to reach a local instability threshold abruptly rather than yielding progressively. The 316L lattice absorbed the least energy (EA ≈ 8.8 J, SEA ≈ 1.2 J/g) but produced a smooth, stable plateau indicating its greater ductility and progressive strut deformation. The multi-material lattice exhibited intermediate values (EA ≈ 14.5 J, SEA ≈ 1.7 J/g), with a moderately stable curve smoother than 17-4PH but less uniform than 316L, consistent with the compliant 316L struts and grip partially, but not fully, suppressing the instability otherwise driven by the stiffer 17-4PH vertical struts. The study showed that the multi-material achieves above-average volumetric energy absorption and comparable SEA indicating that this particular spatial arrangement reshapes and improves the energy-absorption response. The DIC-derived Poisson's ratio evolution showed the multi-material lattice sustaining a deep, stable auxetic contraction over a substantially longer portion of the loading history. This confirms that the 316L network preserves coordinated re-entrant cell-wall rotation, and therefore reliable progressive collapse, well into the deformation history. Finite Element Models were carried out utilizing the material data obtained from the micro-tensile test but failed to reproduce the experimental curve of any system. Artificially extending the material model narrowed this gap, qualitatively reproducing the experimental curve's general shape. However, it remained wavy and oscillating rather than a clean plateau, and full quantitative agreement was not achieved. This work represents a first attempt to characterise multi-material metallic auxetic structures for lightweight energy absorbers. Future work should refine the material model for improved FEM accuracy and extend characterisation to dynamic loading conditions.