Compressive mechanical properties of hierarchically porous 316L stainless steel interpenetrating composites under quasi-static and dynamic loadings

In this study, a hybrid fabrication strategy is proposed to produce hierarchically porous, interpenetrating stainless steel composites with enhanced quasi-static and dynamic mechanical performance under compression. Freeze casting was employed to fabricate unidirectional micro-channels and was combined with additively manufactured reinforcement featuring triply periodic minimal surface (TPMS) geometry. The hierarchical structures of the freeze-casted scaffolds, 3D-printed reinforcements, and hybrid-structured composites were characterized by scanning laser confocal microscopy, scanning electron microscopy (SEM), and projection X-ray microscopy (PXM). Quasi-static compression tests showed that the mechanical properties of the hybrid-structured porous materials can be tuned by varying the repeating units and relative densities of the reinforcements. The measured porosity, elastic modulus, yield strength, plateau stress, densification strain, and energy absorption ranged from 52.3-64.7 %, 1.1-5 GPa, 30.7-56.6 MPa, 72.3-153.7 MPa, 43.8-48.5 %, and 22.6-48.1 MJ/m 3 , respectively. The highest energy absorption was achieved in samples with four repeating units and a relative density of 20 vol.%. The hybrid-structured porous materials exhibited higher plateau stress and energy absorption compared to other porous composites, highlighting the advantages of the interpenetrating architecture and reinforcement design. Under quasi-static compressive loading, the hybrid-structured composites demonstrated uniform collapse without phase separation. Under dynamic loading using direct impact Hopkinson bar (DIHB), the hybrid-structured composites retained their plastic deformation capability and structural integrity, with mechanical properties improved by more than 10%. In summary, this study demonstrates a viable strategy for producing hierarchically porous, interpenetrating metallic composites through the integration of additive manufacturing and freeze casting, offering potential applications in impact protection and lightweight energy-absorbing components.