Design and fabrication of hierarchical honeycomb structure with in-situ foaming

• Hierarchical honeycombs with solid–foam sandwich walls were fabricated using FFF. • Achieved up to 63% higher specific energy absorption than solid PLA designs. • Developed design maps for tailoring structures under allowable stress and energy absorption constraints. Achieving high energy absorption with minimal weight remains a key challenge in structural materials design. Drawing on hierarchical architectures observed in natural systems such as woods and bones, this study introduces a second-order hierarchical honeycomb structure fabricated via fused filament fabrication (FFF) process, incorporating solid polylactic acid (PLA) skins and in-situ foamed PLA cores. A design framework was established to explore the effects of geometric parameters and processing conditions—particularly nozzle temperature—on foam morphology and mechanical performance. Through systematic experiments and numerical simulations, it was demonstrated that the proposed solid–foam hybrid structures significantly enhance specific energy absorption (SEA), with the best design achieving up to 63% improvement over conventional solid PLA honeycombs at equivalent density. Design maps were developed to quantitatively guide structure selection under varying energy absorption and allowable stress constraints. This hierarchical design strategy offers a scalable and tunable pathway for lightweight, impact-resistant structures in advanced engineering applications by tailoring the geometric parameters on each hierarchical level.