## 8 AM Techniques for Crash Boxes
### Hybrid Manufacturing Techniques
The experimental results revealed that the hybrid structure achieved an optimal \(SEA\) increase of 83.02% compared to an empty tube. Moreover, the \(E_{\text{T}}\) of the hybrid structure was 43.40% higher than the sum of its individual components, highlighting the synergistic effect between the lattice and tubular elements. Thin-walled structures exhibit excellent crashworthiness; however, their energy absorption efficiency often diminishes as wall thickness increases, leading to undesirable weight gain. To address this trade-off, the incorporation of foam fillers has emerged as a viable strategy to simultaneously reduce weight and enhance the energy absorption capacity in thin-walled designs. [PERSON] et al. [131] proposed X-shaped rib-reinforced foam-filled tubes, illustrated in Figure 23f, considering both aluminum foam and additively manufactured PLA foam fabricated using FDM as lightweight fillers. The results indicated that tubes that are diagonally filled with PLA foam blocks possessed a better energy absorption capacity. Extending the same concept to lattice structures, [PERSON] et al. [132] examined two configurations of bipyramid octagonal lattice microstructures with equal density, as shown in Figure 23g. The specimens, made from PLA and PA using FDM, are injected with foam in their porous spaces. The results indicated that foam integration raises \(SEA\) in all cases, with PLA Type-1 increasing from 0.1 J/g to 7.6 J/g and Type-2 increasing from 1.2 J/g to 7.4 J/g, confirming foam as the dominant factor in improving energy absorption in hybrid systems. Additionally, PLA consistently outperforms PA in all tested configurations. To leverage the distinct advantages of various AM techniques, [PERSON] et al. [133] introduced a novel foam-filled spiral tube (FFST) inspired by natural spiral structures, as illustrated in Figure 23h. The spiral tubes (STs) and random foam are both additively manufactured, with the STs produced using SLM from stainless steel 316L, while the foam is fabricated using FDM with PLA material. The results reveal intricate interaction mechanisms between the foam and the spiral tube, leading to enhanced energy dissipation. Optimization studies indicate that the FFST design achieves an \(SEA\) of 11.97 J/g, highlighting the effectiveness of combining different AM techniques. The SEA values corresponding to various additive manufacturing techniques used in crash box fabrication, including SLA, MJT, SLM, SLS, FDM, and hybrid manufacturing techniques, are presented in Figure 24, based on findings from recent studies. The mean \(SEA\) values for SLA, SLM, FDM, and hybrid manufacturing techniques are 0.77 J/g, 28.65 J/g, 11.20 J/g, and 13.26 J/g, respectively. SLM-produced crash boxes exhibit the  
Figure 23: Hybrid manufacturing techniques: (**a**) honeycomb hybrid crash box [126]; (**b**) aluminum/PLA hybrid tubes [127]; (**c**) aluminum/nylon hybrid tubes [128]; (**d**) multi-cell tube with lattice structure [129]; (**e**) lattice reinforced thin-walled tubes [130]; (**f**) X-shaped rib-reinforced foam-filled tubes [131]; (**g**) octagonal bipyramid structure [132]; (**h**) foam-filled spiral tube [133]. highest energy absorption capacity, likely due to the superior mechanical properties of metal-based materials used in this technique. In contrast, SLA-produced crash boxes have the lowest \(SEA\), reflecting the brittle nature of photopolymer resins. FDM, which is commonly used for fabricating both polymer and fiber-reinforced polymer crash boxes, shows moderate \(SEA\) values with a relatively high standard deviation of 9.38 J/g, indicating variability in energy absorption performance depending on material selection. Hybrid manufacturing techniques also demonstrate moderate \(SEA\) values, with a standard deviation of 7.25 J/g, suggesting a balance between different material properties.