## 8 AM Techniques for Crash Boxes
### Hybrid Manufacturing Techniques  
Hybrid manufacturing techniques, which integrate various AM techniques or combine AM components with traditionally manufactured parts, offer a promising approach to leveraging the efficiency of crash boxes. By combining various AM techniques, components benefit from the unique advantages each method offers. Furthermore, integrating AM components with conventionally manufactured parts enables the fusion of AM's design flexibility with the material robustness and cost-effectiveness of traditional manufacturing [121]. Moreover, the interaction between AM components and traditional parts enhances overall energy absorption capacity, as the custom geometries of AM elements effectively dissipate impact energy, while traditional materials provide essential structural stability. Several studies employed hybrid manufacturing techniques to enhance the energy absorption capacity of crash boxes [122; 123; 124; 125]. Recent advancements in crash boxes combining aluminum with additively manufactured polymer components have demonstrated significant improvements in crashworthiness. [PERSON] et al. [126] investigated a honeycomb hybrid crash box design consisting of an aluminum circular tube filled with a carbon-fiber-reinforced PLA honeycomb structure manufactured via FDM, as shown in Figure 23a. Three variations of crash box models are examined: circular tube, honeycomb structure, and honeycomb hybrid crash box. The honeycomb hybrid crash box design showed a 17.95% increase in \(E_{\text{T}}\) compared with the sum of the circular tube and honeycomb structure. Further advancing this research direction, [PERSON] et al. [127] investigated aluminum and PLA tubes, multi-cell PLA structures,  
Figure 22: FDM: (**a**) tubes with different infill patterns [117]; (**b**) honeycomb structure [118]; (**c**) cubic lattice structure [119]; (**d**) Voronoi lattice structure [120]. and aluminum/PLA hybrid tubes, as shown in Figure 23b, with PLA structures also produced using FDM. The results revealed that the \(E_{\text{T}}\) of the aluminum/PLA hybrid tube increased by 59% compared to the single-cell aluminum tube. Additionally, the \(E_{\text{T}}\) of the aluminum/PLA hybrid tube exceeded the combined values of the PLA and aluminum tubes as well as the PLA multi-cell structures. Extending the exploration of aluminum-polymer hybrids, [PERSON] et al. [128] further explored aluminum/nylon hybrid tubes, as shown in Figure 23c. The nylon specimens are fabricated using the FDM technique, utilizing nylon 66 as the printing material. The study examined both single-cell and quadruple-cell hybrid tubes, along with pure aluminum tubes. The results revealed that the nylon quadruple-cell tube, along with the interaction effects, exhibited a 45.2% increase in \(SEA\) and a 41.1% increase in \(CFE\) compared to pure aluminum tubes. The incorporation of tubular structures manufactured using traditional methods and lattice structures fabricated through 3D printing is an effective strategy for enhancing the energy absorption performance of crash boxes. [PERSON] et al. [129] introduced a multi-cell tube integrated with a novel lattice structure, as shown in Figure 23d. In this design, the lattice structure is fabricated from aluminum alloy AlSi10 Mg using the Selective Laser Melting (SLM) technique, while the multi-cell tubes are produced from aluminum Al6063-T5 using conventional manufacturing. The results show that the hybrid configuration can absorb up to 30.36% more \(E_{\text{T}}\) compared to the cumulative absorption of the individual components. Building on this approach, [PERSON] et al. [130] proposed a hybrid structure combining lattice-reinforced thin-walled tubes with varying cross-sectional cell counts and gradient densities, as illustrated in Figure 23e. The square aluminum tubes are cut from the Al1060 alloy using electrical discharge machining, and the lattice structures are additively manufactured from AlSi10 Mg using SLM.