# Design of Fiber-Composite/Metal-Hybrid Structures Made by Multi-Stage Coreless Filament Winding
## 2 Materials and Methods
### Additively Manufactured Winding Pins  
The state-of-the-art winding pin in CFW is a sleeve-washer combination, which restricts the vertical directions from which fiber strands can connect to the anchor element. A new adaptive winding pin was developed [44] that adds individually alignable arms to absorb fibers. They were utilized for this study to allow a direct connection from the upper tension cord to the lower base plate without introducing performance-reducing fiber kinks. A disk at the end of each arm prevents the fiber from slipping off. Recessed measuring tips are located at the outer disk surface as they remain accessible even if the pin is completely filled. Using these tips, the position of the pin can be measured, for example, by means of a robotic system. An additive manufacturing process allows the individualization of each winding pin to optimally suit the local fiber net configuration. The number of arms per pin can be adjusted, and their vertical and horizontal orientations do not have to be a regular pattern. In order to be able to transfer the necessary loads from the bolt connection to the fiber net, the winding pins were made of aluminum alloy powder (AlSi10 Mg), utilizing an LPBF machine [51] equipped with a 400 W ytterbium-fiber-laser [52] and application-adjusted settings (Table 2). The dimensional accuracy of the manufactured winding pins is critical to their function as connectors and fiber anchors. It was evaluated by comparing computer tomography (CT) scans to the CAD model (Figures 6 and 7). The tube of the scanner [53] was set to 225 kV at 597 \(\mu\)A. The volume measurement is calculated by filtered back projection, based on the attenuation of the X-rays due to the object geometry and its material density. The maximum geometrical deviation of the winding pins caused by surface irregularities was measured to be below 200 \(\upmu\)m for the majority of the functional surfaces (Figure 6a). Deviations in this range may be induced by residual thermal stresses occurring during the LPBF fabrication process. Larger deviations (Figure 6c) from the target geometry were generated by remnants of the support structure (Figure 7), which is needed to realize overhangs but also to dissipate process heat. Thus, the positioning of the support structure \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline laser power (max. 400 W) & 350 & W \\ laser beam diameter at focal point & 80 & \(\upmu\)m \\ scan speed & 1650 & mm/s \\ hatch distance & 130 & \(\upmu\)m \\ scan vector length & 10 & mm \\ layer thickness & 30 & \(\upmu\)m \\ build platform preheating temperature & 150 & \({}^{\circ}\)C \\ rotation angle increment & 67 & \({}^{\circ}\) \\ fill pattern type & stripes & \\ inert gas & argon & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Machine settings used for the LPBF fabrication of the winding pins. Figure 6: Computer tomography scans of the winding pins. Red areas represent oversized geometry, while blue represent undersized geometry in comparison to the target geometry. (**a**) CT-scan of the 4-arm winding pin, scale range from \(-\)0.2 mm to +0.2 mm; (**b**) Photo of the 4-arm winding pin; (**c**) CT-scan of the single-arm winding pin, scale range from \(-\)0.3 mm to +0.2 mm; (**d**) Photo of the single-arm winding pin. Figure 7: Support structure (blue) needed for the LPBF fabrication process. (**a**) Single-arm winding pin; (**b**) 2-arm winding pin; (**c**) 3-arm winding pin; (**d**) 4-arm winding pin; (**e**) Single-arm winding pin with secondary mounting point. in accordance with the process parameters is decisive for the realization of monolithic components with such small dimensions--the outer disk diameter equals 10 mm. Due to the direct contact of the rovings with the winding pin surfaces, the surface roughness is important to achieve high fiber deposition quality. The surface roughness at the pole of an untreated winding pin is in the range of 30-60 \(\upmu\)m (mean arithmetic height).