# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 2 Additive Manufacturing Technology for Metal Mirrors
### Metal Additive Manufacturing Technology  
Metal AM technologies fall into two categories: (Figure 6) powder-bed fusion (PBF) and directed energy deposition (DED). DED covers 16% of the metal AM market and uses laser beams, electron beams, or electric arcs as energy sources to deposit the melting powder. It uses coaxial or lateral powder feeding methods, which are more practical than PBF's laying powder. Still, the accuracy of DEDed products is not as high as the PBFed. DED is more suitable for manufacturing large and high-performance integral components.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Forming Process** & **AM** & **Forging** & **Casting** \\ \hline Material utilization rate & 2/3 & \textless{}1/10 & 1/5 \\ Design modification time & 1-2 days & 6 months & 3 months \\ Processing time & 1-2 days & 4 months & 6–12 months \\ Consumable & Inert gas & Mold & Mold, gating system \\ Repair rate & Low & Low & High \\ Cost & Low & High & Medium \\ \hline \hline \end{tabular}
\end{table}
Table 2: A comparison of AM and the conventional manufacturing methods for complex structural components [24].  
Figure 5: Process flow chart of AM.  
PBF is ideal for small, ultra-complex monolithic components such as metal mirrors, owing to its products' good surface finish and high density [27,28].  
Figure 7 shows the schematic of SLS. The first step is to coat a layer of powder on the platform, then preheat the powder to near the melting point to reduce the residual stress. After that, the laser beams illuminate specific areas on the cross-section according to the data of the corresponding sliced model's layer. Using the same technique, repeat the process of laying powder and sintering until a part forms. The principles of other PBF techniques are similar to this, repeating the coating and scanning processes. Table 3 presents an overview of the materials and AM technologies for fabricating metal mirrors in recent years. It can be seen that all the mirrors are fabricated using PBF, namely EBM, SLS/DMLS, and SLM. The most common choice is printing AISi\({}_{10}\)Mg via SLM. This section describes several PBF technologies and compares the pros and cons of each process for AM.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**R and D Unit** & **Time** & **Material** & **Technology Type** & **Surface Accuracy** \\ \hline Corning [29] & 2015 & AlSi\({}_{10}\)Mg\({}_{0.3}\) & DMLS & 1.5 nm (RMS) \\ General Dynamics [21] & 2015 & AlSi\({}_{10}\)Mg & DMLS & 43.2 nm (RMS) \\ University of Arizona [30] & 2015 & AlSi\({}_{10}\)Mg & DMLS & 255 nm (PV) \\ University of Arizona [30] & 2015 & Ti6 Al4V & EBM & / \\ Lockheed Martin [31] & 2016 & AlSi\({}_{10}\)Mg & SLM & / \\ Optimax Systems [32] & 2017 & FeNi\({}_{36}\) & SLM & / \\ IOF [33] & 2018 & AlSi\({}_{12}\) & SLM & 12.5 nm (RMS) \\ IOF [34] & 2019 & AlSi\({}_{40}\) & SLM & 7.3 nm (RMS) \\ UKAT [18,35,36] & 2020 & AlSi\({}_{10}\)Mg & DMLS & 16 nm \\ CIOMP [37] & 2020 & AlSi\({}_{10}\)Mg & SLM & 58 nm (RMS) \\ \hline \hline \end{tabular}  
* The examples of \({}^{*}\)/\({}^{*}\) are to research the feasibility of designing and fabricating AM metal mirror. The mirrors’ surface accuracy is relatively poor. Therefore, the authors did not give the surface accuracy of mirrors.  
\end{table}
Table 3: Summary of materials and AM technologies of manufacturing metal mirrors in recent years.  
Figure 6: Classification of metal AM technology.  
Figure 7: Schematic diagram of SLS.