# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 5 Conclusions and Future Trends  
The present review shows significant progress has been made in designing and manufacturing AM metal mirrors and lists the problems encountered in the process and solutions. The designing and manufacturing criteria for the metal mirrors are light weight and high stiffness. Furthermore, the sandwich mirror is the primary design in this field because of its well-known stability and light weight. However, compared with the conventional processing method, the difficulty and time of manufacturing sandwich mirrors can be significantly reduced by AM. In addition, there are several internal structure design methods based on DfAM, such as lattice, TO, and Voronoi, which greatly increase the freedom of mirrors design. SLM has become the most mature and effective metal 3D-printing technology for fabricating metal mirrors now. In the future, material limitations, excessive residual stress in parts, continuous improvement of requirements, a light weight, and change of machining processes are all main factors driving the development and selection of next-generation products. Therefore, further work on AM metal mirror should focus on the following three topics:  
(1) Explore and develop other materials suitable for AM metal mirrors, such as beryllium and beryllium-aluminum alloy, and adopt current metal AM techniques to a broader range of materials. Previously, General Dynamics prefabricated a beryllium-aluminum alloy mirror, but the result was not as expected [21]. We should focus on matching processing technologies and powder materials, such as printing beryllium-aluminum alloy powder with high melting points and high-power electron beams. Moreover, AM technologies can also fabricate other materials' optical components, such as ceramic mirrors, polymer lenses, and liquid lenses [103; 104; 105]. Utilize them in the application of optical instruments and precision engineering as soon as possible;  
(2) High-entropy alloys, magnetic alloys, bulk metallic glasses (BMG), functionally graded materials (FGM), and nano-architected metals are all materials for which advanced research is developing now [106]. Amorphous metals have high elasticity and fracture resistance advantages and are lightweight and beneficial for obtaining high-quality AM products. Moreover, amorphous metal 3D printing technologies, which are ideal for fabricating metal mirrors, can help decrease the internal stress and porosity of the AM products as well as optimize the system's dimensional stability and environmental adaptability. Figure 39 shows a 3D-printed gear of amorphous metal. However, it is a challenging technology that is limited by multiple factors, such as material crystallization due to the high temperatures, residual stress relieving, and laborious post-processing. Therefore, related research is in the exploration stage [107];  
Figure 39: A 3D-printed gear of amorphous metal [108]. (3) The idea of the integration of macro and micro has been advanced to realize a lightweight design. In Section 3.4, researchers from Lockheed Martin have created a metal mirror which is ultra-lightweight and stiff by combining TO with 3D Voronoi. Furthermore, we should give full play to the idea of multiscale optimization in the design of AM metal mirrors, for example, combing the lattice with TO or Voronoi (Figure 40). In the future, this idea will provide strong technical support for AM metal mirrors, as well as the development of related disciplines and industries. Though, few AM metal mirrors are put into practical engineering applications. With the continuous development of the AM parts' densification and surface treatment processes, metal mirrors' mechanical property and dimensional stability will greatly improve. In a few years, AM metal mirrors can be used at the visible level and even the laser level, so they have broad application prospects. Conceptualization, K.Z. ; methodology, J.Z. ; software, C.W. ; formal analysis, H.Q. ; investigation, X.X. ; resources, L.Y. ; writing--original draft preparation, K.Z.