# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Specific Stiffness** & **Thermal Expansion** & **Thermal Distortion Index** & **Thermal Diffusivity** \\
**Material** & **E/p** & \(\mathbf{\alpha}\) & \(\mathbf{\alpha}\)/k & **k/(p\(<\)p)** \\  & **m\({}^{2}\)/s\({}^{2}\)** & **K\({}^{-1}\)** & **m/watt** & **m\({}^{2}\)/s** \\ \hline Aluminum alloy, 6061-T6 & \(255\times 10^{6}\) & \(23.6\times 10^{-6}\) & \(141\times 10^{-9}\) & \(690\times 10^{-3}\) \\ Corning Pyrex 7740 & \(283\times 10^{6}\) & \(3.3\times 10^{-6}\) & \(2.92\times 10^{-6}\) & \(483\times 10^{-9}\) \\ Corning fused silica 7980 & \(330\times 10^{6}\) & \(520\times 10^{-9}\) & \(400\times 10^{-9}\) & \(788\times 10^{-9}\) \\ Corning fused silica ULE\({}^{\text{\textregistered}}\) & \(307\times 10^{6}\) & \(30\times 10^{-9}\) & \(22.9\times 10^{-9}\) & \(776\times 10^{-9}\) \\ Schott Zerodur\({}^{\text{\textregistered}}\) & \(357\times 10^{6}\) & \(20\times 10^{-9}\) & \(68.5\times 10^{-9}\) & \(721\times 10^{-9}\) \\ Expectation & Large & Small & Small & Large \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relative figures of merit for aluminum and glass mirror substrate materials [3]. Figure 2: (**a**) Example of an arched mirror; (**b**) Example of an open-back mirror; (**c**) Example of a sandwich mirror [19]. As shown in Figure 4, the process chain of AM metal mirrors is different from that of conventional mirrors, mainly in the mirror blank prefabricating stage. After the preparing stage, there are two paths: the design for additive manufacturing (DfAM) and design for conventional machining (DfCM). DfAM is driven by design functionality. It can redesign parts, components, and even systems in AM to achieve the overall requirements with fewer materials and better optomechanical integration. Thus, DfAM has become a new paradigm of advanced design and intelligent manufacturing compared to conventional design. The core technology of DfAM is simulation-driven optimization design technology, including creation-based design technology, topology optimization (TO) design technology, lattice design technology, parameter optimization design technology, and simulation analysis technology [23]. The development of DfAM and AM is changing the pattern of metal mirror designing and manufacturing. The porosity and residual stress of the forming parts during the AM process should be carefully dealt with to avoid warpage, fracture, and other side effects. Hence, densification and heat treatment must be carried out after prefabricating. The subsequent optical processing and test stage are the same for metal mirrors based on conventional processing including optical processing, surface modification, and polishing. These steps are all critical to obtain the required surface accuracy and roughness. Figure 3: (a-d) A series of performance comparison images of 5 lightweight structures. It can be seen that the sandwich mirror has the best mirror efficiency of the [19]. Table 2 shows the intercorrelations among material utilization, machining time, repair rate, and cost of several processes in fabricating complex structural parts [24]. In brief, AM has revolutionized the way of manufacturing optical and structural components compared to conventional subtractive manufacturing. Furthermore, it is easier to achieve the optimal structure to meet the requirements and optomechanical integration combining DfAM means. Though few AM metal mirrors are put into actual engineering, the prospect is still comprehensive, especially in optical remote sensing, which is of tremendous research significance and value. Figure 4: Metal mirrors process chain. This paper aims to review the recent advances of AM metal mirrors and provide a picture of current challenges and future trends. First, the AM technologies applicable for metal mirrors are described, and a comparison of the characteristics among these technologies has been made in Section 2. Some research results on lightweight mirror design based on DfAM are listed in Section 3.