# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 4 Challenges Faced in the Development of AM Metal Mirror
### Porosity
#### 4.1.1 Powder Quality  
The quality of the powder directly affects the finished products. The main parameters of powder are particle size, sphericity, fluidity, and density. PBF requires infinitesimal powder, a narrow size range, and a sphericity of 98% or more to manufacture high density and performance parts [27]. The more refined powder, the smaller the pores between particles, so the denser powder layer. The fine powder melts more with the same heat source than coarse powder because of the more prominent surface area.  
Consequently, it lends to the greater sintering drive, which is conducive to obtaining excellent metal parts. Generally, the suitable particle size range for SLM powder is 15-53 \(\upmu\)m. If the range distribution is vast, the surface of powder laying will be delaminated. That will also result in the uneven melting of metal powder and a severe increase in the porosity or surface roughness of formed parts [23]. Powder with the normal distribution of particle size range has the best effect. Owning to the high energy conversion rate and the high energy density of electron beams, the suitable particle size range for EBM powder is 53-150 \(\upmu\)m. Other parameters are similar to those of the SLM.  
The manufacturing of high-quality powder remains a critical challenge due to its high surface area and susceptibility to oxidation. In the process of SLM, the gas elements may form holes and other local defects, resulting in the reduction in densification and mechanical properties of parts. Therefore, the content of oxygen, nitrogen, and other elements shall be strictly controlled. Generally, the mass fraction of oxygen in metal is less than 0.15 wt%, and the mass fraction of nitrogen in metal is less than 0.05 wt% [81]. An assessment of the manufacturing routes of powders and their respective performances during the AM processes is important. The most commonly used is conventional gas atomization (GA), which has the advantages of high capacity, good efficiency, and a low cost [82; 83]. The powder cannot satisfactorily print the high-quality and high precision parts due to deficiencies of varying powder size and high oxygen content. Other ways of mainly making alloy powders are plasma rotating electrode process (PREP) and plasma atomization (PA) [84].  
Figure 32 shows the SEM images of Ti-6 Al-4V powder generated by different methods. PREPed powders are perfectly spherical in shape with smooth surfaces. However, high-quality powders are expensive because of the high cost of the fabrication process, such as PREP, and the low yield of the atomization process. In the PBF process, the solid powders can be reused to reduce cost, although such reuse of powder particles results in an irregular shape and poor surface finish of the final part [85]. Therefore, powders used as feedstock materials must be selected by considering both their quality and cost in association with the corresponding AM process. So, the powder with better sphericity and more uniformity is obtained, which meets the requirements of preparing metal mirrors.