# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 4 Challenges Faced in the Development of AM Metal Mirror
### Dimensional Stability
#### 4.2.2 Process Parameters  
PBF technologies use an energy source to scan the specific areas to melt the powder. There are over 20 process input parameters that need to be set automatically or manually, and possibly more that are not adequately identified, such as scanning speed, scanning path, layer thickness, and laser power. These parameters directly affect the printing speed and the parts' quality [93]. [PERSON] et al. studied the densification mechanism and microstructure evolution law of SLMed Al alloy parts and considered whether the process parameters play a decisive role in the porosity and pore direction [94]. However, the influence of process parameters on formed parts is not a single effect. Only when different  
Figure 34: CT results of AlSi\({}_{10}\)Mg samples: (**a**) deposited state and (**b**) HIPed state [37].  
process parameters cooperate can high-performance parts be obtained. For example, when the laser power is relatively weak and the layer thickness is too large, the alloy powder cannot be completely melted. Therefore, before fabricating high-precision AM parts, stimulation for the whole printing process must be performed to find the best scanning strategy and other parameters [95].  
The first step is to preheat the forming platform and chamber before printing. Then reduce the areas of uneven thickness and avoid significant cross-section changes. The choice of laser power is also crucial. Generally, the central temperature of the laser beams can reach 3500 K, and its choice is related to the melting point of printing powder. The density increases accompany the laser power as the powder absorbs more energy to facilitate total melting. However, the energy densities begin to cause balling at some point, thereby reducing the density and surface roughness. In addition, different laser scanning strategies can be used to minimize the accumulation of residual stress (Figure 35) [96].  
Laser energy density can reflect the influence of scanning speed and laser power on laser energy input. The laser energy density increase can fully melt the power to reduce the pores of the parts. If the laser power density is too high, defects will occur, such as thermal cracks and liquid metal splashes, which can easily happen. A simplified energy density equation has been used by numerous investigators as a simple method for correlating input process parameters to the density and strength of the forming parts. In their simplified model, energy density \(E_{\text{A}}\) can be found using Equation (1) [25].  
\[E_{\text{A}}=\frac{P}{v\times HS} \tag{1}\]  
where \(P\) is laser power, \(v\) is scan speed, and \(HS\) is the scan spacing/hat spacing between parallel scan lines. In this model, energy density increases with increasing laser power and decreases with increasing scanning spacing and velocity. Due to the high thermal conductivity of Al alloy materials, their laser energy is easily consumed during SLM. Therefore, the molten pool temperature and the fluidity decrease results in defects such as pores and cracks. Table 3 gives some of the performance data of AlSi\({}_{10}\)Mg printed with different parameters. Obtaining the appropriate liquid viscosity by adjusting the laser energy density reasonably can possibly restrain the spheroidization effect and the generation of microcracks [97].