# Design and Fabrication Technology of Metal Mirrors Based on Additive Manufacturing: A Review
## 4 Challenges Faced in the Development of AM Metal Mirror
### Heat Treatment  
As mentioned above, residual stress is a critical factor affecting dimensional stability. Like welding, there is much residual stress in metal AM parts. In the process of PBF, each new layer of a part is formed by moving the laser beams through the powder layer, melting the top layer of powder and fusing to the layer below at the same time. The heat flows  
Figure 35: Characteristics of different scanning strategies [23].  
from the pool to the solid metal below, so the temperature gradient of the melting zone is considerable. It is constrained below the solid structure when a new layer is cooled and solidified on the previous layer [98]. Therefore, shear force is generated between layers because of shrinking. Then, holes, delamination, thermal deformation, and cracking may appear, which deteriorates the dimensional accuracy and mechanical properties of end-use parts. To sum up, residual stress is one of the main challenges in the high-quality manufacturing of metal AM products.  
Heat treatment is the most essential and practical measure for reducing residual stress. The main principle is to evenly heat a part and let it soak through the whole part, both the thick sections and thin sections, until it reaches a temperature equilibrium [23; 99; 100]. The heat treatment for SLMed aluminum alloy parts mainly includes annealing and aging. The main purpose of annealing is to reduce residual stress, eliminate defects, and improve ductility. Aging is a strengthening heat treatment, and its main purpose is to further improve the strength of the alloy. Aging can be also divided into artificial aging and natural aging. At present, T6 heat treatment is the most commonly used. The heat treatment temperature and time of different compositions and content of alloy elements vary greatly. If the heating temperature is too high or the holding time is too long, abnormal grain growth and burning easily to occur, which reduces the mechanical properties. Shielding gas needs to be added in order to relieve stress, even in the vacuum furnace in some exceptional cases. Moreover, the excellent design of parts is also the crux of eliminating residual stress, which minimizes the residual stress and maximum dimensional stability.  
The mechanical properties of AlSi\({}_{10}\)Mg after heat treatment can be found in Table 3. Table 3 shows that after T6 heat treatment, yield strength and ultimate tensile strength increase remarkably. However, for other heat treatment experiments, yield strength and ultimate tensile strength decrease. This can be attributed to the Si phase undergoing thermally activated growth during heat treatment at a high temperature. Therefore, the appropriate parameters should be selected before carrying out the heat treatment. In addition, the precipitated Si phase is spherical, which can reduce the local shear and restrain crack initiation and propagation when deformed. In addition, an increase in the Si particle size and a decrease in the number of Si particles reduce the local stress and strain during tensile loading [101]. Generally, proper heat treatment plays a positive role in the later processing procedure for printing mirror blanks.