## 6 Mechanical Properties
### Tensile Strength  
As mentioned above, the IN625 alloy directly formed by the LPBF process consists mainly of a large number of fine \(\gamma\) phases. Therefore, the hardness and corresponding strength of LPBF parts are higher than those prepared by conventional techniques [67]. However, because of the forming characteristics of LPBF, there is large residual stress inside the parts. The plasticity index of LPBF shaped parts without any subsequent treatment is usually difficult to reach the commonly used casting and forging standards. Due to the directional solidification characteristics, there are typical textures that existed in LPBF IN625 alloy parts, which leads to the anisotropy of mechanical properties of the alloy. Thus, not only the hardness but also the tensile properties vary in different directions. The post-treatment can decrease the microstructure anisotropy of the LPBF IN625 alloy and have a significant impact on the mechanical properties. The strength and plasticity of as-built and heat-treated IN625 alloys are shown in Table 2.  
As can be seen from Table 2, the strength of LPBF IN625 alloy parts is generally higher than the wrought part. Although adopting the multi-laser method to increase the construction efficiency, the mechanical properties of the LPBF samples are still comparable with a single laser method and heat treatment at 1048 \({}^{\circ}\)C for 1 h can reduce the strength of the multi-laser specimen and improve the plasticity to a certain extent [61]. The average yield strength and tensile strength of as-built parts obtained by [PERSON] et al. [74] are lower than those by [PERSON] et al. [59], and the elongation after fracture is similar. To one's delight, their tensile properties are similar to those of traditional forged IN625 alloy [75], which is much higher than the minimum tensile properties specified by ASTM B446-03. After annealing [59], the average yield strength decreases, but the tensile strength does not change much. After adding 0.5 wt% multi-wall carbon nanotube (MWCNT), the strength of the IN625 alloy in as-built state increased significantly, but the plasticity also deteriorated accordingly [74]. After solution treatment at 1040 \({}^{\circ}\)C for 2 h and two-step aging treatment (720 \({}^{\circ}\)C for 7 h and 650 \({}^{\circ}\)C for 8 h), the strength of MWCNT-IN625 decreased to a certain extent, but the plasticity was restored, which indicates that it is feasible to achieve the coordination of strength and plasticity of LPBF IN625 parts by heat treatment [74]. According to the research of [PERSON] et al. [75] when the angle between the laser scanning direction and the growth direction of the columnar crystal is changed (0\({}^{\circ}\), 45\({}^{\circ}\), 90\({}^{\circ}\)), the tensile properties of \"horizontal\" and \"vertical\" samples are almost the same, indicating that mechanical properties will not be affected by the scanning strategy. In addition, the study of [PERSON] et al. [76] shows that the \(\delta\) phase precipitated in the process of stress relief annealing at 870 \({}^{\circ}\)C and the carbide precipitated in the solution treatment at 1120 \({}^{\circ}\)C did not cause the high-temperature plastic loss of LPBF IN625 alloy, mainly because of that the deformation of the solution treated alloy at high-temperature is accompanied by dynamic strain aging, while the deformation after stress relief and medium temperature annealing are accompanied by dynamic recrystallization. Therefore, there is a competition between dynamic strain aging and dynamic recrystallization at high-temperature. The former decreases the plasticity while the latter increases the plasticity of the alloy.