## 6 Mechanical Properties  
### Hardness Distribution  
In general, hardness is positively related to the tensile strength of the material and higher hardness means higher strength of the material. Therefore, measuring the hardness of the LPBF samples can provide some references for the strength of the samples. The LPBF IN625 alloy has a large degree of anisotropy in hardness produced by typical anisotropic microstructure, and the plane perpendicular to the building direction exhibits higher hardness due to the higher concentration of the barriers to dislocation under the surface [45; 66]. [PERSON] et al. [71] studied the effects of laser energy density (LED) on the microhardness of LPBF IN625 samples. It was found that when the LED was increased from 400 J/m to 1200 J/m, the microhardness increased from 300 to 350 HV\({}_{0.3}\), but the increase was not very large. However, when 5 wt% of TiB\({}_{2}\) particles were added to the original powder, the hardness of the IN625 formed part was significantly improved due to the change in microstructure. When the LED was increased from 400 J/m to 1200 J/m, the microhardness was increased from 600 to 700 HV\({}_{0.3}\). The maximum average microhardness (293 HBW) of the LPBF IN625 sample with the lowest porosity (0.036%) was obtained by [PERSON] et al. [72] at a scanning speed of 900 mm/s with a laser power of 175 W and a hatching distance of 0.07 mm values. They also pointed out that the reduction of hatching distance can improve the density and hardness of the part, while high scanning speed had the opposite effect. To obtain a hardness suitable for the working environment, many researchers have also studied the influence of heat treatment and second phase particles on the hardness of LPBF IN625 parts. [PERSON] et al. [55] studied the relationship between annealing temperature and hardness of LPBF IN625 samples. Due to the very fine structure and the solid solution strengthening effect of many solute elements, the average hardness of as-built IN625 samples was very high (343 HV), which was even higher than the 305 HV obtained by the traditional forging process. When the annealing temperature was 700 \({}^{\circ}\)C, the hardness of the sample decreased due to the relaxation of residual stress. When the annealing temperature was 800 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, the hardness increased due to the precipitates of the \(\delta\) phase (Ni\({}_{3}\)Nb) in the annealing process. Above 1000 \({}^{\circ}\)C, the \(\delta\) phase was dissolved in the Ni matrix, so the hardness decreased rapidly. [PERSON] et al. [61] printed IN625 alloy using the multi-laser method and found that the average hardness (284.25 HV\({}_{0.5}\)) and grain size (10-100 \(\upmu\)m) of the samples obtained were the same as those reported in the literature (280 HV\({}_{0.5}\)[73]) by a single laser. They also found that heat treatment could reduce the hardness (271.08 HV\({}_{0.5}\)). In the work conducted by [PERSON] et al. [49], the hardness of the LPBF IN625 sample did not have any tendency in the depth direction, and the hardness of the track was higher than that of HAZ. They also found that the heat-treated sample can show a lower microhardness value than the as-built sample [50]. Similar results were obtained by [PERSON] et al. [67], although the microhardness measured in the build (Z) direction changed with the increase in laser power, the change was not significant when considering