## 3 Experiment and Result Analysis
### Mechanical Property Analysis  
The hardness and elastic modulus of the samples were measured using a nanoindentation instrument. The displacement-load curve obtained by nanoindentation shown in Figure 11a includes three stages: loading, holding at maximum load, and unloading. The maximum load was 300 mN, and the maximum penetration depths of the samples at the end were 1841.99 nm, 1889.20 nm, 1926.03 nm, and 1964.27 nm. The maximum hardness was obtained when the laser power was 285 W. The nano-hardness and elastic modulus of the samples under different laser powers are shown in Figure 11b. As the laser power increased from 285 W to 360 W, the nano-hardness gradually decreased, but the change in elastic modulus was not significant. The nano-hardness of the four groups of samples was 3.43 GPa, 3.10 GPa, 3.01 GPa, and 2.09 GPa; the elastic modulus was 205.72 GPa, 205.11 GPa, 203.24 GPa, and 199.91 GPa, respectively.  
According to Section 3.1, as the laser power increased from 285 W to 360 W, the density of the samples gradually decreased, but the porosity gradually increased. The load acting on the surface of the sample caused a significant collapse, resulting in a gradual decrease in the hardness. In addition, according to Section 3.3, as the laser energy density increased with the increase in laser power, heat accumulation and overheating occurred, leading to the coarsening of the microstructure. According to the principle of fine grain strengthening, as the microstructure coarsens, the mechanical properties, i.e., hardness, decrease.  
The mechanical properties of the SLM-formed samples were mainly restricted by the precipitation strengthening mechanism, fine grain strengthening mechanism, grain boundary fraction, and pore defects. Since chemical element segregation and precipitation of components in the morphologically different samples were small, the impact of the precipitation hardening mechanism on the strength difference was negligible. A large number of low-angle grain boundaries can hinder the movement of dislocations, thereby enhancing the strengthening effect through the action of grain boundaries [32]. The pores are caused by the increase in laser energy density as the laser power increases, resulting in an increase in input heat. The resulting spherical evaporation holes have no sharp corners and will not cause severe stress concentration under tensile load [33]. Therefore, when the  
Figure 11: SLM-formed Inconel 718 samples under different laser powers: (**a**) displacement–load curves, (**b**) nano-hardness and elastic modulus Er.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Laser Power** & **310 W** & **335 W** & **360 W** \\ \hline Grain size/\(\upmu\)m & 47.49 & 50.57 & 47.95 \\ Fraction of HAGBs & 35.4\% & 47.0\% & 31.5\% \\ Fraction of LAGBs & 17.8\% & 15.2\% & 20.1\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: Average grain size (d) and grain boundary fraction of Inconel 718 formed by SLM under different laser powers.  
laser power is increased to the maximum, the highest tensile strength and yield strength will be obtained, while the elongation and area reduction will not show obvious trends.