## 4 Discussion
### Quality Assessment and Defect Analysis Comparison  
The defect analysis for NickelAlloy HX correlates directly with the mechanical properties (Table 6). The relative density of 99.8% was achieved by decreasing the pore sizes \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Material** & **Reference** & **Build Orientation** & \begin{tabular}{c} **Yield Strength** \\ **(MPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile Strength** \\ **(MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Elongation (\%)** \\ \end{tabular} \\ \hline EOS NickelAlloy HX & This study & XY & 716 \(\pm\) 5 & 827 \(\pm\) 7 & 19.2 \(\pm\) 1.1 \\ Hastelloy X & [18] & Z & 480 \(\pm\) 10 & 620 \(\pm\) 15 & 40.0 \(\pm\) 2.0 \\ Hastelloy X & [17] & Z & 792 \(\pm\) 1 & 923 \(\pm\) 9 & 12.0 \(\pm\) 0.5 \\ Hastelloy X & [18] & XY & 663 \(\pm\) 12 & 773 \(\pm\) 9 & 22.4 \(\pm\) 1.5 \\ Inconel 718 & [19] & Z & 559 \(\pm\) 15 & 782 \(\pm\) 18 & 31.0 \(\pm\) 2.1 \\ Inconel 718 & [21] & XY & 1211 \(\pm\) 24 & 1406 \(\pm\) 21 & 13.6 \(\pm\) 4 \\ Inconel 625 & [16] & XY & 559 \(\pm\) 20 & 894 \(\pm\) 25 & 30.0 \(\pm\) 3.2 \\ CM247 LC & [22] & Z & 690 \(\pm\) 25 & 791 \(\pm\) 18 & 1.15 \(\pm\) 0.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Mechanical properties comparison of LPBF nickel-based superalloys (as-built condition). and defect density, which were particularly visible in the mechanical analysis and high elongation values. [PERSON] et al. [28] demonstrated that porosity levels below 0.5% are critical for achieving optimal strength-ductility combinations in LPBF nickel alloys, a threshold clearly exceeded in this study. The present study demonstrates that, although volumetric energy density (E) provides a useful first-order guideline for LPBF parameter selection, the complex interplay between laser power (P), scan speed (v), and hatch spacing (h) must be considered to fully understand porosity formation, microstructure evolution, and mechanical anisotropy in EOS Nickel Alloy HX. Our finding that an energy density of \(\approx\)111 J\(\cdot\)mm\({}^{-3}\) (P = 120 W, v = 900 mm\(\cdot\)s\({}^{-1}\), h = 0.06 mm) yields the highest relative density (99.96%) and balanced hardness highlights the benefits of operating in the conduction-mode melting regime, where stable melt pools minimize keyhole porosity and ensure good interlayer bonding ([PERSON] et al. [9]; [PERSON] et al. [10]). However, equating different P\(-\)v-h combinations to the same E can be misleading. For example, specimens processed at 100 W/800 mm\(\cdot\)s\({}^{-1}\)/0.07 mm (E = 179 J\(\cdot\)mm\({}^{-3}\)) showed significantly lower density (98.7%) than those at 120 W/900 mm\(\cdot\)s\({}^{-1}\)/0.06 mm, despite similar E values. We attribute this to the transition from conduction- to keyhole-dominated melting at high E, which leads to unstable vapor cavities that collapse into spherical porosity ([PERSON] et al. [8]; [PERSON] et al. [28]). Conversely, at low E (\(<\)75 J\(\cdot\)mm\({}^{-3}\)), insufficient melt-pool overlap and depth result in lack-of-fusion defects characterized by irregular, interconnected pores ([PERSON] et al. [2]; [PERSON] et al. [8]). Microstructural examination revealed a fine cellular-dendritic substructure (0.5-1.5 \(\upmu\)m spacing) within columnar grains, consistent with rapid solidification rates (10\({}^{5}\)-10\({}^{6}\) K\(\cdot\)s\({}^{-1}\)). The columnar-to-equiaxed grain morphology change between the XZ and XY planes underlies the pronounced tensile anisotropy: higher UTS in XY (827 MPa) versus greater ductility in XZ (A = 50.8%). Similar behavior has been reported for LPBF Inconel 718 and Hastelloy X, where directional solidification aligns grains along the thermal gradient, promoting anisotropic mechanical response ([PERSON] et al. [21]; [PERSON] et al. [20]). Post-build heat treatments (solutionizing plus aging or hot isostatic pressing) could promote recrystallization and homogenization of solute segregation, thereby reducing both microstructural anisotropy and residual porosity ([PERSON] et al. [29]).