## 4 Discussion  
The attainment of a high relative density in LPBF depends on the ability to obtain the right process parameter combinations. The 80 \(\upmu\)m hatch distance varied with scanning speed yielded a high relative density of 99.8% at 1000 mm/s scanning speed. However, when scanning speed increased to 1500 mm/s, cracks became pronounced. The cracks which are believed to be residual stress-induced cracks were, however, not seen when the hatch distance was increased to 140 \(\upmu\)m and the scanning speed was comparatively reduced (550-850 mm/s). The role of scanning speed, as studied by various authors [23, 26, 41], is critical not only in determining the amount of volumetric energy density available for complete melting but also the magnitude of local residual stress build-up. Overly increased scanning speed results in insufficient interaction between laser beam and material because high scanning speed leads to reduced energy density available for complete melting, which would lead to lack of fusion defects. High scanning speeds also lead to a faster cooling rate which may generate cracks due to the high residual stress build-up as already explained. This is believed to have resulted in the cracks seen in Figure 3e,f. No such defect was seen with an increase in hatch distance to 140 \(\upmu\)m. This may be due to the comparatively lower scanning speed (550-850 mm/s) regime which gave sufficient beam-material interaction time to enhance good metallurgical bonding, not only between tracks but also with underlying layers. The lower scanning speed regime (550-850 mm/s) at the higher hatch distance (140 \(\upmu\)m) may have generated considerable local residual stress which was low enough to avoid crack \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Material: G-NiCu30 Nb** & & & \\ \hline  & **UTS (MPa)** & **YS (MPa)** & **EI (\%)** \\ As-Cast [16] & 450 & 170 & 25 \\ LPBF as-built (V) at r.t.p & 739 & 531 & 20 \\ LPBF as-built (H) at r.t.p & 731 & 567 & 16 \\ LPBF as-built + PHT (V) & 674 & 408 & 31 \\ LPBF as-built + PHT (H) & 665 & 417 & 25 \\ \hline \hline \end{tabular} As-built process parameters (tensile specimens): laser power (LP)—200 W, layer thickness (Ds)—30 \(\upmu\)m, hatch distance (vs)—140 \(\upmu\)m, scanning speed (Vs)—850 mm/s; r.t.p—room temperature; PHT—post heat treatment at 1000 \(\lx@math@degree\)C, 1 h; V—vertical orientation-built specimen; H—horizontal orientation-built specimen.  
\end{table} 
Table 4: Summary of compared tensile properties. Figure 10: Tensile properties of as-built specimens (vs: 140 \(\upmu\)m, Vs: 850 mm/s) shown in both horizontal and vertical orientations by stress–strain diagram and bar chart highlighting anisotropy in different building orientations. formation. The building of a highly dense sample of 99.8% relative density with a scanning speed of 850 mm/s was, therefore, possible at 140 \(\upmu\)m hatch distance. Small spherical gas-induced porosity, as observed in both 80 \(\upmu\)m and 140 \(\upmu\)m hatch distances and described by various authors [22; 42], takes its origin from the powder feedstock and from the in-process escape of elements and materials, which results in entrapped gases in the molten pool carried through to the as-built part. The elimination of this porosity can be achieved through the combination of the use of metallic powders of good packing density, good pre-process powder preparation, and highly optimized process parameters [43; 44]. Samples of both as-built 80 \(\upmu\)m and 140 \(\upmu\)m hatch distance showed typical LPBF microstructures with defined melt pools oriented along the building direction. High epitaxial grain growth was observed in both microstructures. Epitaxial grain growth is formed from remelted areas as a result of multiple laser scans or the remelting of previous layers [44; 45]. The LPBF technique generates fine microstructures owing to the high cooling rates.