## 3 Results
### Microstructure
In the micrograph of the island scan stripes specimen, the elongated former \(\beta\) grains are visible in the x-z cross section (Figure 4e,f) and have the same width in the line scan specimen. The lengths of these grains sometimes reach the full height of the specimen. As opposed to the line scan specimen, where \(\beta\) grains are inclined in the y-direction (i.e., the y-z cross section), the elongated prior \(\beta\) grains are oriented vertically and do not continue in the neighboring island (Figure 6b). This is a result of the high degree of porosity in-between the islands, causing the steepest thermal gradient to be along the thickness direction of the island. In the scan direction (Figure 6a) the prior \(\beta\) grains are slightly tilted inward to the center of the islands, indicating a small contribution of the thermal gradient along the x-axis, associated with the order of the parallel stripes scanning of the islands (Figure 1b). The bowl shaped fusion boundaries are visible, elongated along the scan direction, and rounded off in the print direction (y-axis). Figure 6: Optical micrograph of island scan stripes specimen showing the microstructural features in the (**a**) x–z cross section and (**b**) y–z cross section. White arrows mark the locations of the island boundaries. In the micrographs of the island scan modeled specimen (Figure 7), the microstructure at the connections between islands is observed to consist of prior \(\beta\) grain interfaces. Some are wavy, looking like intertwined 50-100 \(\upmu\)m waves, while others are less distinguishable. The macrostructure is very chaotic, due to the different scan strategies, resulting in more erratic prior \(\beta\) grains. The prior \(\beta\) grains are elongated, roughly along the build direction, but differ significantly in width, both within the islands and among islands with different scan strategies. An example is the spiral scans, creating prior \(\beta\) grains tilted either to the center of the island (out-in) or from the center and to the edges of the island (in-out). The contrast resulting from fusion boundaries is less pronounced than for the island scan modeled specimen, while banding structures from selective etching are abundantly present. Some islands show clear layering, while others have none. The microstructures as revealed by SEM are presented in Figure 8 for the three specimens at three different locations (bottom, middle, and top). The microstructure in the top and middle of the SLM specimens is acicular \(\alpha^{\prime}\) martensite, with twinned primary and secondary \(\alpha^{\prime}\) plates. Mostly the martensite in the SLM specimens shows acicular \(\alpha^{\prime}\) structure with primary, secondary, tertiary, and quartic \(\alpha^{\prime}\) martensite, as described by [PERSON] et al. [13]. This is clearly observed for the line scan specimen at the top and in the middle (Figure 8a). In the bottom of the line scan specimen, the microstructure is mainly ultrafine lamellar as revealed by partitioning observed in backscatter imaging, indicating that decomposition of \(\alpha^{\prime}\) into \(\alpha+\beta\) had occurred [14]. The lamellar \(\alpha\) and \(\beta\) forms from the \(\alpha^{\prime}\) martensite by partitioning of Al and V [26]. The lamellar \(\alpha+\beta\) may also have formed indirectly from \(\alpha^{\prime}\); where \(\alpha^{\prime}\) martensite is transformed into \(\beta\) during reheating, from which \(\alpha\) and \(\beta\) lamellar form during cooling, if the cooling after reheating is sufficiently slow. No such \(\alpha^{\prime}\) to \(\alpha+\beta\) transformation was observed in the island scan specimens, where hierarchical \(\alpha^{\prime}\) martensite was the only microstructure constituent observed, as no partitioning was visible (Figure 8b).