# Laser Beam Welding of CubeSat 1U Structure Parts Obtained by Powder Bed Fusion
## 3 Results and Discussions
### Spot Welds Characterization  
The images in Figure 4a,b were obtained by stereoscopy of the weld spots on the connector end produced by SLM and EBM, respectively. In the images, it can be observed that both spot welds have a diameter of approximately 2 mm, and there is a recess in the center of the weld spots. As the weld solidification process started at the border and the grain growth was directed to the center, the recess was associated with liquid-to-solid shrinkage. No surface cracks were observed at the weld spots; however, to assess the presence of internal weld discontinuities, a microstructural analysis of the cross-section was performed. Figure 3: Example of the actual CubeSat structure ready for the welds. Each edge measures 100 mm. Figure 5 shows the micrographs of the cross-sections of the samples produced by SLM (a) and EBM (b) and spot-welded to the grade 2 titanium tube. The fusion zones (FZ), connector side (C), and tube side (T) were marked in the images. In both conditions, isolated spherical pores were observed in the fused zone of the bead weld owing to outgassing [21]. These pores were systematically observed in the weld cross-section, and their location and size indicated the effect of superposed free spacing (gap) as their origin. In the images of Figure 4a, a depression in the center of the weld bead is observed, as indicated by a white arrow, due to the void formed at the end of solidification, as observed in the images of Figure 3. The presence of the depression did not cause any internal defects owing to the stress concentrator, such as the formation of cracks. Figure 4: Laser spot welds for superposed connector to tube geometry: (a) SLM and (b) EBM. Figure 5: _Cont._  
Figure 5c,d shows the microstructures of the fusion zones of the weld beads produced by SLM (a) and EBM (b). In both fusion zones, an acicular martensitic microstructure, \(\alpha^{\prime}\), was observed owing to the high solidification rate (\(\sim\)500 \({}^{\circ}\)C/s), which is characteristic of laser beam welding [22]. The martensitic transformation starts in the prior \(\beta\)-phase grains and then decorates a fine network of crystallographically oriented plates, in agreement with the literature [23]. This martensitic microconstituent increases the hardness to 750 HV (Vickers hardness) [24]. Figure 6 shows the microhardness profiles of the two laser-welded samples produced via SLM and EBM. The microhardness profile began in the AM connectors, ran through the weld bead, and ended in a grade 2 titanium tube (CP-Ti). In general, both the SLM and the EBM cases showed the same profile; the HV started at approximately 420 HV in the connector, then decreased to a path around 340 HV in the fusion zone, and finally dropped to 130 HV in the tube. The sudden drop in hardness in the region between the fusion zone and the tube could eventually configure a stress concentrator zone. Figure 7 shows three representative curves of tensile force versus displacement for specimens SLM and EBM and, for comparison purposes, the F(d) curve for the titanium  
Figure 5: Laser spot welds cross-section macrographs: (**a**) SLM and (**b**) EBM. The letters refer to fusion zone (FZ), connector (C), and tube (T). The arrows indicate, approximately, the laser beam optical axis. Micrographs of FZ: (**c**) SLM and (**d**) EBM. Figure 6: Vickers hardness values as a function of the weld center distance for SLM and EBM cases. Each region is noted at bottom. tube. As can be seen in the figure, the behavior of the welded assemblies differs significantly from the result obtained when the titanium tube was tested separately. This indicates that rupture occurs at the interface between the connector end and the tube long before permanent deformation of the tube occurs. After three tensile tests, the maximum forces were 2114 \(\pm\) 420 and 2021 \(\pm\) 493 N for the SLM and EBM specimens, respectively.