PSI - Issue 79

Daniela Neves et al. / Procedia Structural Integrity 79 (2026) 266–274

273

Table 7 - Defects and their size observed by nanotomography

Volume of the sample [mm 3 ]

Volume of defects [mm 3 ]

Percentage of defects in the volume sample observed [%]

Surface area of defects [mm 2 ]

Batch

2

155.26

1.27

0.817

310.64

3

141.87

3.57

2.518

2134.84

3.5. Fractography The three SEM fractographs (Fig. 7a–c) exhibit characteristic dimple features typical of ductile fracture surfaces. In Fig. 7a), corresponding to a tensile fracture surface, unmelted powder particles are observed, indicating incomplete fusion during the manufacturing process. Fig. 7b), showing an impact fracture surface, reveals numerous entrapped gas pores, which can locally reduce the material's strength. Fig. 7c), which also represents an impact fracture surface, displays unmelted powder and entrapped gas pores, observed across the fracture surface, indicating defects introduced during processing. Nevertheless, the overall dimpled morphology in all three samples demonstrates a ductile fracture behaviour, resulting from the nucleation, growth, and coalescence of microvoids during deformation.

Fig. 7 – SEM Micrographs: a) Batch 1; b) Batch 2; c) Batch 3

4. Conclusions Based on the comprehensive research findings, it is evident that both optical microscopy and scanning electron microscopy SEM consistently identified defects across all batches, including gas porosity, lack of fusion, and unmelted powders. The hardness measurements yielded a microhardness of approximately 500 HV0.3, higher than reported in the literature. In conclusion, the surface roughness increases with a higher proportion of recycled powders. From this research, several key conclusions emerge: Batch 1, composed entirely of 100% virgin powder, demonstrated the lowest porosity, featuring small, predominantly spherical gas pores, and revealed the most stable processing conditions. In contrast, Batch 2, which contains 23% twice-sieved powder, presents fewer defects than Batch 3 but still shows an increase compared to Batch 1. Batch 3, containing 23% three-time-sieved powder, exhibited the highest defect level. These insights underscore the critical importance of powder preparation and recycling treatment in determining the quality of the final components. The impact of powder reuse is profound and cannot be overlooked, as it significantly influences the component's structural integrity. Despite these issues, it is worth noting that the overall defect volume fraction remains impressively low (batches 2 and 3 maintain a commendably low defect volume fraction of 1% to 3%, reinforcing the viability of using recycled materials). Notably, an increase in recycling cycles correlated with a significant rise in defects and elevated surface roughness, underscoring the substantial impact of powder reuse on the final quality of components and, consequently, on their structural integrity.