## 3 Feedstock
### Powder Reuse  
Sustainability in metal powder usage extends beyond incorporating recycled feedstock; it also encompasses the recycling and repurposing of powders that have undergone AM processes. Due to the layer-by-layer nature of the PBF-LB/M process, a considerable amount of metal powder surrounding the solidified part remains unused. To maximize material efficiency and minimize waste, these powders must be reclaimed and reused in subsequent builds. A comprehensive understanding of this aspect of sustainability is essential to fully grasp how powder recycling contributes to the circular economy. Powder particles near the melt pool, but not fully incorporated into the part, as well as those that remain on the building plate or are carried into the overflow tank, can experience varying levels of heat exposure during the build process [60]. This differential exposure can lead to distinct effects on the particles, influencing their shape and flowability. Heat exposure can cause agglomeration or sintering, resulting in larger particles with satellites and increased surface roughness [61]. Furthermore, spattered particles from the melt pool can create agglomerates and oxides in the surrounding powder bed [62]. Although sieving is commonly used to remove larger particles, particle shape and surface chemistry alterations can still influence powder flowability, packing, and, consequently, the quality of the powder bed. A poor-quality powder bed can, in turn, impact the densification of the component produced through PBF-LB/M, and thus its mechanical properties, as observable in Figure 5.  
ASTM F3318-18 [64], which pertains to AlSi10 Mg, and ASTM F3592-23 [60], which provides guidelines for feedstock reuse, both permit the recycling of previously used powder. After each build cycle, the remaining powder may be blended with virgin powder to maintain sufficient quantities for subsequent builds. To ensure compliance with quality standards, regular chemical analysis of the recycled powder is required. Additionally, all the used powder must be sieved using a mesh with appropriate sizing to effectively remove agglomerates and contaminants from the build process. [PERSON] et al. [65] investigated the use of recycled AlSi10 Mg powder in PBF-LB/M, comparing the properties of virgin powder, heat-affected condensate particles, and recycled powder. The study revealed that recycled powder exhibited characteristics similar to those of virgin powder, confirming  
Figure 5: Surface morphology and mechanical behavior comparison between virgin and reused powder in PBF-LB/M of Al–Mg–Sc–Zr alloy [63]. its suitability for use in PBF-LB/M without compromising the quality of the final product. [PERSON] et al. [63] investigate the impact of powder reuse on the properties of Al-Mg-Sc-Zr alloy (Scalmalloy(r)), focusing on key characteristics such as morphology, composition, and particle size distribution in both virgin and reused powders. The study also evaluates the mechanical properties of specimens produced from virgin powder and those after four build cycles, assessing the effects of blending reused and virgin powder on the final material properties. Although the microstructure of the specimens remains largely consistent, the reused powder introduces slightly higher porosity, characterized by small micro-sized pores; however, these do not significantly influence the mechanical properties. The findings indicate that Al-Mg-Sc-Zr powder can be effectively reused for up to four build cycles, provided proper sieving is applied and 40% of the powder is refreshed with virgin material. Another study by [PERSON] et al. [66] evaluated the reuse of AlSi7 Mg0.6 powder in the PBF-LB/M process. The findings showed that the powder could be reused up to five times with minimal degradation, supporting its viability for sustainable and cost-effective manufacturing.