ADVANCED MATERIALS & PROCESSES | MARCH 2026 19 defense sustainment, where powders are accelerated to high velocities and plastically deformed on impact to form dense deposits without melting[2]. This makes it well-suited for coatings, dimensional restoration, and build-ups, and it can also utilize powder fractions that may be less suitable for powder bed fusion (PBF) in certain applications (e.g., out-of-spec size fractions), provided cleanliness and flow requirements are met. While cold spray does not eliminate powder production requirements entirely, it expands the range of viable powder reuse and supports distributed repair workflows where material efficiency and part life extension are key drivers. Collectively, these solid-state routes broaden the upcycling “toolkit” by enabling the direct use of compacted scrap or intermediate solids, lowering processing intensity relative to melting and atomization routes, and supporting closed-loop manu- facturing strategies in which waste can be converted locally into new feedstock or repair material. Solid-state routes offer the advantages of reduced energy demand relative to remelting, limited oxidation and chemistry drift, and often refined microstructure through severe plastic deformation and dynamic recrystallization. These characteristics make them well-suited for closed-loop manufacturing environments, where machining waste can be directly transformed into new components. Melt-based Reprocessing for Powder and Wire AM. Melting- and fusion-based additive processes, such as PBF and directed energy deposition (DED), impose strict requirements on feedstock morphology, chemistry, and cleanliness. As a result, upcycling in these modalities typically involves remelting scrap material followed by powder or wire production through atomization or casting-drawing routes. While technically mature, these approaches are energy-intensive and can offset some of the sustainability benefits of AM unless paired with renewable energy sources, high material utilization rates, or hybrid strategies that blend recycled and virgin feedstocks. Reconditioning and qualification of used powders are increasingly being explored as intermediate solutions rather than full re-atomization. Extrusion- and Pellet-based AM. Pellet-fed and extrusion-based AM systems offer greater tolerance to feedstock variability, enabling direct use of shredded polymer waste, recycled thermoplastics, or metal-polymer composites. These approaches are particularly attractive for large-format AM, tooling, and construction-scale applications, where material cost and throughput outweigh the need for fine feature resolution. PERFORMANCE, QUALIFICATION, AND DESIGN CONSIDERATIONS As upcycled materials transition from laboratory demonstrations and research facilities to functional components, performance validation and qualification emerge as central challenges. Unlike conventionally produced feedstocks, upcycled materials often exhibit greater variability in morphology, cleanliness, and prior processing history. Machining chips may vary in size, surface oxidation, and residual lubricants, while recycled powders can experience changes in particle size distribution, chemistry, or flow behavior over repeated reuse cycles. These variations directly influence process stability, consolidation quality, and defect formation, making performance assurance a critical concern for industrial adoption. Current studies on material upcycling for AM increasingly recognize that conventional qualification frameworks, largely developed around tightly controlled virgin feedstocks, are not always well suited to these emerging material streams. Instead, there is a growing shift toward performance- based qualification, in which acceptance is determined by demonstrated mechanical properties, microstructural integrity, and part-level performance rather than by feedstock background alone. In solid-state upcycling routes, Additive Manufacturing By- products. Powder-based AM processes generate significant amounts of non- reusable powder due to contamination, particle-size drift, or oxidation. Similarly, support structures and failed builds represent another underutilized material stream. Post-consumer and Industrial Waste. Polymeric waste (e.g., PET, ABS, PVC, HDPE, LDPE) and metal scrap from end-of-life components are increasingly being investigated for distributed and large-scale AM applications, especially in construction, tooling, and energy sectors. FROM WASTE TO AM FEEDSTOCK Material upcycling strategies for AM span a spectrum of processing routes, depending on the form of waste material and the requirements of the target AM technology. These pathways can be broadly grouped into solid- state, melt-based, and extrusion-driven approaches, as illustrated in Fig. 1. Solid-state Upcycling Routes. Solid-state AM technologies have emerged as particularly promising platforms for upcycling metallic waste. Processes such as additive friction stir deposition (AFSD), friction-based extrusion, and hybrid solid-state deposition enable direct consolidation of chips, flakes, or compacted scrap without melting. Friction surfacing, for example, deposits material from a consumable rod onto a substrate through frictional heating and severe plastic deformation, creating dense coatings and build-ups that are well- suited for repair and remanufacturing. Similarly, emerging solid-state extrusion approaches, including chip- and billet-based methods such as Enabled Engineering’s SolidStir, use frictional plus thermomechanical deformation during extrusion to consolidate feedstock and produce new forms (e.g., wire, tube, MMC) while avoiding the oxidation, dross formation, and variable chemistry associated with remelting[1]. Cold spray is another increasingly adopted solid-state pathway, particularly for
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