Nanostructured Multi-Material Alloys Synthesised through Hybrid Additive manufacturing/severe Plastic Deformation Approach: Process, Microstructures, Properties, and Opportunities

Additive manufacturing (AM) has transformed metallic component fabrication by enabling precise, layer-wise construction of complex geometries, offering unparalleled design flexibility, reduced material waste, and shorter production times compared to conventional methods. Specifically, powder bed fusion (PBF) and directed energy deposition (DED) AM technologies are pivotal for producing high-performance metallic parts in critical industries such as aerospace, biomedical, and energy. Further, the recent demand for multi-material AM (MMAM) arises from the need to integrate dissimilar alloys, such as 316L stainless steel (316L SS) with excellent corrosion resistance and Inconel 718 (IN 718) with superior high-temperature strength within a single component to meet diverse functional requirements in applications like turbine blades and bio-implants. Advances in MMAM technology have since enabled the direct fabrication of such multi-material structures. However, MMAM fabrication introduces interfacial challenges, including poor interfacial bonding due to thermal expansion mismatch and porosity (0.8–2%) from gas entrapment or incomplete melting that impair mechanical performance. On the other hand, severe plastic deformation (SPD), particularly high-pressure torsion (HPT), offers a solution by eliminating such defects and produces nanostructured materials through extreme torsional shear strains, achieving ultrafine-grained (UFG) or nanostructured (NG) microstructures that significantly enhance strength. Thus, the integration of AM and SPD is critical to address the aforementioned limitations arising from MMAM processing while attaining nanostructured multi-materials with superior properties for demanding applications. Therefore, this chapter investigates the nanostructuring of MMAM 316L SS/IN 718 through HPT processing, focusing on the interfacial microstructures, phase evolution, and hardness. Using advanced characterization techniques—electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and hardness measurements — this study elucidates porosity evolution, microstructure refinement, phase changes, and mechanical property enhancements at the interfacial region. The results demonstrate the efficacy of HPT processing to achieve a pore-free structure after 1 revolution. After 1 revolution, the fusion zone (FZ) at the interface narrowed from 100 μm to