Functionally Graded Metal Metamaterials: Additive Manufacturing, Materials, Architectures, Intelligent Design and Industrial Application—A Review

Functionally Graded Metamaterials (FGMMs) refer to new categories of materials whose design through variation in composition and geometry leads to the attainment of properties that cannot be realized using regular materials. The use of additive manufacturing (AM) offers the necessary flexibility in terms of geometry for creating such designs. This research topic was selected due to an increase in the use of metamaterials and architectural gradients in AM, along with the need to establish the relationship between gradients in design, process, structure, and performance. In this review, a comprehensive study will be conducted regarding metallic FGMMs fabricated using additive manufacturing. The methods to be studied include gradient design, lattice and Triply Periodic Minimal Surface (TPMS) geometries, Directed Energy Deposition (DED), Wire Arc Additive Manufacturing (WAAM), Laser Powder Bed Fusion (LPBF), Electron Beam Melting (EBM), the future of AM, composition gradient in metals, topology optimization, multiscale design, and machine learning. The literature review indicates that material and architectural gradients can produce features such as controlled deformation, lightweighting, energy dissipation, heat dissipation, and multifunctional design. DED and WAAM provide the freedom of grading composition and large-scale manufacturing, whereas LPBF has a higher resolution for lattice and TPMS geometries. Topology optimization and machine learning are helping with designing the materials and architecture with regard to performance, but there are still some constraints due to data dependency, generalizability, manufacturability, and physical constraints. Some of the challenges facing implementation include defects, residual stresses, cracking, intermetallic formation, process variability, and post-processing. Even sustainability depends on the material choice, energy usage, processing, manufacturing volume, and lifecycle. In general, the review points out that there is a need for a design-to-manufacturing approach that brings together materials selection, structural design, process control, and smart design techniques for the development of metallic FGMMs.