Abstract
Metal-reinforced polylactic acid (PLA) composites produced by fused deposition modeling/fused filament fabrication (FDM/FFF) offer thermal, electrical, magnetic, antimicrobial, biomedical, and post-processing functions. However, these benefits are often accompanied by reduced strength, ductility, and fracture reliability because metallic fillers affect both the particle–matrix interphase and the road/layer weld formed during material extrusion. This review proposes a process-specific dual-interface framework that couples classical particle–matrix interphase theory with the transient and directionally organized road/layer interfaces of FDM/FFF. Metallic fillers influence both interface levels through changes in melt viscosity, thermal transport, crystallization, interlayer diffusion, and defect formation. The review compares earlier filler- and application-based classification schemes, explains the selection of Cu, Ag, Mg, Ti, 316L stainless steel, Fe/magnetite, Al, and Cu-alloy systems, and introduces measurable descriptors for wetting, adhesion, road/layer fusion, porosity, and print stability. A cross-scale data-integration workflow and a provisional processability-screening framework are proposed. The resulting process–thermal history–crystallization–interface–property–function framework provides experimentally testable guidance for designing reliable metal-filled PLA composites.