Citation: Costa, J.M.; Sequeiros, E.W.; Vieira, M.F. Fused Filament Fabrication for Metallic Materials: A Brief Review. Materials 2023, 16, 7505. https://doi.org/10.3390/ma16247505 Academic Editors: Pavel Lukᡠc and Feng Qiu Received: 19 September 2023 Revised: 20 November 2023 Accepted: 28 November 2023 Published: 5 December 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Review Fused Filament Fabrication for Metallic Materials: A Brief Review Jose M. Costa 1,2, * , Elsa W. Sequeiros 1,2 and Manuel F. Vieira 1,2 1 Department of Metallurgical and Materials Engineering, Faculty of Engineering, University of Porto, R. Dr. Roberto Frias, 4200-465 Porto, Portugal; ews@fe.up.pt (E.W.S.); mvieira@fe.up.pt (M.F.V.) 2 LAETA/INEGI—Institute of Science and Innovation in Mechanical and Industrial Engineering, R. Dr. Roberto Frias, 4200-465 Porto, Portugal * Correspondence: jose.costa@fe.up.pt Abstract: Fused filament fabrication (FFF) is an extrusion-based additive manufacturing (AM) tech- nology mostly used to produce thermoplastic parts. However, producing metallic or ceramic parts by FFF is also a sintered-based AM process. FFF for metallic parts can be divided into five steps: (1) raw material selection and feedstock mixture (including palletization), (2) filament production (extrusion), (3) production of AM components using the filament extrusion process, (4) debinding, and (5) sinter- ing. These steps are interrelated, where the parameters interact with the others and have a key role in the integrity and quality of the final metallic parts. FFF can produce high-accuracy and complex metallic parts, potentially revolutionizing the manufacturing industry and taking AM components to a new level. In the FFF technology for metallic materials, material compatibility, production quality, and cost-effectiveness are the challenges to overcome to make it more competitive compared to other AM technologies, like the laser processes. This review provides a comprehensive overview of the recent developments in FFF for metallic materials, including the metals and binders used, the challenges faced, potential applications, and the impact of FFF on the manufacturing (prototyping and end parts), design freedom, customization, sustainability, supply chain, among others. Keywords: additive manufacturing; solid-state processes; material extrusion; fused filament fabrication; metallic materials 1. Introduction Additive manufacturing (AM) processes, commonly called 3D printing, are receiving the attention of several industries. These processes allow layer-by-layer construction of complex and customized shaped parts from engineering materials directly from design, without using expensive tooling [1–5]. To accomplish the AM potential, continued research and development on processes and equipment are essential to enable full manufacturing readiness and understanding of the materials [3,4]. AM processes in general, from laser to solid-state processes, can create complex shapes and components from various materials, including plastics, metals, ceramics, and composites, like metal matrix composites (MMC), functionally graded materials (FGM), high entropy alloys (HEA), and others [6–10]. The beam (laser or electron) powder processes are the most used technologies in metal AM. The pertinence and capabilities of this type of process are widely demonstrated and recognized. Thus, this technology has emerged as challenging, since layer-by-layer manufacturing with a heat source leads to columnar grain formation due to the directionality of heat extraction. This microstructure has characteristics dissimilar from those of traditional processes; it usually reveals an anisotropy that degrades mechanical strength in Z-axis directions (or with enhanced strength in the XY-axis), resulting in components that can have unpredictable mechanical behavior, incompatible with parts that require stringent properties [11–14]. The metal AM processes require high-end and digital technology, like hardware, software, and procedures. They are an intrinsic part of a new industrial paradigm to increase efficiency and productivity by ensuring sustainability and the improvement of the circular economy [3,9,15–17]. Materials 2023, 16, 7505. https://doi.org/10.3390/ma16247505 https://www.mdpi.com/journal/materials