PSI - Issue 49

Federico Fazzini et al. / Procedia Structural Integrity 49 (2023) 59–66 / Structural Integrity Procedia 00 (2023) 000 – 000

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Keywords: Additive Manufacturing; Metal Fused Filament Fabrication; 17-4 PH; optimised printing parameters

1. Introduction Metal Additive Manufacturing (AM) is a fabrication process increasingly becoming popular, due to the ease of obtaining functional metal prototypes and ready-to-use parts, short time to market and reduced initial investment costs for customised or small batch production. AM in particular, allows the creation of geometries that would otherwise not be achievable with traditional manufacturing technologies. Metal AM finds extensive use in diverse sectors, encompassing aerospace, automotive, medical, and energy, Chemkhi et al. (2021). At present, its market dominance is primarily attributed to established methodologies, such as Selective Laser Melting (SLM), Electron Beam Melting (EBM), Direct Energy Deposition (DED), and Metal Binder Jetting, also referred to as 3D Printing, Lotfi et al. (2021). In this market, a cutting-edge technology known as Metal Fused Filament Fabrication (MFFF) is gaining substantial momentum. While not entirely novel, the origins of MFFF can be traced back to the late twentieth century when initial research focused on the development of polymeric filaments filled with metal powder, Argawala et al. (1996a). This AM process involves a first step using the FFF process, where a composite filament is extruded and deposited layer by layer to attain the desired geometry based on a Computer-Aided Design (CAD) model of the desired shape. The part is referred to as green part. The employed filament comprises sinterable powders, which make up approximately 60% by volume, and a polymeric binder that constitutes the remaining portion. The polymeric constituent is composed of three distinct parts: the main component, the backbone, and additives. The selection of the ideal binder systems relies on key factors such as powder compatibility, resulting viscosity, and the necessary mechanical properties. These factors directly influence the choice of binders for MFFF applications, Gutierrez et al. (2018). The second step is the debinding process, which depends on the polymers used. There are three different methods available: thermal, solvent, or catalytic methods, which can also be combined. The debinding process is carried out on the green part to remove the polymeric binder system, resulting in a product referred to as the brown part. After debinding, the brown part retains its geometric shape due to the presence of the backbone, although it is often weaker than the green part. Typically, the backbone is removed through a thermal treatment just before the sintering process, Gutierrez et al. (2018). The last step is the sintering process, where the brown part is exposed to temperatures typically around 70 90% of the powder's melting point. During sintering, six different mass transfer mechanisms occur, facilitating the growth of necks between particles and promoting bonding, Gutierrez et al. (2018). This transforms the loose powder into a cohesive bulk material with noticeable linear shrinkage. This process ultimately achieves the desired final density, up to 99%, Zhang et al. (2022), and structural integrity of the metal parts. This lengthy post-processing requires appropriate machinery to carry out debinding and sintering. However, this treatment can be outsourced, reducing the number of assets to be purchased. In fact, many companies offer debinding and sintering services at an affordable price, minimising the equipment investment. The MFFF technique offers substantial advantages over current metal AM methods. It boasts a lower initial investment for machinery, as it can be implemented on any commercial Fused Filament Fabrication machine as long as it is an open machine. Unlike other metal AM processes, there is no need for a protective environment to handle hazardous and costly metal powders. Furthermore, the powders used in loaded filaments do not necessitate particular specifications as seen in PBF technologies, enabling the use of powders not explicitly designed for AM, Galati et al. (2019). This results in reduced material costs and subsequently lowers the production expenses associated with metal components, Tosto et al. (2021). The MFFF method eliminates material waste and exhibits lower energy consumption compared to metal AM technologies that employ energy beams. The Metal Fused Filament Fabrication technology is also known by several designations in the literature, with the most frequently used ones being: Metal Extrusion Additive Manufacturing (MEAM), Metal Extrusion Additive Manufacturing with Highly-filled Polymers (MEAM HP), Fused Deposition of Metals (FDMet). A distinguishing feature of MFFF, is its capability to generate close lattice structures, unlike PBF technologies. These structures effectively reduce component weight, optimise material usage, and decrease production time, Atatreh et al. (2023). Additionally, the use of a ceramic material as a release agent between the part and support structures can significantly reduce the time and cost associated with support structure removal, BASF Process Guidelines Ultrafure Support Layer (2023). The MFFF process enables the simultaneous printing of two different materials obtaining the production of bi-component objects through co-sintering. Notably, Abel et al. (2019) successfully demonstrated the fabrication of yttria-stabilised zirconia and stainless steel 17-4 PH bicomponent objects using this approach.