PSI - Issue 47

P. Ferro et al. / Procedia Structural Integrity 47 (2023) 535–544

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P. Ferro et al./ Structural Integrity Procedia 00 (2023) 000–000

1. Introduction Additive manufacturing (AM) applied to metallic materials developed in the last years with a tremendous rapidity since both industry and researchers intuited its high intrinsic potentiality in producing parts with very complex shapes and cost saving compared to standard manufacturing technologies, above all when dealing with high performance materials such as titanium and nickel-based alloys (Razavi et al., 2018; Ferro, Berto and Romanin, 2020; Ferro et al. 2020). The most studied processes, also known as mainstream commercial metal AM technologies, include Powder Bed Fusion (PBF) (Bobbio et al., 2017), Directed Energy Deposition (DED) (Tan et al., 2020), Binder Jetting (BJ) (Roberts et al., 2020), and other emerging technologies such as Ultrasonic Additive Manufacturing (UAM) (Bourmias Varotsis et al., 2019) and Metal Droplet Printing (MDP) (Liu et al., 2021). A laser or electron beam source is used to consolidate the powder, layer by layer, in PBF and DED processes, which are low-efficiency and high energy demanding heating sources; while ultrasonic energy is deployed in UAM to consolidate the feeding of metal foils/tapes, which also exhibits high energy consumption. Moreover, they require large investments for facilities. Therefore, there is a strong motivation to find out an alternative less expensive and more energy efficient AM solution. In this scenario, Fused Filament Fabrication (FFF) AM could represent a promising alternative to the above-mentioned standard technologies (Agarwala et al., 1996). A polymer-based filament with highly filled metal particles is used to produce the green part using the additive manufacturing approach. The polymer binder is then chemically and/thermically removed to obtain the ‘brown part’ (debinding process) which is finally densified by a sintering heat treatment (Gonzalez-Gutierrez et al., 2018). This AM process is also called Printing-Debinding-Sintering (PDS) (Wang et al., 2022) or even Fused Deposition Modeling and Sintering (FDMS) (Liu et al., 2020), Atomic Diffusion Additive Manufacturing (ADAM) (Galati and Minetola, 2019) or Bound Metal Deposition (BMD) (Watson et al., 2020). Since the energy for deposition is linked to the polymer binder properties and the debinding and sintering heat treatment can be carried out on entire batches, this technology shows great promise in the large-scale manufacturing of metal components at relatively higher fabrication rate and lower manufacturing cost. Despite this, the research efforts on FDMS process for metal AM fabrication are still at the beginning stage. Recent literature reports some pioneering works on stainless steels (SS) or Inconel 718 (IN718). Liu et al. (2020) investigated the mechanical and metallurgical properties of 316L SS parts made by FDMS. Catalyst debinding to achieve the Brown Parts was conducted to remove the POM binder in the Green Parts at 120 °C for 8 h under nitrogen whose rate was 1 L/h. The brown parts were subsequently subjected to a sintering process by first removing the remaining binder apart from POM at 600 °C for 2 h and then sintering at a temperature of 1360 °C for 2 h under the protection of argon gas. In such conditions, the FDMS parts resulted to have a residual porosity that prevented them from reaching the same mechanical properties of selective laser melting additively manufactured components. Thompson et al. (2019) attempted to optimize the process parameters to obtain FDMS 316L sound parts. A density greater than 95% was reached using a chemical and thermal debinding at 750°C for 90 min, a heating rate of 0.2 °C/min in the critical temperature range to give time for volatilization of degraded polymer out of the part and prevented defect formation, and sintering at 1360 °C for 120 min. In this case, mechanical evaluation using 3-point-bending tests proved that deflections similar to conventionally fabricated stainless steels were obtained albeit at lower strength. Wang et al. studied the sintering mechanism for extrusion-based additive manufacturing of 316L SS via molecular dynamics simulation. They in particular highlighted the role of Cr element which diffuses to grain boundary and forms severe grain boundary aggregation due to its lower diffusion activation energy and stronger interactions between atoms, compared to the other alloys elements. Dealing with IN718, in a recent paper Thompson et al. (2022) proved the possibility to produce high density (> 97%) sound FDMS parts by using an optimized practice consisting of thermal debinding and sintering in vacuum atmosphere followed by vacuum sintering at 1280 °C for 4 h. They found that despite the remaining porosity, mechanical properties after heat treatment are comparable to those of conventionally manufactured IN718. FDMS technology is particularly suitable to produce not only mono-materials, as described above, but multi materials by combining more than one filament at the same time, using the same nozzle or even two different nozzles. Depending on hotend and nozzle configurations, the outgoing material can be a mixture, or a juxtaposition of the initial materials as occurs in polymer co-extrusion (Sponchiado et al., 2023). This possibility opens to a huge number of other possibilities to produce new multi-material components, each one characterized by a particular combination of parameters such as the alloys to be coupled and their configuration. In