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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However, it is important to highlight that the mechanical properties achieved by parts obtained through MFFF are lower compared to those obtained through PBF, Gong et al. (2019). This is despite the microstructure obtained in MFFF being characterised by grains of equal size in all directions, Singh et al. (2021). The presence of residual porosity after sintering is responsible for this limitation, as it cannot be completely eliminated. Similarly, it has been observed that the surface roughness, Boschetto et al. (2022), and geometric accuracy are inferior to other metal AM techniques but comparable to traditional metal production technologies such as casting or forging, Galati et al. (2019). Early attempts at printing materials like 17-4 PH steel, encountered challenges such as internal delamination and cracking in the printed parts, Argawala et al. (1996b) and Wu et al. (1999). It was only after the expiring of the Fused Deposition Modelling (FDM) patent, Crump (1992), in 2009 that research interest and investments surged, leading to the publication of some paper exploring the use of various metals. Among these metals, the most commonly employed ones are two types of stainless steel: 17-4 PH and 316L. However, this technology shows many issues in the selection of the proper process parameters for each step, the first step of Fused Filament Fabrication, although known for polymers, is not fitted for filled polymers, which are more difficult to extrude and keep stable as it is filled with metal particles. Therefore, known models regarding the quality of the obtained artefact must be recalibrated for this multistep process, as the aim is to provide a sinterable structure. Moreover, the study investigating the correlation between FFF parameters and the density and mechanical properties of 17-4PH is still lacking in terms of scope and data to reach definitive conclusions. Further research and experimentation are needed. The aim of this research is bridging this knowledge gap in MFFF by optimising the FFF process parameters to maximise the 17-4PH sintered product mechanical properties and geometric and dimensional tolerances. For this purpose, an experimental full factorial campaign was performed. Different deposition speeds, extrusion temperatures and infill strategies (raster angles) are implemented to produce mechanical samples for metallographic and mechanical characterisation. The process parameter effect on voids and defect distribution will be recorded and correlated with static mechanical properties. Regression models are also developed to allow multiple optimisations. The influence of the original filament pattern on material shrinkage, microstructure and void distribution will be assessed by metallographic analyses and related to mechanical properties under compressive and tensile loads. Different sample geometry will be tested in an attempt to define a coupon standard reference for the metal fused filament fabrication technologies. 2. Material and methods 2.1. Printing material and equipment The precipitation hardened stainless steel, 17-4 PH, was selected for the investigation. The stainless steel EN X5CrNiCuNb16-4, commonly referred to as 17-4 PH, belongs to the category of precipitation hardening martensitic stainless steels. Through an appropriate heat treatment, the copper (Cu) content in the alloy undergoes precipitation in the form of finely dispersed, extremely small metallic particles within the steel's crystalline structure. This precipitation phenomenon reinforces the material's matrix, resulting in enhanced mechanical properties, Boniardi et al. (2022). Among various precipitate stainless steels, 17-4 PH is widely used due to its exceptional characteristics, including high mechanical strength, hardness, and corrosion resistance up to temperatures of 300°C. Consequently, it finds extensive applications in the aerospace, naval, chemical and petrochemical industries, as well as the energy sector for manufacturing components such as valves, shafts, bearings, turbine blades, and compressors. Additionally, it is employed in load cells, gears, firearms, tools, springs, and surgical instruments, Boniardi et al. (2022). In this particular study, the filament used was the 17-4 PH BASF Ultrafuse® with a diameter of 1.75mm. This filament is compatible with standard open FFF machine. The composition of the 17-4 PH BASF Ultrafuse® material consists of approximately 90% metal powder and the remaining portion is polyolefin, BASF Technical Data Sheet Ultrafuse 17-4 PH]. The printing process was performed using the Raise3d Pro2 printer, Raise3d Pro2 Series Technical Specifications (2023). The post-processing steps were entrusted to BASF. The first step involved catalytic debinding, where the green 3D printed part was subjected to exposure to NHO3>98% in an oxygen-free environment. This precaution was taken to prevent any formaldehyde, which may be released during debinding, from reacting with O 2 . The catalytic debinding process was considered complete when a minimal debinding loss of 10.5% was achieved, in other words, when the weight of the green part is reduced by 10.5% with respect to the initial one. Subsequently, a sintering cycle was