PSI - Issue 47

P. Ferro et al. / Procedia Structural Integrity 47 (2023) 535–544 P. Ferro et al. / Structural Integrity Procedia 00 (2023) 000–000

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this scenario, the present work describes the first attempt to produce IN718-High Carbon Steel bi-metallic parts by Fused Deposition Modeling and Sintering. The choice of this alloys couple derived by their similar sintering temperatures and the research of new materials with a unique combination of metallurgical and mechanical properties at lower cost. 2. Materials and parts production Cubic specimens, size 10 mm, were produced via co-extrusion of two filaments containing Inconel 718 (IN718) and high carbon steel powder (by The Virtual Foundry), respectively. The composition of the two alloys is reported in Table 1. IN718 filament, diameter 1.75 mm, contained nominally 87 wt% metal and had a density of 3.73 g/cc while high carbon steel (HCS) filament had the same size but contained 79.1 wt% metal resulting in a density of 2.76 g/cc

Table 1. Chemical composition of the two alloys (wt%) Inconel 718 (nominal composition) Ni Cr Nb Mo Co 50-55 17-21 4.75-5.5 2.8-3.3 Max. 1.0 High carbon steel (measured with EDS) Fe C Si Al S & P 85.6 11.5 2.15 0.73 Bal.

It is observed that the high carbon powder belongs to the family hypereutectoid steels. Coextrusion of the two different wires means that both filaments passed through the same nozzle. In this condition the scanning strategy, computed by a dedicated python code, is directly linked to the resulted bi-material configuration (where ‘ configuration ’ means the geometrical disposition of the two alloys inside the specimen). The co-extruded specimen, ‘ left-right’, is obtained by moving the nozzle with a horizontal path (x direction) in the reference system schematized in Fig. 1a. The xy plane coincides with the 3D printer plate. The co-extruded sample, ‘ top-bottom’, is produced by shifting the nozzle up and down (y direction) with respect to the same reference system (Fig. 1b) and finally the ‘crossed’ co-extruded is carried out by combining layer by layer the previous nozzle paths, resulting in a configuration schematized in Fig. 1c. As first tentative, only one sample per scanning strategy was produced by a customized Prusa I3 equipped with an adapted Marlin firmware and a Cyclops hotend using process parameters collected in Table 2. More detail about hardware and software are available in a previous work by Sponchiado et al. (2023). After the 3D printing, the samples underwent a debinding and sintering heat treatment to eliminate the polymer (polylactic acid (PLA)) and consolidate the powder. Heat treatment recipe is shown in Fig. 2. Since the filament’s producer recommended sintering temperatures of 1260 °C and 1300 °C for IN718 and HCS, respectively, it was decided, as first tentative, upon an intermediate value of 1280 °C. It is worth noting that Kloeden et al. (2013) suggested for IN718 sintering temperatures in the range between 1260 and 1290 allowing liquid phase sintering that promotes the highest material density. Inert atmosphere, made of Ar (99.99%, 100 SCCM), was used to prevent oxidation. Moreover, no steel blend was used but the samples were simply placed on a refractory basal layer. Both filaments and samples were investigated by using electron scanning microscope (QUANTA FEG 250) with Energy Dispersive Spectroscopy (EDS). Eventually, a chemical etching (Nital 3%) was used to reveal the microstructure.

Table 2. FDMS process parameters Layer height (mm)

Printing Speed (mm/s)

Nozzle Temperature (°C)

Bed Temperature (°C)

Extrusion width (mm)

0.4

15

210

50

0.8