PSI - Issue 41

Saveria Spiller et al. / Procedia Structural Integrity 41 (2022) 158–174 Saveria Spiller/ Structural Integrity Procedia 00 (2019) 000–000

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green parts. With a thin layer height, the green part is more compact, facilitating the densification in the subsequent treatment (Kurose et al., 2020). Nevertheless, the printing time is longer and the effect on the mechanical properties is not necessarily beneficial. This is thoroughly discussed in Section 5: reducing the layer thickness entails a higher number of interfaces in the part where the adhesion is not flawless. In addition, as depicted in Fig. 4, on the side of the parts, more discontinuity points can evolve in crack initiation points especially when fatigue loading is involved. The second parameter which is worthy to mention is the extrusion temperature. The typical extrusion temperature for PLA is 210°C; for stainless steel, it can reach 290°C (Thompson et al., 2019). The purpose of the printing phase is not to melt the metal powder, nevertheless, the printability of the filament is compromised by the high solid infill, therefore it is necessary to increase the printing temperature to be able to soften and push the material through the nozzle. It is usually beneficial to increase the printing bed temperature as well, from the 60°C suggested for PLA parts to around 100°C. This is beneficial to assure the first layer adhesion and avoid warping, another common issue of FDM (Loh et al., 2020). To increase the first layer adhesion, it is also advisable to print a brim or a skirt around the component. The extrusion flow rate multiplier is a parameter related to the quantity of material extruded at a time. A value higher than 100% means that the material is forcibly pushed through the nozzle. This is an important factor affecting the density of the green part since the over extrusion helps to close the voids between the deposited rasters. It affects also the external appearance of the components (Loh et al., 2020), and it can result in more evident roughness as visible in Fig. 5.

Fig. 5 The effect of different process parameters set on the same part (Godec et al., 2020)

As delicate to set, is the cooling fan speed. An excessive speed provokes the premature solidification of the deposited layers, damaging the adhesion of the following ones. On the other hand, disenabling the cooling fan leads to deformations since the layers are too soft to bear the weight of new material, and the structure collapses (Thompson et al., 2019). In Fig. 6, the effect of the cooling fan speed is shown. Finally, print speed is important for the balance between surface accuracy and printing time. Fast prints are preferred but excessive speed can lead to voids and discontinuities in between the layers. The nozzle diameter affects the feasibility of the extrusion. Different values were used in the literature, ranging from 0.2 mm to 0.8 mm. Smaller diameter allows more detailed designs, but they are also more prone to clogging, totally or partially, introducing under-extrusion defects on the part. Other printing parameters affect significantly the subsequent phases of the process, such as the infill strategy, the raster angle, and the build orientation. General considerations can be found in the work by Loh et al. (2020).

Fig. 6. Effect of the cooling fan speed in Thompson et al. (2019): a) full speed; b) optimized speed; c) disabled fan

4. Debinding and sintering To achieve a dense and homogeneous metallic component, the green part undergoes the debinding and sintering