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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surface and bulk mechanisms. Surfaces transports processes do not contribute to the densification of the components, which in turn is the result of bulk transport mechanisms (German, 2010b). As mentioned in the previous section, the final density of the sintered parts is not merely affected by the sintering process parameter. In the green part, the metal powder should be homogenously distributed, and the solid content of the filament used must be high enough to avoid the collapse of the brown part once the binder is removed. Due to the high temperature reached during the sintering, the parts usually shrink. The main shrinkage cause is related to the voids in the part where the binder was removed (Kurose et al., 2020). In addition, during the sintering phase, the bulk transport mechanisms abovementioned entail atomic rearrangement and volume reduction (German, 2010b). Shrinkage was proven to be anisotropic, with a higher linear volume reduction observed along the building direction (Z-axis). Along the building direction and on the X-Y plane the shrinkage was reported to be respectively 15-17% and 14-15% according to Kurose et al. (2020), and 23% and 18% according to Gong et al. (2019). Based on these experiences, it is possible to compensate for the shrinkage during the design phase. In the last study mentioned, shrinkage anisotropy is ascribed to the effect of gravity during the sintering process (Gong et al., 2019). This is in contrast with what was found by Kurose et al. (2020) since in this work the parts were printed in three different orientations on the building platform, depicted in Fig. 9, but for each layout, the maximum shrinkage was observed along the building direction. Since in the sintering furnace all the specimens laid in the same position, gravity could not be the only cause for the anisotropic shrinkage of the parts. According to the authors, a possible explanation is related to the metal powder distribution in the filament, which is not homogenous. The voids left after the binder is removed are elongated in between the layers, causing the powder particles to be more distant along the building direction. Therefore, in this direction, these voids will experience a higher volume reduction during the heat treatment. Jiang et al., (2022) studied the shrinkage and the deformation of overhang structures obtained with a 316L-polymeric filament. The shrinkage was found dependent on the overhang angle, with higher values for vertical pillars. This difference is again ascribed by the authors mainly to the gravity effect, and it is a step forward toward the production of MEAM lattices. 5. Stainless steel Stainless steels, especially 316L and 17-4 PH, were widely used to investigate the feasibility of MEAM in the production of metallic parts. Some filaments are also available on the market, such as the BASF Ultrafuse ( BASF Ultrafuse 316L ) and Markforged 17-4 ( Mark3D ). As already mentioned, the quality of a specimen manufactured through the SDS process is strongly dependent on the quality of the green part. The majority of the studies performed so far on this topic are focused on the optimization of printing parameters. The green part quality evaluation is usually based on the external appearance of the specimen and the density. Some preliminary tests are sometimes performed on the green parts, such as surface roughness assessment, density measure, and, rarely, mechanical tests under quasi-static loading conditions. This is the case of the work proposed by Godec et al. (2020). A filament containing the 55vol% of 17-4 PH powder was used to print tensile dogbone specimens. A DoE (Design of Experiment) was also performed to define the printing parameters to achieve the strongest specimen. The factors considered were extrusion temperature, flow rate multiplier, and layer thickness. The strongest tensile dog bone was obtained using the following parameters: extrusion temperature 260°C, flow rate multiplier 130%, and layer thickness 0.3 mm. The tensile strength was measured to be 9.95 MPa, which is a demonstration that green parts are not suitable for load-carrying purposes. The use of DoE, coupled with ANOVA analysis to define the significance of the parameters is attracting increasing attention when dealing with AM. Another example of DoE applied to MEAM is offered by Travieso Rodriguez et al. (2020), where the printing process of a filament composed of PLA and wood fibers was optimized. Some other examples with other techniques such as Binder Jetting additive manufacturing can be found (Chen et al., 2016; Shrestha et al., 2017). This tendency is due to the abundance of parameters involved specifically in the printing phase. On the contrary, the post-process parameters are generally more bound for facilities reasons, and sometimes the whole post-process is outsourced to appointed companies. Therefore, there is usually limited exploration regarding the debinding and sintering process. The sintered parts have been studied from both a metallurgical and mechanical point of view. Some of the tests usually performed are the following: density measurement with the Archimedean method (Kurose et al., 2020; Liu et al., 2020; Thompson et al., 2019), Vickers or Rockwell hardness test (Gong et al., 2019; Hassan et al., 2021; Liu