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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printing procedure: first, low viscosity is desirable to keep the minimum extrusion temperature low and reduce the probability of clogs in the nozzle. Moreover, since in a filament-based extrusion process the filament itself acts as a plunger to push the material into the printing head, a suitable stiffness is also required. This should be coupled with good flexibility and strength, otherwise, it is difficult to spool the filament and load it in the printer without breaking it. The compound has to show good adhesion behavior, considering that the metal powder does not contribute to the adhesion of the layers during the printing. Finally, to ensure geometrical accuracy in the part, the cross-section area of the filament should be circular and constant. All those properties are controlled through the composition of the material. A high infill percentage is desirable to enhance the density of the green part and consequently reduce the porosity and the shrinkage of the final part, as pointed out by Cerejo et al. (2021). In this study, the maximum powder content in a stainless steel-binder feedstock was defined using a torque rheometer. The method consists in measuring the torque in the mixer while gradually adding the metal powder. The same torque rheometer method was used by Singh et al. (2021) to obtain an optimal Ti-6Al-4V feedstock. The torque increases until it reaches a point of instability, that corresponds to the maximum solid fraction exploitable to ensure printability. Exceeding the maximum limit leads to a viscous filament with worse flowability. In Fig. 3a, the critical load was registered for 64vol% solid content, but it was preferred to use the precautionary percentage of 59vol%. The interaction between powder particles is the main responsible for the viscosity increase when excessive solid infill is mixed. According to Gloeckle et al. (2020), the filler-filler interaction depends on the powder loading and the particles size. The friction between powder leads to an increase in the minimum extrusion temperature and a lower flow rate.

Fig. 3. (a) torque rheometer method in Singh et al., (2020); (b) thermogravimetric analysis method, in Gloeckle et al. (2020)

To control the viscosity of the filament, the polymeric part composition is crucial. The polymers contained in the filament form the binder systems (Gonzalez-Gutierrez et al., 2018). The binder system components are different polymers with different purposes, and they depend on how the debinding phase is performed. It is a common practice to have a solvent debinding phase first, followed by a thermal debinding, which is indeed the first part of the longer sintering thermal cycle (Gonzalez-Gutierrez et al., 2018). More details will be described in a dedicated section. In this eventuality, the binder system is composed of three parts, referred to as main , backbone , and additives . The main component (50-90%) usually belongs to the family of waxes and it is suitable to be removed using solvents. The backbone (0-50%) is the part of the binder system that survives the solvent debinding to give support to the metal powder during the thermal debinding. Usually, the backbone is composed of polyolefins. Finally, additives are used in low percentages to enhance specific properties of the filament. For example, elastomers are added for better flexibility (Gonzalez-Gutierrez et al., 2018); surfactants like stearic acids to reduce viscosity, and plasticizers to reduce brittleness (Cerejo et al., 2021). A broad example of the optimization of the binder system is proposed by Wagner et al. (2022). Usually, thermogravimetric (TGA) analyses are carried out on the feedstock to determine the degradation temperature of each component of the binder system as done by Gloeckle et al. (2020), and shown in Fig. 3b. With regards to the metal powder, some crucial parameters include the powder size, its morphology, and dispersion homogeneity. The particle size affects the flowability of the filament. In a work by Cerejo et al. (2021), it was observed that using finer powder results in increasing mixing torque, which means a higher viscosity. This is explained as a consequence of the higher interparticle friction. The same conclusion was obtained by Singh et al. (2020), where a model was proposed to predict the pressure drop at the nozzle exit. The pressure is expressed as a function of the nozzle geometry, feedstock viscosity, and feed rate through the orifice. The authors used two particles’ sizes, 13 and 30 µm, discovering that the pressure drop associated with finer powder is higher. This means