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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process. The debinding phase is needed to remove the polymer from the part. This can be done through a heat treatment or a chemical reaction. As mentioned earlier, solvent and thermal debinding are usually coupled. The solvent debinding can be done in a controlled atmosphere using cyclohexane or nitrogen, flowing in a furnace at a controlled rate. The furnace is heated up to facilitate the degradation of the main binder component and the temperature ranges from 60 to 120 °C (Nurhudan et al., 2021). Thermal debinding can be performed in vacuum or under a controlled atmosphere, the maximum temperature reaches 600°C (Nurhudan et al., 2021), and it is usually done in the same furnace where the sintering occurs. Alternatively, catalytic debinding is a faster process thanks to the catalytic degradation of the polymer exposed to acid vapors (Gonzalez-Gutierrez et al., 2018). The dwell time is usually assessed through TGA analysis: the weight loss of the green part is measured as a function of the temperature. When a target value of weight is reached, which approximately corresponds to 99% of the binder weight loss, the debinding phase is considered concluded. To simplify the debinding step, other binder systems were developed and tested. For example, Sadaf et al. (2021) prepared a filament with a 316L metal powder infill content of 65vol% dispersed in a monocomponent binder. The polymer used was polyethylene (LDPE), which was completely removed in a single thermal debinding step. The maximum density achieved by the sintered parts was 93%: this value is coherent with other works (Kurose et al., 2020; Liu et al., 2020), showing that a monocomponent binder is also a viable option. The thermal cycle is shown in Fig. 7. The final process to achieve a fully dense metallic part is the sintering. Sintering is a typical process of powder metallurgy. The brown parts are held at high temperatures for several hours to allow the powder particles to bond together through atomic diffusion. For stainless steel powder, the temperature reaches up to 1360°C (Fig. 8a). Singh et al. (2021) fabricated titanium specimens were sintered at 1250°C for 4h using Argon as a reducing gas, as shown in Fig. 8b; copper parts were sintered in Dehdari Ebrahimi et al. (2018) at 980°C for 50 min, with slightly over sintering at 1080°C for 10 min. Sintering parameters are crucial to enhance the strength of the parts since in this phase, the metal particles bounds form the solid part, compensating for the voids left after the binder removal. The residual porosity is one of the main issues regarding MEAM. It was proved in several works that the pores’ distribution, shape, and morphology strongly affect the mechanical properties. The sintering process starts with a fragile part composed merely of packed metal powder. Before the complete densification, the parts can collapse, and sometimes it is necessary to introduce supports. The main sintering parameters are maximum temperature, holding time, heating and cooling rate. Moreover, a relevant choice regards the use of gas to create a controlled atmosphere. Sintering is sometimes performed in vacuum furnaces. Nevertheless, using protective atmospheres during the process can lead to improvements in the sintered parts’ quality. Using a flow of inert gas (e.g., helium, argon) facilitates the densification keeping the carbon and oxygen levels under control. Reducing atmospheres (e.g., oxygen, hydrogen) are beneficial to prevent oxidation phenomena. More details about the sintering process are described by German, (2010a; 2010b).

Fig. 7. Debinding and sintering cycles used in a) Sadaf et al. 2021; and b) Singh et al. 2021

From a physical point of view, the driving force of material diffusion is surface energy reduction, which is a temperature-sensitive phenomenon. Finer metal powder is reported to have a greater sinterability. To reduce the energy of the surface, the atoms move according to several mass transport mechanisms. They are divided into