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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(B scale) and obtained 60HR for MEAM parts, and more than 80HR for SLM parts. For 17-4 PH instead, Suwanpreecha et al. (2021) found a value of Rockwell microhardness around 35 HRC for all the specimens, independently of the printing orientation. 6. Other metals Additive Manufacturing of titanium alloys is usually of great interest in several fields due to the difficulties in machining titanium with conventional processes. Some titanium alloys are biocompatible, and have several applications in the biomedical field, in the production of implants, tools, or other devices. These components are typically produced in small series and the customization of the geometry and mechanical properties is important. Recently, MEAM has been used to study the feasibility of the process to fabricate components made of titanium alloys. As mentioned before, Gloeckle et al. (2020) and Singh et al. (2020) studied the effect of the infill percentage of Ti-6Al-4V powder in the high filled filament. Singh et al. (2021) used a filament of the same titanium-aluminum alloy with 59vol% of powder in it. Dogbone specimens were printed, sintered, and tested with tensile loading. After the sintering, a relative density of 94.2% was measured. The mechanical properties were found to be similar to the ones obtained from the specimens fabricated via MIM, with a UTS of 875±15 MPa, yield strength of 745±10 MPa, and elongation of 17±3%. Thompson et al. (2021) printed and tested components with a filament containing 55vol% of pure titanium powder. Their research aimed to print pure titanium components to take advantage of titanium corrosion resistance for electrochemical applications. Some interesting strategies were used to increase the density of the green parts: two infill patterns were alternated (rectilinear and concentric), and before debinding, the green parts were pressed at 180°C for 10 min to help close the pores. The higher density was obtained by sintering at 1350°C for 5h, and it was reported to be 95.7%. With this procedure, excessive grain growth was avoided, and the purity of the composition was preserved. Another interesting study is reported by Shaikh et al. (2021). Lattice structures were printed and sintered with a filament containing 59vol% of Ti-6Al-4V, some examples are depicted in Fig. 14. The research was mainly focused on the feasibility of the process, but further studies are required to investigate the mechanical properties of the structures. Lattices are geometrically complicated and therefore difficult to achieve conventionally. AM is suitable to produce metal metamaterials, but there is still little knowledge about the mechanisms of failure of such structures, both under static and cyclic loading (Benedetti et al., 2021). Shaikh et al. (2021) used the parameters used to print bulk titanium components via MEAM to produce lattices, with poor results. New parameters were set and good quality green parts were achieved. In particular, it was necessary to reduce the flow extrusion multiplier due to the precision required in the manufacturing of such structures. For the same reason, the speed of the print was significantly reduced. To increase the density of the struts, a concentric infill strategy was chosen since larger voids were created using the standard rectilinear 0/90° infill pattern. The last important change regards retraction, which was increased. Although it was not mentioned before, it is an important parameter when dealing with FDM since it indicates how much the filament should be retracted during the travel of the nozzle to avoid leakage of melted material.

Fig. 14. Titanium lattices before and after sintering (Shaikh et al., (2021))