PSI - Issue 28

Costanzo Bellini et al. / Procedia Structural Integrity 28 (2020) 667–674 Author name / Structural Integrity Procedia 00 (2019) 000–000

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software of the 3D printer. The machine used was the ARCAM A2X, while the relevant slicing software was the Build Assembler. Then, the machine was prepared for the manufacturing run: the powder reservoirs were filled, and the process parameters were set. As mentioned before, for the all-titanium specimens, two different themes (i.e. sets of parameters) were necessary: the face sheets were built using the “melt” theme, that is suitable for bulk objects, while the core was generated with the “net” theme, that is more indicated for lattice structures. After, the vacuum was drawn in the manufacturing chamber, the electron beam was calibrated, and the manufacturing chamber was preheated at 700 °C. As the imposed temperature was reached, the specimens were built according to the typical sequence of a powder bed additive manufacturing process. At the end of the process, the chamber was cooled down and the specimens were extracted from the unmelted powder and cleaned in a suitable chamber, using pressurized air and a sandblasting machine, while the final depowdering operation was carried out in an ultrasound bath. Some of the produced all-titanium specimens are visible in Fig. 2.

Fig. 2. The all-titanium specimens.

The manufacturing process of the CFRP-titanium hybrid structure was more complex; in fact, a metal lattice parallelepiped was manufactured as described above for the all-titanium specimens, then further steps were required to create the CFRP skins. In fact, for this case, the titanium core was 3D-printed without any skin, that was added to the titanium core at the end of the depowdering process. For this purpose, the prepreg-vacuum bag process was chosen: CFRP prepreg plies were layered on the mould, then a ply of film adhesive was placed on the prepreg, followed by the lattice core, then another ply of adhesive and the other prepreg plies were layered to complete the stack. The prepreg chosen for this research was the Saati CC202 ER450, a plain weave carbon fabric impregnated with epoxy resin, while the adhesive was the Hexcel Hexbond ST 1035, a film epoxy adhesive commonly used in the aeronautic field. The number of prepreg plies was chosen in order to obtain a thickness of about 0.6 mm for the CFRP face sheets, like that one of the titanium ones. This similarity was important to perform a meaningful comparison between the hybrid and the all-titanium specimens. All the layered specimens were covered with the release film and the breather fabric, then the mould was closed with the vacuum bag. After the vacuum was drawn, the mould was positioned in the autoclave for the curing process. Some of the hybrid specimens are visible in Fig. 3.

Fig. 3. The hybrid specimens.