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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was useful to define the best combination of the lattice core and the composite skin parameters for different structure configuration. In the present work, an experimental campaign was carried out, testing the three-point bending behaviour of two types of sandwich panel, both constituted by a metal lattice core, in Ti6Al4V titanium alloy, produced through Electron Beam Melting (EBM) process, but with two different types of skins: in one case it was made of the same additive manufactured metal, in the other one it was made of CFRP. 2. Materials and methods In the present work, hybrid panels with a metal lattice core, produced by additive manufacturing technology, and composite material skins were defined, produced and tested. The results obtained for this kind of structure were compared with those determined for all-metal panels. The characteristics of the lattice core were very important since they influenced the mechanical peculiarities of the whole structure. As concerns the cell type, the octet-truss lattice was chosen. This cell, visible in Fig. 1, is a centred face cubic cell, formed by 12 struts, with an octahedron inside. In this kind of lattice, 12 struts are directed towards each node, as defined by Dong et al. (2015). The cell side was chosen equal to 6 mm, while the truss diameter to 1 mm. The lattice core was produced through EBM (Electron BeamMelting) technology, that is an additive manufacturing process widespread in the aeronautical industry. The titanium alloy adopted in the present research activity was the Ti6Al4V, that is the most used titanium alloy in the aerospace and aeronautic field. For the purpose of this work, the produced lattice core had a section of 30 mm x 9 mm and a length of 270 mm. Being the single cell dimension equal to 6 mm, in order to reach a thickness of 9 mm, one cell and half were considered along this direction.

Fig. 1. The octet-truss cell.

All the produced specimens presented a titanium lattice core, while the skin was made of titanium or CFRP. The EBM process was suitable to produce the structures relevant to the former kind without any subsequent operation, so the all-titanium specimens were produced in a single step. On the contrary, for the other type, ulterior manufacturing steps were required to bond the CFRP skin on the lattice core. As concerns the manufacturing process of the all-titanium specimens, after having defined all the geometric parameters, the first step consisted in the creation of a virtual geometrical model. For this purpose, the Materialise Magics software was adopted, since it was able to produce a lattice structure in a specific volume, by defining the geometry of the volume itself and the type and the dimensions of the cell. For the all-titanium specimens, the lattice core and the skins, having a thickness of 0.6 mm, were drawn as two separate CAD files with an interpenetration between the struts and the skins of 0.2 mm. Two separate files are needed to process the two parts with different process themes (net theme for the lattice core and melt theme for the skin core), the interpenetration ensures that the two parts are well welded at the end of the process. Once the CAD model was ready, it was exported in the slicing