PSI - Issue 7

Theo Persenot et al. / Procedia Structural Integrity 7 (2017) 158–165 Persenot et al. / Structural Integrity Procedia 00 (2017) 000–000

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Fig. 1. a) 3D isometric view of the Computer Assisted Design input. The dimensions indicated are in millimeters; Photographs of an EBM sample b) before and c) after removal of the manufacturing supports.

Nonetheless, the roughness of the surface of as-built EBM samples is quite large and this has a detrimental e ff ect on the mechanical properties of as-built parts (Sun et al. (2016)). For struts assembly like cellular structures, conventional machining techniques cannot be used and this strongly restricts their use in real applications. As a result, there is today a need to identify post-treatments in order to improve the surface state of additively manufactured lattices. The objective of this paper is to characterize the fatigue properties of as-built Ti-6Al-4V single struts manufactured by EBM and representative of elementary elements of lattices and to study the e ff ect of chemical etching on the geometry and fatigue performance of such thin parts.

2. Experimental method

2.1. Sample manufacturing

Fatigue samples were manufactured using an ARCAM A1 EBM machine and a Ti-6Al-4V ELI powder (average diameter = 70 µ m , see composition in Table 1).Two di ff erent powders were used. The first one is the “fresh” powder supplied by ARCAM which conforms to ASTM F 3001 requirements in terms of composition. The second one is a powder which has already been used and recycled for several builds (over 20 di ff erent builds). The sample geometry (Figure 1) was chosen so that the central cylindrical zone (gauge length of 10mm and 2mm diameter) has the same size of a single strut within a lattice structure. The samples were built with the cylinder axis parallel to the building direction; the melting parameters as well as the scanning strategy were defined using the standard “Melt” build theme recommanded by ARCAM for layers with 50 µ m thickness.

Table 1. Chemical composition of the fresh and recycled Ti-6Al-4V powder batch used for this investigation. Elements Al V C Fe O N

H

Ti

Fresh powder (wt%) Recycled powder (wt%)

6.47 6.17

3.93 4.35

0.01 0.05

0.22 0.28

0.09 0.44

0.01 0.01

0.001 0.001

Bal. Bal.

2.2. Chemical etching

The etching treatment consists in dipping the as-built fatigue samples in a fresh and still solution (no stirring) containing 12 mL HF (48%), 40 mL HNO 3 (70%) and 300 mL of distilled water. Two di ff erent etching times were