PSI - Issue 53

Venanzio Giannella et al. / Procedia Structural Integrity 53 (2024) 172–177 Raffaele Sepe / Structural Integrity Procedia 00 (2019) 000 – 000

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most widely used (Sepe et al., 2020, 2021; De Luca et al., 2021; Alfieri et al., 2022). EBM allows producing 3D components from metal powders that are layer-by-layer melted through a high intensity electron beam (Körner, 2016; Gong et al., 2014). Thin layers of powder are continuously distributed, heated and selectively melted during the process, while the building table is lowered at any increment. The process was firstly presented by the Swedish company ARCAM in 1997, subsequently acquired by General Electric, and represents, nowadays, one of the most promising technologies for the manufacturing of lightweight, durable and fully dense parts. EBM is mainly used for titanium and cobalt alloys for the sectors of aerospace, medical, automotive and defense industries (Li et al., 2014), thanks to its excellent properties such as low density and high mechanical strength. The development of the EBM technology in processing titanium alloys for the manufacturing of massive parts has brought the technology to an advanced state of maturity. Indeed, several studies (Pirozzi et al., 2017; Facchini et al., 2009) focused on the mechanical characterization of the Ti6Al4V alloy which is today perhaps the most promising alloy that can be processed via EBM. Research efforts are currently in the direction of understanding the influence of numerous factors (roughness, accuracy, defects distribution, etc.) on the performances of EBM-printed Ti6Al4V components (Sepe et al., 2022; Pirozzi et al., 2017, 2019; Borrelli et al., 2020; Franchitti et al., 2018, 2020), also with reference to lattice structures (Bellini et al., 2021, 2022; Sepe et al., 2022). According to these outcomes, manufacturing imperfections such as deviation of the actual geometry from the nominal CAD geometry can negatively affect the performances of EBM structures with respect to the expected ones (Sepe et al., 2022). Researchers highlighted that, for thin EBM components, surface roughness, dimensional accuracy and building direction can have an impact on the structural strength. With the aim of quantifying the impact of the building direction on the structural strength, this work aimed at assessing the fatigue crack propagation behavior of TiAl64V specimens made by EBM through experimental tests in laboratory conditions. Tests were carried out using standard 8 mm thick Compact-Tension C(T) specimens by considering constant amplitude load with R sets to 0.1 and frequency sets to 5 Hz. The main objective was to study the effects of the building direction on the residual fatigue life and on the fatigue failure mechanisms. 2. Materials and methods An ARCAM A2X machine was used to manufacture samples made of Ti6Al4V. Such a machine has a build envelope of 200 x 200 x 380 mm 3 and is specifically designed to process materials that require high process temperatures such as titanium alloys. The chemical composition of the metal powder used for the manufacturing was listed in Table 1. The machine was used to manufacture samples of material in the shape of small plates. Electrical Discharge Machining (EDM) was used to machine the holes for the pins and the initial notch in such a way to obtain C(T) specimens. No further machining was considered, hence meaning that the specimens could be considered in an “as - built” configura tion. Three measurement systems were used during the tests for cross comparisons: a clip gauge to measure the crack opening, a crack gauge to measure the smallest crack lengths, a strain gauge glued at the back face to measure the local strain. As discussed in the following, this latter measurement system turned out to be the preferred choice. A general overview was reported in Figure 1. Samples were manufactured by considering three building directions, see the right side of Figure 2. Tests were conducted in constant amplitude at a frequency of 5 Hz, with a peak load P max = 6 kN and a load ratio sets to R = 0.1.

Table 1. Chemical composition of the metal powder.

Elements

Al

V

Fe

O

N

H

C

Ti

Wt%

6.40

4.12

0.18

0.14

0.01

0.003

0.01

Balance