PSI - Issue 24

Giulia Morettini et al. / Procedia Structural Integrity 24 (2019) 349–359 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Nomenclature R load ratio σ mlimean Mean fatigue limit σ̌ Nc Standard deviation of number of cycles 1 0 % Fatigue limit and corresponding to 10% of survival probability 9 0 % Fatigue limit and corresponding to 90% of survival probability Scatter index B Slop of the mean Wöhler curves A Intercept of the mean Wöhler curves

2. Materials and method

2.1. Specimen fabrication and process parameter

The knowledge of test specimen condition, process parameters and the method by which the tests were conducted are of the utmost importance. All the Ti-6Al-4V specimens were manufactured by DMLS of EOS optimized metal powders. Specimens were built by EOSINT M290 with the powder that has a chemical composition, reported in Table 1, corresponding to ASTM F1472 and ISO 5832-3 that guarantee its use for surgical implants applications.

Table 1. Chemical composition of Ti-6Al-4V powder from EOS material data sheet. Ti Al V O N C H Fe (Wt.-%) (Wt.-%) (Wt.-%) (ppm) (ppm) (ppm) (ppm) (ppm) Balance 5.5-6.75 3.35-4.5 <2000 <500 <800 <150 <3000

The default set process parameters employed and optimized by EOS, has been adopted to fabricate the test specimens. Among these it is possible to find Laser Power 340 W, Laser Speed 1250 mm/s and Layer Thickness of 0.06 mm. Moreover, to reduce the residual stress and to produce a homogeneous distribution of this, a Stripe Hatching Pattern with 67° rotation was define. In this way from one layer to the next, the components of the stress state are not all alienated in the same direction Fatemi (2017). The specimen geometry and its dimensions are shown in Figure 1. The rectangular cross section was used in order to reduce to the full the residual stress problem Patterson (2017). 2.2. Experimental procedure To perform the fatigue tests, specimens were built with two different build directions (vertical and horizontal) taken also into consideration for the analysis, as shown in the Figure 1. The specimens were removed from the build plate by saw cut. No post-processing machining was carried out, the only operation performed after the detachment of the specimens was the cleaning of the supports by manual removal.