PSI - Issue 13

S.M.J. Razavi et al. / Procedia Structural Integrity 13 (2018) 74–78 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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2. Experimental procedure

The Ti-6Al-4V samples were fabricated using the SLM technique according to the optimized process parameters that guaranteed a density greater than 99.7%. Afterwards, test samples were sandblasted at 6 bar using corundum sand with a mean grain size of 220  m followed by stress relief heat-treatment in non-controlled atmosphere (heating rate: 10.8 °C/min; holding time: 3h at 650 °C; cooling rate: 2 °C/min) to provide a better surface finish with lower residual stresses. Then the samples were cut off the base plate and re-sandblasted at 6 bar using corundum sand. A layer thickness of 60  m was considered for all specimens. The geometries of smooth, semi-circular and blunt V- notched specimens are illustrated in Fig. 1. The thickness of all testing samples was 3 mm. The notch tip radius of semi-circular notches and blunt V notches were 5 mm and 1mm, respectively. The Fatigue tests were carried out using a universal MTS machine (250 kN). All tests have been carried out under load control, using a sinusoidal signal in uniaxial tension with a frequency of 10 Hz and load ratio R = 0 (Razavi et al., 2017; Razavi et al., in press). a b c

Fig. 1. Geometries of the test samples, built axis (Z) (dimensions in mm); (a) smooth sample, (b) circular notched sample, and (c) blunt V notched sample.

3. Results and discussion

Figure 2 show the fatigue test results for smooth and notched samples. Values of stress amplitude related to a survival probability of 50%, the slope of the Wöhler curve and the scatter index T  which is the ratio between the stress amplitudes corresponding to 10% and 90% of survival probability are reported in the above-mentioned figures. Specimens survived over 1 million cycles are considered as run out and marked up with an arrow. It can be noted that the difference between the Wöhler curves are related only to the mean value of the stress amplitude at one million cycles, but not to the scatter index T  . The fatigue strength of smooth samples was 243 MPa, while fatigue strengths 213 MPa and 144 MPa were respectively reported for the double circular notched specimens and blunt V notched specimens at 1 million cycles. By comparing the results from SLM semi-circular notched and smooth specimens it is observed a reduction of about 12% of the mean value of the stress range at 1 million cycles. While for the blunt V-notched specimens this reduction was about 41%. The inverse slope of the three curves is also significantly different, k = 4.74 for the smooth specimens, k = 4.88 for the semi-circular specimens and k = 4.15 for the blunt V-notched specimens. In all cases the scatter index is limited, with T σ = 1.328 for smooth specimens, T σ = 1.332 for semi-circular specimens and T σ = 1.158 for blunt V-notch specimens. From the high values for inverse slope k and fatigue stress, it is evident that SLM additively manufactured specimens demonstrate a comparable fatigue strength compared to the traditionally produced specimens (Berto et al., 2015). It should be mentioned that the notch sensitivity of the SLM Ti6Al4V samples were quite small for semi-circular specimens which can be related to the rough surface of the AM specimens and also to the presence of surface defects.