PSI - Issue 13

J.-P. Brüggemann et al. / Procedia Structural Integrity 13 (2018) 317–321 J.-P. Brüggemann et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Table 1. Room temperature tensile properties of heat-treated and HIP Ti-6Al-4V SLM-specimens with two different particle size distributions. Condition PSD [µm] yield strength [MPa] ultimate tensile strength [MPa]

31.7

1040

1080 1040 1020

heat-treated 1073.15 K

40

962 940 912

31.7

HIP

40 1005 Since no significant influence of the changed PSD regarding the static material properties can be detected, the fracture-mechanical behavior is analyzed. Fig. 3 shows the crack propagation curves for the 1073.15 K heat-treated condition and the condition after hot isostatic pressing.

Fig. 3. Crack growth curves for SLM-processed Ti-6Al-4V ( d (0.5) = 31.7 µm) in the 1073.15 K heat-treated condition and the condition after HIP.

Analogous to Riemer (2015) the same specimen geometry is used for the determination of crack propagation curves. The specimen used are oriented in a way that their building direction (BD) is perpendicular to the crack path (CP) (BD ⊥ CP ሻ according to the investigations of Riemer (2015). Overall, the detected parameters have a high reproducibility, recognizable by the little variation of the crack propagation curves. The determined threshold value Δ K I,th for the heat-treated condition is 3.6 MPa·m 1/2 , the standard deviation of 0.02 MPa∙m 1/2 being an indication of good reproducibility. Hot isostatic pressing can significantly improve the fracture mechanical properties of the Ti-6Al 4V alloy. The threshold for fatigue crack propagation is Δ K I,th = 4.7±0.16 MPa·m 1/2 . A comparison of the fracture mechanical behavior of the powder lot with d (0.5) = 31.7 µm with the experimental data from Riemer (2015) with d (0.5) = 38.9 μm is visualized in Fig. 4. The threshold areas for the powders with mean particle size of 31.7 µm and 38.9 µm by Riemer (2015) are shown for the heat-treated condition (1073.15 K) and the condition after hot isostatic pressing. Comparing both powder lots in the heat-treated condition the threshold for fatigue crack propagation Δ K I,th for the powder with d (0,5) = 31.7 µm is 0.3 MPa·m 1/2 lower in contrast to the data from Riemer (2015). After hot isostatic pressing it is 0.5 MPa·m 1/2 higher. Both results show that there is no significant influence of the particle size distribution on the fracture mechanical behavior, especially on the threshold for fatigue crack propagation Δ K I,th . The small deviation of the threshold values may be due to environmental parameters and accuracies of the test equipment. All these factors have a minimal influence on the experimental determination of the crack propagation curves, which in total can give a slight scattering of the measurement results. Furthermore, the relatively little variation in the determined characteristics is a sign of a good reproducibility and a process-safe processing of Ti-6Al-4V alloy in selective laser melting manufacturing process. Due to the good accordance of the mechanical and fracture mechanics parameters, fatigue tests for the heat-treated condition and the condition after hot isostatic pressing are carried out for selected stress states to determine a Wohler curve for the PSD with d (0.5) = 31.7 μm. Compared to the determined fatigue properties from Leuders et al. (2013), there are no significant differences for the tested stress levels