PSI - Issue 17

Ludvík Kunz et al. / Procedia Structural Integrity 17 (2019) 222–229 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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4. Discussion

DMLS process is characteristic by very large temperature gradients of the order of 10 4 Kcm -1 . As a consequence high residual stresses with high gradients appear in as-built components, Knowles et al. (2012). High stresses, which can reach the yield stress of material, have to be removed by some stress relieving procedure, see e.g. Edwards and Ramulu (2014). The heat treatment, however, influences the microstructure and thus the resistance of materials against propagation of fatigue cracks. Leuders et al. (2013) and Riemer et al. (2015) investigated the fatigue crack propagation in Ti6Al4V at stress ratio R = 0.1. The CT specimens were manufactured with laser power 400 W. The 30  m thick layers were created from powder with average particle size 40  m and the temperature of the building platform was 100 °C. The authors reported that the lowest resistivity to the propagation of fatigue cracks exhibit as built material with  ʹ martensitic microstructure. The threshold for long crack growth was found to be dependent on the specimen orientation. The lowest value 1.4 MPam 1/2 corresponds to the orientation c . For the orientation a the threshold is slightly higher, namely 1.7 MPam 1/2 .

Fig. 7. Primary columnar  grains. Building direction is from bottom to top.

Fig. 8. Microstructure after heat treatment at 380 °C.

Fig. 9. Fatigue growth data for as-built and stress relieved Ti6Al4V at 380 °C.

In this study, the crack growth after stress relieving at temperature 380 °C for 8 h was investigated. The stress state was measured on the lateral sides of the CT specimens which were metallographically polished. Finally, the surface layer of thickness of about 0.1 mm was removed by electropolishing. The residual stresses were determined by X- ray sin2ψ method . Measurement was performed in longitudinal direction, i.e. in direction of the crack growth from the notch and in the transversal direction, i.e. in direction perpendicular to the crack growth. It has been found that the long range residual stresses are negligible. The highest value of principal stress -28 ± 6 MPa was found in