PSI - Issue 17

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

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Ludvík Kunz et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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the longitudinal direction, tangential stress was 6 MPa. It means that the applied heat treatment was sufficient to remove the long range residual stresses.

Fig. 10. Comparison of crack growth curves for different heat treatments and stress ratios 0.1 and 0.8.

Fig. 11. Microstructure after heat treatment at 740 °C.

Fig. 12. Microstructure after heat treatment at 900 °C.

Microstructure of Ti6Al4V after stress relieving at 380 °C consists of primary columnar  grains elongated in the building direction which is obvious from Fig. 7. Their length corresponds to several building layers. The micro structure of the primary  grains transformed during the rapid cooling to acicular  ʹ martensite which is typical structure for as-built material, see e.g. by Vilaro et al. (2011). This as-built microstructure remained unchanged after heat treatment at 380 °C, Fig 8. Unlike the  grains the  ʹ martensite does not exhibit any evident directionality as regards the building direction. Though the microstructure seems to be unchanged due to stress relieving at 380 °C, the crack growth resistance and the threshold is higher than that reported by Leuders et al. (2013) for the as-built material. Comparison with literature data is shown in Fig. 9. The data by Leuders et al. (2013) for the orientation c fits well in Paris region with the data published by Becker et al. (2015), who however did not present the information about specimen orientation. Further, in Fig. 9 Paris equation for as-built material, orientation c by Cain et al. (2015) is shown by the dashed line. From the comparison it can be concluded, that the stress relieving used in this work a) improves the long crack behaviour both in Paris region and notably in the threshold region, b) reduces the scatter of the experimental points namely in the range of high crack growth rates and, c) no influence of the crack growth direction with respect to the direction of building was observed, Fig. 3. From the comparison of Fig. 3 and 4 it is obvious that the increase of temperature of post processing heat treatment from 380 to 740 °C reduces the scatter of data, though the long range residual stresses are already removed at annealing temperature 380 °C. There is also no apparent influence of the laser power and the crack growth and the threshold is not orientation dependent.