PSI - Issue 7

Z.H. Jiao et al. / Procedia Structural Integrity 7 (2017) 124–132 Z.H. Jiao et Al./ Structural Integrity Procedia 00 (2017) 000–000

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4. Discussion The steady stage crack growth rates of different orientation specimens of SLM produced Ti6Al4V under experimental conditions are consistent with each other, shown in Fig. 5. The corresponding fractography of FCG specimens under different conditions is shown in Fig. 7. It is found that all the fractured specimens exhibit similar flat surfaces and different crack growth textures. Fracture surfaces of stress ratio of 0.1 exhibit longer initial (I region) and steady growth (II region) regions and shorter final rupture region (III region) than stress ratio of 0.5, which are in accordance with the results shown in Fig. 5. The fracture surface textures are related to the directions of crack growth and columnar grains. For XY specimens, the crack plane is parallel to columnar crystals as well as crack propagation direction is perpendicular to the columnar crystals. Consequently, the transverse distribution texture along the crack propagation direction is formed. For YZ specimens, the crack propagates along the length of the columnar grains and at last the longitudinal distribution texture along the crack propagation direction is formed. For ZX specimens, crack planes crosscut columnar crystals, so no obvious directionality texture present. In this study, the difference of structure morphology on vertical and horizontal orientations does not have a strong impact on the steady stage FCGR of SLM produced Ti6Al4V. It is in accordance with Haize Galarraga et.al study results [12]. Consistent crack growth rates and flat fracture morphology are obtained in their study and a weak impact of microstructure on steady stage FCGR is concluded.

RT, R =0.1

XY

YZ

ZX

III

I

II

RT, R =0.5

XY

YZ

ZX

III

I

II

400℃, R =0.1

ZX

YZ

XY

400℃, R =0.5

YZ

XY

ZX

Fig. 7. Fractography of FCG specimens of different orientations of SLM produced Ti6Al4V at RT and 400 ℃ . The d a /d N - △ K curves in the lower △ K region under different experimental conditions are shown in Fig. 8. Scatter of d a /d N - △ K curves of different orientations at RT is displayed in Fig. 8 (a). For stress ratio of 0.1, the FCGR of YZ specimen is consistent with ZX specimen and faster than XY specimen; For stress ratio of 0.5, YZ specimen shows the fastest FCGR, ZX specimen behaves in a somewhat intermediate fashion and XY specimen exhibits the slowest FCGR. Fig. 8 (b) shows that there is no obvious scatter among curves of different orientations in the lower △ K region at 400℃. From published literatures, the FCGR of initial stage is closely related to microstructures. Different experiment results are obtained on different orientation specimens with different microstructures. The results of RT in this study are in accordance with V. Cain et al [10]. They concluded that the crack growth rate in initial stage was related to microstructure morphology and the residual stress condition. In comparison, ZX specimen might be