PSI - Issue 7

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

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

expected to display the lowest crack growth rate since the crack path was perpendicular to the columnar grain structure, but it was also dominated by tensile residual stresses perpendicular to the crack plane near the free surfaces, as a result, the crack growth rate of ZX specimen was improved. The residual stress condition relative to the crack plane, was similar for YZ and XY specimens, but the microstructure anisotropy was slightly more favourable towards resisting crack propagation in the case of XY specimen. So XY specimen acquired the slowest crack growth rate. YZ specimen crack path cleaved down the length of the columnar grains (akin to chopping wood along the grain), which led to the fastest crack growth rate. Haize Galarraga et al [12] investigated the crack growth rates in the near-threshold region of XZ and ZX specimen orientation of as- fabricated and β annealed EBM Ti-6Al 4V and found that, for as-fabricated condition, XZ specimen exhibited slower crack growth rate in the near threshold region than ZX specimen. The reason was that, for XZ specimen, the prior β grain boundaries of the alloy act as crack deflectors in angles close to their orientation, decreasing the crack propagation rate in XZ specimen. The columnar grain structure was responsible for the branching of the crack, while the scanning layers contributed to the deflection of the crack front enhancing the tortuosity of the crack path. For ZX specimen, the prior β grain boundaries acted as barriers to the crack growth; the horizontal crack propagated in the same direction as the scanning layers and cut directly through the prior β grain boundaries without considerable interaction with them. For β annealed condition, the FCGR in both orientations was identical due to the equiaxed microstructure of the alloy. Leuders et al [11] also detected the same results. All in all, the anisotropy of FCG behaviour of AM Ti6Al4V alloy mainly exist in the near-threshold region. In this region, the crack is small, correspondingly, the crack tip stress intensity factor is also small. The crack path and stress intensity factor are easy to be affected by the combined contribution of anisotropy of microstructure, the scanning layers as well as residual stresses. For 400℃, the anisotropy of FCG behaviour is not obvious, which may be due to the relief of residual stresses and grain softening at elevated temperature. At present, the experiment data for FCG behaviour is still limited, more work must be conducted to clarify the regularity of anisotropy for FCG behaviour and fracture mechanism.

4x10 -5 5x10 -5 6x10 -5 7x10 -5 8x10 -5

5x10 -5 6x10 -5 7x10 -5 8x10 -5 9x10 -5

(a)

(b)

Ti6Al4V(SLM) 400 ° C R =0.1

Ti6Al4V(SLM) RT R =0.1

R =0.5

R =0.5

XY YZ ZX

XY YZ ZX

XY YZ ZX

XY YZ ZX

3x10 -5

4x10 -5

2x10 -5 d a /d N , mm/cycle

3x10 -5 d a /d N , mm/cycle

10 -5

2x10 -5

6

7 8 9 10 11 12 13 14 Fig. 8. The d a /d N - △ K curves in the low △ K range under different conditions. (a) RT; (b) 400 ℃ . ∆ K , MPa√m 6 7 ∆ K , MPa√m

8 9 10 11 12 13 14

5. Conclusions Tensile and FCG performances at RT and 400 ℃ of SLM produced Ti6Al4V alloy are studied and compared with conventionally manufactured Ti6Al4V alloys. Different orientations of SLM alloy are considered to investigate the anisotropy of static tensile properties and FCG behavior. The results presented in this study lead to the following conclusions: • The static tensile properties at RT and 400 ℃ are highly dependent on the specimen orientation relative to build direction. The FCG resistance is related to specimen orientation in the near threshold region but no relationship with specimen orientation in the steady growth stage at RT. The FCG resistance is much less influenced by specimen orientation at 400 ℃ . • The static tensile strength at RT and 400 ℃ of SLM produced Ti6Al4V reaches even exceeds the strength level of conventionally manufactured Ti6Al4V alloys such as forging, bar and casting. The ductility is in accordance with forging and bar alloys and superior to casting alloy.