Microstructure effects on fatigue crack growth in additively manufactured Ti-6Al-4V

In this work we investigate the relationship between the different microstructural features in additively-manufactured (AM) Direct Metal Laser Deposited Ti-6Al-4V alloys and the observed fatigue and fracture behaviors, using high resolution Digital Image Correlation experiments and microstructural imaging. Fatigue cracks in the AM specimens were observed to periodically propagate parallel to the alpha' laths and deflect at prior beta grain boundaries. Corresponding plastic strain distributions show needle-like regions of high strain that correlated to the alpha' laths, suggesting significant influence of the microstructure on the plastic zone. In many cases, the fatigue cracks also propagated towards voids from unsintered powder or gas entrapment in the vicinity. In several experiments, premature failure of the AM specimens occurred under monotonic fracture or cyclic fatigue loading, which we attribute to the presence of void clusters located near the crack-tip. These fatigue and fracture characteristics were similar for AM specimens of different build-orientations.