Rotating bending fatigue mechanisms of L-PBF manufactured Ti-6Al-4V alloys using in situ X-ray tomography

Lightweight titanium alloys are being increasingly used in high value structures, but in-depth knowledge of the rotating bending fatigue (RBF) mechanism remains insufficient, particularly for additively manufactured components. In response, an in situ RBF test rig was developed for the first time, using time-lapse synchrotron radiation X-ray microtomography (SR-μCT) to identify the failure behavior of laser powder bed fused Ti-6Al-4V alloys. The RBF damage evolution was characterized by combining the non-destructive SR-μCT with post-mortem scanning electron microscopy, and electron backscatter diffraction. Compared with RBF life data and fatigue long crack growth from standard specimens, in situ RBF tests proved to be reliable, in revealing the fatigue mechanism. However, fatigue short cracks in the representative mini RBF testing campaign were found to initiate from microstructural features of outer surfaces, instead of gas pores or lack of fusion defects. It is demonstrated that neither the standard nor the improved Kitagawa-Takahashi diagrams are unable to correlate the fatigue strength to manufacturing defects. The equivalent defect should include the effect of surface microstructural defects, in addition to internal defects, when studying additively manufactured alloys.