Coupling effects of microstructure and defects on the fatigue properties of laser powder bed fusion Ti-6Al-4V

The coupling effects of microstructure and macro-scale defects are significant for the fatigue properties of metallic materials. However, this correlation is still vague for additively manufactured materials. Therefore, the fatigue properties and microstructure of laser powder bed fusion (LPBF) Ti-6Al-4V were studied in this work, it is found that there is an anomalous trade-off relationship between the hardness and the high-cycle fatigue properties. Surprisingly, the softest microstructure has better fatigue resistance, i.e., its tensile strength is reduced by over 23%, while its fatigue strength is increased by ∼28%, compared with the hardest stress-relieved state. Statistical analysis on the fatigue source morphologies indicates that the hard α′ microstructure is sensitive to lack of fusion (LOF)-type defects while the soft lamellar (α + β) microstructure is sensitive to pore-type defects, which may be related to the higher tolerance concerning sharp defects for the soft microstructure relative to the hard one. Through considering the transition of the fatigue cracking mechanism and the effect of the microstructure, a fatigue life prediction model has been developed based on the Murakami theory, which does not only give a sound explanation for the trade-off relationship but is also in good agreement with the fatigue data for different defect characteristics and microstructure states at different applied stress amplitudes.