Build Direction‐Dependent Ratchetting‐Fatigue Interaction of Laser Additively Manufactured Ti6Al4V Alloy

ABSTRACT The microstructural features and manufacturing defects in additively manufactured metallic materials typically exhibit anisotropy, leading to deformation behavior and mechanical properties dependent on the build direction. This study utilized laser powder bed fusion (L‐PBF) technology to vertically and horizontally build Ti6Al4V titanium alloy specimens. Monotonic tensile tests, symmetric strain‐controlled low‐cycle fatigue tests, and asymmetric stress‐controlled ratchetting‐fatigue interaction tests were performed on specimens with vertical and horizontal build directions. The analysis shows that specimens with vertical build direction, owing to their higher yield stress, exhibit higher stress amplitude and lower ratchetting strain at the same strain amplitudes and stress levels, respectively. The fatigue life of vertically built specimens is superior to that of horizontally built ones. Fractographic analysis reveals multiple crack initiation sites in horizontally built specimens across all stress levels, whereas a single crack initiation site is observed in vertically built specimens at higher stress levels. Moreover, with increasing stress levels, both the ratchetting strain and its evolution rate increase, leading to reduced fatigue life and a smaller proportion of crack initiation and stable propagation zones. This demonstrates that under elevated stress levels, specimens predominantly exhibit ratchetting‐induced failure mode.