Short Crack Behavior of an Additively Manufactured Ti–6Al–4V Alloy Under Ultrasonic High Cycle Fatigue Testing

Abstract

The high cycle fatigue (HCF) behaviors of an additively manufactured (AM) Ti–6Al–4V alloy with fully lamellar microstructures processed electron beam powder bed fusion (EB-PBF) and wire-fed electron beam directed energy deposition (Sciaky) routes were compared. Ultrasonic fatigue (USF) testing at the stress ratio of R = −1 was applied to monitor the growth of small cracks initiated at surface micronotches. Crack growth rates lower than 10⁻⁸ (m/cycle) at ΔK = 6 MPa·m^1/2 were measured in samples processed by both methods. The finer α lath thickness (~1 μm) of the Sciaky samples resulted in a slower fatigue crack growth rate than the EB-PBF samples with coarser laths. The interaction of cracks with the lamellar microstructures was characterized by electron backscatter diffraction. Crack propagation largely followed the lath interfaces in the Sciaky samples, whereas cracks cut across colonies in the EB-PBF samples. Different fatigue fracture surface characteristics were observed for the EB-PBF and Sciaky samples.