Multiaxial fatigue behavior of wrought and additive manufactured Ti-6Al-4V including surface finish effect

Additive manufacturing (AM) technology has enabled efficient fabrication of parts with complex geometries and laser-based powder bed fusion (L-PBF) is one of the most commonly used AM fabrication process. In many applications, the loading condition of AM parts is multiaxial. Even under uniaxial loading conditions, due to the geometry complexity or interaction of residual stresses, the stress state may still be multiaxial. Therefore, an understanding of cyclic deformation and fatigue behaviors of AM materials under multiaxial stress states is critical to the expected performance of such parts. These behaviors were investigated in this study using thin-walled tubular specimens of Ti-6Al-4V alloy made of a common PBF process. To compare with the conventional material performance, wrought Ti-6Al-4V alloy was also investigated. The loadings considered included axial, torsion, in-phase axial-torsion, and 90° out-of-phase axial-torsion loads. The surface roughness effect was also studied by considering both the as-built and the machined and polished surface conditions of the AM specimens. The ductility of the AM material was found to be significantly lower than the wrought material due to the martensitic microstructure as well as the presence of defects. AM specimens had significantly shorter lives compared to the wrought specimens under all loading conditions and regardless of the surface finish. However, machining improved the fatigue performance of AM specimens. For all loading conditions brittle fracture of AM specimens was observed with cracking on maximum tensile plane, and ductile fracture of wrought specimens with shear cracking. Consequently, fatigue test results of the wrought material were correlated using a shear-based critical plane model, while the AM specimen test data were correlated based on the maximum principal stress criterion.