Multiaxial fatigue behavior and modelling of additive manufactured Ti-6Al-4V parts: The effects of layer orientation and surface texture

Abstract

Additive manufacturing (AM) technologies have enabled industries to create and manufacture highly intricate parts with designs that have previously been deemed impossible via subtractive means. Analogous to conventionally fabricated parts, additively manufactured (AM) parts are often subjected to cyclic loadings throughout their service life. Consequently, fatigue performance becomes an important consideration in design and qualification. The geometric complexity of AM parts is accompanied by varying thermal histories which affect microstructure, porosity, surface texture, and residual stresses. The aforementioned factors influence fatigue performance and possibly introduce multiaxial stress states even when the loading is uniaxial. Therefore, understanding the fatigue behavior under multiaxial loadings is necessary to assess reliable in-service part performance. In this study, the effects of build layer orientation and surface texture on the multiaxial fatigue behavior of unmachined Ti-6Al-4V parts fabricated via a laser beam powder bed fusion process are investigated. Fatigue tests include axial, torsion, in-phase, and 90° out-of-phase axial/torsion loadings. Results indicate that the fatigue cracks in unmachined Ti-6Al-4V start from surface defects and are oriented along the maximum tensile stress plane. Accordingly, fatigue results are correlated using the tensile-based Maximum Principal Stress (MPS) approach. Finally, the influence of surface defects on the fatigue performance is incorporated by evaluating the surface micro-notches along the MPS using Murakami’s area parameter to obtain an initial crack size, which is then used with the NASGRO crack propagation model to predict fatigue lives.