Surface roughness effect of SLM and EBM Ti-6Al-4V on multiaxial high cycle fatigue

As built Ti-6Al-4V parts obtained by SLM and EBM, often stress-released and HIP-treated, show a very rough surface which affects the fatigue properties. In many cases of complex geometries, surface treatments can not be achieved so that the use of such parts for structural applications requires to understand and to quantify the effect of roughness on fatigue strength. This study presents an experimental and numerical approach to take into account the role of roughness on the high cycle fatigue strength of additively manufactured parts. The specific roughness of "as-built" surfaces was evaluated by several acquisition methods(2D and 3D profilometry, tomography) on "as-built" surfaces and different surface finish (machining and chemical polishing). Fatigue strength of SLM and EBM specimens with an "as built", machined or chemically polished surface were evaluated under fully reverse tension (R-sigma = 1) and torsion (R-tau = 1). Fractography analyses were performed in order to identify defects at the origin of fatigue crack initiation. Finally, a numerical approach based on finite element (FE) computation of real surface topologies was proposed to quantify the effect of the surface roughness on fatigue strength. The surface roughness was modelled from 2D and 3D profilometry and the FE computations were conducted in elasticity and elastoplasticity. The proposed numerical approach combines a non local fatigue indicator parameter (FIP) integrating the high stress gradients at notch root and a statistical description of potential initiation sites by extreme value statistics.