Fatigue limit of Ti6Al4V alloy produced by Selective Laser Sintering

Abstract3D printing is an advanced manufacturing technology for producing metal components, and titanium is a typical alloy that is used in this technique. Some limitations and peculiarity should be considered during the design of components by additive manufacturing. We adopted the most common technique to produce the samples, the selective laser sintering (SLS). In this case the remaining porosity and the surface roughness are affecting negatively the fatigue life. In this study the effects of porosity and surface roughness were studied by performing push-pull tests (R=-1) in a Rumul resonant machine to evaluate the fatigue limit in different conditions. Samples were built by SLS from Ti64 ELI biomedical grade powder. After building, all samples were thermal treated at 670°C to relax residual stresses due to the building process. At this step the microstructure was characterized, it was found to be martensitic (α’). A first lot of samples, as benchmark, was tested in this condition and in the present work are simply called “as built”. Part of the samples were treated by hot isostatic pressing (HIP), by performing this process we obtained the full density, removing the pores still present in the microstructure. The HIP was performed at 920°C, so not only the density was modified by this process, but also the microstructure. The HIP worked as a thermal treatment in the α+β field and the result is that the microstructure is extremely different from the previous condition. It is a lamellar α+β microstructure. To have a significant comparison between the results part of the remaining samples was thermal treated at the same temperature and for the same holding time as for the hipped samples to obtain the same microstructure, maintaining the residual porosity typical of the SLM process. Wohler curves were determined from push-pull test to have a direct comparison of the fatigue performance between the different conditions