High Cycle Fatigue Behavior of Recycled Additive Manufactured Electron Beam Melted Titanium Ti6Al4V

Additive Manufacturing (AM) has become a popular method for producing complex and unique geometries. Many have also been utilizing AM for its short turnaround time and the diverse set of materials available for each specific need. Metal AM has especially gained traction in the aerospace and medical industries for its novel benefits and many studies have been performed to better understand the benefits and limitations of the technology. As additive manufacturing becomes more popular for these application, it is critical we move towards certification of AM produced parts. However, current as-printed components are susceptible to failure at limits far below wrought titanium Ti-6Al-4V and further understanding of the material properties and fatigue life need to be gained. In this study, a high strength Titanium alloy, Ti-6Al-4V is used to print cylindrical fatigue specimens using the Electron Beam Melting (EBM) technology, which is unique for its vacuum build environment and high temperature build chamber. Uniaxial High Cycle Fatigue (HCF) tests have been performed on as-printed, hot-isostatic pressed (HIP) and surface treated cylindrical specimens and the location of crack initiation and propagation has been determined through the use of optical microscopes and scanning electron microscope (SEM). Various effects such as the rough surface exterior, internal and external defects, and build orientation can be linked to fatigue failure, and the performance of post treatments such as HIP, and surface treatments is further explored in this study. Much of the analysis has shown that the rough surface exterior is much more detrimental to the component compared to internal defects produced due to insufficient process parameters. Due to the inaccessibility of features and the related costs associated with machining additively produced components, tool-less surface finishing processes need to be considered to improve overall fatigue life. Surface treatments such as shot peening are performed in this study to improve the surface roughness of as-printed specimens and have been correlated to an improvement in fatigue life. In addition, further comparing the effects of performing HIP versus improving the surface roughness, the improved surface roughness has shown to have a greater effect on improving fatigue life of EBM produced titanium specimens. In addition, the effects of the surface roughness as a stress concentration is confirmed using the Arola-Ramulu Model to estimate the fatigue strength from the specimen surface roughness. The fatigue performance of the EBM specimens is also compared to traditionally manufactured Ti-6Al-4V specimens to further understand the challenges for qualification and provide possible processes to increase fatigue life of additive parts.