Defect Induced Fatigue Behaviors of Selective Laser Melted Ti-6Al-4V via Synchrotron Radiation X-Ray Tomography

As a very promising additive manufacturing (AM) technique, selective laser melting (SLM) has gained considerable attentions due to the feasibility of producing light-weight metallic components directly from virtual design data. On the other hand, high strength, low density and high corrosion resistance Ti-6Al-4V alloy has been a preferred AM used material for the aviation and military industries. However, the fatigue damage behaviors of SLMed or AMed components usually suffer from interior defects such as incomplete fusion and gas pores due to unstable process or unsuitable processing parameters. Therefore, thorough investigations on process-induced and metallurgical defects and its influence on the fatigue behavior is required for robust designs and engineering applications of high performance SLM components. As an advanced characterization approach, synchrotron radiation micro computed X-ray tomography (SR -mu CT) has been recently to investigate the fatigue damage behaviors of critical components with defects. Based on self-developed in situ fatigue testing rig fully compatible with the BL13W1 at Shanghai Synchrotron Radiation Facility (SSRF), several AMed specimens were prepared for in situ fatigue SR - mu CT. The Feret diameter and extreme values statistics were then adopted to characterize the defect size, morphology, population, location and the influence on fatigue life. Fatigue fractography was also examined to further identify the defect to really initiate a fatigue crack. Results show that two types of defects including gas pores and the lack of fusion can be clearly distinguished inside SLM Ti-6Al-4V alloys. Fatigue crack with a typical semi-ellipse usually initiates from the defects at the surface and near the surface. Besides, the defects less than 50 mu m and sphericity of 0.4 similar to 0.65 dominate for the SLM Ti-6Al-4V alloys. It is also found that the larger the characteristic size of the defect, the lower the fatigue life. Current results can provide a theoretical basis and support to predict the fatigue performance of SLM Ti-6Al-4V alloys. Further investigations should be performed on the relationship between the critical defect and fatigue strength by introducing the Kitagawa-Takahashi diagram.