Evaluation of Fatigue Behaviors of SLMed Ti-6Al-4V in High-Cycle and Very-High-Cycle Regimes Considering Effects of Stress Ratio and Surface Roughness

The high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) behaviors of Ti-6Al-4V manufactured by selective laser melting (SLM) were investigated with the consideration of effects of surface roughness and stress ratio. The fatigue testing was performed by an ultrasonic vibration machine with a frequency of 20 kHz. Fatigue cracks initiate from the surface of the as-built SLMed Ti-6Al-4V, but from the subsurface of the surface-polished SLMed Ti-6Al-4V. The size and depth of the defect inducing fatigue crack initiation of as-built SLMed Ti-6Al-4V is much smaller than that of surface-polished SLMed Ti-6Al-4V. Fatigue cracks are much easier to propagate in as-built than surface-polished SLMed Ti-6Al-4V. For surface-polished SLMed Ti-6Al-4V, fatigue cracks have the same difficulty to propagate at R = −1 and 0.5. Analytical models are developed to well predict the fatigue lives of both as-built and surface polished SLMed Ti-6Al-4V under R = −1 and 0.5.