An experimental investigation of fatigue performance and crack initiation characteristics for an SLMed Ti-6Al-4V under different stress ratios up to very-high-cycle regime

The fatigue behavior of SLMed Ti-6Al-4V has been intensively investigated in recent years. However, the effect of stress ratio has not been comprehensively studied, especially in VHCF regime. In this paper, the fatigue performance and crack initiation characteristics up to VHCF regime for an SLMed Ti-6Al-4V at various stress ratios (R = −1, −0.5, 0.1 and 0.5) were investigated and the effects of stress ratio were substantially discussed. The stress amplitude decreases with the increase of stress ratio with the order of: σ_a (R = −1) > σ_a (R = −0.5) > σ_a (R = 0.1) > σ_a (R = 0.5). Internal crack initiation is dominant in HCF and VHCF regimes under the four stress ratios, although surface crack initiation rarely occurs in HCF regime at R = −1 and 0.5. Whether the crack initiates from subsurface or from interior is affected by the applied maximum stress, and the crack initiation is in relation not only to defect size but also to defect location. The nanograins in RA are evidently formed in VHCF regime at R = −1, −0.5 and 0.1, but not at R = 0.5, which is well explained by the NCP model.