Fatigue limit prediction model and fatigue crack growth mechanism for selective laser melting Ti6Al4V samples with inherent defects

Abstract Additive manufacturing-selective laser melting (AM-SLM) technology for Ti6Al4V alloys has been industrialized and widely used for complex components in recent decades. However, with the effect of AM defects (especially the AM internal defects), the quantitative evaluation of the SLM specimen fatigue limits is difficult but necessary. In the present work, lots of high cycle fatigue data of the SLM Ti6Al4V alloy were provided. To estimate the fatigue limits of the specimens whose fatigue cracks initiate from the SLM internal defects under vacuum-like condition, the linear relation between the Ti6Al4V alloy threshold stress intensity factor range and the logarithmic of vacuum pressures was first identified. Based on the physical mechanisms, a new fatigue limit prediction model was presented. Moreover, the fatigue crack propagation behaviour and deformation mechanisms were discussed by using in-situ scanning electronic microscope and transmission electron microscope observation technologies. The results indicate that, for SLM Ti6Al4V alloy fatigue short cracks, inter-granular fracture is more common. The intersecting slip crack propagation behaviour of crack tips is identified. A zigzag crack path can be observed. Finally, with the increase of crack lengths, intra-granular crack propagation is predominant.