Fatigue of additive manufactured Ti-6Al-4V, Part II: The relationship between microstructure, material cyclic properties, and component performance

Part I of these two-part paper series focused on the process and structure relationships, effect of powder feed-stock, fabrication parameters, and post fabrication treatments on the resulting microstructure, defect characteristics, and surface quality of the fabricated Ti-6Al-4V parts. This second part extends the study by evaluating the effect of the aforementioned factors on axial, torsion, and multiaxial fatigue behavior of the additively manufactured (AM) Ti-6Al-4V specimens. Despite the advantages of additive manufacturing techniques discussed in Part I, they are still rarely used in fatigue critical load carrying applications, partly due to insufficient understanding of fatigue behavior and its dependence on variations in material microstructure and defects. This becomes even more challenging when other process characteristics of AM including build orientation, residual stresses, and surface roughness are considered. This paper discusses these effects, as well as machine-to-machine variability and the effects of specimen geometry and size, post heat treatment, and multiaxial stress state. Experimental uniaxial, torsion, and multiaxial fatigue test results recently generated by the authors for laser beam powder bed fusion- produced Ti-6Al-4V alloy are reviewed. The observed behaviors and the influence of the aforementioned effects are then related to the resulting microstructure and defect characteristics discussed in Part I. Fatigue life prediction results for specimens based on the effective defect size calculated by extreme value statistics (EVS) of the internal defects and surface roughness are also presented and compared with experimental data. The observed behaviors and specimen test results are then used for fatigue life analysis and predictions of a link component as an illustrative application example.