Fatigue life assessment of WAAM-processed Ti-6Al-4V

The integrity of a load-bearing structure may degrade during service life due to undetected material defects or accidental damage. Additively manufactured structures are particularly susceptible to fatigue cracking due to stress concentration at possible surface notches or internal material defects. Despite the advantages of arc-based manufacturing techniques, it is still not comprehensively investigated how the manufactured parts fail under cyclic loading. This study aims to investigate the effect of material defects on the high-cycle fatigue behavior of WAAM-fabricated Ti-6Al-4V with dedicated thermal treatment. Furthermore, an emphasis was paid to investigating to which extent the fatigue life prediction model developed and validated for laser beam-welded Ti-6Al-4V joints can be transferred to WAAM-fabricated structures made from this alloy. The model is based on the NASGRO equation in which the short crack growth from the internal defect, such as lack of fusion defect, is taken into account. The results of the study show that by the use of the model, the minimum fatigue life of the fatigue specimens extracted from the WAAM-fabricated structure can be appropriately predicted. Some specimens showed lower predicted fatigue lives in comparison to the experimentally obtained ones. The reason could be due to the effective defect size, which was overestimated for irregular shape defects by analyzing the fracture surfaces of tested specimens. Thus, a conservative prediction of the fatigue life is achieved.