Fatigue of laser powder-bed fusion additive manufactured Ti-6Al-4V in presence of process-induced porosity defects

Abstract

In metal additive manufacturing, process-induced defects are a prime source of failure under cyclic loading at the present time. In this work, X-ray Computed Tomography was used to characterise the defect population inside the gauge volume of near-net shape manufactured dog-bone samples. The result further corroborates that crack initiating defects are at the lower tail of a positive-skewed size distribution, but not necessarily correspond to the largest defect. Contrary to common assumption, defects were not randomly located, but a clear tendency towards the free surface was observed. Then, from these surface defects, fatigue crack growth rates were measured using the replica technique. Although measured cracks were relatively small, e.g. below 1 mm, a similitude to long crack growth rates was demonstrated, when pore size was added to measured crack lengths. Finally, the crack initiation life from porosity was observed to be at least 50% of the total fatigue life based on the first detectable surface crack on the replica. This should be taken into consideration when using fracture mechanics for fatigue life prediction in presence of defects, where it is common to assume crack initiation from the very first cycle.