Fatigue of laser powder-bed fusion additive manufactured Ti-6Al-4V in presence of process-induced porosity defects. (January 2022)

• Record Type: Journal Article
• Authors: Akgun, Emre; Zhang, Xiang; Lowe, Tristan; Zhang, Yanhui; Doré, Matthew
• Is Part Of: Engineering fracture mechanics. Volume 259(2022)
• Publication Date: 2022-01
• Subjects: Additive manufacturing -- Ti-6Al-4V -- Fatigue -- Defects -- Porosity; Fracture mechanics -- Periodicals; Rupture, Mécanique de la -- Périodiques; Fracture mechanics; Periodicals; 620.112605
• DOI: 10.1016/j.engfracmech.2021.108140
• Languages: English
• ISSNs: 0013-7944

Abstract

Highlights: Fatigue of laser powder-bed fusion Ti-6Al-4V was studied under CA axial loading. Process-induced porosity was characterised using X-ray Computed Tomography. Fatigue crack growth rates initiated from process-induced porosity were measured. Similitude to the long crack growth rate was shown for pore plus the crack length. Faster small crack growth was minor compared to wrought or cast Ti-6Al-4V. 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.