Work

Alpha-case promotes fatigue cracks initiation from the surface in heat treated Ti-6Al-4V fabricated by Laser Powder Bed Fusion

Year: 2024

Type: article

Abstract: This research investigates the effect of forming an oxygen-stabilised titanium alpha-case layer on the fatigue properties of Ti-6Al-4V (Ti64) alloy components produced by Laser Powder Bed Fusion (L-PBF). Various post-processing heat treatments with different controlled atmospheres were conducted on samples with as-built surfaces to evaluate how differences in alpha-case layer thickness and hardness influence the material’s susceptibility to surface embrittlement and its overall fatigue performances. The examination involves bulk and sub-surface microstructure analysis, surface characterization by X-Ray computed tomography (XCT), and fatigue testing. Key findings reveal that alpha-case layers can lower the fatigue resistance of L-PBF fabricated Ti64. The presence of a 70 ± 3 μm thick alpha-case layer was found to promote crack initiation. This is emphasized by a higher density of initiated cracks, thus leading to a decrease in fatigue life. Conversely, thinner alpha-case layers were found to have a reduced impact on the fatigue performances, underscoring the critical role of postprocessing heat treatments in modulating the material’s resistance to fatigue. The use of XCT to characterize sample’s surfaces in 3D confirms that fatigue cracks primarily initiate at surface notches, highlighting the predominance of as-built surfaces over microstructure in determining the fatigue resistance of L-PBF Ti64 components.

Source: International Journal of Fatigue

DOI: 10.1016/j.ijfatigue.2024.108621