Experimental study on mechanical properties of laser powder bed fused Ti-6Al-4V alloy under post-heat treatment

Abstract

The mechanical and fatigue resistance of additively manufactured (AM) metals is based on its microstructural features and defects, that can be mitigated by post-processing. Currently, understanding the effect of metallurgical defects on fatigue life is a significant step towards the wider application of AM alloys. Here, the tensile properties of laser powder bed fused (L-PBF) Ti-6Al-4V alloys, with varied annealing temperatures (650–950 ℃) were studied, and fatigue resistance under the optimized annealing temperature was investigated. The results show that after the optimized heat treatment, the high cycle fatigue (HCF) data of this alloy still has a large variation. Surface defects typically act as crack origin sites. The largest defect size of HCF samples was predicted by extreme value statistics, based on observation at the fatigue fracture origin. This was used to establish the relationship between defect size and fatigue limits. The El-Haddad model is proven to better fit the HCF data than the Classical linear elastic fracture mechanics and standard Murakami approach. Finally, a fatigue life prediction model based on a normalized fatigue S-N curve is proposed, to well describe the relationship between critical defect size and loading stress.