Effect of Initial Defects on Fatigue Strength of Ti-6Al-4V Fabricated by 3D Additive Manufacturing

Abstract

Rotating bending fatigue tests were conducted using a Ti-6Al-4V alloy which was additively manufactured by EBM (Electron Beam Melting) method and the relationship between the initial defect and the fatigue strength was investigated. Defects were observed in the microstructure of the EBM Ti-6Al-4V. The size of the defects was up to nearly 100 μm. The fatigue strength of the EBM Ti-6Al-4V was lower than that of the conventionally melted Ti-6Al-4V. The fatigue limit of the EBM Ti-6Al-4V was 300 MPa, while that of the conventionally melted Ti-6Al-4V was 625 MPa. Fatigue fracture surface observation revealed that the fatigue crack initiated from the defect in the EBM Ti-6Al-4V, which results in the lower fatigue strength than the conventionally melted Ti-6Al-4V. The fatigue limit was predicted based on Murakami's method using the estimated maximum defect size in the specimen by extreme value statistics and the hardness. The predicted fatigue limit value was slightly higher than the experimentally obtained fatigue limit.