Anisotropic high cycle fatigue property estimation for laser additive manufactured Ti6Al4V alloy dependence on tomographic imaging of defect population

Abstract

Improvement of fatigue properties of additive manufactured metals is subject to the regulation of process-induced microstructure and defect population. Here, the high cycle fatigue properties of the annealed Ti6Al4V alloy manufactured by two types of laser powder bed fusion (L-PBF) processes, in orthogonal sample orientations, were studied. The fatigue performance of this alloy was also estimated dependence on the investigation of microstructure and defects using optical microscopy, X-ray diffraction, scanning electron microscopy and micro computed tomography. The results demonstrated that the fatigue properties were degraded by the negative effect of prior-β columnar grain boundaries and defects, wherein the surface keyhole governed the pronounced fatigue scatter and anisotropy due to its geometrical parameters and anisotropy severity along the building direction. The projection size √area of keyholes was well applied in the fatigue property estimation. The theorical fatigue limits were estimated individually due to the keyhole anisotropy, and provided conservatively in the improved Kitagawa-Takahashi diagram based on the modified El-Haddad model and the extreme size of keyholes. A relation derived from Paris law was proposed for fatigue life prediction, and to well establish a linear fitting correlating fatigue life with the projection size of surface keyholes and stress amplitude range with the achievement of the alleviation of fatigue anisotropy and scatter. The research provides a reference for the non-destructive estimation of fatigue properties in terms of tomographic imaging of defect population.