The effect of porosity defects on the mid-cycle fatigue behavior of directed energy deposited Ti-6Al-4V

•Porosity-induced mid-cycle fatigue (MCF) behavior of directed energy deposited (DED) Ti-6Al-4V are investigated.•Pore size classification method is quantitatively developed by combining microstructure and da/dN-ΔK curve in high vacuum.•A quantitative study of the fine granular area (FGA) shows the ΔKFGA in the MCF regime is not a material constant.•The stress level, pore size and pore-to-surface distance are important factors affecting fatigue behavior and performance.