Ti-1Al-8V-5Fe (Ti-185) alloy with different iron contents was additively manufactured by electron beam powder bed fusion (EB-PBF), and its microstructure and mechanical properties were investigated. The results show that increasing the Fe powder content from 4.56 wt.% to 5.98 wt.% (within the specification range) converted coarse columnar prior-β grains in as-printed alloy into fine equiaxed ones ((54.2±32.4) μm) by EB-PBF. However, due to subsequent in-situ precipitation, a micron-thick low-solute weak α-phase became prevalent along each equiaxed grain boundary (GB). This drastically decreased the tensile deformation energy of Ti-185 from 6.2′10⁷ J/m³ (columnar grains) to 4.8′10⁷ J/m³ (equiaxed grains), despite a mild increase in strength. Fracture characteristics unveiled that the weak GB α-phase is the main crack initiation site and propagation path.