When Ti-6Al-4V is processed by laser powder bed fusion (L-PBF), acicular martensitic α’-Ti grains are formed within the columnar prior β-Ti grains, resulting in inferior mechanical properties. The application of blended powders in L-PBF enables to tailor the microstructures and obtain a mixture of α’ + β phases. In this work, we demonstrate an effective method to engineer the phase fraction of an L-PBF manufactured Ti alloy using blended powders consisting of Ti-6Al-4V and 3 wt% Fe particles. By varying laser parameters, the as-built microstructures transit from α’ dominated microstructure to a nearly complete β-dominant microstructure. High-speed operando X-ray diffraction during L-PBF processing combined with X-ray fluorescence and EBSD characterization allows for relating microstructure to the spatial distribution of the β-stabilizer Fe under the high cooling rates typical for L-PBF. The as-built microstructure containing large amounts of β phase achieves high strength and enhanced ductility without post-processing heat treatments.