Microstructure anisotropy and its implication in mechanical properties of biomedical titanium alloy processed by electron beam melting

Titanium alloy Ti-6Al-4V processed by electron beam melting (EBM) has a great potential for orthopedic and aerospace applications. However, the process induced porosity and microstructure anisotropy will have a significant impact on the material properties. This work has found that spherical and elongated pores with strong size effect are common characteristics for the as-EBM samples made with horizontal, diagonal, and vertical orientations w.r.t. the substrate. Furthermore, the major axis of the elongated pores is perpendicular to build direction for samples with different build orientations. The microstructure consists of columnar prior β grains delineated by grain boundary α and transformed α/β structures with α’ marteniste and basket weave morphology. Of note is that a high fraction of twin boundaries are prevalent in α (α’) phase. The configuration of the applied load w.r.t. the major axis of the elongated pores is the most significant influencing factor to mechanical properties, while the columnar prior β grain structure is secondary. Fractography reveals that microcracks tend to originate from elongated pores for cleavage fracture. In addition, the co-existing local terrace-like and shallow dimples are attributed to the intergranular crack propagation from the lamella α grain boundaries. Thus, the anisotropy of porosity and microstructure is of significance to enhance mechanical properties in process development.