Insights into the gradient microstructure and mechanical properties of Ti-6.5Al–2Zr–Mo–V alloy manufactured by electron beam freeform fabrication

Herein, the Ti-6.5Al–2Zr–Mo–V (TA15) alloy with gradient characteristics is fabricated via electron beam freeform fabrication (EBF³) technology. The gradient microstructure and mechanical properties are investigated in detail. Specifically, the microstructural evolution containing the morphology and size along the building direction and its impact on microhardness and tensile properties are systematically illustrated. Besides, the numerical simulation is developed for the establishment of the EBF³ temperature field thermal cycling profile to further elaborate the effect of temperature and thermal history on the microstructural evolution. The results reveal that the columnar to equiaxed transition and coarsening of the prior β grains, as well as successive increase and decrease of thickness of α lamellae along the building direction. The formation of fine lamellar α colony within the layer band is associated with the survived α phase when the peak temperature is just below T_β. The gradient microstructure endows the TA15 deposit with different mechanical properties. Specifically, the microstructure with fine α lamellae manifests marginally higher microhardness in the layer-band-free region. The elemental dilution, loss and partitioning result in relatively low microhardness in the region with fairly fine α lamellae immediately adjacent to the substrate. The top region of the TA15 deposit, characterized by fine basket-weave microstructure, is imparted with better comprehensive properties relative to the middle and bottom regions, i.e., 1007.4 ± 30.8 MPa, 915.31 ± 28.7 MPa, and 12.08 ± 1.96% for tensile strength, yield strength and elongation, respectively. The integrated effect of coarsened prior β grains and refined α lamellae allow the middle and bottom regions to exhibit similar tensile properties.