Tailoring bimodal structure for high strength and ductility in pure titanium manufactured via laser powder bed fusion

Laser-powder bed fusion (L-PBF) is being increasingly employed in the fabrication of commercially pure titanium (CP-Ti) components for biomedical applications. However, L-PBF-manufactured CP-Ti parts typically exhibit high strength and reduced ductility owing to the formation of acicular α′ martensite. It is essential to decompose the acicular α′ martensite into the equilibrium α phase through post-heat treatments to achieve superior mechanical properties. In this study, post-heat treatments were applied to L-PBF-fabricated CP-Ti Gr. 1−Gr. 4 samples. The microstructures of the as-fabricated CP-Ti samples were dominated by acicular α′ martensite, which exhibited high strength (>800 MPa) but low ductility (<20%). Full annealing resulted in the formation of equiaxed grains and the disappearance of the columnar structures in the CP-Ti samples. The equiaxed grains contributed to high ductility (>30%) in the Gr. 1 and Gr. 2 samples, but low ductility (<20%) in the Gr. 3 and Gr. 4 samples. This low ductility is associated with the formation of grain-boundary β layers. Well-designed partial annealing led to the formation of bimodal structures consisting of both equiaxed and fine lamellar grains. This specific structure provides both a high strength (>700 MPa for Gr. 2 and,>850 MPa for Gr. 4) and high ductility (>35% for Gr. 2 and Gr. 4), which are superior to those of the samples processed under full annealing conditions. Therefore, tailoring the bimodal structure in the L-PBF-manufactured CP-Ti for both high strength and ductility is promising for biomedical applications.