Role of martensite decomposition for achieving bi-lamellar microstructure in 3D printed Ti-6Al-4V

To obtain bi-lamellar microstructure, the Ti-6Al-4V alloy samples were produced using laser powder bed fusion and subsequently subjected to annealing heat treatment within the α + β region. The influence of annealing temperature (910°C, 960°C, 990°C), annealing time (0.5h  and 4 h), and cooling rate (air, oil, and water quench) on the microstructure evolution, yield strength, ultimate tensile strength, and ductility at room temperature were examined. It was found that annealing at 910°C for 0.5 h with air cooling initiated the decomposition of initial martensite and the formation of β-transformed regions and secondary-α lamellas, while annealing at 990°C resulted in the creation of equiaxed grains with a lamellar microstructure. FESEM analysis revealed the presence of β-nano-precipitates in the meta-stable primary α phase in all annealed samples. Fast cooling in oil and water promoted the retention of a higher fraction of β nano-precipitates and suppressed the formation of secondary-α lamellas. The results showed that extending the annealing time to 4 h at 910°C was not sufficient to achieve achieving complete martensite decomposition. Notably, the decomposed martensite microstructure exhibited unfavorable characteristics in contrast to that of its as-built state.