Microstructural Development of Ti-6Al-4V Alloy via Powder Metallurgy and Laser Powder Bed Fusion

A detailed study was carried out to gain a better understanding of the microstructural differences between Ti-6Al-4V parts fabricated via the conventional powder metallurgy (PM) and the laser powder bed fusion (L-PBF) 3D printing routes. The parts were compared in terms of the constituent phases in the microstructure and their effects on the micro- and nano-hardness. In L-PBF parts, the microstructure has a single phase of martensitic alpha ' with hcp crystal structure and acicular laths morphology, transformed from prior parent phase beta formed upon solidification of the melt pool. However, for the sintered parts via powder metallurgy, two phases of alpha and beta are noticeable and the microstructure is composed of alpha grains and alpha + beta Lamellae. The microhardness of L-PBF processed Ti-6Al-4V samples is remarkably higher than that of the PM samples but, surprisingly, the nano-hardness of the bulk martensitic phase alpha ' (6.3 GPa) is almost the same as alpha (i.e., 6.2 GPa) in PM samples. This confirms the rapid cooling of the beta phase does not have any effect on the hardening of the bulk martensitic hcp alpha '. The high microhardness of L-PBF parts is due to the fine lath morphology of alpha ', with a large concentration of low angle boundaries of alpha '. Furthermore, it is revealed that for the alpha phase in PM samples, a higher level of vanadium concentration lowers the nano-hardness of the alpha phase. In addition, as expected, the compacting pressure and sintering temperature during the PM process led to variations in the porosity level as well as the microstructural morphology of the fabricated specimens, which will in turn have a significant effect on the mechanical properties.

Automated summary

A detailed study comparing the microstructural differences between Ti-6Al-4V parts fabricated via conventional powder metallurgy and laser powder bed fusion routes revealed that L-PBF parts have a higher microhardness with a single phase of martensitic alpha '. On the other hand, parts sintered via powder metallurgy showed two phases with lower microhardness. The rapid cooling of the beta phase had no effect on the hardening of the bulk martensitic hcp alpha ' in L-PBF parts.