Grain Boundary α-Phase Precipitation and Coarsening: Comparing Selective Laser Melted and Conventional Manufactured Ti6Al4V

Investigation into the precipitation and coarsening behavior of grain boundary α-phase (GB-α) in Ti6Al4V (Ti-64) manufactured by selective laser melting (SLM) were carried out and compared with that in as-cast Ti64 in this study. It was found that in SLM Ti64, the GB-α tends to precipitate first at the triple junctions (TJs) of the β-phase grain boundaries during the further annealing heat treatments in the α+β phase regime. With 750 ℃ annealings, the coarsening process of GB-α was controlled by the bulk diffusion mechanism with a small coarsening coefficient, different from as-cast Ti64. The interface reaction which has a large coarsening coefficient was dominant for the GB-α coarsening at 850℃ and 950 ℃ annealings. Such a difference in coarsening behavior at different annealing temperatures in SLM Ti64 was found to be related to the different curvatures of GB-α caused by the different nucleation mechanisms of α lamellae.