Property Improvement of Additively Manufactured Ti64 by Heat Treatment Characterized by In Situ High Temperature EBSD and Neutron Diffraction

Among various off-equilibrium microstructures of additively manufactured Ti-6Al-4V alloy, electron beam powder bed fusion, in which three dimensional metallic objects are fabricated by melting the ingredient powder materials layer by layer on a pre-heated bed, results in a specimen that is nearly free of the preferred orientation of the alpha-Ti phase as well as a low beta phase fraction of similar to 1 wt%. However, when further heat treatment of up to 1050 degrees C was applied to the material in our previous study, a strong texture aligning the hexagonal basal plane of alpha phase with the build direction and about 6% beta phase appeared at room temperature. In this study, to understand the mechanism of this heat treatment, the grain level microstructure of the additively manufactured Ti-6Al-4V was investigated using in situ high temperature EBSD up to 1000 degrees C, which allows the tracking of individual grains during a heat cycle. As a result, we found a random texture originating from the fine grains in the initial material and observed a significant suppression of alpha phase nucleation in the slow cooling after heating to 950 degrees C within the alpha and beta dual phase regime but close to the the beta-transus temperature at similar to 980 degrees C, which led to a coarse microstructure. Furthermore, the texture resulting from phase transformation of the additively manufactured Ti-6Al-4V assuming nucleation at the grain boundaries was modeled, using the double Burgers orientation relationship for the first time. The model successfully reproduced the measured texture, suggesting that the texture enhancement of the alpha phase by the additional heat treatment derives also from the variant selection during the phase transformation and nucleation on grain boundaries.

Automated summary

This study investigated the microstructure of additively manufactured Ti-6Al-4V alloy using in situ high temperature EBSD. It found a significant suppression of alpha phase nucleation during slow cooling after heating to 950°C. The double Burgers orientation relationship was used to model the texture resulting from phase transformation, successfully reproducing the measured texture.