Microstructural and texture evolution during thermo-hydrogen processing of Ti6Al4V alloys produced by electron beam melting

Abstract

The present study was conducted to reveal the effects of building angles and post heat-treatments (2-step Thermo-Hydrogen Processing (THP) and conventional annealing treatment) on the density, microstructure and texture of Ti6Al4V alloy parts produced by Electron Beam Melting (EBM). The results showed that regardless of the building angle; the density, microstructure and crystallographic texture (defined with respect to building angle) of the as-produced samples were identical; having Widmanstätten α structure and columnar β-grains which are parallel to building direction. The main texture component for the α phase was (101¯0)//building direction, and for β phase (001)//building or heat flow direction. The first step of THP, namely, the hydrogenation step, produced a needle-like microstructure and increased the local misorientations due to lattice distortion. On the other hand, after application of the second step of THP, dehydrogenation step, microstructure was refined, particularly α-grains that were larger than 10 μm and located at grain boundaries. Moreover, THP randomized the crystallographic texture since it involves β to α phase transformation, at which one β-grain can produce 12 distinct α-variants. The grain boundary misorientation distributions also changed in accordance with the microstructural changes during the 2-step THP. On the other hand, annealing coarsened the grain boundary and Widmanstätten α phases; moreover, it changed the texture so that the basal planes (0001) rotated 30° around the building direction. © 2020 Elsevier Inc.