Texture evolution as a function of scan strategy and build height in electron beam melted Ti-6Al-4V

Alec I Saville; Sven C Vogel; Adam Creuziger; Jake T Benzing; Adam L Pilchak; Peeyush Nandwana; Jonah Klemm-Toole; Kester D Clarke; S Lee Semiatin; Amy J Clarke

doi:10.1016/j.addma.2021.102118

Texture evolution as a function of scan strategy and build height in electron beam melted Ti-6Al-4V

Addit Manuf. 2021 Oct:46:10.1016/j.addma.2021.102118. doi: 10.1016/j.addma.2021.102118.

Authors

Affiliations

¹ Colorado School of Mines, Golden, CO, USA.
² Los Alamos National Laboratory, Los Alamos, NM, USA.
³ National Institute of Standards and Technology, Gaithersburg, MD, USA.
⁴ National Institute of Standards and Technology, Boulder, CO, USA.
⁵ Air Force Research Laboratory, Dayton, OH, USA.
⁶ University of Tennessee Knoxville, Knoxville, TN, USA.

Abstract

Metal additive manufacturing (AM) enables customizable, on-demand parts, allowing for new designs and improved engineering performance. Yet, the ability to control AM metal alloy microstructures (i.e., grain morphology, crystallographic texture, and phase content) is lacking. This work performs corroborative neutron diffraction and large-scale electron backscatter diffraction (EBSD) measurements to assess crystallographic texture in electron beam melted (EBM) Ti-6Al-4V as a function of scan strategy and build height. Texture components for one raster and two spot melt scan strategies were evaluated using a triclinic specimen symmetry to capture all possible texture components, which were found to be considerably different than previously reported values from studies employing orthotropic specimen symmetry. This finding highlights the importance of a standard method and best practice for assessing textures produced by AM. Texture was found to vary between scan strategies, but changed minimally as a function of build height. Parent phase β-Ti reconstructions obtained from as-built crystallographic orientations revealed spot melt scan strategies produced finer equiaxed/columnar grains with clear 001 _β build direction fiber textures, whereas the raster scan strategy produced large columnar grains and a weaker 001 _β build direction fiber texture. The observed grain morphologies agree with those predicted by solidification theory for the thermal gradients and solidification velocities experienced during the build process. The presence of a strong 001 _β fiber orientation (typical of cubic solidification) produced by spot melting was found to correlate with a previously unreported $01 \bar{1} 2_{α}$ fiber texture in the as-built condition and colony microstructures. The $01 \bar{1} 2_{α}$ fiber texture was weakly observed for the raster scan strategy, and 001 _β oriented grains preferentially transformed into α' martensite with orientations between $1 \bar{1} 00_{α}$ and $11 \bar{2} 0_{α}$ . This shift in product α-Ti orientations has not yet been reported, and further work is recommended to understand these crystallographic signatures in the context of solid-state phase transformations. The presence of the $01 \bar{1} 2_{α}$ fiber texture is proposed as a useful diagnostic for evaluating the solidification or transformed microstructure condition (e.g., grain morphology and texture) of Ti-6Al-4V AM builds via accessible techniques like laboratory X-ray diffraction.

Keywords: Beta-Ti; Crystallographic texture; Reconstructions; Scan strategy; Solidification and solidification modelling; Ti-6Al-4V.

Grants and funding

9999-NIST/ImNIST/Intramural NIST DOC/United States