Variant selection and macrozone in Ti-6Al-4V walls during laser hot wire direct energy deposition

Additively manufactured Ti-6Al-4V (Ti64) wall structures used as various structural aerospace components suffer from the spatial variations in mechanical properties, texture-mediated anisotropy, and macrozones that degrade the mechanical performance, especially fatigue, under certain service conditions. The development of texture and macrozone formation in alpha/beta titanium alloys are closely related to the variant selection during the beta ->alpha phase transformation in the additive manufacturing process. In this study, electron backscatter diffraction (EBSD) was employed to investigate the variant selection mechanism and macrozone formation criteria in single- and multi-wall specimens manufactured with laser hot wire directed energy deposition (LHW-DED). Variant analysis revealed that strong variant selection with a preferred selection of the type 2, <112<overline>0>/60 degrees variant occurs in both single and multiwalls. The primary factor dictating the selection of the alpha/alpha variant boundaries was found to be the cooling rates. The main underlying mechanism governing variant selection to compensate for beta ->alpha transformation strain during solid-state phase transformation was determined to be the triple-alpha variants clustering of Category I. Evidence of thermal stress playing a minor role in variant selection was also observed. Furthermore, location-specific variant analysis revealed that the white bands (microstructural bands with distinct etching contrast) on the walls consist of fewer alpha variants than the bulk. The macrozones formation criteria are proposed and the effect of macrozones on mechanical behavior is discussed in conjunction with the Schmid factor analysis of alpha variants with respect to different loading directions. The density of macrozones was also quantified.