Evolution of Texture from a Single Crystal Ti-6Al-4V Substrate During Electron Beam Directed Energy Deposition
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nderstanding of crystallographic texture and its evolution during processing is vital because it often results in anisotropy of mechanical and sometimes physical properties.[1] This is true for the two-phase titanium alloy Ti-6Al-4V, which exhibits a high-symmetry, body-centered cubic (BCC) b-phase at high temperatures and transforms partially to a low-symmetry hexagonal close-packed (HCP) a-phase at lower temperatures.[2] It is well established that the orientation of the c-axis in the HCP phase relative to loading direction plays a significant role in determining mechanical properties in both the elastic and plastic regimes.[2–5] Titanium alloys develop textures that stem from their processing history, including the melting/solidification
TODD M. BUTLER, S. LEE SEMIATIN, and ADAM L. PILCHAK are with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB, WPAFB, OH, 45433. Contact e-mail: [email protected] CRAIG A. BRICE is with Lockheed Martin Space Systems Company, Littleton, CO, 80125. WESLEY A. TAYON is with the National Aeronautics and Space Administration Langley Research Center, Hampton, VA. Manuscript submitted June 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
method and subsequent thermomechanical treatments.[2,4,5] With regard to casting Ti-6Al-4V, the b-phase forms initially during solidification and generally exhibits preferred h001i columnar growth in the direction of the strongest thermal gradient, i.e., typically perpendicular to the mold wall for an ingot after some degree of equiaxed grain formation.[6–8] The h001i direction represents the family of preferred growth directions in BCC and FCC crystal systems.[9,10] This preferred b-phase growth texture then impacts the resulting a-phase transformation texture at lower temperatures through the well-established Burgers orientation relationship (BOR): ð0001Þa jj f110gb and h1120ia jj h111ib .[2,6,7,11–13] Thus, modification of the high-temperature b-phase texture can play a significant role on the room-temperature strength of Ti-6Al-4V. The formation of strong h001i fiber textures has been widely reported in the additive manufacturing (AM) literature.[14–17] These reports include observations for both electron beam-based[18–20] and laser-based[6,21,22] approaches, in which the b-phase grows with the h001ib parallel to the strongest thermal gradient along the build direction. Researchers at NASA are also developing a novel electron beam DED AM process, referred to as electron beam freeform fabrication (EBF3), to manufacture metal parts using wire feedstock.[23–25] Compared to other AM methods, the EBF3 approach utilizes nearly 100 pct of the feedstock, and power efficiencies of ~95 pct are achieved.[23–27] Similar to other additive techniques, EBF3 also produces builds with h001ib jj build direction.[28] Despite broad understanding of texture formation during conventional AM processes using polycrystalline substrates, little is known regarding the specific influence of the local crystallographic orientati
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