Microstructure and Mechanical Properties of Ta15/Ti60 Graded Structured Materials Fabricated by Directed Energy Deposition

Abstract

A defect-free graded structured material (GSM) was fabricated from TA15/Ti60 dual alloy using the direct energy deposition (DED) technique.  The study focused on analyzing the grain morphology, gradients in chemical composition at the interfaces, and the general microstructure of the deposited GSM specimens.Rigorous characterization of the graded variations along the deposition direction was conducted. Results reveal that columnar grains dominate both TA15 and Ti60 zones in the as-deposited specimens. However, the Ti60 zone features a smaller average grain size, with equiaxed grains nucleating atop columnar grains—a transition driven by the columnar-to-equiaxed transformation (CET) process. The elemental composition of the transition area shows a step-like variation. As the heat treatment temperature rose, the α phase size gradually increased, the aspect ratio decreased and the morphology changed from finer needle shaped to short wider lath shaped and the content of β phase was increased gradually. At the highest temperature (HT4), the acicular secondary α phase was found in β phase of TA15. The changes of structures determine its performance changes: with the rise in temperature, the strength decreased and the elongation rate increased gradually. The specimen exhibited optimal performance after heat treatment at 860℃/0.75h/AC+650℃/2h/AC (HT1). For all the specimens, the fracture was located in the TA15 region, which indicated that the transition zone had good mechanical properties.

Keywords: TA15/Ti60 graded structural material, Direct energy deposition, microstructure, Mechanical properties, annealing treatments