A study of Cu modified Ti–6Al–4V deposits fabricated by wire-arc directed energy deposition: Alloy design, microstructure, and mechanical properties

The addition of alloying elements during additive manufacturing increases the strength but always sacrifices ductility. Therefore, simultaneously enhancing strength and ductility is still the focus today. In this study, Cu was selected as the alloying element, and the optimal Cu addition (1.2 wt%) was re-alloyed and predicted using Thermo-Calc software combined with a single-bead deposition experiment, followed by alloying into feeding wire. Ti–6Al–4V-1.2Cu was compared with commercial Ti–6Al–4V deposits, and it was found that both microstructures consisted of prior β, GB α, and basketweave structures containing α+β lamellae. The introduction of Cu realized the refinement of prior β grains (836 ± 12 μm→592 ± 8 μm), the refinement of α laths (2.56 ± 0.06 μm→1.91 ± 0.04 μm), the coarsening of β laths (0.03 ± 0.005 μm→0.2 ± 0.03 μm), and the refinement of GB α (0.9 ± 0.07 μm → 0.45 ± 0.04 μm). As a β stabilizer, Cu increased the residual β content in the final microstructure (8.89%→18.67%). No formation of brittle Ti2Cu means that only grain refinement strengthening and solid-solution strengthening matter. The yield strength increased from 868.23 MPa to 934.32 MPa (7.61% increase). Among them, the contribution ratio of grain refinement strengthening and solid solution strengthening was the same through quantitative analysis. The ultimate tensile strength increased from 934.97 MPa to 990.97 MPa (6% increase). At the same time, tensile fracture elongation increased from 6.35% to 10.66% (67.87% increase), while the fracture mode transits from brittle cleavage to ductile dimples. Grain refinement of prior β grains, α laths, and GB α, inhibition of the β→α′ martensitic transformation, and local misorientation change of α laths are the main factors in improved ductility. In addition, the UTS and EL results are compared with earlier studies to reveal the prospect of this research.