Effect of Copper Content on Grain Structure Evolution in Additively Manufactured Ti-6Al-4V Alloy

Electron beam additive manufacturing with simultaneous feeding of two dissimilar metal wires was used to obtain Ti-6Al-4V specimens successively alloyed with 0.6, 1.6, 6.0 and 9.7 wt % Cu. The specimens were characterized for microstructure, phases, and mechanical properties. Increasing the copper content in the alloy from 0.6 to 9.7 wt % resulted in the refinement of primary beta-Ti grains and the columnar-to-equiaxed grain transformation owing to the effect of constitutional undercooling on grain nucleation and growth. The grain growth restriction factor was calculated to substantiate the microstructural evolution from columnar to equiaxed grains. Admixing with up to 6.0 wt % Cu resulted in the formation of ultrathin alpha-Ti platelets, while increasing the copper content to 9.7 wt % Cu led not only to further thinning of alpha-Ti platelets but also to the formation of refined alpha '-Ti and alpha ''-Ti phases. Intermetallic Ti2Cu particles were precipitated due to the beta -> Ti2Cu + alpha eutectoid decomposition of primary beta-Ti grains and then plausibly induced heterogeneous nucleation of alpha-Ti platelets. A combined effect of solid solution hardening, precipitation hardening, and grain boundary hardening was achieved and allowed increasing the microhardness, ultimate tensile stress, tensile yield stress, and compression yield stress of Ti-6Al-4V/Cu specimens.

Automated summary

Electron beam additive manufacturing was used to produce Ti-6Al-4V specimens alloyed with different amounts of copper. Increasing the copper content resulted in the refinement of primary grains and transformation of the grain structure. The addition of copper also led to the formation of different phases and increased the mechanical properties of the Ti-6Al-4V/Cu specimens.