Microstructural evolution and mechanical properties of Ti-6Al-4V in situ alloyed with 3.5 wt.% Cu by laser powder bed fusion

Abstract

The present work deals with the fabrication of Ti-6Al-4V in situ alloyed with 3.5 wt.% Cu (Ti-6Al-4V–3.5Cu), its microstructural evolution, and its related tensile properties. Specimens with a relative density of 99 ± 0.1% presented a microstructure that differed from that of pure Ti-6Al-4V in LPBF, which usually exhibits a fully martensitic structure (α′). In Ti-6Al-4V–3.5Cu, apart from α-Ti, β-Ti, and intermetallic Ti2Cu were also observed, generated by in situ martensite decomposition. Initially solidified large columnar β-Ti grains underwent a martensitic transformation to α′. Cyclic reheating caused by layer-wise deposition during LPBF enabled the diffusion of Cu and V, allowing the transformation of α′ to stable α-Ti and the precipitation of β-Ti and Ti2Cu. The α′ decomposition and formation of β-Ti and Ti2Cu contributed to an UTS of 1362 ± 14 MPa and an Rp0.2 of 1313 ± 14 MPa in the horizontal specimen orientation in the as-built state. The elongation at fracture was 3.9 ± 0.6%. In the vertical orientation, the specimens exhibited an UTS of 1305 ± 19 MPa, an Rp0.2 of 1215 ± 23 MPa, and an elongation at fracture of 5.4 ± 1.5%. Stress-relief annealing reduced the UTS and Rp0.2 to 1099 ± 6 MPa and 1014 ± 5 MPa, respectively, with an increase in elongation at fracture to 6.7 ± 0.4% in the horizontal specimen orientation. In the vertical specimen orientation, stress relief annealing resulted in an UTS of 1051 ± 13 MPa and an Rp0.2 of 926 ± 18 MPa. The elongation at fracture was increased to 11.3 ± 1.3%.