Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures

Laser powder bed fusion (L-PBF) was utilized to produce specimens in Ti-6Al-4V, which were subjected to a bi-lamellar heat treatment, which produces microstructures consisting of primary α-lamellae and a fine secondary α-phase inside the inter-lamellar β-regions. The bi-lamellar microstructure was obtained as (i) a direct bi-lamellar heat treatment from the asbuilt condition or (ii) a bi-lamellar heat treatment preceded by a β-homogenization. For the bi-lamellar treatment with β-homogenization, cooling rates in the range 1−500 K/min were applied after homogenization in β-region followed by inter-critical annealing in the α + β region at various temperatures in the range 850-950 °C. The microstructures were characterized using various microscopical techniques. Mechanical testing with Vickers hardness indentation and tensile testing was performed. The bi-lamellar microstructure was harder when compared to a soft fully lamellar microstructure, because of the presence of fine α-platelets inside the β-lamellae. Final low temperature ageing provided an additional hardness increase by precipitation hardening of the primary α-regions. The age hardened bi-lamellar microstructure shows a similar hardness as the very fine, as-built martensitic microstructure. The bi-lamellar microstructure has more favorable mechanical properties than the as-built condition, which has high strength, but poor ductility. After the bi-lamellar heat treatment, the elongation was improved by more than 250 %. Due to the very high strength of the as-built condition, loss of tensile strength is unavoidable, resulting in a reduction of tensile strength of ∼18 %.