Targeted heat treatment of additively manufactured Ti-6Al-4V for controlled formation of Bi-lamellar microstructures. (10th August 2021)

• Record Type: Journal Article
• Authors: Funch, Cecilie V.; Palmas, Alessandro; Somlo, Kinga; Valente, Emilie H.; Cheng, Xiaowei; Poulios, Konstantinos; Villa, Matteo; Somers, Marcel A.J.; Christiansen, Thomas L.
• Is Part Of: Journal of materials science & technology. Volume 81(2021)
• Page Start: 67
• Page End: 76
• Publication Date: 2021-08-10
• Subjects: Additive manufacturing -- Ti-6Al-4V -- Targeted heat treatment -- Bi-lamellar microstructures -- Laser powder bed fusion; Metals -- Periodicals; Materials science -- Periodicals; Materials science; Metals; Periodicals; 620.1105
• DOI: 10.1016/j.jmst.2021.01.004
• Languages: English
• ISSNs: 1005-0302

Abstract

Graphical abstract: Highlights: Bi-lamellar microstructures can be produced in additively manufactured Ti-6Al-4V. Direct bi-lamellar heat treatment performs better than a treatment with β-homogenization. Dramatic increase in tensile elongation after heat treatment. Abstract: 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 %.