Additive manufacturing of strong and ductile Ti-6Al-4V by selective laser melting via in situ martensite decomposition

Novel ultrafine lamellar (alpha + beta) microstructures comprising ultrafine (similar to 200-300 nm) alpha-laths and retained beta phases were created via promoting in situ decomposition of a near alpha' martensitic structure in Ti-6Al-4V additively manufactured by selective laser melting (SLM). As a consequence, the total tensile elongation to failure reached 11.4% while maintaining high yield strength above 1100 MPa, superior to both conventional SLM-fabricated Ti-6Al-4V containing non-equilibrium acicular alpha' martensite and conventional mill-annealed Ti-6Al-4V. The formation and decomposition of alpha' martensite in additively manufactured Ti-6Al-4V was studied via specially designed experiments including single-track deposition, multi-layer deposition and post-SLM heat treatment. The essential SLM additive manufacturing conditions for Ti-6Al-4V including layer thickness, focal offset distance and energy density, under which a near alpha' martensitic structure forms in each layer and then in situ transforms into ultrafine lamellar (alpha + beta) structures, were determined. This is the first fundamental effort that has realized complete in situ martensite decomposition in SLM-fabricated Ti-6Al-4V for outstanding mechanical properties. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.