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

Novel ultrafine lamellar (α + β) microstructures comprising ultrafine (∼200-300 nm) α-laths and retained β phases were created via promoting in situ decomposition of a near α′ 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 α′ martensite and conventional mill-annealed Ti-6Al-4V. The formation and decomposition of α′ 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 α′ martensitic structure forms in each layer and then in situ transforms into ultrafine lamellar (α + β) 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.