In situ tailoring microstructure in additively manufactured Ti-6Al-4V for superior mechanical performance

The "Holy Grail" of metal additive manufacturing is to manufacture reliable high-performance metal parts with no or a minimal need of post processing. However, Ti-6Al-4V parts made by selective laser melting (SLM) often suffer from poor ductility and low toughness because of the predominant acicular alpha' martensite contained in columnar prior-beta grains. In practice, post heat treatment is necessary. To overcome this deficiency, we have explored designing innovative SLM processing routes to turn the unfavoured alpha' martensite, via in-situ decomposition, into lamellar (alpha+beta) microstructures with tuneable characteristic length scales. Such lamellar (alpha+beta) microstructures lead to superior mechanical properties which markedly exceed ASTM standards and outperform the majority of Ti-6Al-4V fabricated by other additive manufacturing processes. Furthermore, we find that the lattice parameter of the beta phase in the (alpha+beta) lamellae falls into a specific range of 3.18-3.21 angstrom. Hence the lattice parameter of beta phase can serve as an indicator to predict whether significant martensite decomposition has taken place in situ in Ti-6Al-4V made by SLM. This work marks an important step forward in the understanding of how to tailor microstructure in situ for the development of high-performance Ti-6Al-4V parts by SLM. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.