Selective Laser Melting of Ti6Al4V: towards superior mechanical properties

A large number of commercially available metal powders are processed using SLM. The objectives of fundamental investigations are 1) to validate the manufacturing viability of standard commercial powders using AM, 2) to compare the metallurgical and mechanical properties of components made using AM and conventional manufacturing techniques and 3) to extrapolate information susceptible to help design new AM-specific materials displaying superior mechanical properties. 

Among the materials studied, titanium alloy Ti6Al4V has generated much interest given its broad industrial applications within established economic markets. Using SLM to achieve similar or superior mechanical properties compared to wrought material properties is a critical performance benchmark. Recent results suggest this is possible.

The conventional production of parts made of Ti alloys generates high costs and considerable waste material [1]. Alternative production routes, such as Selective Laser Melting (SLM), could save material and allow greater level of design freedom [2,3]. Over the recent years, SLM of Ti alloys has greatly evolved: several studies have elucidated the microstructure evolution and the density mechanisms during SLM of Ti alloys [2,3,4]. Parts with mechanical properties comparable to cast and wrought counterparts can now be obtained [5,6]. 

The microstructure that originates from SLM differs in many aspects from that obtained from conventional manufacturing. For additively manufactured Ti6Al4V products, sound mechanical properties require that their microstructure be pore free and comprise suitable metallurgical phases that induce both strong and ductile properties. This ‘strength–ductility’ trade-off  is what poses the greatest challenges to  SLM of Ti6Al4V.

Columnar prior-b grain structure [7]

a' martensite formed inside the prior-b grains [7]

The mechanical properties of the SLM-fabricated Ti6Al–4V depend largely on

1.       The microstructure:

• its constituent phases, (a' martensite, a + b )
• their morphology, (acidular, lamellar, globular, mixed)
• their characteristic length scales,

2.       The grain texture:

• the size
• and orientation of the prior-b grains

AM of Ti64 with SLM usually occurs at powder-bed temperatures <230C. The resulting microstructure often features columnar prior-b grains owing to and aligned along the steep temperature gradients encountered during SLM. It is usually filled with acicular a’ martensite . 

The elongated grain boundaries combined with the presence of acicular a’ martensite favor intergranular failure [9].  In addition, such strongly textured structures promote anisotropic mechanical behavior, causing large discrepancies in mechanical response when subject to external loading along different sample orientations.  That’s why post heat treatment must usually be carried out  to transform the acicular a’ martensite into equilibrium (a + b) microstructures. This also reduces thermal stresses at the same time.

Although post heat treatment improve ductility of SLM-fabricated Ti6Al4V samples, it also coincides with strength reduction as it promotes coarsening of the lamellar (a + b) microstructure . In other words, post heat treatment can improve the tensile ductility but at the expense of strength.

Engineering tensile stress–strain curves of SLM-fabricated Ti-6Al-4V comprising ultrafine lamellar (a + b) structure and a' martensitic structure compared with that of mill-annealed Ti-6Al-4V. [8]

ultrafine lamellar a+b [8]

Recent research shows however that this trade-off can now be avoided. A resent study [7] shows it is possible to locally transform the nonequilibrium submicron acicular a’ martensite into submicron near-equilibrium lamellar (a + b) without triggering noticeable coarsening.  In effect, suitable SLM processing parameters (layer thickness, energy density, and focal offset distance) are chosen to promote in situ transformation of the non equilibrium a’ martensite to near-equilibrium a and b phases in the form of an ultrafine lamellar (a + b) structure without undergoing significant coarsening. The ultrafine (200–300 nm) (a + b) structure offers high yield strength of 1106 MPa and large tensile elongation of 11.4% in the as-fabricated state. These properties make the SLM-fabricated Ti6Al4V with an ultrafine lamellar (a + b) structure comparable with solution treated and aged Ti6Al4V [8] and superior to mill–annealed Ti6Al4V [8].

References
[1] F.H., Froes and M.A. Imam: Key Engineering Materials Cost Vol. 436 (2010), p. 1
[2] L. Thijs, F. Verhaeghe, T. Craeghs, J.V. Humbeeck and J.P. Kruth: Acta Mat. Vol. 58.9 (2010),p. 3303
[3] T. Sercombe, N. Jones, ,R. Day and A. Kop: Rapid Prototyping Journal Vol. 14.5 (2008), p. 300
[4] M.Simonelli, Y.Y. Tse, C. Tuck: Metal. Mater Trans. A Vol. 45.6 (2014), p. 2863
[5] T. Vilaro, C. Colin and J.D. Bartout: Metal. Mater. Trans. A Vol. 41.10 (2011), p. 3190
[6] B Vrancken, L. Thijs, J.P. Kruth and J. Van Humbeeck: J. Alloys Compd Vol. 511 (2012), p.177
[7] W. Xu et al. / Acta Materialia 85 (2015) 74–84
[8] W. XU, S. SUN, J. ELAMBASSERIL, Q. LIU, M. BRANDT, and M. QIAN,  Ti-6Al-4V Additively Manufactured by Selective Laser Melting with Superior Mechanical Properties  2015 The Minerals, Metals & Materials Society
[9] H.K. Rafi, N.V. Karthik, H.J. Gong, T.L. Starr, and B.E. Stucker, J. Mater. Eng. Perform. 22, 3872 (2013).