On the Role of Out-of-Equilibrium Microstructures in Ti-6Al-4V and in Stainless Steel 316L Processed by Selective Laser Melting in Determining their Mechanical Properties

Additive manufacturing processes such as Selective Laser Melting (SLM) appear very promising in view of the economic production of near-net-shape, complex and (almost) fully dense parts from metallic materials such as Ti alloys and stainless steels. Practically, in SLM, a metallic powder is deposited layer-by-layer in a powder bed and then molten locally according to the desired shape. An important feature of this process is that the structure undergoes an ultra-fast cooling once the beam leaves the working zone, thus giving rise to strongly out-of-equilibrium microstructures. In the case of Ti alloy Ti-6Al-4V, in particular, the microstructural anisotropy resulting from the epitaxial growth of the newly deposited layer on the material previously solidified has been shown to exert a very strong influence on the mechanical properties [1].
In the present work, the microstructures and mechanical properties of Ti-6Al-4V and of stainless steel 316L processed by SLM have been characterised in details. Since these two materials exhibit quite different physical behaviours, their careful comparison might shed more light into the role of phenomena such as epitaxial growth, out-of-equilibrium phase transformation and/or internal stresses in determining the mechanical properties of metallic parts processed by SLM.