Comparison of Microstructural Evolution in Ti-6Al-4V Multi-Layer Builds Produced By Three Additive Manufacturing Processes

Additive Manufacturing (AM) processes build solid, near-net shape parts in a layer by layer manner based on 3D computer-aided-design (CAD) models. AM of metallic and alloy components starts with wire or powder feedstock which is heated and melted layer by layer in accordance with slices of the CAD file by an energy source which may be a laser or electron beam, or an electric arc. The molten metal solidifies to form a fully dense layer, and the addition of multiple layers results in a dense, near-net shape part. Microstructural evolution in metal AM parts is influenced by the localised melting in each layer followed by rapid cooling, and the relatively complex thermal history associated with the temporal and spatial thermal excursions as a result of the deposition of subsequent layers and associated phase transformations and their kinetics. As AM processes span a wide range of deposition rates from 0.1 kg/h to several kg/h, the dimensional scale of parts produced by the different processes vary, influencing the associated thermal history, and the resulting microstructures. In this paper, we compare and contrast the microstructural evolution and crystallographic texture development in multi-layer Ti-6Al-4V builds produced by powder-bed processes, Electron Beam Melting (EBM) and Selective Laser Melting (SLM), and wire-feed Electron Beam Free Form Fabrication (EBFFF). Samples of identical geometry were produced using EBM and SLM, using optimal parameters for each process; a T-shaped part was produced by EBFFF. Light optical and scanning electron microscopy and electron back scatter diffraction techniques were used for microstructural and texture characterisation. Despite some generic similarities, the specific characteristics of each AM process, such as the energy source used, the ambient temperature and atmosphere (vacuum or inert gas), build paths and scanning strategies, and the associated thermal history had a dominant effect on the observed microstructures.