The Influence of Iron in Minimizing the Microstructural Anisotropy of Ti-6Al-4V Produced by Laser Powder-Bed Fusion
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LASER powder-bed fusion (L-PBF) of structural alloys is beginning to find widespread use in a variety of applications thanks to the ability to produce net-shaped components with unparalleled design freedom. To sustain the development of this emerging manufacturing technology, a number of studies have investigated how the unique microstructure imposed by L-PBF affects the macroscale mechanical properties of important engineering alloys, in particular those based on titanium such as Ti-6Al-4V. The microstructure of Ti-6Al-4V after L-PBF exhibits complex features that span across several length scales (from the nm to mm scale), the morphology, and MARCO SIMONELLI, NESMA T. ABOULKHAIR, and RICHARD HAGUE are with the Centre for Additive Manufacturing, University of Nottingham, Nottingham, NG8 1BB, UK. Contact e-mail: [email protected] DAVID GRAHAM MCCARTNEY is with the Advanced Materials Group, University of Nottingham, Nottingham NG7 2RD, UK. PERE BARRIOBERO-VILA is with the Institute of Materials Research, German Aerospace Center (DLR), Linder Ho¨he, Cologne, 51147, Germany. YAU YAU TSE is with the Department of Materials, Loughborough University, Loughborough, LE11 3TU, UK. ADAM CLARE is with the Advanced Component Engineering Laboratory (ACEL), University of Nottingham, Nottingham NG7 2RD, UK. Manuscript submitted October 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
arrangement of which are influenced by the printing process, such as the laser power, scanning speed, the spacing between raster paths (also known as hatch spacing), position within the part, the scan strategy, and the environmental conditions (temperature and oxygen concentration). Typically, a hierarchical structure made up of fine martensitic a¢ phase (including primary, secondary, tertiary, and quartic martensite plates) within columnar prior-b grains dominates the microstructure of Ti-6Al-4V in the as-built L-PBF condition.[1] While the metastable a¢ phase can be readily decomposed into a more ductile a + b microstructure by standard post-processing heat treatments,[2,3] the columnar prior-b grains maintain, however, a characteristic elongated morphology. This morphology is an undesirable feature of L-PBF, and more generally in additively made materials. Such columnar prior-b grains are known to negatively affect the mechanical properties of Ti-6Al-4V, giving rise to anisotropy and low fracture toughness and consequently affecting the fracture modes and fatigue resistance of the alloy.[4–8] The formation of columnar prior-b grains and, more specifically, their suppression, via suitable modification of the alloy chemistry of Ti-6Al-4V, is the focus of this present study. The formation of columnar prior-b grains in Ti-6Al-4V is a consequence of epitaxial growth (from
a substrate of previously deposited layers) of the high-temperature b phase during the layer-by-layer deposition.[9] It is now accepted that, in a repeating sequence, as a new layer of powder is melted, the top of the previously deposited layer is also re-melted
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