On the Texture Formation of Selective Laser Melted Ti-6Al-4V
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DUCTION
SELECTIVE laser melting (SLM) is an additive manufacturing (AM) technique based on an infra-red fiber laser that creates solid layers from loose powder material and joins them in an additive manner. In principle, a thin layer of loose powder is initially leveled across a process platform to form a powder bed. Selected areas of the powder bed are then melted and consolidated line by line by a scanning laser. As soon as this layer is formed, new loose powder is deposited and melted on top of the layer below. Repeating this basic principle for multiple additive layers, any 3D component can then be created. SLM not only reduces waste material but also saves energy when compared to traditional processing techniques.[1,2] As the energy delivered by the laser is controllable, SLM has shown the possibility to fabricate near fully dense objects made of a wide range of metals generating interest in the aerospace and biomedical devices applications.[3,4] Nevertheless the microstructure of a/b titanium alloys produced by SLM (e.g., prior b grain morphology) differs significantly from the microstructure of the cast or wrought alloys of the same composition. The specific microstructure of the as-built SLM components is MARCO SIMONELLI, Ph.D Researcher, and YAU YAU TSE, Lecturer, are with the Department of Materials, Loughborough University, Loughborough LE11 3TU, U.K. Contact e-mail: M. [email protected] CHRIS TUCK, Associate Professor, is with the Additive Manufacturing and 3D Printing Research Group, Faculty of Engineering, The University of Nottingham, Nottingham NG7 2RD, U.K. Manuscript submitted October 3, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
responsible for the manifested mechanical anisotropy and low ductility.[5–7] Microstructural studies of the asbuilt components have revealed for example large elongated prior b grains formed along the build direction filled with acicular a¢ martensitic laths.[8–12] It is suggested that when the laser hits the powder bed, the grains in the previous deposited layers and the powder particles of the top layer transform into the b phase field. The b grains then solidify and grow epitaxially along the direction of heat conduction (typically in a columnar way in the build direction). Finally, as the laser moves away across the powder bed multiple a¢ martensitic laths precipitate within the elongated columnar grain of the parent b grain. The origin of similar columnar microstructures has also been discussed in a/b titanium alloys produced by other AM systems.[13–16] Although a number of process parameters such as laser scan speed, hatch spacing, and scan strategy have shown to play an important role in the densification mechanism of the SLM parts,[9,10,17,18] less effort has been spent to understand how these process parameters affect the solidification of the b grains, the b fi a¢ transformation and the texture evolution that takes place at each layer deposition. Therefore the microstructure of the a/b titanium alloys processed by SLM is not yet tailorable and specific post-treatm
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