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Electron beam melting (EBM) is a form of additive manufacturing (AM) that offers fast, customizable, near-net production of metallic parts by depositing material "layer by layer." In EBM, an electron beam scans across a bed of metal powder, thereby melting the powder in the shape of a desired part. The melted powder solidifies into a layer, the layer is covered with a new layer of powder, and the process repeats. Consequently, any location within the build could be rapidly heated above solid-solid phase transition temperatures several times as the beam scans the layer(s) above [1]. These beam-dependent, cyclic thermal profiles are known to strongly affect morphology and microstructure, resulting in anisotropic grain sizes, macro-and micro-porosity, and complex phase morphologies [2,3]. This directly impacts the mechanical properties of AM-built parts. While these complex thermal profiles can be controlled with beam scanning strategies, they are not fully understood. Understanding and controlling AM thermal gradients in space and time opens the opportunity to design unique, tailored, desirable microstructures.

Additive manufacturing has been around for many years, yet the underlying physics of thermal gradients, local pressure environment, and other non-steady state manufacturing conditions are not fully understood. A Multi-University Research Initiative (MURI) is currently ongoing to measure liquid/solid and solid/solid interface stabilities in AM Ti-6Al-4V. Samples were produced with different beamscanning strategies in order to study the role of thermal gradients on the resulting microstructure. The motivation is to determine which beam-scanning strategy leads to desired grain size and texture. Orientation at different length scales (from mm to nm) can be quantified and compared with a combination of techniques including Precession Electron Diffraction (PED), Electron Backscatter Diffraction (EBSD) and Neutron diffraction. This new information will help predict properties of additively manufactured parts.

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