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• Fabrication of highly dense crack free pure tungsten via electron beam additive manufacturing. • "Texture switch" observed along the build direction microstructure. • High temperature tensile properties show material behavior similar to recrystallized tungsten. • High dislocation densities were observed in the as-fabricated samples. • Highly anisotropic behavior due to the underlying crystallographic anisotropy. There is considerable interest in the adoption of additive manufacturing for processing refractory metals. The layer-wise fabrication approach enables opportunities for producing complex geometries which cannot be otherwise be achieved via powder metallurgy. However, the processing science is still in its nascent stages and structure–property relations are relatively unexplored. Fundamental research is needed to further develop the technology and enable the fabrication of refractory metals for high temperature applications. Here we focus on the processing of pure tungsten using electron beam melting additive manufacturing. Experimentally we develop a suitable processing window for achieving high density crack free material. Microstructural analysis reveals that the microstructure generally consists of a columnar structure with a 111 build direction fiber preference, although, fiber switching was observed. Process induced deformation is believed to drive the formation of subgrains whose boundaries exhibit a high dislocation density. High temperature tensile testing reveals that the material exhibits excellent properties closer to that of annealed tungsten. Significant mechanical anisotropy was observed to be present which is likely driven by strong crystallographic texture. [ABSTRACT FROM AUTHOR]

[Display omitted] • Crack-free pure molybdenum is fabricated via electron beam melting AM. • Fiber texture switching is observed across various energy density settings. • The weld pool shape likely drives the fiber selection mechanism. • Columnar grains consist of fine subgrains believed to be due to process stresses. Additive manufacturing (AM) technologies offer novel opportunities for processing difficult to cast refractory materials. Electron beam melting (EBM) AM is particularly attractive as the rapidly moving electron beam can be utilized to heat the powder bed which mitigates against some process induced cracking mechanisms. A great deal of prior work has been done to investigate laser based processing of molybdenum but little EBM focused work currently exists. In this work we investigate EBM processed molybdenum and observe sharp 0 0 1 , 1 1 1 , and mixed 0 0 1 & 1 1 1 crystallographic fibers in the build direction. The apparent preference between these build direction fibers is dependent on the imposed energy density and this is likely explained by the weld pool shape. Detailed microscopy reveals that the observed columnar grains consist of much finer equiaxed low angle boundary subgrains suggesting large process induced stresses leading to appreciable plastic deformation. The implications resulting from this work are that molybdenum may be processed crack-free via EBM AM and that fiber-switching may be controlled, and exploited, towards fabricating components with optimized performance. [ABSTRACT FROM AUTHOR]

Additive manufacturing technologies have emerged as potentially disruptive processes whose possible impacts range across supply chain logistics, prototyping, and novel materials synthesis. Numerous works illustrate the ability to control microstructure in fusion based processes and a few recent authors have even produced single crystals. However, a number of questions remain open regarding the process window which enables printing of single crystals. Furthermore, it has been observed that these additively manufactured single crystals exhibit a preferred 〈 011 〉 secondary orientation parallel to the scanning direction. In this work we investigate the fabrication conditions that enable printing of single crystals via electron beam melting. A space filling design of experiments is utilized to efficiently explore the fabrication space. Single crystals are successfully obtained using both commercially available powders and custom melt alloys. Microstructures obtained via these exploratory experiments exhibited a continuum of columnar structures ranging from weakly textured polycrystals, near single crystal, and fully single crystalline material. Complex geometry experiments are performed to study the grain selection mechanism. We find that the grain selection mechanism is independent of the bulk scale geometry and must therefore be driven by local heat transfer and solidification dynamics. Furthermore, grain selection is shown to be driven by competing driving forces; one which prefers epitaxial growth and another which is driven by the imposed processing conditions. [ABSTRACT FROM AUTHOR]

Additive manufacturing processes supplement traditional material processing routes with unique capabilities which can have profound impacts on component production. Physical prototyping is accelerated and the fabrication of complex components, difficult or impossible to produce conventionally, is realized. Metals research is often focused on identifying process windows to avoid defects which thereby yield desirable properties. In electron beam melting fusion processes, however, precise spatial control of the heat source allows for detailed microstructure manipulation. Design is therefore extended to the microstructure scale offering greater overall flexibility towards engineering high performance components. In this work the role of geometry and beam path sequencing in a powder bed electron beam melting process is investigated. It is observed that by carefully engineering the melting sequence the morphology and texture at the mesoscale can be controlled. Solidification in the build direction, which usually prefers [001] directions, is tilted by control of the heat flux vector which yields large columnar crystals with a strong [011] build direction preference. This newly developed conduction control strategy is demonstrated for producing alternating mesoscale structures in bulk samples. Furthermore, a new scanning strategy is demonstrated which may be suitable for promoting a randomized crystallographic texture during the additive manufacturing process. Unlabelled Image • Scan strategy shown to control build direction orientation of epitaxial columnar grains in a continuous fashion. • Tilting of grains away from preferred [001] direction is achieved by controlling lateral conductive heat flow. • Physics utilized to produce bulk components with alternating mesoscale structure. • Scan strategy devised to enable randomization of build direction texture. [ABSTRACT FROM AUTHOR]