Microstructures and Grain Refinement of Additive-Manufactured Ti- x W Alloys
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THE high level of interest in additive manufacturing (i.e., 3D printing) has made it necessary to develop process–composition–microstructure and composition–microstructure–property relationships[1–3] for a variety of metallic alloys deposited using additive manufacturing techniques. This need, concurrently, has led to a number of research activities and publications on additive manufacturing, with common microstructural features observed, including (depending upon the processing parameters) large columnar grains,[4] compositional fluctuations,[3] unmelted particles or other anomalies,[5] zigzag grains,[6] and
MICHAEL Y. MENDOZA, DAVID A. BRICE, BRIAN W. MARTIN, and MATT R. ROLCHIGO are with the Department of Materials Science and Engineering, Iowa State University, Ames, IA, 50011. PEYMAN SAMIMI, RICHARD LESAR, and PETER C. COLLINS are with the Department of Materials Science and Engineering, Iowa State University, and also with the Center for Advanced Non-Ferrous Structural Alloys (CANFSA), a joint NSF I/UCRC between the Colorado School of Mines, Golden, CO, 80401, and Iowa State University. Contact e-mail:[email protected] Manuscript submitted January 14, 2017. Article published online May 15, 2017 3594—VOLUME 48A, JULY 2017
columnar-to-equiaxed transitions (CETs).[7] To develop the aforementioned relationships, it is necessary to understand at a minimum, and predict if possible, microstructural attributes including grain size, composition fluctuations, unmelted particles, grain orientation/ crystal texture, and any spatial variations in these features. Once these initial attributes have been set, many of the subsequent phase transformations and microstructural configurations are governed (restricted). Yet, there is a solution to this need. The additive manufacturing of metallic structures is fundamentally a melting and solidification problem, albeit one that often deviates from equilibrium processing and one for which composition may fluctuate from the intended composition. The fact that additive manufacturing is essentially a solidification problem at a small volume (0.2 mm3 to 1 cm3) means that there is an opportunity to use theories related to solidification to predict grain structures and sizes, as well as other microstructural attributes of interest (e.g., texture). Additive manufacturing allows for a new exploration of alloy space where conventional processing may not be possible due to difficulties such as solute partitioning or macrosegregation in large ingots. Indeed, such alloys may be possible to process in a compositionally homogeneous manner by additive manufacturing. Some Ti-based alloys are prone to strong METALLURGICAL AND MATERIALS TRANSACTIONS A
solute partitioning in the liquid, but are of general interest for additive manufacturing. This paper explores both this need (e.g., identifying models to predict grain size for additive manufacturing) and opportunity (i.e., new alloy spaces) for grain refinement in titanium-based alloys. Further, it leverages a capability afforded by additive manufacturing
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