Development of Additively Manufactured Molybdenum and Improvement via Lanthanum Oxide Addition

Additive manufacturing (AM) technologies have provided an avenue for processing traditionally difficult to manufacture metals. Manufacturability of one such material, molybdenum, has remained on the forefront of challenges hindering its widespread application. In fact, the current manufacturing methods employed by Framatome to create sintering boats for fissionable fuel out of a molybdenum-lanthanum oxide alloy often result in residual stress buildup, defects, and subsequent part failure. Thus, the use of AM methods could provide a promising path to remediate the expensive operations and tooling needed to manufacture molybdenum parts. To date, work done on the additive manufacturing of molybdenum has focused on small scale parts with processes such as powder bed fusion. Novel investigation of both pure molybdenum and molybdenum alloyed with nanoparticle lanthanum oxide manufactured by a directed energy deposition – laser beam – powder blown (DED-LB-PB) AM process is presented in this work. Importantly, the discovery of process parameter sets corresponding to high densities of molybdenum is accomplished via response surface methodology experimental design. Maximum densities achieved are 96.99% and 99.87% in the pure molybdenum and alloyed molybdenum systems, respectively, thus demonstrating the capability of the DEDLB-PB method for manufacturing commercial parts with future work. Furthermore, microstructural characterization of the AM specimens produced has demonstrated the effectiveness of the nanoparticle lanthanum oxide addition to reduce grain size, reducing millimeter-tall grains to hundreds of microns. Accordingly, grain boundary cracking is reduced significantly, allowing for the creation of larger mechanical samples. The compression testing of these alloyed samples yielded an average strength of 217 MPa, further indicating the possibility of commercial part manufacturability. Chemical analysis data alluded to the loss of lanthana during the DED-LB-PB process. Dimensional stability and accuracy of parts made with the AM method showed relationships to varied parameters of laser power, scan speed, and mass flow, as well as to the addition of lanthana.

Date

2023-08-30

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