Additive Manufacturing of Tungsten & Tungsten Alloys for High Temperature Applications

This study examines the microstructure, cracking defects, and mechanical properties of pure tungsten (W) and W alloys fabricated by additive manufacturing (AM) methods such as Directed Energy Deposition - Laser Beam (DED-LB) and Powder Bed Fusion - Laser Beam (PBF-LB). In DED-LB, cubes were deposited on a refractory metal baseplate in ambient air and an inert atmosphere. Severe microcracking and porosity were observed in all samples when building in ambient air and alloys were selectively oxidized. Under inert atmospheres, pure W was manufactured with no bulk microcracking and 1.7% porosity. Alloying with tantalum led to decreased porosity and increased hardness with again no microcracking observed. The lack of cracking in the inert atmosphere was attributed to the increase in build temperature and reduced oxidation. Micrographs also showed grain refinement with increasing Ta content and a cell structure for W - Ta alloys. W - Refractory Multi Principal Element Alloys (RMPEAs) fabricated by this process were also used to explore the effect of build sequence on the unmelted powder particles and 2000 ◦C strength properties. Results show that for the same energy density, build sequence shows a direct correlation with unmelted particles, where the particles decrease with an increase in the build sequence showing an effect of the baseplate preheat. Furthermore, high-temperature bend tests of the alloy show that the alloy shows a higher strength in comparison with pure W in wrought conditions, although variability and grain boundary contamination from carbon and oxygen are observed. Alternative cracking mechanisms using PBF-LB are explored, where centerline cracking and keyhole cracking are observed. Future work includes decoupling the effect of oxygen and process parameters on centerline cracking in pure W using in-situ radiography studies.