(407c) Direct Ink Writing of Tungsten and Tungsten Alloys for Dense Complex Shapes

Tungsten (W) is a refractory metal that exhibits a unique combination of high melting point, strength, ductility, thermal conductivity, and corrosion resistance. As such, it is integral for use in extremely high temperature applications such as nuclear thermal propulsion and shielding elements of solar probes. However, due to the high melting point (3422°C), W is extremely difficult to melt and cast into various complex shapes. Most W based alloys are made through powder metallurgy, pressing and sintering W powder with the addition of transition metal sintering aids. This limits the complexity of the final fabricated part, limiting the ability to produce intricate components for the desired applications. Current state of the art additive manufacturing techniques for W and W alloys are powder bed fusion and binder jet printing. Both techniques are highly dependent on process conditions, which can introduce flaws such as stress gradients, porosity, and non-uniform shrinkage in the final part.

In this work we propose the use of direct ink writing (DIW) of W and W alloys to fabricate complex shaped components with near full density, high yield strength, and low surface roughness. Further, we utilize atomic layer deposition (ALD) to deposit sintering aids and dopants at low loading directly on the particle surface. A tungsten-based colloidal ink was formulated with an optimized solids fraction volume loading. The ink was then continuously extruded through a nozzle which enables bottom-up fabrication of the desired geometry, such as cylinders or lattices for testing. A design of experiments (DOE) was employed to optimize printing parameters in order to mitigate surface roughness and maintain print uniformity. The green bodies were dried and sintered at 1600°C in a low hydrogen atmosphere. Surface profilometry was used to obtain the sintered surface roughness and evaluate the DOE. The optimized printing parameters were employed to manufacture samples with various sintering aids and dopants, added through conventional mixing or ALD on the powder prior to ink formation. A secondary DOE was employed to evaluate the parameters that affect the final print density. The systematic optimization of the colloidal ink resulted in parts that demonstrated both densification and stability in complex geometry. Addition of the sintering aids promoted densification further at the lower sintering temperature and ALD was shown to be a successful loading method. This work introduces the use of ALD-functionalized W in colloidal gels for the purpose of DIW complex shapes.