Chemical composition gradients within an In-situ alloyed tungsten-tantalum laser powder-bed fusion: modelling and validation

Tungsten remains a material of considerable interest to engineers for nuclear fusion reactor designs, in particular for the plasma-facing components (PFCs) thanks to its outstanding thermal conductivity and unmatched high melting point. However, tungsten remains a difficult to additively manufacture metal, due to its highly brittle nature, leading to cracking during cooling. Tantalum can be alloyed with the tungsten within a laser-based powder bed fusion (LPBF) manufacture by blending of elemental W and Ta powders, in a ratio of approximately 92 wt.% W to 8 wt.% Ta, which allows the LPBF builds to not experience cracking during cooling. However, it becomes of interest to understand any compositional gradients which may occur during in-situ alloying. An 85mm tall cylinder was manufactured via LPBF and was sectioned and analysed for chemical composition variation along the height. Whilst a discrete element modelling (DEM) framework was developed to simulate the powder behaviour during the tipping / pouring and the LPBF spreading motion. Both model prediction and experimental measurement agreed that the W powder segregates in higher concentrations toward the top of the build, whilst the Ta powder segregates to higher concentrations toward the bottom of the build. Experimental measurements suggest a +/- 1% variation from the nominal composition, whilst DEM predictions were a little wider, at +/- 2%. It is noted that a compositional gradient in the build may limit the end-application of LPBF built components due to issues with in-service performance thermally and mechanically, however this would need to be assessed on a case-by-case basis, as blends with smaller compositional gradient or different alloying elements may have varying effects upon performance.