Layerwise rolling in laser powder bed fusion of tungsten refractory materials: Effects and mechanisms

Abstract

Tungsten (W), which possesses the highest melting point of any metal (3422 °C) and thermal stability and mechanical strength, is a material of choice for extremely demanding applications ranging from aerospace and nuclear fusion reactors to high-temperature manufacturing. However, tungsten's use in additive manufacturing processes is limited by its tendency to crack due to its high ductile-to-brittle transition temperature (DBTT). During these processes, tungsten undergoes repeated thermal cycling and often cools below its DBTT, entering a brittle regime that promotes crack initiation and, in turn, compromises the microstructure, mechanical performance, and reliability of fabricated parts. Such cracking is a major challenge across additive manufacturing techniques; however, the present study concentrates on laser powder bed fusion (LPBF) as a representative process to examine this issue. To address this challenge, we introduce a novel approach termed cold rolling assisted laser powder bed fusion (CR-LPBF), wherein each layer of the LPBF build is cold-rolled at a temperature above tungsten's DBTT prior to the onset of cracking. It is hypothesized that this in situ layerwise rolling induces uniform grain subdivision within each layer, thereby reducing the dislocation source spacing (λ) and increasing the dislocation density. The resultant high dislocation density is anticipated to enhance the material's capacity for plastic deformation, inhibit the formation of cracks, and thereby improve the material's overall mechanical properties. Consistent with this expectation, experimental results indicate that incorporating a layerwise cold-rolling step into the LPBF process significantly reduces cracking and refines the grain structure; this combined approach also increases the dislocation density and, in turn, enhances the mechanical performance of the fabricated tungsten parts. To the best of our knowledge, this work constitutes the first successful integration of cold rolling into an LPBF process, thus offering a novel strategy to overcome the limitations of conventional manufacturing. These enhanced properties achieved through the CR-LPBF technique pave the way for broader deployment of tungsten in critical applications involving high temperatures and severe mechanical stresses.