Effects of scanning strategies on the microstructure and mechanical properties of Nb-5W-2Mo-1Zr alloy fabricated by electron beam powder bed fusion

Nb-based alloys are regarded as promising high-temperature structural materials. However, when fabricated by electron beam powder bed fusion (EB-PBF), they typically exhibit coarse columnar grains and limited ductility. The scanning strategy is a critical factor governing microstructure evolution and mechanical performance. In this study, the effects of a discrete spot scanning (DSS) strategy on the microstructure and mechanical properties of an EB-PBF-fabricated Nb-5W-2Mo-1Zr (Nb521) alloy were investigated and compared with those of a conventional line raster scanning (LRS) strategy. The results showed that DSS promoted the formation of hemispherical molten pools with radial heat conduction. This thermal profile, coupled with intermittent remelting, effectively suppressed the epitaxial growth of <001>-oriented long columnar grains and produced short columnar grains with randomized orientations. This strategy also mitigated residual stress, as indicated by an average kernel average misorientation (KAM) value of 0.54, through stepwise constraint loading and localized strain relaxation. Differences in thermal histories also led to distinct precipitation behaviors under the two scanning strategies. Owing to greater thermal accumulation, LRS facilitated the diffusion of interstitial elements, promoting the precipitation of intragranular (Nb, Zr)₂C as well as intergranular ZrO₂ and (Nb, Zr)₂C. In contrast, DSS induced only the precipitation of intragranular (Nb, Zr)₂C, with no grain-boundary phases observed. Both strategies yielded ultimate tensile strengths (510.0 ± 12 MPa for LRS and 477.7 ± 10 MPa for DSS) comparable to those of conventionally thermomechanically processed alloys. Notably, DSS increased the elongation to 14.4 ± 0.6%, corresponding to a 33.3% improvement over LRS. This improvement was attributed to the formation of shorter columnar grains and a more uniform stress distribution.