Operando X-ray scattering reveals ordering-mediated solidification in additive manufacturing

Solidification in fusion-based metal additive manufacturing (AM) occurs under non-equilibrium conditions, often producing microstructures that deviate from classical theories, such as refined grains with high twin density arising from abnormal columnar-to-equiaxed transition (CET). While such feature has been linked to liquid atomic orderings, direct mechanistic evidence has been lacking. Here, we perform operando synchrotron X-ray total scattering measurement with rapid pair distribution function (PDF) analysis to probe atomic structures in AM melt pools. We resolve the evolution of short- and medium-range orderings and connect their selective consumption to distinct solidification pathways in Inconel 718 and other alloys. Our findings not only confirm the importance of icosahedral clusters in controlling solidification behavior, but also suggest a distinct nucleation and growth pathway responsible for abnormal CET. This insight offers opportunities for alloy design and microstructure control in metal AM.