Zhongshu Ren

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.

Aeroacoustic emissions from intense evaporation are widely measured yet often treated as noisy byproducts and used mainly in empirical monitoring. Here, we show that airborne sound encodes physics-governed sub-millisecond fingerprints of vapor-jet dynamics in excessive vaporization, exemplified by vapor keyholes in laser metal processing. From first principles, we develop a vapor-jet-cavity oscillation framework and incorporate it into an aeroacoustic formulation, thereby coupling measured sound to transient cavity depth and oscillation frequency. Reconciled with synchronized multimodal in-situ data, airborne acoustics enable accurate tracking of vapor-cavity properties within tens to hundreds of microseconds. Combined with newly discovered correlations, cavity-jet-acoustic theory recasts the transition from steady, pore-free to pore-shedding vaporizations as a critical-frequency event. Aeroacoustic emissions thus become scalable, physics-guided, and cost-efficient probes of rapidly evolving liquid–vapor systems.

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 functon(PDF) analysis to probe atomic structures in AM melt pools. We characterize 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 show that abnormal CET originates from decomposition of icosahedral clusters into twinned nuclei, offering new opportunities for alloy design and microstructure control in AM.

Porosity defects are currently a major factor that hinders the widespread adoption of laser-based metal additive manufacturing technologies. One common porosity occurs when an unstable vapor depression zone (keyhole) forms because of excess laser energy input. With simultaneous high-speed synchrotron x-ray imaging and thermal imaging, coupled with multiphysics simulations, we discovered two types of keyhole oscillation in laser powder bed fusion of Ti-6Al-4V. Amplifying this understanding with machine learning, we developed an approach for detecting the stochastic keyhole porosity generation events with submillisecond temporal resolution and near-perfect prediction rate. The highly accurate data labeling enabled by operando x-ray imaging allowed us to demonstrate a facile and practical way to adopt our approach in commercial systems.