The interplay between vapour, liquid, and solid phases in laser powder bed fusion.

The capability of producing complex, high performance metal parts on demand has established laser powder bed fusion (LPBF) as a promising additive manufacturing technology, yet deeper understanding of the laser-material interaction is crucial to exploit the potential of the process. By simultaneous in-situ synchrotron x-ray and schlieren imaging, we probe directly the interconnected fluid dynamics of the vapour jet formed by the laser and the depression it produces in the melt pool. The combined imaging shows the formation of a stable plume over stable surface depressions, which becomes chaotic following transition to a full keyhole. We quantify process instability across several parameter sets by analysing keyhole and plume morphologies, and identify a previously unreported threshold of the energy input required for stable line scans. The effect of the powder layer and its impact on process stability is explored. These high-speed visualisations of the fluid mechanics governing LPBF enable us to identify unfavourable process dynamics associated with unwanted porosity, aiding the design of process windows at higher power and speed, and providing the potential for in-process monitoring of process stability.

Roles of texture and latent hardening on plastic anisotropy of face-centered-cubic materials during multi-axial loading.

This study investigates the joint impact of preferred texture and latent hardening on the plastic anisotropy of face centered cubic (FCC) materials. The main result is that both aspects have significant impact on the anisotropy, but the two can either counteract each other or synergistically reinforce each other to maximize anisotropy. Preferred texture results in significant anisotropy in plastic yielding. However, the latent hardening significantly alters the texture-induced anisotropy. In addition, one latent hardening type can cancel out the anisotropy of another type. Consequently, if all dislocation-based latent hardening types are included at the same level as the self-hardening, the result might not reveal the complexity of plastic anisotropy. The present study of the synergistic influence of detailed latent hardening and texture presented helps provide new insights into the complex anisotropic behavior of FCC materials during multi-axial forming.

Grain boundary character distribution of nanocrystalline Cu thin films using stereological analysis of transmission electron microscope orientation maps.

Stereological analysis has been coupled with transmission electron microscope (TEM) orientation mapping to investigate the grain boundary character distribution in nanocrystalline copper thin films. The use of the nanosized (<5 nm) beam in the TEM for collecting spot diffraction patterns renders an order of magnitude improvement in spatial resolution compared to the analysis of electron backscatter diffraction patterns in the scanning electron microscope. Electron beam precession is used to reduce dynamical effects and increase the reliability of orientation solutions. The misorientation distribution function shows a strong misorientation texture with a peak at 60°/[111], corresponding to the Σ3 misorientation. The grain boundary plane distribution shows {111} as the most frequently occurring plane, indicating a significant population of coherent twin boundaries. This study demonstrates the use of nanoscale orientation mapping in the TEM to quantify the five-parameter grain boundary distribution in nanocrystalline materials.

Bridging simulations and experiments in microstructure evolution.

We demonstrate the importance of anisotropic interface properties in microstructure evolution by comparing computed evolved microstructures to final experimental microstructures of 5170 grains in 19 thin aluminum foil samples. This is the first time that a direct experimental validation of simulation has been performed at the level of individual grains. We observe that simulated microstructures using curvature-driven grain boundary motion and anisotropic interface properties agree well with experimentally evolved microstructures, whereas agreement is poor when isotropic properties are used.