Abnormal columnar-to-equiaxed transition in metals processed under non-equilibrium conditions in laser additive manufacturing

NON-TECHNICAL SUMMARY

Solidification plays a pivotal role in metallurgy, as it is a fundamental step in various metalworking and manufacturing processes. The mechanical properties of a metal are influenced not only by its composition and phases but also by its microstructures, such as grain size, orientation, and defects. Therefore, understanding how a liquid metal transforms into solid with varying microstructures during cooling is crucial.

The established understanding of metal solidification is based on classical nucleation theory, which posits that the structural fluctuation in a disordered liquid lead to the formation of small nuclei - clusters of atoms arranged in a specific order. If these nuclei exceed a critical size, they can continue to grow by the diffusion of free atoms from the surrounding liquid onto their surfaces.

However, classical nucleation theory encounters challenges when explaining certain unique microstructures formed under complex processing conditions. It is speculated that the limitations of classical nucleation theory stem from its neglect of pre-existing local orderings in liquid metals. Characterizing the structure of liquid metals and their evolution during cooling under realistic metal processing conditions has been a significant challenge.

To address this, the proposed research will investigate the metal solidification using in situ synchrotron x-ray diffraction technique and multiscale simulations. This study aims to answer three key questions: (1) What are the atomic orders in a liquid metal? (2) How do these local orders affect nucleation and microstructure development? (3) Are there effective methods to tailor these orders in liquid metal to control the microstructure of the solid?

TECHNICAL SUMMARY

In processes involving melting and solidification, the grain structure of a metal sample is known to be influenced by the temperature gradient (G) at the solid-liquid interface and the growth rate (R). A lower G/R ratio tends to favor the development of equiaxed grains, while a higher G/R ratio shifts the growth mode towards dendritic, cellular, and eventually planar structures.

This framework, rooted in classical nucleation-and-growth theory, has successfully explained the microstructures observed in many cast and welded materials. Recently, several research teams have observed a distinct cluster of fine equiaxed grains surrounded by larger columnar grains in Inconel 718 produced using the wire-laser directed energy deposition (DED) technique.

Since the formation of these fine grains under the associated processing conditions cannot be fully explained by classical solidification theory, this phenomenon has been described by the community as an abnormal columnar-to-equiaxed transition (CET). The objectives of this research are to understand the mechanism behind this abnormal CET in metals processed using wire-laser DED and to explore strategies to control this phenomenon.

State-of-the-art operando synchrotron X-ray experiments, along with complementary numerical simulations, will be conducted to investigate the influence of short-range orders in liquid metals and other dynamic structural attributes within the melt pool on the solidification behavior of complex alloys under non-equilibrium conditions. The insights gained are expected to inform and guide the design of alloys and the control of microstructures in wire-laser DED, as well as in other manufacturing processes with similar complexities.

Furthermore, this research is anticipated to fill the knowledge gap in nucleation theory by providing experimental evidence of non-classical nucleation pathways.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.