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Reanalyzed Data Sheds Light on Solidification Process

Feb. 12, 2018
Materials science researchers have discovered a “bias field” that influences crystallization in molten metal

Most metalcasters and casting designers probably feel they understand solidification in their work or research — how metals transition from molten or fluid state into solid form, the variable factors that affect the results, and how to evaluate good solidification versus defective results. Recently two researchers announced they have made a “fundamental discovery” that may expand everyone’s understanding of the solidification process, with potential effects for foundries and diecasters.

Monitoring, or controlling, solidification is a central focus of activity in metalcasting, not only for foundry and diecasting operations but for cast-product designers, for suppliers of pouring and filtration devices, and for software engineers developing programs to simulate metal solidification.

Martin Glicksman, a research professor in materials science and the Allen Henry Chair at Florida Institute of Technology, believes his discovery will change the current understanding of how metals solidify and form crystalline patterns, which in turn may influence how to approach metalcasting to achieve better or more specific results. 

According to the standard view of solidification — a view shared by many scientists and professionals — pattern formation results from “random noise,” sound vibrations or other external influences acting upon solidifying material.

Glicksman, working with Kumar Ankit at the School of Matter, Transport and Energy at Arizona State University, reviewed data he collected two decades ago in NASA experiment involving repeated freezing and melting of high-purity materials in microgravity. The data reveals new information about naturally forming, complex patterns in materials that crystallize.

According to Glicksman, there is an energy field that influences all crystallizing substances. He calls this “the bias field,” and he contends that it is an empirical guide to the development of cellular and branching dendritic microstructures that form during solidification of most metals and alloys.

“In the last phases of melting,” Glicksman reported their observation, “needle-like crystals suddenly changed to spheres, and so for the first time ever, as we watched stationary particles melting in microgravity and observed their rather remarkable shape change, … There must be something more going on than just noise.”

Glicksman and Ankit reported that what they detected is “a subtle internal energy source” (i.e., the bias field) that modulates the speed of the solid/liquid interface on small scales, and ends up creating remarkably complex structures. Their finding has been confirmed theoretically and though advanced simulation methods.

“We were fortunate to perform experiments in microgravity, where the bias field idea was initially suggested to explain the occurrence of unusual melting patterns,” Glicksman stated. “Now we have a sound thermodynamic theory and proof to back that idea up.”

Glicksman and Ankit published their discovery proving the existence of bias fields in the journal Metals.

Because the metal solidification process produces branch-like internal micropatterns that disturb the chemical homogeneity of cast materials, a better understanding of the bias field’s role in their formation will aid casting manufacturers and designers to improve the results for their processes and products. The duo’s new insight also will be influential for welding and joining process technology.

“If we expect improvements in the structure of castings, weldments, and other solidification processes, we’ve got to know and apply the correct physics,” Glicksman stated. “This discovery potentially could lead to metallurgical process improvements.”