Advances on Experimental and Multiscale Modeling of Ultrasonically-Processed Alloys and Nanocomposites

The usage of lightweight high-performance casting components including metal-matrix-nano-composites (MMNCs) is expected to increase significantly as automotive, military and aerospace industries are required to improve the energy efficiency and performance of their products. Al₂O₃ and SiC nanoparticles reinforced A356 matrix composite castings were fabricated via ultrasonic technology. The resulting as-cast MMNCs have superior microstructure characteristics with very low micro-porosity levels, uniform distribution of the nanoparticles within the matrix and significantly higher mechanical properties. Since the ultrasonic energy is concentrated in a small region under the ultrasonic probe, it is difficult to ensure proper cavitation and acoustic streaming for efficient dispersion of the nanoparticles without to determine the suitable ultrasonic parameters via modeling and simulation. The ultrasonic treatment has also significant effects on the as-cast A356 microstructures, which includes grain structure refinement and modification of the eutectic phase. The primary causes are due to ultrasonic cavitation and acoustic streaming. Molten A356 alloy was treated with high power ultrasonic then cast into a steel mold. Ultrasonically-induced cavitation consists of the formation of small cavities (bubbles) in the molten metal followed by their growth, pulsation and collapse. These cavities are created by the tensile stresses that are produced by acoustic waves in the rarefaction phase. The pressure for nucleation of the bubbles (e.g., cavitation threshold pressure) may decrease with increasing the amount of dissolved gases and especially with the amount of inclusions in the melt. The ultrasonically-stirred A356 alloy shows superior microstructures with very low micro-porosity levels.

Modeling and simulation of casting solidification of alloys with UST requires complex multi-scale computations, from computational fluid dynamics (CFD) macroscopic modeling through mesoscopic to microscopic modeling, as well as strategies to link various length-scales emerged in modeling of microstructural evolution. The developed UST modeling approach is based on the numerical solution of Lilley model (that is founded on Lighthills’s acoustic analogy), fluid flow, and heat transfer equations, and mesoscopic modeling of the grain structure. The CFD analysis tool is capable to model acoustic streaming, ultrasonic cavitation, and dispersion of nanoparticles. An efficient three-dimensional (3D) stochastic model for simulating the evolution of dendritic crystals during the solidification alloys in the presence of UST was developed. The model includes time-dependent computations for temperature distribution, solute redistribution in the liquid and solid phases, curvature, and growth anisotropy. 3D mesoscopic computations at the dendrite tip length scale were performed to simulate the evolution of columnar and equiaxed dendritic morphologies as well as columnar-to-equiaxed transition and then compared with predictions obtained with 2D mesoscopic computations. The 3D model can run on PCs with reasonable amount of RAM and CPU time and therefore no parallel computations are needed. It was observed that the 3D columnar dendritic morphologies look different than the 2D columnar dendritic morphologies. However, the predicted 3D equiaxed dendritic morphologies look similar to the 2D equiaxed dendritic morphologies.

BIOSKETCH: Dr. Laurentiu Nastac is currently an Associate Professor as well as a Key FEF Professor and the Director of the Foundry and Solidification Laboratory at the University of Alabama, Metallurgical and Materials Engineering Department, Tuscaloosa, AL, USA. For his teaching and research interests please visit his website: http://mte.eng.ua.edu/people/lnastac/. Dr. Nastac received the Diploma Engineering degree in Metallurgy and Materials Science from the University "Politehnica" of Bucharest, Romania in 1985 and the M.S. and Ph.D. degrees in Metallurgical and Materials Engineering from the University of Alabama, Tuscaloosa in 1993 and 1995, respectively. He has held various engineering, research, and academic positions in Romania and USA (1985-1996). At Concurrent Technologies Corporation (CTC) (1996-2011) he conducted research primarily in the area of advanced manufacturing processes with emphasis on the modeling and simulation of casting and solidification phenomena. In 2009, he was promoted to a CTC Fellow Engineer. In 1999, in recognition of his work on solidification of Ti and superalloy remelt ingots, he received the prestigious "Bunshah Best Paper Award" from the American Vacuum Society, Vacuum Metallurgy Division. More recently, he received the NMC (Navy Metalworking Center) achievement award and 2 CTC awards. Dr. Nastac developed 8 software tools, made over 150 presentations, co-authored 3 patents and over 200 publications in the materials science and manufacturing fields, and edited 6 books, one is a monograph titled “Modeling and Simulation of Microstructure Evolution in Solidifying Alloys” published by Springer New York in 2004 (http://www.springer.com/materials/special+types/book/978-1-4020-7831-6). He is a Key Reader for Met Trans A & B, a member of the Editorial Board of the International Journal of Cast Metals Research and of the ISRN Materials Science, and a member of the TMS Solidification Committee; he served in scientific committees and as an organizer for many prestigious international conferences dedicated to CFD modeling and simulation in materials processing field and for casting and solidification processing area.