Mathematical Modeling of the Solidification Structure Evolution in the Presence of Ultrasonic Stirring

Abstract

LAURENTIU NASTACUltrasonic treatment (UST) was studied in this work to improve the quality of the cast ingots as well as to control the solidification structure evolution. 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. Modeling and simulation of casting solidification of alloys with UST requires complex multiscale 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 the Lilley model (that is founded on Lighthills's acoustic analogy), fluid flow, heat transfer equations, and mesoscopic modeling of the grain structure. The CFD analysis tool is capable of modeling acoustic streaming and ultrasonic cavitation. It is used in this work to study ingot solidification under the presence of ultrasound. The UST model was applied to low-temperature alloys including Al-and Mg-based alloys. Although the predicted ultrasonic cavitation region is relatively small, the acoustic streaming is strong and, thus, the created/survived bubbles/nuclei are transported into the bulk liquid quickly. The predicted grain size under UST condition is at least one order of magnitude lower than that without UST.

Synchrotron quantification of ultrasound cavitation and bubble dynamics in Al–10Cu melts

“…Cavitation is a multi-scale and multi-physics phenomenon [7,9,10,36,37]. It includes many elementary processes such as bubble nucleation, growth, collapse, travelling/movement, and so on.…”

“…Understanding of ultrasonic cavitation mechanisms and cavitation bubbles through experimental investigation can significantly contribute to reducing casting defects such as porosity through effective control of the dissolved hydrogen content [6]. In addition, such studies can provide insights on other cavitation-related phenomena such as fragmentation and deagglomeration [8][9][10], and for the validation of numerical models [9,10].…”

Numerical Modeling of Macrosegregation in Round Billet with Different Microsegregation Models

“…Favourable results have been obtained by applying the described models to quantitatively predict macrosegregation in cast alloys. [12][13][14][15][16] Solute redistribution occurring at a moving solid-liquid interface, calculated by the microsegregation model, is known to play a significant role in the modeling of mac-rosegregation. A few microsegregation models, including the Lever rule, Gulliver-Scheil equation (Scheil), Brody-Flemings (BF) model, Ohnaka model, Clyne-Kurz (CK) model and Voller-Beckermann (VB) model, are reported in previous works.…”

EXPERIMENTAL AND MODELING ANALYSES OF Al- BASED NANOCOMPOSITES MANUFACTURED VIA MOLTEN-METAL AND SOLIDIFICATION PROCESSING ASSISTED BY ULTRASONIC STIRRING TECHNOLOGY

“…The onset of cavitation for this alloy system is around 8.8e-06s [23,24]. The predicted ultrasonic cavitation region is presented in Figure 3a, where the cavitation phase is hydrogen [21].…”