Solidification Processing of Metallic Alloys Under External Fields by Dmitry G. Eskin & Jiawei Mi

Author:Dmitry G. Eskin & Jiawei Mi
Language: eng
Format: epub
ISBN: 9783319948423
Publisher: Springer International Publishing

(5.9)

where μ and μ′ are the shear and volume viscosities; a is the thermal conductivity; and cv and cp are the specific heats at constant volume and pressure, respectively. This dependence demonstrates that very high ultrasonic frequencies would be impractical for melt processing because of their strong attenuation (even without taking into account the shielding effect of the developed cavitation region).

In addition to that, nondimensional analysis of an ultrasonically treated aluminum melt showed that heat conductivity would be the dominant heat transfer process over convection, and the attenuation of the acoustic waves propagation (sound converts to heat) in this medium is significant [56]. On the other hand, the same report showed that in nonmetallic liquids that are good heat insulators the heat dissipation will be controlled by viscous forces (convection).

The interfaces between the liquid phase and suspended particles (nonmetallic inclusions, and crystals) may significantly affect absorption [57]. The attenuation factor increases with the amount of particles and with their fineness. Similar effect is produced by gas bubbles whose interfaces with the melt act as scattering sources. As we will show below, the very same interfaces of gaseous and solid inclusions act as cavitation nuclei and favor the development of cavitation, absorbing additional ultrasonic energy.

Cavitation development in water closely resembles that of liquid aluminum [57]. This allows researchers to experimentally investigate cavitation activity in water using advanced experimental equipment and techniques such as advanced cavitometers and particle image velocimetry (PIV) and subsequently feed numerical models to replicate and validate the cavitation development in liquid aluminum [53].

Tzanakis et al. [52] directly measured cavitation acoustic pressures in liquid aluminum using an advanced high-temperature cavitometer (Fig. 5.5). Results showed that shielding and acoustic damping are more pronounced in liquid aluminum, in contrast to a more uniform pressure regime measured in water. The extent of the cavitation zone was quantified in both tested liquids.

According to well-adopted views on the cavitation threshold , the tensile stress-induced disruptions in liquids are not governed by molecular forces, but rather by the presence of cavitation nuclei such as vapor and gas bubbles, solid gas-adsorbing suspensions, and hydrophobic inclusions.

The cavitation strength is related to the surface tension at the liquid–gas interface and the initial bubble radius. The viscosity μ also markedly influences the cavitation response of the liquid, increasing the cavitation threshold and the critical resonance radius of a cavitation bubble. The cavitation threshold or critical pressure is directly proportional to the ln(μ) [58] or to μ [59].

The dynamic behavior of a single vapor–gas cavity in an uncompressible liquid is described (neglecting gas diffusion to the cavity) by the Noltlingk–Neppiras equation [60, 61]:

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