Cavitation mechanism of ultrasonic melt degassing

Abstract

The real melt always contains non-wettable fine inclusions which are potential nuclei for cavitation and degassing. This paper deals with the nature of ultrasonic degassing and the industrial application of a relevant technology. Keywords: acoustic cavitation; cavitation and degassing nuclei; ultrasonic degassingThe degassing of liquids and low-melting melts under action of ultrasound was among first effects revealed in the 1930s 1-4. We started own investigations on the mechanism of ultrasonic degassing of light alloy melts and began the industrial application of ultrasonicinduced degassing in the 1960s 3'5. These investigations showed that hydrogen could be efficiently removed from A1-and Mg-based melts only when the ultrasonic treatment is accompanied by developing cavitation. It was shown that the ultrasonic degassing of liquid metals differed essentially from that of aqueous solutions and organic liquids. This is due to the different nature of cavitation nuclei and, hence, different conditions required for the origination and development of acoustic cavitation.In the case of water and organic liquids, the cavitation nuclei are represented by solid inclusions and very fine gaseous bubbles. In contrast, only fine solid particles (mainly oxides, e.g. AlzO 3 in aluminium melts) can act as cavitation nuclei in metallic melts. Cavitation and degassing nucleiOwing to its nature, any metallic melt always contains a suspension of submicroscopic particles that are non-wettable by the melt and containing a gaseous phase in surface defects. This 'plankton' produces potential cavitation nuclei. The proportion of free hydrogen on the surface of these particles is very small, less than 0.1%. Nevertheless, this amount is sufficient to initiate cavitation. The transformation of cavitation bubbles into gaseous bubbles depends on the dynamics of cavitation bubbles and the diffusion-induced penetration of dissolved hydrogen into pulsing cavitation bubbles. Hydrogen bubbles thus formed can coarsen and, on reaching a certain size, float up to the surface of a liquid bath.Earlier, we have shown 5'6'8-x° that real melts always contain oxide particles that are non-wettable by the melt and adsorbed hydrogen in surface defects of these particles. Under ultrasonic treatment, the amount of these particles and the content of adsorbed hydrogen become sufficient for simultaneous cavitation and ultrasonicinduced degassing. Diffusion growth of cavitation bubblesThe diffusion growth of bubbles in an ultrasonic field is one of the most interesting features of acoustic cavitation in metallic melts. In the absence of the acoustic field, the gaseous bubble should slowly dissolve owing to the gas diffusion from the bubble to the melt 5'6. The situation changes dramatically when the surface of the bubble pulsates.Under cavitation ultrasonic treatment, the diffusion of gas occurs in a unique direction, from the liquid to the bubble. This directed diffusion, being superimposed with the usual static gas diffusion from the bubble to the liquid, can gr...

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

“…When an external force is applied, e.g. via ultrasound, and cavitation conditions are met above the threshold acoustic pressure amplitude, a bubble of a given size will form and grow, while smaller bubbles will tend to dissolve by diffusing gas back into the melt [6,7,35] that might be happened for some bubbles (not attached to the crucible surface) above the field of view in this experiment.…”

“…Hydrogen is the only known gas with a measureable solubility in molten aluminium or its alloys [32,33], and when the local pressure is altered or supersaturation conditions are met, gas bubbles can form, mostly on existing substrates in the melt [6,7,34], that grow due to the rectified diffusion of dissolved hydrogen into the bubble [6,7,28]. When an external force is applied, e.g.…”

“…Ultrasonic treatment has been employed in solidification processing to achieve structure control [4], grain refinement [5] and degassing [6]. Particularly, its application to light metal (Al and Mg-based) alloys has attracted great interest recently [7].…”

“…Particularly, its application to light metal (Al and Mg-based) alloys has attracted great interest recently [7]. The mechanisms of ultrasonic treatment include formation, oscillation and collapse of cavitation bubbles; thus promoting melt degassing, wetting and dispersion of solidification phases, including refinement of primary intermetallics and dendrite fragmentation [5][6][7][8]. 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].…”

“…The mechanisms of ultrasonic treatment include formation, oscillation and collapse of cavitation bubbles; thus promoting melt degassing, wetting and dispersion of solidification phases, including refinement of primary intermetallics and dendrite fragmentation [5][6][7][8]. 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].…”