Ultrasonic degassing of aluminium alloys: Basic studies and practical implementation

Abstract

Ultrasonic processing is known to be an efficient means of aluminium melt degassing and structure modification with additional benefits of being economical and environmentally friendly. This paper reports on the kinetics of ultrasonic degassing and regassing of foundry aluminium alloys and on pilot-scale degassing trials. Efficiency of ultrasonic degassing is compared with conventional Ar rotary degassing. Direct measurements of hydrogen concentration in the melt by Foseco Alspek-H probe are used along with reduced-pressure test. The effects of ultrasonic processing on porosity are studied using 3D X-ray tomography.

Ultrasonic processing of molten and solidifying aluminium alloys: Overview and outlook

“…In this case the sonicated volume has time for the bubble release while the active phase of ultrasonic degassing happens in the other part of the treated volume. This scheme demonstrated successful degassing of a substantial volume of 150 kg of A356 melt with the degassing efficiency more than 30% achieved in 15 min [8,9]. Similar degassing efficiency was shown by the standard Ar rotary degassing (Foseco Degassing Unit) in the same volume and time.…”

“…This is obviously not the case as the kinetics of ultrasonic degassing and the level of residual hydrogen clearly show. Cavitation-assisted degassing is very quick (minutes) as compared with the natural degassing (tens of minutes) and the reached level is about 50% lower than the quasi-equilibrium hydrogen solubility [3,8,37]. The latter is evidenced by considerable regassing of ultrasonically degassed melt up to the quasi-equilibrium level [37].…”

“…It is important to note that such a low hydrogen concentration cannot be sustained for long and the melt will absorb hydrogen from atmosphere back to the quasi-equilibrium level. This process is called re-gassing and occurs rather quickly in a matter of tens of minutes [8].…”

“…Therefore, the technology should allow for the release of the bubbles. It was shown that intermittent ultrasonication with idle intervals is more efficient than the continuous ultrasonic degassing, and can achieve 90% decrease in hydrogen concentration (degassing efficiency) in relatively small volumes [2,8].…”

“…The actual quasi-equilibrium solubility of hydrogen is a function of humidity, temperature and pressure and varies from day to day, and from alloy to alloy. In most cases the concentration of hydrogen in liquid aluminium ranges from 0.2 to 0.8 cm 3 /100 g, while the desired concentration for producing sound castings is about 0.1 cm 3 /100 g. Ultrasonic degassing is capable of decreasing the amount of dissolved hydrogen to the levels 40-50% below the quasi-equilibrium concentration [3,8]. It is important to note that such a low hydrogen concentration cannot be sustained for long and the melt will absorb hydrogen from atmosphere back to the quasi-equilibrium level.…”