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Efficiency of molten aluminum refining when the melt is exposed to ultrasonic frequency vibrations

The article was provided by our partner company DEMIN SRM GmbH (www.demin-srm.de).

The presence of hundredths and even thousandths of a percent of gaseous and non-metallic impurities in metals and their alloys significantly reduces their strength and ductility. To purify the metals from extraneous impurities of gases, oxides, nitrides and other non-metallic inclusions, a complex of technological operations has been developed, which can be united by the general concept of “refining”. The refining process is essential for improving the quality of metals and alloys.

Liquid metal purification from non-metallic inclusions involves the release of the smallest gas bubbles and particles of oxides, nitrides, sulfides and other compounds, which normally remain in the melt and enter the bar. In recent years, combined methods of refining – adsorption and physical ones – have been increasingly used. When refining by the adsorption method, inert or active gases are introduced into the melt, as well as solids that easily decompose into gaseous products. Due to the low pressure inside these gas bubbles, hydrogen, nitrogen and other gases dissolved in the metal diffuse into them, and solid particles of non-metallic inclusions are adsorbed on the surface of the bubbles. After reaching a significant size, bubbles of refining substances float to the surface of the molten metal. For a sufficiently complete removal of non-metallic inclusions from the melt, it is necessary to pass a large amount of refining substances through the metal, which is not always advisable and possible.

When refining through physical methods, e.g. by evacuation, additional equipment and time are required for metal processing.

Nowadays, ultrasonic methods of metals in the liquid phase treatment are becoming the most attractive and efficient. Ultrasonic vibrations application in metals and alloys production and processing is a well known and theoretically justified techniques. However, the practical application of the ultrasonic degassing effect is currently associated with a variety of unsolved problems and, first, it is a technique of vibrations introducing into the melt.

To solve these problems, we have created a device that makes it possible to influence by ultrasonic vibrations on the liquid metal in the flow, with an adjustable intensity and different vibration amplitude.

To illustrate this, we show the photographs of photos of life size aluminum alloy castings below:

Untreated specimen
Samples exposed to vibration treatment at a frequency of 18.5 kHz for 2 and 5 seconds
 

Photo 1 presents a sample without processing, photos 2, 3 and 4 show samples that were exposed to vibration treatment at a frequency of 18.5 kHz for 2; 5 and 8 seconds respectively.

As it can be seen from the above photographs, the area of the bubbles formed after ultrasonic treatment for 2 seconds ranges from 3 to 5%, and the size of the bubbles is not less than 0.5 mm in diameter. With an increase in the processing time, most of the bubbles grow larger and rise to the surface of the melt.

Gas bubbles that have reached a certain size rise to the surface of the liquid, capturing non-metallic inclusions which are located at the interface boundary of the liquid and gaseous phases. With the existing methods of liquid aluminum filtration through ceramic foam filters it is not a problem to eliminate sufficiently large gas bubbles from the melt.

Melt outgassing degree is the most indicative criterion for determining the refining efficiency. Degassing is reducing the gas content in a liquid, which is in it both in a dissolved state and in the form of bubbles of various sizes. The main characteristics describing the degassing process are the rate of change in the concentration C of the gas in the liquid dC/dt and the quasi-equilibrium concentration of the gas Cp’, i.e. the constant concentration that is established in the liquid in the presence of an ultrasonic field after a certain time period.

The change in the gas content in a liquid in an acoustic field is described by the expression:

С = Ср’ + (Со — Ср’)е-n

where Со is the initial concentration, t is time, р is the parameter determined by acoustic characteristics – sound intensity and acoustic frequency.

There exist two modes of ultrasonic outgassing: pre-cavitation and in the presence of cavitation. In the first case, the rate of concentration change is proportional to the sound intensity, and its dependence on frequency, obtained on the basis of experimental data generalization, has the form: dC/dt = B~ht, where B is a constant inherent in a given liquid, h is the sound frequency, Cp’ does not depend on the sound intensity and frequency.

The effect of acoustic vibrations on the steady-state concentration value is characterized by a dimensionless parameter:

у = (Ср — Ср’)/Ср

where Ср is equilibrium concentration in the absence of sound.

At a static pressure of 1 atmosphere and a temperature of 20 °C, the value of “y” is about 30%. With a static pressure decrease the “y” parameter also increases and at a pressure of 0.5 atm. reaches 70%.

In the presence of cavitation, the rate of concentration change is also proportional to the sound intensity, but it rises with an increase in the latter faster than in pre-cavitation mode, since cavitation accelerates the gas release from a liquid. The Cp’ retains its value corresponding to the specified conditions. Such a mode of cavitation bubbles oscillation can be realized only at very high sound intensity, in which a further increase in intensity causes a decrease in the outgassing rate.

Modern ideas about the ultrasonic outgassing mechanism are associated with the assumption of presence in the liquid of nuclei of stable gas bubbles with special properties enabling them long-term existence even at high static pressures. In environments where solid impurities are present (for example, in liquid metals), the gas phase is also present in the microscopic irregularities on their surfaces. At the exceeding the cavitation threshold sound intensity, new “fragmentation” nuclei can be formed, which appear when the bubbles collapse, so that the total number of nucleated bubbles increases sharply. At the first stage of degassing, gas bubbles vibrate in the acoustic field and increase their size due to the diffusion of the dissolved gas.

The greatest diffusion flux is inherent in those bubbles, the natural oscillation frequency of which coincides with the frequency of sound, therefore, depending on the choice of frequency and on the nature of bubbles distribution by their size, more or less of them are involved in the process of “pumping” into the bubbles of a gas dissolved in a liquid. Thus, at this stage of degassing, the mechanism of “one-way”, or “directed” diffusion exists due to the bubble’s oscillations.

Acoustic microflows accelerate this mass transfer. In case of cavitation, this process limits the growth of the number of bubbles, inhibiting their collapse and thereby reducing the formation of new “fragmented” bubbles. So, during cavitation in molten aluminum for 2.5 periods of a sound wave, directed hydrogen diffusion increases the gas pressure in the bubble by more than four orders of magnitude.

Alongside with diffusion, the growth of the bubbles size can be due to the merging of pairs or groups of bubbles under the action of hydrodynamic forces, the so-called Bjerknes’ forces. At the second stage of ultrasonic degassing, gas bubbles that have reached a certain size rise to the surface of the liquid and are released, which is facilitated in some cases by the bubbles entrainment by acoustic currents and an increase in lifting force due to the sound radiation pressure.

Moreover, ultrasonic degassing of molten metal is accompanied, as a rule, by its refining, i.e., liberation from non-metallic solid inclusions, which are floated by gas bubbles and brought to the surface of the melt.

Our work on the practical application of ultrasonic oscillations in a stream of molten aluminum fully confirmed the theoretical calculations, and the results are close to 100%

Thus, with the application of the degassing method developed by us, a real possibility of using a deeper metal cleaning from non-metallic inclusions appeared.

The ultrasonic degassing using our installation for aluminum alloys casting reduces hydrogen concentration in them by more than eight times, which reduces the likelihood of defects such as porosity, delamination, discontinuities in welded seams in finished products.

The created installation makes it possible to process liquid metals, including cast iron and steel in virtually any conditions – this applies to casting into forms and molds, and continuous casting of metal.