The presence of gases and non-metallic impurities in metals and their alloys, even in hundredths or thousandths of a percent, significantly reduces their strength and plasticity. To purify metals from undesirable impurities such as gases, oxides, nitrides, and other non-metallic inclusions, a complex of technological operations has been developed, which can be collectively termed “refining”. The refining process is of great importance for improving the quality of metals and alloys.
The purification of liquid metal from non-metallic inclusions involves the extraction of the finest gas bubbles and particles of oxides, nitrides, sulfides, and other compounds onto the surface of the melt, which under normal conditions remain in the melt and end up in the ingot. In recent years, combined refining methods, both adsorption and physical, have been increasingly used. In adsorption refining, inert or active gases are introduced into the melt, as well as solid substances that readily decompose into gaseous products. Due to the low pressure inside these gas bubbles, dissolved hydrogen, nitrogen, and other gases diffuse into them, while solid particles of non-metallic inclusions adsorb onto the surface of the bubbles. After reaching significant sizes, the bubbles of refining substances rise to the surface of the molten metal. For 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 feasible or possible.
In refining by physical methods, such as vacuum treatment, additional equipment and time for metal processing are required.
Currently, ultrasonic methods of influencing metal in the liquid phase are becoming the most attractive and effective. The application of ultrasonic vibrations to various technological processes in the production and processing of metals and alloys is well known and theoretically justified. However, the practical application of the ultrasonic degassing effect is currently associated with a number of unresolved problems, primarily related to the method of introducing vibrations into the melt.
In order to address these issues, we have developed an installation that allows for the application of ultrasonic frequency oscillations to the liquid metal stream, with adjustable intensity and various amplitudes of oscillations.
Below, as a visual example, are photographs of ground aluminum alloy castings in their natural size:
On photo No. 1, an untreated sample is shown, while photos 2, 3, and 4 depict samples subjected to vibrations at a frequency of 18.5 kHz for 2, 5, and 8 seconds, respectively.
As can be seen from the photographs, the area of the bubbles formed after ultrasonic treatment for 2 seconds ranges from 3 to 5%, with the bubble size being no less than 0.5 mm in diameter. With increasing treatment time, most of the bubbles coalesce and rise to the surface of the melt.
Gas bubbles, once reaching a certain size, ascend to the liquid surface, carrying non-metallic inclusions located at the interface between the liquid and gaseous phases. With existing methods of aluminum liquid filtration, particularly through foam ceramic filters, addressing the removal of sufficiently large gas bubbles generated by this refining method poses no significant challenge.
The degree of melt degassing is the most indicative criterion for determining the effectiveness of refining. Degassing refers to the reduction of gas content in the liquid, present both in dissolved form and as bubbles of various sizes. The main characteristics describing the degassing process are the rate of gas concentration change in the liquid dC/dt and the quasi-equilibrium gas concentration C’, i.e., the concentration constant established in the liquid in the presence of an ultrasonic field after a certain period of time.
The change in gas concentration in the liquid under acoustic field conditions is described by the expression:
C = C’ + (C0 – C’)e^-nt
where C0 is the initial concentration, t is time, n is the parameter determined by acoustic characteristics – sound intensity and frequency of sound oscillations.
Two ultrasonic degassing modes are distinguished: precavitation and cavitation. In the former case, the rate of concentration change is proportional to the sound intensity, and its dependence on frequency, based on experimental data generalization, is expressed as: dC/dt = B ~ ht, where B is a constant inherent to the liquid, and h is the sound frequency; the value of C’ does not depend on sound intensity and frequency.
The influence of acoustic oscillations on the equilibrium concentration is characterized by the dimensionless parameter:
u = (C – C’)/C’
where C’ is the equilibrium concentration in the absence of sound.
At a static pressure of 1 atmosphere and a temperature of 20°C, the value of “u” is approximately 30%. With decreasing static pressure, the parameter “u” increases, reaching 70% at a pressure of 0.5 atm.
In the presence of cavitation, the rate of concentration change is also proportional to the sound intensity but increases more rapidly with increasing intensity than in the precavitation mode, as cavitation accelerates gas release from the liquid. The value of C’ remains constant, corresponding to the specified conditions. Only at very high levels of sound intensity can a mode of cavitation bubble oscillations be realized, where further intensity growth leads to a decrease in degassing rate.
Modern Concepts of Ultrasonic Degassing Mechanism are associated with the presence of embryos in the liquid, in the form of stable gas bubbles possessing special properties that allow them to exist for a long time even at high static pressures. In environments where solid impurities are present (e.g., in liquid metals), the gas phase is also contained in microscopic irregularities on their surfaces. With sound intensity exceeding the cavitation threshold, new “fragmentary” embryos may form due to bubble collapse, resulting in a sharp increase in the total number of embryo bubbles. During the initial degassing stage, gas bubbles oscillate in the acoustic field and increase in size due to the diffusion of dissolved gas into them.
The greatest diffusion flow is inherent to those bubbles whose own oscillation frequency coincides with the sound frequency. Therefore, depending on the selected frequency and the nature of the bubble size distribution during the “transfer,” a greater or lesser number of bubbles containing dissolved gas in the liquid participate in the process. Thus, at this stage of degassing, a mechanism of “one-sided” or “directed” diffusion operates, driven by bubble oscillations.
Acoustic microcurrents accelerate this mass exchange. During cavitation, this process limits the growth of the bubble population by inhibiting their collapse and reducing the formation of new “fragmentary” bubbles. For example, during cavitation in molten aluminum, directed diffusion of hydrogen increases gas pressure in the bubble by more than four orders of magnitude over 2.5 periods of the sound wave.
In addition to diffusion, bubble growth can be caused by the coalescence of pairs or groups of bubbles under the action of hydrodynamic forces, known as Bjerknes forces. During the second stage of ultrasonic degassing, gas bubbles reaching a certain size rise to the liquid surface and are released, often facilitated by the entrainment of bubbles by acoustic currents and the increase in buoyancy due to sound pressure.
Moreover, ultrasonic degassing of molten metal is usually accompanied by its refining, i.e., the removal of non-metallic solid inclusions, which float with gas bubbles and are expelled to the surface of the melt.
Our practical application of ultrasonic frequency vibrations in molten aluminum flow fully confirms the theoretical calculations, with the agreement of results approaching 100%.
Thus, with the application of our developed degassing method, a real opportunity has arisen to achieve a deeper purification of metal from non-metallic inclusions.
The use of ultrasonic degassing with our installation in casting aluminum alloys reduces hydrogen concentration in them by more than eight times, reducing the likelihood of defects such as porosity, delamination, inconsistency in welded seams, etc., in finished products.
The created installation allows for the processing of liquid metals, including cast iron and steel, under virtually any conditions – this applies to casting into molds, pouring into ingots, and continuous metal pouring.
