Effectiveness of refining molten aluminum by exposing the melt to ultrasonic frequency oscillations
Presence of hundredths and even thousandths of a percent of gas and non-metallic impurities in metals and their alloys significantly reduces their strength and ductility. To clean metals from unwanted gas impurities, oxides, nitrides, and other non-metallic inclusions a set of technological operations has been developed that can be united by a common concept of "refining". The process of refining is vital in improving quality of metals and alloys.
Refining of liquid metal from non-metallic inclusions is performed by exudation of tiny gas bubbles and particles of oxides, nitrides, sulfides, and other compounds which normally remain in the melt and fall into an ingot, to the surface of melt. In recent years, combined methods of refining are becoming increasingly widespread — adsorptive and physical. Refining by adsorption method consists of injecting inert or active gases, and solids easily decomposed into gaseous products, into the melt. Due to low pressure within these gas bubbles, hydrogen, nitrogen and other gases dissolved in the metal, are diffused in them, and solid particles of non-metallic inclusions are adsorbed on the surface of the bubbles. After achieving considerable size, bubbles of refining substances float to the surface of the molten metal. A large number of refining substances have to be passed through the metal for sufficient removal of non-metallic inclusions from the melt, which is not always appropriate and feasible.
Physical refining method, and vacuuming in particular, require additional equipment and time for processing of the metal.
Currently, ultrasonic techniques of affecting metals in their liquid phase are the most appealing and effective. The use of ultrasound oscillations for affecting a number of processes in preparation and processing of metals and alloys is sufficiently well known and theoretically justified. However, practical application of ultrasonic gas removal effect, is faced with a number of unsolved problems, primarily — methods of introducing oscillations into the melt.
In order to solve these problems, we created an installation that can influence liquid metal in the flow with oscillations in the ultrasonic range, with adjustable intensity and varying amplitude.
Below are photographs of thin aluminum alloy castings in full size used as case examples:
Photo №1 shows a sample without processing, while photos 2, 3 and 4 show samples that have been subjected to oscillation processing with frequency of 18.5 kHz for 2; 5 and 8 seconds, respectively.
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Sample without rocessing |
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Samples subjected to oscillation processing with frequency of 18.5 kHz for 2 and 5 seconds |
As seen from the photographs, the area of bubbles formed after ultrasonication for 2 seconds ranges from 3 to 5%, and bubble size is not less than 0.5 mm in diameter. Increasing processing time leads to most of the bubbles becoming larger and moving to the surface of the melt.
Gas bubbles that reach a certain size, rise to the surface of the liquid, capturing nonmetallic inclusions, which are located at the threshold between liquid and gaseous phases. In existing methods of refining of liquid aluminum, in particular through ceramic foam filters, solving the problem of removing rather large gas bubbles produced in the melt during this refining process does not present any difficulty.
The degree of degassing is the most indicative criteria for determining the efficiency of refining. Degassing is the reduction of gas content in a liquid in dissolved state or in the form of bubbles of varying size. Main characteristics describing the degassing process are concentration change rate of gas C in liquid dC/dt and quasi-equilibrium concentration of gas Cp', i.e. constant concentration, maintained in the liquid in the presence of an ultrasonic field after a certain time interval.
Change of concentration of gas in the liquid in an acoustic field is described by the formula:
С = Ср' + (Со - Ср')е-n
where Со is the initial concentration, t — time, р — parameter defined by acoustic characteristics — sound intensity and sound oscillation frequency.
There are two modes of ultrasonic degassing: precavitation and in the presence of cavitation. In the first case the rate of concentration change is proportional to sound intensity, and its dependence on frequency obtained on the basis of experimental data generalization is written as: dC/dt=B ~ ht, where B is a constant characteristic of a given fluid, h is the sound frequency, and value Cp' does not depend on sound intensity and frequency.
Effect of acoustic oscillations on the steady concentration value is characterized by a dimensionless parameter:
у = (Ср — Ср')/Ср
where Ср is the equilibrium concentration in the absence of sound.
At static pressure of 1 atmosphere and temperature at 20°C the value "y" is around 30%. With a decrease in static pressure the parameter "y" is growing and at a pressure of 0.5 atm. it equals 70%.
In the presence of cavitation, concentration change rate is also proportional to sound intensity, but increases with the latter growing faster than in precavitation mode, since cavitation accelerates separation of gas from liquid. Cp' retains its value corresponding to the given conditions. Only at very high levels of sound intensity can such cavitation bubble oscillation mode be used, in which a further increase in the intensity causes a decrease in the rate of degassing.
Modern views on the mechanism of ultrasonic degassing are based on the assumption of the presence of nuclei in the form of stable gas bubbles in the liquid that have special properties that allow them to exist even at high static pressures. In environments with solid impurities (e.g. liquid metals), the gas phase is also present in microscopic irregularities of their surfaces. With sound intensity exceeding cavitation threshold, new "splinter" nuclei can form resulting from the collapse of the bubble, so that the total number of bubble-nuclei increases dramatically. During the first degassing step gas bubbles oscillate in the acoustic field and increase in size due to diffusion of dissolved gas.
The greatest diffusion flux is present in the bubbles, whose natural frequency coincides with the frequency of the sound, so depending on frequency choice and the nature of the bubble size distribution their larger or smaller numbers take part in the "transfer" process of gas dissolved in the liquid. Thus, at this stage of degassing a "one-sided" or "directed" diffusion mechanism is at work due to fluctuations in the bubble.
Acoustic micro-flows accelerate such mass transfer. With cavitation this process limits the increase in the number of bubbles, hindering their collapse and reducing the formation of the "splinter" bubbles. Thus, with cavitation in molten aluminum in 2.5 sound wave periods directional diffusion of hydrogen increases the pressure in the bubble by more than four orders of magnitude.
Along with diffusion the increase of bubble sizes may be due to fusion of pairs or groups of bubbles under the influence of hydrodynamic forces, called Bjerknes forces. In the second stage of ultrasonic degassing gas bubbles that reach a certain size, rise to the surface of the liquid and effervesce, aided in some cases by size increase through acoustic currents and increase of the lift due to acoustic radiation pressure.
Moreover, ultrasound degassing of molten metal is usually accompanied by its refinement, i. e. removal of non-metallic solid inclusions floated by gas bubbles and extracted to the surface of the melt.
Our work on the practical application of ultrasonic oscillations in the flow of molten aluminum fully confirmed the theoretical calculations, with matches close to 100%.
Thus, using our degassing method, there is a real possibility of applying a deeper cleaning of metals from non-metallic inclusions.
Application of ultrasonic degassing using our installation for casting aluminum alloys, reduces the concentration of hydrogen by more than eight times, thereby reducing the probability of occurrence of defects in the finished product, such as porosity, lamination, discontinuities in welds and the like.
This installation allows processing of liquid metals, including iron and steel, in virtually any environment — this also applies to fill in molds and casting into molds, as well as continuous casting of metal.
