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The efficiency of the refining of molten aluminum when exposed to the melt, vibrations of ultrasonic frequency

The presence of metals and their alloys hundredths and even thousandths of a cent gas and non-metallic impurities significantly reduces their strength and ductility. To clean metals from unwanted impurities, gases, oxides, nitrides, and other nonmetallic inclusions developed the complex technological operations, which can combine the common term "refining". The process of refining is of great importance to improve the quality of metals and alloys.

Purification of molten metal from nonmetallic inclusions is to allocate to the melt surface with the smallest gas bubbles and particles of oxides, nitrides, sulfides and other compounds, which under normal conditions remain in the melt and fall into the ingot. In recent years, increasingly using combined methods of refining – adsorption and physical. In the refining absorption method in the melt is injected inert or active gases, and solids, easy to decompose into gaseous products. Due to the low pressure inside these gas bubbles to diffuse them dissolved in the metal hydrogen, nitrogen and other gases, and on the surface of the bubbles adsorbed solid particles of non-metallic inclusions. After reaching a considerable size bubbles refining substances float on the surface of the molten metal. For a fairly complete removal of nonmetallic inclusions from the melt must pass through a metal a large number of refining substances, which is not always appropriate and feasible.

In the physical refining method, in particular, the vacuum requires additional equipment and processing time of the metal.

Currently, ultrasonic impact methods for the metal in liquid phase become the most attractive and effective. Application of ultrasonic oscillations to impact on a number of technological processes for the obtaining and processing of metals and splavain is well known and theoretically justified. However, the practical application of the effect of ultrasonic degassing is currently associated with a number of unsolved problems and, primarily, is a way of introducing oscillations in the melt.

To address these problems, we created installation allows to influence the fluctuations of the ultrasonic range are in the stream of liquid metal, with adjustable intensity and different amplitude.

Below, in the illustrative example shown photographs of thin sections of the casting aluminum alloy life-size:

The sample without treatment
Samples treated with oscillations with a frequency of 18.5 kHz for 2 and 5 seconds

Photo No. 1 shows the sample without treatment, in photo 2, 3 and 4 show the samples treated with oscillations with a frequency of 18.5 kHz for 2; 5 and 8 seconds respectively.

As can be seen from the photos, the area of the formed bubbles after ultrasonic treatment for 2 seconds ranges from 3 to 5%, and the bubble size is not less than 0.5 mm in diameter. With increasing processing time the most part of bubbles coarsens and takes on the melt surface.

The gas bubbles reached a certain size, rise to the liquid surface, capturing nemetallicheskihs inclusions, which are located at the interface of liquid and gaseous phases. The existing methods of filtering molten aluminum, in particular through ceramic foam filters to resolve the question of the removal from the melt is obtained with this method of refining sufficiently large gas bubbles does not represent any complexity.

The degree of degassing of the melt is the most significant criterion to determine the effectiveness of refining. Decontamination is the reduction of gas content in the fluid as dissolved, and in the form of bubbles of different size. The main characteristics describing the process of degassing - the rate of change of concentration With gas-liquid dC/dt and the quasi-equilibrium concentration of gas CP', i.e. a constant concentration, which is installed in the liquid in the presence of ultrasonic field after a certain period of time.

The variation of gas concentration in a fluid in an acoustic field describe by the expression:

C = CP' + (- SR')e-n

where co is the initial concentration, t is time, R is the parameter determined by the acoustic characteristics of the sound intensity and frequency sound vibrations.

There are two modes of ultrasonic degassing: documetation and in the presence of cavitation. In the first case, the rate of change of concentration is proportional to the intensity of sound and its dependence on frequency, obtained on the basis of generalization of experimental data has the form: dC/dt = ~ht, where At is a constant characteristic of the fluid, h is the sound frequency, the value of CP' from the sound intensity and frequency is not affected.

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

y = (CP — CP')/CP

where CP is the equilibrium concentration in the absence of sound.

At a static pressure of 1 the atmosphere and 20°C the value of "y" is about 30%. With a decrease of the static pressure parameter "y" is growing and at a pressure of 0.5 ATM. up to 70%.

In the presence of cavitation, the rate of change of concentration is also proportional to sound intensity, but increases with the increase latest faster than documetation mode because cavitation accelerates the evolution of gas from liquids. The value of CP' at the same time saves a value corresponding to the specified conditions. Only at very high intensity levels of sound can be realized, this mode of oscillation of cavitation bubbles, in which a further increase in intensity causes a decrease in the speed of degassing.

Modern ideas about the mechanism of ultrasonic degassing associated with the assumption of the presence of fluid in the germ, in the form of stable gas bubbles with special properties, which provide them the opportunity of continued existence, even at high static pressures. In environments where solids are present (e.g. in liquid metals), gas phase contains also microscopic unevenness on their surfaces. When the intensity of sound that exceeds the cavitation threshold, can form a new "high" embryos arising from the collapse of the cavitation bubbles so that the total number of bubbles of germs increases dramatically. In the first stage of degassing, the gas bubbles oscillate in the sound field and increase their sizes due to diffusion of diluted gas.

The greatest diffusion flux inherent in those bubbles, the natural frequency of which coincides with the frequency of the sound, so depending on the frequency selection and the nature of the distribution of bubble size in the process of "pumping" in the bubbles dissolved in the liquid gas involved more or less. Thus, at this stage of degassing of the mechanism "one-way" or "directed", diffusion due to oscillations of the bubble.

ss="a-par-osn">Acoustic microflows accelerate such a mass transfer. When cavitation this process limits the growth of bubbles, inhibiting their collapse and thereby decreasing the formation of a new fragmentation of bubbles. So, when cavitation in the molten aluminum 2.5 period of a sound wave directed diffusion of hydrogen increases the pressure of the gas in the bubble more than four orders of magnitude.

Along with the diffusion of larger bubbles may be due to the merging pairs or groups of bubbles under the action of forces of hydrodynamic origin, the so-called Bjerknes forces. In the second stage ultrasonic degassing gas bubbles reached a certain size, go up to the liquid surface and stand out, aided in some cases entrainment of bubbles, acoustic currents and increasing the lift force due to the pressure of the sound radiation.

In addition, ultrasonic degassing of molten metal accompanied, usually, its purification, ie, relief from non-metallic solids that are floated by the gas bubbles and are displayed on the surface of the melt.

The works performed on the practical application of vibrations of ultrasonic frequency in the flow of molten aluminium, fully confirmed the theoretical considerations and convergence results are close to 100%.

Thus, with the implementation of our method, degassing, there is a real possibility of using a deeper cleaning of metal from non-metallic inclusions.

Application of ultrasonic degassing using our installation during casting of aluminum alloys, more than eight-fold reduces the concentration of hydrogen, which reduces the probability of occurrence in the finished product defects, such as porosity, delaminations, discontinuities in welds, etc.

The set-up allows for the processing of liquid metals, including chuguna and steel, almost any conditions – this applies to pouring into molds, and casting ingot casting and continuous casting of metal.