Ultrasonic degassing

For sampling, bottle A is disconnected from the line, evacuated and, via VI, filled up with water. For analysis, bottle A is attached to the burette. Some water is pumped off into bottle E until bottle A contains approximately 1 litre of water. The vacuum of the head space in bottle A at this time is close to the water vapour pressure. Ultrasonic energy is applied over a period of 5 min. Once gas extraction is finished, the vacuum-degassed water in bottle B is added to the water sample in bottle A. Via VI, the water level rises into the burette until atmospheric pressure is reached. The volume of gas can be read on the burette scale [143]. 

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The rate was enhanced using ultrasound when carried out in a semi-sealed cell, which minimised the effects of ultrasonic degassing. An optimum acoustic power was found when using a source at a frequency of 20 kHz. 

Effect of Ultrasonic Degassing of a Melt on Properties of Shape Castings,... 

The improvement of the contribution of diffusion processes to the growth of cavitation bubbles as well as the concepts on the nucleation of cavitation centers in a metallic liquid allow one to consider the cavitation nature of a process of ultrasonic degassing of melts of aluminum and other nonferrous alloys. 

Considering the results of the ultrasonic degassing action on the melt of commercial aluminum in a wide range of ultrasonic intensity, one can note that the dependence given in Figure 11 is characterized by three specific regions. The formation of these regions may be interpreted as the initiation and development of ultrasonic cavitation in a liquid metal [48]. 

Region I, where no ultrasonic degassing occurs, may be called the region of precavitation modes of the treatment. Region II, where the efficiency of degassing initially sharply increases and then gradually stabilizes, refers to cavitation modes of the ultrasonic treatment. 

With further increase in the ultrasonic intensity and achievement of a certain degree of cavitation phenomena development, one can reveal Region III where the efficiency of ultrasonic degassing starts to increase linearly with the increasing ultrasonic intensity. This is the region of developed cavitation with wave resistance of the melt pccc/p0cQ 1.0. 

The analysis of the efficiency of ultrasonic degassing in relation to the modes of the UST allows one to conclude that the degassing threshold coincides with the cavitation threshold in liquid metals, which confirms the cavitation nature of the process of gas removal from a melt under action of powerful ultrasound [48]. 

Similar dependencies are inherent in processes of ultrasonic degassing of the majority of industrial wrought aluminum alloys as well as aluminum alloys for shape casting. 

One of the first successful attempts of ultrasonic degassing of aluminum alloys for shape casting was the development of the process of the ultrasonic treatment in a crucible with the stationary volume of up to 200 kg [3-5,49]. 

The data show that melt fluidity increases by 30% due to ultrasonic degassing of the melt, which is very important for a shape-casting production. Bondarek [18] has confirmed these results, according to his data the ultrasonic treatment in a 45-kg crucible of an AL9 grade alloy not only decreases the hydrogen content and volume proportion of nonmetallic inclusions but disperses the latter (Table 7). 

The temperature of a molten metal is of significant importance for the active occurrence of ultrasonic degassing. The higher is the melt temperature and the lower is its viscosity, p, the higher is the rate of acoustic streams and the easier is the process of gas bubble evolution. However, there is the optimum temperature—temperature increase above 750 °C adversely affects the efficiency of the process due... 

With a sufficiently high level of ultrasonic treatment intensity, the main parameter of the efficiency of the degassing process is its duration. From the industrial experience of operation of ultrasonic degassing setups of the UZD-200 and UZD-200M types, one can reveal the optimum period of the ultrasonic treatment of 50 to 250 kg of the melt ensuring 50%-effective degassing,... 

The efficiency of ultrasonic degassing in a melt flow corresponds to the completeness of the processes of cavitation nucleation, growth and evolution of hydrogen bubbles. So with increasing flow of a melt through an ultrasonic setup, the increase in a number of working ultrasonic sources and in the duration of the residence time of the melt in the cavitation region are required. 

The studies on the ultrasonic degassing of aluminum and its alloys melts [3—5, 48,49,52] show that the significant decrease in the content of hydrogen and oxides in the melt substantially influences basic physico-mechanical properties of the as-cast metal. 

As a result of the ultrasonic degassing of the melt, metal density increases and mechanical properties of castings and ingots are improved—mainly ductility, which is particularly important for loaded castings and for further plastic deformation of ingots. 

The comparison of a number of industrial methods of degassing [5] show that ultrasonic degassing is distinguished for its simplicity and efficiency of the improvement of casting quality (Table 8). 

The effect of ultrasonic degassing of liquid metal on the quality of ingots manifests itself by increased density, decreased coefficient of ultrasonic attenuation, and increased ductility at temperatures of plastic deformation. The data on the ductility of a flat-shaped ingot (1700 x 300 mm) from an AMg6 grade alloy at the temperature of hot deformation of 400 °C are given in Table 10. 

Table 10. Effect of Ultrasonic Degassing on Mechanical Properties of an AMg6 Alloy Flat Ingots at 400 °Ca...

Numerator - the conventional technique denominator - the ultrasonic degassing. 

Ultrasonic degassing is rather ineffective and only applicable for acetonitrile/water mixtures or if you have a really good pump. 

Ultrasonic degassing limits gas bubble accumulation at the electrode... 

Ultrasonic degassing also helps gas-evolving processes. 

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