Ultrasonic Processing

Ultrasonic Processing 

(Russian, ul’trazvukovaia obrabotka), the application of ultrasonic energy, usually with a frequency of 15-50 kilohertz, to substances in production processes.

The ultrasound may be generated by electroacoustic radiators or by whistles or sirens. The main component of a radiator is an electroacoustic transducer, which is generally magnetostrictive or piezoelectric in nature. The transducer is connected to a matching device, which transmits the acoustic energy from the transducer to the medium being treated or worked; the matching device provides the required dimensions of the radiating surface and the required intensity of the ultrasonic field. The matching device may be an expanding (usually for applications involving liquids) or narrowing (usually for applications involving solids) acoustic concentrator, or intensifier (seeACOUSTIC INTENSIFIER); some matching devices are resonant (tuned to a certain frequency) or nonresonant plates. Moreover, the matching device may function as a cutting tool or a tool of some other type in, for example, drilling, welding, or soldering. Sometimes transducers without a matching device are used, as when transducers are built into a pipeline.

The principal uses of ultrasound with respect to solids include the welding of metals, plastics, and synthetic fabrics; the machining of such materials as metals, glass, ceramics, and diamond by, for example, drilling, grinding, or engraving; and the pressure shaping of metals by drawing, stamping, and pressing.

Ultrasonic machining provides a high degree of precision and can be used to form not only ordinary round holes but also complicated holes and curvilinear channels (see). The feeding of ultrasonic energy to, for example, the drill or cutter of a conventional metalcutting machine intensifies the cutting action and reduces the size of the chips produced (seeVIBRATION CUTTING). In the pressure shaping of metals, ultrasonic vibrations improve the deformation conditions and reduce the force required. Ultrasonic surface hardening improves the micro-hardness and wear resistance of a material and reduces the roughness of its surface. In the processes described, the ultrasonic energy is usually transmitted to the drill, the rolls of the rolling mill, the punch of the press, the die, and so on through a concentrator.

The applications of ultrasound involving liquids are based primarily on the phenomenon of cavitation. Some cavitation effects, such as the streaming occurring in the liquid near the cavitation bubbles and the “water hammer” effect resulting from the collapse of the bubbles, are made use of in, for example, soldering, tinning, dispersion, and the cleaning of parts. Other effects, such as the heating and ionization of the vapors within the bubbles, are made use of to initiate and accelerate chemical reactions. To intensify the ultrasonic effects, processes are sometimes conducted at elevated pressures.

In the soldering and tinning of metals, such as aluminum, titanium, or molybdenum, the ultrasonic vibrations remove the oxide film on the surface of the metal and thereby facilitate the process. The use of ultrasonics makes it possible first to tin and then to solder ceramics, glass, and other nonmetallic materials. The ultrasonic energy is fed through a concentrator to a solder bath or to solder placed on the surface of the part.

Ultrasonic cleaning is the most effective method of removing metallic dust, chips, carbon deposits, grease and other contaminants from the surfaces of parts: no more than 0.5 percent of the contaminants remain after ultrasonic cleaning. Some parts having a complex shape or not easily accessible surfaces can be cleaned only by ultrasonic means. The cleaning is generally conducted in tanks with built-in electroacoustic radiators; the cleaning solution usually includes surfactants. To remove burrs from parts, abrasive particles are added to the cleaning solution; the presence of such particles leads to a severalfold increase in the rate of cleaning (seeVIBRATION PROCESSING).

The degassing of liquids is conducted at low ultrasonic intensities (usually below the cavitation threshold). The small gas bubbles suspended in the liquid move toward each other, coalesce, and float up to the surface (seeCOAGULATION). Degassing is applied to melts of optical glass, to liquid aluminum alloys (seeGASES IN METALS), and to other liquids. Ultrasonic techniques are also used in ore concentration by floatation; gas bubbles settle on the mineral particles and float, together with the particles, up to the surface.

Ultrasonic waves have a beneficial effect on the crystallization processes in metals during casting: the structure of the ingot and the ingot’s mechanical properties are substantially improved.

The ultrasonic generators in apparatus used to form emulsions are most often of the whistle or siren type. Suspensions are usually formed in apparatus with magnetostrictive transducers operating at elevated pressures (seeDISPERSION).

An aerosol can be formed from a thin layer of a liquid under the action of ultrasonic waves transmitted through a concentrator serving as an atomizer.

The action of ultrasonic waves on solutions of high polymers can cause substantial depolymerization. This property is made use of, for example, to synthesize various block and graft copolymers and to obtain valuable compounds of low molecular weight from natural polymers (seeMECHANOCHEMISTRY OF POLYMERS).

Ultrasonic processing accelerates many mass-exchange processes (such as dissolution, separation, and impregnation of porous solids) whose rate is limited by the diffusion rate. The effects of high temperature within the cavitation bubbles, the reduction of the thickness of the boundary layer, and the agitation of this layer also intensify chemical and mass-exchange processes, such as chemical absorption, that occur concurrently.

The exposure of gases to ultrasonic waves promotes the coagulation of aerosols and dust, that is, the agglomeration and precipitation of fine particles suspended in the gases. This effect is made use of in, for example, ultrasonic dust removal systems.

The generation of ultrasonic waves in a heated gas (a drying agent) accelerates the drying of porous solids. The ultrasonic energy has several effects conducive to drying. For example, it intensifies evaporation from the free surface of the liquid and gives rise to streaming in capillaries. Ultrasonic drying is usually combined with other drying methods, such as those using infrared radiation or high-frequency electric fields. The ultrasonic generators employed are sirens.

Ultrasonic processing encompasses many electrophysical and electrochemical processing techniques. The further development of ultrasonic processing depends not only on increasing the power output of ultrasonic devices and the volume of space that can be usefully exposed to ultrasonic radiation but also on acquiring a fuller understanding of the physical and physicochemical processes occurring in the ultrasonic field. The practical applications of ultrasonics are continually increasing. For example, ultrasonic techniques are now used in the food-processing industry to clarify wines and liqueurs and in the pharmaceutical industry to produce and sterilize various preparations.

REFERENCES

Fizika i tekhnika moshchnogo ul’trazvuka [book 3]. Moscow, 1970.
Ul’trazvukovaia tekhnologiia. Edited by B. A. Agranat. Moscow, 1974.
Khorbenko, I. G. Ul’trazvuk v mashinostroenii, Moscow, 1974.

S. L. PESHKOVSKII