Ultrasonic Processing of Materials
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ULTRASOUND waves, like all sound waves, consist of cycles of compression and expansion. When ultrasound is applied in a liquid, the compression cycles exert a positive pressure on the liquid and the expansion cycles exert a negative pressure. The pressure increases with increasing intensity of ultrasonic vibration. The injection of high-intensity ultrasonic energy in liquid is capable of producing unique nonlinear effects such as cavitation and acoustic streaming.[1,2] Cavitation, or the formation of small cavities in the liquid, occurs as a result of the tensile stress produced by an acoustic wave in the rarefaction phase. These cavitation cavities continue to grow by inertia until they collapse under the action of compressing stresses during the compression half-period, producing high-intensity shock waves in the fluid. Acoustic streaming is a kind of turbulent flow that is developed near various obstacles (interfaces) due to energy loss in the sound wave. These nonlinear effects can be utilized for the processing of molten metals and for the improvement of the solidification structure for ingots and castings. Ultrasonic vibrations also produce oscillating strain and stress fields in solid materials. Experiments have shown that ultrasonic vibrations above a critical intensity increase the concentration of dislocations and point defects in solids.[1] The density of the defects increases with increasing intensity and time of ultrasonic vibrations. When the density of the dislocations is high enough, an alignment of dislocations occurs, forming small grains in metals and alloys. These alternating stresses and strains induced in solid materials using ultrasonic vibration can be utilized for assisting solid metal forming processes. QINGYOU HAN, Professor, is with the School of Engineering Technology, Purdue University, 401 N. Grant Street, West Lafayette, IN 47906. Contact e-mail: [email protected] Manuscript Submitted October 9, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
In the past half century, high-intensity ultrasound has received much attention by scientists all over the world, especially in the former Soviet Union. Numerous applications have been tested including, for instance, ultrasonic cleaning, welding, machining, ultrasonic atomization, and metalworking. In the past 10 years, the U.S. Department of Energy (DOE) and Oak Ridge National laboratory have been sponsoring a number of projects investigating the use of high-intensity ultrasonic vibrations for material processing, including degassing of molten aluminum, grain refining of alloys during solidification, modifying microstructure during welding, and forming nanostructures in solid metals. Industrial companies are also interested in these areas as well as areas such as processing of particulate-reinforced metal matrix composites (MMCs) and ultrasonic-driven shot peening. This article describes some of the research results obtained in my group on ultrasonic processing of materials.
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ULTRASONIC DEGASSING
Gas porosity is one of the major defects in a
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