Industrial Application of Ultrasonic Vibrations to Improve the Structure of Al-Si Hypereutectic Alloys: Potential and Li
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IT has been long recognized that high-intensity ultrasonic vibrations offer a promising tool to improve the solidified structure of metal, and therefore its mechanical properties. An example is application of ultrasonic treatment during casting of Al-Si alloys. There is a large body of literature which has shown that the introduction of ultrasonic vibrations into molten hypereutectic Al-Si alloys can significantly refine their solidified structure and improve their quality.[1–4] Hypereutectic Al-Si alloys are used in various automotive and aerospace applications due to their high wear resistance, low thermal expansion coefficient, and excellent strength-to-weight ratio. However, these alloys have low ductility, low fracture toughness, and limited workability, mainly because of the formation of coarse particles of primary silicon. Therefore, the ultrasonic treatment of hypereutectic Al-Si alloys is usually aimed at obtaining fine particles of primary silicon uniformly distributed throughout the metal matrix. The practical application of ultrasonic treatment in this area, however, faces a number of challenges three of which will be discussed here. Usually, ultrasonic vibrations are introduced into molten metals through an ultrasonic sonotrode, the tip of which is submerged in the melt. Typically during casting of hypereutectic Al-Si SEGREY KOMAROV, Professor, is with the Graduate School of Environment Studies, Tohoku University, 6-6-02 Aramaki Aza Aoba, Aoba-ku, Sendai 980-8576, Japan. Contact e-mail: komarov@material. tohoku.ac.jp YASUO ISHIWATA, Senior Researcher, is with the Casting Development Center, Nippon Light Metal Co., ltd., CDC, 161 Kambara, Shimizu-ku, Shizuoka 421-3297, Japan. IVAN MIKHAILOV, Engineer, is with the Department of Functional Nanosystems and High-Temperature Materials, National University of Science and Technology MISiS—, Leninskiy prospekt 4, Moscow 119049, Russia. Manuscript submitted December 29, 2014. Article published online March 6, 2015 2876—VOLUME 46A, JULY 2015
alloys, temperatures range from 973 K to 1073 K (700 °C to 800 °C), depending on the alloy composition. Thus, a sonotrode must be made of a refractory material, and remain chemically stable and elastic to efficiently transmit ultrasonic vibrations. Besides, because ultrasonic vibrations produce cavitation, the sonotrode must be resistant to cavitation erosion. This requirement is particularly important, since erosion of the sonotrode can significantly reduce its service life and cause contamination of the metal. Also, the sonotrode material must resist well both thermal shock and hightemperature oxidation because most casting processes are performed in batch or semi-continuous mode in air atmosphere. These requirements impose strict limitations on the choice of material required for use in the treatment of high-temperature molten metal. Thus, the first big challenge is to choose the proper material to manufacture the ultrasonic sonotrode. Several important aspects concerning the high-temperature ultrasonic sonotrode have been prese
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