ABSTRACT
When casting aluminum alloys in ceramic and gypsum-asbestos molds with low thermal conductivity and even when using metal molds (chills), solidification proceeds at a considerably lower rate than in continuous casting. Therefore, the grain structure and properties of mold castings are inferior to that of ingots. However, many parts, such as turbine discs and fans can be fabricated only by precise casting that allows one to simultaneously form a bulky hub and thin-walled blades without machining. Some intricate shapes can also be obtained by chilling. Therefore, the control of the grain structure and properties of cast metal and the filling conditions for thin-walled blades with 0.1mm trailing edge was a scientific and practical challenge. This chapter demonstrates how this problem can be solved by the ultrasonic treatment of the solidifying melt. 245
7.1 Arrangements for the Insonation of a Solidifying Melt in Molds Two methods are regularly used in precise casting: pressurized solidification in autoclaves and vacuum suction of melt in the mold (Eskin et al, 1967). Ultrasonic tools are fitted in the existent casting equipment. These facilities are common in that they offer a relatively simple system for inserting the radiator into the central part of the mold.One system for precise casting is shown in Fig. 7.1(a); this installation was designed for combined casting with pressurized crystallization in an ultrasonic field. The flask is made as an autoclave with a sealed lid and a mass produced magnetostrictive transducer with an oscillatory system mounted in the bottom part. The prepared and dried mold inserted into the flask is put in the autoclave, as shown in Fig. 7.1(b). The ultrasonic generator is switched on, melt is poured into the downgate, the lid is rapidly closed, and the autoclave is pressurized. The length of ultrasonic processing is about one
minute, and the time spent under pressure is about five minutes. After depressurization the lid is opened and the mold and casting removed from the autoclave. In order to prevent the solidification of the metal in the gap between the radiator and the flask, the opening in the flask is covered with a graphite grease. Eskin and Ryzhenkova (1975) developed a modified system that allowed precise casting in ceramic molds up to 92mm diameter (Fig. 7.2). The acoustic decoupling is achieved by winding asbestos cord onto the cylindrical part of the radiator.Ultrasonic precise casting of aluminum alloys in industrial environment indicated that the best results could be achieved after double treatment of the melt-in refining and in crystallization. For this purpose, it is recommended to arrange separate locations for generators at the precise casting line, along with a panel for the distribution of acoustic energy to refining and mold casting installations.Figure 7.3 shows schematic diagrams of vacuum suction systems-ultrasonic magnetostrictive vacuum flasks-in which 50-100mm diameter turbine wheels are cast in gypsum - asbestos molds. These systems differ by the method of feeding metal in the mold: from the top or from the bottom upwards.
