The Influence Of Power Ultrasound On Solidification Behavior Of Pb-sn Alloy

Power ultrasonic is a high technology based on physics, mechanics, electronics, materials and so on, and it is an applied technology that change some aspects of objects or properties, or speed this change by ultrasonic energy. Comparing with general technologies, power ultrasonic treatment has a lot of characteristics that can greatly advance process speed and quality, and accomplish what some general technologies cannot do. So it is applied more and more in modern industry, agriculture, national defence, medicine and environment protect. Power ultrasonic treatment is one of the effective way to improve solidification structure and mechanical properties of metals in the process of metal solidification.Experimental system of power ultrasonic treatment is designed. The material and size of Amplitude Transformer Horn and the Sandwich Piezoelectric Transducer are calculated. Sound field characteristic and heating effect in melts under ultrasonic treatment are discussed. Then the effect of power ultrasonic on solidification process of 5%Pb-Sn alloy is studied. The results show that the transmission of power ultrasonic during solidification process of Pb-Sn alloy not only can greatly refine grains of the primary & phase and the growing dendrite, and disperse it homogeneously, but also refine eutectic (a + B ) phase. Ultrasonic power is increased from 200 W to 600 W, the effect of refining particles is significant.According to the results, the mechanisms of grain refining with power ultrasonic are analyzed by thermodynamics and kinetics of solidification process. After imposing power ultrasonic on metal melts, solidification condition can be changed. Cavitation in melt will close or collapse at a very high speed, then a local zone with high temperature, pressure and powerful shock wave are formed. Powerful shock wave will break down the primary crystal and the growing dendrite, making them particle crystal. With the stirring of acoustic streaming, particle crystal is dispersed in melt, as is equal to increase the number of external nucleus. So crystal grains are refined. Dendrite is molten and broken, so that it also increases the number of nucleation and reduces the average size of paticles.Otherwise, the higher and lower melt temperature is disadvantageous to refinement and improvement on solidification process. When melt temperature is too high, the time of ultrasonic treatment is so longer as to result obvious heating effect, which make the structure coarse due to acoustic absorption. In the other side, the pressure in cavitation bubble will increase, which reduce the intensity of cavitation. On the contrary, when melt temperature is too low, the melt viscidity becomes intensive, so cavitation and acoustic streaming are in no effect.