功率超声对Pb-Sn合金凝固行为的影响

【作者】 冯伟骏；

【导师】 李喜孟； 谭家隆；

【作者基本信息】 大连理工大学 ， 材料学， 2004， 硕士

【摘要】 功率超声技术是以物理、机械、电子、材料等学科为基础的高新技术，它是以超声能量使物体或物性某些状态发生变化，或者使这种改变的过程加快的一门应用技术。与常规处理技术比较，功率超声处理技术具有许多特点，它常常能大幅度地提高处理速度和处理质量，完成一般技术所不能完成的工作。因此，功率超声处理技术在现代工业、农业、国防和医药卫生、环境保护等部门得到越来越广泛地应用。在金属凝固过程中施加功率超声处理技术，是改善金属凝固组织、提高材料机械性能的最有效方法之一。 在功率超声作用下的熔融金属凝固行为研究中，作者首先进行了超声处理实验系统的设计、计算，确定了变幅杆的材质和尺寸，分析了功率超声作用下熔体的声场特性和热效应，研究了功率超声对5％Pb-Sn合金凝固过程的影响。试验结果表明：在Pb-Sn合金凝固过程中导入功率超声，不仅显著细化了初生β相，使之均匀分布在基体上，而且细化了共晶组织(α+β)。超声功率从200W提高到600W时，细化程度明显。 本文从金属凝固过程的热力学和动力学角度初步探讨了功率超声的细化机理。当超声导入金属熔体后，凝固条件被改变，熔体中产生的空化泡将以极高的速度闭合或崩溃，从而在熔体内部局部微小区域产生瞬时高温、高压和强烈的冲击波。强烈的冲击波会击碎初生晶体，使之成为破碎的晶核，在声流的搅拌作用下，又使其弥散地分布于熔池中，相当于增加了外来核心，因而使晶粒得到细化。空化效应产生的高温和枝晶的温升使枝晶熔断，也增加了形核率，从而增加了晶粒数目，减少了平均晶粒尺寸。 另外，研究认为施振温度过高和过低都会明显影响超声波对凝固组织的细化作用。如果施振温度过高，一方面由于超声作用时间较长，加上媒质的吸声效应，则使熔体自身温度提高，造成细化效果减弱；另一方面，由于温度的升高，空化泡内的压力将增大，从而使空化强度减弱，细化效果不明显。如果施振温度过低，熔融合金粘度就会增大，致使声空化和声流效应发挥不了应有的作用。更多还原

【Abstract】 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.更多还原