超声场对铝合金凝固作用机制试验研究

【作者】 杨运猛；

【导师】 胡仕成；

【作者基本信息】 中南大学 ， 机械设计及理论， 2009， 硕士

【摘要】 功率超声形成的能量可以使物质的物理、化学、生物特征或状态发生改变,或者使其速率加快。通过在金属凝固过程施加功率超声可以改变金属凝固过程特性,细化组织,减少成分偏析,提高力学性能等。本论文重点研究了超声场对铝合金凝固过程作用机制及其对凝固组织的影响。首先论述了超声空化效应、声流效应以及超声场衰减特性,探索了功率超声对金属熔体凝固过程作用机制,分析了功率超声对铝熔体凝固过程热力学、动力学以及形核过程和宏观成分偏析的影响,为试验的开展及其结果的分析提供了理论基础。其次通过热分析方法研究了超声场对纯铝凝固过程特性影响。通过热分析方法分析了冷却曲线、冷却速率曲线,确定了金属临界形核、最大形核速率、形核结束以及凝固结束的特征温度点。实验结果表明随着施振超声功率的增加,形核时间t_N减小,且整个凝固时间t_S缩短,经过超声场处理的铝熔体温度场更加均匀,温度梯度减小。通过对动态凝固曲线分析表明纯铝熔体未经超声场处理时为逐层凝固方式,经过超声波处理后转变为体积凝固方式。另外研究了超声场对7050铝合金中Zn、Mg、Cu主要元素宏观分布的影响,在铝合金熔体凝固过程中施加超声场,合金元素的宏观分布趋于均匀,其中Cu元素的负偏析减弱,Mg元素的偏析大大减少,超声振动减小了铝合金中Zn、Mg、Cu元素的宏观偏析。再次分析了超声施振功率与细化晶粒尺寸关系。在铝熔体中导入不同功率超声场,其凝固组织均有显著细化,随着施振超声功率的增大,晶粒细化效果越好,但超声功率增大到一定程度后,晶粒进一步细化效果不再明显。试验证明在铝熔体凝固过程不同温度区间施加超声振动,温度过高、过低都将减弱细化效果,最佳施振温度区间为760～660℃,施振功率240W时平均晶粒尺寸可达67μm。最后对平均晶粒尺寸与超声功率二者分析表明超声功率与细化晶粒尺寸近似满足指数递减关系,为超声细晶技术在工业应用提供了试验依据。最后采用不同超声施振功率对7050合金进行处理,对比不同功率下沿超声传播方向铸件凝固组织的变化,发现铸锭纵向上随着距离的增加凝固组织的晶粒尺寸不断变大,超声细晶效果越来越差,当施振功率240W时细晶最大距离可达265mm。最后分析得出超声功率与细晶距离之间的数学关系式,这对功率超声应用于工业领域具有一定的指导价值。更多还原

【Abstract】 High intensity ultrasonic is based on physics、mechanics、material and so on.it is an applied technology that changes some aspects of objects or properties, or its speed is changed by ultrasonic energy. High intensity ultrasonic treatment is one of the effective ways to improve solidification struction、macro-segregation and mechanical properties of the metal in the process of metal solidification. This paper main study aluminum ultrasonic effect in the process of metal solidification was shown as follows:First of all, it is Describe the power ultrasonic cavitation, acoustic streaming and attenuation characteristics, The effect of power ultrasound on solidification process of melt metal mechanism and aluminum melt thermodynamics、kinetics as well as the nucleation and The macro-segregation of the elements were reserched in the end, which provided a academic basis for the proceeding of experiments and their results analysis.Secondly, The effects of ultrasonic treatment on solidification characteristics was studied bases on the thermal analysis method.The critical nucleus, maximum nucleation rate, characteristic points involving the final temperatures of nucleation and solidification have been determined on the basis of the thermal analysis cooling and cooling rate curves.The experiment results show that ultrasound field has a significant effect on the solidification process of pure Al. Increasing the power of ultrasound can decrease the cooling temperature is increase and decrease the nucleation time so as the overall solidification time is. solidification process mode of aluminum melt without ultrasound treatment is the layer-by-layer,under ultrasonic treatment tend to volume solidification model. and the temperature field can become more uniform and decrease temperature gradient with the application of ultrasonic field.Besides, 7050 aluminum alloy of Zn, Mg, Cu elements of macro-distribution under ultrasonic field were studied.the solute elements in 7050 alloy could be made homogeneous under ultrasonic field,the negative segregation phenomena of Cu was deleted and the segregation of Mg was decreased.The macro-segregation of the elements Zn、Mg and Cu in 7050 Al alloy was decreased with the application of ultrasonic field with different powers during solidification.In the present thesis, The casting ingot structure refining laws of the different ultrasonic powers was analyzed.With increasing of ultrasonic power,more refined structure can be obtained. However, when the ultrasonic power reaches a certain value,the effect of refining grains is not increased significantly.experiment results showed: different temperatures range of the aluminum melt applying vibration,tmperature too high, too low will weaken the effect of refinement,there is an optimal temperature range 760～660℃applying vibration, ultrasonic power 240W and grain size is 67μm .There is a law between ultrasonic power and the average grain size, meeting exponential relationships , which provide an experimental basis for industrial applications of ultrasonic Fine-grained technology.Finally,Using different ultrasonic power on solidification process of aluminum alloy, comparison of different power ultrasound propagation direction along the solidification structure changes in the sample and found that the grain size of solidification structure of continuous decline in fine-grained effect getting worse, Fine-grained distance up to 260mm when ultrasonic power 240W. Came to the conclusion that the mathematical relationship between ultrasonic power and Fine-grained distance in the end, It has certain guide value for power ultrasonic effective treating metal melt used in industrial filed.更多还原