【作者】 蒋日鹏；

【导师】 李晓谦；

【作者基本信息】 中南大学 ， 机械工程， 2014， 博士

【摘要】 摘要：大规格铝合金铸锭成形成性困难是发展我国航空航天和武器装备用大型整体结构件所面临的技术瓶颈。对铝合金凝固过程施加超声波扰动是改善铸造缺陷、提高铸锭品质、提升材料性能的一个新的工艺途径。全面理解与系统掌握超声场对铝合金凝固过程的影响规律与作用机理是实现这一工艺的关键。然而,现有关于超声波处理熔体技术的研究大多以经验和现象描述为主,缺乏定量的表征与本质的揭示。针对于此,本论文以航空航天用高强高韧7×××系铝合金为研究对象,通过实验测试、理论推导、数值模拟等方法,对超声场作用下铝合金熔体的凝固行为及其产生机制进行了深入探讨。论文的主要研究内容与结论如下：1)进行了一系列铝合金熔体凝固传热分析实验。结果表明：施加超声场可以改善温度场分布的均匀性,并且能够明显加快熔体的整体凝固进程,尤其是显著提高固-液相区熔体冷却速率,使其相应的冷却时间缩短约2/3。而另一方面,超声辐射杆本身对铝合金熔体传热的影响非常微小,可以忽略不计。2)研究了不同工艺处理条件对铝合金凝固组织的影响规律。实验证明超声波对铸锭的细晶效果并非辐射杆端面的激冷形核所致,也与从辐射杆中分离出来的钛元素无关,而是主要归功于超声空化促成的形核增殖与谐振效应抑制了晶体生长。3)在归结实验现象的基础上,分别建立了压力过冷模型、毛细浸润模型与初生晶体谐振模型。通过计算与分析,全面揭示了超声场对铝合金熔体凝固过程不同阶段的主导作用机理。在结晶温度区段,超声主要通过空化效应,以压力过冷、异质活化促进熔体形核的方式,提高形核率,实现铸锭晶粒的细化；在固-液相共存温度区段,超声主要通过谐振效应,以晶体振幅增大、界面自由能级提高至相变驱动力减小的方式,控制晶体生长速度,实现铸锭晶粒的细化。通过进一步的计算发现铝合金熔体凝固体系中最易出现谐振的晶粒为1-10μm的初生晶体。4)开展了大规格7050铝合金铸锭的超声波外场辅助铸造工业实验,铸锭的规格分别达到了Φ550mm与1320mm×500mm。实验结果表明：多源超声场的协同作用可以促使铸锭全断面获得均匀的细化效果,晶粒度分别为1级(圆锭)和2级(扁锭),并且能够显著改善溶质元素的晶界偏析与宏观偏析,使其相对固溶率至少提高20%以上,同时还能提高铸锭的成品率与机械性能的均匀性。5)针对1320×500mm规格铝合金扁锭超声波半连铸工艺,以添加源项的方式,建立了声-流-热耦合的三维数值模型。通过Fluent软件与二次开发的子程序,实现对铝合金半连铸多源超声作用下的多物理场耦合过程模拟,获得了不同超声频率和振幅对宏观声场、流场与温度场的影响规律,初步确定了超声参数之间的匹配关系。这些研究工作将为推动铝合金超声波外场铸造技术的发展与工业应用提供工艺基础与理论参考。更多还原

【Abstract】 Abstract:The formation of the qualified structure and performance is difficult in the casting of large size aluminum alloy ingot, which is a technical bottleneck in the development of large-scale integrated parts required for aerospace and weapon equipment in our country. Exerting ultrasonic disturbance to the solidification of aluminum alloy is a novel process approach for improving casting defects, enhancing ingot quality and promoting material performace. A comprehensive understanding and systematic mastering about the effect rules and function mechanism of ultrasonic field on the solidification of aluminum alloy is the key to realize this process. However, most of the comtemporary study had been empirical and phenomenological rather than quantitative expression and essential revealment. Based on this, the7xxx series aluminum alloy with a high strength and high tenacity was taken as the research object in this dissertation. Through the experimental testing, theoretical modeling, numerical simulation and other methods, the solidification beheavior of the aluminum alloy treated with ultrasound and its generation mechanism were discussed throughly. The main research contents and conclusions are as follows:(1)A series of experiments were carried to analyze the heat transfer during the solidification of aluminum alloy. The results show that ultrasonic treatment (UT) can not only improve the uniformity of temperature field distribution, but also speed up the solidification process of the melt obviously, in especial, significantly increase the cooling rate of solid-liquid phase zone melt so that its corresponding cooling time was shortened by about2/3. But on the other side, the ultrasonic radiator itself has little efffect on the heat transfer of the aluminum alloy melt so it can be be negligible.(2)The aluminum alloy melt was treated with different process conditions to study the rules of solidification structure formation of the aluminum alloy. The experiments confirm that the grain-refinement effect of the ingot obtained with UT is neither resulted from nucleation caused by the chilling effect of the radiating face nor the titanium element separated from the radiator, but owes to the nucleation proliferation induced by ultrasonic cavitation and the growth inhibition of crystals caused by resonance effect.(3)Based on the summarization of experimental phenomena, the models for pressure-suppercooling, capillary infiltration and the primary crystal resonance were established respectively. The dominant function mechanism of ultrasonic field on the solidification of Al alloy melt at different solidification stages was revealed comprehensively through the mathematical calculations and theoretical analysis. Researches show that with UT in the temperature range of crystallization, cavitation effect plays a leading role in grain refinement by promoting the nucleation of Al alloy melt in the ways of pressure-supercooling and heterogeneity activation; While with UT in the solid-liquid phase coexistence temperature range, the resonance effect plays a main role in grain refinement by inhibiting the grain growth in the ways that the phase transformation driving force is reduced as the free energy level of crystal interface increases, which is due to the increase of crystal vibration amplitude. Moreover, through further calculation, it is found that primary crystals with a size1～10μm are prone to resonate among the grains in the solidification sytem of aluminum alloy..(4)The industrial experiments of semi-continuous casting of large size7050aluminum alloy ingot with UT were conducted successfully. The size of the ingot reached Φ550mm and1320mm×500mm. Results show that the cooperative treatment of multi-source ultrasonic field can not only implement a homogeneous grain refinement effect of the whole cross section in ingots of which the crystal degree was level1(billet) and level2(slab) repectively, but also can significantly improved the grain boundary segregation and macro-segregation of solute elements. The solution rate of the main alloy elements was increased by more than20%. Meanwhile, the yield of the ingots and the uniformity of mechanical properties are able to be improved.(5)For the technology of ultrasonic semi-continuous casting of aluminum alloy slab with a size of1320mm×500mm, a three-dimensional numerical model based on the coupling of acoustic field, flow field and temperature field was established through adding an extra source term. It was realized by Fluent soft and user subroutine that the coupling process of the multi-field in semi-continuous casting with multi-source ultrasonic field was simulated. The matching rules among the ultrasonic parameters were preliminarily set down from the simulation results of distribution characteristics of macroscopical acoustic fied, flow field and temperature field that obtained with UT of difference frequency and amplitude.These studies will provide technical base and theoretical reference for promoting the development of the casting technology of aluminum alloy with outer ultarsoncid fied and its industrial applications.更多还原