【作者】 汤孟欧；

【导师】 徐骏；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2012， 博士

【摘要】 控制金属材料的凝固过程、提高成形材料的冶金质量是材料科学领域研究的重要方向。半固态加工技术就是利用各种熔体处理方法实现对金属熔体凝固过程中固相大小、形貌和分布情况的控制。目前最常用的熔体处理方法是机械搅拌法和电磁搅拌法,机械搅拌法虽然能实现对熔体的强制均匀凝固,但存在污染金属熔体、熔体处理量小等问题。电磁搅拌法不增加机械装置与金属熔体的接触、减少了对金属熔体的污染、可精确控制搅拌参数、容易与其他工艺对接,因而率先实现工业化生产；然而由于存在趋肤效应,熔体受到的电磁力不均匀使得凝固组织整体一致性差,限制该方法在大尺寸铸件的工业应用。针对这一关键技术难题,本文在深入研究电磁搅拌技术的基础上,自行研制出环缝式电磁搅拌连续铸造技术原型,并采用A357合金在该装置上系统研究了工艺参数对宏观物理场和凝固组织的影响,实现了大体积熔体的强制均匀凝固,为拓展电磁搅拌技术在大体积高合金化铝合金铸造的工业应用提供了一种新的方法。在原有静态环缝式电磁搅拌装置基础上建立了制备A357铝合金半固态浆料过程宏观物理场的三维耦合计算模型,系统研究了环缝宽度和芯棒位置等几何因素对电磁搅拌过程磁场、流场和温度场的影响规律。发现对熔体的搅拌强度随着环缝宽度的减小而增加；随着芯棒向下移动,熔体下部分的轴向循环流被压缩,而上部分的周向流被扩展；这一现象表明,环缝宽度和芯棒位置在提高电磁搅拌作用,增强熔体紊流性质具有关键作用。进一步的优化实验结果表明,在环缝宽度20mm、芯棒位置0mm、搅拌电流17A、搅拌频率30Hz、搅拌时间10s的条件下,可以获得组织细小均匀的A357合金半固态组织。在静态研究基础上,通过建立环缝式电磁搅拌连铸A357铝合金坯料过程宏观物理场的三维耦合计算模型,深入系统地研究了连续冷却条件下,环缝式电磁搅拌对金属熔体的流场和温度场的影响规律,发现在足够的搅拌强度下,芯棒的位置对有效降低液穴高度、控制熔体的凝固过程具有重要的影响。在此基础上自行优化设计和建造了环缝式电磁搅拌连续铸造技术原型装置,其主要技术特征是：采用三相三对极搅拌器；环缝宽度为20-25mm之间,环缝位置处于半径3/4-4/5处,芯棒位置为0mmm。采用该装置铸造出直径200mmm、长度1500mm的具有明显半固态组织特征的A357铝合金坯料。对比试验表明,环缝式电磁搅拌连铸能降低坯料中宏观和微观偏析；其显微组织主要由蔷薇状组织和细小的圆形组织组成,初生相晶粒平均尺寸为122.2μ m。而普通电磁搅拌连铸和无电磁搅拌连铸坯的平均晶粒尺寸分别为257.9um和407.2μm。这一结果表明环缝式电磁搅拌连铸工艺技术实现了熔体分散-汇聚-连续均匀凝固一体化控制,在控制大体积熔体的均匀凝固、制备优质铝合金大直径坯料具有显著的技术优势和工业应用价值。为深入理解环缝式电磁搅拌对增加熔体结构均匀性、控制有效形核率的作用机理,基于热力学第二定律,分析和讨论了电磁搅拌对形核的影响,建立了电磁搅拌磁感应强度、熔体转速与临界晶核半径的数学模型。分析表明电磁搅拌可以减小临界晶核半径,且随着熔体旋转转数增加临界晶核半径减小；可以降低形核功,且.随着转数增加形核功降低；使形核率增加,且随着转数增加形核率增加；随搅拌转数增加界面浓度减低,晶粒密度增加,晶粒尺寸减小,形状因子变大。关键词：连铸；半固态；环缝式电磁搅拌；A357更多还原

【Abstract】 To improve metallurgical quality of metal materials by controlling solidification process is an important direction in materials science. The core of semisolid processing technology is to control the size, morphology and distribution of solid phase in the slurry by melt treatment. The most commonly used methods for melt treatment are mechanical stirring and electromagnetic stirring (EMS). Although mechanical stirring method is able to achieve uniform solidification even in a small volume of melt, there is pollution problem occurs due to the stirrer contacted with molten. It is difficult to dock with the follow-up process, and it is not conducive to bulk melt for uniform solidification. Electromagnetic stirring technology has been applied more broadly on a commercial scale due to its non-pollution, easy process control and continuous production. However, it has some shortcomings. For example, stirring force exerted in the slurry is larger in the external part but smaller in the inner one of the slurry because of the skin effect, which leads to inhomogeneous micro structures. In response to this key technical problem, this paper presents an annulus electromagnetic direct chill casting process to achieve bulk melt uniform solidification. The annulus electromagnetic stirring (AEMS) machine were self-designed and built. A series of experiments on casting A357aluminum alloy have been carried out to investigate the effects of AEMS parameters on the microstructure, and the effects of electromagnetic stirring on solidification. The main work and results are as follows:In this paper, the calculation models of the electromagnetic field、flow field and temperature field in semi-solid slurry preparation by AEMS were established, and effects of annulus gap width and centre pipe location on electromagnetic field、flow field and temperature field were analyzed. The simulation results showed that with annulus gap width decreases, the stirring intensity increases. As center pipe moves down the axial circulation flow is compressed, while the shear flow was expanded. Optimal match of shear flow and circulate flow is of key importance for realizing uniform temperature of bulk melt. The experiment results indicated that when annulus gap width is20mm, centre pipe location is0mm, stirring current is17A, stirring frequency is30Hz and stirring time is10s, semisolid slurry with high quality can be prepared. Based on above research, a comprehensive mathematic model was developed to describe the interaction of the multiple physics fields (electromagnetic field, fluid flow, heat transfer and solidification) during continuous casting with AEMS and optimize the design of continuous casting process with AEMS. Under enough stirring intensity, the location of center pipe can effectively reduce the liquid sump depth and temperature gradient. Technical prototype of continue casting process with AEMS was:three-phase with three pairs of poles (six coils) for stirrer; the annulus gap width of20-25mm; the centre pipe location is Omm and the annulus gap position is3/4-4/5.The experimental device was designed and built. The semi-solid ingots of A357aluminum alloy with diameter of200mm and length of1500mm were processed by continue casting with AEMS. The EDS spectrum analysis shows that the ingots with low macro-and micro-segregation were obtained. Comparison test show that average grain size of primary phase is122.2μm by AEMS. The average grain size of DC and EMS are407.2μm and257.9μm respectively. Spreading-Collecting-Whole controlling idea was achieved. The experimental results show that annulus electromagnetic stirring has significant advantages for industrial applications in bulk melt uniform solidification and preparation large diameter high-quality aluminum billet.Based on the second law of thermodynamics the effect of electromagnetic stirring on nucleation was investigated. A mathematical model between the magnetic induction intensity, rotation speed and the critical radius was developed. Analysis results show that under electromagnetic stirring, critical radius decrease, critical nucleation work decrease and nucleation rate increase. With rotation speed increase, critical radius decrease, critical nucleation work decrease and nucleation rate increase. With rotation speed increase, grain density increase, grain size decrease and shape factor increase.更多还原