亚共晶铝铜合金凝固组织演变及其形貌形成机理的同步辐射研究

发布时间：2018-02-27 07:05

  本文关键词： 同步辐射 铝合金 凝固组织 枝晶生长 结构效应 细化剂　出处：《上海交通大学》2015年博士论文　论文类型：学位论文

【摘要】：微观组织控制是获得高质量、低缺陷铸件的关键所在,特别是对于具有复杂结构(比如变截面、T型结构、薄壁结构)的大型复杂薄壁件尤为重要。众所周知,热场、溶质场以及流场的分布是凝固微观组织形成的关键因素。本文基于热与质对合金结晶的影响,采用同步辐射方法研究凝固微观组织和多场分布的关系。结果显示了Al-Cu合金中的一种新的双胞晶生长模式,TiB2颗粒细化机理以及凝固过程中微观组织形成的特征。采用同步辐射成像技术和传统的电子背散射衍射(EBSD)技术研究了亚共晶Al-Cu合金中的枝晶形态及树枝双胞晶的生长位相。实验结果表明,新型双胞晶空间结构为二维结构,二次枝晶臂与主干成垂直生长。双胞晶形态可以等轴枝晶、柱状枝晶或与正常枝晶臂共存等形式存在。EBSD结果表明,树枝双胞晶尖端和二次枝晶臂的生长方向分别为110和001。树枝双胞晶的形成几率强烈地依赖于Cu浓度。随着合金浓度的逐步提高,双胞晶出现几率逐步提高,并在Al-20 wt%Cu时达到最高值。根据实验数据和理论计算的结果,修正了亚共晶al-cu合金动力学晶体生长形貌图,确定树枝双胞晶为分形枝晶和密集枝晶之间的过渡形态。利用同步辐射成像技术研究了上述所提到的特征结构中亚共晶al-cu合金微观组织的演化过程。根据成像照片呈现的吸收衬度差异与成像的时间相关性,分析了温度场、溶质场和流场对凝固组织演变的影响规律。结果表明,对于凝固空间存在空间结构变化的变截面结构和t型结构,生长界面前沿温度分布呈明显的结构相关性。不同的结构导致不同的溶质富集区域和熔体流动状态,从而造成不同的凝固顺序和组织特点。对于薄壁结构而言,壁厚是凝固组织演变的关键影响因素,提升浇注温度和铸模温度对改善薄壁结构的充填与补缩作用不大。冷却速度随壁厚减小而增加,等轴晶粒均匀度随壁厚减小而减小。为了进一步分析细化凝固组织对均衡组织的影响因素,本文利用x射线小角散射(saxs)研究了添加tib2颗粒对al-15wt%cu熔体结构的影响。研究结果表明,al-cu熔体中的团聚体表现为质量分形特征,且呈现松散的三维团絮状空间结构。在熔点到800℃范围内,tib2颗粒的加入能够破坏这种空间网链状结构,大幅减小熔体团聚体的尺寸。同时,同步辐射成像结果表明,tib2细化剂的加入,可以很好细化凝固组织,消除溶质富集造成的大面积共晶,有效改变凝固区域的溶质和流场分布,提高凝固组织的均衡性。综上所述,本文对亚共晶Al-Cu合金枝晶形态转变机制,特征结构处凝固组织演化规律以及细化剂抑制结构效应作用机理的研究,为调控铸件凝固组织提供了科学依据。研究结果为枝晶形态研究提供了有益补充。
[Abstract]:Microstructure control is the key to obtaining high quality, low defect castings, especially for large and complex thin wall parts with complex structures (such as variable section T structure, thin wall structure). The distribution of solute field and flow field are the key factors in the formation of solidification microstructure. The relationship between solidification microstructure and multi-field distribution was studied by synchrotron radiation method. The results show that a new two-cell crystal growth model, TIB _ 2 particle refinement mechanism, and the characteristics of microstructure formation during solidification of Al-Cu alloy are shown. The dendritic morphology and the growth phase of dendrite in hypoeutectic Al-Cu alloys were studied by synchrotron radiation imaging and traditional electron backscatter diffraction (EBSD) technique. The spatial structure of the new double cell crystal is two dimensional structure, the secondary dendritic arm grows perpendicular to the main stem. The morphology of the double cell dendrite can be equiaxed dendrite, columnar dendrite or coexisting with the normal dendritic arm. The results show that the secondary dendritic arm grows perpendicularly with the main dendritic arm. The growth direction of dendritic tip and secondary dendritic arm are 110 and 001 respectively. The formation probability of dendritic double crystal is strongly dependent on Cu concentration. According to the experimental data and theoretical calculation results, the morphologies of hypoeutectic al-cu alloys were modified. The transition form between dendritic dicellular crystal and dense dendrite is determined. The microstructure evolution of the meso-eutectic al-cu alloy mentioned above has been studied by synchrotron radiation imaging. According to the imaging radiation, the microstructure of the meso-eutectic al-cu alloy has been studied. The difference in absorption contrast presented by the film is correlated with the time of the imaging. The effects of temperature field, solute field and flow field on the evolution of solidification structure are analyzed. The temperature distribution at the front of the growth interface has obvious structural correlation. Different structures lead to different solute enrichment regions and melt flow states, resulting in different solidification sequence and microstructure. The wall thickness is the key factor influencing the evolution of solidification structure. Raising the pouring temperature and casting mold temperature has little effect on improving the filling and shrinkage of thin-walled structure. The cooling rate increases with the decrease of wall thickness. The uniformity of equiaxed grains decreases with the decrease of wall thickness. In this paper, the effect of adding tib2 particles on the structure of al-15wt%cu melt has been studied by means of X-ray small angle scattering. The results show that the aggregates in the melt of al-cu show the fractal character of mass. In addition, in the range of melting point to 800 鈩，

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