半连续铸造铝合金宏观偏析的外场调控及其机理研究

Macrosegregation as a traditional problem has attracted more and more attention due to the increasing demand of large sized aluminum products. As known the formation of macrosegregation is related to the relative movement of liquid phase enriched with solute and solid phase with low concentration of solute (for the case of k<1). This project aims to control macrosegregation by the control of convection during DC casting process with the introduction of external physical fields. The static magnetic field is used to achieve the electromagnetic braking to reduce or eliminate natural convection. The alternating magnetic field is used to generate a forced convection. The rotor-stator type high shear device is used to generate intensive melt shearing to control the intensity and direction of the forced convection. DC casting process, reduction of natural convection, natural convection, forced convection with different intensity and flow direction, measurement of temperature field, separation of floating grains with mesh screen are major methods used in this project to study the macrosegregation behaviour under the influence of external fields, reveal the main macrosegregation mechanism, establish a model describing the macrosegregation behaviour during DC casting process. Based on this, a combined application of intensive melt shearing and alternating magnetic field is used to control the convection during DC casting process and realize the elimination or reduction of the macrosegregation in large sized ingot . The project has great theoretical and practical significance for the development of the theory macrosegregation control under the influence of external fields, improvement of the aluminum alloy ingot quality and the development of large-sized aluminum alloy for the application in the aerospace industries.

随着铝合金大规格产品需求的与日俱增，铝合金的宏观偏析作为一个传统问题，又重新受到重视。宏观偏析与富溶质液相和贫溶质固相相对移动有关(k<1)。本项目拟在半连续铸造过程中引入物理外场，实现流场的控制，从而获得外场对宏观偏析的影响规律。引入静磁场实现电磁制动，消除自然对流；引入交变磁场，实现强制对流；通过转子-定子型强剪切装置实现强制对流强弱和方向的控制。通过对流的抑制，对流的加强和强制对流方向的改变，温度场的测量，浮游晶的网筛隔离等措施研究不同外场下铝合金的宏观偏析行为，获得能够有效抑制宏观偏析的外场条件，揭示外场对宏观偏析的影响机理，建立一种描述外场作用下铝合金半连续铸造过程宏观偏析的模型。在此基础上，引入交变磁场和熔体强剪切处理复合外场实现半连续铸造铝合金大尺寸铸锭宏观偏析的减弱或消除。本项目对于发展外场作用下的宏观偏析控制理论、发展大尺寸航空铝合金具有重要的理论意义与实用价值。

宏观偏析显著降低组织、性能的均匀性，已成为限制大尺寸、高性能铝合金发展的重要因素。本项目设计并制备了熔体强剪切与电磁场作用下的工业级DC铸造装置，并将其应用于研究铝合金铸锭宏观偏析。系统研究了熔体强剪切、磁场、组合场、铸造速度、细化剂等参数对铸锭凝固组织和宏观偏析的影响规律。结合铸造过程中实时测温数据，分析了熔池内的温度分布及冷却特点，进而深入研究了不同条件下铸锭不同区域典型组织的形成机制，揭示了不同外场条件下晶粒细化机理，探讨了光亮晶的形成机制以及其对宏观偏析的影响。项目完成了预定研究内容，基本实现研究目标。.在DC铸造直径300mm的2024铝合金过程中施加熔体强剪切可减弱宏观偏析，减小心部光亮晶，并可使锭坯心部组织比边部更加细小，为大尺寸均质铝合金锭坯的制备提供了新的思路。实时连续测温结果表明，强剪切装置底部吸入纯液相时熔体强剪切仅对凝固前沿产生一定的扰动作用，可以有效减少光亮晶数量，但不会造成晶核的大量破碎与增殖。当底部吸入半固态熔体时才有晶粒破碎和增殖作用。熔池心部的强剪切作用还会使得大量晶核集中在铸锭心部，从而形成比边部更细小的铸态组织。.同时施加电磁场和强剪切可更好的细化凝固组织并提高组织均匀性，可在较高铸造速度下获得不开裂且组织均匀细小的锭坯，为DC铸造效率的提高和组织优化提供了新的解决方案。在组合外场作用下，直径300mm的2024铝合金锭坯的铸造速度可以提高至150mm/min。在添加细化剂条件下，施加强剪切和磁场同样可有效细化晶粒和提高横截面组织均匀性。单独施加外场能获得与添加细化剂相近的细化效果，为减少细化剂用量提供了可能。.本项目对铝合金DC铸造过程中熔池不同位置两相区的温度分布特点及晶粒形成机制的关系、光亮晶、宏观偏析和疏松的形成机理有了更深入的理解。课题组将继续就光亮晶、疏松、结晶相的形成机制进行深入研究，力求进一步提高铝合金DC铸锭冶金质量。