铝合金多源超声波铸造凝固行为的数值模拟与工艺研究

Exerting ultrasonic perturbation in the process of metal solidification is an important way that not only can enhance the casting formability and homogeneity of structure, but also inhibit some cast defects, such as porosity, shrinkage cavities and segregation in ingots. So this technology is significant for the breakthrough of casting technology bottleneck in the development of high-strength, high-toughness, large-size and complex integral aluminum component for the equipments of aviation and weapon. Synergistic perturbation of multi-source ultrasound and adjustment of parameters are a key challenge to accomplish ultrasonic casting process for the large-size ingots. Aim to above problems, numerical simulation of multi-source ultrasound casting is carried out in this project as well as the technological test is studied at an industrialized pilot scale. Solidification behavior of aluminum alloy is verified under the synergistic action of multi-source ultrasonic fields. A multi-field coupling model that include multi-source sound field, flow field, temperature field and stress field is built. Macroscopic distribution characteristics are obtained by numerical simulation on sound, flow and temperature fields in the process of ingot solidification. Through the study, it could reveal the mechanism of action of multi-source ultrasonic fields on solidification of aluminum alloy and clarify the rule of energy transfer between various physical fields. The theory system within the micro scale is established, which based on the corresponding relation between micro-structure of ingot casting and external field conditions of crystal growth, such as the introducing position and pattern of multi-source ultrasound, vibration frequency and amplitude, etc. At last, results of numerical simulation is validated, amended and completed by industrial experiments. All of these would provide theoretical foundation and technical support for speeding up the industrial application of ultrasonic casting technology.

对金属凝固过程施加超声波扰动是提高铸造成形性和组织均匀性、抑制气孔、疏松、偏析与开裂等缺陷的一个重要途径。对于突破航空航天和武器装备所需的高强高韧铝合金大型复杂整体构件的铸造技术瓶颈具有重要意义。施加多源超声协同扰动与参数调控是大型铸件超声铸造工艺实现的关键难题。针对于此，本项目将开展大型铝合金铸锭多源超声波铸造数值模拟与工业化中试规模的工艺实验研究，探明多源超声场协同作用下铝合金的凝固行为，构建包含多源声场、流场、温度场、应力场的多场耦合模型。通过数值模拟，获得铸锭凝固过程中的声场、流场与温度场的宏观分布特征，揭示多源超声场对铝合金凝固过程的作用机理与不同物理能场间的能量传递规律，并在微观尺度内建立晶体生长的外场条件如多源超声的布控位置、方式、超声频率、振幅等参数与铸锭组织对应关系的理论体系，并开展工艺实验验证与完善数值模型，为超声波铸造技术的工业化运用提供理论依据与技术支持。

超声处理金属熔体具有细化晶粒、除气除杂、增强合金元素固溶等效果，可以作为突破大规格优质高强铝合金铸锭制备技术瓶颈的新工艺。然而在大规格铝合金半连续铸造过程中，多源超声波的导入、传播、协同作用机理及其对铸锭组织与成分均匀性的影响规律等研究较少。针对于此，项目以航空航天用铝合金为研究对象，开展多源超声处理熔体、辅助凝固成形工艺的数值模拟与实验研究。主要内容与结果如下：.1）对深液位Φ650mm热顶半连续铸造中超声声压场分布及不同超声空间布控方式下熔池内流场与温度场的分布进行了模拟仿真，结果表明：超声辐射杆声辐射区主要在端面以下，随着超声施振深度的增加，辐射杆侧面会出现多个不连续的小范围空化区域。.2）以添加源项的方式，建立了声-流-热耦合的三维数值模型，并对1100×400mm规格铝合金半连铸多源超声作用下的多物理场耦合过程进行了模拟，获得了不同超声频率、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。.3）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；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。.4）进行了Φ1250、1380mm规格2219合金热顶半连续铸造超声调控探索实验，结果表明：超声施加深度为200mm，距圆心R/2时，铸锭凝固组织在整个横截面的分布更加均匀并且细晶区域的面积增大，宏观偏析程度降低；超声施加深度为300mm，且施加位置由R/2向R/3改变时，可以进一步细化心部晶粒组织，降低心部正偏析程度。.本项目的研究工作将为推动超声波外场辅助铸造技术的发展与工业应用提供工艺基础与理论参考，为航空航天用大规格高品质的铸锭制造提供技术保障。