压铸镁合金缺陷带形成机理及其超声辅助调控研究

Magnesium alloy die castings have been widely used in the automotive industry as load-bearing parts to satisfy the demands of lightweight, energy saving and environmental protection. The defect bands in the microstructure of cold-chamber die cast magnesium alloys do great harm to the mechanical properties of die castings, however, the formation, evolution and corresponding modification and regulation mechanisms of defect bands are still uncertain. In this project, systematic investigation will be conducted on the morphology and distribution of defect bands in magnesium alloy die castings. Based on the establishment of the relationship among the defect bands with externally solidified crystals (ESCs), gas and shrinkage porosities, process parameters and geometric structure of die castings, the solidification behavior of the melt with a certain amount of ESCs will be revealed under the interaction among the flow filed, temperature field and stress field during the high pressure die casting (HPDC) process. Consequently, the formation and evolution mechanisms of defect bands will be clarified. On the basis of the cognition of defect bands in this project, a process with ultrasonic treatment on the melt during the fast shot and pressure intensification phases of the HPDC process is proposed to control the formation of defect bands in magnesium alloy die castings. By investigating the effects of ultrasound on the flow filed, temperature filed, stress field, ESCs and the corresponding solidification behavior of the melt during the HPDC process, the modification and regulation mechanisms of ultrasound on the formation of defect bands, as well as the effect mechanism of ultrasound on the mechanical properties of magnesium alloy die castings will be revealed. The research works in this project have academic and practical significance on the solidification theories and practical HPDC process.

镁合金压铸件作为承力构件被广泛应用于汽车等领域以此满足轻量化、节能环保等需求。冷室压铸镁合金凝固组织中的缺陷带对铸件的力学性能具有较大的危害，然而其形成、演化及调控机理尚未明确。项目系统研究压铸镁合金缺陷带的组织形貌及分布特征，建立缺陷带与压室预结晶、气缩孔、压铸工艺参数及压铸件几何结构特征的对应关系，在此基础上揭示压铸过程流场、温度场、应力场等作用下含一定数量预结晶的合金熔体的凝固行为和机制，阐明缺陷带的形成及演化机理。基于本项目研究对缺陷带的认知，项目提出在压铸过程高速充填及增压阶段应用超声处理控制和改善压铸镁合金缺陷带的形成，研究超声外场对压铸熔体流场、温度场、应力场、压室预结晶颗粒及合金凝固行为的影响，揭示超声对压铸镁合金缺陷带形成的调控机理和力学性能的影响规律。研究工作对合金凝固理论和实际压铸生产具有重要的理论及实际意义。

作为压铸凝固组织中的典型组织特征，缺陷带对镁合金压铸件的力学性能具有重要的影响。本项目围绕压铸镁合金缺陷带的形成、演化及调控机理进行了深入研究，通过系统的压铸镁合金缺陷带组织表征，研究了压铸件不同结构形式、位置及压铸工艺参数对缺陷带形貌及分布的影响，阐明了缺陷带与压室预结晶组织和气缩孔之间的对应关系；通过建立适合镁合金枝晶生长的PF-LBM数值模型，仿真研究了不同温度场、流场（强迫对流与自然对流）下的镁合金枝晶生长行为，揭示了镁合金在凝固过程中的溶质偏析/溶质羽流形成机理及枝晶竞争生长机制；在缺陷带组织表征的基础上，结合压铸过程模拟仿真与理论分析，揭示了压铸过程流场、温度场、应力场等作用下含一定数量预结晶的合金熔体的凝固行为和机制，阐明了压铸镁合金双缺陷带的形成及演化机理，研究表明压铸件内层缺陷带的形成与压室预结晶在压铸件心部的聚集有关，而表层缺陷带的形成与压铸件表面激冷层有关；通过在压铸过程中采用超声辅助工艺，揭示了超声对压室预结晶及缺陷带形成的影响规律，研究得出超声可以一定程度上破碎并分散压室预结晶组织，从而改善内层缺陷带的分布特征；通过固溶、时效处理前后的显微组织表征、力学性能测试以及断裂失效机理分析，揭示了热处理对压铸镁合金AE44微观组织及力学性能的影响规律，研究得出固溶及时效处理后晶界处原本层片状的Al11RE3相发生了熔断和球化，导致合金强度有所下降，而延伸率显著提升，在拉伸过程中裂纹以沿晶和穿晶相结合的方式扩展。项目的研究工作对于丰富合金凝固理论以及实现镁合金压铸件组织与力学性能的调控具有重要的理论及实际意义。