高频振动下消失模壳型铸造镁合金组织性能控制机理

The requirement and properties of the complicated magnesium alloy precision castings are more and more increased with the rapid development of aerospace and automotive industries. The expendable pattern shell casting process is a precision casting method for manufacturing the complicated magnesium alloy precision castings using dry sand, vacuum and vibration compact as well as pouring of the thin-walled ceramic shell, and it combines the advantages of lost foam casting (LFC) and investment casting and can produce the complicated magnesium alloy precision castings with high properties. However, the magnesium alloy castings usually show coarser dendrite and poorer properties, resulting in the larger effect on the application of magnesium alloy castings. Therefore, it has become a key problem for taking effective measures to control the microstructure and properties of magnesium alloy castings. The high frequency mechanical vibration is applied in the solidification of the molten metal of the magnesium alloy in this study based on own characteristics of the expendable pattern shell casting technology to explore a new method in order to improve the microstructure and properties of magnesium alloy castings as the features of convenient, economical and effective. The specific research constents are shown as follows. Firstly, the heat transfer, mass transfer and filling characteristics of the molten metal of the magnesium alloy during the expendable pattern shell casting technology under high frequency mechanical vibration are investigated. Moreover, the evolution law and mechanism, influence factors and optimization design of the microstructure and properties of magnesium alloy are also investigated in the expendable pattern shell casting process under high frequency mechanical vibration. In addition, the characteristic of casting defects of magnesium alloy castings as well as formation mechanism of casting defects are also investigated. Finally, the control mechanism of the microstructure and properties of magnesium alloy in the expendable pattern shell casting process under high frequency mechanical vibration is obtained. The implementation of this study will lay the theoretical basis and technical support for the application of the expendable pattern shell casting technology under high frequency vibration solidification.

航空航天、汽车等行业的迅速发展对复杂镁合金精密铸件的需求越来越大、性能要求越来越高。消失模壳型铸造是一种生产复杂镁合金铸件的精密铸造方法，它利用干砂真空振动紧实、薄壁型壳浇注，综合了消失模铸造和熔模铸造的优势，可获得高品质复杂镁合金铸件。然而，镁合金铸件组织多呈树枝晶、力学性能偏弱，这较大影响了镁合金铸件的应用，采取措施改善镁合金铸件组织性能是扩大镁合金应用的关键问题。本项目结合消失模壳型铸造自身特点，将高频振动应用于消失模壳型铸造镁合金凝固过程，探索一种便捷、经济、有效的细化铸件组织、提高铸件性能的新方法。具体研究高频振动下消失模壳型铸造镁合金熔体传热、传质及充型特征，研究高频振动下消失模壳型铸造镁合金组织性能的演变规律及机理、影响因素及优化设计，研究镁合金铸件缺陷特征及其形成机制，研究镁合金组织性能控制机理。本项目将为镁合金消失模壳型铸造振动凝固新方法的实际应用奠定理论基础和技术支持。

航空航天、汽车等行业的迅速发展对复杂镁合金精密铸件的需求越来越大、性能要求越来越高。消失模壳型铸造是一种适合生产复杂镁合金铸件的精密铸造方法，然而，镁合金铸件组织多呈树枝晶、力学性能偏弱，较大影响了镁合金铸件的应用。本项目结合消失模壳型铸造自身特点，将高频振动应用于消失模壳型铸造镁合金凝固过程，研发了一种便捷、经济、有效的细化铸件组织、提高铸件性能的新方法。研发了一套消失模壳型铸造高频振动试验设备系统，研究了高频振动下消失模壳型铸造镁合金熔体传热、传质及充型特征，研究了高频振动下消失模壳型铸造镁合金组织性能的演变规律及机理，研究了镁合金铸件缺陷特征及其形成机制，最后进行了复杂镁合金铸件的浇注实践验证。研究结果表明：高频振动的施加明显加快了消失模壳型铸造中镁合金金属液的凝固速度，使得镁合金熔体中溶质的扩散更加均匀；高频振动明显提高了消失模壳型铸造镁合金的充型能力，不同浇注温度下相比于未振动充型能力均得到显著提高，且浇注温度越低，充型能力的提高幅度越大，在浇注温度为690℃时，充型长度提高了75.6%，充型面积提高了82.3%。随着振动频率和振幅的增加，镁合金的充型能力逐渐提高；高频振动明显改善了消失模壳型铸造镁合金的显微组织和力学性能，初生相和第二相的晶粒尺寸、形貌及分布均得到显著改善，初生相呈细小等轴晶均匀分布，第二相呈细小颗粒状弥散分布于晶界处。随着振动频率和振幅的增大，初生相和第二相的尺寸和形貌改善效果更加显著，镁合金力学性能和密度不断提高；随着试样壁厚增加，晶粒尺寸和形貌改善效果更加显著，镁合金力学性能和密度提高幅度不断增大；随着浇注温度的升高，晶粒尺寸增大，而树枝晶的数量减少，高频振动有利于树枝晶的消除，但浇注温度过低不利于振动发挥最佳效果；相比未振动条件下，初生相的晶粒尺寸减小了51%，形状系数增大了88%，第二相的长、宽和长宽比分别减小了79.6%、32.7%和69.7%，铸态镁合金的抗拉强度、屈服强度和硬度分别提高了60%，38%和59%；研究了高频振动消失模壳型铸造镁合金浇不足、冷隔、氧化膜、夹杂、孔洞等典型缺陷特征和形成机理；获得了具有充型完整、组织细小致密的复杂镁合金铸件，验证了该新方法的实际应用效果。本项目研究成果将为镁合金消失模壳型铸造振动凝固新方法的实际工业应用奠定理论基础和技术支持。