复杂截面中空镁合金型材分流模挤压变形机理与组织性能控制研究

Hollow magnesium profile with complex cross-section is an important component for achieving lightweight design in the fields of automobile, rail transit, aviation and aerospace. However, the basic mechanisms of plastic deformation, temperature rise, microstructure evolution during extrusion process of this kind of profile are still not comprehensively clarified. This research focuses on the porthole die extrusion of hollow magnesium profile with complex cross section. The deformation mechanism and material flow behavior during the whole extrusion process are investigated, and the multi-objective optimization system for dies and process parameters is established. The variation of temperature field during multi-cycle continuous extrusion is studied, and the scientific methods of the size and layout of liquid nitrogen channels are developed to control the temperature rise and temperature distribution on the cross-section of the profile. The microstructure evolution during extrusion, solution and aging processes are studied. The formation of inhomogeneous microstructure of the magnesium profile is clarified, and the intrinsic relationship between extrusion die, process parameter, microstructure and mechanical property is established. Based on the above research, it is aimed to propose the rational material distribution principle, the temperature and microstructure controlling methods, which can provide theoretical support for the dimension and microstructure control of the hollow magnesium profile with complex cross-section. This research is beneficial for improving the overall quality of hollow magnesium profile, and it can promote the theory of profile extrusion for lightweight material.

复杂截面中空镁合金型材是实现汽车、轨道交通、航空航天等领域轻量化的重要构件，但目前在其挤压变形机理、挤压温升规律、微观组织演化等基础科学问题上仍缺乏深入研究。本项目针对复杂截面中空镁合金型材分流模挤压成形，深入研究挤压全过程的材料变形机理和流动规律，建立挤压模具和工艺参数的多目标优化设计方法；探明多周期连续挤压过程温度场的变化规律，提出模具液氮冷却通道尺寸与布局的科学设计方法，实现对挤压温升和型材截面温度分布的有效调控；研究挤压和固溶时效过程镁合金微观组织的演变机理，揭示型材组织性能不均匀性的形成原因，阐明“工艺模具-微观组织-力学性能”之间的内在联系。在以上研究的基础上，提出材料合理分配原则、温度调控以及组织性能控制等方法，为实现复杂截面中空镁合金型材的控形、控性提供理论基础。项目的实施，有助于提高中空类镁合金型材的整体质量，对于丰富和发展轻合金型材挤压成形理论具有重要意义。

分流挤压是复杂截面中空镁合金型材的主要生产方式，但存在材料流动与挤压变形复杂、型材截面温度分布均匀性差、固态焊合和微观组织控制难度大等问题，严重影响了型材综合性能的提升。本项目阐明了镁合金高温大应变速率条件下的塑性变形机制，构建了高精度本构关系模型，为分流挤压过程数值建模提供了重要基础数据。揭示了复杂截面中空镁合金型材分流挤压过程的材料流动规律和变形机理，提出了材料体积合理分配原则和挤压工艺模具优化设计方法。获得了镁合金型材分流挤压条件下的温升规律，揭示了铸棒梯温加热对挤压温度场和型材截面温度分布均匀性的影响，实现了对挤压温升和型材截面温度分布的有效调控。揭示了镁合金型材分流挤压过程的微观组织演变规律，阐明了型材表层粗晶和纵向焊缝粗晶的形成机制，建立了挤压工艺模具、微观组织和力学性能之间的内在联系。阐明了镁合金型材固溶过程纵向焊缝区晶粒的异常长大机理以及时效析出行为，揭示了型材固溶时效强化机制，获得了最佳固溶时效热处理工艺。提出了室温预拉伸工艺方法，显著抑制了纵向焊缝区晶粒的异常长大，提高了型材组织均匀性及其力学性能。本项目的实施，丰富和发展了镁合金挤压成形理论，对实现复杂截面中空镁合金型材高效率、高质量、低成本的挤压生产具有重要意义。