非对称热挤压变形镁合金织构与塑性调控及织构形成机制研究

Wrought magnesium alloys are emerging as potentially good candidates for numerous applications, especially in transportation vehicles and lightweight enclosures for modern industrial products owing to their good properties, such as low density and high specific strength. However, the disadvantage of wrought magnesium alloys is that they generally exhibit limited ductility due to their hcp crystalline structure and strong basal texture at room temperature. Therefore, it is required to improve their poor ductility at room temperature. In the view of industrial practice, the thermo-mechanical treatment (TMT), such as hot extrusion and hot rolling is one of efficient forming technology to refine grain size and change crystalline textures, which are the key prerequisites for the microstructural capable of deformation. The main aim of the current project will be emphasized on the dynamic recrystallization (DRX) behavior and textural development of wrought magnesium alloy sheets fabracated by asymmetric hot extrusion process, which is expected to be result in a fine-grained microstructure and a weak basal texture. The evolution and formation mechanism of non-basal texture induced by asymmetric hot extrusion will be investigated by the measurements of macro and micro texture and analysis of microstructure. The physical model of dynamic recrystallization texture would be established based on the experimental results and theoretic analysis. The relationship between the mechanical properties, forming ability and microstructural characteristics inclding textures and grain size would be confirmed, which could offer the essential groundwork for controlling and optimizing the asymmetric hot extrusion process in order to improve the mechanical properties at room temperature in magnesium alloy sheet for industrial application.

变形镁合金作为一种新型轻质金属结构材料在现代制造业具有广阔的应用前景。采用传统热变形技术制备的变形镁合金通常会产生强烈的{0001}基面织构，该基面织构的存在是导致镁合金室温塑性低的重要原因之一。针对这一关键问题，本项目提出了非对称热挤压技术制备高塑性变形镁合金板材这一新的方法，即通过热挤压模具孔型的非对称设计，旨在热挤压变形过程中引入较大的切应力，使得镁合金在动态再结晶晶粒细化的同时弱化基面织构。利用细晶强韧化和织构韧化的双重耦合作用，提高镁合金的室温塑性。通过对比研究镁合金在对称和非对称热挤压变形过程中的动态再结晶行为、织构特征及其影响因素，着重揭示基面织构的形成与弱化机制，建立动态再结晶微观组织与取向（织构）形成相关联的物理模型，明确微观组织特征参量与综合力学性能（拉伸性能和成型性能）之间的关联系，丰富和发展镁合金动态再结晶理论，为制备高塑性变形镁合金提供理论与实验基础。

变形镁合金作为一种新型轻质金属结构材料在现代制造业具有广阔的应用前景。采用传统热变形技术(如热轧或热挤压)制备的变形镁合金通常会产生强烈的{0001}基面织构，该基面织构的存在是导致镁合金室温塑性低的重要原因之一。针对这一关键问题，本项目提出了非对称热变形技术制备高塑性变形镁合金板材这一新的方法，在实践中分别采用热挤压模具孔型的非对称设计和热轧进口端推升的方法，旨在热变形过程中引入较大的切应力，使得镁合金在动态再结晶晶粒细化的同时弱化基面织构，利用细晶强韧化和织构韧化的双重耦合作用，提高镁合金的室温塑性。本项目主要研究了（1）镁合金热变形动态再结晶基面织构的形成机制；（2）非对称热变形基面织构的弱化机制及力学性能；（3）强织构镁合金Hall-patch关系式与织构强度的依赖性；（4）强织构镁合金塑性变形的加工硬化机理。在对镁合金动态再结晶基面织构的形成机理的研究基础上，提出了镁合金基面织构形成的“基面织构残余”机制，该机制可以很好地解释镁合金传统热变形基面形成原理和最新提出的非对称热变形工艺（包括异步热轧、ECAP、FSP等）的基面织构弱化原理、具有普适意义。在此基础上，针对传统热轧工艺，提出了两段热轧加工方法，分别用以弱化基面织构和细化晶粒。模拟和实验研究结果均表明，通过非对称热变形可大幅度提高材料内部的剪切应力的比例，从而使得基面取向（或近基面取向）的晶粒得以发生动态再结晶，而动态再结晶后的晶粒取向为随机的，因而基面织构得以弱化，上述研究结果不仅丰富和发展镁合金动态再结晶理论，更为重要的是对于指导镁合金基面织构弱化工艺具有理论指导意义。