基于实时成像的HCP结构材料搅拌摩擦焊材料流变行为研究

The material flow during friction stir welding is crucial to obtaining sound joints. However, due to the invisibility of material flow during friction stir welding, most studies only can predict the material flow pattern by final position of tracer. The visualization of material flow in detail is still limited. As a result, the influential factors of material flow and regulatory mechanism during friction stir welding is still not fully understood. Therefore, in present study, the material flow during friction stir welding of HCP structure materials represented by magnesium alloy will be systematic studied by the X-ray transmission real-time imaging systems, which is an in-situ three-dimensional visualization technique for the material flow research during friction stir welding. The influence of welding parameters, the microstructure of the base material and the geometric of the rotation tools on the velocity/range of material flow and the stress state of the material will be better understood. By the above analysis, the relationship of the material flow, temperature, microstructure and mechanical properties of the joints will be revealed. The defects formation mechanism and non-homogeneous characteristics during friction stir welding also can be studied by the above analysis. In addition, based on the understanding of the plastic deformation behavior during friction stir welding, the regulatory mechanism of material flow during friction stir welding of HCP structure materials will also be investigated. The accurate understanding on the material flow and deformation mechanism during friction stir welding will be beneficial for guiding the design of tools, optimization of the welding parameters and microstructure control of joints.

搅拌摩擦焊接过程中的材料流变行为是获得无缺陷焊缝的关键。但由于搅拌摩擦焊接中材料流动的不可见性，多数研究仍仅基于示踪粒子的焊后终态位置来推断其过程中的材料流动，缺少对材料流变行为直观的研究。因此，搅拌摩擦焊接过程中材料流变行为及调控机制尚未清楚。本项目拟以镁为主的HCP结构材料为对象，借助原位三维可视的X射线实时成像系统来探索搅拌摩擦焊过程中的材料流变行为，深入揭示焊接工艺、母材组织、搅拌头形状对搅拌摩擦焊过程中材料流动速度、流动范围以及受力状况的影响规律，建立材料流变行为、温度分布与焊缝组织性能之间的内在联系，分析搅拌摩擦焊过程中的缺陷形成机理及组织非均匀特性，结合搅拌摩擦焊中材料塑性变形规律阐明HCP结构材料搅拌摩擦焊过程中流变行为调控机理。在此基础上，通过对材料流变行为及调控机制的准确理解，为搅拌头设计，焊接参数优化以及焊缝组织调控提供指导。

通过本项目的研究，深入考察了以镁、锌为代表的HCP结构材料搅拌摩擦焊接过程中材料流动、显微组织演变以及塑性变形机制，阐明了纯镁、纯锌、AZ31、Mg-Gd系稀土镁合金等不同母材种类及显微组织、焊接工艺、导入粉末等因子对搅拌摩擦焊接过程中材料流动以及显微组织演变的影响规律，其研究结果显示c/a、细小导入颗粒、稀土固溶元素、第二相（如长周期堆垛有序结构）等均会影响HCP结构材料搅拌摩擦焊接过程中的材料流动以及塑性变形机制。同时，利用液态CO2冷却等方式，通过考察不同母材搅拌摩擦焊缝组织、织构演变以及力学性能，结合焊接过程中的温度场测定，建立了材料流变行为、温度分布与焊缝组织性能之间的内在联系，分析了搅拌摩擦焊接过程中的缺陷形成机理及组织非均匀特性，为优化HCP结构材料搅拌摩擦焊接工艺及调控焊缝组织，提高焊接质量提供了指导。