变形镁合金搅拌摩擦焊接头的低周疲劳行为及失效机制研究

Wrought Mg alloys joints with high quality can be produced by friction stir welding (FSW). Texture evolution in the weld is the main factors that influence the mechanical properties of the joints. Our experiments showed that although the FSW joints of an as-extruded Mg alloy may exhibit good fatigue resistances at low strain amplitudes, the joints showed apparently lower fatigue resistances at higher strain amplitudes. Meanwhile, the FSW joints did not exhibit the phenomenon of "asymmetric cycle tension-compression deformation behavior" which is relative to the base metal; meanwhile, the phenomenon of "material-extrusion" appeared in the welds of the FSW joints. However the reason for the appearance of the unusual phenomena and their effect are not clear. In this project, AZ31 alloys under as-rolled and as-extruded conditions are to be welded by FSW, respectively, and the low cycle fatigue properties of the FSW joints are to be tested using a fatigue-testing machine. Based on the two unusual phenomena, the low cycle fatigue behavior and its effect mechanism of the FSW joints for wrought magnesium alloys are to be investigated. Microstructures of the joints before and after the fatigue testing are to be analyzed mainly using TEM and EBSD. The effect of texture and twinning on the cyclic deformation behavior of the joints is to be analyzed. The morphologies of fracture surface are to be observed under SEM, and the initiation and propagation modes of fatigue cracks in the joints are to be established. Based on the above analysis, the reason for the appearance of the unusual phenomena and their effect are expected to be elucidated. The key factors and mechanisms that influence the fatigue properties of the FSW joints are expected to be explained. The strengthening methods for the fatigue properties of the FSW joints are to be established. This project would provide an idea for optimizing the FSW process of magnesium alloys; meanwhile, it would provide essential theory and experimental data support for the safe design and reliable service of welded structural members made from magnesium alloys.

搅拌摩擦焊（FSW）可实现变形镁合金的高质量连接，焊缝区织构演变是影响接头力学性能的主要因素。低周疲劳实验表明，中高应变幅下挤压态镁合金FSW接头的疲劳性能明显低于母材；FSW接头不再具有母材的"拉-压不对称循环变形行为"，接头在疲劳过程中出现"材料挤出"现象，其成因还不清楚。本项目拟分别测试挤压态与轧制态AZ31镁合金FSW接头的低周疲劳性能，研究FSW接头的低周疲劳失效特征行为。采用TEM、EBSD等技术对比分析疲劳实验前后FSW接头微观组织的变化情况，分析织构与孪晶对FSW接头循环变形行为的影响。观察疲劳断口形貌，分析裂纹形成与扩展模式。通过上述研究，阐明变形镁合金FSW接头的低周疲劳特征行为的成因，揭示影响FSW接头疲劳失效的主导因素与影响机制，提出强化其疲劳性能的技术路线，为镁合金FSW工艺优化提供思路，为镁合金焊接结构件的安全设计和可靠使用提供理论依据与数据积累。

本项目按照研究计划，设计不同尺寸工具与工艺参数，选取挤压态与轧制态两种AZ31镁合金板材，以及挤压态的Mg-Zn-Y-Zr合金为研究对象，进行搅拌摩擦焊（FSW）研究。通过工艺优化，实现了高强度连接，阐明了搅拌区的微观组织演化情况，并深入研究了接头的低周疲劳性能。结果表明，织构是影响变形镁合金FSW接头强度的主要因素，通过优化焊接工具与工艺参数，可调控FSW接头的织构分布，从而优化力学性能。阐明了FSW接头不再具有母材的“拉—压不对称循环变形行为”以及接头在疲劳过程中出现“材料挤出”现象的根本原因。采用TEM、EBSD等技术对比分析了疲劳实验前后FSW接头微观组织的变化情况，分析了织构与孪晶对FSW接头循环变形行为的影响。通过上述研究，阐明了变形镁合金FSW接头的低周疲劳特征行为主要受搅拌区的织构分布以及变形过程中的孪晶生长控制，提出了通过调控接头织构分布、控制压缩孪晶生产的接头性能强化技术路线，为镁合金FSW工艺优化提供思路。