基于同步辐射X射线三维高精度原位成像技术,识别和统计出工艺稳定的激光复合焊接7020铝合金接头中的气孔数量、形貌、尺寸和空间分布特征,结合气孔统计数据结果和焊缝晶粒大小,定义了7020铝合金激光复合焊接头中影响接头疲劳性能的气孔临界尺寸。利用同步辐射X射线三维原位疲劳实验数据和疲劳断口形貌,探讨了疲劳试样裂纹源处气孔尺寸、应力和疲劳寿命之间的定量关系。同时,基于有限元仿真分析,研究了不同位置下气孔处的应力场状态。最后,通过疲劳裂纹扩展速率实验,揭示了气孔对疲劳裂纹萌生、扩展和试样瞬断的影响。研究结果表明,激光复合焊接头临界气孔尺寸可定为30 μm;同步辐射X射线成像和疲劳断口显示,较大的表面气孔和近表面的气孔较容易萌生疲劳裂纹。仿真研究也表明,气孔周围的应力集中程度随着气孔位置由表面向内部移动呈现出先增大后减小最后趋于稳定的趋势;疲劳裂纹扩展速率数据分布趋势表明,气孔对长裂纹扩展过程的影响较小,可忽略不计,但一般认为对裂纹前缘形貌有较大影响。

With the rapid development of Chinese high-speed railway system, the urgent demand for lighter weight structures is increasing, and aluminum alloys are widely applied into manufacturing the railway train and critical safety components. As a medium strength aluminum alloy, the 7020 aluminum alloy shows a great potential. Hybrid laser welding has currently become one of the most important welding techniques for medium and high strength aluminum alloys. Nevertheless, intrinsic defects such as pores and shrinkages physically determine the fatigue resistance of the welded joint. Based on in situ synchrotron radiation X-ray computed microtomography (SR-μCT), the population, location and size of gas pores within AA7020 hybrid welded joints are firstly identified and counted. The critical size of gas pores, affecting the fatigue properties of welded joints, is acquired by combining the statistical results of the pores and the average grain size of the hybrid weld. Meanwhile, the qualitative relationship between pore size, effective stress and fatigue life is discussed through in situ fatigue life data via SR-μCT and fracture morphology. By using the finite element analysis, detailed works have been performed on the stress state near the pores of different positions inside the joint. Through the simulation analysis, the stress concentration coefficient around the pores firstly increases, then decreases, and finally tends to a stable trend as the location of the pore-like defect is transferred from the surface to the inside. Besides, the influence of porosity on fatigue crack initiation, fatigue crack growth and sudden breaking process is also analyzed using fatigue crack growth experiment. In conclusion, the results show that the critical pore size of hybrid laser welded joint can be qualitatively identified as 30 μm; the SR-μCT and fracture analysis show that larger surface and sub-surface pores are more likely to initiate fatigue cracks, and the fatigue crack propagation experiment further shows that the porosity has very little effect on the long crack growth but significant influence on the crack front.